TEMPERATURE EFFECT ON PYRENE AS A POLARITY PROBE FOR SUPERCRITICAL-FLUID AND LIQUID SOLUTIONS

The effect of temperature on the fluorescence spectrum of pyrene in su percritical and liquid carbon dioxide and liquid organic solvents is s ystematically studied. The Py parameter (intensity ratio of vibronic b ands 1 and 3) is found to increase with the density of supercritical c arbon dioxide in the range from 0.54 to 0.75 g/cm3. This observation i s 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 sup ercritical and liquid carbon dioxide is also found to decrease with te mperature at constant density, which is not consistent with expectatio ns for dispersion forces. Carbon dioxide, which is generally regarded as a nonpolar solvent, shows a temperature effect comparable to that f or polar liquid solvents. The origin of this temperature effect is exa mined in this study by computer simulation using both semiempirical mo lecular 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 so lvents. The temperature dependence of the Py parameter can be qualitat ively explained by these simulation results.