cis–trans Photoisomerizations of 2-(9-anthrylmethylene)-1-indanones were carried out in the solid state. The photoisomerizations proceeded without destruction of the crystal lattices. The crystal structural changes accompanying the photoisomerizations were observed by X-ray diffraction analysis. From the structural correlation between the reactants and products, the single-crystal isomerizations were proved to proceed through one bond flip (OBF), where the indanonyl moiety of the molecule rotates around the isomerizing double bond.