On the Vapor Pressures, Phase Transitions, and Solid‐State Fluorescence of 2‐(2‐Hydroxyphenyl)benzoxazole and 2‐(2‐Hydroxyphenyl)benzothiazole

Abstract

Benzazole derivatives exhibit distinctive photophysical behavior due to excited‐state intramolecular proton transfer (ESIPT), making them promising candidates for optoelectronic applications such as organic light‐emitting diodes (OLEDs) and fluorescent sensors. Understanding their sublimation energetics, phase behavior, and emissive properties is essential for both fundamental studies and materials design. This article reports an investigation on two benzazole derivatives—2‐(2‐hydroxyphenyl)benzothiazole and 2‐(2‐hydroxyphenyl)benzoxazole (HBO)—through studies of thermal analysis, vapor pressure measurements, and fluorescence spectroscopy to establish structure–property relationships. Thermal stability and phase transitions are characterized using simultaneous thermogravimetry‐differential scanning calorimetry (TG‐DSC) and heat‐flux DSC. Vapor pressures are determined using both Knudsen effusion mass loss and mass spectrometry. The derived standard molar enthalpies of sublimation, vaporization, and fusion highlight the presence of heteroatom (S versus O) on intermolecular interactions. Solid‐state fluorescence measurements reveal strong emission in both compounds, with a large Stokes shift—consistent with ESIPT—and complex spectra attributed to solid‐state molecular packing. This comprehensive experimental strategy delivers benchmark thermodynamic and photophysical data, offering new insights into the interplay between molecular structure, thermal behavior, and fluorescence of benzazole derivatives. Such understanding is relevant for the development of advanced optoelectronic materials.

Benzazoles 2‐(2‐hydroxyphenyl)benzothiazole and 2‐(2‐hydroxyphenyl)benzoxazole exhibit tunable optoelectronic properties via excited‐state intramolecular proton transfer. Combining thermogravimetry (TG‐DSC), vapor pressure measurements, and fluorescence spectroscopy, how sulfur (S) versus oxygen (O) substitution dictates their stability, sublimation energetics, and solid‐state emission is revealed—guiding design for organic light‐emitting diodes and sensors.

1. Introduction

Proton transfer reactions—both intramolecular and intermolecular—play a fundamental role in chemical and biochemical processes, including acid–base neutralization and enzymatic reactions.^{[ 1} , ^{2 ]} Often described as the most general and important reactions in chemistry,^{[ 3 ]} these processes are particularly relevant in organic molecules containing hydrogen donor and acceptor groups in close proximity, where intramolecular hydrogen bonds (IHBs) readily form in the ground state. Among these, hydroxyl groups interacting with nitrogen‐, sulfur‐, or oxygen‐containing acceptors are especially common.^{[ 2} , ^{3 ]} In some systems, upon electronic excitation, charge redistribution can trigger a rapid structural reorganization known as electronically excited‐state intramolecular proton transfer (ESIPT).^{[ 2} , ^{4 ]}

ESIPT has attracted considerable scientific interest due to its distinct photophysical properties, such as an unusually large Stokes shift (6000–12 000 cm⁻¹), absence of self‐reabsorption, and efficient population inversion in the proton‐transferred keto form. Its ultrafast nature (femtosecond timescale) allows the process to occur even in the solid‐state.^{[ 5 ]}

Benzazole derivatives constitute a class of ESIPT‐capable organic compounds with a significant potential for optoelectronic applications.^{[ 6} , ⁷ , ^{8 ]} Understanding their volatility,^{[ 9 ]} phase transitions,^{[ 10} , ^{11 ]} and emissive properties^{[ 12 ]} is crucial for their use in organic light‐emitting diodes (OLEDs),^{[ 13} , ¹⁴ , ¹⁵ , ^{16 ]} fluorescent sensors,^{[ 17} , ¹⁸ , ¹⁹ , ²⁰ , ^{21 ]} and other optoelectronic devices, as well as for guiding their fabrication.^{[ 22} , ^{23 ]} In this study, two structurally related benzazoles are investigated: 2‐(2‐hydroxyphenyl)benzothiazole (HBT) and 2‐(2‐hydroxyphenyl)benzoxazole (HBO) whose structural formulas are shown in Table  1 .

Table 1.

Comprehensive information on the studied compounds.

Acronym	Structural formula	CAS
HBO	2‐(2‐hydroxyphenyl)benzoxazole	835‐64‐3
HBT	2‐(2‐hydroxyphenyl)benzothiazole	3411‐95‐8

While the fluorescence of HBO^{[ 24} , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ^{29 ]} and HBT^{[ 30} , ³¹ , ³² , ³³ , ^{34 ]} based fluorophores, such as works by Massue et al.,^{[ 35} , ³⁶ , ³⁷ , ^{38 ]} has been extensively studied in solution, solid‐state investigations remain limited.^{[ 39} , ^{40 ]} Moreover, it is well established that impurities and polymorphism can affect fluorescence behavior and quantum yields.^{[ 41 ]} Yet, these factors are often overlooked in solid‐state fluorescence studies. The present work therefore aims to explore the fluorescence of high‐purity HBO and HBT samples under controlled conditions.

Although some thermodynamic studies have been published on related compounds,^{[ 42} , ⁴³ , ⁴⁴ , ⁴⁵ , ⁴⁶ , ^{47 ]} to the best of our knowledge, only one work available in literature addresses the thermodynamics of HBO and HBT. The work by Silva et al.^{[ 48 ]} focused on the thermochemistry of four phenylbenzazoles using experimental and computational methods, including the determination of sublimation and formation enthalpies. This data is relevant for characterizing of their intra and intermolecular interactions and to evaluate their overall thermochemical behavior. Additionally, it also analyzed the energetic effects associated with the presence of a hydroxyl group on the 2‐phenylbenzazole core, particularly regarding its influence on intramolecular H‐bonding.

However, while the vapor pressures and sublimation thermodynamics of HBO were thoroughly investigated using Calvet microcalorimetry (CM) and Knudsen effusion mass loss (KEML), data for HBT remain incomplete, relying solely on Calvet microcalorimetry. In this context, this work seeks to expand the thermophysical understanding of HBO and HBT by investigating their crystal–crystal (cr‐cr), crystal–liquid (crystal‐l), and liquid–gas (l‐g) phase transitions, complementing existing vapor pressure data, and correlating these properties with their solid‐state fluorescence. Using a recently developed multitechnique approach,^{[ 49 ]} this study provides a comprehensive, self‐validating thermophysical and photophysical characterization of benzazole derivatives, bridging gaps in both fundamental and applied research.

2. Results and Discussion

2.1. Crystal–Crystal and Crystal–Liquid Phase Transitions

Differential scanning calorimetry (DSC) was used to determine the temperatures, enthalpies, and entropies of fusion of both compounds as well as to identify any possible crystal‐crystal phase transitions.

For HBO, two sets of DSC experiments were performed at different heating and cooling rates. The first series, conducted at 10 K min⁻¹, aimed to assess the presence or absence of any cr‐cr transitions. Across two complete heating and cooling cycles from ≈−80 up to 150 °C, only two thermal events were observed—fusion and crystallization. Complementary, powder X‐ray diffraction (PXRD) analysis (Figure S1, Supporting Information) confirmed that the crystalline structure corresponds to the one reported in literature.^{[ 50 ]} The DSC curves are shown in Figure  1 . Subsequently, a series of essays at 2 K min⁻¹ was performed to determine the fusion parameters. These are reported in Table  2 along with the standard molar enthalpy of fusion at 298 K.

Figure 1.

Heating and cooling DSC curves of HBO at 10 K min⁻¹ under a nitrogen flow.

Table 2.

Thermodynamic parameters of fusion of HBO (cr) and HBT (cr, form II) determined via DSC.

Compound	T fus a) [K]	${\Delta }_{\text{cr}}^{\mathrm{l}}{\mathit{\text{H}}}_{\mathrm{m}}^{°}{⟨\hspace{0.17em}\mathit{\text{T}}}_{\text{fus}}⟩$ a) [kJ mol−1]	${\Delta }_{\text{cr}}^{\mathrm{l}}{\mathit{\text{S}}}_{\mathrm{m}}^{°}{⟨\hspace{0.17em}\mathit{\text{T}}}_{\text{fus}}⟩$ b) [J K−1 mol−1]	${\Delta }_{\text{cr}}^{\mathrm{l}}{\mathit{\text{H}}}_{\mathrm{m}}^{°}$[298.15 K]c) [kJ mol−1]
HBO	396.7 ± 0.9	24.3 ± 0.3	61.2 ± 0.9	19.4 ± 1.5
HBT	403.9 ± 0.9	25.7 ± 0.3	63.7 ± 0.9	20.3 ± 1.7

^a)The quoted uncertainty corresponds to twice the combined standard deviation which includes the calibration.

^b)The quoted uncertainty corresponds to the combination of the uncertainties associated with the temperature and enthalpy measurements.

^c)The experimental enthalpies of fusion ${\Delta }_{\text{cr}}^{\mathrm{l}}{H}_{\mathrm{m}}^{\mathrm{o}}$ measured at T _fus were adjusted to T = 298 K with help of the equation: ${\Delta }_{\text{cr}}^{\mathrm{l}}{H}_{\mathrm{m}}^{\mathrm{o}}$(298 K)/(J·mol⁻¹) = ${\Delta }_{\text{cr}}^{\mathrm{l}}{H}_{\mathrm{m}}^{\mathrm{o}}$(T _fus/K) − ${\Delta }_{\text{cr}}^{\mathrm{l}}{C}_{\text{p,m}}^{\mathrm{o}}$ × [(T _fus/K) − 298 K] which was rearranged by with Monte et al.^{[ 61} , ⁶² , ^{63 ]} Here, ${\Delta }_{\text{cr}}^{\mathrm{l}}{C}_{\text{p,m}}^{\mathrm{o}}$was obtained from the gas‐phase molar heat capacity which was derived from statistical thermodynamics and the vibrational frequencies obtained at B3LYP/6‐31G(d) level, scaled by a factor of 0.960 ± 0.022 (refer to Table 5). Uncertainties in the adjustment are estimated to account for 30% of the total enthalpic adjustment.^{[ 62 ]}

The HBT sample was subjected to two consecutive heating and cooling cycles at 10 K min⁻¹ between −80 up and 150 °C. The heating first cycle started at 25 °C, and as illustrated in Figure  2a, the first thermal event corresponds to the melting of the thermodynamically stable form II. Then, when cooling, the melt crystallizes into a metastable form I. Upon reheating, that is, the second cycle that starts at −80 °C in Figure 2a, an exothermic crystal–crystal transition is observed near 110 °C. This corresponds to the thermodynamically irreversible transformation of form I back into the stable form II. Immediately afterwards, the melting of form II is observed. After which, when cooling, the melt recrystallizes into form I and thus finishing the second cycle again at 25 °C. This behavior was confirmed by additional experiments performed at different heating rates (5, 10, and 20 K min^{− 1}), as shown in Figure 2b, which support the monotropic nature of the transition.^{[ 51 ]} It is also important to note that this behavior is recurrent over various cycles. Furthermore, the melting onset of form II remains consistent across cycles, reinforcing its stability. The observed transition is attributed to a structural relaxation of the crystalline lattice.

Figure 2.

a) DSC curves of HBT recorded over two heating and cooling cycles at 10 K min⁻¹ under nitrogen flow. b) DSC profiles of HBT recorded during the second heating cycle at different heating rates (5, 10, and 20 K min⁻¹).

Complementary PXRD and FTIR‐ATR experiments were conducted on the samples (the residual DSC sample and the original one) to further explore the chemical and crystalline nature of the two identified polymorphs. The infrared spectra (Figure  3 ) showed the following: (i) no significant differences in the fingerprint region; (ii) the presence of characteristic N⋅⋅⋅H⋅⋅⋅O interactions; and (iii) the retention of the basic structure. These results suggest that polymorphism arises mainly from differences in points of contact. The PXRD patterns (Figure  4 ) clearly distinguish the two forms: for form II, the diffraction peaks align with those reported in the literature^{[ 52 ]} (COD ID: 7002569), though intensity differences may result from preferred orientation in the powder sample. In contrast, form I exhibits distinct peaks positions, indicating a new polymorphic form not previously described. Complementary NMR analysis confirmed that the samples did not undergo decomposition (Figure S2, Supporting Information).

Figure 3.

Infrared spectra of HBT polymorphs obtained via FTIR‐ATR.

Figure 4.

PXRD diffraction patterns of HBT's form I and form II compared with literature data.^{[ 57 ]}

All thermophysical and fluorescence data related to the crystalline‐to‐liquid (cr–l) and crystalline‐to‐gas (cr–g) transitions discussed in this work refer specifically to form II, identified as the thermodynamically stable polymorph.

Finally, as no crystal–crystal transitions were observed in the original sample and it was identified as the thermodynamically stable one (Form II), a series of essays at 2 K min⁻¹ was carried out to determine the fusion parameters. These are collected in Table 2. Results from individual DSC runs for both samples are given in Table S1, Supporting Information.

2.2. Thermal Stability and Liquid–Gas Transitions

In addition to conventional DSC experiments, the samples were analyzed using open‐crucible, simultaneous TG‐DSC experiments at a heating rate of 10 K min⁻¹, under an argon flow. Both compounds showed thermal stability in the crystalline phase, with no significant mass loss prior to melting, including the absence of water desorption. However, just after the melting process, a continuous mass loss accompanied by an endothermic heat flow change was detected for both samples. The corresponding thermograms are presented in Figure  5 . The shape of the endothermic events suggests a vaporization process, that is, an endothermic mass loss, although the possibility of thermal decomposition cannot be entirely excluded without further analysis. Nonetheless, this behavior is a strong indication that their vaporization enthalpies can be determined using I‐TG, a method previously applied to study the vaporization of small organic molecules.^{[ 53} , ⁵⁴ , ^{55 ]}

Figure 5.

TG‐DSC curves obtained from nonisothermal thermogravimetric (NITG) experiments for a) HBO and b) HBT at 10 K min⁻¹ under an Ar flow of 40 mL min⁻¹.

Using the aforementioned I‐TG technique, a series of experiments were performed to study and to determine their standard molar enthalpies of vaporization. Using this technique, an analog to vapor pressure can be determined. Experimental conditions and the corresponding vaporization parameters are collected in Table  3 and Figure  6 with additional details given in Table S2, Supporting Information.

Table 3.

Clausius–Clapeyron equation parameters and enthalpies of vaporization obtained for HBO and HBT using the I‐TG method determined at the mean temperature and adjusted to 298.15 K.

Compounda)	a	b	r2	$⟨T⟩/\mathit{K}$	${\Delta }_{\mathrm{l}}^{\mathrm{g}}{H}_{\mathrm{m}}^{°}(⟨T⟩)$ [kJ mol−1]	${\Delta }_{\mathrm{l}}^{\mathrm{g}}{H}_{\mathrm{m}}^{°}[298K]$ [kJ mol−1]
HBO	19.7 ± 0.3a)	−8995 ± 113a)	0.9984	435.4	74.8 ± 2.1a)	86.9 ± 2.4b)
HBT	19.5 ± 0.4a)	−9453 ± 140a)	0.9980	432.2	78.6 ± 2.6a)	90.8 ± 2.9b)

^a)The uncertainties quoted correspond to the combined standard uncertainties at the 0.95 confidence level. These were obtained by applying a coverage factor, k, for 10 and 9 degrees of freedom, corresponding to independent measurements for HBO and HBT, respectively.

^b)Obtained by adjusting the standard molar enthalpy of vaporization from the average temperature to the reference temperature of 298 K using ${\Delta }_{\mathrm{l}}^{\mathrm{g}}{H}_{\mathrm{m}}^{\mathrm{o}}$(298 K)/(J·mol⁻¹) = ${\Delta }_{\mathrm{l}}^{\mathrm{g}}{H}_{\mathrm{m}}^{\mathrm{o}}$(<T>/K) – ${\Delta }_{\mathrm{l}}^{\mathrm{g}}{C}_{\text{p,m}}^{\mathrm{o}}$ × [(<T>/K) – 298 K] as derived by Monte et al.^{[ 61} , ^{63 ]} ${\Delta }_{\mathrm{l}}^{\mathrm{g}}{C}_{\text{p,m}}^{\mathrm{o}}$was estimated using the gas‐phase molar heat capacity (derived from statistical thermodynamics and the vibrational frequencies obtained at B3LYP/6‐31G(d) level, scaled by a factor of 0.960 ± 0.022; refer to Table 5). Uncertainties associated with the temperature adjustment are estimated to account for ≈10 % of the total enthalpic adjustment.^{[ 62 ]}

Figure 6.

Graphical representation of ln (dm/dt·T ^1/2) versus 1000/T obtained through the I‐TG method for HBO and HBT.

The presented values indicate that the intensity of the intermolecular interactions follows the same trend observed on the other phase transitions, being higher with when the thiazole moiety is present. Furthermore, these will be subsequently used in thermophysical calculations to verify the internal consistency across the three main phase transitions.

2.3. Vapor Pressures and Sublimation Thermodynamics of HBO and HBT

The study of the temperature dependency of the vapor pressures of HBT was performed using the mass‐loss Knudsen effusion method. A detailed description of the apparatus used and the general procedure are reported in literature^{[ 56 ]} and in the Supporting Information. The measurements of vapor pressure were performed over a temperature range chosen to yield vapor pressures between 0.1 and 1.0 Pa. Regression analysis and the derived thermodynamic parameters are presented in Table  4 and  5 and Figure  7 . The full dataset, including the temperatures, effusion times, and corresponding vapor pressures, is provided in Table S4, Supporting Information.

Table 4.

Parameters of the Clausius–Clapeyron equation and calculated values for the enthalpy and entropy of sublimation at the mean temperature of the experiment for crystalline HBT.

Orificesa)	a	b	r2	$p(⟨T⟩)$ [Pa]	${\Delta }_{\text{cr}}^{\mathrm{g}}{H}_{\mathrm{m}}^{°}(⟨T⟩)$ [kJ mol−1]	[J K−1 mol−1]
$⟨T⟩$ = 361.1 K
S1‐S2‐S3	36.4 ± 0.6	−13 527 ± 213	0.9995	0.347	112.5 ± 1.8	311.4 ± 4.9
M4‐M5‐M6	36.4 ± 0.7	−13 501 ± 264	0.9992	0.373	112.3 ± 2.2	310.8 ± 6.1
L7‐L8‐L9	36.2 ± 0.2	−13 482 ± 67	0.9999	0.356	112.1 ± 0.6	310.4 ± 1.5
Global results	36.4 ± 0.3	−13 503 ± 122	0.9993	0.371	112.3 ± 1.0	310.9 ± 2.8

^a)The uncertainties quoted are the combined standard uncertainties with a 0.95 level of confidence.

Table 5.

Gas‐phase standard (p° = 0.1 MPa) molar heat capacity, $C_{\mathrm{p},\mathrm{m}}^{°}(g)$, molar enthalpy, ${\Delta }_{\mathrm{c}\mathrm{r}}^{\mathrm{g}}{H}_{\mathrm{m}}^{°}$, entropy, ${\Delta }_{\mathrm{c}\mathrm{r}}^{\mathrm{g}}{S}_{\mathrm{m}}^{°}$, Gibbs energy, ${\Delta }_{\mathrm{c}\mathrm{r}}^{\mathrm{g}}{G}_{\mathrm{m}}^{°}$, of sublimation, as well as vapor pressure, p, at T = 298.15 K for crystalline HBT.

${\mathrm{C}}_{\mathrm{p},\mathrm{m}}^{°}[\mathrm{g}]$ a ,b) [J K−1 mol−1]	${\Delta }_{\mathrm{c}\mathrm{r}}^{\mathrm{g}}{C}_{\mathrm{p},\mathrm{m}}^{°}$ [J K−1 mol−1]	${\Delta }_{\text{cr}}^{\mathrm{g}}{H}_{\mathrm{m}}^{°}$ [kJ mol−1]	${\Delta }_{\text{cr}}^{\mathrm{g}}{S}_{\mathrm{m}}^{°}$ [J K−1 mol−1]	${\Delta }_{\text{cr}}^{\mathrm{g}}{G}_{\mathrm{m}}^{°}$ [kJ mol−1]	p [Pa]
219.11	−39.46	114.8 ± 1.0	214.6 ± 2.8	50.8 ± 1.3	1.26 × 10−4

^a)The uncertainties quoted are the combined standard uncertainties (with a corresponding 0.95 level of confidence).

^b)Derived from statistical thermodynamics and the vibrational frequencies obtained at B3LYP/6‐31G(d) level, scaled by a factor of 0.9603.

Figure 7.

Plots of ln(p/Pa) against 1000/T for the small, medium, and large orifices obtained from Knudsen effusion mass loss experiments for crystalline HBT.

Furthermore, Knudsen effusion mass spectrometry experiments were conducted on both HBT and HBO. Detailed results are reported in Tables S5 and S6, Supporting Information. For HBT, a value of (114.8 ± 3.7) kJ·mol⁻¹ at 298.15 K was obtained further validating the effusion results previously described. A scan of the mass spectra of the effusing vapor show that the only relevant signals detected were those of the molecular ion at m/z = 227 and the +1 isotope at m/z = 228.

In the case of HBO, KEMS experiments show that, again, the molecular ion at m/z = 211 and the +1 isotope at m/z = 212 are the only relevant signals, and as such, no decomposition occurs during the sublimation process at the studied temperatures. Additionally, these experiments yielded a value of (102.2 ± 4.7) kJ·mol⁻¹ further validating the effusion and calorimetric results already published in literature.^{[ 48 ]}

Table  6 compiles all the available standard molar enthalpies of sublimation at 298 K for both HBO and HBT (including values obtained in previous studies that use alternative techniques). Additionally, in Figure  8 , all the vapor pressure data for the crystalline phase of both HBO and HBT are shown.

Table 6.

Compilation of the experimental and calculated enthalpies of sublimation of HBO and HBT.

Compound	${\Delta }_{\text{cr}}^{\mathrm{g}}{H}_{\mathrm{m}}^{°}$ [kJ mol−1]	Source
HBO	100.5 ± 1.8	Calvet microcalorimetry[ 48 ]
	105.5 ± 0.6	KEML[ 48 ]
	102.2 ± 4.7	This work (KEMS)c)
	106.3 ± 2.9	This worka)
	104.9 ± 0.6	Recommended valued)
HBT	105.0 ± 2.2	Calvet microcalorimetry[ 48 ]
	114.8 ± 1.0	This work (KEML)b)
	114.8 ± 3.7	This work (KEMS)c
	111.1 ± 3.3	This worka)
	113.1 ± 0.9	Recommended valuee)

^a)Calculated with the following equation: ${\Delta }_{\text{cr}}^{\mathrm{g}}{H}_{\mathrm{m}}^{°}={\Delta }_{\text{cr}}^{\mathrm{l}}{H}_{\mathrm{m}}^{°}+{\Delta }_{\mathrm{l}}^{\mathrm{g}}{H}_{\mathrm{m}}^{°}$which uses experimental data determined using DSC and ITG in this work. The quoted uncertainty is calculated from the uncertainties of both measurements.

^b)Determined using KEML in this work. Refer to Table 5.

^c)Determined using KEMS in this work. Refer to Table S6, Supporting Information.

^d)Calculated from the pondered average of the KEMS value obtained in this work and literature Calvet microcalorimetry and KEML data.^{[ 48 ]}

^e)Calculated from the pondered average of literature Calvet microcalorimetry^{[ 48 ]} and KEML and KEMS data from this work.

Figure 8.

Plots of ln(p/Pa) against 1000/T for HBO and HBT obtained with KEMS and KEML both in this work and in literature.

For HBO, there is a good agreement between KEML and KEMS values obtained in this work, and the data reported previously using Knudsen effusion mass loss and Calvet microcalorimetry. Additionally, the sum of the enthalpies of fusion and vaporization is consistent with these results. This overall concordance supports the validity of the experimental findings, and therefore, all values were considered in the calculation of the recommended value.

In contrast, the case of HBT reveals a discrepancy of nearly 10 kJ·mol⁻¹ between the calorimetric value and the effusion values. The sublimation enthalpy estimated indirectly—by summing the enthalpies of fusion and vaporization—shows better agreement with the value obtained via Knudsen effusion. Nevertheless, the calorimetric value was included in the recommended value, as the combination of direct and indirect methods contributes to increase confidence in the final value.

The recommended values for both compounds are reported in Table 6.

2.4. Solid‐State Fluorescence

The normalized fluorescence spectra of HBO and HBT are shown in Figure  9 along with photographs of the quartz disks with the samples under UV light. For each sample, the spectra have been normalized to the highest emission intensity. In Table  7 , some literature data and results from this work are presented. The fluorescence results for each individual essay are presented in Table S7, Supporting Information.

Figure 9.

a) Normalized relative fluorescence emission spectra of HBO and HBT. b,c) Photographs of the quartz disks with HBO and HBT samples under UV light, respectively.

Table 7.

Results from fluorescence spectroscopy obtained in this work along with some pertinent literature results.

Samplea)	${\lambda }_{\text{exc}}$ [nm]	${\lambda }_{\text{em}}$ [nm]	${\Phi }_F$	Δλ [cm−1]	Source
HBO
Toluene	332	499	0.02	10 100	[39]
EtOH	319	361/483	0.02	10 800	[39]
Solid (KBr pellet)	347	501	0.56	8900	[39]
Solid (powder)	350	504.8	0.54	8761	This work
HBT
DCM	336	513	0.04	10 268	[40]
Solidb)	385	513	0.72	6484	[40]
Solid (powder)	360	514.5	0.49	8341	This work

^a)Determined using the maximum absorption wavelength as the excitation wavelength.

^b)Purified by recrystallization from EtOH.

Most literature cases report fluorescence studied on the femtosecond scale to explore the mechanisms of excitation and fluorescence of these benzazoles.^{[ 25} , ²⁷ , ³⁰ , ^{33 ]} As such, a few important events have been identified but for this work, two are of importance, the keto and enol emissions. The enol emission, although unlikely in most cases, can be identified via an emission with a small Stokes shift whereas the keto triggered emission has a larger Stokes shift that is characteristic of the ESIPT process. One such case is that of^{[ 39 ]} where in ethanol solution both could be identified. However, in most cases, a larger Stokes shift is observed.

Considering the previous analysis the following steps can be identified. The first thing that can be analyzed is the fairly large Stokes shift observed in both samples, which is indicative of the occurrence of ESIPT. On the ground state, the enol form absorbs UV radiation which then promotes the enol tautomer to an excited state. This excitation then triggers an ultrafast proton transfer creating the keto form. This is responsible for both maximum peaks in HBO and HBT.

For HBO, two shoulders/peaks appear at ≈490 and 530 nm. For HBT, a shoulder appears at 540 nm. The first shoulder of HBO at 490 nm could be due to several effects, but the most likely hypothesis is that it is due to some vibronic transition from the excited keto tautomer. Following emission, these vibrationally excited ground‐state molecules would then relax to the vibrational ground state through rapid nonradiative processes. The third peak for HBO could still be due the vibronic transitions/structure, but it is most likely due to solid‐state effects on the fluorescence process, which is our explanation for the second HBT peak as well. The observed shoulders in the emission spectra (at 530 nm for HBO and 540 nm for HBT) are most likely caused by the specific molecular packing in the solid state. π–π interactions and inter/intramolecular hydrogen bonding (both reported in literature^{[ 50} , ^{57 ]}) can lead to distinct emitting states or stabilize specific conformations of the keto tautomer and thus to the presence of these spectral features. Although literature studies are done with derivatives of HBO and HBT,^{[ 28} , ²⁹ , ³² , ³³ , ³⁴ , ³⁵ , ³⁶ , ³⁷ , ^{38 ]} similar conclusions with those samples support the interpretation provided in this work. Time‐resolved fluorescence could help understand the fluorescence mechanism in HBO and HBT and temperature‐dependent studies could provide some clarity on the solid‐state effects on the fluorescence profile of the base moieties.

The fact that these compounds are weakly emissive in solution but display high fluorescence quantum yields in the solid state is a strong indication of aggregation‐induced enhancement. Additionally, the quantum yield values reported in literature for HBO are in agreement with our own. However, this is not the case for those of HBT In the case of You et al.,^{[ 40 ]} the sample used was commercially acquired and subsequently purified via recrystallization with ethanol. However, no information is provided on the final purity of the sample. As such, this step most likely led to the formation of crystals which were then used to make the measurement whereas our measurements were done using a powder.

3. Conclusion

An extensive characterization of the thermophysics of HBO and HBT is reported on this work with all the recommended values for their respective phase transition enthalpies at 298 K collected in Table  8 . This study aimed at studying the interplay between the heteroatom difference in HBO and HBT and correlating it with their thermophysical and photophysical properties. A couple of trends can be observed: 1st HBT has higher phase transition enthalpies than HBO and subsequently, lower vapor pressures in the solid state; 2nd. HBO has a higher fluorescence quantum yield, but they have a similar Stokes shift.

Table 8.

Standard molar enthalpies of phase transition at the reference temperature of 298.15 K.

HBOa , b )
${\Delta }_{\text{cr}}^{\mathrm{l}}{\mathit{\text{H}}}_{\mathrm{m}}^{°}$	${\Delta }_{\mathrm{l}}^{\mathrm{g}}{\mathit{\text{H}}}_{\mathrm{m}}^{°}$	${\Delta }_{\text{cr}}^{\mathrm{g}}{\mathit{\text{H}}}_{\mathrm{m}}^{°}$
19.3 ± 1.5	86.9 ± 2.4	104.9 ± 0.6
HBT
${\Delta }_{\text{cr}}^l{\mathit{\text{H}}}_{\mathrm{m}}^{°}$	${\Delta }_{\mathrm{l}}^{\mathrm{g}}{\mathit{\text{H}}}_{\mathrm{m}}^{°}$	${\Delta }_{\text{cr}}^{\mathrm{g}}{\mathit{\text{H}}}_{\mathrm{m}}^{°}$
20.3 ± 1.7	90.8 ± 2.9	113.1 ± 0.9

^a)All uncertainties are reported with a 0.95 level of confidence.

^b)All the values are reported in kJ mol⁻¹.

As already explored in literature, HBT has shown to have higher intermolecular interactions (van der Waals, dipole–dipole interactions) as the sulfur atom is more polarizable than the oxygen atom. This also translates into a less efficient H‐bond. The S···H bond in HBT is weaker than HBO's O···H bond due to sulfur's lower electronegativity, leaving more residual intermolecular interactions that must be overcome during phase transitions such as vaporization and sublimation. Nevertheless, both samples are chemically and thermally stable over a fairly wide range of temperatures.

Furthermore, the quantum yields of high purity samples of HBO and HBT were measured. The complex nature of inter and intramolecular hydrogen bonding and π‐π stacking are the main reason why HBO's quantum yield is higher than that of HBT.

4. Experimental Section

4.1.

4.1.1.

Characterization and Purity Assessment of Compounds

The molar fraction results were obtained by gas–liquid chromatography with flame ionization detector, performed on a water‐free basis using an Agilent 8860 instrument equipped with a polysiloxane column. The compounds used in this work were commercially sourced and acquired with a minimum fractional purity of 0.99 (molar fractions). Gas–liquid chromatography analysis of both compounds confirmed the high purity required for the thermophysical and fluorescence studies, thus eliminating the need for further purification. The obtained values were 0.9999 for both HBO and HBT and as such no further purification was required.

Thermal Analysis and Isothermal Thermogravimetry

A Netzsch DSC 204 F1 Phoenix, a heat flux differential scanning calorimeter equipped with an intra‐cooler unit, was used to determine the fusion temperatures and enthalpies, as well as to analyze the thermal behavior of the title compounds. The data acquired were processed using the Netzsch Proteus Thermal Analysis software (version 8.0.3). The DSC experiments aimed at determining the fusion temperature and enthalpy for both compounds were conducted under a nitrogen purge flow of 40 cm³ min⁻¹ and a protective flow of 20 cm³ min⁻¹, using hermetically sealed aluminum crucibles and a heating rate of 2 K min⁻¹. Additional DSC runs at different heating rates were carried out to explore the possible presence of multiple crystalline forms and to gain insight into their relative stabilities.

Simultaneous thermogravimetric and calorimetric analysis (TG‐DSC) was performed using a Stanton‐Redcroft 625 instrument. Nonisothermal thermogravimetry (NITG) and isothermal thermogravimetry (I‐TG) experiments were carried out under an argon flow of 40 cm³ min⁻¹ to monitor the sample behavior and study the vaporization process. For both TG‐DSC and I‐TG experiments, a heating rate of 10 K min⁻¹ was applied. Open aluminum crucibles were used, with sample masses ranging from 4 to 6 mg.

Further details on the devices and their respective calibrations are given in the supporting information, and the theoretical background is described elsewhere.^{[ 53} , ^{54 ]}

Fourier Transform Infrared Spectrometry

FTIR measurements were carried out on a PerkinElmer Spectrum Two (ATR module) with four scans per spectrum. Data was analyzed using the PerkinElmer software Spectrum. FTIR analysis was conducted at FCUP|DQB Lab& Services.

Powder X‐Ray Diffraction

Diffraction patterns of HBO and of both forms of HBT were acquired, in the angular range 2θ = 10°–90°, with a Malvern Panalytical X’Pert Pro MPD diffractometer (Cu Kα radiation, λ = 1.54184 Å), operating in Bragg−Brentano geometry and equipped with an ultrafast X’Celerator RTMS detector.

NMR Analysis

¹H‐NMR spectra of the two forms (I and II) of HBT were performed with a JEOL JNM‐ECZ 600 R spectrometer (600 MHz) in CDCl₃, using 5 mm tubes. Spectral data confirmed the structure and identity of the two forms.

Knudsen Effusion Mass Loss/Mass Spectrometry

The temperature dependence of the vapor pressures was studied using the Knudsen effusion mass loss technique (KEML). The apparatus and general procedure have been previously described in the literature,^{[ 56 ]} and additional experimental details are provided in the Supporting Information. The measurements were carried out within temperature ranges chosen to correspond to vapor pressures between 0.1 and 1.0 Pa.

For each effusion experiment, conducted under high vacuum conditions (ca.1·10⁻⁴ Pa), the sublimated mass of the compound, m, over the effusion time, t, was determined by weighing the effusion cells before and after the experiments with a precision of ±0.01 mg. At the experimental temperature T, the vapor pressure, p, was calculated using Equation (1), where A₀ is the area of the effusion orifice, R is the gas constant (R = 8.3144598 J K⁻¹ mol⁻¹),^{[ 58 ]} M is the molar mass of the compound, and w ₀ is the Clausing probability factor. Vapor pressures were calculated using Equation (1).

$$p=(m/A_{\mathrm{o}}{w}_{\mathrm{o}}t)\cdot {(2\pi RT/M)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$$ (1)

Additionally, Knudsen Effusion Mass Spectrometry (KEMS) experiments were performed to investigate the chemical nature of the effusing gas from the Knudsen cell and ascertain if the samples decompose or retain their structure. Details regarding the mass spectrometry apparatus can be found in the literature.^{[ 59} , ^{60 ]} Vapor pressures were calculated using Equation (2).

$$P_x=(K_{\text{instr}}{I}_{\mathrm{n}x}^{+}T)/({\sigma }_x{\gamma }_{\mathrm{n}x}{\alpha }_{\mathrm{n}x})$$ (2)

where P _x is the partial pressure of a given species x, $I_{nx}^{+}$ is the ion intensity of the isotope n of species X, K _instr is the instrumental constant of the ionization source, T is the temperature in K, and σ _x is the ionization cross section which measures the probability of ionization occurring when a molecule of the sample collides with an electron. γ _nx is the gain constant of the electron multiplier detector and α _nx is the isotopic abundance.

Afterward, a regression of ln p versus 1000/T is performed to apply the Clausius–Clapeyron relation and the following equations.

$$ln(P/P^{°})=a-(b/T)$$ (3)

(4)

$${\Delta }_{\text{cr}}^{\mathrm{g}}{S}_{\mathrm{m}}(p,⟨T⟩)={\Delta }_{\text{cr}}^{\mathrm{g}}{H}_{\mathrm{m}}/⟨T⟩$$ (5)

Solid‐State Fluorescence Spectroscopy

The fluorescence spectra and the absolute emission quantum yields (Φ_F) of the samples, in the solid state (loose powder), were measured using the spectrometer C11347‐11 from Hamamatsu, that is equipped with a 150 W xenon lamp (coupled to a monochromator for wavelength discrimination), an integrating sphere as sample chamber, and a multichannel spectroscope for signal detection. The fluorescence spectra were acquired at room temperature (T ≈ 295K) using a quartz lidded Petri dish (Hamamatsu A10095‐03) and are the result of an average of three essays.

Supporting Information

The authors have cited additional references within the Supporting Information.^[64–68]

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

José M. Silva Ferraz: conceptualization (equal); data curation (equal); formal analysis (equal); funding acquisition (equal); investigation (equal); writing—original draft (lead); writing—review and editing (equal). Ana L. R. Silva: data curation (equal); formal analysis (equal); investigation (supporting); supervision (equal); writing—review and editing (equal). Lorenza Romagnoli: data curation (supporting); investigation (supporting); writing—review and editing (equal). Andrea Ciccioli: data curation (supporting); investigation (supporting); writing—review and editing (equal). Vera L. S. Freitas: data curation (equal); formal analysis (equal); supervision (equal); writing—review and editing (equal). Maria D. M. C. Ribeiro da Silva: supervision (supporting); writing—review and editing (equal). Stefano Vecchio Ciprioti: conceptualization (equal); formal analysis (equal); funding acquisition (equal); supervision (lead); writing—review and editing (equal).

Supporting information

Supplementary Material

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.