Self-Assembly of Transition-Metal-Based Macrocycles Linked by Photoisomerizable Ligands:  Examples of Photoinduced Conversion of Tetranuclear−Dinuclear Squares

A series of hetero- and homometallic square complexes bridged by a photoactive 4,4‘-azopyridine (AZP) or 1,2-bis(4-pyridyl)ethylene (BPE) ligand, cyclobis{[cis-(dppf)M](μ-L)2(fac-Re(CO)3Br)}(OTf)4 (M = Pd, L = trans-AZP (5); M = Pt, L = trans-AZP (7); M = Pd, L = trans-BPE (8); M = Pt, L = trans-BPE (10)), cyclo{[cis-(dppf)M](μ-L)2(fac-Re(CO)3Br)}(OTf)2 (M = Pd, L = cis-AZP (6); M = Pd, L = cis-BPE (9)), [cis-(dppf)Pd(μ-trans-AZP)]4(OTf)8 (11), and [cis-(dppf)Pd(μ-cis-AZP)]2(OTf)4 (12), where dppf is 1,1‘-bis(diphenylphosphino)ferrocene and OTf is trifluoromethanesulfonate anion, were prepared by thermodynamically driven self-assembly processes. The photophysical and photochemical properties of these complexes have been investigated, and all of them show a lack of luminescence in room temperature solution. Upon irradiation at 313 or 366 nm, Pd(II)−Re(I)-containing tetranuclear squares 5, 8, and 11 undergo photoisomerization and convert to their corresponding dinuclear complexes 6, 9, and 12, whereas Pt(II)−Re(I)-based squares 7 and 10 show only slow square disassembling processes. The tetranuclear squares can be fully recovered by heating the photoisomerized solution for several hours.