Electromagnetic vs Chemical Interfacial Interactions at the Single-Molecule-Confined Sub-nanometer Molecule-Metal Gap: A Solution-Phase Chiroptical Study.

Molecular chirality can be strongly influenced by plasmonic environments. Observing and distinguishing the physical and chemical origins at plasmon-molecule interfaces, especially at the single-molecule level, have remained a great challenge. Herein, through DNA-mediated sub-nanometer assembly, we achieved the first observation of plasmon-altered circular dichroism from single-molecule chiroplasmonic complexes. Experimental and theoretical results indicated that, under such sub-nanometer confinement, the chiroptical response is predominantly governed by the coupling between the plasmonic field gradient and the molecule's extended electronic distribution. We devised a critical strategy contrasting the chiroptical response under DNA-controlled physical proximity with that induced by direct chemical binding using atomic-site modification as a probe. This comparison allowed us to differentiate the roles of electromagnetic effects from chemical interfacial interactions in reshaping molecular chirality. This work offers a generalizable framework for studying competing mechanisms at plasmon-molecule interfaces, paving the way for the advancement of chiral photonics and optoelectronics at the atomic scale.