Alkaline Earth Metal Ion/Dihydroxy–Terephthalate MOFs: Structural Diversity and Unusual Luminescent Propertiesby Antigoni Douvali, Giannis S. Papaefstathiou, Maria Pia Gullo, Andrea Barbieri, Athanassios C. Tsipis, Christos D. Malliakas, Mercouri G. Kanatzidis, Ioannis Papadas, Gerasimos S. Armatas, Antonios G. Hatzidimitriou, Theodore Lazarides, Manolis J. Manos

Inorganic Chemistry


Inorganic Chemistry / Physical and Theoretical Chemistry


Alkaline Earth Metal Ion/Dihydroxy−Terephthalate MOFs: Structural

Diversity and Unusual Luminescent Properties

Antigoni Douvali,† Giannis S. Papaefstathiou,‡ Maria Pia Gullo,§ Andrea Barbieri,§ Athanassios C. Tsipis,†

Christos D. Malliakas,∥ Mercouri G. Kanatzidis,∥ Ioannis Papadas,⊥ Gerasimos S. Armatas,⊥

Antonios G. Hatzidimitriou,# Theodore Lazarides,*,# and Manolis J. Manos*,† †Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece ‡Laboratory of Inorganic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis,

Zografou 157 71, Athens, Greece §Italian National Research Council (CNR), Institute for Organic Synthesis and Photoreactivity (ISOF), Via P. Gobetti 101, 40129

Bologna, Italy ∥Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ⊥Department of Materials Science and Technology, University of Crete, 71003 Heraklion, Greece #Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece *S Supporting Information

ABSTRACT: Alkaline earth (group 2) metal ion organic frameworks (AEMOFs) represent an important subcategory of MOFs with interesting structures and physical properties. Five MOFs, namely, [Mg2(H2dhtp)2(μ-H2O)(NMP)4] (AEMOF-2), [Mg2(H2dhtp)1.5(DMAc)4]Cl·DMAc (AEMOF-3), [Ca(H2dhtp)(DMAc)2] (AEMOF-4), [Sr3(H2dhtp)3(DMAc)6]·H2O (AEMOF-5), and [Ba(H2dhtp)(DMAc)] (AEMOF-6) (H4dhtp = 2,5-dihydroxy-terepthalic acid; DMAc = N,Ndimethylacetamide; NMP = N-methylpyrrolidone), are presented herein. The reported MOFs display structural variety with diverse topologies and new structural features. Interestingly, AEMOF-6 is the first example of a Ba2+−H2dhtp2− MOF, and

AEMOF-5 is only the second known Sr2+−H2dhtp2− MOF. Detailed photoluminescence studies revealed alkaline earth metal ion-dependent fluorescence properties of the materials, with the heavier alkaline earth metal ions exhibiting red-shifted emission with respect to the lighter ions at room temperature. A bathochromic shift of the emission was observed for the MOFs (mostly for AEMOF-3 and AEMOF-4) at 77 K as a result of excited state proton transfer (ESIPT), which involves an intramolecular proton transfer from a hydroxyl to an adjacent carboxylic group of the H2dhtp 2− ligand. Remarkably, AEMOF-6 displays rare yellow fluorescence at room temperature, which is attractive for solid state lighting applications. To probe whether the alkaline earth metal ions are responsible for the unusual luminescence properties of the reported MOFs, the potential energy surfaces (PESs) of the ground, S0, and lowest energy excited singlet, S1, states of model complexes along the intramolecular proton transfer coordinate were calculated by DFT and TD-DFT methods. ■ INTRODUCTION

Metal organic frameworks (MOFs) are porous crystalline solids constructed from organic molecules (bridging ligands) acting as linkers and metal ions or clusters serving as connecting points or vise versa.1,2 MOFs based on alkaline earth (group 2) metal ions have received much less attention compared to those of transition metal ions, with the Sr2+ and Ba2+ MOFs being still scarce. However, alkaline earth metal ion MOFs (AEMOFs) have several unique attributes such as low density, reduced toxicity, relatively low cost (because of the abundant alkaline earth metal ions), stability in air and in various solvents, etc.3

Furthermore, some AEMOFs have shown quite interesting gas sorption, photochromic, and photoluminescence-sensing properties.3

Received: March 9, 2015

Article © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.5b00539

Inorg. Chem. XXXX, XXX, XXX−XXX

We have currently initiated a research program involving the synthesis and design of new highly luminescent AEMOFs with potential sensing properties for industrial and environmental applications. The first result of these investigations was the discovery of [Mg(H2dhtp)(H2O)2]·DMAc (AEMOF-1). 4 This compound exhibits strong turquoise ligand-based emission arising from radiative deactivation of two closely lying excited states: a locally excited state and a lower lying state populated through excited state intramolecular proton transfer (ESIPT).5

The ESIPT process involves the transfer of a proton in an electronically excited species from a hydroxyl or an amino group to a carbonyl oxygen through a pre-existing strong hydrogen bond (five- or six-membered ring configuration), Scheme 1.

Although this phenomenon has been extensively studied in molecular systems,5 analogous studies in MOFs are scarce.3f,g,4

The energetics of the ESIPT process are known to be sensitive to the environment around the emitting chromophore, thus making such chromophores particularly appealing for sensing applications.5e,f Additionally, the large Stokes shifts associated with the

ESIPT process offer the advantage of facile spectral discrimination between excitation and emission signal, thus eliminating possible errors and misinterpretations caused by the selfabsorption (inner filter) effects. Furthermore, ESIPT sensors often exhibit analyte-specific shifts in emission wavelength in addition to quenching or enhancement, thereby offering the possibility of superior selectivity and direct analyte recognition.6

Indeed, AEMOF-1 showed extraordinary ability for detection of traces of water in various organic solvents via an unusual “turnon” luminescence sensing mechanism that involves enhancement accompanied by a red shift in emission wavelength.4

Building upon our previous studies on AEMOFs exhibiting

ESIPT luminescence, we present herein five new AEMOFs, namely, [Mg2(H2dhtp)2(μ-H2O)(NMP)4] (AEMOF-2), [Mg2(H2dhtp)1.5(DMAc)4]Cl·DMAc (AEMOF-3), [Ca(H2dhtp)(DMAc)2] (AEMOF-4), [Sr3(H2dhtp)3(DMAc)6]·

H2O (AEMOF-5), and [Ba(H2dhtp)(DMAc)] (AEMOF-6).

The compounds display a noticeable structural diversity adopting 2-D (AEMOF-3) or 3-D frameworks (AEMOF-2,