Linear free energy relationship analysis of solvent effect on singlet oxygen reactions with mono and disubstituted anthracene derivatives

Detection of O2(1Δg) emission, λmax = 1270 nm, following laser excitation and steady-state methods were employed to measure the total reaction rate constant, kT, and the reactive reaction rate constant, kR, for the reaction between singlet oxygen and seven mono and disubstituted anthracenes. In most solvents, the rate constants for the quenching of singlet oxygen by monosubstituted anthracenes are in the order of 106 M−1 s−1, whereas for the disubstituted compounds the values are in the order of 107 M−1 s−1 indicating that both, mono and disubstituted anthracenes are good quenchers of singlet oxygen. The reactive rate constants were very close to the total rate constants, implying that quenching is essentially a chemical process. The analysis of solvent effect on kT and kR by using LSER equations indicates that singlet oxygen deactivation by the anthracene derivatives is accelerated by solvents with large values of both, dipolarity/polarizability and Hildebrand solvent parameters, being inhibited by hydrogen bond donor solvents in some cases. Solvent effect analysis support the formation of a loose, zwitterionic like exciplex for 9-methylanthracene and 9-methoxyanthracene, a loose exciplex with partial charge separation for di-substitutes anthracene and a compact exciplex with partial charge separation for anthracene and 9-phenylanthracene.