Pressure-Dependent Luminescence Properties of Gold(I) and Silver(I

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Pressure-Dependent Luminescence Properties of Gold(I) and Silver(I) Dithiocarbamate Compounds Franc-ois Baril-Robert, M. Alex Radtke, and Christian Reber* D
epartement de chimie, Universit
e de Montr
eal, Montr
eal, Qu
ebec H3C 3J7, Canada

bS Supporting Information ABSTRACT: Luminescence spectra for dithiocarbamate complexes of gold(I) and silver(I) display very different luminescence properties. At room temperature, a narrow band with a maximum at approximately 18 000 cm 1 and a broad band with a maximum at approximately 13 000 cm 1 are observed for the gold(I) and silver(I) compounds, respectively. The luminescence from the gold(I) compounds is strongly affected by external pressure, with maxima shifting to lower energy by approximately 120 cm 1/kbar. In contrast, a silver compound shows a small shift to higher energy by approximately +20 cm 1/kbar. These shifts are analyzed in terms of structure and bridging ligand geometry, revealing the important influence of unsupported metal metal interactions on pressure-induced shifts of luminescence bands.

’ INTRODUCTION Coordination polymers with gold(I) gold(I) and silver(I) silver(I) interactions have widely varying luminescence properties that can be influenced through changes in the molecular structure and the environment of the luminophores.1 5 Variations of luminescence energies are often qualitatively rationalized in terms of changes of the metal metal distances. Pressure-dependent luminescence spectroscopy provides the opportunity to continuously vary structures, leading to insight into effects due to shorter metal metal distances, but the technique has been applied only to gold(I) cyanides compounds with an intricate, two-dimensional crystal structure.3,6 8 We report luminescence spectra at variable temperature and pressure of gold(I) and silver(I) complexes with one-dimensional structures, where dithiocarbamate ligands define bridging geometries of gold(I) and silver(I) ions. Dithiocarbamates are an attractive class of ligands, as they form chains with alternating supported and unsupported gold(I) gold(I) interactions, as illustrated in Scheme 1. For these structures, much simpler pressure effects can be expected than for the two-dimensional cyanides, and a clearer quantitative characterization of the tuning possibilities through the gold gold bonds can become accessible. The corresponding silver(I) compounds form shorter one-dimensional segments without unsupported metal metal distances, allowing for an interesting comparison of luminescence properties of dithiocarbamate complexes of d10 metal ions. ’ EXPERIMENTAL SECTION All chemicals and solvents were used as received; all syntheses were performed under aerobic conditions. r 2011 American Chemical Society

Scheme 1. Schematic Structures of (A) Diethyldithiocarbamate (edtc) and (B) Pyrrolidinedithiocarbamate (pdtc) Ligands and (C) 1D Polymeric Chains Present in Gold(I) Dithiocarbamate Compounds

[Au2(dtc)2]n. Synthesis of [Au2(edtc)2]n and [Au2(pdtc)2]n (where edtc and pdtc denote diethyldithiocarbamate and pyrrolidinedithiocarbamate, respectively) was carried out following a method similar to literature procedures.9 Chloro(dimethylsulfide) gold(I) (1 mmol) was dissolved in a minimum of acetonitrile. A second acetonitrile solution containing 1 mmol of either sodium diethyldithiocarbamate trihydrate (Na(edtc) 3 3H2O) or ammonium pyrrolidinedithiocarbamate ((NH4)(pdtc)) is prepared. Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: July 15, 2011 Revised: November 16, 2011 Published: December 13, 2011 2192

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Figure 1. Room-temperature luminescence, solid-state diffuse reflectance, and solution absorption spectra (chloroform) of (A) [Au2(edtc)2]n as well as luminescence and solid-state diffuse reflectance spectra of (B) [Au2(pdtc)2]n.

Table 1. Transition Energies and Energy Difference between Emission Bands and Their Corresponding Reflectance Signals CHCl3 solution (cm 1) reflectance (cm 1)

emission (cm 1) ΔE (cm 1)

[Au2(edtc)2]n

[Au2(pdtc)2]n

34 600 ∼17 000

∼16 000

19 500

19 100

>24 000

>24 000

13 750

14 500

18 150

18 270

3250

1500

1350

830

Rapid precipitation of an orange solid occurs when both solutions are mixed. The precipitate is filtered and washed with acetonitrile. Single crystals of [Au2(edtc)2]n were obtained by slow evaporation of a chloroform solution. Elemental analysis for [Au2(edtc)2]n (Calcd (%) for C10H20N2S4Au2: C, 17.39; H, 2.92; N, 4.06; S, 18.57. Found (%): C, 17.46; H, 2.81; N, 3.77; S, 18.37). [Ag(edtc)]6. The synthesis of [Ag(edtc)]6 was carried out as described in the literature. The structure contains “folded chains” of six silver(I) ions bridged by diethyldithiocarbamate ligands, without unsupported metal metal interactions.10,11 Elemental analysis for [Ag(edtc)] (Calcd (%) for C5H10NS2Ag: C, 23.45; H, 3.93; N, 5.47; S, 25.04. Found (%): C, 22.08; H, 3.35; N, 5.42; S, 22.9). Experimental and calculated powder X-ray diffraction patterns are in excellent agreement. Spectroscopic Measurements. UV vis absorption spectra were measured on a Varian Cary 5E double beam spectrometer. A praying mantis accessory was used for solid state reflectance spectra. Raman and luminescence spectra were measured with a Renishaw 3000 Raman imaging microscope equipped with a

Figure 2. Temperature-dependent luminescence spectra of [Au2(edtc)2]n (A) and [Au2(pdtc)2]n (B). Temperatures are given on the spectra. Temperature-dependent intensities are shown as insets.

Peltier-cooled CCD camera. Excitation sources were a 488 nm argon ion laser for the luminescence experiments and a 785 nm diode laser for the Raman experiments. The microscope was used to focus light onto a spot of approximately 1 μm in diameter and to collect the scattered light. Low-temperature Raman experiments were obtained by coupling a Linkam coldfinger cryostat to the apparatus with liquid nitrogen used as coolant. A Janis closedcycle helium cryostat was used for low-temperature luminescence experiments. Pressure-dependent measurements on solid samples in nujol were made with a diamond-anvil cell (DAC, HighPressure Diamond Optics). The ruby R1 line method12 was used to calibrate the hydrostatic pressure inside the gasketed cell. All pressure-induced phenomena reported here are reversible upon gradual release of external pressure. Density Functional Calculations. The theoretical structure and energy of electronic states of [Au2(dtc)2]1or2 model compounds were determined using Gaussian ‘03 software (Gaussian Inc.).13 Density functional theory optimization calculations were performed using the PBEPBE14 16 functional and LanL2dz basis set as implemented in the software. Structures of the monomers and dimers were constrained to adopt D2 symmetry. Excited state energies were calculated using time-dependent theory (TD-DFT). 13 Isodensity representations of molecular orbitals were generated using GaussView 3.07 software (Gaussian Inc.).13 Spectroscopic Results. Room-Temperature Spectroscopy. Diffuse reflectance spectra of deep orange dithiocarbamate gold(I) 2193

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Figure 3. Temperature-dependent luminescence spectra of [Ag(edtc)]6. Temperatures are (top to bottom): 270, 240, 225, 200, 165, 150, 120, 105, and 90 K.

compounds are shown in Figure 1, and band maxima and energy differences are summarized in Table 1. The spectra are characterized by an intense band with a maximum at ∼19 500 cm 1, a weak shoulder at lower energy (∼16 000 cm 1), and a broad and unresolved intense band at higher energy (>24 000 cm 1). Similarly, room-temperature emission of both compounds consists of an intense peak around 18 200 cm 1 and a weak, broader band at lower energy (∼14 000 cm 1). Solution absorption spectra of [Au2(edtc)2] in chloroform exhibit a single peak at much higher energy (34 600 cm 1) than those observed for the solid. This difference indicates that solid-state polymerization through bridging ligands and metallophilic interactions greatly affects the energies of electronic states. Similar solid-state red-shifts have been observed for MLCT transitions in [Au(CN)2] systems where metallophilic interactions significantly reduce the HOMO LUMO gap and the corresponding excited state energy. Other noteworthy spectroscopic features of the gold(I) compounds include the narrow width of the dominant bands and the small Stokes shifts between luminescence and corresponding absorption bands. Luminescence Spectra at Variable Temperature and Pressure. Emission intensity and energy behavior at variable temperature provide insight into the nature of the emitting state. As shown in Figure 2, emission properties are significantly influenced by temperature. For [Au2(edtc)2]n, a weak low-energy peak and a slightly unsymmetrical intense band at 18 150 cm 1 are observed at room temperature. At lower temperature, a new symmetrical sharp band at approximately 17 250 cm 1 rapidly grows and becomes the major emission peak at approximately 210 K. At temperatures lower than 50 K, the sharp band at approximately 18 000 cm 1 regains some intensity and represents approximately 7% of the total emission intensity at 6 K. Time-resolved measurements of both sharp peaks (Supporting Information) show that, at 5 K, the band at 18 000 cm 1 has a longer lifetime (∼4 μs) than the band at 17 250 cm 1 (∼0.5 μs). This explains the presence of two different emitting states at similar energy. The total luminescence intensity increases by a

Figure 4. Normalized luminescence spectra of [Au(edtc)2]n (A) and [Au(pdtc)2]n (B) at variable pressure. Pressures are given near the band maxima of the spectra.

factor of 20 between 230 and 45 K and decreases by 25% between 45 and 6 K. This decline at low temperature occurs with the reappearance of the sharp peak. A slight shift of the band maximum to higher energy is observed as temperature is increased, most likely due to the thermal dilatation of the Au Au distances, confirming the influence of metallophilic interactions on the temperature-dependent luminescence properties. A new, narrow luminescence band is also observed for [Au2(pdtc)2]n at low temperature and is the only observed band at 5 K. This low-temperature band exhibits a vibronic progression with an interval of approximately 1150 cm 1, corresponding to a ligand centered normal mode observed in Raman spectroscopy. In contrast to [Au2(edtc)2]n, the low-temperature, low-energy narrow band has a symmetric band shape, but at room temperature, the dominant high-energy band is asymmetric. In contrast, the silver(I) compound [Ag(edtc)]6 shows a much simpler luminescence behavior. A broad band is observed, with a maximum at approximately 12 500 cm 1 at room temperature, as shown in Figure 3. The band shifts to higher energy by +2.5 cm 1/K as the temperature is lowered. This variation is linear and reversible, indicating that there are no major structural phase changes occurring as a function of temperature. A comparable shift is not observed for the gold(I) compounds, illustrating the different nature of the emitting state. 2194

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The Journal of Physical Chemistry C Compression of crystals in the diamond anvil cell leads to shorter inter- and intramolecular distances. Our focus is on the gold gold distances. For both compounds, the two different emitting states observed at room temperature are present throughout the pressure range of our experiments. As shown in Figure 4, both luminescence energy and band shape of the gold(I) compounds are affected by pressure. As pressure increases, the band maxima shift from approximately 18 000 cm 1 to approximately 14 000 cm 1. In polymeric gold(I) chains, this strong, pressure-induced red-shift of 120 cm 1/kbar is characteristic of electronic transitions with initial and final states involving the metal metal bonding and antibonding orbitals. For the lower energy, broader band with a maximum at approximately 14 000 cm 1, as observed at high pressure, the energy shift is approximately zero. This band slowly gains intensity as pressure increases, and in [Au2(edtc)2]n, it becomes the dominant luminescence band at pressures above 30 kbar, where the energies of

Figure 5. Pressure-dependent luminescence spectra of [Ag(edtc)]6. Pressures are (top to bottom): 39, 37, 33, 32, 28, 26, 23, 17, and 12 kbar. The sharp peaks near 14 000 cm 1 are due to ruby luminescence, used for pressure calibration.

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both bands are similar. Again, the silver compound shows a simpler behavior, illustrated in Figure 5, with a broad band centered at 12 500 cm 1 and a small blue shift of the luminescence band maximum by +17 cm 1/kbar as pressure increases. No change in band shape occurs, and the spectrum is qualitatively reminiscent of the luminescence band observed for the gold compounds at high pressure. The comparison of silver(I) and gold(I) luminescence spectra under variable conditions indicates that unsupported metal metal interactions lead to a much more varied luminescence behavior. Raman Spectroscopy. The luminescence spectra of the title compounds show distinct temperature variations, but the Raman spectra in Figure 6 show only much smaller changes. For all compounds, Raman peaks become sharper, and small shifts of the maxima by 0.02 to +0.03 cm 1/K occur as temperature is lowered. The spectra do not provide any evidence for temperature-induced structural phase transitions. The Raman spectra of the gold(I) dithiocarbamate compounds show considerable changes with pressure. Peaks become broader with pressure, and at higher pressure, luminescence masks the weak Raman peak intensities. As pressure increases, vibrational frequencies also increase without any sudden variations of peak positions, and therefore no obvious pressure-induced phase transitions are observed in this pressure range. One notable feature of the [Au(edtc)2]n pressure experiments is the significant and nonuniform intensity increase of the peak at 610 cm 1 (Figure 6b, inset) at ∼14 kbar. The width of this peak shows a drastic increase over the same pressure range. We conclude that a subtle structural deformation combined with a possible pressure-induced

Figure 7. Frontier orbitals energy behavior toward dimerization and compression.

Figure 6. Raman spectra (λexc. = 785 nm) at variable temperature (A) and pressure (B) of the [Au2(edtc)2]n compound. Inset shows relative intensity of several Raman peaks (black triangle: 610 cm 1 peak). Temperature and pressures are indicated beside the corresponding spectra. 2195

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Figure 8. Isodensity plots (0.02 atomic units) of [Au2(edtc)2]2 molecular orbitals obtained from DFT calculations (PBEPBE/Lanl2dz): (A) HOMO, (B) LUMO, (C) ligand-centered LUMO+1 and (D) LUMO+3.

Figure 9. Calculated variation of transition energies in [Au2(edtc)2]2 when distorted along the 38 cm 1 (A) and 117 cm 1 (B) normal modes. Open and solid symbols correspond to singlet and triplet excited states, respectively. Circles correspond to MCinterdimer excited states, squares represent MCintradimer states. Other symbols correspond to MLCT excited states.

preresonance enhancement leads to the observed changes of width and intensity without affecting the vibrational frequency.

’ DISCUSSION Excited state distortions can be estimated from emission and absorption band widths. As observed in Figures 1 3 and summarized in Table 1, gold(I) dithiocarbamate complex luminescence spectra exhibit both relatively narrow and broad bands. The narrow bands indicate that an excited-state structural change occurs along normal coordinates of low frequency and that distortions along high-frequency modes are small. The most likely coordinates are those involving metal metal modes. Comparisons of the narrow bands for both compounds in absorption and emission at room temperature suggest that excited-state distortions for the pyrolidinedithiocarbamate compound are slightly smaller than the corresponding excited state in [Au2(edtc)2]n. This explains the apparent asymmetry observed in the room-temperature luminescence spectrum of the diethyldithiocarbamate complex. The narrow bands show large pressure-induced red-shifts characteristic of electronic transitions where aurophilic bonding changes in the excited state. In closed-shell gold(I) complexes, the HOMO normally consists of gold 5dz2 orbitals, while the LUMO usually is an empty metal-centered 6s/p or a ligandcentered orbital with a large contribution from the empty gold 6s/p orbitals. Figure 7 illustrates this situation, with interacting orbitals split into bonding and antibonding pairs. The HOMO has a gold gold antibonding character, while the LUMO adopts a bonding nature. When pressure is applied and neighboring gold

centers get closer together, the HOMO LUMO energy gap is significantly reduced. This behavior of orbital energies leads to an important reduction of corresponding emission energies as seen for the shift of the narrow band in pressure experiments. The lower energy, broad luminescence band at high pressure does not display the large pressure-induced red-shift, and its width suggests the existence of excited state distortions along highfrequency, ligand centered, normal modes. The nature of the emitting state therefore changes, likely through a distortion of the bridged bimetallic unit. This electronic transition likely has a significant ligand structure change in the excited state, compensating for any effect of gold gold distance decrease with pressure. Observed emitting state changes in the gold(I) compounds at variable temperature and the low-temperature lifetime measurements for [Au(edtc)2]n are consistent spectroscopic results. In the gold(I) complexes, sharp peak maxima are separated by approximately 1000 cm 1, and for [Au(edtc)2]n at 5 K, the lifetime of the higher-energy emission is shorter by approximately a factor of 10 than that measured for the low-energy band. At low temperature, two excited states may emit if the two excited states have significantly different geometry. Such a phenomenon is observed for [Au(edtc)2]n. As temperature increases, the energy barrier can be crossed, and only the lower energy state emits. At higher temperatures, nonradiative decay dominates and greatly affects the emission intensity of the longer lived, lower energy excited state. DFT Calculations. Time-dependent density functional theory calculations allow us to determine energies and assignments of electronic transitions at variable gold gold distances. Calculated orbitals involved in electronic transitions obtained from TD-DFT are shown in Figure 8. In closed-shell gold(I) complexes, the HOMO normally consists of the gold 5dz2 orbital, with the z axis along S Au S, while the LUMO can be an empty metal centered 6s/p or a ligand centered orbital with a large contribution from the gold empty 6s/p orbitals. The HOMO orbital consists of the 5dz2 with increasing contribution of the dx2 y2 orbital as the metallophilic interaction increases. Similarly, the 6px contribution to LUMO increases as the gold gold distance decreases. As expected, the calculated HOMO has strong gold 5dz2 character (z axis along S Au S), illustrating the antibonding interaction between metal centers. The LUMO corresponds to a mixture of gold 6s, 5dz2, and ligand centered orbitals. Interestingly, bonding interactions are observed between interdimer metal centers but antibonding intradimer effects. The unoccupied orbital exhibiting intradimer bonding interaction is at higher energy. In this case, the ligand contribution is different and leads to the unexpected gold gold interaction energy order observed in LUMO orbitals. Two different ligand orbitals are positioned between these two orbitals. The calculated transition energy for the lowest-energy absorption is 17 300 cm 1 which is close to the 2196

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The Journal of Physical Chemistry C observed band maximum at 19 500 cm 1. Figure 9 shows that different excited states are close in energy and can even cross along normal coordinates involving Au Au distance changes. This proximity of several excited states can lead to a different emitting state, as illustrated by the observation of a dominant broad band at high pressures. Crystal defects can lead to sites with this emitting state even at ambient pressure, as illustrated by the ambient-pressure spectrum in Figure 4a. Pressure-dependent luminescence spectra of the dithiocarbamate complex of silver(I) and gold(I) show a rich variety of luminescence phenomena. Shifts to lower energy are observed for structures with unsupported metal metal interactions that are more easily compressed than distances between metal ions coordinated to bridging ligands. Luminescence energies for the silver(I) dithiocarbamate are lower than for the gold(I) analogs, and no pressure-induced change of the emitting state is observed below 40 kbar.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Luminescence lifetimes at variable wavelength measured at 5 K. Temperature- and pressuredependent Raman spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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