Unveiling the role of dark states in dynamic control of azopyrrole photoisomerization by light-matter interaction.

Strong light-matter interactions demonstrated considerable potential to control photochemical reactions. Here, we coupled a single cavity mode to the electronic S₀-S₁ transition of azopyrrole E and Z-isomers. This allows us to observe the impact on the photoisomerization process "on-the-go", i.e., capturing a sharp transition in the kinetics when moving from strong to weak coupling. Pumping either at the upper polaritonic state or the uncoupled population shows an acceleration of the photoisomerization process (strong to weak), whereas the opposite is observed when exciting the lower polaritonic state. Excellent correlation between spectral overlap and rate suggests that changes in photochemistry are mediated by relaxation via the dark state manifold. Remaining in the ultra-strong coupling regime affects the reaction kinetics, but without sharp transitions. Our experimental and theoretical findings underline that dynamic transitions between coupling domains might pave the way to a better understanding of how strong coupling modifies photoisomerization reactions.