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

Abstract

AbstractIt is common sense that emission maxima of phosphorescence spectra (λP) are longer than those of fluorescence spectra (λF). Here we report a serendipitous finding of up-converted room-temperature phosphorescence (RTP) withλP < λFand phosphorescence lifetime > 0.1 s upon doping benzophenone-containing difluoroboron β-diketonate (BPBF2) into phenyl benzoate matrices. The up-converted RTP is originated from BPBF2’s Tn(n ≥ 2) states which show typical3n-π* characters from benzophenone moieties. Detailed studies reveal that, upon intersystem crossing from BPBF2’s S1states of charge transfer characters, the resultant T1and Tnstates build T1-to-Tnequilibrium. Because of their3n-π* characters, the Tnstates possess large phosphorescence rates that can strongly compete RTP(T1) to directly emit RTP(Tn) 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.