Schwarzer D, Kutne P, Schröder C, Troe J
Max-Planck-Institut für Biophysikalische Chemie, Am Fassberg, 37077 Göttingen, Germany.
J Chem Phys. 2004 Jul 22;121(4):1754-64. doi: 10.1063/1.1765092.
Intramolecular vibrational energy flow in excited bridged azulene-anthracene compounds is investigated by time-resolved pump-probe laser spectroscopy. The bridges consist of molecular chains and are of the type (CH(2))(m) with m up to 6 as well as (CH(2)OCH(2))(n) (n=1,2) and CH(2)SCH(2). After light absorption into the azulene S(1) band and subsequent fast internal conversion, excited molecules are formed where the vibrational energy is localized at the azulene side. The vibrational energy transfer through the molecular bridge to the anthracene side and, finally, to the surrounding medium is followed by probing the red edge of the azulene S(3) absorption band at 300 nm and/or the anthracene S(1) absorption band at 400 nm. In order to separate the time scales for intramolecular and intermolecular energy transfer, most of the experiments were performed in supercritical xenon where vibrational energy transfer to the bath is comparably slow. The intramolecular equilibration proceeds in two steps. About 15%-20% of the excitation energy leaves the azulene side within a short period of 300 fs. This component accompanies the intramolecular vibrational energy redistribution (IVR) within the azulene chromophore and it is caused by dephasing of normal modes contributing to the initial local excitation of the azulene side and extending over large parts of the molecule. Later, IVR in the whole molecule takes place transferring vibrational energy from the azulene through the bridge to the anthracene side and thereby leading to microcanonical equilibrium. The corresponding time constants tau(IVR) for short bridges increase with the chain length. For longer bridges consisting of more than three elements, however, tau(IVR) is constant at around 4-5 ps. Comparison with molecular dynamics simulations suggests that the coupling of these chains to the two chromophores limits the rate of intramolecular vibrational energy transfer. Inside the bridges the energy transport is essentially ballistic and, therefore, tau(IVR) is independent on the length.
通过时间分辨泵浦 - 探测激光光谱研究了激发态桥连薁 - 蒽化合物中的分子内振动能量流动。桥由分子链组成，类型为(CH₂)ₘ（m 最大为 6）以及(CH₂OCH₂)ₙ（n = 1,2）和 CH₂SCH₂。光被吸收到薁的 S₁带并随后进行快速内转换后，形成激发分子，其振动能量定域在薁一侧。通过探测 300 nm 处薁 S₃吸收带的红边和/或 400 nm 处蒽 S₁吸收带，跟踪振动能量通过分子桥转移到蒽一侧并最终转移到周围介质的过程。为了区分分子内和分子间能量转移的时间尺度，大多数实验在超临界氙中进行，其中振动能量向浴的转移相对较慢。分子内平衡分两步进行。约 15% - 20%的激发能量在 300 fs 的短时间内离开薁一侧。该成分伴随着薁发色团内的分子内振动能量重新分布（IVR），它是由对薁一侧初始局部激发有贡献且扩展到分子大部分区域的简正模式的退相引起的。后来，整个分子发生 IVR，将振动能量从薁通过桥转移到蒽一侧，从而导致微正则平衡。短桥的相应时间常数 τ(IVR) 随链长增加。然而，对于由三个以上元素组成的较长桥，τ(IVR) 在约 4 - 5 ps 时保持恒定。与分子动力学模拟的比较表明，这些链与两个发色团的耦合限制了分子内振动能量转移的速率。在桥内部，能量传输基本上是弹道式的，因此，τ(IVR) 与长度无关。
