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

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.