RECOMBINATION OF 2,3,4,5-TETRAPHENYLPYRROLYL RADICALS IN BENZENE SOLUTION : PHOTOCHROMISM, PARTIALLY DIFFUSION-CONTROLLED REACTION, AND MAGNETIC FIELD EFFECT

Violet-colored 2,3,4,5-tetraphenylpyrrolyl radicals (TPPRs) were remarkably long-lived (more than 10 h) in benzene solution at room temperature. Their decay obeyed the second-order kinetics; two TPPRs recombine to form a colorless dimer. The temperature dependence of the recombination rate was close to that of the reciprocal of solvent viscosity, precluding the presence of any additional activation barrier for the recombination reaction apart from that imposed on the encounter of radicals by solvent viscosity; the rate is limited solely by the steric factor. Although TPPRs come across many times by diffusion, four outspread phenyl groups limit the useful approach of the reactive sites that are on the pyrrolyl ring. In most cases, TPPRs form the nonreactive random encounter radical pair (RERP), which separates again with the diffusion rate. The reactive RERP is formed only when TPPRs approach each other with their reactive sites exactly facing each other. Useful approach is limited to only a small “favorable” solid angle, resulting in the partially diffusion-controlled recombination. A significant retarding effect caused by the external magnetic field was observed on the recombination reaction of the reactive RERP. The effect was not saturated up to 0.5 T and can be explained by the relaxation mechanism with the underlying hyperfine coupling mechanism.