Sergei Arzhantsev

Solvation dynamics in room temperature ionic liquids

Room temperature ionic liquids (ILs) are receiving considerable attention as potential ”green” substitutes for conventional solvents in organic synthesis, separation processes, and electrochemical applications. The basic physical and chemical properties of this new type of solvent are only beginning to be characterized. As part of this effort we have recently used time-resolved fluorescence spectroscopy to measure the solvation times of several ionic liquids. When measurements are performed using a time-correlated single photon counting instrument having time resolution of 25 ps before deconvolution only about half of expected Stokes shift is observed. The observed portion of the dynamics is found to take place in the nanosecond time range and to be highly non-exponential. The slow solvation component displays kinetics typical of glass-forming liquids and tracks solution viscosity in a similar manner in all of the ILs studied.

We have also combined data from a Kerr-gated emission experiment with data from time-correlated single photon counting experiments. The combined data reveals a remarkable dispersion in the temporal response of ionic liquids, which involve dynamics stretching all the way from 100 fs to 10 ns. The amplitude of the fastest portion of dynamics, previously missed, is correlated to the relative size and/or mass of anion.

We also measured rotational dynamics of the solvation probes as well as a few other solutes. Apart from the fact that rotational correlation functions are generally found to be non-exponential, no indication of distinctive frictional effects in ILs was observed, even for ionic solutes. Rotational correlation times appear to approximately follow hydrodynamic predictions in the same manner as in conventional solvents.

Time-resolved fluorescence spectroscopy study of solvation dynamics and charge transfer systems

The connection between polar solvation dynamics as measured by dynamic Stokes shift experiments and the friction on charge transfer reactions has been appreciated since the early 1980. Yet, while many experimental results related to solvent frictional control of electron transfer have accumulated over the years, definitive examples are still relatively rare. The main reason is the difficulty of untangling frictional effects from equilibrium energetic effects. The latter are always large in the case of charge transfer reactions and tend to obscure more subtle frictional effects. To characterize dynamics of charge transfer systems time-resolved emission spectra is collected.

Measurements of the dynamic Stokes shift of DCS (trans-4-dimethlamino-4’-cyanostilbene) in polar solvents show a good correlation with the corresponding dynamics previously reported using the solvatochromic probe C153 (coumarin 153). The time-resolved spectra of DCS are interpreted as simple solvent relaxation rather than any complicated electronic state kinetics.

Another application is time-resolved fluorescence spectroscopy of DTN (4-dimethylamino-4’-cyano-diphenylacetylene) in polar solvents. The DTN molecule undergoes a barrier-less transition between a strongly emissive locally excited (LE) state and a weakly emissive charge transfer (CT) state. This transition causes a large decrease of fluorescence intensity and a simultaneous solvation induced red shift of the emission spectrum. The more complex spectral dynamics observed with this solute provide an interesting contrast to the dynamics observed with the structurally similar solute DCS.