Nawagamuwage Sithara U, Qasim Layla N, Zhou Xiao, Leong Tammy X, Parshin Igor V, Jayawickramarajah Janarthanan, Burin Alexander L, Rubtsov Igor V
Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States.
J Phys Chem B. 2021 Jul 15;125(27):7546-7555. doi: 10.1021/acs.jpcb.1c03986. Epub 2021 Jun 29.
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-, and isotopically substituted, N-, nitrogen atoms, were tested for wavepacket initiation in compounds with alkyl chains of = 5, 10, and 15 methylene units terminated with a carboxylic acid (-a) group, denoted as NC-a and NC-a. The transport was initiated by exciting the azido moiety stretching mode, the ν tag, at 2100 cm (NC-a) or 2031 cm (NC-a). Opposite to the expectation, the ballistic transport speed was found to decrease upon N → N 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 + ν 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 of the azido group with CH 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 initiation: The ν mode relaxes efficiently into the twisting band states and low-frequency acoustic modes, and the ν 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 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 tag and the wavepacket generated predominantly at the CH twisting band. The isotope effect of the transport speed is attributed to a larger contribution of the faster wavepackets for NC-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.
寡聚分子链中振动能量传输的弹道 regime 以恒定的、通常较高的传输速度和高效率发生。这种传输 regime 可以通过用中红外（IR）光子激发链端基来启动。为了更好地理解波包形成过程，测试了两个化学性质相同的端基，即具有正常 N - 氮原子和同位素取代 N - 氮原子的叠氮基，用于在具有由羧酸（-a）基团终止的 = 5、10 和 15 个亚甲基单元的烷基链的化合物中启动波包，分别表示为 NC - a 和 NC - a。通过在 2100 cm（NC - a）或 2031 cm（NC - a）处激发叠氮基部分的拉伸模式 ν tag 来启动传输。与预期相反，发现弹道传输速度在 N → N 同位素编辑后降低。描述并分析了振动波包传输启动的三种机制。第一种机制涉及通过用 IR 光子激发 tag 模式与 ν + ν 组合态的几个强费米共振直接形成波包，而组合态的每个组分都与离域链态混合。第二种机制依赖于端基局部化 tag 到主要局部化的端基态的振动弛豫，该端基态与链带的多个离域态强烈耦合。叠氮基的 ν 与链的 CH 摇摆态的谐波混合允许在摇摆带的一部分内形成波包，表明传输速度很快。第三种机制涉及端基局部化模式到链态的振动弛豫。发现了 ν 启动的两条这样的途径：ν 模式有效地弛豫到扭曲带态和低频声学模式，而 ν 模式弛豫到摇摆带态和低频声学模式。定量评估了由 ν tag 启动的弹道能量传输中三种启动机制的贡献，并将其与实验相关联。我们得出结论，第三种机制在由 ν tag 启动的 5 - 15 个亚甲基单元的烷烃链中的传输中占主导地位，并且波包主要在 CH 扭曲带产生。传输速度的同位素效应归因于 NC - a 中较快波包的更大贡献或扭曲带内波包的不同宽度。该研究提供了对不同传输启动机制的系统描述，并讨论了每种机制的要求和特征。这种分析将有助于设计用于能量管理的新型材料。
