The experimental visualisation of molecular structural changes during both photochemical and thermal reactions by real-time vibrational spectroscopy

Have you ever hoped to observe transition states? Chemists have long desired to monitor the deformation of molecular structures via transition states to understand the mechanisms of complicated reactions. Detailed knowledge of transition states helps find strategies to develop novel reaction schemes for introducing new functionalities to chemicals. Molecular structural changes via transition states can be observed by real-time vibrational spectroscopy using sub-5 fs laser pulses. In this paper, I report the direct observation of time-dependent frequency shifts of relevant molecular vibrational modes, which allowed for the clear visualization of ultrafast structural changes in molecules during bond breaking and bond reformation steps. Various mechanisms for photochemical reactions were clarified using sub-5 fs laser pulses. Moreover, a non-thermal vibrational excitation method for efficiently driving chemical reactions in the electronic ground state in solution with the use of broadband visible sub-5 fs laser pulses has been developed. The respective chemical reaction processes were directly observed, including transition states during not only “photochemical” but also “thermal” reactions. Time-resolved spectroscopy with a time resolution of a few femtoseconds enables observation of real-time vibrational amplitudes of complicated molecules and opens up new ways for clarifying reaction mechanisms and developing new chemical transformations.