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[Hironobu Hayashi](https://orcid.org/0000-0002-7872-3052), Hiroko Yamada

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[Exploring the chemistry of higher acenes: from synthesis to applications](https://mdr.nims.go.jp/datasets/4988ae8e-ada2-4de7-9867-76c75eb1df94)

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Exploring the chemistry of higher acenes: from synthesis to applicationsChemicalScienceREVIEWOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueExploring the chHironobu HayashiHPUaUlpaa2I(2pDuring 2021–2025, he was a recurrent research interests includemacrocycles and their applicationaCenter for Basic Research on Materials,(NIMS), 1-2-1 Sengen, Tsukuba, IbarakiHironobu@nims.go.jpbInstitute for Chemical Research, Kyoto UnJapan. E-mail: hyamada@scl.kyoto-u.ac.jpCite this: Chem. Sci., 2025, 16, 11204Received 31st March 2025Accepted 19th May 2025DOI: 10.1039/d5sc02422frsc.li/chemical-science11204 | Chem. Sci., 2025, 16, 11204–1emistry of higher acenes: fromsynthesis to applicationsHironobu Hayashi *a and Hiroko Yamada *bThis review explores the advancements in the chemistry of higher acenes and their derivatives, with a focuson their synthesis, characterization, and potential applications. Historically, higher acenes have presentedchallenges to study due to their inherent instability and reactivity under ambient conditions. However,innovative synthetic strategies, including on-surface synthesis and the precursor approach, havesignificantly contributed to the ability to synthesize higher acenes even at preparative scales whileevaluating their magnetic and semiconducting properties. Furthermore, ethynylene-bridged aceneoligomers and polymers, known for their extended p-conjugated systems, have shown promise not onlyas semiconducting materials but also as topological materials. As synthetic methods continue to evolveand characterization techniques become more sophisticated, higher acenes offer exciting opportunitiesfor progress in the fields of organic chemistry and materials science, paving the way for advancedapplications in organic electronics.1 IntroductionIn recent years, the advancement of organic electronics hasbeen truly remarkable. The unique characteristics of extremelylightweight and ultra-thin organic devices have paved the wayironobu Hayashi received hishD degree in 2012 from Kyotoniversity, Japan. Aer workings a post-doctoral fellow at theniversity of Geneva, Switzer-and (2012–2014), he wasromoted to assistant professort the Nara Institute of Sciencend Technology (NAIST) in014. He moved to the Nationalnstitute for Materials ScienceNIMS) as a senior researcher in023, and was promoted torincipal researcher in 2025.searcher at PRESTO, JST. Histhe synthesis of acene-basedin organic devices.National Institute for Materials Science305-0047, Japan. E-mail: HAYASHI.iversity, Gokasho, Uji, Kyoto 611-0011,1231for the creation of wearable technologies. In these devices,charge transport within organic semiconductors is facilitatedthrough the overlapping of p-orbitals in conjugated moleculesalong the direction of carrier ow. Thus, organic semi-conducting materials possessing a rigid and planar p-systemhold great promise for providing optimal packing that enhancesp-orbital overlap. Due to their extensive and robust p-systems,higher acenes and their derivatives emerge as promisingcandidates for efficient p-type semiconducting materials.1–5Higher acenes, comprising multiple linearly fused benzenerings, represent a signicant category of polycyclic aromaticHiroko YamadaHiroko Yamada received herPhD degree in 1992 from KyotoUniversity. Aer post-doctoralfellowships at the ArgonneNational Laboratory, USA(1993), and Osaka University(1998–2003), she was promotedto associate professor at EhimeUniversity (2003), and moved tothe Nara Institute of Science andTechnology (NAIST) in 2011.She was promoted to fullprofessor at NAIST in 2012 andmoved to the Institute forChemical Research (ICR), Kyoto University in 2023. During 2006–2010, she was a researcher at PRESTO, JST. Her current researchinterests include the synthesis and morphology control of smallmolecular organic electronic materials for solution processing.© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://crossmark.crossref.org/dialog/?doi=10.1039/d5sc02422f&domain=pdf&date_stamp=2025-06-21http://orcid.org/0000-0002-7872-3052http://orcid.org/0000-0002-2138-5902http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fhttps://pubs.rsc.org/en/journals/journal/SChttps://pubs.rsc.org/en/journals/journal/SC?issueid=SC016025Fig. 1 (a) Bottom-gate, top-contact OFET device structure. (b) Imageof an OFET using a single crystal. Fig. 2 Pristine and functionalized pentacenes.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehydrocarbons.6–8 Substantial research efforts have been directedtowards their synthetic development, derivatization, andapplication as organic semiconductors. Organic eld-effecttransistors (OFETs) are preferred for rapid screening of chargecarrier mobility in newly synthesized organic materials due totheir straightforward fabrication and processing. A typical OFETdevice comprises an active layer (either an organic semi-conductor lm or a single crystal), three electrodes (gate,source, and drain), and a gate dielectric (insulator) (Fig. 1). Thesubstrate and gate electrode are typically composed of highlydoped silicon. Silicon oxide (SiO2) is used as the dielectricinsulating layer, formed by the oxidation of the silicon surface.Organic semiconductors or single crystals are directly depositedonto the dielectric layer, while sometimes self-assembledmonolayers are employed to modify the dielectric surface toimprove its surface energy and roughness. In device operationfor p-type molecules, as the gate voltage increases, holes accu-mulate at the interface between the organic semiconductor andthe dielectric insulator, thus allowing the source–drain currentto be modulated by the gate voltage. Charge carrier mobility isdirectly proportional to the channel current.As described above, higher acenes possess extended p-conjugated systems with rigid and planar structures that facil-itate enhanced charge delocalization. In device operation, p-orbital overlap in the active layer is crucial for achieving optimalperformance. The rigid and planar backbone of higher acenessignicantly contributes to efficient packing and p-orbitaloverlap, thereby enhancing device performance. This featuresets them apart from other organic materials by promotingsuperior charge transport properties, which are vital for organicelectronics like OFETs and organic photovoltaics (OPVs).Additionally, the radical character of higher acenes, whichvaries depending on their length, opens up opportunities fortheir application in carbon-based spintronic devices. In termsof differentiation from other materials, higher acenes possesschemical reactivity that enables modications to their back-bone, signicantly inuencing the packing structure on thesubstrate and the processability of device fabrication. Thisadaptability is essential for achieving optimal device charac-teristics and tailor-made applications. Another advantage ofhigher acenes is their ability to achieve high purity materials,a feature that poses challenges for polymer-based organicsemiconductors, which oen suffer from impurities due tomixtures of varying polymer lengths. Pure materials lead tomore uniform electronic properties and improved device© 2025 The Author(s). Published by the Royal Society of Chemistryreliability. Thus, the use of higher acenes in organic electronicdevices offers numerous benets, including enhanced deviceperformance and the potential for solution-processed devicefabrication. Their capacity for ne-tuning through molecularengineering enables the design of materials with specic elec-tronic properties, further enhancing their versatility and appealin advanced applications. For example, pentacene, whichconsists of ve fused benzene rings, was found to function as anOFET when vacuum-deposited into thin lms, exhibiting anexceptional hole mobility of 1.5 cm2 V−1 s−1 at the time,9leading to explosive progress in research. Later, 6,13-ditriiso-propylsilylethynylpentacene (TIPS-Pen),10 developed by Anthonyand colleagues, emerged as a benchmark compound for organicsemiconductor materials (Fig. 2). Unlike pentacene, whichshows almost no solubility in any media, TIPS-Pen's highsolubility—attributed to its TIPS groups—enables solutionprocessing. The innovative fabrication of highly crystallinelms has shown signicantly enhanced hole mobility, reaching11 cm2 V−1 s−1.11Moreover, in recent years, higher acenes and their deriva-tives, including tetracene, pentacene, and hexacene, have beenidentied as efficient materials for exhibiting singlet ssionphenomena.12–19 To thoroughly explore the intricacies of elec-tron transfer dynamics, extensive research has been conductedon the synthesis of higher acene oligomers integrated withvarious substituents and linkers. Advancing these studies isanticipated to substantially boost power conversion efficiency oforganic photovoltaic cells.A dening characteristic of higher acenes is their remarkablynarrow highest occupied molecular orbital (HOMO)–lowestunoccupied molecular orbital (LUMO) gaps.2–4 As the acenechain length increases, the energy gap between the HOMO andLUMO diminishes signicantly, leading to oxidative instability.This unique electronic feature is attributed to their zigzag-edged structure, in contrast to phenacenes—structuralisomers of higher acenes—that exhibit stability againstoxidation.20–26 This disparity stems from differences in elec-tronic congurations between higher acenes and phenacenes,as illustrated in Fig. 3 and 4. According to Clar's aromatic sextetrule, structures that host the greatest number of aromaticsextets (resonating benzene rings) tend to be more stable.Phenacenes can have these aromatic sextets alternately withoutissue (Fig. 4). In contrast, attempts to arrange aromatic sextetsin higher acenes result in unpaired electrons, rendering higheracenes unstable.Chem. Sci., 2025, 16, 11204–11231 | 11205http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 5 Higher acenes, which are deeply intertwined with GNRchemistry.Fig. 3 Acenes and phenacenes.Chemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineThis zigzag-edged structure of higher acenes is stronglycorrelated with the magnetism observed in zig-zag type gra-phene nanoribbons (ZGNRs).27–33 Higher acenes are regarded asthe narrowest ZGNRs, serving as excellent model systems forunderstanding the edge states, chemical reactivity, and elec-tronic structures of ZGNRs (Fig. 5). Moreover, as demonstratedby the surface-assisted synthesis of armchair-type 7-GNRs (7-Fig. 4 Clar's aromatic sextet rule for nonacene and [9]phenacene.11206 | Chem. Sci., 2025, 16, 11204–11231AGNRs) from brominated anthracene dimers,34 higher aceneshave signicant potential as key compounds in the bottom-upsynthesis of AGNRs with narrow bandgaps.35–37 As such,higher acenes are deeply intertwined with the chemistry ofGNRs and nanographenes. GNRs retain the electronic proper-ties of graphene and are anticipated to possess superior chargetransport properties compared to silicon semiconductors,positioning them as promising candidates for next generationsemiconductors. Given these fascinating features of higheracenes, advancing their synthesis and exploring their applica-tions in organic devices have become increasingly crucial.Despite the intriguing electronic properties and structure ofhigher acenes, their stability decreases as the acene lengthincreases. Furthermore, higher acenes exhibit poor solubility incommon organic solvents owing to their uncomplicated struc-tures. These challenges of poor solubility and instability asso-ciated with higher acenes have impeded the acquisition ofexperimental insights. Until recently, characterizing higheracenes, particularly verifying their formation, primarily reliedon measuring absorption spectra under inert conditions atextremely low temperatures or exclusively using massspectrometry.38–41 As a result, a denitive conclusion based onexperimental evidence regarding the magnetism of higheracenes has remained elusive, leading to ongoing controversy formany years. Establishing the link between the length andmagnetism of higher acenes is one of the most signicantchallenges in this research eld, prompting numerous theo-retical studies. In 2007, Hachmann et al. suggested that higheracenes exhibit a singlet polyradical nature in their ground stateusing an ab initio density matrix renormalization group algo-rithm.42 In 2016, Davidson and Yang applied the particle–particle random-phase approximation with the B3LYP func-tional, suggesting that the 1Ag ground states of acenes up todecacene are predominantly closed-shell, while undecaceneand dodecacene display increasing polyradical character,trending towards open-shell states.43 In 2018, Malrieu et al.predicted spin-symmetry breaking in higher acenes beyonda certain length using density-functional theory (DFT), with theresult dependent on the exchange–correlation potential.44Heptacene (N = 7, where N represents the number of fusedbenzene rings) demonstrated the rst symmetry breaking, withClar sextets positioned on the outermost rings on its le and© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 7 Representative examples of thermal precursors for pentacene.Fig. 8 Visible-light-induced photodecarbonylation to generatepentacene.Fig. 6 Predicted spin density of higher acenes. Adapted withpermission from ref. 44. Copyright 2018 American Chemical Society.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineright sides, linked by tetra-methylene anthracene at the core(Fig. 6). Unpaired electrons, partially localized, extend acrossthe central ring, with notable spin density observed up to N =11. For 7 < N < 11, this conguration persists, with unpairedelectrons distributed over extended tetra-methylene acenes. AtN= 13, the acene can accommodate three Clar sextets, with oneon each of the external rings and one on the central ring. In thecase of N = 14, a naphthalene can be drawn in the central part.At N = 15, pentadecacene is expected to exhibit a tetraradicalnature, with a Clar sextet occupying the central ring. Thus,various theoretical studies have been conducted concerning themagnetism of higher acenes, suggesting the need for furtherexperimental investigations.To explore the intrinsic properties of higher acenes, theirreactive nature must be managed. One promising approachinvolves functionalizing higher acenes with stabilizing andprotective groups. As demonstrated by TIPS-Pen, introducingsuitable substituents can effectively enhance the stability,solubility, and processability of higher acenes, making themmore amenable to solution-based processing. Typically, alky-lated silyl ethynyl groups or bulky substituents are introducedto the acene backbone. Specically, in oligomer synthesis,a protection/deprotection strategy is commonly employed incombination with appropriate substituents. Another methodinvolves the “precursor approach”, where entails synthesizinga higher acene precursor equipped with leaving groups that canbe released in situ to generate higher acenes. Stable and solubleprecursors can be transformed into the corresponding higheracenes under optimal conditions. Successful examples includethermally induced reactions, achieving quantitative conversionfrom precursors to higher acenes through simple thermalannealing, yielding only gaseous byproducts if the precursorpurity is adequate (Fig. 7).45–51 Following the initial report ofa thermal precursor for pentacene by Müllen et al.,50,51 severalthermal precursors have been synthesized. The retro-Diels–Alder reactions of pentacene precursors occur quantitativelyand can be executed in solution, powder, or lm form.Remarkably, pentacene lms aer spin-coating and subsequentretro-Diels–Alder reactions through annealing showed OFETperformance comparable to that of amorphous silicon. Chow© 2025 The Author(s). Published by the Royal Society of Chemistryet al. reported monoketone precursors that release a CO mole-cule via annealing and photoirradiation.45,47In addition, the Strating–Zwanenburg reaction,52 where a-diketone groups undergo visible-light-induced photo-decarbonylation to produce higher acenes, exempliesa precursor method. For the photochemical conversion ofa pentacene precursor, Yamada et al. utilized the Strating–Zwanenburg reaction, converting a 6,13-a-diketone precursor ofpentacene to pentacene upon exposure to visible light atapproximately 460 nm, corresponding to the n–p* absorptionof the diketone moiety (Fig. 8).53,54 This conversion is feasiblesolely through photoirradiation, with the precursor beingthermally stable above 300 °C. Together with substitution ofhigher acene backbones, these precursor approaches propelcontemporary higher acene chemistry forward.The challenges in synthesizing higher acenes, and evalu-ating their optical and physical properties can be effectivelyaddressed using these precursors, which are optimized differ-ently for bulk conditions versus “on-surface synthesis”. Thisreview highlights recent advancements in the synthesis andexploration of the electronic structures of higher acenes underbulk conditions and within ultra-high vacuum (UHV) environ-ments on metal surfaces. Given the compelling optical andelectronic properties of higher acenes, it is essential not only toexplore their synthesis and property evaluation but also toemphasize their material applications such as in OFETs.Indeed, higher acenes and their derivatives show promise inorganic device applications. In this review, we present recentexamples of their utilization in organic devices.2 On-surface synthesis of higheracenesIn recent years, there has been increasing interest in studyinghigher acenes through on-surface synthesis. This methodinvolves sublimating precursor molecules of higher acenes ontoChem. Sci., 2025, 16, 11204–11231 | 11207http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 10 (a) STM image of nonacene–Au interaction. (b) Side and topviews of the corresponding DFT equilibrium geometry. (c) Closed-shell Kekulé (left) and one of the many possible non-Kekulé (right) Clarstructures of nonacene. Adapted with permission from ref. 57. Copy-right 2019 Springer Nature.Chemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinea metal surface and transforming them into higher acenesthrough external stimuli, such as annealing. Conducting thesetransformation reactions under UHV conditions effectivelyaddresses the instability issues associated with higher acenes.Moreover, the structure, band gap, magnetism, and otherproperties of the resulting higher acenes can be evaluated at thesingle-molecule level through in situ observations with scanningtunneling microscopy (STM) and non-contact atomic forcemicroscopy (nc-AFM).55,56 Thus, synthesizing suitable higheracene precursors, in conjunction with on-surface synthesis andprobe techniques, is anticipated to provide denitive insightsinto the magnetism of higher acenes, which has predominantlybeen a subject of theoretical prediction until now.Heptacene and nonacene were synthesized from theirrespective photoconvertible precursors using on-surfacesynthesis. Two a-diketone moieties were introduced into thebackbone of heptacene (7DK2) and nonacene (9DK2) (Fig. 9).57The a-diketone precursors offer stability during sublimationand reactivity upon light exposure, which is crucial for on-surface synthesis since molecules must be evaporated ontoa metal surface. Under UHV conditions, photoirradiation withvisible light (470 nm) of 9DK2 on Au(111) resulted in a rod-likestructure, indicating the formation of nonacene via photoin-duced decarbonylation of 9DK2. Heptacene was synthesizedusing a similar approach. Detailed structural analysis wasconducted using STM and nc-AFM with a CO-functionalized tip.In particular, the AFM images clearly revealed the presence ofseven and nine benzene rings, along with the zig-zag edgecarbon atoms with single hydrogen terminations, conrmingthe formation of pristine heptacene and nonacene.Notably, following photoconversion, lateral protrusions wereobserved near each nonacene, attributed to Au adatoms boundto the nonacene backbone (Fig. 10). This interaction between Auand nonacene, where Au atoms primarily occupy the center ofthe molecules, is consistent with the well-known increase inacene reactivity as their length grows, with the central ringsbeing the most reactive. In fact, Au–acene interactions were alsoevident in heptacene, although less frequently (∼60% of themolecular species are bound to Au adatoms compared to ∼95%for nonacene). These Au–nonacene interactions suggest anenhanced open-shell character in nonacenes. Nonacene can berepresented by either a closed-shell Kekulé structure or a non-Fig. 9 (a) Light-induced formation of heptacene and nonacene. STMand nc-AFM observations of (b) heptacene and (c) nonacene. Adaptedwith permission from ref. 57. Copyright 2019 Springer Nature.11208 | Chem. Sci., 2025, 16, 11204–11231Kekulé open-shell structure.58,59 In the non-Kekulé open-shellrepresentation, an additional stabilizing aromatic p-sextet(totaling two p-sextets) is incorporated into the chemicalstructure, accompanied by unpaired electrons. This non-Kekuléopen-shell structure aligns with the observed interactionsbetween the central ring of nonacene and Au adatoms, indi-cating a shi from a purely closed-shell structure to an open-shell conguration. Theoretical calculations, along with exper-imental investigations of the HOMO–LUMO gap of nonacenevia scanning tunnelling spectroscopy (STS), further support thisopen-shell character.The on-surface synthesis of nonacene, together with theexperimental and theoretical evaluation of its open-shell char-acter, has markedly advanced research on higher acenes.Further combined experimental and theoretical investigationsare anticipated to reveal more details about the ground statenature of higher acenes. Therefore, developing precursors thatproduce even longer acenes is necessary. One distinct advan-tage of on-surface synthesis is leveraging the high catalyticactivity of single-crystal metal surfaces.60 Substituents that areunreactive to external stimuli such as heat or light irradiation inbulk or solution states oen undergo elimination reactions onthe Au(111) surface. These substituents, which exhibit elimi-nation reactions on the Au(111) surface, can serve as protectivegroups for unstable, higher acenes.Echavarren et al. developed a valuable synthetic method forproducing hydrogen-protected higher acenes.61–66 The syntheticprocess involves gold(I)-catalyzed cyclization of 1,7-enynes, ob-tained from a palladium-catalyzed Sonogashira cross-couplingbetween an aryl iodide and key precursors, which then formhydrogen-protected higher acenes through aromatization byeliminating a methanol molecule (Fig. 11). This strategy wasapplied to synthesize hydrogen-protected higher acenes fromheptacene to undecacene (Fig. 12). The parent higher acenescould be generated either thermally, by annealing the sample at520 K for 10 minutes, or using a microscope tip. Notably, thedehydrogenation process is highly efficient, achieving nearlya 100% conversion rate. The formation of the series of higher© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 13 (a) Synthesis of the tridecacene precursor and (b) STM imageof on-surface generated tridecacene. (c) Single point STS spectraacquired over tridecacene, exhibiting a symmetric step-like appear-ance (top) together with numerically differentiated d2I/dV2 spectrashowing the presence of pronounced resonances centred atapproximately ±126 meV. Adapted with permission from ref. 63.Copyright 2023 Wiley.Fig. 12 On-surface generation of undecacene. (a) High-resolutionfilled state STM image of undecacene. Laplace-filtered constantheight, frequency shift nc-AFM images of (b) undecacene and (c)kinked acene. (d and e) STSmeasured transport gap for long acenes onAu(111). (d) Dependence of the gap on the number of fused benzenerings with the inverse proportionality fit (red curve) and exponentialdecay (blue curve). (e) Transport gap plotted as a function of theinverse number of rings with the linear fit. Adapted with permissionfrom ref. 62. Copyright 2018 Wiley.Fig. 11 Synthesis of tetrahydroacenes. Adapted with permission fromref. 62. Copyright 2018 Wiley.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineacenes and their denitive chemical structures were conrmedunambiguously by STM and nc-AFM with a CO-functionalizedtip. Moreover, the series provided the opportunity to analyzethe dependence of the STS-measured transport gap on thenumber of benzene rings (Fig. 12). In the range of 5–10 fused© 2025 The Author(s). Published by the Royal Society of Chemistrybenzene units, both inverse proportionality and the exponentialdecay provided good ts. However, undecacene deviatestowards larger gap values, indicating a saturation in furtherband gap reduction for longer higher acenes. This trendbecomes even more evident when the gap is plotted as a func-tion of inversed number of annulated rings. These ndingscould be rationalized by the expected increased contribution ofthe open-shell conguration to the overall electronicstructure.43Recently, Echavarren, Jelinek, and Godlewski et al. extendedthis synthetic strategy to octahydrotridecacene, synthesizedthrough multiple Sonogashira cross-coupling reactions fol-lowed by a nal four-fold gold(I)-catalyzed [4 + 2] cycloaddition(Fig. 13).63 The surface-assisted reaction of this precursor, ach-ieved by annealing at 270 °C for 15 min yielded tridecacene. Theformation of tridecacene was conrmed by STM observation.STS measurements and spatial orbital mapping, combined withtheoretical modeling, indicated that tridecacene possesses anopen-shell ground state. Importantly, an inelastic signal wasdetected, attributed to spin excitation from the singlet diradicalground state to the triplet excited state, with an estimatedsinglet–triplet gap of approximately 126 meV. This workprovides experimental conrmation of tridecacene's magneticChem. Sci., 2025, 16, 11204–11231 | 11209http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 15 (a) On-surface synthesis of undecacene from an etheno-bridged precursor. (b) Overview STM topography image after anneal-ing the sample at 220 °C. (c) Table showing the statistics of the acenespecies found on the substrate after annealing at 220 °C. Adapted withpermission from ref. 71. Copyright 2022 Springer Nature.Chemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinecharacter. Furthermore, the magnitude of the singlet–tripletband gap was found to be inuenced by surface proximity anddynamic electron correlation.Peña et al. introduced a different kind of precursor, speci-cally epoxy-protected higher acenes.67–70 Essentially, Diels–Alderreactions involving OTf and TMS-modied aromatic speciesand benzofuran create epoxy-protected higher acenes. Thisstrategy was initially used to synthesize a tetracene precursor.67Furthermore, repeated Diels–Alder reactions can extend theprotected state of the fused benzene rings, producing decaceneand dodecacene precursors (Fig. 14).69,70 The surface-assisteddeoxygenation of pentaepoxy-protected precursors throughannealing at 220 °C resulted in the formation of dodecacene.High-resolution STM images with a CO-functionalized tiprevealed 12 lobes corresponding to the 12 benzene rings ofdodecacene. STS experiments provided experimental insightsinto the electronic structure of dodecacene. Interestingly, whilea progressive closing of the gap and its stabilization to about1 eV was observed for decacene and undecacene, the energy gapof dodecacene unexpectedly increased again to 1.4 eV. ThisFig. 14 (a) Synthesis of dodecacene. (b) Development of the energygap of acenes as a function of the number of benzene rings. Adaptedwith permission from ref. 70. Copyright 2023 American ChemicalSociety.11210 | Chem. Sci., 2025, 16, 11204–11231reopening of the gap was interpretated as a possible indicationof an increased poly-radical (tetra-radical) character ofdodecacene.44Eimre, Yamada, Fasel, and Pignedoli et al. discovered thatetheno-bridges can serve as protective groups for higheracenes.71 While bis-etheno-bridged undecacene precursors arethermally stable under bulk conditions, the surface-assistedreaction of these precursors on Au(111) effectively cleaves theetheno-bridges, yielding undecacene (Fig. 15). It is likely thatacetylene molecules are removed as leaving groups via retro-Diels–Alder reactions. The on-surface reactions resulted notonly in the formation of undecacene but also a variety of aceneFig. 16 (a) Synthetic strategy for tridecacene. (b) Magnified constant-current STM image of edge-on and stretched-out conformations ofa tridecacene precursor. Reproduced with permission from ref. 73.Copyright 2024 American Chemical Society.© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fReview Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinespecies, ranging from anthracene to decacene, includingkinked acenes. The cleavage of different C(sp3)–C(sp2) bondsexhibited no clear selectivity, due to the similarity between theC(sp2) atoms in the etheno-bridge and those belonging to theacene backbone.Bettinger, Tonner-Zech, and Gottfried et al. employedetheno-bridged precursors to synthesize tridecacene and pen-tadecacene (Fig. 16).72,73 For tridecacene, STM tip manipulationinitiated a three-dimensional conformational transformation ofthe precursor on Au(111), changing it from an edge-on structureto a stretched-out linear conformer. Subsequent repeated tip-assisted carbon–carbon bond dissociations removed theetheno-bridges, resulting in tridecacene. Notably, annealing at490 K induced the edge-on to stretched-out transformation ofthe precursor without eliminating the etheno-bridges. STSmeasurements showed a HOMO–LUMO gap reduction to1.09 eV, consistent with the gap reopening reported for dodec-acene (Fig. 14).70 There was a discrepancy in the band gapbetween tridecacene derived from etheno-bridges versus epoxy-protected precursors. This difference was attributed to varia-tions in the adsorption site of the molecule on Au(111),74inuenced by preparation steps like annealing and cleavageprocesses, along with the strong inuence of the Au(111)surface on the transport gaps,75 altering the ground state elec-tronic conguration.For pentadecacene, the trietheno-bridged pentadecaceneprecursor with a stretched-out conformation was converted topentadecacene through repeating tip treatments applyinga pulse voltage of 2.5–3.0 V (Fig. 17).72 The STS transport gap wasapproximately 1.12 eV, slightly larger than the 1.09 eV forundecacene and tridecacene. Surprisingly, the STS transportgap does not signicantly change with length. Notably, a spinexcitation feature near the Fermi energy, characteristic ofFig. 17 (a) Synthetic strategy for pentadecacene. (b) dI/dV curves forpentadecacene; two resonance peaks are identified as the HOMO andLUMO. (c) Low-bias dI/dV curve (top) and its corresponding d2I/dV2spectrum. Adapted with permission from ref. 72. Copyright 2025American Chemical Society.© 2025 The Author(s). Published by the Royal Society of Chemistrya singlet open-shell electronic structure,76 was observed.Numerically obtained inelastic tunneling spectra (d2I/dV2)determined the singlet–triplet gap to be 124 meV, closelyaligning with tridecacene's 126 meV.73Increasing the sublimation temperature to 750 K inducedthermal cleavage of the etheno bridges during deposition.Importantly, directly generated pentadecacene reacted with Auatoms (Fig. 18). STM images showed a linear product with sixnoticeable edge indentations at the edges at low bias andpronounced protrusions at high bias, similar to adsorbed open-shell carbon nanostructures.57,77,78 The STM tip was found toeliminate the adatoms; interestingly, removing an Au adatomfrom one side le the opposite side unaffected, unlike typicalacene metal complexes where adatoms are removed simulta-neously.57 Removing a single Au adatom leaves an unpaired p-electron, evidenced by the Kondo effect.79,80 The observation ofpentadecacene's spontaneous complexation with up to six Auatoms on the Au(111) surface reveals the high reactivity of thelong acene, indicating substantial polyradical charactercontributing to the ground state of pentadecacene.42–44,81,82As mentioned above, there have been numerous reports onthe ground state and band gap of higher acenes. The use ofvarious computational levels, methods, and basis functions hasoen led to different theoretical predictions for the electronicstates of the same higher acenes. This discrepancy is especiallypronounced for higher acenes that lie at the boundary betweenopen-shell and closed-shell congurations, which may result incompletely different interpretations. Therefore, it is necessaryto reconcile the discrepancies between theoretical predictionsand experimental measurements of the electronic states ofhigher acenes in order to provide clear guidance regarding thevalidity of computational methods. The synthesis of higheracenes through on-surface synthesis and the direct measure-ment of their electronic states and band gaps via STS, asmentioned above, have dramatically improved this situation.The synthesis and elucidation of the polyradical character ofpentadecacene by Bettinger, Tonner-Zech, Gottfried andcolleagues support previous predictions.44 However, it is alsobecoming apparent that the electronic states of higher acenesprepared by surface-assisted reactions need to consider theFig. 18 (a) (Left to right) STM, constant-current nc-AFM, andconstant-height nc-AFM of the 6Au–pentadecacene complex. (b)Constant-height nc-AFM image with the chemical structure. (c) STMand constant-height nc-AFM image of the 5Au–pentadecacenecomplex, which was obtained by removing a single Au atom from the6Au–pentadecacene complex. Reproduced with permission from ref.72. Copyright 2025 American Chemical Society.Chem. Sci., 2025, 16, 11204–11231 | 11211http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 20 Attempts for cyclacene synthesis by (a) Sttodart and (b) Vittal.Chemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineeffects of the substrate, adsorption position, and even thesynthesis method employed. Thus, the continued improvementof computational methods to accurately interpret experimentalndings will become increasingly important.Despite these successful on-surface syntheses of higheracenes and heteroatom-substituted ZGNRs,27 the investigationinto higher heteroacenes has remained largely unexplored.Eimre, Yamada, Fasel, and Pignedoli et al. reported the on-surface synthesis of 6,12,19,25-tetraazaundecacene froma diethano-bridged precursor (Fig. 19).71 Interestingly, anneal-ing the precursor on Au(111) did not yield 6,12,19,25-tetraa-zaundecacene directly; instead, it produced hydrogenatedtetraazaundecacene and its analog with two edge-fused ve-membered rings. The ve-membered ring-fused tetraazaunde-cacene analog presumably forms through bond cleavage at thebridged head position of the bicyclo[2.2.2]octadieno unit, wherethe terminal part of the resulting fragment re-bonds toa neighboring nitrogen atom. These steps facilitate the removalof two protons from the ethano groups via dehydrogenativearomatization. To circumvent such side reactions (hydrogenpassivation and/or ve-membered ring formation), STM tip-induced deprotection with a voltage ramp (−1.0 to −3.0 V)yielded 6,12,19,25-tetraazaundecacene. STS measurementscomplemented by ab initio simulations revealed its consider-able open-shell character on Au(111). Furthermore, the elec-tronegative nitrogen atoms caused a noticeable shi in energylevel alignment compared to the pristine undecacene.The success of on-surface synthesis and structure–propertyanalysis using STM/nc-AFM for these higher acenes hints at theFig. 19 (a) On-surface synthesis of tetraazaundecacene and itsanalogues. (b) Successive STM topography images illustrating thecleavage process. (c) DFT-calculated equilibrium geometry of a clusterof tetraazaundecacene. (d) Experimental constant-height dI/dV mapsand DFT-calculated molecular orbitals of tetraazaundecacene.Adapted with permission from ref. 71. Copyright 2022 Springer Nature.11212 | Chem. Sci., 2025, 16, 11204–11231potential to synthesize cyclacene, a cyclic molecule that remainselusive (Fig. 20). Cyclacene is considered as the simplest zigzag-type carbon nanobelt, rst proposed in 1954.83 Despitenumerous synthesis attempts, success has been hindered by thestrong ring strain, low stability, and high reactivity of cyclacenein solution. While Stoddart and co-workers managed to producea precursor, they could not achieve cyclacene through reductivearomatization.84–86 In 1996, Vittal and co-workers made anotherattempt, and Wang and co-workers detected [8]cyclacenederivatives by mass spectrometry.87 The pursuit of cyclacenesynthesis thus continues to captivate researchers globally.Recently, Peña, Gross, and colleagues explored cyclaceneformation through on-surface synthesis.88 They synthesizedepoxy-protected cyclacene precursors, akin to strategies used forhigher acenes (Fig. 21). The precursors were deposited ona Cu(111) surface, and STM-based tip manipulation was used toremove oxygen atoms from the epoxy groups. Detailed charac-terization using AFM with CO-functionalized tips suggestedthat up to two oxygens per molecule could be removed, formingan oval-shaped diepoxy-[10]cyclacene. Although cyclacene wasnot fully synthesized, experimental results and DFT calculationspredicted that further deoxygenation might be feasible forlarger diepoxycyclacenes, as employing larger macrocyclescould reduce the energy cost for the deoxygenation.Hayashi et al. extended the ethano-protection strategy tosynthesize azacyclacenes.89 A dehydration condensation reac-tion between anthracene tethered with bis-diketone groups andbenzene-1,2,4,5-tetraamine tetrahydrochloride yielded M[3 + 3](Fig. 22). Although the isolated yield of M[3 + 3] was low (0.8%),the simplicity of the 1H NMR spectrum reecting its highlysymmetrical structure, along with HR-MALDI-TOF MS results,clearly indicated M[3 + 3] formation. Despite M[3 + 3] beingunsuitable for sublimation on metal surfaces due to its largeFig. 21 Synthesis of a precursor of [10]cyclacene and its X-ray crystalstructure. Zoomed-in AFM images of two molecules after tip manip-ulation. Adapted with permission from ref. 88. Copyright 2019 Wiley.© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 22 Structure of M[3 + 3] and its HR-MALDI-TOF MS spectrum.Adapted with permission from ref. 89. Copyright 2022 Wiley.Fig. 23 (a) Stepwise photochemical synthesis of undecacene. (b)Absorption spectra obtained after irradiation (450 nm > l > 350 nm) ofthe undecacene precursor in polystyrene at 8 K. (c) Plot of the energymaximum of the p band (HOMO / LUMO) transition energy in theacene series. Adapted with permission from ref. 96. Copyright 2018Wiley.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinemolecular weight, necessitating new deposition methods, thisresult offers a fresh perspective on azacyclacene synthesis.These ndings suggest that carefully designing precursorsand employing appropriate methods are crucial for realizingpristine cyclacenes and azacyclacenes in the near future.Fig. 24 (a) Heptacene and its dimerization reaction. (b) Solid state 13CCP-MAS NMR spectrum. The sample was stored at room temperaturein an ambient atmosphere for 1 month, and then heated at 300 °C for12 min. (c) Absorption spectra of heptacene obtained by heatinga solution of diheptacene in 1-methylnaphthalene. Adapted withpermission from ref. 97. Copyright 2017 American Chemical Society.3 Organic synthesis of higher acenesAs noted earlier, on-surface synthesis is an excellent method forproducing unstable higher acenes and analyzing their molec-ular structure and properties at the single-molecule level.However, large-scale synthesis, which is essential for practicalapplications, remains a signicant challenging, although someGNRs, which were prepared through on-surface synthesis, wereused for transistors.90–93 Thus, it is crucial to leverage insightsgained from on-surface synthesis to develop organic synthesismethods that support large-scale production.Bettinger et al. have made substantial advancements insynthesizing higher acenes using a-diketone precursors.38–41Converting these precursors to higher acenes in either an Armatrix or poly(methyl methacrylate) (PMMA) has been proveneffective. Acenes ranging from hexacene to nonacene had beensynthesized, and their optical properties had been exploredalongside theoretical calculations. A notable feature of acenes istheir distinctive optical spectrum; for acenes up to heptacene,these spectra are characterized by a single very strong transitionin the UV region (1Bb, b band) and weaker transitions extendinginto the visible range (1La, p band).94 The p band arises from anexcited state predominantly involving the HOMO / LUMOtransition, with its energy linked to the optical gap. Both char-acteristic bands shi bathochromically as the system extends.For octacene and nonacene, an additional strong band has beenobserved. DFT/MRCI computations suggest this is due to anexcited state, labeled D2, with signicant double excitationcontributions.94 Theoretical analysis indicates that this stateshis to lower energies faster than the 1Bb and1La states as theacene chain length increases.95Further exploration of the optical properties of higher acenesled to the synthesis of undecacene.96 Here, the precursor wasphoto-converted to undecacene in a polystyrene matrix undercryogenic conditions (8 K) (Fig. 23). Photoirradiation resulted ina rapid decrease in n–p* transition intensity, alongside a strongsignal at 354 nm and weak signals in the 600–800 nm range.These observations align with the b and p bands of heptacene,© 2025 The Author(s). Published by the Royal Society of Chemistryindicating stepwise photodecarbonylation.38 Although completephotoconversion took time, bands with maxima at 543 nm and1007 nm increased, suggesting undecacene generation. Corre-lating with computed data, the strong band at 543 nm wasattributed to the D2 state, while the weak band at 1007 nm waslinked to the 1La state. Plotting the1La state energies against 1/N(N = number of benzene rings) yielded a straight line, esti-mating an optical gap of 1.23 eV at innite chain length in theacene series.In the early days of higher acene chemistry, thesecompounds were considered inherently unstable, posingsignicant challenges for isolating or bulk synthesizing higheracenes under ambient conditions. Higher acenes were noted fortheir tendency to undergo oxidation in solution and theirproclivity toward dimerization (Fig. 24). However, in 2017, it wasChem. Sci., 2025, 16, 11204–11231 | 11213http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 25 Synthetic strategy for higher acenes and higher acene deriv-atives employed in this study. Adapted with permission from ref. 98.Copyright 2019 Wiley.Fig. 26 (a) Synthetic route for nonacene precursors. (b) Thermaldecarbonylation of precursors at 350 °C for 20 min and 13C CP-MASNMP spectra. Adapted with permission from ref. 99. Copyright 2022Springer Nature.Chemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinediscovered that heptacene could remain in a solid state forextended periods, only gradually undergoing dimerization oroligomerization at room temperature.97 When diheptacene washeated in a 1-methylnaphthalene solution, heptacene wasformed. Weak signals at 753, 682, and 623 nm, along witha shoulder at 792 nm on the long-wavelength side of the 753 nmsignal, persisted at 230 °C for at least 40 minutes, indicatingheptacene's unexpected stability.Jancarik et al. introduced a versatile strategy for synthesizinga wide variety of higher acenes, using (1S,4S)-7,7-dimethoxy-2,3,5,6-tetramethylenebicyclo[2.2.1]heptane as a pivotalcompound (Fig. 25).98 This key compound serves as a precursorfor protected higher acenes via Diels–Alder reactions, and thesubsequent cleavage of the ketals leads to CO-bridged inter-mediates. The target acenes can be quantitatively produced byheating in the solid state at approximately 200 °C. This methodenabled the synthesis of heptacene, dibenzopentacene isomers,and benzohexacene as notable examples.This strategy was further advanced to successfully synthesizenonacene in bulk.99 By performing repeated Diels–Alder reac-tions followed by oxidation with DDQ, isomers of ketal-protected nonacenes were obtained (Fig. 26). Cleaving theketals produced bis-monoketone protected nonacenes. Ther-mogravimetric analysis (TGA) showed that complete conversionoccurs below 190 °C in a single step. Remarkably, nonaceneexhibited thermal stability up to nearly 500 °C. Additionally,storing nonacene at room temperature within a glovebox for twomonths did not lead to degradation or dimerization. Thesendings align with previous observations that both bulk hep-tacene and even thin-lm heptacene remain stable for monthsat room temperature in a nitrogen-lled environment. Thisstability potentially paves the way for applications in OFETs andmolecular spintronics.Research ndings in bulk synthesis and on-surface synthesisof higher acenes have demonstrated that these compounds,previously considered unstable, can exhibit relatively long-termstability in oxygen-free or low-oxygen environments. Hayashiet al. discovered that the interior of a single crystal can serve as11214 | Chem. Sci., 2025, 16, 11204–11231an isolated space, protecting higher acenes from externalinuences.100 By irradiating the interior of a single crystal of thephoto-convertible heptacene precursor with a CW laser (470nm), they observed an increase over time in the intensity of theabsorption spectrum at 623, 682, 753, and 792 (shoulder) nm,indicating heptacene formation via decarbonylation of theprecursor (Fig. 27). This result supports the notion that theinterior of the single crystal functions as an isolated space. Thegenerated heptacenes were shielded by precursor molecules oroxidized heptacenes located in the outer regions of the crystal,thereby enabling access to otherwise inaccessible compoundswithout requiring deoxygenated conditions.In higher acene research, the main focus has traditionallybeen on the synthesis and property evaluation of acenes withxed lengths. Conversely, research on polyacenes, which consistof numerous fused benzene rings, has experienced limitedprogress. In 2023, Kitao and Uemura et al. introduced a strategyfor synthesizing exceptionally long polyacenes using metal–organic frameworks (MOFs).101 MOFs are notable for theirstructural diversity, which allows precise control over pore sizeand shape at the molecular level, providing an ideal environ-ment for encapsulating various guest species and controllingtheir assembly structures.102,103© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 27 (a) Photoconversion reaction. (b) X-ray single crystal structureof the precursor and its optical image. (c) Transmission image of thesingle crystals of the precursor before (left), during (middle), and after(right) photoirradiation, and UV/Vis-NIR absorption spectra recordedduring photoirradiation. Adapted with permission from ref. 100.Copyright 2020 Wiley.Fig. 28 (a) Schematic of polyacene synthesis using an MOF. (b) FTIRspectra of tetracene, pentacene, hexacene, heptacene and poly-acenes. (c) The relative peak area of SOLO to QUATRO vibrationmodes (ASOLO:AQUATRO) plotted against the benzene ring number forthe acene series. (d) ESR spectrum of polyacene. No hyperfine splittingwas observed, suggesting the delocalization of spin density along thepolymer chains. (e) Temperature dependence of the magneticsusceptibility c of polyacene. Reproduced with permission from ref.101. Copyright 2023 Springer Nature.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineIn this study, monomers for polyacenes were introduced intothe pores of the MOF via sublimation, forming host–monomercomposites (Fig. 28).104 Polymerization was achieved by heatingthe composites at 250 °C for 24 hours in a sealed glass tube,resulting in the formation of polyacene precursor polymers. Thespatial constraints of the MOF enabled highly regulated poly-coupling reactions within one-dimensional nanochannels,leading to linearly extended polymeric precursors. The subse-quent dehydro–aromatization reaction, performed by heating at300 °C in an air atmosphere, produced bulk polyacenes. FTIRspectroscopy not only provided evidence for the presence ofpolyacenes but also offered quantitative structural information.The out-of-plane aromatic C–H vibrationmodes are classied asSOLO, DUO, TRIO, and QUATRO, based on the number ofadjacent C–H groups.105 Only the SOLO and QUATRO modeswere detected at 900 and 736 cm−1, respectively. Polyacenelength was analyzed using IR spectroscopy, correlating thepeaks of out-of-plane sp2 C–H vibration modes (SOLO andQUATRO) with the number of benzene rings. Combined withsimulated IR analysis, a linear correlation between the relativepeak area of the SOLO to QUATRO modes and the number ofbenzene rings estimated the mean number (±S.D.) of benzenerings in polyacene from two precursor polymers to be 17.8 ± 3.3and 18.6 ± 3.5, respectively. The biradical character of poly-acenes was evaluated using electron spin resonance (ESR) anda superconducting quantum interference device (SQUID). TheESR spectrum of polyacene revealed a signal with a g value of2.003, attributable to a carbon-centered radical.106 Additionally,SQUID data showed a component with a steep decrease in© 2025 The Author(s). Published by the Royal Society of Chemistrymagnetic susceptibility upon cooling from 70 to 20 K, consis-tent with the Bleaney–Bowers equation.107 This magneticbehaviour, characteristic of open-shell singlet biradical mole-cules, indicates the biradical nature of polyacene.108The bulk synthesis of higher acene opens the door for theirapplication in devices such as OFETs and single crystal (SC)FETs. Here, the intrinsic properties of higher acenes, includingtheir p-conjugation length and electronic properties, determinehow effectively they can facilitate charge delocalization andtransport. Additionally, the instability and insolubility of higheracenes must be overcome to enable their application in devices.The stacking mode and morphology of higher acenes is anothervital factor. These factors greatly inuence OFET characteris-tics. The crystalline order and grain size of the semiconductorlayers can affect the degree of charge carrier mobility. Here,SCFETs can unveil the intrinsic charge transport properties oforganic semiconducting materials due to ordered arrangementof molecules, absence of grain boundaries andminimal defects.Chem. Sci., 2025, 16, 11204–11231 | 11215http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 29 X-ray crystallographic analysis of hexacene. (a) ORTEPdrawing depicting two adjacent hexacene molecules. (b) Layeredarrangement of hexacene molecules on the a–b plane. (c) Arrays ofhexacene along the a-axis. SCFETs based on hexacene. (d) Outputcharacteristics. Inset: a crystal across the electrodes (scale bar, 50 mm).(e) Transfer characteristics recorded at VDS = −80 V. (f) Time-dependent decay of performance under ambient conditions (redsquares) and in a nitrogen atmosphere (blue diamonds). Reproducedwith permission from ref. 109. Copyright 2012 Springer Nature.Fig. 30 (a) Synthetic route for brominated hexacene and pristinehexacene. (b) Vacuum-deposited thin-film OFETs based on bromi-nated hexacene. Output (left) and transfer characteristics (right, VDS =−100 V). (c) Vacuum-deposited thin-film OFETs based on hexacene.Output (left) and transfer characteristics (right, VDS = −100 V). AFMimages of thin films of (d) brominated hexacene and (e) pristine hex-acene. Adapted with permission from ref. 110. Copyright 2018 RoyalChemical Society.Chemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineThe hole mobility obtained from SC FETs reects the potentialof the synthesized molecules.SCs of hexacene were obtained by sublimating the mono-ketone precursor, with in situ formation during sublimationmade possible through thermally triggered decarbonylation(Fig. 29).109 Hexacene SC FETs demonstrated a hole mobility of4.28 cm2 V−1 s−1 and maintained functionality for more than 19days. However, the mobility decreased by 32% even ina nitrogen atmosphere, suggesting potential surface oxidationof the single crystal. These results experimentally unveil thehigh charge transport potential of hexacene attributed toeffective p-overlap, while also indicating the need to enhancestability for practical applications.Hole mobility of the thin lm of hexacene prepared byvacuum deposition was evaluated.110 Halide-substituted acenes,such as pentacene, have shown signicantly superior holemobility (>5 cm2 V−1 s−1) compared to the parent acene (1.4 cm2V−1 s−1),111,112 as the bromine substituent improves crystalpacking. The OFET properties of brominated hexacene werecompared to those of parent hexacene (Fig. 30). Themonoketone-type precursor of hexacene was synthesized, andits complete thermal conversion at 230 °C was conrmed byTGA, IR, and solid-state 13C NMR. OFET devices were fabricatedby vacuum sublimation of hexacene/brominated hexacene tocreate thin lms on an 1,1,1,3,3,3-Hexamethyldisilaza-ne(HMDS)/SiO2/Si substrate, followed by the deposition of goldelectrodes on top of the lms. The best hole mobility achievedfor brominated hexacene was 0.83 cm2 V−1 s−1, with an on/offratio of 5.0 × 104 and a threshold of −52 V. In contrast, thebest mobility for parent hexacene was 0.076 cm2 V−1 s−1, withan on/off ratio of 2.4 × 102 and a threshold of −21 V. Notably,the best hole mobility for spin-coated hexacene, prepared from11216 | Chem. Sci., 2025, 16, 11204–11231a different precursor, was 0.035 cm2 V−1 s−1.113 Thus, bromi-nated hexacene exhibited higher hole mobility, and bettercrystalline lms of hexacene were obtained by thermal deposi-tion. AFM conrmed that the smoother surface of brominatedhexacene contributed to reduced energy loss during holetransport between the source and drain electrodes.Heptacene was expected to exhibit comparable or even betterhole mobility than hexacene, due to its potentially smallerreorganization energy. Heptacene was synthesized from thecorresponding monoketone precursor (Fig. 31).114 TGA indi-cated a thermal conversion temperature of 220 °C. Top-contactOFET devices were fabricated by thermal evaporation of hep-tacene on a HMDS/SiO2/Si substrate. Notably, the highest holemobility was 2.2 cm2 V−1 s−1, with an on/off ratio of 5× 103 anda threshold voltage of −56 V. Thus, heptacene thin lmsexhibited higher hole mobility than hexacene. However, highcontact resistance was evident in the output characteristics ofthe OFET devices. Considering the HOMO level of heptacene(−5.03 eV) and the Au electrode (−4.76 eV),115 along with XRD© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 32 (a) Chemical structure of 2,6-diphenylanthracene. Typical (b)transfer and (c) out-put characteristics of the SCFET device based on2,6-diphenylanthracene. (d) Mobility distribution. Reproduced withpermission from ref. 119. Copyright 2015 Spring Nature.Fig. 31 (a) Heptacene generation from its monoketone precursor. (b)Output (left) and transfer characteristics (right, VDS = −100 V) ofheptacene thin films. Adapted with permission from ref. 114. Copyright2021 Wiley.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineanalysis, the presence of a hole-injection barrier and theformation of small crystals in the heptacene thin lm contrib-uted to the high contact resistance. This is because holesinjected from the top-contact Au electrodes nd it more chal-lenging to reach the channel region near the SiO2 gate insulatorsurface due to the presence of smaller crystals. These thin lmscertainly contain grain boundaries. Thus, using a single crystalof heptacene for OFET fabrication could lead to higher mobility,as observed with hexacene.109These results highlight the signicant potential of higheracenes as organic semicondctors. Notably, the observed holemobility of heptacene thin lms, exceeding 2 cm2 V−1 s−1, isa particularly promising value. Here, analyzing the packingstructure in the fabricated devices is crucial, as vacuum-deposited lms inherently contain grain boundaries, whichdiffer fundamentally from SC FETs. Pristine higher acenes havean inherent tendency to aggregate, which can complicate thepreparation of lms with fewer domains through vacuumdeposition. Maximizing the potential of higher acenes neces-sitates providing feedback for molecular design to optimize thepacking structure and morphology on the substrate, therebyfacilitating further renements.4 Organic synthesis of oligoacenederivativesFor the application of acene-based compounds in organicdevices, the overlap of p-electrons between molecules in thecrystal packing is a crucial factor. The crystal packing geometryand orientation of acenes are particularly favorable for use inOFETs. Consequently, controlling factors such as the packingstructure, orientation, and morphology on the substrate,through the introduction of substituents to the molecule isessential. Importantly, substitutions on higher acenes have thepotential to create promising packing structures, due to theaddition of other intermolecular interactions, that cannot beachieved with pristine higher acenes. Furthermore, soluble© 2025 The Author(s). Published by the Royal Society of Chemistrysubstituents impart solution-processability to higher acenes,which is advantageous for practical applications.Anthracene derivatives, known for their rigid and planarstructure coupled with high air stability, serve as efficient p-typesemiconducting materials.116 Pristine anthracene single crystalsexhibit hole mobilities of 0.02 cm2 V−1 s−1.117 Additionally,extending the p-system at the 2,6-positions of anthracenethrough oligomerization or functionalization has yieldedpromising candidates for p-type FETs, with mobility reachingup to 34 cm2 V−1 s−1 (Fig. 32).118,119As described earlier, acenes longer than anthracene, pos-sessing greater conjugation lengths and thus stronger inter-molecular overlaps, also demonstrate efficient charge–transportproperties. Charge carrier mobilities for tetracene-based OFETsrange between 0.4 and 2.4 cm2 V−1 s−1.120,121 Pentacene,a benchmark compound among organic semiconductors,exhibits hole mobilities ranging from 5–40 cm2 V−1 s−1.11,122,123Hexacene single crystals showed hole mobilities of up to 4.28cm2 V−1 s−1.109 To achieve efficient charge transport, p-systemelongation through oligomerization is a promising strategy.Among various approaches for p-extension, the incorporationof triple-bond systems remains a subject of intense researchinterest in the development of organic electronic devices.124–126The introduction of TIPSethynyl groups to the pentacenebackbone enhances not only stability but also solubility, therebyenabling solution processability and high hole mobility.11Furthermore, ethynylene-bridged systems are widely employedin organic electronic materials. The availability of efficientsynthetic protocols for ethynylene bridging facilitates the easymodication of effective p-conjugation lengths by controllingthe number of acene–ethynylene repeating units, yieldingshape-persistent, rod-like structures.In the case of anthracene, its better stability and relativelyeasier chemical modication compared to higher acenes haveled to numerous studies.116 It is noteworthy that oligomeriza-tion at the 9,10-positions results in large dihedral anglesbetween adjacent anthracenes due to signicant sterichindrance, which disrupts p-conjugation within the molecule.Chem. Sci., 2025, 16, 11204–11231 | 11217http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 33 (a) Chemical structure of di-anthrylene-ethynylene. (b) Singlecrystal X-ray structure. (c) Schematic of device fabrication by using the“organic ribbon mask” technique, and SEM images of devices withdifferent channel lengths by using the technique. Adapted withpermission from ref. 127. Copyright 2008 Wiley.Fig. 34 (a) Chemical structure of ethynylene-bridged anthracenetrimers with solubilizing groups. (b) Output (left) and transfer (right)characteristics of the trimer with n-decyl groups. (c) STM image of thetrimer with n-butyl groups at the n-tetradecane/HOPG interface. (d)Proposed packing model of the trimer with n-butyl groups. (e) STMimage of the trimer with n-decyl groups. (f) Proposed model packingof the trimer with n-decyl groups. Adapted with permission from ref.133. Copyright 2010 Royal Chemical Society.Chemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineNonetheless, effective conjugation extension at the 9,10-posi-tions of an anthracene core can be achieved using suitableconjugated spaces such as ethynylene groups. Hu and co-workers synthesized di-anthrylene-ethynylene, where the ethy-nylene bridge between the anthracene units prevents repulsionbetween hydrogen atoms in adjacent anthracene rings, result-ing in a rigid, planar “H”-type molecular structure (Fig. 33).127SC FETs were fabricated using an organic mask ribbontechnique.128–131 The shorter intermolecular distance (b = 6.04Å) of this dimer along the b axis, compared to that of anthra-cene, coupled with its rigid “H”-type conjugation, effectivelyresulted in the best hole mobility of 0.82 cm2 V−1 s−1 with an on/off ratio of 5 × 105.As part of oligomerization efforts, a series of 9,10-ter-anthrylene-ethynylenes were synthesized andcharacterized.132–134 Specically, the introduction of ethynylene-bridges along with solubilizing groups, such as alkyl and alkoxychains, enabled their evaluation as organic semiconductors insolution processed OFET devices (Fig. 34). For examples, theimpact of different alkyl chains (n-butyl, n-octyl, 2-ethyl-hexyl,and n-decyl) on OFET performance was studied.133 It wasdiscovered that 9,10-ter-anthrylene-ethynylene with 2-ethyl-hexyl groups exhibited the best hole mobility (average holemobility: 1.1± 0.1× 10−2 cm2 V−1 s−1 with an on/off ratio of 105and a threshold voltage ranged between −3 V and −10 V), whilethe variant with n-decyl groups demonstrated similar perfor-mance (average hole mobility: 1.2± 0.1× 10−2 cm2 V−1 s−1 withan on/off ratio of 104). It is known that the initial monolayers ofan organic lm signicantly inuence conduction in OFETdevices.135 Therefore, the two-dimensional organization of thesemolecules on the lm substrate was investigated. The mole-cules were spontaneously physiosorbed on both highly orderedpyrolytic graphite (HOPG) and reconstructed Au(111) surfaces.STM images of the self-assembled structures at the liquid–solidinterface revealed that molecules with n-butyl alkyl chains self-assembled into a quasi hexagonal packing on HOPG, whereasthose with longer alkyl chains formed lamellar rows. This11218 | Chem. Sci., 2025, 16, 11204–11231behavior is attributed to the insufficient van der Waals inter-actions between the shorter n-butyl chains to enable efficientinterlocking. In contrast, due to stronger interactions with themolecular cores, the molecules arranged more closely to formlamellar rows on Au(111), irrespective of alkyl chain length;these layers are assumed to be tilted relative to the substrate.Hayashi et al. examined the impact of further oligomeriza-tion on OFET performance.136 Ethynylene-bridged anthraceneoligomers ranging from monomers to tetramers were synthe-sized from anthraquinone. It was observed that the 3merformed both block-shaped and needle-like crystals. In bothcrystal types, the anthracene backbones are densely packedthrough p–p interactions and CH–p interactions (Fig. 35). Inthe block crystal, the three anthracene planes are nearlycoplanar along the linear molecular axis, forming a face-to-facep-stack structure with slip-stacked packing. Conversely, theneedle-like crystals display anthracene units arranged in© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 35 (a) Ethynylene-bridged anthracene trimers and tetramersemployed in this study. (b) Optical microscope image of needlecrystals of the 3mer. (c) Top and side views of the crystal structure ofthe 3mer in needle crystals. (d) Packing structure and transfer integrals.Reproduced with permission from ref. 136. Copyright 2021 Wiley.Fig. 36 (a and b) Experimental out-of-plane XRD and simulatedpowder XRD patterns of (a) 3mer and (b) 4mer single crystals on a Si/SiO2/OTS substrate. (c and e) Transfer characteristics of the device for(c) 3mer and (e) 4mer at a fixed source/drain voltage, VSD = −40 V. (dand f) Output characteristics of the device for (d) 3mer and (f) 4mer.Reproduced with permission from ref. 136. Copyright 2021 Wiley.Fig. 37 Ethynylene-bridged tetracene dimer formation. Adapted withpermission from ref. 144. Copyright 2011 Royal Chemical Society.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinea twisted formation with a torsion angle of around 36°, alsoadopting a face-to-face packing structure. The correspondingtransfer integral is calculated to be 18.8 meV, with intercolumnvalues between anthracene units being 5.2 meV and 7.8 meV.The 4mer consistently formed needle-like crystals; althougha twisted structure was observed, the crystal quality was insuf-cient to obtain detailed information.The charge transport properties were evaluated using SCFETs. Crystals of the 3mer and 4mer were grown by drop-casting. The XRD patterns of the 3mer and 4mer crystals onthe substrate suggested that the molecular orientations werefavorable for SC FETs, with charge transport occurring throughthe p–p stacking of anthracene units (Fig. 36). Gold source anddrain electrodes were deposited on a needle-type crystal byusing the “gold layer glue technique”.137–143 The 3mer showeda hole mobility of up to 0.14 cm2 V−1 s−1 with an on/off ratio of5.6 × 105 and a threshold voltage of −0.34 V, surpassing thanthat of the 4mer (3.3 × 10−2 cm2 V−1 s−1 with an on/off ratio of4.4 × 105 and a threshold voltage of −6.3 V) (Fig. 35). Thus, the3mer crystals exhibited slightly higher hole mobility than the4mer, despite similarities in packing structure. This superiorperformance is likely related to the reduced number of defectsin 3mer crystals, as 3mer achieves a favorable balance betweeneffective p-conjugation and defect-less crystal formation,compared to the freer rotation of ethynylene units in the 4mer.Considering p-conjugation and effective molecular packing,p-extension—specically using tetracene oligomers instead ofanthracene—appears to be well-suited for OFET applications.However, the instability of higher acenes oen requires multi-step synthesis involving protection/deprotection processes.Barlier et al. discovered a one-pot synthesis of ethynylene-bridged tetracene dimers from 5,12-naphthacenequinone© 2025 The Author(s). Published by the Royal Society of Chemistry(Fig. 37).144 The yields of monomers and dimers were highlydependent on the pH during reductive aromatization, asdeprotection of the silyl group, competing with hydrolysis/reduction steps in a kinetically controlled process, promoteddimer formation.This dimerization reaction was modied to synthesize anethynylene-bridged pentacene dimer.145 The key step involvedChem. Sci., 2025, 16, 11204–11231 | 11219http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 38 (a) Synthesis of ethynylene-bridged pentacene dimers. (b)Optical microscope image of dip-coated dimer films on a Si/SiO2substrate. (c) Out-of-plane XRD patterns of dip-coated dimer films. (d)Representative transfer characteristics of the device at a fixed VSD =−60 V. (e) Representative output characteristics of the device.Reproduced with permission from ref. 145. Copyright 2018 Wiley.Fig. 39 (a) Pentacene oligomers employed in this study. Protectedbuilding blocks are also shown. (b) Absorption spectra of oligomers.R]Si(nC6H13)3. Adapted with permission from ref. 147. Copyright 2012Wiley.Chemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinesuccessfully isolating the soluble and stable pentacene tetraolas an intermediate, providing additional insights into thedimerization mechanism (Fig. 38). Subsequent reductivearomatization of the tetraol with SnCl2 in 3 M HClaq. yielded anethynylene-bridged pentacene dimer as deep-green crystals. X-ray single crystal analysis revealed that two pentacene moie-ties in the dimer exhibit a nearly coplanar arrangement. Eachmolecule effectively interacts with four different neighboringmolecules, contributing to efficient charge transport throughtwo-dimensional electronic interactions in the solid state.Transfer integrals between the HOMOs of stacked pentacenemoieties were calculated to be 46.0 and 53.6 meV. To evaluatecharge transport properties, crystalline lms were obtained bya dip-coated method with a mixture of CS2/hexane (5 : 1, v/v) ata pull rate of 1.2 mm min−1 on Si/SiO2 substrates. Out-of-planeXRD analysis indicated edge-on packing of the dimer on thesubstrate. Hole mobility was measured in a bottom-gate, top-contact FET, exhibiting a value of up to 0.24 ± 0.05 cm2 V−1s−1, comparable to the value obtained from single-crystal fourprobe measurements.Further oligomerization of pentacene is expected to enhanceelectronic communication within the packing structure,improving charge transport properties. Although nucleophilicattacks on pentacene quinones followed by reduction isa straightforward approach for oligomerization, this method ischallenging due to pentacene's reactive nature. Specically,since TIPS-Pen easily decomposes aer desilylation,146protection/deprotection strategies have been employed tosynthesize pentacene oligomers. Lehnherr et al. reported thesynthesis of conjugated pentacene oligomers up to the tetramerusing protected building blocks (Fig. 39).147 As the oligomer11220 | Chem. Sci., 2025, 16, 11204–11231length increases, a corresponding red shi in the lmax isobserved, indicating enhanced conjugation. Notably, trimersand tetramers exhibit remarkably low band gaps comparable tothose of ethynylated heptacenes (ca. 1.3–1.4 eV).148–150 Noemissions were observed for these oligomers. X-ray crystallo-graphic analysis indicated that two pentacene moieties of thedimer adopt a pseudocoplanar arrangement featuring a two-dimensional slipped stacked conguration, forming a stair-case arrangement along the crystallographic b axis. Cofacial p-stacking occurs within the staircase arrangement along theb axis and between adjacent staircases along the crystallo-graphic a axis. This two-dimensional arrangement, coupledwith the ability of the butadiynyl unit to facilitate electroniccommunication, offers potential for three-dimensional elec-tronic interactions in the solid state.Thermal precursor approaches were explored to synthesizepentacene oligomers, circumventing the reactive nature ofpentacene. It was discovered that oligomer precursors could besynthesized from BCOD-fused anthraquinone in a one-potmethod, eliminating several steps, although the formation ofthe dimer was conceivable as seen in the previous report(Fig. 40).151 Notably, the achievable yield of the dimer via one-pot synthesis signicantly reduces labour-intensive multi-stepprocedures required for dimer preparation. The obtaineddimer serves as a crucial building block for further oligomeri-zation, with bulky BCOD moieties enhancing solubility, whichcould be an important factor in this oligomerization. Despitebeing a common strategy for synthesizing ethynylene-bridgedacene oligomers, this specic oligomerization beyond dimersfrom acenequinone had not been previously reported. Thisunexpected oligomerization, revealed through conventionalreactions, provides valuable insights for the efficient synthesisof ethynylene-bridged compounds. Finally, the pentaceneprecursor oligomers were converted to the corresponding pen-tacene oligomers by heating (Fig. 41). The bulk powders of© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 40 Unexpected oligomerization. Reaction yields depended onthe lithiation time for TIPS ethynylene, and the yield corresponding toa 0.5-h-lithiation period is shown.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineBCOD3mer and BCOD4mer were heated at 350 °C and 280 °C,respectively, for 2 hours under vacuum conditions. HR-MALDI-TOF-MS of the annealed solids showed parent ion peaks ofFig. 41 (a) Thermal conversion. (b) Representative normalizedabsorption spectra of BCDO3mer and BCOD4mer before and afterheating in diphenyl ether. Heating conditions: 300 °C, 3 minutes forBCOD3mer, and 260 °C, 5 minutes and then 280 °C, 90 seconds forBCOD4mer under a N2 atmosphere. Adapted with permission fromref. 151. Copyright 2024 Wiley.© 2025 The Author(s). Published by the Royal Society of ChemistryPen3mer and Pen4mer. Solutions of BCOD3mer or BCOD4merin diphenyl ether were subjected to high temperatures viamicrowave irradiations under a N2 atmosphere. BCOD3mer wastransformed into Pen3mer through retro-Diels–Alder reactionsin diphenyl ether solution at 300 °C, and heating at 260–280 °Cconverted BCOD4mer to Pen4mer. The maximum absorptionwavelengths of Pen3mer and Pen4mer were around 770–800 nm, with Pen4mer exhibiting a slight red-shi compared toPen3mer, suggesting that the effective p-conjugation length isessentially achieved at the Pen4mer stage. Considering thatcrystalline dip-coated lms of ethynylene-bridged pentacenedimers exhibited a hole mobility of 0.24 cm2 V−1 s−1,145 animproved charge transport performance is highly expected forPen3mer.Incorporating heteroatoms into higher acenes signicantlyenhances their stability and oen modies their optical andphysical properties, as well as their molecular packing in thesolid state.5,152–158 Among the doping options, nitrogen-dopedhigher acenes can act as p-type, n-type, or ambipolar organicsemiconductors, depending on their structural conguration.Additionally, nitrogen-doped higher acenes were applied notonly in OFETs but also in OPVs and organic light-emittingdiodes (OLEDs).152–162 Here, again, silylethynylene groups oenplay a crucial role in creating soluble and stable organic semi-conductors. Notably, silylethynylated nitrogen-doped pentacene(TIPS-TAP), where nitrogen atoms are incorporated into middlerings of the pentacene structure, has shown exceptionalpromise as a n-type organic semiconductor (Fig. 42).163–168Initially, vacuum-deposited thin lms of TIPS-TAP demon-strated n-type semiconductor behavior with eld-effect mobil-ities ranging from 1.0 to 3.3 cm2 V−1 s−1.166,167 Then, solution-Fig. 42 (a) Molecular structure of TIPS-TAP. (b) Transfer characteris-tics of the device for a thin-film transistor with TIPS-TAP at a fixed VSD= −60 V. (c) Molecular structure of 4Cl-TAP. (d) Transfer character-istics of the device for a thin-film transistor of 4Cl-TAPwith the highestfield effect mobility. Adapted with permission from ref. 166 for (b), andref. 165 for (d). Copyrights 2011 Wiley for (b) and 2018 Wiley for (d).Chem. Sci., 2025, 16, 11204–11231 | 11221http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fChemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineprocessed TIPS-TAP thin lms achieved an electron mobility of7.6 ± 1.6 cm2 V−1 s−1 with a maximum value of 11 cm2 V−1s−1,164 whereas single crystal arrays displayed an electronmobility of 8.0 ± 2.2 cm2 V−1 s−1 with a maximum value of 13.3cm2 V−1 s−1.163 The performance was signicantly boosted bymolecular engineering of TIPS-TAP through halogenation,optimizing the frontier molecular orbitals, molecular vibra-tions, and p–p stacking interactions. Solution-processed n-channel thin-lm transistors using 4Cl-TAP, prepared via dip-coating, attained electron mobilities up to 27.8 cm2 V−1 s−1.165The remarkably high electron mobility of 4Cl-TAP is attributedto the reduced reorganization energy and enhanced electrontransfer integral, a result of modifying TIPS-TAP with fourchlorine substituents.Similar to higher acenes composed solely of carbon andhydrogen, efforts have been made to synthesize longer acenesfeaturing extended p-conjugation in nitrogen-doped deriva-tives. Stability and solubility continue to be crucial concerns,although the introduction of nitrogen atoms enhances stability.Fig. 44 Synthesis of molecular GNRs.Fig. 43 (a) Representative nitrogen-doped higher acenes with TIP-Sethynyl groups. The heptacene derivative (Hep) is also shown asa reference. (b) Time-dependent evolution of UV-vis absorptionspectra of DAH1 (10−5 mol L−1 in dry CH2Cl2) under ambient light andatmospheric conditions. Adapted with permission from ref. 172.Copyrights 2024 American Chemical Society.11222 | Chem. Sci., 2025, 16, 11204–11231Consequently, before synthesizing pristine nitrogen-dopedhigher acenes,89 research has prioritized those with silyle-thynyl groups (Fig. 43), which, as observed in carbon-basedFig. 45 (a) UV-vis absorption and (c) fluorescence spectra of molec-ular GNRs in CHCl3. (b) 5 mM solutions of GNRs in CHCl3 under naturallight and (d) UV light. (e) Cyclic voltammograms of the GNRs in n-Bu4NPF6/CH2Cl2. (f) Time-resolved THz photoconductivity (Ds) ofdifferent GNRs normalized to absorbed photon density followingresonant excitation. Adapted with permission from ref. 184. Copyrights2023 Cell Press.© 2025 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fTable1SummaryofchargetransportpropertiesofhigheracenederivativesintroducedinthisreviewandrepresentativeorganicsemiconductorsexhibitinghighmobilityMolecularstructureMaximummobility(cm2V−1s−1)ActivelayerRef.MolecularstructureMaximummobility(cm2V−1s−1)ActivelayerRef.2.75(p-type)Thinlm19314.7(p-type)Thinlm19416(p-type)Thinlm19521(p-type)Thinlm19618(p-type)SCa1972.2(p-type)SCa1374.1(p-type)Thinlm1980.02(p-type)SCa11734(p-type)SCaa118and1190.82(p-type)SCa1211.1–1.2×10−2(average)(p-channel)Thinlm1330.14(p-type)Thinlm1360.24(p-type)Thinlm1451.5(p-type)Thinlm911(p-type)Thinlm114.28(p-type)SCa1090.83(p-type)Thinlm1102.2(p-type)Thinlm114© 2025 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2025, 16, 11204–11231 | 11223Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fTable1(Contd.)MolecularstructureMaximummobility(cm2V−1s−1)ActivelayerRef.MolecularstructureMaximummobility(cm2V−1s−1)ActivelayerRef.0.038(p-type)0.023(n-type)Thinlm1723.5(n-type)Thinlm19812.6(n-type)SCa1998.6(n-type)SCa2003.0(typicalvalue)(n-type)Thinlm20113.3(n-type)Thinlm16327.8(n-type)Thinlm1650.042(n-type)Thinlm172149�32(estimatedbyopticalpump-THzprobespectroscopy)Thinlm184aSinglecrystal.11224 | Chem. Sci., 2025, 16, 11204–11231 © 2025 The Author(s). Published by the Royal Society of ChemistryChemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fFig. 46 (a) Schematic evolution of the bandgap of acene monomersand polymers with increasing size of acene monomers. The latter caseshows a phase transition between trivial and non-trivial topologicalclasses accompanied by transformation of thep-conjugation. (b) Banddiagrams for ethynylene-linked acene polymers obtained from tight-binding calculations. (c) Chemical sketch of the resulting pentacene-based polymer. (d) Experimental determination of Egap with STS. (e) nc-AFM image of the pentacene-based polymer. (f) Constant-height nc-AFM (top) and STM (bottom) images of a H-terminated pentacenepolymer. (g) STS spectra along the termination at the positionsdepicted by purple dots in (f). Adapted with permission from ref. 202.Review Chemical ScienceOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehigher acenes, effectively enhance stability and solubility.169While stabilizing higher acenes with a single pair of bulkysilylethynyl substituents is feasible, this approach is less suit-able for diazaacenes, wherein the pyrazine units occupy thecentral rings. Higher diazaacenes oen undergo dimerizationthrough [4 + 4] cycloaddition170 at the rings adjacent to thepyrazine units.171 Acknowledging the electronical stabilizationprovided by alkyne groups against cycloaddition type reactions,Bunz et al. developed a stabilization strategy for higher azaa-cenes by incorporating four silylethynyl substituents. WhileTIPS groups are not positioned particularly close to the acenecore, they contribute signicantly to packing and solu-bility.171,172 Intriguingly, the stability of azaheptacenes varieswith the number of nitrogen atoms and TIPSethynyls' place-ment. UV-vis absorption spectra revealed specic p-band char-acteristics for DAH1, DAH2, TAH, and HAH with absorptiononsets at 1074, 1064, 1152, and 1045 nm, respectively, demon-strating a red-shi relative to Hep173 (absorption onset: 950 nm).Further investigation using NMR and UV-vis spectroscopyassessed the impact of nitrogen atom loading on stability, withthe following stability trend DAH1 > TAH > HAHz DAH2 > Hep> DAH3. Remarkably, solutions of DAH1 remained unchangedaer 7 days under ambient conditions, suggesting that theirsubstitution patterns effectively prevent endoperoxide forma-tion, dimerization, and Diels–Alder reactions. These ndingsindicate the importance of precise placement of silylethynylsubstituents and the number of pyrazine units. Bottom-gate/top-contact OFETs based on DAH1 and TAH exhibited thebest electron mobilities of 0.042 and 0.0031 cm2 V−1 s−1,respectively. DAH2 and Hep exhibited ambipolar transportcharacteristics, with DAH2 achieving an electron mobility of0.005 cm2 V−1 s−1 and a hole mobility of 0.0017 cm2 V−1 s−1,while Hep showed an electronmobility of 0.023 cm2 V−1 s−1 anda hole mobility of 0.038 cm2 V−1 s−1. Taking advantage of theirstability, further enhancements in thin lm quality are likely toelevate charge transport properties.From the perspective of enhancing stability, annulationproves to be an effective strategy. By incorporating multipleClar-sextets through sandwiching azaacenes between aromaticunits such as pyrene and coronene,174–178 the stability isimproved owning to the cross-conjugated character of theannulated unit. In addition, benzannulation on both sides ofhigher azaacenes179 also contribute effectively to structuralstabilization.Mateo-Alonso and co-workers have demonstrated an elegantstrategy for synthesizing molecular GNRs containing annulatedazaacenes.180–186 These molecular GNRs are prepared throughthe formation of pyrazine rings via reaction between o-diaminesand o-quinones, providing an efficient means of interconnect-ing building blocks (Fig. 44). With careful selection and place-ment of solubilizing groups that ensure both good solubilityand stability, column chromatography can be used for puri-cation. Remarkably, the longest molecular GNR (NR-147-Q)displayed an atomically precise core containing 920 sp2 atomswith a 35.8 nm long backbone (comprising 147 linearly fusedrings).© 2025 The Author(s). Published by the Royal Society of ChemistryThanks to their high solubility (NR-147-Q: ∼20 mg ml−1 inCHCl3) and stability, detailed structural characterization of themolecular GNRs was possible using NMR, along with opto-electronic and redox characterization (Fig. 45). NR-147-Qshowed a molar absorption coefficient of 1 845 900 M−1 cm−1.Intriguingly, it also displayed uorescence despite its largedimensions, and demonstrated a record uorescence bright-ness value of 250 500 M−1 cm−1, approximately 1 order ofmagnitude higher than that of carbon quantum dots andcomparable to that of state-of-the-art inorganic quantumdots.187 The redox properties of NR-147-Q were determined bycyclic voltammetry, estimating the electrochemical LUMO levelat −3.84 eV, with no oxidation observed in the solvent-supported electrolyte window. For assessing charge carriermobility, optical pump-THz probe spectroscopy was employed.The maximum value of time-resolved THz photoconductivity,normalized to the absorbed photon density (Ds/Nabs), in themolecular GNR thin lms increased with the length of theGNRs, indicating enhanced charge charrier mobility in longerCopyright 2020 Spring Nature.Chem. Sci., 2025, 16, 11204–11231 | 11225http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5sc02422fChemical Science ReviewOpen Access Article. Published on 04 June 2025. Downloaded on 8/18/2025 8:32:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinemolecular GNRs. The intrinsic charge carrier mobility value ofNR-147-Q was estimated to be ∼149 ± 32 cm2 V−1 s−1, consis-tent with those observed in other type of GNRs.188–192As discussed, higher acenes have shown remarkable poten-tial in various applications. Here, we present a table summa-rizing the transistor characteristics of higher acenes, azaacene,and other major organic molecules (Table 1). While mobilityvaries depending on the fabrication method, it is evident thatsome higher acenes exhibit not only high hole mobility but alsoexceptionally high electron mobility. By incorporating suitablesubstituents and optimizing transistor fabrication processes, aswell as improving thin lm and single crystal preparation, evenhigher mobility is expected in the future.5 ConclusionsThe synthesis and evaluation of electronic properties of higheracenes have been compelling research targets because of theirextended p-systems and radical characteristics. Over the pasttwo decades, the combination of on-surface synthesis andadvanced observation techniques such as STM and nc-AFM hasenabled the synthesis of higher acenes with various lengths,providing insights into their electronic properties. Theprecursor method for preparing higher acenes has beeninstrumental in advancing synthesis, while the development ofpristine, functionalized, and heteroatom-doped higher aceneshas expanded their applications in organic devices, includingOFETs, OPVs, and OLEDs. Some higher acene derivatives havedemonstrated exceptional charge transport properties, func-tioning as both p-type and n-type organic semiconductors.Effective stabilization strategies, combined with solubilizinggroups as discussed, signicantly contribute to the fabricationof solution-processed device. The next challenge involvesdeveloping strategies for producing large-scale crystalline lmswith minimal domains and grain boundaries by solution-pro-cessing, a crucial requirement for high-end practicalapplications.Looking ahead, higher acenes are prepared to increasinglyinteract with nanocarbon chemistry. As demonstrated in thisreview, higher acenes, with their rigid structures and extendedp-conjugation, serve as ideal building blocks for nanocarbonmaterials such as GNRs, carbon nanotubes, and giant ladder-type polymers. Exploring the properties of higher acenesbased on their length will deepen our understanding of the edgestates in ZGNRs and could lead to the discovery of interestingnanocarbon materials suitable for applications in semi-conducting and/or spintronics devices. For example,ethynylene-bridged higher acene oligomers and polymers arebroadening their application scope. Recent studies highlightthe signicance of ethynylene-bridged acene systems not onlyas semiconducting materials but also as topological materials,due to their ability to transition between two resonant struc-tures by tuning p-conjugation.202–204 Ecija et al. developeda strategy to synthesize polymers with engineered topologies byadjusting their p-conjugation (Fig. 46).202 The polymer featurestwo resonant congurations, where altering the conjugation ofthe central acene rings from a quinoid to an aromatic structure11226 | Chem. Sci., 2025, 16, 11204–11231simultaneously transforms the cumulene into an ethynylenebridge. These conjugation forms correlate with different topo-logical classes, with transitions depending on the size of therepeating unit. It is fascinating to note how Peierls distortionsand Clar's sextet formalism work synergistically, leading to anincrease in Clar's sextets in the cumulenic form as the bondlocalization decreases. In pentacene polymers, each pentacenecan host a single aromatic ring in a Kekulé representation.Alternatively, a second Clar sextet can form, leaving two elec-trons unpaired, thereby imparting the molecule with partialopen-shell character. The unpaired p-electrons from quinoidrings are likely to be shared along the polymeric chain, favoringthe cumulene over the ethynylene bridge—a competitionmaking the topological inversion point. Analogous to poly-acetylene, a p-bond is observed at the polymer's end in thequinoid–cumulene structure, leaving an unpaired electron,unlike the aromatic-ethynylene structure, which highlightstheir topologically non-trivial and trivial characters, respec-tively. These results indicate that higher acenes are poised toplay a signicant role in the eld of quantum materials.Thus, while higher acenes possess a simple structure, theyexhibit extraordinary potential. The history of higher aceneresearch is long-standing, but recent advancements combiningstate-of-the-art synthetic205 and observational techniques,theoretical calculations, and device fabrication are pushing theeld forward. The molecular structures of higher acenes ob-tained to date, as well as the resulting crystal and packingstructures, are extensive. Therefore, the integration of machinelearning is expected to accelerate the search for more optimalmolecular structures in the future.206,207 In addition, theongoing interaction with nanocarbon chemistry and deviceoptimization heralds a new era of research on higher acenes,with much more to discover and develop.Data availabilityNo primary research results, soware or code have beenincluded, and no new data were generated or analysed as part ofthis review.Author contributionsHH and HY contributed to this article through conceptualiza-tion and writing.Conflicts of interestThere are no conicts to declare.AcknowledgementsThis work was partly supported by JST PRESTO grant no.JPMJPR21AC (HH) and JSPS KAKENHI grant no. JP24K01576(HH), JP20H05833 (HY), and JP25K01751 (HY).© 2025 The Author(s). 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