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

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 ν_N≡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 ν_N═N + ν_N–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 ν_N═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 ν_N≡N initiation: The ν_N═N mode relaxes efficiently into the twisting band states and low-frequency acoustic modes, and the ν_N–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 ν_N≡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 ν_N≡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 of the wavepacket within the twisting band. The study offers a systematic description of different transport initiation mechanisms and discusses the requirements and features of each mechanism. Such analysis will be useful for designing novel materials for energy management.