Photochemical ionogenesis in solutions of zinc octaethyl porphyrin

Publisher Website
Google Scholar
Abstract
Absolute ion yields and the kinetic parameters of ion formation and decay have been determined by transient conductimetry for two photochemical ionogenic reactions of zinc octaethyl porphyrin in a variety of inert organic solvents. One reaction (T–P) involves electron transfer in the encounter complex of the porphyrin triplet state (T) and ground state (P); it is relatively slow (kTP∼108M−1s−1, and solvent insensitive). The second (T–T reaction) involves reactive collision between two triplets; it occurs at the encounter limit (kTT∼1010M−1s−1). Neither rate constant depends on solvent dielectric constant. Reaction yields are very dielectric‐dependent, however, and provide unusually straightforward experimental access to the problem of geminate ion‐pair decorrelation. A two‐parameter model is presented in which the initial photochemically‐formed ion pair is created by electron tunneling in a specific spin state at nontrivial separation. The electron transfer radii for the two reactions are determined by completely independent analysis of the kinetic and yield data to be ∼21 Å, some 7 Å greater than twice the radius of the porphyrin π‐electron system. Following e− transfer, the solvent‐separated components of the geminate pair diffuse in their Coulomb field, undergoing both coherent and incoherent changes in their combined spin state as they do so. They either escape beyond the Coulomb radius to give uncorrelated and conductimetrically active doublet ions, or approach to the critical radius for reverse electron transfer to the ground state (2P) and, subject to a spin selection rule, are annihilated. The dominant mechanism of spin interconversion appears to be spin–orbit coupling to the molecular rotations rather than nuclear hyperfine interactions. The coupled diffusion and spin relaxation equations are solved numerically by the method of finite differences; theoretical yields of free ions show good fit to the data over five orders of magnitude. Recombination of the escaped and randomized radicals occurs at the electrostatically accelerated encounter limit and conforms well to the Debye–Smoluchowski equation. Rate constants in excess of 2×1011 M−1s−1 are observed. A third ionogenic mechanism, direct 2‐photon ionization of the porphyrin, is briefly reported. We also briefly characterize the weakly ionogenic quenching reaction of the prophyrin triplet state with molecular oxygen.