The rotational correlation times of trans,trans-1,4-diphenylbutadiene in n-alcohol solvents over a wide temperature range are reported. They are well represented as linear functions of q/T within a single alcohol, but comparison across a series of alcohols indicates that the boundary condition factor in the modified StokesEinstein-Debye equation decreases as the solvent size increases. The experimental results are compared with the Dote-Kivelson-Schwartz (DKS) quasihydrodynamic free space model, which is based on regular solution theory. The model in its original form is able to qualitatively predict the trend toward smaller boundary condition factor with increasing solvent size but is unable to quantitatively predict the observed dependence of the rotational correlation time either on q at fixed temperature across the alcohol series or as a function of temperature within a single alcohol. Substitution of an empirical measure of the solute free space into the DKS theory results in improved agreement between the theoretical and experimental results. This suggests that the general framework of the DKS is appropriate for both regular and associating liquids but that different measures of free volume are required for each solvent type.

1-(9-Anthryl)-3-(4-N,N-dimethylaniline) propane (ADMA) is known to form an emissive “sandwich heteroexcimer” (SH) in the excited state. The SH state has a distinct emission spectrum that can be characterized as a single Gaussian peak. We have studied preferential solvation of the ADMA SH state in binary hexane−ethanol mixtures by monitoring its peak emission energy. The results are analyzed with a theory of preferential solvation by dielectric enrichment (Suppan, P. J. Chem. Soc., Faraday Trans. 1987, 83, 495). Our analysis demonstrates a significant influence of mixture dielectric nonideality on solvatochromism, and this effect can be misinterpreted as specific solvent−solute interaction if it is not treated properly. We are able to separate the influence of mixture nonideality from the influence of dielectric enrichment, and conclude that specific interactions do not contribute to the observed solvatochromic shift in the ADMA−hexane−ethanol system. The analysis also indicates that solvent−solute interactions in this system can be adequately described with a continuum model. We calculate the composition of the ADMA solvation shell and find that it is enriched in ethanol by ∼50% over the bulk composition. ADMA is identified as an excellent probe of dielectric enrichment in complex environments.

Rotational correlation times of two solutes, trans, trans-1 ,Cdiphenylbutadiene (DPB) and rrans-4-(hydroxymethy1)stilbene (HMS), have been measured in supercritical C02 as a function of density at a reduced temperature of 1.01 (35 "C) across the compressible region. The DPB rotational correlation time increases by 50% upon increasing density from 0.3 to 0.8 g/cm3, whereas the HMS rotational correlation time increases by more than 300% over the same density regime. The results are compared with a model that is developed to describe the influence of local solvent density on the rotational Correlation time of the solute. The model predicts a strong dependence of the rotational correlation time on the local solvent density as the liquid density of C02 is approached. On the basis of a comparison of the data with the model, the experimental results are interpreted as evidence for solute-induced clustering of C02 around HMS as a result of attractive solute-solvent interaction. The relative magnitude of the observed effect in HMS compared with DPB is taken as evidence that the operative interaction occurs between C02 and the hydroxyl group of HMS.Clustering of supercritical fluid solvents around solute molecules is thought to be responsible for enhanced solubility of solutes, particularly in the compressible region of the solvent's phase Considerable the~retical~-~ and experimental8-I3 effort has been aimed at developing a detailed understanding of this phenomenon. Spectroscopic studies have been widely applied to these systems because they are inherently sensitive to local solvent effects around chromophoric solute molecules. Such studies have the potential for distinguishing between theoretical models describing fluid clustering. Solvatochromic studies are most commonly employed, resulting in empirical measures of the local solvent reaction field around the solute.8-11~13-14 However, solvatochromic studies suffer from the fact that compounds which exhibit strong solvatochromic behavior contain highly polar functional groups that often experience charge transfer isomerization, either in the ground or excited electronic states. Thus it is difficult to develop relationships between solute functionality and the strength of solvent-solute interactions through these measurements because only specific solutes exhibit the correct properties to be useful candidates for such examinations.Studies of molecular rotational relaxation are not restricted by this shortcoming. A wide variety of solutes are amenable to this method, offering the possibility of examining the influence of solute functionality on solvent clustering in supercritical fluid solutions. This approach has been applied to the study of solvent-solute interactions in liquid solution^,'^^'^ and a few studies of solute rotational relaxation have been carried out in supercritical fluids solutions to For example Bright and co-workers have observed remarkable variation in the rotational correlation time of PRODAN in supercritical N20 as a function of density.I2 However th...

Dielectric nonideality of a binary solvent system refers to the deviation of the Onsager reaction field function from linearity in the polar mole fraction of the solvent mixture. A dipolar fluorophore dissolved in an ideal dielectric mixture exhibits a solvatochromic shift that is linear in the solvent polar mole fraction in it's solvation sphere. As a result, the "local composition" can be easily determined from the peak shift. Here we identify the conditions under which this linear approximation is appropriate for estimating local compositions around dipolar solutes. In a previous study (Khajehpour, M. H.; Kauffman, J. F. J. Phys. Chem. A 2000, 104, 7151-7159), we have demonstrated the influence of dielectric nonideality on the observed emission peak shifts of the charge-transfer excited state of ADMA [1-(9-anthryl)-3-(4-N,N-dimethylaniline)propane] in hexanesethanol mixtures. The linear approximation fails for this binary solvent, and a more elaborate method of analysis such as Suppan's nonlinearity ratio method must be used to determine the local composition from solvatochromic shifts. In this work, we examine mixture nonideality and dielectric enrichment in hexanetetrahydrofuran and hexane-dichloromethane mixtures. Our analysis demonstrates that the contribution of nonideality to the observed solvatochromic shifts cannot be neglected in these binary solvents. Using Suppan's theory of dielectric enrichment, we have calculated the local composition of ADMA's solvation sphere and find that it is enriched in the polar component by ∼30% over the bulk composition. This calculated value agrees with experimental measures of the local composition based on analysis of solvatochromic shifts using Suppan's nonlinearity ratio method which accounts for dielectric nonideality. The linear approximation overpredicts this composition by as much as 50%, even though these binary solvents are more nearly ideal than the hexane-ethanol system. Following this observation, we have identified conditions under which the linear approximation is justified, and find that for most cases of practical importance the linear approximation will not provide accurate estimates of the local solvent composition from solvatochromic studies. Similarly, solvatochromic shifts can only be accurately predicted from theoretical local compositions if dielectric nonideality is taken into account. These results along with our previous studies indicate that the chargetransfer excited state of ADMA behaves as an ideal dipolar solute.

This article presents an overview of our recent studies of solvation and unimolecular reactivity in liquids and supercritical fluids. We review our research on rotational diffusion and photoisomerization kinetics of diphenylpolyenes in polar liquids and supercritical fluids. These results provide a qualitative demonstration of the influence of solvent quadrupole-solute dipole interactions on chemical reactivity in supercritical CO 2 . The CO 2 quadrupole moment both increases solvent-solute friction experienced by a dipolar diphenylpolyene solute molecule and diminishes the activation barrier of the isomerization reaction via interaction with the polar transition state. We also review our recent studies of quadrupolar solvation and excited-state excimer formation kinetics of ADMA (1-[9-anthryl]-3-[4-N,N-dimethylaniline] propane). The ADMA studies have yielded quantitative confirmation of the importance of quadrupolar solvent-dipolar solute interactions in solvation and reactivity in supercritical CO 2 . The results of these studies allow us to measure the contribution of solvent quadrupole-solute dipole interaction to the solvation energy of the excited-state charge transfer form of ADMA. We also discuss recent evidence indicating that the CO 2 quadrupole moment influences the kinetics of the charge-transfer reaction.