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Novel Luminescent Heterobimetallic Nanoclusters of Copper(I), Silver(I), and Gold(I) Doped in Different Alkali Halide Crystals Xiaobo Li,† Zhong Pan,§ François Baril-Robert,† John C. Ahern,† and Howard H. Patterson*,† †

Department of Chemistry, University of Maine, Orono, Maine 04469, United States Department of Civil Engineering, University of Maine, Orono, Maine 04469, United States

§

S Supporting Information *

ABSTRACT: Heterometallic d10−d10 nano systems of dicyanocuprate(I) with dicyanoaurate(I) or dicyanoargentate(I) doped in different alkali halide hosts (NaCl, NaBr, and KBr) have been studied. In addition to the characteristic emission peaks for each d10 anion cluster, these doped crystals show a new luminescent peak due to the heterometallic interaction between Cu and Au or Cu and Ag. This new long lifetime emission peak occurs at much lower energy than that of single metal doped cases. The relative intensity of these emission peaks depends on the stoichiometric ratios of the corresponding metals doped in alkali halide hosts. Unlike the emission peaks originating from single Cu(I), Ag(I), and Au(I) doped systems, this emission peak from heterometallic interaction is independent of the alkali halide hosts. A spin-forbidden metal− metal-to-ligand charge-transfer transition is proposed, and the Cu−Au emission peak occurs at lower energy than that of Cu−Ag, which is due to the relativistic effect of Au. Density funcational theory and atomistic calculations have been carried out to correlate the theoretical and experimental results.

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INTRODUCTION Closed-shell metallic interaction have been extensively investigated experimentally and theoretically in the last few decades because of their interesting photophysical properties.1 The monovalent compounds of coinage group metals (Au, Ag, and Cu) have been the major focus of these studies. Gold(I) compounds tend to form a supramolecular structure via aurophilic interaction. The bond strength of such interaction is about the energy of a hydrogen bond (10−40 kJ/mol), and this is due to the large relativistic effect of gold.2 For Ag(I) and Cu(I), the relativistic effect is smaller and theoretical calculations indicate that argentophilicity or cuprophilicity is relatively weak, in the order Au(I) > Ag(I) > Cu(I).3,4 Compared to those of Au(I)−Au(I) interaction, reports of Ag(I)−Ag(I) or Cu(I)−Cu(I) interactions are less common, most of which are related to bridging ligand,5−7 restricted network,8,9 or Coulombic effect.10,11 Recently, heterometallic closed-shell metallophilic interactions have attracted more and more attention. These systems include Pt−Pt,12 Au−Tl,13 Au−Hg,14 Au−Ag,15,16 and Au− Cu,17 and they have potential applications as sensors for volatile organic compounds18−20 and LEDs.21 The enhanced photophysical properties in most cases are of great interest for experimental and theoretical chemists, especially the question of whether the synergistic effect originates from the heterometallophilic interaction or ligands or some other factors. Among these bimetallic compounds, the seemingly most favorable heterometallic systems between group 11 metals are actually very few.22−24 Currently reported Au(I), Ag(I), and © 2014 American Chemical Society

Cu(I) mixed-metal interaction systems are all in the bulk state, and the synthesis method is one of the major challenges. Our group has carried out research of Au(CN)2−, Ag(CN)2−, and Cu(CN)2− nanoclusters doped in different alkali halide hosts, as well as in aqueous solution, and has provided evidence for aurophilic, argentophilic, and cuprophilic interactions. These closed-shell d10−d10 homometallic interactions give rise to many interesting photophysical properties, such as the exciplex tuning, thermochromism, and optical memory. Heterometallic nanoclusters for Au(I)−Ag(I) in alkali halide hosts have also been studied, and an atomistic model has been developed to simulate the crystal growth process. We report for the first time heterometallophilic interaction between Cu(CN)2− and Au(CN)2− or Ag(CN)2− nanoclusters doped in different alkali halide hosts. In addition to the emission peaks from a homometallic interaction, new peaks due to Cu(I)−Au(I) or Cu(I)−Ag(I) heterometallic interaction arise at much lower energy. The luminescence pattern is affected by the molar ratio of two corresponding dopants, temperature, and the host alkali halide crystals. Density functional theory (DFT) calculations were carried out to study these alkali halide doped systems structurally and electronically. A spin-forbidden metal−metal-to-ligand charge transfer (MMLCT) transition was proposed for this heterometallic interaction. These mixed-metal nanosystems have Received: March 18, 2014 Revised: May 1, 2014 Published: May 9, 2014 11886

dx.doi.org/10.1021/jp5026976 | J. Phys. Chem. C 2014, 118, 11886−11894

The Journal of Physical Chemistry C

Article

potential application for LEDs, solid-state lasers, and tunable laser materials.

Scheme 1

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EXPERIMENTAL METHODS Synthesis and Characterization. All chemicals were purchased from Sigma-Aldrich. Single crystals of KCu(CN)2 were prepared as in ref 25 by slow evaporation of solution of CuCN with NaCN or KCN. The doped crystals were also grown by slow evaporation of an aqueous solution (10 mL) with different stoichiometric ratios of KCu(CN)2 and KAg(CN)2 or KAu(CN)2 with NaCl (3 g), NaBr (4.5 g), or KBr (6 g). Crystals were harvested before complete evaporation of the solution. Copper, silver, and gold contents were determined by atomic absorption spectroscopy using an 857-Smith-Hieftje 11/ 12 spectrophotometer and Cu, Ag, and Au analytical lamps operating at 324.8, 328.1, and 267.6 nm, respectively. Reflectance spectra were recorded by an Ocean Optics usb4000 spectrometer coupled to halogen and helium arc lamps via a fiber optic probe. All crystalline materials were ground to improve reflectivity. Fine powder of the corresponding alkali halide was used as a blank. The reflectance results were converted to absorbance according to Kubelka−Munk theory. Photophysical Measurements. Steady-state photoluminescence spectra were recorded with a QuantaMaster-1046 photoluminescence spectrophotometer from Photon Technology International. The instrument is equipped with two excitation monochromators and a single-emission monochromator with a 75 W xenon lamp. Low-temperature steady-state photoluminescence measurements were achieved by using a Janis St-100 optical cryostat equipped with a Honeywell temperature controller. Liquid nitrogen was used as coolant. Lifetime measurements were conducted using an Opolette (HE) 355 II UV tunable laser with a range of 210−355 nm. The laser has a Nd:YAG flashlamp pumped with a pulse repetition rate of 20 Hz and an average output power of 0.3 mW. The detection system is composed of a monochromator and photomultiplier from a Jobin Yvon Ramanor 2000 M Raman spectrometer. Data were collected by a Le Croy 9310C dual 400 MHz oscilloscope. A low-temperature shroud with Honeywell temperature control was used for lifetime measurements at liquid nitrogen temperature. The decays were averaged over 1000 sweeps and fitted using a curve-fitting method in Igor Pro 6.0. Computational Details. Theoretical structure and energy of electronic states of the mixed dimers composed from [Cu(CN)2]− and [Ag(CN)2]− or [Au(CN)2]− units doped in NaCl were determined using Gaussian 09 software (Gaussian Inc.).26 Scheme 1 shows the structures of eight possible mixedmetal dimers. There are different bridging modes for these dimers, including three chloride-bridging dimers, two chloridecyanide-bridging dimers, and three cyanide-bridging dimers. The optimal structures were calculated by density functional theory using B3LYP27,28 functional and SDD29 basis set as implemented in the software. Models for doped dimers enclosed by 18 alkali ions at a fixed position corresponding to the studied alkali halide salt were used. Stationary point charges30,31 were added to surround the fixed alkali ions and model the remaining ionic sites within a 4 × 4 × 5 supercell model. Excited-state energies were calculated using timedependent density functional theory (TD-DFT).32−34 Isodensity representations of molecular orbitals were generated using GaussView 3.07 software (Gaussian Inc.). These calculations

were performed on the University of Maine supercomputer. Structural optimization was performed for the models, and conventional single-point TD-DFT was then carried out on the optimized structure.

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RESULTS AND DISCUSSION Synthesis. NaCl and KBr doped crystals are cubic and transparent, whereas NaBr doped crystals are white in color and brittle and flaky in texture. Table 1 gives the concentration ratio

Table 1. Cu-to-Au and Cu-to-Ag Molar Ratio in Different Alkali Halide Hosts

a

Cu:Aua

NaCl (Cu:Au)

NaBr (Cu:Au)

KBr (Cu:Au)

A B C Cu:Aga

1.46:1 0.85:1 3.41:1 NaCl (Cu:Ag)

1.20:1 0.61:1 3.74:1 NaBr (Cu:Ag)

1.06:1 0.38:1 3.16:1 KBr (Cu:Ag)

A B C

1.63:1 0.72:1 3.44:1

1.13:1 0.69:1 3.73:1

1.01:1 0.38:1 3.11:1

Original molar ratio of Cu to Au or Ag: A, 1:1; B, 1:3; C, 3:1.

for all the metals in doped alkali halide hosts. Compared to the initial stoichiometric ratio, the metal−metal molar ratio of the harvest crystals shows alkali halide host dependence. For Cu− Au and Cu−Ag codoped in NaCl and NaBr crystals, the Auand Ag-to-Cu content ratios are less than those of the prepared ones. However, the KBr doped crystals have a molar ratio similar to that of the initial preparation. Because of the lattice size of these alkali halides, KBr has the largest volume for the occupancy of Au and Ag cyanide ions, whereas NaCl and NaBr have a smaller lattice size, and the concentrations of Ag and Au are less than those in KBr. This concentration difference is also reflected in the photoluminescence results (vide infra). Diffuse Reflectance. The reflectance results were converted to absorbance, as shown in Figure 1. The absorption of Cu(CN)2− and Au(CN)2− or Cu(CN)2− and Ag(CN)2− doped in alkali halides are no different from only Cu doped in 11887



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Figure 1. Absorption spectra of KCu(CN)2, KCu(CN)2 with KAg(CN)2, and KCu(CN)2 with KAu(CN)2 doped in different alkali halides. Absorption data are converted from reflectance results using Kubelka−Munk theory.

corresponding hosts. The previously reported absorbance for Ag(CN)2− doped in alkali halide was not observed. This is due to the intense Cu absorption peaks that suppress the transitions from either Ag or Au, e.g., the molar absorption coefficient of Cu(CN)2− is much larger than those of Au(CN)2− and Ag(CN)2− in doped alkali halide hosts. Our group has shown that Cu(CN)2− in an alkali halide host possesses a linear structure and with the surrounding four halides, [Cu(CN)2X4]5−, has a D4h symmetry.35 For the Cu(I) center, five d orbitals split to A1g, B1g, B2g, and Eg orbitals, and the excited 4s orbital has only A1g symmetry; the metal-centered absorption is from the metal-centered 3dz2 → 4s transition. The cyanide ligands in a doped crystal also provide empty π* orbitals for the Cu(I) d electrons to undergo MLCT. As shown in Figure 1, for Cu(CN)2− doped in NaBr and NaCl, there are two major absorption peaks, with the higher-energy peak assigned to a metal-centered transition and the lower-energy peak to a MLCT transition, which produces intense and long lifetime luminescence. The absorption maxima blue shifts when the host is varied from NaCl to NaBr and KBr, and these two transitions become less resolved. This is because in the NaCl host, the Cu(I) ion is on-center; however, for NaBr and KBr, the Cu(I) ion is shallow off-center and deep off-center, respectively.36 Such distortion from a highly symmetrical D4h structure leads to loss of symmetry; thus, more transitions occur in this region. Photoluminescence. As shown in Figure 2, at 77 K, for KCu(CN)2 and KAu(CN)2 codoped in NaCl, these mixed nanoclusters exhibit multiple emission peaks and emission tunability, which means the luminescence pattern varies with different excitations. For comparison, the luminescence spectra of only KCu(CN)2 or KAu(CN)2 doped in NaCl are also given. The mixed Cu and Au doped system clearly show emission peaks from both Cu and Au, with peaks at 343, 375, and 420− 445 nm from Cu and the peak at 380 nm from Au. Because KCu(CN)2 and KAu(CN)2 are both luminescent around 380 nm, the excitation profile of this peak for the mixed-metal doped system is simply a combination of Cu and Au doped in NaCl. Interestingly, a new emission peak at lower energy occurs at 530 nm, and this is also observed in the cases of NaBr and KBr hosts. Figure 3 gives the emission spectra with 530 nm maxima and the corresponding excitation spectra for all three alkali halide hosts. Unlike other luminescent peaks from either Cu or Au, the lowest-energy emission peak shows no host

Figure 2. Luminescent spectra of KCu(CN)2 with KAu(CN)2, KCu(CN)2, and KAu(CN)2 doped in NaCl at 77 K.

Figure 3. Emission peak at 530 nm and corresponding excitation peak of KCu(CN)2 and KAu(CN)2, codoped in NaCl, NaBr, and KBr at 77 K.

dependence, i.e., it always emits at 530 nm at 77 K. The other peaks slightly shift in different hosts; for example, the peak at 343 nm in NaCl shifts to 358 nm in NaBr and 336 nm in KBr. Because the 530 nm peak was not observed in Cu(CN)2− or Au(CN)2− doped systems, we suggest that this emission peak originates from a heterometallophilic interaction between the metal center of Cu(CN)2− and Au(CN)2−. The lifetime of this green emission peak is 14.4, 12.8, and 16.2 μs for NaCl, NaBr, and KBr, respectively, indicating a spin-forbidden parentage of this peak. Thus, this peak is due to a spin-forbidden metal− 11888



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metal-to-ligand-charge-transfer transition. A previous study of unsupported gold(I)−copper(I) interactions also ascribed the luminescence peak to a MMLCT origin.23 The excitation profile for 530 nm shown in Figure 3 is substantially similar to the excitation for Au(CN)2− doped in different alkali halide hosts, suggesting the MMLCT has a contribution from Au that is higher than that from Cu. This will be discussed in more detail in Calculations (vide infra). These crystals are also luminescent at room temperature (RT). Table 2 gives the Table 2. Emission and Excitation Maxima for Mixed Cu(CN)2− with Au(CN)2− Doped in Different Alkali Halides at RT and 77 K RT

77 K

host

λem (nm)

λex (nm)

λem (nm)

λex (nm)

NaCl

340 370 380 − 518 343 364 − 518 318 342 380 412 518

267 267 273 − 338 258 258 − 338 252 252 260 263 333

343 375 380 445 530 352 386 452 530 336 375 385 445 530

263 274 260, 335 343 330 259 259, 334 346 332 253 265 265, 306 345 329

NaBr

KBr

emission peaks at 77 K, and all these peaks have a slight blue shift at room temperature (Supporting Information). This has been ascribed to a metal−metal separation distance change. The bond distance contraction caused by a temperature decrease enhances electron coupling between the metal center and ligand. Therefore, the highest occupied molecular orbital− lowest unoccupied molecular orbital (HOMO−LUMO) energy gap is lowered and the emission peak red shifts at 77 K compared that at room temperature, which is seen for many closed-shell metal compounds.37,38 The luminescence pattern of these Cu−Au systems is dependent on the concentration ratio of Cu and Au as well as the alkali halide hosts. Figure 4 gives the 2D luminescence mapping of Cu(CN)2− and Au(CN)2− with different molar ratios doped in KBr at 77 K. To compare the relative intensity, the value for the Cu(CN)2− monomer peak at 336 nm is set to 1; the intensity maxima values are given in Table 3. With the increase of Au, the 390 nm peak, which is mostly from Au(CN)2− nanoclusters, becomes more and more intense. Interestingly, the emission peak intensities between 433 and 455 nm have the same trend as the 390 nm peak. This might be due to the increase of Au content in the alkali halide hosts, which leaves less sites available for Cu(CN)2− to occupy; therefore, the Cu(CN) 2 − nanoclusters became more “crowded”. This is similar to increasing the concentration of Cu leading to an enhancement of the lower-energy peaks in our previous study.35 However, the emission peak from heterometallophilic interaction at 530 nm shows a different trend. In the Cu:Au 1:1 ratio case, this green emission peak is the most intense. This implies the possibility of heterometallic interaction for the Cu:Au 1:1 ratio is greater than that in the 1:3 and 3:1 cases and also the percentage of mixed-metal

Figure 4. Luminescence 2D mapping of KCu(CN)2 codoped with KAu(CN)2 in KBr crystals with different concentration ratios.

Table 3. Emission Peak Relative Intensities of Cu(CN)2− and Au(CN)2− Doped in NaCl, NaBr, and KBr with Different Concentrationsa KBr

NaCl

NaBr

a

Cu:Au molar ratio

∼390 nm

433−455 nm

530 nm

1:1 1:3 3:1 1:1 1:3 3:1 1:1 1:3 3:1

0.95 2.04 0.21 0.52 0.61 0.094 0.74 1.38 0.17

0.52 1.54 0.10 0.058 0.024 − 0.21 1.24 0.026

0.37 0.22 0.13 0.22 0.83 0.093 0.41 2.09 0.33

The intensity of Cu(CN)2− monomer emission peak is set to 1.

nanoclusters for the 1:1 case is higher than that for the 1:3 and 3:1 cases. For the NaCl doped Cu−Au crystals, less Au is doped in these crystals; consequently, the 530 nm emission peak is relatively weak in all cases and the Cu−Au 1:3 ratio 11889



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the relativistic effect of Au. Gold has a larger relativistic effect than any other element with Z < 100,41 which leads to an interaction between Cu and Au that is stronger than that between Cu and Ag. The Cu(CN)2− and Au(CN)2− nanocluster has a smaller HOMO−LUMO energy gap and emits at 530 nm, whereas the Cu(CN)2−−Ag(CN)2− excimer emits at 493 nm. This is observed for all NaCl, NaBr, and KBr doped crystals, as shown in Figure 6, as for the Cu(CN)2−−Au(CN)2− case.

gives the highest relative intensity. NaBr doped crystals show the same trend: the higher the Au concentration, the stronger the relative intensity for the heterometallic emission peak. In these two types of crystals, the Au concentration is relatively low; in addition, the excitation profiles and our calculation results indicate that the HOMO for the Cu−Au mixed transition is mostly from Au. This leads to the crystals with the highest gold concentration showing the most intense 530 nm emission. However, the intensity of the low-energy emission peak is even stronger than that of the Cu(CN)2− monomer, and this suggests that the NaBr host favors the formation of mixed Cu(CN)2− and Au(CN)2−, which was also observed for Ag(CN)2− and Au(CN)2− doped in different alkali halide hosts.39 For the Ag(CN)2− and Cu(CN)2− case, a similar result was observed, except the luminescence patterns are more complicated. This is due to the fact that [Ag(CN)2−] doped in different alkali halide hosts also exhibits multiple emission peaks, as we reported.40 Figure 5 shows the luminescence

Figure 6. Emission peak at 493 nm and corresponding excitation peak of KCu(CN)2 and KAg(CN)2 codoped in NaCl, NaBr, and KBr at 77 K.

As with the Cu−Au case, these Ag(CN)2− and Cu(CN)2− nanoclusters doped in different alkali halide hosts also show a concentration dependence. The concentration results of Cu− Ag doped crystals have the same trend as the Cu−Au case. For NaCl and NaBr with a smaller lattice size, less Ag(CN)2− is doped in these hosts. The Ag-to-Cu 3:1 cases give the most intense 493 nm emission peak. However, the KBr doped crystals exhibit a different trend; they have results roughly similar to those of the starting molar ratio, giving the strongest green emission for the Ag to Cu 1:1 case. Figure 7 provides the 2D luminescence mapping of Cu(CN)2− and Ag(CN)2− with different molar ratios doped in KBr at 77 K. This is also shown in Cu(CN)2− and Ag(CN)2− doped in NaCl and NaBr crystals (Supporting Information). These luminescence results are consistent with the Au(CN)2− and Cu(CN)2− doped in different alkali halide hosts. Calculations. Because the low-energy emission peaks are due to heterometallic interactions, density functional theory calculations for eight possible mixed dimers (Cu−Au and Cu− Ag) shown in Scheme 1 doped in NaCl were conducted to study the nature of the transition between two metal cyanide nanoclusters. The microsecond lifetime of the emission peak indicates the origin of a spin-forbidden transition. Optimized structures of the ground state and first triplet state for these dimers were obtained (Supporting Information). Table 4 gives the bond distances for Cu(CN)2− and Au(CN)2− doped in a NaCl host. The Cu−C bond distance is generally 1.91 Å (even shorter in cyanide bridging cases), which is slightly shorter than that of bulk KCu(CN)2 and KCu(CN)2, and a slightly longer CN bond distance, about 1.20 Å.25,42 This is probably due to the surrounding halide ions with fully occupied electrons. The d

Figure 5. Emission peak at 530 nm and corresponding excitation peak of KCu(CN)2 and KAg(CN)2 codoped in NaCl, NaBr, and KBr at 77 K.

spectra of Ag(CN)2− and Cu(CN)2− codoped in NaCl and the single-doped cases. The lower-energy emission peak at 493 nm is not observed for either Ag(CN)2− or Cu(CN)2−, which is similarly assigned to an origin of heterometallic interaction. This emission peak has a long lifetime (13.5, 4.7, and 5.7 μs for NaCl, NaBr, and KBr respectively) and is assigned as phosphorescence. The Cu−Ag luminescence peak energy is higher than the Cu−Au emission, which can be explained by 11890



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electrons in the center copper atom are pushed away, and this enhances the π back bonding to the cyanide ligand. Therefore, the Cu−C distance contracts and the CN bond expands. For single-crystal KAu(CN)2, the Au−C distance is 2.12 Å and the CN distance is 1.17 Å.43 Similar to the Cu(CN)2− results, Au(CN)2− doped in NaCl also shows an increase of Cu−C separation and a decrease for CN separation. Interestingly, the bond distance results for bridging and nonbridging metal− carbon and carbon−nitrogen exhibit a different trend. The bridging metal−carbon separations are shorter than those of nonbridging ligands, whereas the bridging cyanides have longer bond distances. This can be explained by the fact that the bridging cyanide is close to both metals and electrons occupy an antibonding LUMO, leading to an elongation. Consequently, the metal−carbon bond distances are shortened. Similarly, the bond distances for Cu(CN)2−−Ag(CN)2− doped in NaCl give the same results (Supporting Information). Table 5 gives the contribution of gold, copper, and corresponding cyanide ligands of the HOMO for all eight Table 5. Population Analysis for Cu(CN)2− and Au(CN)2− Doped in the NaCl Host in the HOMO and LUMO HOMO

LUMO

Figure 7. Luminescence 2D mapping of KCu(CN)2 codoped with KAg(CN)2 in KBr crystals with different concentration ratios.

dimer

C−N (Å)

1.911 1.910 1.910 1.909

1.201 1.201 1.201 1.200

μ(ClCNb)

1.891b/ 1.899n 1.850b/ 1.848n 1.907

1.199b/ 1.197n 1.196b/ 1.189n 1.201

1.892b/ 1.896n

1.198b/ 1.197n

μ(CNa) μ(CNb) μ(CN2) b

Cu−C (Å)

μ(Cl) μ(Cl90) μ(Cl2) μ(ClCNa)

Au(CN)2− Au−C (Å)

C−N(Å)

2.036 2.035 2.023 2.001b/ 2.023n 2.018

1.196 1.196 1.195 1.192b/ 1.193n 1.194

2.021

1.195

2.000b/ 1.965n 2.005b/ 2.020n

1.192b/ 1.193n 1.192b/ 1.193n

Cu (%)

CN (%)

Au (%)

CN (%)

X (%)

6.72 5.69 0.42 3.55 16.36 40.77 61.90 12.56 5.14 7.38 6.51 1.06 26.91 0.32 2.06 0.35

1.42 1.29 0.04 0.77 1.11 5.92 6.76 2.69 3.49 2.74 16.02 2.24 47.66 1.81 16.78 2.62

58.77 60.05 82.58 79.33 57.16 23.30 3.24 67.62 37.89 46.48 28.69 8.52 11.86 6.21 4.15 7.65

8.11 8.07 3.52 3.14 8.93 3.91 0.10 3.37 17.59 17.39 37.03 71.77 3.00 62.80 44.06 72.26

20.42 20.43 8.50 9.48 14.90 16.63 18.57 10.57 1.64 2.35 4.67 7.22 5.14 7.19 8.94 8.60

possible dimers. The population results show that the HOMOto-LUMO transition is clearly a MLCT transition. The excitation profile of the 530 nm peak due to heterometallic interaction is very similar to the excitation of Au(CN)2−, suggesting the HOMO of the mixed-metal system is mostly from the gold atom. So we can exclude the possibility of the two mixed Cu(CN)2−−Au(CN)2− dimers with single cyanide bridging between two moieties. Because the emission is involved with two metal centers, the LUMO should have more electron density on the copper unit. In addition, the emission of this green peak also exhibits independence of the alkali halide host, which indicates the contribution from the halide should be relatively small. Consequently, the μ(X2), μ(XCNa), and μ(CN2) dimers with less electron density on copper and the μ(X) and μ(X90) dimers with higher halide population are all ruled out from the possible structures for the d10−d10 heterometallic interaction. The remaining μ(XCNb) dimer gives the most reasonable population results for a mixed Cu(CN)2−−Au(CN)2− configuration. Figure 8 gives the isodensity surface of the HOMO and LUMO for Cu(CN)2− and Au(CN)2− μ(ClCNb) dimer doped in NaCl. The HOMO of this dimer consists most of 3dz2 orbitals of gold σ

Table 4. Calculated Bond Distances for Cu(CN)2− and Au(CN)2− Doped in the NaCl Host in the Ground State Cu(CN)2−

dimer μ(X) μ(X90) μ(X2) μ(XCNa) μ(XCNb) μ(CNa) μ(CNb) μ(CN2) μ(X) μ(X90) μ(X2) μ(XCNa) μ(XCNb) μ(CNa) μ(CNb) μ(CN2)

Bridging. nNonbridging.

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Table 6. Population Analysis for Cu(CN)2− and Ag(CN)2− Doped in the NaCl Host in the HOMO and LUMO HOMO

LUMO

dimer

Cu (%)

CN (%)

Ag (%)

CN (%)

X (%)

(X) (X90) (X2) (XCNa) (XCNb) (CNa) (CNb) (CN2) (X) (X90) (X2) (XCNa) (XCNb) (CNa) (CNb) (CN2)

17.48 15.54 57.68 16.34 11.77 49.98 28.49 39.97 6.21 7.62 5.37 12.50 47.38 8.58 82.33 6.21

3.86 3.49 12.24 3.48 0.52 6.64 3.19 7.21 3.32 2.94 20.65 33.44 25.03 1.06 0.83 54.29

49.90 52.04 13.77 66.03 61.53 18.34 41.80 35.58 65.28 70.14 35.61 29.50 12.56 63.93 3.95 2.74

7.76 8.08 0.04 4.41 9.90 2.23 9.11 4.17 4.35 2.52 26.51 13.50 6.35 4.76 1.93 20.28

16.17 16.17 14.46 7.97 14.60 13.97 11.72 11.70 1.06 1.63 4.54 6.03 4.02 1.93 1.22 10.19

Figure 8. Isodensity of HOMO (top) and LUMO (bottom) of μ(XCNb)Cu(CN)2− and Au(CN)2− dimer. (Cu, pink; Au, yellow; C, gray; N, blue; Cl, green; and Na, purple).

antibonding interacting with cyanide and chloride ions in the x and y directions. The LUMO is composed of the π* orbitals of cyanide ligands from both units, and the interaction between Cu(CN)2− and Au(CN)2− moieties are stronger than those in the HOMO. This indicates a MMLCT transition, and the interaction from the chloride ion adds some ligand-to-ligand charge-transfer character to this transition. However, for the Cu and Ag mixed-metal interaction, the luminescence results suggest a different pathway for the transition. The Cu(CN)2− moiety has more contributions in the HOMO; however, the LUMO is mostly from the ligand of the Ag(CN)2− unit. Table 6 gives the contribution of silver, copper, and corresponding cyanide ligands in the HOMO for all 8 possible dimers. Similar to the rationale for Cu(CN)2− and Au(CN)2− dimers, the most possible configuration for Cu(CN)2− and Ag(CN)2− is the μ(CN2) dimer, and the HOMO and LUMO isodensity surface is shown in Figure 9. Calculations of the ground state and the first triplet state for all Cu(CN)2−−Au(CN)2− and Cu(CN)2−−Ag(CN)2− dimers were carried out and the optimal structures were obtained. The Cu−Au and Cu−Ag distances in the ground and first triplet state are given in Table 7, which shows the mixed-metal separations for all dimers decrease in the excited state. This implies that the heterometallophilic interaction is stronger than in the ground state. This is typical excimer and exciplex behavior, and the calculated results confirm our conclusion of the formation of mixed-metal excimers. Table 8 gives the theoretical absorption and emission energy of all eight mixed dimers (Cu−Au and Cu−Ag) via TD-DFT. The absorption energy is calculated from the ground singlet, and the emission energy is given by the first triplet excited state. Compared to those of the Cu(CN)2−, Au(CN)2−, or Ag(CN)2− monomer, the calculated absorption and emission energies are

Figure 9. Isodensity of HOMO (top) and LUMO (bottom) of μ(XCNb)Cu(CN)2− and Ag(CN)2− dimer. (Cu, pink; Ag, white; C, gray; N, blue; Cl, green; and Na-purple).

at lower energy, which suggests the heterometallic interaction has a smaller energy gap. This is consistent with the experimental results for excitation and emission of these d10 cyanide monomers.

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CONCLUSION In this paper we have reported for the first time photophysical studies of d 10 −d 10 heterometallic interaction between dicyanocuprate(I) and dicyanoargentate(I) as well as 11892



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Table 7. Metal−Metal Distances for Cu(CN)2−−Au(CN)2− and Cu(CN)2−−Ag(CN)2− Doped in the NaCl Host in the Ground State and the First Triplet Excited State ground state Cu−M* distance (Å)

excited state Cu−M distance (Å)

μ(X)

6.369

6.125

μ(X2) μ(X90) μ(XCNa) μ(XCNb) μ(CNa) μ(CNb) μ(CN2) μ(X) μ(X2) μ(X90) μ(XCNa) μ(XCNb) μ(CNa) μ(CNb) μ(CN2)

4.188 6.363 4.102 3.983 6.031 6.238 3.979 6.295 4.240 6.293 4.149 4.047 5.953 6.167 4.051

4.323 6.316 3.399 3.510 5.990 5.135 3.197 6.242 4.333 6.249 3.466 3.580 5.943 5.269 3.274

cluster Cu− Au

Cu−Ag

Cu−Ag

dimer

absorption (nm)

emission (nm)

μ(X) μ(X90) μ(X2) μ(XCNa) μ(XCNb) μ(CNa) μ(CNb) μ(CN2) μ(X) μ(X90) μ(X2) μ(XCNa) μ(XCNb) μ(CNa) μ(CNb) μ(CN2)

309 283 305 314 262 336 328 305 295 294 294 304 283 297 281 292

434 332 397 455 474 393 388 479 416 414 331 430 462 332 388 433

ASSOCIATED CONTENT

S Supporting Information *

2D luminescence mapping of KCu(CN)2 codoped with KAu(CN)2 or KAg(CN)2 in NaCl, NaBr, and KBr crystals at room temperature and 77 K; calculated bond distances for Cu(CN)2− and Ag(CN)2− doped in NaCl host in ground state. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-0315877). We acknowledge Professor Scott Collins for his support with reflectance spectroscopy. We thank Mr. David LaBrecque for general assistance.

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REFERENCES

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Table 8. Calculated Absorption and Emission Energy for Cu(CN)2−−Au(CN)2− and Cu(CN)2−−Ag(CN)2− Doped in the NaCl Host Cu−Au

Article

dicyanoaurate(I) compounds doped in different alkali halide hosts. In addition to luminescence peaks originating from each cyanide compound, a new low-energy emission occurs for Cu(CN)2−−Au(CN)2− and Cu(CN)2−−Ag(CN)2− doped in NaCl, NaBr, and KBr. This green emission peak is independent of the hosts, whereas the relative intensity is affected by the alkali halides because of their different lattice sizes. Long lifetime results for the low-energy emission peaks indicate a spin-forbidden parentage. DFT calculations for the ground state and the first triplet state give bond distance and electron density information suggesting an MMLCT transition. The μ(CN2) dimer with one bridging cyanide from each moiety is the most possible configuration for heterometallic interaction. These dimers have a shorter metal−metal bond distance, suggesting stronger interaction in the excited state, which is consistent with the excimer and exciplex behavior. 11893



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