Syntheses, structures, and photochemical properties of (μ3-O)tris{bis(μ-carboxylato)}trimanganese complexes with naphthylacetate ligands with relevance to artificial solar energy-harvesting systems

Two new trinuclear Mn complexes containing naphthalene moieties, [Mn3(μ3-O)(μ-O2CCH2-1-naph)6(py)3] (1) and [Mn3(μ3-O)(μ-O2CCH2-2-naph)6(py)3] (2) (naph-1-CH2CO2H = 1-naphthylacetic acid, naph-2-CH2CO2H = 2-naphthylacetic acid, py = pyridine), were synthesized. The X-ray crystallography of 1, the EXAFS analyses of 1 and 2, and the magnetic susceptibility data for these complexes showed that they are trinuclear complexes containing a [Mn3(μ3-O)(μ-O2CR)6] core with mixed-valence Mn3(II, III, III) centers. CVs exhibited an oxidation peak ascribed to MnIII3/MnIIMnIII2 and a reduction peak ascribed to MnIIMnIII2/MnII2MnIII in CH2Cl2. In the fluorescence spectra, the fluorescence intensities of 1 and 2 were smaller than those of the naphthylacetic acids and depended on the dielectric constant of solvent. These results suggested that there is electron transfer between the manganese centers and the naphthyl moieties. The fluorescence decays of 1 and 2 in CH3CN were also faster than the decays in the toluene solutions; this result parallels the lower intensity of the static fluorescence spectra in solvents with a higher dielectric constant. The fluorescence decay curves of 1 and 2 were fitted by two lifetimes, τ1 and τ2, suggesting that electron transfer occurs from the monomer (τ1) and the excimer (τ2) of the naphthyl moieties to the manganese center.