Architecture-Dependent Vibrational Energy Transport in Molecular Chains
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.