Solvent-solute interactions probed by picosecond transient Raman spectroscopy: band assignments and vibrational dynamics of S1 trans-4,4'-diphenylstilbene

Abstract

We present the ground-and excited-state Raman spectra of trans-4,4'-diphenylstilbene (DPS). We analyze the spectra on the basis of comparison with the Raman spectra of the ground-and excited-state and anion radical spectra of trans-stilbene and biphenyl. The excited-state Raman spectra of DPS in methylene chloride and dioxane exhibit mode-specific, solvent-dependent dynamics. Specifically, the intensities of several vibrational modes associated with the biphenyl portion of DPS are solvent dependent. We attribute the change in intensity to a variation in the Franck-Condon overlap between SI and S, caused by differences in the planarity of the biphenyl portion of DPS in the two solvents. The lower viscosity solvent, methylene chloride, results in a more planar SI structure of DPS than does dioxane. However, the rate at which the SI geometry achieves equilibrium is slower in methylene chloride than it is in dioxane. This result suggests that dielectric stabilization of SI DPS by the solvent, not viscosity, controls the conformational dynamics.

Time-Resolved Raman Studies of Photoionization of Aromatic Compounds in Polar Solvents: Picosecond Relaxation Dynamics of Aromatic Cation Radicals

“…22,26,33,35,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] In this section, we first focus on the temporal changes in the center wavenumbers of the cation Raman bands. The slow increases in the cation Raman intensities are discussed in the next subsection.…”

“…33,46,47,49,56 For studying intermolecular vibrational relaxation in solutions, analyses of the changes in center wavenumbers and bandwidths of picosecond time-resolved Raman bands have been recognized as one of the useful method. 22,26,33,35,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] Iwata and Hamaguchi have analyzed the high-wavenumber shifts and the band narrowings of the picosecond time-resolved Raman bands of trans-stilbene in the S 1 state. 38,39 The time dependence of the peak position of the olefinic CdC stretching band was found to be a good indicator of the vibrational temperature change in the solute.…”

“…33 The high-wavenumber shifts and the band narrowings of the vibrational bands, which occur on a picosecond time scale, have been observed in other vibrationally hot polyatomic neutral molecules. 26,35,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] All the dynamics have been considered to be associated with intermolecular vibrational relaxation of hot neutral solutes toward thermal equilibria with solvents.…”

“…58 However, the time scale of the intermolecular vibrational relaxation of the cation radicals (17 ps in acetonitrile and 13 ps in ethyl acetate in Figure 7) is roughly the same as that of the intermolecular vibrational relaxation of the polyatomic neutral molecules reported so far. 33,35,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][59][60][61][62] This suggests that the solute-solvent interaction has only a minor effect on the vibrational relaxation observed as the shifts of picosecond vibrational bands. This behavior can be explained by a very rapid cooling model.…”

Sub‐picosecond Raman spectrometer for time‐resolved studies of structural dynamics in heme proteins

“…This method has provided valuable information on myoglobin (Mb) and hemoglobin [14,16,17] but required very long acquisition times and the use of rapidly degrading dyes such as coumarin. Another system [37] based on two picosecond dye lasers synchronously pumped by a mode-locked master Nd : YAG laser yielded a 5-ps time jitter and 1-MHz repetition rate, parameters which are not suitable for ultrafast studies of heme proteins. In the last decade, a large number of TR 3 studies [18,19,31 -33] were performed with a system based on picosecond Ti : sapphire laser [38] having a temporal resolution of about 2 ps.…”