Two-dimensional spectroscopic study of biological photosynthetic systems and the solar cell functional materials

Abstract

This thesis focuses on the experimental and theoretical studies of molecular electronic dynamics by two-dimensional (2D) electronic spectroscopy in biologically relevant systems. The primary energy transport and charge separation in natural photosynthetic complexes such as light-harvesting complex II, Photosystem II Reaction Center and Fenna-Matthews-Olson (FMO) complex were studied. The energy-transfer and charge-separation pathways and the associated timescales were identified from the experiments in conjunction with theoretical modeling and a global fitting approach. In the FMO complex, the measured timescale of the electronic coherence was on the order of 60 fs at ambient temperature. No evidence was found that suggests that this plays a functional role for the process of natural energy transport. In a separate study, the exciton and free-carrier dynamics in hybrid lead perovskite thin films were investigated by 2D electronic spectroscopy after excitation with a 750 nm-laser. The ultrafast exciton dissociation and free-carrier scattering processes were identified in these measurements.

Moreover, theoretical study was undertaken of the dynamics of an electronic wave packet in the vicinity of a conical intersection (CI). It was found that the numerical simulation time can be significantly reduced by transforming the molecular vibrational modes into the bath and treating the full resulting non-Markovian dynamics numerically exactly. The presence of the CI was identified by the excited state absorption in the 2D spectrum. Furthermore, the impact of vibrational coherence on the electronic wave-packet dynamics was also studied near to the CI. It was shown that vibrational coherence is one of crucial factors to determine the quantum efficiency of the wave packet transfer at the CI.