Advances in technology have enabled the possibility to construct devices smaller than the mean-fee path of an electron. This technology continues to press towards the molecular level where energy level quantization, phase coherences, and the interaction between the electronic and nuclear degrees of freedom determine the properties of the device. The central theme in designing molecular electronic components is the manipulation and control of charge flow through single molecules and molecular assemblages.

Our work ion this area has been to develop a solid theoretical understanding the electronic dynamics which occur in electronically excited organic semiconducting molecules. Moreover, we are interested in how the internal molecular vibrations come into play in the relaxation pathways following some initial excitation.

In the case of organic light-emitting diodes, the initial excitation is the injection of mobile charge carriers into the material which can recombine to form luminescent excitonic states. One important question that we have addressed over the past number of years is how the spin of a given electron/hole influences the overal quantum efficiency of an organic LED device. Based purely upon spin statistics, this quantum efficiency has a theoretical upper bound of 25%.i.e only 1 in every 4 charge carrier pairs injected into the material has can form the correct spin combination (singlet) to form an emissive state. The bulk of the experimental evidence indicates that singlet populations are significantly enhanced in these systems and that this enhancement is a universal property reflecting the quasi-one-dimensional nature of the excited states. Our theoretical models support this observation as well as provide a rationalization of why this enhancement should scale (universally) with the persistence length of a given conjugated domain.

Conjugated polymer heterojunctions are also of significant interest since these systems can be used to produce pn-junction diodes and photovoltaic cells. Organic semiconducting materials offer a unique advantage over their inorganic counterparts since their HOMO and LUMO levels can be easily tuned by chemical modification.

Charge transfer can occur at the junction of these materials. Depending upon the band off-sets and the exciton binding energy heterojunction materials can be efficient photovoltaics (exciton binding energy < band offset ) or efficient LEDS (binding energy > band offset). Our work here has focused upon predicting the time-resolved photoemission in heterojunction materials since this is a direct experimental measure of the excited state population dynamics in these systems.

Finally, predicting and understanding electronic current flow through molecules from an atomistic and first-principles stand-point presents a significant theoretical challenge in that it requires the extension of standard quantum chemical methods that are well suited for bound and stationary state problems to solve non-stationary and continuum scattering problems.

The primary theoretical difficulty (setting aside the experimental ones!) is that we are dealing with an open quantum system which may be in a steady-state but is certainly not at equilibrium once an electrical current passes through the system. Current quantum chemical treatments have historically focused upon closed quantum systems and hence are ill-suited at best for studying transport through a molecular junction. Consequently, there is a clear need for both theoretical and computational tools capable of handling electronic transport through meso- and nano-scale structures, including electronic correlation effects, the effects of inelastic scattering, a and structural changes with in the molecular junction. Our work over the next few years will be to develop systematic treatments which address steady-state quantum dynamics in molecular junctions. While most theoretical work here has been on molecular wires and tend to neglect electronic many-body interactions, our work focuses upon the inclusion of electronic correlation and polarization effects along with phonon-mediated processes in molecular semi-conductors.