Torsion-Induced Nonradiative Relaxation of the Singlet Excited State of <i>meso</i>-Thienyl Bodipy and Charge Separation, Charge Recombination-Induced Intersystem Crossing in Its Compact Electron Donor/Acceptor Dyads

We prepared a series of meso-thienyl boron-dipyrromethene (Bodipy) derivatives to investigate the spin–orbit charge transfer intersystem crossing (SOCT-ISC). The photophysical properties compounds were studied by steady-state and femtosecond/nanosecond transient absorption spectroscopy, as well density functional theory (DFT) computations. Different from meso-phenyl Bodipy analogues, are weakly fluorescent. Based on femtosecond DFT computations, we propose that torsion thienyl group distortion core (19.7 ps) in S1 state lead conical intersection potential energy surface an efficient nonradiative decay channel (408 ps), which is responsible for observed weak fluorescence compared analogue. increased quantum yield (from 5.5 14.5%) viscous solvents supports this hypothesis. With electron donor 4′-hydroxylphenyl moiety attached unit, fast separation (CS, 15.3 recombination (CR, 238 processes outcompete torsion-induced induce ISC through SOCT-ISC mechanism. triplet donor/acceptor dyad highly dependent solvent polarity (ΦT = 1.9–45%), mechanism, triplet-state lifetime up 247.3 μs. Using donor–acceptor showing photosensitizer, triplet–triplet annihilation (TTA) upconversion was with 6.0%.