3-pyridylazoindolium ion as water-soluble photoswitches with acid-alkali discoloration properties

Abstract

Three new water-soluble photoswitches (PH, PM, and PB) were synthesized via assembling N-methylpyridinium to 3-pyridylazoindole scaffold which we previously researched. The n-π* absorption of the 3-pyridylazoindolium exhibited a significant red-shift compared to their unmodified state, which meant the N-methyl photoswitches might achieve photoisomerization upon the irradiation of visible light. In both organic and aqueous solvents, the photoswitches PB exhibited excellent reversible photoswitching and regular thermal stability. In addition, the n-π* transition of compound PB red shifted 20 nm compared to its unmodified state (3). Under simulated biological conditions, the presence of glutathione (GSH) not only almost without impact on the photoisomerism of PB in aqueous solution, but also did not affect the optical fatigue resistance. Besides, because of the N-methylpyridinium of the scaffold compound PB, the adhesion of molecule and hydroxide ions was significantly enhanced and achieved discoloration towards acid-alkali. These results indicated that modification of N-methyl not only is an excellent strategy for enhancing the application potential of 3-pyridylazoindole photoswitches in aqueous phase, but also achieving detection of alkaline strength by 3-pyridylazoindole scaffold in aqueous solution.