The coherence length of the Frenkel excitons (N_coh) is one of the most critical parameters governing many key features of supramolecular J-aggregates. Determining experimentally the value of N_coh is a nontrivial task since it is sensitive to the technique/method applied, causing discrepancies in the literature data even for the same chemical compound and aggregation conditions. By using a combination of different experimental techniques including UV-vis-NIR, fluorescence emission, time-resolved photoluminescence, and transient absorption spectroscopies, we determined N_coh values for J-aggregates of a cyanine dye. We found that the absorption spectroscopy alone - a widely used technique- fails in determining right value for N_coh. The correct approach is based on the modification of photoluminescence lifetime and nonlinear response upon aggregation and careful analysis of the Stokes shift and electron-phonon coupling strength. This approach revealed that N_coh of JC-1 J-aggregates ranges from 3 to 6.

Plasmonic catalysis in the colloidal phase requires robust surface ligands that prevent particles from aggregation in adverse chemical environments and allow carrier flow from reagents to nanoparticles. This work describes the use of a water-soluble conjugated polymer comprising a thiophene moiety as a surface ligand for gold nanoparticles to create a hybrid system that, under the action of visible light, drives the conversion of the biorelevant NAD⁺ to its highly energetic reduced form NADH. A combination of advanced microscopy techniques and numerical simulations revealed that the robust metal–polymer heterojunction, rich in sulfonate functional groups, directs the interaction of electron-donor molecules with the plasmonic photocatalyst. The tight binding of polymer to the gold surface precludes the need for conventional transition-metal surface cocatalysts, which were previously shown to be essential for photocatalytic NAD⁺ reduction but are known to hinder the optical properties of plasmonic nanocrystals. Moreover, computational studies indicated that the coating polymer fosters a closer interaction between the sacrificial electron-donor triethanolamine and the nanoparticles, thus enhancing the reactivity.

Plexcitonic systems based on metal nanostructures and molecular J-aggregates offer an excellent opportunity to explore the intriguing interplay between plasmonic excitations and excitons, offering unique insights into light–matter interactions at the nanoscale. Their potential applications in photocatalysis have prompted a growing interest in both their synthesis and the analysis of their properties. However, in order to construct a high-performing system, it is essential to ensure chemical and spectral compatibility between both components. We present the results of a study into a hybrid system, achieved through the coupling of gold nanobipyramids with organic molecules, and demonstrate the strengthened photochemical properties of such a system in comparison with purely J-aggregates. Our analysis includes the absorbance and photoluminescence characterization of the system, revealing the remarkable plexcitonic interaction and pronounced coupling effect. The absorbance spectroscopy of the hybrid systems enabled the investigation of the coupling strength (g). Additionally, the photoluminescence response of the J-aggregates and coupled systems reveals the impact of the coupling regime. Utilizing fluorescence lifetime imaging microscopy, we established how the photoluminescence lifetime components of the J-aggregates are affected within the plexcitonic system. Finally, to assess the photodegradation of J-aggregates and plexcitonic systems, we conducted a comparative analysis. Our findings reveal that plasmon-enhanced interactions lead to improved photostability in hybrid systems.

We investigate, both experimentally and theoretically, photoluminescence (PL) spectra of J-aggregates (J-agg) covering silver nanorods (AgNRs). When these hybrid nanoparticles (h-NPs), J-agg@AgNRs, hereafter referred to as h-NPs, are deposited on the surface of a spherical microcavity (MC), their non-resonantly excited PL spectra show a series of sharp resonances in the broad spectral range of 550–650 nm, characterized by Fano-type lineshapes. We develop a semi-phenomenological model based on coupled-oscillator equations, which explains the observed spectra of the h-NPs/MC system. In particular, we show that the complex Fano-type lineshapes are due to effective interaction of various whispering gallery modes of the microsphere, coupled to the h-NP emitters. Our experimental results and their understanding qualify the investigated hybrid structure as a sensitive and controllable system, suitable for applications.