Fluorescence quenching of meso‐substituted anthracene derivatives by molecular oxygen

Rate constants of fluorescence quenching by molecular oxygen, kSq, of a series of meso‐substituted anthracene derivatives have been determined in toluene and acetonitrile at room temperature. They adopt values between 0.4 and 3.4 × 1010M−1 s−1 in toluene and values between 0.7 and 5.5 × 1010M−1 s−1 in acetonitrile. The ratio k (toluene)/k (acetonitrile) in all cases nearly equals the inverse ratio of the solvent viscosities η(acetonitrile)q/η(toluene) = 0.61. Additionally, singlet oxygen (1O2) efficiencies for quenching of the singlet state ϕ and the triplet state ϕ have been measured in both solvents. No significant decrease of the efficiencies of 1O2 formation in the polar solvent compared to the non‐polar solvent has been observed for the meso‐substituted anthracenes. In both solvents ϕ varies approximately linear with 1/kSq. The weak dependence of (1 ‐ ϕ )k = k (3O2) on the free enthalpy of electron transfer suggests the participation of charge transfer in the quenching process, but small and almost identical slopes of Hammett‐plots of the rate constants of fluorescence quenching in both solvents indicate that there is only little contribution of charge transfer interactions if the rate constant of fluorescence quenching is below the diffusion limit.