Viscosity dependence of rubrene fluorescence quenching by organic radicals via energy transfer

The kinetics of rubrene (5,6,11,12-tetraphenylnaphthacene) fluorescence quenching by energy transfer to the radicals of N,N,N′,N′-tetramethyl-para-phenylenediamine (TMPD⁺˙), tetracyanoquinonedimethane (TCNQ⁻˙) and galvinoxyl (2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy; GAL˙), have been measured in solvents of different viscosities. At room temperature the observed quenching rate constants vary between 0.12 and 7.8 × 10¹⁰ M⁻¹ s⁻¹, always exceeding the diffusion rate constants given by the simplest version of Smoluchowski theory combined with the rate constants for dipole–dipole energy transfer. This discrepancy is explained by accounting for the non-stationary stage in the Förster kinetics, even though the experimental fluorescence decays are apparently mono-exponential.