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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.

We describe results of computer simulations and sub-picosecond time-resolved fluorescence experiments on the solvation dynamics of 1-aminonapthalene and coumarin 153 in acetonitrile and methanol. Both the simulations and experiments point to the importance of fast, inertial components in the solvation response in these systems. Where direct comparisons between the experiment and simulation are possible, the agreement is found to be quite good. We also consider application of a simple theory for the solvation response in order to make some predictions about the likely importance of the inertial component in a variety of common solvents. 9This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to IP: 128.32.208.2 On: Wed, 13 Nov 2013 23:36:09 C(t ) = a t exp(-to2t 2 / 2) + a 2 exp(-t / x2) + a3 exp(-t/'t 3) Amplitudes are given in parenthesis. The column marked "c gives the average of the exponential time constants and the biexponential parameters (if applicable).

The excited-state dynamics of polymer-bound J-aggregates formed in aqueous mixtures of pseudoisocyanine (PIC) chloride and poly(viny1 sulfonic acid sodium salt) (PVS) (MW -1 1 200) have been studied by picosecond time-correlated single-photon counting and picosecond polarized pump-probe spectroscopy. At a concentration of 40 NM PIC and 5 X 10-4 g/dL PVS, the absorption spectrum of the J-aggregate is characterized by a J-band at 565 nm with a fwhm of ~5 0 0 cm-I. For this particular mixture, a dye molecule is bound to each of the so3groups on the polymer chains. The physical size of the aggregate is therefore determined by the number of polymer residues (-87) per chain. The fluorescence lifetime and fluorescence quantum yield of these J-aggregates are 17 f 3 ps and 0.022 f 0.003, respectively. The lifetime is independent of the excitation intensity. From an analysis of the photophysical parameters, we infer a coherence size of -5 f 1, which is smaller than the physical size of the aggregate. The pump-probe signal at 565 nm is entirely due to bleaching and consists of a fast component, with a decay time comparable to the fluorescence lifetime, and a slow component. The decay kinetics of the induced bleaching are independent of the excitation intensity. The signal at 558 nm consists of an absorption component at early times and a slowly decaying bleaching component at long times. The anisotropy at 565 nm was constant over the 160-ps time range of the signal. The kinetics are rationalized in terms of a model involving the singlet exciton states of the J-aggregate and a long-lived bottleneck state. The induced absorption at early times for excitation on the blue edge of the J-band is consistent with a one-exciton to two-exciton transition.