Halogen-substituent effect on the spectroscopic properties of 2-phenyl-6-dimethylaminobenzothiazoles.

• 7-Halogenated derivatives of 6-dimethylamino-2-phenylbenzothiazoles were prepared. • The halogenated derivatives have the deformed dimethylamino group by steric repulsion. • The halogenated derivatives showed blue-shifted electronic absorption and fluorescence. • Intersystem crossing from the fluorescence state suffers from heavy atom effect. 6-Dimethylamino-2-phenylbenzothiazole (1 -H) is a push-pull benzothiazole fluorophore mimicking the firefly oxyluciferin structure. We newly prepared 7-chloro and 7-bromo derivatives of 1 -H and its 4-acetyl derivative (2 -H), and their spectroscopic and photophysical properties were investigated. The halogenated derivatives showed the blue-shifted electronic absorption maxima and fluorescence emission maxima compared to 1 -H and 2 -H, resulted from the deformations of the NMe 2 groups and the electron withdrawing properties of the halogen groups. In addition, the halogen substitutions accelerate intersystem crossing by heavy atom effect, resulting in a decrease in fluorescence quantum yields. Interestingly, however, the halogenated derivatives of 2 -H still showed moderate fluorescence quantum yields. The halogenation effect is one of the guides to design push-pull benzothiazole fluorophores for tuning fluorescence properties. [ABSTRACT FROM AUTHOR]