Charge transfer effects on the efficiency of singlet oxygen production following oxygen quenching of excited singlet and triplet states of aromatic hydrocarbons in acetonitrile

Rate constants for quenching by molecular oxygen of excited singlet and tri plet states, k(S)(O2) and k(T)(O2), respectively, are reported for 12 aroma tic hydrocarbons in acetonitrile. Measured values of k(S)(O2), except in th e case of fluoranthene for which k(S)(O2) = 6.6 x 10(9) dm(3) mol(-1) s(-1) , are in the range (2.3-4.3) x 10(10) dm(3) mol(-1) s(-1), i.e., close to 4 .5 x 10(10) dm(3) mol(-1) s(-1), the average value obtained for k(d), the r ate constant for diffusions controlled reactions of oxygen with aromatic hy drocarbons in acetonitrile. Values of k(T)(O2) vary from 0.24 to 5.6 x 10(9 ) dm(3) mol(-1) s(-1) Thus, k(T)(O2)/k(d) was found to be less than one-nin th for 11 compounds. The efficiencies of singlet oxygen production during o xygen quenching of the excited singlet and triplet states, f(Delta)(S) and f(Delta)(T), respectively, were also measured, as were the oxidation potent ials of the hydrocarbons in acetonitrile. Values offs, were shown to be zer o within experimental error for eight compounds and in the range of 0.27 +/ - 0.05 for the ether four compounds. Three different methods, which gave go od agreement, were used to measure values of f(Delta)(T) which were found t o vary from 0.41 in the case of acenaphthene to 0.85 for anthracene. The fr action of excited singlet states quenched by oxygen which result in triplet states f(T)(O2) was also measured for all compounds and found to vary from 0.49 to 1.0. Combination of the total quenching rate constants with the fr actional efficiencies allows separate net quenching rate constants to be ob tained for the various oxygen quenching pathways in acetonitrile. The reaso ns for variations in these net quenching rate constants and thus in the fra ctional efficiencies for quenching by the various quenching pathways are di scussed. Quenching of both excited singlet and triplet states by energy tra nsfer and by charge-transfer assisted pathways are established. The logarit hm of the net rate constants for quenching of the triplet states without en ergy transfer to oxygen for ii of the aromatic hydrocarbons shows a linear dependence on the free energy for full charge transfer from the triplet sta te, with a slope which indicates that the transition states for this quench ing pathway only have about 13.5% charge-transfer character.