Electrical Voltage Control of the Pressure-Induced Spin Transition at

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Electrical Voltage Control of the Pressure Induced Spin Transition at Room Temperature in the Microporous 3D Polymer [Fe(pz)Pt(CN)] 4

Georgiy Georgievich Levchenko, Gennadiy V. Bukin, Hennagii Fylymonov, Quanjun Li, Ana Belen Gaspar, and Jose Antonio Real J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00885 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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The Journal of Physical Chemistry

Electrical Voltage Control of the Pressure Induced Spin Transition at Room Temperature in the Microporous 3D Polymer [Fe(pz)Pt(CN)4]

Georgiy Levchenko,*a,c Gennadiy Bukin,d Hennagii Fylymonov,e Quanjun Li,a Ana Belén Gaspar,*b José Antonio Real*b

aState

Key Laboratory of Superhard Materials , International Centre of Future Science, Jilin

University, Changchun 130012, Chine bInstitut

de Ciència Molecular/Departament de Química Inorgànica, Universitat de València,

Catedràtric Beltrán 2, E-46980 Paterna, València, Spain cDonetsk

Physical – Technical Institute named after A.A. Galkin NANU, Kiiv 03028, Ukraine

dDonetsk

Institute of Physics and Engineering named after A.A. Galkin, Donestk 83114, Ukraine

eDonetsk

National University, Donetsk 83050, Ukraine

*Corresponding Authors’ e-mails: [email protected], [email protected], [email protected]

Abstract Fine control and direct monitoring of the spin crossover properties driven by pressure at room temperature are reported for the porous three-dimensional coordination polymer {Fe(pz)[Pt(CN)4]} by using a homemade pressure cell that transforms a DC voltage into pressure. The pressure induced spin state switching is steadily driven through a piezoelectric ceramic element, which transmits pressures in the 0.001-0.035 GPa range in the voltage interval 1-4 kV. While, the spin state is easily monitored through changes in the optical spectra of the title compound. The results demonstrate that {Fe(pz)[Pt(CN)4]} responds to as small pressure variations as 0.001 GPa (10 atmospheres), thereby probing its efficacy to work as an effective pressure sensor. Introduction Molecular based multi-property materials are being a very active focus of research worldwide. Future technologies, transport, pollution control, energy storage to cite a few will demand materials gathering diverse physical, chemical and structural properties at one. Moreover, a synergetic coupling and/or cooperative behavior among properties, dexterity to perform parallel functions, ability to store, sort and code information are desirable characteristics.

1,2

Porous

coordination polymers (PCP) also called metalorganic frameworks (MOF) are a class of molecular materials constructed from organic and inorganic building blocks using the reticular chemistry, in other words, the covalent bond. Many of the PCP reported show a permanent porosity greater than ACS Paragon Plus Environment

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50% of its crystal volume and a high surface area (from 1000 to 10.000 m2/g), which exceed by large those values observed for traditional porous materials like zeolites and carbons. These characteristics have created prospects to be exploited in storage of fuels (H2 and CH4), capture of pollutants (CO2, NO2, SO2), as catalyst, as drug delivers, etc. Advanced PCP exhibit apart from the porous properties another physical property, for example, spin state switching, magnetic coupling, proton or electron conduction, fluorescence, mechanical motion, etc.3-7 The spin state switching (ST) or spin crossover phenomenon (SCO) 8-15 have mainly been reported on PCP based on iron(II)metallocyanate inorganic building blocks. 16-18 The molecular Fe(II) switches produce outputs such as changes in absorbance, refractive index, crystal and molecular structure, magnetic and dielectric responses. It then becomes possible to associate a piece of information with each of the low spin (LS) and high spin (HS) states. To date the most studied compound due to its unique electronic, magnetic, optical and structural properties has been the Hofmann-like porous 3D coordination polymer {Fe(pz)[Pt(CN)4]} (pz = pyrazine).19-23 It undergoes a very cooperative thermally induced spin state switching at room temperature accompanied by hysteresis and a pronounced change of color from red (low spin state, LS) to yellow (high spin state, HS).19 The compound adsorbs in a reversible manner hydroxilic solvents, gases (CO2, N2, H2, SO2, CS2),20,23 organic molecules (pyrazine (pz), pyridine (py), furan, pyrrole, and thiophene)22 as well as halogens (I2, Br2 and Cl2).21 The spin transition properties are strongly influenced by the chemical nature and size of the molecules hosted in the pores. On the other hand, the spin state switching at room temperature can be induced as well by pressure. 24 Pressure has been shown to be useful tool to enhance the pore size and sorption capabilities of selected PCP.

25,26

However, in other cases application of pressure promotes the release and

restricts the absorption and diffusion of guest molecules through the porous crystalline framework. 27

Recently, the study of the temperature induced spin transition (TIST)

8-15

аt various hydrostatic

pressures as well as the studies of pressure induced spin transition (PIST)

28-31

at constant

temperature for {Fe(pz)[Pt(CN)4]} have been reported (Figure S1, S2). 24 These studies demonstrated that TIST is inhibited at a critical pressure of c.a. 0.4 GPa due to the diffusion of the silicon oil, used as pressure transmission media, inside the porous structure of the compound. Hence, in these conditions, the compound is fully paramagnetic above this critical pressure. Contrary to the TIST analysis, the PIST studies of {Fe(pz)[Pt(CN)4]} shows no hysteresis, in accordance with the calculated value of the cooperative interaction parameter Γ 32 which is 1.5 times smaller in the case of PIST. This behavior differs from that exhibited by several related two- and three-dimensional Hofmann like Fe(II) porous polymers. A possible explanation accounting for the decrease of cooperativity in the PIST regime in {Fe(pz)[Pt(CN)4]} is based on its more rigid structure or/and in the fact that the experiments were conducted at the temperature where the compound is in the metastable state determined by the its hysteresis loop about room temperature. In any case, this ACS Paragon Plus Environment

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PIST study demonstrated that {Fe(pz)[Pt(CN)4]} potentially could act as a molecular pressure sensor in the room-temperature region since a small variation of pressure is followed by an abrupt spin state change of the material. In this contribution, we experimentally demonstrate that the compound {Fe(pz)[Pt(CN)4]} can operate as a molecular pressure sensor for pressures as small as 0.001 GPa (10 atmospheres) at room temperature. To implement this hypothesis, a specific pressure cell, where pressure can be created both mechanically and by electrical voltage, has been designed and constructed. The DC voltage is applied to a piezoceramic material in contact with the compound. The piezoceramic element transmits a small pressure to the material thus. The spin state switching at room temperature is driven smoothly under a current of 1-4 kV (0.001-0.035 GPa). Results High pressure cell Figure 1 depicts a scheme of the constructed high-pressure cell. Pressure inside the cell is created by means of two methods: i) the traditional one using a cylinder-shaped piston where pressure is created mechanically with a hydraulic press; and a new one ii) that creates pressure by applying an electrical voltage to piezoceramic elements located inside the press. The pressure transmitted to the material is the result of the sum of both pressures created distinctly. Importantly, the spin state change in the compound driven by pressure is detected by recording the UV-visible absorption spectra. To do so, the cell press has been specifically adapted with an optical chamber made of hardened beryllium bronze with an axial hole. The pistons, the optical windows and the multiplier are made of sapphire and the piezoceramic elements have a form of rings with a hole that permits a beam of light to pass through. Pressure induced spin state switching experiments (PIST) In previous work it has been demonstrated that at 293 K the compound {Fe(pz)[Pt(CN)4]} undergoes a complete HS-LS spin transition characterized by a critical pressure P½ = 0.06 GPa measured at a LS molar fraction, γLS, equal to 0.5 (Figure S2). The course of the HS-LS transition has been monitored following the variation of the optical absorption spectra of the Fe(II) and more precisely the intensity of the LS 1А1 → 1Т1 d-d electronic transition band (Figure S3 and S4). 24 In the present experiments an initial pressure of 0.06 GPa is fixed using a traditional hydraulic press. Subsequently, two cycles of experiments have been performed increasing and decreasing the pressure smoothly by applying an electrical voltage to the piezoceramic elements. The penetration spectrum of the compound is recorded at each voltage investigated. Figure 2a and 2b depict the 3 ACS Paragon Plus Environment


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dependence of the optical density (D) with the wavelength both in the increasing and decreasing voltage mode, respectively. The optical density is expressed as the difference between the HS and the LS states, ∆D. For convenience of measuring the absorption changes in the spectrum caused by pressure variation, the spectrum of the HS and LS states were normalized to zero value at 400 nm by subtraction of the background signal of the equipment (spectrometer with pressure cell). At zero voltage the optical intensity measured reflects a value of γLS = 0.5. As voltage (U) increases the optical intensity of the 1А1 → 1Т1 band of the Fe(II) ions in the LS state increases reaching its maximum at 4 kV (γLS = 0.65). Conversely, the optical intensity decreases as the voltage diminish. Figure 2c illustrates the observed lineal dependence of γLS with the voltage in the interval of 0-4 kV. These values of γLS have been plotted as in blue (increasing voltage) and red (decreasing voltage) asterisks in the γLS vs. P graph (figure 2d) in order to obtain the correlation between the applied voltage and the pressure created. It is shown in Figure 3. The maximum DC electric voltage applied, 4 kV, provokes an increase of pressure of 0.035 GPa (35 atmospheres), which lifts the γLS to the value of 0.65. This relatively small variation of pressure induces a notable increase in the γLS due to the abrupt and cooperative spin transition exhibited by the compound {Fe(pz)[Pt(CN)4]}. By the same token, even small variations of pressure such as 0.01 GPa (0.5 kV) are detectable by the compound. Conclusion To the best of our knowledge it is the first time that a DC electrical voltage is used to induce a spin state switching driven by pressure in a metal coordination compound. To do so, a new device for obtaining smoothly changed and modulated high pressure is designed and the smooth variation and modulating the pressure in high-pressure cell cylinder- piston type is realized using electrical voltage. The experiments presented here have demonstrated the capability of the material {Fe(pz)[Pt(CN)4]} to work as molecular pressure sensor. Indeed, the compound {Fe(pz)[Pt(CN)4]} can sense small variations of pressure at room temperature (0.001 GPa), which are monitored easily as changes in its optical absorption spectrum. In comparison with other cyanide based Fe(II) spin crossover polymers previously reported {Fe(pz)[Pt(CN)4]} shows the lowest value of the critical pressure, P½ = 0.06 GPa, for the PISPT at room temperature. In this respect, the two-dimensional compounds {Fe(3-Fpy)2[M(CN)4]} (M(II) = Ni, Pd and Pt), {Fe(phpy)2[Ni(CN)4]}

35

feature critical pressures of

33

{Fe(3-Clpy)2[Pd(CN)4]}

34

and

0.3, 0.6 and 1.4 GPa, respectively.

Furthermore, the three-dimensional compound {Fe(pmd)(H2O)[Ag(CN)2]2}·H2O

36

shows a value

of the critical pressure, 0.6 GPa, similar to that observed for the series of 2D based on the ligand 3Fpy. In summary, {Fe(pz)[Pt(CN)4]} is by far the compound most sensitive to pressure of the series of spin crossover cyanide based coordination polymers currently investigated. In addition the ACS Paragon Plus Environment

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high-pressure cell constructed is amenable to be adapted by replacement of the piezo-elements and the multiplier. This opens new possibilities to investigate the pressure induced spin state switching in coordination compounds by applying DC or AC electrical voltage, which could disclose new phenomenology and potential civil applications.37, 38 Experimental section Materials: {Fe(pz)[Pt(CN)4]} was synthesized as a microcrystalline powder and characterized as described previously. 18,19 Absorption Spectroscopy in the Visible Region under Hydrostatic Pressure: Full absorption spectra were recorded between 330 and 840 nm by using a Carl Zeiss PGS-2 spectrometer. The sample was in the form of a thin transparent layer of microcrystalline powder placed between colorless Scotch tape layers (with diameters not larger than 0.2 mm). The hydrostatic pressure cell made of hardened beryllium bronze with NaCl as the pressure-transmitting medium operates in the pressure range 105 Pa < p < 1.4 GPa (accuracy ≈ 0.01 GPa). The pressure is controlled both by hydraulic press and electrical voltage. The smooth change of the pressure by piezoelements is 0.035GPa when the applied voltage is 4 kV. The pressure was monitored using the pressure dependence of ruby. Supporting Information View of the crystal structure of the 3D porous coordination polymer {Fe(pz)[Pt(CN)4]}. Optical density of the {Fe(pz)[Pt(CN)4]} in LS and HS states at atmospheric pressure. Analysis of the UVVisible spectra of the {Fe(pz)[Pt(CN)4]} at room temperature and at 0.8 GPa and at atmospheric pressure and 77 K. Acknowledgements We thank “The Thousand Talents Program for Foreign Experts”, Project WQ20162200339 (Chine), the Spanish Ministerio de Economía y Competitividad (MINECO) and FEDER funds CTQ201678341-P and Unidad de Excelencia María de Maeztu MDM-2015-0538, the Generalitat Valenciana (PROMETEO/2016/147) and the Ukrainian State Fund of the Fundamental Investigations F-71 (project F-71/61-2017). References [1] Ratner, M. A Brief History of Molecular Electronics. Nat. Nanotechnol. 2013, 8, 378-381.

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[29] Guionneau, P.; Collet, E. Piezo- and Photo-Crystallography Applied to Spin-Crossover Materials, in Spin-Crossover Materials: Properties and Applications (Ed.: M. A. Halcrow), 1st ed., John Wiley & Sons, Chichester, 2013, 507-526. [30] Gaspar, A. B.; Rotaru, A.; Mólnar, G; Shepherd, H. Pressure Effect Investigations on Spin Crossover Coordination Compounds. C. R. Chimie, 2018, 21, 1095-1120. [31] Levchenko, G.; Khristov, A.; Kuznetsova, V.; Shelest, V. Pressure and Temperature Induced High Spin-Low Spin Phase Transition: Macroscopic and Microscopic Consideration. J. Phys. Chem. Solids, 2014, 75, 966-971. [32] Slichter, C. P.; Drickamer, H. G. Pressure‐Induced Electronic Changes in Compounds of Iron J. Chem. Phys. 1972, 56, 2142-2160. [33] Levchenko, G.; Bukin, G. V.; Terekhov, S. A.; Gaspar, A. B.; Martínez, V.; Muñoz, M. C.; Real, J. A. Pressure-Induced Cooperative Spin Transition in Iron(II) 2D Coordination Polymers: Room-Temperature Visible Spectroscopic Study. J. Phys. Chem. B, 2011, 111, 8176-8182. [34] Martínez, V.; Arcís-Castillo, Z.; Muñoz, M. C.; Gaspar, A. B.; Etrillard, C.; Létard, J. F.; Terekhov, S. A.; Bukin, G. V.; Levchenko, G.; Real, J. A. Thermal-, Pressure- and Light-Induced Spin-Crossover Behaviour in the Two-Dimensional Hofmann-Like Coordination Polymer [Fe(3Clpy)2Pd(CN)4]. Eur. J. Inorg. Chem. 2013, 813-818. [35] Gaspar, A. B.; Levchenko, G.; Terekhov, S.; Bukin, G.; Valverde-Muñoz, F. J.; Muñoz-Lara, F. J.; Seredyuk, M.; Real, J. A. The Effect of Pressure on the Cooperative Spin Transition in the 2D Coordination Polymer {Fe(phpy)2[Ni(CN)4]}. Eur. J. Inorg. Chem. 2014, 429-433. [36] Galet, A.; Gaspar, A. B.; Muñoz, M. C.; Bukin, G. V.; Levchenko, G.; Real, J. A. Tunable Bistability in a Three‐Dimensional Spin Crossover Sensory and Memory Functional Material. Adv. Mater. 2005, 17, 2949-2953. [37] Diaconu, A.; Lupu, S.-L.; Rusu, I.; Risca, I.-M.; Salmon L.; Molnár G.; Bousseksou A.; Demont P.; Rotaru, A. Piezoresistive Effect in the [Fe(Htrz)2(trz)](BF4) Spin Crossover Complex. J. Phys. Chem. Lett. 2017, 8, 3147-3151. [38] Boukheddaden, K.; Ritti, M. H.; Bouchez, G.; Sy, M.; Dirtu, M. M.; Parlier, M.; Linares, J.; Garcia, Y. Quantitative Contact Pressure Sensor Based on Spin Crossover Mechanism for Civil Security Applications. J. Phys. Chem. C 2018, 122, 7597-7604.


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Figure 1. Schematic diagram of the high pressure cell (HPC) used for the creation of the modulated pressure by applying a DC electric voltage (U). 1 - casing of HPC, 2 - optical chamber made of hardened material with axial hole, 3 - fixed piston made of sapphire, 4 - optical window from sapphire, 5 - sample in the form of a thin layer of microcrystals {Fe(pz)[Pt(CN)4]}, 6 - pressure sensor from ruby crystal, 7 - a movable piston from sapphire, 8 - multiplier from sapphire; 9 piezoceramic elements in the form of rings with a hole to pass a beam of light, 10 - fixing nut.


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Figure 2. PIST in {Fe(pz)[Pt(CN)4]} at room temperature by applying electrical voltage to the piezoelements of the HPC. Dependence of the optical density ΔD as a function of the wavelength at different voltages: a) increasing voltage; and b) decreasing voltage; c) Dependence of the low spin fraction (γLS) with increase and decrease of the electrical voltage (U) applied to the piezoelements; d) Dependence of the γLS with pressure. For c) and d) blue open circles and red open triangles correspond to increasing and decreasing voltage.


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Figure 3. Correlation between pressure and the DC voltage (U) applied to the piezoelements inside the high-pressure cell (HPC) up to 4 kV. Blue open circles and red open triangles correspond to increasing and decreasing voltage.


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Table of Contents Graphic

A DC electrical voltage is used to induce a spin state switching driven by pressure in the porous coordination polymer {Fe(pz)[Pt(CN)4]}. Inside the high pressure cell the DC voltage is applied to a piezoceramic material in contact with the compound. The spin state switching is driven smoothly under a current of 1-4 Kv (0.001-0.035 GPa). The experiments performed have demonstrated that the compound {Fe(pz)[Pt(CN)4]} can sense small variations of pressure as low as 0.001 GPa (10 atmospheres), which are monitored easily as changes in its optical spectra.

293 K

Fe

Pt


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Table of contents (TOC) A DC electrical voltage is used to induce a spin state switching driven by pressure in the porous coordination polymer {Fe(pz)[Pt(CN)4]}. Inside the high pressure cell the DC voltage is applied to a piezoceramic material in contact with the compound. The spin state switching is driven smoothly under a current of 1-4 Kv (0.001-0.035 GPa). The experiments performed have demonstrated that the compound {Fe(pz)[Pt(CN)4]} can sense small variations of pressure as low as 0.001 GPa (10 atmospheres), which are monitored easily as changes in its optical spectra. 293 K

Fe

Pt


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Electrical Voltage Control of the Pressure-Induced Spin Transition at

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