Luminescence Mechanochromism Induced by Cluster Isomerization

Article pubs.acs.org/IC

Luminescence Mechanochromism Induced by Cluster Isomerization Brendan Huitorel,† Hani El Moll,† Marie Cordier,‡ Alexandre Fargues,§ Alain Garcia,§ Florian Massuyeau,∥ Charlotte Martineau-Corcos,⊥,# Thierry Gacoin,† and Sandrine Perruchas*,†,∥ †

Laboratoire de Physique de la Matière Condensée and ‡Laboratoire de Chimie Moléculaire, CNRS, Ecole Polytechnique, 91128 Palaiseau Cedex, France § Institut de Chimie de la Matière Condensée de Bordeaux, CNRS, 87 Avenue du Docteur A. Schweitzer, 33608 Pessac Cedex, France ∥ Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France ⊥ MIM, Institut Lavoisier de Versailles, UMR CNRS 8180, Université de Versailles St. Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France # CEMHTI-CNRS, UPR 3079, 1D Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France S Supporting Information *

ABSTRACT: Luminescent mechanochromic materials exhibiting reversible changes of their emissive properties in response to external mechanical forces are currently emerging as an important class of stimuli-responsive materials because of promising technological applications. Here, we report on the luminescence mechanochromic properties of a [Cu4I4(PPh3)4] copper iodide cluster presenting a chair geometry, being an isomer of the most common cubane form. This molecular cluster formulated [Cu4I4(PPh3)4]·2CHCl3 (1) exhibits a highly contrasted emission response to manual grinding, and, interestingly, the optical properties of the ground phase present striking similarities with those of the cubane isomer. In order to understand the underlying mechanism, a comparison with two related compounds has been conducted. The first one is a pseudopolymorph of 1 formulated as [Cu4I4(PPh3)4]·CH2Cl2 (2), which exhibits luminescent mechanochromic properties as well. The other one is also a chair compound but with a slightly different phosphine ligand, namely, [Cu4I4(PPh2C6H4CO2H)4] (3), lacking mechanochromic properties. Structural and optical characterizations of the clusters have been analyzed in light of previous electronic structure calculations. The results suggest an unpreceded mechanochromism phenomenon based on a solid-state chair → cubane isomer conversion. This study shows that polynuclear copper iodide compounds are particularly relevant for the development of luminescent mechanochromic materials.

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INTRODUCTION

intriguing properties, an in-depth understanding of mechanochromism mechanisms is strongly needed. Compared with mechanochromic organic dyes, with an increasing number of examples associated with the aggregationinduced emission properties,19 mechanochromic metal-based compounds are less represented and are mainly based on gold20 and platinum21 coordination complexes. More recently, some copper-based compounds have been reported.22,23 Actually, in addition to their rich photoluminescence properties,24 they are particularly attractive from an economic point of view, with copper being abundant and less expensive compared with noble metals. The development of stimuli-responsive materials based on copper complexes is therefore particularly appealing. The mechanochromic properties of polynuclear copper compounds have been reported, in particular those of copper iodide clusters. Among them, the [Cu4I4L4] (L = organic ligand) cubane clusters constitute an unprecedented family of luminescent mechanochromic compounds by exhibiting great changes of their luminescence properties upon mechanical

Stimuli-responsive materials displaying modulation of the luminescence properties are currently attracting considerable attention because of their promising technological applications as smart photoactive systems.1−6 Among them, mechanochromic luminescent materials that exhibit reversible change of their emission wavelength in response to external mechanical forces (grinding, pressuring, etc.) constitute relevant candidates for the development of data recording devices, motion or stress sensors, and systems dedicated to security issues.7−16 Despite the increasing number of reported pressure-sensitive luminescent compounds based on organic dyes17 and, to a much lesser extent, on transition-metal complexes,18 the understanding of the underlying mechanism is not trivial and hampers deep comprehension. This difficulty can be attributed to the amorphous nature of the crushed phases, which renders structural characterization difficult. In some cases, this becomes even tougher when the mechanical solicitation only alters a small fraction of the compound that is responsible for the observed optical modifications. However, in order to rationally design and develop new stimuli-responsive materials with © 2017 American Chemical Society

Received: July 25, 2017 Published: September 26, 2017 12379

DOI: 10.1021/acs.inorgchem.7b01870 Inorg. Chem. 2017, 56, 12379−12388

Article

Inorganic Chemistry stress.23 In addition to mechanochromism, these compounds can also exhibit luminescence thermochromism providing multiple stimuli-responsive properties, particularly attractive for the development of multifunctional materials. Related to applicative perspectives, their use as phosphors for lighting devices has recently been demonstrated.25 The molecular structure of these [Cu4I4L4] clusters presents a cubane geometry formed by four copper and four iodine atoms, which alternatively occupy the corners of a distorted cube, with the ligands coordinated to the copper atoms. An isomer of the cubane form exists that consists of an open form called the chair (or step) configuration, with two Cu−I bonds being suppressed, as represented in Figure 1. Even if the

Grinding was realized manually in an agate mortar, and the resulting samples are labeled with the letter G for ground. [Cu4I4(PPh3)4]·2CHCl3 (1). The cubane isomer [Cu4I4(PPh3)4] (0.250 g, 0.14 mmol) was dissolved in chloroform at room temperature. Colorless crystals were obtained by allowing the solution to stand at room temperature for several hours. Yield: 94% (0.265 g, 0.13 mmol). For 1 (just removed from the solvent). 1H NMR (300 MHz, (CD3)2SO): δ (ppm) 7.39−8.04 (m, 60H, C6H5), 8.31 (s, 2H, CHCl3). For 1G. 1H NMR (300 MHz, (CD3)2SO): δ (ppm) 7.40−8.00 (m, C6H5). Anal. Calcd (wt %) for C72H60P4Cu4I4·1.5CHCl3: C, 44.36; H, 3.12. Found for 1: C, 44.59; H, 3.05. Anal. Calcd (wt %) for C72H60P4Cu4I4·0.5CHCl3: C, 46.55; H, 3.26. Found for 1G: C, 46.37; H, 3.14. [Cu4I4(PPh3)4]·CH2Cl2 (2) was synthesized according to a previous report.28 Anal. Calcd (wt %) for C72H60P4Cu4I4: C, 47.75; H, 3.34. Found for 2: C, 47.75; H, 3.15. Found for 2G: C, 47.77; H, 3.15. [Cu4I4(PPh2C6H4CO2H)4] (3). 4-(Diphenylphosphino)benzoic acid (65 mg, 0.21 mmol) and CuI (97 mg, 0.51 mmol) in acetonitrile (10 mL) were mixed and heated at 100 °C for 12 h. After cooling to room temperature, yellowish crystals were collected by filtration. Yield: 86% (92 mg, 0.05 mmol). 3 is not soluble in common organic solvents. Anal. Calcd (wt %) for C76H60O8P4Cu4I4: C, 45.94; H, 3.04. Found: C, 45.79; H, 3.04. Characterizations. Liquid 1H NMR spectra were recorded on a Bruker Avance II spectrometer at room temperature, operating at a radio frequency of 300 MHz. 1H NMR spectra were internally referenced from peaks of residual protons in deuterated solvents. Elemental analyses (C and H) were performed by the Service de microanalyse de l’ICSN, CNRS, Gif-sur-Yvette, France. The solid-state static 63Cu NMR spectra were recorded on a Bruker Avance III 750 MHz spectrometer (B0 = 17.6 T) using the wide-band uniform rate and smooth truncation32 Carr−Purcell−Meiboom−Gill33 (WURST-Q-CPMG)34 pulse sequence in static condition, with an interpulse delay of 150 μs. The WURST pulse had a length of 50 μs and a sweep of 1 MHz. 1H SPINAL-6435 decoupling was applied during acquisition. The recycle delay was 1 s. The number of transients was about 10000 for each sample and the number of echoes 40. The chemical shifts were referenced to CuI at 0 ppm. Three spectra with different offsets were coadded to ensure full excitation of the pattern. The samples were packed in 4-mm-o.d. rotors. The 1H magic-anglespinning (MAS) and 13C and 31P cross-polarization (CPMAS) NMR spectra were recorded on an Avance Bruker 500 spectrometer (B0 = 11.7 T). The samples were packed in 3.2- or 4-mm-o.d. rotors and spun at 20 kHz (1H) and 10 kHz (13C and 31P). The 1H and 13C chemical shifts were referenced to tetramethylsilane at 0 ppm. The 31P chemical shifts were referenced to a solution of H3PO4 at 0 ppm. 1H SPINAL-64 decoupling was applied during the signal acquisition. Luminescence spectra were recorded with a SPEX Fluorolog FL 212 or with a Fluoromax-4 spectrofluorimeter (Horiba Jobin Yvon). Low-temperature measurements were realized with a liquid-nitrogen circulation cryostat SMC TBT Air Liquid model C102084 on a SPEX spectrofluorimeter. The absolute internal quantum yields (Φ) were measured by using the Fluoromax-4 integrating sphere. The error evaluation of the measured values is about 10%. Emission lifetimes (τ) were recorded with a time-resolved photoluminescence setup consisting of a frequency-tripled regenerative amplified femptosecond Ti:sapphire laser system from Spectra Physics to obtain λex = 266 nm and a C7700 streak camera from Hamamatsu coupled to an Acton SP2300 imaging spectrograph for luminescence detection. Data were analyzed by exponential curve fitting using Origin software. Powder X-ray diffraction (PXRD) diagrams were recorded on an X’Pert Philips diffractometer (40 kV and 40 mA) with Cu Kα radiation (λ = 0.154056 nm). The calculated patterns were obtained from single-crystal data using Mercury software.

Figure 1. General representation of the two isomers of the [Cu4I4L4] (L = phosphine ligand) copper iodide cluster with the chair and cubane geometries.

discovery of the two isomers was almost concomitant, the chair isomer has been far less studied compared with the cubane isomer, especially with phosphine-based ligands.26,27 Only a few photophysical investigations on chair compounds coordinated by phosphine ligands have been reported by us28 and others29 along with an example with chelating N,P ligands.30 Here, we report on the luminescence mechanochromic properties of the chair isomer of [Cu4I4(PPh3)4]. This mo l e c u l a r co p pe r i o d i d e c l u s t e r , f or m u l a t e d a s [Cu4I4(PPh3)4]·2CHCl3 (1), exhibits a highly contrasted emission response to manual grinding. Interestingly, the optical properties of the ground phase present striking similarities with those of the cubane form, suggesting isomer conversion activated by a mechanical stimulus. In order to understand the underlying mechanism, a comparison with two related compounds has been conducted. The first one is a pseudopolymorph of 1 formulated as [Cu4I4(PPh3)4]·CH2Cl2 (2), which exhibits luminescent mechanochromic properties as well. The other one is also a chair compound but with a slightly different phosphine ligand bearing a carboxylic acid group, namely, [Cu4I4(PPh2C6H4CO2H)4] (3), lacking mechanochromic properties. Structural and optical characterizations of the clusters in light of previous electronic structure calculations allow one, through analysis of the structure−property relationships, to go further into the understanding of the mechanism of this unpreceded mechanochromism phenomenon based on cluster isomerization.

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EXPERIMENTAL SECTION

Synthesis. All manipulations were performed with standard air-free techniques using Schlenk equipment, unless otherwise noted. Solvents were distilled from appropriate drying agents and degassed prior to use. Copper(I) iodide, triphenylphosphine (PPh 3 ), and 4(diphenylphosphino)benzoic acid (PPh2C6H4CO2H) were purchased from Aldrich and used as received. The cubane isomer of [Cu4I4(PPh3)4] were synthesized according to literature procedures.31 12380


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Figure 2. (a) Crystalline structure of 1 viewed along the c axis. CHCl3 solvent molecules are omitted for clarity. (b) Molecular structure of [Cu4I4(PPh3)4]. Cu−Cu: 2.815(1) and 3.489(1) Å. Cu−I: 2.585(1), 2.715(1), 2.662(1), 2.703(1), and 2.532(1) Å. Cu−P: 2.226(1) and 2.241(1) Å. I−Cu−I: 102.99(2), 110.60(2), and 119.50(2), 99.83(2)°. Single-crystal X-ray diffraction (SCXRD) analyses were realized by mounting the crystals on a Kapton loop using paratone oil. All data were collected at 150 K on a Nonius Kappa CCD diffractometer using a Mo Kα (λ = 0.71073 Å) X-ray source and a graphite monochromator. The cell parameters were initially determined using more than 50 reflections. Experimental details are described in Table S1. The crystal structures were solved with SIR9736 and SHELXT201437 and refined with SHELXL-201438 by full-matrix least squares using anisotropic thermal displacement parameters for all nonhydrogen atoms. All hydrogen atoms were placed in geometrically calculated positions. Details of the crystal data and structure refinements are summarized in Table S1. CCDC 1555174 and 1555175 are for 1 and 3, respectively.

within the range of values reported for this family of compounds.26,28 Compound 1 presents two short Cu−Cu bond lengths [2.815(1) Å] and a longer one [3.489(1) Å] at the center of the cluster. Considering the van der Waals radii of copper(I) (1.40 Å),39 the [Cu4I4] chair core can be described as two binuclear [Cu2I2] subunits in relatively weak interaction but within significant cuprophilic (d10−d10) bonding interaction.40 Mechanochromic Properties. At room temperature, 1 is a white crystalline powder under daylight that emits a sky-blue light under UV irradiation. The mechanochromic luminescence properties of 1 are revealed upon grinding, which causes a noticeable color change of the emission shifting from sky blue to a much more intense yellow (Figure 3), while the compound

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RESULTS AND DISCUSSION Synthesis and X-ray Diffraction Characterization. 1 was synthesized by recrystallization of the cubane form in chloroform and obtained as colorless crystals (see the Experimental Section). In agreement with 1H NMR and elemental analysis, chloroform solvent molecules are included in the structure, giving the formula [Cu4I4(PPh3)4]·2CHCl3. Note that a pseudopolymorph of the [Cu4I4(PPh3)4] chair has already been reported and synthesized by refluxing CuI and triphenylphosphine in chloroform26 or by a mechanosynthesis method,29 but in those two cases, the reported crystalline structure is solvent-free. By using our procedure of recrystallization in dichloromethane, another solvate formulated as [Cu4I4(PPh3)4]·CH2Cl2 (2) can be obtained,28 but the solvent molecule could not be crystallographically characterized because of high disorder. Note that these compounds (1 and 2) are insoluble in common organic solvents (acetone, ethanol, tetrahydrofuran, ethyl acetate, and cyclohexane). From SCXRD analysis, 1 crystallizes in the monoclinic space group C2/c and the unit cell content is shown in Figure 2a. The structure can be described as an assembly of columns of complexes running along the b axis, and the orientation of the complexes changes from one column to another. The molecular structure depicted in Figure 2b presents a chair geometry, with two copper atoms having a tetrahedral CuI3P environment and two others a planar CuI2P configuration. The coordinated PPh3 ligands present a classical geometry with Cu−P values similar to those of the reported phosphine-based copper iodide derivatives (selected bond lengths and angles are listed in Table S2). The Cu−I bond distance and I−Cu−I angle mean values are also

Figure 3. Photographs of a crystalline powder of cluster 1 partially ground (center of the sample) under daylight at room temperature and under UV irradiation at 365 nm (UV lamp) at room temperature (295 K) and in liquid nitrogen (77 K).

maintained its white body color. Upon exposure to a CHCl3 solvent for a few hours, the initial sky-blue emission of 1 is recovered, and subsequent grinding leads to the yellow emission again. The grinding process also modifies the luminescence thermochromic properties of 1. Its thermochromism is characterized by a change of the sky-blue room temperature emission to a greener one upon cooling (77 K). This emission color change is different after the grinding process, with the yellow emission becoming purple at low temperature (Figure 3). Upon warming to room temperature, the original emissions are recovered, indicating reversibility of the thermochromism phenomenon. From liquid 1H NMR and elemental analyses 12381


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Figure 4. Temperature dependence of solid-state luminescence spectra of 1 before and after grinding (1G) with emission indicated by a solid line (λex = 350 nm) and excitation spectra by a dotted or dashed line from 290 to 77 K.

appearance of an additional emission band. Upon solvent exposure (CHCl3), the initial blue emissive state of 1 is restored, testifying to the reversibility of the mechanical effect (spectra in Figure S2). Structural Characterizations. In order to identify structural modifications induced by the mechanical solicitation, PXRD analyses have been conducted, and the corresponding patterns are presented in Figure 5. The pattern of 1 matches the calculated one, with slight differences attributed to some structural disorder induced by CHCl3 solvent departure in agreement with the 31P NMR analysis (see the solid-state NMR analysis below). The high intensity of the two peaks at 22.5 and

(see the Experimental Section) of the ground compound, namely, 1G (G for ground), no chemical change occurs upon grinding. Only CHCl3 solvent departure is detected, but this is not related to the grinding process (only speeded up) because compound 1 spontaneously loses these solvent molecules included in its structure upon air exposure without a change of its emission properties. Photoluminescence Characterizations. Solid-state emission and excitation spectra have been recorded for 1 before and after grinding (1G), from room temperature to 77 K (Figure 4). Corresponding data are reported in Table S3. At 290 K, the emission spectra of 1 display a single unstructured broad emission band, centered at 470 nm (λex = 350 nm), in accordance with the sky-blue light observed. When the temperature is lowered, a red shift and an intensity increase of the emission band are observed along with a narrowing of the bandwidth. At 77 K, the emission band is centered at 510 nm. The excitation profiles are similar to maxima at 350 nm. The mechanochromic properties of cluster 1 are characterized by a drastic red shift of the emission wavelength upon grinding from λmax = 470 to 570 nm at 290 K (black curve in Figure 4), with the initial weak blue-sky color being replaced by an intense yellow one. This agrees with an enhancement of the luminescence quantum yield from Φ = 5 to 16% (λex = 350 nm). Emission lifetimes have been determined before and after grinding at room temperature (Table S3). 1 has a singleexponential decay with τ = 0.20 μs, whereas 1G presents a biexponential decay with a similar value of τ1 = 0.26 μs and a longer one of τ2 = 1.84 μs. When the temperature is lowered, a new emission band appears at higher energy (namely, HE for high energy as opposed to the other one at lower energy, labeled LE) for 1G, whereas for 1, only a red shift of the band was observed. At 77 K, the two bands are centered at 430 and 570 nm (Figure 4) and the combination of the blue and yellow emissions leads to the purple light observed in liquid nitrogen (Figure 3). The intensity of the LE band increases upon cooling from 290 to 200 K and then decreases with a concomitant increase of the HE bands (Figure 4). There is no band shift associated with the relative change of the intensity of the two emission bands. The excitation profiles of the two emission bands are similar to maxima around 350 nm. The Stokes shift between excitation and emission maxima of the LE band of 12460 cm−1 is thus much larger compared to that of the HE band of 5730 cm−1 at 77 K. The grinding completely modifies the luminescence properties of the compound in the emission wavelength and the thermochromic behavior with the

Figure 5. PXRD pattern of cluster 1 before and after grinding (1G) and for comparison the calculated one from SCXRD data. 1G·CHCl3 corresponds to the ground sample after CHCl3 solvent exposure. Insets: Photographs of the corresponding samples under UV excitation (365 nm). 12382


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Figure 6. Solid-state NMR spectra of 1 before and after grinding (1G): 1H, 13C CPMAS, and 31P and static 63Cu solid-echo NMR spectra.

NMR spectrum is actually very sensitive to the ligand geometry. The 63Cu NMR patterns are particularly broad, as expected, because of large quadrupolar moments,32−34 but the characteristic shape of the chair geometry41 is maintained in the 1G phase. These results confirm preservation of the chair structure after the grinding process with a slight modification of the ligand geometry. Discussions. Before considering the mechanochromic properties, the origin of the luminescence properties of the pristine chair isomer (1) should be discussed. From previously reported DFT calculations,28 the highest occupied molecular orbital (HOMO) of the chair [Cu4I4(PPh3)4] is mainly composed of Cu 3d and I 5p orbitals (Figure 7 and the

30.2° is due to the preferential orientation of the crystallites because the sample was not ground to prevent a mechanochromism effect. After grinding (1G), the few left broad diffraction peaks are still assignable to the structure of 1. These results indicate partial amorphization of the compound upon grinding, which is accompanied by a further loss of CHCl3 molecules (elemental analysis see the Experimental Section). Upon CHCl3 solvent exposure of 1G, a narrowing of the peak width is observed (1G·CHCl3 in Figure 5), indicating recrystallization of the compound, which is in accordance with the optical reversibility of the mechanochromism. To probe structural modifications at the local scale, solidstate NMR characterizations have been conducted for 1 and 1G. The 1H, 13C CPMAS, and 31P and 63Cu NMR spectra are reported in Figure 6. The 13C peaks corresponding to the phenyl groups of the ligands are broadened after grinding, which is in agreement with the amorphization of the compound revealed by the PXRD analysis. The 1H NMR signal is narrower after grinding, which can be attributed to enhanced hydrogenatom mobility due to more disorder and less constraints in the ground sample. The solid-state 31P CPMAS NMR spectra present several combinations of overlapping quartets resulting from the one-bond J couplings between the 31P and two 65,63Cu isotopes.41 Note that we observe more resonances than the two expected quartets from the two independent phosphorus atoms because of the structural disorder induced by solvent departure. The 31P chemical shift values (taken as the barycenter of the NMR patterns) of −12 ppm are characteristic of the chair geometry, whereas the signal of the cubane isomer is generally found around −25 ppm.23e,28 Upon grinding, a broadening of the peaks is also observed along with a slight decrease in the chemical shift, indicating ligand geometry modification. The 31P

Figure 7. HOMO, LUMO, and LUMO+24 of the [Cu4I4(PPh3)4] chair isomer. Color code: Cu, blue; I, orange; P, purple.

molecular orbital diagram in Figure S13). The 24 lowest unoccupied molecular orbitals (LUMOs), which are in a narrow energy range, are largely localized on the phosphine ligands (∼90%) and correspond to combinations of the π* orbitals of the phenyl groups. Far above this block of unoccupied orbitals lies an orbital of a symmetry having a 12383


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From the different experimental results and theoretical considerations, the origin of the mechanochromic properties of 1 can be attributed to the chair → cubane solid-state conversion. The schematic drawing in Figure 8 illustrates this phenomenon. The main evidence supporting this mechanically induced isomerization is the luminescence properties of the ground phase (1G), which are characteristic of the cubane form and, in particular, the appearance of luminescence thermochromism, which eliminates a mechanism based on just an increase of the Cu−Cu interactions, as reported for the cubane clusters.23,45 The emission spectra of the [Cu4I4(PPh3)4] cubane isomer at different temperatures, reported by us a few years ago,31 are shown in Figure S4 for comparison. The similarity of the two emissions bands (HE and LE) is striking. The excitation profile of the emission band also presents similar features. As was already mentioned and illustrated by the optical properties of compound 3, the thermochromism is specific to the cubane form. From PXRD and solid-state NMR analyses of 1G, no signal related to the cubane isomer is observed, but an amorphous phase is clearly detected. This amorphous phase formed upon grinding must contain the cubane form but in a low amount, being below the detection threshold of the NMR spectroscopy. The higher emission quantum yield of the cubane isomer allows one to detect its emission easily upon grinding. Additional proofs are given by the emission lifetime measurements, revealing the appearance of a new species with a longer lifetime value after grinding, which is consistent with the cubane cluster formation.46 The isomer conversion is supported by the very close relative stability of the two isomers, as reported from DFT calculations.28 They are indeed both synthesized at room temperature in solution, with the only difference being the solvent nature (toluene for the cubane vs chloroform). Note that such isomerization has already been observed upon solvent exposure. The cubane → chair isomer conversion has been reported using different solvents, and the reverse isomerization was only possible by a seeding process.29 A related isomerization phenomenon has also been reported for copper iodide compounds with pyridine-based ligands for which the cubane [Cu4I4(4-picoline)4] → [CuI(4-picoline)∞] polymeric chain interconversion occurs upon toluene or pentane exposure.47 Indeed, the polymeric double-zigzag chain can be described as a polymer of chair isomers. Another example of cubane → chair reversible transformation is found in the literature, but in that case, it occurred between coordination polymers with mixed nitrogen- and sulfur-based ligands, upon heat treatment and acetonitrile exposure.48 The isomerization mechanism for 1 is also consistent with the general observation of structural flexibility within the copper iodide compound family.49 Note that compound 3 does not display mechanochromic properties; the emission does not present a significant change upon grinding (Figures S7 and S8). In comparison, the other [Cu4I4(PPh3)4] solvate, 2, obtained from a dichloromethane solution, exhibits mechanochromic properties similar to those observed for 1. As shown in Figure S9, the nonemissive powder exhibits an intense yellow emission upon grinding, which can be correlated with the emergence of an amorphous phase as revealed by PXRD and solid-state NMR analysis (Figures S10 and S11). This ground phase exhibits luminescence thermochromic properties just as 1G does (Figure S9). The similar crystalline structures of 1 and 2 can explain their similar mechanochromic behavior. The only difference between 1 and 2 is the absence of emission for pristine 2 at room temperature.

strong Cu 4s/4p character, which is bonding along the four copper atoms (LUMO+24; see Figure 7). This cluster-corebased orbital is too high in energy to be reachable. Therefore, the observed emission (λmax = 470 nm) is attributed to LUMO(0+23)−HOMO transitions corresponding to a mixed 3 X,MLCT charge-transfer transition (T2 excited state). In addition to their structural differences, the cubane isomer differs from the chair one by displaying luminescence thermochromism.28−30 Indeed, the thermochomic behavior is characterized as a variation in the temperature of the relative intensities of two emission bands that correspond to two different radiative transitions (LE and HE).42,43 One transition is similar to that of the chair isomer, which is the 3X,MLCT (T2), and the other one is of a different nature and implies the in-phase a symmetry orbital of copper-bonding character, which is called 3CC for cluster-centered transition (T1). In contrast with the chair isomer, this cluster-core-based orbital of the cubane is significantly low in energy to be close to the π*based orbital block. This proximity allows thermal population of this LUMO+24 orbital from the block of LUMOs. Thermal equilibrium between the two excited states (T2 and T1) results in the intensity variation of the corresponding two different emission bands (LE and HE) and is, consequently, at the origin of the luminescence thermochromism observed. Note that, because of its copper bonding character, the excited state of the 3 CC transition endorses a great stabilization in energy upon excitation accompanied by a substantial shortening of the Cu− Cu distances. In the case of the chair isomer, this triplet state is too high in energy to be reachable because it is bonding along only three Cu−Cu contacts compared to six in the more compact cubane geometry (Figure 8). The single emissive

Figure 8. Simplified energy diagram illustrating stabilization of the copper bonding orbital through chair-to-cubane isomerization. The Cu−Cu interactions are represented in pink.

transition explained the lack of luminescence thermochromism for the chair isomer in comparison to the cubane one. This feature, already observed for examples of the chair [Cu4I4L4] forms,28−30,44 is indeed illustrated by compound 3 formulated as [Cu4I4(PPh2C6H4CO2H)4]. This compound was synthesized by reacting copper iodide with 4-(diphenylphosphino)benzoic acid, a triphenylphosphine bearing one carboxylic acid group per ligand (see the Experimental Section). The geometrical characteristics of the chair structure of 3 are similar to those of 1 (Figure 9 and Table S2). At room temperature, 3 displays a weak emission (ϕ < 1%) centered at 550 nm. No thermochromism is exhibited for this compound because only an increase of the emission band intensity is observed by lowering the temperature (Figure S8). 12384


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Figure 9. (a) Crystalline structure of 3 viewed along the a axis. (b) Molecular structure of [Cu4I4(P(C6H4CO2H)3)4]. Cu−Cu: 2.719(1) and 3.196(1) Å. Cu−I: 2.529(1), 2.583(1), 2.652(1), 2.659(1), and 2.718(1) Å.

However, this difference is erased at low temperature because both compounds exhibit an intense emission centered at 500 nm. The nonemissive behavior of 2 compared to 1 can be related to slight differences in the molecular structures (longer Cu−Cu distances for 1 in Table S2), which may impact the nonradiative phenomenon. This particular feature displayed by 2 leads to a great contrasted mechanochromic effect. Indeed, by going from a nonemissive state (Φ < 1%) to a highly emissive one (Φ = 13%), a relatively rare OFF−ON effect is displayed. With regard to the mechanochromism mechanism of [Cu4I4(PPh3)4], the last concern is the driving force of the chair → cubane solid-state isomerization. As was already mentioned, the two isomers present very close stabilities because the chair isomer compensates for its two less Cu−I bonds by stronger bonds associated with a less strained constraint geometry compared to the cubane one. The grinding process is thus energetically sufficient to close the [Cu4I4L4] open chair structure, leading to the cubane geometry, but this isomerization occurs because the crystalline structure permits these changes. The labile solvent molecules in the crystalline structure (partially lost upon air exposure and grinding) may give enough freedom for the cluster to change its geometry, and also the weak intermolecular contacts (only weak CH···C contacts; Figure S3) lead to a crystal packing that can be easily altered. In comparison, the lack of mechanochromism for 3 can be explained by the high cohesion of its crystalline structure based on a strong hydrogen-bonding network (Figure S5) and also by steric hindrance of the bulky carboxylic groups, which may favor the chair geometry.

shows that polynuclear copper iodide compounds are particularly relevant for the development of luminescent mechanochromic materials. Their great structural diversity and flexibility modulating subtly the metallophilic interactions lead to a great range of optical properties. The extreme sensitivity of these noncovalent interactions plays a key role, giving access to a wide range of stimuli-responsive materials.

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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.7b01870. DFT and other characterization data (PDF) Accession Codes

CCDC 1555174−1555175 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

*E-mail: [email protected]. Phone: (+33) (0)2 40 37 63 35. ORCID

Thierry Gacoin: 0000-0001-6774-3181 Sandrine Perruchas: 0000-0002-6341-8629

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CONCLUSION In this study, the luminescent mechanochromic properties of a copper iodide cluster presenting chair geometry are reported. A great change in the color (100 nm redshift) and intensity of the emission properties occurs upon mechanical solicitation, which is directly visualized by the naked eye. In addition, a rare OFF− ON effect is also presented that enlarges the variety of mechanochromic properties of this family of compounds. The mechanism at the origin of the mechanochromism is attributed to the chair → cubane isomer conversion. This isomerization phenomenon occurring in the solid state upon manual grinding is, to our knowledge, unprecedented. More generally, this study

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the CNRS and Ecole Polytechnique for funding. Financial support from the TGIR-RMN-THC Fr3050 CNRS for conducting the research is gratefully acknowledged. C.M.-C. is grateful for financial support from Contract ANR12-JS08-0008 and from the Institut Universitaire de France. The CHARM3AT labex is thanked for the postdoctoral funding of H.E.M., and J.-Y. Saillard and S. Kahlal are thanked for DFT calculations. 12385


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