Impact of High Pressure on Metallophilic Interactions and Its

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Impact of High Pressure on Metallophilic Interactions and Its Consequences for Spectroscopic Properties of a Model Tetranuclear Silver(I)−Copper(I) Complex in the Solid State Katarzyna N. Jarzembska,*,† Radosław Kamiński,† Kamil F. Dziubek,‡,§ Margherita Citroni,‡,∥ Damian Paliwoda,⊥ Krzysztof Durka,# Samuele Fanetti,‡,∥ and Roberto Bini‡,∥,○ Department of Chemistry, University of Warsaw, Ż wirki i Wigury 101, 02-089 Warsaw, Poland LENS − European Laboratory for Non-linear Spectroscopy, via N. Carrara 1, 50019 Sesto Fiorentino, Italy § Department of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland ∥ “Ugo Schiff” Department of Chemistry, University of Florence, via della Lastruccia 3, 50019 Sesto Fiorentino, Italy ⊥ Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States # Department of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland ○ Institute for the Chemistry of Organometallic Compounds, Italian National Council for Research, CNR-ICCOM, via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy †

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S Supporting Information *

ABSTRACT: Structure−property relationships were investigated via combined highpressure spectroscopic and X-ray diffraction techniques for a model luminescent Ag2Cu2L4 (L = 2-diphenylphosphino-3-methylindole) complex in the crystalline state. The experimental results were contributed by theoretical calculations, compared with the previously evaluated light-induced geometrical changes, and discussed in the context of available literature to date. To the best of our knowledge, this is the first study of this kind devoted to a coinage-metal complex for which the argentophilic interactions are crucial. High-pressure X-ray diffraction and optical spectroscopy experiments showed close correspondence between structural changes and optical properties. The unit-cell angles, absorption edges, emission maxima, decay lifetimes and silver−copper bond trends, all change around 2−3 GPa. A blue-shift to red-shift switch when increasing the pressure was observed for both absorption and emission spectra. This is unique behavior when compared to the literature-reported coinage metal systems. It also occurred that the pressure-induced structural changes differ notably from the geometrical distortions observed for the excited state. Interestingly, shortening of the Ag−Ag bond itself does not ensure the red shift of the absorption and emission spectra. All the optical spectroscopy data are suggestive of an important role of defects, likely related to the lack of a hydrostatic pressure transmitting medium, for pressures higher than 3 GPa.

1. INTRODUCTION Multicenter transition-metal coordination complexes have gained a lot of attention owing to their rich electronic and luminescent properties.1,2 If these properties can be manipulated using external stimuli, such materials may find important applications in optoelectronics, e.g., as memory devices, sensors, light-emitting diodes (LEDs), etc. In order to sensibly control the desired properties, their origin has to be identified, profoundly studied, and analyzed under different conditions. In general, the solid-state luminescence properties of molecular materials depend on their molecular structure and packing mode of molecules.3,4 In the case of transition metal complexes, especially the multicenter d8 or d10 systems, it turns out that short metal−metal contacts usually determine the nature of the lowest laying emissive states, and so are crucial to the understanding of physical properties of the respective materials.1 Thus, the studies on the nature of these © XXXX American Chemical Society

metallophilic interactions, their role in the photoexcitation and emission processes, as well as, structure-response to external factors, such as pressure, and its consequences for the luminescence properties, cannot be overestimated. It is important not only regarding the recent wide interest in mechanochromic luminescent materials,5,6 or different technological applications of such controllable materials, but also from the fundamental science point of view. To achieve a molecular-level understanding of the correlation between structural modifications and the corresponding emissive properties, single-crystal X-ray diffraction experiments should be combined with spectroscopic investigations at high-pressure conditions. Such characterizations are feasible thanks to the diamond-anvil cells (DACs), which Received: May 1, 2018

A

DOI: 10.1021/acs.inorgchem.8b01196 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

2.2. X-ray Diffraction Data Collection and Refinement. Room temperature, ambient-pressure single-crystal X-ray diffraction measurement of Ag2Cu2L4 was carried out using classical Bruker AXS single-crystal diffractometer equipped with an APEX II CCD detector, TXS rotating anode (Mo−Kα radiation, λ = 0.71073 Å) and multilayer optics. Data collection strategy determination and optimization, unit-cell determination, raw diffraction image integration, absorption correction and data scaling were all performed using the appropriate algorithms implemented in the APEX3 diffractometer software.25 The structure was solved using a charge-flipping method26−28 as implemented in the SUPERFLIP program,29 and refined with the JANA package30 within the independent atom model (IAM) approximation. A series of high-pressure X-ray diffraction experiments were performed at high-pressure ID27 beamline31 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, using monochromatic X-ray beam (λ = 0.3738 Å, E = 33 keV) focused down to about 5 × 5 μm2 (fwhm) spot size. A single crystal of Ag2Cu2L4 with the largest dimension of about 50 μm was loaded into a high-pressure membrane-driven diamond-anvil cell (mDAC, culet diameter of 600 μm)32 supported by Boehler-Almax seats with an opening angle of 74°. The sample crystal was loaded together with a ruby chip into the high pressure chamber built of two opposite diamond anvils and stainless steel gasket in-between. Helium was used as a pressure transmission medium, whereas the ruby fluorescence method was applied in order to monitor the pressure value inside the cell.33 Room-temperature high-pressure diffraction data sets were collected using the vertical-axis ω-scans with a MAR165 CCD-type detector. Because of the severe radiation damage of the crystal, only six data sets, up to about 3.5 GPa, were recorded (namely at about 0.5, 1.0, 1.6, 2.2, 2.7, and 3.5 GPa). It also has to be noted that the last data set is of considerably lower quality. Raw images were later imported into the CRYSALISPRO software34 and subsequently integrated, scaled and corrected for absorption. Crystal structures at various pressure points were refined using the SHELX program package,35 taking the initial coordinates from the ambient-pressure crystal structure model. A number of restraints were used to fix the organic part of the molecule (i.e., bond lengths and planarity restraints) as a rigid body. Because of the relatively large number of atoms in the molecule, low crystal symmetry and limited completeness (resulting from the mDAC construction obscuring the access to the sample), such an approach helps to stabilize the refinement of a structural model. Moreover, bond lengths and planarity restraints have been applied only to the organic ligands, while the rigid bodies themselves were allowed to shift and rotate during the refinement. Therefore, whereas in our model the particular ligands’ geometries are fixed, the overall molecule “shell” accommodates to the surrounding changes under high pressure conditions. The described procedure is additionally supported by the fact that small organic aromatic ligands should unlikely change their geometry in the considered pressure range. Many of the atomic displacement parameters (ADPs) were refined using isotropic restraints available in the SHELX program. It should be noted here, that none of above restraints were applied for the metal atoms in the core of the complex. Reflections, intensities of which clearly deviate from the values calculated from the model (i.e., these most affected by the strong parasite diamond diffraction, or so-called “diamond dips”36) were omitted from the refinement. Finally, it is worth noting that, despite experimental limitations during high-pressure data collection, the described treatment is known to provide reliable results and allows for quantitative analysis of crystal structures.37 Final data collection and reduction parameters are summarized in the Supporting Information. All sets of raw diffraction frames and the associated data are available online under the following DOI: 10.18150/repod.5075600 (Repository for Open Data, Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw, Warsaw, Poland). All the final refinement statistics are summarized in Supporting Information. The respective CIF files can be retrieved from the Cambridge Structural Database (CSD).38

enable accurate pressure control. There are some recent papers that report the emission evolution of mechanochromic compounds under hydrostatic pressure,7−15 or pressureinduced structural changes of coinage-metal complexes (including metal−organic framework structures with argentophilic interactions),16−19 however, only few combine detailed structural and luminescence studies.20−22 Furthermore, most of them deal with copper- or gold-based complexes. In this contribution we continue studies on a model literature-reported luminescent tetranuclear silver(I)−copper(I) complex (hereafter referred to as Ag2Cu2L4; L = 2diphenylphosphino-3-methylindole ligand; Scheme 1) in the Scheme 1. Schematic Representation of the Studied Ag2Cu2L4 (L = 2-Diphenylphosphino-3-methylindole Ligand) Tetranuclear Silver(I)−Copper(I) Complexa

a

Short Ag−Ag and Ag−Cu interactions are emphasized with bold magenta lines.

solid state.23,24 In our previous article23 we have captured the structure of the excited state and analyzed its nature as well as the origin of the observed luminescence. It occurred to us, among others, that the argentophilic interaction was crucial for the observed optical properties. In the excited state, the Ag−Ag bond significantly strengthened due to the ligand-to-metal− metal-bond charge transfer. The character and structure of the excited state were defined thanks to the use of the advanced photocrystallographic methods, i.e., the time-resolved (TR) Laue technique supported by the time-dependent density functional theory (TDDFT) computations and quantummechanics/molecular-mechanics (QM/MM) modeling. Having this extensive knowledge, we decided to investigate structure−property relationships of the complex in the crystalline state by applying high pressure. This way we expected to impose some distortions on the tetranuclear metal core and, thus, affect the spectroscopic properties. The question was whether we can control the changes made, and what effect these will have on the luminescence lifetime and emission spectrum. For that purpose, we have combined highpressure crystallographic technique with high-pressure timeresolved spectroscopic methods. To the best of our knowledge, this is the first joint study of this character devoted to a coinage-metal complex for which the argentophilic interactions are crucial.

2. EXPERIMENTAL PART 2.1. Synthesis and Crystallization. The Ag2Cu2L4 complex was synthesized according to the procedure found in the literature.24 The solvent-free crystal form was obtained through crystallization from methylene dichloride (CH2Cl2) instead of the originally used chloroform, as reported by us previously.23 B


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Figure 1. Selected weak interactions of two copper fragments of the molecule in the crystal lattice of the solvent-free form of Ag2Cu2L4 at ambient pressure.

Table 1. Selected Bond Distances and Valence Angles for Structures Determined at Several Hydrostatic Pressures in Comparison with Previous TR Laue Diffraction Results23 a bond distances, d/Å pressure, P/GPa 0.0c 0.45(1) 0.98(1) 1.57(2) 2.17(1) 2.71(4) 3.47(4) ΔHP GS (90 K)d ES (90 K)d ΔTRd pressure, P/GPa 0.0c 0.45(1) 0.98(1) 1.57(2) 2.17(1) 2.71(4) 3.47(4) ΔHP GS (90 K)d ES (90 K)d ΔTRd

Ag1−Ag2

Ag1−Cu1

3.042(1) 3.017(2) 2.975(3) 2.930(2) 2.883(2) 2.821(4) 2.773(4) −0.221(4) 3.0345(2) 2.66(3) −0.38(3)

2.799(1) 2.805(3) 2.776(3) 2.764(3) 2.758(3) 2.772(5) 2.757(6) −0.027(5) 2.7640(2) 2.82(3) +0.06(3)

Ag1···Cu2 3.452(1) 3.457(3) 3.471(4) 3.435(3) 3.412(4) 3.412(6) 3.428(8) −0.040(7) 3.4655(2) 2.88(3) −0.59(3) valence angles, θ/deg

Ag2···Cu1

Ag2−Cu2

3.247(1) 3.254(4) 3.235(4) 3.170(4) 3.092(4) 3.058(7) 3.089(9) −0.189(7) 3.2110(2) 2.93(3) −0.29(3)

2.813(1) 2.810(4) 2.779(4) 2.759(4) 2.782(5) 2.803(7) 2.760(9) −0.009(8) 2.8043(2) 2.70(3) −0.10(3)

Ag1−Cu1···Ag2

Cu1···Ag2−Cu2

Ag2−Cu2···Ag1

Cu2···Ag1−Cu1

59.86(3) 59.17(7) 58.74(8) 58.70(7) 58.72(7) 57.6(1) 56.3(2) −2.2(1) 60.495(4) 54.33(1) −6.16(1)

123.79(4) 124.5(2) 125.6(2) 126.7(1) 127.9(2) 129.8(2) 131.4(3) +6.0(2) 124.072(7) 125.80(1) +1.72(1)

57.01(3) 56.41(7) 55.53(8) 55.16(7) 54.33(7) 52.9(1) 51.9(1) −4.1(1) 56.720(4) 56.09(1) −0.63(1)

117.30(4) 117.69(7) 117.53(8) 117.17(7) 117.24(8) 117.9(1) 118.7(2) +0.6(1) 116.724(7) 123.09(1) +6.36(1)

a ΔHP = parameter differences computed for structures at ambient and 2.7 GPa pressure points, ΔTR = literature values for differences between ES and GS geometries; both shown in bold). In all tables, and in the text, the “−” is used for “shorter” metal···metal interactions, whereas “···” is used for the “longer” ones. Values for 3.5 GPa pressure point are shown in italics so, as to emphasize the lower data quality. bValues in this table are given with error bars and are not rounded. cValues derived from the room temperature data measured at the home source. dData extracted from our previous literature report (time-resolved Laue diffraction done at 90 K).23

2.3. High-Pressure Spectroscopy. Luminescence spectra and emission decays were collected using an in-house-build setup available at the LENS facility (Florence, Italy), available through the LaserlabEurope collaboration network. Several of the Ag2Cu2L4 single crystals were loaded into mDAC (culet diameter of 400 μm) filling the available space entirely (stainless steel gasket hole diameter of 150 μm). The IIa class diamonds were used in the luminescence measurements, being characterized by the ultralow fluorescence and no IR band due to atomic nitrogen impurities. To avoid parasitic emission from the ruby chip, the pressure inside mDAC was monitored by comparing the absorption frequencies with those measured in the IR experiment, in which the ruby fluorescence was used for pressure calibration.39 The IR-light absorption experiments

were performed on a Bruker IFS-120HR FTIR spectrometer locally modified for high-pressure experiments.40,41 Primary laser beam was produced by the optical parametric generator (OPG; EKSPLA PG401) pumped by the third-harmonic (355 nm) of the mode-locked Nd:YAG laser (EKSPLA PL2143A, 20 ps, 10 Hz), which provided about 20 ps laser pulses of 420 nm wavelength, with a bandwidth of about 6 cm−1 (the excitation wavelength was chosen based on the collected absorption spectrum; 420 nm appeared to be efficient for inducing sample’s emission). The beam was focused on the sample enclosed in mDAC, and the luminescence signal was collected in backscattering geometry, focused subsequently on the 1/4 m monochromator coupled with the photomultiplier tubes (PMTs; ET 9235QB, Hamamatsu R943−02), and analyzed with the 1 GHz C


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Inorganic Chemistry

Figure 2. TDDFT-evaluated UV−vis spectra (DFT(PBE0)/LANL2DZ level of theory) for the ground- (a) and excited-state (b) geometries determined in the previous study.23 Oscillator strengths are marked as green bars; blue-line envelope (normalized) was drawn using default GAUSSSUM program65 settings. Thin red dotted line in panel b is the same as the blue line in panel a, drawn for comparison. (10 GS/s on each channel) oscilloscope (Rohde & Schwarz RTO1014). The time-resolution of the experiment amounted to about 6−10 ns (fwhm) due to the PMT response. The luminescence lifetimes were extracted from the intensity decay data by means of the least-squares fitting. All luminescence decays were analyzed using the FITYK program.42 The near-UV−vis spectra were measured with a homemade setup specifically designed for absorption measurements in the mDAC. A xenon lamp (Hamamatsu L10725) constituted the source. It was attenuated by UV-fused-silica neutral-density filters with varying optical densities, and filtered by a short-pass filter with cutoff wavelength at 650 nm and by a long-pass filter with cutoff wavelength at 400 nm to reject light out of the desired spectral range. The light was focused onto the sample and collected by a couple of Al-coated 90° off-axis parabolic mirrors (with reflected focal length of 50.8 mm), and then focused by another Al-coated 90° off-axis parabolic mirrors into a 1/8 m monochromator (Newport 77250), with F/number = F/ 3.7. The light is dispersed by a ruled grating (Newport 77305) with 600 lines/mm blazed at 700 nm, and collected on a CCD (Hamamatsu S9971−1006UV). The resulting spectral resolution was better than 3 nm. Data sets from spectroscopic experiments with associated description are available online under the following DOI: 10.18150/repod.5075600 (Repository for Open Data, Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw, Warsaw, Poland). 2.4. Theoretical Computations. TDDFT computations of the spectroscopic transitions43,44 were performed for each pressure point molecular structure in the GAUSSIAN package45 using the PBE0 functional46,47 and LANL2DZ basis set.48−51 The experimental geometries were not optimized prior to calculations, however, in all cases the C−H bond distances were extended to neutron-normalized values.52,53 All input files (including the X−H standardization) were prepared using the CLUSTERGEN program.54 Topological analysis of electron density, based on Bader’s quantum theory of atoms in molecules (QTAIM),55 was accomplished with the DENPROP program.56

among others, originates from significant structural changes observed upon light excitation.23 Furthermore, metallophilic interactions play a crucial role in the analyzed ligand-to-metal charge transfer (LMCT) process, especially the central Ag−Ag bond, on which the lowest unoccupied molecular orbital (LUMO) is majorly located. Upon absorption of near-UV light the silver centers become less positively charged and the distance between them shortens by 0.38(3) to 2.66(3) Å, which is comparable to the shortest ground-state Ag···Ag contacts ever observed in the known crystals of silvercontaining compounds.57−61 It should also be noted that to date the argentophilic interactions have been far less studied than the aurophilic, or cuprophilic equivalents. As it has already been shown in literature62−64 and also in our previous paper,23 crystal symmetry and packing may have a considerable effect on the excited-state (ES) geometry, including the inner parts of the molecules, such as the silver−copper core in the case of Ag2Cu2L4. Crystal packing is also expected to play important role in high-pressure studies. Because of the low crystal symmetry (the triclinic P1̅ space group) the two parts of the Ag2Cu2L4 complex molecule do not form identical contacts in the crystalline state (Figure 1). A given Cu1 fragment (i.e., the Cu1 atom and the attached organic ligands) faces another Cu1 moiety. Similarly, the Cu2 fragments point at one another. This results in an anisotropic behavior of the metal core when excited−unequal metal centers’ shifts and bond distance changes23 (see Table 1). As a consequence, the “zigzag”-shaped ground-state (GS) metal core becomes more rhombic-like when laser-irradiated. Assuming that the geometry of the Ag2Cu2L4 complex molecule changes toward that of the excited state when pressed, red-shift of the absorption spectrum (and presumably some red-shift of the emission maximum) would be expected. Indeed, the theoretical UV−vis spectra calculated using the TDDFT method for the 90 K ground-state molecular geometry, and for that of the excited state, indicate the significant red-shift of the spectrum (Figure 2). In the case of the ground state, the maximum of the low-energy band is located around 450 nm, which is in line with the experimental

3. RESULTS AND DISCUSSION The Ag2Cu2L4 system constitutes an interesting subject for structure−property studies due to several factors. First, it is luminescent and characterized by a large Stokes shift, which, D


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Inorganic Chemistry

Figure 3. Pressure-induced evolution of the unit cell parameters of the Ag2Cu2L4 crystal structure.

data obtained for solution sample,24 whereas for ES it reaches about 620 nm (this gives also an idea of the emission spectrum maximum and agrees well with experiment). In view of the above, it is valuable to investigate the structure evolution under high pressure to see how the laser-induced geometry changes compare with these imposed by high pressure, and to study how the latter correlate with the spectroscopic properties’ trends observed at different pressure. 3.1. High-Pressure Structural Studies. The selected Ag2Cu2L4 crystal was placed in mDAC, which was then loaded with helium as a hydrostatic pressure medium. This way it was possible to successfully measure, solve and refine crystal structures from ambient pressure to 3.5 GPa with a step of about 0.5 GPa at room temperature. Because of severe radiation damage crystal structures above this limit could not be sensibly evaluated. It should also be noted here that the quality of the derived crystal structures decreases with the increased pressure and is lowest for the 3.5 GPa pressure. Therefore, the results obtained at 3.5 GPa are given mostly for the illustrative purposes, whereas they will not be deeply analyzed due to their lower certainty. The obtained unit-cell parameters for each pressure point are shown in Figure 3. While the unit-cell dimensions (a, b, c) decrease monotonically with the elevated pressure, this is not the case for the unit cell angles. The trends for the α and β angles are, however, qualitatively similar. These angles increase at lower pressures, whereas the opposite behavior is observed from around 2 GPa. The α angle is slightly more pressureinfluenced than β, but the amplitudes of these changes do not exceed 1°. In turn, the trend for the γ angle differs most among this group. This angle decreases between 1 and 2.7 GPa, whereas rises from the latter point to 3.5 GPa. Again, the

overall changes do not exceed 1°. Furthermore, the variations of angles do not affect the pressure evolution of the unit cell volume much, which smoothly decreases to 85% of its initial volume, from about 3623.2(5) Å3 at ambient pressure to 3066.97(7) Å3 at 3.5 GPa (Supporting Information). The volume-pressure data were additionally fitted to the third-order Birch−Murnaghan equation-of-state using the EOS-FIT7 software,66 yielding an isothermal bulk modulus, K0, of 6(1) GPa and its first derivative, K′, equal to 11(2). This indicates that the unit-cell compressibility is comparable to that observed for molecular crystals of aromatic hydrocarbons, such as benzene or naphthalene, which is supported by rather similar interactions present in the solid state in these cases.67−70 In summary, the parameters a, b, and c and the unit-cell volume indicate gradual structural changes in the same direction, whereas the angle values suggest that around 1.5 GPa−2.2 GPa there might appear some variations in the observed molecular geometry and/or spectroscopic properties’ trends in the analyzed pressure range. When analyzing the molecular geometry at different pressure, considering the structural data quality, we shall focus our attention on the metal atoms constituting the central core of the complex. First, the heaviest atoms in the structure should have more precisely evaluated positions, and, second, they are most important regarding the Ag2Cu2L4 spectroscopic properties. The geometrical parameters characterizing the metal core at different pressure together with the respective parameters evaluated in our previous work for the GS and ES at 90 K, given for comparison purposes, are presented in Table 1. E


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Inorganic Chemistry

Table 2. Electron Density (ϱ) and Laplacian (∇2ϱ) Values at BCPs of Selected Core Metal···Metal Interactions Computed at Several Hydrostatic Pressuresa ϱ(rBCP)/e·Å−3 [∇2ϱ(rBCP)/e·Å−5] pressure, P/GPa 0.0 0.5 1.0 1.6 2.2 2.7 3.5 ΔHP GS (90 K) ES (90 K) ΔTR

Ag1−Ag2

Ag1−Cu1

Ag2−Cu2

0.125 [1.407] 0.131 [1.502] 0.142 [1.683] 0.156 [1.892] 0.171 [2.139] 0.195 [2.513] 0.216 [2.852] +0.091 [+1.446] 0.127 [1.435] 0.298 [3.380] +0.170 [+1.945]

0.175 [1.204] 0.173 [1.172] 0.180 [1.282] 0.186 [1.350] 0.188 [1.383] 0.189 [1.362] 0.188 [1.409] +0.012 [+0.204] 0.185 [1.346] 0.176 [1.0.95] −0.009 [−0.250]

0.169 [1.164] 0.170 [1.165] 0.178 [1.283] 0.186 [1.384] 0.181 [1.302] 0.173 [1.182] 0.186 [1.330] +0.017 [+0.166] 0.172 [1.193] 0.208 [1.733] +0.037 [+0.540]

a Respective values computed based on 90 K GS and ES geometries taken from the previous work23 are also presented (ΔHP = parameter differences computed for structures at ambient and 3.5 GPa pressure points, ΔTR = values for differences between ES and GS geometries; both shown in bold). Properties were computed at the same level of theory (DTF(PBE0)/LANL2DZ).

Figure 4. Overlay of metal cores: (a) ambient pressure (green) vs 2.7 GPa (magenta), (b) 90 K ground state (green) vs excited state (red), and (c) 2.7 GPa (magenta) vs excited state (red). For panels a and c drawn with the 3.5 GPa crystal structure see Supporting Information.

the Cu2···Ag1−Cu1 angle does not start to increase until 1.6 GPa. At lower pressure range this angle first slightly increases, reaching maximum at about 0.5 GPa, and then decreases until around 1.5 GPa. An overall picture of the pressure-induced changes is shown in Figure 4a, which presents the superposition of molecular cores at ambient and higher (2.7 GPa) pressure. The shortening of the Ag−Ag bond is clearly visible, although the overall “zigzag” pattern is retained. This stays in contrast to the 90 K GS and ES overlay (Figure 4b), where more significant deformation of the core is observed leading to its more pronounced contraction and rhombic-like shape.23 Finally, the comparison of the 90 K ES geometry with the high-pressure one (Figure 4c) indicates that, indeed, the two geometries are not quite alike. The above observations can be explained by the nature of the processes initiating the described changes, and by crystal packing. In the case of the electronic excitation, structural changes are more emphasized at the Cu2 fragment of the molecule, which creates closer contacts with the neighboring species than the Cu1 moiety in the GS crystal structure (Figures 1 and 5a,b). As a consequence, the Ag1···Cu2 contact shortens the most, exceeding even the Ag1−Ag2 distance change. Also, the Cu2 atom shifts most notably upon irradiation, whereas Cu1 shifts the least. The core becomes rhombic-shaped and more compact, as all Ag···Ag and Ag···Cu contacts become shorter than the sum of the respective van der Waals radii. It is important to note here, that in this case charge transfer takes place, which, among others, contributes to the strengthening of the metal···metal contacts.

It appears that various metal···metal bonds or contacts behave differently with gradually rising pressure. The argentophilic interaction distance constantly decreases with pressure, which is from 3.042(1) Å at ambient pressure to 2.821(4) Å at 2.7 GPa (2.774(4) Å at 3.5 GPa). Although the latter value indicates significant strengthening of the Ag−Ag bond, its length is still noticeably greater than the respective result derived for the ES at 90 K, i.e., 2.66(3) Å. In turn, the pressure-induced behavior of the silver−copper contacts is more complex. As may have been to some extent expected, under pressure these contacts behave qualitatively similarly in pairs. This is understandable, since two of them can be considered bonds leading to the characteristic Cu1−Ag1− Ag2−Cu2 “zigzag” motif in GS, while the other two, i.e., Ag1··· Cu2 and Ag2···Cu1, are longer than the sum of the respective van der Waals radii (Table 1, Table 2). In the former case, at lower pressure range, the bond distance decreases, while inbetween 1.6 and 2.2 GPa the trend reverts (reaching the maximum around 3 GPa, assuming the 3.5 GPa value is meaningful). For the longer Ag1···Cu2 and Ag2···Cu1 interactions the interatomic distances shorten from 1 GPa to about 2−2.7 GPa, and then most possibly lengthen slightly. Taking a closer look at the geometrical results, one may conclude that, in general, there is a change in the trend between 2 and 3 GPa, which may suggest that in this region some spectroscopic properties may also vary. The dependence of valence angles of the tetranuclear metal core is more monotonic and straightforward. Both Ag···Cu−Ag angles decrease with pressure, whereas values of the Cu···Ag−Cu angles, in general, increase. It should be noted, however, that F


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Inorganic Chemistry

total size of the core and its planarity were defined and calculated. The first one constitutes the total area of two triangles formed by the Cu1, Ag1, Ag2 and Cu2, Ag1, Ag2 atoms. The second parameter is the angle between planes defined by these triples of atoms. The first parameter changes monotonically from 8.0(1) to 7.3(5) Å2 for the ambient and 3.5 GPa pressure points, respectively. This indicates about 9% reduction of the initial area induced by pressure. In turn, the planarity parameter first rises from 11.66(4) at ambient pressure to 13.9(2)° for 1.0 GPa, and then decreases to 9.9(5)° for 3.5 GPa. These results indicate that the metal core first becomes slightly more bent, while then flattens again, while its size gradually contracts. It should, however, be stressed, the changes of planarity are not large. Additionally, it can be concluded that the changes of the core are much less pronounced when applying pressure, than it was the case in the former TR diffraction studies. Indeed, during the electronic excitation the core shrinks by about 17%, whereas the planarity parameter goes down to around 6.6°. 3.2. Electron Density Topology Vs Pressure. Considering that the charge density properties of a given molecule are directly related to its spectroscopic behavior, at this point it is valuable to take a closer look at the bond topological properties derived via the Bader’s QTAIM method for the theoretical charge density distribution of Ag2Cu2L4 at different pressure. The electron density and Laplacian computed at the bond critical points (BCPs) for the selected metal···metal interactions are presented in Table 2. In the case of the Ag− Ag bond high-pressure induces the distance shortening accompanied by both almost linear increase of the electron density and its Laplacian values. Since these two charge density parameters are well correlated with the bond distance, the less pronounced bond shortening leads to the weaker increase of the mentioned values, when compared to the respective results obtained for the ES geometry characterized by the very short silver−silver bond distance. Quite a different situation is observed for both Ag−Cu bonds (in the case of two longer silver−copper contacts the BCP is either not present, or very close to the QTAIM catastrophe situation associated with very low electron density value in this region). The electron density at BCP of the Ag1−Cu1 bond, in general, increases with pressure, whereas for Ag2−Cu2 it rises at first and then drops down (not taking into account the 3.5 GPa data set where the value increases again). The analogous trend is observed for the Laplacian values. This observation stays partially in contrast to the TR Laue results (Table 2). Whereas for the Ag2−Cu2 bond the electron density and its Laplacian the change direction is preserved (ΔHP = +0.017 e·Å−3 and +0.166 e·Å−5, ΔTR = +0.037 e·Å−3 and +0.540 e·Å−5), for the Ag1−Cu1 interaction the trend is reversed (ΔHP = +0.012 e·Å−3 and +0.204 e·Å−5, ΔTR = −0.009 e·Å−3 and −0.250 e·Å−5). As the topological analysis is based on the calculated electron density distribution, it explicitly takes into account the differences between ground and excited states. All this shows the charge transfer plays a very important role here (as expected), while the geometrical changes induced by the hydrostatic pressure cannot account for that. Therefore, both charge density properties and molecular geometry changes indicate that the molecular core at high pressure and that of the ES differ considerably. Obviously, one has to keep in mind the limitations of the computed values, since these strongly depend on the refined geometries.

Figure 5. Hirshfeld surfaces with a dnorm property mapped71,72 generated for the room temperature ground state geometry of Ag2Cu2L4 at ambient (top) and 2.7 GPa (bottom) pressure points: (a, c) the Cu1 and (b, d) Cu2 side of the complex molecule (views along Cu···Cu direction). For more information, see the Supporting Information.

In contrast, in the case of the pressure data, the Cu1 fragment seems to be more affected. This goes along with our intuition, as the slightly larger space around this site (overall further interatomic contacts; Figures 1 and 5a,b) is the first thing to be reduced when pressed. Thus, the Cu1 part of the complex is more pressure-influenced than it is the case for the initially slightly more tightly packed Cu2 moiety. This is reflected in the more emphasized Ag1−Cu1 and Ag2···Cu1 distances shortening, reaching 0.027(5) Å and 0.189(7) Å values, correspondingly, when compared to the respective contacts involving the Cu2 metal center (Table 1). It should be also stressed that the most pronounced distance changes under high pressure are observed for the Ag−Ag bond and then for the just mentioned Ag2···Cu1 contact. The illustrative Hirshfeld surface representation71,72 of pressure-induced changes in the crystal packing of both sides of the complex species at 3.5 GPa vs ambient pressure is shown in Figure 5, whereas the respective maps for the remaining pressures are available from the Supporting Information, Figure 3S. The qualitative comparison of Hirshfeld surfaces generated for both sides of the complex along with the increased pressure shows that at lower pressure range there is significantly more close contacts at the Cu2 side, whereas at around 1.6−2.2 GPa this number becomes very much comparable for the Cu1 and Cu2 fragments. Finally, at the highest pressures short contacts predominate at the Cu1 side. Such a picture suggests that more attractive/stronger interactions are found between the Cu2 fragments. By applying high pressure, closer contacts are also gradually imposed at the Cu1 side. However, at the highest pressure the stronger forces at the Cu2 side become repulsive when too short, which results in the overall closer contacts finally observed at the Cu1 side. Collective geometrical parameters of the core may also be of some use in characterizing changes occurring along with the increased pressure. Therefore, two parameters describing the G


Article

Inorganic Chemistry

Figure 6. Absorption (a) and emission (b) spectra for the Ag2Cu2L4 complex for various high-pressure points. Only selected absorption spectra are shown (see also Supporting Information). Arrows in the absorption spectra plot indicate pressure increase for low-pressure (0.3−2.6 GPa; solid lines) and higher pressure (3.3−16.5 GPa; dashed lines) regimes. Emission spectra are smoothed and shown with different heights for clarity (the lower the height, the higher the pressure; selected pressure values are shown in the plot).

Figure 7. Emission maxima positions (a) and longer emission decay lifetime values (τl) (b) vs pressure. Full squares denote compression, whereas the empty ones denote decompression.

3.3. High-Pressure Spectroscopy. A very interesting issue concerns the possible relation between the previously described structural changes and the spectroscopic properties of the Ag2Cu2L4 complex in the solid state. In the case of absorption spectra a blue-shift of the absorption edge is clearly visible upon compression up to 2.6 GPa, while above this pressure a quite significant increase of the low energy absorption wing is suggestive of an inversion of the shift direction, or of a consistent intensification and/or broadening of the absorption band (Figure 6a). However, due to the impossibility of accessing full absorption pattern a more

elaborate identification and interpretation of these phenomena is not accomplishable. As far as emission is concerned, the evolution of two parameters was investigated, i.e. emission maxima and lifetimes at different pressures. Similarly to the absorption edge case, the luminescence maxima shift toward shorter wavelengths at lower pressures, namely up to about 3.0 GPa, above which the trend reverts for higher pressures (Figure 6b). This evolution is not reversible on decompression, with the emission maximum not shifting even when the pressure is completely released (Figure 7a). This behavior is suggestive of irreversible stressH


Article

Inorganic Chemistry

Table 3. Four Lowest-Energy Singlet−Singlet Transitions Calculated Using the TDDFT Method (DTF(PBE0)/LANL2DZ) for GS and ES Solid-State Geometries of the Studied Complex at 90 K and at Room Temperature under Different Pressuresa energy, E/eV/wavelength, λ/nm (oscillator strength, f × 104) pressure, P/GPa 0.0 0.5 1.0 1.6 2.2 2.7 3.5 GS (90 K) ES (90 K)

HOMO → LUMO 2.74/452 2.55/487 2.62/472 2.63/472 2.78/446 2.48/500 2.30/540 2.67/464 2.01/617

(28) (185) (181) (199) (133) (162) (23) (154) (39)

HOMO−1 → LUMO 2.77/448 2.68/463 2.73/454 2.75/451 2.84/437 2.54/480 2.36/524 2.73/453 2.03/611

(429) (196) (208) (159) (319) (63) (75) (281) (610)

HOMO−2 → LUMO 2.97/417 2.79/444 2.79/445 2.84/437 2.97/418 2.79/437 2.48/500 2.87/432 2.18/569

(189) (154) (121) (155) (133) (138) (186) (143) (329)

HOMO−3 → LUMO 3.01/412 2.87/432 2.89/429 2.97/417 3.05/406 2.84/408 2.55/486 2.96/419 2.26/549

(9) (33) (19) (45) (9) (90) (125) (33) (31)

a

Only major orbital contributions to computed excitations are given.

transitions also the contribution from the Cu d orbitals cannot be neglected, being especially manifested in the case of the LUMO orbital. For the purpose of the current study, the TDDFT calculations were run for the isolated complex geometries up to 3.5 GPa, so as to gain some idea about the possible qualitative spectroscopic properties’ evolution based on the experimental geometries, and to compare these with the experimentally obtained spectroscopic data. In all the cases, the four lowest-energy electronic transitions are of the abovedescribed nature, however, they differ in the respective computed energies. The obtained UV−vis spectra are shown in the Supporting Information, while the corresponding lowest-energy singlet−singlet transition energies and oscillator strengths are gathered in Table 3. The computational findings suggest that, from 0.5 to 2.2 GPa, one should expect some blue shift in the absorption spectra, while above that pressure, the spectrum should start to be more spread and red-shifted. In general, the calculated shifts are overestimated, especially the red-shift values, which results from the TDDFT shortages and the fact that calculations were performed for isolated molecules and not for the crystal. Surely, the 2.7 and 3.5 GPa geometries are also of lower quality, which affects the respective electronic transition energies. Nevertheless, the described trend is clearly manifested, and these results are qualitatively comparable to the experimental findings. It should be also noted that, as the 3.5 GPa geometry significantly differs from that of the ES, the red-shift is still not that pronounced.

induced sample modifications, which may be ascribed to the lack of a hydrostatic pressure-transmitting medium in the case of spectroscopic experiments. The emission signal can be reproduced only using more than one Gaussian function without altering the most important information: the emission maximum first blue-shifts by about 20 nm, then red-shifts by about 25−30 nm up to 6.5 GPa, and then remains constant up to the maximum pressure reached in the experiment (15.0 GPa). Regarding the fluorescence decay, we were able to reproduce only the ambient pressure experimental data by a monoexponential decay function, whereas we had to add a second contribution to the decay function when the pressure was applied to the sample. Therefore, the luminescence time constants were extracted from the intensity decay data by means of the least-squares fitting of the following function: I(t) = Ale−t/τl+Ase−t/τs+B, where τl and τs are the lifetimes, Al and As are scale constants, and B accounts for the vertical-axis offset (subscripts “l” and “s” denote longer and shorter lifetimes, respectively). The time constant of the more rapid decay contribution, τs, added when pressure is applied, is not appreciably changing with pressure with a value of about 0.2(1) μs throughout the investigated pressure range (0−15 GPa) (Figure 5S, Table 2S). It should be noted here that this contribution must be maintained in the fit of the data collected at ambient pressure after the entire compression−decompression cycle. The other, slower, contribution,τl, exhibits a certain dependence on pressure (Figure 7b and Table 2S), increasing from 0.8 μs at ambient pressure to a maximum value of ∼1.0 μs around 4 GPa. At higher pressure, its value drops suddenly to about 0.5 μs not changing appreciably with further compression and pressure release. It is important to remark that these values are independent of the emission and excitation wavelength investigated, thus indicating that the same relaxation paths are observed in all the cases. The missed observation of the fast decay at ambient pressure and the almost constant value of the corresponding time constant with raising pressure make the occurrence of a retarded fluorescence unlikely but instead indicate the formation of a subpopulation where the relaxation is driven by defects. 3.4. TDDFT Results. In our previous paper, we have described the origin of the observed emission as the ligand-tometal−metal-bond charge transfer.23 Indeed, there are four lowest-energy transitions, each occurring majorly from a different indole ligand (the respective MOs consist mainly of p atomic orbitals centered on carbon and nitrogen atoms of a given ligand) to the pσ-silver−silver bond. In these electronic

4. CONCLUSIONS The current contribution describes the first combined highpressure spectroscopic and crystallographic studies of a complex, Ag2Cu2L4, exhibiting silver···silver and silver···copper metallophilic interactions in the solid state. It occurred that the pressure-induced structural changes differ notably from the geometrical distortions observed for the excited state. Although the Ag−Ag bond distance is reduced in both cases, it is far shorter for the ES geometry. Furthermore, in the case of ES, all Cu···Ag contacts become shorter than the sum of the respective van der Waals radii leading to the final rhombic-like shape of the tetranuclear metal core, whereas under high-pressure conditions the metal core contracts, however, it preserves its “zigzag” shape (it seems the overall core changes are more isotropic in the hydrostatic pressureinduced case, despite anisotropic changes of the molecule’s environment as a whole; see Figures 7S and 8S73). Such I


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Inorganic Chemistry findings result from the crystal packing and charge transfer accompanying the excitation, which under high pressure does not take place to that extent. The low crystal symmetry and the bulky ligands additionally make it more difficult to control, or predict the geometrical changes that occur under high pressure. However, as was shown, via analysis of the intermolecular contacts one may deduce where to expect more significant distortions. Interestingly, shortening of the Ag−Ag bond itself does not ensure the red shift of the absorption and emission spectra. Furthermore, the observed significant blue shift at pressures up to about 2.2−3 GPa is followed by the gradual red shift, however, the absorption edge, as well as, the emission maximum around 10 GPa does not notably differ from that measured at the ambient pressure. In general, the spectral changes amplitudes are comparable to that reported for the previously studied copper compounds.16,20,21 Additionally, redshift of emission spectrum upon the applied high pressure is more commonly observed in the literature than blue-shift. Furthermore, no blue-shift to red-shift switch when increasing the pressure was observed to date for similar systems. In the case of luminescence decay of Ag2Cu2L4, a significant shortening of the longer emission lifetimes is observed at high pressures in respect to the ambient pressure conditions. It can also be concluded, that all the optical spectroscopy data are suggestive of an important role of defects, likely related to the lack of a hydrostatic pressure transmitting medium, for pressures higher than 3 GPa.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.N.J. would like to acknowledge the SONATA Grant (2014/ 15/D/ST4/02856) of the National Science Centre in Poland for financial support. K.D. thanks the Warsaw University of Technology for financial support. K.N.J. and R.K. thank the Laserlab-Europe consortium for providing access to the highpressure spectroscopic equipment (Proposal No. LENS002118; EC’s Seventh Framework Programme, Grant Agreement No. 284464). The high-pressure X-ray diffraction experiments were performed on the ID27 beamline of the European Synchrotron Radiation Facility (ESRF), Grenoble, France (Proposal No. CH-4536). K.N.J., R.K., and K.F.D are grateful to Paraskevas Parisiades and Mohamed Mezouar (Grenoble, France) for providing assistance during these experiments. The authors thank the Wrocław Centre for Networking and Supercomputing in Poland (Grant No. 285) for providing computational facilities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01196. Crystal structure data collection and refinement information, additional plots and figures regarding the X-ray diffraction high-pressure data analysis, Hirshfeld surfaces, auxiliary plots and tables for spectroscopic data (absorption spectra, emission lifetimes), and theoretically computed UV−vis spectra (PDF) Accession Codes

CCDC 1834266−1834272 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*(K.N.J.) E-mail: [email protected]. ORCID

Katarzyna N. Jarzembska: 0000-0003-4026-1849 Radosław Kamiński: 0000-0002-8450-0955 Kamil F. Dziubek: 0000-0001-7577-4527 Margherita Citroni: 0000-0001-8555-1263 Damian Paliwoda: 0000-0003-0020-0515 Krzysztof Durka: 0000-0002-6113-4841 Samuele Fanetti: 0000-0002-5688-6272 Roberto Bini: 0000-0002-6746-696X J


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