[Impulse photoconductance of solutions of chlorophyll and its analogs. IV. Absolute quantum yield of ion radical formation during photooxidation of chlorophyll a by n-benzoquinone].

The values of absolute quantum yield phi of the formation of free ion-radicals during the illumination of alkohol solutions of chlorophyll alpha (Chl) and rho-benzoquinone (Q) at room temperature were obtained by the method of impulse photoconductance. With an increase of the dielectric constant epsilon of the solvent from approximately 6 to approximately 25 phi increases by two orders ( approximately 10(-3)--approximately 10(-1). That obtained relationship phi (epsilon) is explained by epsilon effect on the efficiency of dissociation of "solvent-shared" ion-radical pair Chls+. Os-. The comparison of experimental data and theoretically expected ones allowed the estimation of some parameters to be obtained which characterize the ion-radical pair: interionic distance (10 A), the dissociation velocity constant ( approximately 10(5)--10(8) s-1), the velocity constant of reverse electron transfer (10(8) s-1), the life time approximately 10(-8) s).