  Intermixed light-matter quasiparticles - polaritons - possess unique optical
properties owned 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. Here, we show that a nanoscale system, comprised of a plasmonic
nanoprism strongly coupled to excitons in J-aggregated form of organic
chromophores, experiences modified excited state dynamics and therefore
modified photo-chemical 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.
