Solvent and Solvent Density Effects on the Spectral Shifts and the Bandwidths of the Absorption and the Resonance Raman Spectra of Phenol Blue

Abstract

We have measured the absorption and the resonance Raman spectra of a solvatochromic dye, phenol blue, in liquid and supercritical solvents. We have found anomalous solvent dependence of the absorption bandwidth in liquid solvents: the width has apparently no correlation with the absorption peak shift. On the other hand, we have found good linear correlation between the absorption peak shift and the peak position of the resonance Raman bands (the CdN and the CdO stretching modes). The relative intensities of the Raman bands and the bandwidth of the CdN stretching mode also show correlation with the absorption peak shift. Incorporating these Raman data, the anomalous bandwidth of the absorption spectrum is explained by the change of the intramolecular vibrational contribution to the absorption bandwidth due to the electronic structure change by the solvent, which cancels the change of the solvent contribution. We have estimated the solvent reorganization energy assuming linear dependence of the intramolecular contribution on the absorption band center and neglecting the solvent reorganization energy in alkanes such as ethane and cyclohexane. In liquid solution, the estimated solvent reorganization energy is correlated fairly well with the absorption peak shift. Solvent dependence of the Raman bandwidth of the CdN stretching mode resembles the solvent dependence of the solvent reorganization energy estimated in this way. Relatively large bandwidths of both absorption and resonance Raman spectra have been observed in supercritical solvents compared with those in liquid solvents of a similar absorption peak shift. We interpreted this as due to the small refractive indices of the supercritical solvents relative to the liquid solvents; the large refractive indices of the liquid solvents only make the absorption peak shifts without broadening the absorption spectra.

NMR Spin–Spin Coupling Constants in Polymethine Dyes as Polarity Indicators

“…There are a few XRD and NMR‐ and IR/Raman‐spectroscopy‐based measurements that discuss the solvent‐dependence of molecular structure and provide direct evidence for such geometrical changes 13. 42, 43 Hybrid QM/MM approaches, which account for polarization effects of the molecules owing to surrounding homogeneous and heterogeneous media, allow us to model such environment‐dependent structural modifications in the molecular probes.…”

Energy Flow in Push–Pull Chromophores: Vibrational Dynamics in para‐Nitroaniline

“…In general, the absorption and vibrational spectra of push-pull molecules exhibit inhomogeneous broadening attributed to solvent-solute interactions, which are particularly strong in polar solvents. [13,14,[30][31][32] However, RREP studies by Moran and Kelley found little evidence for inhomogeneous broadening in PNA, except in the hydrogen-bonding solvent methanol. [17] Perhaps in PNA, inhomogeneous broadening is a relatively minor effect, in comparison to other push-pull chromophore systems.…”

“…In particular, conformational broadening of the absorption lineshape for the CT transition may offer a mechanism by which to test the influence of molecular structure on the electronic and vibrational dynamics of the system. [13,14,[30][31][32] In the ground electronic state, the potential surface of PNA is flat along the NO 2 coordinate (w NO2-twist % 30-60 cm…”

Vibrational energy relaxation of azulene in the S2 state. II. Solvent density dependence

“…The high-pressure apparatus for the measurement of fluorescence in supercritical fluids is described elsewhere. [24][25][26][27][28] We used two different high pressure cells for the pressure higher or lower than 35 MPa. The cell for the lower pressure is made of titanium and has four quartz windows of larger aperture.…”