Stimuli-responsive fluorescent dyes for electrochemically tunable multi-color-emitting devices

The preparation and characterization of two fluorescence dyes are demonstrated that can respond to the various external stimulations including solvent, acid, base and electrical potentials through changing the multi-colored fluorescent emission. The fluorescent dyes are designed as 2-(2-thiophen-3-olyl)benzothiazole derivatives consisting of di- or tri-thiophene with long alkyl chain, namely, T₂-BT and T₃-BT, respectively. T_n-BTs exhibit excited state intramolecular proton transfer (ESIPT) phenomenon in the organic solvent that do not involve the oxygen atom such as chloroform, dichloromethane and toluene, where the ESIPT induces red-shift of the emission spectra compared to the original emission. T_n-BTs also display clearly multi-colored emission in the cationic, neutral and anionic states upon the addition of the acid or base. Based on this feature, the white emission system was achieved with T₂-BT through the addition of the 0.5 equivalent of tetrabutylammonium hydroxide. Furthermore, multi-colored electrofluorochromic (EFC) cells based on single molecules (T₂-BT or T₃-BT) were successfully implemented in the low electrical bias (-3 V–3 V).