Competition of Several Energy-Transport Initiation Mechanisms Defines the Ballistic Transport Speed

• Published in: The journal of physical chemistry. B 2021-07, Vol.125 (27), p.7546-7555
• Main Authors: Nawagamuwage, Sithara U, Qasim, Layla N, Zhou, Xiao, Leong, Tammy X, Parshin, Igor V, Jayawickramarajah, Janarthanan, Burin, Alexander L, Rubtsov, Igor V
• Format: Article
• Language: English
• Subjects: B: Soft Matter, Fluid Interfaces, Colloids, Polymers, and Glassy Materials
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.1c03986

The ballistic regime of vibrational energy transport in oligomeric molecular chains occurs with a constant, often high, transport speed and high efficiency. Such a transport regime can be initiated by exciting a chain end group with a mid-infrared (IR) photon. To better understand the wavepacket formation process, two chemically identical end groups, azido groups with normal, 14N3-, and isotopically substituted, 15N3-, nitrogen atoms, were tested for wavepacket initiation in compounds with alkyl chains of n = 5, 10, and 15 methylene units terminated with a carboxylic acid (-a) group, denoted as 14N3Cn-a and 15N3Cn-a. The transport was initiated by exciting the azido moiety stretching mode, the νNN tag, at 2100 cm–1 (14N3Cn-a) or 2031 cm–1 (15N3Cn-a). Opposite to the expectation, the ballistic transport speed was found to decrease upon 14N3 → 15N3 isotope editing. Three mechanisms of the transport initiation of a vibrational wavepacket are described and analyzed. The first mechanism involves the direct formation of a wavepacket via excitation with IR photons of several strong Fermi resonances of the tag mode with the νNN + νN–C combination state while each of the combination state components is mixed with delocalized chain states. The second mechanism relies on the vibrational relaxation of an end-group-localized tag into a mostly localized end-group state that is strongly coupled to multiple delocalized states of a chain band. Harmonic mixing of νNN of the azido group with CH2 wagging states of the chain permits a wavepacket formation within a portion of the wagging band, suggesting a fast transport speed. The third mechanism involves the vibrational relaxation of an end-group-localized mode into chain states. Two such pathways were found for the νNN initiation: The νNN mode relaxes efficiently into the twisting band states and low-frequency acoustic modes, and the νN–C mode relaxes into the rocking band states and low-frequency acoustic modes. The contributions of the three initiation mechanisms in the ballistic energy transport initiated by νNN tag are quantitatively evaluated and related to the experiment. We conclude that the third mechanism dominates the transport in alkane chains of 5–15 methylene units initiated with the νNN tag and the wavepacket generated predominantly at the CH2 twisting band. The isotope effect of the transport speed is attributed to a larger contribution of the faster wavepackets for 14N3Cn-a or to the different breadth