Antiaromatic character of cycloheptatriene-bis-annelated indenofluorene framework mainly originated from heptafulvene segment

Fully π-conjugated polycyclic hydrocarbons with antiaromatic character have attracted research attention because of their unique properties such as narrow energy gaps, and thus should find application as optical and electronic materials. Although antiaromatic 16π-electron frameworks can be constructed by the incorporation of multiple seven-membered rings in a fused fashion to install methylenecycloheptatriene (heptafulvene) segments, the development of corresponding benzo[1,2:4,5]di[7]annulene (BDA)-containing π-conjugated systems remains challenging due to the difficulty of their molecular design and synthesis. In this study, we develop an unprecedented chemical structure of cycloheptatriene-bis-annelated indenofluorene, which possesses both BDA and indenofluorene frameworks in a fused fashion. Physical measurements and X-ray analyses, along with theoretical calculations, indicated that antiaromaticity appeared in the BDA framework. By using the conjugated polycyclic hydrocarbon possessing both seven-membered and five-membered rings, this study provides fundamental insight into the strong antiaromatic nature of heptafulvene-based BDA framework.

unit, the formation of cycloheptatrienyl cation by one-electron oxidation of heptafulvene leads to aromatic stabilization. Thus, the thin-film of 1 showed hole-transporting characteristics in organic field-effect transistor (OFET) devices [44][45][46] . However, irrespective of these electronically complemental properties, fundamental studies to directly investigate the antiaromatic character between the ID and BDA frameworks have not been carried out so far. Therefore, we sought to construct an unprecedented π-conjugated polycyclic hydrocarbons by introducing multiple five-membered and seven-membered rings together in a conjugated molecule. Based on this strategy, we combined ID and BDA, resulting in the molecular framework of dicyclohepta[cd,ij]-s-indacene (DCHI), and synthesized the benzene-fused cycloheptatriene-bis-annelated indenofluorene 2 ( Fig. 1). Herein, we describe the synthesis, structure, properties, and OFET characteristics of 2.

Results and Discussion
Although the formation of the DCHI framework by photochemical reaction of o-diethynylbenzene derivatives was previously inferred 48 , it was disproven based on theoretical calculations of the spectroscopic data 49 and reconsidered 50 . Therefore, the synthesis of this framework was heretofore unknown. With these precedents in mind, we used an olefin metathesis reaction as a key step to construct the seven-membered ring framework. The synthetic route to 2 starting from compound 3 is shown in Fig. 2. The Suzuki coupling reaction between 3 and allylboronic acid pinacol ester in the presence of [Pd(allyl)Cl] 2 afforded di-allylated compound 4, which was reacted with allylmagnesium bromide to give the tetra-allylated compound 5. The construction of seven-membered rings was achieved by olefin metathesis using the second generation Grubbs' catalyst to give key intermediate 6 in 95% yield 51 . We here report that compounds 5 and 6 were isolated as a mixture of diastereomers. Then, treatment of 6 with the Burgess reagent provided dehydrated compound 7 52 , and subsequent oxidation with 2,3-dichloro-5,6 -dicyano-p-benzoquinone (DDQ) produced the target compound 2 in 50% yield. Note that the presence of the hexyl group is essential to impart enough solubility of 2 to enable characterization by NMR and other spectroscopic measurements. Detailed synthetic procedures and characterization data of the new compounds are summarized in the Methods section. The NMR spectra of 2 are shown in Supplementary Figs 1 and 2. As shown in Supplementary Fig. 2, the 1 H NMR spectrum of 2 in CDCl 3 showed a downfield shift of the benzene protons by 0.42-0.62 ppm as compared to the indenofluorene derivative IF(Me)-TA (structure shown in Supplementary  Fig. 2), indicating the increased contribution of aromatic character in the benzene ring for 2. The proton signals in the seven-membered ring were observed in the upfield region between 5.95 and 7.29 ppm. Since the 1 H NMR signals for the seven-membered ring of the antiaromatic BDA framework are reported to appear in a similar δ range of 4.88-6.30 ppm 41 , contribution of the antiaromatic character in the seven-membered ring of 2 is expected. Thermogravimetric analysis (TGA) of 2 showed a 5% weight-loss temperature of 443 °C under a nitrogen atmosphere ( Supplementary Fig. 3(a)). The differential scanning calorimetry (DSC) profile of 2 showed endothermic and exothermic peaks at 234 and 250 °C, respectively, during the first heating process (Supplementary Fig. 3(b)). On the other hand, no clear peaks were detected during the second heating process, and an insoluble black solid was observed after the DSC measurements. Given that only a slight weight loss was seen in TGA up to 443 °C, we considered that 2 melted at 234 °C, and intermolecular reactions occurred in the melting state to give unidentifiable insoluble products. Since the indenofluorene derivatives include a p-quinodimethane core, it is known that these molecules show biradical character originating from the aromatization of the central benzene ring 53 . Thus, the singlet biradical characters (y) of 2(Me) and IF were estimated using the occupation numbers of the spin-unrestricted Hartree-Fock natural orbitals. As a result, 2(Me) displayed a moderate singlet biradical character (y = 0.49), which is larger than that of IF (y = 0.30). Since the calculated spin density for 2(Me) shows the largest amplitude at the fused carbons between the five-and seven-membered rings and is distributed to the seven-membered ring (Fig. 3), the biradicaloid electronic structure is thought to be stabilized by the spin delocalization at the seven-membered rings. The theoretically estimated singlet-triplet energy gap (ΔE S-T ≡ E T − E S ) of 2(Me) at the UB3LYP/6-31 G(d,p) level is a large positive value of 92.5 kJ mol −1 , indicating that the biradical structure of 2 has an exclusively singlet nature, which is consistent with the observation of sharp 1 H NMR signals ( Supplementary Fig. 1).
The electrochemical behavior of 2 was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements in CH 2 Cl 2 containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as a supporting electrolyte. All potentials were calibrated against a ferrocene/ferrocenium (Fc/Fc + ) couple as the standard. As shown in Fig. 4, the CV of 2 revealed three oxidation and two reduction processes, and the redox potentials were determined from DPV. From the first oxidation potential (E ox1 ) and first reduction potential (E red1 ) and the assumption that the energy level of Fc/Fc + is −4.8 eV below the vacuum level [54][55][56] , the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels (E HOMO and E LUMO ) of 2 were estimated to be −4.69 and −3.46 eV, respectively. Based on these values, the HOMO-LUMO energy gap of 2 is calculated to be 1.23 eV. Interestingly, these E HOMO and E LUMO values are significantly different from those of IF(Me)-TA (E HOMO = −5.84 eV and E LUMO = −3.99 eV) 30 . In order to understand the origin of this phenomenon, density functional theory (DFT) calculations of 2(H), IF, and BDA at the B3LYP/6-311 + G(d, p) level were performed. As shown in Fig. 5, the theoretically estimated E HOMO and E LUMO values of 2(H) and IF qualitatively mimic the experimental values. Although the HOMO and LUMO orbitals of 2(H) are delocalized over the π-conjugated backbones, both the E HOMO and E LUMO values of 2(H) are between those of BDA and IF and closer to those of BDA. These results indicate that the electronic structure of the 2(H) molecule is formed from a hybrid of those of BDA and IF, and that the contribution of the electronic character of BDA seems to be dominant.  To investigate the photophysical properties, a UV-vis-NIR absorption measurement of 2 in a CH 2 Cl 2 solution was performed. As shown in Fig. 6, the absorption spectrum of 2 includes three intense bands with absorption maxima (λ max ) at 279 (ε = 55000 M −1 cm −1 ), 498 (36000 M −1 cm −1 ), and 692 (26000 M −1 cm −1 ) nm and weak bands at 866 and 997 nm. A time-dependent (TD)-DFT calculation at the B3LYP/6-31 G (d,p) level indicated that these intense bands were mainly assigned to HOMO − 1 to LUMO + 1, HOMO to LUMO + 2, and HOMO to LUMO transitions, respectively (see the Electronic Supplementary Information). Considering that 2(Me) possesses a moderate singlet biradical character (Fig. 3), the weak bands in the NIR region are assignable to the   57,58 . Compared to the solution spectra, a new broad band was observed in the thin-films. This phenomenon is attributed to the intermolecular electronic interactions of π−π stacked backbones, which is favorable for carrier transport in thin-film devices (discussed later). The molecular structure of 2 was unambiguously determined using X-ray crystallographic analysis of crystals grown by the slow evaporation of hexane/CHCl 3 solutions. As shown in Fig. 7(a), the π-conjugated framework of compound 2 holds a nearly planar structure: the deviations of carbon atoms constituting the BDA core from the mean plane of this core are less than 0.04 Å, and the dihedral angles between the mean planes of central BDA and the outer benzene rings are 4.9°, as shown in Fig. 7(a). The structure of 1 was reported to show clear bond alternation in the p-quinodimethane core due to the increased contribution from resonance form 1a (structure shown in Supplementary Fig. 4), which in turn is attributed to the presence of the outer fused benzene rings 44 . On the other hand, the extent of the bond alternation for the BDA framework in 2 was rather small, with C-C bond lengths varying within the range of 1.37-1.44 Å (Fig. 7(b)). To further assess the degree of bond alternation, we calculated the harmonic oscillator model of aromaticity (HOMA) values 59 based on the reported X-ray information. As shown in Supplementary Fig. 5, the HOMA value of the BDA framework in 1 was determined to be 0.31. In contrast, the HOMA value of the BDA framework in 2 was calculated to be as high as 0.81, supporting the small degree of bond alternation and resulting delocalized electronic structure. A similar trend was also observed for the antiaromatic ID framework: the absence of fused benzene rings in compound A leads to an increase in the HOMA value ( Supplementary Fig. 5). These results indicate that pristine antiaromatic frameworks show large HOMA values; thus, the BDA framework in 2 is expected to show antiaromatic character. Interestingly, this HOMA value is larger than that of the indenofluorene framework in 2 (0.56), indicating that the contribution of 16π (BDA) and 6π × 2 (two outer benzene rings) is larger than that of 20π (indenofluorene) in the electronic structure of 2.
To investigate the electronic contribution of the π electrons in 2, we performed an electron localization function (ELF) estimation 60 . The ELF isosurface plot (Fig. 7(c)) showed that the π electron pairs are uniformly distributed over the entire molecule. This result clearly indicates that the π-conjugation is delocalized over the entire π conjugated framework in 2, despite the different HOMA values of BDA and IF frameworks.
Based on this molecular structure, we estimated the aromaticity of the π-conjugated framework of 2(H) by the nucleus independent chemical shift (NICS). The results of the NICS(1.7) πzz -XY-scans 61 are summarized in On the other hand, the indenofluorene core composed of rings C and D in 2(H) exhibits an aromatic character, which is in contrast with the result of pristine indenofluorene IF, in which the five-membered ring shows an antiaromatic character 33 . Since framework 2(H) contains both five-and seven-membered rings, this calculated aromatic character of the C ring is considered to result from increased electron density owing to electrical polarization between these rings. This was confirmed by the calculation of the electrostatic potential of 2(H), which was similar to that calculated for benz[a]azulene, as shown in Supplementary Fig. 6. These results are in agreement with the aforementioned findings based on the HOMA values, indicating the large electronic contribution of the BDA framework. To further support the antiaromaticity of the BDA framework in 2, we conducted anisotropy of the current-induced density (ACID) analysis 62 . Compound 2 showed continuous paratropic ring currents in the BDA framework (Fig. 9). On the other hand, diatropic ring currents were seen in the C 5 -C 6 frameworks. This result clearly indicates that 2 has local antiaromaticity in the BDA framework and local aromaticity in the C 5 -C 6 frameworks, consistent with the results of NICS calculation. In the molecular packing diagram, 2 takes a herringbone π-stacked motif with minimum intermolecular π-π distances of 3.42 Å (Supplementary Fig. 7(a)). On the basis of the calculation by the Amsterdam Density Functional (ADF) program at the PW91/TZP level, the transfer integrals for hole transport (t HOMO ) and electron transport (t LUMO ) between adjoining molecules were estimated. As summarized in Supplementary Fig. 7(b), 2 showed large t HOMO and t LUMO of 91.4 and 111.6 meV, respectively, between facial-stacked molecules, and, thus, the construction of charge-carrier transporting pathways is expected along the stacking direction. Reflecting the high thermal stability of 2, the thin-films for OFET measurements could be prepared by vacuum deposition onto hexamethyldisilazane (HMDS)-modified Si/SiO 2 substrates. The atomic force microscopy (AFM) image of this film exhibited interconnected micrometer-sized grains ( Supplementary Fig. 8(a)). X-ray diffraction (XRD) of the thin-film showed clear diffractions, indicating the formation of crystalline structures in thin-films. According to the X-ray crystal structure ( Supplementary Fig. 7(a)), the peak at 2θ = 5.2° can be indexed as a (001) diffraction peak with a d spacing of 17.0 Å, implying that the molecules are aligned with the crystal c-axis perpendicular to the SiO 2 surface. To evaluate the charge-transport characteristics of the thin-films, OFET devices with bottom-gate bottom-contact configuration were fabricated. As shown in Fig. 10, this device showed hole-transporting characteristics with a field-effect hole mobility of 3.0 × 10 −5 cm 2 V −1 s −1 with a current on/ off ratio of 10 5 . On the other hand, electron-transporting behavior was not observed for 2. This p-type response is explained by the high-lying HOMO energy level of 2. Importantly, taking the intrinsic electron-transporting behavior of the indenofluorene chromophore into consideration, embedding the BDA framework significantly influences the type of charge carriers. This trend is in good agreement with the estimated antiaromaticity of the BDA framework.

Summary
In conclusion, in order to directly investigate the antiaromatic character between the ID and BDA frameworks, we successfully synthesized new polycyclic hydrocarbon 2, which contains a fused dicyclohepta[cd,ij]-s-indacene framework in the molecule. Electrochemical and photophysical measurements revealed that 2 has a relatively high HOMO energy level and a narrow HOMO-LUMO energy gap, and these properties mainly come from the BDA  framework. Investigation of the X-ray crystal structure and theoretical calculation indicates that the contribution of antiaromatic character for the BDA framework is dominant in the molecule. These results clearly demonstrate that the unique character of 2 originated from the heptafulvene-based BDA framework, and we can conclude that the development of new antiaromatic compounds possessing the DCHI or BDA frameworks will pave the way to the fundamental understanding of antiaromaticity. Considering that 2 shows hole-transporting characteristics, fine-tuning of molecular design would aid the development of high-performance electronic materials. Further investigation on the development of such compounds to reveal the structure-property-semiconducting performance relationship is currently underway in our group.

Methods
General Information. Column chromatography was performed on silica gel. KANTO Chemical silica gel 60N (40-50 μm). Thin-layer Chromatography (TLC) plates were visualized with UV light. Preparative gel-permeation chromatography (GPC) was performed on a Japan Analytical LC-918 equipped with JAI-GEL 1H/2H. 1 H and 13 C NMR spectra were recorded on a JEOL JNM-ECS400 or JEOL JNM-ECA600 spectrometer in CDCl 3 with tetramethylsilane (TMS) as an internal standard. Data are reported as follows: chemical shift in ppm (δ), multiplicity (s = singlet, d = doublet, t = triple, m = multiplet, br = broad), coupling constant (Hz), and integration. UV-vis-NIR spectra were recorded on a Shimadzu UV-3600 spectrophotometer. All spectra were obtained in spectrograde solvents. TGA and DSC were performed under nitrogen at a heating rate of 10 °C min −1 with a Shimadzu TGA-50 and a Shimadzu DSC-60, respectively. Cyclic voltammetry was carried out on a BAS CV-620C voltammetric analyzer using a platinum disk as the working electrode, platinum wire as the counter electrode, and Ag/AgNO3 as the reference electrode at a scan rate of 100 mV s −1 . High-resolution mass spectrum (HRMS) was obtained atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI) methods using a Thermo scientific LTQ Orbitrap XL. Elemental analyses were performed on PerkinElmer LS-50B by the elemental analysis section of the Comprehensive Analysis Center (CAC) of ISIR, Osaka University. The surface structures of the deposited organic film were observed by atomic force microscopy (Shimadzu, SPM9600), and the film crystallinity was evaluated by an X-ray diffractometer (Rigaku, SmartLab). X-ray diffraction patterns were obtained using Bragg-Brentano geometry with CuKα radiation as an X-ray source with an acceleration voltage of 45 kV and a beam current of 200 mA. The scanning mode was set to 2θ-θ scans between 2°-30° with scanning steps of 0.01°.

Synthetic information.
Unless stated otherwise, all reagents were purchased from commercial sources and used without purification. Synthetic procedure of 3 was shown in Supplementary Fig. 9, and the corresponding characterization data was summarized in the Electronic Supplementary Information.

Synthesis of 2.
To a solution of 7 (22 mg, 0.042 mmol) in CH 2 Cl 2 (3.2 mL), a solution of DDQ (9.6 mg, 0.042 mmol) in CH 2 Cl 2 (1.0 mL) was added slowly at −40 °C. After stirring for 20 min, the reaction was allowed to warm to ambient temperature. The solvent was removed under reduced pressure and the residue was washed with acetone to give 2 (11 mg, 50%) as a dark green solid. Recrystallization from hexane/CHCl 3 gave a pure compound.