Ultrafast dynamics of functionalized surfaces, air/water interfaces, and water in minerals measured with two-dimensional infrared vibrational echo spectroscopy [electronic resource]
Keywords: Chemistry.

In the past decade, ultrafast two-dimensional infrared vibrational echo spectroscopy (2D IR) has matured as a powerful technique for directly observing molecular dynamics in liquids, organic and aqueous solutions with femtoseconds (fs) to picoseconds (ps) resolution. In contrast, the application of 2D IR methods on investigating surficial and interfacial molecular dynamics remains a challenging topic. Understanding the non-bulk dynamics on material surfaces or at interfaces are fundamental to fields from heterogeneous catalysis to tribology. However, the very small amount of surficial and interfacial chromophores, usually less than ~10-13 mol of molecules within the laser spot, requires us to design highly sensitive 2D IR experiments for capturing the extremely weak vibrational echo signals. Alkyl chain monolayers immobilized on oxide surfaces are among the most popular strategies for surface functionalization. In Chapter 3, I started by examining the conformational dynamics of self-assembled monolayers (SAM) immobilized on silica substrates using transmission mode 2D IR spectroscopy. The SAMs comprised of alkyl chains of which the terminal sites away from the substrate are chemically labelled with rhenium tricarbonyl headgroups. The rhenium carbonyl complex's CO symmetric stretch acts as the strong vibrational mode to generate echo signals with a long vibrational lifetime of ~20 ps. These features allow us to measure spectral diffusion dynamics in sufficiently long time ranges up to tens of picoseconds with good signal-to-noise ratios. The headgroup spectral diffusion dynamics report on the fast gauche-trans conformational dynamics of the underlying alkyl chains. The same type of headgroup was functionalized on a series of monolayers with different alkyl chain structures to determine how major structural variables, including the chain length, the chain density and the presence of interlinking siloxane network among chains (-Si-O-Si-O-) impact on the chain dynamics. To expand the scope of 2D IR measurements to SAMs organized on metallic substrates, 2D IR methods operating in external reflection mode were developed. Comparing the monolayers with the same rhenium complex headgroup and the same alkyl chain length (11 carbons), the spectral diffusion dynamics of SAMs on SiO2 surface are faster (~40 ps) than the dynamics of thiol SAMs on gold surface (~90 ps). We suggest that the slower dynamics are due to the chains in thiol SAMs packing in a more ordered nearly-all-trans conformation, while the faster dynamics of SAMs on silica originate from the less ordered chain packing that contains more gauche defects and free volumes. Collaborated molecular dynamics simulations were applied to reproduce the spectral diffusion dynamics using time-dependent Stark effect models. Simulations confirm that the spectral diffusion dynamics are induced via Stark effect mechanisms where the alkyl chain motions cause intermolecular electric fields projected by the polar headgroups to fluctuate. SAMs with higher alkyl chain gauche defect content among the C-C bonds have more frequent chain conformational fluctuations and faster dynamics. The reflection mode method was advanced to acquire direct time-dependent 2D IR spectra of Langmuir monolayers assembled at the air/water interface for the first time, without the aid of sum-frequency generation process. On the surface of a dielectric such as water, the low reflectivity leads to reducing the local oscillator and therefore enhancing the heterodyne detected vibrational echo signal. An organometallic surfactant, TReF18, with the rhenium tricarbonyl vibrational probe being the hydrophilic part and the octadecyl hydrocarbon chain monolayers acting as the hydrophobic part was spread on water surface at two surface densities, 60 Å2 and 90 Å2 per surfactant, in the liquid expanded phase. The carbonyl spectral diffusion report that water hydrogen bond rearrangement dynamics slow from 1.5 ps in bulk water to 3.1 ps for interfacial water. Another spectral diffusion term at a rate of 42.2 ps was also observed and attributed to slower fluctuations of the number of hydrogen bonds formed between water and the three carbonyls of TReF18. At the higher surface density, two types of TReF18 minor structures are observed in addition to the main structure observed in the low surface density. 2D IR chemical exchange spectroscopy further reveals these structures interconvert in 30 ps. Moreover, the population amplitudes of the minor structures are heterogeneously distributed across the water surface and can evolve through hours to reach macroscopic structural equilibrium. We demonstrate that the enhanced reflection method united with an advanced mid-infrared pulse-shaping laser system can take usable 2D IR spectra on the monolayer within 8 seconds. This enables the simultaneous monitoring of the ultrafast molecular dynamics and the ultraslow real-time structural fluctuation. In the last chapter, I utilized 2D IR methods and polarization-selective pump-probe (PSPP) spectroscopy to investigate molecular dynamics confined in rocks. The systems include water molecules confined in two types of calcium sulfate hydrate minerals, bassanite (CaSO4•0.5H2O) and gypsum (CaSO4•2H2O). The water molecules in gypsum are highly ordered. The 2D IR spectrum show mostly pure homogeneous broadening. PSPP measurements observe only inertial orientational relaxation. In contrast, water molecules in bassanite's tubular channels are dynamically disordered. 2D IR spectra contain a significant amount of temperature-dependent inhomogeneous broadening caused by different water hydrogen-bonding configurations. At 298 K, water dynamics cause spectral diffusion to sample a portion of the inhomogeneous line width on the time scale of ∼30 ps, while the rest of inhomogeneity appears as static on the tens of picoseconds timescale of the measurements. Water angular motions in bassanite exhibit temperature-dependent diffusive orientational relaxation in a restricted cone of angles with the cone half angle being 24°. The experiments were made possible by eliminating the vast amount of scattered light produced by the granulated powder samples using phase cycling methods.