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Wide-Range UV-to-Visible Excitation of Near-Infrared Emission and Slow Magnetic Relaxation in LnIII(4,4′-Azopyridine-1,1′-dioxide)[CoIII(CN)6]3– Layered Frameworks

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journal contribution
posted on 19.12.2018, 13:10 by Szymon Chorazy, Mikolaj Zychowicz, Shin-ichi Ohkoshi, Barbara Sieklucka
Trivalent lanthanide ions combined with two molecular linkers, organic 4,4′-azopyridine-1,1′-dioxide (apdo), and inorganic hexacyanidocobaltate­(III), gave a series of magnetoluminescent coordination polymers, [{LnIII(apdo)­(H2O)4}­{CoIII(CN)6}]·2H2O (Ln = Nd, 1; Tb, 2; Dy, 3; Er, 4; Tm, 5; Yb, 6). They are hybrid organic–inorganic layered frameworks composed of cyanido-bridged {Ln2(μ-NC)4Co2} squares linked by Ln–apdo–Ln bridges into a coordination network of a mixed 4- and 8-metal ring topology. Lanthanide­(III) complexes, [LnIII(μ-apdo)2(H2O)4(μ-NC)2]+, of a distorted dodecahedral geometry are isolated by diamagnetic [CoIII(CN)6]3– and apdo linkers. As a result, 16 reveal field-induced slow relaxation of magnetization, with typical temperature-dependent relaxation of a single-ion origin for NdIII-containing 1, DyIII-containing 3, and YbIII-containing 6. The related alternate-current magnetic data were precisely analyzed, indicating the multiple magnetic relaxation pathways, including a direct process, strong quantum tunneling of magnetization, non-negligible Raman processes, and crucial two-phonon Orbach thermal relaxation. The thermal energy barriers of the Orbach process, ΔE/kB, are 15.1(9) K with τ0 = 9.8(9) × 10–6 s at Hdc = 4500 Oe, 16.1(8) K with τ0 = 9.0(9) × 10–5 s at Hdc = 1500 Oe, and 17.3(6) K with τ0 = 3.2(7) × 10–6 s at Hdc = 700 Oe, for 1, 3, and 6, respectively, proving the single-molecule magnet (SMM) behavior. Because of the presence of [Co­(CN)6]3–, 16 show strong UV absorption, while the chromophoric apdo leads to the strong absorption in the visible range. As a result, the visible 4f/3d metal-centered emission is quenched, but the near-infrared luminescence from NdIII and YbIII is observed in 1 and 6, respectively. It is realized by Co-to-Ln metal-to-metal, and apdo-to-Ln ligand-to-metal energy transfers; thus, broad UV-to-visible excitation can be explored. Compounds 16 form a novel family of functional bimetallic assemblies, incorporating NIR-emissive SMMs as presented for NdCo (1) and YbCo (6) derivatives.