Rational design of NIR fluorescence probes for sensitive detection of viscosity in living cells

Developing near-infrared (NIR) fluorescence probes for detection of intracellular viscosity is still sufficiently challenging. In this work, three kinds of D-A-D type naphthyl and 2,1,3‑benzoxadiazol hybrid NIR dyes functionalized with amino (NY1), N‑methylamino (NY2) and N,N‑dimethylamino (NY3) groups for intracellular micro-viscosity detection were designed and synthesized. All the probes exhibited very weak NIR emission in low viscosity environment and obvious fluorescence enhancement with the increased viscosity. Different substituent groups had a high impact on the photophysical properties and response sensitive of the probes to viscosity. The structure-property relationships were systematic investigated. The results showed that stronger electron-donating ability and larger steric effect of N,N‑dimethylamino led to a narrower energy gap and more sensitive to viscosity environment. Therefore, NY3 exhibited higher signal noise ratio for viscosity detection and was successfully applied for imaging the changes of intracellular micro-viscosity. This work provides an efficient way to design powerful NIR fluorescence probes for viscosity detection.