Ultrafast transient absorption spectroscopy : 1) Dynamics of carbonyl stretching vibration in metal carbonyl. 2) Applications to photosynthetic systems
Date of Issue2017
School of Physical and Mathematical Sciences
This thesis is focused on the setup and application of ultrafast transient absorption spectroscopy. Ultrafast transient absorption spectroscopy is a four-wave-mixing technique which is also known as ultrafast pump probe spectroscopy. The information on the mechanistic and kinetic details of molecular systems can be provided through this technique. In the first part of the thesis (Chapter 2), the dynamics of carbonyl stretching vibration in Mn2(CO)10 was studied by ultrafast infrared pump probe spectroscopy. Three vibrational frequencies at 1985 cm-1, 2016 cm-1 and 2047 cm-1 are assigned to four fundamental carbonyl stretching modes, and two of four modes are corresponding to the 2016 cm-1 frequency. The two lower vibrational frequencies 1985 cm-1 and 2016 cm-1 are due to the perpendicular carbonyl stretches while the highest frequency 2047 cm-1 is due to the coaxial carbonyl stretches. The lifetimes of these carbonyl stretching vibrations are obtained through the fit and analysis in terms of two-component exponential decays modified Gaussian function with the temporal response. In the second and major part of the thesis (Chapter 3, 4, 5, and 6), we describe the application of ultrafast transient absorption spectroscopy to the study of photosynthetic systems. Photosynthesis is the most common light reaction process in nature, which is carried out in chloroplast. Thylakoids, which are organized into sacs of green inner membranes in the chloroplast, are membranes-bound compartments and the sites of the light reaction of photosynthesis. The light reaction function mainly depends on four major protein complexes (photosystem II, photosystem I, cytochrome b6f complex and adenosine triphosphate synthase) which localize at the thylakoid membranes. Usually, the pigments for photosynthesis are bound in these components. The pigments absorb the light energy and transfer the excitation energy to the reaction centres which also contain the pigments. Chlorophylls are the most important pigments in nature. They widely exist in the plant, bacterial and algae and absorb visible light in the red and blue wavelengths but reflect the green and near-green portions. The typical ultraviolet-visible absorption bands of chlorophylls are localized at two regions: the Soret Band and the Q bands. The Soret bands correspond to the Sn states of chlorophylls while the Q bands are corresponding to the S2 and S1 states of chlorophylls. The energy transfer dynamics between the Soret band and the Q band in chlorophyll a and b was investigated by our pump probe spectrometer. It is found that although the energy gap between the Soret band and the Q band in chlorophyll b is smaller than that in chlorophyll a, the lifetimes of the BQ energy transfer processes in chlorophyll b are longer than that in chlorophyll a. Most of chlorophylls in nature are bound in the light-harvesting antenna pigment-protein complex (LHC) II which is a supramolecule associated with the core of photosystem II in photosynthetic systems. LHC II exists as a trimer and plays an important role in the light absorption and the efficient electron and excitation energy transfer in photosynthetic reactions. Transient absorption measurements were carried out for three types of LHC II to study the carotenoid to chlorophyll energy transfer dynamics. The excitation energy absorbed by carotenoids is transferred to both chlorophylls via the Qx bands of chlorophylls in a short time scale of few hundreds of femtoseconds in native and wild type LHC II, but in H120L mutant LHC II the excitation energy primarily is guided into chlorophyll a via the Qx bands of both chlorophylls. The energy transfer among the chlorophylls occurs in the subsequent picoseconds and tens of picoseconds. Photosystem (PS) I complex is an important component in photosynthetic systems. In photosynthesis, PS I absorb the light energy and supply the negative redox and transfer the excitation energy as an intermediator. The energy transfer dynamics among bulk antenna pigments of PS I excited at 650 nm was measured by our transient absorption spectrometer. The excitation energy absorbed by chlorophyll b is transferred from the blue side of bulk pigments to bulk chlorophylls and red chlorophyll. The red chlorophylls absorbing the light with wavelengths at around 715 nm, 730 nm and 750 nm were observed to accept the excitation energy from the 682 nm chlorophyll pool at time scales of 1.93 ps and 10.9 ps.