Solvation Dynamics In Ionic Liquids And Their Polar Solvent Mixtures

Solvation dynamics in ionic liquids (ILs) is typically biphasic, consisting of a fast component completed in1ps and the rest extending up to the nanosecond domain. To observe the fast dynamics, an instrument for broadband fluorescence upconversion spectroscopy (FLUPS) is developed. With tilted1340nm gate pulses, photometric and time-zero calibration, highly time-resolved (80fs), broad spectral range (425-750nm) and completely background free spectra can be obtained.The full dynamic Stokes shift is measured by combining the techniques of FLUPS and time-correlated single photon counting (TCSPC). In this way, the complete solvation response function of coumarin153(C153) is determined in21imidazolium, pyrollidinium, and assorted other ILs. The fast component is found to account for10%-40%of the response. The time constant associated with this component is correlated to ion reduced mass, indicating that it is caused by the ions’inertial motions. A much slower component, which relaxes over a broad time range, completes the solvent relaxation. Its origins are connected to diffusive, structural reorganization, based on the fact that its time is well correlated to the IL viscosity. A simple dielectric continuum model is introduced to investigate the relationship between dielectric relaxation and solvation dynamics. The dielectric continuum model is found to over-estimate the speed of solvation by factors of2-4.To obtain additional perspective on the connection between solvation and dielectric relaxation, mixtures of an ionic liquid,1-butyl-3-methyl-imidazolium tetrafluoroborate ([Im41][BF4]), and two polar solvents, acetonitrile and water, are also studied. The physical properties of both mixtures vary systemically with the volume fraction of [Im41][BF4]. By using C153as a "standard" probe, the solvation response function is examined and no clear evidence is found to confirm the suspicion of preferential solvation. Both solvation and rotational times are nicely correlated to solution viscosity. Experimental dielectric data over the frequency range200MHz-89GHz are used to predict the solvation response and further test the dielectric continuum model. In the case of acetonitrile+IL mixtures, the accuracy of continuum model predictions was comparable to that in neat ILs:the fast component is well predicted while the speed of the slow part is overestimated. The quality of these predictions was equally good at high and low acetonitrile content. In contrast, the continuum model totally failed in the IL+water mixtures at high water content.The dielectric continuum model as applied above predicts the same dynamics for all dipolar solutes. To further test this prediction the solvation response of another solvatochromic probe,4-aminophthalamide (4AP), is measured in four ionic liquids. The4-AP response functions are systematically slower than those of C153in the same ionic liquids. The origin of this difference was thought to arise from the effect of solute motion on solvation. A correction for solute motion using measured rotational correlation functions significantly reduces the differences observed between C153and4AP. Comparisons between literature data on4-dimethylamino4’-cyanostilbene (DCS) and C153support the use of this rotational correction. The rotationally corrected solvation response functions of both C153and4AP can be reproduced using dielectric continuum predictions by allowing the conductivity used in the dielectric modeling to differ from experimental values.In addition to experimental studies the dielectric continuum model is investigated both numerically and analytically. An analytical method for inverting a multi-exponential representation of the solvation response to obtain a description of the permittivity expressed as a sum of a conductivity term+multiple Debye terms. The computed conductivity of C153in neat ILs is found to be systematically smaller than the bulk value, while the predicted permittivity agrees well with the data from bulk dielectric measurements. This analytical approach also reveals a simple relationship between the integral solvation time <τsolv> predicted by the continuum model and the static conductivity σ0. Furthermore, a more general derivation of the same correlation is provided and data on C153solvation in34neat ILs are presented to support this prediction and provide the empirical counterpart:ln(<τsolv>/ps)=4.37-0.92ln(σ0/Sm-1).