Photophysical, electrochemical, and spectroelectrochemical investigation of electronic push-pull benzothiadiazole fluorophores

Abstract

Two electronic push-pull fluorophores consisting of a benzothiadiazole core and a terminal N,N-dimethylamino electron donating group were prepared. The effect of the terminal electron withdrawing group (-NO2 and -CN) on the spectroscopic, electrochemical, and spectroelectrochemical properties were examined. The fluorophores were solvatochromic with Stokes shifts upward of 9000 cm-1 being observed in aprotic solvents of varying polarity. It was found that the fluorophores fluoresced appreciably (86 % > Φfl > 18 %) in hexane, toluene, diethyl ether, dichloromethane, ethyl acetate, and THF. The fluorescence was quenched in acetonitrile, acetone, and DMSO. The fluorophores also fluoresced appreciably in thin films when embedded in poly(methyl methacrylate) and poly(dimethylsiloxane) matrices. Protonating the terminal amine with trifluoroacetic acid quenched the intramolecular charge transfer band, although the fluorophores remained fluorescent. The fluorophores could also be reversibly oxidized electrochemically. The resulting oxidized state was visually different than the neutral form. Electrochemical oxidation also led to reversible changes in both the fluorescence wavelength and intensity.