Fig. 1.

Comparison of the ground-state chemical or biochemical PT reaction and the excited-state intramolecular proton transfer (ESIPT). (a) The PT step in the ground-state transformation. Reaction starts after the donor-acceptor coupling with the H-bond by thermal activation of vibrational modes. The product is released or it participates in coupled reactions. The release of product is an indicator of the reaction rate. (b) The ESIPT reaction. The intramolecular H-bond exists but the thermal energy is insufficient for overcoming the reaction barrier. Here both reactant and reaction product can be highly fluorescent, so that the temporal depopulation of reactant and population of the product can be observed directly. The reaction starts from the supply of energy of electronic excitation to normal N state, and the emissions from initial N* and product T* excited states allow observing its occurrence, rate and reversibility in real time. The reaction product after decay to the ground T state is transformed to initial N state in a ground-state back reaction.