Dual Emission in the Near-Infrared and Visible Regions from a Mixed Cyanido-Bridged EuIII/NdIII(4-OHpy)-CoIII Layered Material

Coordination polymers (CPs) with a dual emission spanning from the visible (vis) to near-infrared (NIR) regions of the electromagnetic spectrum are used for optical sensors, medical diagnostics, and telecommunication technologies. We herein report the synthesis, structural characterization, and optical response of heterometallic cyanido-bridged layered {[EuxNdy(4-OHpy)2(H2O)3][Co(CN)6]} CPs, where 4-OHpy = 4-hydroxypyridine, with a multicolor emission profile across the vis and NIR regions. The crystals show an efficient energy transfer (ET) from the 4-OHpy ligand and the [Co(CN)6] ions to the Eu3+ and Nd3+ ions, resulting in an enhanced photoluminescence (PL) efficiency. We study the ET with steady-state and time-resolved PL, reporting an ET between the Ln3+ centers. The excitation-dependent emission of the mixed Ln3+ CPs and the control over the PL lifetime yield new insights into the optoelectronic properties of these materials.

R esearch on the development of materials with unique optical and magnetic properties 1−6 has been the driving force for the development of an exciting new class of multifunctional supramolecular materials based on coordination polymers (CPs) for emerging optoelectronic devices. In fact, in the last decades, luminescence-based optical sensing has gained ground because it is a low-cost, nondestructible, highly versatile, and sensitive method. The materials for fluorescent sensing consist of emissive species, which are mainly transition-and/or lanthanide-metal ions. 7−10 The sharp emission line widths from the f−f states of lanthanides, which span from the visible (vis) to near-infrared (NIR) electromagnetic spectrum, combined with the multiemissive transitions, which give access to multiple sensing wavelengths, have attracted the interest of the materials community. 11,12 A new generation of luminescent materials incorporating the unique properties of lanthanide ions and the synthetic flexibility of CPs has emerged, paving the way for novel emerging technologies in the fields of medical theragnostics, imaging, and telecommunication. 11−17 Especially for the case of materials with mixed lanthanide ions, dual and bimodal (UV/ vis/NIR) emissive CPs, 18−24 have been synthesized, accelerating the development of advanced technological applications in the areas of clinical diagnostics and ratiometric thermometers. 25−31 The dual emission in CPs usually originates from mixed lanthanide ions, which provide the final dual emission bands. The distribution of the emissive centers in the CP is not trivial, and it has been the center of research over the years. 32 Hence, the atactic distribution of lanthanide ions in the CP crystal structure directly influences its optical response and therefore its sensing efficiency. 33−35 Recently, the role of the lanthanide ion, combined with pyridine derivatives and the red emissive linker [Co III (CN) 6 ] 3− , has been investigated. 36,37 With regard to emissive multifunctional 2D materials, the cases of Dy III (4-OHpy)-Co III , 38 Tb III (4-OHpy)-Co III38b and the mixed lanthanide Tb III 0.5 Dy III 0.5 (4-OHpy)-Co III38b have been shown. These compounds were shown to be multifunctional materials combining dual photoluminescence (PL) and single-molecule magnetism properties. For all cases, it was found that visible emission was switchable by selected wavelengths of UV excitation light. 39 Therefore, we would like to step forward and focus our synthetic efforts on novel CP materials with broad-band emission covering both the vis and NIR spectral regions. Hence, we investigated the synthesis, physicochemical characterization, and optical properties of the mixed lanthanide Eu III Nd III (4-OHpy)-Co III systems. The combination of Eu 3+ , which shows a pronounced emission in the low-energy part of the vis region, with a characteristic sharp NIR emission of Nd 3+ at 1025 nm allowed us to decouple the radiative transitions of the two metal ions, giving rise to broad-band flexible sensing materials. Therefore, we synthesized five layered cyano-bridged CPs, where three contain two lanthanide ions (Eu 3+ and Nd 3+ ) based on the stoichiometric ratio of the reaction of {Eu 0.2 Nd 0.8 (4-OHpy) 2 Figure 1a. The type of coordination polyhedron around the Eu 3+ center was evaluated using SHAPE 39 software; the so-named continuous shape measures approach allows one to numerically estimate how far a real coordination sphere of a metal center deviates from an ideal polyhedron. Of the accessible 8-coordinate polyhedra for metal ions, the triangular dodecahedron is the most appropriate for the description of the eight donor atoms around the Eu 3+ metal center (Table S3). The same conclusion is also reached by applying the angular criteria proposed by Kepert. 40 The 6coordination of the Co 3+ center in the hexacyanocobaltate(III) anion comprises three bridging and three terminal cyanido groups, forming an octahedral {Co III C6} coordination sphere, with the trans C−Co III −C angles being in the range 175.7(4)− 176.2(4)°.
There is significant bending of the intermetallic cyanido bridges, as revealed by the Eu�N�C angles of 158.5(8)°( Eu�N1�C1), 150.8(9)°(Eu�N4�C4), and 170.7(8)°( Eu�N5�C5). This is likely due to the restrictions imposed on the Eu�N5�C5�Co atoms to lie on the symmetry plane; further distortions are also necessary in order to allow for the proper coordination geometry on the Eu 3+ and Co 3+ metal centers (Figures 1 and S2 and S3). The cyanide-bridged polymeric structure is organized in layers that coincide with the mirror planes of the structure parallel to the ac plane and along the b axis (at b = 0.25 and 0.75). Pairs of symmetry-related ligands of 4-OHpy emerge from both sides of the layers, hampering polymerization in the third dimension along the b axis (Figure 1c). The coordinated H 2 O molecules, the terminal cyanido groups, and the pyridine NH groups of the ligands of 4-OHpy, which occur because of the presence of the dominant tautomer of the organic ligand, 38 form strong intermolecular hydrogen bonds within each layer, as well as between adjacent layers, toward a robust 3D assembly (Table S4 and Figure 1c). No lattice solvent (crystallization) H 2 O molecules have been found.
The polycrystalline CP powders have been further probed structurally with PXRD. In Figure S1, the experimental XRD   Figure S4). The ratio of the mixed lanthanide CPs has been determined with microwave plasma atomic emission spectrometry (MP-AES), revealing that the ratio of the Eu 3+ and Nd 3+ precursor salts is translated quantitatively to the final CPs.
The optical properties of the polycrystalline samples have been studied thoroughly with both solid-state absorption and PL spectroscopy. The absorption profile of the solids has been probed with diffuse-reflectance spectroscopy (DRS), as shown in Figure 1a. The absorption spectra of all of the samples are dominated by strong absorption bands from both [Co(CN) 6 ] 3− and 4-OHpy, which can be assigned to spin-and parityforbidden electronic transitions. 41,42 Hence, the absorption band at 330 nm originated from a singlet-to-singlet π → π* transition ( 1 S 0 → 1 S 1 ) and a d−d transition of Co 3+ ( 1 A 1g → 1 T 1g ), whereas the higher-energy transition bands at 280 nm correspond to 1 S 0 → 1 S 2 and 1 A 1g → 1 T 2g from 4-OHpy and the Co 3+ ion, respectively. In addition, the absorption tail down to 470 nm arises from spin-forbidden transitions of both 4-OHpy and Co 3+ .
The mixed lanthanide CPs are of particular interest both both Eu 3+ and Nd 3+ are optically active and the PL spectrum has strong emission in both the vis and NIR windows. The structure of the CPs implies that the Ln III ions are well isolated in the structure and connected only with [Co(CN) 6 ] 3− and 4-OHpy; thus, a direct Ln III -to-Ln III ET should be forbidden. The first approach is to assume that the emission in CPs with only Eu 3+ or Nd 3+ is driven by ET from 4-OHpy and [Co(CN) 6 ] 3− to the metal ion. In the case of mixed Ln III CPs, the mechanism is rather more complicated because the photogenerated excitons from [Co(CN) 6 ] 3− can be transferred to both Eu 3+ or Nd 3+ centers. Delving into the emission mechanism, we probed the excitation profile of the main transitions of both metals, 5 D 0 → Inorganic Chemistry pubs.acs.org/IC Communication 7 F 2 at 614 nm (Eu III ) and 4 F 3/2 → 4 I 11/2 at 1025 nm (Nd 3+ ) of compound 2. The excitation spectra in Figure 2c show all of the electronic states that contribute to the emission of Eu III at 614 nm and in Figure 2d for the Nd 3+ at 1025 nm accordingly. In both excitation spectra, the broad absorption feature centered at 350 nm reflects the ET from 4-OHpy to Ln III . Interestingly, the high absorption cross sections of the 7 F 0 → 5 L 6 and 7 F 0 → 5 D 2 transitions of Eu 3+ at 395 and 465 nm, respectively, point toward an efficient intraband relaxation in the metal, which leads to a strong emission at 614 nm. The same trends are followed in Nd 3+ , probing the excitation profile of the 4 F 3/2 → 4 I 11/2 at 1065 nm (Figure 2d). In the case of Nd 3+ emission, the intraband relaxation appears to be the dominant mechanism, mainly because of the high oscillator strength of the ground state 4 I 9/2 absorption inside the 4f 3 electronic configuration. The hypersensitive character of the 4 G 5/2 multiplet is translated to multiple peaks in the vis region at 512, 524, and 582 nm, in line with the absorption spectra (Figure 2a). 45 It is worth noticing the absorption feature around 355 nm, which arises from the 4 D 5/2 multiplet; despite its strong character in the excitation spectra, the DRS measurements could not resolve it. The traditional relatively low absorption strength of the 4 D 5/2 multiplet in Nd 3+ is in contrast with the excitation spectra, in which it seems to be the most efficient emission pathway. Hence, we assume that the intraband absorption adds constructively to ligand-to-metal ET for the NIR emission. Interestingly, the excitation spectra of Nd 3+ show a peak at 395 nm, which is not correlated with the electronic structure of the metal, pointing out that a hot exciton from another CP site contributes to the emission at 1024 nm. The energy gap of 3.14 eV matches with the 7 F 0 → 5 L 6 transition from Eu 3+ , suggesting that the photogenerated carriers from Eu 3+ are efficiently transferred to the Nd 3+ emissive sites. Elucidating the metal− bridge−metal ET, we collected the excitation profiles of the mixed Ln III CPs shown in Figure 3a. We notice that when increasing the amount of Eu 3+ in the material, the peak at 395 nm rises up linearly while the strength of the peak that is correlated with the 4 D multiplet from Nd 3+ fades. The inset in Figure 3a follows the peak intensity of these two transitions, indicating that the peak originating from the electronic states of Nd 3+ decreases, lowering the amount of Nd 3+ in the mixed Ln 3+ CPs, and, on the other hand, the peak at 395 nm grows at the same rate as the Eu 3+ content, so we can surmise that it is correlated with the 7 F 0 → 5 L 6 transition. In order to further support our model, time-resolved PL data have been collected in all of the CPs 1−5, probing the 5 D 0 → 7 F 2 emission at 614 nm in Figure 3b. In line with the excitation spectra, the lifetime measurements suggest that the ET to Nd 3+ ions originates from europium high energy states. The lifetime traces show a direct dependence of the Nd 3+ metal centers on the Eu 3+ emission. The recombination rate of the 5 D 0 → 7 F 2 transition decreases, while the CPs are loaded with Nd 3+ , further supporting the ET mechanism with exciton lifetimes spanning from 1.4 μs for pure Eu 3+ -based CPs to 41 μs for compound 1. Moreover, the dual emission profile of the mixed Ln 3+ CPs is excitation-sensitive; thus, we describe in Figure 3c the relative percentages of the emission in both the IR and vis regions correlated with the excitation energy. In Figure 3c, we show the normalized excitation sensitivity of compound 2, indicating that in certain excitation energies we can selectively excite the Nd and/or Eu emissive center, gaining control over the relative dual emission of the CPs.
In summary, we report a new series of layered cyanidobridged CPs, with heterometallic emissive centers and dual photoluminescence in both the IR and vis spectral regions. Using excitation spectroscopy and time-resolved spectroscopy, we elucidated the photochemical mechanism, demonstrating tunable emission rates by adjusting the Nd 3+ percentage in the crystals. The wide spectral coverage, together with control of the emission profile of the CPs, paves the way for the next generation of materials that can be deployed in optical sensing and imaging devices. ■ ASSOCIATED CONTENT
Detailed experimental procedures, crystallographic data, PXRD patterns, MP-AES analysis, and FTIR spectra for all of the samples (PDF) which is cofunded by the Republic of Cyprus and the European Regional Development Fund.