Proton Transfer Pathways of 2,2'-Bipyridine-3,3'-diol in pH Responsive Fatty Acid Self-Assemblies: Multiwavelength Fluorescence Lifetime Imaging in a Single Vesicle.

Fatty acids are known to form different supramolecular aggregates in aqueous solutions depending on the pH of the medium. The dynamics of the transformation of oleate micelles into oleic acid/oleate vesicles has been investigated using a pH-sensitive intramolecular proton transfer fluorophore, 2,2'-bipyridine-3,3'-diol [BP(OH)]. Different prototropic forms of BP(OH) exist in different pH values of the system, and thus, the ground state and the excited state dynamics of BP(OH) have been modulated in these confined media. The formation of different tautomeric forms of BP(OH) in oleate micelles (at basic pH) is confirmed using time-resolved emission spectra and fluorescence anisotropy measurements. The hydrophobic environment provided by these assemblies reduces the water-assisted nonradiative decay channels and lengthens the fluorescence lifetime of BP(OH). The rotational relaxation time in the micellar assembly is higher than that in the vesicle, which may be due to the higher microviscosity sensed by the fluorophore in the micelle. Besides, we have shown for the first time that BP(OH) can be used as a membrane-bound fluorophore, using fluorescence lifetime imaging microscopy (FLIM). A broad distribution in the size of the vesicle is observed from the FLIM image. Further, we have used multiwavelength FLIM to collect the FLIM images of a single vesicle at different emission wavelengths, and the lifetime distribution obtained from the FLIM images at different emission wavelengths in a single vesicle correlates well with the lifetime values obtained from the ensemble average measurements in the bulk solution.