Styrylcyanine-based fluorescent probes with red-emission and large Stokes shift for the detection of viscosity

Abstract

Abstract Based on the mechanism of intramolecular charge transfer (ICT), two new styrylcyanine dyes , DPA-1 and DPA-2 , composed of an electron-rich N -phenylaniline and a cationic benzothiazene connected with ethylene(s) bridge, were designed and synthesized. These two dyes exhibited red-emission (653/607 nm) and simultaneously impressive large Stokes shift (111/92 nm) due to intramolecular charge transfer effect, twisted geometry and extended conjugation system. When their solution viscosity increased from 1.01 cP to 234 cP in the water-glycerol system, the fluorescence intensity of the synthetic dyes was enhanced by 81-fold and 64-fold, respectively. Additionally, a favorable linear relationship between the fluorescence intensity and the environmental viscosity was observed within the above viscosity range for the obtained samples (R 2 > 0.99), which led to the establishment of a method for the quantitative determination of the solution viscosity. Such dyes with improved photophysical properties, including emission wavelength and Stokes shift, could be used as promising candidates for intracellular viscosity detection. Moreover, the mechanism of fluorescence emission of the resultant products toward viscosity was further investigated. Of the two probes studies, DPA-1 is better than DPA-2 in terms of emission wavelength, Stokes shift and the sensitivity of the fluorescence intensity to viscosity.