Photocontrol of the Supramolecular Chirality Imposed by Stereocenters in Liquid Crystalline Azodendrimers

Abstract

A series of liquid crystalline dendrimers have been synthesized by peripheral functionalization of PPI dendrimers (generations having 16 or 32 functional groups) with photochromic azobenzene units that contain stereocenters at the terminal alkyl chains. Complete functionalization was checked by NMR and MALDI-TOF mass spectrometry. The mesomorphic properties were analyzed by optical microscopy, DSC, X-ray diffraction and AFM. The dendrimers exhibit a smectic phase that can be frozen at room temperature depending on the thermal history of the sample. Dendrimers did not exhibit a CD response in solution. However, chiroptical properties were detected in films and these were assigned to a supramolecular chirality arising from aggregation of azobenzene units and imposed by the stereocenters. The chiroptical properties depend to a small extent on the thermal history of the sample and can be modulated with light. For example, irradiation with UV light erases the CD signal of the films due to trans−cis isomerization, but this eventually reverts to the initial CD signal by cis−trans back isomerization and aggregation. Irradiation with 488 nm circular polarized light (CPL) modulates the CD signal depending on the handedness of the light and generation of dendrimeric core. Irradiation with l-CPL reinforces the supramolecular chirality imposed by stereocenters in both dendrimers. However, irradiation with r-CPL switches the CD signal in films of the generation dendrimer having 16 functional groups, G2-AZO-C4(S), but this does not occur in films of the higher generation dendrimer, G3-AZO-C4(S).