Competition of Several Energy-Transport Initiation Mechanisms Defines the Ballistic Transport Speed (2021)

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, <sup>14</sup>N<sub>3</sub>-, and isotopically substituted, <sup>15</sup>N<sub>3</sub>-, nitrogen atoms, were tested for wavepacket initiation in compounds with alkyl chains of <i>n</i> = 5, 10, and 15 methylene units terminated with a carboxylic acid (-a) group, denoted as <sup>14</sup>N<sub>3</sub>C<i>n</i>-a and <sup>15</sup>N<sub>3</sub>C<i>n</i>-a. The transport was initiated by exciting the azido moiety stretching mode, the ν<sub>N≡N</sub> tag, at 2100 cm<sup>-1</sup> (<sup>14</sup>N<sub>3</sub>C<i>n</i>-a) or 2031 cm<sup>-1</sup> (<sup>15</sup>N<sub>3</sub>C<i>n</i>-a). Opposite to the expectation, the ballistic transport speed was found to decrease upon <sup>14</sup>N<sub>3</sub> → <sup>15</sup>N<sub>3</sub> 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<sub>N═N</sub> + ν<sub>N-C</sub> 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<sub>N═N</sub> of the azido group with CH<sub>2</sub> 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<sub>N≡N</sub> initiation: The ν<sub>N═N</sub> mode relaxes efficiently into the twisting band states and low-frequency acoustic modes, and the ν<sub>N-C</sub> 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<sub>N≡N</sub> 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<sub>N≡N</sub> tag and the wavepacket generated predominantly at the CH<sub>2</sub> twisting band. The isotope effect of the transport speed is attributed to a larger contribution of the faster wavepackets for <sup>14</sup>N<sub>3</sub>C<i>n</i>-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.