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Research

There are a few major themes around which most of these problems can be organized:

  • Electron transfer, electron transport, and electron dynamics
  • Molecular assemblies, packing, and interactions
  • Quantum dynamics, and its relation to environmental baths and decoherence
  • Organic devices, both single molecule and adlayer-based
  • Energy applications of several sorts

Within this group of themes, projects arrange themselves, either by our becoming aware of a common interest theme within our own group or within the NU department, or spurred on by some signal advance in the literature.

Specific projects, at present, include:

1a. Molecular conductance junctions. In itself a large group of projects, essentially involving use of single molecules as components of electrical circuits. Areas of current interest include vibrational substructures within conductance spectra, accurate calculations of conductance spectra (beyond mean field), interference phenomena in molecular conductance, switching dynamics within molecules, optical interactions with molecular conductance, laser control of molecular conductance, generalization to semiconductor electrode systems and photovoltaics, strong coupling interactions (Kondo and Coulomb blockade regimes), chemical modifications and dynamic switching. Senior collaborators here include Seideman, Hersam, Marks, Schatz at Northwestern, Nitzan in Tel Aviv, Galperin at San Diego, Mikkelsen in Copenhagen, van der Zandt in Delft, Joachim in Toulouse, Troisi in Warwick.

1b, 2, 4. Electron transfer reactions, photovoltaics, conductive polymer systems, and organic electronics. In this area, we consider electron dynamics within molecular assemblies, rather than single molecules. Fundamentally, this is a matter of model building, quantum chemistry, structure/function analysis, molecular design, and collaboration with experimental groups in predicting, producing, interpreting, and applying molecule-based devices.

Specific projects include effects of adlayers on injection at interfaces, molecular exciton fission, “smart matrix” effects in photovoltaics, molecular design to reduce the Onsager radius in functional photovoltaics, spin dynamics and spin teleportation, design of n-type conductive polymers, role of disorder in molecular charge transport in organic solids.

Senior collaborators on these projects include Marks, Weiss, Wasielewski, Hersam, Schatz at Northwestern; Nitzan in Tel Aviv; Bredas, Kippelen and Marder at Georgia Tech; Durrant at Imperial College in London; Michl at Colorado.

3. Quantum Dynamics of Molecular Systems. This is a fundamental problem in physical chemistry: how to describe system/bath interactions, and the effects of the environment (solvent, surface, molecular cluster, distant molecular ligands) on the quantum dynamics of electrons in particular regimes. The fundamental physical chemistry problem here is clear: one really wants to treat the environment classically and the system quantum mechanically – doing this properly represents a major challenge to contemporary chemical physics.

We have taken two quite different approaches. Working with Ronnie Kosloff’s group, we are focusing on what is called the stochastic surrogate Hamiltonian model for the bath: here the bath is represented essentially as a set of spins, that are linearly coupled to the degrees of freedom in the system. The spins themselves decohere stochastically, thus permitting relaxation to occur in the system, and allowing dynamics as well as energetics in the bath. This approach has some substantial formal and physical advantages.

The second approach is based on more ad hoc schemes. The simplest is the Redfield model, in which particular degrees of freedom in the system are designated as those which couple to the bath, the bath correlations are largely neglected, and simple density matrix methods (including Nitzan’s steady state trick) can be used to calculate dynamics. This scheme is very useful in many systems, but is clearly not robust – bath dynamics, particularly sluggish relaxation in the bath, are not found in this model, which therefore can underestimate particular decoherence processes.

Finally, in particular cases where bath effects are expected to be uncomplicated, we use a very formal procedure originally developed by Lindblad and based on the convexity of the density matrix space. This formal technique permits direct study of how particular system/bath coupling forms will affect dynamics in the system, and is quite rigorous.

By combining these methods in what we hope is a judicious way, the group is now focusing on quantum dynamics of molecules in condensed phase, in processes ranging from conical intersections to exciton fission to hot injection.

Collaborators in this area include George Schatz at Northwestern; Ronnie Kosloff in Jerusalem; Abraham Nitzan in Tel Aviv; Gil Katz at Afikim; Arthur Nozik at NREL.

2. Dynamics of Molecules in Crowded Conditions: Surface Induced Chirality, Diffusion in Adlayers, and Crowding. The theme here is how ensembles of molecules differ, due to spatial constraints, steric interactions, and nonbonded forces, from individual isolated molecules. Polymer dynamics are an exemplary case, but our focus is chiefly on how chemical processes such as binding, photochemistry, reaction dynamics, and transport are affected by crowding.

The methods here involve both molecular dynamics and Monte Carlo simulation studies, as well as statistical density functional theory and model building. Specific projects include formation of chiral clusters in two dimensions, electron transfer dynamics in crowded adlayer systems (including applications in membrane electron transfer and biodynamics), reactions in hybrid molecule/quantum dot nanostructures, artificial enzymes, mechanically bonded species and their force/extension curves, molecular lubrication.

Collaborations here include Stoddart, Szleifer, Olvera, Wasielewski, Van Duyne, Mirkin, Weiss, Grzybowski at Northwestern; Glotzer at Michigan; Whitesides at Harvard; Mujica at Arizona State.

5. Plasmon/Molecule Interactions, Quenching, and Enhancement, Design for Functionality. The interaction of molecular entities with plasmon excitations in metals is an extremely active field at Northwestern: beautiful work is being done in the Mirkin, Hupp, Odom, Seideman, Weiss, Schatz, and Van Duyne laboratories. While collaborations are underway with all of these folks, our particular focus here involves utilizing plasmon excitation to enhance photo processes in molecules, particularly simple linear absorption and charge transfer spectroscopy. Applications to photovoltaics are obvious, but the fundamental question of design for function is the real challenge: in addition to all the design properties of the molecule (bonding patterns, optical spectra, stability, orientation, binding, vibronic interactions), one has to deal with the design of the plasmonic entity (composition, shape, orientation, coating). This is a relatively new field both for chemistry and for my group, so there are fewer publications in this area – but it is one to which we will be paying substantial attention, in close collaboration with both experimental and theoretical groups.

Collaborators include Schatz, Weiss, Mirkin, Van Duyne, Seideman, Hersam at Northwestern; Nitzan in Tel Aviv; Mikkelsen at Copenhagen.


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