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 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.

💡 Research Summary

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This paper presents a comprehensive structural investigation of the prototypical plexcitonic system formed by the cyanine dye 5,5′,6,6′‑tetrachloro‑1,1′‑diethyl‑3,3′‑di(4‑sulfobutyl)‑benzimidazolocarbocyanine (TDBC) and silver nanoprisms. Plexcitons are hybrid light‑matter states that arise when the electronic transition of an emitter couples strongly to a localized surface plasmon resonance (LSPR) of a metal nanoparticle, giving rise to upper and lower polariton branches. While the optical signatures of strong coupling (Rabi splitting, polariton dispersion) have been extensively studied, the actual molecular geometry at the metal interface—crucial for determining coupling strength, disorder, and energy‑transfer pathways—has remained largely unknown.

The authors combine three complementary techniques: high‑field ¹H NMR (including NOESY), resonance Raman and THz‑Raman spectroscopy, and density‑functional theory (DFT) calculations (B3LYP‑def2‑SVP‑D3BJ) together with periodic slab simulations (VASP RPBE). They examine three distinct states of TDBC: isolated monomers in methanol, J‑aggregates in water, and the Ag‑bound plexcitonic assemblies. By directly comparing the spectra of these reference states, they identify a set of spectroscopic fingerprints that report on molecular conformation, packing geometry, and adsorption‑induced distortions.

Key findings from NMR: In the monomeric solution the two sulfobutyl side chains (designated H₂ and H*₂) give rise to strong NOESY cross‑peaks with each other, indicating that both chains lie on the same side of the chromophore (an asymmetric conformation). In the J‑aggregate, these cross‑peaks disappear, and new cross‑peaks appear between H₂ and neighboring protons on adjacent molecules, revealing an up‑down alternating arrangement of the side chains (a symmetric conformation). The plexcitonic sample reproduces the J‑aggregate pattern, confirming that the aggregate adsorbs onto the silver surface without losing its alternating packing.

Raman and THz‑Raman results: Resonance Raman spectra (457–528 nm excitation) show the characteristic C=C stretch and CH deformation modes of TDBC, but the plexciton exhibits additional peaks in the 1450–1600 cm⁻¹ region that are absent in the isolated forms, indicating hybrid vibrational‑photonic modes. THz‑Raman (10–400 cm⁻¹) reveals pronounced low‑frequency modes at ~120, 190, and 250 cm⁻¹ that are strongly enhanced in both the J‑aggregate and the plexciton. DFT calculations assign these modes to (i) inter‑molecular slip‑stack motions, (ii) collective lattice vibrations of the aggregate, and (iii) metal‑induced charge‑transfer distortions. Notably, the 190 cm⁻¹ mode shifts by ~10 cm⁻¹ and gains intensity upon adsorption, directly linking the low‑frequency vibrational response to the strength of the exciton‑plasmon coupling.

Computational insights: Geometry optimizations confirm that the monomer adopts an asymmetric conformation (both side chains on the same side), while the lowest‑energy J‑aggregate conformer features alternating side‑chain orientation. Periodic slab calculations of TDBC adsorbed on Ag(111) give a binding energy of ~‑1.2 eV and a modest charge transfer (~0.15 e) from the dye to the metal. The average dye‑metal distance shortens from ~4.1 Å in the symmetric aggregate to ~3.2 Å in the asymmetric monomer, explaining the observed ~15 % increase in Rabi splitting for the latter. The calculations also reproduce the experimentally observed low‑frequency THz‑Raman modes, validating the assignment of these features to collective slip‑stack and metal‑induced motions.

The authors discuss the broader implications of structural disorder. The breadth and asymmetry of the NOESY and THz‑Raman peaks serve as quantitative markers of static disorder in side‑chain orientation and inter‑molecular spacing. According to recent Green’s‑function models, such disorder mixes bright polariton states with a manifold of dark excitons, leading to a “disorder‑induced crossover” from under‑damped coherent dynamics to over‑damped relaxation. The present work provides the first experimental route to measure this disorder directly, thereby enabling predictive control of dark‑state participation in plexcitonic systems.

In summary, the paper establishes: (1) the precise 3‑D geometry of TDBC monomers and J‑aggregates; (2) the structural rearrangement that occurs when the aggregate binds to a silver nanoparticle; (3) a set of spectroscopic signatures (NOESY cross‑peaks, low‑frequency THz‑Raman modes, and new resonance Raman features) that can be used as non‑destructive probes of plexcitonic structure; and (4) a clear correlation between molecular geometry, binding distance, and the magnitude of exciton‑plasmon coupling. These insights provide a much‑needed structural benchmark for designing plexcitonic devices with tailored coupling strengths, controlled disorder, and optimized energy‑transfer pathways, paving the way for advanced applications in polaritonic chemistry, nanoscale lasing, and quantum information processing.