Charge transfer at the plasmon-molecule interfaces controls the collective chiroptical responses.

Molecular excitations interacting with metal plasmons have far-reaching implications in numerous fields. Electronic excitation of chiral molecules or chiral supramolecular assemblies at the plasmon-molecule interfaces is particularly of great interest, but remains elusive so far. Here, we construct chemically-adsorbed, optically-resonant complexes of chiral molecules and plasmonic nanoparticles featuring atomic control over the adsorption of molecules on nanoparticle surfaces. Interfacial charge transfer between molecules and metals is revealed to cause delocalized chiral molecular excitations and intermolecular couplings. The delocalization of molecular excitons significantly impacted the chiroptical responses, leading to spectral shifting, broadening, and signal intensity variations of circular dichroism. Furthermore, the dominant influence of interfacial charge transfer is identified compared to the near-field electromagnetic couplings. These findings provide new insights into the optical chirality at the plasmon-molecule interfaces and inspire approaches for the quantum manipulation of chiral molecules, opening up avenues for emerging applications.