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

Abstract

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. © The Royal Society of Chemistry and Owner Societies 2009.