The nature of the different environmental sensitivity of symmetrical and unsymmetrical cyanine dyes: an experimental and theoretical study

Symmetrical and unsymmetrical cyanine dyes are used in different applications due to their different fluorogenic behaviors toward bio-macromolecules and micro-environments. In the present paper, computational studies on these dyes reveal that the potential energy of the electronic excited state is controlled by C–C bond rotational motion, which causes mainly nonradiative deactivation, according to the activation energies for the rotation. The rotations of different C–C bonds in the molecules have quite different rotational activation energies. Symmetrical dyes (Cy) possess an obviously higher rotating energy barrier as well as a larger energy gap compared to unsymmetrical dyes (TO). The C–C bond rotation close to the quinoline moiety of unsymmetrical thiazole orange (TO) allows the dye to possess the lowest energy barrier and also the lowest energy gap. This rotation plays a major role in reducing fluorescence quantum yields and providing a low fluorescent background in the free states of the unsymmetrical cyanine dyes. The results might provide a foundation for the interpretation of the behavior of the dyes and are useful for the future design of new cyanine fluorophores.