Synthesis, characterization and photophysical properties of luminescent non-symmetric 4-pyridyl benzothiadiazole derivatives

Two nonsymmetrical π-extended 2,1,3-benzothiadiazoles (BTDs) bearing a 4-pyridyl moiety (BTD-4pyr and BTD-Et4pyr) were synthesized by sequential cross coupling reactions. The structural difference between the two dyes is the presence of a triple bond between the BTD core and the 4-pyridyl moiety in BTD-Et4pyr. The compounds architecture is similar to previously described selective mitochondrial biomarkers. Both compounds exhibit large Stokes shifts (93–137 nm), high fluorescence quantum yields and linear fluorescence response in the nanomolar range. The existence of the triple bond decreased in about 35% the fluorescence measured from BTD-Et4pyr in respect to BTD-4pyr. Solid-state fluorescence of the dyes were measured, producing considerable smaller Stokes shifts than the ones observed in solutions (74 nm for BTD-4pyr and 66 nm for BTD-Et4pyr). X-ray crystallography analysis of BTD-4pyr indicated a quinoid character for the BTD ring and a nonplanar relation between the BTD core and the aryl/heteroaryl groups. DFT calculations pointed out that the LUMO electron density is concentrated over the BTD core. In relation to HOMO, the electron density is distributed mainly at the methoxyphenyl group, in the hydrocarbon fraction of the BTD core and in ethynyl group for BTD-Et4pyr.