Substituent-modulated excited-state dynamics and solvent-sensitized nanosecond triplet harvesting in donor–acceptor TADF emitters

Abstract

Extensive research has focused on modifying molecular structures to harvest triplets efficiently through the thermally activated delayed fluorescence (TADF) mechanism. However, much less attention has been given to how solvent environment and chemical substitution simultaneously influence the excited-state dynamics and overall emissive performance. In this work, we examined the effects of solvent environment and chemical substitution on the excited-state dynamics of donor–acceptor TADF emitters. Time-correlated single-photon counting shows that substituent modification significantly alters the excited-state lifetimes. 2Ac–2CF₃Ph, the compound with the highest quantum yield, was chosen for in-depth solvent analyses. Dexter triplet–triplet energy transfer from solvent (toluene) to emitter is manifested as a ∽3 ns risetime component in the emission of 2Ac–2CF₃Ph, but no such rise was detected in cyclohexane or Zeonex. Comparison with TD-DFT-derived energy levels, ab initio molecular dynamics, spin–orbit coupling analysis, and a diffusion-controlled Dexter transfer model supports a mechanistic picture in which toluene triplets undergo Dexter triplet–triplet energy transfer to high-lying triplet states of the emitter, followed by ultrafast high-level reverse intersystem crossing from the triplet manifolds that repopulates S₁. The diffusion-controlled time-dependent Smoluchowski model yields an effective Dexter encounter radius that tracks emitter size. Our results demonstrate that intrinsic molecular design and solvent-mediated triplet harvesting act in concert to control the observed photodynamics in our system.