Investigation of structural, spektroskopic and static/frequency dependent nlo properti̇es of transition metal complexes containing 4-methoxy-pyri̇di̇ne-2-carboxylic acid ligand

This thesis focuses on the investigation of nonlinear optical (NLO) properties of metal-organic complexes. The study specifically aims to synthesize, characterize, and analyze the structural, electronic, and optical properties of new complexes derived from 4-Methoxypyridine-2-carboxylic acid (4-4MeOpca) ligand with Co(II) and Cd(II) metals. The potential applications of these complexes in optoelectronic devices, laser technologies, data storage, and biomedical imaging have been emphasized. Metal-organic complexes are hybrid structures formed by the combination of organic ligands and metal ions. These structures play a significant role in both fundamental science and applied research due to their adaptability in chemical and physical properties. Transition metals are prominent in NLO material design because of their broad electronic configurations and versatile coordination geometries. In this study, Co(II) and Cd(II) complexes were selected due to their high coordination capacities with ligands and stable complex structures. 4-Methoxypyridine-2-carboxylic acid (4-4MeOpca) is a ligand capable of forming strong and stable complexes due to its electron-withdrawing and binding regions. The symmetry and geometric arrangements of the bonds formed by this ligand with metal ions are critically important for optimizing nonlinear optical properties. A series of experimental and theoretical methods were employed to analyze the structural and electronic properties of the synthesized complexes. These methods include X-ray diffraction (XRD), infrared (IR) spectroscopy, and ultraviolet-visible (UV-Vis) spectroscopy for molecular structure and spectral analysis. Additionally, density functional theory (DFT) was applied to calculate the optimized geometric structures, IR, and UV-Vis spectra, frontier molecular orbital energies, MEP surfaces, and static/frequency-dependent nonlinear optical properties of the complexes. Addtionaly, Hirshfeld surface analysis was used to detail intermolecular interactions. The XRD analyses of Co(II) and Cd(II) complexes revealed relatively high symmetry in these structures, indicating low first-order hyperpolarizability (β) values. In metal-organic coordination complexes, coordination geometry is a critical factor that determines the chemical and physical properties of the complex. The Co(II) and Cd(II) complexes examined in this study exhibit the same coordination geometries, with a small bond lengths and angles differecies. The Co(II) and Cd(II) complexes typically displays a six-coordination geometry, characterized by an octahedral arrangement. The d7 and d10 electronic configurations of the Co(II) and Cd(II) ions, disturbs the octahedral symmetry, enhancing the nonlinear optical properties of the complexes. The carboxylic acid and pyridine groups of the ligand bind to the Co(II) and Cd(II) center through multidentate coordination, forming a strong bond. This arrangement strengthens the ligand-metal charge transfer and significantly increases the second-order hyperpolarizability (γ) values. The 4-4MeOpca ligand exhibited strong interactions with metals through its multidentate binding sites in the carboxylic acid and pyridine groups. IR spectroscopy elucidated the nature of metal-ligand bonds and the binding modes of the complexes. UV-Vis spectroscopy confirmed the presence of ligand-to-metal and metal-to-ligand charge transfer transitions, demonstrating a broad optical absorption spectrum for the complexes. The HOMO energy of the Co(II) complex was calculated as -6.2478 eV, the LUMO energy as -1.7714 eV, and the energy gap as 4.4764 eV. For the Cd(II) complex, the HOMO energy was -7.0438 eV, the LUMO energy was -1.8324 eV, and the energy gap was 5.2114 eV. These results indicate that the Co(II) complex has higher reactivity, whereas the Cd(II) complex exhibits a chemically more stable structure. Hirshfeld surface analysis visualized intermolecular interactions in the crystal structures of the complexes. The O∙∙∙H interactions were found to play significant roles in both Co(II) and Cd(II) complexes by the contributions of 39.5% and 38.6% to the overal interactions. The static first-order hyperpolarizability (β) of the Co(II) complex was calculated as 0.0002×10-30 esu, and its second-order hyperpolarizability (γ) was 47.364×10-36 esu. For the Cd(II) complex, the first-order hyperpolarizability (β) was 0.0002×10-30 esu, and the second-order hyperpolarizability (γ) was 20.829×10-36 esu. The static first-order hyperpolarizability (β) of the 4-4MeOpca ligand was calculated as 2.7062×10-30 esu, and its second-order hyperpolarizability (γ) was 11.492×10-36 esu.These results suggest that an increase in the γ parameters after the complex formation. So, the Co(II) and Cd(II) complexes are potentialy suitable materials for the effective third order NLO materials. When the second-order hyperpolarizability (γ) parameters of the Co(II) and Cd(II) complexes are compared, it is observed that the Co(II) complex has a significant advantage. The γ value of the Co(II) complex at (-2w;w,w,0) is calculated to be 1521.4 ×10-36 esu, while the value for the Cd(II) complex is found to be 413.94×10-36 esu. These results indicate that the Co(II) complex exhibits superior performance in nonlinear optical (NLO) properties and is particularly more effective in applications such as second harmonic generation (SHG). The high γ value of the Co(II) complex is attributed to the strong effect of metal-ligand charge transfer interactions. Additionally, the SHG values of the Co(II) complex are said to result from a more uniform distribution of charge density around the metal center in the molecule. In conclusion, the Co(II) complex is preferred for applications such as electro-optic modulators and frequency converters due to its high γ values, while the Cd(II) complex provides an advantage in various optical applications due to its first-order NLO properties. These findings reveal that both complexes have potential for specific applications in NLO material design. This study comprehensively investigated the nonlinear optical (NLO) properties of Co(II) and Cd(II) complexes synthesized with the 4-4MeOpca ligand. The results demonstrate that these complexes have promising potential in NLO material design. In particular, the ligand-metal interactions and geometric arrangements of the complexes were found to be critical in optimizing NLO activity. Co(II) complexes exhibited higher nonlinear optical activity due to the high spin multicipility of d7 electronic configuartion. These differences allow for the selection of specific applications in material design. The integration of theoretical and experimental approaches in examining the structural and electronic properties of metal-organic complexes highlights their significant contributions to the development of future optoelectronic devices. The findings provide a foundation for designing and synthesizing next-generation NLO materials. Furthermore, the potential applications of these complexes in biomedical imaging, laser technologies, and data storage present a strong motivation for future research.