2,6-Diphenylpyridine-based fluorophores: Synthesis, photophysical properties and effects of protonation

A new series of 2,6-diphenylpyridine-based fluorophores with different type of substituents (R = Br, H, OMe, NMe2 and NPh2 for 2–6) were prepared, and their photophysical properties in neutral and protonated state were investigated. With the increasing of the donor ability of substituents in 2–6, their UV/vis and emission spectra show a successive red shift. In addition, a large intramolecular charge transfer (ICT) was observed in 5 and 6, which can be confirmed by time-dependent density functional theory (TD-DFT) calculation, and X-ray crystallographic analysis for 5. All compounds studied in this work show proton-sensitive fluorescence. It was found that 2 and 3 were capable of ratiometric emission signaling of protons, 4 and 6 showed proton-induced quenching in their fluorescence, while 5 revealed a proton-induced ON–OFF–ON switching of its emission.