Reverse intersystem crossing from higher triplet to excited singlet in 2,2′-bipyridine-3,3′-diol phototautomer

Abstract

The S 1′→S 0′ fluorescence of 2,2′-bipyridine-3,3′-diol was detected on red probe pulse excitation of the tautomeric triplet (T 1′), which was generated via S 1′→S 1′ double proton transfer followed by S 1′→T 1′ intersystem crossing. The excitation spectrum of the two-step laser-induced fluorescence corresponds satisfactorily to the transient T n′←T 1′ absorption spectrum in the 600–800 nm region. It is demonstrated that the T 2′(n,π*)→S 1′(π,π*) reverse intersystem crossing is responsible for the generation of S 1′ on excitation of T 1′. The probability of reverse intersystem crossing was estimated to be 0.0024.