Electronic structure of helicenes, C2S helicenes, and thiaheterohelicenes

The structures of linearly and ortho-fused helical polyaromatic hydrocarbon oligomers and polymers and their isoelectronic thiophene variants are studied using density functional theory (DFT). Structural and optical absorption data are compared with experiments where possible and excellent agreement is obtained. The results are interpreted with reference to orbital interaction diagrams. Infinite helicene tends to adopt a symmetry close to 6(1). C2S helicene is predicted to have an approximately 263 symmetry leading to an interdigitated S center dot center dot center dot S network parallel to the helical axis. Thiaheterohelicene has an approximately 7(2) helical structure. Periodic boundary condition (PBC) calculation and highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap extrapolation at the B3LYP/6-31G* level indicate a smaller band gap for helicene compared to phenacene. This difference is mainly due to the gap reducing effects of the transannular pi-pi interactions across the helical pitch in helicene. The calculated band gap is much smaller for linear thienoacene than that for isomeric C2S helicene due to the lack of effective conjugation pathway for the latter system. While thiaheterohelicene is structurally between the two large gap systems, helicene and C2S helicene, its band gap is significantly lower than either of the two.