Solubilization Sites and Orientations in Microheterogeneous Media. Studies Using Donor-Acceptor-Substituted Azobenzenes and Bichromophoric Solvatochromic Molecules

Article abstract of DOI:10.1021/ja00204a025

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, k_ct, 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 (k_ct) 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 k_ct 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.