Synthesis and liquid crystal properties of a new class of calamitic mesogens based on substituted 2,5‐diaryl‐1,3,4‐thiadiazole derivatives with wide mesomorphic temperature ranges

Abstract

Liquid crystals based on substituted 2,5‐diaryl‐1,3,4‐thiadiazole derivatives (1a–1f, 3a and 3b) and 1,3,4‐oxadiazole analogues (2a–2f, 4a and 4b) were synthesised and characterised by ¹H, ¹³C nuclear magnetic resonance, Fourier transform infrared, mass spectrometry, high‐resolution mass spectrometry techniques and elemental analyses. The X‐ray crystal structure of 1e revealed that it contains tilted lamellar arrangement of molecules in the crystalline solid. The liquid crystal properties have been investigated by polarised‐light optical microscopy, differential scanning calorimetry and in‐situ variable‐temperature X‐ray diffraction. All compounds (except 2e and 2f) exhibited thermotropic liquid crystal behaviours with various mesophases (smectic A and C, nematic N or soft crystal E phases). Notably, the 1,3,4‐thiadiazole derivatives consistently have wider mesomorphic temperature ranges than those of the respective 1,3,4‐oxadiazole analogues. The solutions of all compounds in CH₂Cl₂ individually displayed one or two absorption bands with λ _max values at 297–355 nm and emitted with λ _max values at 363–545 nm and quantum yields of 0.12–0.73. Structure–property relationships of these compounds are discussed in the contexts of their molecular structures and weak intermolecular interactions.

Keywords:

• heterocyclic liquid crystals
• phase transition
• microwave‐assisted synthesis
• structure‐property relationship

Acknowledgements

This work was financially supported by grants from the National Natural Science Foundation of China (Project No. 20772064). SSYC acknowledges the award of a research assistant professorship from The University of Hong Kong (HKU).

Notes

Crystal data for 1e: C₂₄H₂₉N₃O₃S, M _r = 439.57, triclinic, P‐1, a = 7.040(2) Å, b = 8.992(3) Å, c = 18.102(5) Å, α = 78.75(1)°, β = 81.81(1)°, γ = 88.17(1)°, V = 1112.3(6) ų, Z = 2, T = 133 K, ρ _calcd = 1.312 g cm⁻³, μ _Mo = 1.765 mm⁻¹, F(000) = 468, total number of unique reflections 5218 (R _int = 0.041). Numbers of parameters = 281, R1 = 0.046 and wR2 (all data) = 0.1138. Crystallographic data (excluding structure factors) for the structure reported in this work have been deposited in Cambridge Crystallographic Data Center with CCDC numbers: 660493. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: [phone]; Email: [email protected]).