Photoisomerization of Polyionic Layer-by-Layer Films Containing Azobenzene

Abstract

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)benzenesulfonamido]-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 Å was measured for PDDA/PAZO, while the PEI-containing system resulted in a 36 Å thick bilayer. We used quartz crystal microbalance (QCM) measurements and UV−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 UV−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 π−π* band, which is associated with the trans form of the azo molecules. The photoreaction apparently did not reach completion because the π−π* 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 π−π* 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 × d, where n is the refractive index and d is the overall thickness of the film.

Ultrathin polymer coatings by complexation of polyelectrolytes at interfaces: suitable materials, structure and properties

“…The silanol groups at the surface of glass and quartz, as well as of silicon wafers due to the thin surface layer of oxide, can be easily deprotonated by base. Treatment with various functionalized silanes, such as trialkoxy-3-aminopropylsilane 113,114,143,200,219,227,286,388,395) , provides an additional, "easy" access to positively charged surfaces. In a similar way, treatment of gold with functionalized thiols or disulfides, respectively, enables the tailoring of both positively (e. g. when applying cysteamine 119,211,277,296,387) ) or negatively (e. g. when applying mercaptoundecanoic acid 115,116,228,269,286,396,397) ) charged surfaces.…”

Polar orientation of a pendant anionic chromophore in thick layer-by-layer self-assembled polymeric films

“…21 The effect of polycation type-branched poly͑ethyleneimine͒ and linear poly͑diallyldimethylammonium chloride͒-on the orientation of the azo group in polycation/poly͕1-͓p-͑3Ј-carboxy-4Ј-hydroxyphenylazo͒benzenesulfonamido͔-1,2-ethandiyl͖ ͑PCBS͒ LbL films was studied using absorbance measurements although NLO effects were not probed. 22 A lyotropic, cationic polyionene and strong polyanions were used to make highly ordered films consisting of up to 50 bilayers, but SHG was not studied. 23 In most of these studies where film thickness was measured, it was found that the film thickness and chromophore content-the latter characterized by absorbance measurements-scaled linearly with the bilayer number.…”

Azobenzene Polymers for Photonic Applications

“…The aggregation and photochemical behavior of the azo chromophore (absorbance spectrum, isomerization rate, etc.) vary depending on the nature of the counterpolymer (Dante et al, 1999) (and of course, is affected by any ionic ring substituent). These may be viewed as undesirable matrix effects, or as a way to tune the chromophore response.…”