Alkyl acid hydrazide-containing liquid crystalline triphenylenedicarboxyimides

ABSTRACT Seven hydrazide-containing triphenylenedicarboxyimides (TDIs) bearing four β-octyloxy side chains and one alkyl acid amide group at the imide nitrogen have been synthesised. These TDIs exhibit enantiotropic mesophases with broad mesomorphic temperature ranges and high clearing temperatures between 258°C and 318°C. Notably, TDI-NHCO-C4H9 with a n-butyl side chain exhibits a wide mesomorphic temperature range of over 300°C extending from below room temperature to 303°C. In contrast, its branched isomer TDI-NHCO-C(CH3)3 carrying a bulky tert-butyl chain displays a hexagonal columnar mesophase between 105°C and 318°C with the highest clearing temperature within the series, indicating that branching leads to a stabilisation of the mesophase. On the other hand, the approximately 83°C increase in the clearing temperature of hydrazide TDI-NHCO-C4H9 relative to imide TDI-C4H9 lacking the hydrazide function indicates a significant enhancement of mesophase stability upon introducing the hydrazide functional group. However, variable-temperature infrared studies revealed the presence of weaker hydrogen bonds below the clearing temperature, which do not significantly contribute to the stabilisation of the columnar mesophase. Instead, the primary stabilisation mechanism is attributed to the synergistic effect of π–π interactions and dipole–dipole interactions. GRAPHICAL ABSTRACT

inter-disk stacking distance within a single column, thereby promoting molecular orbital overlap and ultimately enhancing charge transport efficiency.
Recently, we conducted a study on hydrazidefunctionalised triphenylenedicarboxyimides (TDIs) that contained a benzoic acid hydrazide group with different substitutes in the end benzene ring [41].We observed that the hydrogen bonding was not strong enough to exist in their liquid crystalline phases, and the position of the lateral substituent attached to benzene ring has a significant impact on the self-assembly due to its effect on the overall dipole moment.In this work, we further explore the structure and liquid crystalline property relationship of new discotic hydrazides TDI-NHCO-C n H 2n + 1 (n = 1, 3,4,7,11,15) and TDI-NHCO-C(CH 3 ) 3 with an end linear or branched alkyl chain (Scheme 1), respectively, by replacing the benzoic acid amide group at imide end with the analogous alkyl acid amide.

Materials
TPC 8 anhydride was synthesised according to literature procedure [42].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.

Instrumentation
1 H and 13 C NMR spectra were recorded using Varian 400 M or Bruker 600 MHz spectrometer at 298 K with CDCl 3 as solvent and tetramethylsilane (TMS) as internal standard.Chemical shifts were reported in units (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) are reported in Hertz (Hz).The following abbreviations were used to explain the multiplicities: s = singlet, brs = broad singlet, d = doublet, t = triplet, dd = doublet of doublet, m = multiplet.Infrared spectra were recorded on a Bruker VERTEX70 spectrometer (KBr disc) at room temperature.High-resolution mass spectra were obtained with a Varian 7.0T FTMS mass spectrometer.UV-vis absorption spectra were recorded at room temperature using a Perkin Elmer Lambda 950 spectrophotometer.Fluorescent emission spectra were recorded on a HORIBA Fluoromax-4p fluorescence spectrophotometer.Polarised optical microscopy (POM) images of the samples sandwiched between two glass slides were recorded on the Zeiss, AXIO Scope A1, with a hot stage (Mettler, FP80HT) and controller (Linkam Scientific, T95-STD).A TA-Discovery differential scanning calorimetry (DSC) was used to determine the phase transitions and associated enthalpy changes; heating and cooling rates were 10°C min −1 under a nitrogen flow.Thermal gravimetrical analysis (TGA) was conducted on a TA-Discovery 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.

Synthetic procedures
The target TDIs having four β-octyloxy side chains and one alkyl acid amide chain of different lengths at the imide nitrogen were prepared from the triphenylene dicarboxylic acid anhydride TPC 8 anhydride, the syntheses of which have been previously reported from our laboratory [42].As shown in Scheme 1, a mixture of TPC 8 anhydride (0.15 mmol), alkyl acid hydrazide (0.30 mmol)and toluene(10 mL)were refluxed at 130°C for 12 hours.After cooling, the reaction mixture was concentrated and the residue was purified by silica gel chromatography to afford the desired compounds in high yields.The structures of all TDIs were characterised by 1 H and 13 C NMR, and high resolution MALDI-TOF-MS spectra.Detailed synthesis procedures and compound characterisation data for the target TDIs and the triphenylene dicarboxylic acid anhydride are provided in the experimental section.For comparison, analogue TDI-C 4 H 9 having the similar structure as TDI-NHCO-C 4 H 9 but lacking the hydrazide function was also prepared from triphenylene 2,3-dicarboxylic ester and butylamine according to the literature procedure [30,43,44]. TDI-NHCO-CH

Structure characterisation
All the 1 H NMR spectra of hydrazides show three wellresolved singlets in aromatic region, corresponding to three sets of protons attached to triphenylene core.Additionally, the signal of the N-H proton appears in the range of 7.71 to 7.97 ppm in the 1 H NMR spectrum.Based on the two-dimensional (2D) NOESY experiments (Figure S2 in the SI), the protons of the CH 2 O groups on the octyloxy chains in TDI-NHCO-CH 3 were clearly identified at 4.01 and 4.26 ppm, respectively.The NOE correlations between these CH 2 O protons and two distinct sets of aromatic protons (H b and H c ) neighbouring to octyloxy side chains were observed.Moreover, one NOE cross-peak observed between proton H b and another downfield aromatic proton allowed for the identification of the signal corresponding to aromatic proton H a , which is located next to the carbonyl group.Notably, the chemical shifts of these protons exhibit distinct concentration-dependent changes.In dilution experiments shown in Figure 1, the signals for three sets of triphenylene protons experience significant upfield shifts with increasing concentration.This effect can be attributed to the marked shielding effects from neighbouring molecules via molecular π-stacking aggregation in solution [20], consistent with previous observations for benzoic acid hydrazide [41].Specifically, the H a protons are significantly upfield-shifted (from δ = 8.29 to 8.08 ppm) relative to H b and H c protons, reflecting their relative positions above the π-system of the neighbouring disc.Conversely, the signal of N-H proton (H d ) showed a gradual downfield shift from 7.84 to 7.95 ppm concomitantly with increasing concentration, indicating the formation of hydrogen bond [15].

Mesomorphism
The liquid crystalline properties of target TDIs were investigated by DSC, POM, and variable temperature XRD.TGA of the target TDIs was performed under nitrogen atmosphere, as shown in Figure S3 in SI.The temperature at which compounds lose 5% of their weight is above 320°C, indicating that these TDIs show great thermal stability.Figure S4 in the SI shows the characteristic pseudo focal-conic textures of the prepared TDIs in their columnar mesophases observed by POM on the cooling run.Their DSC curves of heating and cooling cycles are shown in Figure S5 in SI.The phase transition temperatures and the associated enthalpy changes are summarised in Table 1.When compared with the parent TPC 8-anhydride, which cleared into the isotropic liquid at 229°C and melted to a mesophase at 140°C in the cooling DSC curve, all target TDIs show higher clearing temperatures and wider mesomorphic temperature ranges.
As shown for TDI-NHCO-CH 3 in Figure 2(a), two exothermal peaks at 258°C (ΔH, 24.2 kJ mol −1 ) and 141°C (ΔH, 78.9 kJ mol −1 ) corresponding to isotropic liquid-to-mesophase transition and mesophase-tocrystalline transition were detected upon cooling from the isotropic liquid in its DSC curves.The POM studies revealed that between 141°C and 258°C this TDI exhibited linear defect textures with homeotropic domains, which is often observed in Col h phase.During subsequent heating, an endothermal crystalline to mesophase transition at 159°C with a higher enthalpy of 86.8 kJ mol −1 , followed by one endothermal mesophase-toisotropic liquid transition at 260°C (ΔH, 29.9 kJ mol −1 ) were found.The XRD recorded at 200°C (Figure 2(b)) confirmed the presence of a Col h phase made by POM and DSC, although only a single intense (100) reflection at a d-spacing of 18.7 Å could be detected in the small-angles region.The lattice parameter a calculated from the (100) reflection corresponding to the intercolumnar distance is found to be 21.6 Å.In the high-angle region a diffuse halo at 4.2 Å characteristic of the molten chains h ch , and an outer diffused reflection at 3.5 Å corresponding to the ordered stacking of triphenylene rings into columns h π were observed [45][46][47].
The length of the terminal alkyl chain on the hydrazide moiety has been shown to influence liquid crystalline properties, as seen from TDIs TDI-NHCO-CH 3 to TDI-NHCO-C 15 H 31 , all possessing an end linear alkyl chain.Upon cooling, an increase in the number of carbon atoms within the linear alkyl chain, from 1 to 3 in TDI-NHCO-C 3 H 7 , led to a rise in the clearing temperature from 258°C to 301°C.Simultaneously, it also induced a significant decrease in the melting temperature from 141°C to 38°C.Consequently, this change resulted in the broadening of the mesomorphic  mesophases was observed and XRD confirmed the existence of a Col h mesophase (Figure 2(d)).Interestingly, TDI-NHCO-C(CH 3 ) 3 shows the highest clearing temperature of the series at 318°C in the cooling process, which is almost 15°C higher than its linear isomer TDI-NHCO-C 4 H 9 .Besides, its melting temperature is also higher than that of TDI-NHCO-C 4 H 9 .These results suggest that the stabilisation of both the crystalline and mesophase may be attributed to the efficient filling of the space facilitated by the tert-butyl group within the compound.
On further increasing the length of alkyl chain longer than that of alkoxy chains, the clearing temperature started decreasing to 289°C and 262°C for two TDIs TDI-NHCO-C 11 H 23 and TDI-NHCO-C 15 H 31 with an alkyl chain of 11 or 15 carbon atoms, each of which shows two types of columnar mesophases between the crystalline and isotropic liquid phases.As shown in the DSC curves for TDI-NHCO-C 11 H 23 in Figure 2(e), it cleared at 289°C with an enthalpy of 11.3 kJ mol −1 on cooling from the isotropic liquid phase and exhibited a linear defect texture between 289°C and 101°C, which is typical for the formation of a Col h phase.On cooling to 101°C, a weak transition with an enthalpy of 2.3 kJ mol −1 appeared and no clear textural change could be observed, showing that the TDI entered another mesomorphic state (Figure 3(c,d)).During subsequent heating, the crystal melted at about 40°C and two transitions at 104°C (ΔH, 1.5 kJ mol −1 ) and at 291°C (ΔH, 11.5 kJ mol −1 ) were observed.XRD patterns of TDI-NHCO-C 11 H 23 obtained from 200 to 40°C (Figure 2(f)) during the cooling process are similar and each exhibits a sharp and strong peak in the low-angle region that was attributed to a (100) reflection, and two weak peaks, corresponding to the alkyl halo and π-stacking distances.Like TDI-NHCO-CH 3 , the latter peaks can be indexed as h ch and h π reflections of a 2D hexagonal lattice, thus indicating a characteristic diffraction of the two mesophases.Table 2 summarises the data obtained from XRD experiments for the TDIs under investigation.A careful examination of the XRD data obtained at 200 and 40°C confirmed the subtle distinctions between the two mesophases, despite both exhibiting similar lattice spacings.In comparison to the latter, the d value of the (100) reflection in the former increases from 18.7 to 19.3 Å.As a result, the hexagonal lattice constant a calculated from the sharp (100) reflection corresponding to the intercolumnar distance increases from 21.6 (at 200°C) to 22.3 Å (at 40°C), revealing the occurrence of interpenetration or folding of the side chains in the high temperature phase.Conversely, a minor reduction in the π-π stacking distance from 3.44 to 3.38 Å is observed with decreasing temperature, and the presence of a reflection at 3.4 Å indicates an ordered π-stacking of the discs within the columns at low temperature.Based on the XRD analysis and combined with the DSC measurements and POM results, it was more difficult to identify the type of mesophase because of the fact that shearing under mechanical stress was not possible.This mesophase is thus denoted as 'Col x ' phase in analogy to similar phases that have been reported for alkoxynitrotriphenylenes [48] or alkoxybromotriphenylenes [49].Similar behaviour was observed in  In comparison to imide TDI-C 4 H 9 , which contains only an N-alkyl chain at the imide end, hydrazide TDI-NHCO-C 4 H 9 exhibits an approximately 83°C higher clearing temperature, indicating improved stability of the mesophase upon the introduction of the hydrazide functional group.Moreover, while imide TDI-C 4 H 9 crystallises at 47°C, hydrazide TDI-NHCO-C 4 H 9 remains in a mesophase state without any sign of crystallisation upon cooling, leading to a wide mesophase temperature range of over 300°C.On the other hand, variable temperature IR was carried on TDI-NHCO-CH 3. Figure 4 shows a set of FT-IR spectra in the form of a thin solid film on top of a KBr plate at different temperatures upon cooling.Notably, a very clear change can be seen upon cooling from 130°C (below the clearing temperature) to room temperature, where N-H stretching absorption bands gradually shifted towards lower frequencies with decreasing temperature.At 130°C, the N-H stretching absorption bands are observed at 3350 cm −1 .As the temperature decreased, a significant shift of about 77 cm −1 was observed from 3350 cm −1 at 130°C to 3273 cm −1 at the room temperature.It can be concluded that the hydrogen bonds in TDI-NHCO-CH 3 remain in the low temperature range and are insufficiently strong to maintain stability in the mesophase.This would also mean that the higher clearing temperatures for these studied TDIs can be attributed to π-π interactions in conjunction with other interactions, such as dipoledipole interactions.These observations are consistent with those previously reported benzoic acid hydrazide [41].

Conclusions
In summary, we synthesised a series of new discotic liquid crystalline hydrazide-functionalised triphenylenedicarboxyimides bearing one alkyl acid amide chain of varying lengths at the imide nitrogen, and investigated their mesophase characterisation.Within the homologous series, the length of the terminal alkyl tail has significant influence on the phase transition temperatures and mesomorphic behaviour of these TDIs.Specifically, the clearing temperature displays a regular pattern of initial increase followed by decrease as the length of the alkyl chain is increased.Notably, a wide mesomorphic temperature range of over 300°C was determined to be from below room temperature to 303°C for TDI-NHCO-C 4 H 9 with a n-butyl chain.In contrast, its branched isomer TDI-NHCO-C(CH 3 ) 3 carrying a bulky tert-butyl side chain formed a hexagonal columnar mesophase between 105°C and 318°C with the highest clearing temperature within the series, indicating that branching enhances the stabilisation of the mesophase.On the other hand, the incorporation of a hydrazide functional group into TDI-NHCO-C 4 H 9 significantly stabilises the mesophase when compared to imide TDI-C 4 H 9 without the hydrazide function.This enhancement is evidenced by an approximately 83°C increase in the clearing temperature.However, variabletemperature infrared investigations suggest that weaker hydrogen bonds existing below the clearing temperature do not significantly contribute to the stabilisation of the columnar mesophase.Instead, the synergistic effect of π-π interactions and other interactions, such as dipole-dipole interactions, play a more dominant role in stabilising the columnar phase.
h : hexagonal columnar phase; Col x : unidentified columnar phase; 2θ and d hkl are the experimentally measured diffraction angles [ o ] and distances [Å]; d hkl = λ/(2sinθ); I corresponds to the relative intensity of the reflections (given for the sharp reflections); hkl are the Miller indices of the reflections, h ch : liquid-like lateral distances between molten chains; h π : average distances between stacked triphenylene units.Lattice parameter a = p 4=3 ð Þ× d 100 ; lattice area A = a 2 sin 60°; lattice volume V = a 2 sin 60° × h π (h ch if h π is not observed); number of molecules per slice of column (Z) = (√3×N a ×ρ×a 2 ×h π )/2M; N a : Avogadro number; ρ: density (1000 kg•m −3 ); M: molecular weight in kg•m −3 .case of TDI TDI-NHCO-C 15 H 31 , and there are substantial decreases in the phase transition temperatures due to the presence of the longer alkyl chain.The calculated lattice parameter a reflects the intercolumnar distance and varies between 21.6 and 22.8 Å for the TDIs studied.A gradual increase in the intercolumnar distance is observed for the former four TDIs TDI-NHCO-C n H 2n + 1 (n = 1, 3, 4, 7) with alkyl chain shorter than or similar in length to octyloxy chains as the alkyl chain length increases.Notably, TDI-NHCO-C 7 H 15 with an alkyl chain of seven carbon atoms, exhibits a larger lattice parameter a of 22.7 Å than TDI-NHCO-C 11 H 23 , which has a longer undecyl chain but a smaller intercolumnar distance of 21.6 Å. TDI-NHCO-C 15 H 31 carrying the longest alkyl chain of 15 carbon atoms shows an almost identical a value of 22.8 Å to TDI-NHCO-C 7 H 15 .This observation suggests the possibility of greater interpenetration or folding of the side chains in the latter two TDIs, which have alkyl chains longer than octyloxy chains.

Figure 4 .
Figure 4. (Colour online) Temperature-dependent FT-IR spectra of compound TDI-NHCO-CH 3 in the form of a thin solid film on top of a KBr plate upon cooling.

Table 2 .
X-Ray diffraction data for all mesomorphic TDIs.