Engineering the Charge‐Transfer State to Facilitate Spin–Orbit Charge Transfer Intersystem Crossing in Spirobis[anthracene]diones

Spiro conjugation has been proposed to dictate the efficiency of charge transfer, which could directly affect the spin–orbit charge transfer intersystem crossing (SOCT‐ISC) process. However, this process has yet to be exemplified. Herein, we prepared three spirobis[anthracene]diones, in which two benzophenone moieties are locked in close proximity and differentially functionalized to fine‐tune the charge transfer state. Its feasibility for SOCT‐ISC was theoretically predicted, then experimentally evaluated. Through fine‐tuning the spiro conjugation coupling and varying the solvent dielectric constants, ISC rate constants were engineered to vary in a dynamic range of three orders of magnitude, from 7.8×108 s−1 to 1.0×1011 s−1, which is the highest ISC rate reported for SOCT‐ISC system to our knowledge. Our findings substantiate the key factors for effective SOCT‐ISC and offer a new avenue for the rational design of heavy atom free triplet sensitizers. The highest SOCT‐ISC rate constant (1.0×1011 s−1) is achieved in spirobis[anthracene]diones solution. ISC rate constants are adjustable within 3 orders of magnitude by fine‐tuning the charge transfer state through spiro conjugation coupling and solvent dielectric constants.