Synthesis and mesomorphism of mixed ether-ester tail triphenylene discotic liquid crystals with long alkyloxy peripheral chains

Abstract

Symmetrical and asymmetrical triphenylene discotic liquid crystals with two kinds of different peripheral chains, sym-TP(OC₁₁H₂₃)₃(O₂CR)₃ and asym-TP(OC₁₁H₂₃)₃(O₂CR)₃, (R= CH₂OC₂H₅, CH₂OC₃H₇, CH₂OC₄H₉, CH₂OC₅H₁₁, C₃H₇, C₄H₉, C₅H₁₁, C₆H₁₃, C₇H₁₅) were synthesized. Their thermotropic liquid crystalline properties were studied by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The results showed that the asymmetrical compounds had higher melting and clearing points than that of their corresponding symmetrical compounds. For the same series of compounds, TP(OC₁₁H₂₃)₃(O₂CR)₃, their melting points decrease and clearing points increase gradually with the lengthening of ester chains. Most of the b-oxygen containing esters of triphenylene derivatives, TP(OC₁₁H₂₃)₃(O₂CR)₃, (R=CH₂OC₂H₅, CH₂OC₃H₇, CH₂OC₄H₉, CH₂OC₅H₁₁), symmetrically or asymmetrically attached on triphenylene cores, have higher melting and clearing points than those of triphenylene derivatives, TP(OC₁₁H₂₃)₃(O₂CR)₃, (R=C₄H₉, C₅H₁₁, C₆H₁₃, C₇H₁₅), with the same length of peripheral chains. The triphenylene derivatives with longer peripheral chains have shown mesophase at room temperature.

Keywords

triphenylene / discotic liquid crystal / columnar mesophase / ether-ester mixed tail / beta-oxygen effect / molecular symmetry

Discotic liquid crystal, which was first discovered by Chandrasekhar in 1977 [1], is different from conventional rod-like thermotropic liquid crystals in molecular geometry, phase behavior, and optic-electric property, and show huge potential application as organic semiconductors. Therefore, it has attracted increasing attention since its discovery [2–11]. Generally, the discotic liquid crystal molecule consists of a flat-like polycyclic rigid aromatic core and six or more peripheral flexible alkyl chains [6-12]. So there are two different methods for the synthesis of new discotic liquid crystals: change aromatic cores with different size and shape or modify the peripheral chains. On one hand, the aromatic core can be modified by introducing hetero-atoms instead of carbon atoms, such as triphenylene and hexaazotriphenylene, and aromatic core size with different number of carbon atoms, for example, triphenylene and hexabenzocoronene. On the other hand, peripheral chains can be modified by variation of chain length, or introducing hydrophilic, hydrophobic, hydrogen bonding, perfluorinated chains, metal-containing chains, and polymerisable functional groups [13–16]. Mesomorphism and charged carrier mobility behaviors of discotic liquid crystals depend both on the aromatic cores and peripheral chains. After the rigid core of the discotic liquid crystal is selected, the properties can be changed by peripheral chain modification. The discotic mesogens, triphenylene derivatives have played an important role because of their chemical stability, structure variation, well developed synthetic methods, availability of high purity samples, and high charged carrier mobility as organic semiconductors [17] reported in the literature. In order to investigate the influence of soft chain on mesomorphism of triphenylenes, many research groups have prepared triphenylenes with different functional groups [18–22]. For example, we have prepared symmetrical and asymmetrical triphenylenes with ether functional group and studied the influence of ether chain length on mesomorphism [23,24]. Kumar [25] has reported symmetrical and asymmetrical hexa-substituted ether-ester mixed tail triphenylenes and the correlation of chemical structure and physical property. Up to now, most of the reported studies are concentrated on short chain triphenylenes, such as pentyl, or hexyl. There are very few reports on longer chain triphenylene derivatives, maybe due to their disadvantage of having lower synthetic yields, low melting points and difficulty for purification through recrystallization from organic solvents. On the other hand, longer chain triphenylenes possess the advantage of being liquid crystal states at room temperature. The applicable liquid crystals in fields of optoelectrical display and organic semiconductors should be in liquid crystal state with broad temperature ranges, such as-50 to 70°C, and have high thermo-, photo- and chemical stability. Therefore, synthesis and development of discotic liquid crystals with longer chain are of importance in both the point of views of academic and industry application. Then, in order to know the structure-property correlation of long chain triphenylene derivatives, based on previous reported results [23-25], we designed and synthesized two series of symmetric and asymmetric triphenylene derivatives with ether-ester mixed tails, and investigated the molecular symmetry, ester chain length, and beta-oxygen-atom effects on mesomorphism. We hope the information accumulation on the structure-property relationship can contribute to the practical applications of triphenylene derivatives. Scheme 1 describes the synthesis routes of target triphenylene derivatives.

Experimental

Instruments and characterization methods

¹H NMR of triphenylene derivatives were recorded with Bruker-Avance-600 spectrometer, Me₄Si as the internal standard. Elemental analysis was measured with Carlo Erba 110b micro elementary analysis instrument. The optical textures of discotic liquid crystals were observed with XP-201 polarized optical microscope, with XP-201 heating plate, and XPR-201 temperature controller. The photomicrograph was recorded after the sample in two untreated glasses slowly cooled from isotropic liquid. Phase transition temperature and enthalpies were measured with differential scanning calorimetry (TA-DSC Q100), with heating and cooling rate at 10°C/min under N₂ atmosphere. The thermal stability of samples was measured by thermal gravimetry analysis (TGA) with a TA-TGA Q500 instrument.

Chemical reagents and organic solvents

Chemical reagents and organic solvents are commercial products and used without purification unless otherwise indicated. Butanoyl chloride, C₃H₇COCl, was prepared by the reaction of butanoic acid with benzoyl chloride, by controlling the refluxing and distillation rate. Other acid chlorides were synthesized directly from the reaction of acids between SOCl₂ under refluxing and distillation.

2-alkoxyacetic chloride, ROCH₂COCl were prepared as follows [26]. ROH was carefully reacted with sodium, and then 2-chloroacetic acid was added for further reaction. Through acidic aqueous work-up, the product was purified by vacuum distillation. The prepared β-alkoxy-acetic acid reacted with excess SOCl₂, refluxed for 6 h with two drops of DMF as catalyst. After the excess SOCl₂ was distilled out, the acid chloride was collected by vacuum distillation. Product yield was in the range of 72%-83%.

Intermediates synthesis

Synthesis of 2, C₆H₄(OMe)(OC₁₁H₂₃), 2-methoxy-1-undecyloxybenzene

C₆H₄(OMe)(OH), C₁₁H₂₃Br, anhydrous K₂CO₃ were added to dry ethanol, catalytic amount of KI added and refluxed 12 h with thorough stirring. After workup, the product 2 was collected as yellow oil, with a yield of 98%.

Synthesis of s-3 and a-3, 2,6,10-tri(methoxy)-3,7,11-tri(undecyloxy)triphenylene and 2,6,11-tri(methoxy)- 3,7,10-tri(undecyloxy)triphenylene

s-3 and a-3 were prepared as reported in Ref. [27]. To the mixture of 2 and dichloromethane, 3 drops of concentrated sulfuric acid were added as catalyst, and anhydrous FeCl₃ was added carefully in three portions. After workup with cold methanol, the triphenylene derivatives s-3 and a-3 were isolated as a mixture. After separation of the mixture by silica column chromatograph, s-3 and a-3 were isolated as pure form with a yield of 31% and 50%, and with a melting point of 74°C and 45°C, respectively.

Synthesis of s-4 and a-4, 2,6,10-trihydroxy-3,7,11-tri(undecyloxy)triphenylene, 2,6,11-trihydroxy-3,7,10- tri(undecyloxy)-triphenylene

s-4 and a-4 were synthesized as reported [28,29]. First, the reaction of PPh₃ and metal Li in dry THF produced Ph₂PLi and PhLi, and then dry tert-butyl chloride was added carefully for reaction with PhLi. After that, pure s-4 or a-4 was added and further refluxed 6 h with stirring under N₂ for the demethylation reaction. After acidic aqueous workup, s-4 or a-4 was further purified with silica gel column chromatograph and collected with a yield of 87% or 88%, and a melting point of 76°C and 134°C, respectively.

Compound s-4, sym-TP(OH)₃(OC₁₁H₂₃)₃

¹H NMR (CDCl₃, TMS, 600 MHz) d: 0.88 (t, J = 6.84 Hz, 9H), 1.15-1.38 (m, 30H), 1.39-1.47 (m, 6H), 1.48-1.68 (m, 12H), 1.86-2.00 (m, 6H), 4.25 (t, J = 6.57 Hz, 6H), 5.92 (s, 3H), 7.78 (s, 3H), 7.91 (s, 3H).

Compound a-4, asym-TP(OH)₃(OC₁₁H₂₃)₃

¹H NMR (CDCl₃, TMS, 600 MHz) d: 0.88 (t, J = 6.81 Hz, 9H), 1.16-1.38 (m, 26H), 1.39-1.46 (m, 6H), 1.47-1.71 (m,16H), 1.86-2.01 (m, 6H), 4.26 (t, J = 7.29 Hz, 6H), 5.88 (s, 3H), 7.72 (s, 2H), 7.78 (s, 1H), 7.92 (s, 1H), 7.94 (s, 2H).

The synthesis of target molecules s-5 and a-5

The sym-TP(OC₁₁H₂₃)₃(O₂CR)₃ and asym-TP(OC₁₁H₂₃)₃(O₂CR)₃ were synthesized by the reaction of trihydroxy-tri(undecyloxy)triphenylene, s-4 or a-4 with the corresponding acid chloride in the presence of weak organic base, such as triethyl amine. After weak acidic aqueous workup, the product was purified through SiO₂ column chromatograph, and collected in white wax form, with a yield of 52%-78%.

Compound s-5b, sym-TP(OOCCH₂OC₃H₇)₃(OC₁₁H₂₃)₃

¹HβNMR (CDCl₃, TMS, 600 MHz) d: 0.89 (t, J = 6.99 Hz, 9H), 1.04 (t, J = 7.41 Hz, 9H), 1.19-1.35 (m, 36H), 1.35-1.43 (m, 6H), 1.44-1.53 (m, 6H), 1.73-1.87 (m, 12H), 3.70 (t, J = 6.78 Hz, 6H), 4.08 (t, J = 6.42 Hz, 6H), 4.46 (s, 6H), 7.43 (s, 3H ), 7.87 (s, 3H ); Anal. calcd for C₆₆H₁₀₂O₁₂: C 72.93, H 9.39; found C 72.97, H 9.43. The spectra data of s-5a, s-5c and s-5d are similar with s-5b.

Compound s-5g, sym-TP(OOCC₅H₁₁)₃(OC₁₁H₂₃)₃

¹HβNMR (CDCl₃, TMS, 600 MHz) d: 0.89 (t, J = 6.99 Hz, 9H), 0.99 (t, J = 7.17 Hz, 9H), 1.22-1.36 (m, 36H), 1.36-1.42 (m, 6H), 1.42-1.47 (m, 6H), 1.47-1.52 (m, 12H), 1.77-1.92 (m, 12H), 2.65 (t, J = 7.62 Hz, 6H), 4.12 (t, J = 6.60 Hz, 6H), 7.55 (s, 3H), 7.90 (s, 3H); Anal. calcd for C₆₉H₁₀₈O₉: C 76.67, H 10.00; found C 76.57, H 10.05. The spectra data of s-5e, s-5f, s-5h, s-5i are similar with s-5g.

Compound a-5b, asym-TP(OOCCH₂OC₃H₇)₃(OC₁₁H₂₃)₃

¹HβNMR (CDCl₃, TMS, 600 MHz) d: 0.89 (t, J = 6.84 Hz, 9H), 1.03 (t, J = 5.94 Hz, 9H), 1.19-1.37 (m, 36H), 1.37-1.46 (m, 6H), 1.46-1.56 (m, 6H), 1.69-1.80 (m, 6H), 1.80-1.93 (m, 6H), 3.69 (t, J = 6.72 Hz, 6H), 4.12 (t, J = 6.45 Hz, 2H), 4.17 (t, J = 6.36 Hz, 2H), 4.20 (t, J = 6.27 Hz, 2H), 4.43 (s, 2H), 4.45 (s, 2H), 4.49 (s, 2H), 7.58 (s, 1H ), 7.70 (s, 2H ), 7.74 (s, 1H), 7.76 (s, 1H), 8.03 (s, 1H); Anal. calcd for C₆₆H₁₀₂O₁₂: C 72.93, H 9.39; found C 72.92, H 9.47. Compound a-5a, a-5c, a-5d showed similar spectra feature with a-5b.

Compound a-5h, asym-TP(OOCC₆H₁₃)₃(OC₁₁H₂₃)₃

¹HβNMR (CDCl₃, TMS, 600 MHz) d: 0.88 (t, J = 6.87 Hz, 9H), 0.95 (t, J = 5.73 Hz, 9H), 1.18-1.35 (m, 36H), 1.36-1.44 (m, 18H), 1.45-1.55 (m, 12H), 1.75-1.93 (m, 12H), 2.73-2.57 (t, J = 6.00 Hz, 6H), 4.17 (t, J = 6.57 Hz, 6H), 7.70 (s, 1H), 7.77 (s, 1H), 7.79 (s, 1H), 7.87 (s, 1H), 7.89 (s, 1H), 8.02 (s, 1H); Anal. calcd for C₇₂H₁₁₄O₉: C 77.01, H 10.16; found C 76.76, H 10.14. Compound a-5e, a-5f, a-5g, and a-5i had spectra character of compound a-5h.

Results and discussion

Synthesis

We synthesized the key intermediates s-3 and a-3, in statistic mixture (in rate of 1∶3) through oxidative cyclic trimerization of 1,2-dialkoxybenzene with anhydrous FeCl₃. The triphenylene mixture was separated by SiO₂ column chromatograph, and s-3 and a-3 were collected as pure form before the next demethylation reaction step, which resulted in s-4 and a-4. The selective demethylation reagent in this step is LiPPh₂, which was reported in-situ preparation through reaction of BuLi with HPPh₂ [30]. Because HPPh₂ is highly toxic and expensive, here instead we used metal Li reduction of Ph₃P in dry THF for producing Ph₂P-Li and Ph-Li. The by-product PhLi in this step was further decomposed by its reaction with dry tert-butyl chloride [28]. To guarantee the complete demethylation of 3, excess Ph₂PLi was used, in our case five equivalents of Ph₂PLi was used, and 4 was prepared in high yield.

The target discotic triphenylene derivatives 5 with ether-ester mixed tails were prepared by the reaction of 4 with acid chloride in the presence of weak organic base triethylamine. The β-oxygen containing acid chlorides, ROCH₂COCl, was not commercially available and prepared by the nucleophilic substitution reaction of sodium alkoxide with 2-chloroacetic acid, and further reacted with excess thionyl chloride.

Spectroscopic characterization of synthesized compounds

Triphenylene derivatives were purified with silica column chromatography and recrystallization from organic solvent, such as methanol, ethanol or acetone. The pure compounds were fully characterized by 600 MHz NMR and elemental analysis. In the high resolution NMR spectra, the chemical shift values of Ar–H are in the range of 7-8, and symmetrical compounds showed two single peaks (3H for each peak), while asymmetrical compounds exhibited six peaks (1H for each peak). Compound s-4 and a-4 have three hydroxyl groups attached on the triphenylene core, and s-4 showed a single peak at 5.9 (3H for–OH), while a-4 had three single peaks separated around 5.9. All triphenylene derivatives showed their aliphatic C–H protons δ-values in the range of 0.9-4.0. The methylene of alkoxy-acetyl group (ROCH₂CO–) containing triphenylenes (5a-5d) had its signs at a much lower field, about 4.45: the symmetrical derivative showed a single peak (6H), while the asymmetrical one exhibited three single peaks (2H for each peak).

Liquid crystalline properties of triphenylenes

Intermediates triphenylenes 3 and 4 did not show a liquid crystalline phase, as they contained very short methoxy or hydroxyl groups on aromatic core. The liquid crystalline properties of target triphenylene 5 (5a-5i) were investigated by DSC, TGA and POM. Table 1 and Table 2 summarize the liquid crystalline properties of triphenylenes with β-oxygen-containing ester chains (5a-5d) and with aliphatic ester chains (5e-5i), respectively. s-5e has no mesophase, due to the obvious length difference of the periphery (C11 and C4) [31] and molecular symmetry. s-5f is in liquid state at room temperature and DSC measurement did not find its phase transition even cooling to-50°C. All other triphenylenes showed liquid crystalline phases.

Thermal stability and mesophase characteristic features of triphenylenes 5

Figure 1 shows four TGA curves of typical triphenylenes, which demonstrated that these ether-ester mixed chain triphenylene derivatives are thermally stable until 350°C. Because the clearing points of triphenylenes were lower than this decomposing temperature, the DSC measurement and POM observation are investigated in the thermal stable state of 5.

The mesophase characteristics of 5 were studied with polarizing optical microscopy, and Fig. 2 shows four typical microphotographs of triphenylenes. The optical textures showed here are fan-shaped, which are common to discotic columnar phase. Figure 2(d) shows the texture structure of a-5g, which displayed its homeotropic alignment behavior. The self-assembled discotic columnar molecular stacking benefits in charged carrier mobility along the column axis and must promote the photoconductivity of discogens. Compounds 5a-5i possess high thermal stability and moderate clearing temperature, self organized homeotropic alignment behaviors of discotic liquid crystals which favor physical property measurement and devices fabrication [32].

The influence of molecular symmetry, peripheral chain length and β-oxygen atom on mesomorphism

The thermotropic liquid crystalline properties of all triphenylenes were investigated by DSC with heating and cooling rate of 10°C/min under N₂ atmosphere, and the DSC traces of four triphenylene discogens as shown in Fig. 3. Compound a-5i displayed polymorphism, its DSC trace showed three peaks in the heating run: the first peak represents K₂→K₁ transition; the second, K₁→Col transition; the third, Col→Iso transition. On the cooling run it just showed two peaks: the Iso→Col and Col→K₂ transitions. Analyzing the data summarized in Tables 1 and 2, some other triphenylenes can be found possessing polymorphism, supercooling phenomena and being in liquid crystalline states at room temperature.

Figure 4 and Fig. 5 show the phase diagrams of two series triphenylene derivatives, one with β-oxygen-containing ester chains, the other one with aliphatic ester chains. Analyzing the phase diagrams, one can seen that with the ester chains length n increase, the melting points decrease and clearing points increase, and mesophase ranges broaden. When discotic liquid crystals were heated up to their melting points, the periphery transformed from solid crystalline packing to isotropic liquid; further heated to their clearing points, the aromatic discotic cores change from ordered columnar stacking into disordered isotropic liquid. From this point of view that the ester chain length increases, its melting points will be lowered. The ester chain length increased gradually when its length reached—C₁₁H₂₃, the six peripheral chains gradually became equal in length, and molecular symmetry is improved, therefore the columnar intermolecular stacking becomes more stable, and clearing point increased. The phase transition temperature trend found here is different from the reported result in literature [33].

Further analysis of Fig. 4 and Fig. 5, the phase diagrams of the mesogens, directly showed that asymmetrical compounds have higher clearing points than that of their symmetrical counterparts. These results are different from Kumar’s report [25] about C₁₈H₆(OC₅H₁₁)₃(O₂CR)₃, in which symmetrical triphenylenes showed higher clearing points than that of the asymmetrical ones. It is worth noting that in Kumar’s report, triphenylenes have three short peripheral alkyl chains (C₅H₁₁); the discogens prepared in this report have three long alkyl chains (C₁₁H₂₃). The difference in molecular structure would result in the physical property variation.

Comparing the phase behaviors of the two series, triphenylene discogens with β-oxygen substituted ester chains have higher clearing points than that of their aliphatic ester counterparts. It is also explained by the beta-oxygen substituted peripheral chains that the intermolecular interaction increased compared with the aliphatic acid derivatives [13-16].

Figure 6 and Fig. 7 describe the molecular stacking model of two symmetrical and asymmetrical triphenylenes of each other on top. Compared to all ether tail triphenylene systems, TP(OR)₆, with just the aromatic π-π interactions promoted the columnar molecular self assembly. However, in the system of triphenylenes connecting ester groups, they have both aromatic π-π interactions and dipole-dipole interaction of the ester groups, and make the intermolecular columnar stacking more stable. The asymmetrical isomers may have stronger intracolumnar intermolecular interaction, such as the dipole interaction, and therefore could cause their clearing points to be higher. The β-oxygen containing ester derivatives showing higher clearing points than that of the aliphatic ester derivatives can be explained by the same reason [13-16]. However, the oxygen atom in the other place of soft chain (not the beta position) would have opposite effect [34].

Conclusion

In this paper, we have reported the synthesis of symmetrical and asymmetrical ether-ester mixed-tail triphenylene derivatives with long ether chains (C11). The asymmetrical discogens have exhibited higher melting points and clearing points than their symmetrical counterparts. With the ester chain lengthening, the homologous discogens have shown decreased melting points, increased clearing points, and broadened columnar mesophase ranges. The triphenylene derivatives with β-oxygen ester chains have displayed higher clearing points and more stable Col phases. Because these triphenylene discogens contain long alkyloxy chains, most of them are in columnar mesophase at room temperature, which is in favor of the charge carrier mobility and electronic devices fabrication.

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