SOLVENT/SOLUTE INTERACTIONS PROBED BY PICOSECOND TRANSIENT RAMAN SPECTROSCOPY

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1994

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Ohio State University

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Picosecond transient Raman spectroscopy provides valuable information about the microscopic interactions that occur between a solute, molecule and its solvent environment. In this study, we have chosen 1,4-diphenyl-1,3-butadiene(DPB) as the probe molecule and the linear alkanes from pentane to dodecane as the solvents. Given the virtual degeneracy of the 21A8 and 11Bu states of DPB in solution, variation of the solvent properties provides valuable information about solvent/solute dynamics, in general. we have obtained the S1 Raman spectra of DPB in the linear alkanes series at two different probe wavelengths (630 and 670 nm). We observe bands assignable to both the 21Ag and 11Bg states in solution. The Spectra exhibit broad (>50cm−1) Raman bands attributed to motions involving the olefin portion of the molecule. We note that the relative intensities of bands assigned to olefinic stretching motions change as the solvent viscosity increases, and we also observe mode specific, solvent dependent changes in the S1 spectra with delay. Based on the S1 Raman spectra of 1,4-diphenyl-1,3-cyelopenladiene (DPCP), a ``stiff” DPB analogue, we attribute the large bandwidths in the S1 Raman spectra of DPB to a distribution of s-trans conformers in solution. The changes in the relative intensities of particular bands with respect to solvent viscosity are believed to arise from differing population distributions of s-trans conformers in solution that are dependent on solvent viscosity. The mode specific, solvent dependent changes that occur with delay are attributed to vibrational relaxation of DPB via resonance energy exchange of the excess vibrational energy with the solvent.

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Author Institution: Department of Chemistry, The Ohio State University

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