Structure controlled solvatochromism and halochromic fluorescence switching of 2,2′-bipyridine based donor-acceptor derivatives

Simple donor-acceptor derivatives were prepared by substituting alicyclic amines into the 2,2 '-bipyridine (Bpy) core unit and they exhibited a substituent structure dependent solution and solid state fluorescence, solvatochromism, and halochromic fluorescence switching in solution and solid state. Structural studies showed the formation ofs-transconformation with the internal cancellation of dipoles in solid state. However, the protonation of the pyridine unit produceds-cisconformation. Pyrrolidine (Bpy-Pyr) and piperidine (Bpy-Pip) substituted Bpy derivatives showed strong solid state fluorescence compared to other compounds. Substituent structure dependent reversible fluorescence switching was observed upon exposure to trifluoroacetic acid (TFA) and ammonia (NH3). Interestingly, theN-methyl piperazine (1MP) substituted compound (Bpy-1MP) showed comparatively strong fluorescence under acidic conditions compared to basic conditions. In contrast,Bpy-Pyrshowed strong fluorescence under basic conditions compared to acidic conditions. The absorption studies also revealed different changes with respect to pH for both compounds. The subtle structure-controlled fluorescence modulation with pH could be of potential interest for bio-imaging and optical devices.