No abstract

Steady-state and time-resolved emission measurements of the solvatochromic probe coumarin 153 are used to study solvation of a dipolar solute in nondipolar solvents such as benzene and 1,4-dioxane. Contrary to the predictions of dielectric continuum theories, the Stokes shifts (or nuclear reorganization energies) that accompany electronic excitation of this solute are substantial in such solvents (∼1000 cm-1). The magnitudes of the shifts observed in both nondipolar and dipolar solvents can be consistently understood in terms of the relative strength of the interactions between the permanent charge distributions of the solute and solvent molecules. (Information concerning these charge distributions is derived from extensive ab initio calculations on the solute and 31 common solvents.) The dynamics of solvation in nondipolar solvents, as reflected in the time dependence of the Stokes shifts, is qualitatively like that observed in polar solvents. But, whereas the dynamics in polar solvents can be rather simply modeled using the solvents dielectric response as empirical input, no simple theories of this sort are currently capable of predicting the solvation dynamics in nondipolar solvents.

Subpicosecond fluorescence anisotropy measurements are used to characterize the rotational dynamics of coumarin 153 (C153) in 35 common solvents and eight solvent mixtures at room temperature. The rotational anisotropy decays of C153 are generally nonexponential as a result of the non-Markovian nature of the friction on its rotational motion. Rotational correlation times are observed to be larger in polar solvents than in nonpolar solvents of the same viscosity. This difference is examined in the context of theories of dielectric friction, which relate the extra friction in polar solute/solvent systems to long-range dipole−dipole interactions. Since the latter interactions have been thoroughly characterized via dynamic Stokes shift measurements for the same solute/solvent combinations studied here, the present data provide a unique opportunity to test general concepts of dielectric friction. Contrary to expectations, the departures from simple hydrodynamic behavior cannot be modeled using only theories of rotational dielectric friction. More important than dielectric friction is the role that the relative solute/solvent size plays in determining the extent of solute−solvent coupling. Once this size dependence is approximately accounted for, the remaining departures from simple hydrodynamic behavior are relatively small in all solvents. In polar aprotic solvents, solvation data indicate that dielectric friction effects should be rather modest (10−20% of the total friction). In these solvents no clear correlation is found between dielectric friction predictions and the observed solute−solvent coupling. However, in normal alcohol solvents the effects of dielectric friction are predicted to be large and well beyond the scatter in the experimental data. No evidence for such an important dielectric friction contribution is observed in these solvents, in spite of the fact that long-time components of the solvation dynamics do appear to be present in the rotational friction.

Progress in understanding, diagnosis, and treatment of coronary artery disease (CAD) has been hindered by our inability to observe cells and extracellular components associated with human coronary atherosclerosis in situ. The current standards for microstructural investigation, histology and electron microscopy, are destructive and prone to artifacts. The highest resolution intracoronary imaging modality, optical coherence tomography (OCT), has a resolution of ~10μm, which is too coarse for visualizing most cells. Here we report a new form of OCT, termed μOCT that has an order of magnitude improved resolution. We show that μOCT images of cadaver coronary arteries provide clear pictures of cellular and subcellular features associated with atherogenesis, thrombosis, and response to interventional therapy. These results suggest that μOCT can complement existing diagnostic techniques for investigating atherosclerotic specimens today and may in the future become a useful tool for cellular and subcellular characterization of the human coronary wall in vivo.