Growth and Property Investigations of Two Organic – Inorganic Hybrid Molecular Crystals with High Thermal Stability : 4-Iodoanilinium perchlorate 18-crown-6 and 4-Iodoanilinium borofluorate 18-crown-6

Two new organic–inorganic hybrid molecular single crystals, 4-Iodoanilinium perchlorate 18-crown-6 (1) and 4-Iodoanilinium borofluorate 18-crown-6 (2), with large sizes and high thermal stability were successfully synthesized by solution method. Their structures, phase purities, thermal stability, dielectric, absorption and fluorescence spectra were systematically investigated for potential applications. Compounds 1 and 2 crystallize in orthorhombic crystal system, in same space group, namely Pnma. The thermal measurements shown 1 and 2 maintain high thermal stability up to 150 ◦C. The temperature dependency of dielectric constant was studied, and no distinct anomaly was observed. The band gap were calculated to be 3.38 eV and 3.57 eV for 1 and 2, respectively, slightly smaller than those of layer perovskite (benzylammonium)2PbCl4 semiconducting materials, which have potential applications in optoelectronic detection field. The investigations throw light on the semiconductor properties of organic–inorganic hybrid crown type material and provide two types of crown compounds with high thermal stability.

However, most of the phase-transition/decomposition temperatures of hybrid molecular crystals of crown system were reported occurred at low temperatures.From a practical point of view, such transition take place at low temperature imposes important limitation for potential applications.Therefore, in this work, two organic-inorganic hybrid supramolecular crystals with high phase-transition temperatures, 4-Iodoanilinium perchlorate grown by solution method.Their physical properties, including in structures, phase purities, phase transitions, dielectric, absorption and fluorescence spectra were firstly systematically investigated for potential applications.

Materials and Methods
Compared with traditional/routine methods (such as Czochralski method, flux method and Bridgman method), the solution method have many advantages, including low cost, low growth temperature, easy observations, easy to produce large size crystals and so on.Therefore, in this work, crystals 1 and 2 were obtained by solution method-that is after the solute were dissolved in solvent, by naturally volatilizing solvent at room temperature, the solution become oversaturate solution, and then crystals 1 and 2 could be grown slowly from the above oversaturate solution.All reagents and solvents in the syntheses were of reagent grade and used without further purification.Stoichiometric ratio of 18-crown-6, 4-iodoaniline and HClO 4 /HBF 4 were dissolved in acetone solutions to synthesize crystals 1 and 2. The synthetic process and molecular configurations of the two crystals are shown in Scheme 1.
However, most of the phase-transition/decomposition temperatures of hybrid molecular crystals of crown system were reported occurred at low temperatures.From a practical point of view, such transition take place at low temperature imposes important limitation for potential applications.Therefore, in this work, two organic-inorganic hybrid supramolecular crystals with high phase-transition temperatures, 4-Iodoanilinium perchlorate 18-crown-6 (C6H7IN + •ClO4 -•C 12H24O6) (1) and 4-Iodoanilinium borofluorate 18-crown-6 (C6H7IN + •BF4 -•C12H24O6) (2), were successfully grown by solution method.Their physical properties, including in structures, phase purities, phase transitions, dielectric, absorption and fluorescence spectra were firstly systematically investigated for potential applications.

Materials and Methods
Compared with traditional/routine methods (such as Czochralski method, flux method and Bridgman method), the solution method have many advantages, including low cost, low growth temperature, easy observations, easy to produce large size crystals and so on.Therefore, in this work, crystals 1 and 2 were obtained by solution method-that is after the solute were dissolved in solvent, by naturally volatilizing solvent at room temperature, the solution become oversaturate solution, and then crystals 1 and 2 could be grown slowly from the above oversaturate solution.All reagents and solvents in the syntheses were of reagent grade and used without further purification.Stoichiometric ratio of 18-crown-6, 4-iodoaniline and HClO4/HBF4 were dissolved in acetone solutions to synthesize crystals 1 and 2. The synthetic process and molecular configurations of the two crystals are shown in Scheme 1.
Scheme 1.The synthetic process and molecular configurations of crystals 1 and 2.
Infrared spectra were obtained using a Nicolet Magna-IR 560 infrared spectrometer and KBr pellets in the 4000-400 cm −1 region to confirm the phase purity.Infrared spectra were obtained using a Nicolet Magna-IR 560 infrared spectrometer and KBr pellets in the 4000-400 cm −1 region to confirm the phase purity.
Single-crystal X-ray data of the as-grown crystals were collected on a Bruker SMARTAPEX II CCD with Mo-Kα radiation (λ = 0.71073 Å) at 153 K.The structures of crystals 1 and 2 were solved by direct methods and refined by full-matrix method based on F 2 by means of SHELXLTL software package.Besides, phase purities were also checked by powder X-ray diffraction (XRD), using a Bruker-AXS D8 ADVANCE X-Ray diffractometer with Cu-Kα1 radiation (λ = 1.54186Å) in the ranges of 10 • -50 • (2θ) with a time setting of 0.1 s per step and a step length of 0.002 • .
Thermal measurement is commonly used to detect whether a compound displays a phase transition triggered by temperature.In this work, the Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TG) data were recorded using a NETZSCH STA 449F3 instrument from −150 • C to 250 • C with a heating rate of 10 • C/min under nitrogen at atmospheric pressure in aluminum crucibles.
The variable-temperature dielectric response is another common method for detecting phase transitions.The complex dielectric permittivity ε (ε = ε − iε") was measured on pressed-powder pellets that were covered by silver conducting glue.An Impedance E4990A analyzer was used to record the variability of ε of crystals 1 and 2 in the frequency between 1 kHz and 500 kHz from The optical properties, including absorption and fluorescence measurements were carried out at room temperature.By grinding the air-dried crystals into fine powder, the polycrystalline samples were prepared to measure the UV-vis absorption spectra on a UV-2700 spectrometer with an integrating sphere over the spectral ranges of 175-850 nm.The photoluminescence (PL) spectra was performed by employing an FLSP-920 fluorescence spectroscopy (Edinburgh Instruments) using a Xenon lamp with 300 nm excitation the as-grown single crystals.

Crystal Growth
Large transparent and colorless single crystals of 1 and 2 were obtained by slow evaporation solution, as shown in the inset of Figure 1.Single-crystal X-ray data of the as-grown crystals were collected on a Bruker SMARTAPEX II CCD with Mo-Kα radiation (λ = 0.71073 Å) at 153 K.The structures of crystals 1 and 2 were solved by direct methods and refined by full-matrix method based on F 2 by means of SHELXLTL software package.Besides, phase purities were also checked by powder X-ray diffraction (XRD), using a Bruker-AXS D8 ADVANCE X-Ray diffractometer with Cu-Kα1 radiation (λ = 1.54186Å) in the ranges of 10°-50° (2θ) with a time setting of 0.1 s per step and a step length of 0.002°.
Thermal measurement is commonly used to detect whether a compound displays a phase transition triggered by temperature.In this work, the Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TG) data were recorded using a NETZSCH STA 449F3 instrument from −150 °C to 250 °C with a heating rate of 10 °C/min under nitrogen at atmospheric pressure in aluminum crucibles.
The variable-temperature dielectric response is another common method for detecting phase transitions.The complex dielectric permittivity ε (ε = ε′−iε′′) was measured on pressed-powder pellets that were covered by silver conducting glue.An Impedance E4990A analyzer was used to record the variability of ε′ of crystals 1 and 2 in the frequency between 1 kHz and 500 kHz from −175 °C to 100 °C.
The optical properties, including absorption and fluorescence measurements were carried out at room temperature.By grinding the air-dried crystals into fine powder, the polycrystalline samples were prepared to measure the UV-vis absorption spectra on a UV-2700 spectrometer with an integrating sphere over the spectral ranges of 175-850 nm.The photoluminescence (PL) spectra was performed by employing an FLSP-920 fluorescence spectroscopy (Edinburgh Instruments) using a Xenon lamp with 300 nm excitation the as-grown single crystals.

Crystal Growth
Large transparent and colorless single crystals of 1 and 2 were obtained by slow evaporation solution, as shown in the inset of Figure 1.

Crystal Structure and XRD
Crystallographic data and structure refinements of crystals 1 and 2 are listed in Table 1.Both the two crystals crystallized in orthorhombic with space group of Pnma.The lattice parameters a, b, c and cell volume V were calculated to be 15.975

Crystal Structure and XRD
Crystallographic data and structure refinements of crystals 1 and 2 are listed in Table 1.Both the two crystals crystallized in orthorhombic with space group of Pnma.The lattice parameters a, b, c and cell volume V were calculated to be 15.975The crystal structure of compound 1 was described as a representative.As depicted in Figure 2a, the asymmetric unit consists of one [(4-Iodoanilinium)(18-crown-6)] + cation and one ClO 4 − anion.2), which further stabilizes the supramolecular structure of crystal 1.Furthermore, as checked by PLATON software, no C-H…π and π…π interactions existed in the supramolecular structure.Besides, phase purities of the as-grown crystals were also checked by powder X-ray diffraction (XRD), as shown in Figure 3.The XRD patterns of crystals 1 and 2 at room temperature (298 K) matches well with the patterns simulated from the single-crystal structures measured at 153 K, except the peaks positions at 153 K were found to shift downward than those measured at room temperature, which compositing the regularity that the lattice parameters increased with the temperature increasing.In addition, the good matching of XRD patterns at the two temperatures, also confirm the thermal stability of crystals 1 and 2 in the temperature ranges of 153 K and 298 K.It is notable that at room temperature, the peaks intensities of crystal 2 are much stronger than those of crystal 1, indicating the crystallinity of crystal 2 is better than crystal 1 at room temperature.Besides, phase purities of the as-grown crystals were also checked by powder X-ray diffraction (XRD), as shown in Figure 3.The XRD patterns of crystals 1 and 2 at room temperature (298 K) matches well with the patterns simulated from the single-crystal structures measured at 153 K, except the peaks positions at 153 K were found to shift downward than those measured at room temperature, which compositing the regularity that the lattice parameters increased with the temperature increasing.In addition, the good matching of XRD patterns at the two temperatures, also confirm the thermal stability of crystals 1 and 2 in the temperature ranges of 153 K and 298 K.It is notable that at room temperature, the peaks intensities of crystal 2 are much stronger than those of crystal 1, indicating the crystallinity of crystal 2 is better than crystal 1 at room temperature.

Thermal Measurements
As depicted in Figure 4, the DSC and TG curves do not show any peak anomaly over the temperature ranges from −150 °C to 150 °C, indicating no phase transition was detected by thermal measurements in this temperature ranges, shown high thermal stability.To be precise, the phase

Dielectric Properties
The temperature dependence ε' of crystals 1 and 2 taken at 1, 10, 100 and 500 kHz were illustrated in Figure 5.As can be seen, ε' were found to decrease with increasing frequency, especially at relative high temperatures, and no observable dielectric anomaly was observed in their measured temperature ranges.

Dielectric Properties
The temperature dependence ε' of crystals 1 and 2 taken at 1, 10, 100 and 500 kHz were illustrated in Figure 5.As can be seen, ε' were found to decrease with increasing frequency, especially at relative high temperatures, and no observable dielectric anomaly was observed in their measured temperature ranges.

Optical Properties
The optical ultraviolet-visible absorption spectra were carefully performed to understand the optical and semiconducting properties, as shown in Figure 6.The absorption edges of compounds 1 and 2 located in ultraviolet ranges are assigned to 335 nm and 325 nm, respectively.The energy band gap Eg can be calculated by fitting the Tauc equation, and determined to be 3.38 eV and 3.57 eV, respectively, slightly smaller than those of layer perovskite (benzylammonium)2PbCl4 (3.65 eV) [3] and its analogues with the general formula (R-NH3)2PbCl4 (3.64 eV) [35], and comparable to 3.20 eV for anatase-TiO2 and 3.29 eV for ZnO.As it is known, a narrow energy band gap will be an advantage in optoelectronic application due to the expansion of active wavelength band.

Optical Properties
The optical ultraviolet-visible absorption spectra were carefully performed to understand the optical and semiconducting properties, as shown in Figure 6.The absorption edges of compounds 1 and 2 located in ultraviolet ranges are assigned to 335 nm and 325 nm, respectively.The energy band gap E g can be calculated by fitting the Tauc equation, and determined to be 3.38 eV and 3.57 eV, respectively, slightly smaller than those of layer perovskite (benzylammonium) 2 PbCl 4 (3.65 eV) [3] and its analogues with the general formula (R-NH 3 ) 2 PbCl 4 (3.64 eV) [35], and comparable to 3.20 eV for anatase-TiO 2 and 3.29 eV for ZnO.As it is known, a narrow energy band gap will be an advantage in optoelectronic application due to the expansion of active wavelength band.

Dielectric Properties
The temperature dependence ε' of crystals 1 and 2 taken at 1, 10, 100 and 500 kHz were illustrated in Figure 5.As can be seen, ε' were found to decrease with increasing frequency, especially at relative high temperatures, and no observable dielectric anomaly was observed in their measured temperature ranges.

Optical Properties
The optical ultraviolet-visible absorption spectra were carefully performed to understand the optical and semiconducting properties, as shown in Figure 6.The absorption edges of compounds 1 and 2 located in ultraviolet ranges are assigned to 335 nm and 325 nm, respectively.The energy band gap Eg can be calculated by fitting the Tauc equation, and determined to be 3.38 eV and 3.57 eV, respectively, slightly smaller than those of layer perovskite (benzylammonium)2PbCl4 (3.65 eV) [3] and its analogues with the general formula (R-NH3)2PbCl4 (3.64 eV) [35], and comparable to 3.20 eV for anatase-TiO2 and 3.29 eV for ZnO.As it is known, a narrow energy band gap will be an advantage in optoelectronic application due to the expansion of active wavelength band.As illustrated in Figure 7a,b, the emission peaks appear at about 525 nm for compound 1, and 425 nm and 550 nm for compound 2. The intensities of emission peaks of 2 are slightly larger than that of 1.The broad emission spectra (from 400 nm to 650 nm for both 1 and 2) suggest that near-edge defect levels related to surface states take important part in the emission process [36].
Fitted by biexponential decay, the time-resolved PL decay curves of compounds 1 and 2 were calculated, as presented in Figure 7c,d.The faster decay time (τ 1 ) and slower decay time (τ 2 ) correspond to trap-assisted recombination on surface and free carrier recombination in bulk, respectively.From decay curves, both faster and slower decay times of compound 1 were slightly longer than those of compound 2.
1, and 425 nm and 550 nm for compound 2. The intensities of emission peaks of 2 are slightly larger than that of 1.The broad emission spectra (from 400 nm to 650 nm for both 1 and 2) suggest that near-edge defect levels related to surface states take important part in the emission process [36].
Fitted by biexponential decay, the time-resolved PL decay curves of compounds 1 and 2 were calculated, as presented in Figure 7(c) and (d).The faster decay time (τ1) and slower decay time (τ2) correspond to trap-assisted recombination on surface and free carrier recombination in bulk, respectively.From decay curves, both faster and slower decay times of compound 1 were slightly longer than those of compound 2.

Discussion
Two organic-inorganic hybrid molecular crystals, C6H7IN + •ClO4 − •C 12H24O6 and C6H7IN + •BF4 − •C 12H24O6 were successfully grown by solution method.Their physical properties, including in structures, phase purities, thermal stability, absorption and fluorescence properties were systematically investigated for potential applications.Crystal structure analyses and thermal measurements showed compounds 1 and exhibited similar crystal packings and maintain high thermal stability to 150 °C.The dielectric constants as a function of temperature were investigated, with no distinct dielectric constant anomaly were observed.For compounds 1 and 2, the absorption edges located in ultraviolet ranges are assigned to 335 nm and 325 nm; the Eg were calculated to be 3.38 eV and 3.57 eV, respectively, slightly smaller than the data 3.65 eV of layer perovskite (benzylammonium)2PbCl4, indicating their potential applications in optoelectronic detection field.All the results throw light on the semiconductor properties of crown type compounds and provide two crown compounds with high thermal stability.

Scheme 1 .
Scheme 1.The synthetic process and molecular configurations of crystals 1 and 2.

Figure 2 .
Figure 2. Crystal structure of crystal 1 at 153 K. (a) Asymmetric unit for crystal 1, light blue dashed lines indicate the N-H . . .O hydrogen bonds; (b) Unit cell of crystal 1 viewed along the b + c axis.Hydrogen atoms are omitted for clarity.

Figure 3 .
Figure 3. Calculated and experimental X-ray diffraction patterns of crystals 1 and 2.

Figure 3 .
Figure 3. Calculated and experimental X-ray diffraction patterns of (a) crystal 1 and (b) crystal 2.
9, x FOR PEER REVIEW 6 of 10 decomposition temperatures are detected to be larger than 150 °C and 175 °C for 1 and 2, respectively.

Figure 4 .
Figure 4. DSC and TG curves of crystals 1 and 2.

Figure 5 .
Figure 5. Dielectric constants of crystals 1 and 2 as a function of temperature under a frequency ranges of 1 kHz to 500 kHz.

Figure 5 .
Figure 5. Dielectric constants of (a) crystal 1 and (b) crystal 2 as a function of temperature under a frequency ranges of 1 kHz to 500 kHz.

Figure 5 .
Figure 5. Dielectric constants of crystals 1 and 2 as a function of temperature under a frequency ranges of 1 kHz to 500 kHz.

Figure 7 .
Figure 7.The photoluminescence (PL) spectra and fluorescence decay curves of crystals 1 and 2.

Table 1 .
Summary of crystallographic data for crystals 1 and 2 at 150 K.

Table 1 .
Summary of crystallographic data for crystals 1 and 2 at 150 K.

Table 3 .
Phase transition temperatures and characteristics of a series of crown-ether-organic compounds.

Table 3 .
Phase transition temperatures and characteristics of a series of crown-ether-organic compounds.