Exciton dynamics

I am interested in how light is absorbed and transported in molecular aggregates. During my PhD I have mostly focused on developing and applying theoretical models to study exciton transport in natural and synthetic materials (For a non scientist intro to what an exciton is, see the video I’ve posted on this page).

Understanding which physical principles govern exciton transport and what molecular parameters can be tuned to increase transport efficiency is of paramount importance in the design of new materials such as solar cells, detectors etc.

Topics of interest

Exciton dynamics in photosynthetic complexes


Light-harvesting complex of green sulfur bacteria (sketch)

Exploring the possibility that we can learn from natural photosynthetic systems to develop optimal materials for energy transfer is an exciting problem. These systems are highly complex and their intricate structures have been optimized by natural evolution over millions of years. To this day, the exact molecular packing in most photosynthetic complexes remains unknown. Furthermore, there are many factors such as energy and structural disorder, temperature and pH which may influence energy transport.

I have been working on using mixed ab-initio open quantum system approaches as well as electrodynamics to study excitons in the light-harvesting-complex of green sulfur bacteria, a sketch of the bacterium is shown in the figure on the right.

Some publications on this topic:

  • S. Valleau, Semion K. Saikin, Davood Ansari-Oghol-Beig, Masoud Rostami, Hossein Mosallaei, and Alán Aspuru-Guzik. Electromagnetic study of the chlorosome antenna complex of Chlorobium-tepidum, ACS Nano: 8, 3884 (2014)
  • J. Huh, S. K. Saikin, J.C. Brookes, S. Valleau, T. Fujita, and A. Aspuru-Guzik. Atomistic study of energy funneling in the light-harvesting complex of green sulfur bacteria, Journal of the American Chemical Society: 136, 2048 (2014)

Exciton dynamics in synthetic aggregates


Brickstone packing model for a J-aggregate

Many synthetic materials have been created in time with the goal of maximizing exciton transport efficiency. The possibility of synthesizing these systems in a controllable way enables one to tune the process to optimize the transport properties. In this context one type of materials which have been studied extensively are J-aggregates. These aggregates are formed of fluorescent dies and have a distinct spectral signature, a red shifted (respect to the monomers) narrow absorption band known as the J-band. They are characterized by strong intermolecular couplings and their exciton states are delocalized over many molecular sites. I have studied exciton transport in a particular class of J-aggregates, thin-film J-aggregates. These aggregates can be synthesized layer by layer in a controllable fashion. A sketch of an example of molecular packing in J-aggregates is shown in the figure. In particular I investigated the role of disorder in determining the diffusion constant for exciton transport within these thin-film layers.

For more details, see this paper:

Open quantum system methods and ab-initio


From QM/MM to open quantum system approaches

Open quantum system approaches are useful to tackle the dynamics of large molecular systems. In fact, they provide a formalism which allows one to focus on a specific part in more detail, the system, while the rest of the degrees of freedom can be treated as a bath. Similarly, quantum mechanical/molecular mechanical approaches (QM/MM) allow one to treat the most important part of a large system using quantum mechanics and the rest classically using molecular dynamics.

Often, one is interested in incorporating the information which can be obtained from QM/MM into an open quantum system model through a system bath correlation function so as to capture the bath and its interaction with the system in atomistic detail. I am interested in exploring how the quantities extracted from QM/MM can be incorporated into the open quantum system approach consistently.

More details can be found in this paper: