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Excited state quantum dynamics in molecular aggregates: a unified description from biology to devices

Final Report Summary - ESTYMA (Excited state quantum dynamics in molecular aggregates: a unified description from biology to devices)

In this project we study the dynamics of charge and excited state in condensed phase with two classes of systems in mind (i) organic semiconductors (molecules or polymers) and (ii) biological light harvesting complexes. In both cases the time evolution of the excited state is difficult to study because the motions of the electron are strongly coupled to some of the nuclear motions and weakly coupled with a large number of low frequency vibrations of the environment. This project focuses on the development of appropriate methodologies to study such problems and to apply such new methods to a subset of research questions of interest in materials science and biophysics.

The essence of the methodology is based on tackling the description of the system at two levels: (i) a detailed level containing all the information that state-of-the-art computational methodology can provide on few specific systems and (ii) a phenomenological model, i.e. a very simplified version of the system, that should capture its essence and enable the prediction of new properties or materials. Traditionally, the two approaches are chosen by different teams of scientists. Bringing them under the same roof allows the development of more useful phenomenological models and a stronger interpretation of the detailed chemical models. Virtually all key findings of this project have been obtained combining descriptions of the same phenomena at different model “resolutions”.

We have studied in detail the interaction between excited states localized on neighbouring molecule in a crystal discovering that the thermal motion of the molecules influences the dynamics of the excited states. We then developed a phenomenological (generic) model of exciton transport revealing under what condition one can expect very long range coherent dynamics of the exciton.

We have studied the process of generation of free charges from excited states in organic solar cells, again building simple models from detailed ones. From the theory we identified a particular orbital feature that can be used to design new electron acceptors. We verified the validity of the theory considering a large set of experimental systems.

We developed a particularly simple model that captures the quantum dynamics of an excess charge in a molecular semiconductor without numerically intensive calculation. Because of its simplicity it was possible to build a “map” of molecular semiconductors that enables the rapid search of better materials.

In the study of light harvesting system we have looked at all key components of the problem individually (i) the strength of the interaction between chromophore interacting (ii) the interaction of the excited state with the local environment and (iii) with the intra-chromophore degrees of freedoms. We have used this detailed information and the known structure of a large number of light harvesting complexes to build a “universal” model of biological light harvesting.