In Fig. , a plasmonic array cavity (panel (i)) was used to modify the photochemistry of the photocatalytic decomposition of methyl orange from Ref. 211. The methyl orange can become reactive when the adjacent TiO 2 undergoes UV irradiation and ionizes the methyl orange, which can then react with other radicals in solution and break down. The coupling of this pathway to the plasmonic array allows for the formation of waveguide-plasmon polaritons which can increase visible light absorption and decrease radiative damping, 211 which can alter the reaction rate. In order to control the decomposition reaction using a cavity, the array nature of the cavity (in panel c(i)) allows the lattice period, and thus the cavity frequency, to be adjusted. This ability to selectively modify the lattice period was used in panel c(ii) to demonstrate the frequency-dependent modification of a photocatalytic decomposition reaction rate. In addition to adjusting the cavity frequency and coupling strength, the pump excitation frequency can also be adjusted to selectively control reaction outcomes as demonstrated by the work shown in Fig. , adapted from Ref. 31. When the cavity-coupled system (i) was pumped at the frequency of the lower polariton instead of at the bare reactant frequency, the photoluminescence spectra was dominated entirely by the TICS product (ii-iv) instead of a mixture of reactant and product signals. This selectivity based on pumping frequency was further enhanced by increasing the molecule concentration and thus the light-matter coupling strength (ii-iv). Photochemistry has also been shown to be modifiable in the weak coupling regime in the work of Fig. , adapted from Ref. 116. The decrease in excitation lifetime (ii) due to the Purcell effect caused a reduction of population transfer from the singlet to triplet excited state (i) which ultimately reduced the rate of excited state tautomerization.