Rotational dynamics of neutral red: Do ionic and neutral solutes experience the same friction?

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Abstract
The rotational dynamics of neutral and cationic forms of the phenazine dye neutral red has been studied in n-alcohols, amides, and aprotic solvents using picosecond time-resolved fluorescence depolarization spectroscopy. While both the neutral and cationic forms of neutral red experienced more or less the same friction in alcohols, the cationic form experienced 16%–26% more friction in amides and aprotic solvents exceptions being formamide and propylene carbonate (PC). The results were analyzed in terms of the Stokes–Einstein–Debye (SED) hydrodynamic theory and dielectricfriction theories of Nee–Zwanzig and van der Zwan–Hynes. Both the Nee–Zwanzig and van der Zwan–Hynes dielectricfriction theories overestimate the dielectricfriction contribution for the neutral form of neutral red in alcohols. The rotational dynamics of neutral form of neutral red in N, N-dimethyl formamide (DMF), N, N-dimethyl acetamide (DMA), N, N-dimethyl propionamide (DMP), and dimethyl sulphoxide (DMSO) is adequately described by the hydrodynamic model with the stick boundary condition. However, it overestimates the friction experienced in formamide, and to a certain extent in PC wherein for both forms similar reorientation times were observed. As the cations are strongly solvated by amides only 60%–70% of the friction experienced in DMF, DMA, and DMP can be accounted for by the SED theory.