Suppression of photo-oxidation of organic chromophores by strong coupling to plasmonic nanoantennas

Abstract

Intermixed light-matter quasi-particles-polaritons-have unique optical properties owing to their compositional nature. These intriguing hybrid states have been extensively studied over the past decades in a wide range of realizations aiming at both basic science and emerging applications. However, recently, it has been demonstrated that not only optical but also material-related properties, such as chemical reactivity and charge transport, may be significantly altered in the strong coupling regime of light-matter interactions. We show that a nanoscale system, composed of a plasmonic nanoprism strongly coupled to excitons in a J-aggregated form of organic chromophores, experiences modified excited-state dynamics and, therefore, modified photochemical reactivity. Our experimental results reveal that photobleaching, one of the most fundamental photochemical reactions, can be effectively controlled and suppressed by the degree of plasmon-exciton coupling and detuning. In particular, we observe a 100-fold stabilization of organic dyes for the red-detuned nanoparticles. Our findings contribute to understanding of photochemical properties in the strong coupling regime and may find important implications for the performance and improved stability of optical devices incorporating organic dyes.

Fig. 1. Sketch and schematic diagram of photobleaching reaction in a strongly coupled system.

(A) Graphic sketch of the system under study. J-agg, J-aggregates. (B) Dark-field (DF) scattering spectra of the strongly coupled hybrid system (red), scattering spectrum from uncoupled J-aggregates (orange), and uncoupled individual plasmonic nanoprism (blue). The inset shows a SEM image of the corresponding nanoprism. Scale bar, 100 nm. a.u., arbitrary units. (C) Schematic diagram of a photobleaching reaction in the uncoupled molecular system and its possible modification in the strong coupling regime (light gray lines indicate upper and lower polaritonic states). ROS, reactive oxygen species; CT, charge transfer; ET, energy transfer.

Fig. 2. Photobleaching as a function of Rabi splitting.

(A to E) Evolution of the DF scattering spectra of strongly coupled hybrid systems with different initial Rabi splitting energies of 209, 200, 188, 170, and 154 meV, respectively. Insets show the corresponding SEM images of the plasmonic silver nanoprisms. Scale bars, 100 nm. (F) Uncoupled J-aggregates as a function of irradiation/bleaching time under broadband excitation. (G) Photobleaching kinetics for the strongly coupled hybrid systems with different Rabi splitting energies of 209 meV (red), 200 meV (brown), 188 meV (violet), 170 meV (green), and 154 meV (blue), compared to the uncoupled bare J aggregates (orange). (H) Photobleaching rates for hybrid systems versus uncoupled J-aggregates as a function of Rabi splitting energy. The inset shows a zoomed-in plot of the photobleaching rates for the coupled systems.

Fig. 3. Photobleaching as a function of plasmon-exciton detuning.

(A to D) Evolution of the DF scattering spectra of strongly coupled hybrid systems with time under broadband excitation for different blue (A and B) and red (C and D) detunings of plasmon resonance with respect to the exciton resonance of J-aggregates. Insets show corresponding SEM images of the individual silver nanoprisms. Scale bars, 100 nm. (E) Photobleaching kinetic curves and (F) the corresponding bleaching rates of the strongly coupled hybrid systems with different plasmon-exciton detunings. Note the logarithmic scale of the vertical axis in (F).

Fig. 4. Photobleaching as a function of excitation wavelength.

(A to C) DF scattering spectra of three identical hybrid systems at three different excitation resonances of 532, 568, and 640 nm, respectively. The insets in (A) to (C) show corresponding SEM images of the individual silver nanoprisms. Scale bar, 100 nm. (D) Photobleaching kinetics for the three nearly identical hybrid systems excited using 532-nm (green solid line), 568-nm (orange solid line), and 640-nm (red solid line) light sources. The corresponding dashed lines show the data for uncoupled J-aggregates. (E) Photobleaching rates of strongly coupled hybrid systems as a function of excitation wavelength for zero-detuned hybrids. The orange solid line shows a representative example of experimental DF spectrum for the coupled system. Calculated absorption and scattering spectra are also shown by the blue and yellow lines, respectively. FDTD, finite-difference time-domain; Exp., experimental scattering.

Fig. 5. Scheme of photobleaching mechanism.

(A) Schematic representation of a strongly coupled system and transitions between various states. (B) SF as a function of Rabi splitting. Circles represent experimental results and are normalized by the bleaching rate of uncoupled J-aggregates. The solid blue line represents the theoretical dependence of the SF on the Rabi splitting. (C) Schematic representation of the photobleaching process as a function of plasmon-exciton detuning. The visibility/transparency of arrows represents high/low probability of the corresponding transition, demonstrating that highly red-detuned particles are much more stable than highly blue-detuned ones. (D) SF as a function of plasmon resonance frequency. Circles represent experimental results and are normalized by the bleaching rate of uncoupled J-aggregates. The solid blue line represents the relative SF (normalized by the SF at zero detuning) calculated using Eq. 4 at Ω_R = 200 meV. Note the logarithmic scale and different vertical axes for the experimental and theoretical data.