High-pressure studies of rotational reorientation dynamics: The role of dielectric friction

Abstract

Picosecond, time-resolved, fluorescence depolarization spectroscopy is used to measure the rotational reorientation times of rhodamine 6G (R6G) and p-terphenyl (PTP) as a function of solvent viscosity. The viscosity is varied either by changing the solvent or by changing the pressure in a single solvent. The differences between the two molecules, PTP and R6G, provide a means of evaluating the role of solute structure and solute–solvent interactions on the dynamics of rotational reorientation. The rotational behavior of PTP is well described by simple hydrodynamic models as embodied by the Stokes–Einstein–Debye equation. In contrast, the rotational reorientation dynamics of the charged molecule R6G are not well described by these models. It is demonstrated that dielectric friction plays an important role in governing the rotational motion of charged molecules in polar solvents. When the solvent dielectric properties are varied, the dielectric friction model accurately predicts the observed experimental trends under a wide variety of experimental conditions. This model is also shown to explain anomalous effects previously attributed to the presence of solute–solvent hydrogen bonded complexes.

Fluorescence-Based Investigation of the JÁnossy Effect Anomalous Wavelength Dependence

“…We tested the set-up and the analysis procedure by measuring the fluorescence lifetime s e of Rh6G at 10 À4 mol=L in ethylene glycol and in ethanol, finding in both cases s e ¼ 3.6 AE 0.1 ns, in agreement with values reported in the literature for the case of negligible energy transfer [14,15]. This ensured that energy transfer effects could be neglected for our experimental geometry [15].…”

Rotational Dynamics of Nonpolar and Dipolar Molecules in Polar and Binary Solvent Mixtures

“…The dielectric friction can be modeled using continuum theories of NeeZwanzig (NZ) (Nee and Zwanzig, 1970), which treats the solute as a point dipole rotating in a spherical cavity, Alavi-Waldeck (AW) (Alavi and Waldeck, 1991b;1993) model which is an extension of the NZ theory where the solute is treated as a distribution of charges instead of point dipole and the semiempirical approach of van der Zwan and Hynes (vdZH) (van der Zwan and Hynes, 1985) in which fluorescence Stokes shift of the solute in a given solvent is related to dielectric friction. Conversely, the results of neutral and nonpolar solutes deviate significantly from the hydrodynamic predictions at higher viscosities (Waldeck et al, 1982;Canonica et al, 1985;Phillips et al, 1985;Courtney et al, 1986;Ben Amotz and Drake, 1988;Roy and Doraiswamy, 1993;Williams et al, 1994;Jiang and Blanchard, 1994;Anderton and Kauffman, 1994;Brocklehurst and Young, 1995;Benzler and Luther, 1997;Dutt et al, 1999;Ito et al, 2000;Inamdar et al, 2006). These probes rotate much faster than predicted by the SED theory with stick boundary condition and are described by either slip boundary condition or by quasihydrodynamic theories.…”

“…In the two limiting cases of hydrodynamic stick and slip for a nonspherical molecule, the value of C follows the inequality, 0< C ≤ 1 and the exact value of C is determined by the axial ratio of the probe. It is observed that the experimentally measured rotational reorientation times of number of the nonpolar solutes (Waldeck et al, 1982;Canonica et al, 1985;Phillips et al, 1985;Courtney et al, 1986;Ben Amotz and Drake, 1988;Roy and Doraiswamy, 1993;Williams et al, 1994;Jiang and Blanchard, 1994;Anderton and Kauffman, 1994;Brocklehurst and Young, 1995;Benzler and Luther, 1997;Dutt et al, 1999;Ito et al, 2000;Inamdar et al, 2006) could be described by the SED theory with slip boundary condition (subslip behavior). For a homologous series of solvents such as alcohols or alkanes, the normalized reorientation times decreased as the size of the solvent is increased.…”

“…Note that the probes experience reduced friction as the size of the solvent increases. A number of probes have been studied (Phillips et al, 1985;Courtney et al, 1986;Ben Amotz and Drake, 1988;Roy and Doraiswamy, 1993;Williams et al, 1994;Jiang and Blanchard, 1994;Anderton and Kauffman, 1994;Brocklehurst and Young, 1995) in alcohols and alkanes, wherein faster rotation of the probe in alcohols is observed compared to alkanes, which has been explained as due to higher free volume in alcohols compared to alkanes with the help of DKS theory. If there were no electrical interaction between the coumarins and alcohols, a faster rotation of the coumarins would have been observed in alcohols compared to alkanes, but an opposite trend has been observed that indicates the presence of electrical friction (Dutt and Raman, 2001).…”

“…Objections to this kind of interpretations arise from the fact that in bulk solution, the solvent molecules are expected to exchange (solute-solvent hydrogen bonding dynamics) on a much faster time scale compared to the rotational dynamics. Later, the slower reorientation times of polar molecules in polar solvents have been interpreted using dielectric friction theories (Phillips et al, 1985;Dutt et al, 1990;Alavi et al, 1991b,c;Dutt and Raman, 2001;Gustavsson et al, 2003). Dielectric friction on a rotating solute arises because the polar molecule embedded in a dielectric medium polarizes the surrounding dielectric.…”