Metal-ligand directed assembly of copper(II) ions and chelidonate (CDO) ligands into a metal-organic assembly has resulted in the proper orientation and positioning of CDO ligands for [2 + 2] photodimerization in the solid state. Upon exposure to ultraviolet light, pairs of adjacent CDO ligands photoreact via single-crystal-to-single-crystal cycloaddition to form novel cage dimers within the metal-organic assembly, which can be isolated successively as the organic sodium salt.