Structural and Thermal Behaviour of the Pyrrolidinium Based Ionic Liquid Crystals [C10mpyr]Br and [C12mpyr]Br

The ionic liquid crystals N-decyl-N-methylpyrrolidinium bromide, [C(10)mpyr]Br, and N-dodecyl-N-methylpyrrolidinium bromide, [C(12)mpyr]Br have been synthesized. X-ray diffraction single crystal structure analyses {[C(10)mpyr]Br: monoclinic, P2(1)/n, no. 14, Z = 12, Pearson code mP588, a = 19.446(2) angstrom, b = 7.0659(5) angstrom, c = 36.844(4) angstrom, beta = 90.63(1)degrees, 4317 unique reflections with I-o > 2 sigma (I-o), R-1 = 0.0372, wR(2) = 0.0822, GooF = 0.897, T = 298(2)K; [C(12)mpyr]Br: monoclinic, P2(1)/n, no. 14, Z = 12, Pearson code mP660, a = 20.457(8) angstrom, b = 7.0796(16) angstrom, c = 39.335(14) angstrom, beta = 96.96(3)degrees, 4158 unique reflections with I-o > 2 sigma (I-o), R-1 = 0.0431, wR(2) = 0.0753, GooF = 0.823, T = 298(2)K)} reveals a layer structure for both compounds. Sheets composed of hydrophilic structure regions built by positively charged pyrrolidinium head groups and bromide anions alternate with hydrophobic areas formed by interdigitated long alkyl chains belonging to pyrrolidinium cations with opposite orientation. Thermal analyses together with polarizing optical microscopy show that both compounds are thermotropic liquid crystals adopting smectic mesophases. For [C(10)mpyr]Br the solidification temperature is dramatically depressed and the liquid crystalline state is preserved metastably at room temperature and well below (similar to -80 degrees C). In contrast [C(12)mpyr]Br solidifies around room temperature (similar to 23 degrees C).