Photoinduced electron transfer in meso-nitrophenyl-substituted porphyrins and their chemical dimers

It is shown that steric interactions of volume substituents in the β-positions of pyrrole rings and the nitro group in mono-and di-meso-phenyl-substituted of octaethylporphyrins and their chemical dimers containing the electron-acceptor NO2 group in the ortho-position of the phenyl ring at 295 K favor the direct overlap of molecular orbitals of the interacting subunits, resulting in the efficient quenching of fluorescence due to the direct electron transfer from the S1 level to the lower-lying state via the “through-space” mechanism. The electron transfer in these compounds in nonpolar media (the rate constant k et S =(3.2–9.5)×109 s−1 is nonadiabatic, whereas in strongly polar solvents (k et S =2×1011 s−1) the adiabatic effects can be manifested. In compounds containing the NO2 group in meta-or para-positions of the phenyl ring, the nonadiabatic electron transfer from the S 1 level occurs less efficiently both in polar [k et S =(0.2–5)×1010 s−1] and nonpolar media [k et S =(0.1–1.0)×107 s−1]. In this case, the electron transfer involves molecular orbitals of phenyl (the “through-bond” mechanism), and its efficiency depends on the orbital electron density in the meta-and para-positions of the phenyl ring. Based on the experimental data obtained and analysis of the electron transfer within the framework of the Marcus theory, the energy scheme of relaxation processes of the electronic energy in the compounds under study involving charge transfer states is suggested.