Novel cyanine dyes as fluorescent pH sensors: PET, ICT mechanism or resonance effect?

Cyanine and related polymethine dyes have been widely used as fluorescent and phosphorescent dyes for biological investigations. But the mechanism of this kind of cyanine dye as pH sensor is still not clear. In this paper, two groups of cyanine dyes were designed and synthesized, each including two asymmetry cyanine dyes and one symmetry dye. The UV–vis and fluorescent spectra of compounds Ia–b and IIa–b were recorded in phosphate buffers with different pH value. It was found that these compounds could be used as a kind of pH sensors with high pKa value. The calculation on structures or conformations of compounds Ia–b, IIa–b and Ic before and after protonation revealed that the planar conjugations were different between non-protonic and protonic forms. It implied that the change from PET to ICT system was possible in the process of binding between polymethine cyanine dyes and proton.