Research On Liquid Crystal Compounds Based On Azobenzene Groups And Its Luminescence Mechanisms

Since the advent of compounds based on azobenzene, it caught great concernbecause its unique properties of the photoisomertzation. The azobenzene derivativessystems usually have a good optical memory and optical features that make it showgreat potential in many areas, such as self-assemble, the backlight date storage, lightswitch, lithography diffraction grating. In addition, in the aggregation inducedfluoresecence (AIEE) field, the aggregate morphology is very self-evident.Azobenzene introduced in the system which has some with similar AIEE propertieswill certainly cause the system to produce more interesting optical properties. On thisbasis, we synthesized compound B8-OXD which containing azobenzene andoxadiazole, We studied the hydrogen bonding mode of the rod-like molecules B8by1H NMR, studied the liquid crystal properties of B8and B8-OXD by POM, DSC,XRD, observed the photoluminescence phenomenon of B8-OXD by PL, UV-vis, inorder to understand the light-emitting mechanism by TEM test.The main results of the paper are as follows:1. B8molecules associated by intermolecular hydrogen bonding and assembledinto supramolecular aggregates by double intermolecular hydrogen bondings. Duringthe heating process of B8, we observed typical fan-shaped texture and schlieren ofsmectic C phase by POM, so as the nematic phase in the heating process of B8-OXD.The disappearance of hydrogen leads to the LCD temperature range narrows betweenB8and B8-OXD, this result confirmed the presence of intermolecular hydrogen bondsis conducive to the stability of the liquid crystal phase. The POM and XRD inferredthe layered arrangement of B8molecules, the interlayer spacing is29.3, molecularlength is37.8, so we can calculate the molecules in the layer arrangementinclination angle of B8is51°.2. Study by photoluminescence transition mechanism of B8-OXD, we believethat the photoluminescence mechanism of B8-OXD is as follows: The azobenzenewere almost entirely in the trans configuration when there was no light (a small amount of cis), theoretical simulation confirmed that B1-OXD is forbidden transitionwhen the azobenzene were all trans configuration, so B1-OXD will not luminous, sodoes the B8-OXD. With the UV irradiation time extend the fluorescence emissionenhanced. This phenomenon mainly because two factors: First, the occurrence ofcis-trans isomerization of azobenzene reduce the trans configuration result in theincrease of the cis configuration, transition forbidden lifted resulting in fluorescenceenhancement; The second is light cause part of aggregates depolymerization result inthe fluorescence enhancement. The weakened of the fluorescence after a certainperiod of UV light mainly due to the formed of ordered aggregates by nanoparticles,this made the energy gap decreases, resulting in the occurrence of level conversion, sothe light-emitting weakened. The light-emitting of B8-OXD in solution state also hasgreat dependence on the solvent polarity. When B8-OXD is in excited state will occurintramolecular charge transfer, that cause the molecular polarizablity, the interactionobetween the polarized molecules and the solvent molecules will be stronger when thesolvent polarity increase, so cause the excitation energy reduce, resulting in red shiftof the emission peak. The stokes displacement in different solvents will increase whenthe solvent polarity increase, this indicates that the excited state of the solutemolecular dipole distance is greater than the ground state, so the interaction betweensolvent molecules and solute molecules stronger when the solute molecules are inexcited state.