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Takumi Miura, Takashi Kitao, Keigo E. Yamada, Yee Seng Chan, [Hironobu Hayashi](https://orcid.org/0000-0002-7872-3052), Hiroko Yamada, Takashi Uemura

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[Accessing Single‐Molecule Properties of Heptacene Using a Metal‐Organic Framework](https://mdr.nims.go.jp/datasets/c5de2f85-12a8-414b-9920-39eaf96d16b7)

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Accessing Single‐Molecule Properties of Heptacene Using a Metal‐Organic FrameworkChemistry—A European JournalResearch Articledoi.org/10.1002/chem.202501787www.chemeurj.orgAccessingSingle-MoleculePropertiesofHeptaceneUsingaMetal-Organic FrameworkTakumi Miura,[a] Takashi Kitao,[b] Keigo E. Yamada,[a] Yee Seng Chan,[c] Hironobu Hayashi,[d]Hiroko Yamada,[e] and Takashi Uemura*[a]Acenes, classic polycyclic aromatic hydrocarbons composed oflinearly fused benzene rings, represent a model system forexploring the physical properties of 1D π -conjugated structures.Isolating individual molecules at the single-molecule level pro-vides a means to investigate their intrinsic properties, which istypically done by dissolving in solutions. However, this approachbecomes increasingly difficult to apply for higher acenes owingto their high insolubility and instability. Thus, the electronicstructure of higher acenes has long been a subject of intensediscussion. This paper introduces a method to encapsulate hep-tacene within a metal-organic framework (MOF) through in situphotochemical conversion of a precursor molecule. The trans-formation reaction of the precursor is significantly acceleratedupon inclusion. This approach stabilizes otherwise unstable hep-tacene by suppressing undesirable side reactions through thespatial constraint. The bulk production of heptacene in an iso-lated state enables its exploration through various analyticaltechniques, providing insights into its single-molecule propertiesand leading to the first observation of its fluorescence. More-over, experimental and theoretical studies reveal the electronicground state of pristine heptacene.Acenes, consisting of linearly annulated benzene rings, haveattracted significant research attention owing to their diversephysical properties.[1–5] As the number of benzene ringsincreases, acenes exhibit exceptional optical, electronic, andmagnetic functions, rendering them promising for use invarious technological domains.[6,7] Investigating the proper-ties of single acene molecules is key to understanding themechanisms underlying the functionalities of acenes.[8] Con-ventionally, single-molecule properties are studied in solution.[a] Dr. T. Miura, Dr. K. E. Yamada, Prof. T. UemuraDepartment of Applied Chemistry, Graduate School of Engineering, TheUniversity of Tokyo, 7–3–1 Hongo, Bunkyo-ku, Tokyo 113–8656, JapanE-mail: uemurat@g.ecc.u-tokyo.ac.jp[b] Dr. T. KitaoNanocarbon Material Research Institute, National Institute of AdvancedIndustrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi,Tsukuba, Ibaraki 305–8565, Japan[c] Dr. Y. S. ChanDivision of Materials Science, Nara Institute of Science and Technology(NAIST), 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan[d] Dr. H. HayashiCenter for Basic Research on Materials, National Institute for MaterialsScience (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan[e] Prof. H. YamadaInstitute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto611-0011, JapanSupporting information for this article is available on the WWW underhttps://doi.org/10.1002/chem.202501787© 2025 The Author(s). Chemistry – A European Journal 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 work isproperly cited and is not used for commercial purposes.However, research on acenes longer than hexacene, referred toas higher acenes, has been hindered by their poor solubility andinstability.[9–13] Although the introduction of solubilizing andstabilizing substituents can address this problem, such modifi-cations compromise the inherent properties of the molecules.[14]On-surface synthesis has recently emerged as a promisingalternative for producing single-molecule higher acenes withoutany substituents, providing valuable insights into the propertiesof individual molecules.[15,16] However, the strong electroniccoupling between the acenes and underlying metal substratesmay affect their physical properties.[10,17–19] Moreover, higheracenes can only exist under ultra-high vacuum conditions onthe surface because of their chemical instability, as explained byClar’s aromatic π -sextet rule. Furthermore, the limited reactionareas result in an exceedingly small yield of products, restrictingthe application of various characterization techniques.To address these challenges, this study introduces a novelstrategy based on host–guest chemistry to investigate thesingle-molecule properties of unstable higher acenes. Metal-organic frameworks (MOFs), which are nanoporous materialscomposed of metal ions and organic ligands, have gainedincreasing attention for potential application in numerous fields,such as gas storage, separation, catalysis, and drug delivery.[20–24]In particular, the tunable nanospaces of MOFs offer an ideal com-partment for controlling the assemblies of guest species.[25–28]Additionally, their crystalline arrangement ensures that thenanochannels are uniform across the entire material. Therefore,confining molecules within MOF nanochannels can help createa single-molecule-like state akin to that in solution while main-taining a bulk solid phase, thus expanding the scope of possibleanalytical methods.As a proof of concept, we attempted to encapsulateheptacene into the nanochannels. However, heptacene isChem. Eur. J. 2025, 31, e202501787 (1 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbHwww.chemeurj.orghttps://orcid.org/0000-0002-9397-4319https://orcid.org/0000-0002-5527-7424https://orcid.org/0000-0002-7872-3052https://orcid.org/0000-0002-2138-5902https://orcid.org/0000-0002-1357-3196mailto:uemurat@g.ecc.u-tokyo.ac.jphttps://doi.org/10.1002/chem.202501787http://creativecommons.org/licenses/by-nc/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fchem.202501787&domain=pdf&date_stamp=2025-06-22Chemistry—A European JournalResearch Articledoi.org/10.1002/chem.202501787Figure 1. Schematic of in situ conversion of DEH to heptacene within an MOF.insoluble and unstable, rendering its direct insertion into MOFsunfeasible.[29–31] Therefore, we synthesize heptacene within anMOF by introducing 7,16-dihydro-7,16-ethanoheptacene-19,20-dione (DEH), a soluble and stable precursor, into the nanochan-nels and then inducing its photochemical reaction (Figure 1).The reactivity of DEH molecules within the MOF nanochannels isconsiderably higher than in its crystalline solid state. Comparedwith that in solution, the lifetime of heptacene is extendedthrough the suppression of oxidation and dimerization reactionswithin the nanopores, permitting extensive characterizations.The bulk production of isolated heptacene molecules enablesthe first experimental observation of its fluorescence, providinginsights into the elusive electronic ground state of pristine hep-tacene. Overall, the proposed method can help fully leveragethe functionalities of chemically unstable molecules and expandour understanding of the nature of higher acenes.Initially, we synthesized bulk heptacene through the solid-state conversion of neat DEH. A photoconversion experimentwas conducted under reduced pressure and at room tem-perature using a light-emitting diode as the light source. Awavelength of 467 nm was selected owing to its proximity to then–π* transition of the diketone group of DEH. A transparent KBrfilm containing dispersed DEH powder was prepared to ensurethorough illumination of the sample and facilitate deeper lightpenetration. Fourier-transform infrared (FT-IR) spectroscopy wasconducted to analyze the product structure. The out-of-plane(opla) sp2 C─H vibration modes were classified as SOLO, DUO,TRIO, or QUATRO, based on the number of adjacent C─H groups.As the number of terminal C─H groups remained unchangedafter the transformation reaction, the conversion ratios of DEHwere determined by analyzing the relative peak intensities ofthe α-diketone moiety (1735 cm−1) and opla aromatic C─H vibra-tion QUATRO mode (742 cm−1).[32,33] The reaction efficiency washighly dependent on the light intensity. At an irradiation powerof 50 mWcm−2, the conversion reaction proceeded slowly withthe reaction ratio of DEH being only 30% after photoirradiationfor 90 minutes (Figure S1). Complete conversion of DEH in thesolid state was achieved by increasing the light power from 50to 200 mWcm−2, as evidenced by the disappearance of the peakfor the α-diketone moiety of DEH in the FT-IR spectra (Figure S1).However, along with the characteristic peaks for heptacene, anadditional peak was detected at 2950 cm−1. Based on the sim-ulated spectra (Figure S2), this peak was attributed to the sp3C─H vibration mode of diheptacene, indicating that part of theheptacene dimerized to form diheptacene owing to its inherentdiradical character.[34] For the cycloreversion of diheptacene, theproduct was annealed at 300 °C under vacuum, yielding pureheptacene in the bulk state (Figure S1).[35]To obtain heptacene in the single-molecule state, we usedan MOF, [Al(OH)(L)]n (L = dicarboxylate), with 1D nanochannelsas the host. This system afforded precise control over the poresize by varying the dicarboxylate ligand, L.[36] Additionally, thisMOF did not exhibit any absorption at the irradiation wave-length. Considering the molecular size of DEH (18 × 7 × 6Å3), [Al(OH)(bpdc)]n (1; bpdc = 4,4-biphenyldicarboxylate; poresize = 11 × 11 Å2) could accommodate single DEH moleculeswithin its nanochannels (Figure 2a). DEH was introduced intothe MOF by soaking the host crystals in a saturated acetonesolution containing DEH at room temperature for 72 hours. Theresulting light-yellow solid was filtered and washed with CHCl3and acetone to remove the surface-adsorbed DEH, affording thenanocomposite of 1 with DEH (1⊃DEH). The powder X-ray diffrac-tion (PXRD) pattern of 1⊃DEH, compared with that of the originalhost, indicated that the crystal structure was maintained uponthe encapsulation of DEH (Figure 2b). The absence of diffractionpeaks corresponding to DEH confirmed the lack of depositionof DEH on the crystal surface of 1. This observation was alsosupported by scanning electron microscopy (SEM) and parti-cle size distribution measurements (Figures S3, S4). Furthermore,the gas adsorption isotherm of 1⊃DEH showed a decrease inits adsorption capacity compared with that of 1 (Figure S5).These results confirmed the full encapsulation of DEH withinthe MOF nanochannels. To evaluate the loading amount of DEHin 1, DEH was extracted using CDCl3 after dissolving 1 in 1 Msodium ethylenediaminetetraacetate aqueous solution. Dimethylterephthalate was used as the internal standard. Proton nuclearChem. Eur. J. 2025, 31, e202501787 (2 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 39, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501787 by National Institute For, Wiley Online Library on [24/08/2025]. 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 LicenseChemistry—A European JournalResearch Articledoi.org/10.1002/chem.202501787Figure 2. a) Crystal structure of 1 (Al, pink; O, red; C, gray; H, white). b) PXRD patterns of DEH (purple), 1 (black), 1⊃DEH (blue), and 1⊃heptacene (red).magnetic resonance (1H-NMR) analysis revealed that the numberof DEH molecules per unit cell of 1 was 0.04 (Figure S6; detailscan be found in the supporting information).1⊃DEH powder was placed between two glass slides andexposed to light at 50 mWcm−2, resulting in the quantita-tive formation of the nanocomposite of 1 including heptacene(1⊃heptacene). During irradiation, the sample color changedfrom light yellow to light brown, suggesting the conversion ofDEH to heptacene with an extended conjugated structure. ThePXRD pattern of 1⊃heptacene was consistent with that of thepristine host, confirming that the structure of 1 was preservedduring the irradiation (Figure 2b). Furthermore, SEM imagesshowed that the size and morphology of the MOF particlesremained unchanged after irradiation (Figure S3). These resultsconfirmed the incorporation of heptacene within the nanochan-nels of MOF 1. The consumption ratio of DEH was determinedthrough the 1H-NMR analysis of DEH extracted from the MOFbefore and after photoirradiation. As 90 minutes of irradiation,the resonance peaks corresponding to DEH significantly dimin-ished (Figure S6). This observation demonstrated the formationof heptacene through quantitative conversion, suggesting thatthe geometrical constraint of the host MOF effectively sup-pressed dimerization within 1.[35] Notably, DEH within the MOFparticles underwent complete conversion at a low power of 50mWcm−2, in contrast to the bulk DEH under identical conditions(only 30% conversion). In the bulk condition, the DEH powderwas dispersed in a transparent KBr film, effectively minimizinglight scattering and ensuring more efficient light exposure tothe sample. Despite this, 1⊃DEH showed higher conversion effi-ciency, likely owing to the difference in the aggregation stateof DEH. The transformation reaction of DEH necessitates sub-stantial structural changes, which are restricted owing to thedense packing of DEH molecules in the bulk crystalline state(Figure 3a).[37] In contrast, DEH molecules in the MOF were iso-lated in the single-molecule state, which likely facilitated theconversion reaction. Molecular dynamics (MD) simulations of1⊃DEH provided deeper insights into the effect of the nanocon-finement on the conversion reaction. Upon encapsulation, thedihedral angle between the two aromatic regions bridged bythe diketone moiety increased from 125° to 152° via the host–guest π–π interactions, thereby lowering the reaction barrier forα-diketone cleavage (Figure 3b).[15,18,38,39]Although the dissolution of DEH enables high-efficiencytransformation reactions, the resulting heptacene in solu-tions inevitably undergoes immediate dimerization and/oroxidation.[37] In contrast, the bulk heptacene prepared by thesolid-state conversion of DEH was found to be stable, with noundesirable side reactions observed for at least one month(Figure S1). Recent studies have demonstrated such high sta-bility of higher acenes in the solid state,[40–42] attributable tothe robust intermolecular packing that prevents oxygen accessand suppresses dimerization, as suggested by the aggregationstructure of heptacene (Figures S7, 8). Entrapment within MOFscan stabilize otherwise unstable higher acenes through thegeometrical constraint of the nanopores. Consequently, despitebeing in a single-molecule state, similar to that in solution, hep-tacene can be handled as a solid. Thus, the proposed approachrepresents a promising route to access the single-moleculephysical properties of heptacene using various characterizationmethods.To demonstrate this advantage, we investigated the stabil-ity of heptacene within the MOF. When 1⊃heptacene was storedin an N2 environment under dark conditions, the absorbance ofheptacene remained nearly constant over time, attributable tothe suppression of dimerization (Figure S9). Moreover, heptaceneremained detectable for several hours even under ambient air,owing to the restricted access of oxygen to the heptacenemolecules. In contrast, heptacene in solution and polymer matri-ces has been noted to rapidly decompose through dimerizationand/or oxidation reactions.[37,41] These findings highlight that thecombination of the precursor method and host–guest chemistryprovides a promising approach for stabilizing typically unsta-ble heptacene, enabling the exploration of its single-moleculeoptoelectronic properties.The ultraviolet/visible/near IR (UV/vis/NIR) spectrum of bulkheptacene exhibited the S0 to S1 transition band (p-band)from 600 to 850 nm (Figure 4a).[37,43] Additionally, broad bandswere detected around 850–1100 nm, attributable to the charge-transfer exciton coupling between nearest-neighbor heptacenemolecules.[44] The absorption spectrum of 1⊃heptacene alsoshowed the S0 to S1 transition band of heptacene. The energygap of heptacene in 1 was calculated to be 1.46 eV from theedge of the absorption band, similar to the value (1.50 eV)reported for heptacene dispersed in the polymer matrix.[43] AChem. Eur. J. 2025, 31, e202501787 (3 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 39, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501787 by National Institute For, Wiley Online Library on [24/08/2025]. 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 LicenseChemistry—A European JournalResearch Articledoi.org/10.1002/chem.202501787Figure 3. a) Crystal structure of DEH containing dichloromethane as the synthetic solvent.[37] b) MD structure of DEH confined within the nanochannels of1 (Al, pink; O, red; C, gray; Cl, light green; H, white).blue shift in the maximum absorption wavelength was observedfor 1⊃heptacene (775 nm) compared to bulk heptacene (811 nm).The peaks of p-band were sharpened compared with those ofthe bulk heptacene. Moreover, intermolecular exciton couplingbands were not observed. These results indicate that the absorp-tion bands originated from electronically decoupled single hep-tacene molecules, as supported by the modeled structure of1⊃heptacene (Figure 4b).[43,45,46]The isolation of heptacene within the MOF enabled thedetection of its emission properties for the first time. In gen-eral, unsubstituted π -conjugated materials, including acenes,suffer from aggregation-induced quenching in the solid state;therefore, their emission characteristics are typically investigatedby dissolving in solutions.[47] However, higher acenes with-out any peripheral substitutions are insoluble and chemicallyunstable,[48,49] and to the best of our knowledge, their emissionproperties have not yet been reported. When emission measure-ments for 1⊃heptacene were conducted at room temperature,no emission was detected, consistent with the results for bulkheptacene. According to the energy gap rule, the emission quan-tum yield of acenes decreases as the number of benzene ringsincreases. Therefore, the emission intensity of heptacene waslikely below the detection limit.[48,49] Low-temperature measure-ments are effective for substances with low quantum yieldsowing to reduced molecular motion and nonradiative deacti-vation. Thus, we performed emission measurements at 77 K.Emission signals were successfully detected from 860 to 1000 nm,unlike the results for bulk heptacene (Figure 4c).[31,50] Theobserved emission wavelength was comparable with the valuepredicted by time-dependent density-functional theory calcula-tions of closed-shell heptacene, confirming that the emissionwas attributable to fluorescence from the S1 to S0 states ofheptacene (Figure 4d).According to theoretical predictions, the open-shell diradicalcharacter of acenes intensifies with increasing acene length.[51,52]To clarify the electronic structures, electron spin resonance (ESR)measurements of higher acenes with solubilizing substituentshave been conducted in solutions, revealing the presence ofradical species. However, the corresponding temperature depen-dence remains elusive owing to the thermal instability of theseacenes.[53,54] Indeed, variable temperature ESR of the bulk hep-tacene dispersed in a KBr film has failed to clarify its electronicstructure owing to the occurrence of pyrolysis before the sin-glet to triplet thermal transition (Figure S10). Scanning tunnelingspectroscopy is a powerful method to investigate the electronicstructures of single unsubstituted higher acenes.[15] However,strong electronic interactions between acene molecules andmetal surfaces could induce undesirable changes in the elec-tronic structures.[18] Therefore, elucidating the intrinsic electronicstructures of higher acenes remains a formidable task. The spec-troscopic analysis of single heptacene molecules within the MOFhighlighted the electronic ground state of pristin heptacene. Theabsorption and fluorescence wavelengths of 1⊃heptacene weresimilar to predictions for a closed-shell configuration (Figures 4d,S11). This result demonstrates that the ground state of heptacenewith seven fused benzene rings within the MOF is predominantlyclosed-shell singlet.In conclusion, we have demonstrated that the conversion ofthe precursor within the MOF is an effective strategy for elucidat-ing the single-molecule properties and electronic ground stateof unstable higher acenes. Precursor conversion within MOFsenables the synthesis and in-depth characterization of aceneslarger than heptacene. As our technique yields bulk quanti-ties of isolated higher acenes, we believe that this approachhelps enhance our understanding of the effects of intermolec-ular interactions on the remarkable properties of these higherChem. Eur. J. 2025, 31, e202501787 (4 of 6) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 39, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501787 by National Institute For, Wiley Online Library on [24/08/2025]. 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 LicenseChemistry—A European JournalResearch Articledoi.org/10.1002/chem.202501787Figure 4. a) UV/vis/NIR absorption spectra of DEH (orange), bulk heptacene (green), 1 (black), 1⊃DEH (blue), and 1⊃heptacene (red). b) MD structure of1⊃heptacene, showing the nanoconfinement of single heptacene molecules within the MOF nanochannels (Al, pink; O, red; C, gray; H, white). c) Emissionspectra of bulk heptacene (black) and 1⊃heptacene (red) at 77 K (excitation wavelength of 705 nm). d) Schematic of potential energy surfaces ofclosed-shell heptacene, calculated at RCAM-B3LYP-D3(BJ)/6–311++G(3d2f,3p2d). f denotes the oscillator strength of transition.acenes, such as their diradical (and even polyradical) characterand singlet fission.[9,10,42,55,56] Moreover, this methodology can beextended to other unstable and insoluble compounds, pavingthe way for their diverse applications.AcknowledgmentsThis work was supported by the JST CREST (JPMJCR20T3) andPRESTO (JPMJPR21A7) programs, Ogasawara Foundation for thePromotion of Science and Engineering, and a Grant-in-Aid forScientific Research (JP24K01276) and the Japan Society for thePromotion of Science (JSPS) Fellows (JP23KJ0539) from theMinistry of Education, Culture, Sports, Science, and Technol-ogy, Government of Japan. 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elax Accessing Single-Molecule Properties of Heptacene Using a Metal-Organic Framework  Acknowledgments  Conflict of Interest  Data Availability Statement  2025-07-09T17:30:46+0530 Preflight Ticket Signature