pH is a fundamental physiological parameter related to myriad applications, therefore great efforts are continuously made to develop high performance pH indicators. Here, we exhaustively investigated the pH response of an acridine analog, 9-acridinecarboxylic acid with steady state absorption and fluorescence spectroscopies, picosecond resolved fluorescence spectroscopy, and femtosecond resolved transient absorption spectroscopy. The fluorescence intensity ratio (R_450/525) of 9-acridinecarboxylic acid showed a 7-fold change in the physiological pH range (pH ∼ 5–8), and it can be served as an excellent ratiometric and lifetime pH indicator. Analysis of the picosecond and femtosecond-resolved spectra combined with theoretical calculation revealed that 9-acridinecarboxylic acid existed in zwitterionic form at acidic condition with emission centered at 480 nm and lifetime of about 2.1 ns and anionic form at alkaline condition with emission centered at 430 nm and lifetime of about 6.6 ns. As pH decreased, the protonation of heterocyclic N atom resulted in the redshift of fluorescence peak and shortening of the fluorescence lifetime. Moreover, 9-acridinecarboxylic acid exhibited good selectivity, photostability and excellent biocompatibility and was successfully applied for intracellular pH imaging by ratiometric fluorescence.

pH 值是与众多应用相关的基本生理参数，因此人们不断努力开发高性能的 pH 指示剂。在这里，我们详尽地研究了吖啶类似物、9-吖啶羧酸与稳态吸收和荧光光谱、皮秒分辨荧光光谱和飞秒分辨瞬态吸收光谱的 pH 响应。荧光强度比（R _450/525) 的 9-吖啶羧酸在生理 pH 值范围内 (pH ∼ 5–8) 显示出 7 倍的变化，它可以作为一种出色的比率和寿命 pH 指示剂。对皮秒和飞秒分辨光谱的分析结合理论计算表明，9-吖啶羧酸在酸性条件下以两性离子形式存在，发射中心在 480 nm，寿命约为 2.1 ns，在碱性条件下以阴离子形式存在，发射中心在 430 nm寿命约为 6.6 ns。随着pH值的降低，杂环N原子的质子化导致荧光峰红移，荧光寿命缩短。此外，9-吖啶羧酸表现出良好的选择性，