Analyte-Triggered Excited-State Intramolecular Proton Transfer- Delayed Fluorescence: A General Approach for Time-Resolved Turn-On Fluorescence Imaging

Abstract

The research of delayed fluorescence (DF) has been a hot topic in biological imaging. However, the development of analyte-triggered small molecule DF probes remains a considerable challenge. Herein a novel excited-state intramolecular proton transfer-delayed fluorescence (ESIPT-DF) approach to construct analyte-stimulated DF probes was reported. These new classes of ESIPT-DF luminophores were strategically designed and synthesized by incorporating 2-(2'-hydroxyphenyl)benzothiazole (<b>HBT</b>), a known ESIPT-based fluorophore, as acceptor with a series of classic donor moieties, which formed a correspondingly twisted donor-acceptor pair within each molecule. Thereinto, <b>HBT-PXZ</b> and <b>HBT-PTZ</b> exhibited significant ESIPT and DF characters with lifetimes of 5.37 and 3.65 μs in the solid state, respectively. Furthermore, a caged probe <b>HBT-PXZ-Ga</b> was developed by introducing a hydrophilic d-galactose group as the recognition unit specific for β-galactosidase (β-gal) and ESIPT-DF blocking agent and applied to investigate the influence of metal ions on β-gal activity on the surface of <i>Streptococcus pneumoniae</i> as a convenient tool. This ESIPT-DF "turn-on" approach is easily adaptable for the measurement of many different analytes using only a predictable modification on the caged group without modification of the core structure.