Rotational dynamics of nondipolar probes in alkane-alkanol mixtures: Microscopic friction on hydrogen bonding and nonhydrogen bonding solute molecules

Rotational dynamics of two structurally similar nondipolar probes; 2,5-dimethyl-1,4-dioxo-3,6-di- phenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) has been studied in mixtures of squalane-1-butanol with the idea of finding out the role of size, chemical composition, and viscosity of the solvent on the friction experienced by hydrogen bonding (DPP) and nonhydrogen bonding (DMDPP) solute molecules. Although the reorientation times of both the probes followed a power law dependence on the solvent viscosity, DPP is found to rotate two to three times slower than DMDPP due to solute-solvent hydrogen bonding. The observed size effects of DMDPP have been modeled using the quasihydrodynamic theory of Gierer-Wirtz (GW). The rotational dynamics of DPP, however, follows stick hydrodynamics in the butanol rich region due to solute-solvent hydrogen bonding. But at higher concentrations of squalane, DPP gets preferentially located in a cagelike structure formed by butanol molecules and even this DPP-1-butanol complex experiences microscopic friction. (C) 2000 American Institute of Physics. [S0021-9606(00)50348-1].