Determination of Rate Constants and the Mechanism of Phototransformations of Metalloporphyrins by Solving the Inverse Photokinetic Problem

Fluorescence kinetics of some substituted Zn- and Mg-porphyrins in solid polymeric films upon excitation by two-step-wise rectangular laser pulses and at 293 K are studied by methods of the inverse kinetic problem. The experimental nonmonotonic kinetic curves are approximated by simulated ones within the framework of a six-level energy scheme describing reversible interconversions of two complexes. The maximum correspondence between experimental and simulated curves is obtained by iterative optimization using the Nelder–Mead algorithm. Based on statistically estimated values of rate constants and parameters of the considered models, an interpretation of the observed kinetics is presented and a conclusion is drawn that the interconversions are caused by the process of reversible extraligation of the central metal ion; the process occurs in the excited triplet T 1 state.