Investigation of Photoinduced Processes in Water-Soluble Cationic Ni-Porphyrin by the Resonance Raman Scattering Method

Abstract

By the resonance Raman scattering spectroscopy (RRS) method, photoinduced processes involving cationic Ni-5,10,15,20-tetrakis(4-N-methylapyridiniumyl)porphyrin (Ni(TMpy-P4)) in a phosphate water buffer have been investigated. Excitation into different regions of the Soret band at wavelengths of 397, 423, 441.6, and 457.9 nm permitted separate investigation of the behavior of the 4- and 6-coordinated forms of Ni(TMpy-P4). It has been found that Ni(TMpy-P4) is characterized by two photoinduced processes: the deactivation of initially 4-coordinated Ni(TMpy-P4) is followed by the addition to it, as axial ligands, of two water molecules from the environment to form a nonequilibrium 6-coordinated form Ni(TMpy-P4)(H₂O)₂ in the ground state, whereas in the process of deactivation of the excited 6-coordinated complex there occurs a dissociation of the axial water ligands to form a nonequilibrium 4-coordinated complex Ni(TMpy-P4) in the ground state. Some of the 4-coordinated molecules of Ni(TMpy-P4) are deactivated through the excited B _1g ^*-state without undergoing extracoordination. Quantitative analysis of the RRS spectra using the global fitting method has shown that in the course of deactivation ∼33% of photoexcited 6-coordinated molecules of Ni(TMpy-P4)(H₂O)₂ retain their axial ligands, relaxing through the “dark” long-lived intermediate state of the 6-coordinated form.