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(P)) are longer than those of fluorescence spectra (λ(F)). Here we report a serendipitous finding of up-converted room-temperature phosphorescence (RTP) with λ(P) < λ(F) and phosphorescence lifetime > 0.1 s upon doping benzophenone-containing difluoroboron β-diketonate (BPBF(2)) into phenyl benzoate matrices. The up-converted RTP is originated from BPBF(2)’s T(n) (n ≥ 2) states which show typical (3)n-π* characters from benzophenone moieties. Detailed studies reveal that, upon intersystem crossing from BPBF(2)’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-π* 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.