Dynamic quenching of the dual fluorescence of molecules

Mathematical relations describing the properties of spontaneous steady-state dual fluorescence under conditions of dynamic quenching of excited states by foreign impurities are derived. It is shown that, in the case of a kinetic character of the reaction, the initial form of the dye and its photoproduct are quenched, the intensity ratio of the fluorescence bands of the initial form and the product linearly increasing with the quencher concentration. Analysis performed is applicable to a wide range of photoreactions accompanied by the dual fluorescence (charge transfer, proton transfer, complexation, etc.). The properties of the fluorescence, absorption, and dual fluorescence excitation for 3-hydroxyflavone in acetonitrile under conditions of dynamic quenching by the TEMPO spin quencher with a concentration below 1.25 × 10−2 M are studied. 3-Hydroxyflavone is characterized by the excited-state intramolecular proton transfer and by the fluorescence spectrum consisting of two well-spaced bands. The observed dependences of the intensity of both fluorescence bands on the quencher concentration correspond to the theoretical conclusions. The Stern-Volmer constants calculated from the experimental data on the assumption of diffusion quenching of the excited states are 858 and 1141 M−1 for the normal and tautomeric fluorescence bands, respectively. The experimental results reveal the kinetic character of the excited-state proton transfer in 3-hydroxyflavone in acetonitrile.