Tuning Transition Electric and Magnetic Dipole Moments: [7]Helicenes Showing Intense Circularly Polarized...

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 (Φ_f < 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 (k_f = 0.02 ns^-1) and a large TMDM for the S₁ → S₀ transition (|| = 2.37 × 10^-20 erg·Gauss^-1), which are more than 10 times greater than those of unsubstituted [7]helicene (k_f = 0.001 ns^-1, || = 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 (|g_CPL| = 1.3 × 10^-2, Φ_f = 0.17) in the solution phase.