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[Hironobu Hayashi](https://orcid.org/0000-0002-7872-3052), Noburu Tsunoda, Shoma Kasahara, Chie Negoro, Yee Seng Chan, [Naoki Aratani](https://orcid.org/0000-0002-3181-6526), [Hiroko Yamada](https://orcid.org/0000-0002-2138-5902)

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[Photochemical Synthesis of 2,6‐Linked Anthracene Oligomers without Introducing Extra Substituents](https://mdr.nims.go.jp/datasets/146af61c-e8a2-4991-9476-81bcd9bee946)

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Photochemical Synthesis of 2,6‐Linked Anthracene Oligomers without Introducing Extra Substituentswww.eurjoc.orgPhotochemical Synthesis of 2,6-Linked AnthraceneOligomers without Introducing Extra SubstituentsHironobu Hayashi,* Noburu Tsunoda, Shoma Kasahara, Chie Negoro, Yee Seng Chan,Naoki Aratani, and Hiroko Yamada*Photoconvertible precursors of 2,6-linked anthracene oligomers(trimer, tetramer, and pentamer) are synthesized through repeatedSuzuki–Miyaura cross-coupling reactions. Upon exposure of theseprecursors to light at 450 nm, which corresponds to the n–π*transition of diketone moieties, yellow precipitates are formed,suggesting the conversion to the corresponding anthraceneoligomers from the precursors. The disappearance of 1H nuclearmagnetic resonance peaks after photoirradiation indicates theformation of anthracene oligomers with less solubility. High-resolution (HR)mass spectrometry evidently indicates the oligomerformations, and infrared (IR) spectral analysis for thin films showthe disappearance of peaks originating from the carbonyl groups,also supporting the conversion. The absorption spectra afterphotoirradiation indicate a red shift in the absorption peaksaccompanying oligomer formations, suggesting an extension ofthe π-conjugated system. The low solubility of the resulting anthra-cene oligomers in organic solvents highlights the effectiveness ofthis synthetic strategy using the photoconvertible precursor. Thisstudy provides a practical method to synthesize acene oligomerswithout the introduction of extra substituents.1. IntroductionAnthracene has been widely used as an organic semiconductor,[1–5]fluorescence material,[6] and molecular building block[7–13] due toits rigid and planar structure with high air stability. Compared tolinear [n]acenes,[14–18] anthracene and its derivatives possess anappropriate highest occupied molecular orbital level and bettersolubility. More importantly, the ease of chemical modificationto tune the packing structure has facilitated the derivatizationsand functionalizations of anthracene, resulting in several effi-cient p-type semiconductors.[1,4,5] Pristine anthracene singlecrystal exhibits hole mobilities of 0.02 cm2 V�1 s�1.[19] Specifically,π-elongation at 2,6-positions of anthracene, achieved througholigomerization or functionalization, presents a promising strategyfor preparing p-type organic field-effect transistor (OFET) materialsdue to the most extended π-conjugation and highest planarity.For instance, 2,6-diphenylanthracene exhibited an excellent holemobility of 34 cm2 V�1 s�1.[4,5] Its simple molecular structure withno additional solubilizing groups facilitates effective molecularpacking leading to high hole mobility. Therefore, utilizinganthracene oligomers as a planar backbone could potentiallyimprove π–π interactions in the packing, resulting in superior OFETproperties. However, π-extension of anthracene at the 2,6-positionswithout introducing extra solubilizing groups poses a challenge.Indeed, 2,6-dianthracenylanthracene (3mer) and even those withhexyl groups at the edges of anthracene exhibited extremely lowsolubility in organic media, although they demonstrated compa-rable hole mobility (0.3 cm2 V�1 s�1) to amorphous silicon.[20]In this study, we employed the photochemical precursormethod to synthesize 2,6-linked anthracene oligomers, includingtrimer (3mer), tetramer (4mer), and pentamer (5mer), withoutany functional groups (Figure 1). In particular, 4mer and 5merare among the longest 2,6-linked anthracene oligomers withoutsolubilizing groups. Due to their low solubility, conventionalsynthesis methods are not applicable, and if the synthesis waspossible, the purification would remain challenging.Photochemical precursor method utilizes the Strating–Zwanenburg reaction,[21] whereby α-diketone groups undergovisible-light-induced photodecarbonylation at the final stage togenerate anthracenes (Figure 2). The irradiation of α-diketone-type precursors at the n–π* absorption (≈450–500 nm) leads tothe release of two molecules of CO, enabling the quantitativepreparation of the corresponding acenes in solutions orfilms.[14,22–25] Therefore, purification of the resulting products isunnecessary if the purity of the precursor is sufficient, as thereaction produces only gaseous byproducts. In addition, photo-chemical precursor molecules are often more soluble and stablethan their resultant products.H. Hayashi, C. NegoroCenter for Basic Research on MaterialsNational Institute for Materials Science (NIMS)1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, JapanE-mail: HAYASHI.Hironobu@nims.go.jpN. Tsunoda, Y. S. Chan, N. ArataniDivision of Materials ScienceNara Institute of Science and Technology (NAIST)8916-5 Takayama-cho, Ikoma, Nara 630-0192, JapanS. Kasahara, H. YamadaInstitute for Chemical ResearchKyoto UniversityGokasho, Uji, Kyoto 611-0011, JapanE-mail: hyamada@scl.kyoto-u.ac.jpSupporting information for this article is available on the WWW under https://doi.org/10.1002/ejoc.202500490© 2025 The Author(s). European Journal of Organic Chemistry published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial License, which permits use,distribution and reproduction in any medium, provided the original workis properly cited and is not used for commercial purposes.Eur. J. Org. Chem. 2025, 28, e202500490 (1 of 5) © 2025 The Author(s). European Journal of Organic Chemistry published by Wiley-VCH GmbHResearch Articledoi.org/10.1002/ejoc.202500490http://www.eurjoc.orghttps://orcid.org/0000-0002-7872-3052http://orcid.org/0000-0002-3181-6526https://orcid.org/0000-0002-2138-5902mailto:HAYASHI.Hironobu@nims.go.jpmailto:hyamada@scl.kyoto-u.ac.jphttp://creativecommons.org/licenses/by-nc/4.0/http://doi.org/10.1002/ejoc.202500490http://crossmark.crossref.org/dialog/?doi=10.1002%2Fejoc.202500490&domain=pdf&date_stamp=2025-06-16Here, precursors of anthracene oligomers with α-diketonegroups were synthesized through multiple Suzuki–Miyauracross-coupling reactions, and these precursors were then con-verted to the corresponding anthracene oligomers by simplephotoirradiation. The optical properties of these anthraceneoligomers were investigated. Through this study, wedemonstrated a practical method to synthesize acene oligomerswithout the introduction of extra substituents.2. Results and DiscussionMolecular structures of the photochemical precursors (3merDK,4merDK, and 5merDK) are depicted in Scheme 1. To ensuresufficient solubility, each precursor was designed to incorpo-rate two anthracenes bearing α-diketone group.[22] Initially,essential building blocks for synthesizing these photochemicalprecursors were prepared. Vinylene carbonate (VC) groups wereintroduced to brominated anthracenes (2-bromoanthracene and2,6-dibromoanthracene) through Diels–Alder reactions, yieldingcompounds 2 and 3, in alignment with previous reports.[23,26]Subsequently, pinacoloronic esters were introduced to com-pound 3 via palladium-catalyzed reactions to form compound5. Similarly, a coupling reaction between compound 4 andbis(pinacol)diboronic acid with palladium catalysis producedcompound 6 and its byproduct of anthracene dimer with bis(pi-nacol)boronic esters (compound 7).[27–30] Synthesis of 3merDKwas initiated using the Suzuki–Miyaura cross-coupling reactionbetween compound 2 and compound 6, resulting in an anthra-cene trimer bearing VC-introduced anthracenes at 2,6-positionsFigure 1. Anthracene oligomers were synthesized in this study.Figure 2. Anthracene formation via visible-light-inducedphotodecarbonylation.Scheme 1. Precursor synthesis for anthracene oligomers.Eur. J. Org. Chem. 2025, 28, e202500490 (2 of 5) © 2025 The Author(s). European Journal of Organic Chemistry published by Wiley-VCH GmbHResearch Articledoi.org/10.1002/ejoc.202500490 10990690, 2025, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejoc.202500490 by National Institute For, Wiley Online Library on [27/08/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://doi.org/10.1002/ejoc.202500490(compound 8) with 57% yield. VC groups were deprotected usingNaOH aqueous solution to yield compound 9 with 87% yield.Finally, Swern oxidation of compound 9 afforded 3merDK with50% yield. Similarly, 4merDK was synthesized through a couplingreaction with building blocks (compound 2 and compound 7),yielding a key intermediate for anthracene tetramer (compound10) with 28% yield. Deprotection of the VC groups led to bis-diolcompound 11, and Swern oxidation yielded 4merDK. For synthe-sizing 5merDK, a key intermediate (compound 12) was pre-pared via a Suzuki–Miyaura cross-coupling reaction. Subsequentreactions mirrored those used for 3merDK and 4merDK, withthe Swern oxidation of bis-diol compound 14 yielding5merDK. Notably, 3merDK, 4merDK, and 5merDK all exhibitedreasonable solubility in organic solvents such as dichloromethane(DCM), chloroform, tetrahydrofuran, N,N-dimethylformamide, anddimethyl sulfoxide, which is in contrast to 3mer shows minimalsolubility in organic media.The UV–vis absorption spectra of 3merDK, 4merDK, and5merDK in DCM are shown in Figure 3. These precursors dis-played vibrational peaks characteristic of anthracene, along withbroad peaks at ≈450–500 nm, attributed to the n–π* transitionof the α-diketone moiety.[22,23] Given that the anthracene moietydoes not absorb light beyond 400 nm, photoirradiation at 450–500 nm can selectively excite the n–π* transition in the precur-sors, thereby converting them to the corresponding anthraceneoligomers. Note that 4merDK shows a red shift in absorptionmaxima of longer wavelengths, which corresponds to the pres-ence of directly linked anthracene dimer in the system.The photoconversion of these precursors in CDCl3 was exam-ined in nuclear magnetic resonance (NMR) tubes. Prior to photo-irradiation, the solution was bubbled with Ar gas for 30min. Ablue LED (470� 10 nm, 200 mW cm�2) or a metal halide lamp(>390 nm) served as the light source for photoconversion.In the case of 5merDK, the 1H NMR spectrum before photoirra-diation showed several peaks in the aromatic region, indicatingreasonable solubility (Figure 4). A notable peak at 5.2 ppm, cor-responding to bridgehead protons of the α-diketone group,confirmed the presence of precursors. Upon photoirradiation,the 1H NMR signals of 5merDK completely vanished after10min, with no signals from the product detected after photo-conversion, indicating the low solubility of the resultant productsin CDCl3. Yellow precipitates formed in the NMR tube aremost likely insoluble compounds like 5mer, generated duringphotoirradiation. HR matrix-assisted-laser-desorption/ionizationtime-of-flight mass spectrometry detected parent ion peaksat m/z= 882.3287 (calcd. for C70H42= 882.3281 [M]þ) for 5mer(see Supporting Information). Similarly, 3merDK and 4merDKwere also photoconverted in NMR tubes to the correspondingoligomers, 3mer and 4mer, respectively (Figure S1, SupportingInformation), with clear parent ion peaks observed, confirmingsuccessful photoconversion from precursors to anthraceneoligomers (see Supporting Information).The conversion of these materials during photoirradiationwas further investigated in DCM, judging from changes in absorp-tion spectra (Figure 5). Upon photoirradiation of 5merDK, newpeaks at 447 and 484 nm emerged, with the broad peak corre-sponding to the n–π* transition of the α-diketone moiety disap-pearing (Figure 5b). Photoirradiation afforded precipitates of5mer, affecting the baseline of the absorption spectra. Priorreports showed oligo(2,6-anthrylene)s with solubilizing groupsat the 9,10-positions of anthracene, where pentamer absorptionbands at the long wavelength were well-resolved and observedat 425, 448, and 483 nm.[31] These findings further support thesuccessful photoconversion in this study. Similar photoconver-sions were performed for 3merDK and 4merDK, resulting inlong-wavelength absorption maxima at 460 nm for 3mer and478 nm for 4mer (Figure S2, Supporting Information). The degreeof red shift in absorption maxima upon oligomerization agreeswell with anthracene oligo(2,6-anthrylene)s with solubilizingFigure 3. Absorption spectra of photochemical precursors in DCM.Figure 4. 1H NMR spectra changing upon photoirradiation to 5merDK inCDCl3. The corresponding photochemical reaction is also shown.Eur. J. Org. Chem. 2025, 28, e202500490 (3 of 5) © 2025 The Author(s). European Journal of Organic Chemistry published by Wiley-VCH GmbHResearch Articledoi.org/10.1002/ejoc.202500490 10990690, 2025, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejoc.202500490 by National Institute For, Wiley Online Library on [27/08/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://doi.org/10.1002/ejoc.202500490groups,[31] indicating small but effective π-conjugation betweenrepeated anthracene units (Figure 5c). The obtained anthraceneoligomers exhibited no significant degradation over time, indicat-ing those stabilities under ambient conditions.Solubility of these oligomers in organic solvents promoted usto perform the solution-processable film fabrication of anthra-cene oligomers, specifically targeting 5mer film preparation.3mer was previously deposited on substrates by sublimation;[20]however, oligomers such as 5mer faced challenges in sublimationdue to there large molecular weight. In this study, a solutionof 5merDK in DCM was firstly drop-casted on a glass substrate.Photoirradiation of the drop-cast films was then performedusing a blue LED (470� 10 nm, 200 mW cm�2). Upon photoirra-diation, the film exhibited an absorption peak at ≈480 nm(Figure 6a), consistent with photoconversion from a DCMsolution (Figure 5b). IR spectroscopy directly verified photocon-version; the IR spectra of thin films showed the disappearance ofthe characteristic C═O stretching band at 1786 cm�1 correspond-ing to α-diketone groups upon photoirradiation (Figure 6b),indicating efficient photoreaction even in thin-film states. Thisphotochemical approach demonstrates the capability to prepareorganic thin films previously limited by solution processing dueto poor solubility or by vacuum deposition due to the largemolecular weight. These results significantly contribute to devel-oping methods for preparing various types of organic thinfilms, paving the way for applications in organic devices suchas OFETs and organic photovoltaics (OPVs).[14,25]3. ConclusionIn summary, this study has successfully demonstrated a photo-chemical approach to synthesize 2,6-linked anthracene oligomerswithout introducing solubilizing groups, achieving notableadvancements in both synthesis methodology and materialapplication. Through the strategic design of photochemical pre-cursors (3merDK, 4merDK, and 5merDK), we addressed solubilitychallenges and facilitated effective π-conjugation betweenanthracene units. In contrast to the utilization of the thermal con-version method,[8,14,32–34] the ability to convert these precursorsinto anthracene oligomers through selective photoirradiationhighlights the versatility of the Strating–Zwanenburg reactionfor creating advanced organic materials. The observed solubilityof precursors in various organic solvents emphasizes thesignificance of α-diketone moiety incorporation in enhancingsolubility and facilitating solution-processable film fabrication.Furthermore, the successful photoconversion to the correspond-ing anthracene oligomers, 3mer, 4mer, and 5mer in both solutionand thin-film states, confirmed by UV–vis and IR spectroscopy aswell as NMR analysis, provides compelling evidence for the meth-od’s efficacy. Thus, this photochemical approach not only opensnew avenues for the synthesis of π-conjugated systems but alsocontributes significantly to the material science field by enablingFigure 5. a) Images of color change before (left), during (middle), andafter (right) photoirradiation with a blue LED lamp. b) Absorption spectraof 5merDK during photoirradiation. c) Normalized absorption spectra of5mer, 4mer, and 3mer obtained by photoirradiation for 90 s (5mer and4mer) and 120 s (3mer).Figure 6. a) Normalized absorption and b) IR spectra of 5merDK film ona glass substrate. Red line: after photoirradiation. Black line: beforephotoirradiation.Eur. J. Org. Chem. 2025, 28, e202500490 (4 of 5) © 2025 The Author(s). European Journal of Organic Chemistry published by Wiley-VCH GmbHResearch Articledoi.org/10.1002/ejoc.202500490 10990690, 2025, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejoc.202500490 by National Institute For, Wiley Online Library on [27/08/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://doi.org/10.1002/ejoc.202500490the fabrication of organic thin films that are challenging to pro-duce through conventional methods. This advancement haspotential applications in organic electronics, including OFETsand OPVs, where such films can greatly enhance device perfor-mance and broaden the scope of organic materials utilized intechnological innovations.[35,36]AcknowledgementsAuthors would like to thank Ms. Y. Nishikawa (NAIST) and Ms. A.Fujihashi (Kyoto University) for the mass spectrometry measure-ments, Ms. A. Maeno (Kyoto University) for NMR measurements.This work is partly supported by JST PRESTO grant no.JPMJPR21AC (HH), JSPS KAKENHI grant Nos. JP18K14190 (HH),JP24K01576 (HH), JP23K26480 (NA), 20H05833 (HY), 25K01751(HY), and FOUNDATION for NARA INSTITUTE of SCIENCEand TECHNOLOGY (HH), the Sasagawa Scientific Research Grantfrom The Japan Science Society (HH), Iketani Science andTechnology Foundation (HH), the Nagase Science andTechnology Foundation (HY), JSPS Bilateral Program grant no.JPJSBP120243209 (HY), the Tokyo Ohka Foundation for ThePromotion of Science and Technology (HY), JST the establishmentof University fellowships towards the creation of science technologyinnovation, grant no. JPMJFS2123 (SK) and JST SPRING grant no.JPMJSP2110 (SK), Research Fellow of Japan Society for thePromotion of Science grant no. JP25KJ1609 (SK), and ARIM ofMEXT grant no. 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See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://doi.org/10.1002/ejoc.202500490 Photochemical Synthesis of 2,6-Linked Anthracene Oligomers without Introducing Extra Substituents 1. Introduction 2. Results and Discussion 3. Conclusion