Phototautomerization of 3-hydroxyflavone in the lowest triplet state

Selective excitation of benzil in the presence of 3-hydroxyflavone (3HF) in fluid solution results in diffusional triplet energy transfer from benzil to 3HF, and the Tn←T1 absorption spectrum of 3HF (acceptor) was determined by transient absorption spectroscopy. It is demonstrated that red probe pulse excitation of the lowest triplet state (T1) of the normal tautomer yields green fluorescence from the phototautomer in the lowest excited singlet state (S′1). Generation of S′1 upon the excitation of T1 may be ascribed to an efficient Tn→T′n proton transfer followed by Tn′→T′2 relaxation and T′2→S′1 reverse intersystem crossing.