Structure and Emissive Properties of Heterobimetallic Ln–Au Coordination Polymers: Role of Tb and Eu in Non-aurophilic [ⁿBu₄N]₂[Ln(NO₃)₄Au(CN)₂] versus Aurophilic Ln[Au(CN)₂]₃·3H₂O/3D₂O Chains

Abstract

This investigation is focused on comparing photophysical properties between two series of lanthanide-dicyanoaurate coordination polymers that contain and lack aurophilic interactions, respectively. Luminescence and crystallographic studies have been carried out on five different coordination polymer chain frameworks: the non-aurophilic [(n)Bu4N]2[LnxGd1-x(NO3)4Au(CN)2] (Ln = Eu, Tb; x = 0.01, 0.02, 0.04, 0.08) and[(n)Bu4N]2[EuxTb1-x(NO3)4Au(CN)2] (x = 0.25, 0.5, 0.75), as well as the analogous solid-solutions of aurophilic LnxGd1-x[Au(CN)2]3·3H2O and EuxTb1-x[Au(CN)2]3·3H2O. The single-crystal structures of M[Au(CN)2]3 ·3H2O (M = Eu, Gd) are also reported for comparison. In the aurophilic frameworks the close proximity of gold(I) centers on neighboring chains allows for Au-Au interactions to take place that facilitate energy transfer between lanthanides. Terbium- and europium-doped aurophilic frameworks show energy transfer between one of the lanthanide ions and dicyanoaurate centers as observed via luminescence measurements. In the non-aurophilic frameworks the [(n)Bu4N] cations separate the Au-Au chains, thereby preventing interaction between them, and preventing energy transfer. By preparing the aurophilic EuxTb1-x[Au(CN)2]3·3D2O frameworks, it was shown that the O-H vibrational energy in the hydrated (aurophilic) samples can partially quench the Ln signal.

Lanthanide Photoluminescence in Heterometallic Polycyanidometallate-Based Coordination Networks

“…In addition, they are photoluminescent in the visible range due to their metal-to-ligand charge transfer (MLCT) transition [27,28]. They were found to be attractive for the sensitization of lanthanide(3+) luminescence, which was precisely investigated for the family of classical three-dimensional [Ln III (H 2 O) 3 ][M I (CN) 2 ] 3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy; M = Ag, Au) coordination polymers (Figure 1, Table 1) [55,56,57,58,59,60,61,62,63,64,65,66,67,68,69]. These compounds are composed of the alternately arranged Ln III and M I (M = Ag, Au) centres bonded through cyanide bridges (Figure 1a).…”

“…Moreover, the composition-dependent visible photo- luminescence was presented for the mixed trimetallic LaAgAu and TbAgAu compounds [62,63]. For Ce 3+ and Sm 3+ , the very weak energy transfer from 4d/5d to 4f metal centres was detected, resulting in the complex UV-Vis emission signal of the mixed Ln 3+ and [M I (CN) 2 ] − origins [66,67,68,69]. In the case of Sm 3+ , even the reverse Sm(III) to Au(I) energy transfer could be postulated [69].…”

“…The combination of dicyanidoaurates(I) with lanthanide(3+) ions in the non-aqueous solution hampered the crystallization of the 3D network, and gave cyanido-bridged ( n Bu 4 N) 2 [Ln III (NO 3 ) 4 ] [Au I (CN) 2 ] (Ln = Ce, Nd, Sm, Gd, Eu, Tb, Dy) zig-zag chains (Figure 2) [67,68,69,70]. For this coordination topology, unlike to the related 3D LnAu networks, there is no efficient energy transfer between [Au I (CN) 2 ] − and Ln III complexes, and both entities act as two separate emissive sources.…”

“…This was explained by the spatial separation of the 1D bimetallic LnAu coordination polymers by the [ n Bu 4 N] + cations. It causes the lack of aurophilic Au–Au interactions (Figure 2a), playing the presumably important role in the ET process within the 3D LnAu networks [67]. Therefore, taking advantage of the intrinsic properties of both luminescent 5d and 4f (Sm, Eu, Tb, Dy) metal complexes embedded in such cyanido-bridged chains, the multi-coloured composition-dependent emission, including the white light, was fruitfully achieved (Figure 2b) [67,68,69,70].…”

“…It causes the lack of aurophilic Au–Au interactions (Figure 2a), playing the presumably important role in the ET process within the 3D LnAu networks [67]. Therefore, taking advantage of the intrinsic properties of both luminescent 5d and 4f (Sm, Eu, Tb, Dy) metal complexes embedded in such cyanido-bridged chains, the multi-coloured composition-dependent emission, including the white light, was fruitfully achieved (Figure 2b) [67,68,69,70]. Moreover, the related trimetallic mixed 5d-4f-4f’ compounds enabled the adjustment of the emission colour to the very broad range of the visible spectrum between red colour characteristic of EuAu chains, green emission typical for TbAu chains, up to the deep violet detected for CeAu analogues, for which mainly the UV-violet Ce III -centred emission was found (Figure 2b) [68,69].…”