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

Abstract

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 of s-trans conformation with the internal cancellation of dipoles in solid state. However, the protonation of the pyridine unit produced s-cis conformation. 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, the N-methyl piperazine (1MP) substituted compound (Bpy-1MP) showed comparatively strong fluorescence under acidic conditions compared to basic conditions. In contrast, Bpy-Pyr showed 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.