"Synthesis, characterization and luminescent properties of new Eu-MOFs"

GERMÁN E. GOMEZ; MARÍA CELESTE BERNINI; ELENA V. BRUSAU; GRISELDA E. NARDA; MASSAD WALTER A.

Córdoba

Encuentro; XI "Encuentro Latinoamericano de Fotoquímica y Fotobiología (ELAFOT)"; 2012

Universidad Nacional de Río Cuarto

Traditional inorganic and organic luminescent materials have been extensively explored. Mixed oxides containing rare ? earths ions i.e., are well ? known for their narrow emission and high colour purity. In turn, the interest in organic luminescent materials has been mainly motivated by their applications in organic light emitting diodes (OLEDs) in which a thin film of a luminescent organic material needs to be incorporated between two conductors for such electronic devices [1][2]. Metal-organic frameworks (MOFs) [3][4] that result from the self-assembly of organic connectors and metal ions, are certainly very promising as multifunctional luminescent materials because both inorganic and organic moieties can provide the platforms to generate luminescence. Different organic linkers such as carboxylates, phosphonates or sulfonates have been useful to construct MOFs based in lanthanides (Ln-MOFs). Recently, Ln-MOFs have received special attention due to their unusual coordination characteristics and exceptional optical and magnetic properties arising from 4f electrons [5]. In the present work, the luminescent properties of three new Eu(III) ? based MOFs (Eu-MOFs) ?[Eu2(C4H4O4)3(H2O)2].H2O (I), [Eu2(C6H8O4)3(H2O)2] (II) and [Eu2(C10H8O4)3(H2O)] (III)? are reported. The hybrid compounds have been synthesized under different hydrothermal conditions and characterized by single-crystal X-ray diffraction, thermal analysis and FTIR spectroscopy. Luminescent measurements show that the three Eu-MOFs emit red light, upon irradiation with UV-Vis light source. The characteristic signals are observed in the 590-700 nm region, corresponding to the 5D07FJ (J=1?4) transitions. The 5D0 → 7F2 emission is a typical electric dipole transition and strongly varies with the local symmetry of the Eu3+ ion, while the 5D0 → 7F1 one, corresponds to a parity-allowed magnetic dipole transition, which is practically independent of the host material. Therefore, the intensity ratio (R) of 5D0 → 7F2 to 5D0 → 7F1 is sensitive to the symmetry around the Eu3+ ion. The R values of 4.5 and 4.0 for Eu-MOF II and III, respectively, are higher than the value of 3 for Eu-MOF I indicating that the Eu3+ occupies a site of higher symmetry in the latter one. These values are lower than those reported by Gao et al. for Eu3+ hybrid materials [6]. The luminescence decay profiles are fitted with single exponentials and the lifetimes are in the ms range, which is in agreement with the Eu3+ room temperature luminescence decay time [6]. Acknowledgements: Authors thank to CONICET (PIP 2008-01360), SECyT-UNSL, SECyT-UNRC. References: [1] Guo, H. et al. Adv. Mater. 22, 4190, 2010. [2] Koen, B. Chem Rev. 109, 4283, 2009. [3] Biradha, K. et al. J. Crys.Growth Des. 11, 875, 2011. [4] Janiak, C. Dalton Trans.2781, 2003. [5] Cui, Y. et al. Chem.Rev. 112, 1126, 2012. [6] Guo, X. et al. J. Photochem. and Photobiol. A: Chem. 200, 318, 2008.