Comparison of ZE isomerization in Langmuir-Blodgett layers and in solution

Z⇄E isomerization processes in the Langmuir-Blodgett (LB) monolayer and in the bulk of the solution were compared. The molecules studied are crown ethers with the electro- and photoactive azobenzene moiety as part of the macrocyclic ring. The rates of the photoinduced  E→Z isomerization and of the spontaneous thermal  E→Z reaction were calculated on the basis of the electrochemical and spectroscopic measurements. Isomerization was found to be faster in the monolayer assembly than in the bulk of the solution. On the other hand, the activation energies of the thermal  E→Z isomerization in the film and in the heptane solution were comparable. Entropy of the isomerization reaction is larger in the film indicating the lower stability of the Z form upon immobilization. The higher rate of isomerization in monolayers compared to what was observed in solutions was explained as due to shorter intermolecular distances and uniform orientation of molecules in the monolayer. These features, unique for well-organized monolayers, allow for the cooperative domino type behavior of molecules leading to a faster isomerization process than observed in the bulk of the solution.