Low-level detection of water in polar aprotic solvents using an unusually fluorescent spirocyclic rhodamine

Abstract

Rhodamine, a well-known fluorescent building block, widely explored for its versatile functionalization and stimulus-induced switching between a non-fluorescent, spirocyclic form and a fluorescent, non-spirocyclic form is used for various applications. In this work, we have synthesized monomeric and dimeric rhodamines linked with N-methylpropane-1,3-diamine, sMRh, and dipropylenetriamine, sDRh, and they are present in the spirocyclic form. Surprisingly, we could observe fluorescence with the maxima at 490 and 484 nm in acetonitrile from both sMRh and sDRh, respectively, even in the spirocyclic form, which has not been reported in the literature to the best of our knowledge. The fluorescence originating from the spirocyclic form of rhodamine was quite sensitive to the trace amounts water (<1%, volume/volume) present in polar aprotic organic solvents such as acetonitrile, tetrahydrofuran, DMSO, and DMF. The lowest detection limits calculated using the Stern–Volmer equation were 0.02% in acetonitrile, 0.05% in THF, 0.09% in DMSO, and 0.08% in DMF. The presence of water in organic solvents could be identified by the emission maxima and ratiometric fluorescence changes between the spirocyclic and non-spirocyclic forms. The sensing initially seemed to be driven by a hydrogen bonding interaction between the probe and water, followed by photoinduced electron transfer from water to the probe. The existence of hydrogen bonding was studied using ¹H NMR spectroscopy. This probe opens the possibility for the detection of water with respect to multiple fluorescence parameters such as wavelength and intensity.