Theoretical Study on the Competition Mechanism between ESIPT and TICT in N-H Indanone Derivatives

A series of amino (NH)-type hydrogen bond (H-bond) compounds comprising indanone derivatives were investigated, which exhibit distinct photophysical behavior compared with conventional hydroxyl (OH)-type H-bond systems. While these compounds similarly display mono- or dual-fluorescence characteristics, they exhibit remarkably low emission quantum yields (Φ < 9%). Through comprehensive analyses of H-bond dynamics, electronic structures, and excited state dynamics/kinetics, we demonstrated a barrier-dependent dynamic competition between excited state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT). Crucially, the nonradiative pathway along the TICT coordinate was conclusively associated with intersystem crossing. This mechanism successfully elucidates the commonly observed but often overlooked low emission quantum yields in most NH-type ESIPT systems. Intriguingly, our findings preliminarily disclose that smaller substituents R are actually unfavorable for lowering the torsional barrier.