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-dime thyl-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 squal ane-1-butanol with the idea of finding out the role of size, chemical compo sition, and viscosity of the solvent on the friction experienced by hydroge n 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 effect s of DMDPP have been modeled using the quasihydrodynamic theory of Gierer-W irtz (GW). The rotational dynamics of DPP, however, follows stick hydrodyna mics 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 P hysics. [S0021-9606(00)50348-1].