Evidence for Preferential Solvation in the Cyclohexane/<i>n</i>‑Butanol Binary Solvent System

Abstract

We report on the rotational diffusion and vibrational population relaxation dynamics of the polycyclic aromatic hydrocarbon (PAH) perylene in a series of cyclohexane/<i>n</i>-butanol binary solvent systems. The molecular scale heterogeneity of this binary system is seen in both types of data. The rotational diffusion results show that in neat <i>n</i>-butanol and neat cyclohexane perylene reorients as an oblate rotor, but for all binary solvent systems examined this chromophore reorients as a prolate rotor. The perylene ring breathing mode is nearly degenerate with the <i>n</i>-butanol terminal methyl group rocking mode and vibrational population relaxation data for the perylene ring breathing mode reveal a substantial decrease in the relaxation time constant with the addition of small amounts of <i>n</i>-butanol to cyclohexane. This finding, in concert with the rotational diffusion data, indicates that perylene is solvated preferentially by <i>n</i>-butanol in cyclohexane/<i>n</i>-butanol binary solvent systems. The implication of this finding is that the cyclohexane/<i>n</i>-butanol binary solvent mixture is not homogeneous on nanometer length scales