Spectroscopic characterization of hydroxylated nanoballs in methanol

In this report, the photophysical properties of self-assembled [Cu ₂(5-OH-bdc)₂L₂]₁₂ [where (5-OH-bdc)^2- = 5-hydroxybenzene-1,3-dicarboxylate and L is a dimethyl sulfoxide, methanol, or water ligand] hydroxylated nanoballs (OH-nanoball) were examined in methanol using optical absorption and steady-state and time-resolved fluorescence methods. The optical spectrum of the OH-nanoball is dominated by ligand absorbance at 305 nm and a weaker Cu²⁺-to-ligand charge-transfer transition at ∼695 nm, which are distinct from the absorption of either the free ligand (∼312 nm) or Cu²⁺(NO ₃)₂ (>750 nm) in methanol. The corresponding emission spectrum of the OH-nanoball originates from the emission of the ligand and is centered at ∼360 nm with a shoulder at ∼390 nm. The emission from the OH-nanoball is significantly quenched relative to the free ligand [Φ_5-OH-H2bdc = 0.014 and Φ_OH-nanoball = (5.6 ± 0.5) × 10^-5]. The addition of bases such as imidazole results in an increase in the emission intensity of the OH-nanoball solution, indicating dissociation of the [Cu₂(5-OH-bdc)₂L ₂]₁₂ units. Although the mechanism of (5-OH-bdc) ^2- quenching within the OH-nanoball is not clear, it is likely due to interactions between the ligand π system and the Cu d orbitals. Fluorescence polarization studies further suggest that the OH-nanoball retains a spherical shape in solution. This is evident by the fact that the fluorescence anisotropy of the nanoball is nearly identical with that of the free ligand, suggesting rapid energy transfer (homogeneous fluorescence resonance energy transfer) between ligands within the OH-nanoball.