Solvent-Solute Excitation Energy Transfer between 1-Butyl- 3-Methylimidazlium Chloride Ionic Liquid and Rose Bengal.

MARTÍN MIRENDA ; KARIN GUTKOWSKI

Mendoza

Conferencia; 21st International American Photochemical Society Conference; 2011

We study the solvent-solute excitation energy transfer phenomenon between the Ionic Liquid (IL) 1-butyl-3-methylimidazolum chloride, [bmim][Cl], and Rose Bengal (RB). Absorption, steady-state fluorescence measurements, and anisotropy calculations were performed on RB-IL solutions for different RB concentrations. Absorption measurements of the RB-[bmim][Cl] show a characteristic RB band with no evidence of RB aggregation nor solvent-solute associations. The RB does not absorb in the blue region. Increasing RB concentration Fluorescence spectrum of pure [bmim][Cl],  recorded at lexc = 400 nm shows an emission maximum in 470 nm. When the dye is added to the system, the fluorescence intensity of the IL decreases while a characteristic RB emission band appears, showing a clear isostilbic point in 560 nm (figure on the right). The presence of RB fluorescence when only the IL is excited is indicative of a solvent-solute energy transfer process. The calculated efficiencies for the total excitation energy transfer and the non-radiative excitation energy transfer, show that the non-radiative mechanism dominates at low RB concentration (diffusional events in the system are totally discarded due to the high viscosity of the IL at room-temperature). Steady-state emission anisotropy spectra were calculated for the pure [bmim][Cl] and for the  [bmim][Cl]-RB system with the aim to explain the mechanism involved in the observed energy transfer process. The solutions were excited at two different wavelengths so the dye or the IL is irradiated selectively. The experiments reveal that the solvent-solute energy transfer phenomenon is mediated by excitation energy migration trough different IL domains.