Ballistic transport of high-frequency vibrational quanta via oligo(p-phenylene) chains is examined experimentally and computationally. The transport was initiated by exciting an end group with a mid-IR photon and observing energy arrival to the opposite chain end using dual-frequency two-dimensional infrared spectroscopy. Multiple wavepackets were detected propagating in the chain with different speeds. A portion of energy was delivered with an extremely high speed reaching 8.6 km/s, which exceeds the speed of sound in common metals (~5 km/s) and polymers (~2 km/s). Moreover, the transports initiated by different end groups were found to feature different speeds. The efficiencies of energy injection into the chain and transport along the chain are found to be very high and dependent on the extent of conjugation across the structure, leading to an opportunity of their controlling by tuning the degree of conjugation in the chain. Different mechanisms of wavepacket formation in the chain are described and characterized. The first mechanism involves direct formation of a wavepacket via excitation with IR photons of several strong Fermi resonances of the initially excited mode, tag, with the combination state involving chain states. The second mechanism relies on vibrational relaxation of an end-group-localized tag into a mostly end-group state that is coupled strongly to multiple states of a chain band. The third mechanism involves vibrational relaxation of an end-group-localized mode into chain states. The three mechanisms were analyzed for the oligo(p-phenylene) chains as well as for the transport via alkyl chains initiated with an azido end group with normal 14N3 and isotopically substituted 15N3 nitrogen atoms. The study opens avenues for developing materials with controllable energy transport properties for heat management, schemes with efficient energy delivery to hard-to-reach regions, including transport against thermal gradients.

Unidirectional coherent energy transport via oligomeric chains

ACS Fall 2021

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