SOLVENT EFFECTS ON PRERESONANCE RAMAN-SCATTERING IN TCNQ SOLUTIONS

The preresonance Raman spectrum of tetracyanoquinodimethane (TCNQ) in methylene chloride/cyclohexane solution was measured at excitation wavelengths from 455 to 620 nm. Using cyclohexane Raman modes as internal standards, the intensities of the v1 and v4 vibrations of CH2Cl2 were found to be greater in the neat binary solution than in the TCNQ solution, at most excitation wavelengths. The reduced intensity is not accounted for by self-absorption of the scattered radiation or by dispersion in the refractive index which contributes to the local field correction. The excitation profile of the 330-cm-1 mode of TCNQ was also measured in acetonitrile/cyclohexane solution and compared to that in methylene chloride/cyclohexane. The main difference in the TCNQ Raman cross section in the two binary solvents was that due to the shift in the origin of the TCNQ electronic transition. Local field and other models for solvent effects on electronic transition moments cannot account for the observed perturbations of solvent and solute Raman intensities. The observed intensity changes may be the result of specific solute-solvent interactions, not considered by existing theories of solvent effects on Raman intensities. © 1990 American Chemical Society.