Band-Selective Ballistic Energy Transport in Alkane Oligomers: Toward Controlling the Transport Speed

Abstract

Intramolecular transport of vibrational energy in two series of oligomers featuring alkane chains of various length was studied by relaxation-assisted two-dimensional infrared spectroscopy. The transport was initiated by exciting various end-group modes (tags) such as different modes of the azido (ν(N≡N) and ν(N═N)), carboxylic acid (ν(C═O)), and succinimide ester (νas(C═O)) with short mid-IR laser pulses. It is shown that the transport via alkane chains is ballistic and the transport speed is dependent on the type of the tag mode that initiates the transport. The transport speed of 8.0 Å/ps was observed when initiated by either ν(C═O) or νas(C═O). When initiated by ν(N≡N) and ν(N═N), the transport speed of 14.4 ± 2 and 11 ± 4 Å/ps was observed. Analysis of the vibrational relaxation channels of different tags, combined with the results for the group velocity evaluation, permits identification of the chain bands predominantly contributing to the transport for different cases of the transport initiation. For the transport initiated by ν(N≡N) the CH2 twisting and wagging chain bands were identified as the major energy transport channels. For the transport initiated by ν(C═O), the C-C stretching and CH2 rocking chain bands served as major energy transporters. The transport initiated by ν(N═N) results in direct formation of the wave packet within the CH2 twisting and wagging chain bands. These developments can aid in designing molecular systems featuring faster and more controllable energy transport in molecules.

Surface-enhanced, multi-dimensional attenuated total reflectance spectroscopy

“…Many useful information can be extracted from such diagrams which are not straightforward to be obtained by methods with only a single spectra axis. Examples of such information are for instance spectral diffusion 25 , chemical interaction and exchange 26 , energytransfer 27 or vibrational coupling 28 . Figure 2 shows a schematic depiction of 2D IR spectroscopy in ATR geometry and in pump-probe configuration.…”

Ballistic Energy Transport in Oligomers

“…The nearest and nextnearest cell interactions were included in the Hamiltonian ( Figure 8A). 42 Ten lowest-energy optical bands for alkane chains are shown in Figure 8B. Bands of the same motion type are plotted with the same color.…”

“…For chains with a finite length, the number of states in each band pair equals the number of CH 2 groups in the chain; the chain states are evenly spaced along the abscissa in Figure 8B. 42 If a superposition state involving a specific chain band is formed in the chain as a spatially bell-shaped wavepacket, it will propagate with time along the chain with the group velocity dictated by the involved states. The group velocity for a narrow range of frequencies centered at ω 0 (q 0 ) is determined as V(q 0 ) = (∂ω/∂q)| q=q 0 .…”

Electron transfer across a thermal gradient

“…An important driving factor in this growing interest is the development of experimental capabilities that greatly improve on the ability to gauge temperatures (and "effective" temperatures in nonequilibrium systems) with high spatial and thermal resolutions (29-43) and to infer from such measurement the underlying heat transport processes. In particular, vibrational energy transport/ heat conduction in molecular layers and junctions has recently been characterized using different probes (6,19,(44)(45)(46)(47)(48)(49)(50)(51)(52).The interplay between charge and energy (electronic and nuclear) transport (53-60) is of particular interest as it pertains to the performance of energy-conversion devices, such as thermoelectric, photovoltaic, and electromechanical devices. In particular, the thermoelectric response of molecular junctions, mostly focusing on the junction linear response as reflected by its Seebeck coefficient, has been recently observed (61-65) and theoretically analyzed (2,20,64,(66)(67)(68)(69)(70)(71)(72)(73)(74)(75)(76)(77).…”