Investigation of nanostructure in room temperature ionic liquids using electronic excitation transfer

Donor-donor electronic excitation transfer among 9-phenylanthracene (9PA) chromophores was measured in the room temperature ionic liquid (RTIL) 1-methyl-3-octylimidazolium chloride using time dependent fluorescence depolarization. 9PA, which is uncharged and nonpolar, will partition into the organic regions of the RTIL. The excitation transfer rate, which is sensitive to the distribution of chromophores in the RTIL, is modeled using different spatial configurations of 9PA molecules in the RTIL solution. The models are an isotropic distribution (random distribution) and a clustered sphere model to represent hydrophobic regions of a nanostructured environment. Model calculations were performed to demonstrate the sensitivity of excitation transfer to different distributions of chromophores. When compared to the experiment, the isotropic model can adequately match the data. From a Bayesian analysis of the sensitivity of the excitation transfer to the models for the spatial distribution of chromophores, an upper limit of 6 Å radius is placed on the size of hydrophobic domains in the RTIL.