Steady-state and time-resolved emission spectroscopy with 25 ps resolution are used to measure equilibrium and dynamic aspects of the solvation of coumarin 153 (C153) in a diverse collection of 21 room-temperature ionic liquids. The ionic liquids studied here include several phosphonium and imidazolium liquids previously reported as well as 12 new ionic liquids that incorporate two homologous series of ammonium and pyrrolidinium cations. Steady-state absorption and emission spectra are used to extract solvation free energies and reorganization energies associated with the S0 <--> S1 transition of C153. These quantities, especially the solvation free energy, vary relatively little in ionic liquids compared to conventional solvents. Some correlation is found between these quantities and the mean separation between ions (or molar volume). Time-resolved anisotropies are used to observe solute rotation. Rotation times measured in ionic liquids correlate with solvent viscosity in much the same way that they do in conventional polar solvents. No special frictional coupling between the C153 and the ionic liquid solvents is indicated by these times. But, in contrast to what is observed in most low-viscosity conventional solvents, rotational correlation functions in ionic liquids are nonexponential. Time-resolved Stokes shift measurements are used to characterize solvation dynamics. The solvation response functions in ionic liquids are also nonexponential and can be reasonably represented by stretched-exponential functions of time. The solvation times observed are correlated with the solvent viscosity, and the much slower solvation in ionic liquids compared to dipolar solvents can be attributed to their much larger viscosities. Solvation times of the majority of ionic liquids studied appear to follow a single correlation with solvent viscosity. Only liquids incorporating the largest phosphonium cation appear to follow a distinctly different correlation.

Dynamic Stokes shift measurements of the solvatochromic probe trans-4-dimethylamino-4'-cyanostilbene were used to measure the solvation response of five imidazolium and one pyrrolidinium ionic liquid at 25 degrees C. The Kerr-gated emission and time-correlated single-photon-counting techniques were used to measure spectral dynamics occurring over the time ranges of 100 fs-200 ps and 50 ps-5 ns, respectively, and a combination of data sets from these two techniques enabled observation of the complete solvation response. Observed response functions were found to be biphasic, consisting of a sub-picosecond component of modest (10-20%) amplitude and a dominant slower component relaxing over times of a few picoseconds to several nanoseconds. The faster component could be correlated to inertial characteristics of the constituent ions, and the slower component to solvent viscosity. Dielectric continuum calculations of the sort previously used to predict solvation dynamics in dipolar liquids were shown to work poorly for predicting the response in these ionic liquids.

The electronic relaxation and isomerization mechanism of trans-azobenzene after the S 2 (ππ*) r S 0 photoexcitation were investigated in solution by steady-state and femtosecond time-resolved fluorescence spectroscopy. In the steady-state fluorescence spectrum, two bands were observed with their peaks at ∼390 nm (∼25 750 cm -1 ) and ∼665 nm (∼15 000 cm -1 ). These fluorescence bands showed good mirror images of the S 2 (ππ*) r S 0 and S 1 (nπ*) r S 0 absorption bands, so that they were assigned to the fluorescence from the S 2 (ππ*) and S 1 (nπ*) states having "planar" structures. The lifetimes of the S 2 and S 1 states were determined as ∼110 fs (S 2 ) and ∼500 fs (S 1 ) by time-resolved measurements. The quantum yield of the S 2 f S 1 electronic relaxation was evaluated by comparing the intensity of the S 2 and S 1 fluorescence, and it was found to be almost unity. This implies that almost all molecules photoexcited to the S 2 (ππ*) state are relaxed to the "planar" S 1 (nπ*) state. The present fluorescence data clarified that the isomerization following S 2 (ππ*) photoexcitation takes place after the S 2 f planar S 1 electronic relaxation and that the rotational isomerization pathway starting directly from the S 2 (ππ*) state does not exist. It was thus indicated that the isomerization mechanism of azobenzene is the inversion isomerization occurring in the S 1 state, regardless of difference in initial photoexcitation. The relaxation pathways in the S 1 state were also discussed on the basis of spectroscopic and photochemical data.

Physical properties of 4 room-temperature ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various perfluorinated anions and the bis(trifluoromethylsulfonyl)imide (Tf2N-) anion with 12 pyrrolidinium-, ammonium-, and hydroxyl-containing cations are reported. Electronic structure methods are used to calculate properties related to the size, shape, and dipole moment of individual ions. Experimental measurements of phase-transition temperatures, densities, refractive indices, surface tensions, solvatochromic polarities based on absorption of Nile Red, 19F chemical shifts of the Tf2N- anion, temperature-dependent viscosities, conductivities, and cation diffusion coefficients are reported. Correlations among the measured quantities as well as the use of surface tension and molar volume for estimating Hildebrand solubility parameters of ionic liquids are also discussed.

Solvation and rotational dynamics of coumarin 153 (C153) were measured in a series of phosphonium ionic liquids consisting of the trihexyl(tetradecyl)phosphonium cation and the anions Br-, Cl-, dicyanamide-, bis(trifluoromethylsulfonyl)imide-, and BF4 -. None of these liquids display a prominent ultrafast solvation component comparable to the one observed in imidizalium ionic liquids. The solvation dynamics observed on the nanosecond time scale are also 5-fold slower than those in imidazolium liquids of comparable viscosities. Both rotation and solvation are highly nonexponential functions of time and can be represented by stretched-exponential decays with exponents near 0.5. Characteristic times of these processes track solvent viscosity. A number of similarities between the dynamics observed in these ionic liquids and supercooled glass-forming liquids are noted.

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Electronic structure calculations, steady-state electronic spectroscopy, and femtosecond time-resolved emission spectroscopy are used to examine the photophysics of trans-4-(dimethylamino)-4'-cyanostilbene (DCS) and its solvent dependence. Semiempirical AM1/CI calculations suggest that an anilino TICT state is a potential candidate for the emissive state of DCS in polar solvents. But observation of large and solvent-independent absorption and emission transition moments in a number of solvents (M(abs) = 6.7 +/- 0.4 D and M(em) = 7.6 +/- 0.8 D) rule out the involvement of any such state, which would have a vanishingly small transition moment. The absorption and steady-state emission spectra of DCS evolve in a systematic manner with solvent polarity, approximately as would be expected for a single, highly polar excited state. Attempts to fit the solvatochromism of DCS using standard dielectric continuum models are only partially successful when values of the solute dipole moments suggested by independent measurements are assumed. The shapes of the absorption and emission spectra of DCS change systematically with solvent polarity in a manner that is semiquantitatively reproduced using a coupled-state model of the spectroscopy. Kerr-gate emission measurements show that the emission dynamics of DCS down to subpicosecond times reflect only solvent relaxation, rather than any more complicated electronic state kinetics. The spectral response functions measured with DCS are well correlated to those previously reported for the solvation probe coumarin 153, indicating DCS to be a useful alternative probe of solvation dynamics.