The structure and dynamics of organic dye molecules included in β-cyclodextrin are studied using spectroscopic measurements and theoretical models. The effect of the charge and the size of the guest molecule on the properties of the host-guest complex is probed by comparing complexes formed with three different chromophores: resorufin (anion), oxazine-118 (cation), and oxazine-725 (cation) in aqueous solutions of cyclodextrin. The binding is characterized using absorption and calorimetry titrations. The structure of the complexes is analyzed by molecular modeling using empirical force field and semiempirical quantum theory calculations. Time-resolved polarization spectroscopy is used to investigate the rotational dynamics of different chromophores bound to β-cyclodextrin. Internal motion of the guest and overall rotational tumbling of the complex are observed for resorufin and oxazine-118. Modeling the internal motion of the chromophore as diffusion in a cone provides the mean square diffusion angle inside the cavity. It is found that the relative host-guest size determines the character of intermolecular host-guest dynamics.

Spectroscopic and molecular dynamics (MD) studies of organic dye molecules in polar solvents are performed to investigate the nature of solute−solvent interactions. Experimental and MD data demonstrate that positively charged molecules rotate more slowly than neutral and negatively charged solutes in polar aprotic solvents. MD simulations of the resorufin, resorufamine, and thionine molecules in DMSO solvent are analyzed to reveal differences in the origins of the frictional forces experienced by each solute molecule. It is demonstrated that specific associations (hydrogen bonds) are formed between the DMSO molecules and both the neutral and the cation solute molecules. No specific solute−solvent association is observed for the negative solute molecule. According to a force separation analysis, the calculated friction on the cation is mostly Coulombic in nature, while it is mostly collisional (mechanical) for the anion case.

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Rotational diffusion of organic ions in electrolyte solution is studied via optically heterodyned polarization spectroscopy and molecular dynamics (MD) simulations. Significant differences between the behavior of organic cation and anion species were observed in the experiments. While the rotational relaxation time of the anion normalized by the viscosity of the solution increases with the electrolyte concentration, the normalized relaxation time of the cation decreases with increasing electrolyte concentration. The experimental data are analyzed by using a continuum theory approach and MD simulations. It is demonstrated that one must include ion pairs to describe the dynamics of the anion, but the analysis of the relaxation of the cation does not require ion pairing. MD simulations show that the difference in the dynamics of the anion and cation in electrolyte solution is caused by the different ability of free anion, free cation, and ion paired species to associate with the solvent (DMSO).