Fluorescent Probes with Variable Intramolecular Charge Transfer: Constructing Closed-Circle Plots for Distinguishing D 2 O from H 2 O.

It is difficult to distinguish between H ₂ O and D ₂ O due to their very similar properties. Triphenylimidazole derivatives with carboxyl groups ( TPI-COOH-2R ) show intramolecular charge transfer that responds to polarities and pH of solvents. Here, a series of TPI-COOH-2R with very high photoluminescence quantum yields (73-98%) were synthesized to distinguish D ₂ O from H ₂ O by the method of wavelength-changeable fluorescence. In a mixed THF/water solution, the increase of H ₂ O and D ₂ O contents will separately induce different pendulum-type fluorescence variations and form plots of closed circles with the same starting and ending points from which a THF/water ratio that displays the most different emission wavelengths (up to 53 nm with an LOD of 0.064 vol %) can be determined to further distinguish D ₂ O from H ₂ O. This is proved to be originated from the various Lewis acidities between H ₂ O and D ₂ O. The results of theoretical calculations and experiments suggest that, for different substituent groups in TPI-COOH-2R , an appropriate electron-donating effect is beneficial to distinguish between H ₂ O and D ₂ O, while the electron-pulling effect is adverse. Moreover, because the potential hydrogen/deuterium exchange does not affect the as-responsive fluorescence, this method is reliable. And this work provides a new strategy for the design of fluorescent probes for D ₂ O.