Intramolecular charge-transfer process of jet-cooled (p-cyanophenyl)pentamethyldisilane: roles of the torsional motion and the Si-Si bond change

To investigate the intramolecular charge-transfer (ICT) process of (p-cyanophenyl)pentamethyldisilane (CPDS), laser-induced fluorescence, dispersed fluorescence, and two-color resonance enhanced two-photon ionization spectra were measured in a jet-cooled isolated condition. Dual fluorescence of CPDS was observed from a ground vibrational level in the locally excited pipi state. Similar to an emission from the charge-transfer (CT) state in solution, one of the dual emissions of the isolated molecule in the jet was assigned as the CT emission. A significant vibrational dependence on the ICT process was found as exciting vibronic levels of the molecule. It was identified that the promoting mode of the ICT process is a torsional motion of the disilanyl group with respect to the phenyl ring. It was also revealed that an effective appearance energy of the CPDS cation via the CT state is much lower than that via the locally excited pipi state suggesting that the electronic configuration of the CT state is similar to that of the cation. On the basis of an electronic configuration of the cationic state, that of the CT state was suggested to be of the (sigma(Si)(-)(Si), 2ppi) type.