Self-assembly triangular and square rhenium(I) tricarbonyl complexes: A comprehensive study of their preparation, electrochemistry, photophysics, photochemistry, and host-guest properties

A series of self-assembly macrocyclic compounds featuring fac-Re(CO)(3)X (X = Cl or Br) as corners and linear bipyridyl bridging ligands have been prepared and characterized. Depending on the lengths as well as the bonding angles of the bridging ligands, the resulting geometries of these macrocyclic complexes are squares {[ClRe(CO)(3)(mu-DPB)](4) (3) and [ClRe(CO)(3)(mu-AZP)](4) (4)}, triangles {[BrRe(CO)(3)(mu-BPDB)](3) (6) and [BrRe(CO)(3)(mu-BPDDB)](3) (7)}, or a dimeric species {[ClRe(CO)(3)(mu-BPET)](2) (5)}. A general mechanism for the self-assembly processes involving soluble intermediates is proposed. The photophysical properties of these macrocyclic compounds are dominated by the characteristics of the lowest excited states which vary from metal-to-ligand charge transfer (MLCT) to ligand-localized pi --> pi* or n --> pi* transitions for the different molecules. Square 3 and triangles 6 and 7 are luminescent in room-temperature solution while square 4 and dimer 5 are nonemissive. An energy transfer mechanism from the MLCT excited state to the lowest nonemissive n --> pi* excited state is attributed to the lack of emission in square 4. The emission from square 3 is assigned to (MLCT)-M-3 character. In the cases of triangles 6 and 7, emissions from the (1)pi - pi* state were observed, as evidenced by their short lifetimes and structured emission bands. The large strain imposed on the triangular structures of 6 and 7 results in these molecules being photoactive. Photolysis of 6 or 7 at 313 nm is observed to break the triangular structure to form a polymeric structure. Square 4 exhibits reversible multielectron redox properties. Square 3 is also demonstrated to be a very effective host for nitro-substituted aromatic compounds.