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

The S1′→S0′ fluorescence of 2,2′-bipyridine-3,3′-diol was detected on red probe pulse excitation of the tautomeric triplet (T1′), which was generated via S1′→S1′ double proton transfer followed by S1′→T1′ intersystem crossing. The excitation spectrum of the two-step laser-induced fluorescence corresponds satisfactorily to the transient Tn′←T1′ absorption spectrum in the 600–800 nm region. It is demonstrated that the T2′(n,π*)→S1′(π,π*) reverse intersystem crossing is responsible for the generation of S1′ on excitation of T1′. The probability of reverse intersystem crossing was estimated to be 0.0024.