Photochemical behaviour of an acid-terminated azopolymer in solution and in Langmuir-Blodgett films

Abstract

Trans-cis-trans isomerization processes of an acid-terminated azopolymer have been investigated in mono and multilayered Langmuir-Blodgett (LB) films and the results compared with the behaviour of this azopolymer in solution. UV-vis spectroscopy was used to determine the apparent extent of photoisomerization at the photostationary state, as well as the order and rate constant of the cis → trans thermal isomerization, whilst structural changes of the LB films during photoisomerization have been monitored by AFM microscopy. From these studies it was concluded that the trans → cis photoisomerization extent largely depends on the free volume around the azobenzene chromophores, with the smaller apparent extent of photoisomerization obtained for a well-organized 10-layer LB film. Data also revealed a first-order kinetics for the thermal cis → trans isomerization process of the azopolymer both in solution and in LB films. The cis → trans isomerization process depends on the balance between the stress produced by the larger cross-section of cis-isomers inside the system and the interactions between cis-isomers and neighbouring molecules. The slowest cis → trans isomerization process takes place in a 35-layer LB film, whilst LB films with well-packed azobenzene layers show faster cis → trans isomerization processes. © 2009 Elsevier B.V. All rights reserved.