Manipulating room-temperature phosphorescence by electron–phonon coupling

Designing and optimizing efficient organic room-temperature phosphorescent (RTP) materials remains a captivating yet challenging endeavour due to the inherent difficulties in generating and stabilizing triplet excitons. Here, we report a suite of highly efficient phosphors characterized by near-unity intersystem crossing (ISC) yields. Surprisingly, upon doping these dyes into a polyvinyl alcohol matrix, their phosphorescence quantum yields (<i>Φ</i> _P) spanned a wide range from 2.7% to 69.6%, governed by the position of the methyl substituent. Theoretical calculations and experimental results indicate that the variation in phosphorescence efficiency is primarily due to the strong electron-phonon coupling caused by the positional variation of the methyl substituents, rather than common factors such as ISC or energy levels. These findings provide a new insight into the design of high-performance organic RTP dyes.