Isomerization and acid-base behavior in polyion complex Langmuir-Blodgett films

The nature of an azobenzene moiety incorporated into a polyion complex Langmuir-Blodgett (LB) film over a range of subphase pH values was investigated by UV-visible spectroscopy. The azobenzene unit was substituted with both carboxyl, and hydroxyl groups and the charge per polymer segment was varied by altering the pH of the subphase. The trans isomer of the azobenzene is present in films transferred from a neutral pH subphase. At a higher pH where both the carboxyl and hydroxyl groups are ionized in aqueous solution, the complex LB film formed from this subphase exhibits the cis isomer of the singly charged polymer segment. By comparison with a water-soluble polyion-surfactant complex for which the cis isomer is not observed, it is concluded that the LB film has considerable rigidity which stabilizes the cis isomer. The severely limited photoisomerization of the trans isomer in the film is further indication of the restricted molecular motion within the polyion complex LB film.