Experimental and theoretical investigations of an organic nonlinear optical material p-toluidinium picrate – A comparative study

Abstract

The structure analysis of p-toluidinium picrate (PTP) crystal was performed by X-ray diffraction method and the spectral characterization was performed by FT-Raman spectroscopic technique. The optimization of PTP structure was carried out by using the four common functionals (B3LYP-D3BJ, B3LYP, CAM-B3LYP, and M05-2X). The obtained structural parameters were compared with the crystallographic data. The B3LYP-D3BJ functional was chosen for further analysis, since it allowed reliable reproduction of experimental bond lengths and angles (correlation coefficients > 0.98). The vibrational assignment of the fundamental modes was proposed on the basis of potential energy distribution (PED) calculations. The stability of the molecule due to hyper-conjugative interaction and charge delocalization was studied by natural bond orbital (NBO) analysis. The ultraviolet–visible (UV–Vis) spectrum of PTP was recorded in the region 200–600 nm and the electronic properties such as highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were measured by time-dependent density functional theory (TD-DFT) approach. The small energy gap between the HOMO and LUMO is an indicator of intramolecular charge transfer which is responsible for nonlinear optical (NLO) properties. The first order hyperpolarizability result confirmed the nonlinear optical activity of the molecule. Molecular electrostatic potential (MEP) analysis was performed to predict the reactive sites of the molecule. The theoretical results showed good agreement with the experimental values. The Hirshfeld surface analysis was made to find out the various intermolecular non-covalent interactions in the PTP molecule.