Tunable Spectral Properties and Excitonic Couplings of Bacteriochlorophyll-like Dimer Aggregates

Natural photosynthetic systems achieve remarkable energy transfer efficiency through the highly arranged network of integrated chromophores, where strong intermolecular excitonic interactions boost and direct the energy and electron migrations within the system. This article systematically explores how geometric arrangements of bacteriochlorophyll-like (BChl) dimers modulate excitonic couplings and spectral characteristics using a Frenkel exciton Hamiltonian (FEH) model coupled with multiconfigurational SA-RASSCF/MS-RASPT2 monomeric wave functions. Through extensive analysis of over 11,000 BChl dimeric configurations, we demonstrate how intermolecular distances, translation, and rotations around different axes drive transitions between H-, J-, X-, and (+)-aggregate types, with their distinct spectral and energetics landscape. Our results reveal significant deviations from classical dipole-dipole approximations in closely stacked dimers, highlighting the necessity of employing the FEH framework to describe Coulomb interaction between spatially extended transition densities. Rotational symmetry-breaking (e.g., pitch and yaw rotations) is shown to amplify or diminish coupling strength, with some spatial dispositions that mimic natural light-harvesting 2 design. Additionally, macrocycle curvature in BChl monomers imposes excitonic band asymmetry, thus introducing another path to fine-tune the system's properties. The comprehensive dataset outlines fundamental concepts and design principles for efficient artificial light harvesting and advanced exciton-based materials, offering potential for organic optoelectronic applications.