Solution and Solid-State Optical Properties of Trifluoromethylated 5-(Alkyl/aryl/heteroaryl)-2-methyl- pyrazolo[1,5-a]pyrimidine System

This paper describes the photophysical properties of a series of seven selected examples of 5-(alkyl/aryl/heteroaryl)-2-methyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidines (3), which contain alkyl, aryl, and heteroaryl substituents attached to the scaffolds of 3. Given the electron-donor groups and -withdrawing groups, the optical absorption and emission in the solid state and solution showed interesting results. Absorption UV–Vis and fluorescence properties in several solvents of a pyrazolo[1,5-a]pyrimidines series were investigated, and all derivatives were absorbed in the ultraviolet region despite presenting higher quantum emission fluorescence yields in solution and moderate emission in the solid state. Moreover, the solid-state thermal stability of compounds 3a–g was assessed using thermogravimetric analysis. The thermal decomposition profile showed a single step with almost 100% mass loss for all compounds 3. Additionally, the values of T0.05 are considerably low (72–187 ◦C), especially for compound 3a (72 ◦C), indicating low thermal stability for this series of pyrazolo[1,5-a]pyrimidines.


Introduction
According to the Web of Science, there have been about nine hundred publications on photophysical properties and organic compounds in the last five years [1]. This shows the importance of synthesizing organic compounds with these photophysical characteristics, which have drawn considerable attention and have been widely used in industrial and scientific fields [2].
For many organic molecules to exhibit outstanding photophysical properties, in most cases, a combination of factors is required, which are related mainly to their structural properties. These properties may involve the polarization of the chemical scaffolds due to the presence of electron-donating (EDG) and electron-withdrawing groups (EWG) [3], chain arrangements, and conformations (stereochemistry) [4][5][6], as well as the presence of charge-transfer bands, such as intramolecular charge transfer transitions (ICT) [7,8].
For these reasons, this study sought to evaluate and study, for the first time, the photophysical properties of pyrazolo-pyridimine derivatives, more specifically, the compounds For these reasons, this study sought to evaluate and study, for the first time, the photophysical properties of pyrazolo-pyridimine derivatives, more specifically, the compounds named 5-(alkyl/aryl/heteroaryl)-2-methyl-7-(trifluoromethyl)pyrazolo[1, 5a]pyrimidines (3), where the synthetic approaches have already been mostly described in the literature [13,20,21], although there is still a lack of studies on the absorption and emission properties of these derivatives, both in solution and in the solid state. Given this context, UV-Vis absorption analysis and steady-state fluorescence emission properties, both in liquid and the solid state, will be discussed and studied. Furthermore, the solvent polarity on absorption and emission effects and the thermal stability in the solid state will also be discussed and presented (Scheme 1).

General
Unless otherwise indicated, all common reagents and solvents were used as obtained from commercial suppliers without further purification. The 1 H, 13 C, and NMR spectra were acquired on a Bruker Avance III 600 MHz (3a-g) spectrometer for one-dimensional experiments with 5 mm sample tubes at 298 K and digital resolution of 0.01 ppm in CDCl3 as the solvent, using TMS as the internal reference, and the atoms numbering according to Figure 1. All spectra can be found in the Supplementary Information (Figures S1-S8). All melting points were determined using coverslips on a Microquímica MQAPF-302 apparatus and are uncorrected. The HRMS analyses were performed on a hybrid highresolution and high-accuracy (5 mL L −1 ) micrOTOF-Q mass spectrometer (Bruker Scientifics, Billerica, MA, USA) at Caxias do Sul University (Brazil).

General
Unless otherwise indicated, all common reagents and solvents were used as obtained from commercial suppliers without further purification. The 1 H, 13 C, and NMR spectra were acquired on a Bruker Avance III 600 MHz (3a-g) spectrometer for one-dimensional experiments with 5 mm sample tubes at 298 K and digital resolution of 0.01 ppm in CDCl 3 as the solvent, using TMS as the internal reference, and the atoms numbering according to Figure 1. All spectra can be found in the Supplementary Information (Figures S1-S8). All melting points were determined using coverslips on a Microquímica MQAPF-302 apparatus and are uncorrected. The HRMS analyses were performed on a hybrid high-resolution and high-accuracy (5 mL L −1 ) micrOTOF-Q mass spectrometer (Bruker Scientifics, Billerica, MA, USA) at Caxias do Sul University (Brazil).
Photochem 2022, 2, FOR PEER REVIEW 2 For these reasons, this study sought to evaluate and study, for the first time, the photophysical properties of pyrazolo-pyridimine derivatives, more specifically, the compounds named 5-(alkyl/aryl/heteroaryl)-2-methyl-7-(trifluoromethyl)pyrazolo[1, 5a]pyrimidines (3), where the synthetic approaches have already been mostly described in the literature [13,20,21], although there is still a lack of studies on the absorption and emission properties of these derivatives, both in solution and in the solid state. Given this context, UV-Vis absorption analysis and steady-state fluorescence emission properties, both in liquid and the solid state, will be discussed and studied. Furthermore, the solvent polarity on absorption and emission effects and the thermal stability in the solid state will also be discussed and presented (Scheme 1).

General
Unless otherwise indicated, all common reagents and solvents were used as obtained from commercial suppliers without further purification. The 1 H, 13 C, and NMR spectra were acquired on a Bruker Avance III 600 MHz (3a-g) spectrometer for one-dimensional experiments with 5 mm sample tubes at 298 K and digital resolution of 0.01 ppm in CDCl3 as the solvent, using TMS as the internal reference, and the atoms numbering according to Figure 1. All spectra can be found in the Supplementary Information (Figures S1-S8). All melting points were determined using coverslips on a Microquímica MQAPF-302 apparatus and are uncorrected. The HRMS analyses were performed on a hybrid highresolution and high-accuracy (5 mL L −1 ) micrOTOF-Q mass spectrometer (Bruker Scientifics, Billerica, MA, USA) at Caxias do Sul University (Brazil).

Photophysical Measurements in the Solid State
For the absorption and UV-Vis measurements in the solid state, derivatives 3a-g were treated as powder, and the baseline in the solid state was obtained using a barium sulphate standard (BaSO 4 ; Wako Company ® , Richmond, VA, USA). The diffuse reflectance spectra (DRUV) were measured using an integrating sphere attachment on a Shimadzu UV-2600 spectrophotometer in the 250-700 nm range.
The fluorescence emission spectra in the solid state were measured in the 300-700 nm range using the Horiba Yvon-Jobin Fluoromax Plus (Em/Exc; slit 5.0 mm) instrument. Fluorescence quantum yields (Φ f ) in the solid state were determined by comparing the integrated area to the corrected fluorescence spectrum of compounds with the integrated area to the corrected fluorescence spectrum of a standard compound (in this case, sodium ascorbate − Φ f = 55%), as reported elsewhere [23].
Fluorescence lifetimes in the solid state of related compounds were recorded using the time-correlated single-photon counting (TCSPC) method with DeltaHub controller and Horiba spectrofluorometer. Data were processed with the DAS6 and Origin ® 8.5 software (Northampton, MA, USA) using mono-exponential fitting of raw data. NanoLED (1.0 MHz; pulse width < 1.2 ns; 284 nm excitation wavelength) was used as a source of excitation.

Thermogravimetric Analysis
Thermogravimetric analyses (TGA) were performed using a TGA Q5000 instrument (TA Instruments Inc., New Castle, DE, USA) at a heating rate of 10 • C min −1 , from 40 • C to 600 • C under a N 2 flux of 25 mL min −1 . The masses were approximately 1 mg for all samples. Data analysis was performed using the OriginPro 8.5 software (Northampton MA, USA). The confirmation of calibration of apparatus before analysis was done with Differential scanning calorimetry (DSC) analyses were carried out using a Q2000 DSC calorimeter (TA Instruments, New Castle, DE, USA) equipped with an RCS refrigeration accessory and with N 2 as purge gas (50 mL min −1 ). The heating rate used was 5 • C min −1 . The calibration of instruments in standard DSC mode was verified with indium (99.99%). The masses of the samples (1-5 mg) were weighed on a Sartorius balance (M500P) with a precision of ±0.001 mg. All samples were subjected to three heating-cooling cycles, as follows: 25 to 250 • C.
All products were fully characterized with 1 H NMR and the melting point showed spectral data typical for these compounds and also in agreement with the literature [13,20,21]. Until now, an unpublished compound (3f) was also characterized by 1 H-and 13 C NMR and HRMS. For instance, in the NMR chemical shifts assignment, compound 3f presented a chemical shift at 7.60 ppm at the 1 H NMR spectrum, which was assigned to the pyrimidine H-6; a signal at 6.77 ppm was assigned to the pyrazole H-3, a signal at 2.65 ppm referred to the unique methyl substituent, and a signal at 8.41 and 8.32 ppm was assigned to the p-phenyl substituted aromatic ring. The same compound 3f showed chemical shifts in the 13  All products were fully characterized with 1 H NMR and the melting point showed spectral data typical for these compounds and also in agreement with the literature [13,20,21]. Until now, an unpublished compound (3f) was also characterized by 1 H-and 13 C NMR and HRMS. For instance, in the NMR chemical shifts assignment, compound 3f presented a chemical shift at 7.60 ppm at the 1 H NMR spectrum, which was assigned to the pyrimidine H-6; a signal at 6.77 ppm was assigned to the pyrazole H-3, a signal at 2.65 ppm referred to the unique methyl substituent, and a signal at 8.41 and 8.32 ppm was assigned to the p-phenyl substituted aromatic ring. The same compound 3f showed chemical shifts in the 13 C{ 1 H} NMR spectrum as a singlet at 157.8 (C-2), 152.1 (C-5), 149.1 (C-3a), 102.4 (C-6) and 98.7 (C-3) ppm, and a quartet for C-7 and CF3 group appearing at 134.1 ppm with J = 37.1 Hz and 119.4 ppm with J = 274.8 Hz, respectively, due to the 13 C-19 F scalar coupling.

Solution Analysis
Regarding the photophysical properties of pyrazolo derivatives 3a-g, the photophysical properties of all compounds in different solvent polarities (toluene, CHCl3, CH3CN, THF, EtOH, and DMSO) were analyzed. For exemplification purposes, the spectral profile of derivative 3b in all solvents studied is illustrated in Figure 2, and the absorption parameters of compounds are listed in Table 1; all UV-Vis absorption spectra are listed in the Supplementary Information (Figures S9-S14).
In general, all derivatives showed electronic transition bands in the UV region and can be attributed to π → π* and n → π* type transitions, which are characteristics of this type of heterocyclic and aromatic skeleton, according to the literature [12,23,24,34]. As seen in Figure 2, the derivatives studied show a similar absorption behavior according to the nature of the solvent. Additionally, by analyzing the UV-Vis spectra in the ground state of related compounds, small changes according to the solvent property are also observed, and some spectral changes occur due to the presence of electron-donor oracceptor substituents (Table 1).
By comparing the electronic effect of the substituent on the aromatic moiety (3c-OCH3 and 3f-NO2 units), very subtle shifts can be observed in the other solvents investigated, revealing that there is no significant change in the ground state (Table 1).

Solution Analysis
Regarding the photophysical properties of pyrazolo derivatives 3a-g, the photophysical properties of all compounds in different solvent polarities (toluene, CHCl 3 , CH 3 CN, THF, EtOH, and DMSO) were analyzed. For exemplification purposes, the spectral profile of derivative 3b in all solvents studied is illustrated in Figure 2, and the absorption parameters of compounds are listed in Table 1; all UV-Vis absorption spectra are listed in the Supplementary Information (Figures S9-S14).
In general, all derivatives showed electronic transition bands in the UV region and can be attributed to π → π* and n → π* type transitions, which are characteristics of this type of heterocyclic and aromatic skeleton, according to the literature [12,23,24,34]. As seen in Figure 2, the derivatives studied show a similar absorption behavior according to the nature of the solvent. Additionally, by analyzing the UV-Vis spectra in the ground state of related compounds, small changes according to the solvent property are also observed, and some spectral changes occur due to the presence of electron-donor or -acceptor substituents (Table 1). Regarding fluorescent emission properties, derivatives 3a-g were investigated in the same solvent polarities used in the UV-Vis analysis, and the data regarding the emission peaks (λem), quantum fluorescence yield (QY), and Stokes shifts (SS) are presented in Table  1. The normalized fluorescence emission spectra of derivatives in all solvents are  By comparing the electronic effect of the substituent on the aromatic moiety (3c-OCH 3 and 3f-NO 2 units), very subtle shifts can be observed in the other solvents investigated, revealing that there is no significant change in the ground state (Table 1).
Regarding fluorescent emission properties, derivatives 3a-g were investigated in the same solvent polarities used in the UV-Vis analysis, and the data regarding the emission peaks (λ em ), quantum fluorescence yield (QY), and Stokes shifts (SS) are presented in Table 1. The normalized fluorescence emission spectra of derivatives in all solvents are presented in the Supplementary Information (Figures S15-S20). Regarding the fluorescence lifetime measurements of the derivatives in solution, time-resolved measurements were not made because the proper NanoLED source was unsuitable for this analysis.
In general, derivatives 3a-g have emission bands located in the blue to green range. As with the UV-Vis absorption analysis, compound 3b was chosen as an example, and the fluorescence emission spectra in all solvents and natural/UV light solution photography are listed in Figure 3. According to the spectra in Figure 3c, the solvent polarity does not show any significant changes in the emission peaks of compound 3c. As for compound 3f (containing NO 2 group), more visible changes are observed, mainly in the protic medium ( Supplementary Information-Figure S24). We can attribute this to a difference in the stabilization of the structures in the excited state, primarily in the presence of electronwithdrawing groups and the secondary H-bonding interactions in ethanol solution.  As for the Φf values, the compounds presented higher QYs; this may be associated with a greater stabilization and solvation of these molecules in the singlet excited state (Table 1) and dependence on the substituent electronic property. Finally, moderate to large SS were observed for all derivatives in the solvents studied, and this can be attributed to the vibrational relaxation or dissipation and solvent reorganization, which can decrease the separation of the energy levels of the ground and excited states (Table 1).  As for the Φ f values, the compounds presented higher QYs; this may be associated with a greater stabilization and solvation of these molecules in the singlet excited state (Table 1) and dependence on the substituent electronic property. Finally, moderate to large SS were observed for all derivatives in the solvents studied, and this can be attributed to the vibrational relaxation or dissipation and solvent reorganization, which can decrease the separation of the energy levels of the ground and excited states (Table 1).

Aggregation-Induced Emission Behavior
In a generalized way, the aggregation-induced emission (AIE) phenomenon describes the behavior of a molecule that shows dim or no emission in dilute solution but muchenhanced emission in aggregates or the solid state [35,36]. The fluorescence emission behaviors of the selected compounds 3b, 3c, and 3f were examined in the THF-H 2 O mixture (0-90% water fraction) to confirm the possibility of AIE characteristics. Studied compounds emit a blue to green region under a UV lamp with 365 nm in THF solution ( Figure 3). All fluorescence emission spectra in the THF-H 2 O mixture of compounds 3c and 3f are listed in the Supplementary Information (Figures S23 and S24).
Interestingly, the fluorescence emission of derivatives 3b, 3c, and 3f is sensitive to solvent polarity; thus, we aimed to explore their emission behavior in THF as an aprotic water-miscible solvent. The emission responses of compound 3b upon adding different amounts of water to THF solution is presented in Figure 4. With the increase of water content (0-90% v/v), a great decrease in the emission peak intensities was observed, and the fluorescence intensity as a function of water content showed a slightly bathochromic shift. Tigreros and co-workers previously described similar behavior in a study with pyrazolo derivatives containing a triphenylamine substituent [16]. Thus, the AIE properties were not observed, and this decrease in the emission intensities of derivatives can be directly attributed to an aggregation phenomenon (J-or H-aggregate types) as the water fraction increases. Consequently, this result demonstrates that (trifluoromethyl)pyrazolo-based probes can act as possible fluorescent sensors for small amounts of acid or protic molecules.
Photochem 2022, 2, FOR PEER REVIEW 9 In a generalized way, the aggregation-induced emission (AIE) phenomenon describes the behavior of a molecule that shows dim or no emission in dilute solution but much-enhanced emission in aggregates or the solid state [35,36]. The fluorescence emission behaviors of the selected compounds 3b, 3c, and 3f were examined in the THF-H2O mixture (0-90% water fraction) to confirm the possibility of AIE characteristics. Studied compounds emit a blue to green region under a UV lamp with 365 nm in THF solution ( Figure 3). All fluorescence emission spectra in the THF-H2O mixture of compounds 3c and 3f are listed in the Supplementary Information (Figures S23 and S24).
Interestingly, the fluorescence emission of derivatives 3b, 3c, and 3f is sensitive to solvent polarity; thus, we aimed to explore their emission behavior in THF as an aprotic water-miscible solvent. The emission responses of compound 3b upon adding different amounts of water to THF solution is presented in Figure 4. With the increase of water content (0-90% v/v), a great decrease in the emission peak intensities was observed, and the fluorescence intensity as a function of water content showed a slightly bathochromic shift. Tigreros and co-workers previously described similar behavior in a study with pyrazolo derivatives containing a triphenylamine substituent [16]. Thus, the AIE properties were not observed, and this decrease in the emission intensities of derivatives can be directly attributed to an aggregation phenomenon (J-or H-aggregate types) as the water fraction increases. Consequently, this result demonstrates that (trifluoromethyl)pyrazolo-based probes can act as possible fluorescent sensors for small amounts of acid or protic molecules.

Solid-State Analysis-First Evidences
A solid-state absorption and fluorescence emission spectroscopy analysis in powder was performed as the (trifluoromethyl)-pyrazolo derivatives 3a-g present fluorescence emission in the solid state. The reflectance spectra of the compounds revealed similar absorption peaks compared to the solution study, in which we observed the broadening of the absorption bands ( Supplementary Information; Figure S25).
The fluorescence emission data of derivatives 3a-g in the solid state are listed in Table 2, and all spectra are presented in Figure 5a. Thus, compared to the spectra in solution, the derivatives presented emission peaks very close to the values obtained in organic solvents ( Table 1). The variations in emission peaks observed in the solid state can be attributed to a change in the molecular arrangement in the absence of the solvent, which may be favored by π-π stacking interactions. The QY values observed in the solid state for derivatives 3a-g are smaller than those observed in the solution, which may be directly related to the solid-state arrangement. a Excitation at a less-energy absorption peak using sodium salicylate as standard (Φ f = 55%); b Stokes shifts: ∆λ = λ em − λ abs = 1/λ abs − 1/λ em ; c Using excitation by NanoLED source at 284 nm; d,e Determined by [23].

Solid-State Analysis-First Evidences
A solid-state absorption and fluorescence emission spectroscopy analysis in powder was performed as the (trifluoromethyl)-pyrazolo derivatives 3a-g present fluorescence emission in the solid state. The reflectance spectra of the compounds revealed similar absorption peaks compared to the solution study, in which we observed the broadening of the absorption bands (Supplementary Information; Figure S25).
The fluorescence emission data of derivatives 3a-g in the solid state are listed in Table  2, and all spectra are presented in Figure 5a. Thus, compared to the spectra in solution, the derivatives presented emission peaks very close to the values obtained in organic solvents ( Table 1). The variations in emission peaks observed in the solid state can be attributed to a change in the molecular arrangement in the absence of the solvent, which may be favored by π-π stacking interactions. The QY values observed in the solid state for derivatives 3a-g are smaller than those observed in the solution, which may be directly related to the solid-state arrangement. Compared with the solution study, solid-state fluorescence lifetime measurements were conducted, and lifetime decay plots and the τf, radiative (kr) and non-radiative (knr) values for derivatives 3a-g are presented in Figure 5b and Table 2, respectively. It is possible to note a variation in the τf values according to the electronic nature of the molecule, which is attributed to the non-influence of the solvent in the excited state and a greater ordering of the molecules in the solid state (Table 2). In addition, we can evidence a decrease in the radiative (kr) rates with an increase in the non-radiative (knr) rates, and this is probably evidenced by a relaxation of the vibrational levels of the molecules and restricted motion.

Thermal Stability in the Solid State
The solid-state thermal stability of compounds 3a-g was accessed using TGA, and the results are summarized in Table 3, where T0.05 expresses the temperature at which 5.0% of mass loss occurred and Td is the temperature of maximum decomposition rate (i.e., the peak of the derivative curve). The order of thermal stability was established in terms of Compared with the solution study, solid-state fluorescence lifetime measurements were conducted, and lifetime decay plots and the τ f , radiative (k r ) and non-radiative (k nr ) values for derivatives 3a-g are presented in Figure 5b and Table 2, respectively. It is possible to note a variation in the τ f values according to the electronic nature of the molecule, which is attributed to the non-influence of the solvent in the excited state and a greater ordering of the molecules in the solid state (Table 2). In addition, we can evidence a decrease in the radiative (k r ) rates with an increase in the non-radiative (k nr ) rates, and this is probably evidenced by a relaxation of the vibrational levels of the molecules and restricted motion.

Thermal Stability in the Solid State
The solid-state thermal stability of compounds 3a-g was accessed using TGA, and the results are summarized in Table 3, where T 0.05 expresses the temperature at which 5.0% of mass loss occurred and T d is the temperature of maximum decomposition rate (i.e., the peak of the derivative curve). The order of thermal stability was established in terms of T 0.05 as follows: 3a < 3b < 3d < 3e < 3g < 3c < 3f. The TGA curves for compounds 3a, 3e, and 3f are presented in Figure 6, and the other results, including DSC/TGA/DTG curves for compounds 3b and 3d, are shown in the Supplementary Information (Figures S26-S33). It is possible to note from Figure 6 and the other curves that the thermal decomposition occurs in a single step with almost 100% of mass loss. Additionally, the values of T 0.05 are considerably low, especially for compound 3a, indicating low thermal stability for this series of pyrazolo[1,5-a]pyrimidines. Regarding T 0.05 and structure relations, no direct correspondence between molar masses and thermal stability was observed for the entire series. More important than the molar mass of the compounds was the nature of the R substituent. Nonetheless, more detailed explanations for the observed order of thermal stability would require further analysis. From the values of T 0.05 in Table 3 and the melting temperatures of compounds 3a-g, it is worth noticing that the majority of the compounds presented considerable mass loss (5%) below their melting point, narrowing possible applications to the solid state. T0.05 as follows: 3a < 3b < 3d < 3e < 3g < 3c < 3f. The TGA curves for compounds 3a, 3e, and 3f are presented in Figure 6, and the other results, including DSC/TGA/DTG curves for compounds 3b and 3d, are shown in the Supplementary Information (Figures S26-S33). It is possible to note from Figure 6 and the other curves that the thermal decomposition occurs in a single step with almost 100% of mass loss. Additionally, the values of T0.05 are considerably low, especially for compound 3a, indicating low thermal stability for this series of pyrazolo[1,5-a]pyrimidines. Regarding T0.05 and structure relations, no direct correspondence between molar masses and thermal stability was observed for the entire series. More important than the molar mass of the compounds was the nature of the R substituent. Nonetheless, more detailed explanations for the observed order of thermal stability would require further analysis. From the values of T0.05 in Table 3 and the melting temperatures of compounds 3a-g, it is worth noticing that the majority of the compounds presented considerable mass loss (5%) below their melting point, narrowing possible applications to the solid state.

Conclusions
The synthesis in yields of 50-98% and photophysical behavior of a series of seven examples of 5-(alkyl/aryl/heteroaryl)-substituted 2-methyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine core (3) was achieved, where one new compound (3f) was obtained and fully structurally characterized. The optical properties in solution and the solid state of this geminated system 3 were also successfully investigated. In the photophysical evaluation of the molecules, transition bands were observed in the UV region, and moderate to higher values in the quantum fluorescence yields for the derivatives 3a-g. Regarding the solvent polarity variation, the changes vary

Conclusions
The synthesis in yields of 50-98% and photophysical behavior of a series of seven examples of 5-(alkyl/aryl/heteroaryl)-substituted 2-methyl-7-(trifluoromethyl)pyrazolo[1,5a]pyrimidine core (3) was achieved, where one new compound (3f) was obtained and fully structurally characterized. The optical properties in solution and the solid state of this geminated system 3 were also successfully investigated. In the photophysical evaluation of the molecules, transition bands were observed in the UV region, and moderate to higher values in the quantum fluorescence yields for the derivatives 3a-g. Regarding the solvent polarity variation, the changes vary according to the electronic nature of the molecules evaluated in the presence or absence of the substituent. Furthermore, photophysical analysis in the solid state and AIE phenomena were also evaluated. For this series of pyrazolo[1,5a]pyrimidines, regarding T 0.05 and structure relations, no direct correspondence between molar masses and thermal stability was observed for the entire series. Additionally, it is worth noticing that most of the compounds presented considerable mass loss (5%) below their melting point, narrowing possible applications to the solid state.