Synthesis, structural spectroscopic and nonlinear optical properties of new metallic complexes containing pyridine-2-carboxylic acid derivatives

This thesis focuses on the synthesis of transition metal complexes containing pyridine-2-carboxylic acid derivatives and detailed investigation of the structural, electronic and nonlinear optical (NLO) properties of the obtained complexes. The fact that all the obtained metal-organic coordination compounds were synthesized for the first time and the above-mentioned studies were carried out for the first time for these complexes reflects the originality of this thesis work. In particular, the limited number of studies in the literature on transition metal complexes containing both pyridine-2-carboxylic acid derivatives and secondary neutral ligands indicates that the present study is an important study that contributes to the search for new materials with effective NLO properties. The combination of electron donating and withdrawing groups in the ligand structures is aimed to strengthen intermolecular interactions and improve optoelectronic properties. In this context, the structural, electronic and optical properties of new complexes formed with Zn(II), Cu(II), Co(II) and Mn(II) transition metal ions were investigated in detail by experimental and theoretical methods. The analysis of the crystal structures was not limited to the determination of the atomic coordinations, but also considered the effects of these coordinations on the electronic transitions. The geometrical parameters obtained from X-ray diffraction data were compared with the theoretical results obtained by the density functional theory (DFT) method to test the efficiency of the DFT method on these complexes. The integrated use of both experimental and theoretical methodologies in this study will provide researchers with insights on important points such as density functional, basis set and spin state during the theoretical investigation of new complexes with similar structures. In this context, [Cu(6Fpca)2(H2O)] and [Zn(6Clpca)2(H2O)2] complexes have distorted octahedral; [Mn(6Brpca)2 (Dmdpy)].CH3OH and [Zn(4Clpca)(3OHpca)(H2O) 2] complexes exhibit distorted octahedral; [Co(4Clpca)2 (H2O)2] and [Cu(6Clpca) 2(H2O)2] complexes exhibit tetragonal pyramidal coordination geometry. FT-IR data played an important role in determining the coordination patterns of the carboxylate groups. O-H stretching vibrations were observed in the range of 3188-3449 cm-1, while the Δν values between the asymmetric and symmetric vibrations of COO- groups ranged between 318-473 cm-1, supporting the single-toothed bonding via the carboxylate atom in the pyridine ring. Moreover, the shift of the pyridine ring fading vibrations in the range of 986-1036 cm-1 confirms that the ligands have an effective electronic interaction with the metal centers and metal covalent bonding via the N atom in the pyridine ring. Considering UV-Vis studies, the ligand-metal charge transfer (LMCT) bands observed in the 300-400 nm range, especially in Cu(II) and Mn(II) complexes, play a critical role in understanding the photophysical and nonlinear optical characters of the complexes. For example, in the [Mn(6Brpca)2(Dmdpy)].CH3OH complex, intra-ligand π→π* and n→π* transitions were detected at 206, 225, 263 and 275.6 nm. Similarly, π→π* intra-ligand charge transitions at 263 nm and LMCT transitions at 369 nm were reported in the [Cu(6Fpca)2(H2O)] complex. In the study, quantum chemical calculations were also performed to confirm the experimental results and the electronic properties of the molecules were evaluated in detail by TD-DFT, boundary molecular orbital analysis and natural bond orbital (NBO) calculations. Through these analyses, properties such as charge distribution within the complex, electronic structure stability of the complexes and high-energy conjugative interactions within the complex were investigated in detail and the relationship between the NLO properties was explained. Among the HOMO-LUMO energy differences, the lowest value was determined to belong to the [Mn(6Brpca)2 (Dmdpy)]-CH3OH complex (2.425 eV), while the highest difference was calculated for the [Zn(6Clpca) 2(H2O)2] complex (4.640 eV). Significant correlations were obtained between these energy differences and NLO parameters. In the theoretical part of the thesis, quantum chemical calculations carried out by DFT and TD-DFT methods provided important data on the electronic structures and NLO potentials of the complexes. In particular, the static <β> value of the ncomplex [Mn(6Brpca)2(Dmdpy)].CH3OH (6Brpca: 6-bromopyridine-2-carboxylic acid, Dmdpy: 4,4-dimethyl-2,2-bipyridyl carboxylic acid) is 62. 498×10-30 esu, the frequency-dependent <β> value was calculated as 11.448×10-30 esu, and the <γ> value was calculated as 387.88×10-36 esu statically and 680.035×10-36 esu in the frequency-dependent case. These values exceed the classical NLO reference molecules and indicate that this complex can be used with high efficiency in second and third order NLO applications. The Cu(II) complexes gave remarkable responses in terms of both <β> and <γ> parameters, suggesting their potential for use in various NLO applications. In the [Cu(6Clpca)2(H2O)] (6Clpca: 6-chloropyridine-2-carboxylic acid) complex, the <β> parameter was determined as 10.40 ×10-30 esu and the <γ> parameter was determined as 4.0259×10-36 esu. Although Zn(II) and Co(II) complexes exhibit relatively low NLO parameters, the gamma <γ> value calculated at 532 nm in the [Co(4Clpca)2(H2O)2] complex is as high as 433.93×10-36 esu, indicating the second-order high polarizability effect of crystal symmetry and orbital arrangements. The static <β> value of the [Zn(4Clpca)(3OHpca)(H2O)2] (4Clpca: 4-chloropyridine-2-carboxylic acid, 3OHpca: 3-hydroxypyridine-2-carboxylic acid) complex was calculated as 4.5200×10-30 esu while the frequency dependent <β> value was calculated as 4.720×10-30. Similarly, static and frequency-dependent <γ> values were calculated as 39.106×10-36 esu and 2258.9×10-36, respectively. As a result of all these calculations, the beta value is higher than its frequency dependent value, while the <γ> value is higher in frequency dependent calculations. This shows us that while the beta value is low in centrally symmetric structures, the gamma value does not depend on the centrally symmetric structure. Moreover, the relationship between the stable crystal systems and the observed high NLO responses was closely related to the arrangement of ligand-metal coordinations. In particular, the high <β> and <γ> values obtained in [Mn(6Brpca)2(Dmdpy)]-CH3OH, [Zn(3OHpca)(4Clpca)2.H2O] and [Cu(6Fpca)2(H2O)] complexes indicate that such structures can be used as active components in NLO applications. This finding supports an approach that suggests simultaneous evaluation of the spin state of the transition metal and the ligand geometry in the logical ligand selection step in future studies. In conclusion, this thesis is an interdisciplinary study in which metal-organic coordination compounds containing pyridine-2-carboxylic acid derivatives and the transition metals Zn(II), Co(II), Mn(II) and Cu(II) have been investigated not only in terms of coordination chemistry but also in terms of functional materials development. These results prove that the complexes obtained can have advanced optical properties if key choices such as ligand selection, metal ion selection, coordination geometry and spin state of the metal ion are made carefully. These complexes are particularly promising for second and third order NLO applications. These findings are expected to contribute scientifically to the realization of advanced molecular designs in the future and the development of next generation NLO materials with high performance, especially in areas such as electronics, laser technologies, data storage systems and photonic switching.