The time evolution of the donor phosphorescence following pulsed laser excitation is used to monitor direct energy transfer at 16 K between the triplet states of donor molecules (4,4rsquo;hyphen;dibromobenzophenone) and acceptor molecules (1,4hyphen;dibromonaphthalene) doped into polymer films (polystyrene). The correspondence between the experimental data and the theoretical description provided by Inokuti and Hirayama is not satisfactory. Better agreement between theory and experiment is obtained by introducing an improved configuration average, which is valid at higher concentrations, and a nonuniform density of acceptor sites, which is a consequence of the polymer structure. The nonuniform density reveals a fundamental difference between the properties of a small molecule glass and a polymer glass. This difference can be quantified by the effective fractal dimension, which equals 3 for the small molecule glass and is found to equal 2.2 for this system.
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