  Manipulation of chemical reactivity often involves changing reagents or
environmental conditions. Alternatively, strong coupling between light and
matter offers a way to tunably hybridize their physicochemical properties and
thereby change reaction dynamics without synthetic modifications to the
starting material. Here, we theoretically design a polaritonic (hybrid
photonic-molecular) device that supports ultrafast tuning of reaction yields
even when the catalyst and its reactant are spatially separated across several
optical wavelengths. We demonstrate how photoexcitation of a `remote catalyst'
in an optical microcavity can control photochemistry of a reactant in another
microcavity. Harnessing delocalization across the spatially separated compounds
that arises from strong cavity-molecule coupling, this intriguing phenomenon is
shown for the infrared-induced \textit{cis} $\rightarrow$ \textit{trans}
conformational isomerization of nitrous acid (HONO). Indeed, increasing the
excited-state population of the remote catalyst can enhance the isomerization
efficiency by an order of magnitude. The theoretical proposal reported herein
is generalizable to other reactions and thus introduces a versatile tool to
control photochemistry.
