Two novel two-photon excited fluorescent pH probes based on the A-π-D-π-A system for intracellular pH mapping

Dynamic monitoring the pH fluctuations is critical for many industrial and biological processes and hence many researchers devoted themselves to develop novel sensors for monitoring pH variations. Herein, two novel two-photon excited fluorescent (2PEF) pH probes abbreviated as L1 and L2, based on the A-π-D-π-A system were facilely synthesized by one-step solid-state reaction at room temperature. Probes L1 and L2 exhibited linear response over the weakly acidic pH range (4.33-6.73 for L1, 2.99-5.28 for L2) and remarkable emission ratio enhancement with the pKa of 5.12 and 3.85, respectively. Interestingly, the position of the nitrogen atom had influence on the pH-dependent single-photon excited fluorescence, two-photon absorption (2 PA) across-section and linear region of pH response in probes L1 and L2. Furthermore, the probes exhibited favorable two-photon photophysics, high sensitivity, low cytotoxicity, and capability of mapping intracellular pH distribution. Therefore, probes L1 and L2 could act as practical tool for the sensitively detection of pH in weakly acidic conditions and tracking pH difference in living cells via two-photon fluorescence imaging.