Computational Investigations of the Ground and Excited State Properties of Porphyrin-Based Light-Harvesting Array

Tetrapyrroles have found immeasurable utility in nature as multi-purpose pigments within intricate macromolecular light-harvesting assemblies that produce chemical potential from solar-energy input. The mechanism of solar energy conversion in these assemblies in part relies on quantum mechanical properties of tetrapyrroles and the interactions among them. This dissertation describes an effort to model the ground and excited state properties, as well as light-induced processes such as light absorption and excited state energy transfer, in covalently or noncovalently bound tetrapyrrole-containing pigment arrays.;Chapter 1 provides a broad survey of natural and artificial light harvesting, and presents a short overview of the computational approaches utilized in this work to model the ground and excited state properties and processes in pigment assemblies.;Chapter 2 describes a Kohn-Sham density functional theory (KS-DFT) and timedependent DFT (TD-DFT) study of unique panchromatic absorption exhibited by a series of strongly coupled perylene-accessorized porphyrin arrays, PMI NP (N = 1, 2C, 2T, 3, 4). These systems are found to exhibit large conformational flexibility, which impacts both their ground and excited state electronic properties. Panchromatic absorption by these arrays arises from the presence of many conformations with vastly different electronic properties and absorption profiles in solution.;Chapter 3 focuses on the development of a model to simulate the wavepacket dynamics associated with excited state energy transfer (EET) in a prototypical weakly-coupled covalently-linked zinc-freebase porphyrin dyad. A stepwise procedure connecting results from KS-DFT, TD-DFT, density functional based tight binding molecular dynamics, and wavepacket dynamics simulations with an extended Huckel Hamiltonian was developed to simulate EET on a pico-second time scale with computational effort far reduced relative to more rigorous methods.;Chapter 4 highlights efforts to understand and model noncovalent interactions in a porphin dimer. Twenty-nine different exchange-correlation functionals were tested for the calculation of inter-porphin interaction energies in a face-to-face orientation at 3-8 A separation distances. The calculated potential energy curves were benchmarked against second order Moller-Plesset perturbation theory (MP2), and spin-component scaled MP2 (SCS-MP2) calculations. The findings demonstrate the importance and effectiveness of various dispersion correction schemes for calculations of noncovalent interactions in weakly-bound tetrapyrrole arrays.;Chapter 5 summarizes the results obtained from the electronic structure calculations of various pigment arrays described in Chapters 2-4. Overall, the work described in this dissertation demonstrates the utility of KS-DFT, TD-DFT, and quantum dynamics approaches based on an extended Huckel Hamiltonian for modeling various challenging aspects of light harvesting, especially light absorption and EET processes. Moving forward, these methods should continue to provide useful insights into the complex nature of light harvesting in tetrapyrrole-based pigment aggregates.