A room-temperature liquid crystalline polymer based on discotic 1,3,4-oxadizole

Abstract

A side-chain polymer POXD and the corresponding monomer MOXD based on discotic 1,3,4-oxadiazole have been synthesized and characterized. Both POXD and MOXD exhibit liquid crystalline behaviors at room temperature with excellent thermal stability, while the mesomorphic temperature range of POXD is much wider than that of MOXD. In addition, they are photoluminescent in solution, in the solid state and in liquid crystalline phases. Notably, the fluorescent intensity of POXD is almost 12 times that of MOXD in both the solid state and liquid crystalline phases.

---

1,3,4-Oxadiazole-based liquid crystals may exhibit various mesophases such as nematic, smectic, blue phases, and columnar mesophases, and have been attracting continuous interest during the past decade.^1,2 Furthermore, 1,3,4-oxadiazole derivatives can be used as emissive and/or electro-transporting materials in organic light-emitting devices due to the electron deficiency, high photoluminescence quantum yield, and excellent thermal and chemical stability.³ Among the reported liquid crystalline 1,3,4-oxadiazoles, much attention has been paid to the rod-like or bent-shaped molecules, and the relationship between the molecular structures and liquid crystalline properties has been investigated intensively. In contrast, the examples of discotic mesogenic 1,3,4-oxadiazoles are relatively limited due to lack of enough structural skeletons to construct this type of compound.⁴

Liquid crystalline polymers have many advantages such as self-organizing characteristics, enhanced mechanical and thermal stabilities, good process ability and simple fabrication over the conventional polymeric materials.⁵ In particular, the 1,3,4-oxadiazole based liquid crystalline polymers may display fluorescent property with good electron accepting ability. To date, a variety of 1,3,4-oxadiazole based liquid crystalline polymers, including main-chain,⁶ side-chain⁷ and mesogen-jacketed types,⁸ have been developed for different purpose. Generally, most of these polymers exhibit relatively narrow mesomorphic temperature ranges with high melting points due to the rod-like 1,3,4-oxadiazole mesogens. This has greatly hindered the development in practical application, and also made it difficult to explore the structure–property relationship of this kind of materials.

Herein, we describe the synthesis and liquid crystalline behaviors of a side polymer POXD and the corresponding monomer MOXD derived from a discotic 1,3,4-oxadiazole unit. The synthetic route is shown in Scheme 1. Both POXD and MOXD exhibit liquid crystalline behaviors at room temperature region. The photoluminescence behaviors of POXD and MOXD in solution, in solid state, and in liquid crystalline phases have also been studied in this work. To our best knowledge, this is the first example of liquid crystalline polymer containing discotic 1,3,4-oxadiazole units.

Scheme 1 Synthetic route of the monomer MOXD and polymer POXD.

As seen in Scheme 1, compounds 2 and 3 were synthesized in a single reaction of 3,4,5-trihydroxy methylbenzoate with 1-bromodecane. The yields of 2 and 3 are 40% and 48%, respectively. Compound 4 is a key intermediate to synthesize the monomer MOXD. Similar compound to 4 had ever been synthesized by a four-step reaction from 3,4,5-trihydroxy methylbenzoate in literature,⁹ which needs the protection of two of the hydroxyl groups and cleavage the protective group. In this work, we have developed a direct method to synthesize 4 by etherification of the hydroxyl group of 2 with 1-bromodecanol in DMF at 60 °C using K₂CO₃ as base, which needs only two steps of reaction from the same starting materials, and has a higher overall yield with simple workup. The ester group of 4 was hydrolyzed with KOH in refluxing EtOH to yield the hydroxyl acid 5 in 94% yield. Esterification of 5 with methacryloyl chloride at 0 °C in dry CH₂Cl₂ in presence of pyridine led to a mixed ester anhydride, which was then heated in a solution of pyridine in water to afford 6 in 72% yield according to the reported procedures.^9,10 3,4,5-Tridecyloxybenzohydrazide 7 (ref. 11) was prepared in 94% yield by reaction the ester 3 with hydrazine in ethanol. Refluxing 7 with 3,4,5-tridecyloxybenzoic acid chloride yielded the hydrazide 8, which was used without purification in the cyclisation reaction to give the monomethacylate functionlized monomer MOXD with an overall yield of 65%. Radical polymerization of MOXD initiated with AIBN was carried out in THF, and the resulting polymer POXD was purified by silica column chromatography to separate the unreacted monomer.

The molecular structure of MOXD was characterized by ¹H NMR, ¹³C NMR and high resolution mass spectroscopy (HRMS). From the ¹H NMR spectrum (Fig. 1a) of MOXD, the characteristic vinyl proton signals of the methacryl group at 5.50 and 6.10 ppm can be seen clearly, and the peak integrations and chemical shifts of all the protons are in excellent agreement with its expected structure. In contrast, the ¹H NMR spectrum (Fig. 1b) of POXD showed that the vinyl proton signals of the methacryloyl group from the monomer have disappeared completely from the spectrum, indicating that the unreacted monomer has been fully removed from the purified polymer.

Fig. 1 ¹H NMR spectra of (a) MOXD and (b) POXD.

The thermal stability of MOXD and POXD was investigated by thermogravimetric analyses (TGA) under nitrogen atmosphere. The TGA curves of the solid samples of MOXD and POXD are given in the ESI (Fig. S1†), and the onset decomposition temperatures are listed in Table 1. Both of them showed no weight loss at 25–300 °C, indicative of the excellent thermal stability. The liquid crystalline behaviors of MOXD and POXD were investigated by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The transition temperatures and enthalpy values derived from DSC results are listed in Table 1.

Table 1 Phase transition temperature and corresponding enthalpies (kJ mol⁻¹) of MOXD and POXD

Compd	Phase transitions^a	T^b[°C] (ΔH [J g^−1])	Tdec^c [°C]
a Cr = crystalline phase; ND = discotic nematic phase; Colho = hexagonal columnar phase; Iso = isotropic melt.b Peak transition temperature in the DSC thermograms obtained during the second heating and first cooling cycles at 10 °C min^−1.c Onset of decomposition determined by TGA.
MOXD	Cr1 → Cr2	−21.0 (3.5)	327
	Cr2 → ND	14.7 (2.5)
	ND → Iso	27.9 (0.2)
	Iso → ND	24.6 (1.4)
	ND → Cr1	−4.3 (−6.5)
	Cr1 → Cr2	−27.0 (−5.2)
POXD	Cr1 → Cr2	−25.9 (0.1)	366
	Cr2 → Colh	24.1 (17.9)
	Colh → Iso	54.6 (2.8)
	Iso → Colh	49.0 (−2.7)
	Colh → Cr2	4.1 (21.2)
	Cr2 → Cr1	−29.1 (−0.1)

---

At room temperature, the sample of the monomer MOXD placed between two glass slides can be easily sheared to show birefringent texture, indicating that the compound is in the liquid crystalline state at room temperature. The sample transforms into an isotropic liquid upon heating to 28 °C. Upon cooling, little birefringent drops first appear from the isotropic melt and then fully form a threaded Schlieren texture with two and four brush defects and marbled textures (Fig. 2a) typical for a discotic nematic phase.¹² The N_D–I transition enthalpy (0.2 J g⁻¹ (0.25 kJ mol⁻¹)) may also support the assignment of discotic nematic phase, in which the N_D–I transition enthalpy is usually <1 kJ mol⁻¹.¹³ Further cooling to −4 °C results in the crystallization of the mesophase. The POM observations are reasonably consistent with the results of DSC (Fig. S2a†).

Fig. 2 The texture of (a) MOXD at 18 °C, ×100; (b) POXD at 29.6 °C, ×100, in the cooling cycle.

The polymer POXD is a sticky solid at room temperature. In the microscopic study, typically pseudo focal-conic textures with a large area of homeotropic domains (Fig. 2b) are observed both in the heating and cooling cycles, suggesting that the mesophase is an enantiotropic hexagonal columnar phase.^4e,11,14 In addition, POXD exhibits the liquid crystalline behaviors at room temperature region, and the mesomorphic temperature range is much wider than that of MOXD. The DSC curves (Fig. S2b†) of POXD exhibited clearly two peaks and a crystal-to-crystal transition in both the heating and cooling scans, which is well in line with the POM observations.

The UV-vis absorption and photoluminescent spectra of MOXD, and POXM in dichloromethane solution at room temperature are presented in Fig. 3. All compounds exhibited a strong absorption λ_max at ca. 310 nm, while the polymer POXD showed two additional weak shoulders at 260 nm and 265 nm. They all emitted a strong blue emission with a λ_max at ca. 391 nm. The shapes of the photoluminescent spectra, the emission λ_max, the intensity of emission peaks and the photoluminescent quantum yields (MOXD: 51%; POXD: 64%) of these compounds are very similar, indicating that the emission was mainly attributed to conjugated mesogenic core, the molecular weight and peripheral alkoxy chains had negligible effect on the optical properties.

Fig. 3 Absorption and photoluminescent spectra of MOXD and POXM in CH₂Cl₂ (1 × 10⁻⁵ M) at 25 °C. The excitation wavelength was 300 nm.

The emission spectra of MOXD and POXD in solid state and liquid crystalline phases are presented in Fig. 4. For POXD, the fluorescent emission λ_max in solid state at −78 °C is blue-shifted (5 cm⁻¹) with respect to that in the columnar phase at room temperature. Compared to the slight spectral shift, the fluorescent intensity increased dramatically from columnar phase to solid state. Similar results are also observed in the case of MOXD. The fact that the fluorescent intensity increases from liquid crystalline state to solid state may be ascribed to the different extent of forming self-quenching aggregates in different states.¹⁵ This phenomenon often occurred in the compounds with an intramolecular donor–acceptor structure,¹⁶ compounds like 1,3,4-oxadiazoles without strong electron-donating groups were found to show the self-quenching aggregates for the first time. By comparison of the emission spectra of MOXD and POXD, it is found that the fluorescent intensity increases significantly both in solid state and liquid crystalline phases. The fluorescent intensity of POXD is almost 12 times of that of the monomer MOXD in solid state as well in liquid crystalline state.

Fig. 4 Fluorescence spectra of MOXD and POXD in solid state at −78 °C and liquid crystalline phases at room temperature.

Conclusion

A novel side-chain polymer and the corresponding monomer based on discotic 1,3,4-oxadiazole have been reported in this work. Both of them are room temperature liquid crystals with excellent thermal stability. The polymer facilitated to form an ordered hexagonal columnar phase a with wider temperature range than that of the monomer. Both the polymer and the momomer are photoluminescent in solution, in solid state and in liquid crystalline state, while the different states have a great effect on their optical behaviors. In CH₂Cl₂ solution, they all display a fluorescent emission with similar quantum yields. In contrast, the intensity of the polymer is almost 12 times of that of the monomer in both solid state and liquid crystalline state.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21272130) and the Open Project of State Key Laboratory of Elemento-organic Chemistry (no. 201211), which are gratefully acknowledged.

Notes and references

1. J. Han, J. Mater. Chem. C, 2013, 1, 7779.
2. (a) E. Girotto, J. Eccher, A. A. Vieira, I. H. Bechtold and H. Gallardo, Tetrahedron, 2014, 70, 3355; (b) I.-H. Chiang, C.-J. Long, H.-C. Lin, W.-T. Chuang, J.-J. Lee and H.-C. Lin, ACS Appl. Mater. Interfaces, 2014, 6, 228; (c) A. A. Viera, E. Cavero, P. Romero, H. Gallardo, J. L. Serrano and T. Sierra, J. Mater. Chem. C, 2014, 2, 7029; (d) T. E. Frizon, A. G. Dal-Bó, G. Lopez, M. M. S. D. Paula and L. D. Silva, Liq. Cryst., 2014, 41, 1162; (e) E. Westphal, D. H. da Silva, F. Molin and H. Gallardo, RSC Adv., 2013, 3, 6442.
3. C.-H. Shih, P. Rajamalli, C.-A. Wu, M.-J. Chiu, L.-K. Chu and C.-H. Cheng, J. Mater. Chem. C, 2015, 3, 1491.
4. (a) K. T. Lin, H. M. Kuo, H. S. Sheu and C. K. Lai, Tetrahedron, 2013, 69, 9045; (b) J. Han, Q. Geng, W. Chen, L. R. Zhu, Q. Wu and Q. C. Wang, Supramol. Chem., 2012, 24, 157; (c) E. Westphal, I. H. Bechtold and H. Gallardo, Macromolecules, 2010, 43, 1319; (d) S. Qu and M. Li, Tetrahedron, 2007, 63, 12429; (e) C. R. Wen, Y. J. Wang, H. C. Wang, H. S. Sheu, G. H. Lee and C. K. Lai, Chem. Mater., 2005, 17, 1646.
5. (a) M. Watanabe, K. Tsuchiya, T. Shinnai and M. Kijima, Macromolecules, 2012, 45, 1825; (b) M. Tokita, A. Ikoma, T. Ishii, S. Kang and J. Watanabe, Macromolecules, 2012, 45, 4857; (c) M. Petr and P. T. Hammond, Macromolecules, 2011, 44, 8880; (d) I. Tahar-Djebbar, F. Nekelson, B. Heinrich, B. Donnio, D. Guilon, D. Kreher, F. Mathevet and A. J. Attoas, Chem. Mater., 2011, 23, 4653; (e) Z. Li, Y. Zhang, L. Zhu, T. Shen and H. Zhang, Polym. Chem., 2010, 1, 1501; (f) C. Xing, J. W. Y. Lam, K. Zhao and B. Z. J. Tang, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 2960.
6. (a) L. Marin, M. D. Damaceanu and D. Timpu, Soft Mater., 2009, 7, 1; (b) R. Balamurugan and P. Kanan, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 5760; (c) D. Acierno, E. Amendola, S. Bellone, S. Concilio, P. Iannelli, H. C. Neitzert, A. Rubino and F. Villani, Macromolecules, 2003, 36, 6410; (d) C. J. Tsai and Y. Chen, J. Polym. Sci., Part A: Polym. Chem., 2002, 40, 293.
7. (a) M. Kawamoto, H. Mochizuki, T. Ikeda, H. Lino and J. Hanna, J. Phys. Chem. B, 2005, 109, 9226; (b) H. Mochizuki, T. Hasui, M. Kawamoto, T. Ikeda, C. Adachi, Y. Taniguchi and Y. Shirota, Macromolecules, 2003, 36, 3457.
8. (a) Q. Yang, Y. Xu, H. Jin, Z. Shen, X. Chen, D. Zou, X. Fan and Q. Zhou, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 1502; (b) Y. Xu, Q. Yang, Z. Shen, X. Chen, X. Fan and Q. Zhou, Macromolecules, 2009, 42, 2542.
9. V. Percec, C. H. Ahn, T. K. Bera, G. Ungar and D. J. P. Yeardley, Chem.–Eur. J., 1999, 5, 1070.
10. V. Percec, M. R. Imam, M. Peterca and P. Leowanawat, J. Am. Chem. Soc., 2012, 134, 4408.
11. J. Han, X. Y. Chang, L. R. Zhu, M. L. Pang, J. B. Meng, S. S. Y. Chui, S. W. Lai and V. A. L. Roy, Chem.–Asian J., 2009, 4, 1099.
12. H. K. Bisoyi and S. Kumar, Chem. Soc. Rev., 2010, 39, 264.
13. K. Praefcke, in Physical Properties of Liquid Crystals: Nematics, ed. D. Dunmar, A. Fukuda and G. Luckhurst, INSPEC, London, 2001.
14. I. Dierking, Color Plates, in Texture of Liquid Crystals, WILEY-VCH. Vrlag, GmbH&CoKGaA, Weinheim, FRG, 2003.
15. (a) H. F. Hsu, M. C. Lin, W. C. Lin, Y. H. Lai and S. Y. Lin, Chem. Mater., 2003, 15, 2115; (b) I. A. Levitsky, K. Kishikawa, S. H. Eichhorn and T. M. Swager, J. Am. Chem. Soc., 2000, 122, 2474.
16. X. Zhang, Z. C. Li, K. B. Li, S. Lin, F. S. Du and F. M. Li, Prog. Polym. Sci., 2006, 31, 893.

---

Footnote

† Electronic supplementary information (ESI) available: Synthetic details and characterization data for all new compounds, TGA and DSC thermographs of MOXD and POXD. See DOI: 10.1039/c5ra05983f

---