Uranyl-Organic Coordination Compounds Incorporating Photoactive Vinylpyridine Moieties: Synthesis, Structural Characterization, and Light-Induced Fluorescence Attenuation.

The fluorescence of uranyl originated from electronic transitions (S ₁₁ -S ₀₀ and S ₁₀ -S _0v , v = 0-4) of the ligand-to-metal charge transfer (LMCT) process is an intrinsic property of many uranyl coordination compounds. However, light-induced regulation on fluorescence features of uranyl hybrid materials through photoactive functional groups is less investigated. In this work, the photoactive vinyl group-containing ligands, ( E)-methyl 3-(pyridin-4-yl)acrylate and ( E)-methyl 3-(pyridin-3-yl)acrylate, have been used in the construction of uranyl coordination polymers in the presence of 1,10-phenanthroline (phen). Five compounds (UO ₂ ) ₃ (μ ₃ -O)(μ ₂ -OH) ₂ (L ₁ ) ₂ ( phen) ₂ (1), (UO ₂ ) ₃ (μ ₃ -O)(μ ₂ -OH) ₃ (L ₁ )( phen) ₂ (2), (UO ₂ ) ₃ (μ ₃ -O)(μ ₂ -OH) ₃ (L ₂ )( phen) ₂ (3), [(UO ₂ ) ₂ (μ ₂ -OH) ₂ (L ₂ ) ₂ ( phen) ₂ ]·2H ₂ O (4), and (UO ₂ )Zn(SO ₄ )(phen)(H ₂ O)(OH) ₂ (5) were obtained under hydrothermal conditions. Compounds 1-4 are polynuclear uranyl structures with abundant π-π interactions and hydrogen bonds contributed to the 3D crystal packing of them. As model compounds, 1 and 3 are selected for exploring photoresponsive behaviors. The emission intensities of these two compounds are found to decrease gradually over the exposure time of UV irradiation. X-ray single crystal structural analysis suggests that the fluorescence attenuation can be explained by the slight rotation of pyridinyl groups around the carbon-carbon double bond during UV irradiation, which is accompanied by the change of weak interactions, i.e., π-π interactions and hydrogen bonds in strength and density. This feature of light-induced fluorescence attenuation may enable these two compounds to act as potential photoresponsive sensor materials.