Temperature dependence of the nearly diffusion-controlled fluorescence quenching by oxygen of 9,10-dimethylanthracence in liquid solution

The rate constant, k_q, for the fluorescence quenching of 9,10-dimethyl-anthracene (DMEA) by oxygen in n-hexane and methylcyclohexane (MCH) was measured as a function of temperature at 0.1 MPa. It was found that the activation energy for the quenching, E_q^≠, was 6.6 ± 0.1 and 8.4 ± 0.3 kJ mol⁻¹, smaller than that for the solvent viscosity, E_η^≠, 7.1 ± 0.2 and 10.6 ± 0.1 kJ mol⁻¹, in n-hexane and MCH, respectively. The difference between E_q^≠ and E_η^≠ was interpreted to arise as a result of the quenching being not fully but nearly diffusion-controlled. The contribution of diffusion to the quenching was analyzed quantitatively from the temperature dependence of the solvent viscosity together with that of the radial distribution function at the closest approach distance with hard spheres. The results were shown to be consistent with those obtained from the high-pressure study reported previously.