Two pH-responsive fluorescence probes based on indole derivatives

Two new pH fluorescent probes 2,8-bis((E)-2-(1,1-dimethyl-1H-benzo[e]indol-2-yl)vinyl)-6H,12H-5,11-methanodibenzo[b,f] [1,5]diazocine (1) and 2,8-bis((E)-2-(3,3-dimethyl-3H-indol-2-yl)vinyl)-6H,12H-5,11-methanodibenzo[b,f] [1,5]diazocine (2) were designed and synthesized via ethylene bridging of the Troger's Base (TB) framework and indole derivatives. As the protonation of nitrogen atoms of indole derivatives, both probes displayed the significant pH-dependent spectral properties. The probing behavior toward pH variations indicated that probes exhibited excellent pH dependent behavior in the pH ranges of 2.07-7.01 (1) and 1.96-7.01 (2), especially possessing good linear response in the pH ranges of 2.50-4.00. The emissions of 1 can be reversibly switched between brilliant yellow and dark states by protonation and deprotonation. Also, the yellow-green fluorescence emission of 2 could be quenched upon protonation. Such behaviors enable them to work as turn-off fluorescent pH sensors in the solution state. Furthermore, protonation and deprotonation on indole moiety nitrogen of the probe 1 and 2 provided a sensing mechanism for pH changes. In addition, both probes performed good photostability, high selectivity and excellent reversibility.