EFFECT OF SENSITIZER HEAVY-ATOMS ON SINGLET OXYGEN GENERATION EFFICIENCY

The quenching rate constants of triplets by molecular oxygen (k(q)T) and the efficiency of singlet oxygen generation per quenching encounter (S(DELTA)) were measured for various pi,pi* and n,pi* triplet sensitizers containing heavy atoms using nanosecond laser photolysis and time-resolved O2((1)DELTA(g)) phosphorescence. For pi,pi* triplets (tetrahalo-p-benzoquinones and haloanthracenes), introduction of the heavy atom has practically no influence on S(DELTA). For n,pi* triplets (haloacetophenones), S(DELTA) values decrease sharply with increasing number of heavy atoms; for example, for 4-bromo- and 2,4'-dibromoacetophenone in benzene, S(DELTA) = 0.10 and 0.032, respectively. An analysis of the mechanism of triplet quenching by oxygen was carried out, taking into account intersystem crossing between singlet and triplet states of the encounter complex. Expressions for S(DELTA) and k(q)T were obtained which qualitatively describe the experimental data. A reasonable explanation for the trends is that for pi,pi* triplet sensitizers intersystem crossing from the triplet to the singlet state of the encounter complexes with oxygen is faster than dissociation of the triplet encounter complex to ground-state sensitizer and oxygen. In contrast, for n,pi* triplets, the rate constant for dissociation of the triplet encounter complex to the ground-state reagents must be much larger than that for intersystem crossing to the singlet encounter complex. This regularity explains the low S(DELTA) values in these cases.