Re(CO)5Cl readily reacts with pyridine-derivatized porphyrins to give molecular squares or planar dimers featuring fac-Re(CO)3(Cl) corners. Chelating ligands, in sequential combination with pyridine-derivatized porphyrins, can be used to obtain related cofacial porphyrin assemblies. Films and membranes displaying molecular-scale porosity can be generated from squares by: (1) van der Waals aggregation, (2) layer-by-layer assembly chemistry based on zirconium-phosphonate links, or (3) polymerization at liquid-liquid interfaces. In addition to molecular sieving, films, membranes, and free assemblies have been used for chemical sensing, oxidative catalysis, and light-to-electrical energy conversion. The assemblies have also been used to investigate fundamental aspects of light-induced energy and electron transfer.