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The synthesis, structure, and photophysical properties of a new family of trinuclear CuRe2 chromophore-quencher complexes having the general form [Cu(pyacac)2(Re(bpy')(CO)3)2](OTf)2 (where pyacac = 3-(4-pyridyl)-acetylacetonate and bpy' = 4,4'-5,5'-tetramethyl-2,2'-bipyridine (tmb, 1), 4,4'-dimethyl- 2,2'-bipyridine (dmb, 2), 2,2'-bipyridine (bpy, 3), 4,4'-dichloro-2,2'-bipyridine (dclb, 4), and 4,4'-diethylester- 2,2'-bipyridine (deeb, 5)) are reported. Time-resolved emission data acquired in room-temperature CH2CI2 solutions revealed excited-state lifetimes of 14.9 ± 0.7, 8.1 * 0.4, 8.2 ± 0.4, 5.6 ± 0.3, and 5.0 ± 0.3 ns for complexes 1-5, respectively. The emission in each case is assigned to decay of the Re'-based 3MLCT excited state; the lifetimes are all significantly less than the corresponding BeRe2 model complexes, which were also prepared and characterized. Electron transfer was found to be significantly endothermic for all five CuRe2 complexes: this fact, coupled with the ca. 10 Å donor-acceptor distance and favorable spectral overlap between the 3MLCT emission profile and absorptions of the Cu" center, implicates dipolar energy transfer as the dominant quenching pathway in these compounds. Gaussian decorivolution of the ground- state absorption spectrum of Cu(phacac)2 (phacac = 3-phenylacetylacetonate) allowed for a differential analysis of the spectral overlap between the donor emission spectra and the two observed ligand-field transitions of the Cu" ion. Energy transfer was found to occur preferentially to the lower energy ligand-field band due primarily to more favorable dipole orientation as measured by the κ2 term from Färster theory. These results, supported by time-dependent DFT calculations on Cu(phacac)2, indicate enhanced dipolar coupling to the dxz→dxy, transition of the Cu" center and thus represent an orbitally specific energy-transfer process occurring in this system. [ABSTRACT FROM AUTHOR]

The syntheses, physical, and photophysical properties of a family of complexes having the general formula [M2(L)(mcb)(Ru(4,4′-(X)2-bpy)2)](PF6)3 (where M = MnII or ZnII, X = CH3 or CF3, mcb is 4′-methyl-4-carboxy-2,2′-bipyridine, and L is a Schiff base macrocycle derived from 2,6-diformyl-4-methylphenol and bis(2-aminoethyl)-N-methylamine) are described. The isostructural molecules all consist of dinuclear metal cores covalently linked to a RuII polypyridyl complex. Photoexcitation of [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)]-(PF6)3 (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the ³MLCT excited state of the RuII chromophore but with a lifetime (τobs = 5.0 ± 0.1 ns) and radiative quantum yield (Φr ≈ 7 × 10-4) that are significantly attenuated relative to the ZnII model complex [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6) (τobs = 730 ± 30 ns and Φr = 0.024, respectively). Quenching of the ³MLCT excited state is even more extensive in the case of [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3), whose measured lifetime (τobs = 45 ± 5 ps) is >104 shorter than the corresponding model complex [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (τobs = 1.31 ± 0.05 µs). Time-resolved absorption measurements on both Mn-containing complexes at room-temperature revealed kinetics that were independent of probe wavelength; no spectroscopic signatures for electron-transfer photoproducts were observed. Time-resolved emission data for complex 4 acquired in CH2Cl2 solution over a range of 200-300 K could be fit to an expression of the form knr = k0 + A·exp{-ΔE7kBT} with k0 = 1.065 ± 0.05 × 107 s-1, A = 3.7 ± 0.5 × 1010 s-1, and ΔE= 1230 ± 30 cm-1. Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a reorganization energy of λ 0.4-0.5 eV which is too small to be associated with charge separation in this system. This result coupled with the lack of enhanced emission at temperatures below the glass-to-fluid transition of the solvent and the absence of visible absorption features associated with the Mn2II core allows for a definitive assignment of Dexter transfer as the dominant excited-state reaction pathway. A similar conclusion was reached for complex 3 based in part on the smaller driving force for electron transfer (ΔG0ET = -0.1 eV), the increase in probability of Dexter transfer due to the closer proximity of the donor excited state to the dimanganese acceptor, and a lack of emission from the compound upon formation of an optical glass at 80 K. Electronic coupling constants for Dexter transfer were determined to be ~10 cm-1 and ~0.15 cm-1 in complexes 3 and 4, respectively, indicating that the change in spatial localization of the excited state from the bridge (complex 3) to the periphery of the chromophore (complex 4) results in a decrease in electronic coupling to the dimanganese core of nearly 2 orders of magnitude.… [ABSTRACT FROM AUTHOR]