Journal of Optoelectronics and Biomedical Materials Vol. 10, No. 3, July - September 2018 p. 73 - 82 HIRSHFELD SURFACE ANALYSIS, TOPOLOGICAL FEATURES AND NONLINEAR OPTICAL PROPERTIES OF PHTHALONITRILE DERIVATIVE: LOW TEMPERATURE EXPERIMENTAL CHARGE DENSITY AND QUANTUM CHEMISTRY STUDIES H. BENAISSI a , M. DRISSI a , S. YAHIAOUI a , Y. MEGROUSS a , A. CHOUAIH a* , F. HAMZAOUI b a Laboratory of Technology and Solid Properties, University Abdelhamid Ibn Badis of Mostaganem, 27000 Mostaganem, Algeria b LPFM Académie de Montpellier, France The Hansen-Coppens multipolar formalism was used to investigate the experimental electron density (ED) distribution of phthalonitrile derivative from single crystal X-ray data. The crystal displays C–H⋅⋅⋅O and O–H⋅⋅⋅N intra- and intermolecular interactions. The ED topological investigation of the explored molecule was carried out, from that the electron density   () and its Laplacian ∇ 2   () at the bond critical points (b.c.p.) have been calculated. The molecular dipole moment and the electrostatic potential were calculated and compared with those obtained using theoretical calculations. The NLO behavior of the title compound was examined by computing the molecular polarizabilities α and β. (Received May 6, 2018, Accepted August 6, 2018) Keywords: Electron density, Dipole moment, Electrostatic potential, Hirshfeld Surface, Hyperpolarizabilty. 1. Introduction As organic materials for electronics and optics, molecular π-conjugated nonlinear optical (NLO) compounds are attractive materials in crystal engineering beside those of inorganic NLO materials [1,2]. These compounds have several applications such as semiconductors, nonlinear optic and photovoltaic materials [3-6]. NLO organic molecules, in general, have an electron- donating unit (e.g. Nitro or Cyano groups), an electron-withdrawing unit (e.g. NR2 or OR groups) and a transmitter group with double or triple chemical bonds (e.g. π-conjugated aromatic rings) [7]. In addition, incorporating heteroatoms (N, O, S etc.) into the structure of NLO molecules leads to better stability, chemical robustness and increase charge transfer. From this perspective, as important class of organic molecules, phthalonitrile derivatives have various applications in photovoltaic, NLO and other electronic and biological fields [8-12]. As described previously, molecular compounds with NLO properties are constituted by an electron donor (D) and an electron acceptor (A) groups in the two extremities with a conjugated π- electrons system between them. Then, the electronic charge is withdrawn from the donor to the acceptor group by means of the conjugated system. Therefore, a molecular dipole moment occurs from the polarization of the conjugated system giving rise to a charge transfer within the molecule. These kind of compounds are called push–pull materials [13,14]. In the last two decades, several studies on experimental charge density in aromatic push-pull molecules have been reported for their intramolecular charge transfer [15-19]. For this purpose, we have chosen 4-[(E)-(2- hydroxyphenyl) iminomethyl] phenoxy} benzene-1,2-dicarbonitrile, which appears a good candidate for nonlinear optical applications. Besides investigating charge transfer and low temperature experimental charge density in this molecule, we have examined other molecular properties such as topological properties, the dipole moment and the electrostatic potential of the * Corresponding author: aek_chouaih@yahoo.fr