Ultrafast Intramolecular Photophysical Processes in Molecules of Biological Importance by Time-Resolved Fluorescence

Abstract

The elementary processes that lead to ultrafast chemical transformations occur at the fundamental time scale of molecular vibrations, which ranges from 10 to 300 fs. Ultrafast photochemical and photophysical processes such as intramolecular proton transfer, barrierless charge transfer, and internal conversion from an electronic state higher than S1 occur on tens of femtoseconds. Ultrashort light pulses generated by Ti:sapphire laser brought all of these processes within experimental reach, and the femtosecond time-domain spectroscopy has provided wealth of information on the ultrafast chemical reaction dynamics in condensed phases. The observation and control of the wave packet motion directly in time domain is one of the most intriguing tasks in modern spectroscopy, and now we can observe the wave packet dynamics manifested in the oscillations of various time-resolved spectroscopic signals. Most of all, time-resolved fluorescence (TRF) with a resolution higher than the vibrational period of interest is the most direct and ideal way to observe the wave packet motion in a specific state, as it probes the excited state dynamics exclusively. Recent advances in TRF experiment has achieved a time resolution of ~30 fs, which allows direct observation of wave packet oscillations up to ~1000 cm-1. In this thesis, I first give a brief account of coherent nuclear wave packet dynamics and the important classes of ultrafast photochemical/physical processes in Chapter 1. An overview of experimental methods for femtosecond TRF is presented in Chapter 2. Femtosecond time resolution can be achieved by optical gating of the fluorescence based on nonlinear methods, for instance, fluorescence up-conversion by sum frequency generation and Kerr gating by optical Kerr effect. By means of TRF with sub 50 fs time resolution, I have investigated ultrafast intramolecular photophysical processes in molecules of biological importance that could not be studied previously. In Chapter 3, intramolecular charge transfer (ICT) dynamics including reaction coordinates, structural changes, and reaction rates has been studied experimentally and theoretically. I report the ICT dynamics of laurdan investigated by TRF at extreme time resolution of 30 fs. A single high-frequency coherent nuclear wave packet motion on the product potential surface is observed through the modulation of the fluorescence intensity in time. Theory and experiment show that this vibrational mode involves large displacement of the carbon atoms in the naphthalene backbone, which indicates that the naphthalene backbone coordinates are strongly coupled to the ICT reaction of laurdan, not the twisting or planarization of the dimethylamino group. Vibrational coherence in electronic transition has been noted especially in biochemical systems. In Chapter 4, I report a unique and distinct coherent nuclear wave packet dynamics in each Q state of free base tetraphenylporphyrin (H2TPP). The ultrafast internal conversion dynamics of H2TPP is investigated by means of femtosecond TRF at time resolution of 55 fs. The instant, serial internal conversion from the B to Qy and Qx states within 80 fs generates the coherent wave packets in each Q state. Together with theoretical calculations, I suggest that vibrational modes observed here involve the out-of-plane vibrations of the porphyrin ring that are strongly coupled to the internal conversion of H2TPP. In Chapter 5, I report the ultrafast lifetimes of B and Q states in ferrous cytochrome c (cyt c) as an extension of my study on porphyrins. Highly time-resolved fluorescence is measured by fluorescence up-conversion. Numerical simulation of the TRF signals is performed by applying the third-order nonlinear response theory and a model transition frequency correlation function. Comparison of the experiment and simulation leads to the conclusion that the B band lifetime of ferrous cyt c is 30 fs while the Q band lifetime is determined to be 120 fs.