Fluorescent probes are recognised tools in the various fields, including in studies of biochemical processes, diagnosis of disease, and detection of hazardous compounds, among others. For biological and medicinal applications, fluorescent near-infrared (NIR) probes are more adequate, due to the reduced interference with the self-fluorescence of the biological material, in addition to less harming effects induced by light at wavelengths longer than 600 nm.

With the aim of contributing to the development of NIR probes with applications in biomedicine, our research group is committed to the development of new water-soluble benzo[a]phenoxazine derivatives and the evaluation of their photophysical and biological potential. Herein we report the photophysical behaviour in anhydrous ethanol of four synthesised benzo[a]phenoxazinium chlorides, possessing the (3-chloropropyl)amino and/or (4-ethoxy-4-oxobutyl)amino groups at 5- and 9-positions of the polycyclic system.