Ultrafast time-resolved infrared spectroscopy of molecular monolayers and solute-solvent complexes [electronic resource]
Keywords: Chemistry

Ultrafast time-resolved infrared spectroscopy has been a powerful tool in resolving and studying ultrafast motions in bulk chemical and biological systems. The utility of ultrafast time-resolved infrared spectroscopy is illustrated through two studies of solute-solvent complexes. The same experimental methods used to study bulk systems are then extended to study surface systems through the development of both surface molecular probes and new spectroscopic techniques. Ultrafast polarization and wavelength selective IR pump-probe spectroscopy is used to measure the inertial and long time orientational dynamics of pi-hydrogen bonding complexes. The complexes studied are composed of phen-d-ol (phenol-OD) and various pi-base solvents with different electron donating or withdrawing substituents. The inertial motion is found to be insensitive to the strength of the hydrogen bond, but highly sensitive to the local solvent structure as reported on by inhomogeneous line broadening. The local solvent structure therefore acts as the controlling influence in determining the extent of inertial orientational relaxation, and thus the angular potential. Variation in the pi-hydrogen bond strength is of secondary importance. Hydrogen bonded complexes between phenol and phenylacetylene are studied using ultrafast two-dimensional infrared (2D IR) chemical exchange spectroscopy. Phenylacetylene has two possible pi-hydrogen bonding acceptor sites (phenyl or acetylene) that compete for hydrogen bond donors in solution at room temperature. The chemical exchange process occurs in ~5 ps, and is assigned to direct hydrogen bond migration along the phenylacetylene molecule. The observation of direct hydrogen bond migration can have implications for macromolecular systems. 2D IR vibrational echo spectroscopy and heterodyne detected transient grating (HDTG) spectroscopy (an ultra-sensitive analog of pump-probe spectroscopy) are developed as means of study of the structural and vibrational dynamics of surfaces. The surfaces studied are silica surfaces functionalized with a transition metal carbonyl complex, tricarbonyl (1,10)-phenanthroline rhenium chloride. The functionalization process produces chromophore surface density of 1-2 × 10^14 per cm squared. The high surface density achieved indicates that energy transfer between molecules on the surface could impact the experimental observables probed in 2D IR and HDTG spectroscopy. The theory of excitation transfer induced spectral diffusion has been developed and is capable of calculating the effect of the energy transfer on any spectroscopic observable through a master equation approach. Initial estimates of surface structural dynamics, based on both experimental 2D IR data and theoretical calculations, showed sub-100ps structural dynamics in the molecular monolayers even without the presence of solvent. Furthermore, solvent is shown to accelerate the structural dynamics in a manner that is different from that of bulk solution. Additional surface density dependent experiments indicate the negligible nature of excitation transfer even in these dense systems. The functionalized molecular monolayers are found to have a ~40 ps structural dynamics relaxation time in the absence of solvent. Further investigation of the effects of solvents on the RePhen(CO)3Cl monolayers has been carried out. Immersion in solvent is found to change the infrared spectrum, structural dynamics and vibrational dynamics in ways that differ from the changes evidenced in the bulk. The monolayers were immersed in both solvents that can dissolve RePhen(CO)3Cl and those that cannot. For both hexadecane and D2O, which cannot dissolve the headgroup, the structural dynamics of the monolayer are slowed by the presence of solvent while the vibrational dynamics are not impacted. Polar organic solvents, which can dissolve the headgroup, accelerate the dynamics. Dimethylformamide (DMF) is found to have a particularly strong effect on the structural dynamics of the monolayers, accelerating the timescale from 40 ps to 15 ps, yet DMF has little impact on the vibrational dynamics. Chloroform is found to enhance the vibrational lifetime of the CO symmetric stretch of the RePhen(CO)3Cl headgroups in the monolayer by 50%. These results indicate that the properties of thin films can be modified by the presence of solvent, even in the case when the solvent is repelled by the monolayer.