Substituent Effect on the Properties of pH Fluorescence Probes Containing Pyridine Group

Abstract A series of 4–styrylpyridine (SYP) pH fluorescence probes with different substituents (–NH 2 , –OH, –H, –Br, –NO 2 ) on para–benzene ring were developed. These compounds exhibited good pH–dependent performance that their spectra red–shifted upon protonation of pyridine N–atom and blue–shifted upon deprotonation. These sensors utilized the substituent on the para–benzene ring as electron donor and worked by intramolecular charge transfer (ICT) under acidic pH conditions. The pH detection range and pKa value of them were tuned by the electron–donor ability of substituents. The p K a had a significant linear correlation with the Hammett constant (σ p ) of substituent, the p K a was larger when the Hammett constant of substituent was larger.