Monitoring structural fluctuations of discotic liquid crystal during phase transitions.

[Display omitted] • Monomeric and dimeric structure of discotic liquid crystal (DLC) A8 is optimized using density functional theory (DFT). • Phase transition of DLC A8 (Isotropic → Discotic nematic → Columnar → Crystalline) have been analysed with the help of temperature dependent FT-IR and FT- Raman spectra. • Combination of experimental (FT-IR and FT-Raman) and computational results allow us to elucidate the dynamics of the molecules during the phase transitions. • AIM analysis has predicted the strength and nature of the inter and intra molecular hydrogen bonding in DLCs. The discotic liquid crystal 4-((2, 3, 4-tris (octyloxy) phenyl) diazenyl) benzoic acid , hereafter referred as DLC A8, exhibited in dimeric form has been studied using a combination of quantum chemical approaches and vibrational spectroscopy. This study investigates the structural alteration of DLC A8 associated with phase transition. The phase transitions of DLC A8 are Iso → Discotic nematic → Columnar → Crystalline, which have been investigated using differential scanning calorimetry (DSC) accompanied with polarized optical microscopy (POM). Monotropic columnar mesophase was observed during the cooling cycle while discotic nematic mesophase was observed in both the heating and cooling cycles. Density functional theory (DFT) along with IR and Raman spectroscopic techniques were utilized to study the dynamics of molecules during phase transition. To predict the most stable conformation of the molecule, one-dimensional PES scans have been performed along 31 flexible bonds using DFT/B3LYP/6-311G++(d,p) method. Vibrational normal modes were analyzed in detail, taking potential energy contribution into account. The spectral analysis of FT-IR and FT-Raman was done by deconvoluting the structural sensitive bands. The agreement between the calculated IR and Raman spectra and the observed FT-IR and Raman spectra at room temperature confirms our theoretically predicted molecular model of investigated discotic liquid crystal. Moreover, our studies have unraveled the existence of intact intermolecular H-bonding of dimers throughout the phase transitions. [ABSTRACT FROM AUTHOR]