Nanoscale Plasmon–Exciton Strong Coupling‐Enhanced Photocatalysis: Role of Optical Dark States

ABSTRACT The interaction between plasmon and molecular transitions has emerged as a cornerstone in nanophotonics, offering fundamental insights and enabling plethora of practical applications. In this study, we harnessed the unique properties of strongly coupled plexcitons to overcome key limitations in plasmonic photocatalysis, particularly faced due to inadequate availability of hot charge carriers because of their ultrafast recombination and short mean free path. Using gold nanorods and their plexcitonic hybrids with cyanine dye, we demonstrate significantly enhanced catalytic activity in the case of the plexciton system compared to the bare plasmonic counterpart. Ultrafast nonlinear spectroscopy revealed that in the case of plexciton, the excitation energy from the plexcitonic branches transitions into optical dark states, which act as reservoirs, gradually replenishing polariton modes. This mechanism mitigates the rapid charge carrier recombination typically observed in plasmonic photocatalysis, extending the availability of hot charge carriers. This demonstration that strong light–matter coupling can directly manipulate catalytic reaction dynamics in free‐space nanosystems, without relying on complex cavity architectures, represents a major step toward the practical realization of light–matter hybrid‐driven chemistry. The prospect of chemistry under plexciton strong coupling is enormous and opens new possibilities to realize complex redox processes in a sustainable way.