AbstractThe single‐crystal‐to‐single‐crystal (SCSC)EŕZphotoisomerization of TA in the supramolecular solid CECR‐[Zn(TA)2(bpy)]·H2O (CECR =C‐ethylcalixresorcinarene, HTA = tiglic acid, and bpy = 2,2′‐bipyridine, is induced by 458 nm light, indicating a red‐shift of the photo‐active wavelength on introduction of the aromatic bpy ligand compared with the previously studied reaction of CECR‐[Zn(TA)2(H2O)2] 4H2O. Theoretical calculations show that the initial excitation involves the bipyridyl ligand, which acts as an intramolecular photosensitizer for the isomerization process. The reaction is topotactic and illustrated