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.