Liquid-Crystal and Fire-Retardant Properties of New Hexasubstituted Cyclotriphosphazene Compounds with Two Schiff Base Linking Units

A series of new hexasubstituted cyclotriphosphazene compounds (4a–j) consisting of two Schiff base linking units and different terminal substituents was successfully synthesized and characterized. The structures of these compounds were confirmed using Fourier Transform Infra-Red (FTIR), Nuclear Magnetic Resonance (NMR), and CHN elemental analysis. Polarized optical microscopy (POM) was used to determine their liquid-crystal behavior, which was then further confirmed using differential scanning calorimetry (DSC). Compounds 4a–i with heptyl, nonyl, decyl, dodecyl, tetradecyl, hydroxy, 4-carboxyphenyl, chloro, and nitro terminal ends, respectively, showed the liquid-crystal properties, whereas compound 4j with the amino group was found to be non-mesogenic. The attachment of an electron-donating group in 4j eventually give a non-mesogenic product. The study of the fire-retardant properties of these compounds was done using the limiting oxygen index (LOI). In this study, polyester resin (PE) was used as a matrix for moulding, and the LOI value of pure PE was 22.53%. The LOI value increased to 24.71% when PE was incorporated with 1 wt.% of hexachlorocyclotriphosphazene (HCCP), thus indicating that HCCP has a good fire-retardant properties. The result showed that all the compounds have good agreement in their LOI values. Compound 4i with a nitro terminal group gave the highest LOI value of 28.37%.


Introduction
Hexachlorocyclotriphosphazene (HCCP) is an inorganic phosphorus-nitrogen compound consisting of alternating phosphorus and nitrogen atoms, bound to each other through alternating single and double bonds [1]. HCCP derivatives are excellent models for structure-activity studies and their multiarmed rigid ring allows the exploration of new discotic molecules in the field of liquid crystals [2,3]. Liquid crystal is an intermediate phase between liquid and solid, a state of matter that has both the properties of isotropic liquid and of solid crystal. As temperature increases, the solid absorbs some heat and melts into a liquid-crystal phase whereby molecules can move freely, disrupting the positional order without changing the orientational order [4].
To study the behavior of the HCCP compound, which consists of inorganic backbones as well as organic side chains, small changes in the structure must be made [5,6]. Due to the high reactivity of the P-Cl bond, the corresponding substitution method allows the introduction of a wide range of substituents [7,8] and hence provides substituted cyclotriphosphazene derivatives with different chemical and physical properties [9,10]. Such compounds have been used in commercial applications [11][12][13][14] especially in liquid-crystal and fire-retardant materials, where it has compounds, 4a-i [30][31][32][33].

Scheme 3.
Reduction of compound 4i-j [34]. All the synthesized intermediates (1a-e, 2a-i, and 3) and final compounds 4a-j were then characterized using Fourier Transform Infra-Red (FTIR), Nuclear Magnetic Resonance (NMR), and CHN elemental analysis. Liquid-crystal properties of these compounds were determined using polarized optical microscope (POM) and further confirmed using differential scanning calorimetry (DSC). In this study, the limiting oxygen index (LOI) test is used to determine the minimum amount of oxygen required to support the combustion of a sample. The compact data of these intermediates and compounds are summarized in Section 3.3.

FTIR Spectral Data of the Intermediates and Final Compounds
The IR data for intermediates 1a-e with heptyl, nonyl decyl, dodecyl, and tetradecyl chains, respectively, showed a similar pattern to the absorption bands at 2850 and 2930 cm −1 for the symmetrical and asymmetrical Csp 3 -H stretching, 2733 cm −1 for the C-H stretching of the aldehyde, 1690 cm −1 for C=O stretching, 1600 cm −1 for C=C aromatic stretching, and 1250 cm −1 for C-O stretching. The absence of the O-H stretching at 3300 cm −1 indicated that the insertion of the alkyl groups was a success.
These intermediates 1a-e and other commercially available aldehydes such as 4-hydroxybenzald ehyde, 4-formylbenzoic acid, 4-chlorobenzaldehyde, and 4-nitrobenzaldehyde were used in the reaction with 1,4-phenylenediamine to form the Schiff base intermediates, 2a-i. The IR data of intermediates 2a-i showed similar absorption bands with two bands at 3320 and 3450 cm −1 for the N-H stretching. Other bands at 1610, 1550, and 1165 cm −1 were assigned to the C=N, C=C, and C-N stretching, respectively. The absorption band for C-O stretching at 1250 cm −1 can also be observed for intermediates 2a-g. Only intermediates 2a-e displayed the absorption bands of asymmetrical and asymmetrical Csp 3 -H stretching at 2850 and 2920 cm −1 , while intermediates 2f and 2g showed a band at 3210 cm −1 for O-H stretching. An additional band at 1698 cm −1 for C=O stretching was observed for 2g and band at 790 cm −1 for C-Cl stretching for 2h. The band for aldehydic C-H stretching was not observed in any of the intermediates, 2a-i, which confirmed the success of the condensation reaction.
The substitution reaction of HCCP with p-hydroxybenzaldehyde formed intermediate 3.
The main absorption bands for 3 were observed at 2730 (H-C=O), 1700 (C=O), 1595 (C=C), and 1205 cm −1 (C-O). The absorption bands from the cyclotriphosphazene ring, P=N stretching, appeared at 1151 cm −1 while P-O-C bending was located at 944 cm −1 . Intermediate 3 was reacted with 2a-i to afford final compounds with two Schiff base linking units, 4a-i.
According to the IR spectra of compounds 4a-j (Figure 1), the presence of the alkoxy moieties in compounds 4a-e was substantiated by the appearance of the absorption bands at 2851 and 2920 cm −1 (Csp 3 -H stretching). The absorption bands at 1168, 1248, and 1509 cm −1 were assigned to the stretching of C-N, C-O, and aromatic C=C, respectively. The stretching of P=N and P-O-C bending of the cyclotriphosphazene ring in the compound was observed at 1192 and 983 cm −1 , respectively. Only compound 4g showed the absorption band for carbonyl (C=O) of the 4-carboxyphenyl substituent at 1673 cm −1 , while C-Cl bending at 823 cm −1 was observed in compound 4h. On the other hand, the reduction reaction of compound 4i to form 4j was completed with the appearance of two absorption bands at 3312 and 3478 cm −1 for the N-H stretching in the amino group. The stretching corresponding to C=N linkage at 1616 cm −1 in the IR spectra of compounds 4a-j indicated that the Schiff base formation was successful.

NMR Spectral Data of Final Compounds
Compound 4a was used to represent the structure confirmation in the series. The structure of compound 4a with complete atomic numbering is shown in Figure 2.

NMR Spectral Data of Final Compounds
Compound 4a was used to represent the structure confirmation in the series. The structure of compound 4a with complete atomic numbering is shown in Figure 2. 1 H NMR spectrum of compound 4a ( Figure 3) showed two different azomethine proton signals, which were observed in the downfield region at δ 8.66 (H5) and δ 8.55 ppm (H10), respectively. Six doublets integrating to 12 aromatic protons appeared in the range of δ 7.03-7.95 ppm and were assigned to the para-substituted aromatic protons. The presence of the azomethine groups caused the signals of H3 and H12 to be the most deshielded aromatic protons. Meanwhile, the signals for H7 and H8 almost overlapped as the protons experienced similar chemical environment. Moreover, proton signals of the aliphatic chains were observed in the upfield region, with the oxymethylene protons (H15) more deshielded among other methylene protons due to their close proximity to the

NMR Spectral Data of Final Compounds
Compound 4a was used to represent the structure confirmation in the series. The structure of compound 4a with complete atomic numbering is shown in Figure 2. 1 H NMR spectrum of compound 4a ( Figure 3) showed two different azomethine proton signals, which were observed in the downfield region at δ 8.66 (H5) and δ 8.55 ppm (H10), respectively. Six doublets integrating to 12 aromatic protons appeared in the range of δ 7.03-7.95 ppm and were assigned to the para-substituted aromatic protons. The presence of the azomethine groups caused the signals of H3 and H12 to be the most deshielded aromatic protons. Meanwhile, the signals for H7 and H8 almost overlapped as the protons experienced similar chemical environment. Moreover, proton signals of the aliphatic chains were observed in the upfield region, with the oxymethylene protons (H15) more deshielded among other methylene protons due to their close proximity to the Six doublets integrating to 12 aromatic protons appeared in the range of δ 7.03-7.95 ppm and were assigned to the para-substituted aromatic protons. The presence of the azomethine groups caused the signals of H3 and H12 to be the most deshielded aromatic protons. Meanwhile, the signals for H7 and H8 almost overlapped as the protons experienced similar chemical environment. Moreover, proton signals of the aliphatic chains were observed in the upfield region, with the oxymethylene protons (H15) more deshielded among other methylene protons due to their close proximity to the neighboring electronegative oxygen atom. Therefore, the signal of these protons appeared at δ 4.07 ppm as a triplet while that of the other methylene protons (H16-H20) were observed in the region of δ 1.30-1.79 ppm. A triplet in the most upfield (δ 0.89 ppm) region was assigned to H21. neighboring electronegative oxygen atom. Therefore, the signal of these protons appeared at δ 4.07 ppm as a triplet while that of the other methylene protons (H16-H20) were observed in the region of δ 1.30-1.79 ppm. A triplet in the most upfield (δ 0.89 ppm) region was assigned to H21. The 13 C NMR spectrum of compound 4a as shown in Figure 4a indicates that 4a has 21 carbon signals in the side arms. These signals consist of two azomethine, six aromatics, six quaternary, six methylene, and one methyl carbons. Further complete assignment of all carbons was done using DEPT experiments. DEPT 90 gave information on methine carbon (CH) and DEPT 135 gave information on the methylene carbon (CH2) which appeared as the negative signal while methine (CH) and methyl (CH3) carbon showed positive signals.  The 13 C NMR spectrum of compound 4a as shown in Figure 4a indicates that 4a has 21 carbon signals in the side arms. These signals consist of two azomethine, six aromatics, six quaternary, six methylene, and one methyl carbons. Further complete assignment of all carbons was done using DEPT experiments. DEPT 90 gave information on methine carbon (CH) and DEPT 135 gave information on the methylene carbon (CH 2 ) which appeared as the negative signal while methine (CH) and methyl (CH 3 ) carbon showed positive signals.
Molecules 2020, 25, x FOR PEER REVIEW 6 of 26 neighboring electronegative oxygen atom. Therefore, the signal of these protons appeared at δ 4.07 ppm as a triplet while that of the other methylene protons (H16-H20) were observed in the region of δ 1.30-1.79 ppm. A triplet in the most upfield (δ 0.89 ppm) region was assigned to H21. The 13 C NMR spectrum of compound 4a as shown in Figure 4a indicates that 4a has 21 carbon signals in the side arms. These signals consist of two azomethine, six aromatics, six quaternary, six methylene, and one methyl carbons. Further complete assignment of all carbons was done using DEPT experiments. DEPT 90 gave information on methine carbon (CH) and DEPT 135 gave information on the methylene carbon (CH2) which appeared as the negative signal while methine (CH) and methyl (CH3) carbon showed positive signals.  Based on the DEPT 90 spectrum (Figure 4b), peaks at δ 159.38 and 158.83 ppm were assigned to two azomethine carbons, C5 and C10. The aromatic carbon signals were observed at δ 115.46, 122.13, 122.32, 129.28, 130.53, and 130.81 ppm which can be assigned to C12, C8, C7, C3, C13, and C2, respectively. Six carbon signals, which were absent in the DEPT 90 spectrum were assigned for the quaternary carbons (C1, C4, C6, C9, C11, and C14). C1 and C14 were in the most deshielded region at δ 162.09 and 162.02 ppm, respectively, due to electronegativity effect of the oxygen atom. On the other hand, C4 and C11 were assigned at δ 136.49 and 135.76 ppm, respectively, as these carbons were attached to the less electronegative atoms among other quaternary carbons. The two carbons attached to nitrogen atoms, C6 and C9, were assigned to peaks at δ 150.63 and 149.14 ppm, respectively. Six methylene carbons of the heptyl chains (C15-C20) showed negative signals in the DEPT 135 spectrum (Figure 4c). C15 adjacent to the oxygen atom was observed at δ 68.64 ppm. Five methylene carbons of the aliphatic chains showed signals at δ 22.31 (C20), 25.88 (C19), 28.76 (C18), 29.16 (C17), and 31.58 ppm (C15). The signal at δ 13.91 ppm disappeared in the DEPT 90 spectrum but appeared in the positive region of DEPT 135 spectrum, confirming that this carbon signal belongs to the methyl carbon (C21).
The assignment of all proton ( 1 H-1 H) correlations between proton and neighboring proton was further confirmed using a COSY ( 1 H-1 H) NMR experiment. According to the COSY ( 1 H-1 H) spectrum ( Figure 5), H5 and H6 did not show any correlation with other protons, which indicated that there are the azomethine protons. The correlation of the aromatic protons can be seen between H2 and H3, H7 and H8, and H12 and H13, respectively. However, the correlations between methylene protons of the heptyl chains could be observed between H15 and H16, H16, and H17, and H18-H21. Based on the DEPT 90 spectrum (Figure 4b), peaks at δ 159.38 and 158.83 ppm were assigned to two azomethine carbons, C5 and C10. The aromatic carbon signals were observed at δ 115. 46, 122.13, 122.32, 129.28, 130.53, and 130.81 ppm which can be assigned to C12, C8, C7, C3, C13, and C2, respectively. Six carbon signals, which were absent in the DEPT 90 spectrum were assigned for the quaternary carbons (C1, C4, C6, C9, C11, and C14). C1 and C14 were in the most deshielded region at δ 162.09 and 162.02 ppm, respectively, due to electronegativity effect of the oxygen atom. On the other hand, C4 and C11 were assigned at δ 136.49 and 135.76 ppm, respectively, as these carbons were attached to the less electronegative atoms among other quaternary carbons. The two carbons attached to nitrogen atoms, C6 and C9, were assigned to peaks at δ 150.63 and 149.14 ppm, respectively. Six methylene carbons of the heptyl chains (C15-C20) showed negative signals in the DEPT 135 spectrum (Figure 4c). C15 adjacent to the oxygen atom was observed at δ 68.64 ppm. Five methylene carbons of the aliphatic chains showed signals at δ 22.31 (C20), 25.88 (C19), 28.76 (C18), 29.16 (C17), and 31.58 ppm (C15). The signal at δ 13.91 ppm disappeared in the DEPT 90 spectrum but appeared in the positive region of DEPT 135 spectrum, confirming that this carbon signal belongs to the methyl carbon (C21).
The assignment of all proton ( 1 H-1 H) correlations between proton and neighboring proton was further confirmed using a COSY ( 1 H-1 H) NMR experiment. According to the COSY ( 1 H-1 H) spectrum ( Figure 5), H5 and H6 did not show any correlation with other protons, which indicated that there are the azomethine protons. The correlation of the aromatic protons can be seen between H2 and H3, H7 and H8, and H12 and H13, respectively. However, the correlations between methylene protons of the heptyl chains could be observed between H15 and H16, H16, and H17, and H18-H21.  carbons at δ 159.38 (C5) and 158.83 (C10) ppm, respectively. The 1 H-13 C connectivity in the aromatic region could be observed between H2 and C2 at δ 130.81 ppm; H3 and C3 at δ 129.28 ppm; H7 and C7 at δ 122.32 ppm; H8 and C8 at δ 122.13 ppm; H12 and C12 at δ 115.56 ppm; and H13 with C13 at δ 130.53 ppm. Moreover, H15 at δ 4.07 ppm was correlated with C15 at δ 68.64 ppm. The connectivity of methylene protons (H16-H20) with their corresponding carbons (C16-C20) was also observed in the HSQC spectrum. A triplet at δ 0.89 ppm assigned to H21 for the methyl protons showed connectivity with its methyl carbon (C21) at δ 14.06 ppm in the most upfield region. All the data obtained from the COSY ( 1 H-1 H) and HSQC ( 1 H-13 C) spectra are summarized in Table 1.
Molecules 2020, 25, x FOR PEER REVIEW 8 of 26 Assignment of the protonated carbons in compound 4a was done using HSQC NMR experiment ( Figure 6). The azomethine protons at δ 8.66 (H5) and δ 8.55 ppm (H10), showed correlations with the carbons at δ 159.38 (C5) and 158.83 (C10) ppm, respectively. The 1 H-13 C connectivity in the aromatic region could be observed between H2 and C2 at δ 130.81 ppm; H3 and C3 at δ 129.28 ppm; H7 and C7 at δ 122.32 ppm; H8 and C8 at δ 122.13 ppm; H12 and C12 at δ 115.56 ppm; and H13 with C13 at δ 130.53 ppm. Moreover, H15 at δ 4.07 ppm was correlated with C15 at δ 68.64 ppm. The connectivity of methylene protons (H16-H20) with their corresponding carbons (C16-C20) was also observed in the HSQC spectrum. A triplet at δ 0.89 ppm assigned to H21 for the methyl protons showed connectivity with its methyl carbon (C21) at δ 14.06 ppm in the most upfield region. All the data obtained from the COSY ( 1 H-1 H) and HSQC ( 1 H-13 C) spectra are summarized in Table 1.    The 31 P NMR spectrum of compound 4a (Figure 7a) showed only a singlet at δ 8.20 ppm. The peak of this compound was shifted to the upfield region as compared to that of HCCP with six electron-withdrawing chlorine atoms (Figure 7b), indicating that all the phosphorus had been substituted with the same side arms. The 31 P NMR spectrum showed the chemical shifts of the hexasubstituted cyclotriphosphazene series experience more shielding. This behavior was attributed to the molecular structure of the hexa-series that contains six side arms with high electron density. As a result, greater shielding effect was observed.   (Figure 7a) showed only a singlet at δ 8.20 ppm. The peak of this compound was shifted to the upfield region as compared to that of HCCP with six electron-withdrawing chlorine atoms (Figure 7b), indicating that all the phosphorus had been substituted with the same side arms. The 31 P NMR spectrum showed the chemical shifts of the hexasubstituted cyclotriphosphazene series experience more shielding. This behavior was attributed to the molecular structure of the hexa-series that contains six side arms with high electron density. As a result, greater shielding effect was observed. Furthermore, other homologues with alkylated terminal chains (compounds 4b-e) showed similar splitting patterns and chemical shifts in the 1 H and 13 C NMR spectra with compound 4a but only differed in terms of the number of protons and carbons in the alkyl chains. Compounds 4f-j with small terminal substituents (OH, Cl, COOH, NO2, and NH2) showed two singlets of azomethine and six doublets for aromatic protons in the 1 H NMR spectra. There was a slight difference in the chemical shift values of the signals of each compound due to the chemical environment, Furthermore, other homologues with alkylated terminal chains (compounds 4b-e) showed similar splitting patterns and chemical shifts in the 1 H and 13 C NMR spectra with compound 4a but only differed in terms of the number of protons and carbons in the alkyl chains. Compounds 4f-j with small terminal substituents (OH, Cl, COOH, NO 2 , and NH 2 ) showed two singlets of azomethine and six doublets for aromatic protons in the 1 H NMR spectra. There was a slight difference in the chemical shift values of the signals of each compound due to the chemical environment, electronegativity effect, and bond angles. Only compounds 4f and 4g showed the hydroxyl and carboxyl proton peaks in the 1 H NMR spectra.

Determination of Liquid-Crystal Properties Using POM
The phase textures of intermediate and final compounds were determined using POM. In this study, all the intermediates (1a-e, 2a-j, and 3) were found to be non-mesogenic with no liquid-crystal behavior. Meanwhile, compounds 4a-i with two Schiff base linking units were found to exhibit liquid-crystal phase while compound 4j was found to be non-mesogenic. Observation under POM (Figure 8) showed that compound 4a exhibited the thread-like nematic phase with four-point brushes in the cooling cycle. Further cooling changed the nematic phase into SmA phase before it became a crystal phase.
liquid-crystal phase while compound 4j was found to be non-mesogenic. Observation under POM (Figure 8) showed that compound 4a exhibited the thread-like nematic phase with four-point brushes in the cooling cycle. Further cooling changed the nematic phase into SmA phase before it became a crystal phase.
The phase transitions of compounds 4b-d showed the focal conic fan texture of SmA phase in the heating and cooling cycles, as illustrated in Figure 9. Meanwhile, compound 4e exhibited two different types of phase transition in both heating and cooling cycles. Upon cooling, a focal conic fan of SmA phase was formed from the isotropic phase and further cooling resulted in the formation of a broken focal conic fan of SmC phase which might be due to the stacking of molecules. The texture is illustrated in Figure 10.
For compounds 4f-i, schlieren texture of the nematic phase with four-point brushes was clearly observed under POM (Figures 11 and 12) in the heating and cooling cycles. Upon cooling, compound 4f showed the formation of droplets from the isotropic phase which became the nematic phase with a thread-like texture, as shown in Figure 11. However, compound 4j without any liquid-crystal properties only displays the phase transition of crystal to isotropic phase in heating cycle and isotropic to crystal phase in cooling cycle.  The phase transitions of compounds 4b-d showed the focal conic fan texture of SmA phase in the heating and cooling cycles, as illustrated in Figure 9. Meanwhile, compound 4e exhibited two different types of phase transition in both heating and cooling cycles. Upon cooling, a focal conic fan of SmA phase was formed from the isotropic phase and further cooling resulted in the formation of a broken focal conic fan of SmC phase which might be due to the stacking of molecules. The texture is illustrated in Figure 10.
For compounds 4f-i, schlieren texture of the nematic phase with four-point brushes was clearly observed under POM (Figures 11 and 12) in the heating and cooling cycles. Upon cooling, compound 4f showed the formation of droplets from the isotropic phase which became the nematic phase with a thread-like texture, as shown in Figure 11. However, compound 4j without any liquid-crystal properties only displays the phase transition of crystal to isotropic phase in heating cycle and isotropic to crystal phase in cooling cycle.

Determination of Thermal Transitions Using DSC
Texture observation under POM with controlled temperature helped to determine the type of mesophases present in the molecules. Only compounds exhibiting mesophase(s) were sent for DSC measurements. The DSC thermograms are used to confirm the phase transition temperatures and enthalpy change involved during the transition in the heating and cooling cycles. The phase transitions and its corresponding enthalpy change of compounds 4a-i with two Schiff base linking units is summarized in Table 2.  The DSC thermogram of compounds 4a and 4e shows three endotherms or curves with their enthalpy values, in both heating and cooling cycles. Upon heating of compound 4a, the curves were attributed to the phase transitions from crystal to SmA and nematic before reaching a clearing temperature at 235.89 • C. Similar phase transitions were also observed in the reversed order of the cooling cycle. For compound 4e, the phase transitions were from the crystal to SmC and then SmA before it became the isotropic phase with the melting point observed at 147.75 • C and the clearing temperature at 197.81 • C.
Compounds 4b-d and 4f-i exhibited only two curves in the DSC thermogram which indicates that these compounds have only one liquid-crystal phase. The DSC thermogram of compounds 4b-d displayed the transition from crystal to SmA and isotropic phase in the heating cycle. Meanwhile, compounds 4f-i with substituents such as hydroxy, 4-carboxyphenyl, chloro, and nitro terminal chains in the side arms also showed two curves in the DSC thermogram, the transition from crystal to the thread-like nematic, and isotropic phase. In the cooling cycle, these two curves of phase transitions were also observed in the reverse order. The DSC thermogram of compound 4f is shown in Figure 13. In addition, the DSC thermogram of compounds 4a-e and 4g-i are provided in the Supplementary Materials Section. The existence of endotherm appears in the DSC thermogram for each of the mesophase transition, which corresponds to the texture of liquid crystal, which is observed under POM. Molecules 2020, 25, x FOR PEER REVIEW 15 of 26 Figure 13. DSC thermogram of compound 4f.

Structure-Properties Relationship
The study on the relationship between structure and liquid-crystal mesophase behavior is very important in designing new liquid-crystal materials with desirable properties for future applications.
Based on the POM observation, all the final compounds in the series with a Schiff base linking unit that bore a different terminal alkoxy function were mesogenic and the wider of the thermal mesomorphic range increased with increasing chain length. Compound 4a with heptyl chains showed smectic and nematic phases with the texture of thread-like four-point brushes. As the length of the alkyl chain increased, the tendency for formation of the nematic phase was reduced. This trend was observed for compounds 4b-e which exhibited only the smectic phase since the flexibility of the long chains tends to decrease the clearing temperatures, Tc. Smectic phases are more ordered than the nematic phase. Nematic phase has the positional order but no orientational order and closest to the isotropic phase. Hence, effective molecular packing is essential for the formation of stable phases. Moreover, the polarisability of the molecules also increased with an increase of the alkoxy chain length in the compounds. This enhanced the cohesive forces between the sides and the core of the molecules, thus increased the tendency to form the smectic layer [35]. Molecules with smectic phase behavior are likely to favor a lamellar packing, which is due to higher Van der Waals interactions and intertwining possibility between the alkoxy chains. Thus, the nematic orientation cannot be adopted as the chain length increased and resulted in only a smectic phase in compounds 4b-e.
In this work, compounds 4a-e exhibited the smectic A phase while compound 4e showed an additional of smectic C. The presence of smectic C phase in compound 4e is due to the tilted analogue of the smectic A phase [36]. In smectic C, molecules are aligned with their long axes tilted relative to the layer normal in which the molecules are stacked. Meanwhile, molecules in smectic A are arranged in layers, with long axes perpendicular to the layers which can slide over one another [37].
Small terminal substituents, such as hydroxy or carboxy, do not always induce liquid-crystal behavior [38]. In contrast, Sudhakar et al. (2000) reported that small molecules often exhibit one or more liquid crystalline phases since these molecules have geometrical anisotropy and high polarisability [39]. In this study, compound 4f with the -OH terminal group and compound 4g with -COOH substituent showed the formation of the nematic phase. Meanwhile, compound 4h with

Structure-Properties Relationship
The study on the relationship between structure and liquid-crystal mesophase behavior is very important in designing new liquid-crystal materials with desirable properties for future applications.
Based on the POM observation, all the final compounds in the series with a Schiff base linking unit that bore a different terminal alkoxy function were mesogenic and the wider of the thermal mesomorphic range increased with increasing chain length. Compound 4a with heptyl chains showed smectic and nematic phases with the texture of thread-like four-point brushes. As the length of the alkyl chain increased, the tendency for formation of the nematic phase was reduced. This trend was observed for compounds 4b-e which exhibited only the smectic phase since the flexibility of the long chains tends to decrease the clearing temperatures, T c . Smectic phases are more ordered than the nematic phase. Nematic phase has the positional order but no orientational order and closest to the isotropic phase. Hence, effective molecular packing is essential for the formation of stable phases. Moreover, the polarisability of the molecules also increased with an increase of the alkoxy chain length in the compounds. This enhanced the cohesive forces between the sides and the core of the molecules, thus increased the tendency to form the smectic layer [35]. Molecules with smectic phase behavior are likely to favor a lamellar packing, which is due to higher Van der Waals interactions and intertwining possibility between the alkoxy chains. Thus, the nematic orientation cannot be adopted as the chain length increased and resulted in only a smectic phase in compounds 4b-e.
In this work, compounds 4a-e exhibited the smectic A phase while compound 4e showed an additional of smectic C. The presence of smectic C phase in compound 4e is due to the tilted analogue of the smectic A phase [36]. In smectic C, molecules are aligned with their long axes tilted relative to the layer normal in which the molecules are stacked. Meanwhile, molecules in smectic A are arranged in layers, with long axes perpendicular to the layers which can slide over one another [37].
Small terminal substituents, such as hydroxy or carboxy, do not always induce liquid-crystal behavior [38]. In contrast, Sudhakar et al. (2000) reported that small molecules often exhibit one or more liquid crystalline phases since these molecules have geometrical anisotropy and high polarisability [39]. In this study, compound 4f with the -OH terminal group and compound 4g with -COOH substituent showed the formation of the nematic phase. Meanwhile, compound 4h with chlorine at the terminal end also exhibited the nematic phase. The result agreed with the literature data which demonstrated that compounds with the polar group are more likely to exhibit nematic phase compared to compounds with a methyl group or hydrogen atom at the terminal chain [40]. Chlorine is a polar substituent which possesses a strong dipole moment, thus enhances the stability of the lattice and melting temperatures [41].
The effect of the NO 2 and NH 2 groups at the terminal end in promoting the mesophase properties was also highlighted. The nematic phase exhibited by compound 4i was due to the high polarity of NO 2 group which increases the molecular aromaticity and polarisability of the molecules [42]. On the other hand, compound 4j was found to be non-mesogenic. This phenomenon might be due to the properties of the NH 2 group as an electron-donating group, which increases the repulsive interactions between adjacent aromatic rings. As a result, the mesophase transitions cannot be induced. Galewski and Coles (1999) also reported that the attachment of an electron withdrawing at the terminal end of a compound induces the liquid-crystal properties, while compounds with electron-donating groups did not show any liquid-crystal properties [43].
In this research, the effect of the Schiff base (-C=N-) linking unit was also studied. When the Schiff base linking unit conjugated with the phenylene rings, the length of the molecules increased which enhanced the anisotropic polarisability and the flexibility of the compounds [44,45]. Thus, the rigidity and linearity of its constituents were maintained and impart various property alterations to mesomorphic materials.
The Schiff base linkage provides a stepped core structure which can maintain the linearity of the molecules to provide high stability [46]. This induces mesophase formation whereby the phase transition temperatures and the physical properties are usually governed by the linking group [47]. This was proven when all the compounds, 4a-i in this series (two Schiff base linking units) except for compound 4j with the terminal NH 2 substituent, showed liquid-crystal behavior. Schiff base offers the possibility of controlling the alignment and orientation of their molecules which can generate liquid-crystal materials [48]. The POM observation of compounds 4a-j is summarized in Table 3. All the hexasubstituted cyclotriphosphaze compounds 4a-j were further tested for their fire-retardant properties. All the samples were prepared using 1 wt.% to achieve the highest fire retardancy with less additive usage. Their effect is to reduce the initiation of a fire by delaying the spread of flame and provide resistance to ignition. LOI was used to determine the fire-retardant properties of the sample. This test was conducted by suspending the sample vertically inside a closed chamber where this chamber was equipped with oxygen and nitrogen gas inlets so that the atmosphere in the chamber can be controlled. The sample was then ignited from the top and the atmosphere was adjusted to determine the minimum amount of oxygen required to burn a sample at a certain time.
In this study, polyester resin has been used as a matrix for moulding. As shown in Table 4, the LOI value of pure polyester resin was determined as 22.53%. When the polyester resin was incorporated with 1 wt.% of HCCP in the matrix, the LOI value increased to 24.71%. HCCP material is well known as the compound that has high thermal stability and fire retardancy. This behavior was due to its hexa-functionality and high phosphorus content [49]. All compounds showed good agreement in the LOI results where they could be considered to be fire-retardant materials. Generally, materials with LOI value above 26% are considered to have fire-retardant property and will show self-extinguishing behavior [50,51]. The best result was achieved for compound 4i with nitro (-NO 2 ) and compound 4h with chlorine (-Cl) groups at the terminal end, with LOI values of 28.37% and 27.90%, respectively. This might be due to the electron-withdrawing properties of both the nitro and chlorine groups which induced the properties of fire retardancy. These groups can release the electron from their resonance effects due to their corresponding P-N bonds. Thus, the P-N synergistic effect was enhanced, and they exhibited both the condensed and gas phase action [52]. Similar trends can be observed for the compounds attached to hydroxy, carboxy, and amino groups at the terminal end. However, the LOI values decreased as the alkyl chain length increased. Moreover, the LOI values of the alkylated compounds were lower compared to those of compounds with the hydroxy, carboxy, and amino groups at the terminal end. The longer the alkyl tails are attached to the cyclotriphosphazene ring, the more combustible the target materials [53,54].

Instruments
In this research, Thin Layer Chromatography (TLC) is used to monitor a reaction progress and identify a product in a mixture. The ratios used were 5:95, 10:90, 15:85, 20:80, and 25:75 of ethyl acetate:hexane. Meanwhile, all the synthesized intermediates and compounds were characterized using FTIR spectroscopy (PerkinElmer, Waltham, MA, USA), NMR spectroscopy (Bruker, Coventry, UK), and CHN elemental analysis (PerkinElmer, Waltham, MA, USA). Moreover, the mesophase texture of these compounds was determined using POM (Linkam, London, UK) and their transition was further confirmed using DSC (PerkinElmer, Waltham, MA, USA). In addition, LOI (S.S. Instruments Pvt. Ltd., Delhi, India) was used to determine the minimum amount of oxygen needed to support the combustion of a sample. The sample was prepared by mixing 1 wt.% of the final compound with polyester resin. About 1 wt.% of methyl ethyl ketone peroxide (MEKP) curing agent was added to the mixture and stirred until the sample is homogeneous and then poured into the moulds. The samples were cured for 5 h in an oven at 60 • C and left overnight at room temperature before it was burned using LOI testing. The LOI test was performed using an FTT oxygen index, according to BS 2782: Part 1: Method 141 and ISO 4589 with the dimension of 120 mm × 10 mm × 4 mm. The minimum value of oxygen obtained was expressed as a percentage and the LOI results were calculated according to the equation given below: where: C F is the oxygen concentration of the final test, k is the factor obtained from the manual book Fire Testing Technology (ISO 4589), and d is the oxygen concentration increment.