High Pressure Study of Viscosity Effects on the Luminescence of Some S-Tetracyanobenzene Eda Complexes

Abstract

High pressure fluorescence studies from 0 - 10 kilobars have been performed on electron donor-acceptor (EDA) complexes of s-tetracyanobenzene (TCNB) with a series of aromatic hydrocarbons. Four solvents were used, 2,2,4,4,6,8,8-heptamethylnonane (HMN), methylcyclohexane (MCH), 2,6,10,14-tetramethylpentadecane (TMPD), and a mixture of MCH and HMN. A viscosity range from 0.006 to 10,000 poise was covered at two temperatures, 0(DEGREES)C and 25(DEGREES)C. As pressure (viscosity) increased the fluorescence spectrum shifted from one dominated by emission from the equilibrium (EQ) excited singlet state to one dominated by Franck-Condon (FC) singlet emission. Lifetime measurements for the complexes of o-xylene and p-xylene with TCNB yielded the two radiative rates (k(,EQ) and k(,FC)) as well as the rate of relaxation from FC to the EQ excited state (k(,RE)). k(,RE) was found to correlate well with viscosity and to be independent of temperature at constant viscosity, indicating that the relaxation process is diffusion controlled.