Theoretical Chemical Physics

I am primarily interested in the dynamics of molecules in their excited electronic states. We use both computational and theoretical approaches to answer the following questions: 1. Which nuclear motions are responsible for accommodating the energetics of the transition by either absorbing excess energy in an exothermic transition or supplying the energy deficit in an endothermic transition? 2. What is the role of quantum coherence, localization, and relaxation in determining not only the transition rates, but also the transport, and spectroscopic properties of excitonic and charge-transfer states, especially in organic semiconducting systems? 3. How do these effects affect the observed dynamics, material properties, or electronic behavior in systems such as organic light-emitting diodes, photovoltaic cells, and DNA chains.

A Excited state dynamics in conjugated polymers

Organic semiconductors are quite different than their inorganic counter parts in that most of their electronic properties are determined by local molecular electronic interactions rather than by delocalized states. The impact of local level interactions is driven home at the interface between domains of semiconducting polymers in which there is an off-set between the respective HOMO and LUMO orbital energies. Here our work has largely focused upon developing robust semi-empirical descriptions of charge and energy transfer between co-facially stacked conjugated polymers. One distinguishing aspect of our work is that we have focused upon how the electronic dynamics are modulated and tuned by the intramolecular vibrational motions of the polymers. Typically, this is accomplished within the Marcus-Hush approach; however, we showed recently that when the non-adiabatic coupling is sufficiently strong, such approaches can give quite different relaxation pathways than if the explicit vibrational coordinate dependence in the coupling is included. My group has also developed a new time-convolutionless approach for including non-Markovian vibrational dynamics into the calculation of phonon-driven electronic transition probabilities. Lastly we have begun to explore a projection operator approach for determining sub-sets of phonon modes that are mostly responsible for the tuning, coupling, and energetic accommodation of electronic transitions.

Energy transport and relaxation in DNA

The electronic transport properties of DNA has attracted considerable attention over a number of years. The possibility that electron transfer in DNA chains may be linked to several biological processes has spurred nearly countless experimental and theoretical studies. From a biophysical standpoint, DNA also has a remarkably large photo-absorption cross-section for UVB (290-320 nm) light, making it susceptible to carcinogenic mutations. With this in mind, is important to understand how excitation energy is transported along the chain, the role of intra-strand stacking effects and interstrand couplings. Our guiding principle in this is that the interbase electronic interactions are modulated by the global structural dynamics, but that the DNA molecular dynamics are not affected by the excited states. Thus, we can bring to bear an arsinal of computational and theoretical tools ranging from molecular dynamics and TD-DFT to analytical lattice models to develop a complete theoretical understanding of this system. Quantum dynamics of small clusters We have recently developed a novel quantum hydrodynamic based approach for computing the quantum vibrational energies of small atomic clusters. Our numerical approach combines a a de Broglie/Bohm description of the quantum equations of motion with a Baysian sampling algorithm for the atomic density function. We are currently usig this approach to study quantum contributions to the thermodynamics of small rare gas clusters (Ne)n especially in thermal regions close to the bulk melting transition.