Synthesis, characterization, and induction of stable anisotropy in liquid crystalline photo-addressable PPI dendrimers

A series of liquid crystalline azobenzene codendrimers with light-driven properties has been prepared by chemical modification of the third generation of an amine-terminated poly(propylene imine) (PPI) dendrimer. These photoactive dendrimers were obtained by grafting the photochromic 4-cyanoazobenzene and/or liquid crystalline 4-cyanobiphenyl units onto the periphery of PPI. The synthesized dendrimers include different percentages of each functional unit randomly distributed in the dendritic shell. The chemical structures of the dendrimers were characterized by several techniques to determine their composition. In particular, MALDI-TOF analysis provided detailed information about the chemical composition of the dendrimers. The photo-induced orientation of azobenzene groups in thin films of these dendrimers was studied. Illumination with either blue (488 nm) or UV (350 nm) linearly polarized light at room temperature gave rise to a stable dichroism and moderate birefringence values when 80% (molar ratio) of the units are azobenzene. In addition, pre-irradiation of the films with unpolarized UV light gave rise to higher birefringence values.