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

The design and regulation of phosphor are attractive but challenging due to the spin-forbidden intersystem crossing (ISC) process. Here, a new perspective on the enhancement of ISC was proposed and demonstrated. Different from current strategies, the ISC yield (ΦISC) was enhanced by decreasing the fluorescence radiative transition rate constant (kF) via rational molecular designing, rather than boosting the spin-orbit coupling by decorating molecular skeleton with heavy atom, heteroatom, or carbonyl. The kF of the designed molecule in this case was associated with the substituent position of the methoxy group, which alter the distribution of the front orbitals. The S0→S1 transition of these compounds evolved from bright state to dark state gradually with the variation of substituent position, accompanied by the decrease of kF and increase of ΦISC. The fluorescence emission was switched to phosphorescence emission successfully by regulating the kF. This work provided an alternative strategy to design efficient room-temperature phosphorescence material.