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

It is shown that steric interactions of volume substituents in the P-positi ons of pyrrole rings and the nitro group in mono- and di-meso-phenyl-substi tuted of octaethylporphyrins and their chemical dimers containing the elect ron-acceptor NO2 group in the ortho-position of the phenyl ring at 295 K fa vor the direct overlap of molecular orbitals of the interacting subunits, r esulting in the efficient quenching of fluorescence due to the direct elect ron transfer from the S-1 level to the lower-lying state via the "through-s pace" mechanism. The electron transfer in these compounds in nonpolar media the rate constant k(et)(S) = (3.2-9.5) x 10(9) s(-1) is nonadiabatic,where as in strongly polar solvnts (k(et)(S) = 2 x 10(11) s(-1)) the adiabatic ef fects can be manifested. In compound containing the NO2 group in meta- or p ara-positions of the phenyl ring, the nonadiabatic electron transfer from S -1 level occurs less efficiently both in polar [k(et)(S) = (0.2-5) x 10(10) s(-1)] and nonpolar media [k(et)(S) = (0.1-1.0) x 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 densi ty in the meta- and pare-positions of the phenyl ring. Based on the experim ental data obtained and analysis of the electron transfer within the framew ork of the Marcus theory, the energy scheme of relaxation processes of the electronic energy in the compounds under study involving charge transfer st ates is suggested. (C) 2000 MAIK "Nauka/Interperiodica".