Investigation of structure-property relations of synthesized single crystal Cu(II) complex

Investigation of the electronic, optical and fluorescence properties of pyridine-2-carboxylic acid (picolinic acid, pic) and its derivatives, whose donor N atom forms bifunctional coordination to transition metal ions at low oxidation level and O atom forms bifunctional coordination to donor transition metal ions at high oxidation level, in many application areas such as metal complexes, materials science and medicinal chemistry is important for optoelectronic technology and pharmaceutical applications. This is because these compounds allow easier access to the delocalized π electrons of π systems, resulting in higher-order nonlinear optical properties. In this context, compounds with large NLO parameter values are significant as a material for potential applications in photonic and optoelectronic devices due to their fast reaction times and easy processability. In general, studies in this field require the synthesis and design of NLO motifs with high NLO properties and the importance of optics for the characterization and fabrication of active reactive materials. The synthesis and design of new active materials with NLO properties are gaining significant importance in optoelectronic technology due to possible applications in optical switching, information storage, etc. It is also known that the theoretical calculation method as well as the experimental technique used in the determination of structure-property relationships of new materials by experimental and theoretical methods is important. From a theoretical perspective, there are known challenges in the development of DFT methods, especially in accurately describing intermolecular interactions such as charge transfer interactions, van der Waals forces, and transition states. However, it is important for reliable results in structure-property relationships that new functionals be used in theoretical calculations in future studies, in addition to the widely used functionals. The DFT methods chosen in this study allow for further investigation of the structure-property relationships for a new Cu(II) complex and enable optimal molecular analysis through the comparison of experimental and computational results regarding the relationship between molecular structure and properties. Given its importance as mentioned above, different derivatives of picolinic acid (such as 6-methylpicolinic acid, 6-bromopicolinic acid) are being studied both structurally and spectroscopically. To date, studies have investigated the crystal structures and thermal properties of 6-bromopicolinic acid complexes with Co(II), Ni(II), Mn(II), Zn(II), and Cu(II). Additionally, crystal structure, electrochemical, and cytotoxicity studies have been conducted for the Ru(II) bipyridine complex of 6-bromopicolinic acid. In this context, due to a gap in the literature, the Cu(II) 2,2′-bipyridyl complex of 6-bromopicolinic acid has been synthesized. 2,2′-Bipyridyl is an organic compound formed by connecting two pyridine rings with a methyl bridge. This compound can form numerous coordination complexes, especially with transition metals. 2,2′-Bipyridyl, which has various application areas, can be used in the synthesis of electronic and photophysical materials. This thesis presents a detailed structure-property relationship of the [Cu(II) (6-Brpic)_2 (bpy)] complex, providing a comprehensive perspective both experimentally and theoretically. The complex was synthesized using 6-bromopicolinic acid and 2,2′-bipyridyl ligands, and its single crystal structure was obtained via X-ray diffraction technique. Additionally, the Hirshfeld surface analysis technique was utilized to measure the dimensions of the molecules and understand the intermolecular interactions. The vibrational, electronic, and optical spectral properties of this newly synthesized complex were examined through FT-IR, UV-Vis, and fluorescence spectra. Furthermore, absorption and emission spectra were recorded at concentrations ranging from 10 to 50 μM. Structure-property relationships corresponding to experimental parameters were investigated using five different DFT methods (HCTH, M06L, TPSSTPSS, B3LYP, CAM-B3LYP) with four different functionalities (GGA/meta-GGA/hybrid-GGA/hybrid-meta-GGA). Moreover, natural bond orbital (NBO) analysis was conducted using these methods to determine the coordination environment for the Cu(II) complex and the intermolecular interactions. Using DFT methods, the HOMO and LUMO energy values of the Cu(II) complex were determined, and based on these values, the chemical hardness (η), chemical softness (S), electronegativity (χ) represented as the negative of the chemical potential (μ), electrophilicity index (ω), and nucleophilicity index (φ) were calculated for the complex. The total electric dipole moment (μ), refractive index (n), static/dynamic isotropic and anisotropic linear optical parameters (<α> and ∆α), nonlinear optical properties (β and γ) of the Cu(II) complex, which has application potential in optoelectronic technology, were examined using different DFT methods both statically and dynamically (λ=532 nm, ω=0.0856 a.u.). Additionally, the linear optical susceptibility (χ(1)) and third-order nonlinear optical susceptibility (χ(3)) values were calculated. For all theoretical calculations, the Gaussian 16 program was used. Furthermore, the SWizard program was utilized to calculate the contributions of molecular orbitals in electronic transitions, and the percentage contributions of atoms/atom groups were determined using the Chemissian program. In this study, for the synthesized Cu(II) complex, calculations such as the linear correlation coefficient (R2) for structural parameters, along with the R2 parameter for vibration frequencies, the percentage of mean percentage deviation (%MPD), mean absolute deviation (MAD), optimal scaling factor (λ), root mean square (RMS), and also %MPD and MAD calculations for electronic absorption wavelength parameters, were performed to uncover the structure-property correlation between the findings of the DFT methods used and the experimental data. According to the R2 parameter obtained from DFT methods, the order for bond lengths in the synthesized Cu(II) complex was found as HCTHTD-B3LYP>TD-CAM–B3LYP>TD-HCTH>TD-TPSSTPSS and TD-M06L>TD-TPSSTPSS>TD-B3LYP>TD-CAM–B3LYP>TD-HCTH. These results indicate that the λabs performance of TD-HCTH is inferior compared to other functionals, with the best performance being achieved for TD-M06L. In conclusion, the static/dynamic isotropic and anisotropic values in the gas phase were found to be highest with the HCTH method and lowest with the CAM-B3LYP method. Similarly, the highest static/dynamic first and second-order polarizability values in the gas phase were obtained with HCTH, and the lowest with CAM-B3LYP. These results have turned out to be significantly larger than the prototype pNA and urea values used. In summary, based on these findings, it is considered that the Cu(II) complex has potential application possibilities in optoelectronic technology depending on its 2nd and 3rd order nonlinear optical (NLO) parameters.