Steady state phosphorescence at 4.2 K is monitored for different concentrations of 4-methylbenzophenone (MBP) doped into polystyrene. At concentrations less than 20%, the spectra are characteristic of separated molecules embedded in a disordered matrix. As the concentration increases to 20%, the vibronic bands shift to longer wavelengths and become slightly sharper. At concentrations above 30%, the spectra have additional features due to aggregates, which appear to be characteristic of a segregated phase of MBP. Triplet excitation energy transport in glassy MBP and MBP doped into a polystyrene matrix is monitored for different excitation energies and temperatures using time-resolved spectroscopy. These experiments reveal that spectral diffusion at liquid helium temperatures is not thermally activated, and that the spectral diffusion efficiency increases abruptly at a critical mole fraction of acceptors, 0.054, which corresponds to a critical distance of 9.8 Å. Although low-dimensional energy transfer dynamics has been revealed in other studies of disordered systems, the results indicate that spectral diffusion in these systems is controlled by a three-dimensional exchange interaction and the emission of phonons and/or vibrons. © 1990.
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