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Scientists have investigated the structural changes of a discotic liquid crystal during its phase transitions using a combination of quantum chemical approaches and vibrational spectroscopy.
Scientists from Indiana University in Bloomington, Indiana, USA and University of Lucknow in Lucknow, India, investigated the structural changes of a discotic liquid crystal during its phase transitions. The study, published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, focuses on 4-((2, 3, 4-tris (octyloxy) phenyl) diazenyl) benzoic acid, referred to as DLC A8, which exists in a dimeric form (1).
The team used a combination of quantum chemical approaches and vibrational spectroscopy to analyze the structural alteration of DLC A8 during phase transition. The phase transitions of DLC A8 are Iso → Discotic nematic → Columnar → Crystalline, which were investigated using differential scanning calorimetry (DSC) accompanied by polarized optical microscopy (POM). The researchers observed a monotropic columnar mesophase during the cooling cycle while discotic nematic mesophase was observed in both the heating and cooling cycles.
DSC and POM are two techniques used for studying chemical structures. DSC measures the heat flow associated with a physical or chemical change in a material as a function of temperature or time. When used properly, this technique helps determine the phase transition temperature of a material, such as the melting point or glass transition temperature. POM is a nondestructive technique that uses polarized light to observe the optical properties of materials. It can provide information about the crystallographic orientation, birefringence, and morphology of a material, as well as the presence of defects or impurities. Together, DSC and POM can provide a comprehensive understanding of the structural changes that occur during phase transitions in materials.
Density functional theory (DFT) along with infrared (IR) and Raman spectroscopic techniques were used to study the dynamics of molecules during phase transition. To predict the most stable conformation of the molecule, one-dimensional potential energy surface scans were performed along 31 flexible bonds using the 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.
DFT is a quantum mechanical method used to study the electronic structure and energetics of materials. It can be used to predict the most stable conformation of a molecule during a phase transition. IR and Raman spectroscopic techniques are used to measure the vibrational modes of molecules. These modes are sensitive to changes in molecular structure and can provide information about the molecular dynamics during a phase transition. The spectral analysis of IR and Raman spectroscopy can be used to identify the characteristic vibrational bands associated with different functional groups in a molecule. By comparing the experimental spectra with the theoretically predicted spectra using DFT, the structural changes during phase transitions can be identified.
The study revealed that intact intermolecular hydrogen bonding of dimers exists throughout the phase transitions. The agreement between the calculated IR and Raman spectra and the observed FT-IR and Raman spectra at room temperature confirmed the team's theoretically predicted molecular model of investigated discotic liquid crystal.
According to the authors, their findings could provide important insights into the properties and potential applications of discotic liquid crystals, which have a range of technological applications, including in electronic devices and sensors (1).
This research highlights the importance of understanding the structural changes of materials during phase transitions and demonstrates the value of a multidisciplinary approach in investigating complex materials.
(1) Chaudhary, R.; Yadav, A.; Bahota, A. S.; Singh, S.; Fonseca, J. d. C.; Ayala, A. P.; Prasad, V.; Tandon, P.Monitoring Structural Fluctuations of Discotic Liquid Crystal During Phase Transitions. Spectrochimica Acta Part A: Mol. Biomol. Spectrosc. 2023, 295, 122619. DOI: 10.1016/j.saa.2023.122619