Study on pH Fluorescent Probe Based on Exocyclic Ring-Fused Boron Dipyrromethene Fluorophore

Compound d was prepared by the improved three-step synthetic route using cyclohexene as starting material. After treatment of d with excess LiAlH4 at high temperature, the obtained e was reacted with various kinds of aromatic aldehydes including 4-N, N-dimethylaminobenzaldehyde, 4-hydroxybenzaldehyde, 4-methoxybenzaldehyde, benzaldehyde and 4-formyl-benzo-15-crown-5, respectively, in the presence of CF3COOH and oxidated with DDQ, followed by treatment with NEt3 and BF3.Et2O to provide five kinds of BODIPY derivatives fused with exocyclic rings. Their absorption and fluorescence properties were investigated. The fluorometric titrations were performed to examine their responsive abilities to pH in CH3OH-H2O (1:1, V/V). The results indicate that BODIPY 1 and 2 can be applied as a fluorescent probe for pH sensing in the acid and alkaline region, respectively, and switching between the fluorescence “ON/OFF” states in BODIPY 1 or 2 can be repeated by modulation of protonation/deprotonation process without decomposition and formation of precipitates. Moreover, the design principle and sensing mechanism of two probes are also preliminarily discussed.