Mechanistic Insights into Ultrafast Excitation Energy Transfer in Hybrid Light-Harvesting System of BODIPY Derivatives and LH2 from Rhodobacter sphaeroides 2.4.1 Co-assembled in Lipid Bilayers

To further explore the intrinsic mechanism of excitation energy transfer (EET) in a hybrid light-harvesting system composed of a natural antenna complex and fluorophores, the peripheral light-harvesting complex LH2 from Rhodobacter (Rba.) sphaeroides and a bistyryl-BODIPY derivative (B6) were co-assembled into phospholipid bilayers, followed by experimental and theoretical investigations. The singlet EET efficiency from B6 to B850 BChls of LH2 was determined by fluorescence excitation profile, exceeding 60%. The ultrafast dynamics study reveals that EET from optimally aligned B6 monomers to B850 BChls occurs in ∼7 ps, while EET from randomly dispersed B6 molecules occurs in ∼90 ps. Combined with CD and Raman spectroscopy, along with DFT and molecular dynamics simulations, a structural mechanism is proposed. In the co-assembled system, pairs of B6 molecules insert into the grooves between adjacent beta-subunits surrounding the B850 ring of LH2, with a closest distance of 21 Å from the B850 BChls. These results provide molecular-level insights and design principles for biomimetic hybrid light-harvesting systems.