Theory

Long-Range Energy Transfer

theory_fig1

Exciton system coupled to bath, ground state, and energy sink or trap.

Our group is interested in understanding the implications of architecture and organization on energy transport.  Recently, we modeled energy transfer in large assemblies of LH2-like complexes. In order to facilitate simulations of such a large system, we used a simplified, Markovian quantum master equation that neglects quantum coherence and treats the coupling between pigments by simple dipole-dipole interactions only.  Even with these assumptions, our model provides strong evidence that many of the salient features of the organization and structure of the PSU in purple bacteria may be explained based solely on electrostatic interactions within a quantum framework. For instance, we correctly predict the optimal number of pigments per ring, the energy spectrum of individual LH2 pigment-protein complexes (PPCs), and the timescale of transfer between rings. More importantly, our model clearly shows that the role of quantum mechanics extends beyond increasing transfer efficiency, as it provides remarkable robustness to both spectral and spatial disorder. For instance, when changing the energetic disorder from 0 to 100 cm-1, the surface describing the optimal transfer time as function of the dephasing rate and trap rate changes dramatically. However, in the quantum case, the surface stays nearly unchanged, indicating that quantum delocalization spreads the excitation over the system and is therefore less susceptible to kinetic traps. One of the most important findings of this work is that the energy and extent of system-bath interactions need not be controlled or tuned at each pigment site in order for near perfect efficiency to be achieved. Rather, a single, global system-bath coupling strength can be set along with a single trap rate to give nearly the same efficiency as if each of many hundreds of parameters are tuned in search of a global optimum. This design principle is important for fabricating a biomimetic device where high tenability at the molecular level may not be readily achievable.

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Classical versus quantum simulation showing the effect of energetic disorder on optimal transport parameters (global dephasing rate and trap rate). Quantum case is far more robust to disorder (i.e. little change in optimal surface).

E. Harel, Long Range Excitonic Transpot in a Biomimetic System Inspired by the Bacterial Light-harvesting Apparatus, J. Chem. Phys. 136, 174104-1 (2012) PDF

 

GAMERS Theory

see E. Harel, Four-dimensional Coherent Electronic Raman Spectroscopy, J. Chem. Phys., 146, 154201 (2017) PDF