Excited state intramolecular proton transfer reaction revisited: S1 state or general reversibility?

The excited state intramolecular proton transfer (ESIPT) reaction reversibility was reexamined both experimentally and theoretically on the example of a series of 3-hydroxychromone (3HC) derivatives. The experimental reversibility criteria for the excited state adiabatic photoreactions based on dynamic quenching of two-banded 3HC fluorescence were re-deduced. Quenching experiments in the series of 3HC with the specially selected quencher, which does not absorb on the excitation wavelength, 2,4-dinitrotoluene, did not reveal the excited state reversibility in any of the examined dry aprotic solvents of different polarity. Quantum-chemical modeling including the excited state molecular geometry analysis, estimation of the ground and excited state H-bonds energies with the application of the “atoms in molecules” (AIM) approach and calculation of energetic profiles of the forward and back proton transfer reactions in the S0 and S1 states allows us to conclude there are no serious prerequisites for the excited state reversibility of the intramolecular proton transfer reaction.