Influence of halogen substitution on crystal packing, molecular properties and electrochemical sensing

Abstract

Halogen atoms are commonly present in organic molecules and halogen functionalization is of great significance in organic synthesis, to achieve desired material and biological functions. In crystal engineering, the halogen bond is an intriguing interaction utilized to design molecular solids. Herein we report the development of halogen functional red tri-halo Schiff bases/Anils 1–4 in which the nature of halogen atoms is rationally changed, to explore the impact of halogen atoms on molecular packing and physicochemical properties. The trichloro (1), tribromo (2) and triiodo (3) Anils crystallize in the same centrosymmetric triclinic crystal systems and the 2 and 3 are isomorphous. The molecular solids exhibit similar 2-dimensional packing induced by halogen atoms, however, the molecules differ significantly in the third-dimensional packing propagated by π-stackings. It is observed that π-π interactions are stronger in triiodo Anil. To further understand the effect of the nature of halogen on molecular packing a mixed dichloro-iodo Anil (4) was synthesized. The introduction of different halogens on the Anil skeleton greatly alters the packing behavior and the molecules pack in monoclinic P21/n space group. 4 undergoes halogen bonded interactions to arrange as zig-zag solid, further stabilized by π-stackings. The Hirshfeld studies validate the formation of a significant sigma hole on a para-iodine atom in 3 (0.0161 a.u), while the tip of p-chlorine in 1 is not electron-deficient (−0.0128 a.u.), which also reflects in more contribution of halogen-halogen interactions in 3 than 1 and 2, in their fingerprint plots. Theoretical and experimental studies indicate lower band gap values in triiodo form 3 than others. The framework analyses of the molecular solids indicate nature of intermolecular contacts is predominantly dispersive. The physicochemical property investigation of the Anils validates relatively better chemical and thermal stability and optical properties of the trihalo form. Solubility studies of the molecular solids 1–4 validate their hydrophobic nature and the iodo functional groups enhance the hydrophobicity of the molecules, while the solubility investigations of these solids in DCM and toluene indicate higher solubilities with relatively lower values for iodo-functionalized molecules. The molecular solids have been employed for electrochemical sensing of picric acid with the limit of the detection value of 0.72, 0.84, 0.74 and0.73 ​μM for 1–4, respectively.