UC Irvine

Solvent dipolar reorientations following excitation of DANCA in glycerol at various temperatures were investigated by using multifrequency-phase fluorometry. The time evolution of the emission spectrum was determined by performing lifetime measurements at a number of wavelengths across the emission band and then reconstructing the spectrum at selected times after excitation. At low temperatures, a large displacement of the time-resolved spectrum was apparent. The spectral center of gravity was observed to move faster at short times and then to exhibit a long decay. The spectral width was also observed to increase at short times and then to decrease at much longer times. Several models have been considered to account for the major features of our experimental results. A two-state model agreed well with the data only in the high-temperature regime; at lower temperatures, the data were not consistent with the characteristic features of the two-state model. In the high-temperature regime, we obtained a reorientation rate for the glycerol molecules using the two-state model. Other models which consider a continuous translation of the emission spectra as a function of time also failed to account for the experimental results. Instead, a good qualitative agreement was found with the model proposed by Weber in which the charge separation of the excited fluorophore is followed by reorientation of the solvent molecules around the individual monopoles. © 1987, American Chemical Society. All rights reserved.