Solvent and temperature effects on the deactivation pathways of excited ion pairs produced via photoinduced proton transfer.

The kinetics of the energy dissipation pathways of the excited ion pairs formed upon light absorption of the hydrogen bonded complex between N-methyl-3-hydroxynaphthalimide (3HONI) and 1-methyl-imidazole (MIm) has been studied in a wide temperature range in solvents of various polarities. In polar media such as butyronitrile, three emitting species are found which are assigned as hydrogen bonded, solvent separated and free ion pairs. In contrast, less polar solvents do not favour free ion pair formation, and reversible transition occurs between the excited hydrogen bonded (HBIP) and solvent separated (SSIP) ion pairs around room temperature. Systematic steady-state and time-resolved fluorescence measurements combined with computer modelling enabled the deduction of the kinetic parameters for all excited state deactivation processes in solvents of moderate polarity. The Arrhenius parameters of the interconversion between the ion pairs, the A-factor for the radiationless deactivation of SSIP and the radiative rate constant of HBIP diminish with the increasing dielectric constant of the media. The enthalpy and the entropy change in the SSIP formation from HBIP become less negative in more polar solvents. Going from ethyl acetate to CH2Cl2 the most profound decrease (more than three orders of magnitude) is observed for the A-factor of SSIP --> HBIP reaction.