Dynamics study of artificial light harvesting systems by time-resolved spectroscopy

Abstract

The remarkable character of the natural light harvesting system that converts absorbed photon energy with high efficiency in spite of the disordered environment attracted the attention of many researchers, and many studies were conducted on the natural light harvesting system. However, even with relatively simple organisms, natural light harvesting systems are so complex that it is difficult to understand their efficient internal energy and electron transfer processes. For this reason, a number of artificial mimics of light harvesting systems have been synthesized which are composed of only light absorbing units and energy/electron acceptor. In addition, artificial light harvesting system consisting of multiple chromophores has recently attracted much interest as building blocks of molecular optoelectronic devices, such as photovoltaic solar cells. For the development and optimization of such devices, it is crucial to understand the physics and chemistry of dynamic processes. Thus, many artificial light harvesting systems have been extensively studied using various time-resolved spectroscopic techniques. In this dissertation, energy transfer in a light harvesting system composed of multiple chromophores and charge separation in a molecular system composed of organic electron donor and acceptor molecules were studied using transient absorption spectroscopy and two-dimensional electronic spectroscopy. Transient absorption spectrum provides information on the dynamic processes that occur in the photosynthetic system under study. In addition, the evolution of non-emissive states and dark states can be investigated. Two-dimensional electronic spectrum has enabled us recording time-resolved correlation spectra between absorption and emission frequencies. Therefore, the two-dimensional spectroscopy is specialized in observing the excited-state dynamics depending on excitation wavelength. First, the correlation between the structural parameters and energy transfer efficiency of the light-harvesting complex composed of multiple chromophores was studied by transient absorption spectroscopy and simulation. Based on the simulations on model multi-chromophore systems (MCSs) for four cases having different structural heterogeneity and the TA measurements on multi-porphyrin dendrimers, we revealed the followings on the energy transfer efficiencies and the structural parameters of the MCSs: (i) energy transfer efficiency is enhanced by homo-FRET only when there exists structural heterogeneity in the MCSs

(ii) the enhancement of energy transfer efficiency is directly related to the distribution of the energy transfer efficiency of an individual donor, of which the width is governed by the distribution of orientation factors and donor-acceptor distances

(iii) energy transfer efficiency is enhanced by the increase of the number of donors, especially by the increase of the number of the nearest donors. Based on these lessons learned from this study, we can explain why the conclusions from previous studies seemingly conflict with each other. Next, the charge separation dynamics in a molecule composed of diphenylamine (electron donor) and perylene monoimide (electron acceptor) were investigated using vis-to-NIR transient absorption spectroscopy and two-dimensional electronic spectroscopy. By observing the absorbance change in the visible and near-infrared regions, it was confirmed that charge separation occurs only in the solvent with strong polarity, and that the charge separation was accelerated as the excitation wavelength decreased. The results show that excess energy can accelerate the intramolecular charge separation.