Synthesis, spectral characterization, Z-scan and NLO detection sensitivity and applications of various DFT calculation methods on a new Zn(II) complex

Abstract

The synthesized Zn(II) complex of 3−/6-methylpicolinic acid (3−/6-MepicH) was structurally characterized by mass spectrometry (MS) as well as ¹H and ¹³C NMR spectroscopy. The spectral features were investigated by the FTIR and UV–Vis spectra. To elucidate third−order nonlinear optical (NLO) characteristic, Z-scan study was fulfilled. Theoretical characterizations were carried out using different featured DFT methods. Moreover, the values of external electric field (E), polarization (P), and electric displacement (D) for the Zn(II) complex were computed employing DFT/M06-L, CAM-B3LYP, and HSEh1PBE functionals. Similarly, the linear optical (LO) susceptibility and polarization parameters χ⁽¹⁾/P⁽¹⁾ as well as the second- and third-order nonlinear optical (NLO) tensors χ⁽²⁾/P⁽²⁾ and χ⁽³⁾/P⁽³⁾ were derived at the same computational levels. Furthermore, the refractive index (n) and optical band gap were determined within the UV–Vis spectrum. Subsequently, the Zn(II) complex's static and dynamic LO and NLO properties were examined at the DFT/M06-L, CAM-B3LYP, and HSEh1PBE levels. The electronic band gap (E_g), dipole moment and polarizability values were obtained at 4.37 eV (experimental) and 4.777 eV (DFT/M06-L), 8.009 D (DFT/M06-L), <α(0,0)> 43.134 × 10⁻²⁴ esu and <α(−ω;ω)> 38.954 × 10⁻²⁴ esu, respectively. From the Z-scan experiments, the complex exhibited a third-order nonlinear optical susceptibility (χ⁽³⁾) of 62.5068 × 10⁻⁴ and a second-order molecular hyperpolarizability (γ) of −6532.41 × 10⁻²⁸ esu. Using the DFT/M06-L method, the values of <γ(0,0,0,0)>/<γ(−ω;ω,0,0)> and <γ(−2ω;ω,ω,0) > third-order nonlinear optical susceptibilities of the complex in ethanol were determined as 70.427 × 10⁻³⁶, 94.085 × 10⁻³⁶ and 2599.27 × 10⁻³⁶ esu, respectively. The significant nonlinear optical responses from Z-scan measurements and DFT-based calculations reveal that the compound exhibits promising characteristics for advanced optical sensing applications, including intensity-dependent detectors and environmentally responsive phase-modulating sensors.