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, N-14(3-), and isotopically substituted, N-15(3-), 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 (14)N(3)Cn-a and (15)N(3)Cn-a. The transport was initiated by exciting the azido moiety stretching mode, the nu(N=N) tag, at 2100 cm(-1) ((14)N(3)Cn-a) or 2031 cm(-1) ((15)N(3)Cn-a). Opposite to the expectation, the ballistic transport speed was found to decrease upon N-14(3) -> N-15(3) 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 nu(N=N) + nu(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 nu(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 nu(N=N) initiation: The nu(N=N) mode relaxes efficiently into the twisting band states and low-frequency acoustic modes, and the nu(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 nu(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 nu(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 (14)N(3)Cn-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.