Heptacene: Characterization in Solution, in the Solid State, and in

Article pubs.acs.org/JACS

Heptacene: Characterization in Solution, in the Solid State, and in Films Ralf Einholz,† Treliant Fang,‡ Robert Berger,§,∥ Peter Grüninger,⊥ Andreas Früh,⊥ Thomas Chassé,⊥ Reinhold F. Fink,⊥ and Holger F. Bettinger*,† †

Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Flux Research Institute LLC, 11444 Rothschild Place, Dublin, California 94568, United States § Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, 35032 Marburg, Germany ∥ FIAS, Goethe-Universität, Ruth-Moufang-Strasse 1, 60438 Frankfurt am Main, Germany ⊥ Institut für Physikalische und Theoretische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany ‡

S Supporting Information *

ABSTRACT: Acenes comprise an important class of organic semiconducting materials. As graphene nanoribbons of ultimate width, they are valuable atom-precise model systems for studying the properties of this form of nanoscale carbon materials. Heptacene is the smallest member of the acene series that could only be studied under matrix isolation conditions. Its existence in bulk had never been positively confirmed, despite efforts dating back more than 70 years. We report that the reduction of 7,16heptacenequinone produces a mixture of two diheptacene molecules. The diheptacenes undergo thermal cleavage to heptacene at high temperatures in the solid state. Monitoring this cycloreversion by solid state 13C cross-polarized magic angle spinning NMR reveals that solid heptacene has a half-life time of several weeks at room temperature. The diheptacenes are valuable precursors for generating films of heptacene by vapor phase deposition that can be studied below or at room temperature.

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INTRODUCTION The acene family (Chart 1a) comprises a class of molecular materials that is central to the development of organic

However, acenes larger than pentacene are problematic to handle due to their instability and poor solubility,25,26 and indeed, only acenes up to hexacene could be synthesized and isolated in bulk.27−29 Clar, a pioneer in acene chemistry, claimed achievement of the first synthesis of heptacene (1, Chart 1b) in 1942 by heating dihydroheptacene using a copper bronze in a CO2 stream at 310 °C.30 He described heptacene as poorly soluble with green color in boiling 1-methylnaphthalene, extremely reactive, and difficult to obtain in pure state even after repeated sublimation.30 Already in 1943, Marschalk31 disagreed with some of the reported properties of Clar’s dihydroheptacene intermediates and doubted the successful preparation of heptacene. In a joint paper, Clar and Marshalk reassigned some of the UV−vis bands previously assigned to heptacene to benzo[a]hexacene instead.32 Despite other occasional reports,33−35 heptacene was considered elusive until Mondal et al.36 succeeded in obtaining unambiguous evidence for its existence by generating heptacene photochemically via the Strating−Zwanenburg37 reaction in a poly(methyl methacrylate) (PMMA) matrix from the bridged α-diketone 2. Heptacene was characterized by its optical

Chart 1. (a) Structure of the Acene Series and (b) Structure of Heptacene and Orientation Used throughout This Work

electronics.1−6 Some of the members, in particular tetracene, pentacene, and their derivatives, have been successfully employed in organic photovoltaic devices, organic light emitting diodes, and organic field effect transistors.7−9 The acenes are zigzag graphene nanoribbons10 (GNRs) of ultimate width that share these predictions of exciting electronic and magnetic properties,11−17 making them promising materials for applications such as spintronics18 and plasmonics.19 The favorable charge transport characteristics of the acenes result, inter alia, from the low reorganization energy.1,20−22 This decreases with acene length and thus makes long acene molecules particularly promising for organic electronics.23,24 © 2017 American Chemical Society

Received: December 23, 2016 Published: March 20, 2017 4435

DOI: 10.1021/jacs.6b13212 J. Am. Chem. Soc. 2017, 139, 4435−4442

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Journal of the American Chemical Society

Scheme 1. Photochemical and Reductive Meerwein−Ponndorf−Verley (MPV) Synthesis of Diheptacenes (3a and 3b) from αDiketone 2 and Heptacene-7,16-quinone 4, Respectivelya

a

In both cases, 7,16-dihydroheptacene (5a) and 6,17-dihydroheptacene (5b) are formed as byproducts.

established method for the synthesis of acenes such as anthracene and pentacene.53−56 This modified Meerwein− Ponndorf−Verley (MPV) reduction reaction, which can be accelerated by the addition of CBr4 or CCl4,35,55−57 does not stop at the dihydro−dihydroxyacene stage but proceeds to the acene if secondary alcoholates are employed.53 When we subjected 7,16-heptacenequinone 4 to MPV reduction in the presence of 0.1 equiv of CBr4, we could not obtain heptacene.35 Instead, an orange solid was isolated that could be identified by high resolution atmospheric pressure photoionization mass spectrometry (HR-APPI-MS) as diheptacenes 3. Solution phase (CDBr3) 1H NMR spectroscopy reveals a singlet at 5.30 ppm and a multiplet at 5.27 ppm. These are assigned to the bridgehead H atoms of a diheptacene isomer with high symmetry (3a, D2h, 5.30 ppm) and one with low symmetry (3b, Cs, 5.27 ppm) based on HMBC and HSQC NMR spectroscopy (see SI for details). The bridgehead aliphatic carbon atoms resonate at 54.0 (3a) and 54.4 (3b) ppm in CDBr3 solution.58 Due to poor solubility, the two isomers 3a and 3b could not be separated. Additional aliphatic signals at 4.35 and 4.30 ppm in the 1H NMR spectrum are assigned to 7,16-dihydroheptacene (5a) and 6,17-dihydroheptacene (5b) (Scheme 1), respectively, on the basis of HMBC and HSQC NMR spectroscopy (see SI for details). Their aliphatic carbon atoms resonate at 38.7 (5a) and 38.5 (5b) ppm in CDBr3 solution.59 The mole fraction of 5 is up to 20% based on solution phase 1H spectroscopy. Optical absorption and fluorescence spectroscopy allowed readily identifying the isomers of lower symmetry 3b and 5b within the product mixture as their tetracene subunits show characteristic electronic absorption (420, 450, and 481 nm) and emission bands (488, 522, 562 nm) in dichloromethane

absorption spectrum that had disappeared after 4 h due to reaction with atmospheric oxygen.36 The Strating−Zwanenburg reaction allowed synthesis of a large variety of acenes in solution or as films on surfaces,38 and generation of the parent acenes up to nonacene in cryogenic noble gas matrices.39−41 However, it failed for synthesis of hexacene or heptacene in solution.40,42−44 Instead of monomeric heptacene, its covalent dimers, diheptacenes 3a and 3b (Scheme 1), are formed presumably due to facile photodimerization,43 a reaction that is well-known for smaller acenes.45,46 Heptacene could only be obtained with stabilizing silyl ethynyl groups attached, a significant achievement first reported by Anthony et al.47 in 2005. Kinetically stabilized heptacenes were synthesized by other groups in the following years,48−50 and even nonacene derivatives were reported.51,52 Until now, heptacene could only be observed spectroscopically in stabilizing matrices, and no convincing evidence for its existence in a pure state at room temperature was available. We show that the diheptacenes can undergo a clean cycloreversion to heptacene in the solid state at elevated temperatures. Heptacene slowly undergoes cyclodimerization to diheptacene in the solid state at room temperature in the dark. The high temperature cracking of diheptacene under vacuum allows for the first time the preparation of heptacene films by physical vapor deposition (organic molecular beam deposition). Such films are stable at room temperature for an extended period of time. The thermal cycloreversion of diheptacenes measurement of an optical absorption spectrum in solution.

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RESULTS AND DISCUSSION Synthesis and Characterization of Diheptacenes 3. Reduction of acenequinones with aluminum trialkoxides is an 4436


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Figure 1. (a) Optical absorption, emission (λex = 450 nm), and excitation spectra (λem = 522 nm) in DCM of the products obtained from MPV reduction of 7,16-heptacenequinone (4). (b) Optical absorption and excitation spectra of product mixture from MPV reduction of 4 in DCM at different emission wavelengths. (c) Optical absorption, emission (λex = 363 nm), and excitation spectra (λem = 412 nm) in DCM of the products obtained from photodecomposition of α-diketone 2. (d) Comparison of optical absorption spectra in 1,2,4-trichlorobenzene of the products obtained from MPV reduction of 7,16-heptacenequinone (4) and isolated dihydroheptacenes (5) before and after reaction with maleic anhydride. (e) Highest occupied (left, HOMO) and lowest unoccupied (right, LUMO) molecular orbitals of diheptacene 3b computed at the B3LYP/TZVP level of theory.

The different fluorescence behavior of 3a and 3b follows from Kasha’s rule. In 3b, the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) are confined to the tetracene unit (Figure 1e). The S1 state of tetracene and that of 3b are due to a HOMO → LUMO excitation, and hence, tetracene-like fluorescence is observed for 3b. Similar arguments were employed by Berg et al. when differentiating between the two pentacene photodimers by fluorescence spectroscopy.46 The two dihydroheptacene isomers 5a/5b can be separated from the diheptacenes 3a/3b. After dissolution of the product mixture in boiling 1-methylnaphthalene, 5a and 5b precipitate as red solid during slow cooling to room temperature overnight as confirmed by 1H NMR (see SI for details). Their UV−vis spectrum differs slightly but markedly from that of the initial product mixture (see Figure 1d). The bands of the tetracene chromophore of 5b (424, 452, and 482 nm) quickly disappear upon treatment of the dihydroheptacenes with maleic anhydride while those of the anthracene chromophore (330, 348, 365, 386 nm) persist unchanged. Note that the absorption

(DCM) (Figure 1a). The excitation spectrum detected at λem = 522 nm is identical with the optical absorption spectrum in the long wavelength regime of the tetracene subunit absorption. However, it differs slightly at shorter wavelengths where the anthracene subunits absorb. This difference can be ascribed to the presence of higher symmetry isomers 3a/5a with only anthracene units that do not show fluorescence at 522 nm. Accordingly, measuring the excitation spectrum at λem = 410 nm reveals the absorption of the anthracene band of the symmetric isomers 3a/5a (Figure 1b). In contrast, a low content of low symmetry isomers is obtained photochemically from α-diketone 2 as described previously,43 and the corresponding mixture shows different UV−vis and fluorescence spectra in DCM (Figure 1c). Excitation at λex = 363 nm produces an emission spectrum with peaks at 393, 412, and 436 nm that is clearly due to an anthracene chromophore. The excitation spectrum monitored at λem = 412 nm differs from the absorption spectrum due to the presence of the lower symmetry isomers that do not fluoresce at this wavelength. 4437


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Figure 2. Solid state 13C CP-MAS NMR spectra (rotational side bands are marked by asterisks). (a) Product mixture obtained by MPV reduction of 4. (b) Heptacene obtained from sample displayed in part a after heating to 300 °C for 12 min. (c) Same sample as in part b after storage at room temperature in ambient atmosphere for 1 month. (d) Same sample as in part c after heating again (300 °C/12 min).

dimers based on the increased signal at 137.0 and 54.6 ppm (see Figure 2c). On the basis of the signal intensities, it is concluded that the major part of the sample still consists of heptacene. The dimers that formed over the four-week period can be split again into heptacene by heating (300 °C for 12 min) as demonstrated by the NMR spectrum of the sample (see Figure 2d). This series of solid state NMR experiments demonstrates that heptacene can exist at room temperature for an extended period of time and that it slowly transforms to dimers upon storage. Optical spectroscopy further supports the conclusion that heptacene forms from the dimers upon heating. Subliming (10−5 mbar, 330 °C) a sample of the diheptacene mixture intimately mixed with a 10-fold amount of copper powder to dehydrogenate residual dihydroheptacenes and freezing of the products in an argon matrix produced an UV−vis spectrum of heptacene that was identical to that obtained from photobisdecarbonylation of 2 (Figure 3a).40 No other species were detected by optical spectroscopy. Attempts to measure the optical spectrum of heptacene in a 100 mm cylindrical cuvette by heating the diheptacenes 3 in 1methylnaphthalene solution were successful only if the solvent was rigorously degassed (see Supporting Information for details). At around 150 °C weak signals at 753, 682, and 623 nm can be detected (Figure 3b). In addition, a shoulder (792 nm) at the long wavelength side of the 753 nm absorption is also observed. The intensity of these bands increases upon increasing the temperature to 230 °C. The signals persist at this temperature for at least 40 min, but they almost immediately

bands are identical to those reported by Clar and Marschalk for dihydroheptacenes in 1,2,4-trichlorobenzene.32 Thermal Cycloreversion of Diheptacenes to Heptacene. The thermal cycloreversion of covalent anthracene photodimers is well-established.60,61 Computations (UM062X/6-31G*) by Zade et al. arrive at ΔrG0298.15 = 31 kcal mol−1 for cycloreversion of an isolated molecule of 3a to two heptacene molecules and at a barrier of ΔrG⧧298.15 = 37 kcal mol−1 for this stepwise reaction.62 This prompted us to investigate the cycloreversion by solid state 13C CP-MAS NMR spectroscopy. The MPV reduction sample shows (Figure 2a) prominent signals in the aromatic (138.3, 130.3, 126.2, 122.7 ppm) and aliphatic (54.7 ppm) regions in addition to a very weak signal due to dihydroheptacenes 5 (31.7 ppm). After heating this sample in a solid state NMR rotor under an argon stream to 300 °C for 12 min, the 13C CP-MAS NMR spectrum (Figure 2b) shows a sharpening of the signal in the aromatic region (129.4 and 126.6 ppm, accompanied by a shoulder around 124 ppm). Most importantly, the bridgehead carbon signals essentially vanished upon heating. The presence of only 13C signals in the aromatic region along with the disappearance of bridgehead signals shows that cleavage of heptacene dimers has resulted in a compound that only contains sp2 hybridized carbon atoms. The most straightforward explanation is the formation of heptacene (1) upon heating. Keeping the sample within the NMR rotor and storing this rotor for four weeks under ambient conditions show that after this period of time some of the heptacene has reacted back to 4438


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Figure 3. (a) Comparison of optical absorption spectra of heptacene (Ar, 30 K) obtained from cycloreversion of diheptacenes in the presence of copper and from photobisdecarbonylation of α-diketone 2 (top trace shifted for ease of comparison). Note that there is residual intensity due to 2 in the range 450−500 nm in the top trace. (b) UV−vis spectrum of heptacene obtained by heating a solution of diheptacenes in 1-methylnaphthalene. (c) Peak fit of the p band region of heptacene. (d) Normalized optical spectra of pentacene, hexacene, and heptacene in the p band region measured in 1-methylnaphthalene at elevated temperatures.

Figure 4. (a) Comparison of the p band region of heptacene obtained from photodecomposition of α-diketone (black) and cycloreversion of diheptacene (red) with the calculated Franck−Condon profile at 0 K (blue, Lorentzian line broadening with full width at half maximum of 200 cm−1 with the 0−0 transition shifted to 727 nm (13 755 cm−1). (b) Energies of excited states 13B2u, 11B2u, and 21Ag with respect to the ground state of the n-acenes as computed at the MCCEPA level of theory.

disappear upon contact of the solution with air. In comparison to an Ar matrix, the 1-methylnaphthalene solution causes a bathochromic shift of peak maxima by 400−500 cm−1 in the spectrum. The signal shape, including the long wavelength shoulder, can be fit to four Gaussians with Lorentzian contributions (Figure 3c). The UV−vis spectra of the acenes

up to heptacene could be measured in solution under very similar conditions (Figure 3d). Interpretation of the Optical Spectrum of Heptacene. The longest wavelength electronic singlet transition in the absorption spectrum of the acenes ranging in size from anthracene to hexacene, called p band in Clar’s63 or 1La in Platt’s64 nomenclature, is due to the HOMO−LUMO 4439


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Figure 5. (a) Variation of the optical spectra of a heptacene film deposited on sapphire at 16 K (in the absence of argon) with deposition time. (b) Variation of the optical spectra of the heptacene film upon slowly warming from 16 to 295 K. (c) Optical spectra of heptacene films deposited on sapphire at 298 K.

transition (11Ag → 11B2u, see Chart 1 for axis convention).65 The band is characterized by vibrational fine structure that is typical of the acene series. In particular, the 0−0 transition is always the most intensive feature of the band system. One of us noted previously that the optical spectrum of heptacene produced from the photoprecursor 2 does not follow this rule: in addition to the strongest feature at 727 nm, an additional feature of lower intensity at 768 nm was observed in argon matrix.40 This was also observed in other media, such as xenon matrix40 or organic glasses.66 As discussed above, the solution spectra at elevated temperatures also show a long wavelength shoulder. The observation that the spectra obtained from 2 by photobisdecarbonylation within the matrix and by thermal decomposition of 3 followed by sublimation of heptacene are identical excludes the possibility that the weak feature at 768 nm originates from matrix effects associated with, e.g., incomplete planarization of heptacene during photobisdecarbonylation under matrix isolation conditions. It rather indicates that the feature could be intrinsic to the optical spectrum of heptacene. Moreover, similar features were not observed in the absorption spectra of octacene and nonacene,41 and hence, heptacene appears unique in this regard. To analyze this further, we have computed the vibrational fine structure of the 11Ag → 11B2u transition of heptacene using harmonic vibrational force fields.67,68 These were obtained for the ground state at the B3LYP/TZVP level of theory with analytic second derivatives and for the excited state at the TDB3LYP/TZVP level with numerical derivatives of analytical first derivatives (see SI for computational details). The calculated

Franck−Condon profile (0 K) shows that the 0−0 transition is the strongest feature of the p band also of heptacene (Figure 4a). The computed vibrational fine structure is overall in good agreement with the experimental spectrum obtained at 10 K in solid argon. The main vibrational progression is assigned to a totally symmetric mixed C−C stretch/C−H bending mode v′11 at about 1395 cm−1 (1110) and 2790 cm−1 (1120) in excess of the 0−0 transition. Another prominent signal due to an ag symmetric mode appears at about 0−0 + 1179 cm−1 (1610). An explanation for the weak feature at 768 nm could be an electric dipole transition to a state that is forbidden in the Franck−Condon approximation but gains intensity, for instance, due to vibronic coupling with the 11Ag → 11B2u transition. A DFT/MRCI study of Marian et al.69 computed the 21Ag state of octacene to be significantly lower in energy than the 11B2u state. In the polyene series, it is well-established that a state (1Ag), to which a transition is electric dipole forbidden, falls below the lowest energy electric dipole allowed transition of π → π* nature (to 1Bu).70,71 This is also true for polycyclic aromatic hydrocarbons, e.g., terrylene.72,73 As the 21Ag states of heptacene were not considered previously in DFT/MRCI studies,69,74 we have performed computations at the multiconfiguration coupled electron pair approximation75 (MCCEPA) level of theory using complete active space selfconsistent field reference wave functions with six electrons in six orbitals [CASSCF(6,6)] (see SI for details). The excitation energies for the 11B2u, 21Ag, and 13B2u states of the acene series (Figure 4b) indicate that for heptacene the 21Ag state drops below the 1B2u state by 0.17 eV. Similar findings for the relative 4440


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existence of heptacene in substance that was doubted and disputed over the last 74 years, and it opens new avenues for acene research. For example, as heptacene can readily be generated from diheptacenes at high temperature, organic vapor phase deposition under high vacuum conditions is possible. This will allow preparation of heptacene films on numerous substrates, fundamental investigations by various techniques of surface science spectroscopy and microscopy, and applications in charge transport systems.

order of these states were also obtained in a very recent paper by Yang et al.76 Hence, the hypothesis that the band at 768 nm could be due to the 11Ag → 21Ag transition appears to be supported by theory. Note that the small computed singlet− triplet energy splitting (see Figure 4b) in favor of the 1Ag state was previously reported by Bendikov et al.77 and is supported by others.14,76,78,79 We conclude that it appears plausible that the additional weak feature in the absorption spectrum of heptacene originates from a transition, which is electric dipole forbidden in the Franck−Condon approximation, to a state that is slightly lower in energy than the first state to which a transition is electric dipole allowed within the Franck−Condon approximation. Further confirmation could come from fluorescence spectroscopy experiments at cryogenic temperatures. As the fluorescence quantum yield of heptacene is presumably quite low,65 such experiments are currently beyond our technical capabilities, but are planned for the future. Alternatively, twophoton transitions, which are electric dipole allowed in the Franck−Condon approximation, could provide further information on the suggested 21Ag state. Film Formation of Heptacene. The thermal cycloreversion of 4 for obtaining parent heptacene has the advantage over the photobisdecarbonylation method that now heptacene films can readily be grown by vapor phase deposition. In this way (10−6 mbar/330 °C), heptacene films were deposited on sapphire at 16 K in the absence of argon gas and investigated by UV−vis spectroscopy in transmission mode. Instead of the resolved p band obtained in argon, a broad, poorly structured band with a maximum of absorption at roughly 800 nm and an onset of absorption at around 900 nm was obtained (Figure 5a). An increase of the film thickness at 16 K did not change the spectra. Annealing the film to 77 K resulted in a slight shift of the absorption maximum to 805 nm, while the onset of absorption shifted beyond 1000 nm (Figure 5b). Further increase of the temperature resulted in a gradual shift of the maximum to 840 nm at room temperature (Figure 5b). The onset of absorption did not shift further, but the intensity of the p band gradually decreased with increasing temperature. Extended deposition at room temperature (Figure 5c) resulted in a film with an absorption maximum at 924 nm that did not show any spectral change over a period of 48 days under vacuum in the dark. The experiments indicate that heptacene can persist under reduced pressure in the absence of oxygen at room temperature in the form of thin films for an extended period of time, although some degree of dimerization cannot be excluded. One of us has previously reported that heptacene photogenerated in argon produced a similarly broad unstructured band at 80 K after careful evaporation of the argon matrix gas.40 However, at room temperature only a continuously increasing baseline was observed, and this was interpreted as disappearance of heptacene due to dimerization.40 It appears that the difference might be primarily due to the significantly larger amount of heptacene deposited in the current experiment.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available for of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.6b13212. Experimental and computational details, synthetic protocols, NMR spectra of compounds 3a/b and 5a/b, and Cartesian coordinates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Holger F. Bettinger: 0000-0001-5223-662X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work in Tübingen was supported by the Deutsche Forschungsgemeinschaft (DFG). The authors acknowledge support by the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/467-1 FUGG. We are very grateful to Bruker Daltonics for measuring high resolution APPI mass spectra. Dr. Klaus Eichele and Dr. Markus Kramer are gratefully acknowledged for measuring the solid state NMR spectra. R.B. acknowledges financial support by the DFG within the SFB 1083 (project B8) and thanks the Center for Scientific Computing (CSC) Frankfurt as well as the Hochschulrechenzentrum Marburg for computer time. T.F. would like to express his gratitude to the late Dr. Orville Chapman for his inspiration and encouragement. H.F.B. thanks Dr. Michael Winkler for many stimulating discussions. It is to his memory that we dedicate this paper.

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REFERENCES

(1) Ortmann, F.; Radke, K. S.; Günther, A.; Kasemann, D.; Leo, K.; Cuniberti, G. Adv. Funct. Mater. 2015, 25, 1933. (2) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012, 112, 2208. (3) Pron, A.; Gawrys, P.; Zagorska, M.; Djurado, D.; Demadrille, R. Chem. Soc. Rev. 2010, 39, 2577. (4) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452. (5) Anthony, J. E. Chem. Rev. 2006, 106, 5028. (6) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. Rev. 2004, 104, 4891. (7) Sirringhaus, H. Adv. Mater. 2014, 26, 1319. (8) Roncali, J.; Leriche, P.; Blanchard, P. Adv. Mater. 2014, 26, 3821. (9) Xiang, D.; Wang, X.; Jia, C.; Lee, T.; Guo, X. Chem. Rev. 2016, 116, 4318. (10) Narita, A.; Feng, X. L.; Müllen, K. Chem. Rec. 2015, 15, 295. (11) Salem, L.; Longuet-Higgins, H. C. Proc. R. Soc. London, Ser. A 1960, 255, 435.

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CONCLUSIONS In summary, we could show that heptacene can form by thermally induced cycloreversion from diheptacenes in the solid state. Quite unexpectedly, heptacene can persist in the solid state for an extended period of time and only slowly undergoes dimerization/oligomerization at room temperature in the solid state. Our observations resolve the controversy over the 4441


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Journal of the American Chemical Society (12) Nakada, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 17954. (13) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Phys. Rev. Lett. 2006, 97, 216803. (14) Hachmann, J.; Dorando, J. J.; Avilés, M.; Chan, G. K.-L. J. Chem. Phys. 2007, 127, 134309. (15) Plasser, F.; Pašalić, H.; Gerzabek, M. H.; Libisch, F.; Reiter, R.; Burgdörfer, J.; Müller, T.; Shepard, R.; Lischka, H. Angew. Chem., Int. Ed. 2013, 52, 2581. (16) Kivelson, S.; Chapman, O. L. Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 28, 7236. (17) Karakonstantakis, G.; Liu, L.; Thomale, R.; Kivelson, S. A. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 224512. (18) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Nature 2006, 444, 347. (19) Bursi, L.; Calzolari, A.; Corni, S.; Molinari, E. ACS Photonics 2014, 1, 1049. (20) Brédas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. Rev. 2004, 104, 4971. (21) Winkler, M.; Houk, K. N. J. Am. Chem. Soc. 2007, 129, 1805. (22) Yelin, T.; Korytar, R.; Sukenik, N.; Vardimon, R.; Kumar, B.; Nuckolls, C.; Evers, F.; Tal, O. Nat. Mater. 2016, 15, 444. (23) Watanabe, M.; Chen, K.-Y.; Chang, Y. J.; Chow, T. J. Acc. Chem. Res. 2013, 46, 1606. (24) Würthner, F. Angew. Chem., Int. Ed. 2001, 40, 1037. (25) Tönshoff, C.; Bettinger, H. F. Top. Curr. Chem. 2013, 349, 1. (26) Bettinger, H. F.; Tönshoff, C. Chem. Rec. 2015, 15, 364. (27) Clar, E. Ber. Dtsch. Chem. Ges. B 1939, 72B, 1817. (28) Marschalk, C. Bull. Soc. Chim. Fr. 1939, 6, 1112. (29) Watanabe, M.; Chang, Y. J.; Liu, S.-W.; Chao, T.-H.; Goto, K.; Minarul Islam, M.; Yuan, C.-H.; Tao, Y.-T.; Shinmyozu, T.; Chow, T. J. Nat. Chem. 2012, 4, 574. (30) Clar, E. Ber. Dtsch. Chem. Ges. B 1942, 75, 1330. (31) Marschalk, C. Bull. Soc. Chim. Fr. 1943, 10, 511. (32) Clar, E.; Marschalk, C. Bull. Soc. Chim. Fr. 1950, 444. (33) Bailey, W. J.; Liao, C.-W. J. Am. Chem. Soc. 1955, 77, 992. (34) Boggiano, B.; Clar, E. J. Chem. Soc. 1957, 2681. (35) Fang, T. Ph.D. Thesis. University of California, Los Angeles, 1986. (36) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Am. Chem. Soc. 2006, 128, 9612. (37) Strating, J.; Zwanenburg, B.; Wagenaar, A.; Udding, A. C. Tetrahedron Lett. 1969, 10, 125. (38) Suzuki, M.; Aotake, T.; Yamaguchi, Y.; Noguchi, N.; Nakano, H.; Nakayama, K.-i.; Yamada, H. J. Photochem. Photobiol., C 2014, 18, 50. (39) Bettinger, H. F.; Mondal, R.; Neckers, D. C. Chem. Commun. 2007, 5209. (40) Mondal, R.; Tönshoff, C.; Khon, D.; Neckers, D. C.; Bettinger, H. F. J. Am. Chem. Soc. 2009, 131, 14281. (41) Tönshoff, C.; Bettinger, H. F. Angew. Chem., Int. Ed. 2010, 49, 4125. (42) Mondal, R.; Adhikari, R. M.; Shah, B. K.; Neckers, D. C. Org. Lett. 2007, 9, 2505. (43) Einholz, R.; Bettinger, H. F. Angew. Chem., Int. Ed. 2013, 52, 9818. (44) Mondal, R.; Okhrimenko, A. N.; Shah, B. K.; Neckers, D. C. J. Phys. Chem. B 2008, 112, 11. (45) Fritzsche, J. J. Prakt. Chem. 1867, 101, 333. (46) Berg, O.; Chronister, E. L.; Yamashita, T.; Scott, G. W.; Sweet, R. M.; Calabrese, J. J. Phys. Chem. A 1999, 103, 2451. (47) Payne, M. M.; Parkin, S. R.; Anthony, J. E. J. Am. Chem. Soc. 2005, 127, 8028. (48) Chun, D.; Cheng, Y.; Wudl, F. Angew. Chem., Int. Ed. 2008, 47, 8380. (49) Kaur, I.; Stein, N. N.; Kopreski, R. P.; Miller, G. P. J. Am. Chem. Soc. 2009, 131, 3424. (50) Qu, H.; Chi, C. Org. Lett. 2010, 12, 3360. (51) Purushothaman, B.; Bruzek, M.; Parkin, S. R.; Miller, A.-F.; Anthony, J. E. Angew. Chem., Int. Ed. 2011, 50, 7013.

(52) Kaur, I.; Jazdzyk, M.; Stein, N. N.; Prusevich, P.; Miller, G. P. J. Am. Chem. Soc. 2010, 132, 1261. (53) Coffey, S.; Boyd, V. J. Chem. Soc. 1954, 2468. (54) Bruckner, V.; Karczag Wilhelms, A. K.; Kormendy, K.; Meszaros, M.; Tomasz, J. Tetrahedron Lett. 1960, 5. (55) Bruckner, V.; Karczag Wilhelms, A.; Kormendy, K.; Meszaros, M.; Tomasz, J. Acta Chim. Acad. Sci. Hung. 1960, 22, 443. (56) Bruckner, V.; Tomasz, J. Acta Chim. Acad. Sci. Hung. 1961, 28, 405. (57) Gaylord, N. G.; Stepan, V. Collect. Czech. Chem. Commun. 1974, 39, 1700. (58) Purushothaman, B.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2010, 12, 2060. (59) Athans, A. J.; Briggs, J. B.; Jia, W.; Miller, G. P. J. Mater. Chem. 2007, 17, 2636. (60) Donati, D.; Guarini, G. G. T.; Sarti-fantoni, P. Mol. Cryst. Liq. Cryst. 1981, 69, 241. (61) Grimme, S.; Diedrich, C.; Korth, M. Angew. Chem., Int. Ed. 2006, 45, 625. (62) Zade, S. S.; Zamoshchik, N.; Reddy, A. R.; Fridman-Marueli, G.; Sheberla, D.; Bendikov, M. J. Am. Chem. Soc. 2011, 133, 10803. (63) Clar, E. Chem. Ber. 1949, 82, 495. (64) Platt, J. R. J. Chem. Phys. 1949, 17, 484. (65) Nijegorodov, N.; Ramachandran, V.; Winkoun, D. P. Spectrochim. Acta, Part A 1997, 53, 1813. (66) Einholz, R. Dr. rer. nat. Thesis. Universität Tübingen, 2016. (67) Berger, R.; Fischer, C.; Klessinger, M. J. Phys. Chem. A 1998, 102, 7157. (68) Jankowiak, H. C.; Stuber, J. L.; Berger, R. J. Chem. Phys. 2007, 127, 234101. (69) Marian, C. M.; Gilka, N. J. Chem. Theory Comput. 2008, 4, 1501. (70) Hudson, B. S.; Kohler, B. E. Chem. Phys. Lett. 1972, 14, 299. (71) Schulten, K.; Karplus, M. Chem. Phys. Lett. 1972, 14, 305. (72) Deperasińska, I.; Zehnacker, A.; Lahmani, F.; Borowicz, P.; Sepioł, J. J. Phys. Chem. A 2007, 111, 4252. (73) Karabunarliev, S.; Baumgarten, M.; Müllen, K. J. Phys. Chem. A 1998, 102, 7029. (74) Bettinger, H. F.; Tönshoff, C.; Doerr, M.; Sanchez-Garcia, E. J. Chem. Theory Comput. 2016, 12, 305. (75) Fink, R.; Staemmler, V. Theor. Chim. Acta 1993, 87, 129. (76) Yang, Y.; Davidson, E. R.; Yang, W. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, E5098. (77) Bendikov, M.; Duong, H. M.; Starkey, K.; Houk, K. N.; Carter, E. A.; Wudl, F. J. Am. Chem. Soc. 2004, 126, 7416. (78) Casanova, D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2009, 11, 9779. (79) Chakraborty, H.; Shukla, A. J. Phys. Chem. A 2013, 117, 14220.

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Heptacene: Characterization in Solution, in the Solid State, and in

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