A Butterfly-Shaped Pyrene Derivative of Cholesterol and Its Uses as a Fluorescent Probe

A butterfly-shaped pyrene derivative of cholesterol, namely, N,N′-(ethane-1,2-diyl)-bis­(N-(2-(chol-amino)­ethyl)­pyrene-1-sulfonamide) (ECPS), has been designed and synthesized. Solvent effect studies revealed that in good solvents such as n-hexane, benzene, and 1,4-dioxane, the profile of the fluorescence emission of the compound is characterized by pyrene monomer emission, but in poor solvent such as water, the emission is dominated by pyrene excimer emission. Quantitatively speaking, the ratio of the excimer emission to monomer emission changes from 50 to 0 when ECPS is dissolved in water and n-hexane, respectively. In contrast, for a commonly used polarity probe pyrene, the ratio of I₃/I₁ varies only from ∼0.6 to ∼1.7, where I₃ and I₁ stand for the intensities of the fluorescence emission at peak 3 and peak 1, respectively. This value suggests that a more powerful discriminating ability of the new compound in polarity sensing. Furthermore, unlike the main components of the compound, pyrene and cholesterol, its main chain is composed of multiple hydrophilic structures, and it is this structure that makes the emission of the compound in organic solvents sensitive to the presence of water. Accordingly, the applicability of the compound in determination of the trace amount of water in some organic solvents was evaluated. As expected, the detection limit of the compound toward water in acetonitrile reaches 7 ppm, a result never reached before. Furthermore, the fluorescence emission of the compound is also sensitive to viscosity variation. Therefore, it is assumed that ECPS may be used both as a polarity probe and a viscosity probe. On the bases of a series of steady-state and time-resolved fluorescence, as well as dynamic light scattering studies, a structural model was proposed to rationalize the fluorescence behavior of the compound in different solvents and its polarity and viscosity probing performances.