Dynamic internal cavities of dendrimers as constrained media. A study of photochemical isomerizations of stilbene and azobenzene using poly(alkyl aryl ether) dendrimers

Dendritic microenvironments defined by dynamic internal cavities of a dendrimer were probed through geometric isomerization of stilbene and azobenzene. A third-generation poly(alkyl aryl ether) dendrimer with hydrophilic exterior and hydrophobic interior was used as a reaction cavity in aqueous medium. The dynamic inner cavity sizes were varied by utilizing alkyl linkers that connect the branch junctures from ethyl to n-pentyl moiety (C ₂G3-C ₅G3). Dendrimers constituted with n-pentyl linker were found to afford higher solubilitiesof stilbene and azobenzene. Direct irradiation of trans-stilbene showed that C ₅G3 and C ₄G3 dendrimers afforded considerable phenanthrene formation, in addition to cis-stilbene, whereas C ₃G3 and C ₂G3 gave only cis-stilbene. An electron-transfer sensitized trans-cis isomerization, using cresyl violet perchlorate as the sensitizer, also led to similar results. Thermal isomerization of cis-azobenzene to trans-azobenzene within dendritic microenvironments revealed that the activation energy of the cis- to trans-isomer was increasing in the series C ₅G3 < C ₄G3 <C ₃G3 <C ₂G3. With the change in the alkyl linker from C ₅ to C ₂, significant changes in the activation parameters were observed.