Synthesis of fluorescent water-soluble functionalised benzo[a]phenoxazinium salts

In order to develop long-wavelength functionalised oxazine dyes with good water solubility and high photostability for biological applications, a series of novel side-chain functionalised benzo[a]phenoxazinium salts were synthesised and characterised. These polycyclic cationic compounds showed strong fluorescence in ethanol and water (pH 7.4), with an emission wavelengths higher than 637 nm, as well as high quantum yields and moderate Stokes’ shifts. The photostability of the fluorophores synthesised, under irradiation at 419 nm, was good to excellent in ethanol and moderate in water at physiological pH.