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

Abstract

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 Me2 N group or an electron-withdrawing NC group onto the quinoxaline unit (denoted as Me2 N-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 (ΦFL ) were achieved. In particular, the maximum ΦFL of Me2 N-QTTH was 0.43 in benzene (NC-QTTH: ΦFL =0.30), which is more than 20 times larger than that of a pristine tetrathia[9]helicene (denoted as TTH; ΦFL =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 ΦFL values thus enabled us to show the excellent CPL properties. The value of anisotropy factor gCPL (normalized difference in emission of right-handed and left-handed circularly polarized light) was estimated to be 3.0 × 10(-3) for NC-QTTH.