Mechanism of Quenching of Triplet States by Molecular Oxygen: Biphenyl Derivatives in Different Solvents

Abstract

The bimolecular rate constants kTO2for oxygen (O2(3∑g-)) quenching and the efficiencies fΔTwith 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 kTO2and fΔTare inversely correlated, and both parameters exhibit pronounced sensitivity to the oxidation potential (EMOX) 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, kd, it is established that kTO2/kdvalues are comparable when the electrostatic interaction energy of charge transfer complexes are taken as 0, 3, and 20 kJ mol-1for 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.