No abstract

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 (µ-DPB)] 4 (3) and [ClRe(CO) 3 (µ-AZP)] 4 (4)}, triangles {[BrRe(CO) 3 (µ-BPDB)] 3 (6) and [BrRe(CO) 3 (µ-BPDDB)] 3 (7)}, or a dimeric species {[ClRe(CO) 3 (µ-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 π f π* or n f π* 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 f π* excited state is attributed to the lack of emission in square 4. The emission from square 3 is assigned to 3 MLCT character. In the cases of triangles 6 and 7, emissions from the 1 π-π* 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.

A series of transition-metal-containing molecular "loops" and "squares" has been prepared via a directed-assembly approach and characterized. The molecular loops were prepared from the reaction of cis-(PEt(3))(2)Pt(OTf)(2) with bis(4-pyridyl)-functionalized free-base salen-type ligands. Zn(II)-metalation of the salen-type ligands in the molecular loops converts the loops to molecular squares. Alternatively, the squares can be obtained by the directed assembly of cis-(PEt(3))(2)Pt(OTf)(2) and bis(4-pyridyl)-functionalized Zn(II)-salen-type ligands. A concentration-dependent dynamic equilibrium between cyclic species was observed when bis(3-pyridyl)-functionalized free-base salen-type ligand was employed in the reaction. Zn(II) or Cr(III) metalation of the free-base ligand shifted the equilibrium to the single dimeric species. The incorporation of multiple reactive metal sites into a single, cavity-containing supramolecular structure points toward catalytic applications for these new assemblies.

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Two structurally simple and easily synthesized luminescent anion receptors featured with an amide-type anion binding site and rhenium(I) tricarbonyl pyridine signaling units have been developed, and they display outstanding sensitivity and selectivity toward a variety of anionic species. These complexes are highly emissive in solution. Upon anion binding, the emission intensity was significantly quenched. The sensitivities of these complexes are so high that the emission intensity can be effectively quenched by as much as 10% even in the presence of only 10(-8) M cyanide or fluoride anions. The ability of formation of intramolecular hydrogen bonding between the amide protons and central pyridine is believed to be responsible for the observed high selectivity.