Abstract
The rates of Z-E isomerization of azobenzene and seven 4-substituted derivatives (CH3, CF3, OCH3, n-Butyl, Ot-Butyl, Br, F) have been investigated at 25 °C in ethanol, in methanol, in methanol/water mixtures and in aqueous solutions of ionic and non-ionic micelles by recording in the dark the spectral changes observed after exposure to UV light of the samples. The competitive contributions of the rotation vs. inversion mechanisms by which the isomerization of azobenzenes can occur have emerged from V-shaped Hammett plots. The obtained rate constants can be used as polarity indicators of the different micro-heterogeneous media with two main advantages with respect to other probes. Firstly they are not affected by hydrogen bonding between the substrate and the environment, as in the case of push-pull azobenzenes. Secondly, the derived substrate-independent Hammett ρ values allow to separate the effects of orientation of the probe from the polarity of the supramolecular aggregate.
Highlights
IntroductionIncorporation of molecular probes into aqueous micelles allows measurement of parameters such as the critical micelle concentration (cmc), the degree of water penetration into the surfactant aggregates and the fluidity of the micellar core
Incorporation of molecular probes into aqueous micelles allows measurement of parameters such as the critical micelle concentration, the degree of water penetration into the surfactant aggregates and the fluidity of the micellar core
In this study we have investigated the Z-E isomerization of azobenzene and seven 4-X substituted azobenzenes, bearing either an electron withdrawing or an electron donating group
Summary
Incorporation of molecular probes into aqueous micelles allows measurement of parameters such as the critical micelle concentration (cmc), the degree of water penetration into the surfactant aggregates and the fluidity of the micellar core. Most probes have ARKAT USA, Inc. been chosen for their selectivity towards specific micellar domains, such as specific affinity for the hydrocarbon core formed by the surfactant tails (e.g., pyrene, naphthalene and anthracene) or towards the interface composed primarily by the amphiphile head groups (e.g., pyrenesulfonic acid, pyrenebutyric acid, and 1-anilinonaphthalene-8-sulfonate).[1]. Azobenzene and its derivatives are photoreactive molecules that undergo reversible photoisomerization from the generally more stable trans (E) isomer to the less stable cis (Z) isomer upon UV irradiation. The strong geometrical difference between the extended trans form and the compact cis form in the gas phase and in solution may provide important details on the distribution and the dynamics of the azobenzene guest into hosts such as cyclodextrins[6] and crown ethers[7] or on the conformational changes of polypeptide chains.[8]
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