Near-infrared pH probes based on phenoxazinium connecting with nitrophenyl and pyridinyl groups.

Three phenoxazinium compounds attached with o -nitrophenyl ( 3a ), m -nitrophenyl ( 3b ) and p -nitrophenyl ( 3c ) groups were prepared, and the pH promoted emission spectra were used to evaluate the deprotonated-protonated equilibrium between phenoxazinium and phenoxazine. They showed nearly ON-OFF emission responses at 650–850 nm around pH 8.0–10.8, and the fluorescence related p K a s of 3a–c were 8.7, 9.2, and 8.9, respectively. Two phenoxazinium compounds attached with pyridinyl groups ( 7a–b ) were further prepared, the pH promoted emission spectra were influenced by both the deprotonated-protonated equilibrium between phenoxazinium and phenoxazine, and the equilibrium between pyridinyl group and pyridinumyl group. They exhibited more strong red and near-infrared emission at 600–850 nm, probe 7a gave p K a ,1 = 3.71 and p K a ,2 = 9.68, and that of probe 7b was 2.75. Moreover, the fluorescent imaging experiment indicated that probe 7a was a lysosome biomarker for V79 and HeLa cells. [ABSTRACT FROM AUTHOR]