Molecular rotors with long alkyl chains as fluorescent viscosity sensors for hydrocarbon and silicone oil fluids

Three different tetraphenylethylene derivatives (TPE18, TPE8, TPE1) with different-length alkyl chains (octadecyl, octyl, methyl) were prepared, and their viscosity (η) sensitivities were examined for hydrocarbons and silicone oils. Both TPE18 and TPE8 dissolved in the fluids very well, except for the case of TPE8 in silicone oils, while TPE1 remained undissolved. In order to examine the viscosity sensitivity, the hydrodynamic fluorescence (FL) emission behaviors of TPE18 and TPE8 were investigated. The FL intensity (I) significantly increased as the η of the fluid increased. The relationship between I and η was expressed by the Förster-Hoffmann equation log I = C + x log η. TPE18 and TPE8 exhibited extremely high x values of 0.63 and 0.70, respectively, for higher alkanes (η 1.34–84.0 cP); the corresponding x values were 0.73 and 0.63 for lower alkanes (η 0.29–1.70 cP). The slight difference in the x value between the two derivatives was due to the difference in their molecular affinity to the fluids. On the other hand, TPE18 dissolved well in silicone oils, while TPE8 did not dissolve completely. The x value of TPE18 was determined to be 0.16, which was much smaller than those for hydrocarbon fluids.