Pyrylazo Dye: A\nNovel Azo Dye Structure with Photoinduced\nProton Release and Highlighted Photophysical Properties in Biological\nMedia

A straightforward\nmethod for synthesizing a stable, photoreactive,\nand fluorescent-probe azo dye molecule is presented, highlighting\nthe influence of azo and pyrylium groups within the electronic structure\nof the novel dye. This compound, named the pyrylazo molecule, is synthesized\nthrough the chemical reaction between 2,4,6-trimethylpyrylium and\na 4-methoxybenzenediazonium species. The methyl group at the para\nposition of the pyrylium readily reacts with the diazonium molecule,\nforming a stable protonated pyrylium-azo dye (N-protonated pyrylazo).\nThe pyrylazo structure can easily change into its N-deprotonated form\nupon introduction of a weak base, such as an amine, promoting significant\nspectral shifts in the visible absorption and fluorescence bands.\nBecause of that and other photochemical properties, this novel dye\nhas shown significant potential for applications in photoinduced processes\nand biological contexts, particularly in Coulombic interactions with\nmicelles and animal cells. In contrast to other nonfluorescent azo\ndyes, the singlet excited state of pyrylazo is deactivated through\na radiative process in organized media, as evidenced by its behavior\nduring micelle media, cell membrane permeation, and fluorescence emission\nin the cytoplasm. Nanosecond-transient absorption spectroscopy reveals\na reversible photoinduced proton release process occurring in the\nexcited singlet state, suggesting that the excited states of pyrylazo\nmay play roles in transport through ion channels, artificial photosynthesis,\nand photoinduced protein folding. These promising applications underscore\nthe pyrylium-azo structure as a novel dye with remarkable photochemical\nand photophysical properties not observed in other azo dye molecules\nreported before.