A direct observation of up-converted room-temperature phosphorescence in an anti-Kasha dopant-matrix system

It is common sense that emission maxima of phosphorescence spectra (lambda(P)) are longer than those of fluorescence spectra (lambda(F)). Here we report a serendipitous finding of up-converted room-temperature phosphorescence (RTP) with lambda(P) < lambda(F) and phosphorescence lifetime > 0.1 s upon doping benzophenone-containing difluoroboron beta-diketonate (BPBF2) into phenyl benzoate matrices. The up-converted RTP is originated from BPBF2's T-n (n >= 2) states which show typical (3)n-pi* characters from benzophenone moieties. Detailed studies reveal that, upon intersystem crossing from BPBF2's S-1 states of charge transfer characters, the resultant T-1 and T-n states build T-1-to-T-n equilibrium. Because of their (3)n-pi* characters, the T-n states possess large phosphorescence rates that can strongly compete RTP(T-1) to directly emit RTP(T-n) which violates Kasha's rule. The direct observation of up-converted RTP provides deep understanding of triplet excited state dynamics and opens an intriguing pathway to devise visible-light-excitable deep-blue afterglow emitters, as well as stimuli-responsive afterglow materials. The emission maximum of phosphorescence is normally larger than that of fluorescence. Here, authors report up-converted room-temperature phosphorescence materials that emit from higher-lying triplet states with 3n-pi* characters and large phosphorescence decay rate constants, violating Kasha's rule.

Automated summary

The emission maximum of phosphorescence is usually longer than that of fluorescence. In this study, researchers report a serendipitous finding of up-converted room-temperature phosphorescence (RTP) with emission maximum smaller than that of fluorescence. This is achieved by doping benzophenone-containing difluoroboron beta-diketonate (BPBF2) into phenyl benzoate matrices. The up-converted RTP originates from BPBF2's higher-lying triplet states with (3n-pi*) characters and large phosphorescence rates, violating Kasha's rule.