Tuning Aza-BODIPY Scaffold: Development of pH-sensitive and Hydrophilic Derivatives for Potential Bioimaging and Chemo sensor Applications

In this study, the synthesis, characterization, optical properties, and TD-DFT studies of 20 near-infrared (NIR) fluorophores, as well as the pH sensing capabilities of four compounds, are presented. Aza-BODIPY fluorophores have a central boron atom complexed with two fluorine atoms, comprising a donor-acceptor-donor (D-A-D) structure. The structure of the fluorophores was characterized by ¹H, ¹³C, and ¹⁹F NMR, as well as IR spectroscopy. The fluorophores exhibited absorbance and emission in the NIR region between 600 nm and 870 nm. The fluorophores with N,N-dimethylamino modification showed a significant red shift in absorbance spectra (beyond 800 nm), leading to the appearance of two absorbance maxima in all optical profiles (DMSO, Ethanol, 50 mM HEPE, and 1X PBS). Moreover, density-functional theory (DFT) studies were used to understand the energy levels of the frontier HOMO/LUMO orbitals. When evaluated during in vivo biodistribution studies, the fluorophores were analyzed for potential bio-targetability, but yielded inconclusive results, as they were highly hydrophobic and accumulated in highly dense adipose-containing organs (kidneys, pancreas, and adrenal glands) rather than achieving a longer residence time in the aqueous systemic circulation. However, the pH profile for four Aza-BODIPY fluorophores was established to demonstrate their pH response under acidic and alkaline conditions, establishing four new derivatives synthesized and characterized within the Aza-BODIPY scope, demonstrating a platform to leverage the pH sensitivity to be applied to future works and to improve the hydrophilicity of the Aza-BODIPY fluorophores.