Facile Synthesis and Solid-State Properties of Liquid-Crystalline Polypeptides Bearing Biphenyl Mesogens and Alkyl Tails

A series of liquid-crystalline 4-alkyloxybiphenyl-poly(γ-propyl- l -glutamate) conjugates (PPLG n -g-BPC m , n = 10, 54, 110, and 274, m = 4, 6, and 12) with end-on biphenyl mesogens and variable alkyl tails is synthesized via a facile copper-mediated alkyne–azide [2+3] 1,3-dipolar cycloaddition. 1 H NMR and FTIR analyses reveal a quantitative conjugation efficiency for each sample. Variable-temperature FTIR results indicate an irreversible α-helical to β-sheet transition of the PPLG 10 -g-BPC 6 in the solid state, whereas PPLG n -g-BPC m samples with a longer main-chain length ( n ≥ 54) only adopt an α-helical conformation in the temperature range between 25 and 200 °C. DSC, polarized optical microscopy (POM), and wide-angle X-ray scattering (WAXS) results collectively reveal that the PPLG n -g-BPC m samples undergo a reversible crystalline to liquid-crystalline phase transition depending on the main-/side-chain lengths. The PPLG n -g-BPC m ( n ≥ 54, m ≤ 6) samples form highly stable smectic (SmC or SmE) phases with a calculated tilt angle of 73–90°.