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 (TPEBe-X; X=I(-), ClO(4)(-) and PF(6)(-)) 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 TPEBe-ClO(4) and TPEBe-PF(6), that of TPEBe-I is quenched, thus demonstrating the phenomena of aggregation-induced emission and aggregation-caused quenching. TPEBe-I works as a light-up fluorescent sensor for Hg(2+) 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 HgI(2) as well as the induction in aggregate formation by the complexation of Hg(2+) with the S atom of the benzothiazolium unit of TPEBe-I. A solid film of TPEBe-I was prepared that can monitor the level of Hg(2+) in aqueous solution with a detection limit of 1 μM.