A Twisted Phosphor: Breaking T-1 Energy Conservation in Dopant-Matrix Organic Phosphorescence Systems

In the exciton model of organic compounds proposed by Kasha, energy levels of T-1 states are insensitive to microenvironment because of negligibly small transition dipole moments of T-1 states. This phenomenon of T-1 energy conservation holds true in most organic systems. Here a serendipitous finding of a twisted organic phosphor that breaks T-1 energy conservation in dopant-matrix phosphorescence systems is reported. Specifically, the twisted phosphor exhibits distinct phosphorescence colors and T-1 energy levels when doped into different organic matrices under ambient conditions. Time-dependent density functional theory calculations reveal that, when the twisted phosphor is in its freely rotating form, the increase of twisted angle leads to the increase of its T-1 energy level. After being planarized by alkyl cyclization, the phosphor shows the recovery of T-1 energy conservation. Owing to its sensitive T-1 level, the twisted phosphor functions as label-free and visual probe for direct observation of polymer phase separation.