Piezochromic Topology Switch in a Coordination Polymer

Feb 7, 2017 - reversible transformations at 1.93 GPa from blue phase α to green phase β and at 2.39. GPa to colorless phase γ. The clearly visible ...
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Piezochromic Topology Switch in a Coordination Polymer Michał Andrzejewski and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan, Poland S Supporting Information *

ABSTRACT: Abrupt color changes coupled to a giant strain in the crystal of coordination polymer CoCl2bpp (bpp = 1,3-bis(4-pyridyl)propane) mark piezochromic reversible transformations at 1.93 GPa from blue phase α to green phase β and at 2.39 GPa to colorless phase γ. The clearly visible shape and color changes are ideal for calibrating discrete pressure magnitudes associated with these phase transitions. The crystal spectra have been measured and the structures have been determined in situ under pressure in a diamond-anvil cell. In phase α (of monoclinic space group P21/m) and phase β (orthorhombic space group Pnmm) the tetrahedral Co-coordination is stepwise modified within the 1D chain topology, but in phase γ (triclinic space group P1)̅ the Co2+ cations become octahedrally coordinated and the polymer topology transforms to the 2D sheets.

S

propanol alcohols over 1-butanol.47 Usually porous MOFs with 3D frameworks are considered for engineering materials of exceptional elastic properties. However, the elastic behavior of simple 1D (-metal-linker-)n flexible systems has not been characterized so far. Therefore, we have undertaken this study on the pressure response of 1D CoCl2bpp coordination polymer (Figure 1). We have chosen the bpp ligand because it is conformationally flexible, according to our survey of the Cambridge Structural Database,48 and the cobalt cation was selected as the coordination center because it is a textbook example of colorful complexes.49,50 Indeed most

ince its inception, the supramolecular chemistry has been intensively developed with new versatile applications in science and technology.1,2 Particularly interesting properties have been revealed for metal−organic frameworks (MOFs).3−5 Different coordination numbers of metal cations and a huge variety of organic linkers combine into a multitude of structural motifs in MOFs.6,7 Their structure−property relations are used for engineering pharmaceutical,8 catalyst,9 electronic,10 magnetic,11 and porous materials.12 Novel methods for preparing MOFs are environmentally friendly, for example, the dry or liquid-assisted mechanosyntheses.13−15 High pressure is a method capable of efficiently generating a variety of effects, such as polymorphism,16,17 amorphization,18−20 piezochromism,21−23 and others.24−26 Reversible27−30 and permanent31,32 transitions and their different applications were reported; also the “hidden” polymorphs obtained only by in situ recrystallization in their stability region were found.33,34 The experimental techniques developed in recent few years opened new perspectives for producing functional materials under extreme conditions, for example, novel magnetic35,36 or photomagnetic phases.37 Several 3D MOF architectures were studied as a function of pressure and temperature in the search for unusual elastic properties, such as negative compression and negative thermal expansion.38−42 For example, it was postulated that the negative linear compression of crystal structures is characteristic of the wine-rack type of frameworks.43 Pressure induces chemisorption in an MOF based on Co and benzotriazolide-5-carboxylate ligand, where the nucleophilic addition of guest molecules changes the 5-fold to 6-fold Co-coordination.44 A mixed system Cd(II)-MOF composed of two different organic ligands, elastic 1,3-bis(4pyridyl)propane (bpp) and rigid 1,3,5-benzenetricarboxylate, on cooling responds with positive, zero, and negative thermal expansion.45 Moreover, it was shown that flexible porous compounds with the bpp linker may selectively adsorb CO2 over N2 gases46,47 as well as MeOH, EtOH, 1-propanol, and 2© XXXX American Chemical Society

Figure 1. (a) CoCl2bpp under ambient conditions: The Co2+ cation is tetrahedrally coordinated by two chloride anions and two pyridine rings of bpp linkers into 1D polymers in phases α and β. (b) Co2+ cation octahedrally coordinated by four chloride anions and two pyridine rings in 2D sheets of phase γ. Received: January 4, 2017 Accepted: February 7, 2017 Published: February 7, 2017 929

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a strongly discontinuously shrinks, while b and c elongate and the plate visibly becomes narrower and longer (Figure 3). The most remarkable structural change in phase β is the considerable shortening of Co···Cl contacts between neighboring chains (Figure 4). It appears that all phases are stabilized by CH···Cl hydrogen bonds formed between the adjacent chains (Figure 2 and Figure S4 in the SI). At ambient pressure the shortest CH···Cl distance is 2.92(2) Å (commensurate with the sum of van der Waals radii of H and Cl, of 1.2 and 1.75, respectively, according to Bondi52), and this distance is squeezed to 2.726(4) Å within phase α. At 2.39 GPa the colorless γ phase is formed, stable to 5 GPa at least. The β-to-γ transition is also marked by a strong discontinuous strain of the unit cell and by a visible further elongation of the crystals (Figure 3, Movie S2 in SI). At the transition to phase γ the Co cation becomes 6-fold octahedrally coordinated by four Cl and two pyridyl ligands, compared with 4-fold coordination by two Cl and two pyridyl ligands in phases α and β (Figures 2−4). The structure of phase γ is stabilized by additional CH···Cl bonds to the pyridyl rings. However, the propyl bridge flips so strong that apart from the central C(8)H2 in the propyl tether, acting as an H donor to the CH···Cl bond, in phases β and γ, also other groups, methylene C(7)H2 and arene C(5)H, are involved as H donors, too (Figure S5 in the SI). The abrupt color changes in CoCl2bpp (Movies S1 and S2 in the SI) are clearly related to the discontinuous structural phase transitions. The corresponding solid-state absorption spectra are shown in Figure 5. The absorption of phase α is dominated by strong broad bands from 425 to 700 nm. In phase β the overall absorption of these bands is lower by ∼0.05 only; however, a clear division between two regions, of considerably increased (approximately doubled) absorbance below 500 nm, lower absorbance by ca. 23% from 500 to 600 nm, results in the

recently piezochromic properties were revealed in MOF [CoBdcDabcoH2O], labeled AMU-1.51 Herein we report spectacular piezochromic transitions reversibly switching over the blue, green, and colorless phases of the CoCl2bpp crystal. These clearly visible pressure-induced changes of color and crystal shape coincide with the transforming Co-coordination and framework topology in the structure. We have studied these remarkable effects by X-ray diffraction and VIS spectroscopy at high pressure and low temperature. Under ambient conditions CoCl2bpp crystallizes in phase α (Table 1) in the form of blue plates elongated along direction Table 1. Selected Structural Data of CoCl2bpp Polymorphs at 296 K (cf. Table S2 in the SI) phase

α

β

γ

pressure (GPa) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z/Z′ Dx (g·cm−3)

0.0001 P21/m 5.1936(1) 12.9884(2) 10.4964(2) 90 93.579(2) 90 706.67(2) 2/0.5 1.542

2.10 Pnmm 4.0498(12) 13.585(2) 10.800(8) 90 90 90 594.3(5) 2/0.25 1.833

2.51 P1̅ 3.632(4) 14.5490(18) 10.740(9) 90.05(4) 84.07(9) 90.03(3) 564.4(7) 2/1 1.931

[010] down the polymeric chains (Figure 2). Phase α is stable at room temperature and down to 100 K at least (Table S2 in the SI) and at high pressure up to 1.93 GPa at 296 K. Above this pressure the green β phase is formed, stable to 2.39 GPa (Table 1). At the α-to-β phase transition the unit-cell parameter

Figure 2. Crystal structures of CoCl2bpp phases α, β, and γ. Contacts CH···Cl, shorter than the sum of van der Waals radii,52 are indicated by green dotted lines. Detailed information about intermolecular distances is listed in Table S4, and their compression is illustrated in Figure S4 in the SI. 930

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Figure 3. (a) Unit-cell volume of CoCl2bpp phases as a function of pressure (top). The insets show the sample in phases α, β, and γ as well as phases β and γ coexisting in one crystal, where the Co2+ coordination changes from tetrahedral in phase β to octahedral in phase γ (cf. Movie S2 in SI). For plotting the Co2+ coordination volume (bottom), the six atoms involved in the octahedron in phase γ have also been included for phases α and β (cf. Figure 4). (b) Relative changes of the unit-cell parameters as a function of pressure (cf. Figure S4 in SI).

Figure 4. (a) Dimensions of the Co2+ coordination polyhedra plotted as a function of pressure. The insets show the Co2+ cation and its coordinating ligands (Cl green, N blue) with the plotted dimensions indicated and (b) three 1D chains and the corresponding 2D-sheet fragment in phase γ.

is important for such applications that CoCl2bpp is stable in open air and in many solvents, for example, alcohols, water, and oils. Both phase transitions are first-order in character, and they are accompanied by a giant strain, which is indicated by the abrupt changes of the unit-cell parameters between the phases (Figure 3). Despite the first-order character of the transition, the quality of recovered single crystals did not visibly deteriorate after repeatedly increasing and releasing pressure between the ambient pressure and 5 GPa. It is noteworthy that the transition between phases α and β proceeds rapidly in the crystal, while the transition front between phases β and γ gradually progresses through the sample, which clearly contains both of these phases, easily distinguishable by their different color (Figure 3, Movie S2 in

color change of the sample. In phase γ the absorbance is reduced to below 0.1, with the bands between 490 and 580 nm being the highest mild absorption feature, consistently with the γ-phase sample becoming practically colorless. It is characteristic of CoCl2bpp that both the absorbance magnitude and wavelengths of the absorbance bands hardly change within the phases, but the main hypochromic changes in their spectra occur at the transitions. Owing to this feature the CoCl2bpp crystal is an ideal indicator of the pressure values by the visual inspection, without sophisticated spectroscopic equipment. The CoCl2bpp crystal is also suitable for various types of optoelectronic sensors for indicating exact pressure values as well as for optical devices controlled by the pressure stimuli. It 931

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It can be concluded that the piezochromism of CoCl2bpp is mainly associated with the phase transitions of this crystal and with the abrupt changes in the Co-cations coordination, whereas only very gentle monotonic changes take place within the phases. Therefore, this behavior exemplified by CoCl2bpp can be termed as transformational piezochromism, as opposed to the monotonic piezochromism observed in AMU-1.51 The CoCl2bpp crystal can be applied as a marker of two pressure points, suitable for electronic detection and direct visual observations. Owing to the remarkable shape changes, the CoCl2bpp crystals can activate switches and operate mechanical microdevices, while the color changes can be applied in optoelectronic transducers. CoCl2bpp can be also used as an independent pressure calibrant for manometers with tenso- and piezometric sensors. The clearly visible transition front between phases β and γ can be employed for precisely measuring the volume changes of the DAC chamber. The structural mechanism underlying this spectacular property can facilitate the prediction and synthesis of new transformational polychromic indicators at other pressure values by naked eyes. It is planned to find other materials like CoCl2bpp to form a library of similar pressure markers.

Figure 5. Hypochromic effect in the VIS absorbance of CoCl2bpp transforming between phases α, β, and γ with increasing pressure.

SI). This feature allows an ultrafine monitoring of isobaric compression of the DAC chamber volume. According to our survey of the CSD,48 the intermolecular Co···Cl distances usually differ in length by ∼0.8 Å, similarly, as observed in CoCl2bpp phase α (Figure 4). High pressure enforces equal lengths of distances d3 and d4, which increases the symmetry of the coordination scheme and therefore can be considered as a primary chemical cause of the phase transition to the higher-symmetry orthorhombic phase β. The columns of CoCl2 groups along [100] acquire the pseudosymmetry C2v (2mm). On the transition to phase γ the symmetry of columns CoCl2 still increases to pseudo D2v (mmm); however, the symmetry of bpp linkers is much lower, and their enhanced intermolecular interactions reduce the crystal symmetry to the triclinic system (Table 1, Figure 2). It occurs that the volume change in connection with the Cocoordination transforming between the tetrahedral and octahedral schemes is larger than the overall volume compression (Figure 3). In this respect the Co-coordination environment is the softest structural region, while the structure part constituted of the linkers is more resistant to the compression. The compression of the linkers involves their intermolecular contacts CH···Cl conformational changes (Figure S5 in the SI) and the atomic thermal vibrations reduced by pressure.53 It appears that this latter factor plays the main role in the similar tetrahedral-to-octahedral Zn-coordination transformation below 130 K in the analogous structure ZnCl2Bipy54 (Bipy = 4,4′-bipyridine). The abrupt transformations of Co-coordination in CoCl2bpp also provide a better understanding of the different character of monotonically proceeding compression of another Co-coordination complex, AMU-1. 51 In AMU-1 the Co2+ cation is octahedrally coordinated already under the ambient conditions; therefore, this most dense coordination scheme cannot be abruptly compressed. A strikingly different response to temperature was observed in the close analogue of CoCl2bpp, namely the crystal of ZnCl2Bipy.54 It contrasts with no anomalous effects in CoCl2bpp on cooling down to 100 K (Table S2 in the SI). However, the temperature-induced transition in ZnCl2Bipy is similar to those pressure-induced in CoCl2bpp in the change of the polymer topology and in the tetrahedral-to-octahedral change of the metal-cation coordination. No color change associated with this phase transition in ZnCl2bipy were reported.



EXPERIMENTAL DETAILS CoCl2bpp was previously synthesized by gel crystallization during comprehensive research on 1D, 2D, and 3D bpp/MCl2 systems.55 To obtain single crystals, we attempted different methods and we have successfully synthesized CoCl2bpp by diffusion between the solutions of CoCl2·6H2O (from POCH) and 1,3-bis(4-pyridyl)propane (bpp, Sigma-Aldrich) as precursors, used as supplied. After a few days on the test tube walls blue single crystals of CoCl2bpp were found together with cosynthesized pink CoCl2(bpp)2 crystals (Figure S6 in the SI). High-pressure experiments were performed in a modified Merrill-Bassett diamond-anvil cell (DAC).56 Steel foil, initially 0.3 mm thick, preindented to 0.15 mm, with a 0.5 mm aperture was used as a gasket. The crystal was fixed inside the DAC chamber with a cellulose fiber, and anhydrous isopropanol was used as a pressure-transmitting medium.57 The pressure in the DAC was calibrated by the ruby fluorescence method with 0.03 GPa accuracy.58,59 Two phase transitions were initially detected in a membrane DAC, when abrupt crystal color and shape changes were observed (Movies S1 and S2 in the SI). Highpressure single-crystal X-ray diffraction experiments were performed on a four-circle Xcalibur diffractometer with a CCD EOS detector.60 For data collections and their initial reduction CrysAlisPro software was used.61 Structures were solved by direct methods by program Shelxs and refined by fullmatrix least-squares on F2 using program Shelxl incorporated in OLEX2.62,63 Propylene and pyridyl H atoms ideally located from molecular geometry at 0.97 and 0.93 Å, respectively, were included in the models. Phase β was solved in nonconventional space group to maintain the same orientation and conveniently compare the corresponding structures of phases α and β. Subsequent single-crystal X-ray diffraction measurements confirmed that at 1.93 GPa the crystal in phase β assumes the orthorhombic structure with the unit-cell dimensions approximately preserved (Table 1, Figure 2). The subsequent transition at 2.39 GPa to phase γ induces a triclinic strain resulting in the splitting of reflections. Several experiments with fresh samples were tried to obtain a single-crystal sample of phase γ, but they all scattered weakly and reflections were split. Nonetheless, a reliable structural model of phase γ could be 932

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refined for space group P1̅, albeit with some constraints and isotropic thermal parameters of all atoms. According to the symmetry relation, the sample in phase γ becomes a pseudomerohedral twin; hence diffraction images were processed with the twin option of program CrysAlis. The unit cell of phase γ was not reduced to facilitate comparisons between phases. Despite the dominant scattering of inorganic [CoCl2]n sheets, the crystal twinning and reduced data completeness of the triclinic phase γ, the structural model clearly revealed the positions of organic linkers bpp as well as the reliable dimensions of octahedron CoCl6, albeit with relatively high R factor of the refinement. Consequently, there is some uncertainty regarding the positions of bpp atoms, their thermal vibrations, and possible disordering. Low-temperature experiments were performed on the same diffractometer with an Oxford Cryostream attachment. Selected crystallographic data are listed in Table 1 (cf. Table S2 in the SI). High-pressure VIS spectra were measured with a Jasco S-650 spectrometer operated at the 5 nm resolution, in the two-beam mode, with the probing-beam condenser (focusing the beam at the DAC chamber and then expanding it back to the spectrometer pathway) and the prerecorded-empty DAC baseline subtraction. After recording the baseline, for these measurements one culet in the DAC chamber was covered with a tight mosaic of single crystals 0.1 mm thick, and isopropanol was used as the hydrostatic medium.



ACKNOWLEDGMENTS The authors are grateful to Prof. Andrzej Maciejewski for helpful discussion and for granting access to the spectrometer. This work was supported by the Polish National Science Center, research grant Preludium 2014/15/N/ST5/00748 and Etiuda 2016/20/T/ST5/00177.



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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00019. Figure S1. Single crystal of CoCl2bpp compressed in a diamond-anvil cell. Figure S2. CoCl2bpp VIS spectra in the function of pressure up to 3.5 GPa. Figure S3. Spectrum of CoCl2bpp phases divided into 4λ ranges for integrating the signal. Figure S4. Unit-cell parameters of CoCl2bpp as a function of pressure. Table S1. Integrated signal. Table S2. Detailed crystallographic data of Co2Cl2dpp studies in the function of pressure and temperature. Table S3. Dimensions of coordination polyhedron around Co. Figure S5. CH···Cl hydrogen bonds in CoCl2bpp. Figure S6. Diffusion method used in a synthesis of CoCl2bpp. Figure S7. Shortest nonbonding CH···Cl contacts in CoCl2bpp structures as a function of pressure. Table S4. Geometry of CH···Cl hydrogen bonds in CoCl2bpp in the function of pressure. (PDF) Movie S1. A file with phase transitions. (AVI) Movie S2. A file with a β-to-γ phase transition. (AVI)



Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrzej Katrusiak: 0000-0002-1439-7278 Notes

The authors declare no competing financial interest. Full crystal data have been deposited in the Cambridge Crystallographic Database Centre as supplementary publication numbers CCDC: 1512415−1512429. Their copies can be obtained free of charge from www.ccdc.cam.ac.uk. 933

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DOI: 10.1021/acs.jpclett.7b00019 J. Phys. Chem. Lett. 2017, 8, 929−935