Photoresponsive Iodine-bonded Liquid Crystals Based On Azopyridine Derivatives With A Low Phase-transition Temperature

Halogen bonding interactions in the formation of liquid crystalline phases have been recognized in recent years.Halogen bond has a superior high directionality contrasted with hydrogen bond,which provides a more rigid junction with a tunable interaction strength by selecting the appropriate halogen atoms.Secondly,its interaction strength can be tuned by choosing suitable halogen atoms that take part in the bond formation without significantly changing the electronic structure of the compound.Azopyridine derivatives(AnAzPy)are equipped with photoresponsive azo groups,which have a trans-cis isomerization upon light irradiation.They also contain self-assembled pyridine groups,which can form various non-covalently bonded complexes,for example,halogen bond,hydrogen bond and ionic bond ect.It is a distinguished intermediate for medicine and sensitive material.Here,we report a novel series of iodine-bonded liquid crystals using 1,2-diiodotetrafluorobenzene(1,2-DITFB)and AnAzPy,showing a smectic A phase and concurrent photoresponsive behaviour.These were characterized by using a polarizing optical microscope(POM),differential scanning calorimetry(DSC)and UV-vis absorption spectroscopy.The formation of iodine bonding in the complexes was confirmed by X-ray photoelectron spectroscopy(XPS)and Raman spectroscopy.Importantly,these iodine-bonded complexes demonstrated a low liquid crystal temperature range(30-50 ?C)among those reported for photoresponsive halogen-bonded liquid crystals.The molar ratio of the iodine-bonded donor and acceptor was 1:1 upon the self-assembly of the supramolecular complex molecule,as indicated by 1D-WAXD experiments of mixed samples of 1,2-DITFB and AnAz Py with different molar ratios.This study offers a new family of photoresponsive halogen-bonded liquid crystals and broadens the potential applications in their associated systems.The microstructure of iodine-bonded compounds and original materials AnAzPy was investigated by scanning electron microscope(SEM).There was no liquid crystal phase in bromine-bonded compounds due to the weak interaction between AnAzPy and 1,2-dibromotetrafluorobenzene(1,2-DBrTFB).Finally we also studied other iodine-bonded compounds with iodopentafluorobenzene(IPFB),1,4-diiodotetrafluorobenzene(1,4-DITFB)and 1,3,5-triiodobenzene(TBMB),and investigated their microstructures.