Solvent Effects on Spin-Orbit Charge-Transfer Intersystem Crossing in Aryl-Substituted Boron-dipyrromethene Donor-Acceptor Dyads

In heavy-atom-free organic molecules, the rate of triplet generation through charge recombination, as dictated by the El-Sayed rule, can be enhanced by 10¹-10² times compared with the rate of spontaneous spin flipping between π-π* orbitals. This mechanism is known as the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). Within the framework of the SOCT-ISC mechanism, facilitating the generation of charge-separated (CS) states and suppressing the spin-allowed direct charge recombination to the ground state are pivotal for maximizing the efficiency of generating localized triplet states. Herein, a series of orthogonal aryl-substituted boron-dipyrromethene dyads were studied by time-resolved spectroscopy to unravel the multichannel competitive relationships in the SOCT-ISC mechanism. The energy level of the electron donor and the stabilization of the solvent effect to the charge-transfer state are reflected in the Gibbs free energy changes of the electron transfer and recombination reactions, leading to significantly different triplet quantum yields. Additionally, solvation-induced electronic coupling changes in excited states lead to the fact that the spin-allowed charge recombination rate cannot be well simply predicted by the Marcus inverted region but has to consider the specific excited-state dynamics in optimizing the proportion of triplet generation channels based on charge recombination.