Color-Tunable and White-Light Luminescence in Lanthanide

Jun 23, 2017 - (4) Coordination polymers incorporating lanthanides have been used extensively for their emissive color tuning(5-10) and white-light(11...
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Color-Tunable and White-Light Luminescence in Lanthanide− Dicyanoaurate Coordination Polymers Ryan J. Roberts, Debbie Le, and Daniel B. Leznoff* Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada S Supporting Information *

ABSTRACT: The new lanthanide−dicyanoaurate coordination polymers [nBu4N]2[Ln(NO3)4Au(CN)2] (Ln = Sm, Dy) and Sm[Au(CN)2]3·3H2O were prepared and structurally characterized and their luminescence spectra described. The emissions of solid-solutions of [nBu4N]2[Ln(NO3)4Au(CN)2] (Ln = Ce, Sm, Eu, Tb, and Dy) were explored with an emphasis on their capacity for luminescent color tuning and white-light emission via the selection of composition, excitation wavelength, and temperature. Specifically, the binary solid-solutions [nBu4N]2[Ce0.4Dy0.6(NO3)4Au(CN)2] and [nBu4N]2[Sm0.75Tb0.25(NO3)4Au(CN)2], and the ternary solid-solutions [nBu4N]2[Ce0.2Sm0.6Tb0.2(NO3)4Au(CN)2] and [nBu4N]2[Ce0.33Eu0.17Tb0.5(NO3)4Au(CN)2], were prepared and examined in terms of suitability for color-tuning capacity. These results showcase that the emission from the [nBu4N]2[Ln(NO3)4Au(CN)2] framework has the capacity to be tuned to extremes corresponding to deep reds (CIE coordinates 0.65, 0.35), greens (0.28, 0.63), and deep blue/violet (0.16, 0.06) as well as white (0.31, 0.33). Conversely, the emission of the Sm[Au(CN)2]3·3H2O framework, when doped with the green phosphor Tb(III), changes only slightly because of the predominantly Au(I)-based emission and Sm(III) → Au(I) energy transfer.

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INTRODUCTION The use of lanthanides in materials, especially in their application toward luminescent properties, has attracted a considerable amount of attention and research.1 More recently, lanthanides have found use in coordination polymers, wherein they are combined with bridging ligands to form extended network structures.2 Lanthanide-containing coordination polymers can be advantageous since they are often solutionprocessable and also can be influenced by the choice of bridging ligand, typically organic in nature and often acting to sensitize the lanthanide to increase light absorption, thereby increasing emission intensity.3 Alternatively, transition metals are also used as sensitizers, perhaps most notably yttrium in lanthanidedoped YAG systems, wherein they generally have a better energy match and larger spin−orbit-coupling to enable energy transfer, thus offering some benefits when compared to organic bridging ligands.4 Coordination polymers incorporating lanthanides have been used extensively for their emissive color tuning5−10 and white-light11−15 emission properties, which have potential applications as the phosphor coating in white-light-emitting diodes (pc-WLEDs).4 Dicyanoaurate, [Au(CN)2]−, is a metal-containing bridging ligand that forms coordination polymers readily, as well as often displays closed shell Au···Au interactions, otherwise known as aurophilic bonding;16 such systems are often found to be luminescent and sensitively dependent on the interatomic Au··· Au geometry17 and distance.18 Indeed, one of the more © 2017 American Chemical Society

extensively studied coordination polymers of dicyanoaurate is the aurophilic-interaction-rich Ln[Au(CN)2]3·3H2O series, in which the dicyanoaurate bridging unit interacts with the lanthanide center, often acting as a sensitizer for emission from the lanthanide center.19−28 We recently reported a new series of dicyanoaurate−lanthanide coordination polymers, [nBu4N]2[Ln(NO3)4Au(CN)2] (Ln = Ce, Nd, Eu, Gd, Tb),29,30 which lacks the aforementioned aurophilic interactions. A study investigating the doped systems [nBu4N]2[EuxTb1−x(NO3)4Au(CN)2] and EuxTb1−x[Au(CN)2]3·3H2O illustrated the difference between the two frameworks, wherein the former’s emission color could be tuned from red through yellow to green, whereas in the latter, energy transfer permitted only red europium-based emission to be observed regardless of the terbium doping level.31 This aurophilicity-free system provides an opportunity to sample the complete color range available to dicyanoaurate− lanthanide materials. Since only minimal energy transfer exists as shown in the [nBu4N]2[EuxTb1−x(NO3)4Au(CN)2] example, the luminescence spectra are additive, allowing for the emission color to be tuned via lanthanide composition. For a full exploration into the extent of color tuning possible in this system, a wider range of emissive colors is desirable; specifically, a blue phosphor is needed in order to extend the emission Received: March 20, 2017 Published: June 23, 2017 7948

DOI: 10.1021/acs.inorgchem.7b00735 Inorg. Chem. 2017, 56, 7948−7959

Inorganic Chemistry

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gamut beyond the red/yellow/green available with the europium−terbium solid-solutions. Recently, we reported the cerium analogue of the [nBu4N]2[Ln(NO3)4Au(CN)2] series (LnAu),30 which exhibits a broad, blue emission that provides an additional point to which to extend the potential gamut available for color tuning. Since the three phosphors corresponding to Ln = Ce, Eu, and Tb comprise a set of color primaries, we can project the maximum size of the possible color gamut as described by the CIE color-space diagram (Figure 1).

Article

EXPERIMENTAL SECTION

General Procedures and Materials. All reactions were carried out in air. [nBu4N][Au(CN)2]· 1 H2O was synthesized according to the 2 literature procedure.32 All other materials were obtained from commercial sources and used as received. For solid-solutions, compounds are specified on the basis of the nominal concentrations of their respective mother liquors. Infrared spectra were recorded on a Thermo Nicolet Nexus 670 FTIR spectrometer equipped with a Pike MIRacle attenuated total reflection (ATR) sampling accessory (germanium crystal: 4000−700 cm−1). Raman spectra were collected on a Renishaw inVia Raman microscope equipped with either a 785 or 514 nm laser. Microanalyses (C, H, N) were performed by Paul Mulyk at Simon Fraser University on a Carlo Erba EA 1110 CHN elemental analyzer. Elemental analyses for heavy elements (Ne−Am), i.e., Ln and Au, were determined with the Panalytical Epsilon 3XLE energy dispersive X-ray fluorescence (EDXRF) spectrometer; results are reported with respect to the amount of gold in the material. Syntheses. Synthesis of [nBu4N]2[Sm(NO3)4Au(CN)2] (SmAu). A solution of [nBu4N](NO3) (31 mg, 0.1 mmol) and [nBu4N][Au(CN)2]· 1 H2O (50 mg, 0.1 mmol) in 2 mL of ethyl acetate was added 2 to a solution of Sm(NO3)3·6H2O (43 mg, 0.1 mmol) in 1 mL of ethyl acetate in a 3 mL vial. The vial was sealed, and after 1 day, colorless crystals of [nBu4N]2[Sm(NO3)4Au(CN)2] (SmAu) were obtained by filtration. Yield: 0.074 g, 65%. Anal. Calcd for C34H72N8AuO12Sm· H2O: C 35.50%, H 6.48%, N 9.74%. Found: C 35.55%, H 6.69%, N 10.00%. XRF (nominal): Sm1Au1. Found: Sm0.97Au1. IR (ATR, cm−1): 2958 (s), 2933 (m), 2874 (m), 2182 (νC≡N, m), 1493 (s), 1311 (vs), 1153 (w), 1027 (m), 886 (w), 818 (m), 740 (m). Raman (514 nm, cm−1): 2953 (s), 2921 (vs), 2865 (vs), 2730 (m), 2607 (s, br), 2190 (νC≡N, w), 1699 (m, vbr), 1510 (w), 1445 (m, br), 1312 (w), 1125 (w), 1047 (w), 1029 (m), 902 (vw), 866 (vw), 794 (vw), 734 (vw), 304 (vw), 249 (vw). Synthesis of Sm[Au(CN)2]3·3H2O (SmAu3). A solution of K[Au(CN)2] (87 mg, 0.3 mmol) in 2 mL of water was added to a solution of Sm(NO3)3·6H2O (47 mg, 0.1 mmol) in 1 mL of water. The solution was sealed, and after several days, pale yellow crystals of Sm[Au(CN)2]3·3H2O (SmAu3) were collected by filtration. Yield: 0.039 g, 41%. Anal. Calcd for C9H6N9Au3O3Sm: C 7.56%, H 0.63%, N 8.82%. Found: C 7.65%, H 0.55%, N 8.65%. XRF (nominal): Sm1Au3. Found: Sm0.98Au3. IR (ATR, cm−1): 3578 (m), 3520 (m), 2151 (νC≡N, s), 1607 (m). Raman (514 nm, cm−1): 2738 (m, br), 2545 (m, br), 2172 (νC≡N, vs), 1589 (w, br), 681 (vw), 462 (vw), 342 (w), 221 (vw). Synthesis of [nBu4N]2[Dy(NO3)4Au(CN)2] (DyAu). The synthesis is analogous to that of SmAu; however, Dy(NO3)3·6H2O (48 mg, 0.1 mmol) was used, and colorless crystals of [nBu4N]2[Dy(NO3)4Au(CN)2] (DyAu) were obtained by filtration. Yield: 0.082 g, 72%. Anal. Calcd for C34H72N8AuDyO12·(H2O): C 35.13%, H 6.42%, N 9.64%. Found: C 35.12%, H 6.57%, N 9.88%. XRF (nominal): Dy1Au1. Found: Dy1.03Au1. IR (ATR, cm−1): 2959 (s), 2935 (m), 2974 (m), 2186 (νC≡N, m), 1493 (s, br), 1322 (vs), 1154 (w), 1030 (m), 884 (w), 818 (w), 744 (m). Raman (514 nm, cm−1): 2955 (s), 2921 (vs), 2868 (vs), 2732 (w, br), 2196 (νC≡N, s), 1632 (vw), 1515 (w), 1461(w), 1443 (w), 1321 (w), 1127 (w), 1047 (w), 1033 (m), 906 (w), 879 (w), 797 (vw), 741 (w), 702 (vw), 327 (w), 250 (w). Synthesis of [nBu4N]2[Sm0.75Tb0.25(NO3)4Au(CN)2] (SmTbAu). The synthesis is analogous to that of SmAu; however, a combination of Sm(NO3)3·6H2O (33 mg, 75 μmol) and Tb(NO3)3·6H2O (11 mg, 25 μmol) was used, and colorless crystals of [ n Bu 4 N] 2 [Sm0.75Tb0.25(NO3)4Au(CN)2] (SmTbAu) were obtained by filtration. Yield: 0.093 g, 82%. Anal. Calcd for C34H72N8AuO12Sm0.75Tb0.25· (H2O)1.5: C 35.16%, H 6.51%, N 9.65%. Found: C 35.15%, H 6.55%, N 9.92%. XRF (nominal): Sm0.75Tb0.25Au1. Found: Sm0.66Tb0.31Au1. IR (ATR, cm−1): 2958 (s), 2935 (m), 2874 (m), 2183 (νC≡N, m), 1493 (s, br), 1311 (vs), 1153 (w), 1027 (m), 886 (w), 818 (w), 739 (m). Raman (785 nm, cm−1): 2932 (w), 2873 (w), 2197 (m), 1445 (m), 1324 (w), 1130 (m), 1052 (w), 1035 (s), 910 (m), 879 (w), 800 (w), 740 (w), 706 (w), 321 (m), 257 (m).

Figure 1. CIE color-space and coordinates of the emissive colors of LnAu materials (Ln = Ce, Sm, Eu, Tb, and Dy), and (left) actual emissive colors of LnAu materials illustrating the potential color gamut available to the framework.

In order to harness this coordination polymer system, which is isostructural for [nBu4N]2[Ln(NO3)4Au(CN)2] (Ln = Ce, Nd, Eu, Gd, and Tb29,30), and to fully explore the range of materials possible as well as the gamut of emission colors in comparison to Ln[Au(CN)2]3·3H2O, we have also synthesized and characterized two new pure members of [nBu4N]2[Ln(NO3)4Au(CN)2] (Ln = Sm and Dy), as well as Sm[Au(CN)2]3·3H2O. With these new materials in hand, in combination with previously synthesized materials, an exploration of several representative mixed composition members of the [nBu4 N]2[Ln(NO3) 4Au(CN)2] (LnAu) and Ln[Au(CN)2]3·3H2O series (LnAu3), specifically bimetallic solidsolutions of blue/yellow [nBu4N]2[Ce0.4Dy0.6(NO3)4Au(CN)2] and orange/green [nBu4N]2[Sm0.75Tb0.25(NO3)4Au(CN)2], and ternary [ n Bu 4 N] 2 [Ce 0.2 Sm 0.6 Tb 0.2 (NO 3 ) 4 Au(CN) 2 ] and [nBu4N]2[Ce0.33Eu0.17Tb0.5(NO3)4Au(CN)2], and Sm0.75Tb0.25[Au(CN)2]3·3H2O for comparison were prepared and their emissive properties reported herein. The set of isomorphous [nBu4N]2[LnxLny′Lnz″(NO3)4Au(CN)2] solid-solutions allows for the tuning of the emission color nearly throughout the range inscribed on the CIE diagram in Figure 1. The additional tunable parameters of excitation wavelength and temperature, both of pure compounds and solid-solutions, to further expand and refine colors available from this palette created by lanthanide solid-solution/[Au(CN)2]− systems were also investigated. 7949

DOI: 10.1021/acs.inorgchem.7b00735 Inorg. Chem. 2017, 56, 7948−7959

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Inorganic Chemistry Synthesis of Sm0.75Tb0.25[Au(CN)2]3·3H2O (SmTbAu3). The synthesis is analogous to that of SmAu3; however, a combination of Sm(NO3)3·6H2O (33 mg, 75 μmol) and Tb(NO3)3·6H2O (11 mg, 25 μmol) was used, and colorless crystals of SmTbAu3 were obtained by filtration. Yield: 0.066 g, 69%. Anal. Calcd for C9H6N9Au3O3Sm0.75Tb0.25: C 7.56%, H 0.63%, N 8.81%. Found: C 7.65%, H 0.55%, N 8.65%. XRF (nominal): Sm 0.75 Tb 0.25 Au 3 . Found: Sm0.83Tb0.14Au3. IR (ATR, cm−1): 3578 (m), 3520 (m), 2152 (νC≡N, s), 1607 (m). Raman (785 nm, cm−1): 2176 (νC≡N, vs), 2170 (νC≡N, m), 852 (w), 677 (w), 468 (vw), 347 (m), 329 (w), 225 (vw), 186 (vw), 153 (vw), 116 (w). Synthesis of [nBu4N]2[Ce0.4Dy0.6(NO3)4Au(CN)2] (CeDyAu). A solution of [nBu4N](NO3) (0.030 g, 0.1 mmol) in 1 mL of ethyl acetate was added to a solution of Ce(NO3)3·6H2O (19 mg, 40 μmol) and Dy(NO3)3·6H2O (27 mg, 60 μmol) in 2 mL of ethyl acetate in a 40 mL glass vial, causing some precipitate to form. The resulting solution was diluted to 35 mL of ethyl acetate, until the precipitate was dissolved. To this solution, a solution of [nBu4N][Au(CN)2]· 1 H2O 2 (0.050 g, 0.1 mmol) in 1 mL of ethyl acetate was then added. After a few days, colorless crystals of [nBu4N]2[Ce0.4Dy0.6(NO3)4Au(CN)2] (CeDyAu) were collected by filtration. Yield: 0.043 g, 38%. Anal. Calcd for C34H72N8AuCe0.4Dy0.6O12·(H2O)1.5: C 35.13%, H 6.50%, N 9.64%. Found: C 35.12%, H 6.57%, N 9.88%. XRF (nominal): Ce0.40Dy0.60Au1. Found: Ce0.26Dy0.57Au1. IR (ATR, cm−1): 2958 (s), 2936 (m), 2874 (m), 2181 (νC≡N, m), 1493 (s, br), 1307 (s), 1155 (w), 1026 (m), 886 (w), 816 (w), 739 (w). Raman (514 nm, cm−1): 2924 (s), 2867 (s), 2193 (w), 2173 (w), 1443 (m), 1314 (w), 1127 (w), 1048 (m), 1029 (m), 905 (w), 875 (w), 740 (w), 307 (w), 250 (w). Synthesis of [nBu4N]2[Ce0.2Sm0.6Tb0.2(NO3)4Au(CN)2] (CeSmTbAu). The synthesis is analogous to that of CeDyAu; however, a combination of Ce(NO3)3·6H2O (9 mg, 20 μmol), Sm(NO3)3· 6H2O (27 mg, 60 μmol), and Tb(NO3)3·6H2O (9 mg, 20 μmol) was used, and colorless crystals of [nBu4N]2[Ce0.2Sm0.6Tb0.2(NO3)4Au(CN)2] (CeSmTbAu) were obtained by filtration. Yield: 0.066 g, 58%. Anal. Calcd for C34H72N8AuCe0.2O12Sm0.6Tb0.2·(H2O): C 35.51%, H 6.48%, N 9.74%. Found: C 35.55%, H 6.45%, N 9.68%. XRF (nominal): Ce0.20Sm0.60Tb0.20Au1. Found: Ce0.19Sm0.49Tb0.31Au1. IR (ATR, cm−1): 2959 (s), 2934 (m), 2874 (m), 2183 (νC≡N, m), 2147 (νC≡N, w), 1493 (s, br), 1311 (vs), 1153 (w), 1027 (m), 885 (w), 818 (m), 740 (m). Raman (785 nm, cm−1): 2932 (w), 2871 (w), 2196 (w), 1517 (w), 1446 (m), 1318 (w), 1131 (m), 1052 (w), 1035 (s), 910 (m), 879 (w), 739 (w), 705 (w), 310 (w), 255 (m). Synthesis of [nBu4N]2[Ce0.33Eu0.17Tb0.5(NO3)4Au(CN)2] (CeEuTbAu). The synthesis is analogous to that of CeDyAu; however, a combination of Ce(NO3)3·6H2O (17 mg, 40 μmol), Eu(NO3)3· 6H2O (10 mg, 20 μmol), and Tb(NO3)3·6H2O (30 mg, 60 μmol) was used, and colorless crystals of [nBu4N]2[Ce0.33Eu0.17Tb0.5(NO3)4Au(CN)2] (CeEuTbAu) were obtained by filtration. Yield: 0.085 g, 62%. Anal. Calcd for C34H72N8AuCe0.33O12Eu0.17Tb0.50·(H2O)3: C 34.39%, H 6.62%, N 9.44%. Found: C 34.10%, H 6.58%, N 9.74%. XRF (nominal): Ce0.33Eu0.17Tb0.50Au1. Found: Ce0.24Eu0.20Tb0.57Au1. IR (ATR, cm−1): 2960 (m), 2936 (m), 2974 (m), 2183 (νC≡N, m), 1493 (s, br), 1311 (s), 1152 (w), 1028 (m), 886 (w), 818 (w), 743 (w). Raman (785 nm, cm−1): 2926 (w), 2872 (w), 2198 (w), 2178 (w), 1446 (m), 1323 (w), 1132 (m), 1052 (m), 1037 (s), 910 (m), 882 (w), 744 (w), 707 (w), 326 (w), 314 (w), 257 (w). Luminescence Experiments. Solid-state photoluminescence spectra were collected on a Horiba Jobin Yvon Fluorolog-3 fluorimeter equipped with a xenon arc lamp and TBX single photon counter. The powdered samples were loaded into a standard NMR tube which was in turn loaded into a coldfinger Dewar flask equipped with a quartz window. For 77 K spectra, liquid nitrogen was used as the cryogen. Excitation spectra were corrected for lamp response. X-ray Crystallography. Single crystals of SmAu, SmAu3, and DyAu were mounted on a MiTeGen sample holder using paratone oil, and the intensity data was collected at room temperature on a Bruker SMART APEX II diffractometer equipped with a Mo Kα (λ = 0.7109 Å) source. The data was processed with the Bruker APEX II software

suite and solved with the intrinsic phasing option. Subsequent refinements were performed using ShelXle.33 Hydrogen atoms were placed in geometrically calculated positions. Diagrams were prepared using ORTEP-334 and POV-RAY.35 Additional crystallographic information can be found in Table 1, and Table S1 in the Supporting

Table 1. Selected Bond Lengths and Angles of SmAu, DyAu, and SmAu3 Ln−N (Å) N−C (Å) C−Au (Å) N−Ln−N (deg) Ln−N−C (deg) N−C−Au (deg) C−Au−C (deg)

SmAu

DyAu

SmAu3

2.545(8) 1.105(11) 1.978(7) 74.4(4) 172.6(1) 174.5(1) 173.2(7)

2.470(7) 1.141(12) 1.968(8) 74.6(4) 174.3(9) 172.3(1) 174.5(6)

2.596(10) 1.109(18) 2.005(13) 74.9(3) 161.0(1) 175.1(11) ≡180

Information. Supplementary crystallographic data for DyAu, SmAu, and SmAu3 is available free of charge courtesy of The Cambridge Crystallographic Data Centre CCDC: 1482863−1482865, respectively. Powder X-ray diffractograms of CeDyAu, SmTbAu, CeSmTbAu, and CeEuTbAu (Figure 2a) as well as SmTbAu3 (Figure 2b) were collected using a Bruker SMART APEX II equipped with an Incoatec IμS Cu Kα source (λ = 1.540 56 Å). Samples were mounted on MiTeGen sample holders using paratone oil and were exposed as the ϕ axis was spinning (6 deg s−1), for a period of 60 min.

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RESULTS AND DISCUSSION Syntheses and Structures. Sm[Au(CN) 2 ] 3 ·3H 2 O (SmAu3) and Sm0.75Tb0.25[Au(CN)2]3·3H2O (SmTbAu3) were synthesized analogously to previous LnAu3 compounds.31 An aqueous solution containing 3 equiv of K[Au(CN)2] was added to an aqueous solution containing 1 equiv of the respective Ln(NO3)3·6H2O salt or salts, whereupon hexagonal crystals of the products were collected after several days. The non-cerium-containing LnAu compounds [n Bu 4N]2 [Dy(NO3)4Au(CN)2] (DyAu), [nBu4N]2[Sm(NO3)4Au(CN)2] (SmAu), and [ n Bu 4 N] 2 [Sm 0.75 Tb 0.25 (NO 3 ) 4 Au(CN) 2 ] (SmTbAu) were synthesized analogously to previous LnAu compounds,29 wherein an ethyl acetate solution of 1 equiv each of [nBu4N](NO3) and [nBu4N][Au(CN)2]· 1 H2O was added to 2 an ethyl acetate solution of the respective Ln(NO3)3·6H2O salt or salts, whereupon block-shaped crystals were collected after several days. Cerium-containing LnAu compounds [nBu4N]2[Ce0.4Dy0.6(NO3)4Au(CN)2] (CeDyAu), [nBu4N]2[Ce0.2Sm0.6Tb 0.2 (NO 3 ) 4 Au(CN) 2 ] (CeSmTbAu), and [ n Bu 4 N] 2 [Ce0.33Eu0.17Tb0.5(NO3)4Au(CN)2] (CeEuTbAu) were prepared similarly,30 however in more dilute conditions. All compounds are reported on the basis of the nominal concentrations of their respective mother liquors. The single crystal X-ray structures of DyAu and SmAu were determined and confirmed to belong to the isomorphous set of previously reported [nBu4N]2[Ln(NO3)4Au(CN)2] (Ln = Ce, Nd, Eu, Gd, and Tb) materials.29,30 The single crystal X-ray structure of SmAu3 (previously incorrectly described)23 was also determined and found to be a member of the isomorphous set of Ln[Au(CN)2]3·3H2O (Ln = La, Ce, Pr, Nd, Eu, Gd, and Tb)20−22,27,30,31 materials (Table S1). For the blended materials SmTbAu, CeDyAu, CeSmTbAu, CeEuTbAu, and SmTbAu3, powder X-ray diffractograms were collected, which confirmed the phase purities of [nBu4N]2[Ln(NO3)4Au(CN)2] and Ln[Au(CN)2]3·3H2O systems, respectively (Figure 2). 7950

DOI: 10.1021/acs.inorgchem.7b00735 Inorg. Chem. 2017, 56, 7948−7959

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Figure 2. (a) Powder X-ray diffractograms of CeDyAu, SmTbAu, CeSmTbAu, and CeEuTbAu, with accompanying simulated diffractogram of DyAu. (b) SmTbAu3 with accompanying simulated diffractogram of SmAu3.

The structures of the LnAu materials show a zigzag-motif 1D coordination polymer composed of 10-coordinate, nitratebound lanthanide(III) centers (eight nitrato-oxygens and two N-cyano units) bridged by linear dicyanoaurate anions (Figure 3a). The anionic 1-D zigzag chain is surrounded by tetrabutylammonium cations. The cyanoaurate gold centers are well-separated from each other, both within and between

the chains; thus, there are no aurophilic interactions in these materials. The structure of LnAu3 is composed of a 3-D coordination polymer that can be described as a Kagomé network in which the lanthanide(III) center is nine-coordinate tricapped trigonal prismatic with six N-cyano donors originating from dicyanoaurate units, and is capped by three bound water molecules (Figure 3b). The network is 2-fold interpenetrated, supported by aurophilic interactions on the order of 3.1−3.35 Å between the networks (Table 1). Methodology of Color Tuning. For the LnAu series, we have shown that the emission has the capacity to be tuned by the relative composition of x in [nBu4N]2[EuxTb1−x(NO3)4Au(CN)2], from the characteristic red emission of Eu(III) to orange, yellow, chartreuse, and eventually the characteristic green of Tb(III) (x = 1, 0.75, 0.50, 0.25, and 0, respectively). In contrast, the analogous members of the LnAu3 series, EuxTb1−x[Au(CN)2]3·3H2O (x = 0.75, 0.50, and 0.25), did not show this emission tunability, and only a Eu(III)-based emission was observed as low as x = 0.25.31 This is consistent with the original studies of the luminescence properties of EuAu3, wherein it was found that the [Au(CN)2]− anion efficiently sensitizes Eu(III) emission at low temperature (and no [Au(CN)2]− emission was observed),24,36 and the later study of TbAu3 where it was found that the [Au(CN)2]− anion sensitizes Tb(III) emission, but to a lesser degree.27,37 To expand the scope of Ln(III) phosphors available to incorporate into blended systems, new pure compounds of [nBu4N]2[Dy(NO3)4Au(CN)2] (DyAu) and [nBu4N]2[Sm(NO3)4Au(CN)2] (SmAu), as well as its aurophilic analogue Sm[Au(CN)2]3·3H2O (SmAu3), were synthesized. The new compounds of SmAu and DyAu together with the previously reported LnAu series (Ln = Ce, Eu, and Tb)29,30 give the potential to span a variety of emitted colors by controlling the degree of lanthanide dopant, and also the temperature and the excitation wavelength (Figure 1). Given the large parameter space in the LnAu series that could be sampled in these materials, several compositions of solid-solutions were explored at 298 and 77 K at excitation wavelengths that best illustrate the maximum possible gamut

Figure 3. (a) One-dimensional chain of [nBu4N]2[Dy(NO3)4Au(CN)2] with [nBu4N]+ cations removed for clarity. (b) Threedimensional network of Sm[Au(CN)2]3·3H2O with H2O molecules removed for clarity. Dy, green; Sm, orange; Au, gold; O, red; N, blue; C, gray. 7951

DOI: 10.1021/acs.inorgchem.7b00735 Inorg. Chem. 2017, 56, 7948−7959

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Figure 4. (a) Luminescence spectra of DyAu at 298 K with superimposed spectra (bottom) to show relative intensity. (b) CIE plot with photograph of DyAu illuminated by an ultraviolet lamp (λmax = 370 nm, inset). Experimental parameters and CIE coordinates can be found in Table S2.

spectra of DyAu remain similar apart from the increase in intensity of the Au(I) bands relative to that of the Dy(III) bands (Figure S1). The bands in the blue region of the spectrum appear to be composed of two components: one, the most intense, is located at λmax = 392 nm, and a shoulder is located at 420 nm which is consistent with Au(I)-based 1 MLCT.29,31 Overall, upon changes in excitation wavelengths and temperature, the color gamut of this system ranges from the yellow end of white [λex = 364, T = 298 K, CIE(x, y) = 0.33, 0.39] to the blue/purple end of white [λex = 333, T = 77 K, CIE(x, y) = 0.27, 0.22] (Figure 4b, and Table S2), mainly attributable to the increased intensity of the blue bands associated with gold(I) upon cooling. The four excitations mapped in Figure 4b illustrate the ability of two independent levers to control the overall emission in DyAu. First, the selection of wavelength can be used to bias the emission toward either the gold- or dysprosium-based phosphor, which results in emissions on the blue and yellow sides of white, respectively. Alternatively, lowering the temperature increases the relative intensity of the gold(I) (blue) emission, which in turn blue-shifts the emission, resulting in an overall bluer possible palette than at ambient temperature and also an emission that is nearly purely white. The use of a broader (i.e., non-monochromated) excitation source centered around λ = 370 nm and at ambient temperature on DyAu results in an overall yellow emission (Figure 4b, inset). This is consistent with the predominantly dysprosium-based emission that is observed at this temperature (Figure 4a) Since DyAu as a pure material emits primarily in the blue and yellow region of white depending on the temperature and excitation wavelength, it was desirable to see if this system could be further extended into the blue region, especially at room temperature with a non-monochromated excitation source. For this reason, cerium was chosen as a dopant in the DyAu system as it is a well-known blue phosphor, and its pure analogue CeAu has been reported recently.30 The addition of Ce(III) as a 40% dopant in DyAu affords [nBu4N]2[Ce0.4Dy0.6(NO3)4Au(CN)2] (CeDyAu); it is noted that since the combination of cerium and dysprosium represents the largest range in terms of lanthanide radii available in the series, this synthesis implies that any

achievable in each material. In addition to this, a color image of the emission of each material at ambient temperatures under a broader (non-monochromated) UV excitation source centered at λmax = 370 nm was taken to provide a view of the overall emission color under broader excitation sources. The compounds and solid-solution blends chosen to be examined here can be grouped into three different categories to illustrate the effect of doping on the resulting color gamut. The first category explores the pure DyAu system (which has a predominantly yellow- to blue-white emission) and the result obtained when the blue-emissive Ce(III) is doped into it. Since the pure DyAu system emits yellow at room temperature under broadband UV, the addition of a blue phosphor was chosen to test whether a “cooler” white emission could be tuned. The second category explores the new pure materials SmAu and SmAu3 (which emit orange/purple) and the effect of doping green-emissive Tb(III) into these systems on the overall emission color. These two systems were chosen as analogous sets to the EuTbAu and EuTbAu3 that were previously studied, to see if there are any advantages in replacing the red primary from Eu(III) with Sm(III), and also to examine the trends in the analogous aurophilic materials. Finally, the ternary doped systems of blue−orange−green CeSmTbAu and blue−red− green CeEuTbAu were explored to compare the overall gamut of these “RGB” systems. The goal is less a detailed photophysical study of the materials (which is in progress separately) and more an understanding of the levers that can be wielded to control the final observed emission color. Luminescence of DyAu and CeDyAu. The luminescence of DyAu is primarily Dy(III)-based at room temperature, with well-defined f−f bands characteristic of Dy(III) (Figure 4a, yellow), the primary emission bands of which are located at λ = 572.5 and 480.5 nm, with the most intense of the several excitation bands located at λ = 364 nm. At room temperature, a Au(I)-based emission is also observed with a broad λmax = 373 nm and a corresponding excitation band of λmax = 322 nm, albeit at a much lower intensity as compared to that of Dy(III) (Figure 4a, bottom). The emission and excitation profile of the Au(I) bands is interrupted by Dy(III)-based excitation (Figure 4a, blue). As a result, the assignment of the true λmax, which usually occurs at ≈ 405 nm near ambient temperature in these systems, may be affected. Upon cooling of the system to 77 K, the overall 7952

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Figure 5. (a) Luminescence spectra of CeDyAu at 298 K with superimposed spectra (bottom) to show relative intensity. (b) CIE plot with photograph of CeDyAu illuminated by an ultraviolet lamp (λmax = 370 nm, inset). Experimental parameters and CIE coordinates can be found in Table S2.

Figure 6. (a) Luminescence spectra of SmAu at 298 K with superimposed spectra (bottom) to show relative intensity. (b) CIE plot with photograph of SmAu illuminated by an ultraviolet lamp (λmax = 370 nm, inset). Experimental parameters and CIE coordinates can be found in Table S2.

emission, and overall the emission shifts further into the blue region. Under illumination from a UV-lamp at ambient temperature, the material emits an overall blue color (Figure 5b, inset). The color gamut of CeDyAu is similar to that of DyAu; however, most of the resulting emissions are blue or bordering on blue/white with the exception of the 298 K spectrum of λex = 451 nm [CIE(x, y) = 0.33, 0.40] which is closer to the yellow edge of white (Figure 5b and Table S2). This illustrates the effectiveness of the addition of Ce(III) to enhance the blue emission of LnAu systems. Luminescence of SmAu, SmAu 3, SmTbAu, and SmTbAu3. The 298 K luminescence spectra of SmAu are qualitatively much less intense than those of other members of the LnAu series. Notably, at this temperature, Au(I) bands predominate over Sm(III) bands (Figure 6a, violet); this is unusual in the LnAu series. The most prominent excitation band has λmax = 396 nm, but it is interrupted by a Sm(III) excitation band at 403.5 nm giving rise to what appears to be a second band at 407.5 nm; the “true” maximum of the Au(I)-1MLCT almost certainly lies between these two points, in line with other LnAu compounds reported herein and in the

combination of the known members of LnAu (Ce, Nd, Sm, Gd, Eu, Tb, and Dy) should be compatible in a solid-solution. The 298 K emission spectra of CeDyAu are still largely Dy(III)based and are similar to those of DyAu (Figure 5a, yellow). The emission in the blue region of the spectrum (Figure 5a, blue) has a λmax of 397.5 nm, which is similar to the Ce(III) emission (CeAu = 393 nm, [nBu4N]3[Ce(NO3)6] = 390 nm)30 but is also reminiscent of Au(I)-based bands (TbAu = 400 nm, SmAu > 395 nm). However, the excitation spectrum is very similar to that of CeAu, with an analogously small Stokes shift and poorly resolved features.30 At 77 K (Figure S2), emissions in the blue region overall become more intense relative to the yellow Dy(III)-based emissions, resulting in an overall emission profile that is much more blue compared to that of the parent DyAu material. At ambient temperatures, depending on excitation wavelength, either a yellow or blue emission can be selected, as in DyAu. However, since the blue band is much more intense in CeDyAu as compared to that of the pure DyAu, the result is, at ambient temperature, a much bluer emission. Upon a decrease in temperature, these blue bands, as in the case of DyAu, become more intense relative to the dysprosium-based yellow 7953

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Figure 7. (a) Luminescence spectra of SmAu3 at 298 K. (b) CIE plot with photograph of SmAu3 illuminated by an ultraviolet lamp (λmax = 370 nm, inset). Experimental parameters and CIE coordinates can be found in Table S2.

Figure 8. (a) Luminescence spectra of SmTbAu at 298 K with superimposed spectra (bottom) to show relative intensity. (b) CIE plot with photograph of SmTbAu illuminated by an ultraviolet lamp (λmax = 370 nm, inset). Experimental parameters and CIE coordinates can be found in Table S2.

previous literature.29−31 The corresponding excitation spectrum of the 396 nm emission band is generally broad with several “negative peaks” corresponding to Sm(III) excitations (Figure 6a, red). The peaks corresponding to the Sm(III)-based emission (Figure 6a, red) are largely unremarkable; however, they can be generated by exciting the Au(I) component (λex = 397 nm) and through either an energy transfer or reabsorption mechanism producing the characteristic Sm(III) emission (λ = 561, 593.5, 641.5, and 702 nm). The 77 K luminescence spectra of SmAu, even more so than at 298 K, are predominated by the Au(I)-based emission with a corresponding λ max = 420.5 nm [Au(I)- 1 MLCT]; the asymmetry in this peak is likely due to underlying Sm(III) excitation bands (Figure S3, violet). Upon cooling, there is also the emergence of a new band centered at λ ≈ 630 nm tentatively assigned to Au(I)-3MLCT. The characteristic albeit weak Sm(III)-based emission can be isolated with a judicious choice of excitation wavelength (Figure S3, red); however, the excitation spectrum is poorly defined, likely because of interference from underlying Au(I)-based transitions. The color gamut of SmAu extends from the red/orange [λex = 480, CIE(x, y) = 0.59, 0.40] through salmon to eventually

magenta [λex = 380, CIE(x, y) = 0.28, 0.12] (Figure 6b and Table S2). Under illumination from a UV lamp at ambient temperature, the compound is overall orange/pink in emission (Figure 6b, inset), which is consistent with the combination of both a blue gold(I)-based emission and an orange samariumbased emission. Again, the choice of excitation wavelength and temperature has drastic consequences on the emission of the material, depending on whether the blue gold(I)-based or the orange samarium-based emission is selected. Lowering the temperature enhances the gold(I)-based bands, resulting in emission in the pink and purple region of the CIE diagram (Figure 6b). As a comparison to the previously reported EuTbAu3 system, the analogous aurophilic Sm(III) Sm[Au(CN)2]3· 3H2O (SmAu3) and the Tb(III)-doped Sm0.75Tb0.25[Au(CN)2]3·3H2O (SmTbAu3) materials were prepared, specifically with an interest in the effect of possible energy transfer on the luminescence and thereby the system’s ability for tuning of emission color. The 298 K emission spectrum of SmAu3 is composed of a strong Au(I)-based peak at 404 nm and several weaker bands characteristic of Sm(III) emission bands (Figure 7a). The excitation spectrum at 298 K has the characteristic bands as 7954

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Figure 9. (a) Luminescence spectra of SmTbAu3 at 298 K. (b) CIE plot with photograph of SmTbAu3 illuminated by an ultraviolet lamp (λmax = 370 nm, inset). Experimental parameters and CIE coordinates can be found in Table S2.

In this case, the choice of excitation wavelength should drastically affect the emission color, effectively turning on or off specific phosphors. While various colors spanning orange/pink from the primarily Sm(III)-based emission [λex = 403.5 nm, CIE(x, y) = 0.42, 0.29] through white [λex = 375 nm, CIE(x, y) = 0.31, 0.28] to blues [λex = 353.5 nm, CIE(x, y) = 0.21, 0.26] and violet [λex = 312 nm, CIE(x, y) = 0.16, 0.05] can be generated by selection of excitation wavelength at 298 K, the 77 K spectra tend to be limited to blues [λex = 380.5 nm, CIE(x, y) = 0.19, 0.16], purples [λex = 375.5 nm, CIE(x, y) = 0.21, 0.12], and violets [λex = 309.5 nm, CIE(x, y) = 0.16, 0.05] (Figure 8b, Table S2). Under illumination from a broadband UV lamp at ambient temperature, the overall emission of SmTbAu is nearly white (Figure 8b, inset). Using this broader excitation source clearly illustrates the three phosphors (emitting blue, green, and orange) acting in concert, giving rise to an overall near-white emission. Upon comparison of the Tb(III)-doped SmTbAu gamut to that of the parent SmAu, the introduction of Tb(III) has effectively increased the potential value of y in the CIE coordinate system and decreased the potential value of x in these systems, allowing for whites, blues, deeper purples, and violet, which were inaccessible by the undoped parent. An important note in the case of SmTbAu, however, is that while all phosphors [Au(I), Sm(III), and Tb(III)] are evident in the luminescence spectra and hence yield a larger effective gamut as compared to SmAu, the relative intensities of most of these emissions are insignificant compared to the dominant blueemissive feature. This is, however, not the case in SmAu, where the effective gamut is smaller, but the range of relative intensities is much narrower. This is likely due to the introduction of additional energy-transfer pathways, giving rise to the partial quenching of specific transitions in favor of others. It is worthwhile to compare the result of doping terbium into the nonaurophilic SmAu with that of its aurophilic analogue SmAu3, specifically in regards to energy transfer, in the same vein as the previously reported terbium-doped analogues, i.e., EuTbAu and EuTbAu3.31 The 298 K emission spectrum of Sm0.75Tb0.25[Au(CN)2]3· 3H2O (SmTbAu3) is overall very similar to that of SmAu3, namely, a large Au(I)-based band at 406 nm and several much weaker peaks at lower energies corresponding to a combination of Sm(III) and Tb(III) emissions (Figure 9a). The excitation

seen in other aurophilic LnAu3 compounds of 366.5 and 322 nm.19 The partially defined Au(I)-based peak at 391.5 nm however is much sharper in profile compared to the others and is similar in wavelength to pure Sm(III)-based excitations (389 nm in SmAu, Figure 6a) which would imply samarium-togold(I) energy transfer. The 77 K spectra of SmAu3 are very similar to the 298 K spectra; the Au(I)-based peak shifts to 435 nm, and a shoulder appears at ≈ 475 nm, consistent with Au(I)-based 1MLCT and 3MLCT, respectively, found in other LnAu3 systems.19 The color gamut upon the change in excitation wavelength and temperature of SmAu3 is limited compared to that of SmAu as it predominantly shows a blue Au(I)-based emission (similar to other LnAu3 systems), and the small amount of intensity variation manifests itself as differing shades of violet [CIE(x, y) = (0.21, 0.11), T = 298 K; (0.18, 0.06), 77 K] (Figure 7b). Given this, at room temperature under illumination from a UV lamp, SmAu3 would best be described as pink, as opposed to the more characteristic orange of Sm(III) (Figure 7b, inset). Overall, SmAu 3 and other previously studied LnAu 3 materials are generally poor candidates for emissive colortuning materials. Specifically in the case of SmAu3, the possibility of energy transfer of the form Sm(III) → Au(I) also hinders the ability for its emission color to be tuned, since samarium emission is decreased in favor of gold(I)-based emission. For an extension of the gamut of SmAu beyond the orange to pink emission that the pure material is capable of, terbium was added as a dopant. This also provides a frame of comparison to the previously studied EuTbAu system. The 298 K luminescence spectra of SmTbAu are almost entirely composed of one intense Au(I)-based peak λ = 370 nm (Figure 8a, blue), which is partially interrupted by characteristic Sm(III) and Tb(III) excitations. Accompanying these peaks is a broad band at 409 nm similar to that observed in SmAu, which is a more typical λmax value for a Au(I)-based 1MLCT peak at 298 K. Characteristic Tb(III) (Figure 8a, green) and Sm(III) (Figure 8a, red) excitation and emission bands can be observed, however at much lower relative intensities (Figure 8a, bottom overlay). The 77 K spectra of SmTbAu are largely unchanged from 298 K (Figure S5) except for the appearance of a new band with λmax = 433 nm, which is typical of Au(I)-based 1 MLCT at 77 K. 7955

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Figure 10. (a) Luminescence spectra of CeSmTbAu at 298 K with superimposed spectra (bottom) to show relative intensity. (b) CIE plot with photograph of CeSmTbAu illuminated by an ultraviolet lamp (λmax = 370 nm, inset). Experimental parameters and CIE coordinates can be found in Table S2.

correspond to Au(I)-based 1MLCT and Ce(III) 5d → 4f, respectively.30 Additional information can be gleaned from the excitation spectra of the shoulder, wherein excitations corresponding to Ce(III) generate a shoulderless Au(I)-based emission, which suggests energy transfer of the form Ce(III) → Au(I). At much lower relative intensities, excitation and emission peaks characteristic of Tb(III) (Figure 10a, green) and Sm(III) (Figure 10a, red) are evident. As seen above, the cooling of CeSmTbAu to 77 K increases the intensity of the blue-emissive peak: a result of increased Au(I)- or Ce(III)based emissions, or a combination thereof. Upon excitation from a UV lamp at ambient temperature, the overall emission of CeSmTbAu appears nearly white (Figure 10b, inset) illustrating that significant portions from each of the blue, orange, and green phosphors are activated, which are not represented in the spectra from monochromated light. The overall gamut of CeSmTbAu at room temperature includes primarily Sm(III)-based pinks [λex = 403.5 nm, CIE(x, y) = 0.46, 0.33], Tb(III)-based near-white cyans [λex = 370 nm, CIE(x, y) = 0.23, 0.24; and λex = 380 nm, CIE(x, y) = 0.23, 0.23], and Au(I)/Ce(III)-based blue [λex = 310 nm, CIE(x, y) = 0.16, 0.06] (Figure 10b) depending on the excitation wavelength. Similar to what was found for SmTbAu, upon cooling to 77 K the overall color shifts toward blue/violet, and the resulting gamut at this temperature is decreased. The emission spectra at 77 K are predominated by Au(I)/Ce(III) emissions with little contribution from Sm(III) and Tb(III) resulting in blue/violet [λex = 308 nm, CIE(x, y) = 0.16, 0.06] and purples [λex = 380 nm, CIE(x, y) = 0.18, 0.07; and λex = 374.5 nm, CIE(x, y) = 0.18, 0.05]. For a comparison and evaluation of the key differences between samarium and europium as near-red phosphors in ternary doped LnAu systems, the cerium-loaded analogue CeEuTbAu was investigated. The 298 K spectra of [nBu4N]2[Ce0.33Eu0.17Tb0.5(NO3)4Au(CN)2] (CeEuTbAu) display relatively strong emissions from all four potential phosphors (in stark contrast to the aforementioned CeSmTbAu), in order of relative intensity, of Eu(III), Tb(III), and Au(I)/Ce(III). Characteristic Eu(III)-

spectrum is nearly identical to that of SmAu3 with two broad bands at 366 and 325 nm that are likely Au(I) in nature and a sharp peak at 393 nm that is not fully resolved which is tentatively assigned to a Sm(III) excitation. There is also a broad feature centered around ≈ 485 nm. The 77 K emission spectrum of SmTbAu3 is also very similar to that of SmAu3 and to the 298 K spectrum, with the Au(I)-1MLCT again shifting to λmax = 433 nm (Figure S6). The emission color when illuminated by a UV lamp at ambient temperature would be best described as primarily pink (Figure 9b, inset) which is very similar to that of the parent material SmAu3 (Figure 7a). Overall, the addition of Tb(III) into the SmAu3 system did little to change the overall color of emission (Figure 9b, compare to Figure 7b). The 298 K spectrum of SmTbAu3 has CIE coordinates of (0.21, 0.15) as compared to (0.21, 0.11) for SmAu3. The 77 K spectrum of SmTbAu3 is even more similar, with CIE coordinates of (0.17, 0.05) compared to that of SmAu3 with values of (0.18, 0.06). Upon comparison of this result to that of SmTbAu, there is a stark contrast in the ability of the SmAu3 series to be color-tuned through excitation selection. Since the Ln(III) components of SmAu3 and SmTbAu3 have such a small impact on the overall color of emission, the effect of doping Ln(III) has a correspondingly small impact on the color gamut of these systems. This result, along with the effect of energy transfer in the EuxTb1−x[Au(CN)2]3·3H2O system,31 again accentuates that the compounds in the LnAu3 series in general make poor hosts for emissive color-tuning materials. Luminescence of CeSmTbAu and CeEuTbAu. For a comparison of the maximum gamut of emissive colors possible in the LnAu series, and for a further expansion of the emissive color gamut into the blue region, the binary lanthanide solidsolution SmTbAu and EuTbAu series, which both show tunability via the terbium and samarium/europium loadings, respectively, were doped with cerium. As an example, the 298 K spectrum of [nBu4N]2[Ce0.2Sm0.6Tb0.2(NO3)4Au(CN)2] (CeSmTbAu) is predominantly Au(I)and Ce(III)-based (Figure 10a, violet) with corresponding λmax = 398.5 nm and a shoulder at approximately 372 nm. These 7956

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Figure 11. (a) Luminescence spectra of CeEuTbAu at 298 K with superimposed spectra (bottom) to show relative intensity. (b) CIE plot with photograph of CeEuTbAu illuminated by an ultraviolet lamp (λmax = 370 nm, inset). Experimental parameters and CIE coordinates can be found in Table S2.

When the comparison to the other “RGB” CeSmTbAu system is made in terms of accessible emission colors, several important factors need consideration. While neither CeSmTbAu nor CeEuTbAu has an ideal green emission because of overlapping excitation bands of Sm(III) and Eu(III), respectively, pure reds can be obtained in CeEuTbAu. With the effect of temperature as a desirable parameter, CeEuTbAu generally displays a trend toward the center of the CIE diagram (i.e., whiter) made by the extremes of the 298 K spectra, while CeSmTbAu upon cooling to 77 K yields almost exclusively a blue emission.

based emission and excitation bands (most prominently 591, 617, and 682 nm; and 395.5, 464, and 534.5 nm, respectively) are prominent in the overall luminescent landscape of CeEuTbAu (Figure 11a, red). Characteristic Tb(III)-based emission and excitation bands (most prominently 488.5, 542, and 581 nm; and 353.5, 369.5, 380, and 488 nm, respectively) are also observed (Figure 11a, green), and by careful selection of the excitation wavelength, a combination of both Eu(III) and Tb(III) emissions can be generated (Figure 5a, yellow). Several broad emissions also exist in the blue region with λmax = 368, 403.5, and 439.5 nm. Broad peaks in this region are typical of Ce(III) (5d → 4f) and/or Au(I)-1MLCT. Upon cooling to 77 K, several changes occur. Unlike previous examples, the emission spectra become overall less blue, as several of the higher energy peaks associated with a combination of Au(I) and Ce(III) shift to the typical values associated with Au(I) at 77 K of λex ≈ 430 nm (Figure S8). Additionally, at 77 K, a new energy-transfer pathway of the form Au(I) → Eu(III) becomes available, partially quenching Au(I)-based emissions in favor of Eu(III) emission.36 Thus, the emission color of CeEuTbAu when illuminated by a UV lamp at ambient temperature is nearwhite or slightly yellow (Figure 11b, inset); in a similar fashion to CeSmTbAu, a significant portion of the red, green, and blue phosphors are activated to produce a near-white-light emission. The overall luminescent color gamut of CeEuTbAu has a large spread (Figure 11b). The 298 K spectra contain the red Eu(III)-based emission at 298 K [λex = 464.5 nm, CIE(x, y) = 0.65, 0.35], through yellow from a combination of Eu(III)/ Tb(III) [λex = 384 nm, CIE(x, y) = 0.38, 0.41], and eventually to blue originating from Au(I)/Ce(III) [λex = 322 nm, CIE(x, y) = 0.18, 0.11; and λex = 312 nm, CIE(x, y) = 0.17, 0.10]. The gamut of the 77 K emission spectra is in general less extreme than its 298 K counterpart, comprising values that are in general closer to white; the most red of these are similar to those of the 298 K Eu(III)-based spectra [λex = 465 nm, CIE(x, y) = 0.63, 0.36], the pink/white Eu(III)-based spectra [λex = 383.5 nm, CIE(x, y) = 0.39, 0.31], the near-white Eu(III)/ Tb(III) [λex = 380.5 nm, CIE(x, y) = 0.35, 0.34], and the purple Au(I)-based [λex = 360 nm, CIE(x, y) = 0.26, 0.16].

■

CONCLUSION In this work, the [nBu4N]2[Ln(NO3)4Au(CN)2] framework was probed for its ability to be tuned in terms of its emission color. With these materials in hand, a series of binary and ternary doped systems were explored with an emphasis on the tunability of emission on the basis of composition, excitation wavelength, and temperature. The pure material [nBu4N]2[Dy(NO3)4Au(CN)2] presents a nearly white emission at ambient temperature; doping of Ce(III) into this system {[nBu4N]2[Ce0.4Dy0.6(NO3)4Au(CN)2]} succeeds in blue-shifting the overall luminescence. The other pure materials [nBu4N]2[Sm(NO3)4Au(CN)2] and Sm[Au(CN)2]3·3H2O have a combination of orange and blue phosphors, resulting in an overall coral and pink color, respectively. Incorporating the green Tb(III) phosphor into these systems {[nBu4N]2[Sm0.75Tb0.25(NO3)4Au(CN)2] and Sm0.75Tb0.25[Au(CN)2]3·3H2O} results in materials with red-, green-, and blue-emissive species, potentially allowing for white-light emission; this was realized in the former species but, because of energy transfer, was absent in the latter. Incorporating Ce(III) into [nBu4N]2[Sm0.75Tb0.25(NO3)4Au( C N ) 2 ] a n d t h e p r ev io u sly r ep o rt ed [ n B u 4 N ] 2 [EuxTb1−x(NO3)4Au(CN)2] systems results in three independently emissive lanthanide centers emitting red, green, and blue. Of these two, [nBu4N]2[Ce0.2Sm0.6Tb0.2(NO3)4Au(CN)2] is the closest to white-light emission under broadband UV excitation; however, [nBu4N]2[Ce0.33Eu0.17Tb0.5(NO3)4Au(CN)2] has a larger gamut available via excitation wavelength and temper7957

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Article

Inorganic Chemistry

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ature. More detailed photophysical studies of these materials are in progress. Thus, overall the [nBu4N]2[Ln(NO3)4Au(CN)2] framework has demonstrated the capacity for the emission color to be tuned throughout a large gamut of colors, from the deep reds of Eu(III) to the bright green of Tb(III) and the deep blues of Au(I) and Ce(III). In addition to the large gamut of emissive colors accessible to these materials, several compositions have white-light emission as a result of their combined red, green, and blue phosphors present.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00735. Crystallographic tables, 77 K luminescence spectra, and luminescence experimental parameters with CIE values (PDF) Accession Codes

CCDC 1482863−1482865 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: dleznoff@sfu.ca. ORCID

Daniel B. Leznoff: 0000-0002-3426-2848 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support via a Discovery Grant.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00735 Inorg. Chem. 2017, 56, 7948−7959