Site-Selective Excitation of “Exciplex Tuning” for Luminescent

Sep 29, 2010 - Site-selective excitation of the different nanoclusters results in the ... ions provides the rationale for calling these doped systems ...
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J. Phys. Chem. C 2010, 114, 17401–17408

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Site-Selective Excitation of “Exciplex Tuning” for Luminescent Nanoclusters of Dicyanoargentate(I) Ions Doped in Different Alkali Halide Crystals Franc¸ois Baril-Robert,† Xiaobo Li,† David A. Welch,† Benjamin Q. Schneider,† Michael O’Leary,† Christie L. Larochelle,‡ and Howard H. Patterson*,† Department of Chemistry, UniVersity of Maine, Orono, Maine 04469, and Department of Physics and Astronomy, Franklin & Marshall College, Lancaster, PennsylVania 17604 ReceiVed: June 4, 2010; ReVised Manuscript ReceiVed: August 31, 2010

Luminescent nanoclusters of linear dicyanoargentate(I) ions doped in different alkali halide single crystals (NaF, NaCl, NaBr, and KCl) have been investigated. Site-selective excitation of the different nanoclusters results in the appearance of emission bands which are assigned to different luminescent {[Ag(CN)2]-}n nanoclusters in the host lattices. The nanoclusters of [Ag(CN)2]- in the alkali halides display energy tunability over a wide wavelength interval. The ab initio DFT calculations show that for nanoclusters, the first excited state has a deeper potential and shorter internuclear separation than the nanocluster ground state, indicative of excimer/exciplex behavior. Molecular modeling indicates how the distribution of nanoclusters varies with the host lattice. This emission energy tunability makes these systems attractive candidates for potential applications, such as tunable solid state lasers, photocatalysts, and photosensitizers for water splitting. Introduction Compounds of closed-shell d10 ions have attracted growing interest from experimental and theoretical chemists due to their tendency to form supramolecular aggregates with metallophilic interactions.1-4 The electronic spectra and X-ray structure5-9 of these complexes clearly provide evidence for the presence of closed-shell metal-metal interactions, as well as excimer/ exciplex formation.10-15 The luminescence exhibited by Au(I) compounds is useful in a variety of applications such as detection of volatile organic compounds,16 ion sensors,17 oxygen sensors,18 and molecular light emitting devices.17,19,20 Applications of Ag(I) compounds include the use of silver halides as photographic materials, and the use of silver compounds as photocatalysts,21 conductors, semiconductors, and photoconductor.22-25 Also, when silver or gold dicyanide clusters are doped in A-zeolites, photodecomposition rates with the pesticide carbaryl are 40 to 60 times faster than those with carbaryl alone.26 These applications provide the motivation for our continuing interest in the photophysics and photochemistry of d10 complexes. Our recent studies of cyano complexes of Ag(I) have shown that the photoluminescence properties are related to the formation of metal-metal bonded excimers and exciplexes. Although luminescent exciplexes are well-known molecular entities in the photochemistry and photophysics of organic systems,27 they are less common in the inorganic literature.28 In addition, most reported inorganic exciplexes are not luminescent, although some luminescent Pt(II) exciplexes have been reported.29,30 We have contributed to this field by discovering the optical phenomenon of “exciplex tuning” which is the tuning of the emission in Ag(CN)2- doped alkali halide crystals to various bands in the ultraviolet and visible regions with each band corresponding to a different [Ag(CN)2-] excimer or exciplex.12-14,31 It should be noted that in contrast to pure complexes which * To whom correspondence should be addressed. † University of Maine. ‡ Franklin & Marshall College.

exhibit bulk properties, doping these systems in alkali halide lattices essentially constitutes the creation of nanosystems32,33 with the presence of dimers, trimers, and higher order oligomers in NaCl or KCl systems.14 A correlation between the luminescence and Raman bands for both pure and doped crystals containing the dicyanoargentate(I) and dicyanoaurate(I) ions provides the rationale for calling these doped systems nano.31 We have also reported that luminescent bonded excimers and exciplexes are present in aqueous and methanolic solutions.10,34 We have investigated the possible role of exciplex formation in the photocatalytic activity of Ag(I) doped ZSM-5 zeolites for the decomposition of nitric oxide.35 In this latter case the results of EXAFS have been used in support of the excitedstate exciplex assignments. Recently we reported the discovery of inorganic excimers and exciplexes in a Cu(CN)2-/KCl system in which we propose that the bonding in the exciplex occurs via a cyanide bridge between the copper atoms.36 This is different from the study of dicyanoargentate ions doped in KCl crystals in which only metal-metal exciplex behavior is present. Each of the copper(I) dicyanide exciplexes found in the KCl host crystals can be selectively excited and have different emission energies. In this paper we report for the first time a spectroscopic and theoretical study of dicyanoargentate(I) excimers and exciplexes formed in different alkali halides such as NaF, NaCl, NaBr, and KCl (Figure 1). When the linear [Ag(CN)2]- ion is substituted into an MX host lattice Ag+ substitutes for M+ and CN- substitutes for X-. The choice of the alkali halide host lattice affects the nuclearity, electronic composition, and multiplicity of the different excimers and exciplexes. Larger lattices favor the formation of linear trimers versus bent trimers and the presence of the neighboring Cl- or CN- ions will affect the energies and the composition of existing excimers/exciplexes. These experimental and theoretical results herein provide a basis for predicting the exciplex tunability of these types of nanosystems. This tunability in the emission energies makes these systems attractive candidates for potential applications,

10.1021/jp105155a  2010 American Chemical Society Published on Web 09/29/2010

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Baril-Robert et al. SCHEME 1

Figure 1. Illustration of a (µ-CN)2 dicyanoargentate(I) dimer embedded in a NaCl lattice.

TABLE 1: Silver Loading (% w/w (0.01%) for Samples of [Ag(CN)2]- Ions Doped in Different Alkali Halides sample

NaF

NaCl

NaBr

KCl

A B C

0.01 0.02

0.19 0.26 1.70

0.05 0.24 0.43

0.01 0.26 0.58

such as tunable solid state lasers, photocatalysts, and photosensitizers for water splitting. Experimental and Computational Methods Synthesis and Characterization. Single crystals of [Ag(CN)2]- ions doped in alkali halides (NaCl, NaBr, and KCl) were grown by slow evaporation of aqueous solutions (20 mL) containing 3 g of the corresponding alkali halide salts and various amounts (0.01-0.1 g) of K[Ag(CN)2]. In a similar way, potassium silver dicyanide (0.005-0.05 g) and 0.8 g of sodium fluoride were dissolved in 20 mL of water in nalgene glassware. Crystals were harvested before complete evaporation of the solution. Silver content was determined by atomic absorption spectroscopy, using a Model 857-Smith-Hieftje 11/12 spectrophotometer and an Ag analytical lamp operating at 328.1 nm. Silver loadings are presented in Table 1. For a specific host, the final silver loadings roughly correlate to the starting K[Ag(CN)2] amount and allow for a tunability of the material dopant concentration. The crystal growing process for doping Ag(CN)2- in different alkali halide hosts was repeated and atomic absorption, luminescence, and Raman analysis give reproduced results. We have reported previously atomic absorption measurements run in triplicate for different crystals of Ag(CN)2-/KCl harvested at the same time. These results showed no variability of the Ag content to two significant figures.13 Photophysical Measurements. Steady-state photoluminescence spectra were recorded with a Model QuantaMaster-1046 photoluminescence spectrophotometer from Photon Technology International. The instrument is equipped with two excitation monochromators and a single emission monochromator with a 75W 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 or liquid helium was used as coolant. Lifetime measurements were performed with 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 meter monochromator with

an Oriel model 77360 photomultiplier tube and a Stanford Research Systems model SR445 350 MHz preamplifier. The data were collected with 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 with a curve fitting routine in Matlab 7.10. The data were corrected for zero baseline and time zero was defined by the excitation pulse. Time-resolved spectra were obtained by integrating to obtain 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 was achieved with a silicon diode temperature sensor, 25 heater coil, and a Lakeshore Model 331 temperature controller. Uncertainties in the lifetimes are on the order of 0.01 µs for the short components and 0.1 µs for the long components. Room temperature Raman spectra were collected by a Renishaw Raman imaging microsope system 1000 using a diode laser operating at 735 nm. 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. Computational Details. The theoretical structure and the energy of electronic states of the [Ag(CN)2]- monomer and dimers doped in alkali halide (NaCl, NaF, NaBr, and KCl) lattices were determined with Gaussian’03 software (Gaussian Inc.).37 Density functional theory optimization calculations were performed with B3LYP38,39 functional and SDD40 basis set as implemented in the software. Models for doped monomeric ions consist of [Ag(CN)2X4]5- ions (X ) F, Cl, or Br) enclosed by 18 alkali ions at fixed position corresponding to the studied alkali halide salt. Stationary point charges41,42 were added to surround the fixed alkali ions and model the remaining ionic sites within a 4 × 4 × 4 supercell model. In a similar way, dimeric species models consist of [Ag2(CN)4X6 or 7]-8 or -9 ions surrounded by alkali ions and point charges yielding a 4 × 4 × 5 supercell model. Structures of the monomer and the 6 potential dimers are shown in Scheme 1. Excited state energies were calculated by using time-dependent theory (TD-DFT).43-45 Isodensity representations of molecular orbitals were generated with GaussView 3.07 software (Gaussian Inc.). These calculations were performed on the University of Maine supercomputer. Tight structural optimization was performed for models and conventional single point TD-DFT were then carried out on the optimized structure.

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Figure 2. Representation of the 10 × 14 × 5 atoms alkali halide slab (A) and of a dimer at the kink (B).

To simulate the creation of nanoclusters during crystal growth, a surface model was developed that relates the growth of these clusters to impurity additions at “perfect” kink sites on a crystal surface.46 Representations of the surface slab and of a dimer present at the kink are shown in Figure 2. For each host system, 1 monomerization, 11 dimerization, and 157 trimerization reactions were tested by considering all oligomerization pathways derived when requiring next-nearest-neighbor spacing between the kink silver ion and at least one other silver ion. The energy of these reactions was determined by comparing the energy of the optimized structures with and without a monomer unit at the kink. Surface optimizations were performed with the General Utility Lattice Program (GULP).47 A periodic (001) surface slab with dimensions of 10 × 14 × 5 atoms that has about half of its top layer removed to create two surface steps that together form a kink site was employed. Optimizations were carried out under constant pressure with the average error in the energy convergence being about 1 kJ/mol. Interatomic two-body Madelung-Buckingham potentials (eq 1) that contain a Coulombic interaction term, an exponential repulsion term, and an r-6 dispersion term were employed.

Uij(rij) ) Aije-rij/Fij -

cij rij6

+

qiqj 4πε0rij

(1)

Host anion and silver ion polarization were represented with a core-shell model; host cation polarization effects were not accounted for, and cyanide polarization was approximately represented with atomic charge assignments. Intramolecular interactions were represented with 1-2 stretching and 1-3 bending terms while intramolecular Coulombic interactions were not employed. Formal charges were assigned to the highly ionic host lattice ions while APT (atomic polar tensor) charges found by DFT calculations were used for the silver, carbon, and nitrogen ions. Two body interaction terms for all the studied ions were taken from the literature (Supporting Information) while the LorentzBerthelot combination rules were applied to generate intermolecular C-Ag, N-Ag, C-Br, N-Br, C-Cl, N-Cl, and K-Ag terms. Intramolecular silver dicyanide terms were taken from Jones’ spectral analysis.48 Information on the silver-carbon bond distance suggests a value of 2.06 Å be used.49,50 The value of the carbon-nitrogen bond distance in these works, however, is unusually low, so the value of 1.17 Å assigned in the NaCN and KCN studies, which closely matches our quantum simulation results for the embedded monomer, was assigned. Results and Discussion Raman Spectroscopy. As seen in Figure 3, the Raman spectra of crystals of [Ag(CN)2]- ions doped in different alkali

Figure 3. Room temperature Raman spectra (corrected for baseline) of pure potassium silver dicyanide and of the silver dicyanide anions doped in different alkali halides (νCN stretch region).

halides show peaks in the νCN region. Frequency and peak count depends on the nature of the alkali halide. For sodium chloride doped with silver dicyanide, only one peak corresponding to the CtN stretch mode is seen at 2156 cm-1. This is at slightly higher frequency than the corresponding mode in pure potassium silver dicyanide (2147 cm-1). Due to the similarity between these two spectra, we propose that the cyanide ions adopt a similar µ1 terminal mode of coordination. This implies that, in the NaCl host, only chlorides serve as bridging ions and we expect to mainly see (µ2-Cl) 0°, 90° and/or (µ2-Cl)2 dimers and oligomers. Raman spectra of silver dicyanide doped in NaBr exhibits a peak similar to the one seen in [Ag(CN)2]:NaCl (2154 cm-1) and two extra broader bands at lower frequency (2027 and 2100 cm-1). These new bands are assigned to bridged cyanide modes. Multiple peaks corresponding to a CtN stretch vibrational mode are seen in the Raman spectrum of silver dicyanide doped in potassium chloride. Even though one peak is seen at 2148 cm-1, the majority of the peaks exhibit a red shift (2093, 2107, and 2114 cm-1) compared to the peak seen for the NaCl host. These three peaks are sharp and correspond to species with bridging cyanides. Two weak high-frequency peaks are seen for [Ag(CN)2]-:NaF at 2143 and 2169 cm-1 and may be characteristic of silver dicyanide anions in an asymmetric cavity. To accommodate the rather large silver dicyanide ions, the small NaF lattice may create extra vacancies via lattice defects. Reflectance Spectroscopy. Absorption transitions were studied by UV-vis reflectance spectroscopy on ground powder of all materials. As shown in Figure 4 for [Ag(CN)2]- anions doped in NaBr, spectra exhibit relatively unresolved bands at high energy (λ < 350 nm). As a general trend, there is an increase in band intensities and appearance of new bands at lower energy when the silver loading increases. These lower energy bands are in agreement with the idea of larger oligomers at higher

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Figure 4. Room temperature reflectance spectra of silver dicyanide anions doped in sodium bromide with dopant concentration increasing from A to B to C.

concentration. This phenomenon has already been observed for silver dicyanide anions in solution.10,34 Steady-State Luminescence. In these materials, mixtures of different nanoclusters (nuclearity, geometry) with different photophysical properties give rise to excitation wavelength sensitivity of the emission spectra. This phenomenon is called emission tunability14 and is seen for [Ag(CN)2]- ions doped in all the studied alkali halides. As seen in Figure 5, depending on excitation wavelength, emission spectra can be composed of a single peak or multiple peaks. Moreover, the energy difference between the excitation and emission maxima is not constant from one band to the other and this energy separation is not proportional to the excitation or emission wavelength. Excitation and emission maxima energies and peak counts are sensitive to the host and hence to the coordinating halide ion and/or the lattice constant. Moreover, by promoting energy transfer between near clusters, a high concentration of dopant could alter photophysical properties of the material. This potential phenomenon was studied by lifetime measurements and is discussed in the appropriate section. To better understand the nature of these multiple peaks and evaluate potential overlapping between these bands, excitation/ emission matrices were collected for all compounds (see the Supporting Information). A low temperature (77 K) excitation/ emission matrix of a [Ag(CN)2]-:NaCl (0.19%) single crystal is shown in Figure 6. From these 3D representations, it was possible to extract the relative intensities and specific excitation and emission wavelengths for every emissive species. Emission and excitation maxima of the different peaks and their assignments are given in Table 2. Excitation maxima range from 225 to 300 nm but most exhibit excitation in the 260-290 nm range. Emission maxima span a larger range (285-500 nm). In most cases, 3 to 4 different {[Ag(CN)2]-}n emissive species can be seen in the same sample. The high concentration case of silver dicyanide ions doped in sodium chloride materials exhibits up to 5 different clusters. The energy differences between excitation and emission maxima are greatly variable (5 000 to 16 000 cm-1) and indicate a wide variability in the excimeric behavior of the different nanoclusters. As a general rule, lower energy bands tend to be more intense at higher concentration. We believe that this behavior arises from the formation of larger nanoclusters at a higher concentration of [Ag(CN)2]- dopant. In a dimer, the two dz2(Ag) filled orbitals mix into one Ag-Ag bonding and one destabilized antibonding orbital. A similar behavior can be seen for the π*(CN) empty orbitals resulting in a net decrease in the HOMO-LUMO gap and in the energy of the corresponding electronic transition.

Figure 5. Excitation (dotted) and emission (full) spectra of silver dicyanide ions doped in different alkali halides. Excitation or emission wavelengths are labeled beside the corresponding spectrum.

Figure 6. Emission spectra (λex ) every 6 nm) of a single crystal [Ag(CN)2]-:NaCl (0.19 wt %) at 77 K showing four different emission peaks: A, λex ) 225 nm, λem ) 287 nm; B, λex ) 275 nm, λem ) 318 nm; C, λex ) 272 nm, λem ) 347 nm; D, λex ) 260 nm, λem ) 398 nm.

Assignments were carried out with the following assumptions. Atomistic models (vide infra) showed that nanocluster formation energies favor the formation of (µ2-X)2 and (µ2-X)(µ2-CN) bridging modes. Other configurations were not significantly present. Moreover, the NaCl host favors the formation of bent

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TABLE 2: Characteristic 77 K Luminescence Wavelengths (Excitation, Emission) for the Different Species in Different Alkali Halidesa λem (nm)

λex (nm)

∆E (cm-1)

Assignment

287 318 347 358 398 395 426 316 404 350 503 302 319 399 372 287 409 345

225, 239 w 275 272 288 260 289 263 269 257 271 277 225 272 257 301 230 253 281

9600, 7000 4900 7150 6800 13 350 9300 14 550 5500 14 150 8300 16 200 11 350 5400 13 850 6350 8650 15 100 6600

monomer dimer A trimer A-A 90° tetramer A-A-A 90° dimer B tetramer A-A-A linear trimer A-B dimer A dimer B trimers A-A 90° trimer B-B monomer dimer A dimer B trimer A-A linear monomer dimer B trimer A-A 90°

NaCl

NaF

NaBr

KCl

a Assignments for the different λex/λem are obtained from comparison with DFT calculation energies and atomistic model population analysis. Bridging modes: A ) (µ-X)2; B ) (µ-X)(µ-CN).

TABLE 3: Observed τ(µs) and λem (nm) for the Lifetime Measurements of Dicyanoargentate Ions Doped in Different Alkali Halides at Different Concentrations (See Table 1)a samples A

B

C

NaCl 0.2/2.7 0.5/3.5 1.7/9 0.4/15.5 0.5/6.3 15.5 0.8/11 3.7/44 NaF N/A N/A 0.2/2.0 0.6/9.1 NaBr N/A 0.3/2.2 0.7/18 KCl N/A N/A >100 0.9/9.4

λem (nm)

assignment

(315, 325, 330) 350 390 440 315 387 325 400 412 346

dimer (µ-Cl)2 trimer (µ-Cl)2-(µ-Cl)2 90° dimer (µ-Cl)(µ-CN) trimer (µ-Cl)2-(µ-Cl)(µ-CN) dimer (µ-F)2 dimer (µ-F)(µ-CN) dimer (µ-Br)2 dimer (µ-Br)(µ-CN) dimer (µ-Cl)(µ-CN) trimer (µ-Cl)2-(µ-Cl)2 90°

a Samples were excited with a 266 nm pulsed laser. Lifetime components of dual exponential decay are separated by the “/” symbol.

90° trimers while linear trimers were preferred in alkali halides with a larger lattice constant (NaBr, KCl). Raman spectroscopy showed that clusters in the NaCl and NaF hosts adopt the halide bridged configuration [(µ2-X)2] and that cyanide bridged clusters [(µ2-X)(µ2-CN)] are present in KCl and NaBr. DFT calculations (vide infra) showed that dimers adopting a (µ2-X)(µ2-CN) arrangement exhibit a small excitation red shift compared to monomer species and have a large energy difference between excitation and emission maxima. In comparison, the (µ2-X)2 configuration exhibits a stronger excitation red shift and a much smaller energy difference between maxima. Lifetime and Time-Resolved Measurements. To evaluate energy decay processes between the different clusters, lowtemperature emission lifetimes were measured for selected emission peaks (Table 3). All bands exhibit double exponential decays with the longer component approximately ten times longer than the shorter one. Lifetimes range from 200 ns up to more than 100 µs. These long lifetimes indicate that emission bands arise from spin forbidden (triplet to singlet) transitions. Multiple lifetimes for a single band indicate the presence of multiple emissive species with similar emission energy but with different deactivation properties. Such a phenomenon could arise

Figure 7. Lifetime measurements (λex ) 266 nm; λem ) 300 nm) of [Ag(CN)2]- doped in NaBr with concentration increasing from bottom to top.

from crystal defects or overlapping bands. Moreover, as seen in the [Ag(CN)2]-:NaCl materials, the lifetime of a specific band varies with concentration, further indicating the influence of neighboring clusters and species on the emission lifetime. Time-resolved experiments showed that all but one of the samples exhibited an emission decay upon 266 nm pulsed excitation with the absence of any rise time. This indicates that if there is energy transfer between clusters, it is extremely fast. As shown in Figure 7, the high concentration [Ag(CN)2]-:NaBr sample does show evidence of energy transfer in the form of a rise time in the luminescence decay of both emission bands. This rise time is relatively short and no rise was observed in the lower concentration samples of [Ag(CN)2]-:NaBr. The concentration dependency of this result can be explained by the fact that energy transfer can occur between fluorophores that are in close proximity to one another, a situation that is more likely to occur in the higher concentration sample. In a previous publication, this type of behavior was described by a simple diffusion model.36 Also a kinetic model has been presented previously13,36 to address potential photophysical interactions between crystals having different degrees of oligomerization. Time-resolved measurements can be used to study possible energy pathways between the excitations characteristic of the different [Ag(CN)2]- nanoclusters, such as dimers and dimers, as well as dimers and trimers. Ab Initio Calculations. To ascertain electronic and structural effects of the alkali halide host on silver dicyanide nanoclusters, density functional theory calculations were performed for the monomeric species in four different alkali halides (NaCl, NaF, NaBr, and KCl) and six different dimeric species in NaCl. Geometries of the doped nanocluster singlet ground state and first triplet excited state were obtained. As shown in Figure 8 (and the Supporting Information), the HOMO of the monomeric species doped in sodium chloride consists of the silver dz2 orbital exhibiting σ antibonding interactions with the cyanide ions in the z direction and with the chloride ions in the x and y directions. The LUMO consists mainly of the cyanide ion π* empty orbitals with some delocalization in the 5px or 5py empty orbitals of the metal center. Similar to previous theoretical studies,51 this leads to an electronic transition with a strong MLCT character. The presence of chloride electronic density in the HOMO orbital shows the influence of the host in these transitions. The halide ions affect

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Baril-Robert et al. TABLE 4: Calculated Emission and Excitation and Phosphorescence Wavelengths for Different Species of [Ag(CN)2]-:NaCl. The Energy Difference Is Given in Wavenumbers

Figure 8. Orbital isodensity representation of the HOMO (1) and LUMO (2) of the monomeric species (A), (µ-Cl)2 (B), and (µ-CN)2 (C) in the sodium chloride host. Orbitals of other configurations are given in the Supporting Information.

the transition’s energy and give a small LLCT (halide to cyanide charge transfer) character to the transition. Similarly to the monomeric species, the dimer HOMO orbital corresponds to 5dz2 of the metal centers with small antibonding between the two moieties. The dimers’ LUMO orbitals are still analogous to the cyanide π* observed for the monomeric species, but the exact nature of the interactions between the two moieties differs from one geometry to the other. In the (µ2-Cl)2 case, the π*(CN) are aligned along the Ag-Ag axis allowing a weak interaction between the two monomers. In the (µ2-CN)2 case, moieties interact via a relatively complex π network. Interestingly, although the bridging mode in the (µ2-Cl)(µ2-CN) case could be considered as intermediate between (µ2-Cl)2 and (µ2-CN)2 configuration, the LUMO orbital exhibits the strongest excited state interaction between monomeric moieties. In this case, a σ interaction between the bridging cyanide ion and the second monomer is seen. In dimers bridged by a single chloride ion (the (µ2-Cl) 0° and 90°), a weak σ* interaction is seen between the silver dicyanide complexes and the bridging ion. The (µ2CN) LUMO orbital shows weak π delocalization between the cyanide π* orbital and the p orbitals of both silver centers. Optimized structures for the singlet ground state and the first triplet excited state have been calculated for the monomeric species and all dimers (see the Supporting Information). In the sodium chloride host, ground state AgsC distances are generally in the 2.11-2.13 Å range and the CtN distance is consistently around 1.19 Å. These values are slightly longer than what is observed in crystallographic studies of the pure potassium silver dicyanide compounds [AgsC ) 2.12(14) Å; CtN ) 1.17(20) Å]52 and other simple silver dicyanide salts [AgsC ) 2.05-2.07 Å; CtN ) 1.13-1.14 Å].49 We suspect that by altering the metal center orbital, equatorial halide ligands are responsible for this small elongation. Calculated silver chloride distances (2.92-3.05 Å) are highly variable depending on the dimer configuration. This discrepancy arises from the relatively weak metal-ion interaction compared to the Coulombic interaction between chloride anion and neighboring host sodium cations.

compd

λem (nm)

λex (nm)

∆Eex-em (cm-1)

monomer (µ-Cl)2 (µ-Cl)(µ-CN) (µ-CN)2 (µ-Cl) 0° (µ-Cl) 90° (µ-CN)

330.9 331.4 370.1 334.2 333.2 330.4 371.6

273.5 291.2 284.0 266.3 291.9 289.6 286.5

6342 4166 8192 7629 4246 4264 7993

In general, the first triplet excited state structure exhibits shorter AgsC distances and longer CtN distances than the ground state. This is in compliance with the excitation of one electron from a dz2(Ag) to a π*(CN) orbital. Behavior of the Ags Ag separation is sensitive to the dimers’ structure. In the (µ2Cl)2 dimer, excitation shortens the metalsmetal distance from 3.969 Å to 3.598 Å. This behavior was seen for all 6 types of dimers with the (µ2-CN)2 compound showing the biggest reduction of the AgsAg separation (from 4.128 Å to 3.522 Å) and the (µ2-Cl) 0° showing the smallest effect (from 5.850 Å to 5.722 Å). In all cases, the AgsAg separation is still longer than the sum of the silver van der Waals radii (∼3.45 Å) even in the excited states. A wide variety of AgsCl distances (2.85-3.13 Å) are observed. Elongation or contraction depends on the position of the chloride ion in the dimer. Due to a global contraction of the AgsAg distance, AgsCl with a bridging chloride ion will shorten while bonds to a terminal chloride ion elongate. Almost all dimers preserve their general structure in their first triplet excited state except for the (µ2-Cl)(µ2-CN) dimer. When excited, the bridging cyanide ion in the (µ2-Cl)(µ2-CN) dimer changes its coordination mode from (η1,η2) to (η1,η1) in which the carbon atom serves as a bridge. This comes with a rotation of the cyanide in the Ag/Ag/C plane and a shortening of the AgsC along the newly formed σ bond (from 3.098 Å to 2.267 Å). Theoretical electronic transition energies found by TD-DFT are given in Table 4. In all our calculations, theoretical ground state structure 1Γ f 1Γ absorption energies are lower than any excitation energies found experimentally (∼12%). Except for the (µ2-CN)2 dimer, all dimer MLCT transitions are at lower energy than those for the monomer by approximately 2000 cm-1. Further oligomerization is expected to follow this red-shift trend. The energy difference between the calculated excitation and emission can be used to evaluate the experimental energy difference between excitation and emission maxima. It is interesting to correlate this ∆Eex-em with structural variation between the ground and the excited state. Dimers with the highest energy difference (∼8000 cm-1) all contain bridging cyanides while chloride bridged dimers display the smallest theoretical energy differences (∼4000 cm-1). It has been observed that the bridging cyanide mode varies upon excitation due to bonding interaction between the cyanide π* orbital and both metal centers. Much bridging cyanide motion occurs and results in a large energy difference between excitation and emission. Ground state and triplet excited state optimization of monomeric species in different alkali halides was carried out to evaluate potential host effects on structural properties and luminescence energies. Structural parameters are reported in Table 5. In NaCl, KCl, and NaBr hosts, silver dicyanide ion bond lengths are similar with the exception of Ag-X. Interest-

Luminescent Nanoclusters of Linear [Ag(CN)2]- Ions TABLE 5: Structural Features for a Monomeric Species in Different Alkali Halides host 1

GS Γ 1 NaBr GS 3 Γ 1 KCl GS 3 Γ 1 NaF GS NaF (F- vacancy) 1GS NaF (Na+ vacancy) 1GS NaCl

3

AgsC (Å)

AgsX (Å)

CtN (Å)

2.114 2.031 2.174 2.056 2.125 2.042 1.761 1.866 2.034; 2.049

3.029 2.962 3.186 3.122 3.260 3.176 2.532 2.458; 2.642 2.633

1.194 1.225 1.200 1.229 1.197 1.227 1.129 1.153 1.173; 1.181

ingly, the Ag-Cl distance is significantly longer (∼0.2 Å) in KCl compared to NaCl. The larger lattice in KCl allows for longer Ag-Cl bonds. The aforementioned structural parameter seems to depend mostly on the lattice size with little to no effects from the ligand type. This observation further confirms the relatively weak silver halide interaction in these materials. Similarly to the Ag-X bond, the energy of the first spin and symmetry allowed MLCT transition follows a lattice parameter trend. The calculated absorption energy for a monomer doped in the “small” NaCl lattice is at lower energy (λ ) 273.5 nm) than that for a monomer doped in larger NaBr (λ ) 265.2 nm) and KCl (λ ) 260.0 nm). These observations suggest that a smaller metal-ion separation lowers the MLCT transition energy. This is in agreement with the proposed dz2 f π* nature of the transition. Halide ions interact in a σ* fashion with the metal d orbital. Stronger bonding interaction (shorter Ag-X distances) will increase the HOMO energy and, therefore, reduce the HOMO-LUMO gap and the corresponding transition energy. The optimized structure of silver dicyanide in a sodium fluoride lattice exhibits questionable structural features. Several bond lengths show signs of a great axial compression caused by the surrounding alkali halide ions. The relatively small NaF lattice cannot accommodate [Ag(CN)2]- ions in a compact configuration. Silver dicyanide could induce crystal defects in its surroundings in order to create a larger doping site. Removal of an axial sodium or equatorial fluoride allowed for bond elongation, but even then, structural parameters are smaller than what was observed in other alkali halides. We suspect that this incorporation of dicyanoargentate ions in a NaF lattice causes some of the spectroscopic anomalies that were observed (no monomers observed in the luminescence and the split Raman peak). Theoretical Nanocluster Distribution. Atomistic calculations based on two-body Madelung-Buckingham potentials were carried out for the silver dicyanide deposition reactions on alkali halide slabs. This model represents the initial step in doped clusters growth. In NaCl, KCl, and NaBr, the most favorable dimerization reaction is that which produces a (µ2Cl)2 cluster, but the energy of this reaction is very sensitive to the host lattice. Table 6 lists the lowest oligomerization energy seen for (µ2-Cl)2 dimerization and trimerization and presents a room temperature Boltzmann distribution analysis that considers all possible dimer surface configurations. The bridging selectivity for (µ2-Cl)2 and tendency toward oligomerization increase in the order of NaBr < KCl < NaCl. It should be noted that surface hydration is not included in the model; crystallization from solution may bring variation in selectivity. A simple dimer configurational analysis gives a very different distribution. When compared to the formation of a (µ2-CN)2 dimer, the 2 (µ -Cl)2 dimerization reaction has a relative energy for NaCl, KCl, and NaBr of -23, -18, and -8 kJ/mol, respectively. This theoretical selectivity toward the (µ2-Cl)2 bridging mode sharply

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17407 TABLE 6: Oligomerization Energies Relative to the Energy of Monomerization and Dimer Bridging Types Distribution oligomerization reaction (kJ/mol)

NaCl KCl NaBr

dimer bridging type distribution

dimerization

trimerization

(µ-Cl)2

(µ-Cl)(µ-CN)

(µ-CN)2

-23 -14 -3

-27 -15 -17

99% 95% 86% 18%a

1% 5% 12% 45%a

0% 0% 2% 36%a

a The simple configurational distribution applies to all three host systems.

Figure 9. Illustration of the emission tunability by site-selective excitation (λex ) 224, 254, and 272 nm) for [Ag(CN)2]- doped in KCl, NaCl, and NaBr.

contrasts with results obtained for bulk-state calculations using an 8 × 8 × 8 atom supercell. In bulk NaCl, KCl, and NaBr, the relative energy of the (µ2-Cl)2 versus (µ2-CN)2 dimers is 14, 8, and -2 kJ/mol, respectively. Hence the chloride salts show opposite results in terms of surface equilibrium and bulk equilibrium, representing opposing kinetic and thermodynamic products for cluster geometry. Trimerization reactions (see Table 6), like dimerization reactions, show the lowest energies when the deposited [Ag(CN)2]- ion aggregates in the (µ2-Cl)2 bridging mode with a neighboring silver dicyanide dimer. The dimeric precursor bridging mode did not show any significant influence on the reaction energy. Lowest trimerization energies are lower than that of the lowest dimerization energies, but the extent of this is very sensitive to the host lattice. These atomistic results give good agreement with the experimental spectral data. The discovery of tetramers in only NaCl agrees with the strong thermodynamic drive toward oligomerization predicted by our model. According to our model, all high nuclearity clusters must derive from a dimer precursor. Thus, the simplicity and low intensities of the [Ag(CN)2]-:NaBr emission spectra can be explained in terms of the very poor drive toward dimerization calculated. In terms of cluster bridging, (µ2-Cl)2 clusters predominance in NaCl and NaBr have been predicted here in agreement with luminescence and Raman assignments. Conclusions This paper describes an experimental and theoretical study of energy tunability and excimer/exciplex behavior for linear dicyanoargentate(I) ions doped in different alkali halides (NaF,

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NaCl, NaBr, and KCl). To illustrate energy tunability, we show in Figure 9 the exciplex tuning by site-selective excitation of the emission spectra of [Ag(CN)2]-:KCl, [Ag(CN)2]-:NaBr, and [Ag(CN)2]-:NaCl crystals at 77 K with different excitation wavelengths. Emission bands span a broad energy range. Density functional theory calculations were carried out for the [Ag(CN)2]- monomeric ion in an environment of four different alkali halides (NaF, NaCl, NaBr, and KCl) as well as for six different dimer species in NaCl. These calculations show that the first triplet excited state structure exhibits shorter AgsAg and AgsC distances and longer CtN distances than the ground state. These results are consistent with exciplex/ excimer behavior for [Ag(CN)2]- nanoclusters in alkali halides. Atomistic calculations based on two-body MadelungBuckingham potentials were performed to predict the nuclearity, configuration, and multiplicity of the nanoclusters in the different alkali halides. These theoretical results were in good agreement with the luminescence spectra and the Raman spectra in the νCN stretch region of [Ag(CN)2]- doped in the different alkali halides. Acknowledgment. This work was supported by the National Science Foundation (CHE-0315877). We would like to acknowledge Professors Carl Tripp and Scott Collins for their support with Raman and reflectance spectroscopy, respectively. We thank Mr. David LaBrecque for general assistance. Supporting Information Available: Room temperature and 77 K luminescence spectra of all compounds, a description of the atomistic model, and detailed DFT results. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412. (2) Fernandez, E. J.; Laguna, A.; Lopez-De-Luzuriaga, J. M.; Monge, M.; Pyykko¨, P.; Runeberg, N. Eur. J. Inorg. Chem. 2002, 750. (3) Pyykko¨, P.; Tamm, T. Organometallics 1998, 17, 4842. (4) Pyykko¨, P. Chem. ReV. 1997, 97, 597. (5) Rawashdeh-Omary, M. A.; Omary, M. A.; Fackler, J. P., Jr. Inorg. Chim. Acta 2002, 334, 376. (6) Fernandez, E. J.; Gimeno, M. C.; Laguna, A.; Lopez-de-Luzuriaga, J. M.; Monge, M.; Pyykko¨, P.; Sundholm, D. J. Am. Chem. Soc. 2000, 122, 7287. (7) Che, C.-M.; Tse, M.-C.; Chan, M. C. W.; Cheung, K.-K.; Phillips, D. L.; Leung, K.-H. J. Am. Chem. Soc. 2000, 122, 2464. (8) Catalano, V. J.; Malwitz, M. A.; Noll, B. C. Chem. Commun. 2001, 581. (9) White-Morris, R. L.; Olmstead, M. M.; Balch, A. L.; Elbjeirami, O.; Omary, M. A. Inorg. Chem. 2003, 42, 6741. (10) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P., Jr. J. Am. Chem. Soc. 2001, 123, 11237. (11) Omary, M. A.; Patterson, H. H. Inorg. Chem. 1998, 37, 1060. (12) Omary, M. A.; Patterson, H. H. J. Am. Chem. Soc. 1998, 120, 7696. (13) Omary, M. A.; Hall, D. R.; Shankle, G. E.; Siemiarczuk, A.; Patterson, H. H. J. Phys. Chem. B 1999, 103, 3845. (14) Patterson, H. H.; Kanan, S. M.; Omary, M. A. Coord. Chem. ReV. 2000, 208, 227. (15) Che, C.-M.; Lai, S.-W. Coord. Chem. ReV. 2005, 249, 1296. (16) Mansour, M. A.; Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329. (17) Lai, S.-W.; Che, C.-M. Top. Curr. Chem. 2004, 241, 27. (18) Mills, A.; Lepre, A.; Theobald, B. R. C.; Slade, E.; Murrer, B. A. Gold Bull. 1998, 31, 68.

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