Experimental and Theoretical Investigations on Quadratic and Cubic Optical Nonlinearities of Cu(II), Co(II), Cd(II), Hg(II), Mn(II), and Zn(II) Transition Metal Coordination Complexes

A series of Cu(II), Co(II), Cd(II), Hg(II), Mn(II), and Zn(II) complexes (1-6) of (1-methyl-1H-imidazol-2-yl)(phenyl)methanone ligand (L) have been prepared and structurally characterized using single-crystal X-ray diffraction. A study of electrochemical properties has been conducted by cyclic voltammetry. Transition metal coordination complexes are known to have promising optical nonlinearity behavior. So, to explore the quadratic and cubic nonlinear optical (NLO) phenomena of title transition metal coordination structures, several experimental and theoretical investigations have been introduced, and their results have been evaluated. The second-harmonic generation (SHG) and third-harmonic generation (THG) techniques by means of the Maker fringe setup have been performed to analyze and evaluate the quadratic (χ(2)) and cubic (χ(3)) susceptibilities on thin films of 1-6 at 1064 nm. The maximum one-photon absorption (OPA) wavelengths (λmax) of 1-6 have been measured utilizing UV-vis spectral analysis. To acquire the electric dipole moment (μ), static dipole polarizability (α), and first hyperpolarizability (β) values of 1-6, we have taken advantage of the density functional theory (DFT) at the B3LYP level. The time-dependent Hartree-Fock (TDHF) that is known as a very productive quantum mechanical procedure has been chosen to achieve the static second hyperpolarizabilities (γ) and dynamic α, β, γ, χ(2), and χ(3) for 1-6. The measured data on the χ(2), χ(3), and (λmax) results for 1-6 have been confronted with their corresponding calculated values from the TDHF approach and the configuration interaction (CI) method including all doubly occupied molecular orbitals (MOs). Our calculation results of microscopic dipole polarizabilities and hyperpolarizabilities for 1-6 have also been crosschecked with similar structures given in the previous works and outcomes of several reference samples. The second- and third-order NLO efficiencies produced theoretically and experimentally of the title materials have been outlined, revealing that the complexes with closed-shell electronic states have turned out increments of second- and third-order susceptibility responses. This work has also elucidated that the substitutions of metallic cations (Cu2+, Co2+, Cd2+, Hg2+, Mn2+, and Zn2+) have created favorable impacts upon χ(2) and χ(3) conclusions. Furthermore, we have surveyed the first and second frontier MOs and their energy gap values via DFT.