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

TL;DR

This work combines high-field NMR, resonance and THz-Raman spectroscopy, and DFT to map the molecular geometry of TDBC in monomeric, J-aggregate, and plexcitonic states on Ag surfaces. It finds that plexcitons retain many aggregate-like vibrational features but exhibit interfacial disorder and partial planarization due to adsorption, with a minority population of monomer-like species bound to the surface. A key outcome is the identification of spectroscopic fingerprints, such as NOESY cross-peaks and low-/high-frequency Raman modes, that diagnose aggregation geometry and adsorption-induced distortion. These findings provide a structural benchmark for geometry-dependent photophysics in exciton–plasmon hybrids and inform design rules for tuning plexcitonic interfaces.

Abstract

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 strength of the interaction 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. In this work, we present a comprehensive structural characterization of one of the most widely studied plexcitons formed by the cyanine dye 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) and silver nanoprisms using a combination of NMR, THz-Raman spectroscopy, and DFT calculations. By comparing the signals from the monomeric and aggregated forms of TDBC with that 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 THz-Raman modes in the 10-400 cm$^{-1}$ region are sensitive indicators of aggregation geometry and adsorption. We find that isolated 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. This work establishes the molecular geometry of a prototypical TDBC-silver plexciton, providing a structural benchmark for understanding geometry-dependent photophysics in hybrid exciton-plasmon systems.

Figures (16)

• Figure 1: Possible geometries for J-aggregates are a) brickwork, b) ladder and c) staircase.
• Figure 2: The chemical strucuture of TDBC. We label only non-equivalent atoms, and use the $*$ symbol to denote mirror atoms. We do not show hydrogens explicitly and will refer to them in the text according to the number of the carbon they are attached to.
• Figure 3: Normalized absorption spectra of TDBC in water (blue), TDBC in methanol (red), Ag nanoparticles (black), and the plexciton (purple). Dotted lines indicate the laser excitation wavelengths. For Raman measurements, the red, pink, and blue lines correspond to excitation wavelengths of 647, 577, and 441 nm, respectively, while the green line denotes the THz-Raman excitation wavelength at 633 nm.
• Figure 4: 1H spectra of TDBC in methanol (red), in a 6:4 (v/v) MeOH–H$_2$O mixture (black) and in water (blue). The H$_2$O peak is indicated by an asterisk, while the MeOH peaks are marked with a circle. The insets show chemical shift changes and aggregation induced broadening.
• Figure 5: NOESY spectrum of TDBC in the aliphatic--aliphatic region. (a) Molecular structure of TDBC, with one half highlighted. (b) NOESY spectrum of TDBC in MeOH. (c) NOESY spectrum of TDBC in 6:4 (v/v) MeOH--H$_2$O, with the appearance of cross-peaks which were not present in the monomeric TDBC.