Mixed-Ligand Terbium Terephthalates: Synthesis, Photophysical and Thermal
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Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 72–80 Contents lists available at SciVerse ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journa l homepage: www.elsevier.com/locate/jphotochem Mixed-ligand terbium terephthalates: Synthesis, photophysical and thermal properties and use for luminescent terbium terephthalate thin film deposition a,∗ b b b Valentina V. Utochnikova , Oksana Pietraszkiewicz , Małgorzata Kozbiał´ , Paweł Gierycz , b c Marek Pietraszkiewicz , Natalia P. Kuzmina a Material Science Department, Lomonosov Moscow State University, Leninskie gory, 1, 3, 119992, Moscow, Russia b Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka, 44/52, Warsaw, Poland c Chemistry Department, Lomonosov Moscow State University, Leninskie gory, 1, 3, 119992, Moscow, Russia a r t i c l e i n f o a b s t r a c t Article history: Addition of potassium terephthalate K2tph to an aqueous solution of a mixture of ter- Received 4 September 2012 bium nitrate and ancillary ligands L resulted in the formation of mixed-ligand terbium Received in revised form terephthalates Tb2tph3(TPPO)8(H2O)4 (II), Tb2tph3(TOPO)2(H2O)4 (III), Tb2tph3(diglyme)2(H2O) (IV), 21 December 2012 Tb2tph3(tetraglyme)4(H2O)3 (V), Tb2tph3(Phen)2(H2O)2 (VI). The composition of the mixed-ligand Accepted 29 December 2012 complexes (MLCs) was confirmed by elemental analysis, Raman spectroscopy, thermal analysis, and Available online xxx luminescent spectroscopy. According to the thermal analysis data, II–V exhibit lower thermal stability in comparison with the terephthalate hydrate Tb tph (H O) (I). MLCs II, IV, and V are able to eliminate Keywords: 2 3 2 4 ancillary ligands, followed by formation of anhydrous Tb2tph3, while in case of III and IV ligand elimi- Terbium terephtalate nation is accompanied by partial thermal decomposition of Tb2tph3. Solubility tests showed that MLC II Mixed-ligand complexes Luminescent thin films exhibits sufficient solubility in acetonitrile for thin film deposition. Thus thin films of II were deposited by spin-coating, and luminescent thin films of Tb2tph3 were obtained for the first time by thermal treatment of II films. © 2013 Elsevier B.V. All rights reserved. 1. Introduction with appropriate morphology is the most probable explanation for the poor brightness of Tb-carboxylate based OLEDs. The present 2 ∼ Unique luminescent properties of lanthanide coordination com- maximum brightness of 350 cd/m is still 35% lower than that pounds, such as their ability to exhibit quantum yields up to observed for Tb-diketonate containing OLEDs [19–22], and is the 100% and quasi-monochromatic emission, makes them useful as most probable explanation for the lack of an active research in the luminescent materials. However compounds, which are among field of OLEDs based on aromatic carboxylates since 2002 [23–27].  the most widely used today, namely -diketonates and pyra- Lately, great efforts has devoted to the design of lanthanide zolonates, exhibit rather low UV stabilities [1–3]. At the same time complexes with rigid aromatic polycarboxylate ligands: dicarboxy- a number of lanthanide aromatic carboxylates Ln(RxC6H5−xCOO)3 lates (terephthalates, phthalates, isophthalates, imidazole-4,5- (“benzoates”) with unique photophysical properties [4–6] and dicarboxylates, 3,5-pyrazoledicarboxylates) and tricarboxylates intriguing structural features [7–9] have been disclosed recently. (1,2,3-benzenetricarboxylates, 1,3,5-benzenetricarboxylates) [28]. Substituted benzoic acid anions are attractive as ligands as they Dicarboxylic terephthalic acid (1,4-benzenedicarboxylic acid, are efficient sensitizers of lanthanide luminescence [4,5], their car- H2tph) is known not only as a rigid building block, but also as boxylate groups interact strongly with the oxophilic lanthanides, an excellent light-harvester for lanthanide luminescence for both the delocalized –electron system provides a strongly absorbing europium and terbium [3,29,30,32]; recently white light emitting 3+ 3+ chromophore [4,5,10]. These advantages result in the appearance materials based on heterometallic terephthalate of Ce , Eu and 3+ of highly efficient and very stable lanthanide benzoate lumines- Tb have been reported [31]. cence [11,12]. However the low volatility and solubility, caused by The formation of mixed-ligand complexes (MLCs) with ancil- their polymeric structure [3,13–15], prevents their deposition as lary neutral ligands is a well-known modification of lanthanide films, which is a prerequisite for the use in optoelectronic devices, carboxylate structures, solubilities, and both luminescent and ther- such as, e.g. OLEDs [16–18]. This difficulty of thin film deposition mal properties [5,29]. Little attention, however, has been paid to mixed-ligand complexes (MLCs) of the lanthanide terephthalates. The difficulties encountered in the preparation of MLCs are most ∗ likely due to the rigidity of the 3D networks formed by lanthanides Corresponding author. Tel.: +7 4959393836. E-mail address: [email protected] (V.V. Utochnikova). with dicarboxylate ligands (Ln2(Carb)3) [33,34]. To the best of our 1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2012.12.021 V.V. Utochnikova et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 72–80 73 Table 1 Neutral ligands L, reagent ratios, and elemental analysis results for MLC synthesis. L [Tb3+]:[tph2−]:[L] Composition Triphenylphosphine oxide (TPPO) 2:3:8 Tb2tph3(TPPO)8(H2O)4 (II) calcd. for Tb2C168H140O24P8: C, 66.46; H, 4.21; found: C, 65.63; H, 4.27; P O 4:3:8 2:3:12 Trioctylphosphine oxide (TOPO) 2:3:8 Tb2tph3(TOPO)2(H2O)4 (III) CH3 calcd. for Tb2C49H73O17P: C, 53.95; H, 7.46; found: C, 53.39; H, 7.75; O P CH3 CH 3 4:3:1 2:3:10 a Diglyme 2:3: Tb2tph3(diglyme)2(H2O) (IV) O O O calcd. for Tb2C36H42O19: C, 39.43; H, 3.86; found: C, 39.65; H, 3.33; H3C CH3 a Tetraglyme 2:3: Tb2tph3(tetraglyme)4(H2O)3 (V) O O O O O calcd. for Tb2C64H106O35: C, 43.84; H, 6.09; H3C CH3 found: C, 44.26; H, 6.41; o-Phenanthroline (Phen) 2:3:8 Tb2tph3(Phen)2(H2O)2 (VI) calcd. for Tb2C48H32N4O14: C, 47.78; H, 2.67%; N, 4.64; found: C, 47.18; H, 2.66; N, 4.11 N N a Dibutyl ether diethylene glycol (dbdg) 2:3: Tb2tph3(H2O)4 (I) O O O calcd. for Tb2C24H20O16: C, 32.67; H, 2.28; found: C, 32.43; H, 2.21; H7C4 C4H7 a Ethyl orthoformate (etof) 2:3: Tb2tph3(H2O)4 (I) H3C CH3 calcd. for Tb2C24H20O16 C, 32.67; H, 2.28; found: C, 32.28; H, 2.32; O H3C O O H3C a L was used as a solvent. Otherwise ethanol was used as a solvent. knowledge, information on mixed-ligand terephtalates is limited to 99.9%, potassium hydroxide, triphenylphosphine oxide, tri- one example of a europium MLC, [Eu2tph3(Phen)2(H2O)2]n [35,36]. octylphosphine oxide, o-phenanthroline, and ethylene glycol Thus, this work is devoted to the synthesis and characterization dimethyl ether (diglyme) were purchased from Aldrich, and dieth- of lanthanide mixed-ligand terephthalates on example of terbium ylene glycol dibutyl ether was purchased from Acros. All other compounds and to the evaluation of benzene dicarboxylates as the solvents used were of analytical reagent grade and purchased from luminescent materials. Ancillary ligands of different classes (aro- Aldrich. Ethylene glycol tetramethyl ether (tetraglyme) was pur- matic diimine, phosphine oxides, and polyesters) were selected for chased from Aldrich and purified by vacuum distillation. corresponding MLC syntheses (Table 1). Elemental analyses were performed with a Vario EL III Her- aeus instrument. Raman spectra were recorded using a Renishaw 2. Experimental InVia spectrometer. UV–vis spectra were recorded with a Shimadzu UV-3100 spectrophotometer; corrected luminescence spectra and The following commercially available chemicals were used luminescence decays were recorded with a Fluorolog 3 spectro- without further purification: terbium(III) nitrate pentahydrate fluorimeter. XRD analyses were performed on a Rigaku D/MAX 2500 74 V.V. Utochnikova et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 72–80 (CuK␣). Thermal analyses were carried out in nitrogen atmosphere The first one is based on the introduction of the ancillary ligand L ◦ ◦ at a heating rate of 10 /min within 20–720 C on a TG-DSC111 into the structure of carboxylate hydrates, accompanied by partial thermoanalyser, SETARAM, France CALVET type, with samples in or complete substitution of coordinated water molecules. The sec- platinum vessels, 75 L volume. ond route includes consecutive steps: terbium chloride or nitrate transformation into soluble mixed-ligand compound followed by a ligand exchange reaction with soluble carboxylate salts. 2.1. Syntheses It is obvious that in case of terbium terephalate hydrate the branched system of intermolecular binding and, as consequence, Terbium terephthalate Tb2tph3(H2O)4 (I) was synthesized by extremely low solubility prevents MLC formation according to first · the reaction of stoichiometric amounts of Tb(NO3)3 5H2O and method (1). Even the second route bears the risk of precipitation of K2tph (from H2tph and KOH). The products were dried in air at insoluble I instead of MLC formation via reaction (2b). room temperature. To check the suitability of these approaches to the synthesis of I calcd. for Tb2C24H20O16: C, 32.67; H, 2.28; found: C, 32.22; H, 2.20; MLCs of terbium terephthalate, we selected a set of ancillary lig- ands of different classes, which, on one hand, possess a high donor activity and, on the other hand, have quite bulky structures to sup- 2.2. Mixed ligand complex syntheses port the transformation of the branched 3D polymeric structure of terephthalates into a soluble form of the MLCs. The ancillary lig- Method 1.