Structure, Luminescence and Temperature Detection Capability of [C(NH2)3]M(HCOO)3 (M = Mg2+, Mn2+, Zn2+) Hybrid Organic–Inorganic Formate Perovskites Containing Cr3+ Ions

Metal-organic frameworks are of great interest to scientists from various fields. This group also includes organic–inorganic hybrids with a perovskite structure. Recently their structural, phonon, and luminescent properties have been paid much attention. However, a new way of characterization of these materials has become luminescence thermometry. Herein, we report the structure, luminescence, and temperature detection ability of formate organic–inorganic perovskite [C(NH2)3]M(HCOO)3 (Mg2+, Mn2+, Zn2+) doped with Cr3+ ions. Crystal field strength (Dq/B) and Racah parameters were determined based on diffuse reflectance spectra. It was shown that Cr3+ ions are positioned in the intermediate crystal field or close to it with a Dq/B range of 2.29–2.41. The co-existence of the spin-forbidden and spin-allowed transitions of Cr3+ ions enable the proposal of an approach for remote readout of the temperature. The relative sensitivity (Sr) can be easily modified by sample composition and Cr3+ ions concentration. The luminescent thermometer based on the 2E/4T2g transitions has the relative sensitivity Sr of 2.08%K−1 at 90 K for [C(NH2)3]Mg(HCOO)3: 1% Cr3+ and decrease to 1.20%K−1 at 100 K and 1.08%K−1 at 90 K for Mn2+ and Zn2+ analogs, respectively.

Among various materials, the perovskite-like metal-organic frameworks (MOFs) containing formate anions (HCOO − ) exhibit unique features, such as ferroelectricity, multiferroicity, and luminescence [1,10,15]. Particularly, the group of Cr-based materials shows strong luminescence and weak concentration quenching [2,10,15]. Nevertheless, temperature-dependent luminescence is one of the most outstanding phenomena. The temperature change induces the change in energy level populations, which makes formatebased compounds containing Cr 3+ ions sufficient materials for non-contact temperature sensing [10].
The optical properties of the transition metals (TM), including chromium trivalent ions, can be affected by the crystal field (CF) strength [13,14,16]. The change in the CF strength

Structural Properties
The phase purity of all samples was confirmed by the XRD patterns with a simulation of the single-crystal structural data of [GA]Mn 1−x Cr x (HCOO) 3 (Figure 1). The samples with Mn 2+ and Zn 2+ crystallized in the orthorhombic Pnna crystal structure [31], and the details of the crystal structure of analogs with Mg 2+ remain unknown. In general, the formate in-connected MnO 6 framework crates cavities occupied by GA + cations (see Figure 2). The right-shifting of the diffraction lines was observed due to the partial replacement of Mn 2+ (CR = 81 Å), Mg 2+ (CR = 86 Å), and Zn 2+ ions (CR = 88 Å) by Cr 3+ ions (CR = 75.5 Å). The crystal radius (CR) was obtained from Shannon [32]. No additional phases were detected, which indicates that the Cr 3+ ions were substituted by the cation M.
The Raman spectra of the [GA]M 1−x Cr x (HCOO) 3 series, where M = Mg 2+ , Mn 2+ , Zn 2+ , and x = 0, 0.01, 0.03, 0.05, are marked in Figure 3a as GAMg, GAMn, and GAZn, respectively. All spectra are very similar and are consistent with the reported orthorhombic Pnna symmetry of all crystals [31,[33][34][35]. However, some differences can be seen in the band shifts and the number of components, which are due to the different sizes, masses, and electronegativity of the metal cations that build the crystals. All these parameters affect the sizes of unit cells, causing Raman bands for [GA]M(HCOO) 3 3 , are also observed for NH stretching vibrations above 2850 cm −1 (Figure 3d), which further indicate the weakest hydrogen bonds in the [GA]Mg(HCOO) 3 crystal and stronger for [GA]Zn(HCOO) 3 . The upshift of 7.1 cm −1 when Zn 2+ ions are replaced by Mg 2+ was evidenced by bands associated with vibrations of oxygen atoms directly coordinated by metal ions, i.e., ν 2 + ν 5 that have been assigned to symmetric C-O stretching vibrations coinciding with C-H in-plane bending modes, respectively (Figure 3b) [36]. A much weaker upshift is observed for the stretching    Figure 3a as GAMg, GAMn, and GAZn, respectively. All spectra are very similar and are consistent with the reported orthorhombic Pnna symmetry of all crystals [31,[33][34][35]. However, some differences can be seen in the band shifts and the number of components, which are due to the different sizes, masses, and electronegativity of the metal cations that build the crystals. All these parameters affect the sizes of unit cells, causing Raman bands for   3 based on data presented in [31]. The dashed lines present HBs between GA + cations and the manganese-formate framework.

Optical Properties and Temperature Detection
The diffuse reflectance spectra (DRS) of representative samples [GA]M1−xCrx(HCOO)3, where M = Mg 2+ , Mn 2+ , Zn 2+ , and x = 0.05, are shown in Figure 4. The intensity of the DRS spectrum is influenced by many factors, such as the size and position of crystallites [10]. Therefore, the DRS is used only for characterizing the localization of the energy levels of Cr 3+ ions in each compound and the effect of the concentration of Cr 3+ ions on the spectrum's shape. Two primary broad bands localized around 16,828 cm −1 (594 nm) and 22,522 cm −1 (444 nm) for Zn-samples, 17,130 cm −1 (583.8 nm) and 23,162 cm −1 (431.7 nm) for Mg-samples, 17,050 cm −1 (586.5 nm) and 23,162 cm −1 (431.7 nm) for Mn-samples can be distinguished in Figure 3. These two bands are assigned to the spin-allowed transitions 4 A2g → 4 T1g and 4 A2g → 4 T2g of Cr 3+ ions. In addition, a very weak and sharp peak centered at approximately 14,550 cm −1 (687.3 nm) is associated with the spin-forbidden transition from the 4 A2g ground state to the 2 E excited level. It was found that when the concentration of Cr 3+ increases, the position of the 4 A2g → 2 E lines slightly changes ( Figures S1 and S2). However, for the Zn-compounds, the 4 A2g → 2 E absorption peak is invisible ( Figure S3).
Noticeably, in the spectrum of Mn-samples, very weak and sharp peaks appeared at 29 240 cm −1 (342 nm) and 24 570 cm −1 (407 nm), which are attributed to the absorption of The introduction of Cr 3+ ions into the crystal structure of [GA]M(HCOO) 3 (M = Mg 2+ , Mn 2+ , Zn 2+ ) at such low concentrations causes very subtle effects on the spectra, not exceeding 1 cm −1 . This confirms that aliovalent doping up to 5 mol% does not cause significant structural changes in the orthorhombic Pnna structure.

Optical Properties and Temperature Detection
The diffuse reflectance spectra (DRS) of representative samples [GA]M 1−x Cr x (HCOO) 3 , where M = Mg 2+ , Mn 2+ , Zn 2+ , and x = 0.05, are shown in Figure 4. The intensity of the DRS spectrum is influenced by many factors, such as the size and position of crystallites [10]. Therefore, the DRS is used only for characterizing the localization of the energy levels of Cr 3+ ions in each compound and the effect of the concentration of Cr 3+ ions on the spectrum's shape. Two primary broad bands localized around 16,828 cm −1 (594 nm) and  Figure 3. These two bands are assigned to the spin-allowed transitions 4 A 2g → 4 T 1g and 4 A 2g → 4 T 2g of Cr 3+ ions. In addition, a very weak and sharp peak centered at approximately 14,550 cm −1 (687.3 nm) is associated with the spin-forbidden transition from the 4 A 2g ground state to the 2 E excited level. It was found that when the concentration of Cr 3+ increases, the position of the 4 A 2g → 2 E lines slightly changes ( Figures S1 and S2). However, for the Zn-compounds, the 4 A 2g → 2 E absorption peak is invisible ( Figure S3).  The crystal field Dq, Racah, B, and C parameters were calculated for Cr 3+ -doped samples (see Table 1) by using the same methodology as presented in reference [10]. Crystal field strength (CFS) Dq/B parameter is in the range of 2.29-2.39 for GAMn and 2.23-2.41 for GAMg samples. These results mean that Cr 3+ ions are located in the intermediate ligand field, and energy separation between 2 E and 4 T2g excited levels is not significant. The Dq/B parameter is slightly higher, around 2.41-2.43 for CAZn analogs. The calculated values of Dq/B are similar to those reported recently for DMANaCr (2.29) [15]. However, for some EA and DMA analogs (EANaCr 2.18 [7], EANaAlCr 2.21 [7], DMAKCr 2.21 [37], EAKCr 2.21 [37]) reported Dq/B values are much lower than for the investigated perovskites. On the other hand, the formate perovskites with AM + cation comprising Cr 3+ ions exhibit a strong crystal field (AMNaCr 2.743 [38], AMNaAlCr 2.55 [38]). The emission spectra of investigated hybrid organic-inorganic formates [GA]M1−xCrx(HCOO)3 (M = Mg 2+ , Mn 2+ , Zn 2+ , and x = 0.01, 0.03, 0.05) recorded at 80 K consists of the intense and narrow emission lines of Cr 3+ ions located at 686 nm and 698 nm attributed to the spin-forbidden 2 E → 4 A2g transitions ( Figure 5). The broad emission band, Noticeably, in the spectrum of Mn-samples, very weak and sharp peaks appeared at 29,240 cm −1 (342 nm) and 24,570 cm −1 (407 nm), which are attributed to the absorption of Mn 2+ ions from 6 A 1 ground state to 4 E, 4 T 2 , and 4 A 1 , 4 E excited levels, respectively. The intensity of these bands decreases as the content of Cr 3+ increases ( Figures S1-S3).
In addition, the intense band located at around 46,729 cm −1 (214 nm) can be assigned to host absorption, and it moved to 44,643 cm −1 (224 nm) for the Mn-based sample. What is more, the bad is much broader because of overlapping with the Mn-O charge transfer band (CTB) ( Figure S4).
The crystal field Dq, Racah, B, and C parameters were calculated for Cr 3+ -doped samples (see Table 1) by using the same methodology as presented in reference [10] 2+ , and x = 0.01, 0.03, 0.05) recorded at 80 K consists of the intense and narrow emission lines of Cr 3+ ions located at 686 nm and 698 nm attributed to the spin-forbidden 2 E → 4 A 2g transitions ( Figure 5). The broad emission band, which spans from 700 nm to 1000 nm, assigned to the spin-allowed transition from the 4 T 2g excited level to the 4 A 2g ground state is also observed [11,13,16,39]. As can be seen in Figures 5b,d and S5-S7, the emission intensity of GAMg and GAMn samples increased with the concentration of dopant ions, while the intensity of 1% Cr 3+ and 5% Cr 3+ in the GAZn analog are comparable. The samples with 3% of Cr 3+ are out of the trend. However, the nature of this behavior is unspecified. The collation of the representative samples [GA]M 1−x Cr x (HCOO) 3 (M = Mg 2+ , Mn 2+ , Zn 2+ , and x = 0.05) showed that the most intense luminescence exhibits a sample comprising Mg 2+ ions. The emission intensity of Mn 2+ and Zn 2+ samples is significantly less. The substitution of different metal M 2+ ions in the crystal structure of guanidine formate have an impact on the intensity relationships between spin-forbidden and spin-allowed transition of Cr 3+ ions. Only for the GAMg compound 2 E → 4 A 2g is emission more intense than spin-allowed transition; for GAMn and GAZn analogs, 4 T 2g → 4 A 2g transition dominates. It is worth noting that no emission of Mn 2+ ions was detected, probably due to energy reabsorption by chromium ions. the crystal structure of guanidine formate have an impact on the intensity relatio between spin-forbidden and spin-allowed transition of Cr 3+ ions. Only for the GAM pound 2 E → 4 A2g is emission more intense than spin-allowed transition; for GAM GAZn analogs, 4 T2g → 4 A2g transition dominates. It is worth noting that no emis Mn 2+ ions was detected, probably due to energy reabsorption by chromium ions. The emission spectra in the function of temperature were recorded within the of 80-300 K with 10 K steps. As can be seen in Figure 6 and Figure S8, the main comp of the photoluminescence spectra belongs to the spin-allowed transitions of Cr 3 Only for the GAMg sample containing 1% dopant, the 2 E emission is much more i The emission spectra in the function of temperature were recorded within the range of 80-300 K with 10 K steps. As can be seen in Figures 6 and S8, the main component of the photoluminescence spectra belongs to the spin-allowed transitions of Cr 3+ ions. Only for the GAMg sample containing 1% dopant, the 2 E emission is much more intense than the band located at 795 nm. Generally, 2 E → 4 A 2g emission quenches significantly with increasing temperature, while the 4 T 2g → 4 A 2g emission of Cr 3+ is more stable. It is due to the thermally stimulated energy transfer from 2 E to 4 T 2g energy level. Obtained results confirmed the occurrence of the intermediate ligand field in the nearest environment of Cr 3+ ions. The mechanism of Cr 3+ luminescence quenching is a well-known phenomenon in the literature and assumes crossing the 4 T 2g excited state parabola with the 4 A 2g one [11,13,16,39]. The significant dependence of photoluminescence intensity on temperature may be an interesting behavior that can be exploited for non-contact temperature readout based on luminescence. Figure 7 demonstrates a schematic representation of the approach to temperature determination. In this model, the Fluorescence Intensity Ratio (FIR) parameter can be defined as a ratio of the 2 E → 4 A2g (spectral range 670-710 nm marked as I1) to the 4 T2g → 4 A2g (spectral range 750-1050 nm represented as I2) transition of Cr 3+ ions, respectively. The significant dependence of photoluminescence intensity on temperature may be an interesting behavior that can be exploited for non-contact temperature readout based on luminescence. Figure 7 demonstrates a schematic representation of the approach to temperature determination. In this model, the Fluorescence Intensity Ratio (FIR) parameter can be defined as a ratio of the 2 E → 4 A 2g (spectral range 670-710 nm marked as I 1 ) to the 4 T 2g → 4 A 2g (spectral range 750-1050 nm represented as I 2 ) transition of Cr 3+ ions, respectively. on luminescence. Figure 7 demonstrates a schematic representation of the appr temperature determination. In this model, the Fluorescence Intensity Ratio (FIR) p ter can be defined as a ratio of the 2 E → 4 A2g (spectral range 670-710 nm marked the 4 T2g → 4 A2g (spectral range 750-1050 nm represented as I2) transition of Cr 3+ i spectively.   (Figure 8), and the highest value of FIR was obtained for the GAMg: 1% Cr 3+ sample. To further comparison of the observed changes in thermometric parameters and to compare their features, the absolute (S a ) and relative (S r ) sensitivities were calculated as follows: and where dFIR represents the change of fluorescence intensity ratio at temperature change ∆T.
The collation of S a and S r changes of the investigated hybrid organic-inorganic perovskites are presented in Figures 9 and S9. Generally, the S a and S r values are the highest at the 80-120 K range and decrease with increasing temperature. However, the sensitivity changes with sample composition and concentration of Cr 3+ ions. For GAMg: Cr 3+ compounds, the significant relative sensitivity exhibits sample with the lowest concentration of dopant ions and equals 2.08%K −1 at 90 K. With increasing Cr 3+ ions concentration, the S r decreased to around 1%K −1 . Substitution of Mg 2+ by Mn 2+ caused a decrease of sensitivity to 1.20%K −1 , but the optimal Cr 3+ ions concentration was determined to be 3%. Similar trends are observed for GAZn: for Cr 3+ analogs, however, the changes of S r with chromium ions concentration are negligible, and the highest S r is 1.08%K −1 at 90 K for GAZn: 1% Cr 3+ . Additionally, the repeatability of the thermal sensing performance of representative samples was verified by the circling heat/cool process ( Figure S10). It can be seen that only a small variation from the initial value was observed, and the temperature parameter ∆ is reversed and repeated overheating/cooling cycles. ilar trends are observed for GAZn: for Cr 3+ analogs, however, the changes of Sr with chromium ions concentration are negligible, and the highest Sr is 1.08%K −1 at 90 K for GAZn: 1% Cr 3+ . Additionally, the repeatability of the thermal sensing performance of representative samples was verified by the circling heat/cool process ( Figure S10). It can be seen that only a small variation from the initial value was observed, and the temperature parameter ∆ is reversed and repeated overheating/cooling cycles.   ilar trends are observed for GAZn: for Cr 3+ analogs, however, the changes of Sr with chromium ions concentration are negligible, and the highest Sr is 1.08%K −1 at 90 K for GAZn: 1% Cr 3+ . Additionally, the repeatability of the thermal sensing performance of representative samples was verified by the circling heat/cool process ( Figure S10). It can be seen that only a small variation from the initial value was observed, and the temperature parameter ∆ is reversed and repeated overheating/cooling cycles.   In fact, only one optical thermometer based on hybrid organic-inorganic formate perovskites [EA] 2 NaCr 0.21 Al 0.79 (HCOO) 6 with a sensitivity S r of 2.84%K −1 at 160 K is known [10]. Obtained values of relative sensitivities were compared with the S r values of other inorganic and hybrid organic-inorganic luminescent thermometers ( Table 2). The results show that investigated [GA]M 1−x Cr x (HCOO) 3 (M = Mg 2+ , Mn 2+ , Zn 2+ , and x = 0.01, 0.03, 0.05) has the potential to be applied as a low-temperature luminescent thermometer. Table 2. Collation of exemplary luminescent thermometers with their highest relative sensitivity (S r ) at working temperature (T) 1 .

Conclusions
Three series of samples [C(NH 2 ) 3 ]M(HCOO) 3 (Mg 2+ , Mn 2+ , Zn 2+ ) doped with 1%, 3%, and 5% of Cr 3+ ions were synthesized using the low-diffusion synthesis method. Their structural, phonon, and luminescent properties were investigated in detail. It was shown that the incorporation of Cr 3+ ions into the crystal structure of investigated hybrid organicinorganic perovskites does not affect the phase purity of the samples. Based on diffuse reflectance spectra, crystal field strength (Dq/B) and Racah parameters were determined. It was found that Cr 3+ ions are located in the intermediate crystal field or close to it with a Dq/B range of 2.29-2.41. The investigation of sample composition showed that the highest emission intensity exhibits GAMg: 5% Cr 3+ sample, while the lowest one GAZn: 5% Cr 3+ . The presence of both the spin-forbidden and spin-allowed transitions of Cr 3+ ions at a broad temperature range enables the characterization of these materials as luminescence thermometers. It turned out that the relative sensitivity of S r depends on the sample composition and concentration of Cr 3+ ions. The highest relative sensitivity S r = 2.08%K −1 at 90 K has [GA]Mg(HCOO) 3