In this study we have compared energy and electron transfer reactions in termolecular systems using a nanosecond diffuse reflectance laser flash photolysis technique. We have previously investigated these processes on silica gel surfaces for bimolecular systems and electron transfer in termolecular systems. The latter systems involved electron transfer between three arene molecules with azulene acting as a molecular shuttle. In this study we present an alternative electron transfer system using trans β-carotene as an electron donor in order to effectively immobilise all species except the shuttle, providing the first unambiguous evidence for radical ion mobility. In the energy transfer system we use naphthalene, a structural isomer of azulene, as the shuttle, facilitating energy transfer from a selectively excited benzophenone sensitiser to 9-cyanoanthracene. Bimolecular rate constants for all of these processes have been measured and new insights into the factors determining the rates of these reactions on silica gel have been obtained.

The photosensitized generation and subsequent decay of singlet oxygen in supercritical fluid xenon has been studied as a function of pressure and temperature. It has been found that the rate constant for quenching of singlet oxygen by ground state oxygen, kqO, increases as the pressure increases and decreases as the temperature increases. At 298 K, the value of kqO increases from (1.27 to 1.76) × 103 dm3 mol−1 s−1 as the pressure increases from 9.8 to 39.2 MPa; at 355 K the values of kqO drop to 6.2 × 102 and 1.54 × 103 dm3 mol−1 s−1 at these same pressures. It has also been found that the fractional contribution of the oxygen quenching to the overall singlet oxygen decay rate increases with increasing pressure, showing greater variations at high temperatures, and decreases with increasing temperature. The measured volume of activation was found to decrease with increasing pressure, and shows a small but systematic decrease with decreasing temperature, particularly at lower pressures.

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) × 109 dm3 mol−1 s−1. Efficiencies of singlet oxygen production, fΔT, 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.

Triplet state and radical cation formation is observed following laser excitation of anthracene, phenanthrene and naphthalene (and their derivatives) adsorbed on silica gel. Energy- and electron-transfer reactions of these compounds with co-adsorbed azulene have been studied using a time-resolved diffuse reflectance laser flash photolysis technique. Triplet energy transfer from the arene derivative to azulene and electron transfer from azulene to the arene radical cation have been investigated in order to distinguish between diffusional and energetic control in these systems. Energy and electron transfer can be studied independently due to differing absorption properties and energy dependencies of production of the triplet states and radical cations. Transient decay kinetics for both electron and energy transfer have been modelled using two different rate constant distributions: a log Gaussian and a symmetrical Lévy stable distribution. The latter model has also been demonstrated to be applicable to the decay of radical cations in the absence of an electron donor, which cannot be adequately described by the Gaussian model. Energy-transfer rates between the arene derivatives and azulene have been found to be close to the diffusion-controlled limit; however, in most cases, the rate of electron transfer is considerably lower. A correlation between the bimolecular rate constant and free energy of electron transfer has been found, indicating a Marcus inverted region. Compounds with bulky substituents show a further reduction in the rate of electron transfer, suggesting that an additional steric factor is involved in this process.

Data is presented on the quenching of 9,10-dicyanoanthracene by benzene derivatives in acetonitrile. The quenching occurs via a charge transfer mechanism with the quenching rate constants exhibiting a Rehm–Weller dependence on the free energy change of the electron transfer reaction. The quenching of the prompt fluorescence brings about an increase in the delayed fluorescence of DCA as a result of intersystem crossing in the exciplex, and a modified Wilkinson's plot has been used to determine the efficiency of triplet formation during the quenching of DCA fluorescence by benzene derivatives. We suggest that intersystem crossing yields in the exciplex are unity, and variations in triplet state yields as a result of singlet state quenching reflect partitioning between exciplex formation and solvent-separated radical ion pair (SSRIP) formation. The data clearly show competition between exciplex formation and SSRIP formation, with the latter becoming dominant when the free energy for electron transfer exceeds the solvent reorganisation energy.

Energy and electron transfer reactions between co-adsorbed molecules on silica gel have been studied using nanosecond time-resolved diffuse reflectance laser flash photolysis. The systems under investigation are anthracene and 9-carboxylic acid anthracene co-adsorbed with azulene, which undergo both triplet–triplet energy transfer and electron transfer from azulene to the anthracene radical cation following laser excitation. The decay traces have been analysed using a model which assumes a log gaussian distribution of rate constants and the methodology behind the optimisation of the fitting parameters is described. Bimolecular rate constants for energy and electron transfer between anthracene (and its derivative) and azulene have been obtained. Ground state association between anthracene and azulene has been observed, and an equilibrium constant for the process determined. The kinetic data is corrected for these ground state association effects which reduce the free azulene concentration. For both systems and for both the energy and electron transfer processes, analysis of the quenching data yields the same quenching constant. This indicates that the rate of reaction of anthracene (and the 9-carboxylic acid anthracene) on silica gel is predominantly governed by the rate of diffusion of the quencher.