Solvent Effects\non Spin–Orbit Charge-Transfer\nIntersystem Crossing in Aryl-Substituted Boron-dipyrromethene Donor–Acceptor\nDyads

In\nheavy-atom-free organic molecules, the rate of triplet generation\nthrough charge recombination, as dictated by the El-Sayed rule, can\nbe enhanced by 10<sup>1</sup>–10<sup>2</sup> times compared\nwith the rate of spontaneous spin flipping between π–π*\norbitals. This mechanism is known as the spin–orbit charge-transfer\nintersystem crossing (SOCT-ISC). Within the framework of the SOCT-ISC\nmechanism, facilitating the generation of charge-separated (CS) states\nand suppressing the spin-allowed direct charge recombination to the\nground state are pivotal for maximizing the efficiency of generating\nlocalized triplet states. Herein, a series of orthogonal aryl-substituted\nboron-dipyrromethene dyads were studied by time-resolved spectroscopy\nto unravel the multichannel competitive relationships in the SOCT-ISC\nmechanism. The energy level of the electron donor and the stabilization\nof the solvent effect to the charge-transfer state are reflected in\nthe Gibbs free energy changes of the electron transfer and recombination\nreactions, leading to significantly different triplet quantum yields.\nAdditionally, solvation-induced electronic coupling changes in excited\nstates lead to the fact that the spin-allowed charge recombination\nrate cannot be well simply predicted by the Marcus inverted region\nbut has to consider the specific excited-state dynamics in optimizing\nthe proportion of triplet generation channels based on charge recombination.