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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.

Electronic absorption, emission, and photochemical data are reported for a series of M(CO)4L complexes, where M = Cr, Mo, or W and L = 2,2'-bipyridine, 1,10-phenanthroline, or a derivative diimine ligand. Low-energy ligand field (LF) and intense metal-to-ligand charge-transfer (MLCT) transitions are observed in the electronic absorption spectra. The energy positions of the MLCT transitions are extremely sensitive to the nature of ligand substituent and solvent medium. Each complex exhibits dual emission features at 298 K in the 500-850-nm region and two low-lying Mx*(L) transitions are implicated in the radiative decay process. Quantum efficiencies for photosubstitution ( ) have been determined following excitation into the low-lying excited states. The photoreaction efficiences depend substantially on the irradiation wavelength; e.g., for W(CO)4(bpy) in benzene LF excitation at = 395 nm yields = 1.2 X 10-2, whereas MLCT excitation at = 514 nm yields = 5.4 X 10'1 2345 6.Photosubstitution data indicate that a LF state is largely responsible for the photochemistry in these M(CO)4L complexes. The photoefficiencies following MLCT excitation at 514 nm are only slightly temperature-dependent, indicating that either the MLCT state is intrinsically photoactive or another excited state lies close in energy and contributes to the photochemistry. The suggestion of photoreaction from the low-lying LF triplet state ( -* 3E) is discussed. An excited-state scheme relating the photochemical and emission data is presented. assigned to originate from a low-energy MLCT state of considerable triplet character.8 The emission was noted to be extremely (1) (a) Chaisson, D.