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

Abstract

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 pKas 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 pKa,1 = 3.71 and pKa,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.