Deoxyribonucleoside-modified squaraines as near-IR viscosity sensors.

Deoxyribonucleoside-modified squaraines were synthesized by Sonogashira coupling reactions using an unsymmetrical, terminal alkynylated benzothiazolium squaraine dye. These non-natural nucleosides exhibited fluorescent 'turn-on' properties in viscous conditions with an enhancement of >300-fold. The viscosity-dependent fluorescence enhancement was attributed to a combination of hampering both molecular aggregation and intramolecular bond rotation of the squaraine probe. Fluorescence microscopy allowed visualization of highly viscous regions during various stages of cellular mitosis.