A series of water-soluble A–π–A′ typological indolium derivatives with two-photon properties for rapidly detecting HSO3−/SO32− in living cells.

It is believed that HSO₃⁻ and SO₃²⁻ play important roles in several physiological processes. However, probes with two-photon absorption to detect HSO₃⁻ or SO₃²⁻ in living cells are still limited. Herein, a series of novel indolium derivatives (L1–L4) with an A–π–A′ structure was designed and synthesized as ratiometric probes to detect HSO₃⁻/SO₃²⁻in vitro. L3 and L4 display a colorimetric and ratiometric fluorescence dual response to HSO₃⁻/SO₃²⁻ with a very fast (∼15 s) and high specificity, as well as low detection limits (∼22 nM). Furthermore, their detection is also carried out by using a two-photon excited fluorescence method. A nucleophilic addition reaction is proposed for the sensing mechanism, which is supported by MS, ¹H NMR, and density functional theory (DFT) investigations. Importantly, L3 was successfully used for detecting intrinsically generated intracellular HSO₃⁻/SO₃²⁻ in cancerous cells under one- and two-photon excited fluorescence imaging. [ABSTRACT FROM AUTHOR]