  The formation of hybrid light-molecule states (polaritons) offers a new
strategy to manipulate the photochemistry of molecules. To fully exploit its
potential, one needs to build a toolbox of polaritonic phenomenologies that
supplement those of standard photochemistry. By means of a state-of-the-art
computational photochemistry approach extended to the strong-coupling regime,
here we disclose various mechanisms peculiar of polaritonic chemistry: coherent
population oscillations between polaritons, quenching by trapping in dead-end
polaritonic states and the alteration of the photochemical reaction pathway and
quantum yields. We focus on azobenzene photoisomerization, that encompasses the
essential features of complex photochemical reactions such as the presence of
conical intersections and reaction coordinates involving multiple internal
modes. In the strong coupling regime, a polaritonic conical intersection arises
and we characterize its role in the photochemical process. Our chemically
detailed simulations provide a framework to rationalize how the strong coupling
impacts the photochemistry of realistic molecules
