ESIPT study in the higher excited states by fluorescent methods

Photophysical and photochemical processes in organic molecules are directly related to the singlet electronic S 1 state and studied usually without taking into account highest excited states. At the same time the role of high-lying singlet states is not reduced only to processes of intramolecular conversion, they can participate in different intra- and intermolecular processes. Here we consider analytically, in general, how the properties of dually fluorescing molecules are affected in the case where their second fluorescence band is formed as a result of excited state photoreaction and where fluorescence is excited in the range of the S 1 and S n electronic absorption bands. It is shown that, upon excitation of molecular objects via high-lying singlet states, the yield of reaction products can be increased in a number of cases. Then, if fluorescence of initial molecules and their photoproducts is detectable, the probabilities of reactions can be determined via high-lying singlet states. Presented experimental data demonstrate the role of high-lying singlet states of molecules from 3-hydroxyflavone family, in the excited state of which reactions of formation of tautomers as a result of internal proton transfer (ESIPT) can take place. This fact reveals a new opportunity for enlargement yield of photoreactions excited via the Sn states, and in some cases this may be explored practically.