Abstract
The objective of this study is to investigate under what circumstances Förster theory of electronic (resonance) energy transfer breaks down in molecular aggregates. This is achieved by simulating the dynamics of exciton diffusion, on the femtosecond timescale, in molecular aggregates using the Liouville–von Neumann equation of motion. Specifically the focus of this work is the investigation of both spatial and temporal deviations between exciton dynamics driven by electronic couplings calculated from Förster theory and those calculated from quantum electrodynamics. The quantum electrodynamics (QED) derived couplings contain medium- and far-zone terms that do not exist in Förster theory. The results of the simulations indicate that Förster coupling is valid when the dipole centres are within a few nanometres of one another. However, as the distance between the dipole centres increases from 2 nm to 10 nm, the intermediate- and far-zone coupling terms play non-negligible roles and Förster theory begins to break down. Interestingly, the simulations illustrate how contributions to the exciton dynamics from the intermediate- and far-zone coupling terms of QED are quickly washed-out by the near-zone mechanism of Förster theory for lattices comprising closely packed molecules. On the other hand, in the case of sparsely packed arrays, the exciton dynamics resulting from the different theories diverge within the 100 fs lifetime of the trajectories. These results could have implications for the application of spectroscopic ruler techniques as well as design principles relating to energy harvesting materials.
Highlights
Excitation energy transfer (EET), sometimes referred to as resonance energy transfer, is an important process exploited throughout nature
The focal point of this work is the investigation of both spatial and temporal deviations between exciton dynamics driven by electronic couplings calculated from Förster theory and those calculated from the unified theory of EET
For the case of the brickstone lattice, the electronic coupling landscapes derived from both Förster theory and quantum electrodynamics (QED), between the central molecule of the lattice and each of the other molecules is shown in figure 2
Summary
Excitation (or electronic) energy transfer (EET), sometimes referred to as resonance energy transfer, is an important process exploited throughout nature. When the donor returns to its ground state, the acceptor becomes excited This is a quantum mechanical process that involves the exchange of a virtual photon between (valence) electrons of the two species. Due to the importance of EET in the development of solar energy harvesting technologies, it is not surprising that enormous effort has been undertaken to understand the mechanisms behind such efficient photochemical systems [3,4,5,6] This has resulted in a wide range of synthetic structures that are able to mimic some features of natural antenna complexes [7,8,9,10]. A great deal work has been carried out on energy transfer in organic materials such as thin films or molecular aggregates and quantum dot arrays [11, 12]
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