archives@tulane.edu / The development of nanocomposite materials with desired heat management properties, including nanowires, layered semiconductor structures, and self-assembled monolayer (SAM) junctions, attracts broad interest. Such materials often involve polymeric/oligomeric components and can feature high or low thermal conductivity, depending on their design. For example, in SAM junctions made of alkane chains sandwiched between metal layers, the thermal conductivity can be very low, whereas the fibers of ordered polyethylene chains feature high thermal conductivity, exceeding that of many pure metals. Generally, there are two classes of vibrational energy transport regimes which have been identified in molecules. There is diffusive energy transport, which involves hopping of vibrational energy between localized states in the Brownian-like style or motion, and ballistic transport, which involves free propagation of a vibrational wave packet through delocalized vibrational modes. Previous studies demonstrated fast vibrational energy transport in a variety of oligomer chains, including 3.9 Å/ps speed in perfluoroalkanes, 5.5Å/ps in PEGs, and 14.4 Å/ps in polyethylene oligomers. The fast speeds suggest that the observed transport regime was ballistic to significant distances. Although the speeds of transport observed were impressive, an in-depth explanation for the different speeds wa The work in the present dissertation is aimed at understanding the fundamental reasons as to why different speeds of vibrational energy transport can be achieved through structurally complex chains. Relaxation assisted two-dimensional infrared spectroscopy is used to investigate experimentally the energy transport in a variety of molecular chains. In these experiments, the transport in a molecule is initiated by exciting an IR-active group (a tag) and recorded by another mode in the molecule (a reporter) via the influence of the excess energy on its frequency. The energy transport time can be measured from the tag to the reporter, and the transport speed through the molecule is evaluated. Experiments on a series of alkane chains with different tag modes showed that different speeds of ballistic transport could be achieved using different methods of initiation. To understand why different speeds of transport occur, a detailed analysis of the vibrational chain states and the intramolecular vibrational relaxation pathways of the tag modes were performed. It was concluded that different tags populate different chain bands, which support different speeds of energy transport. Similar experiments were performed on PEG oligomers and, interestingly, the speed of transport was the same in several case of initiation. The detailed analysis of the transport showed that the presence of oxygen heteroatoms in the backbone weakens the site coupling of the PEG chain states, leading to localization of the vibrational modes. Detailed analysis of the energy transport between the PEG chain states along with modeling based on solving the quantum Liouville equation for a system of coupled states demonstrated that a switch from ballistic to directed diffusive transport occurs in PEG chains for longer chains. To better understand the role of mode localization on the transport mechanism and to see if ballistic transport across an alien molecular group in otherwise uniform chain is possible, vibrational energy transport was studied in a series of alkane chains infused with different small functional groups. The functional group which disrupts the chain state delocalization was shown to disrupt the ballistic transport, and the one which preserves the delocalization appears to not disrupt the transport. The studies presented in this dissertation provides an in-depth description of the vibrational energy transport in a variety of different molecular chains. Such information can be useful in the development of materials with customized energy transport properties. / 1 / Layla Qasim

1 / Jianan Tang

acase@tulane.edu / 1 / Joel Leger

Studying the vibrational energy transfer pathways and dynamics in molecules is important for different areas of chemical research, with the range of potential applications in nanotechnology, organic chemistry and biochemistry. Two transport regimes, ballistic and diffusive, are recognized for the transport in molecules; while the latter is typically slow the former can be fast and efficient. The diffusive transport regime was observed in numerous compounds, whereas there are only a few cases where the ballistic transport has been suggested in molecules. The subject of this dissertation is to identify the factors influencing the ballistic transport, including the molecular chain organization, thermodynamic conditions and end-group functionalities. The research involves several types of oligomeric chains, such as polyethyleneglycol, perfluoroalkane, and alkane chains. The experiments were performed using the relaxation-assisted two-dimensional infrared spectroscopy method, which permits measuring the energy transport time between two vibrating groups. The transport via all three chain types was found to occur with a constant speed, although different speeds were found in different chain types. The fastest speed of 14.4 Å/ps was found in linear alkanes, the slowest, 3.9 Å/ps, in perfluoroalkanes. The difference in the transport speed was attributed to involvement of different chain optical bands. The temperature dependence of the transport speed and efficiency in perfluoroalkanes demonstrated that the ballistic transport, dominating at low temperatures, is switched to the diffusive transport at elevated temperatures; the observation was supported by the theoretical modeling. The energy transport in several compounds lacking periodic structure was found to occur with an effective speed of 1.2-1.4 Å/ps, which approximately matches the speed of passing one bond-length per mean lifetime of the excited vibrational mode (1 ps). This speed was found to be 3-10 fold smaller than the transport speed via oligomeric chains. Moreover both regimes, diffusive and ballistic, were distinguished within the same compound: the transport was ballistic via the chain and diffusive within the bulky end group. The two transport times were found additive, confirming the ballistic nature of the through-chain transport. This study develops a detailed picture of energy transport in molecules and provides new opportunities for designing molecular and nanoscale materials with tailored energy transport properties, potentially useful for making novel elements for molecular electronics. / 1 / Natalia I. Rubtcova

Three types of nonlinear optical spectroscopies, ultrafast two-dimensional infrared (2DIR) spectroscopy, transient infrared (IR) absorption/pump-probe spectroscopy, and sum-frequency generation (SFG) vibrational spectroscopy, are used to investigate molecular structure and dynamics in two distinct classes of materials. First, 2DIR and pump-probe spectroscopies are used to study ultrafast rotational and vibrational energy relaxation in dense gaseous and supercritical fluid solutions, special solvation effects near the critical point, and the evolution of cooperative, liquid-phase dynamics as a function of density for two different solvent systems. 2DIR’s demonstrated capabilities offer a unique tool for identifying co-existing free rotor and liquid-like populations within the same fluid sample, evaluating the adequacy of isolated binary collision (IBC) relaxation descriptions in dense gas and near-liquid density fluids, and learning about how solute-solvent intermolecular properties separately influence rotational and vibrational relaxation in these dynamic and heterogeneous environments. Analysis of the density-dependent 2DIR and pump-probe spectra of a quasi-free rotor (asymmetric stretch rovibrational band of N2O) in SF6 and Xe provides timescales for rotational energy relaxation rates (1 – 3 collisions), but much slower vibrational energy relaxation rates. A critical slowing effect on the rate of rotational relaxation is identified, and liquid-like solvation is observed in dense gaseous solutions at state points lower than the critical density. Solvent-dependent differences in energy relaxation and IBC model breakdown, as well as applications of this 2DIR methodology to other high density and supercritical solution dynamics and descriptions are discussed. In a second nonlinear spectroscopy project, SFG is used to study the role of substrate type, gold or silver, and surface roughness on the parity odd-even effect in n-alkanethiolate (n = 10 – 16) self-assembled monolayers (SAMs), materials of potential importance to molecular scale electronics. SFG methyl vibrational transition intensities, frequencies, and linewidths display parity and metal dependence attributable to the orientational differences of the interfacial ethyl group, which inverts for SAMs on Ag substrates relative to SAMs on Au. Substrate roughness, an often-underreported experimental parameter, is shown here to affect the extent of odd-even methyl orientation anisotropy, and this SFG analysis establishes a new roughness limit for the appearance of odd-even effects on Ag substrates.

The research presented in this thesis makes use of theoretical/computational techniques to calculate nonlinear spectroscopic signals and molecular volumes. These techniques have become more practical with advances in computational resources and now are an integral part of research in these areas. Preliminary results allude to the power of these techniques when applied to relevant problems and suggest that much progress can be made in understanding the complex nature of nonlinear spectroscopic signals and molecular volume contributions. The nonlinear spectroscopy work involves writing the quantum mechanical response functions in terms of classical time correlation functions which are amenable to calculation using classical molecular dynamics. The response functions reported in this thesis include the fifth order response function, probed in the fifth order Raman experiment, and the third order response function probed in the two dimensional infrared experiment. The molecular volume calculations make use of modern algorithms used in molecular dynamics simulations to calculate the full thermodynamic volumes of molecules.

The original research outlined in this dissertation involves the use of novel theoretical and computational methods in the calculation of molecular volume changes and non-linear spectroscopic signals, specifically two-dimensional infrared (2D-IR) spectroscopy. These techniques were designed and implemented to be computationally affordable, while still providing a reliable picture of the phenomena of interest. The computational results presented demonstrate the potential of these methods to accurately describe chemically interesting systems on a molecular level. Extended system isobaric-isothermal (NPT) molecular dynamics techniques were employed to calculate the thermodynamic volumes of several simple model systems, as well as the volume change associated with the trans-cis isomerization of azobenzene, an event that has been explored experimentally using photoacoustic calorimetry (PAC). The calculated volume change was found to be in excellent agreement with the experimental result. In developing a tractable theory of two-dimensional infrared spectroscopy, the third-order response function contributing to the 2D-IR signal was derived in terms of classical time correlation functions (TCFs), entities amenable to calculation via classical molecular dynamics techniques. The application of frequency-domain detailed balance relationships, as well as harmonic and anharmonic oscillator approximations, to the third-order response function made it possible to calculate it from classical molecular dynamics trajectories. The finished theory of two-dimensional infrared spectroscopy was applied to two simple model systems, neat water and 1,3-cyclohexanedione solvated in deuterated chloroform, with encouraging preliminary results.