Mechanistic Understanding of Room Temperature Phosphorescence in Pure Organic Host-Guest Systems: Efficient Emission of Molecules Lacking n→π * States

Abstract

Host-guest doping offers an effective strategy for fabricating and tuning purely organic room temperature phosphorescence (RTP) materials. Herein, four doped materials were prepared employing 8-methoxy-3,6-dimethyl-4-phenyl-1-(piperidin-1-yl)-isoquinoline-7-carbonitrile (IQL) as the guest and different benzophenone derivatives as host matrices to achieve efficient RTP. Three IQL-doped materials exhibit strong dual fluorescence-phosphorescence emission with high phosphorescence quantum yields (≥10.2%) and long lifetimes (≥0.19 s), despite IQL lacks suitable (n, π * ) states capable of decaying into the emissive (π, π * ) T 1 state through an El-Sayed-allowed ISC process. Theoretical studies revealed that doping IQL into a host matrix containing lone-pair electrons enables IQL to "borrow" the (n, π * ) state of the host, thereby circumventing the El-Sayed-disallowed ISC process from the (π, π * ) guest S 1 state to the (π, π * ) guest T 1 state. Furthermore, the doped materials' phosphorescence quantum yields vary non-monotonically with the substituent effects of the host molecules, since both electron-withdrawing and strongly electron-donating groups are unfavorable for phosphorescence, due to the intricate interplay between the host and guest orbital energy levels. This finding provides a useful protocol for tuning the electronic properties of host-guest materials.