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Solubilization Sites and Orientations in Microheterogeneous Media. Studies Using Donor-Acceptor-Substituted Azobenzenes and Bichromophoric Solvatochromic Molecules
Shin, Dong-Myung,Schanze, Kirk S.,Whitten, David G.
, p. 8494 - 8501 (2007/10/02)
A series of p-donor-p'-acceptor-substituted azobenzenes (1-7) has been prepared and their reactivities examined in a variety of homogeneous and microheterogeneous solutions.The series includes several bichromophoric azobenzenes containing a second phenyl or p-nitrophenyl substituent (3-7).Absorption spectra of the trans-azobenzene chromophore are highly solvatochromic in homogeneous solution as are the observed rate constants, kct, for thermal isomerization of photogenerated cis isomers.Good correlations are observed for both properties with a number of empirical solvent polarity parameters, especially the Taft-Kamlet "?*".In oil-continuous reversed micelle solutions (Aerosol OT (sodium bis(2-ethylhexyl)sulfosuccinate)/oil/water), most of the azobenzenes show absorption spectra characteristic of hydrocarbon (nonpolar) solutions but high isomerization reactivity (kct) consistent with a polar environment.The results are analyzed to assess the extent of equilibration between oil and the polar-nonpolar interface or water pool for the isomers of the different azobenzenes.In water-continuous microheterogeneous media (micelles, vesicles, biomembranes), azobenzenes exhibit red-shifted spectra for the azobenzene chromophore and high kct values, consistent with solubilization at very polar sites.The bichromophoric compounds, especially 5, show evidence of differential solvent polarity for the two chromophores, consistent with an interface-spanning solubilization site.
