Energy migration study of random immobile anthracene derivatives by time-resolved fluorescence anisotropy decays

We describe time-resolved fluorescence anisotropy measurements for a simple 2-substituted anthracene derivative (2-An-M) as a function of concentration in poly(methyl methacrylate) (PMMA) films. The anisotropy decays via energy migration among 2-An-M molecules in the matrix. The data were interpreted in terms of the survival probability of the initially excited chromophores and are in good agreement with the plot predicted using R0 = 1.8 nm, the value of the Förster radius determined independently by the spectral overlap method. For all concentrations, we found r0 = 0.20 +/- 0.01 for the initial anisotropy at time zero, whereas the residual anisotropy, r(infinity), was concentration-dependent and higher in magnitude than the theoretically predicted value, 4% of r0. The higher residual anisotropy values may originate from the possibility that not all of the anthracene molecules are involved in the energy migration process. Similar anthracene anisotropy experiments were performed on anthracene-labeled poly(isoprene-b-methyl methacrylate) (PI-PMMA). The results show an increased depolarization rate for samples containing a higher fraction of polymers labeled at the junction with anthracene chromophores.