Investigation of structural, spectroscopic and optical properties of some stilbazolium derivatives

In this thesis, theoretical investigations have been carried out on stilbazolium derivatives 2-[2-(2,4-dimethoxy-phenyl)-vinyl]-1-ethyl-pyridinium iodide (DMPI), (E)-4-(3-ethoxy-2-hydroxystyryl)-1-methyl pyridinium iodide (3ETSI), 2-[2-(4-methoxy-phenyl)-vinyl]-1-methylstilbazolium iodide (4MESI) and 2-[2-(4-Diethylamino-phenyl)-vinyl]-1-methyl-pyridinium iodide (DEASI) molecules using density functional theory (DFT). Furthermore, the optical properties of the molecules have been analyzed with the use of both theoretical and experimental data.All theoretical calculations were performed using Gaussian 09 Revision D.01 package program. In this study, the optimized structures of molecules on the ground state were obtained theoretically using B3LYP/LANL2DZ and HSEH1PBE/LANL2DZ levels. Theoretical vibration frequencies of the optimized molecular structure were calculated in the infrared region and PED analyses were performed. Additionally, the compatibility between the theoretical and experimental frequencies was examined. Subsequently, the HOMO (highest occupied molecular orbital) and LUMO (lowest occupied molecular orbital) energies, as well as the energy band gaps, electronegativity, chemical hardness, and softness parameters were obtained. In previous experimental investigations, the 13C-NMR and 1H-NMR spectra of DMPI, 3ETSI, 4MESI, and DEASI were studied in DMSO-d6 solvent. Consequently, the molecules were re-optimized in the DMSO solvent effect using the B3LYP/LANL2DZ and HSEH1PBE/LANL2DZ theoretical methods. The 13C-NMR and 1H-NMR chemical shift values were then calculated and compared with the experimental results. These chemical shift values were obtained according to the values calculated with the same method and set in DMSO solvent effect of TMS (tetramethylsilane) molecule. The calculations performed at B3LYP/LANL2DZ and HSEH1PBE/LANL2DZ levels have confirmed the existence of charge transfer motion in molecules through Natural Bonding Orbital (NBO) analysis. The molecular electrostatic potential (MEP) surface map provides information about the electronic density in a region, leading to its use in determining possible hydrogen bond interactions and potential interaction areas around a molecule. The calculations revealed that the region exhibiting the highest negative potential was centered around the electronegative iodine (I) atom, while the positive potential region was centered around the hydrogen atom of the CH3 groups. In the ultraviolet-visible region, the optical band gap, absorption coefficient, refractive index, extinction coefficient, dielectric constant, and loss factor of materials are essential parameters for optical applications in technology. To determine these properties, the examination of transmittance, absorption, and reflection spectra is necessary. According to the experimental studies of the molecules in the literature, DMPI and 3ETSI have transmittance spectra in solid phase, while DEASI and MESI have absorption spectra in methanol solvent. The experimental absorption, transmittance, and reflection spectra were obtained using fundamental optical equations. The molar absorption coefficient spectra have been obtained by means of theoretical calculations in the ultraviolet-visible region. These spectra were calculated in the gas phase for DMPI and 3ETSI, and in the effect of methanol solvent for DEASI and MESI. Thereafter, theoretical absorption spectra were produced by normalizing them according to the experimental absorption spectra, respectively. Theoretical transmittance and reflection spectra were also determined using the basic optical equations. The optical band gaps of DMPI and 3ETSI were determined by the Tauc method in previous experimental studies; however, no similar studies were reported for DEASI and MESI. In this study, the experimental optical band gaps of DEASI and MESI and the theoretical optical band gaps of all molecules were calculated by the Tauc method. It was found that the theoretically calculated optical band gaps were in close agreement with the experimental ones. However, experimental studies have not yet investigated other optical properties. Accordingly, the spectra of these other optical properties were obtained using the transmittance and reflection spectra found in previous experimental studies and theoretical calculations. The Urbach energy, indicative of structural irregularity and defectiveness, was calculated from the graphs of the logarithmic absorption coefficient. The dispersion and oscillator energies were determined according to the Wemple-DiDomenico model. The compatibility of theoretical energy values with the experimental results was observed. The optical moments, which play a pivotal role in the interaction between light and material, were also obtained from these calculated energies. Optical conductivity, a critical parameter in the material sciences, is determined by the refractive index and the absorption coefficient. The examination of optical conductivity properties was carried out by using these parameters. The dielectric constant of a material is comprised of two parts: real and imaginary. The real part of the dielectric constant is indicative of the material's capacity for energy storage, while the imaginary part is indicative of the energy loss in the medium. These dielectric parameters were calculated using the refractive index and extinction coefficient. The surface and volume energy loss functions are important quantities that express the rate of energy loss for electrons passing through a material. These parameters are determined using the real and imaginary part spectra of the dielectric constant. Due to their high second-harmonic generation efficiency and transparency in the ultraviolet-visible region, organic nonlinear optical (NLO) materials are essential for many optoelectronic and photonic applications, thus increasing interest in high second-order nonlinear optical materials. Ionic organic materials exhibit excellent NLO properties due to weak Van der Waals forces and hydrogen bonding with π conjugated electrons. Among the various types of organic crystals that have been studied to the present, stilbazolium salts have been found to be the most effective and are of great interest for NLO and terahertz applications. A notable property of stilbazolium derivatives is their strong Coulomb interactions and high dipole-dipole interaction between the stilbazolium cation and the counter anion, resulting from the delocalized π-electron system. Additionally, these derivatives exhibit a significant third-order NLO capacity, due to their strong intramolecular charge transfer property. For this purpose, DMPI, 3ETSI, 4MESI, and DEASI molecules were synthesized and crystal growth was performed in experimental studies in literature. Then, their spectroscopic, electrical, mechanical, thermal, linear, and nonlinear optical properties were experimentally investigated. As a result of these studies, it was determined that the compounds are suitable for optoelectronic and NLO applications due to their transparency in the visible region and low dielectric loss. In the experimental studies, the polarizability properties of all compounds except 3ETSI were determined by dielectric measurement. Subsequently, applying the Penn and Clausius-Mossotti equations, polarizability values were obtained for only the DMPI. In this thesis, polarizability values for 4MESI and DEASI are also reported, based on experimental measurements and the same equations. Furthermore, polarizability values for all compounds were calculated from the Lorenz-Lorentz equation according to the experimental refractive index at 633 nm. Theoretical polarizability values were also calculated at the HSEH1PBE/LANL2DZ and B3LYP/LANL2DZ levels to compare with the experimental results. In experimental studies, the nonlinear optical (NLO) properties of DMPI and 3ETSI were investigated by Z-scan technique, while the investigation of 4MESI and DEASI compounds was lacking. The Z-scan technique was used to determine the nonlinear refractive index, third-order optical susceptibility, and second-order high polarizability values of DMPI and 3ETSI. In this thesis, the second-order high polarizability at the HSEH1PBE/LANL2DZ and B3LYP/LANL2DZ levels were theoretically calculated for all compounds. Then, using these theoretical results and applying the formulas in the Z-Scan technique, the third-order optical susceptibilities and nonlinear refractive index were calculated, respectively. A comparison of the experimental and theoretical results of the polarizability and NLO parameters of the compounds reveals that these values greater than the values of paranitroaniline (pNA) and potassium dihydrogen phosphate (KDP), which are commonly used as prototype compounds. As a result, these compounds are considered promising candidates for a variety of fields, such as integrated photonics, electro-optical modulation, optical communication, high-speed optical information processing, laser technology, optoelectronics, photonics, terahertz, and NLO applications. Theoretical calculations have indicated that the stilbazolium derivatives DMPI, 3ETSI, 4MESI, and DEASI have the potential to exhibit good NLO properties, as observed in previous experimental studies utilizing dielectric measurement and Z-Scan techniques.