Temperature-dependent rotational relaxation of nonpolar probes in mono and diols: Size effects versus hydrogen bonding

The rotational reorientation times of two nonpolar probes, 2,5-dimethyl-1,4-dioxo-3, 6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) have been measured in 1-decanol and ethylene glycol as a function of temperature using steady-state fluorescence depolarization technique. Although both the probes are structurally similar and have almost identical volumes, the experimentally measured reorientation times of DMDPP are longer in ethylene glycol compared to 1-decanol whereas an exactly opposite trend has been observed for DPP. The faster rotation of DMDPP in 1-decanol has been attributed to the larger size of 1-decanol which is three times bulkier than ethylene glycol and hence offers a reduced friction. This pattern has been mimicked using the quasihydrodynamic theories of Gierer-Wirtz and Dote-Kivelson-Schwartz in a qualitative way. The slower rotation of DPP in 1-decanol compared to ethylene glycol is due to the solute-solvent hydrogen bonding which increases the effective volume of the probe more in the case of 1-decanol than ethylene glycol. (C) 2000 American Institute of Physics. [S0021-9606(00)50708-9].