materials Article Multidimensional Ln-Aminophthalate Photoluminescent Coordination Polymers Carla Queirós 1 , Chen Sun 2 , Ana M. G. Silva 1 , Baltazar de Castro 1 , Juan Cabanillas-Gonzalez 2, * and Luís Cunha-Silva 1, *   Citation: Queirós, C.; Sun, C.; Silva, A.M.G.; de Castro, B.; Cabanillas- Gonzalez, J.; Cunha-Silva, L. Multidimensional Ln-Aminophthalate Photoluminescent Coordination Polymers. Materials 2021, 14, 1786. https://doi.org/10.3390/ma14071786 Academic Editor: Andrei S. Potapov Received: 24 February 2021 Accepted: 1 April 2021 Published: 4 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 LAQV/REQUIMTE & Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto, 4169-007 Porto, Portugal; carla.queiros@fc.up.pt (C.Q.); ana.silva@fc.up.pt (A.M.G.S.); bcastro@fc.up.pt (B.d.C.) 2 Madrid Institute for Advanced Studies, IMDEA Nanociencia, Calle Faraday 9, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain; chen.sun@imdea.org * Correspondence: juan.cabanillas@imdea.org (J.C.-G.); l.cunha.silva@fc.up.pt (L.C.-S.); Tel.: +34-912998784 (J.C.-G.); +351-22-040-2653 (L.C.-S.) Abstract: The development of straightforward reproducible methods for the preparation of new photoluminescent coordination polymers (CPs) is an important goal in luminescence and chemical sensing fields. Isophthalic acid derivatives have been reported for a wide range of applications, and in addition to their relatively low cost, have encouraged its use in the preparation of novel lanthanide- based coordination polymers (LnCPs). Considering that the photoluminescent properties of these CPs are highly dependent on the existence of water molecules in the crystal structure, our research efforts are now focused on the preparation of CP with the lowest water content possible, while considering a green chemistry approach. One- and two-dimensional (1D and 2D) LnCPs were prepared from 5-aminoisophthalic acid and Sm 3+ /Tb 3+ using hydrothermal and/or microwave-assisted synthesis. The unprecedented LnCPs were characterized by single-crystal X-ray diffraction (SCRXD), powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM), and their photoluminescence (PL) properties were studied in the solid state, at room temperature, using the CPs as powders and encapsulated in poly(methyl methacrylate (PMMA) films, envisaging the potential preparation of devices for sensing. The materials revealed interesting PL properties that depend on the dimensionality, metal ion, co-ligand used and water content. Keywords: photoluminescence; isophthalate; lanthanides; coordination polymers 1. Introduction Luminescence phenomena include photoluminescence (PL) and chemiluminescence, with fluorescence and phosphorescence being particular examples of PL. The photolumines- cent properties of the materials essentially depend on: (i) fluorescence/phosphorescence intensity, (ii) energy absorption, (iii) photon absorption associated to electromagnetic radia- tion and (iv) nonradiative relaxation processes [1]. In the case of coordination polymers (CP), PL is also dependent on the ligand selected for the preparation of the materials. Considering these facts, a rational selection of the ligand can promote an efficient ab- sorption and transfer of energy to an excited level of the Ln metal centers, leading to a sensitization of the Ln ions-antenna effect—corresponding to an increase in lumines- cence efficiency [2–5]. This occurrence allows the increase of the luminescence intensity of inorganic salts, generally limited by the low absorbance associated with forbidden f –f transitions. Lanthanide-based coordination polymer (LnCP) luminescence, among other prop- erties, can be improved or changed: using selected organic ligands, such as aromatic carboxylates [6–11], or introducing co-ligands, such as phenanthroline, bipyridine, 1,2- bis(4-pyridyl)ethylene and others [12–18]. These strategies are extremely important when Materials 2021, 14, 1786. https://doi.org/10.3390/ma14071786 https://www.mdpi.com/journal/materials