Switching Singlet Exciton to Triplet for Efficient Pure Organic Room-Temperature Phosphorescence by Rational Molecular Design

[Image: see text] The design and regulation of phosphors are attractive but challenging because of the spin-forbidden intersystem crossing (ISC) process. Here, a new perspective on the enhancement of the ISC is proposed and demonstrated. Different from current strategies, the ISC yield (Φ(ISC)) is enhanced by decreasing the fluorescence radiative transition rate constant (k(F)) via rational molecular designing rather than boosting the spin–orbit coupling by decorating the molecular skeleton with a heavy atom, heteroatom, or carbonyl. The k(F) of the designed molecule in this case is associated with the substituent position of the methoxy group, which alters the distribution of the front orbitals. The S(0) → S(1) transition of these compounds evolves from a bright state to a dark state gradually with the variation of the substituent position, accompanied by the decrease of k(F) and increase of Φ(ISC). The fluorescence emission is switched to phosphorescence emission successfully by regulating the k(F). This work provides an alternative strategy to design efficient room-temperature phosphorescence material.