[Role of singlet-excited and triplet states in chlorophyll photooxidation]

Effect of p-benzoquinone concentration [Q] on the yield of ion-radicals during photooxidation of chlorophyll a with p-benzoquinone was studied by the method of impulse photoconductivity. It has been shown that in the studied range [Q] = 10(-6)--10(-1) M the relationship delta n ([Q]) has a bell-like shape with the maximum at [Q] approximately 10(-3) M. From the relationship obtained the ratio beta = delta ns/delta nt of the contributions of singlet--excited (Chl) and triplet (Chl) states of the pigment into the total yield of ion-radicals is evaluated: for [Q] less than or equal to 10(-1) M beta less than or approximately 10(-3). Primary contribution of the triplet state to the formation of ion-radical is explained, first, by a larger, as compared to the singlet-excited state, life time of the state Chl in solution and second, by the fact that the ion-radical pair 3(Chl+Q-) in the triplet state regenerates less effectively to Chl+Q, than the pair (Chl+Q-).