Effect of the Counterion on Light Emission: A Displacement Strategy to Change the Emission Behaviour from Aggregation-Caused Quenching to Aggregation-Induced Emission and to Construct Sensitive Fluorescent Sensors for Hg2+ Detection

In this paper, a simple strategy to change the emission behaviour of luminogenic materials was developed. Tetraphenylethene (TPE)-functionalised benzothiazolium salts with different counteranions (TPEBeX; X=I-, ClO4- and PF6-) were designed and synthesised. All the luminogens show weak red emission in the solution state that originates from intramolecular charge transfer from TPE to the benzothiazolium unit. Whereas aggregate formation enhances the light emission of TPEBeClO4 and TPEBePF6, that of TPEBeI is quenched, thus demonstrating the phenomena of aggregation-induced emission and aggregation-caused quenching. TPEBeI works as a light-up fluorescent sensor for Hg2+ in aqueous solution with high sensitivity and specificity owing to the elimination of the emission quenching effect of the iodide ion by the formation of HgI2 as well as the induction in aggregate formation by the complexation of Hg2+ with the S atom of the benzothiazolium unit of TPEBeI. A solid film of TPEBeI was prepared that can monitor the level of Hg2+ in aqueous solution with a detection limit of 1 mu M.