Switching behavior and optical absorbance of azobenzene-functionalized alkanethiols in different environments

Photoswitching of an azobenzene-functionalized alkanethiol has been investigated in different environments. It is found that in solution, in a polymer matrix, in thin layers adsorbed on quartz or silica gel, and in a self-assembled monolayer (SAM) on a rough gold surface the azo compound shows reversible trans to cis photoisomerization. In contrast, molecules in microcrystalline films are optically inactive. In these systems we observe a blue-shift of the π,π*-absorption band from about 350 nm to 308 nm, which is attributed to H-aggregate formation. This was confirmed by investigation of the compound in different aqueous ethanolic solutions. When the water content is high enough the absorption pattern and maxima of the H-aggregates are identical to those of the non-switching systems. Furthermore, it is shown that both trans and cis isomers of the azo derivative exist in an equilibrium with the H-aggregates which depends on the solvent composition. These findings suggest that also in well-ordered SAMs of azobenzene-functionalized alkanethiols formation of aggregates impedes photoswitching by intermolecular coupling.