Study Of Electron And Energy Transfer Modulation In Molecules Using Time-resolved Vibrational Spectroscopy
Electron transfer is one of the fundamental process occurring in many chemical reactions. Electron transfer process has been under intensive study for many applications, for example artificial photosynthesis, where electrons from photo-excited chromophore molecules are harnessed to produce solar fuels in various forms. Transition metal complexes, such as ruthenium and rhenium complexes, play an important role in the continuing development of artificial photosynthetic devices. The electron transfer process in chromophores involving transition metal complexes often occurs on an ultrafast time scale from sub-ps to ns. To resolve such dynamics, ultrafast spectroscopic techniques are required. A variety of ultrafast techniques, such as time-resolved infrared spectroscopy and multi-pulse transient absorption spectroscopy, were used in this study to unravel the excited state electron transfer dynamics in a series of Re(I) complexes. Transition metal complexes often feature excited states that involve only partial electron transfer between the electron donating and accepting ligands, even for ligands with strong electron donating and accepting properties. It is often difficult to design a compact complex feature a full electron transfer excited state. Therefore, part of the work presented in this thesis was dedicated to the study of the electron transfer extent in the excited states of a series of [Re(N,N)(CO)3L]+ compounds, where N,N stands for electron accepting and L stands for electron donating ligands. By carefully designing the structure and redox properties of both the electron donor and acceptor, we demonstrated that essentially a full-electron charge transfer excited state can be prepared, while the designed Re(I) complex is still compact. To further extend the understanding of the electron transfer in transition metal complexes, modulation of the electron transfer rate in a compact Re(I) complex was studied. By perturbing the electron transfer process with a femtosecond mid-IR pulse, we showed that a 28% increase of the electron transfer rate was achieved. This study demonstrated the possibility of using a small energy mid-IR quanta to change the energy conversion process in a chromophore. Vibrational energy transfer in molecules is another important process in nature. Detailed understanding of the vibrational energy transfer on a molecule level is fundamentally important and essential for the development of molecular optical devices. It was recently discovered that the transport of vibrational energy in molecules can be fast and efficient due to its ballistic character. To understand the mechanism of the ballistic energy transport, experiments with several series of oligomers were performed using a relaxation-assisted two-dimensional infrared method. The energy transport speed was found to be dependent on transport initiation method and the transport pathways for different cases of initiation were identified. Detailed analysis on the chain band structure, group velocity and vibrational relaxation dynamics is presented.