Molecular Control of Light–Matter Interactions in Plasmonic Nanogap Cavities

Plasmonic nanogap cavities enable extreme confinement of optical fields, and consequently enhanced light-matter interactions. However, light-matter coupling of molecules within these structures remains difficult to control, because the coupling strength is sensitive to the orientation of the molecular transition dipole moment relative to the cavity field. Here, we demonstrate a molecular design strategy for engineering preferential dipole alignment in nanoparticle-on-mirror plasmonic nanogap cavities. A series of ruthenium(II) polypyridyl emitters, bearing diazonium-forming surface anchoring groups at different positions on the ligands, were electrochemically grafted onto aluminum oxide-coated gold surfaces and integrated into plasmonic nanogap cavities. The binding group placement was designed to impose distinct transition dipole angles relative to the surface and, consequently, relative to the dominant cavity electric field. Photoluminescence and time-correlated single-photon counting measurements showed that cavity integration produces strong emission enhancement and large Purcell factors, with maximum Purcell enhancement increasing systematically as the estimated transition dipole moment becomes more closely aligned with the cavity field. Finite-element electromagnetic simulations reproduced the observed angular trend and demonstrated that spatial heterogeneity in the nanogap field gave rise to broad distributions of excited-state decay rates. These results establish molecular design as a practical strategy for engineering light–matter interactions and identify surface attachment chemistry as a scalable route to tuning light–matter coupling in plasmonic nanogap cavities.