The field of excitonics is related to the understanding, control and harnessing of electronic excitations in nanoscale environments. One of the goals of our group is the development of novel theoretical approaches for studying excitonic energy transfer in systems such as photosynthetic complexes, organic photovoltaic materials, j-aggregates and nano-materials such as quantum dot assemblies. We are carrying out fundamental work in the development of top-down and bottom-up approaches to the theoretical description of excitonic transfer processes in natural and artificial systems. From these studies we would like to define basic physical principles underlying the efficient energy transfer which could in turn be employed to design new devices.

Our work is synergically driven by a fruitful collaboration with experimentalists at MIT (Bulovic and Nelson groups) at Chicago University (Engel group) and at Univ. of Oregon (Marcus group). Further we are actively involved in the RLE Center for Excitonics.

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Open Quantum Systems and its Application to Energy Transfer
Learning from Photosynthesis

Recent ultrafast experiments have shown evidence of long-lived, preserved coherences in photosynthetic energy transfer for up to 600 femtoseconds. Our research group is studying energy transfer in photosynthetic complexes using the theory of open quantum systems. We have developed a novel way of partitioning the contributions from different physical processes to global quantities such as the energy transfer efficiency. With our methodology, we find that the contribution of quantum coherence to the energy transfer efficiency in these complexes is of about 10% at room temperature. Further, we have found that an interplay of excitonic coherence and the fluctuating phonon environment is responsible for the high transport efficiency commonly observed in these complexes, suggesting the term environment-assisted quantum transport (ENAQT). In addition, we have developed a method to quantify the contribution of wavelike dynamics to the efficiency and thus to the biological functioning.

We have also investigated the importance of using markovian rather than non-markovian master equations to study this system and found that non-markovian approaches can recover more oscillations in the coherences of the density matrix.

Synthetic Aggregates

J-aggregates are highly efficient light absorbing materials which find a variety of applications ranging from their use in optical devices to employing them as sensors in biological membranes. Our goal is to understand how disorder / geometry and molecule specific parameters can enhance or in turn suppress transport in these systems.

We have employed a mixed open quantum system / ab-initio model to model exciton transport in thin-film J-aggregates.

Realistic Description of Excitonic Energy Transfer from First Principles
Complementary to the top-down master equation approach, our group is developing approaches for the understanding of energy transfer phenomena based on sophisticated quantum chemistry methods and semi-classical approaches. In particular we are using couple QM/MM methods to describe exciton transport from a microscopic perspective in various photosynthetic complexes.

The Bacteriochlorophyll Antennas

The Chlorosome antennas are large nanoscale light-harvesting aggregates of bacteriochlorophylls found in green bacteria and attached to the cytoplasmic side of the cell membrane. The light they absorb is then transferred to a complex called the Fenna Matthews Olson complex and finally to the reaction center.

This entire process is highly efficient and these Chlorosomes have the longest exciton diffusion lengths of currently known pigments at room temperature. We are interested in understanding how this efficiency can be related to the microscopic and macroscopic properties of these aggregates. To this end we are developing on one side, a mixed quantum classical model (Molecular dynamics simulations coupled to electronic structure methods) and on the other, a classical electromagnetic model (in collaboration with the Mosallaei group at Northeastern University) coupled to ab-initio determination of molecule specific parameters for the Chlorosome rod elements.

The Fenna Matthews Olson Complex

Recently, we have employed a mixed molecular dynamics / quantum mechanics approach to study the Fenna Matthews Olson complex, a photosynthetic protein complex found in green sulphur bacteria living at the bottom of the ocean. This complex is located in the cell at the interface between the Chlorosome and the reaction center and functions as a funnel transferring energy efficiently from the Chlorosome to the reaction center.

These simulations reproduced the experimentally observed long-lived coherences as well as the absorption, linear and circular dichroism spectra without the need to introduce any ad-hoc parameters such as reorganization energy.

Quantum Process Tomography to Probe Excitonic and Vibrational Coherences
Molecular aggregates may sustain long lived excitonic coherences, which can be relevant for natural and artificial energy transfer processes. Hence, quantum effects might be important. In order to fully characterize these systems quantum mechanically, we need quantum tomographic protocols. We developed Quantum Process Tomography (QPT) as a quantum information processing tool to fully characterize few-chromophore arrays by exploiting nonlinear spectroscopy based on the photon-echo and pump-probe techniques. Using this technique we will be able to distinguish between vibrational and electronic coherences and understand which of these are actually observed in 2D spectra.

The following scheme describes how quantum process tomography works:

The following picture illustrates a system which can be studied with quantum process tomography:

2D Spectroscopy as a Tool to Probe Excitonically Coupled Chromophores
In collaboration with the group of Andrew Marcus (University of Oregon), we have developed a novel non-linear optical approach based on fluorescence to resolve structural/conformational details of excitonically coupled chromophores. One of the unique advantages of this approach is the possibility of extending the technique towards single-molecule studies. For example, one could follow the energy transfer dynamics of isolated clusters of quantum dot arrays, toward the design of better excitonic-based devices. A schematic of the experimental set up can be seen in the following figure:

Representative Publications
  1. Masoud Mohseni, Patrick Rebentrost, Seth Lloyd, and Alán Aspuru-Guzik. Environment-assisted Quantum Walks in Photosynthetic Energy Transfer. The Journal of Chemical Physics 129 (November 6, 2008): 174106.
  2. Patrick Rebentrost, Masoud Mohseni, Ivan Kassal, Seth Lloyd, and Alán Aspuru-Guzik. Environment-assisted Quantum Transport. New Journal of Physics 11 (March 3, 2009): 033003.
  3. Joel Yuen-Zhou, Jacob J. Krich, Masoud Mohseni, and Alán Aspuru-Guzik. Quantum State and Process Tomography of Energy Transfer Systems via Ultrafast Spectroscopy. Proceedings of the National Academy of Sciences 108, no. 43 (October 12, 2011): 17615–17620.
  4. Geoffrey A. Lott, Alejandro Perdomo-Ortiz, James K. Utterback, Julia R. Widom, Alán Aspuru-Guzik, and Andrew H. Marcus. Conformation of Self-assembled Porphyrin Dimers in Liposome Vesicles by Phase-modulation 2D Fluorescence Spectroscopy. Proceedings of the National Academy of Sciences 108, no. 40 (September 22, 2011): 16521–16526.
  5. Sangwoo Shim, Patrick Rebentrost, Stéphanie Valleau, and Alán Aspuru-Guzik. Atomistic Study of the Long-Lived Quantum Coherences in the Fenna-Matthews-Olson Complex. Biophysical Journal 102, no. 3 (February 2012): 649–660.