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Unique Influence of Cholesterol on Modifying the Aggregation Behavior of Surfactant Assemblies: Investigation of Photophysical and Dynamical Properties of 2,2′-Bipyridine-3,3′-diol, BP(OH)2 in Surfactant Micelles, and Surfactant/Cholesterol Forming Vesicles

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journal contribution
posted on 07.08.2014, 00:00 by Surajit Ghosh, Jagannath Kuchlyan, Subhajit Roychowdhury, Debasis Banik, Niloy Kundu, Arpita Roy, Nilmoni Sarkar
The binding and rotational properties of an excited-state intramolecular proton transfer (ESIPT) fluorophore, 2,2′-bipyridine-3,3′-diol, BP­(OH)2 has been investigated in alkyltrimethylammonium bromide containing (CnTAB, n = 12, 14, and 16) micelles and alkyltrimethylammonium bromide/cholesterol (CnTAB (n = 14 and 16)/cholesterol) forming vesicles using fluorescence-based spectroscopy techniques. The formation of thermodynamically stable unilamellar self-assemblies of alkyltrimethylammonium bromide/cholesterol are characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. Individually, aqueous solutions of all these alkyltrimethylammonium bromide form micelles after certain surfactant concentration (critical micelle concentration, cmc) of surfactant, whereas cholesterol molecules are insoluble in water. But with the variation of the cholesterol-to-surfactant molar ratio (Q = [cholesterol]/[surfactant]), uniform distribution of vesicular aggregates in aqueous solution can be obtained. The micelle-to-vesicle transition of surfactant solution upon addition of cholesterol also influences the steady state emission profile, fluorescence lifetime, and rotational dynamics of BP­(OH)2 molecule. The diketo tautomer of BP­(OH)2 molecule gets stabilized as the concentration of surfactant increases in aqueous solution. Fluorescence lifetime and rotational time constant of the BP­(OH)2 molecule are also influenced by the variation of alkyl chain length of surfactant molecule. The emission quantum yield (Φ) is also found to be sensitive with surfactant concentration, variation in chain length of surfactants, and it saturates after the cmc of surfactants. The rigid and restricted microenvironment of vesicle bilayer enhance the lifetime and also rotational relaxation of BP­(OH)2 significantly. The rotational behavior of BP­(OH)2 in surfactant/cholesterol self-assemblies is also explained by using analytical parameters related to time-resolved anisotropy following two-step process and wobbling in a cone models.

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