Exploration of charge transfer interaction, terahertz analysis, Z-scan and nonlinear optical properties of morpholinium 3,5-dinitrosalicylate (M35DNS): a spectroscopic and computational approach

The structural, vibrational, and NLO properties of a morpholinium 3,5-dinitrosalicylate (M35DNS) crystal determined using density functional theory (DFT) to elucidate the charge transfer interaction and the influence of ionic hydrogen bonds are reported. Quantum chemical calculations are used to probe the electronic and optical properties of the given crystal. The elongation of experimentally measured C₁₃O₄ and N₂₁–H₂₃ bonds is observed, which clearly explains the involvement of amino and carbonyl groups in the formation of strong N–H⋯O hydrogen bonding in the crystal. The observed simultaneous and intense activity of 8b, 14, and 18a modes in Raman and IR substantiates the charge transfer interaction in the M35DNS crystal. The low-frequency vibrational modes of the M35DNS crystal are examined using terahertz time-domain spectroscopy (THz-TDS) and terahertz-Raman spectroscopy (THz-RS) studies. Furthermore, the energy gap of 3.82 eV indicates the electron transfer from the occupied orbitals to the unoccupied orbitals in the salicylate region. Natural bond orbital (NBO) analysis predicts that the interaction between the LP (1) (C₁₄) → LP* (1) (C₁₃) shows a strong stabilization energy of 406.29 kJ mol⁻¹, substantiating the intramolecular charge transfer in the molecule. The lowest energy gap substantiates the charge transfer in the M35DNS crystal, and the chemical softness is computed to be 0.1310 eV. The dipole moment, polarizability and first order hyperpolarizability of M35DNS are estimated as 8.76 D, 2.91 × 10⁻²³ electrostatic units (e.s.u.) and 8.9 × 10−30 e.s.u., respectively. The second harmonic generation (SHG) signal of the M35DNS crystal is measured by the powder method, which is 0.02 times the urea standard. The two-photon absorption coefficient was estimated to be 0.66 × 10⁻¹¹ m W⁻¹, mainly due to the D–π⋯A molecular structure and the optical limiting threshold estimated to be 2.58 × 10¹³ W m⁻², enabling this material to be a potential candidate for optical limiting applications. The observed NLO properties of the M35DNS crystal can be useful for photonic, optoelectronic devices and NLO applications.