The realization of the WH rules can be easily rationalized by inspecting the molecular orbital (MO) correlation diagram of the ring closure of 1,3,5-hexatriene. The diagram for the excited state conrotatory mechanism for 1,3,5-hexatriene to form cyclohexa-1,3-diene is shown in Figure . The singly occupied frontier MOs (FMOs) in the lowest excited state of the open form (f3 and f4 * ) correlate with the two orbital counterparts of the closed structure (p3 * and p2, respectively), i.e., the lowest excited states of reactant and product are correlated through the conrotatory pathway, providing an energetically favorable path for the photoreaction to occur. In other words, for the photoreaction to be feasible, the FMOs of the open form ground state must have the proper bonding and anti-bonding character, that is, the HOMO must possess π-type contributions from the two lateral double bonds that convert into single bonds hv, conrotatory upon photocyclization, while the LUMO must hold π*-type character from the central double bond that transforms into a single bond. Although this is relatively obvious when analyzing the MO diagram of simple prototype systems as in Figure , there are many examples in the literature where such chemical intuition is insufficient when analyzing the photoswitching activity of more complex molecular scaffolds. to produce the closed form. Whether electrocyclization can occur, however, is strongly dictated by the specific structural and electronic properties of the substituents at the central ethene unit (backbone substitutions) and at the two thiophenes (side groups). Note that the description of photocyclization reactions via FMO energy diagrams constitute a substantial simplification, which might not capture the mechanistic details of the photochemical dynamics. Still, simple but intuitive arguments can be very useful, as exemplified by the WH rules.