The rate constants, , for oxygen quenching in acetonitrile of the triplet states of biphenyl and nine of its derivatives and the efficiencies of formation thereby of singlet oxygen, , have been measured as have the oxidation potentials of these derivatives. The rate constants decrease from 12.6 × 109 to 0.88 × 109 dm3 mol-1 s-1 as the oxidation potentials of the biphenyls rise from 1.30 to 2.11 V vs SCE while varies in the opposite direction rising from 0.31 to 0.84 with increasing oxidation potential. The mechanism of quenching via singlet and triplet complexes is discussed. The energy of the charge-transfer state, involving electron transfer to oxygen, relative to the energy of the locally excited triplet state, is established as important in determining and . The free energy of activation for charge-transfer-assisted quenching by oxygen via singlet and triplet channels is shown to have a linear dependence on the free energy change for full charge transfer, but the indications are that quenching is via singlet and triplet charge-transfer complexes with only partial (about 13.5%) charge-transfer character.

The bimolecular rate constants for oxygen (O2(3Σg -)) quenching and the efficiencies f Δ T with which singlet oxygen (O2*(1Δg)) is thereby produced are reported for a range of substituted biphenyl triplet states in acetonitrile, benzene, and cyclohexane. The magnitudes of and f Δ T are inversely correlated, and both parameters exhibit pronounced sensitivity to the oxidation potential ( ) of the biphenyl derivative and to the solvent polarity. It has been observed that the quenching rate constant increases as the oxidation potential of the biphenyl derivative decreases and increases as the solvent polarity increases whereas the efficiency of singlet oxygen production increases with the oxidation potential and decreases with increasing solvent polarity. When solvent viscosity changes are allowed for by calculating the diffusion controlled rate constant, k d, it is established that /k d values are comparable when the electrostatic interaction energy of charge transfer complexes are taken as 0, 3, and 20 kJ mol-1 for acetonitrile, benzene, and cyclohexane, respectively. An improved charge transfer mediated mechanism of quenching based on singlet and triplet channels for oxygen quenching is invoked to discuss these results with the triplet channel only operating when charge transfer is favorable. However, to get a good fit to the data, it is necessary to introduce direct formation of singlet oxygen production from the singlet encounter complexes in competition with charge transfer assisted singlet oxygen production. The free energy of activation for charge transfer assisted quenching by oxygen via singlet and triplet channels is shown to have a linear dependence on the free energy change for full charge transfer, but the indications are that quenching is via singlet and triplet charge transfer complexes with only partial charge transfer character being 12.5%, 14.5%, and 17% in acetonitrile, benzene, and cyclohexane, respectively. An explanation is offered as to why the less polar solvents show the larger fractional charge transfer in the transition states involved in the quenching mechanism.

Rate constants for quenching by molecular oxygen of excited singlet and triplet states, k S O 2 and k T O 2 , respectively, are reported for 12 aromatic hydrocarbons in acetonitrile. Measured values of k S O 2 , except in the case of fluoranthene for which k S O 2 ) 6.6 × 10 9 dm 3 mol -1 s -1 , are in the range (2.3-4.3) × 10 10 dm 3 mol -1 s -1 , i.e., close to 4.5 × 10 10 dm 3 mol -1 s -1 , the average value obtained for k d , the rate constant for diffusioncontrolled reactions of oxygen with aromatic hydrocarbons in acetonitrile. Values of k T O 2 vary from 0.24 to 5.6 × 10 9 dm 3 mol -1 s -1 . Thus, k T O 2 /k d was found to be less than one-ninth for 11 compounds. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, f ∆ S and f ∆ T , respectively, were also measured, as were the oxidation potentials of the hydrocarbons in acetonitrile. Values of f ∆ S were shown to be zero within experimental error for eight compounds and in the range of 0.27 ( 0.05 for the other four compounds. Three different methods, which gave good agreement, were used to measure values of f ∆ T which were found to vary from 0.41 in the case of acenaphthene to 0.85 for anthracene. The fraction of excited singlet states quenched by oxygen which result in triplet states f T O 2 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 fractional efficiencies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways in acetonitrile. The reasons for variations in these net quenching rate constants and thus in the fractional efficiencies for quenching by the various quenching pathways are discussed. Quenching of both excited singlet and triplet states by energy transfer and by charge-transfer assisted pathways are established. The logarithm of the net rate constants for quenching of the triplet states without energy transfer to oxygen for 11 of the aromatic hydrocarbons shows a linear dependence on the free energy for full charge transfer from the triplet state, with a slope which indicates that the transition states for this quenching pathway only have about 13.5% charge-transfer character.

Photophysical properties of 10 substituted methoxybenzene or vinyl linked benzo-crown-ether−2,2‘-bipyridyl ruthenium(II) complexes are reported. The lifetimes of the excited triplet metal to ligand charge transfer states, 3MLCT, of the complex ions are in the range 0.85 ± 0.12 μs with two exceptions where the lifetime drops to about half this value. The rate constants, k q, for quenching in acetonitrile of the 3MLCT states of these ruthenium complex ions by molecular oxygen, O2(3Σg -), and the variations in the efficiency, , with which excited singlet oxygen, O2*(1Δg), is thereby produced are reported. The quenching rate constants are in the range 2.2−4.2 × 109 dm3 mol-1 s-1, and efficiencies of singlet oxygen production are in the range 0.21−0.74. Those complexes with the highest values of k q tend to be those with lowest values; that is, k q and show a reasonable inverse correlation. The product k q gives the rate constant for oxygen quenching with energy transfer to oxygen, , and k q − gives the rate constant, , for oxygen quenching by any path which does not lead to energy transfer. The values of , , and k q are compared with those in the literature which are mainly available for organic sensitizers of singlet oxygen. The similarities and differences between these two classes of compounds are discussed taking into account the fact that ruthenium complex ions are likely to show enhanced intersystem crossing, due to the heavy atom effect and the likely dependence of and , and thereby the efficiencies of singlet oxygen production, on the energies of the excited 3MLCT states and on steric factors.

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In this study we report on the photophysical properties of some [RuL(CN)4](2-) complex ions where L = 2,2'-bipyridine (bpy), 5,5'-dimethyl-2,2'-bipyridine (dmb), 1,10-phenanthroline (phen), 1-ethyl-2-(2-pyridyl)benzimidazole (pbe), 2,2':6',2'''-terpyridine (tpy) and [RuL3](2+) where L = bpy or phen. Measurements were carried out in H2O and D2O. The effect of the deuterium isotope effect on the lifetime of these complexes is discussed. It has also been found that the presence of cyano groups has a pronounced effect on the lifetime of the excited metal-to-ligand charge transfer ((3)MLCT) of these complexes. Quenching of the (3)MLCT states by oxygen is reported in H2O and D2O. The rate constants, k(q), for quenching of the (3)MLCT states of these ruthenium complex ions by molecular oxygen are in the range (2.55 to 7.01) x 10(9) M(-1) s(-1) in H2O and (3.38 to 5.69) x 10(9) M(-1) s(-1) in D2O. The efficiency of singlet oxygen, O2((1)Delta(g)), production as a result of the (3)MLCT quenching by oxygen, f(Delta)(T), is reported in D2O and found to be in the range 0.29-0.52. The rate constants, k(q)(Delta), for quenching of singlet oxygen by ground state sensitizers in D2O is also reported and found to be in the range (0.15 to 3.46) x 10(7) M(-1) s(-1). The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T1(3)Sigma) and (3)(T1(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O2((1)Delta(g)), and O2((3)Sigmag-). The balance between CT and nCT deactivation channels which is described by the relative contribution p(CT) of CT induced deactivation is discussed. The kinetic model proposed for the quenching of pi-pi* triplet states by oxygen can also be applied to the quenching of (3)MLCT states by oxygen.

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, kq, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1-7.7) x 10(9) dm3 mol(-1) s(-1). Efficiencies of singlet oxygen production, fDeltaT, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10-0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, kq, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, kq were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, kq1, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, kq3, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin-orbit coupling constant of the central metal.

Photophysical properties in dilute acetonitrile solution are reported for a number of iridium(III) and rhenium(I) complexes. The nature of the lowest excited state of the complexes under investigation is either metal-to-ligand charge transfer ((3)MLCT) or a ligand centred ((3)LC) state. Rate constants, k(q), for quenching of the lowest excited states by molecular oxygen are in the range 1.5 x 10(8) to 1.4 x 10(10) M(-1) s(-1). Efficiency of singlet oxygen production, f(Delta)(T), following oxygen quenching of the lowest excited states of these complexes, are in the range of 0.27-1.00. The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition between a non-charge transfer (nCT) and a CT deactivation channel. The balance between CT and nCT deactivation channels, which is described by the relative contribution p(CT) of CT induced deactivation, is discussed. The kinetic model is found to be successfully applied in the case of quenching of the excited triplet states of coordination compounds by oxygen in acetonitrile, as was proposed for the quenching of pi-pi* triplet states by oxygen.