The synthesis and characterization of photoresponsive dendrimers

The synthesis of dendrimers containing photoresponsive azobenzene moieties at the central core and within the interior of the structure has been accomplished via the convergent approach to dendrimer synthesis. The dendrimers exhibited the expected photoresponsive behavior as detected by UV-Visible spectroscopy. Irradiation with 350 nm light results in the photoisomerization of the azobenzene units from the extended trans to the more compact cis configuration. The thermal cis-trans isomerization for dendrimers containing azobenzene central linkers was determined to follow first-order reaction kinetics. It appears the photoresponsive behavior of the azobenzene moieties are not affected by incorporation into the dendrimer structure up to the fourth generation. ^ Similar results were obtained for multiple azobenzene dendrimers where three photoactive units are radially configured about the central core. With each azobenzene moiety capable of E/Z isomerization, these materials can exist as four photoisomers. All photoisomers have been detected in solution using 1H NMR and UV-Visible spectroscopies. Since the Z isomer of azobenzene is more polar than the E, the different properties of each photoisomer based on the increasing Z content was observed by thin-layer chromatography. Dendrimers which contain six azobenzenes chromophores positioned throughout the interior of the dendrimer structure have also been prepared. Low generation hexakis(azobenzene) dendrimer exhibited similar photoresponsive behavior as the tris(azobenzene) dendrimers. However, an increase in the activation energy for the thermal isomerization was observed for the second generation dendrimer. These results suggested the presence of a dendritic effect. Hexakis(azobenzene) dendrimers were found to exist as seven isomeric states in solution as detected by 1H NMR spectroscopy and each state could be observed by thin-layer chromatography. ^ We have prepared a series of dendrons containing chiral branching units which have been isolated into individual generational shells of the dendritic structure. We have studied the effect of the subunit position on the optical activity of the dendrons. Based on the chiroptical data obtained, the observed increase in optical activity of the materials is solely due to constitutional differences within the dendrons as not macromolecular asymmetry.