Band-selective ballistic energy transport in alkane oligomers: toward controlling the transport speed

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.