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^-, 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 TPEBe-ClO₄ and TPEBe-PF₆, 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 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 TPEBe-I. A solid film of TPEBe-I was prepared that can monitor the level of Hg²⁺ in aqueous solution with a detection limit of 1 μM. From darkness to light! A series of tetraphenylethene-functionalised benzothiazolium salts with different counteranions (TPEBe-X) was developed. By varying the counteranion from I^- to ClO₄^- or PF ₆^-, TPEBe-X changes its emission behaviour from aggregation-caused quenching to aggregationinduced emission (see figure). TPEBE-I shows a light-up response to Hg²⁺.