Benzothiazoles are known to possess
a number of biological activities
and therefore are considered to be an important scaffold in the design
and synthesis of pharmacophores. In this study, an improved synthesis
method for novel fluorescent benzothiazole-based cyclic azacyanine
(CAC) dyes bearing different electron-donating/withdrawing groups
on their scaffold is presented. The improved method enabled us to
increase the synthesis yield for the previously reported CACs. More
importantly, it allowed us to synthesize new CAC dyes that were not
synthesizable with the previously reported method. The synthesized
dyes were characterized by
1
H and
13
C NMR spectroscopy,
elemental analysis, and mass spectrometry and their optical (absorption
and fluorescence) properties were investigated. All of the synthesized
CACs were found to be displaying strong absorption within the range
of 387–407 nm. The spectral shifts observed in the absorption
and fluorescence measurements suggested that the spectroscopic and
optical properties of CACs can be directly modulated by the nature
of the electron-donating/withdrawing substituents. The fluorescence
quantum yields (QYs) of the unsubstituted (parent CAC) and substituted
CACs were also measured and compared. The fluorescence QY of CACs
with electron-donating substituents (methoxy or ethoxy) was found
to be at least four times higher than that of the parent CAC with
no substitutions.