Molecular Diffusion and Fluorescence Energy-Transfer Studies in Thin Surfactant Films

Abstract

Thin surfactant films have been formed by dip-coating glass slides in a solution of reversed micelles containing titanium isopropoxide. The alkoxide is slowly hydrolyzed in the presence of reversed micelles since hydrolysis competes with hydration of surfactant polar groups. Adhesion of the surfactant on the glass slide is assisted by incompletely hydrolyzed alkoxide through the following possible mechanism: the alkoxide adheres by -Si-0-Ti-bonds and the surfactant follows by hydrophobic attraction to the isopropyl groups. The films are transparent and visually uniform. Three surfactants, forming reversed micelles, have been investigated: one nonionic, Triton X-100; one anionic, AOT; and one cationic, hexadecyldimethylbenzylammonium chloride. The environment provided by the surfactant film has been studied by fluorescence probing. In particular, we have analyzed pyrene excimer formation as well as energy transfer between pyrene, acting as donor, and coumarin-153 or N-n-heptyl-4-(((dimethylamino)phenyl)ethenyl)-pyridinium bromide, acting as acceptor. In Triton and hexadecyldimethylbenzylammonium chloride films, pyrene excimer formation is diffusion-controlled while in AOT films excimer largely comes from pyrene aggregation. Pyrene excimer formation capacity decreases in the presence of cosulubilized poly(viny1 methyl ether) chains. Energy transfer data indicate that coumarin-153 is randomly distributed in the films, but N-n-heptyl-4-(((dimethylamino)phenyl)ethenyl)pyridinium bromide is not randomly distributed but shows a tendency to aggregate. Generally speaking, the benzyl-group-bearing surfactants form fluid structures while AOT may provide a restricted low dimensional environment.

Photophysical Studies in AOT Films Deposited on Fused Silica Slides

“…The systematic decrease of the C 2 /C 1 ratio in column 5 can cient to satisfactorily describe any fluorescence decay probe understood by consideration of the nature of Eq. [1]. file.…”

“…Equation [1] represents a universal decay model applicable to all cases where the probability of quenching is the sum of all quenching probabilities for an excited luminophore. This refers to the case when an excited luminophore can, in principle, be quenched by any quencher present.…”

“…It was estimated around tant films can be used as transparent matrices for photophysi-150 nm. cal studies (1,2), as models of biological membranes, as Absorption measurements were made with a Perkin-Ellayers for fine lubrication and coating (3)(4)(5), and as hosts mer Lambda 15 spectrophotometer, fluorescence measureof functionalized species aiming to new applications such ments with a home-assembled spectrofluorometer using as sensors, photoactive layers, non-linear optics, and nano-Oriel parts, and time-correlated fluorescence measurements particle formation (6)(7)(8)(9)(10).…”

“…The analysis of the luminescence decay profiles in the salt (AOT) in cyclohexane both in the absence and in the presence of quenchers was done with stretched exponentials, presence of water, (i.e., of reverse micelles), a thin transpar-according to the following model (1,12,13): ent visibly uniform film is formed. We have studied such a film by using fluorescent probes and quenchers both hydro-I(t) Å I 0 exp(0k 0 t)exp(0C 1 t f / C 2 t 2 f ) 0 õ f õ 1 [1] philic and hydrophobic.…”

“…The excitation wavelength was 450 nm while emission was monitored with a cutoff filter at 600 nm. Absorption spectra for luminophore and quencher as well as a luminescence spectrum for Ru(bpy) of the luminophore in the original reverse micellar solution Equation [1] is fitted to the experimental decay profile and thus f, C 1 , and C 2 can be calculated and used to find K(t). In order to present K(t), we usually tabulate its value at the first time channel, named K 1 , and its value at the last time channel (1000 total number of channels), named K L .…”