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-pi-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 (similar to 15 s) and high specificity, as well as low detection limits (similar to 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, H-1 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.