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J. Phys. Chem. C 2009, 113, 5952–5959
Luminescent Studies of “Exciplex Tuning” for Nanoclusters of Dicyanocuprate(I) Ions Doped in Potassium Chloride Crystals Haiyan Lu,†,‡ Renante Yson,‡,§ Xiaobo Li,‡ Christie Larochelle,| and Howard H. Patterson*,‡ College of Chemistry, Jilin UniVersity, Changchun, 130012 P. R. China, Department of Chemistry, UniVersity of Maine, Orono, Maine 04469, Department of Chemistry, UniVersity of NeVada, Reno, Reno, NeVada 89557 and Department of Physics, Franklin & Marshall College, Lancaster, PennsylVania 17604 ReceiVed: October 8, 2008; ReVised Manuscript ReceiVed: January 14, 2009
Nanoclusters of dicyanocuprate (I) ions doped in potassium chloride crystals have been studied. Several emission bands are observed in this system and each emission band can be tuned by site-selective spectroscopy. These bands are assigned to different [Cu(CN)2]- luminescent nanoclusters in the host lattice. The Cu+ ions substitute for K+ ions in the lattice and the CN- ions substitute for Cl- ions in the host with dimer and trimer nanoclusters bonded together by CN- bridges. This is unlike KAg(CN)2 or KAu(CN)2 substituted in a KCl lattice, in which Ag-Ag or Au-Au bonding directly occurs. Ab initio calculations show that the LUMO is a bonding MO while the HOMO is antibonding. Extended Hu¨ckel calculations indicate the formation of the LUMO excited-state well is deeper than the ground-state HOMO potential well and with a shorter internuclear separation characteristic of exciplex behavior. Time-resolved luminescence measuremence at room temperature and 78 K demonstrate that, upon pulsed excitation, energy transfer occurs between the dimer and trimer nanoclusters. We conclude that the presence of different luminescent Cu(CN)2- nanoclusters in different environments results in tunability over a wide wavelength range. Introduction Cu(I) compounds have been studied extensively because of their interesting and efficient luminescence properties.1-16 The luminescence of these compounds has been studied in a variety of Cu(I) compounds and in different systems was found to arise from a variety of mechanisms, such as metal-to-ligand charge transfer (MLCT),17 charge transfer to solvent (CTTS)18,19 and metal centered transitions of the types 3d10 f 3d94s1 and 3d10 f 3d94p1 on Cu(I).18 Many studies on Cu(I) compounds have been carried out for tetranuclear Cu(I) clusters of the type Cu4X4L4 (where X ) halogen and L ) pyridine, amine, or phosphate).17,20,21 Also, many investigations have focused on synthesizing highly luminescent Cu(I)-phenanthroline complexes because of their possible practical applications including energy conversion and storage.22-24 Felder et al.25 have recently studied the luminescent properties of phenanthroline complexes of Cu(I) and found that these complexes displayed relatively intense metal-to-ligand charge transfer (MLCT) emission bands. While exciplex formation in organic compounds has been recognized for many years,26 it is only in the past decade or so that exciplexes have been found in inorganic compounds.18,19,27-33 In Cu(I) compounds, for example, Horvath and Stevenson have investigated the excited-state interactions of [Cu(CN)2]- ions with halide ions in aqueous solutions.19 They found that the most strongly luminescent emitting species were exciplexes formed when an excited dicyanocuprate ion couples with one or more ground-state halide ions.34 The best example for cuprophilicity, or Cu-Cu interactions, has been reported in tetrameric haloamine clusters of Cu(I). A Cu-Cu bond distance * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. † Jilin University. ‡ University of Maine. § University of Nevada, Reno. | Franklin & Marshall College.
of 3.12 Å was observed in this study providing an explanation forthelow-energyluminescenceofthisCu(I)clustercompound.10-12 Previous work on Ag(CN)2-/KCl in our laboratory has led to the conclusion that inorganic excimers and exciplexes will form between dicyanoargentate ions doped in alkali halide host lattices.28,35 These were the first reported homoatomic exciplexes found in coordination compounds. These exciplexes could be selectively excited to tune the emission energies; we refer to this phenomenon as exciplex tuning. In this paper, we report the discovery of inorganic excimers and exciplexes in the Cu(CN)2-/KCl system in which we propose that the bonding in the exciplex occurs via a cyanide bridge between the copper atoms. This is different than the studies on dicyanoargentate ions doped in KCl crystals,28,32,33,36 in which only metal-metal exciplex behavior is present. Our results show that each of the copper (I) dicyanide exciplexes found in these KCl host crystals can be selectively excited and have different emission energies. This tunability in the emission energies makes these systems attractive candidates for potential applications, such as tunable solid state lasers, photocatalysts, and photosensitizers for water splitting. Experimental Methods Synthesis and Characterization. K[Cu(CN)2] was synthesized by slight modification of a literature procedure.37,38 A solution of KCN (10.0 g, 99.9% pure, Alfa-Aesar) in 50 mL of distilled H2O was added slowly to a suspension of CuCN (7 g, 99.9% pure, Alfa-Aesar) in 50 mL of H2O. The mixture was heated to 60 °C while stirring until all the solids dissolved. The resulting solution was filtered while hot to remove any suspended particles, and the filtrate was allowed to evaporate slowly at room temperature. Crystals of K[Cu(CN)2] were harvested after 2 days. Single crystals of KCu(CN)2/KCl were grown by slow evaporation of a saturated aqueous solution containing 10.0 g
10.1021/jp808910s CCC: $40.75 2009 American Chemical Society Published on Web 03/20/2009
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Figure 1. Infrared spectrum of a pure single crystal of KCu(CN)2 and a single crystal of KCu(CN)2/KCl (batch 2) at room temperature.
of KCl and 0.4 g of KCu(CN)2 at ambient temperature. The first batch of KCu(CN)2/KCl crystals was harvested after 1 day of evaporation. After harvesting the first batch of crystals (hence forth referred to as “batch 1”), the remaining solution (mother liquor) was allowed to evaporate further under ambient conditions. The second batch of KCu(CN)2/KCl crystals (batch 2) was harvested after 4 days. The copper content was determined by atomic absorption spectroscopy using a Model 857-Smith-Hieftje 11/12 spectrophotometer and a Cu analytical lamp operating at 328.1 nm. The standards for the atomic absorption analysis were prepared from a 1000 ppm Cu standard (Cole-Parmer Company) in 1% HNO3. All infrared spectra were recorded on a Bomem MB-series FTIR spectrometer equipped with a liquid N2-cooled mercury cadmium telluride (MCT) detector. Typically, 100 scans were coadded at a resolution of 4 cm-1. A total of 100 scans were collected for each spectrum, and each scan required 6 s. Luminescence and Lifetime Measurements. Steady-state photoluminescence spectra were collected using a Photon Technology International model QuantaMaster-1046 spectrophotometer equipped with a 75 W xenon lamp. Wavelengths were selected with two excitation monochromators and a single emission monochromator. Excitation spectra were corrected for spectral variation of the lamp using rhodamine B as a quantum counter. Liquid nitrogen and helium were used as the coolant in a model LT-3-110 Heli-Tran cryogenic liquid transfer system. Lifetime measurements were performed using a NanoUV diode-pumped solid state laser manufactured by JDS Uniphase. The laser is frequency doubled twice to give an output of 0.8 ns pulses at 266 nm with a repetition rate of 7.1 kHz. The laser outputs an average power of about 1 mW. The detection system is comprised of a Oriel Cornerstone 1/4 meter monochromator with an Oriel model 77360 photomultiplier tube and a Stanford Research Systems model SR445 350 MHz preamplifier. The data were collected using a LeCroy Waverunner model LT262 1 GHz oscilloscope capable of recording one data point every nanosecond. The decays were averaged over 1000 sweeps on the oscilloscope and the data were fitted using a curve fitting routine in Matlab 6.0. The data were corrected for zero baseline, and time zero was defined by the excitation pulse. Time resolved spectra were obtained by integrating to get the area under the decay curve. Variable temperature measurements were made with the sample mounted on a copper sample blank in a Janis model ST-100 sample in vacuum cryostat. Temperature control
Figure 2. Exciplex tuning by site-selective excitation: emission spectra of a KCu(CN)2/KCl (batch 2) crystal at 77 K with different excitation wavelengths. Intensities are not comparable between different spectra.
was achieved with a silicon diode temperature sensor, 25 Ω heater coil, and a Lakeshore model 331 temperature controller. Computational Details. Single-point density functional theory (DFT) ground-state calculations were performed on the models using the Becke three-parameter hybrid exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) as implemented in Gaussian 03. Molecular structures for the [Cu(CN)2]- were taken from the X-ray diffraction results published elsewhere.37,38 All distances, angles, and dihedral angles were kept frozen in the calculations. The Dunning/ Huzinaga valence double-ζ (DV95) basis sets were used for C and N while the Stuttgart/Dresden effective core potentials were used for Cu. Graphical representation of HOMO and LUMO were generated using GaussView, and population analyses were done using the QMForge program. Extended Hu¨ckel calculations were carried out for the ground and first excited states of the [Cu(CN)2-]2 monomers, dimers and trimers using ICON-EDIT. Default parameters for Cu, C, and N were used as provided by the program. Bond distances were taken as reported in the literature for KCu(CN)2.37,38 Potential energy diagrams of the ground-state and first excitedstate were generated by calculating the energy of the dimer and trimer as the separation of the Cu(CN)2- units was varied. Also, the effect of the KCl lattice on Cu(CN)2- monomers and dimers was calculated in comparison to the energies of the [Cu(CN)2-]n (n ) 1 and 2) in the absence of a KCl lattice. Results and Discussion Synthesis. Colorless crystals of [Cu(CN)2-] doped in KCl (batch 1 and batch 2) were harvested from the mother liquor
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Figure 3. Corrected excitation spectra for a single crystal of Cu(CN)2doped in KCl, batch 2. The emission peaks corresponds to emission maxima of bands A, B, and C at 77 K.
Lu et al.
Figure 5. Emission spectrum at 78 K of batch 1 excited by 266 nm and excitation spectra of three emission peaks at 343, 408, and 458 nm respectively.
Figure 6. Time-resolved measurements at 78 K for a [Cu(CN)2-]/ KCl single crystal doped with 1.40% Cu. The excitation wavelength is 266 nm and the gate width is 25 µs for all spectra. The delay times are (bottom to top): 0, 25, 50, 75, and 100 µs.
Figure 4. Emission spectra of pure KCu(CN)2 and two batches of KCu(CN)2/KCl crystal at 4 K when excited at 275 nm.
and analyzed for copper content. Atomic absorption spectroscopic analysis showed that the amount of Cu (by weight) are 0.98% and 1.40% for batches 1 and batch 2, respectively. The reason why batch 1 has a lower copper content compared to batch 2 is a direct result of the synthetic procedure. As the solvent slowly evaporates, the solution becomes more saturated with respect to KCl until eventually crystals start to form. Dicyanocuprate ions can get incorporated in the lattice effectively doping the KCl crystal. During the early stage of evaporation, the solution contained a low concentration of [Cu(CN)2-] ions relative to KCl. After harvesting the first batch of crystals, the remaining mother liquor contains a higher concentration of [Cu(CN)2-] relative to KCl. This leads to KCl crystals grown with a higher copper content in batch 2 than batch 1 crystals. FTIR Spectrum. The infrared spectra of crystals of [Cu(CN)2-] doped in KCl host show multiple peaks in the νC-N region. Figure 1 shows the infrared spectrum of a batch 2 doped crystal with peaks at 2057, 2080, 2096, ∼2113, and 2125 cm-1. In comparison, the IR spectrum of a single K[Cu(CN)2] crystal, also shown in Figure 1, consists of only two strong peaks in
the νC-N region.39 One peak appears at 2080 cm-1 and is assigned to the ν1 fundamental mode (C-N symmetric stretch), and the other peak at 2110 cm-1 is assigned to the asymmetric C-N stretch vibration ν6. The ν6 peak shifts to ∼2125 cm-1 in aqueous solution,40 whereas the ν1 peak energy is unchanged. The structure of solid K[Cu(CN)2] consists of Cu(CN)2- in a bent conformation bonded to two neighboring Cu(CN)2- via a cyanide bridge. In solution and in KCl, we hypothesize that the Cu(CN)2- oligomer breaks into monomer, dimer, and trimer [Cu(CN)2-]n units with the 2057, 2096, and 2113 peaks assigned to the oligomer species. Finally, the possibility of combination bands is small due to the low doping level of Cu(CN)2- in KCl. The very same crystals used in the infrared experiments were also used for the luminescence experiments. Photoluminescence Spectra. The emission spectra at 77 K for a single crystal from batch 2 using a variety of excitation wavelengths are shown in Figure 2. We observe that the emission profile is strongly dependent on the excitation wavelength, with emission peaks at wavelengths of 350, 408, and 455 nm found upon excitation at different wavelengths as indicated in the figure. For the discussion in this paper, we will refer to these emission bands as bands A, B, and C, respectively. Since previous work has shown that the strong luminescence in d10 systems is a result of metal-metal interactions, we conclude that the different emission bands arise from *[Cu(CN)2-]n exciplexes in the doped crystals.
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TABLE 1: Assignment of the Luminescence Bands of K[Cu(CN)2-]/KCl Crystals from Comparison with ab Initio and Extended Hu¨ckel HOMO-LUMO Calculated Energy Differences band
A
assigment em λmax , nm (eV) exc , nm (eV) λmax average energy of excitation and emission, nm (eV) ab initio, nm (eV) extended Hu¨ckel, nm (eV) a
B
C
-
-
monomer 320-360 (3.87--3.44) 250-270 (4.96-4.59) 294 (4.22)
[Cu(CN)2 ]2 dimer 380-420 (3.26-2.95) 290-325 (4.28-3.81) 346 (3.58)
[Cu(CN)2 ]3, trimer 440-470 (2.82-2.64) 330-360 (3.76-3.44) 391 (3.17)
260 (4.77) 281 (4.42), 244a (5.09)
448 (2.77) 329 (3.77), 346a (3.58)
633 (1.96) 364 (3.41)
a
a
Corresponds to a KCl environment.
TABLE 2: Lifetime Measurements for Batch 2a band A band B band C
τ,µs, 78 K
τ,µs, 295 K
15.7 26.5 45.6
11.0 23.5 28.2
NA(0) kET, s-1, 78 K 2.76 × 103 1.04 × 103
NA(0) kET, s-1, 295 K 3.20 × 103 1.08 × 103
a The lifetimes for band A were obtained from fitting the raw data to a single exponential. Lifetimes for bands B and C were obtained from the parameters from the fit to the model given by eqs 1-3.
Figure 3 shows the corrected excitation spectra for batch 2 crystals at 77 K. In each spectrum, the emission was monitored at the wavelength that corresponds to the peak of each of the three emission bands, as indicated in the figure. The distinct profiles of the different emission bands accounts for the tunability of the luminescence in these crystals. In Figure 4, we compare the emission spectra of single crystals from batches 1 and 2 to the emission from a single crystal of KCu(CN)2 at 4 K upon excitation by 275 nm light. We note that batches 1 and 2 show the three aforementioned bands A, B, and C, whereas pure KCu(CN)2 shows only band B. We also observe that the intensity of band B is significantly lower in batch 2 than in batch 1, whereas bands A and C have similar relative intensities in both crystals. This result is reproducible and the trend was verified with different crystals from the same batches. The presence of three distinct emission bands with different excitation profiles in the steady-state luminescence results indicate the presence of three types of emitting exciplexes, *[Cu(CN)2-]n with different n. The assignment of the luminescence bands of K[Cu(CN)2-]/KCl crystals is given in Table 1. Lifetimes/Energy Transfer Model. Lifetime and timeresolved studies were performed using laser excitation at a wavelength of 266 nm. At this excitation wavelength, all three bands are seen in the emission spectrum at 78 K, as shown in Figure 5. In batch 1, the lifetimes for all three bands are monoexponential at all temperature values. In batch 2, however, the lifetimes for band A are single exponential at all temperature values from 78 to 295 K, whereas the lifetimes for both bands B and C show a significant rise time, followed by a monoexponential decay at long times. The rise times in the luminescence decays of the lower energy bands (B and C) in batch 2 leads to the conclusion that when this crystal is excited at 266 nm these bands are at least partially populated through energy transfer from the higher energy band (band A). This energy transfer is also seen in the time-resolved spectra for this crystal. Figure 6 shows the energy transfer from bands A to C. As the delay time after the laser pulse is changed by 25 µs increments, band A decreases in intensity while band C increases in intensity. This energy transfer can be modeled using a simple diffusion model.41 The excitation spectra show that the oligomers responsible for emission band A will absorb at 266 nm (at a
rate wA). These excited species can then undergo a radiative decay process (with rate constant kA), then they could undergo an energy transfer process to either bands B or C (with rate constants kET,B and kET,C, respectively), or they can undergo some nonradiative decay process (with rate constant kNR). The model also allows for the possibility of direct excitation of bands B (wB) and C (wC). The emission from bands A, B, and C will then be described by the following differential equations:
dNA ) wA - kANA - kET,BNA - kET,CNA - kNRNA dt dNB ) wB + kET,BNA - kBNB dt dNC ) wC + kET,CNA - kCNC dt If we assume that the excitation takes the form of a delta function, we can solve for NA, NB, and NC
NA(t) ) NA(0) exp[-(kA + kET,B + kET,C + kNR)t] NA(0)kET,B × kB - kA - kET,B - kET,C - kNR exp[-(kA - kET,B - kET,C - kNR)t] - CB exp[-kBt]
(1)
NA(0)kET,C × kC - kA - kET,B - kET,C - kNR exp[-(kA - kET,B - kET,C - kNR)t] - CCexp[-kCt]
(2)
NB(t) )
NC(t) )
Here, CB and CC are constants determined by the initial conditions NB(0) and NC(0)
CB )
NA(0)kET,B - NB(0) kB - kA - kET,B - kET,C - kNR
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Figure 7. Raw data with a fit to the model (eq 1) for bands B (upper curve) and C (lower curve) at 295 K (left) and 78 K (right).
TABLE 3: Lifetimes for Batch 1 band A band B band C
τ, µs, 78 K
τ, µs, 295 K
15.7 16.1 26.1
10.1 24.0 29.3
This solution indicates that band A should have a monoexponential lifetime equal to the inverse of the sum of the rate constants, which is what is seen experimentally (see Table 2). It should also be noted that the lifetime decreases with increasing temperature, as is expected, since kNR is expected to increase with increasing temperature. Equation 1 can be rewritten as:
NA(0)kET,B [exp(-t/τA)] - CB exp(-kBt) kB - 1/τA (3) ) D exp(-t/τA) - CB exp(-kBt)
NB(t) )
where D is a constant and τA is the lifetime of band A. Equation 2 could be similarly rewritten. This model indicates that we should expect to see a risetime in the luminescence decay of bands B and C. As stated earlier, we do observe a risetime in batch 2 (higher concentration) but not in batch 1 (lower concentration). This can be explained by considering the excitation and emission data for both batches. The laser wavelength used for the lifetime studies is 266 nm. We see in Figure 3 of the excitation spectra for each of the three emission bands that direct excitation of all three bands is possible at this wavelength. However, in batch 1, bands B and C absorb much more strongly at this wavelength than in batch 2. Furthermore, band A emits at wavelengths between 325 and 360 nm in both samples. In batch 1, the overlap between the absorption of bands B and C and the emission of band A is insignificant compared to this same spectral overlap in batch 2. This leads us to conclude that in batch 2, energy transfer contributes significantly to the population of bands B and C, while in batch 1, direct excitation dominates, explaining the monoexponential lifetimes in this batch. We can fit the decays from bands B and C in batch 2 to this model (Figure 7). The fits were performed using the lifetimes obtained for band A at 78 and 295 K. The lifetimes for bands B and C are given in Table 3, as obtained from the fit. The lifetimes all decrease with increasing temperature, as expected since nonradiative quenching processes will increase as the temperature rises. Energy transfer rates were also calculated for bands B and C. The rate of transfer to band B is seen to be higher than the rate of transfer to band C. This is expected as
Figure 8. Emission spectrum of batch 1 at 295 K excited by 266 nm and excitation spectra of two emission peaks at 334 and 410 nm, respectively.
we see in the excitation spectra that band B absorbs more strongly in the region of spectral overlap with band A. Also, the rate of energy transfer in band B increases slightly with increasing temperature, whereas the rate of energy transfer to band C remains essentially the same over the temperature range studied. This can be understood by considering the excitation spectra at both 77 and 295 K, as shown in Figures 5 and 8, respectively. As the temperature is increased, the excitation spectrum for band C remains relatively unchanged in the region of spectral overlap with band A, while with band B, we see an increase in the spectral overlap. This would lead to the increased energy transfer rate observed for band B. Lifetimes for batch 1 are given in Table 3. We see that, as the temperature increases from 77 to 295 K, the lifetime of band A decreases (as expected). It is interesting to compare these
Figure 9. HOMO (above) and LUMO (below) of Cu(CN)2- monomer.
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J. Phys. Chem. C, Vol. 113, No. 15, 2009 5957 TABLE 4: Extended Hu¨ckel Calculation Results for Different [Cu(CN)2-] Oligomers species [Cu] [Cu]/KCl [Cu]2/KCl [Cu]2 *[Cu]2 [Cu]3 *[Cu]3
dcu-N, Å
H-L gap, eV
2.04 1.52 1.46 1.52 1.48
4.42 5.09 3.38 3.71 3.61 3.41 3.32
* Corresponds to the first excited state.
Figure 10. HOMO (above) and LUMO (below) of Cu(CN)2- dimer.
values with those from batch 2: the lifetimes are the same at 77 K, but the lifetime in batch 1 decreases more upon increasing the temperature than in batch 3. Also, the lifetimes of bands B and C actually increase upon increasing the temperature. Both of these results can be explained by an increase in the rate of energy transfer with increasing temperature. We see from the excitation spectra for batch 1 at room temperature (Figure 8) both an increase in the spectral overlap with the emission energies of band A and a decrease in the relative absorption of bands B and C at the excitation wavelength of 266 nm. Therefore, the rate of energy transfer should increase with increasing temperature, leading to a corresponding increase in the lifetimes of bands B and C. Ab Initio Calculations. Single-point ab initio density functional calculations were performed on [Cu(CN)2-] oligomers. The geometry was obtained from published crystallographic data. Bond angles and bond lengths were frozen during the calculation. The HOMO-LUMO gap of the monomer was found to be 4.77 eV. In Table S1, population analysis is given for the monomer which shows that for the monomer the HOMO consist of 69% Cu in character with ∼30% contribution from the cyanide ligands. The LUMO is almost entirely Cu in character (93% Cu). Figure 9 shows a representation of the HOMO and LUMO of the [Cu(CN)2-] monomer. Similarly, a single point DFT calculation was performed on the dimeric [Cu(CN)2-]2 species. The solid state structure of K[Cu(CN)2] 37,38 showed that each [Cu(CN)2-] exists in a bent conformation that is connected to another [Cu(CN)2-] species
via a cyanide bridge. No direct metal-metal interaction exists in the compound. Figure 10 shows the HOMO and LUMO contour diagrams for the [Cu(CN)2-]2 dimer. The population analysis is given in Table S2. The HOMO-LUMO gap for the dimeric species is smaller than the HOMO-LUMO gap for the monomeric species. Single point DFT energy calculations were also performed on the trimeric [Cu(CN)2-]3 species. The structural parameters were taken from published data where the individual [Cu(CN)2-] units exists in a bent conformation, and each [Cu(CN)2-]2 unit is connected to the next one via a cyanide bridge. There was no metal-metal interaction in this compound. The calculated HOMO-LUMO gap of the [Cu(CN)2-]3 species is lower than the HOMO-LUMO gap of the dimeric species. From these simple results, it can be seen that the HOMO-LUMO gap tend to decrease as the oligomer gets longer. It can be seen for the dimer the HOMO has mostly Cu1 character (76%), while the LUMO has mostly Cu2 character, which is suggestive that the primary electronic transition for a [Cu(CN)2-]2 dimer is a metal-centered transition. Population analysis for the trimer in Table S3 gave results consistent with that of the dimer. Extended Hu¨ckel Calculations. Extended Hu¨ckel calculations were carried out for different [Cu(CN)2-]n oligomers as a function of internuclear separations as well as when the oligomers are doped in a KCl lattice. The results are summarized in Table 4 for free and doped oligomers. The ground-state and first excited-state calculations for free oligomers of [Cu(CN)2-] units indicate that the Cu-N bonding and the electronic transition energies are both sensitive to the number of ions in the oligomers. For example, ground-state calculations for oligomers reveal that as “n” increases from 1 to 2 and 3, the binding energy increases from 0 to 1.91-3.83 eV, and the HOMO-LUMO gap decreases from 4.42 to 3.71-3.41 eV. Table 4 also shows that for any given [Cu(CN)2-]n oligomer, the excited-state has a deeper potential well (higher binding energy) and shorter Cu-N equilibrium distance than the corresponding ground state. For example, the HOMO groundstate dimer has a binding energy of 1.91 eV, whereas the corresponding value for the LUMO excited-state is 2.67 eV. It is therefore concluded that, for all [Cu(CN)2-]n oligomers, Cu-N bonding is stronger in the first excited-state than in the corresponding ground state. Stronger Cu-N bonding in the first excited-state than the ground-state is an indication of the formation of *[Cu(CN)2-]n excimers and exciplexes. The example shown in Figure 11 illustrates the strong Cu-N bonding and low electronic energy for exciplexes compared to ground-state oligomer. Ground-state EH calculations have also been performed for a monomer and a dimer doped in a modeled KCl host (3 × 3 × 3 lattice). As shown for a monomer doped in KCl in Figure
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Figure 12. Structure of Cu(CN)2- monomer doped in KCl host. Green and pink spheres represent Cl-and K+ ions respectively. The Cu+, C, and N atoms are represented by the dark red, gray and blue spheres.
Figure 11. Extended Hu¨ckel calculations of the energy of groundstate and first excited-state of [Cu(CN)2-]2 dimer (above) and trimer (below).
12, the potassium and chloride ion distance is fixed at 3.32 Å. One of the potassium cations is replaced by copper(I) ion, and two of the neighboring chloride ions are replaced by cyanide ions. According to Table 4, doping of the [Cu(CN)2-]2 dimer in a KCl lattice results in a reduction of the energy difference between the HOMO and LUMO. This gives a possible explanation for observing lower energy excitation bands in the doped system. Table 4 also shows that in the doped system, as one proceeds [Cu(CN)2-] f [Cu(CN)2-]2, the HOMO-LUMO gap decreases from 5.09 to 3.38 eV due to the presence of groundstate interactions. In Table 1, the ab initio and EH results for the HOMO-LUMO energy differences for monomer, dimer and trimer units are compared with the experimental emission and excitation energies in eV units. If the highest energy emission is assigned to a monomer, then the EH calculated energies are in good agreement with the experimental energies for monomer, dimer and trimer. Exciplex Tuning. Figure 2 shows different emission bands over a broad range of wavelengths. Here, different oligomers give rise to different emissions in a single KCl crystal and the emission can be tuned simply by changing the excitation wavelength. The emission bands are assigned to various excimers and exciplexes formed in the studied crystals with a formula of *[Cu(CN)2-]n. The results here represent further examples of the “exciplex tuning” phenomenon, which was described earlier41 for doped crystals of [Ag(CN)2-]/KCl, [Au(CN)2-]/KCl and solutions of both K[Au(CN)2] and K[Ag(CN)2]. Exciplex tuning in a doped crystal of [Cu(CN)2-]/ KCl can be achieved by site-selective excitation (Figure 3) and
by varying the dopant concentration in a manner similar to that described for doped crystals of [Au(CN)2-]/KCl.39 Site-selective excitation is used to resolve the different emission bands from one another (Figures 2 and 3), whereas varying the dopant concentration is used to maximize the relative intensity of a given exciplex band (Figure 4). For example, [Cu(CN)2-]/KCl crystals with the highest Cu content (batch 2) show three emission bands at 350, 408, and 455 nm upon excitation at 262 and 274 nm (Figure 2). Exciplex tuning of the dicyanocuprate(I) emission is seen in various doped crystals, pure crystals, and solutions. The tuning action is achieved by varying the excitation wavelength and temperature in any of these media, the dopant concentration and host alkali halide crystal in doped crystals, the counterion
Figure 13. Exciplex tuning of the dicyanocuprate (I) emission in different media. Bands shown are (a) 1.00 × 10-5 M KCu(CN)2 solution in methanol (298 K λex ) 275 nm); (b) 2.00 × 10-3 M KCu(CN)2 solution in water (298 K λex ) 320 nm); (c) KCu(CN)2/KCl doped crystal (4 K λex ) 275 nm); (d) KCu(CN)2/KCl doped crystal (77 K λex ) 282 nm); (e) KCu(CN)2 pure crystal (ambient temperature, λex ) 322 nm); (f) KCu(CN)2/KCl doped crystal (ambient temperature, λex ) 335 nm).
Luminescent Studies of Exciplex Tuning in pure crystals, and the concentration and solvent in solutions. Although effective tuning can be achieved in any of these systems alone, the tuning range can be expanded over a wider emission energy range if one combines the results of the various media. To illustrate, we show in Figure 13 selected emission spectra of various dicyanocuprate(I) species with different conditions spanning an energy range of ∼6000 cm-1 with this approach. Similar tuning was achieved for the dicyanoaurate(I) emission.42,43 The various emission bands seen in Figure 8 are assigned to *[Cu(CN)2-]n exciplexes with different “n” configuration, and/or geometry. The exact identity of each exciplex cannot be determined with a great certainty. A reasonable assignment is suggested in Table 1 based on correlating the trends of luminescence energies seen for various dicyanocuprate(I) species (in doped crystals with different dopant concentrations, and pure crystals), with the infrared data and the electronic energies for various [Cu(CN)2-]n oligomers obtained from electronic structure calculations described above. Conclusions This study illustrates interesting luminescence behavior of dicyanocuprate(I) doped in a KCl host lattice. Three luminescence bands are observed in the [Cu(CN)2-]/KCl system, whereas only one band is observed in the pure KCu(CN)2 system excited with the same wavelengths. The luminescence in the [Cu(CN)2-]/KCl system can be tuned by varying the excitation wavelength or by varying the dopant concentration. By increasing the dopant concentration of [Cu(CN)2-], our spectroscopic data predict that the oligomer size increases. At the highest dopant concentration, the Cu-Cu bond distance is shorter than the corresponding value for the pure KCu(CN)2. This is similar to what we have reported for KAu(CN)2 doped in KCl and indicates that with increasing dopant level there is decreased back-bonding and higher νCN frequencies.41 We demonstrate efficient tunability of ∼6000 cm-1 in various Cu(CN)2- systems. Because observed luminescence bands are due to different aggregations of [Cu(CN)2-] units, infrared spectroscopy can be correlated with the measured photoluminescence spectra. Three emission bands observed for a single crystal of the batch 2 of the [Cu(CN)2-]/KCl system show peaks in the νC-N region of the infrared spectrum giving a reasonable correlation among infrared and luminescence spectroscopic results. Extended Hu¨ckel calculation predictions give good agreement with the observed experimental results.Time-resolved spectra suggest the presence of energy transfer from the [Cu(CN)2-]n oligomers responsible for the higher-energy bands to the oligomers responsible for the lower-energy bands. Acknowledgment. This work was supported by the National Science Foundation (CHE-0315877). We thank Mr. David LaBrecque for his assistance. Supporting Information Available: Additional information available as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dias, H. V. R.; Diyabalanage, H. V. K.; Rawashdeh-Omary, M. A.; Franzman, M. A.; Omary, M. A. J. Am. Chem. Soc. 2003, 125, 12072. (2) Omary, M. A.; Rawashdeh-Omary, M. A.; Diyabalanage, H. V. K.; Dias, H. V. R. Inorg. Chem. 2003, 42, 8612.
J. Phys. Chem. C, Vol. 113, No. 15, 2009 5959 (3) Yam, V. W.-W.; Cheung, K.-L.; Cheng, E. C.-C.; Zhu, N.; Cheung, K.-K., Dalton Trans. 2003, 1830. (4) Dias, H. V. R.; Diyabalanage, H. V. K.; Eldabaja, M. G.; Elbjeirami, O.; Rawashdeh-Omary, M. A.; Omary, M. A. J. Am. Chem. Soc. 2005, 127, 7489. (5) Liu, X.; Guo, G.-C.; Wu, A. Q.; Cai, L.-Z.; Huang, J.-S. Inorg. Chem. 2005, 44, 4282. (6) Vitale, M.; Ford, P. C. Coord. Chem. ReV. 2001, 219-221, 3. (7) Yam, V. W.-W. Pure Appl. Chem. 2001, 73, 543. (8) Ford, P. C.; Cariati, E.; Bourassa, J. Chem. ReV. 1999, 99, 3625. (9) Ma, Y. G.; Che, C.-M.; Chao, H.-Y.; Zhou, X. M.; Chan, W.-H.; Shen, J. C. AdV. Mater. 1999, 11, 852. (10) Hollingsworth, G.; Barrie, J. D.; Dunn, B.; Zink, J. I. J. Am. Chem. Soc. 1988, 110, 6569. (11) Barrie, J. D.; Dunn, B.; Hollingsworth, G.; Zink, J. I. J. Phys. Chem. 1989, 93, 3958. (12) Shin, K. S. K.; Barrie, J. D.; Dunn, B.; Zink, J. I. J. Am. Chem. Soc. 1990, 112, 5701. (13) Li, D.; Che, C. M.; Kwong, H. L.; Yam, V. W.-W., J. Chem. Soc., Dalton Trans. 1992, 3325. (14) Anpo, M.; Shioya, Y.; Yamashita, H.; Giamello, E.; Morterra, C.; Che, M.; Patterson, H. H.; Webber, S.; Ouellette, S.; et al. J. Phys. Chem. 1994, 98, 5744. (15) Rasmussen, J. C.; Toftlund, H.; Nivorzhkin, A. N.; Bourassa, J.; Ford, P. C. Inorg. Chim. Acta 1996, 251, 291. (16) Henary, M.; Wootton, J. L.; Khan, S. I.; Zink, J. I. Inorg. Chem. 1997, 36, 796. (17) Kyle, K. R.; Ford, P. C. J. Am. Chem. Soc. 1989, 111, 5005. (18) Horvath, A.; Wood, C. E.; Stevenson, K. L. Inorg. Chem. 1994, 33, 5351. (19) Horvath, A.; Stevenson, K. L. Coord. Chem. ReV. 1996, 153, 57. (20) Dossing, A.; Ryu, C. K.; Kudo, S.; Ford, P. C. J. Am. Chem. Soc. 1993, 115, 5132. (21) Kyle, K. R.; Ryu, C. K.; Ford, P. C.; DiBenedetto, J. A. J. Am. Chem. Soc. 1991, 113, 2954. (22) Liang, S.-W.; Li, M.-X.; Shao, M.; Miao, Z.-X. Inorg. Chem. Commun. 2006, 9, 1312. (23) Mao, H.; Zhang, C.; Xu, C.; Zhang, H.; Shen, X.; Wu, B.; Zhu, Y.; Wu, Q.; Wang, H. Inorg. Chim. Acta 2005, 358, 1934. (24) Yu, J.-H.; Xu, J.-Q.; Yang, Q.-X.; Pan, L.-Y.; Wang, T.-G.; Lu, C.-H.; Ma, T.-H. J. Mol. Struct. 2003, 658, 1. (25) Felder, D.; Nierengarten, J.-F.; Barigelletti, F.; Ventura, B.; Armaroli, N. J. Am. Chem. Soc. 2001, 123, 6291. (26) Gordon, M.; Ware, W. R., Eds.; The Exciplex; Academic Press: New York, 1975; p 372. (27) Clodfelter, S. A.; Doede, T. M.; Brennan, B. A.; Nagle, J. K.; Bender, D. P.; Turner, W. A.; LaPunzina, P. M. J. Am. Chem. Soc. 1994, 116, 11379. (28) Omary, M. A.; Patterson, H. H. J. Am. Chem. Soc. 1998, 120, 7696. (29) Omary, M. A.; Patterson, H. H. Inorg. Chem. 1998, 37, 1060. (30) Pettijohn, C. N.; Jochnowitz, E. B.; Chuong, B.; Nagle, J. K.; Vogler, A. Coord. Chem. ReV. 1998, 171, 85. (31) Cunningham, C. T.; Cunningham, K. L. H.; Michalec, J. F.; McMillin, D. R. Inorg. Chem. 1999, 38, 4388. (32) Omary, M. A.; Hall, D. R.; Shankle, G. E.; Siemiarczuk, A.; Patterson, H. H. J. Phys. Chem. B 1999, 103, 3845. (33) Patterson, H. H.; Kanan, S. M.; Omary, M. A. Coord. Chem. ReV. 2000, 208, 227. (34) Horvath, A.; Stevenson, K. L. Inorg. Chem. 1993, 32, 2225. (35) Omary, M. A.; Webb, T. R.; Assefa, Z.; Shankle, G. E.; Patterson, H. H. Inorg. Chem. 1998, 37, 1380. (36) Rawashdeh-Omary, M. A.; Omary, M. A.; Shankle, G. E.; Patterson, H. H. J. Phys. Chem. B 2000, 104, 6143. (37) Cromer, D. T. J. Phys. Chem. 1957, 61, 1388. (38) Staritzky, E.; Walker, D. I. Anal. Chem. 1956, 28, 419. (39) Lorenzelli, V.; Gesmundo, F. Atti. Accad. Naz. Lincei, Rend., Classe. Sci. Fis., Mat. Nat. 1963, 35, 524. (40) Penneman, R. A.; Jones, L. H. J. Chem. Phys. 1956, 24, 293. (41) Hettiarachchi, S. R.; Rawashdeh-Omary, M. A.; Kanan, S. M.; Omary, M. A.; Patterson, H. H.; Tripp, C. P. J. Phys. Chem. B 2002, 106, 10058. (42) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H. J. Am. Chem. Soc. 2000, 122, 10371. (43) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P. J. Am. Chem. Soc. 2001, 123, 11237.
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