Thermotropic liquid–crystalline properties of 4,4′-dialkoxy-3,3′-diaminobiphenyl compounds and their precursors

ABSTRACT A series of 4,4ʹ-dialkoxy-3,3ʹ-diaminobiphenyl compounds were synthesised by three-step procedure that involves alkylation, nitration and reduction reactions. Their chemical structures were characterised by FTIR, 1H and 13C spectroscopy and elemental analysis. Their thermotropic liquid–crystalline (LC) properties were examined by a number of experimental techniques including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarising optical microscopy (POM) and variable temperature X-ray diffraction (VT-XRD). The 4,4ʹ-dialkoxy-3,3ʹ-dinitrobipheyl compounds, precursors to the diamine compounds, were also examined for their thermotropic LC properties. POM studies of focal conic textures and VT-XRD of the 3,3ʹ-diaminobiphenyl derivatives having flexible alkyl chains (C6–C12) exhibited the smectic A (SmA) phase independent of the length of alkyl chains. Similarly, the 3,3ʹ-dinitrobiphenyl derivatives containing alkyl chains C7, and C9–C11 exhibit the SmA phase, those containing C8 formed the smectic C (SmC) phase and C12 formed both the SmA and smectic B (SmB) phases, respectively. The 3,3ʹ-diaminobiphenyl derivatives had excellent thermal stability in the temperature range of 237–329°C, while those of 3,3ʹ-dinitrobiphenyl derivatives were in the temperature range of 270–321°C. The 3,3ʹ-diaminobiphenyl derivatives emitted UV light both in chloroform and acetonitrile. Graphical Abstract


Introduction
Aromatic diamines are an interesting class of condensation monomers that are commonly used in the synthesis of aromatic polyamides and polyimides. In general, polymers from meta-oriented monomers have better solubility than those from para-oriented monomers. For example, the stark contrast in the solubility of polyterephthalamides and polyisophthalamides in organic solvents. Poly(m-phenylene isophthalamide)s are soluble in polar aprotic solvents, but poly(p-phenylene terephthalalamide)s are completely insoluble in organic solvents. Similarly, fully aromatic polyamides from 4,4ʹ-diaminobiphenyl (4,4ʹ-DABP) also known as benzidine are rather insoluble in organic solvents. The same is true for fully aromatic polyimides based on 4,4ʹ-DABP. The 3,3ʹ-DABP, an isomer of 4,4ʹ-DABP, is an attractive monomer for the synthesis of polyamides and polyimides that improve the solubility of these polymers in organic solvents and other attractive physical properties. [1,2] Although polyimides, among all glassy polymers, have been favoured and developed for gas separation membranes for three decades, one of the recent development of polyimides or related polymer structures have received great attention in the field of gas separation is the thermally rearranged polymers (TRPs) of polybenzoxazoles from ortho-hydroxypolyimides. [3][4][5][6][7][8][9] For example, the ortho-hydroxypolyimide obtained from the 3,3ʹ-dihydroxybenzidine and any aromatic dianhydride can undergo rearrangement in the solid state simply under the controlled thermal treatment at 450°C into polybenzoxazole. The thermally driven interconversion produces a new generation of materials having outstanding properties, particularly those related to the separation of gases. Similarly, the 3,3ʹ-diamino-4,4ʹdihydroxybiphenyl is an isomer of 3,3ʹ-dihydroxybenzidine, which has also drawn attraction in the synthesis of TRPs such as polybenzoxazoles, that render the occurrence of thermal rearrangement at low temperatures. Gas permeability (particularly CO 2 ) of polybenzoxazole is found to be much better than those of 3,3ʹdihydroxybenzidine. [10] In addition, it has been used in the synthesis of new polyimides with desirable properties such as good UV absorption, low water absorption, low surface energy and good mechanical properties. [11] In parallel efforts, we are interested in various aromatic and aliphatic diamino-compounds for the synthesis of poly(pyridinium salt)sa class of main-chain ionic polymers that are usually prepared by ring-transmutation polymerisation reaction of bispyrylium salts and diamines and by metathesis reaction. Depending on the chemical structures of the bispyrylium salts and diamines, they can be π-conjugated or non-conjugated and exhibit a number of interesting properties. These properties include thermotropic and lyotropic liquid crystallinity in polar protic and aprotic solvents and even amphotropic liquid-crystal phases and light-emitting properties in both solution and solid states. [12][13][14][15][16][17] This class of ionic polymers are useful for biosensors. [18] For example, the π-conjugated poly(pyridinium salt), prepared from bispyrylium salt and benzidine, has been found to be a sensitive fluorescence biosensor for homogeneous deoxyribonucleic acid (DNA) detection. [19] Recently, we reported that the dispersion of single-walled carbon nanotubes in dimethyl sulfoxide occurred with poly(pyridinium) salts via non-covalent interactions. [20,21] Herein, we report the design and synthesis of 4,4ʹalkoxy-3,3ʹ-diaminobiphenyl (DA1-9) from 4,4ʹ-biphenol in three steps (Scheme 1) and characterise their chemical structures by various experimental techniques included Fourier transform infrared (FTIR), Fourier transform nuclear magnetic resonance (FTNMR) and elemental analysis. The characterisation of their thermotropic liquid-crystalline (LC) phases is performed by a number of experimental techniques including the differential scanning calorimetry (DSC), polarising optical microscopy (POM) and variable temperature X-ray diffraction (VT-XRD). With the exceptions of DA1 and DA9, they exhibit strongly birefringent smectic A (SmA) phases. Interestingly, some of the nitro-compounds DN3-8, precursors to DA3-8, also exhibit thermotropic SmA, SmC and SmB phases. In the latter case, the nature of their thermotropic LC phases is dependent on the number of carbon atoms in the alkyl chain. To our knowledge, diaminobiphenyl derivatives and dinitrobiphenyl derivatives in the present study have never been reported to form thermotropic LC phases. Note here that diamino-compounds are relatively uncommon to exhibit LC phases [22], while nitro-compounds are found abundantly to exhibit LC phases. [23,24] However, Krowczynski et al. [25] reported that two compounds, 4-octyloxy-1,2-diaminobenzene and 4-decyloxy-1,2diaminobenzene, formed the SmA phase with unusually small optical birefringence through a network of interconnected hydrogen-bonded rings. Weak birefringence is related to small shape anisotropy of these one-ring only [25] molecules. The optical spectroscopic properties of DA1-9 both in chloroform (relatively non-polar) and acetonitrile (polar aprotic) solvents are also studied using UV-visible absorption and fluorescence spectroscopy techniques.

Results and discussion
2.1 Synthesis of DA1-9 All of these compounds were prepared from 4,4ʹ-biphenol by three-step procedures. These procedures are alkylation, nitration and reduction steps. The yields in the reduction step were in the range of 60-77%. However, the alkylation of 3,3ʹ-dinitro-4,4ʹ-biphenol followed by reduction procedures did not produce the desired products in our hands.
2.2 Thermal properties, variable temperature XRD and optical textures. [26][27][28][29] DN1 exhibited a crystal-liquid transition (T m ) at 224°C with ΔH = 129.8 J/g in the first heating cycle of its DSC thermogram. Correspondingly, it had a cooling exotherm at 213°C with similar heat of crystallisation. In the second heating and subsequent cooling cycles, there were a T m and a crystallisation exotherm, respectively. The reported melting point (Mp) of the compound is found to be 224°C. [30] DA1 exhibited a T m at 206°C with ΔH = 144.7 J/g and low-temperature crystal-crystal transition at 184°C in the first heating cycle. Correspondingly, it had a cooling exotherm at 191°C with similar heat of crystallisation. In the subsequent second heating and cooling cycles, there were a T m and a crystallisation exotherm, respectively, with somewhat enthalpies of melting and crystallisation ( Figure S1).
The reported Mp of the compound is found to be 195°C. [30] DN2 exhibited a small low-temperature endotherm at 82°C and large endotherm at 95°C in the first heating cycle of its DSC thermogram. In the subsequent cooling cycle, there was one small exotherm at 85°C and another large exotherm at 81°C. In the subsequent second heating cycle, there was a large endotherm similar to the first heating cycle. In the second cooling cycle, there were two exotherms, which were identical to those in the first cooling cycle ( Figure S2a). The variable temperature (VT-XRD) in conjunction with POM studies ( Figure S3) indicated that large endotherm was related to the T m . Additionally, X-ray diffraction patterns showed only a transition from isotropic (95.6°C) to crystal (94.3°C). Therefore, it did not exhibit thermotropic LC properties. In supplementary information (SI), Figure S2b shows the DSC thermograms of DA2 obtained at heating and cooling rates of 10°C/min. In each of the first and second heating cycles, there were two endotherms for this compound.
In the cooling cycles, there were distinctly three exotherms. The VT-XRD in combination of POM studies (Figures 1 and 2) suggested that low-temperature endotherm was related to the crystal-to-SmA transition (T m ) and the high-temperature endotherm related to SmA-to-isotropic (T i ) transition. In the cooling cycle, the two high-temperature exotherms were related to the isotropic-to-SmA transition and then SmA-to-crystallisation transition. The lowest-temperature exotherm presumably was related to a crystal-crystal transition.
DN3 showed three endotherms in its first heating cycle (Figure 3(a)) of its DSC thermogram, but it showed two endotherms in the second heating cycle. There were two exotherms in each of the cooling cycles. In the second heating cycle, the endotherm at circa (ca.) 78°C was related to the T m and the endotherm at ca. 92°C related to T i resulting in a mesophase range of 14°C. The presence of lowest-temperature endotherm at ca. 61°C in the first heating cycle and the absence of this endotherm in the second heating cycle suggested the transition was presumably related to crystal-crystal transition. It showed the typical focal conic texture (not shown) as determined from the POM studies. The VT-XRD studies ( Figure 4) confirmed the SmA mesophase for this compound. DA3 exhibited two endotherms (85°C and 110°C) in the first heating cycle and three exotherms (107°C, 70°C and 31°C) in the first cooling cycle. In the subsequent second heating cycle, there were three endotherms (40°C, 82°C and 110°C) and correspondingly, three exotherms (107°C, 69°C and 31°C) in the second cooling cycle (Figure 3(b)). The endotherm at 85°C was related to the T m and that at 110°C was related to T i resulting in a mesophase range of 25°C. The lowesttemperature endotherm at 40°C was related to the polymorphism. It also showed the typical focal conic texture (not shown) as determined from the POM studies. The VT-XRD studied ( Figure S4) confirmed the SmA mesophase for DA3.
DN4 showed one large (83°C and 81°C) and one small (96°C and 96°C) endotherm in the first and second heating cycles, respectively, of its DSC thermograms. In their cooling cycles, two exotherms appeared at 92°C and 65°C ( Figure S5). Its large endotherm corresponded to the T m and small endotherm to T i resulting in a mesophase of temperature range 13°C. Its VT-XRD studies (Figures 5 and 6) and the POM studies suggest that DN4 exhibited a SmC phase (tilt angle of 20°with respect to the layer normal) with Schlieren texture. DA4 displayed a broad, large endotherm in the first heating cycle, but displayed two endotherms in the second heating cycle. In the cooling cycles, there were two major exotherms. The lowertemperature exotherm was associated with crystallisation with shoulder peak(s) presumably polymorphism phenomenon ( Figure S5). Its VT-XRD studies (Figures 7 and 8) including the POM studies suggest that DA4 exhibited SmA with the bâtonnets texture.
In the first heating cycle of DSC thermogram, DN5 showed clearly three endotherms (63°C, 76°C and 97°C ), but in the second heating cycle it showed two endotherms (75°C and 97°C). In the first and second cooling cycles, it showed two exotherms located at peak maxima 93°C and 62°C. The absence of lowest-temperature endotherm was presumably related to the crystal-crystal transition. The endotherm at 75°C corresponded to its T m and that of at 97°C was related to its T i that resulted in a mesophase range of 22°C ( Figure S6a). These results were confirmed by VT-XRD studies and POM textures ( Figure S7). DA5 exhibited three endotherms (66°C, 83°C and 112°C) in the first heating cycle and three endotherms (58°C, 82°C and 112°C) in the second heating cycle of its DSC thermograms. In its first cooling cycle it showed four exotherms (107°C, 48°C, 40°C and 37°C), but three exotherms (107°C, 47°C and 41°C) in the second cooling cycles ( Figure S6b). The highest-temperature endotherm was related to the SmA to T i and highertemperature endotherm to T m and lowest-temperature endotherm to crystal-crystal transition. These statements were corroborated with the VT-XRD and POM studies. Figure 9 shows the diffraction patterns (a-b) of DA5 indicative of SmA, which were supportive from the optical textures of typical focal conic textures.
In the first heating cycle of DSC thermogram, DN6 showed clearly two major endotherms (65°C and 99°C). Additionally, the low-temperature endotherm contained a shoulder peak at 74°C. In the second heating cycle it showed a glass transition, T g at 10°C, a cold-crystallisation exotherm and three endotherms (63°C, 73°C and 99°C). In the first and second cooling cycles, it showed two exotherms located at peak maxima 96°C and 62°C. The endotherm at 73°C corresponded to its T m and that of at 99°C was related to its T i that resulted in a mesophase range of 26°C ( Figure S8a). These results were confirmed by VT-XRD studies and POM textures ( Figure S9). DA6 exhibited two endotherms (79°C and 112°C) in the first heating cycle and two endotherms (81°C and 112°C) in the second heating cycle of its DSC thermograms. In its first cooling cycle it showed two exotherms (107°C and 48°C) and two exotherms (107°C and 47°C) in the second cooling cycles, respectively ( Figure S8b). The high-temperature endotherm was related to the SmA to T i and low-temperature endotherm to T m transitions. These statements were corroborated with the VT-XRD and POM studies. Figure S10 shows the diffraction patterns (a-b) of DA7 indicative of SmA, which were supportive from the optical textures of typical focal conic textures. In the first heating cycle the DSC thermogram of DN7 showed clearly two endotherms (76°C and 99°C). In the second heating cycle it showed a cold-crystallisation exotherm prior to the low-temperature endotherm at 76°C and a high-temperature endotherm at 99°C. In each of the cooling cycles, it showed three exotherms located at peak maxima 96°C, 62°C and 27°C. The endotherm at 76°C corresponded to its T m and that of at 99°C was related to its T i that resulted in a mesophase range of 23°C ( Figure S11a). These results were supported by VT-XRD studies and POM textures ( Figure S12). DA7 exhibited three endotherms (77°C, 85°C and 111°C) in the first heating cycle and three endotherms (73°C, 85°C and 111°C) in the second heating cycle of its DSC thermograms. In its first cooling cycle it showed two exotherms (107°C and 55°C) and two exotherms (107°C and 56°C) in the second cooling cycles, respectively. The low-temperature exotherm was associated with a weak shoulder peak ( Figure S11b) that indicated the existence of polymorphism. The highest-temperature endotherm was related to the SmA to T i , higher-temperature endotherm to T m , and low-temperature endotherm to crystal-crystal transitions. It had a mesophase range of 26°C. These results were consistent with the VT-XRD and POM studies. Figure S13 shows the diffraction patterns (a-b) of DA7 indicative of SmA, which were supportive from the optical textures of typical focal conic textures.
DN8 exhibited large and small endotherms at peak maxima of 72°C and 99°C, respectively, in the first heating cycle of its DSC thermogram. Correspondingly, a large and a small endotherms at peak maxima 68°C and 99°C, respectively, appeared in the second heating cycle. However, a cold crystallisation exotherm preceded a large endotherm. In its first cooling cycle, there were three exotherms at peak maxima 95°C, 60°C and 31°C. Three exotherms at identical peak maxima appeared in the second cooling cycle ( Figure S14a). The VT-XRD and POM studies ( Figure 10) revealed that the large endotherm was related crystal to SmB (T m ) and the small endotherm indicated its T i . These studies further revealed that the highest-temperature exotherm in cooling cycle indicated the transition from the isotropic liquid to SmA, higher-temperature exotherm corresponded to the transition from SmA to SmB and lowtemperature exotherm to the transition from SmB to crystallisation. DA8 also exhibited a large and a small endotherms at peak maxima 83°C and 110°C, respectively, in the first heating cycle of its DSC thermogram. Correspondingly, a large and a small endotherms at peak maxima 80°C and 110°C, respectively, appeared in the second heating cycle. Before the large endotherm in each of the heating cycles there was an indistinct endotherm due to crystal-crystal transition. In its first cooling cycle, there were two exotherms at peak maxima 105°C and 34°C. Two major exotherms at peak maxima  of 104°C and 36°C and a minor exotherm at ca. 50°C appeared in the second cooling cycle ( Figure S14b). In combination of VT-XRD and POM studies ( Figure 11) it was found that the large endotherm at ca. 84°C was related to T m and the small endotherm at 110°C related to T i . Additionally, its mesophase was confirmed to be the homeotropic texture of SmA.
DN9, is a branched isomer of DN4, did not form a LC phase. It had a T m at 72°C in the first heating cycle and a T c at 51.9°C in the first cooling cycle (( Figure S15). These results were supported by VT-XRD and POM studies ( Figure S16). DA9 was a liquid at rt and did not form LC phase. Figure 12 shows the transition temperatures as function of number of carbon atom in the alkoxy groups for DN1-8 and DA1-8 reveals an odd-even effect that attenuates on increasing chain length, which are in excellent agreement with those reported by Imrie and Taylor. [32] The different phases and phase transitions of DN2-9 and DA2-8 are summarised for convenience in Table 1. All of DN3-8 and DA2-8 showed relatively low T m values above which they formed highly birefringent smectic LC phases with the exception of DA8 that exhibited a homeotropic aligned SmA phase even in the absence of magnetic field. They also exhibited relatively low T i values. Both the relatively low T m values and low T i values were related to the presence of flexible alkyl chains in their chemical structures. As expected, in their DSC thermograms the T m transitions underwent high degrees of supercooling, but T i transitions underwent low degrees of supercooling with the exception of DA4 that underwent relatively high degree of supercooling. They generally tended to exhibit crystal-crystal transitions prior to their melting transitions because of the presence of flexible alkyl chains in their chemical structures. DN1 and DN2 did not form LC phases, but DN3-DN8 (C 7 -C 12 ) formed the SmA phase. These results suggested that the C 1 and C 6 groups being short did not induce LC phase in these two compounds. DA1 (C 1 ) did not form LC phase due to short chain, but DA2-8 (C 6 -C 12 ) did form the SmA phase. One additional comment is that DN4 formed the SmC phase, but DA4 formed the SmA phase. These results are in agreement with McMillan's model for SmC phase. According to this model, DN4 contains the more strongly polar -NO 2 than -NH 2 group and hence gives rise to a stronger transverse component of the associated dipole causing tilting of the molecules. [26] In contrast, DN3, DN5, DN6 and  specific compound. Comparing DN4 (C 8 ) and DN9 (branched C 8 ), it was found that branching of alkyl chain was not conducive for the formation of LC phase in the series of dinitrobiphenyl derivatives. Similarly, comparison DA4 (C 8 ) and DA9 (branched C 8 ), the same trend was found to be true in the series of diaminobiphenyl derivatives. Although the literature is replete with many chemical structural diversities that exhibit LC phases, [33][34][35][36] the simple dinitro-and diaminobiphenyl derivatives in the present study that exhibited LC properties are quite interesting and hence expanding the repertoire of LC materials.

Thermal stabilities of DN1-9 and DA1-8
The thermal stabilities for all of the compounds were studied by TGA analyses and determined as the temperatures (°C) at which a 5% weight loss for each the compounds occurred at a heating rate 10°C/min in nitrogen. Despite the presence of flexible alkyl chains, TGA thermograms of DN1-9 (Figure 13(a)) show relatively high thermal stabilities that are in the range of 270-321°C and gradually increase with the increase in carbon number in the alkyl chain. Similarly, DA1-8 also show relatively high thermal stabilities that are in the range of 237-329°C (Figure 13(b)) and gradually increase with the increase in carbon number in the alkyl chain.

Optical properties
Because of the presence of two amino groups and biphenyl moieties, optical properties of DA1-8 were examined by UV-Vis and photoluminescence spectrometer in both relatively non-polar solvent such as chloroform and polar solvent such as acetonitrile. In the UV-Vis absorption spectrum, DA2 and DA5 both showed identical two absorption peaks at 230 and 310 nm (not shown). All of the compounds in acetonitrile emit UV light at 357 nm (<400 nm) when excited at 300 nm of light. Figure 14 shows the excitation and emission spectra of DA2 and DA5 in acetonitrile. Both of them clearly show identical the emission of light of 357 nm when excited at 300 nm of light. They also exhibited identical excitation spectrum in the same solvent that consists of two peaks at 214 and 300 nm. In chloroform, they exhibited identical λ em at 351 nm (<400 nm), which is hypsochromically shifted by 6 nm when compared with those in acetonitrile. This phenomenon is known as positive solvatochromic effect. [37,38]

Conclusions
We presented the synthesis of DA1-9 prepared from 4,4ʹ-biphenol in three-step procedure that involves alkylation, electrophilic aromatic substitution and reduction reactions. Their chemical structures including their precursors were determined by spectroscopic techniques and elemental analysis. Their thermal properties including thermotropic LC properties were examined by a number of experimental techniques that included DSC, TGA, POM and VT-XRD studies. Similarly, the precursors of DA1-9, DN1-9 were also examined for thermotropic LC properties.
All the compounds DA2-8 (C 6 -C 12 ) exhibited the SmA phase based on observation of a typical focal conic texture or homeotropic texture using POM and verified by VT-XRD. DA1 (C 1 ) did not form the LC phase because of its short chain. DA9 (branched C 8 ) also did not form LC phase, branching of alkyl chain was not conducive to the formation of LC. DN3-8 (C 7 -C 12 ) exhibited the SmA phase with the exception of DN4 and DN8. DN4 showed the SmC phase that is consistent with the McMillan's model of tilted phase. [26] DN8 exhibited both the SmA and SmB as observed by POM and VT-XRD. DN1 (C 1 ) and DN2 (C 6 ) did not form the LC phase because of their relatively short alkyl chains. DN9 (branched C8) also did not form an LC phase, since branching of alkyl chain was not conducive to the formation of LC. DA1-8 had excellent thermal stabilities in the range of 237-329°C and DN1-9 also had thermal stabilities in the range of 270-321°C. The DA1-8 emitted UV light in chloroform (non-polar) and acetonitrile (polar) as measured by luminescent spectrometer. For example, DA5 emitted at 357 nm in acetonitrile when excited at 300 nm. In chloroform, it emitted at 351 nm wavelength when excited at 260 nm light. They showed a positive solvatochromic effect upon changing from non-polar to polar solvent. Since they are diamine compounds, they are suitable for the synthesis of many classes of neutral polymers including polyamides, polyimides and ionic polymers such as poly (pyridinium salt)s.

Instrumentation
The FTIR spectra of the compounds were recorded with a Shimadzu IRPrestige FTIR analyser with their neat films on KBr pellets. The 1 H and 13 C Nuclear magnetic resonance (NMR) spectra of all of the compounds in CDCl 3 or acetone-d 6 were recorded by using VNMR 400 spectrometer operating at 400 and 100 MHz at room temperature. Elemental analysis was performed in Atlanta Microlab Inc., Norcross, GA. Their UV-Vis absorption spectra in organic solvents were recorded with a Varian Cary 3 Bio UV-Vis spectrophotometer also at room temperature (rt). DSC measurements of all of the compounds were conducted on TA module DSC Q200 series in nitrogen at heating and cooling rates of 10°C/min. The temperature axis of the DSC thermograms was calibrated before using the reference standard of high purity indium and tin. Their TGA were performed using a TGA Q50 instrument at a heating rate of 10°C/min in nitrogen. Optical textures of compounds DA1-9 and DN1-9 were taken using Olympus POM (BX51) equipped with crossed polariser from sample microscope slides placed in Mettler heating stage (FH90) for controlled heating and cooling. Textures were recorded during cooling at a rate of 1°C/min. X-ray scattering measurements of the compounds were done using Rigaku Screen Machine (λ = 1.54187 Å) at Kent State University, Ohio (X-ray source is from MicroMax-003 microfocus sealed tube generator and detector is the Mercury 3 CCD, 75 mm diameter). The sample to detector distance is 76.25 mm, which is calibrated with silver behenate. Samples were contained in quartz capillary tubes (1 mm) and then flame sealed. The sample holder is Linkam-CAP, X-ray measurements were taken at different temperatures from isotropic to crystal phase. Data analyses were performed by using Fit2D and background used was the empty capillary. The UV-Vis absorption spectra of the compounds dissolved in organic solvents were recorded using Varian Cary 50 Bio UV-visible spectrophotometer in quartz cuvettes at rt. Their photoluminescence spectra in various organic solvents were recorded using a Perkin-Elmer LS-55 luminescence spectrometer with a xenon lamp as a light source.

Synthesis of 4,4ʹ-dialkoxybiphenyl (DABP1-9)
All of these compounds were synthesised by Williamson reaction of 4,4ʹ-biphenol with the corresponding iodomethane or n-bromoalkanes or 1bromo-2-ethylhexane in presence of K 2 CO 3 in acetonitrile on heating to reflux. The typical procedure was described as follows.      [31] and crystalline phases at 68.1°C and 46.6°C, respectively (taken on cooling from the isotropic phase, and (c) and (f) were taken before sample was heated to isotropic).   4,4ʹ-Di-2-ethylhexyloxybipheyl (DABP9). This compound was purified by silica gel column chromatography using ether as eluent. The obtained solid product was used in the next step.

Synthesis of 4,4ʹ-dialkoxy-3,3ʹ-dinitrobiphenyl (DN1-9)
These compounds were prepared from the nitration reaction of the corresponding DABP1-9 with HNO 3 in acetic acid. The procedure for the preparation of 4,4ʹ-dimethoxy-3,3ʹ-dinitrobipheyl (DN1) was described as follows. To 8.6 g (40 mmol) of DABP1 was added to 100 mL acetic acid in a reaction flask at room temperature, followed by slow dropwise addition of 4.7 mL of concentrated HNO 3 (104 mmol). The contents of the reaction flask were heated to 100°C for 24 h. At the end of the reaction, contents of reaction flask were cooled to rt and poured into ice-cooled water. The product was precipitated out and then collected by vacuum filtration to give the crude product. It was washed with distilled water several times air-dried. It was then recrystallised from toluene and vacuum dried to produce 6.5 g. Yield = 53%. IR ( Under the similar conditions, DN2-DN9 could not be prepared in our hands. However, they were prepared by the general procedure using a mixture of 70% HNO 3 (aq) and 90% HNO 3 (aq) in acetic acid. The typical procedure was described for DN4 as an example. To an amount of 9.0 g (21.9 mmol) of DABP4 was added 125 mL acetic acid in a roundbottomed flask. To this flask, a mixture of 6.5 mL of 70% HNO 3 (aq) and 0.5 mL of 90% HNO 3 (aq) that was diluted with a few mL of acetic acid that was added slowly at rt. Then the contents of the reaction flask were heated to 100°C for 24 h. At the end of the reaction, contents of the reaction flask were cooled to rt and poured into ice-cooled water. The solid product was precipitated out, then filtered and washed several times with distilled water until the filtrate became neutral as tested with the litmus paper. It was then air-dried to yield 6.3 g of crude product, which was recrystallised from acetonitrile. Yield = 50%. IR (KBr) ν