Substitution Engineering in Triarylborane-Based Aza[7]helicenes for Photophysical Property Tuning

In this work, we have designed and synthesized a series of donor-acceptor-type triarylborane-based aza[7]helicenes, diphenanthro[9,10-c:9',10'-g]carbazole (A7H), to experimentally and theoretically explore the effect of substitution pattern on the photophysical properties. For NBPh-A7H, in which 4-(dimesitylboryl)phenyl is attached at N atom, the aza[7]helicene moiety mainly contributes to HOMO. However, the aza[7]helicene unit is involved in both HOMO and LUMO for oBPh-NH-A7H, oBPh-NMe-A7H, and especially oB-NMe-A7H, in which 4-(dimesitylboryl)phenyl or dimesitylboryl is introduced at the ortho-position of the N atom. As a consequence, oBPh-NH-A7H, oBPh-NMe-A7H, and oB-NMe-A7H are featured by a hybrid locally excited (LE) and charge-transfer (CT) excited state. Despite the twisted intramolecular charge transfer (TICT) nature for the excited state of NBPh-A7H, its emission is always dominated by two vibrational bands that may result in a LE state without contribution from the boryl group. Moreover, the thorough substitution engineering made it possible to achieve intense fluorescence in both solution and solid state. The compound oBPh-NH-A7H can show high Φ_Fs in various solutions (0.42-0.61) and solid state (0.66). In addition, this compound can display intense CPL with |g_lum| and B_CPL up to 1.94 × 10^-3 and 69.1 M^-1 cm^-1, respectively, suggesting its great potential utility as a promising CPL emitter.