FLUORESCENCE QUENCHING OF N,N'-BIS(2,5-DI-TERT-BUTYLPHENYL)-3,4/9,10-PERYLENEBIS (DICARBOXIMIDE) (DBPI) BY MOLECULAR-OXYGEN

The effect of the solvent viscosity on the fluorescence quenching of N,N'-bis (2,5-di-tert-butylphenyl)-3,4:9,10-perylenebis(dicarboximide) (DBPI) by molecular oxygen was investigated by measuring the fluorescence decay. The solvent viscosity was changed by using different solvents at 0.1 MPa and by application of hydrostatic pressure. The quenching rate constant k(q) in non-polar solvents increased with increasing solvent viscosity eta, whereas in polar alcoholic solvents it decreased with increasing solvent viscosity and followed a function of A eta(-alpha) with alpha = 0.48 +/- 0.12. When the viscosity was changed by application of hydrostatic pressure, k(q) of DBPI was also found to follow a function of A eta(-alpha) with alpha values of 0.71 +/- 0.02 and 0.58 +/- 0.08 in methanol and methylcyclohexane respectively. However, in n-hexane, the k(q) values exhibited a convex shape vs. viscosity. The activation energies associated with the fluorescence quenching of DBPI were calculated to be E(a) = 9.87 +/- 1.2, 14.1 +/- 0.24 and -3.96 +/- 0.6 kJ mol(-1) in methanol, methylcyclohexane and n-hexane respectively. The negative activation energy and the convex shape of the viscosity dependence of k(q) obtained in n-hexane indicate complex formation between DBPI* and molecular oxygen prior to fluorescence quenching. The large molecular size of DBPI does not increase the magnitude of k(q) or influence its dependence on eta.