Time-resolved spectroscopy is a powerful tool to investigate conformational changes in organic molecules. Several works
involved these methods for studying tautomerism in 1.For aprotic solutions, times of ∼50-60 ps have been assigned
to the emission lifetimes of relaxed keto tautomers and their rotamers formed after an ultrafast ESIPT
(in less than 100 fs).The formation of twisted structures takes place in∼2-5 ps, depending on the viscosity of solvent. The latterparameter affects also nonradiative deactivation of 1, increasing its lifetime in triacetin (viscosity η298K ) 17 cP) to 155 ps. On the other hand, the fluorescence lifetime of 1 in neutral and alkaline water has been found to be shorter than 10 ps. It originates from anionic species, which has also a very low fluorescence quantum yield in this media. In addition to this time constant, another component in nanosecond time scale has been detected, although its contribution in the emission signal is very small.This component was attributed to an open conformer with intermolecular hydrogen bonding of 1 to water molecules.