Vibrational Cooling in Oligomeric Viologens of Different Sizes and Topologies
Vibrational Cooling in Oligomeric Viologens of Different Sizes and Topologies
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不同尺寸和拓扑结构的低聚紫罗碱的振动冷却
DOI：
10.1021/acs.jpcb.8b12165
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Piotrowiak, Piotr
中科院分区：
文献类型：
--
作者：
Liu, Mengdi;Kawauchi, Takehiro;Iyoda, Tomokazu;Piotrowiak, Piotr
在一组具有甲基紫精重复单元 (MV2+) 的同源二聚体和三聚体中研究了振动冷却。单还原紫精 (MV+•) 的 D1 激发态通过圆锥形交叉点发生快速、<500 fs 的衰减，从而可以制备具有 1.7 eV 的大量过剩能量的振动热 D0 基态，这相当于~800 K 的初始有效温度。泵浦探针光谱用于监测特征 D0→D1 热吸收带的消失，该吸收带出现在比稳态光谱更长的波长处。 “冷”MV+•与溶剂平衡。假设地面的振动激发最初局限于相同的单还原紫罗碱重复单元，该单元被光学激发到局域电子 D1 态，尽管在激发态中可能已经发生了某种程度的重新分布。观察到的冷却速率取决于低聚物的尺寸和拓扑结构，线性三聚物的热化速度明显快于支链三聚物。实验结果通过调和近似下进行的分子动力学模拟得到了证实。这些系统中的热平衡动力学似乎与振动过剩能量在长达〜4 nm的距离上的主要弹道初始传播一致，并且表明支链三聚体中的等效路径之间存在干扰。
Vibrational cooling was investigated in a set of homologous dimers and trimers with methyl viologen repeat units (MV2+). The rapid, <500 fs decay of the D1excited state of monoreduced viologen (MV+•) via a conical intersection allows the preparation of a vibrationally hot D0ground state with a large excess energy of 1.7 eV, which is equivalent to the initial effective temperature of ∼800 K. Pump-probe spectroscopy was used to monitor the disappearance of the characteristic D0→ D1hot absorption band, which appears at longer wavelengths than the steady-state spectrum of “cold” MV+•in equilibrium with the solvent. It is assumed that the vibrational excitation of the ground is initially confined to the same monoreduced viologen repeat unit, which was optically excited to the localized electronic D1state, although some degree of redistribution may occur already in the excited state. The observed cooling rates depend on the size and topology of the oligomer, with the linear trimer exhibiting significantly faster thermalization than the branched one. The experimental results were corroborated by molecular dynamics simulations carried out in the harmonic approximation. The dynamics of the thermal equilibration in these systems appears to be consistent with primarily ballistic initial propagation of the vibrational excess energy over distances as large as ∼4 nm and suggests the presence of interference between the equivalent pathways in the branched trimer.