TDBC-Silver Plexciton Overview

• TDBC-silver plexciton is a hybrid quantum system formed by strong coupling between TDBC J-aggregate excitons and silver nanoprisms, yielding mixed light–matter polaritonic states.
• The system exhibits sensitivity to molecular geometry, interfacial binding, and aggregation, which dictate its Rabi splitting and ultrafast energy transfer properties.
• Its complex exciton–plasmon coupling, disorder effects, and spectroscopic signatures provide practical benchmarks for optimizing nanophotonic device performance.

A TDBC-silver plexciton is a hybrid quantum system formed by strong coupling between the collective excitonic transition of a J-aggregate composed of the cyanine dye 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) and a localized surface plasmon resonance supported by a silver nanostructure (notably, Ag nanoprisms). The resultant plexcitonic states are characterized by mixed light–matter polaritonic eigenmodes with properties highly sensitive to the molecular geometry at the metal–dye interface, local electromagnetic field configuration, aggregation state, and structural disorder. These systems serve as benchmarks for studying tailored photophysics in nanoscale hybrid materials, with implications for ultrafast spectroscopy, nanophotonic devices, and strong-coupling-enhanced energy transfer mechanisms (Baños-Gutiérrez et al., 29 Jan 2026, Babaei et al., 2018, Huang et al., 17 Jan 2025).

1. Molecular Geometry and Interfacial Binding

The ground-state structural configuration of TDBC at a silver interface fundamentally determines the plexciton’s photophysical properties. Density-functional theory (DFT, B3LYP-D3/def2-SVP) optimizations reveal that the isolated TDBC monomer adopts an asymmetric conformation: the benzimidazole rings possess a dihedral angle $\phi_{\rm ring} = 57^\circ\pm1^\circ$, with both sulfobutyl chains positioned on the same side of the chromophore, minimizing steric hindrance and maximizing solvation in methanol. Key internal parameters are summarized below:

Parameter	Value	Comment
C–C (methine bridges)	1.38 Å	polymethine bond
C–N (benzimidazole)	1.35 Å	aromatic core
S–C (sulfobutyl)	1.78 Å	S–alkyl bond
O–C–C (sulfonate tail)	113°	tetrahedral sulfonate
$\phi_{\rm ring}$	57°	ring–ring dihedral

In aqueous media, TDBC forms J-aggregates with a slip-stacked, quasi-2D “staircase” motif. NOESY NMR cross-peaks (e.g., H\textsubscript{11}–H\textsubscript{13} and H\textsubscript{11}–H\textsubscript{14}) reveal an alternating up–down side-chain arrangement enforced by π–π stacking (mean $d_{\pi-\pi}\approx3.6$ Å; lateral slip ≈4 Å). Upon adsorption onto a silver (Ag(111)) nanoprism surface, the terminal sulfonate moieties of TDBC J-aggregates form strong bidentate O-Ag bonds ($d_{\rm O–Ag}\approx2.32\pm0.05$ Å; adsorption energy ≈ –22 kcal mol⁻¹ per dye). At the interface, the aggregate motif is preserved but the π-system becomes more planar, facilitating optimal exciton–plasmon overlap (Baños-Gutiérrez et al., 29 Jan 2026).

2. Exciton–Plasmon Coupling Framework

The low-energy manifold of a TDBC–silver plexciton is captured by the Hamiltonian: $$H = \hbar\omega_p a^\dagger a + \sum_{j=1}^N \hbar\omega_x b^\dagger_j b_j + \sum_{j=1}^N g_j \left(a^\dagger b_j + b_j^\dagger a\right)$$ where $a^\dagger, a$ describe the plasmon mode at frequency $\omega_p$ (Ag nanoprism: $\approx2$ eV), and $b_j^\dagger, b_j$ label the bright-state J-aggregate transitions at $\omega_x \approx1.95$ eV. Coupling $g_j$ depends on the transition dipole $\mu_j$, orientation angle $\theta_j$ (relative to local field), and distance $d_j$ from the metal, scaling as $$g_j \propto \sqrt{ \hbar\omega_p/(2\epsilon_0 V_{\rm eff}) } \cos\theta_j \exp(-k\,d_j)$$.

The collective coupling $g = \sqrt{ \sum_j g_j^2 }$ produces two polariton branches with energies

$$E_\pm = \tfrac12\hbar(\omega_p + \omega_x) \pm \tfrac12 \sqrt{4g^2 + \hbar^2(\omega_p - \omega_x)^2}$$

Strong-coupling is realized for $g\gtrsim$ 100 meV, giving observable Rabi splitting (Baños-Gutiérrez et al., 29 Jan 2026, Babaei et al., 2018). Disorder and deviations from ideal aggregation modulate $g_j$ through distributions in $\theta_j$ ($90^\circ\pm20^\circ$) and $d_j$, with exponential quenching for dyes $>$5 Å from the metal surface.

3. Spectroscopy and Photophysical Signatures

NMR and Raman Spectroscopy

Distinct NMR and THz–Raman fingerprints elucidate the molecular packing and perturbations at the metal interface. The isolated monomer’s $^1$H NMR features sharp aromatic (H\textsubscript{1,6,9} ≈ 7.31 ppm) and separate aliphatic resonances, while J-aggregation broadens all signals, shifts the aromatic region by Δδ ≈ –0.02 ppm, and introduces new NOESY cross-peaks (Table below).

Environment	Cross-peaks	Signature
Monomer	none	Asymmetric side chains
J-aggregate	H\textsubscript{11}–H\textsubscript{13/14}	Alternating up–down geometry
Plexciton	Broad, congested	Large interface complex, microheterogeneity

THz–Raman bands in the 10–400 cm⁻¹ range provide packing and periodicity information:

$\nu$ (cm⁻¹)	Monomer	J-aggregate	Plexciton
140	1.00	0.99	1.08
221, 274	–	1.40, 0.66	–
324	–	5.26	3.79
339, 355	–	0.58, 1.85	1.26, –

The broadening and suppression of certain bands (e.g., 140 cm⁻¹, FWHM up 30%) in the plexciton confirm the disruption of aggregate periodicity upon metal binding (Baños-Gutiérrez et al., 29 Jan 2026).

Plexciton Dispersion and Wave Propagation

The dispersion relations, deduced via transfer-matrix calculations in the Kretschmann configuration for layered Ag/TDBC interfaces, reveal two antisymmetric branches: $$\omega_\pm(k_x) = \frac{\omega_{\rm spp}(k_x) + \omega_x}{2} \pm \frac{1}{2} \sqrt{ \left[\omega_{\rm spp}(k_x) - \omega_x\right]^{2} + {\Omega_R}^2 }$$ The Rabi splitting $\hbar\Omega_R$ ranges from 204–378 meV, corresponding to periods $T_R = 2\pi/\Omega_R = 11$–20 fs. Propagation characteristics are as follows:

Parameter	Range	Notes
Phase velocity	0.4–0.9 c	LP (lower polariton) fastest
Propagation ($L_p$)	0.4–6 μm	Longer for LP than UP

Poynting-vector analysis indicates that energy is exponentially localized at the Ag/TDBC interface, with penetration depths of 10–15 nm in Ag, and 90–180 nm in the excitonic layer (Babaei et al., 2018).

4. Theoretical and Computational Models

A detailed quantum description employs an effective two-level system (TDBC exciton) coupled to a lossy plasmonic mode, with additional vibrational (phonon) interactions. The Hamiltonian includes electronic ($\omega_x$), plasmonic ($\omega_p$), exciton–plasmon (coupling $g$), and exciton–vibration coupling ($\lambda_k$) terms: $$H = \hbar\omega_x |e\rangle\langle e| + \hbar\omega_p a^\dagger a + \hbar g\,\sigma_x (a + a^\dagger) + |e\rangle\langle e| \sum_k \hbar\lambda_k (b_k + b_k^\dagger) + H_{\text{baths}}$$ The hierarchical equations of motion (HEOM) approach enables non-perturbative treatment of the dissipative system. Key optical observables—linear absorption and two-dimensional electronic spectra (2DES)—exhibit a Fano-to-Rabi evolution as $g/\gamma_p$ increases. For $g \gg \gamma_p$ (e.g., $g \sim 1\,200$ cm⁻¹, $\gamma_p \sim 800$ cm⁻¹), pronounced Rabi splitting ($\Delta = 2g \sim 2,400$ cm⁻¹) appears, with vibrational bath effects filling in the dip and broadening line shapes (Huang et al., 17 Jan 2025).

2DES in the strong-coupling regime displays diagonal peaks at $$(\omega_\tau,\omega_t) \approx (\omega_\pm,\omega_\pm)$$ with oscillatory “breathing” (period $T_{\text{Rabi}} \sim 2.6$ fs), a hallmark of coherent Rabi oscillations between polaritonic branches.

5. Role of Structural Disorder and Surface-Induced Effects

Substantial energetic and orientational disorder arises due to distributions in intra-core torsions (32°–68°), side-chain orientations, and inhomogeneous coupling strengths ($g_j$). This heterogeneity, analyzed via DFT and modeled by Green’s function formalism, leads to broader absorption features (linewidth $\sim$0.15 eV) and disorder broadening ($\sigma \sim 50$ meV). Experimentally, a Rabi splitting of $\sim$0.36 eV is observed, notably wider than in microcavity polaritons. The presence of uncoupled, monomer-like TDBC subpopulations—which act as relaxation traps—is observed by selective Raman excitation (441 nm), further reducing exciton coherence lengths and polariton lifetimes (10–50 fs) within the plexcitonic assembly (Baños-Gutiérrez et al., 29 Jan 2026).

6. Photophysical Functions, Energy Transfer, and Applications

Preservation of J-aggregate geometry at the interface sustains long-range exciton delocalization (∼10–20 monomer units), yielding strong ensemble coupling ($g\sim100$ meV) and robust upper/lower polariton branch formation. However, planarization and partial loss of aggregate order near the metal surface promote ultrafast nonradiative decay (via charge transfer to Ag), restricting polariton coherence lifetimes to tens of femtoseconds.

Potential design strategies for optimizing device performance entail:

• Surface passivation/co-adsorbed ligands: Suppress monomeric species at the interface, preserving extended J-aggregate order.
• Silica or polymer capping: Enhance aggregate rigidity and reduce disorder-induced linewidth broadening.
• Nanoparticle anisotropy tuning: Manipulate field-enhancement vs. aggregate structure (edge versus face adsorption), with implications for plexcitonic field localization and spectral purity.

These engineering approaches facilitate rational control over energy transfer, coherence, and relaxation pathways in plexcitonic materials, with implications for ultrafast photonics, quantum information, and energy harvesting (Baños-Gutiérrez et al., 29 Jan 2026).

7. Summary and Structural Benchmarks

The TDBC–Ag plexciton is a prototypical model for understanding how site-specific chemical structure—dihedral angles, side-chain configurations, interfacial adsorption—and mesoscale order/disorder modulate hybridization and relaxation in strongly coupled exciton–plasmon systems. The identification of characteristic NMR, Raman, and THz–Raman modes, together with disorder-driven broadening and photophysical signatures, establishes concrete structural benchmarks for advancing molecular-level design of plexcitonic platforms with tailored optical response, lifetimes, and energy transfer efficiency (Baños-Gutiérrez et al., 29 Jan 2026, Huang et al., 17 Jan 2025, Babaei et al., 2018).