Negatively Curved Nanographene with Heptagonal and [5]Helicene Units

Negatively curved nanographene (NG) 4, having two heptagons and a [5]helicene, was unexpectedly obtained by aryl rearrangement and stepwise cyclodehydrogenations. X-ray crystallography confirmed the saddle-shaped structures of intermediate 3 and NG 4. The favorability of rearrangement over helicene formation following radical cation or arenium cation mechanisms is supported by theoretical calculations. NG 4 demonstrates a reversible mechanochromic color change and solid-state emission, presumably benefiting from its loose crystal packing. After resolution by chiral high-performance liquid chromatography, the circular dichroism spectra of enantiomers 4-(P) and 4-(M) were measured and showed moderate Cotton effects at 350 nm (|Δε| = 148 M–1 cm–1).

C urved nanographenes (NGs), obtained by incorporating nonhexagonal rings into the "honeycomb" framework, have become exciting targets in different scientific fields. 1−4 The resulting surfaces, which can be classified as having positive or negative Gaussian curvatures, furnish NGs with unique properties. 1 NGs containing pentagons have positive curvature and possess bowl-shaped structures. Thus, corannulene-based NGs can be considered as subunits of fullerene, 5,6 and they have been used as caps in the chemical synthesis of carbon nanotubes; 7 bischrysenyl molecules with fused pentagons exist as stable open-shell singlet biradicals and hold promise for quantum computing. 8,9 On the other hand, seven-or eight-membered rings induce negative curvature. Saddle-shaped NGs containing heptagons often demonstrate dynamic stereochemistry, which can lead to fluorescence quenching in solution. 10 Moreover, the syntheses and studies of heptagon-incorporating NGs could stimulate the investigation of elusive carbon nanostructures, such as carbon schwarzites and Mackay crystals. 11−14 Compared with their positively curved counterparts, negatively curved NGs with heptagons are rare and deserve more attention.
The first reported heptagon-embedded NG, [7]circulene was achieved by Yamamoto in 1983 by intramolecular photocyclization and reductive coupling. 15,16 Since then, several methods to construct seven-membered carbocycles, including ring expansion, 17,18 cyclotrimerization, 19 and intramolecular cyclization, 20−26 have been developed. Among these bottom-up approaches, multiple intramolecular cyclizations by the Scholl reaction are particularly powerful, as demonstrated by the grossly warped NG reported by Scott and Itami. 27 However, the outcome of the Scholl reaction is sometimes surprising, even counterintuitive, since it can be accompanied by migrations and rearrangement processes. 28−32 During our attempts at synthesizing π-extended [7]helicene 2 through the oxidative cyclodehydrogenation of precursor 1, we unexpectedly obtained a negatively curved NG 4 with two seven-membered rings and a [5]helicene substructure through an aryl rearrangement and cyclodehydrogenation sequence (Scheme 1). The structures of both intermediate 3 and final NG 4 were unambiguously identified by X-ray crystallography analysis. Density functional theory (DFT) calculations revealed that rearrangement via spiro-ring formation was favored over helicene formation by either radical cation or arenium cation mechanism. NG 4 is fluorescent in the solution and solid states and demonstrated a reversible change in color and emission between crystalline and amorphous powders, presumably due to its loose crystal packing. Because of the incorporation of heptagons and a [5]helicene, NG 4 was twisted into a saddleshaped curvature with an experimental isomerization barrier of 25.4 kcal mol −1 , enabling the separation of its enantiomers by high-performance liquid chromatography (HPLC) with a chiral column and subsequent circular dichroism (CD) measurements.
Further reacting intermediate 3 with DDQ/TfOH in dry DCM at 40°C under argon furnished product 4 in 71% yield. MALDI-TOF MS analysis of 4 displayed an intense signal at m/z = 736.31 ( Figure S14, calculated value for C 58 H 40 : 736.31). Although this mass is the same as that of π-extended [7]helicene 2, the 1 H and 13 C NMR spectra of 4 suggested a highly unsymmetrical structure (Figures S15−S16).
Crystals of 1, 3, and 4 suitable for single-crystal analyses by X-ray diffraction could be obtained by slow diffusion of ethanol into their chloroform or dichloromethane solutions (Figures 1A−C, S21−S22). Excitingly, NG 4 revealed a unique structure with two heptagons as well as a [5]helicene (marked by black arrows in Figure 1A) with a torsion angle of 35.2°( atoms 1−2−3−4). Due to the two heptagonal subunits, NG 4 was negatively curved into a saddle shape with out-of-plane deformed benzenoid rings ( Figure 1B). This saddle was 9.79 Å wide and 3.36 Å deep from the upper part, while it was 9.31 Å wide and 2.79 Å deep from the lower part. P/M enantiomers of 4, denoted 4-(P) and 4-(M) (highlighted in blue and red, respectively), were observed, and they stacked in an alternating manner to form pairs of enantiomers ( Figure 1C). The intermolecular distances of the P/M enantiomer pairs were measured as d 1 = 3.90 Å and d 2 = 3.83 Å, suggesting that 4 was loosely packed, reflecting its highly twisted structure.
Assisted by its crystal structure, 2D NMR analyses, and simulated 1 H NMR spectrum (Figures S17−S20), all the aromatic proton peaks in the 1 H NMR spectrum of NG 4 were assigned ( Figure 1D). Notably, while the chemical shift of proton 20 was still at 8.37 ppm, the signal of proton 21 was shifted upfield to 6.42 ppm. This was in line with the shielding effect expected from the curved surface. Nucleus-independent chemical shift (NICS) calculations of NG 4 at the GIAO-B3LYP/6-311G (d,p) level ( Figure S36 To understand the rearrangement during the cyclodehydrogenation of precursor 1 leading to NG 4 instead of πextended helicene 2, DFT calculations were performed at the (U)ωB97X-D/6-31+G(d,p)//(U)ωB97X-D/6-31G(d) level of theory using the SMD solvation model with DCM. According to the literature, 29,39,40 two possible intermediates, radical cation R1 and arenium cation A1, were proposed for the key step for comparing the energy barriers in the "rearrangement" and "helicene" pathways. Radical cation intermediate R1 can undergo C−C bond formation at either the ipso or ortho position of the central benzene ring, with respect to a naphthylphenyl group (Scheme 2A). The C−C ipso Journal of the American Chemical Society pubs.acs.org/JACS Communication bond formation leading to spiro intermediate R2 was found to proceed through a transition state (TS) with a lower energy (TS R1R2 = 19.9 kcal/mol) than that toward R6 by C−C ortho bond formation (TS R1R6 = 20.9 kcal/mol). Subsequent H atom abstraction (R3), 1,2-migration of the phenyl group (R4), and deprotonation afforded rearranged intermediated R5 with the formation of a new C−C bond. R5 could undergo further dehydrogenative couplings to afford 3 and 4. This mechanism is analogous to that involving a 1,2-aryl migration in the oxidative coupling of tetraarylpyrrolopyrroles. 41 Similarly, the energy barriers were also calculated through arenium intermediate A1, and the rearrangement was again more favorable than helicene formation (Scheme 2B). The possible mechanisms involving arenium cation intermediates with protonation on the central benzene ring were excluded due to higher energies in transition states ( Figure S35 for more details). Therefore, the rearrangement occurs in the first step of the dehydrogenation and is favored over helicene formation in both the radical cation and arenium cation mechanisms. While the absorption spectrum of 3 exhibited a maximum (λ abs ) at 380 nm, the λ abs of 4 was red shifted to 392 nm, in agreement with its extended π-conjugation ( Figure S23 Interestingly, when the DCM was evaporated from the NG 4 solution, a nanocrystalline powder was formed, displaying yellow fluorescence with an emission peak at 550 nm ( Figure  2A). Inspired by its loose crystal packing and structural flexibility, the mechanochromic behavior 42 of 4 was explored. When the nanocrystalline powder of 4 obtained from DCM was ground into an amorphous powder, the emission peak was red shifted to 610 nm, which was almost identical to that in solution. The amorphous powder could be transformed into yellow nanocrystals by treatment with DCM vapor for 10 min. This grinding-fuming sequence could be reversibly repeated for 5 cycles without obvious fatigue ( Figure S30). Notably, 4 formed a slightly different crystal when crystallized from THF solution, showing an emission peak at 565 nm. The shifting of emission wavelength was caused by the presence of solvent molecules, which was confirmed by the X-ray analysis (Figure  Journal of the American Chemical Society pubs.acs.org/JACS Communication S31, CCDC: 2021692). These results indicated that the mechanochromic behavior of 4 was caused by different molecular packing modes in the solid states, which was also supported by different diffraction patterns in powder X-ray diffraction (PXRD) measurements ( Figure 2B). Such reversible mechanochromic behavior is unusual in curved nanographenes with only fused hydrocarbons, 43,44 suggesting potential applications of 4 in mechanosensors, security papers, optical storage, etc. 45,46 The isomerization barrier between 4-(P) and 4-(M) was calculated by DFT to be 26.1 kcal mol −1 ( Figure 3A) and experimentally determined as 25.4 kcal mol −1 (Figures S32−  S33), which was higher than those of previously reported heptagon-bearing NGs 17,18,22,27 but similar to other [5]carbohelicenes. 47−50 4-(P) and 4-(M) could indeed be separated by chiral HPLC (Figure S34). CD spectra of the first HPLC fraction demonstrated a negative Cotton effect at 350 nm with a moderate value (|Δε| = 148 M −1 cm −1 ), while the enantiomer that eluted second displayed a mirror image CD curve with a positive Cotton effect, which was in good agreement with the simulated spectra calculated by timedependent DFT (TD-DFT) ( Figure 3B). The first and second HPLC fraction could thus be assigned to 4-(M) and 4-(P), respectively. The hole−electron analysis of the major transitions 36,51 (first, second, and ninth excited states in Figures S38−S39) suggest that the CD signal at >450 nm can be attributed to the twisted π-backbone of the whole molecule, while the configuration of [5]helicene structure is more related to the CD signal at 350 nm.
In summary, the oxidative cyclodehydrogenation of precursor 1 provided unprecedented NG 4 with a negative curvature through an aryl rearrangement, which was unambiguously revealed by X-ray crystallography. With two heptagonal and one [5]helicene substructure, NG 4 was highly twisted into a saddle shape with two enantiomers, 4-(P) and 4-(M), which could be resolved by chiral HPLC. According to theoretical studies, the energy barriers of the rearrangement pathways are lower than those for helicene formation in both the radical cation and arenium cation mechanisms, rationalizing the experimental results. The unexpected rearrangement observed in this study has inspired us to systematically investigate the Scholl reactions of related precursors. Moreover, it facilitates the design of modified precursors leading to π-extended helicenes and other curved NGs in subsequent studies.