Direct observation of the structural deformation in the intramolecular charge-transfer reaction by means of transient infrared spectroscopy

分子内电荷转移（ICT）反应是光化学反应中的重要过程之一。结构变形，特别是围绕连接电荷供给和接受基团的单键的扭转变形，一直是理解ICT反应的中心问题之一。本文用瞬态红外吸收光谱法测定了氰基苯基二硅烷（CPDS）分子电荷转移（CT）态的平衡结构。我们记录了CT态在CH伸缩区的瞬态红外光谱。结果表明，在CT状态下，二硅烷基基团位于与苯环相同的平面内，而在激光激发LE状态下，二硅烷基基团位于与苯环垂直的平面内。该结果清楚地表明，所提出的扭转变形实际上发生在ICT反应中。这是ICT反应中扭转变形的第一个直接证据。在ICT反应中，除了溶质本身的结构变形外，溶剂分子的再取向过程也非常重要。CT状态通过溶剂的重新取向而在能量上稳定。在解释CT发射的大斯托克斯位移时，经常讨论这种稳定性。在本研究中，我们观察到瞬态红外光谱在OH和CN伸缩区的CPDS-水簇。结果表明，ICT反应分两步进行，如LE→ CT 1 → CT 2。在OH伸缩的情况下，观察到ν_的大位移<OH>，而ν_的位移<CN>很小。这意味着从CT 1到CT 2态的转变是水分子相对于CPDS的重新取向过程，而不是电子转变。我们的观察应该为我们提供一个微观拯救的新视角。

The intramolecular charge-transfer (ICT) reaction is one of important processes in photochemical reactions. The structural deformation, especially a torsional deformation around a single bond which connects a charge donating- and accepting groups, has been one of the central issues in understanding the ICT reaction. In the present study, we have determined the equilibrium structure of the charge-transfer (CT) state of cyanophenyldisilane (CPDS) molecule by means of the transient infrared absorption spectroscopy. We recorded the transient IR spectra of the CT state in the CH stretching region. It was revealed that the disilanyl group lies in the same plane as the phenyl ring in the CT state, while the disilanyl group is in a perpendicular plane to the phenyl ring in the laser-excited LE state. This result clearly showed that the torsional deformation proposed actually happens in the ICT reaction. This is the first direct evidence of the torsional deformation in the ICT reaction. In addition to the structural deformation of a solute itself, re-orientation processes of solvent molecules are also very important in the ICT reaction. The CT state is energetically stabilized by the re-orientation of solvents. Such stabilization is frequently discussed in interpreting a large Stokes shift of the CT emission. In the present study, we observed transient IR spectra in the OH and CN stretching region of CPDS-water cluster. It was found that the ICT reaction proceeds in two-step, like as LE→CT1→CT2. In the case of the OH stretch, a large shift of ν_<OH> was observed, while that of ν_<CN> was very small. This means that a transition from the CT1 to CT2 states is a re-orientation process of water molecule with respect to CPDS rather than an electronic transition. Our observation should provide us with a new insight of the microscopic salvation.