Quantum chemistry study of the second harmonic generation χ(2) tensor in chloranilic acid-based crystals.

Yathreb Naghmouchi,1,2 François Mairesse,1 Wajda Smirani,2 and Benoît Champagne1

1  Laboratory of Theoretical Chemistry, University of Namur
2 Laboratory of Materials Chemistry, Faculty of Sciences of Bizerte

E-mail : Yathreb.naghmouchi@unamur.be   

This research concentrates on the study of a series of charge-transfer crystals between chloranilic acid (C6H2O4Cl2) and organic bases (amines and pyridine). The target property of this work is the second harmonic generation (SHG) second-order nonlinear optical (NLO) response described by the χ(2) tensor and the main objective is to investigate the relationships between the NLO responses of the crystals, the atomic charge distributions, and the supramolecular structural features of these systems.   

All computations were carried out at the density functional theory (DFT) level with periodic boundary conditions (PBC), employing the CRYSTAL17 [1]  suit of software. The work steps were the following. I) A full geometry optimization (atomic fractional coordinates as well as cell parameters) of the systems structures (starting from the single crystal X-ray diffraction structures), employing the ωB97X exchange-correlation functional (XCF). II) The calculation of the χ(1) and χ(2) tensors at the Coupled Perturbed Hartree–Fock/Kohn–Sham level (CPHF/KS) with the ωB97X (XC) functional. For dynamic fields, different damping factors were employed in order to check and screen the electronic transitions. III) Topological analysis of the wavefunctions were performed in the framework of Bader’s quantum theory of atoms in molecules (QTAIM) [2] using the TOPOND14 [3] software:  Bader charge analyses were carried out, as they are a valuable tool to evaluate the charge transfer between the closely packed acid and base molecules. The topology of the electron density was also scrutinized in order to understand the nature of the multiple intermolecular interactions as well as to estimate their strength and directionality. 


[1] Dovesi, R.,  Saunders, V. R., Roetti, C., Orlando, R., Zicovich-Wilson, C. M., Pascale, F., Civalleri, B., Doll, K., Harrison, N. M., Bush, I. J., D’Arco, P., Llunell, M., Causà, M., Noël. Y, Maschio, L., Erba, A., Rerat, M., Casassa, S., CRYSTAL17 User's Manual (University of Torino, Torino, 2017). 
[2] Bader, R. F. W., Atoms in Molecules: A Quantum Theory (Clarendon Press, Oxford, 1994).

[3] Gatti, C.,  Casassa, S., TOPOND14 User’s Manual (CNR-ISTM of Milano, September 12, 2017).