Finally, we must explicitly comment on a key difference between photochemically and chemically driven processes; whereas chemically driven processes take place on a single, continuous potential energy surface, photochemical processes take place on at least two surfaces, the ground and excited states. 55 Thus, whereas chemically driven processes are "downhill" over the full cycle (Figure ), 56 photochemical processes begin with a large "uphill" step (Figure ). Subsequent steps ultimately dissipate most of the energy provided by the incident photon as heat. We make this point explicitly because, as commented above, it is often said that a monomer unit becomes "activated" by reaction with a "fuel" molecule to drive self-assembly, alongside energy diagrams in which the reaction of the monomer with the "fuel" is shown as energetically uphill. This misleading image seems to be inspired by equivalent photochemical processes; in the schematic assembly of A and B it is reasonable to describe B as activated compared with A. However, this is not the case in the context of chemically driven processes and can serve to confuse. The above brief discussion glosses over many details relating to photochemical processes, including the relationship between the operation of photochemically driven motors and the "power stroke" associated with their rotation, which is irrelevant in the case of chemically driven systems. All we intend to highlight here is that, despite these added complications, the key question remains; is the photochemical process coupled to the process of interest or not? If it is not, then this part of the system is unaffected and "at equilibrium".