Multiple Electron and Electronic Energy Transfer Dynamics Relevant to Light Harvesting and Catalytic Reactions
Electronic excitation energy transfer (EET) and electron transfer (ET) are of the fundamental importance to biochemical processes and solar energy conversion. The goal of this thesis is to develop and apply theoretical and computational tools to understand the mechanisms and kinetics of EET and ET in selected molecular systems with the aim to control and boost their efficacy. Specifically, this thesis focuses on three subjects: (1) computational study of energy transfer and its pathways in Ruthenium based metal organic frameworks (MOFs), (2) developing a general analytical model to describe the kinetics of energy transfer/electron transfer in condensed media, and (3) developing theoretical frameworks to describe multiple electron/exciton transfer processes.
In the study of energy transfer in Ruthenium based MOFs, we found that the excitation transport kinetics was well described by a Dexter (exchange) triplet-to-triplet incoherent multi-step hopping mechanism. The sensitive distance dependent rate for Dexter energy transfer in different MOF structures establishes unique energy transport pathways. For example, both one- and three-dimensional exciton-hopping networks were found in mixed Ru/Os MOFs. As such, Dexter energy transfer may potentially be helpful for spatially directing excitation energy along specific direction, for example, towards reaction centers, and an amenable for designing high efficiency energy transfer materials.
Significant amount EET processes happen in condense media and the nature of energy migration kinetics depends heavily on the donor (D) and acceptor (A) distribution in the media, especially for organic photovoltaic devices. The EET in the condense phase allow us to study the impact of ordered, partially ordered, and disordered DA distribution on the solar energy harvesting efficiency. To better account for the EET in condense phase, we developed a general analytical model for the description of the time-dependent luminescence decay emphasizing on the actual D-A spatial distribution. Applications of the developed model have been made to investigate the long-range excitation energy transfer in disordered polymer systems. By fitting the experimental transient luminescence spectra, we found that the derived EET kinetics showed better agreement with experimental observed luminescence decay both in short and long times, a significant improvement over the earlier models by Inokuti and Hirayama. Our model is more reliable in a wide range of time and acceptor density and can also be used for electron transfer.
The frontiers of ET and EET are moving from single particle one-step reactions to coupled multi-particle and multi-step processes. To understand the leading features that mediate the two-electron transfer in catalysis, we developed a two-electron transfer superexchange model that focuses on the roles of these features including (1) the one- and two-electron virtual intermediate states that mediate the ET, (2) the number of virtual intermediates with system size, and (3) the multiple classes of pathways interferes. Key questions, including how bridge structure and energetics influence multi-electron superexchange and interference between singly- and doubly-oxidized (or reduced) bridge virtual states are investigated. We found that even simple linear donor-bridge-acceptor systems have pathway topologies that resemble those seen for one-electron superexchange through bridges with multiple parallel pathways. The simple model two-electron transfer systems studied here exhibit a richness that is amenable to experimental exploration by manipulating the multiple pathways, pathway crosstalk, and changes in the number of donor and acceptor species. The features that emerge from these studies may assist in developing new strategies to deliver multiple electrons in condensed phase redox systems, including multiple-electron redox species, multi-metallic/multi-electron redox catalysts, and multi-exciton excited states.
Finally, to understand the role of structural and environmental disorder on incoherent ET, we developed a perturbative model based on the kinetic master equation to examine incoherent ET in non-equilibrium and non-Markovian regime. The developed method provides an effective way to explicitly investigate how a general (non-Gaussian) fluctuation in ET rate can modify the ET kinetics. Applications of this method have been made to study the ET kinetics for donor-bridge-acceptor systems with the structural and environmental fluctuations. Changing in ET kinetics with structural fluctuations of different nature was examined. Dominant fluctuation characters that significantly boost or reduce ET rate were identified. These findings may be helpful in designing efficient ET materials and provide strategies in modulating ET rate.
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