What can you learn from a molecular probe?. New insights on the behavior of C343 in homogeneous Solutions and AOT reverse micelles

CORREA N.M.*; LEVINGER N.E.

JOURNAL OF PHYSICAL CHEMISTRY B

American Chemical Society

Lugar: Washington DC, USA; Año: 2006 vol. 110 p. 13050 - 13061

1089-5647

AbstractThe behavior of C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media, and in aqueous and nonaqueous reverse micelles (RMs). In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from absorption and emission bands showing that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on hydrogen bonding properties of the solvent. Specifically, in the ground state the molecule displays a bathochromic shift with the polarity-polarizability (p*) and the H-bond acceptor (b) ability of the solvents and a hypsochromic shift with the hydrogen donor ability (a) of the media. The carboxylic acid group causes C343 to display greater sensitivity to the b than to the p* polarity parameter; this sensitivity increases in the excited state, while the dependence on a vanishes. This demonstrates that C343 forms a stable H-bond complex with solvents with high H-bond acceptor ability (high b) and low H-bond donor character (low a). Spectroscopy in nonpolar solvents reveals J-aggregate formation. With information from the Kamlet-Taft analysis, C343 was used to explore RMs composed of water or polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/isooctane using absorption, emission, and time resolved spectroscopies. Sequestered polar solvents included ethylene glycol (EG), formamide (FA), N,N dimethylformamide (DMF) and N,N dimethylacetamide (DMA). Dissolved in the AOT RM systems at low concentration C343 exists as a monomer, and when introduced to RM samples in its protonated form, C343 remains protonated driving it to reside in the interface rather than the water pool. The solvathochromic behavior of the dye depends the specific polar solvent encapsulated in the RMs, revealing different types of interactions between the solvents and the surfactant. EG and water H-bond with AOT sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. In aqueous AOT RMs, C343 interacts with neither the sulfonate group nor the water, perhaps due to intramolecular H-bonding in the dye. DMF and DMA interact primarily through dipole-dipole forces, and the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but retains its H-bond network present in bulk solvent. Surprisingly FA appears to be the only polar solvent other than water forming a ?polar solvent pool? with macroscopic properties similar to the bulk.