Stimulus-Responsive Azobenzene Supramolecules: Fibers, Gels, and Hollow Spheres

Novel,
stimulus-responsive supramolecular structures in the form
of fibers, gels, and spheres, derived from an azobenzene-containing
benzenetricarboxamide derivative, are described. Self-assembly of
tris­(4-((E)-phenyldiazenyl)­phenyl)­benzene-1,3,5-tricarboxamide
(Azo-1) in aqueous organic solvent systems results in
solvent dependent generation of microfibers (aq DMSO), gels (aq DMF),
and hollow spheres (aq THF). The results of a single crystal X-ray
diffraction analysis of Azo-1 (crystallized from a mixture
of DMSO and H2O) reveal that it possesses supramolecular
columnar packing along the b axis. Data obtained
from FTIR analysis and density functional theory (DFT) calculation
suggest that multiple hydrogen bonding modes exist in the Azo-1 fibers. UV irradiation of the microfibers, formed in aq DMSO, causes
complete melting while regeneration of new fibers occurs upon visible
light irradiation. In addition to this photoinduced and reversible
phase transition, the Azo-1 supramolecules display a
reversible, fiber-to-sphere morphological transition upon exposure
to pure DMSO or aq THF. The role played by amide hydrogen bonds in
the morphological changes occurring in Azo-1 is demonstrated
by the behavior of the analogous, ester-containing tris­(4-((E)-phenyldiazenyl)­phenyl)­benzene-1,3,5-tricarboxylate (Azo-2) and by the hydrogen abstraction in the presence of
fluoride anions.