UV‐Vis Spectra of π‐π‐Molecular Complexes. XV. Photochemical (E)‐(Z)‐Isomerization of Substituted Stilbenes and Stilbene‐like Compounds in the Presence of π‐AcceptorsThe reversible photochemical (E)‐(Z)‐isomerization of 1,2‐diarylsubstituted ethylenes in the presence of π‐acceptors in liquid solutions and in PMMA‐films leads to the equilibrium states of the relevant molecular complexes. The longest wavelength absorption maxima of the molecular complexes of discrete (E)‐ and (Z)‐donors, respectively, in the visible range of the UV‐Vis‐spectrum have values for Δλmax ⩽ 125 nm. These potential photochromic properties are impaired by a simultaneous irreversible photodehydrocyclization process, in which the 1,2‐diarylethylenes are transformed into their dihydro‐cyclo‐products, followed by oxidation by a π‐acceptor, forming the respective dehydrocyclization products. This reaction can be useful in the formation of condensed hydrocarbons and their derivatives.