Supramolecular architecture and SHG activity of organic crystals formed between the amidinothiourea and nicotinic acid

Abstract

The single crystals of [1-(diaminomethylene)thiouron-1-ium] pyridine-3-carboxylate were grown using a solution growth technique. Depending on the concentration of the 1: 1 amidinothiourea: nicotinic acid solution, two different forms of 1: 1 crystal (orthorhombic non-centrosymmetric, P212121,(1), and monoclinic centrosymmetric, C2/c, (2)) were obtained. These phases differ in ordering, the orthorhombic phase is ordered, and in the monoclinic phase the 1-(diamino-methylene)-thiouron-1-ium cation is disordered and has two non-equivalent orientations with occupations of around 0.80 and 0.20. This ordering/disordering is due to the rate of crystallization: slow crystallization from a dilute solution leads to an ordered orthorhombic phase and quick crystallization from a saturated solution leads to a disordered monoclinic phase. Multiple recrystallizations of 1 and 2 from heavy water gives deuterated analogs 1d and 2d The conformation of the 1-(diaminomethylene)thiouron-1-ium cation in both phases is very similar and slightly non-planar. In the orthorhombic phase the plane of the carboxylate group is more twisted in relation to the plane of the pyridine ring than in the monoclinic phase. The oppositely charged components of the crystals, i.e., 1-(diaminomethylene)thiouron-1-ium cations and pyridine-3-carboxylate anions interact each other forming three-dimensional supramolecular architecture. Both compounds were also characterized by IR and Raman spectroscopy. Assignment of the vibrational bands were supported by the DFT calculations and the isotropic frequency shift. The SHG efficiency of the powdered orthorhombic phase relative to potassium dihydrogen phosphate (KDP) was found to be 1.70.