Endoplasmic Reticulum‐Targeting Delayed Fluorescent Probe for Dual‐mode Nitroreductase Sensing

Organic thermally activated delayed fluorescence (TADF) materials, known for their long‐lived emission properties, are highly sought after for background‐free imaging of selective analytes in time‐resolved modes. However, their practical application faces significant challenges, including the air sensitivity of triplet states, lack of organelle specificity, and the absence of precise analyte recognition centres. These limitations hinder their effectiveness in detecting key cancer biomarkers such as nitroreductase (NTR). Herein, we present the development of donor (triphenylamine)‐acceptor (quinoxaline)‐based probes, TPQS and TPNS, which are functionalized with a sulphonamide unit to offer endoplasmic reticulum specificity. TPQS exhibits delayed fluorescence, attributed to a minimal singlet‐triplet energy gap, as confirmed by time‐resolved fluorescence measurements. Additionally, a nonfluorescent probe, TPNS, is synthesized by introducing a nitro group into the sulphonamide unit of the TPQS backbone, serving as a recognition centre for NTR. Upon reacting with NTR, TPNS displays a “turn‐on” luminescence and delayed fluorescence, enabling dual‐mode detection of NTR through both confocal fluorescence imaging and time‐resolved fluorescence imaging (TRFI) in cancer cells. These findings underscore the potential of delayed fluorescent emitters for the sensitive and specific detection of cancer biomarkers in complex biological environments.