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Issue 17, 2014
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Multifunctional micro- and nanosized metal–organic frameworks assembled from bisphosphonates and lanthanides

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Abstract

Phase-pure and highly crystalline [Ln(Hpmd)(H2O)] materials [where Ln3+ = Eu3+ (1), Gd3+ (2) and Tb3+ (3); H4pmd = 1,4-phenylenebis(methylene)diphosphonic acid] were prepared by using three distinct approaches: (i) conventional hydrothermal synthesis (180 °C, 3 days); (ii) microwave-assisted heating (50 W irradiation power, 40 °C, 5 seconds; nano-sized aggregates with sizes ranging between 100 and 150 nm); and (iii) ultrasound-assisted synthesis which, for highly diluted reactive mixtures (Ln3+ : H4pmd : H2O of 1 : 1 : 7200), permitted the preparation of isolated nano-crystals at ambient temperature with 5 minutes of ultrasonic irradiation. Compounds were structurally characterized by powder X-ray diffraction (Rietveld refinement and variable-temperature studies), thermogravimetry, vibrational spectroscopy, elemental analysis and electron microscopy (SEM and EDS). The magnetic behavior of 1 and 2 was investigated between ambient temperature and ca. 2 K revealing that the Ln3+ cations act as isolated centers. A strategy to calculate the vibrational spectra of MOF structures and based on the known embedded-clusters approach is proposed and applied to [Ln(Hpmd)(H2O)]. This allowed the identification, along with deuteration of the materials, of the vibrational modes of the confined water molecule in the structure. Compound 1 was tested in the methanolysis of styrene oxide at 55 °C: it is shown that microcrystalline 1 (1-m) does not possess significant catalytic activity; on the other hand, the nano-sized counterpart (1-n) exhibits relatively high catalytic activity and excellent selectivity to 2-methoxy-2-phenylethanol (100% yield within 48 h of reaction time). Photoluminescence studies both at ambient and low temperatures showed, on the one hand, that bulk materials are composed of a single lanthanide site and, on the other hand, that the organic linker is a suitable sensitizer of Tb3+ (absolute quantum yield of ca. 14% for 3). It is further demonstrated that the coordinated water molecule has a pivotal role in the quenching of the photoluminescence of Eu3+ in 1: deuteration of the material results in a ca. 3.4 times improvement of the decay time (e.g., at 300 K the lifetime improves from 0.58 ± 0.01 ms to 1.98 ± 0.01 ms).

Graphical abstract: Multifunctional micro- and nanosized metal–organic frameworks assembled from bisphosphonates and lanthanides

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Publication details

The article was received on 25 Oct 2013, accepted on 25 Nov 2013 and first published on 25 Nov 2013


Article type: Paper
DOI: 10.1039/C3TC32114B
Citation: J. Mater. Chem. C, 2014,2, 3311-3327
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    Multifunctional micro- and nanosized metal–organic frameworks assembled from bisphosphonates and lanthanides

    S. M. F. Vilela, D. Ananias, J. A. Fernandes, P. Silva, A. C. Gomes, N. J. O. Silva, M. O. Rodrigues, J. P. C. Tomé, A. A. Valente, P. Ribeiro-Claro, L. D. Carlos, J. Rocha and F. A. Almeida Paz, J. Mater. Chem. C, 2014, 2, 3311
    DOI: 10.1039/C3TC32114B

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