Eu-MOFs with 2-(4-Carboxyphenyl)imidazo[4,5-f]-1,10-phenanthroline and Ditopic Carboxylates as Coligands: Synthesis, Structure, High Thermostability, and Luminescence Properties

Hydrothermal reactions of europium(III) salt with 2-(4-carboxyphenyl)imidazo[4,5-f]-1,10-phenanthroline and dicarboxylic acid as coligands-benzene-1,4-dicarboxylic acid, 4,4'-biphenyldicarboxylic acid, 2,5-dibromoterephthalic acid, and naphthalene-1,4-dicarboxylic acid-lead to four europium fluorescent materials (1-4). Structural analyses reveal that 1-4 have binuclear 3D metal-organic frameworks with different channels, void volumes, and conjugated structures tuned by ditopic carboxylates. There are no latticed and coordinated water molecules occurring in 1-3, while the free water molecules fill in 1D channels of 4. 4' was readily obtained via water removal of 4. Thermal analyses of all compounds show the high thermal stability of the main framework up to 450 degrees C. Optical studies indicate that 1-4 and 4' show the characteristic red luminescence emission of the Eu-III ion in the visible regions at room temperature. On the basis of emission spectra, their luminescence lifetimes were determined. In particular, compound 4' shows a longer lifetime (t = 0.942 ms) and significantly enhanced quantum yield (39%) compared with those of 1 (11%, 0.770 ms), 2 (4%, 0.414 ms), 3 (18%, 0.807 ms), and 4 (26%, 0.858 ms).