Photoisomerization of polyionic layer-by-layer films containing azobenzene

In this work we employed the layer-by-layer adsorption technique for deposi tion on solid substrates of polyionic films containing photoactive azobenze ne groups. We investigated two systems, each having the same polyanion but using a different polycation. Poly {1-4[4-(3-carboxy-4-hydroxyphenylazo)ben zene-sulfonamido]-1,2-ethanediyl sodium salt} (PAZO) was employed as the ph otoactive polyanion; poly(diallyldimethylammonium chloride) (PDDA) and poly (ethyleneimine) (PEI) were used as the polycations. Our phenomenological da ta show dramatic differences in the behavior of the two systems, although t he same experimental conditions were employed in both cases. The assembly o f the multilayers was monitored by ellipsometry and X-ray reflectivity via thickness measurements. We observed a considerable difference in the bilaye r thickness in the two systems. An average polycation/polyanion bilayer thi ckness of 5 Angstrom was measured for PDDA/PAZO, while the PEI-containing s ystem resulted in a 36 Angstrom thick bilayer. We used quartz crystal micro balance (QCM) measurements and W-visible spectroscopy to monitor the adsorp tion process. QCM measurements showed an influence of the polycation in the polyanion adsorption process of the PAZO molecules. In particular, PEI app ears to promote complexation and aggregation of the negatively charged poly ion. Aggregates, mainly in the J form, were detected in both PDDA/PAZO and PEI/PAZO systems by W-visible spectroscopy. We induced trans-to-cis photois omerization of the azobenzene groups by UV light (340 nm), and we followed the photoreaction by the decrease in the intensity of the pi-pi* band, whic h is associated with the trans form of the azo molecules. The photoreaction apparently did not reach completion because the pi-pi* band did not comple tely disappear. We found also that the polycations have a significant influ ence on the molecular orientation of the azobenzene groups in the film and on the photoisomerization kinetics. The kinetics of photoisomerization were not monoexponential, indicating the coexistence of different processes. We investigated also the cis-to-trans reverse isomerization. In particular, w e observed a partial recovery of the pi-pi* band after thermal relaxation t hat was more significant in the PDDA-containing system. By contrast, cis-to -trans isomerization induced by blue light (460 nm) was not observed. UV li ght irradiation was responsible for reversible changes in the optical thick ness of the films, defined as n x d, where n is the refractive index and d is the overall thickness of the film.