Tuning Transition Electric and Magnetic Dipole Moments: [7]Helicenes Showing Intense Circularly Polarized Luminescence
Hiromu Kubo, Takashi Hirose, Takuya Nakashima, Tsuyoshi Kawai, Jun‐ya Hasegawa, Kenji Matsuda
Abstract
Helicenes are promising candidates for chiral optoelectronic materials because of their helically twisted π-conjugated system. However, the emission intensity of unsubstituted helicenes is very weak (Φf < 0.05) due to a small oscillator strength for the S1 → S0 transition. In this work, we investigated the substitution position of the [7]helicene framework so that the S1 → S0 transition has a large transition magnetic dipole moment (TMDM) and is partially symmetry-allowed. A [7]helicene derivative thus designed showed a large fluorescence emission rate (kf = 0.02 ns–1) and a large TMDM for the S1 → S0 transition (|m| = 2.37 × 10–20 erg·Gauss–1), which are more than 10 times greater than those of unsubstituted [7]helicene (kf = 0.001 ns–1, |m| = 0.045 × 10–20 erg·Gauss–1). As a result, we achieved the [7]helicene derivative whose dissymmetry factor of CPL and fluorescence quantum yield were both high (|gCPL| = 1.3 × 10–2, Φf = 0.17) in the solution phase.