Photoisomerization of polyionic layer-by-layer films containing azobenzene

In this work we employed the layer-by-layer adsorption technique for deposition on solid substrates of polyionic films containing photoactive azobenzene groups. We investigated two systems, each having the same polyanion but using a different polycation. Poly {1-4[4-(3-carboxy-4-hydroxyphenylazo)benzene-sulfonamido]-1,2-ethanediyl sodium salt} (PAZO) was employed as the photoactive polyanion; poly(diallyldimethylammonium chloride) (PDDA) and poly(ethyleneimine) (PEI) were used as the polycations. Our phenomenological data show dramatic differences in the behavior of the two systems, although the same experimental conditions were employed in both cases. The assembly of the multilayers was monitored by ellipsometry and X-ray reflectivity via thickness measurements. We observed a considerable difference in the bilayer thickness in the two systems. An average polycation/polyanion bilayer thickness of 5 Angstrom was measured for PDDA/PAZO, while the PEI-containing system resulted in a 36 Angstrom thick bilayer. We used quartz crystal microbalance (QCM) measurements and W-visible spectroscopy to monitor the adsorption process. QCM measurements showed an influence of the polycation in the polyanion adsorption process of the PAZO molecules. In particular, PEI appears to promote complexation and aggregation of the negatively charged polyion. 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 photoisomerization 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, which is associated with the trans form of the azo molecules. The photoreaction apparently did not reach completion because the pi-pi* band did not completely disappear. We found also that the polycations have a significant influence 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, we observed a partial recovery of the pi-pi* band after thermal relaxation that was more significant in the PDDA-containing system. By contrast, cis-to-trans isomerization induced by blue light (460 nm) was not observed. UV light irradiation was responsible for reversible changes in the optical thickness of the films, defined as n x d, where n is the refractive index and d is the overall thickness of the film.