N-Alkylation of 2-methoxy-10H-phenothiazine revisited. A facile entry to diversely N-substituted phenothiazine-coumarin hybrid dyes

[Display omitted] •N-Alkylation of 2-methoxy-10H-phenothiazine was optimized.•Three novel NIR-emissive phenothiazine-coumarin hybrid dyes were synthesized.•Phenothiazine-coumarin hybrid dyes act as ratiometric fluorescent probes for HClO.•Fluorogenic hypochlorite-mediated oxidation was deciphered by RP-HPLC-fluorescence/MS. N-Alkylation of 2-methoxy-10H-phenothiazine, a valuable building block for the synthesis of bioactive compounds and reaction-based fluorescent probes, has been revisited aimed at introducing a substituent easily convertible into cationic or zwitterionic side chains. We focused our attention on the 3-dimethylaminopropyl group since its derivatization through reactions with various alkyl halides or sultones is a well-established and effective way to enhance polarity of diverse hydrophobic molecular scaffolds. This two-step functionalization approach was applied to the synthesis of novel phenothiazine-coumarin hybrid dyes whose spectral features, especially their NIR-I emission, have been determined in aqueous media with the ultimate goal of identifying novel fluorescent markers for bioanalytical applications, including fluorogenic detection of reactive oxygen species (ROS) through selective S-oxidation reaction of phenothiazine scaffold.