Photophysical and spectroscopic studies of Indigo derivatives in their keto and leuco forms

A comprehensive spectroscopic and photophysical study of the keto and leuco forms of indigo and three other ring-substituted derivatives in solution was performed. The characterization involves absorption, fluorescence, and triplet-triplet absorption spectra, making it possible to obtain the quantum yields for fluorescence (phi(F)), singlet-triplet intersystem crossing (phi(ISC)), internal conversion (phi(IC)), and lifetimes for fluorescence (tau(F)) and triplet decay (tau(T)). For the case of the keto forms, pulse radiolysis experiments have revealed the existence of a triplet acceptor (from energy transfer from different donors) for the indigo, purple, and indirubin compounds. It is shown that with the keto form the major deactivation pathway involves internal conversion from the lowest singlet excited state to the ground state whereas with the leuco form there is competition between internal conversion, triplet formation, and fluorescence deactivation processes. Furthermore, leuco forms present much higher Stokes shifts compared with keto ones, suggesting an excited-state geometry different from the ground-state geometry, possibly involving rotational photoisomerization.