In Situ Synthesis of Phenoxazine Dyes in Water: A Cutting-Edge Strategy to "Turn-On" Fluorogenic and Chromogenic Detection of Nitric Oxide

The synthesis of phenoxazine dyes was revisited in order to access these fluorescent N,O-heterocycles under mild conditions. The combined sequential use of nitrosonium tetrafluoroborate (NOBF4) and triphenylphosphine enables the facile conversion of bis(3-dimethylaminophenyl) ether into the methyl analog of popular laser dye oxazine 1. The ability of nitrosonium cation (NO+) to initiate the domino reaction resulting in pi-conjugated phenoxazine molecules under neutral conditions, then led us to explore the feasibility of expanding it in aqueous media. Thus, we explored the use of reactive signaling molecule nitric oxide (NO) as a biological trigger of phenoxazine synthesis in water. The implementation of a robust analytical methodology based on fluorescence assays and HPLC-fluorescence/-MS analyses, have enabled us to demonstrate the viability of this novel fluorogenic reaction-based process to selectively yield an intense "OFF-ON" response in the near-infrared (NIR-I) spectral region. This study is an important step towards the popularization of the concept of "covalent-assembly" in the fields of optical sensing, bioimaging and molecular theranostics.