Synthetic Control of the Excited-State Dynamics and Circularly Polarized Luminescence of Fluorescent "push-Pull" Tetrathia[9]helicenes

A series of fluorescent "push-pull" tetrathia[9]helicenes based on quinoxaline (acceptor) fused with tetrathia[9]helicene (donor) derivatives was synthesized for control of the excited-state dynamics and circularly polarized luminescence (CPL) properties. In this work, introduction of a quinoxaline onto the tetrathia[9]helicene skeleton induced the "push-pull" character, which was enhanced by further introduction of an electron-releasing Me₂N group or an electron-withdrawing NC group onto the quinoxaline unit (denoted as Me₂N-QTTH and NC-QTTH, respectively). These trends were successfully discussed in terms of by electrochemical measurements and density functional theory (DFT) calculations. As a consequence, significant enhancements in the fluorescence quantum yields (Φ_FL) were achieved. In particular, the maximum Φ_FL of Me₂N-QTTH was 0.43 in benzene (NC-QTTH: Φ_FL=0.30), which is more than 20 times larger than that of a pristine tetrathia[9]helicene (denoted as TTH; Φ_FL=0.02). These enhancements were also explained by kinetic discussion of the excited-state dynamics such as fluorescence and intersystem crossing (ISC) pathways. Such significant enhancements of the Φ_FL values thus enabled us to show the excellent CPL properties. The value of anisotropy factor g_CPL (normalized difference in emission of right-handed and left-handed circularly polarized light) was estimated to be 3.0×10^-3 for NC-QTTH. Controlled push-pull: A series of fluorescent "push-pull" thiahelicenes based on quinoxaline-fused tetrathia[9]helicene derivatives was synthesized to control the excited state dynamics and circularly polarized luminescence properties. The maximum Φ_FL values of 0.43 and 0.30 were obtained in benzene.