Synthesis and characterization of a three-ring bent-core compound

Abstract

To investigate the vacuum-deposited films of the banana-shaped mesogens, we prepared a three-ring bent-core (that is, banana-shaped) compound m- bis(4- p- octoxysryrenyl)benzene (m-OSB) and used it as a prototype for studying the related film physics. Herein the synthesis and the structural characterization of m-OSB are reported. 13C and 1H NMR spectra and IR spectrum confirm that m-OSB has a symmetric and all-trans conformation. The results of differential scanning calorimetry, polarizing optical microscopy and X-ray diffractometry indicate that no liquid crystalline phase but two crystalline phases are present in this material: phase I (T<66°C) and phase H (66°C<T<157°C); an isotropic state is observed at above 157°C. Combining the X-ray diffraction and the electronic diffraction, we assign the room-temperature crystalline phase I to orthorhombic system and P212121 space group with cell parameters of a = 7.43 A, b = 6.34 A, c = 72.07 A, and α = β = γ= 90°. Molecular modeling reveals that the molecules in the unit cell adopt a layered structure with an antiferro-electric alignment. The structure of phase II is very similar to that of phase I. Nonlinear optical measurement shows that m-OSB is active for the second harmonic generation (SHG). Such characterization of the bulk material is necessary for the understanding of the growth and microstructures in the films.