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 (λ) are longer than those of fluorescence spectra (λ). Here we report a serendipitous finding of up-converted room-temperature phosphorescence (RTP) with λ < λ and phosphorescence lifetime > 0.1 s upon doping benzophenone-containing difluoroboron β-diketonate (BPBF) into phenyl benzoate matrices. The up-converted RTP is originated from BPBF's T (n ≥ 2) states which show typical n-π* characters from benzophenone moieties. Detailed studies reveal that, upon intersystem crossing from BPBF's S states of charge transfer characters, the resultant T and T states build T-to-T equilibrium. Because of their n-π* characters, the T states possess large phosphorescence rates that can strongly compete RTP(T) to directly emit RTP(T) 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.