Excited-State Isomerization of Leuco Indigo

The photoreaction of indigo and two other derivatives in its reduced (leuco) form was investigated by absorption and fluorescence (steady-state and time-resolved) techniques. The fluorescence quantum yield (phi(F)) dependence with the UV irradiation time was found to increase up to a value of phi(F) approximate to 0.2-0.3 (after 16 min) for indigo and phi(F) = 0.2 (at similar to 150 min) for its derivative 4,4'-dibutoxy-7,7'-dimethoxy-5,5'-dinitroindigo (DBMNI). With a model compound, where rotation around the central C-C bond is blocked, the phi(F) value was found constant with the UV irradiation time. Time-resolved fluorescence revealed that initially the decays are fitted with a biexponential law (with 0.12 and 2.17 ns), ending with an almost monoexponential decay (similar to 2.17 ns). Quantum yields for the isomerization photoreaction (phi(R)) were also obtained for indigo and DBMNI with values of 0.9 and 0.007, respectively. The results are rationalized in terms of a photoisomerization (conversion) reaction occurring in the first excited singlet state of trans to cis forms of leuco indigo.