The effect of temperature on the fluorescence spectrum of pyrene in supercritical and liquid carbon dioxide and liquid organic solvents is systematically studied. The Py parameter (intensity ratio of vibronic bands 1 and 3) is found to increase with the density of supercritical carbon dioxide in the range from 0.54 to 0.75 g/cm3. This observation is consistent with the fact that dispersion forces, which represent the major interaction between pyrene and carbon dioxide, depend inversely on the sixth power of distance. However, the Py parameter of both supercritical and liquid carbon dioxide is also found to decrease with temperature at constant density, which is not consistent with expectations for dispersion forces. Carbon dioxide, which is generally regarded as a nonpolar solvent, shows a temperature effect comparable to that for polar liquid solvents. The origin of this temperature effect is examined in this study by computer simulation using both semiempirical molecular orbital and molecular mechanics methods. On the basis of these simulations, a strong electrostatic attraction arises between pyrene and carbon dioxide which is similar in magnitude to that with polar solvents. The temperature dependence of the Py parameter can be qualitatively explained by these simulation results.
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