Direct Observation of the Intramolecular Structural Change and the Solvent Reorientation Process in the Intramolecular Charge-Transfer Reaction

Abstract

The intramolecular charge-transfer (ICT) reaction is characterized by intra- and inter-molecular structural changes. A well-known example of the former is a twist around the single bond between the charge accepting and donating groups of the reactant itself. The latter is a reorientation of solvent molecules against the reactant to stabilize the energy of the system. Detailed information about such structural change should provide us with an insight for a well understanding of the ICT reaction. Thus, we have carried out transient infrared (IR) absorption spectroscopy of jet-cooled (p-cyanophenyl) pentamethyldisilane (CPDS) and its solvated clusters. We have succeeded in observing transient IR spectra of the CT state, exhibiting very distinct patterns compared with those of the electronic ground state. Combined with results of quantum chemical calculations, we determined the equilibrium structure of the CT state of CPDS monomer. Our result is clear evidence that the twist occurs in the ICT process, as expected. In addition, we found a step-wise process in the case of solvated clusters based on our transient IR spectra. This process was related to the solvent reorientation in the ICT reaction. The observed spectra enabled us to determine very precise information about the reorientation process such as the initial and final orientations of the solvent molecule(s). These findings should provide us with a deep insight for understanding the ICT reaction.