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

The rotational dynamics of neutral and cationic forms of the phenazine dye neutral red has been studied in rr-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 cheery and dielectric friction theories of Nee-Zwanzig and van der Zwan-Hynes. Both the Nee-Zwanzig and van der Zwan-Hynes dielectric friction theories overestimate the dielectric friction 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, lr-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. (C) 1998 American Institute of Physics. [S0021-9606(98)52038-7].