Promoting effects of the hydroxymethyl group on the fluorescent signaling recognition of anions by thioureas

A series of novel fluorescent naphthylthioureas 1-4 with hydroxymethyl groups was designed and synthesized. Upon complexation with anions, 1-4 showed strong fluorescence enhancements in the order: 1>2>3≈4, which is consistent with the number of hydroxymethyl groups contained in their structures. Hydroxymethyl groups have an important influence on the compounds' trans-trans or trans-cis conformations, and their action to promote the fluorescence signaling recognition of the thioureas for anions might be caused by their preorganizing the intramolecular protons of the receptor in favor of sites of the trans-trans conformation ready for hydrogen bond formation with the anions. Thioureas 1 to 4 had favorable selectivities for certain anions, which relied on the net charge and Brφnsted basicity of the anions.