Mechanism of quenching of triplet states by molecular oxygen: Biphenyl derivatives in different solvents

The bimolecular rate constants k(T)(O2) for oxygen (O-2((3)Sigma(g)(-))) qu enching and the efficiencies f(Delta)(T) With which singlet oxygen (O-2*((1 )Delta(g))) is thereby produced are reported for a range of substituted bip henyl triplet states in acetonitrile, benzene, and cyclohexane. The magnitu des of k(T)(O2) and f(Delta)(T) are inversely correlated, and both paramete rs exhibit pronounced sensitivity to the oxidation potential (E-M(OX)) of t he biphenyl derivative and to the solvent polarity. It has been observed th at the quenching rate constant increases as the oxidation potential of the biphenyl derivative decreases and increases as the solvent polarity increas es whereas the efficiency of singlet oxygen production increases with the o xidation potential and decreases with increasing solvent polarity. When sol vent viscosity changes are allowed for by calculating the diffusion control led rate constant, k(d), it is established that k(T)(O2/kd) values are comp arable when the electrostatic interaction energy of charge transfer complex es are taken as 0, 3, and 20 kJ mol(-1) for acetonitrile, benzene, and cycl ohexane, respectively. An improved charge transfer mediated mechanism of qu enching based on singlet and tripler channels for oxygen quenching is invok ed to discuss these results with the triplet channel only operating when ch arge transfer is favorable. However, to get a good fit to the data, it is n ecessary to introduce direct formation of singlet oxygen production from th e singlet encounter complexes in competition with charge transfer assisted singlet oxygen production. The free energy of activation for charge transfe r assisted quenching by oxygen via singlet and tripler channels is shown to have a linear dependence on the free energy change for full charge transfe r, but the indications are that quenching is via singlet and tripler charge transfer complexes with only partial charge transfer character being 12.5% , 14.5%, and 17% in acetonitrile, benzene, and cyclohexane, respectively. A n explanation is offered as to why the less polar solvents show the larger fractional charge transfer in the transition states involved in the quenchi ng mechanism.