Face-to-face π-π interactions and electron communication boosting efficient reverse intersystem crossing in through-space charge transfer molecules

The excited state dynamics and critically regulated factors of reverse intersystem crossing (RISC) in through-space charge transfer (TSCT) molecules have received insufficient attention. Here, five molecules of through space/bond charge transfer inducing thermally activated delayed fluorescence (TADF) are prepared, and their excited state charge transfer processes are studied by ultrafast transient absorption and theoretical calculations. DM-Z has a larger ∆EST, leading to a longer lifetime of intersystem crossing (ISC), resulting in the lowest photoluminescence quantum yield (PLQY). Oppositely, ISC and RISC are demonstrated to take place with shorter lifetimes for TSCT molecules. The face-to-face π-π stacking interactions and electron communication enable DM-B and DM-BX to have an efficient RISC, increasing the weight coefficient of RISC from 1.7% (DM-X) to close to 50% (DM-B and DM-BX) in the solvents, which make DM-BX and DM-B to have a high PLQY. However, partial local excitation in the donor center is observed and the charge transfer is decreased for DM-G and DM-X. The triplet excited state (DM-G) or singlet excited state (DM-X) mainly undergoes inactivation through a non-radiative relaxation process, resulting in less RISC and low PLQY. This work provides theoretical hints to enhance the RISC process in the TADF materials.