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.