Solvation dynamics and structure in room-temperature ionic liquids investigated with ultrafast infrared spectroscopies [electronic resource]
Keywords: Chemistry.

Room-temperature ionic liquids (RTILs), which refers to salts that are liquid at room temperature, form a class of materials that have been intensively studied in recent years. A major motivator for this explosion in research is their many special properties and proposed applications. Many of these applications rely on their properties as solvents, particularly their ability to simultaneously solvate solutes of disparate natures. These properties have been linked to the existence in certain classes of RTILs of local domains with differing properties, such as polar and apolar domains. This ordering, which occurs when one ion, usually the cation, has a large hydrophobic moiety, such as an alkyl chain, is an additional constraint imposed on both liquid ordering and the charge alternation which is observed due to the ionic nature of the material. Varying the chain length of such cations not only changes the degree of polar-apolar ordering, but also bulk properties such as viscosity. My work seeks to understand and characterize ion solvation in such ionic liquids, and especially how and to what extent the details of solvation structure and dynamics are affected by the change in bulk properties or growing influence of additional ordering. To this end, the dynamics of four 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide RTILs with carbon chain lengths 2, 4, 6, and 10 were studied by measuring the orientational and spectral diffusion dynamics of the vibrational probe SeCN‒. Vibrational absorption measurements, two-dimensional infrared (2D IR) spectroscopy, and polarization-selective pump-probe (PSPP) experiments were performed on the CN stretch of the ion. The PSPP experiments yielded triexponential anisotropy decays, which were analyzed with the wobbling-in-a-cone model. The slowest decay, the complete orientational randomization, slows with increasing chain length in a hydrodynamic trend consistent with the increasing viscosity. The shortest time scale wobbling motions are insensitive to chain length, while the intermediate time scale wobbling slows mildly as the chain length increases. Spectral diffusion from the RTIL structural fluctuations was characterized through 2D IR. The faster structural fluctuations are relatively insensitive to chain length. The slowest structural fluctuations slow substantially when going from a 2 carbon chain a 4 carbon chain and slow further, but more gradually, as the chain length is increased. The main conclusion is that there exists a complex hierarchy of motions in terms of their spatial extent and their timescale. The fastest motions, which tend to be the most local, are also the ones most insensitive to bulk properties. The largest scale motions tend to be the ones most consistent with hydrodynamics. Some intermediate scale motions can be surprisingly sensitive to details of the structure which can be related to the growth of polar-apolar ordering, where other intermediate scale motions exhibit very weak trends. A complete understanding of the structure and dynamics of these ionic liquids, especially when considering the effect of various structural modifications, must then consider the richness of motions and timescales and how they are influenced by structural differences, as well as how they relate to the specific process that is being optimized through this structural modification. In order to obtain data of sufficient quality and detail to allow such modeling to be done successfully, the way these ultrafast experiments are performed, particularly the 2D IR experiments, had to be improved in both speed and freedom from distortion. To this end, I constructed a 2D IR interferometer based on Fourier-domain pulse-shaping. The resulting phase control and stability allows the experiments to be performed free of distortions. It also allows repeating the experiment with various configurations of imparted phase, known as "phase-cycling". Phase-cycling has four immediate benefits: it permits the experiment to be performed in a semi-collinear geometry, further eliminating possible distortions; it allows many unwanted terms such as scatter to be very effectively suppressed; it allows the spectrum of the excitation pulses to be advantageously controlled; and it eliminates the need for chopping beams which alone doubles the speed of acquisition. In addition, the programmatic generation of the first two pulses via pulse-shaping eliminates moving parts from that portion of the experiment, greatly speeding acquisition. This has allowed many previously inaccessible systems to be studied using this technique and was a tremendous aid in performing the experiments described here.