Synthesis of 4-Chloro-1,3-Diazobenzene Bent-Cores Liquid Crystal and Characterizations of Its Mesogenic Behaviors and Photosensitivity

Abstract

Azobenzene-based bent-core liquid crystals demonstrate a variety of mesomorphic behaviors and photochromic properties which are desirable for optical switching. In this study, a series of novel compounds were synthesized by adding azo functional groups and chlorine substituent to the central bent-cores to form a 4-chloro-1,3-dizaophenylene bent-core. Fouriertransform infrared spectroscopy (FTIR), 1 H and 13 C nuclear magnetic resonance (NMR), mass spectrometry (MS), differential scanning calorimetry (DSC), polarized optical microscopy (POM), and ultraviolet-visible spectroscopy (UV-Vis) were performed to evaluate the structure, mesogenic properties, and photosensitivity of these synthesized compounds. The experimental results show that these compounds exhibit a broad temperature window up to 63.8 °C for nematic phase. In addition, the enhancement of photonic properties of these compounds was exemplified by the high conversion ratio and the rapid rate of trans -cis photoisomerization of compound 4c. The cis fraction of 4c can reach 0.81. At 95 °C, 4c in nematic phase became isotropic liquid under UVirradiation in 3 seconds and can be restored to nematic under natural visible light in 5 seconds. At room temperature, 4c when dissolved in ethyl acetate solution can reach photostationary state in 10 seconds. Quantum mechanics modeling confirms that using azos instead of esters as the central linkages can effectively reduce the molecular dipole moment, which appears to promote favorable mesogenic behaviors and photonic characteristics. Moreover, varying the carbon number in the terminal alkyl chains can alter molecular dipole, especially the polarizability anisotropy, of which the variation is strongly correlated with the phase transition temperature and temperature range of nematic phase. These findings suggest that 1) changing azo group position can effectively alter the molecular dipole; 2) reducing molecular dipole interaction can promote favorable photonic properties of azobenzene bent-core liquid crystal. This study linking the mechanistic details with mesogenic behaviors provides a novel approach to improve the material design for photonic applications.