Charge Transfer Reactions

Our research is concerned primarily with mechanistic studies of charge transfer reactions as well as the biomimetic chemistry of light harvesting and small molecule activation. Electron and energy transfer processes impact a vast number of research areas of modern chemistry. Detailed understanding of these reactions is not only important from a theoretical perspective; with a firm mechanistic foundation, it may be possible to design and build synthetic enzymes that catalyze highly specific redox reactions, design new catalysts for alkane activation that feature electron transfer initiation, as well as synthesize photosynthetic reaction center mimics and artificial light-harvesting antenna systems, important first steps toward the development of both new solar energy storage technologies and molecular electronic devices. The following related research areas highlight some of these current interests.

Electronic Coupling and Long-Distance Electron Transfer
When an electron donor and acceptor are separated by an insulating medium such as a hydrocarbon or a protein, charge transfer events generally occur by a tunneling mechanism. The charge-transfer rate in such systems depends critically on the magnitude of donor-acceptor electronic coupling and is thought to be intimately related to the nature of reactant-product electronic states, the donor, acceptor, and bridging medium redox properties as well as the orbital symmetry for these species; a key goal of this work is to delineate the relative and absolute importance of these factors.

Novel Porphyrin-Containing Molecular Arrays and Polymers
New synthetic approaches developed by our group enable the fabrication of novel chromophore arrays that mimic key biological light-harvesting antenna systems. For example, the porphyrin array shown below models the salient spectroscopic features of the core light harvesting complex of the purple photosynthetic bacteria. We are currently exploring a variety of other highly coupled porphyrin-chromophore molecular architectures for their potential to model important biological multichromophoric assemblies and serve as key light-harvesting and excitation-transfer components in molecular devices and materials.

New Porphyrin-Based Redox Catalysts
A variety of experimental and theoretical studies predict that extremely electron deficient porphyrins will find particular utility in the development of catalysts for the activation of small molecules. We have recently synthesized a series of novel porphyrin macrocycles that are highly substituted with perfluorocarbon moieties. These unusual macrocyclic ligands impart exceptional electronic characteristics to the porphyrin central metal ion; such metalloporphyrin systems may serve as a platform for the development of Co, Fe, and Mn catalysts that mimic cytochrome P450's ability to utilize dioxygen for the selective oxidation of hydrocarbons.

The Role of Nuclear Motion in Fast Electron Transfer Reactions
We are utilizing ultrafast vibrational spectroscopy as a direct experimental probe to help decipher how molecular motions are coupled to actual ET events. Our work focuses on simple donor-acceptor complexes that undergo fast charge separation and/or charge recombination reactions (kET > 2 x 1012 sP1). These model systems allow pair-wise examination of reaction center-type chromophores and enable us to explore whether specific vibrational modes can activate chromophores for ET and accept energy in early photosynthetic events; the participation of such modes may be crucial to the highly efficient trans-membrane charge separation that occurs during biological energy transduction.