Visible and NIR Photoluminescence Properties of a Series of Novel Lanthanide–Organic Coordination Polymers Based on Hydroxyquinoline–Carboxylate Ligands

A series of novel two-dimensional (2D) lanthanide coordination polymers with 4-hydroxyquinoline-2-carboxylate (H₂hqc) ligands, [Ln­(Hhqc)₃(H₂O)]<i>_n</i>·3<i>n</i>H₂O (Ln = Eu (<b>1</b>), Tb (<b>2</b>), Sm (<b>3</b>), Nd (<b>4</b>), and Gd (<b>5</b>)) and [Ln­(Hhqc)­(ox)­(H₂O)₂]<i>_n</i> (Ln = Eu (<b>6</b>), Tb (<b>7</b>), Sm (<b>8</b>), Tm (<b>9</b>), Dy (<b>10</b>), Nd (<b>11</b>), Yb (<b>12</b>), and Gd (<b>13</b>); H₂ox = oxalic acid), have been synthesized under hydrothermal conditions. Complexes <b>1</b>–<b>5</b> are isomorphous, which can be described as a two-dimensional (2D) hxl/Shubnikov network based on Ln₂(CO₂)₄ paddle-wheel units, and the isomorphous complexes <b>6</b>–<b>13</b> feature a 2D decker layer architecture constructed by Ln-ox infinite chains cross-linked alternatively by bridging Hhqc^– ligands. The room-temperature photoluminescence spectra of complexes Eu^III (<b>1</b> and <b>6</b>), Tb^III (<b>2</b> and <b>7</b>), and Sm^III (<b>3</b> and <b>8</b>) exhibit strong characteristic emissions in the visible region, whereas Nd^III (<b>4</b> and <b>11</b>) and Yb^III (<b>12</b>) complexes display NIR luminescence upon irradiation at the ligand band. Moreover, the triplet state of H₂hqc matches well with the emission level of Eu^III, Tb^III, and Sm^III ions, which allows the preparation of new optical materials with enhanced luminescence properties