The ratiometric dual-fluorescence of near infrared absorbing aminocyanine dyes: A mechanistic study

A systematic spectral analysis is presented for a heptaaminocyanine dye (Cy7-NCY) and pentaaminocyanine dye (Cy5-NHY). The dyes displayed near infrared and ratiometric dual-fluorescence in viscous environments. Computational studies on these dyes revealed the origin of dual-fluorescence and the nature of the difference in viscous-sensitivity for these dyes. In low-viscous solvents, the amino group can be described as a rotor and was decoupled with the polymethine chain by photo-excitation, displaying an emission peak with long wavelength. In high-viscous solutions, on the contrary, the amino group is almost parallel to the parent polymethine chain and the lone-pair of the amino group is in an appropriate orientation for efficient overlap with the polymethine chain, resulting in an emission peak with short wavelength. The overall balance of the rotating ability for the substituted amino group and the polymethine chain affects the competition between radiative and non-radiative transition processes in a viscous environment. (C) 2017 Elsevier Ltd. All rights reserved.