Structure and time-resolved phosphorescence spectra of crystalline and glassy 2-bromobenzophenone

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Microsecond-level time-resolved photoluminescence spectra of 2-bromobenzophenone are measured on crystalline (at 1.6, 52, and 300  K) and glassy (at 1.6 and 95  K) samples. For each of these physical situations the characteristic decay times are determined for the 0–0 band of monomer emission from metastable (blue 0–0 band) and global (red 0–0 band) excited states of the 2-benzophenone molecule in the crystal and also for the bimolecular excimer emission. The time dependence of the intensity of the red 0–0 band at 52  K in the crystal is found to be nonmonotonic, a fact that provides yet another beautiful demonstration of the presence of two excited states and is indicative of thermally activated nonradiative transitions from the metastable excited state to the global. The corresponding kinetics is well described by a system of equations for joint radiative decay. To gain a better understanding of the nature of the anomalies of the phosphorescence, the crystal and molecular structures of 2-benzophenone at 100 and 200  K are determined by the single-crystal x-ray diffraction method. Within the temperature interval 100–300  K the crystal structure of 2-benzophenone remains monoclinic, with symmetry space group P2₁/a (Z=4). The coefficient of linear thermal expansion is anisotropic and of a magnitude (10⁻⁵−10⁻⁴  K⁻¹) typical for organic molecular crystals. The conformation parameters of the molecule vary relatively strongly with temperature; the behavior of the C–Br and C=O bonds is most interesting: they become shorter with increasing temperature, possibly indicating a weakening of the weak hydrogen bonds of the peripheral oxidizer atoms with the acceptors of the neighboring molecules. ©2009 American Institute of Physics