Fourth-order extension of multichromophoric Förster energy transfer

Excited state energy transfer (EET) is an essential process in many systems with optical functionality, such as photosynthetic systems, photovoltaic materials, and light-emitting diodes. Describing EET in large open quantum systems, such as molecular aggregates, is difficult, especially in intermediate parameter regimes. In the limit where the population transfer between molecular aggregates is incoherent, one can make use of the recently implemented time domain Förster resonant energy transfer method to determine the population transfer rates between the aggregates. In this study, we present a fourth-order perturbation theory that extends the Time Domain Multichromophoric Förster Resonance Energy Transfer (TD-MCFRET) method and allows for the more precise calculation of population transfer rates between molecular aggregates in intermediate regimes of coupling and system memory. The fourth-order correction can also be used to improve the accuracy of FRET models. We present a computationally tractable way of determining these higher-order corrections and demonstrate their relevance and validity for two bacterial photosynthetic systems: LH2 and chlorosomes. This new theoretical refinement may improve the interpretation of various experimental techniques such as absorption, fluorescence, and two-dimensional electronic spectroscopies and will deepen our understanding of EET in photosynthetic systems, synthetic devices, and beyond.