Mechanochemical synthesis and characterization of a nickel(II) complex as a reversible thermochromic nanostructure

Abstract

In this study, a new thermochromic Ni(II) complex with [(C₃H₇)₂NH₂]₂[NiCl₂(H₂O)₄]Cl₂·2H₂O formula has been synthesized in the solid phase. The complex undergoes a reversible phase transition temperature at thermochromic temperature (T_c), 120 °C, changing its color from green-yellow to dark blue upon heating. The structure and the thermochromic behavior of the complex have been studied using Fourier transform infrared spectroscopy (FTIR), elemental analysis, diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), thermal analysis (TG/DTG/DSC), X-ray diffraction spectroscopy (XRD) and magnetic susceptibility measurements. As the thermochromic reverse process takes place gradually, the spectra analysis and visual inspection before and after transition temperature enables us to monitor the structural changes under heating.

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1 Introduction

Studying compounds exhibiting a highly specific physical response to external stimuli or subtle changes in the environment is an eminent area of chemical synthesis science. The most common of these are photochromic, thermochromic and electrochromic materials, where the stimuli are irradiation by light, change in temperature and an applied electric field, respectively.¹ Amongst them, thermochromism offers potential for technological applications, for example, in thermometers (fever indicators, gadgets, design applications), temperature sensors, color filters and displays.^2–5 Generally, thermochromic materials are inorganic, organic, organometallic or metal complexes which change their optical properties and colors by changing temperature.^6–8 Typically, the thermochromic effect occurs over a range of temperatures, which can be observed as a gradual color change so called continuous thermochromism while at a specific transition temperature, discontinuous thermochromism, causes a structural phase change. As this phase change is governed by the thermodynamics of the system, it can be first or second-order in nature, and may be reversible or irreversible.⁹ Thermochromism in metal complexes is usually due to a solid to solid phase transition, which may be the result of the changes in coordination geometry around the metal center, the coordination number or the coordinated ligands. Studying thermochromic metal complexes set the scene for more discussion of progress and attention because of their numerous applications including quick response code carriers, thermochromic windows, security markers for monetary notes, government documents, temperature sensors for safety and quality control applications, writable/rewritable optical data storage, thermal printing and novelty items such as color-changing cutlery and clothing.^9–14 So far a series of thermochromic nickel complexes containing amine and diamine based ligands have been synthesized and characterized by different techniques such as X-ray single crystal, solid state NMR (Nuclear Magnetic Resonance), IR, EPR and calorimetric measurements.^15,16 Among them, a group of Ni(II) thermochromic complexes of the type A₂MX₄, have been reported by Ferraro and Sherren where X is a halogen atom, M is a first-row transition element, and A = alkali metal, ammonium ion, or a substituted ammonium ion.¹⁷ As the mechanism of color transition has been widely investigated by many research groups, it has been generally accepted that the change of molecular conformation causes the color change and the observed thermochromism in Ni(II) complexes is due to a change in geometry and coordination numbers from an octahedral to a tetrahedral geometry.¹⁸ Choosing a synthetic method, solid state chemical synthesis is a promising one in the field of synthetic chemistry which has been widely used to synthesize various solid state metal salts as it avoids complicated experimental operations and harsh reaction conditions. Specifically in thermochromic compounds, during repeated heating-cooling cycles, irreversible solvent evaporation would occur which is a detriment to the cycle life of the thermochromic systems or devices.¹⁹ Besides the mentioned advantages regarding solid state synthesis being superior than solution method, nickel(II) chloride complexes are known to be thermodynamically unstable in aqueous solution. Either very high Cl⁻ concentration or non-aqueous solvents are needed for effective Ni(II)–Cl coordination.²⁰ Here, we have used a simple solid state synthesis method due to the mentioned advantages in order to synthesize a thermochromic Ni(II) complex of the type [R_xNH_4−x]₂NiCl₄, where R is diisopropyl. Through this study, the long reverse time, enough for analyzing the compound, enables us to monitor the changes in the structure during the thermochromic reaction.

2 Experimental

2.1. Materials

[NiCl₂(H₂O)₄]·2H₂O, diisopropylamine and hydrochloric acid for synthesis and analysis were purchased from Merck and Aldrich companies respectively and used as received without further purification.

2.2. Instrumentation

FT-IR spectra of the products were taken in a Magna 550 Nicolet FTIR spectrophotometer using KBr pellet technique from 400 cm⁻¹ to 4000 cm⁻¹ at ambient temperature. The pellets were dried for 1 h before taking the spectra. The compounds were analyzed for carbon, hydrogen, nitrogen by vario El. III CHNOS Elemental Analyzer. X-ray diffraction patterns at about 25 °C and 120 °C were taken in a Philips X'Pert PRO X-ray diffractometer (XRD) equipped with graphite monochromatized Cu-K_α radiation from 0 to 80 (2θ) at room and T_c temperature. DRS of the compounds were taken in a Shimadzu (MPC-2200) UV-Vis spectrophotometer fitted with a double-beam reflectance attachment at two different temperatures. The morphology and microanalysis of the ground mixture is observed using a Hitachi S4160 field emission scanning electron microscope (FESEM). Vibrating sample magnetometer (VSM), Meghnatis Daghigh Kavir Co., was used to evaluate the magnetic parameters of the Ni(II) complex. The determinations of the thermochromic temperature (T_c) and the melting point were measured on Electrothermal 9200 melting point apparatus. The apparatus was standardized by checking melting points of several standard solids. In order to determine thermal stability and decomposition steps of the complex, thermogravimetry (TG) and differential thermogravimetry (DTG) and Differential Scanning Calorimetry (DSC) were carried out in a Netzch (model STA-409) thermoanalyzer at a heating rate of 10 °C min⁻¹ using Pt-crucible and α-Al₂O₃ as reference material in the temperature range of 25–600 °C. Experiments were carried out under a constant flow of nitrogen (50 mL min⁻¹) environment.

2.3. Preparation of diisopropylammonium chloride

For preparation of diisopropylammonium chloride (dipHCl), 11.85 mmol (1.2 g) of diisopropylamine reacts readily with 4.38 mmol hydrochloric acid (2.19 mL, 2 M) to give a colorless solution. Brick shaped crystals of dipHCl were obtained from this solution.

2.4. Preparation of [(C₃H₇)₂NH₂ ]₂[NiCl₂(H₂O)₄]Cl₂·2H₂O

The transparent dipHCl, 4.35 mmol (0.60 g), was grinded with 2.18 mmol (0.52 g) [NiCl₂(H₂O)₄]·2H₂O in 2 : 1 mole ratio in an agate mortar with a pestle at room temperature for about 4 hours. The product is generally green-yellow and extremely hygroscopic. Until 110 °C, it keeps the color, but in temperature range 100–115 °C, it becomes light green, gradually the color changes to blue, while, in temperature range between 120–140 °C, it will be completely dark blue, the color changes to its original yellow color indicating that the thermochromic process is reversible and after repeating heating-cooling cycle for several times, the process is the same. Anal. calc. for C₁₂H₄₄Cl₄NiN₂O₆: C, 28.12; H, 8.59; N, 5.46. Found: C, 28.24; H, 8.60; N, 5.38.

3 Results and discussion

The thermochromic complex has been synthesized by treating [NiCl₂(H₂O)₄]·2H₂O and diisopropylammonium chloride salt, through a mechanochemical reaction. At room temperature, the compound is green-yellow and it transforms to a blue colored compound under heating about 120 °C. Once the compound gets cold to room temperature, it returns gradually to its original green-yellow color. The thermochromic phase transition of the complex is associated with the change of the Ni(II) coordination number from octahedral to tetrahedral structure and there is no significant changes in the cation part. As the ligand strength for Cl⁻ and H₂O is close to each other based on spectrochemical series, they are in high competition in coordinating to nickel metal center. Through heating, the water molecules are replaces by Cl⁻ atoms and change the color of the complex gradually to dark blue until all the H₂O molecules are replaced. While, cooling to room temperature, the complex absorbs atmosphere water and returns to its original form which is yellow. With the aim of understanding the mechanochemical thermochromic reaction, some techniques were employed for the characterization of the synthesized compound which are brought here to discuss.

3.1. Infrared spectroscopy

Fig. 1 shows the IR spectra of dipHCl (a), the complex at room temperature (b) and at thermochromic temperature (c). As can be seen in Fig. 1a, the dipHCl shows N–H stretches in 3426 cm⁻¹, N–H bending at 1590 cm⁻¹ and the N–H wagging at 838 cm⁻¹ and 805 cm⁻¹. Also, C–N stretches are observed at 1153 cm⁻¹ and 1101 cm⁻¹. The region from 2836 cm⁻¹ to 2977 cm⁻¹ is due to C–H stretching symmetric and asymmetric vibrations. Moreover, the peaks in the region from 1395 cm⁻¹ to 1472 cm⁻¹ correspond to methyl and methylene symmetric and asymmetric bending vibrations. Comparison of the vibration spectra of the Ni(II) complex (Fig. 1b) to dipHCl (Fig. 1a) indicates that a strong absorption band maximum at 587 cm⁻¹ appears in Fig. 1b which is due to wagging modes of water molecules.^23,24 In this complex, the absorption band at 1614 cm⁻¹ in the cation shifts to lower frequency at 1590 cm⁻¹ which belongs to NH₂ bending vibration. Besides, in the complex form, two weak nearby absorption bands at 3398 cm⁻¹ and 3497 cm⁻¹ appears. As can be seen in Fig. 1c, for the heated compound, there are less splitting in the same region due to water molecule losses and weakening the N–H bond strength. Thus, a broad single absorption band at 3444 cm⁻¹ appears.

Fig. 1 FT-IR Spectra of the dipHCl (a), complex at room temperature (b) and at thermochromic temperature (c).

3.2. Elemental analysis

CHN analysis confirms the presence of carbon, hydrogen and nitrogen in the synthesized sample which is in great agreement with the proposed complex formula.

3.3. XRD studies

X-ray powder diffraction pattern are depicted in Fig. 2 for [NiCl₂(H₂O)₄]·2H₂O (a), dipHCl (b), the complex below (c) and (d) the phase transition temperature. [NiCl₂(H₂O)₄]·2H₂O crystallizes in the monoclinic crystal system with a space group of C2/m.²⁵ Synthesized dipHCl XRD patterns is exactly the same as dipHCl patterns reported in literatures which crystallizes in the orthorhombic crystal system with a space group of P2₁2₁2₁.²⁶ The maximum intensity of XRD peak for dipHCl is at 2θ = 22.44. Regarding Fig. 2c which shows the patterns for the complex at room temperature, both reactants' patterns are present in the complex, however, some peak positions have been changed and also some slight changes in their intensities are observed. The complex pattern is somehow resembles to orthorhombic crystal system like dipHCl. The obtained XRD patterns for the product before transition temperature can assign four diffraction peaks at 2θ = 16.03, 16.53, 22.54 and 36.88 (Fig. 2c), which the second one is the reference peak at maximum intensity. After heating the complex to thermochromic temperature, 120 °C, obvious changes in the patterns were detected. In other words, as the reaction progresses a new set of reflections appears, corresponding to the blue product and some significant changes in the intensities and positions of the corresponding diffraction maxima are observed. For the complex at elevated temperature, four diffraction peaks at 2θ = 23.52, 25.99, 28.22 and 36.89 (Fig. 2d) are assigned which the last one is the reference peak. After heating, the intensities of all the peaks diminishes, however, a single peak at 2θ = 23.52 with maximum intensity appears. This indicates that the unit cell parameters are almost the same, however, the internal structures of the complex in these two phases are completely different. The literature predicts that continued heating after dehydration causes the complex to undergo a second reaction of structural transformation from distorted octahedral to tetrahedral. The average size of the complex crystallite was estimated by the Scherrer equation.²¹ It was found that the complex crystallite size at room temperature is 30.249 nm, while after heating to thermochromic temperature the size diminishes to 27.136 nm.

Fig. 2 Powder XRD patterns of NiCl₂(H₂O)₄]·2H₂O (a), dipHCl (b), complex at room temperature (c), complex at high temperature (d).

3.4. Diffuse reflectance spectra analysis

Fig. 3 shows the visible absorption spectrum of the complex at room temperature (yellow line) and thermochromic temperature (blue line). At room temperature, it shows a peak in the blue/violet region (band I) which is logical as the compound is yellow. In other words, the UV-Vis spectrum was roughly consistent because the highest absorbance is at approximately 436 nm, which is the wavelength of green light. Since our major absorption band is in the red and violet region of the spectrum, the compound is expected to be yellow, when in fact it is solely green. The absorption in the red region of the spectrum is also expected to be greater than the absorption in the violet region of the spectrum. As it is clear, there is no significant absorption band in the 600–800 nm for the compound. At thermochromic temperature, beside an absorption band at 452 nm, a doublet absorption band forms at T_c, with absorption maxima at 656 nm and 700 nm (band II), typical of a tetrahedral [NiC1₄]²⁻.^18,22,23 As it is expected the absorption band at high temperature is more intense because the compound is tetrahedral and has no identical center. Actually, band (I) is represented by an obvious absorption peak at 436 nm (RT) which is expected from the ³A^(F)_2g → ³T^(P)_1g transition of the octahedral complex Ni(II). By increasing the temperature from room temperature to 120 °C (T_c), the band position (band I) shifts to the higher wavelength (about 16 nm) and the red-shift is completely detectable. At high temperature the intensities of peaks become more obvious and heightened as the identical center has been removed in the new structure. These adsorptions shown in band (II) should be attributed to the blue colored [NiCl₄]²⁻ complex in the regular tetrahedral symmetry with the transition of ³T^(F)₁ → ³T^(P)₁.

Fig. 3 UV-Vis spectra changes for the complex at room temperature (yellow line) and thermochromic temperature (blue line). The up left inlet shows the photographs of the corresponding sample.

However, the appeared color of the Ni(II) complex at about 100–115 °C (light green) indicates that this transition should not be completed at this temperature range.^19,24 Color photographs of the solid sample is visually observed and taken during heating/cooling cycle is illustrated in Fig. 4.

Fig. 4 Photographs of reversible thermochromic process for the complex at different temperatures.

3.5. Thermal analysis (TG/DTG/DSC)

Fig. 5a and b illustrate TG and DTG traces of the synthesized complex [(C₃H₇)₂NH₂]₂[NiCl₂(H₂O)₄]Cl₂·2H₂O in flowing nitrogen environment. The temperature range for each step and the considered weight losses are summarized in Table 1. Under this condition, three endothermic peaks are observed from ambient temperature to 228 °C. The total weight loss at 65–102 °C (8.35%) is approximately the same as the observed ones at 102–132 °C (7.66%) and 132–228 °C (7.08%). The weight losses in this region correspond to the liberation of 6 molecules of H₂O that in each step the complex liberates 2 water molecules. The first one is attributed to lattice water molecules and its intensity is higher than the two others which are attributed to coordinated water molecules to the system (the first rise is sharp). Above 270 °C (270–374 °C), TG curve exhibits gradual large loss in weight corresponding to amine loss and attains a complete plateau region from 380 °C to 460 °C. The decomposition in the temperature range of 300–600 °C involves complex processes of dehydrochlorination and dechlorination. At about 450 °C the compound completely decomposes to NiCl₂. Dissociation of NiCl₂ with loss of both Cl atoms along with oxidation of Ni to NiO occurs over a wide temperature range higher than 500 °C.

Fig. 5 TG (a) and DTG (b) curves of [(C₃H₇)₂NH₂]₂[NiCl₂(H₂O)₄]Cl₂·2H₂O at a heating rate of 10 °C min⁻¹ in nitrogen.

Table 1 The temperature range of each TG step and the considered weight losses

% weight loss	Temperature range (°C)
8.35%	65–102
7.66%	102–132
7.08%	132–228
44.84%	270–374
4.70%	476–598

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Differential scanning calorimeter is primarily used to determine the energetics of phase transitions and conformational changes and allows quantification of their temperature dependence. We decided to apply this technique in order to improve the knowledge concerning the mechanism of the thermochromic reaction. For having a better understanding of phase transition temperature, TG and DSC plots are depicted in a single diagram (Fig. 6). It can be seen that there is three endothermic peaks in the range of 50–120 °C which shows the three step water losses. Thereafter a relatively small exothermic peak appears in the range 120–140 °C where the TG curve somehow remains flat. Therefore the complex undergoes a phase transition in this temperature range. Visual inspection reveals that the transition is associated with a color change from yellow to dark blue. An endothermic peak in the range of 220–230 °C corresponds to the melting of the [(C₃H₇)₂NH₂]₂NiCl₄ which is in agreement with the melting point estimated by melting point apparatus. Significant changes were observed at higher temperatures, 260–340 °C, which there is no intermediate decomposition steps of the coordinated chlorides and amine.²⁰ Both ammonium cations de-structured away from the complex to gases by-products with one broad step decomposition (three small endothermic peaks) in this region.

Fig. 6 TG-DSC heating curves for 13.86 mg complex.

3.6. SEM micrographs

Morphologies of the compound at room temperature form (yellow color) and thermochromic temperature form (blue color) are illustrated in Fig. 7 and 8. The measurements of their size distribution provide consistent results as shown in Fig. 7b and 8b. The SEM images show that the compound at room temperature exhibits spherical shapes with diameter mostly ranging from 55 nm to 60 nm. However, at transition temperature, it exhibits ellipsoid shape that the particles are loosely aggregated and their assemblies' exhibit irregular shape. More specifically, their sizes distributes in 35–55 nm region. The average sizes are about 25 nm and 65 nm for room and elevated temperature, respectively.

Fig. 7 SEM image of the complex at room temperature (a) and size distribution diagram (b).

Fig. 8 SEM image of the complex at thermochromic temperature (a) and size distribution diagram (b).

The variation in shape and the increase in size of the compound can be attributed to the structural changes from yellow form to blue form upon heating. Overall, by comparing the two SEM images, this result can be taken that upon heating the particles started to be larger which removes the uniformity among the particles. This non-uniformity is clearly notable in Fig. 7b. Moreover, the size distribution graphs somehow confirm this fact as indicating a completely different size distribution of the particle in Fig. 8b.

3.7. Magnetic susceptibility measurements

The magnetization measurement for the compound was performed on a vibrating sample VSM apparatus and the resultant M–H curves are shown in Fig. 9 and 10 at room temperature and high temperature, respectively. Applied field region from −2000 to 2000 Oe is enlarged in the right bottom of each diagram. The magnetization of the sample increases with increasing magnetic field linearly throughout passing through the origin. Neither a maximum in the magnetization, nor a hysteresis loop and remnant magnetization in the field-dependent magnetization is observed, indicating Ni(II) complex in so-called paramagnetic state. There is no sign of saturation of the magnetization for this sample as paramagnetic materials usually require high magnetic field to saturate. The value of magnetization sharply increases with the applied field and it does not attain saturation in the presence of relatively strong magnetic field of even 8 kOe. The magnetic susceptibility (χ_M) for the complex at room temperature and thermochromic temperature have been estimated 4396 × 10⁻⁶ (cgs mol⁻¹) and 5220 × 10⁻⁶ (cgs mol⁻¹). The experimental magnetic moments (μ_exp) were calculated from the following equation:

μ_exp = 2.83(χ_AT)^1/2

where χ_A is the atomic magnetic susceptibility.^8,25 Above transition temperature, color change from yellow to deep blue occurs accompanied by drastic changes in magnetic moment which is 3.23 μ_B at room temperature and increases suddenly to 4.05 μ_B. The magnetic susceptibilities for the green-yellow solid at room temperature show values typical for octahedral Ni(II) complexes which are in the range of 2.9–3.4 μ_B at room temperature.^26,27 This increase in magnetic susceptibility at elevated temperature is primarily due to the larger moment associated with the tetrahedral coordination geometry.²⁸

Fig. 9 Magnetization curve (VSM) of the complex at room temperature.

Fig. 10 Magnetization curve (VSM) of the complex at thermochromic temperature.

4 Conclusions

A thermochromic Ni(II) complex has been facilely prepared by grinding transition Ni(II) chloride and dipHCl in an agar mortar pellet. The as-prepared complex exhibited a stable and significant thermochromic behavior over a wide working temperature ranging from room temperature to about 140 °C. This compound shows a reversible thermochromic behavior, green-yellow at room temperature, spring green at ∼100 °C and dark blue at 120 °C, which was confirmed by visual observation and spectral measurements. Obtained spectra analysis and results before and after transition temperature enables us to have a better understanding of the thermochromic process. These analysis all support the view that the anion of the yellow form possesses an octahedral structure, while the blue form contains discrete [NiCl₄]²⁻ anions. That is to say, a solid to solid phase transition causes the thermochromism in this complex, which is the result of the changes in coordination geometry around the metal center. The compound can be applied on the objects by coating techniques or fabricated into films or adhesive tape by mixing with an appropriate matrix. By the way, as the resultant complex exhibits reversible thermochromism and high thermal stability, it may also be used to detect local temperature in microchannel of microfluidic devices.

Acknowledgements

The authors are grateful to University of Kashan for supporting this work by Grant no. 26736/14.

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