Metal–Organic Framework Optical Thermometer Based on Cr³⁺ Ion Luminescence

Abstract

Metal–organic frameworks with perovskite structures have recently attracted increasing attention due to their structural, optical, and phonon properties. Herein, we report the structural and luminescence studies of a series of six heterometallic perovskite-type metal–organic frameworks with the general formula [EA]₂NaCr_xAl_1–x(HCOO)₆, where x = 1, 0.78, 0.57, 0.30, 0.21, and 0. The diffuse reflectance spectral analysis provided valuable information, particularly on crystal field strength (Dq/B) and energy band gap (E_g). We showed that the Dq/B varies in the 2.33–2.76 range depending on the composition of the sample. Performed Raman, XRD, and lifetime decay analyses provided information on the relationship between those parameters and the chemical composition. We also performed the temperature-dependent luminescence studies within the 80–400 K range, which was the first attempt to use an organic–inorganic framework luminescence thermometer based solely on the luminescence of Cr³⁺ ions. The results showed a strong correlation between the surrounding temperature, composition, and spectroscopic properties, allowing one to design a temperature sensing model. The temperature-dependent luminescence of the Cr³⁺ ions makes the investigated materials promising candidates for noncontact thermometers.

Introduction

Over the past few years, perovskite materials with the general formula ABX₃ have become a significant object of study. One of the most noteworthy groups of these materials is hybrid compounds containing organic cations A (e.g., ammonium and methylammonium), divalent metal ions B (e.g., Pb²⁺ and Sn²⁺), and halide ligands X (e.g., I^– and Cl^–).^1,2 Hybrid compounds have been particularly useful in thin-film photovoltaic devices.^3,4 Due to their specific properties, such as ferroelectricity,⁵⁻⁷ colossal magnetoresistance,^8,9 magnetocaloric effect,^10,11 and unique optical properties,^1,12,13 they can be implemented in various applications. The characteristics of the perovskite-like materials can be significantly tuned by replacing the A and B ions and X linkers.¹⁴⁻¹⁶

Formate-based metal–organic frameworks (MOFs) with perovskite structures have attracted a lot of attention due to their various properties, such as multiferroicity, ferroelectricity, luminescence, and magnetic effects.¹⁷⁻²¹ The bimetallic compounds with the general formula [A]₂M^IM^III(HCOO)₆, where A = EA⁺ (ethylammonium), DMA⁺ (dimethylammonium), M^I = Na⁺ and K⁺, and M^III = Cr³⁺, Al³⁺, and Fe³⁺, exhibit unique, especially temperature-induced properties.^14,16,19 The origin of this phenomenon is related to order–disorder phase transitions and changes in energy level populations caused by the change in temperature.

Bimetallic perovskite-like MOFs also exhibit interesting luminescence properties.^14,22−24 Particularly, the subgroup of chromium-based phosphors is noteworthy due to its strong temperature sensitivity and weak concentration quenching.^22,23 The heterometallic MOFs containing chromium ions exhibit temperature-dependent luminescence properties and may be used for noncontact temperature detection.¹⁴

The spectroscopic properties of the transition metal (TM) ions, such as Cr³⁺, are strongly dependent on the local environment, particularly the type of crystal field and temperature.^14,25−27 Trivalent chromium ions exhibit two main emission bands: narrow spin-forbidden ²E_g → ⁴A_2g transitions around 700 nm and broad spin-allowed ⁴T_2g → ⁴A_2g transitions near 750 nm. The narrow emission occurs in a strong crystal field, whereas broad emission takes place when the material exhibits a weak crystal field.^28,29 The materials with intermediate crystal field strength exhibit both types of emissions. Furthermore, the change of temperature strongly affects the intensities of bands, which was reported as a potentially useful feature for temperature sensing.²⁷ The concentration of the Cr³⁺ ions affects not only the intensity of emission but also may influence the dominant emission band due to the change of the crystal structure.²⁶

Luminescence noncontact thermometry is a novel scientific approach for temperature measurements that has attracted a lot of attention.³⁰⁻³⁶ Noncontact temperature sensing has a high potential for application in industrial, scientific, biomedical, and technological fields due to a variety of advantages, such as micro- and nanosized implementation possibility, high accuracy, and fast response. The general measurement mechanism is mainly based on thermally induced changes in the quality of luminescence spectra, such as peak intensities, positions, or decay lifetimes.^31,37 The most promising approach is based on the examination of the parameters known as fluorescence intensity ratio (FIR or Δ), which is calculated by comparing intensities of two emission peaks.²⁷ FIR-based methods are particularly useful due to the minimalization of an influence of measurement conditions.^27,36

The temperature calculation can be based on the emissions originating from individual dopants or codopants incorporated into the structure of the material. Luminescent materials used for temperature sensing can be divided into several groups according to their size (nano- and microthermometers), change of the wavelength (down- and up-converting materials), and number of emission centers (single and dual emission centers). Another subgroup of thermometric materials is compounds containing rare-earth (RE) metal ions.^27,38

The application of host materials exhibiting luminescence properties enables the analysis based on the emission peaks of both host and dopant. The vast majority of research involves inorganic host materials containing RE metal ions as dopants.^13,39−41 Thermal sensing solutions based on transition metal ions are not so popular; however, promising results have been reported for double perovskites codoped with vanadium ions.¹³

Another noteworthy approach is the incorporation of trivalent chromium ions, which can be a valuable direction for RE-free luminescence thermometer design.⁴²⁻⁴⁴ The potential of implementation of Cr³⁺ ions for high thermal sensing has been reported for several inorganic materials.^13,45,46 However, up to date, such solutions based on MOFs have not been proposed.^13,43,44 Another promising approach is using mixed systems containing chromium ions as well as lanthanide ions.^30,47

Herein, we report the preparation and structural and optical characteristics of the first MOF-type luminescence thermometers based solely on spectroscopic properties of the Cr³⁺ ions, i.e., [EA]₂NaCr_xAl_1–x(HCOO)₆, where x = 1, 0.78, 0.57, 0.30, 0.21, and 0. The investigated materials exhibit significant temperature-dependent emission, simultaneously related to chromium ion concentration. In this work, we attempt to describe the effect of the composition of the material and the strength of the crystal field on the spectroscopic properties. Particular attention is given to the possibility of the implementation of investigated materials as noncontact luminescence thermometers. To achieve this purpose, we performed spectroscopic analysis in a broad temperature range.

Experimental Section

Materials and Instrumentation

All precursors (analytical grade) were commercially available and were used without further purification. The synthesis was performed on an Ertec Magnum II microwave reactor with a standard Teflon vessel. The powder X-ray diffraction (XRD) patterns were obtained on an X’Pert Pro X-ray diffraction system equipped with a PIXcel detector, a focusing mirror, and Soller slits for CuKα radiation (λ = 1.54056 Å). The Raman spectra were measured using a Bruker MultiRAM spectrometer with 2 cm^–1 resolution. A 1064 nm wavelength YAG:Nd laser was used as an excitation source. The diffuse reflectance spectra were obtained using a Varian Cary 5E UV–VIS–NIR spectrometer. The temperature-dependent emission spectra were obtained with a Hamamatsu PMA-12 photonic multichannel analyzer combined with a BT-CCD sensor. As an excitation source, a 405 nm laser diode was used. The temperature was controlled by a Linkam THMS600 stage. For lifetime measurements, a Ti-sapphire laser pumped with Nd:YAG was used as the excitation source. To record decay profiles, the digital oscilloscope Tektronix MDO3052 was used. The compositions of samples were determined with energy-dispersive X-ray spectroscopy (EDS) measurement using a FEI NOVA NanoSEM 140 scanning electron microscope.

Synthesis

A series of [EA]₂NaCr_xAl_1–x(HCOO)₆, where x = 1, 0.78, 0.57, 0.30, 0.21, and 0, were prepared using the microwave-assisted solvothermal method. Exemplarily, to grow [EA]₂NaCr_0.78Al_0.22(HCOO)₆ crystals, 3.2 mmol (0.8526 g) of CrCl₃·6H₂O, 0.8 mmol (0.3900 g) of Al(ClO₄)₃·9H₂O, 4 mmol of EA·HCl (0.3262 g), and 8.8 mmol of HCOONa (0.5985 g) were dissolved in 15 mL of water. Afterward, 25 mL of N-ethylformamide and 5 mL of 98% HCOOH were added to the prepared solution. The mixture was subsequently transferred to a microwave reactor containing a Teflon vessel. The reaction was maintained at 140 °C for 16 h and then cooled to room temperature. The solution was kept undisturbed for 24 h. Next, obtained crystals were separated from the mother liquid and dried at 50 °C. The obtained crystals of the [EA]₂NaAl(HCOO)₆ sample were colorless. In contrast, crystals of materials containing Cr³⁺ ions were purple or dark purple depending on the chromium concentration. The exact quantities of precursors used for the preparation together with nominal and experimentally determined compositions of each sample are listed in Table S1. The performed EDS analysis provided information about the real concentration of Cr³⁺ ions, which is used in the further part of the paper.

Results and Discussion

Structural Properties

Both [EA]₂NaCr(HCOO)₆ (EANaCr) and [EA]₂NaAl(HCOO)₆ (EANaAl) crystallize in the monoclinic, polar space group Pn. Previously published results have shown that the space group transformation into a P2₁/n space group occurs at around 373 K for EANaCr and 369 K for EtANaAl.⁶ The crystal structure in both phases exhibits a perovskite-like topology composed of alternatively distributed octahedral units of CrO₆/AlO₆ and NaO₆. A 3D metal-formate framework comprises voids occupied by EA⁺ cations (see Figure 1). In the high-temperature P2₁/n phase, organic cations are dynamically disordered over two independent positions that are occupied with roughly 50% probability. In the low-temperature Pn phase, due to the strengthening of hydrogen bonds, the metal-formate framework distorts, voids shrink, and the thermal motions of EA⁺ cations are suppressed. The arrangement of EA⁺ dipole moments in the ordered Pn phase is responsible for the polar properties of EANaCr and EANaAl.⁶

Figure 1.

Crystal structure of EANaCr in the high-temperature P2₁/n and low-temperature Pn phases. H atoms are omitted for clarity.

The cell volume in the [EA]₂NaCr_xAl_1–x(HCOO)₆ series is dependent on the type of metal ion; the decrease in volume was reported, while the Cr³⁺ ions were replaced with Al³⁺ ions. Such a phenomenon is related to the different ionic radii of Cr³⁺ and Al³⁺ (0.615 and 0.535 Å, respectively).⁶ The unit cell volumes of the investigated series change within the range from 1065.6 to 1078.9 ų. Detailed unit cell parameters are presented in Table S2 and Figure S1. The comprehensive structural and vibrational studies on both EANaCr and EANaAl have been published previously.^6,7

The phase purity of prepared materials was confirmed by XRD measurements. The collation of obtained patterns measured for the series of samples is presented in Figure S2. The increasing concentration of Cr³⁺ ions leads to a change in XRD patterns, where the most visible change takes place in a range of 20.5–21.5°. No additional diffraction lines were detected, which indicates that the Cr³⁺ ions can be substituted by the Al³⁺ ions within the full range of concentrations. The similar structural properties of the investigated materials significantly expand the field of their possible implementation.

Raman Studies

Raman spectra of investigated compounds contain bands related to internal vibrations of HCOO^– and EA⁺ ions as well as lattice vibrations. The specific assignments of observed Raman bands, as well as more complex structural description for combined hybrid materials containing EA⁺ and HCOO^– ions of isostructural compounds, were described in the literature; thus, no particular attention will be given to this matter.^6,7

The collation of the room-temperature Raman spectra for a series of [EA]₂NaCr_xAl_1–x(HCOO)₆ compounds is presented in Figure S3a. As one can see, the change of the composition parameter x affects qualitatively the spectra. The disappearance of some peaks and/or changes in intensity is accompanied by an increase in Cr³⁺ concentration. For instance, the increasing amount of Cr³⁺ causes a decrease in the intensity of bands at 227, 290, 308, 630, 936, 1352, and 1682 cm^–1 followed by the emergence of bands at 245, 1340, 1383, and 1672 cm^–1. Some of them strongly change the relative intensity (Figure S3b,c). These subtle differences are related to the slightly different phonon properties of the Cr³⁺/Al³⁺–O bonds and the changes in the local environments in the samples composed of the mixed CrO₆/AlO₆ octahedra. Additionally, a particular change is observed around 1352 cm^–1 (Figure 2a). The variety of the peak intensities might be implemented to estimate the concentration of the Cr³⁺ ions in the sample. The exemplary intensity as a function of concentration is presented in Figure 2b. Due to the high accessibility and measurement simplicity of the Raman spectra, the calculation of the ion concentration may be a useful analytical tool for material science.⁴⁸⁻⁵⁰ Therefore, this topic may be an interesting subject for further investigation.

Figure 2.

(a) Change of the normalized Raman spectra for a 1352 cm^–1 peak corresponding to the C–H in-plane bending mode (υ₅); (b) relation between 1352 cm^–1 peak intensity and Cr³⁺ concentration with linear fitting.

Diffuse Reflectance Spectra

The comparison of the diffuse reflectance spectra of the prepared compounds is presented in Figure 3a. A series of obtained spectra contain two main broad bands localized at about 17,452 cm^–1 (573 nm) and 24,213 cm^–1 (413 nm) corresponding to spin-allowed ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g transitions, respectively. Low-intensity and narrow lines at around 14,577 cm^–1 (686 nm) are attributed to the spin-forbidden transition from the ⁴A_2g ground state to the ²E excited level. The intensity of the DRS spectrum is related to several factors, e.g., size and position of crystallites. Thus, performed measurements are used in a qualitative rather than quantitative manner. The change in the spectrum shape is caused by the decrease in spectrum components’ overlapping. In fact, each part of the spectrum assigned to ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g transitions contains two bands, in which overlapping creates the final shape of the band, which is particularly visible for a sample containing 100 mol % chromium ions.

Figure 3.

(a) Diffuse reflectance spectra of a series of [EA]₂NaCr_xAl_1–x(HCOO)₆ (x = 1, 0.78, 0.57, 0.30, 0.21, and 0) compounds measured at 300 K; (b) change of the energy band gap (E_g) and Dq/B parameters as a function of Cr³⁺ ion concentration.

The [EA]₂NaAl(HCOO)₆ sample does not exhibit absorption in the given range due to a lack of optically active chromium ions. The comprehensive description of absorption spectrum deconvolution has been described by Stręk et al.⁵¹

In addition, spectral changes like band broadening or maximum peak shift were observed at the absorption spectra (Figure S4).

The Kubelka–Munk function was used to calculate the energy of band gaps (E_g) in the examined materials.⁵² This determination is based on the graphical examination of the following function:

1

where R denotes the measured diffuse reflectance. The comparison of the prepared graphical analysis is presented in Figure S5a–f. The estimated values of band gaps are presented in Figure 3b. The nonlinear decrease in E_g value is observed across the entire range of Cr³⁺ concentrations due to the substitution of the smaller Al³⁺ ion (0.535) by the larger Cr³⁺ (0.615) one. The sample containing only Al³⁺ ions exhibits the maximum value of the band gap energy (5.09 eV), whereas the compound based only on Cr³⁺ ions shows a minimum value of 4.38 eV. The reduction of the E_g value due to an increase in Cr³⁺ ion concentration has been reported for similar compounds containing a methylammonium cation.¹⁴

The obtained diffuse reflectance spectra were used to determine the crystal field (Dq), as well as the Racah parameters (B and C). The calculations were conducted following the previously reported methodology.^16,22 The detailed procedure of crystal field parameter determination is presented in Instruction 1 (Supporting Information). The summary of the calculation values is presented in the Supporting Information (Table S3). The analysis of the Dq/B ratio allows the determination of the crystal field strength. According to the Tanabe–Sugano diagram, the ²E_g and ⁴T_2g levels are overlapped for the Dq/B ratio equal to 2.3, which separates strong (Dq/B > 2.3) and weak (Dq/B < 2.3) crystal fields. Additionally, samples exhibiting a Dq/B value close to 2.3 can be assigned to the so-called intermediate crystal field. The sample containing the lowest concentration of Cr³⁺ ions (21 mol %) exhibits the strongest crystal field (Dq/B = 2.76). The increasing concentration of Cr³⁺ ions leads to a reduction of the Dq/B value (Figure 3b). The lowest value of the Dq/B parameter (2.33) was calculated for the sample containing 100 mol % chromium ions. Thus, the series of investigated compounds can be mainly described as exhibiting strong crystal fields. However, the sample containing 100% Cr³⁺ ions exhibits an intermediate crystal field. The comparison of two samples containing 21 and 100 mol % Cr³⁺ with energy diagrams and representative emission spectra is presented in Figure 4. The obtained results confirm the crystal field strength reduction by increasing the chromium concentration, which was reported previously.¹⁴

Figure 4.

Comparison of the Dq/B values of two samples containing 21 and 100 mol % Cr³⁺ ions as well as energy diagrams of Cr³⁺ ions with the obtained emission spectra (normalized).

Luminescence and Temperature Dependency

The emission spectra of the obtained compounds were measured within the range of 80–400 K with 10 K steps. The used excitation wavelength was 405 nm since this energy corresponds well with the ⁴A_2g → ⁴T_1g transition. The collation of the excitation and emission spectra of [EA]₂NaCr_0.78Al_0.22(HCOO)₆ is presented in Figure S6. Due to fast nonradiative relaxation, the energy transfer from ⁴T_1g to ⁴T_2g and ²E_g levels occurs.⁶ The emission spectra of investigated compounds are strongly sensitive to the environmental temperature. At low temperatures, several narrow bands are present, where the strongest one is located at 686.4 nm (named the R₁ line). Moreover, the band at 684.2 nm (named R₂) can be observed. There are also additional Stokes bands localized at 696.8, 706.5, 727.2, and 752.3 nm. At higher temperatures, the intensities of the R₁ and R₂ bands significantly decrease. The intensities of bands at 727.2 and 752.3 nm also simultaneously decrease, although this process is not as progressive as a reduction of more intensive bands.

As the temperature increases, the emission spectra expand and create a wide band with a maximum at 752.3 nm. A progressive increase in temperature leads to a reduction of the emission intensity. The maximum intensities of this band occur at 210, 190, 185, and 150 K for samples containing 100, 78, 57, and 21 mol % chromium ions, respectively.

This emission is assigned to the spin-allowed ⁴T_2g → ⁴A_2g transitions. The formation of this broad band depends on the concentration of the Cr³⁺ ions. Therefore, the sample of [EA]₂NaCr_0.21Al_0.79(HCOO)₆ does not exhibit such a property and, on the other hand, the sample of [EA]₂NaCr(HCOO)₆ shows the strongest emission from the ⁴T_2g level.

The luminescence properties of transition metal ions, such as Cr³⁺, originate from d–d electronic transitions, which are not shielded by the outer orbitals, unlike the 4f orbitals in trivalent lanthanide ions.⁵³ Consequently, the spectroscopic characteristics of Cr³⁺ ions are strongly affected by the crystal field strength of the matrix material; thus, the emission type, range, and thermal quenching rate can be tuned by changing the matrix type.⁵³⁻⁵⁶ The sensitivity of spectroscopic properties to the change of crystal field parameters plays a particular role in research on TM-based luminescence thermometers.^13,14,57 The irradiation with 405 nm wavelength excites a higher ⁴T_1g level. The nonradiative energy transfer causes the increase in the population of the lowest vibrational level of ²E_g. At low temperatures, the ²E_g → ⁴A_2g transition is dominant. An increase in temperature leads to a thermal population of the ⁴T_2g level, which causes the occurrence of the broad spin-allowed ⁴T_2g → ⁴A_2g transition. The emission of the Cr³⁺ ions may be influenced by the local ion’s symmetry changes, which causes a distortion of the excited state parabola.⁵⁸ Due to the occurrence of the crossing point between ground and excitation state parabolas, the increasing temperature may cause the depopulation of the excitation states via nonradiative transitions.

The same behavior was previously reported in methylammonium-based materials exhibiting different crystal field properties, particularly the Dq/B parameter.¹⁴ The overall emission of prepared compounds quenches rapidly due to an increase in temperature to around 300 K. The concentration of Cr³⁺ ions affects the maximal temperature when any emission is detectable, which is 270 and 330 K for samples containing 21 and 100 mol % Cr³⁺ ions, respectively.

It is worth noting that the sample of [EA]₂NaCr(HCOO)₆ exhibits low intensity and a very broad band within a range of 770 to 870 nm at a low temperature (80–130 K). This behavior was not detected in samples containing a lower concentration of Cr³⁺ ions. Such an occurrence of the additional band may indicate surface defects that can be related to a high concentration of chromium ions. A similar additional emission peak was reported for hybrid perovskites, e.g., CH₃NH₃PbCl₃ or MHyPbCl₃, and it has been assigned to the recombination of photoexcited carriers in defects.^59,60

Additionally, the decay profiles for samples containing 21–100 mol % Cr³⁺ were measured (Figure 5a). To calculate the time components of the decay (τ₁ and τ₂ parameters), double exponential fitting was performed with the following equation:

2

where I₀ is the initial luminescence intensity, A₁ and A₂ are the pre-exponential coefficients, τ₁ and τ₂ are first and second time components, respectively, and t is the time.

Figure 5.

(a) Decay profiles of [EA]₂NaCr_xAl_1–x(HCOO)₆ (x = 1, 0.78, 0.57, 0.30, and 0.21) measured at 77 K; (b) change of time parameters as a function of Cr³⁺ concentration.

The influence of the chromium ions on the decay curve shape is observed. The highest values of τ₁ and τ₂ were calculated for the [EA]₂NaCr_0.21Al_0.79(HCOO)₆ sample and were equal to 1.35 and 2.26 ms, respectively. The increase in chromium ion concentration leads to the nonlinear reduction of the τ₁ and τ₂ parameters (Figure 5b). The most significant decrease occurs between 21 and 30 mol % Cr³⁺ ion concentrations, and then the change of the time parameters is closer to linear. The lowest values of τ₁ (0.069 ms) and τ₂ (0.176 ms) were calculated for the [EA]₂NaCr(HCOO)₆ sample. The values of the time parameters for all of the measured samples are listed in Table S4.

Luminescence Thermometry

The observed changes in the intensity and character of the emission in the aftermath of temperature change make the investigated metal–organic frameworks interesting materials for luminescence thermometry. It was recently suggested that hybrid materials with a perovskite-type architecture containing Cr³⁺ ions exhibit a sufficient optical response and physicochemical properties to be implemented into noncontact temperature sensing solutions.^{13,45,46,57,61}

Temperature sensing originates from the coexistence of at least two temperature-dependent transitions. In the case of [EA]₂NaCr_xAl_1–x(HCOO)₆ compounds, the change in intensities of two bands, assigned to ⁴T_2g → ⁴A_2g and ²E_g → ⁴A_2g, serves as the basis for temperature estimation. The thermal evolution of the emission spectra for the sample containing 100 mol % Cr³⁺ is presented in Figure 6a,b. The results for the remaining samples are presented in Figure S7. To demonstrate the temperature sensing performance, the thermometric parameter Δ (FIR) was calculated. It is described as the ratio of the integrated intensities of the considered bands. For the calculations, two regions were chosen, namely, 660–718 nm for ²E_g → ⁴A_2g (denoted as I₁) and 718–970 nm for ⁴T_2g → ⁴A_2g (denoted as I₂). The comparison of the Δ value as a function of the temperature for a series of investigated materials is presented in Figure 6c.

Figure 6.

(a) Temperature-dependent emission spectra and (b) thermal evolution of the intensity measured for the [EA]₂NaCr(HCOO)₆ sample; (c) thermometric parameter (Δ); (d) absolute sensitivity (S_a); (e) relative sensitivity (S_r) and (f) influence of the Cr³⁺ ion concentration and temperature on the relative sensitivity.

The increase in temperature causes the general reduction of Δ; however, for samples containing 100 and 57 mol % Cr³⁺ ions, the increase in the calculated value is observed within ranges of 80–130 and 80–100 K, respectively. The highest initial value of Δ was calculated for a sample containing 21 mol % Cr³⁺. Within a range of 200–300 K, the calculated values of the thermometric parameters for all samples are comparable. The slightly different shape of the initial part of the plot for the material containing 100% Cr³⁺ ions may be related to the additional, low-intensity broad band associated with a possible self-trapped exciton (STE), whose existence was reported for a wide variety of perovskite materials.⁶²⁻⁶⁴ The observed emission decreases rapidly due to the increase in temperature.

To further demonstrate the luminescence thermometry performance, the absolute (S_a) and relative (S_r) sensitivities were calculated. The parameters are described by the following equations:¹³

3

and

4

where dΔ represents the change of thermometer parameter Δ (Figure 6c) at temperature change dT. The collations of absolute sensitivities for a series of investigated materials are presented in Figure 6d. The absolute sensitivities initially increase with the growth of temperature. The maximum value of S_a is 0.09 K^–1 at 120 K for a sample containing 21 mol % Cr³⁺ ions. A progressive increase in temperature causes a significant decrease in S_a value. Similarly, the relative sensitivity values increase with temperature (Figure 6e). The maximum value of S_r is 2.84% K^–1 and is reached again for a sample containing 21% chromium ions at a temperature of 160 K. The calculated temperature measurement uncertainty (δT) for the [EA]₂NaCr_0.21Al_0.79(HCOO)₆ sample at 160 K was 0.40 K. An increasing temperature leads to a simultaneous decrease in S_r parameter. The final value of relative sensitivity for most of the samples is 1.04–1.33% K^–1. In addition, the dependency of chromium ion concentration on the useful temperature sensing range and relative sensitivity can be observed. The increasing concentration of Cr³⁺ ions causes the change of the maximal relative sensitivity toward a higher temperature (Figure 6f). The possible modulation of the optimal sensing range by tuning the Cr³⁺ concentration significantly expands the possibility of system optimization.

To initially demonstrate a practical potential of luminescence thermometry, the exemplary thermometric solution based on one of presented compounds has been prepared (Figure 7). The obtained materials were investigated as luminescence thermometers for low-temperature (T < −30 °C) systems. Luminescence thermometers have to be cooled by contact with an object, whose temperature is to be measured. The very first tests were performed with the system based on the temperature gradient present in a copper pipe, one of which end is submerged in liquid nitrogen (Figure 7a,b). Crystals of [EA]₂NaCr(HCOO)₆ were attached to the surface of the pipe with a thermal paste.

Figure 7.

Exemplary thermometric system: (a) thermal image and (b) picture of the experimental setup; (c) recorded emission spectra for individual points with calculated FIR.

Thermometric calculations were performed with the model determined with presented temperature-dependent luminescence properties. The obtained photoluminescence spectra (λ_exc. = 405 nm) were used to calculate the integrated intensity ratio (FIR parameter) and then compared to the model temperature relation (Figure 7c).

The presented result shows the undeniable potential of temperature-sensitive luminescent materials. Among various advantages, one of the most significant is the possibility to record the spectrum even in a presence of hoar frost. In addition, even expensive commercially available thermal imaging cameras are able to operate mainly in a temperature regime above −40 °C. The implementation of functional and durable luminescence thermometers is a significant matter for more developed investigation. Investigated organic–inorganic phosphors exhibit sufficient stability during heating–cooling intervals (Figure S8). Performed measurements consist of several consecutive heating and cooling processes. A significant change in the FIR value has not been observed, so the materials can be reused in thermometric systems. Measured materials do not exhibit phase transitions within the thermometric range, which additionally improves the stability in nonconstant temperature conditions. It is worth noting that possible practical usefulness may be significantly improved by protecting solutions, such as resin impregnation.

The obtained results, especially considering the luminescence thermometry performance, are remarkable and provide valuable information about the relationship between the composition, crystal field strength, and optical properties. Achieved values of relative sensitivity, up to 2.84% K^–1, are comparable to conventional inorganic lanthanide-doped materials.^13,65−67 The comparison of the S_r parameters of some reported luminescence thermometers is shown in Table 1.

Table 1. Collation of Exemplary Luminescence Thermometers with Relative Sensitivity (S_r) at a Given Temperature (T)^a.

compound	Sr(% K–1)	T (K)	ref.
Sr2MgAl22O36:Cr3+	1.7	310	(46)
La2MgTiO6:Cr3+, V4+	1.96	165	(13)
La2MgTiO6:Eu3+	3.0	77	(65)
SrGdLiTeO6:Mn4+, Eu3+	4.9	550	(68)
Li5Zn8Al5Ge9O36:Mn2+	8.489	323	(69)
Sr2GeO4:Pr3+	9.0	22	(70)
ZnGa2O4:Cr3+	2.8	310	(45)
[EA]2NaCr0.21Al0.79(HCOO)6	2.84	160	this work
Tb0.9Eu0.1(pia)	3.27	300	(71)
Tb0.95Eu0.05(btb)	2.85	14	(72)
Tb0.957Eu0.043cpda	16.0	300	(73)
FIR-8⊂DMASM	2.98	341	(74)

^aH2pia, 5-(pyridin-4-yl)isophthalic acid; H3btb, 3,5-tris(4-carboxyphenyl)benzene; H3cpda, 5-(4-carboxyphenyl)-2,6-pyridinedicarboxylic acid; DMASM, 4-[p-(dimethylamino)styryl]-1-methylpyridinium.

The results show that formate-based hybrid compounds with the perovskite-like architecture [EA]₂NaCr_xAl_1–x(HCOO)₆ are promising materials for luminescence thermometry. The performed analysis shows that low-concentration chromium-doped materials may be promising compounds for highly sensitive and lanthanide-free temperature sensors.

Conclusions

The microwave-assisted solvothermal method was used to successfully synthesize a series of MOFs with the general formula [EA]₂NaCr_xAl_1–x(HCOO)₆, where x = 1, 0.78, 0.57, 0.30, 0.21, and 0. XRD measurements confirmed the phase purity and the ability to obtain mixed structures across the whole concentration range. The Raman spectra confirmed the expected composition of the prepared materials and provided preliminary information on how the phonon properties change as the Cr³⁺ concentration increases. The emission properties of synthesized compounds were found to be strongly temperature-dependent below 250 K. This feature and the coexistence of temperature-sensitive bands, spin-forbidden ²E_g → ⁴A_2g and spin-allowed ⁴T_2g → ⁴A_2g, let one to perform calculations in order to evaluate the possible implementation of investigated materials as luminescence thermometers. The relative and absolute sensitivities of the studied compounds are satisfactory and are comparable to those of known inorganic materials. The highest relative sensitivity (2.84% K^–1, δT = 0.40 K) was achieved for a sample of [EA]₂NaCr_0.21Al_0.79(HCOO)₆ at 160 K. The results also showed that the concentration of Cr³⁺ ions has a big impact on luminescence outputs. The temperature sensing range and temperature of the maximal relative sensitivity can be precisely tuned by modifying the sample composition, substantially increasing the utility of the developed luminescence thermometer. According to the results of the investigation, chromium-based organic–inorganic perovskites could be promising materials for noncontact temperature sensing. The presented practical implementation of the investigated compounds shows the potential of this particular type of sensor. The development of thermosensitive materials containing transition metal ions could pave the way for lanthanide-free, low-cost, and efficient contactless luminescence thermometry solutions.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.2c19957.

• Quantities of precursors; unit cell parameters; crystal field parameters and electron transition energies; time parameters of decays; change of unit cell parameters; XRD; Raman spectra; normalized DRS spectra; band gap determined with the Kubelka–Munk function; excitation–emission spectra; temperature dependence on the luminescence spectra; thermometric stability in 100–160 K intervals for the representative sample; thermal stability and crystal field parameter calculation (PDF)

The authors declare no competing financial interest.

Notes

Experimental data: The Raman and diffuse reflectance spectra, band gap and crystal field strength, decay profiles and lifetimes, temperature-dependent luminescence and emission maps, thermometric parameters, exemplary system’s luminescence characteristics, unit cell characteristics, powder XRD data, Kubelka–Munk function with band gap estimation, excitation and emission spectra, and stability of PL are available at 10.5281/zenodo.7505524.