Molecular structure, binding, and disorder in TDBC-Ag plexcitonic assemblies

Plexcitonic assemblies are hybrid materials composed of a plasmonic nanoparticle and molecular or semiconducting emitters whose electronic transitions are strongly coupled to the plasmonic mode. This coupling hybridizes the system modes into upper and lower polariton branches. The interaction strength depends on the number of emitters and on their orientation and spatial arrangement relative to the metallic surface. These structural factors have profound consequences for the ensuing photoexcited dynamics. Despite the extensive spectroscopic work on plexcitonic systems, direct understanding of the molecular geometry at the metal interface remains limited. We present a comprehensive structural characterization of a model plexciton formed by the cyanine dye 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) and silver nanodisks using NMR, THz-Raman spectroscopy, and density functional theory calculations. By comparing the signals from the monomeric and aggregated forms of TDBC with those of the plexciton, we identify shared spectral fingerprints that reveal how molecular packing is modified when the aggregate adsorbs on the silver surface. We observe Raman modes specific to plexciton systems and identify NOESY cross-peaks in the aliphatic region that, along with several Raman modes, are sensitive indicators of aggregation geometry and adsorption. We find that TDBC monomers adopt an asymmetric conformation in which both sulfobutyl chains lie on the same side of the chromophore, while J-aggregates adopt a symmetric up-down alternation of the chains from molecule to molecule, which becomes distorted and loses long range periodicity when adsorbed on Ag nanodisks. This work constrains the molecular geometry and interfacial arrangement of a prototypical TDBC-silver plexciton, providing a structural benchmark for understanding geometry-dependent photophysics in exciton-plasmon systems.