Experimental and DFT Studies on Thermochromism Induced Binary HBLC Mixture
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Novel linear double hydrogen-bonded liquid crystals (HBLCs) were derived from diglycolic acid (DGA, a non-mesogenic compound) and 4-n-alkoxybenzoic acids (nOBA, n = 7 and 8 mesogenic compounds). Hydrogen bond (H bond) formation and its vibrational stretching frequencies had been calculated by experimental and theoretical IR spectroscopy. The calculated band gap energy (4.96 eV) using a UV-Vis spectrum clearly reveals the coincidence of highest occupied molecular orbital-lowest unoccupied molecular orbital band energy of the present HBLC mixture. Further, X-ray diffraction (XRD) studies at room temperature confirm the monoclinic nature of the HBLC mixture. Mesophases and their transition temperature were studied by a polarized optical microscope (POM) and differential scanning calorimetry (DSC). The order of the phase transition was evaluated by the thermal analysis. Due to the rotary motion of molecules, nematic phase (threaded texture) with thermochromic effect was observed. The induced thermochromism in the present HBLC and its possible color recording applications were discussed. Molecular descriptors (using computational density functional theory (DFT)) of the present mixture indicate the hardness and softness of the HBLC mixture. Natural bond orbital (NBO) studies revealed the O–H…O stabilization energy in the present HBLC mixture. Also, the lone pair (LP)-to-π* transition confirms the existence of intermolecular hydrogen bonding in the HBLC mixture. The calculated band gap energy of the DGA + nOBA HBLC mixture is a more useful parameter to identify suitable hydrogen-bonded liquid crystal material for photonic applications. Mulliken analysis shows clear evidence of the charge distribution in different molecules of the HBLC system.
KeywordsThermochromic nematic POM DSC DFT Binding energy
One of the authors (V. N. Vijayakumar) received a financial support rendered by the Department of Science and Technology (No. SERB/F/7454/2013-2014, dated 21 February 2014), New Delhi; the Department of Atomic Energy (No. 34/14/14/2016-BRNS/34039, dated 22 April 2016); and the Board of Research in Nuclear Science (DAE-BRNS) (TNSCST/RFRS/VR/19/2018-2019/7666, dated 6 June 2019), and an infrastructural support provided by the Bannari Amman Institute of Technology, Sathyamangalam.
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