Beyond the 2/3 Approximation: A Multiscale Evaluation of the FRET Orientation Factor in Nonfullerene Acceptors

Abstract

Fluorescence resonance energy transfer (FRET) plays a key role in exciton migration within organic optoelectronic devices, with the orientation factor (κ²) being one of its most critical yet poorly characterized parameters. Conventional approaches often assume statistical κ² values (e.g., 2/3 or 0.476), which may lead to inaccurate estimates of Förster radii, transfer rates, and exciton diffusion lengths. In this work, we introduce a novel multiscale computational strategy that combines molecular dynamics simulations and density functional theory to evaluate κ² for two widely used nonfullerene acceptors, IT-4F and Y6. Coulombic interactions were calculated using the transition charges from the electrostatic potentials (TrESP) method and compared with the dipole-dipole (DD) approximation, revealing that the latter produces large errors at shorter intermolecular distances (<20 Å). The calculated κ² distributions exhibit a broad dispersion, with mean values significantly higher than those assumed in dynamic or static averaging regimes, exceeding 0.9 for IT-4F. These elevated orientation factors lead to larger Förster radii and enhanced FRET rates, suggesting that conventional approximations systematically underestimate exciton transport efficiency. Our findings emphasize the need for system-specific κ² estimations to achieve more reliable modeling of exciton dynamics and reveal a correlation with the system planarity that can be identified by computing simplified molecular descriptors. Ultimately, providing valuable insights for the design and optimization of high-performance organic photovoltaic devices for sustainable energy production.