DFT study of electro-optical properties and molecular spectroscopy of heteroatom substituted terphenyl core LCs and comparison with their unsubstituted analogues.

contextDue to high birefringence, nematogenic behavior, and polarizability anisotropy, terphenyl core-based liquid crystals are useful in high-performance display technologies, photonics, and terahertz applications. These potential applications drew researchers' attention towards terphenyl core-based liquid crystalline molecules. The mesomorphic behavior and optical properties of a series of homologous liquid crystalline compounds belong to 4'-alkylthio-4"-isothiocyanoterphenyl with lateral fluorine substitution as well as oxygen and alkene derivatives were previously analyzed and compared with their unsubstituted analogs which demonstrated that the presence of sulfur and oxygen atom enhances the birefringence and nematogenic character. Due to the high birefringent and nematic phase behavior of these molecules, the present study aims to evaluate their electro-optical properties and molecular spectroscopy by using density functional theory (DFT). The results reveal that the electro-optical and nonlinear optical properties of heteroatom-substituted terphenyls are higher than their unsubstituted analogs. The global reactive parameters and thermochemical parameters of pentylthio-(5S_T) molecule exhibit favorable result. The nonlinear optical (NLO) parameters of all investigated molecules are significantly higher than the reference molecules indicating their potential for NLO-based applications like telecommunications and optical interconnections. Pentyloxy-(5O_T) molecule exhibits the most favorable NLO response. Vibrational peaks and their associated modes of vibration in IR and Raman spectra are analyzed to understand their molecular dynamics, and the rotational constants provide information on the molecular rotational characteristics and anisotropy of investigated molecules. In UV-Vis spectroscopy, pentyloxy-(5O_T) and ethylthio-(2S_T) exhibit strong absorption characteristics. The analysis of absorption band, oscillator strength, and their associated HOMO-LUMO transitions gives insight to electronic excitation properties and potential opto-electronic applications.

methodsThe electro-optical properties and molecular spectroscopy (IR and Raman) are evaluated by using DFT with two distinct functionals B3LYP and M06-2X together with 6-311G(d,p) as primary choice of basis set in Gaussian 09 software. UV-Vis spectroscopy utilizes the time-dependent density functional theory (TD-DFT) and configuration interaction singles (CIS) method with functional CAM-B3LYP and M06-2X. The vibrational assignment is analyzed by using potential energy distribution (PED) obtained with VEDA in IR and Raman spectra, while molecular electronic and related properties were analyzed using Multiwfn.