Heptasubstituted triphenylene 2,3-dicarboxylic esters discotic liquid crystals with three different types of peripheral substitution

ABSTRACT Hexasubstituted triphenylene (1a) containing four alkoxy and two adjacent methoxycarbonyl tails can be smoothly brominated or nitrated at its α-position(s) of two electron-rich rings to give a series of bromides and nitro compounds through electrophilic aromatic substitutions. Positions of bromo or nitro groups in target compounds were unambiguously established by combined analysis of 2D nuclear magnetic resonance experiments including nuclear Overhauser effect and heteronuclear multiple-bond correlation which indicated that monobromination or mononitration occurred preferentially at position 8 or 5 of 1a, respectively. Calculations of molecular geometry on 2a–e reveal that the α-Br or α-NO2 group(s) would induce different degrees of helical deformation in the triphenylene core and the degree of non-planarity decrease in the following order: 2c > 2b > 2e > 2a > 2d. In contrast to highly twisted 2b–c and 2e, which did not form a mesophase, the formation of a hexagonal columnar phase at room temperature even down to −50°C was observed and confirmed by polarising optical microscopy and X-ray diffraction for sterically less hindered, monosubstituted 2a and 2d. Graphical Abstract


Introduction
Triphenylene-based compounds are the most widely studied functional materials in the family of discotic liquid crystals (DLCs) owing to their remarkable self-assembling and charge-transporting properties [1][2][3][4][5][6][7][8][9][10][11]. The nature of functional groups attached to the aromatic core of a discotic mesogen has been shown to strongly influence liquid crystalline properties [12][13][14][15]. The well-studied electronrich hexaalkoxytriphenylene (HAT) usually exhibits hexagonal columnar mesophases over relatively narrow temperature ranges above room temperature [13,16]. Williams and co-workers have shown that the addition of electronwithdrawing substituents such as cyano groups into the electron-rich triphenylenes can lead to an increase in the molecular dipole moment, which help to promote the formation and stabilisation of columnar phases by energetically favourable dipole-dipole interactions between molecules within the columns [17]. Several studies on DLCs highlight the importance of other electron-withdrawing groups such as halo [17], nitro [17], dicarboxylic ester [18] and imide [19][20][21] in promoting stable mesophases. Moreover, our recent research indicated that suitable peripheral functionalisation of the DLCs with a combination of electron-donating and electron-withdrawing functional groups at appropriate location not only results in significant differences in mesogenic behaviour of the molecules but also renders the molecules with the interesting optoelectronic properties, including large Stokes shifts and low-lying lowest unoccupied molecular orbitals energy levels [19].
Previous synthetic routes involve the introduction of electron-withdrawing groups, which are achieved by direct substitution into the triphenylene core through classic electrophilic aromatic substitutions [22][23][24][25][26][27][28]. Traditionally, electrophilic substitution at the βor 2position on unsubstituted triphenylene is favoured over the substitution at the αor 1-position, presumably because of the severe steric hindrance. However, the most reported electron-rich hexaalkoxyltriphenylene has been proven to be a useful precursor for the synthesis of α-substituted triphenylene discotics in the bay regions (1, 4, 5, 8, 9 and 12 positions) of the core. Previous work by Bushby [22], Praefcke et al. [27] and Kumar et al. [24] on mono-and di-functionalised HAT indicated that functionalisation of the nucleus at the α-position is important not only to change the molecular conformation in response to increased polarity and enhanced mesophase stability but also to induce the colour of molecule. More importantly, it was also reported that steric crowding imparted by αsubstituent(s) of a triphenylene molecule would induce torsion of the planar triphenylene core, which has already been demonstrated by the formation of a strong helical twist in central rigid cores of 1,12-diiodotriphenylene [29] and HAT2-NO 2 [22] from the results of X-ray crystal structure analysis. Accordingly, introducing various bulky substituents at the α-position of a HAT with six β-side chains is an effective strategy to create conditions for molecule chirality and further preparation of chiral DLCs. Halogen and nitro substituents were chosen as promising candidates (Scheme 1).
Moreover, almost all the literature-reported triphenylene derivatives were confined to having either a single type of peripheral substitution pattern, or two types of peripheral substitution. Relatively few DLCs having more than two types of peripheral substituents are known because of synthetic problems [30]. In our effort to further examine the effects of substituents on liquid crystalline behaviours, we are interested in the synthesis of a more diverse series of triphenylene-based molecules having three different types of peripheral substitution. Herein, we report the synthesis and characterisation of series of α-substituted triphenylene 2,3dicarboxylic esters through electrophilic aromatic substitution based on our previous work. The target mixed-substituent compounds have also been investigated to explore the effects of α-substitute on mesomorphic and optical properties of these molecules.

Materials
All reactions involving Grubbs' second-generation catalyst were carried out under an atmosphere of ethylene (1 atm) using standard Schlenk techniques. Toluene was freshly distilled over sodium with the use of diphenyl ketone as an indicator under nitrogen. Dichloromethane was dried by anhydrous calcium chloride and distilled over Scheme 1. Structures of HAT6-X (X = F, Cl, Br and NO 2 ) reported and target triphenylene 2,3-dicarboxylic ester with α-substituent(s). phosphorus pentoxide. Grubbs' second-generation catalyst was synthesised according to literature procedure [31,32] and CuI was purchased from Shanghai Aladdin Reagent Co., Ltd. Tetraisopropyl titanate [Ti(O i Pr) 4 ] was provided by Shanghai Darui Fine Chemicals Co., Ltd, and 1 mL Ti (O i Pr) 4 was dissolved in 9 mL dry toluene at a concentration of 0.096 mg/mL which served as the stock. Symmetrical diarylacetylenes were synthesised according to the literature procedures [33]. All other chemicals were purchased commercially and used as received unless indicated otherwise. Column chromatography was performed on silica gel (200-300 mesh) and all reported yields are isolated yields. All target compounds were recrystallised twice from CH 2 Cl 2 /CH 3 OH prior to spectral and phase behaviour analysis.

Instrumentation
1 H NMR (nuclear magnetic resonance; 400 MHz) and 13 C NMR (100 MHz) were run on Varian 400M spectrometers with CDCl 3 as solvent and tetramethylsilane (TMS) as internal standard. Chemical shifts were reported in units of ppm by assigning TMS resonance in the 1 H spectrum as 0.00 ppm and CDCl 3 resonance in the 13 C spectrum as 77.0 ppm. All coupling constants (J values) were reported in hertz. The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, dd = doublet of doublet and m = multiplet. Fourier-transform infrared spectra were recorded on a Bruker VERTEX70 spectrometer (KBr disc). High-resolution mass spectra were obtained with a Bruker BioTOF-Q mass spectrometer. UV-vis absorption spectra were recorded at room temperature using a PerkinElmer Lambda 950 spectrophotometer. Photoluminescence (PL) spectra were measured on Horiba Fluorolog-4 spectrophotometer and luminescence quantum yields were measured by the optical dilute method by Crosby and Demas with [Ru(bpy) 3 ]Cl 2 in degassed solution as standard (Φ lum = 0.042, excitation wavelength at 436 nm) [34]. Polarising optical microscopy (POM) images of the samples sandwiched between two glass slides were recorded on the OLYMPUS BX41 with hot stage of HCS302-GXY, and INSTEC STC200 temperature controller. A TA-Discover calorimeter was used to determine melting points, the transition temperatures and transition enthalpies; heating and cooling rates were 10°C min −1 under a nitrogen flow. Thermal gravimetrical analysis (TGA) was carried out on a TA TGA-Q500 instrument at a heating rate of 10°C min −1 under a nitrogen flow. Temperature-variation X-ray diffraction (XRD) experiments were performed on a Rigaku SmartLab 3 X-ray diffractometer equipped with a TCU 110 temperature control unit. The sample temperature was controlled within ±1 K. The X-ray sources (Cu Kα, λ = 0.154 nm) were provided by 40 kW ceramic tubes.

Synthesis of 1a
A dry, 25-mL Schlenk tube equipped with a stir bar was charged with diarylacetylene (0.125 mmol), Grubbs' second-generation catalyst (10 mg, 0.0125 mmol, 10 mol %) and CuI (1.2 mg, 0.00625 mmol, 5 mol %) under 1 atm ethylene pressure (using balloon). Toluene solution of Ti(O i Pr) 4 (18.5 μL, 0.00625 mmol, 5 mol %) and dry toluene (5 mL) was added and the reaction mixture was stirred at 80°C for 24 h under 1 atm ethylene pressure (using balloon). After cooling to room temperature, dimethyl acetylenedicarboxylate (89 mg, 0.625 mmol) was added under air conditions and the resulting mixture was stirred at 100°C for another 24 h in the sealed vessel. After cooling to room temperature, the reaction mixture was filtered through a short pad of silica gel eluting with CH 2 Cl 2 . The volatiles were removed in vacuo. The residue was dissolved in 10 mL dichloromethane and a solution of FeCl 3 (4 equiv., 81 mg) in 1 mL nitromethane is added dropwise at 0°C under nitrogen atmosphere. After the starting material had disappeared by thin-layer chromatography (TLC) monitoring, the reaction was quenched by the addition of a few drops of methanol and extracted with CH 2 Cl 2 (3 × 15 mL). The organic layer was washed with brine, dried over MgSO 4 and concentrated. The crude product was purified by silica gel column chromatography (elution with 7:1 petroleum ether-ethyl acetate) to give the final product.

Synthesis of 2a
1a (30 mg, 0.04 mmol) was dissolved in 1.5 mL CH 2 Cl 2 and cooled to 0°C. Bromine (27 mg, 0.167 mmol), dissolved in 2 mL of CH 2 Cl 2 , was added dropwise over a period of 20 min. The reaction was stirred for another 1.5 h at 0°C before saturated sodium bisulphite was added and stirred for a further 5 min. Then the reaction mixture was extracted with CH 2 Cl 2 (3 × 15 mL). The organic layer was washed with brine, dried over anhydrous MgSO 4 and concentrated. The crude product was purified by silica gel column chromatography (elution with 8:1 petroleum etherethyl acetate) to give 16.5 mg of 2a.

Synthesis of 2b-c
1a (30 mg, 0.04 mmol) was dissolved in 1.5 mL CH 2 Cl 2 . Bromine (27 mg, 0.167 mmol) was added dropwise over a period of 20 min at 25°C. The reaction was stirred another 1 h at 25°C before saturated sodium bisulphite was added and stirred for a further 5 min. Then the reaction mixture was extracted with CH 2 Cl 2 (3 × 15 mL). The organic layer was washed with brine, dried over anhydrous MgSO 4 and concentrated. The crude product was purified by silica gel column chromatography (elution with 8:1 petroleum ether-ethyl acetate) to give 10.8 mg of 2b and 16.1 mg of 2c.

Synthesis of 2d-e
1a (30 mg, 0.04 mmol) was dissolved in 1.5 mL dichloromethane and 0.5 mL nitromethane. Concentrated nitric acid (0.1 mL) was added in one portion at 0°C. The reaction was stirred for another 1 h at 0°C before saturated sodium bisulphite was added and stirred for a further 5 min. Then the reaction mixture was extracted with CH 2 Cl 2 (3 × 15 mL). The organic layer was washed with brine, dried over anhydrous MgSO 4 and concentrated. The crude product was purified by silica gel column chromatography (elution with 8:1 petroleum ether-ethyl acetate) to give 9.8 mg of 2d and 17.3 mg of 2e.

Synthesis
Scheme 2 shows the synthetic route to the targeted compounds. The synthesis of parent triphenylene 2,3dicarboxylic esters (1a) has been previously reported in our group and was achieved in a one-pot manner through a cascade reaction involving enyne metathesis of readily available diarylacetylene, cycloaddition with dienophile, and subsequent aromatisation and oxidative cyclodehydrogenation [18]. This previously developed approach allows the rapid assembly of disc-shaped molecules having two different types of peripheral substitution. Concentration of bromine and temperature plays an important role in the bromination of 1a. If 1a is treated with diluted bromine in CH 2 Cl 2 at 0°C for 1.5 h, a monobromide (2a) is mainly obtained showing five aromatic hydrogen singlets in the 1 H NMR spectrum. A trace (less than 1%) of the dibromide (2b) was monitored by TLC. Treatment of 1a with diluted bromine at 25°C for 1 h gave mainly a mixture of dibromide (2b) and tribromide (2c). After carefully performing column chromatography, 2b and 2c can be obtained in 30% and 41% isolated yield, respectively. Unlike the nitration of hexaalkoxyltriphenylene, the reaction of 1a with concentrated nitric acid was unsuccessful in a solvent mixture of diethyl ether and acetic acid. In contrast, treatment of 1a with concentrated nitric acid in a solvent mixture of dichloromethane and nitromethane at 0°C for 1 h could provide mononitro compound (2d) and dinitro compound (2e) in 31% and 41% isolated yield, respectively; the product of trinitration however was not obtained. All the synthesised compounds were fully characterised by NMR and high resolution mass spectrometry (HRMS) spectrometry. Further, positions of bromo or nitro group were established by combined analysis of 2D NMR experiments including NOE (nuclear Overhauser effect), heteronuclear single quantum coherence and HMBC (heteronuclear multiplebond correlation).

Structure elucidation
In C 6 symmetric HAT6, there is only one possible site for further monosubstitution, but in our parent compound 1a all β-positions are blocked by four alkoxy groups and two adjacent methoxycarbonyl tails. It would be reasonable to assume that 1a possess reactive sites capable of bromination or nitration and the electrophilic aromatic substitution can occur on the α-position of two electron-rich rings which bears two alkoxy substituents. Thus, elucidating the position of the monosubstituted group in products 2a or 2d is not straightforward for the positions of substitution group having two choices: position 5 or 8.
The key HMBC and Nuclear Overhauser effect spectroscopy (NOESY) correlations in monosubstituted products 2a and 2d are summarised in Scheme 3. For 2a, the signals for the hydrogens at positions 1 and 4 can be assigned on the basis of HMBC correlation between the proton signal and the carbon of carbonyl. The signals for the hydrogens at positions 5 and 12 can also be assigned on the basis of the NOE interactions between the aromatic protons and the CH 2 O signals of the side chains at δ around 4.20 ppm. This evidence together with two NOE interactions between positions 1 and 12 and between positions 4 and 5 enables the structure confirmed as drawn and indicates that the monobromination occurred at position 8 of 1a. Figure S1 shows the aromatic region of the 1 H NMR spectrum of 2d in CDCl 3 at various concentrations. The five different aromatic signals indicated 2d is mono-nitrated. The upfield three aromatic signals for the hydrogens on the two electron-rich rings can also be distinguished from the two downfield aromatic signals on the basis of the NOE interactions between the aromatic protons and the CH 2 O group of the side chains. Although only one important HMBC correlation was observed between one downfield signal at δ = 8.69 ppm and the carbon of carbonyl, this signal can be assigned to hydrogen at position 1 on the electron-deficient ring. The fact that only one NOE interaction between the hydrogen at position 1 and the hydrogen at position 12 observed indicated that the mononitration occurred at position 5 of 1a and another downfield signal at δ = 8.57 ppm can be assigned to hydrogen 4 on the electron-deficient ring. The factors that causes the remarkable difference in site selectivity between monobromination and mononitration of triphenylene 2,3-dicarboxylic esters are not clear and the similar examples of unexpected product formation in electrophilic aromatic substitution have been observed by Cammidge and co-workers [23,28]. The structures of other bromides 2b-c and nitro product 2e were established by similar techniques as discussed earlier. Thus, the dibromination and the tribromination occurred at positions 8 and 12 of 1a and positions 5, 8 and 12 of 1a, respectively. The dinitration occurred at positions 5 and 12 of 1a to afford a symmetric product 2e, which can be seen from the two equally intense aromatic hydrogen singlets in the 1 H NMR spectrum.

Geometry analysis
The best way to explore the predicted helical deformation of the core for these α-substituted compounds is through single-crystal X-ray crystallography. However, all attempts to obtain single-crystal X-ray structures of 2a-e were unsuccessful, hence the calculated geometries of 2a-e were determined with the B3LYP/6-31 +G* method, as shown in Figure S7. It was observed that the deviation of the α-substituent(s) from plane due to the steric crowding in the α-position results in a bending of the central triphenylene cores to different extents. Thus, the structures of 2a-e are clearly in a non-planar conformation and the dihedral angles ω for the twist of the three outer rings from the central ring are used to evaluate the degree of non-planarity. For monobromide 2a, the calculations predict a twist of the nucleus and a dihedral angle of ω = 22.4°; for dibromide 2b, the calculations predict two dihedral angles (ω 1 = 25.0°and ω 2 = 15.0°) and for tribromide 2c, the calculations predict three dihedral angles (ω 1 = 20.2°, ω 2 = 27.1°and ω 3 = 33.1°). Similarly, the calculations on 2d-e suggest the O-N-O plane of the α-nitro group is almost orthogonal to the triphenylene plane, which is consistent with earlier reported X-ray crystal observation of HAT2-NO 2 [22]. In particular, for mononitrated 2d, the predicted dihedral angle (ω = 14.3°) is lower than that for monobromide 2a (Figure 1). It is clear that the degree of non-planarity decrease in the following order: It was observed that the α-hydrogens opposite to Br group in these compounds 2a-c are shifted downfield to a different extent, and especially H 4 in dibromide 2b suffers a dramatic downfield shift to 9.8 ppm. Computational studies ( Figure S7) reveal for 2b a dipolar C 5 -Br···H 4 -C 4 interaction with a spatial distance of 2.5 Å that is below the van der Waals distance, which could be responsible for the observed downfield shift of H 4 in the 1 H NMR spectrum.
In addition, 1 H NMR studies revealed minor upfield shifts in the spectra of 2d with increasing concentration, which indicates that intermolecular π-π interactions or self-assembly is occurring in solution. In this case, the signals of all protons in aromatic region are shifted upfield to a different extent with increasing concentration. It was found that protons H 1 and H 4 on the electron-deficient ring shift more than the other aromatic protons as the concentration increased from 1.27 to 12.67 mM ( Figure S1). The observed changes can be explained by the increased polarity of molecule caused by incorporation of NO 2 ; thus, the offset antiparallel π-stacked assembling is suggested for 2d owing to the energetically favourable dipole-dipole interactions between molecules [17]. The upfield shifts of protons are a result of shielding from neighbouring molecule's aromatic ring current by cofacial stacking. The offset would cause the protons H 1 and H 4 to be directly above the neighbouring core and the other protons to be further away, which would result in the different variations in chemical shifts. Besides, the 1 H NMR spectrum of 2d in CDCl 3 at the concentration of 6.34 mM showed the signals of both hydrogens H 1 and H 4 are split into two unequally singlets with coupling constants of about 7 Hz, but the splitting could not be interpreted as the result of an ortho coupling ( Figure S1). This similar splitting was also observed in the spectra of HAT6-F and was ascribed to a through-space coupling between the fluorine and hydrogen [35].

Mesomorphism
The phase behaviour of the novel triphenylene derivatives was investigated by different techniques such as POM, differential scanning calorimetry (DSC) and variable temperature powder XRD. Data obtained from the heating and first cooling cycles of DSC are listed in Figure S4. These investigations show that only 2a and 2d form columnar mesophase between glasses when slowly cooled from the isotropic melt, whereas other compounds did not show mesomorphism. POM images of 2a and 2d ( Figure 2) display fan-shaped texture, which is a characteristic of discotic columnar mesophase. TGA measurement ( Figure S2) showed both compounds 2a and 2d exhibit better thermal stability with the temperatures of 5% weight loss in the range of 290-360°C. The thermal decomposition temperature with 5% weight loss for 2a and 2d was approximately 300°C and 350°C, respectively; thus, the thermostability of 2d is about 50°C higher than 2a.
The DSC traces of 2a and 2d (Figure 3(a,b)) each exhibited a single peak for the transition between the isotropic liquid and the mesophase both on second heating and cooling. The mesophase is stable down even to −50°C on cooling, while no apparent lowtemperature glass transition was observed, which means both the 2a and 2d are room temperature DLCs. Figure 3(c,d) showed the XRD trace of 2a and 2d measured at different temperatures, respectively. The XRD data of the liquid crystalline phases of 1a, 2a and 2d are summarised in Table 1. Powder XRD diffractograms of each of two compounds at 25°C showed  broad reflections corresponding to the alkyl halo and to the π-stacking distance (∼3.5 Å), as well as one intense peak at low angle which is assigned as the (100) reflection. The (110) and (200) reflections were also observed at 2 o = 2-12°for these two samples. The corresponding d-spacing based on these angles were calculated and the values were in accordance with the ratios of 1:1/√3:1/4. Therefore, the columnar hexagonal phase could be confirmed for 2a and 2d. The calculated Lattice parameter a for 2a and 2d was approximately 20 , which is consistent with these single molecular diameters.
A general comparison of the phase transitions of 2a and 2d with parent compound 1a illustrates the effect of α-substituents. Symmetric 1a exhibits hexagonal columnar mesophases over a temperature range of 41-175°C, whereas 2a and 2d exhibited a columnar mesophase in the range of <−50°C to 120°C and < −50°C to 149°C, respectively. It was obvious that unsymmetric 2a and 2d shows a 55°C and 26°C decrease in clearing point as compared to 1a. Examination of the phase transition enthalpies of clearing (Col h -I) in the heating run ( Figure S4) shows that both 2a and 2d form more disordered Col h phase than parent 1a. At the same time, melting transition temperatures in two compounds are typically lowered to a greater extent than the clearing temperatures and no crystallisation can be observed even down to −50°C on cooling from the isotropic phase, which results in a broadening of the liquid crystalline phase range. There are two reasons for the depression of melting and clearing temperatures: (1) incorporation of α-substituents into the molecule breaks the molecule symmetry and disfavours crystallisation entropically and the stability of the mesophase; (2) twisting of the nucleus due to the backfolding of the side chains at the α-position depress the π-π stacking of triphenylene core into columns. Thus, suitable bulky substituent at the αposition of compounds such as 2a and 2d induces helical deformation of the central core without destroying the mesophase, whereas multiple α-substituents in 2b-c and 2e result in the stronger twisting of the molecules and lead to the loss of mesomorphism.
Relative to 2d, 2a shows a lower clearing point and this may be attributed to the stronger steric bulk of Br atom and higher degree of non-planarity.
Furthermore, the mesogenic behaviour of 2a and 2d can be compared to reported HAT6-Br [27] and HAT6-NO 2 [35], respectively. HAT6-Br shows the melting transition at 37°C and the clearing transition at 83°C in the first heating run. HAT6-NO 2 has melting and clearing transitions at <25°C and 136°C, respectively. Compound 2d has a little higher clearing transition temperature at 149°C and no crystallisation can be observed even down to −50°C on cooling from the isotropic phase, thus leading to a broader columnar phase than HAT6-NO 2 . The same trend with an even stronger effect is observed with the 2a and HAT6-Br, the former shows a 37°C increase in clearing point and a significant widening of mesomorphic range when compared to the latter. Apparently, the higher molecular dipole moment in 2a and 2d favours the stability of the mesophase, which is consistent with earlier reports that the clearing point is related to the electron-withdrawing character of the substituents [17].

Photophysical properties
The optical properties of compounds 2a-e were investigated in solution by measuring UV-vis ( Figure S8) and fluorescence spectroscopy ( Figure S9) and the data are shown in Table 2. The UV-vis absorption spectroscopy of bromides 2a-c show absorption maxima centred at 294, 296 and 302 nm, respectively, all of which are little red-shifted compared to 290 nm of parent compound 1a. It reveals that absorption maximum redshift gradually becomes large with the increase of the number of Br groups. The largest shift of absorption maximum of  12 nm is exhibited by the tribromide 2c. On the contrary, a minor blueshift, not the expected large redshift of the absorption, is observed in both the nitro compounds 2d and 2e (283 and 284 nm, respectively) relative to 1a. This phenomenon is consistent with the aforementioned fact that the α-nitro group in compound 5 prefers an orthogonal orientation to the triphenylene plane and there is no effective conjugation between nitro group and the aromatic π system. In addition, it was also observed that the fluorescence quantum yields of 2a-e are low compared with planar 1a, which can be ascribe to the ring deformation and the effects of the heavy halogen atoms [36]. The emission spectra of 2a-e ( Figure S9) are nearly identical, but 2a-c show a clear and minor blueshift with the increase in the number of bulky Br groups. Their PL spectra in solid state ( Figure S10) were also measured and the emission maximum of both 2d and 2e are about 20 nm red-shifted relative to those found in solution. In contrast, the emission maximum of 2a and 2b-c diminishes and are about 3 and 10 nm blueshifted relative to those found in solution, respectively. The difference indicates that stronger twisting of rings in 2b-c decrease the fluorescence quenching and increase the fluorescence quantum yield, which reflect the fact that molecular π-stacking is sterically inhibited in the solid state [37].

Conclusion
In conclusion, we have shown that modification of the our developed discogen triphenylene 2,3-dicarboxylic ester (1a) by electrophilic bromination or nitration at α-position provides a simple strategy for preparing helically deformed molecules and modifying mesophase behaviour. Positions of bromo or nitro group in target compounds were unambiguously established by combined analysis of 2D NMR experiment including NOE and HMBC, which indicated that monobromination or mononitration occurred preferentially at position 8 or 5 of 1a, respectively. Calculations of molecular geometry on 2a-e reveal that the α-Br or -NO 2 group(s) would induce different degrees of helical deformation in the triphenylene core due to the steric strain in the αposition and the degree of non-planarity decrease in the following order: 2c > 2b > 2e > 2a > 2d. Our research indicates that suitable bulky monosubstituent at the α-position of compounds such as 2a and 2d induces slight twist of the central core without destroying the mesophase, whereas multiple α-substituents in 2c-d and 2e result in the stronger helical deformation of the molecules and lead to the loss of mesomorphism. Thus,both monobromo and mononitro products 2a and 2d were found to exhibit an enantiotropic phase at room temperature, which has been identified by XRD and POM as a hexagonal columnar phase. The DSC traces of these two heptasubstituted compounds having three different types of peripheral substitution indicate that their melting and clearing temperatures are both decreased significantly when compared to the precursor 1a, which was attributed to the reduced molecule symmetry and twisting of the nucleus caused by the additional α-substituents. However, the crystallisation of 2a and 2d is fully suppressed on cooling so that the formation of a broader columnar phase at room temperature even down to −50°C was observed for each of these monosubstituted compounds.