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

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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.