Solvent Effects and Solvation Dynamic

Most chemical reactions and biological functions take place in solution. This simple fact has spurred a tremendous research effort aimed at elucidating the effects of the solvent on the chemical or biochemical reactivity. A complete understanding of liquid chemistry requires knowledge of the detailed molecular pathways that are involved in the reaction dynamics. The elucidation of a set of unifying principles describing chemical dynamics in liquids represents a challenge to both experimentalists and theoreticians. The amorphous and dynamic nature of the liquid state leads to reaction systems of exceptional complexity.
Any chemical or photochemical reaction in the condensed phase starts off by a redistribution of charge in the chromophore. In particular, light offers a clean way to selectively trigger reactions, both in energy and in time. The first step of any photo induced process is a redistribution of charge in the absorbing atom or molecule. While this modifies the field of forces within the molecule and leads to its transformation (e.g. via bond breaking, isomerisation, etc.), it also changes the field of forces between the molecule and the solvent species, forcing the latter to rearrange on a time scale that may be shorter or concurrent with that of intra molecular reorganisation. It is this rearrangement of the solvent species that has been termed “solvation dynamics” and is illustrated in Fig. 1. As the solute is excited instantaneously, the solvent species find themselves at time t=0 in a relatively high energy configuration due to the change of field of forces. Subsequently, they begin to move and rearrange to reach their new equilibrium configuration (fig. 1). The nuclear motion can be broadly classified into rotational and translational motions. With water as solvent, rotational motion would also include libration, while translations would include the intermolecular vibrations due to extensive hydrogen bonding. The two specific motions, libration and intermolecular vibration are of relatively high frequency and are expected to play a dominant role in the initial part of the solvation.
The role of solvation dynamics in Chemistry and Biology is obvious. Since the solvent species around a solute form a structure (disordered, or coordinated to solvent species) that determines the outcome of chemical and/or biological events, the solvent species are not just “spectator” species, but are part of the chemical or biochemical process. This manifests itself by both steric and electrostatic effects. For disordered media, the solvation process is usually expressed via the solvation time correlation function:

                                           

Where E(0) and E(∞) are the energy of the system at the beginning and at the end of the dynamics (i.e. corresponding to initial polarization P(0) and final polarization P(∞) in fig. 1), while E(t) is the energy at some intermediate time during the dynamics. The correlation function typically exhibits a multi exponential decay with time scales of <200 fs, some 100s of fsec and a few psec to several tens of psec, depending on the solvent, which are due to bulk polarization, damped rotations and diffusive rotation and translation, respectively. There arises some ambiguity, as nearly all solvation studies are carried out using dye molecules as solutes, which themselves contain many high frequency internal modes and the distinction with high frequency intermolecular modes is difficult to make.

 

  Furthermore, the structure of the solvent shell in the ground state depends on that of the dye molecule, and the dynamics that one observes on ultra short time scales on the excited state reflects a departure from this specific initial structure.
In our group, we use a wide range of techniques to identify solvent effects and disentangle the intra- and intermolecular relaxation processes of photoexcited species in different solvents:

1. Ultrafast fluorescence up-conversion techniques, which consist in recording the fluorescence spectrum of the dye in the course of solvent reorganisation on the femtosecond-picosecond time scale (fig. 1). This is one of the most direct and successful ways of visualising solvation dynamics in real-time, as it involves only 2 electronic states (the ground and the excited state) in the process. The above expression of the solvation response function can easily be inferred from such measurements. Because we are interested in small solutes or amino-acid residues, all of which absorb mainly below 300 nm, we have implemented a broad band fluorescence up-conversion set-up in the deep UV.
2. Photon echo techniques (in particular, the photon echo peak shift (PEPS) technique): For a solute in a solvent the homogeneous width is a consequence of the fluctuations of the phase relationship between ground and excited states, due to the coupling to the solvent. Even without population relaxation, the transition can be perturbed by solvent fluctuations. Measuring this loss of phase coherence (i.e. the decay due to pure dephasing) is a probe of the solute-solvent coupling and a measure of solvent reorganisation times. Inhomogeneous contributions come from the fact that the different solutes “see” a somewhat different environment. Photon echo techniques are powerful tools to measure, both the pure dephasing times and the inhomogeneous width and are, for that reason, much used in studies of solvation dynamics. Here also an extension into the UV below 300 nm has been implemented.
3. Pump-probe transient absorption spectroscopy consists in probing the ongoing dynamics by projecting it to higher excited states of the solute (by excited state absorption) or dumping it back to its ground state (by stimulated emission). The main limitation to this technique is the superposition of bleach, induced absorption and stimulated emission signals, and the fact that one probes intramolecular dynamics to retrieve the intermolecular ones, making the analysis of the dynamics rather complicated. While these optical techniques are unique in delivering time-domain information, and information about the energetics of the system, none of them actually permits a visualization of the molecular structure of the solvation shell around the solute. Therefore, an experimental method is desired that provides direct structural information on the dynamics of the solvation shell around a solute and the formation of the new local structure around it.
4. Ultrafast X-ray absorption spectroscopy is ideal in this respect and we have implemented it recently to look at the solvation shell changes of atomic iodide in water, after removal of the electron from the anion. We could visualise for the first time the changes from a hydrophilic (iodide) to a hydrophobic (iodine) interaction.