Langmuir−Blodgett Films of Photochromic Polyglutamates. 9. Relation between Photochemical Modification and Thermotropic Properties

Abstract

Thermotropic polyglutamates with photochromic azobenzene side groups are polymers of the “hairy rod” type. They form LB multilayer assemblies, in which the rod-like macromolecules are oriented in the dipping direction. The initial LB films have a well-defined bilayer structure in which the rod-like azobenzene moieties are tilted toward the α-helical backbones and form H-aggregates. The change of aggregation and orientational order of the azobenzene chromophores is studied for polyglutamates with different spacer lengths upon irradiation and annealing. The lamellar order and in-plane anisotropy of the chromophores is irreversibly lost on UV irradiation. New ordered structures with a more symmetric distribution of the side chains around the main chain helix, a modified spacing, changed aggregation, and a different in-plane anisotropy are established after subsequent vis irradiation or annealing. The established supramolecular order strongly depends on the spacer length. A shorter spacer (C2) between the backbone and the photochromic moiety causes an isotropic in-plane distribution of the chromophores, whereas in the case of longer spacers (C4 or C6) an arrangement of the chromophores parallel to the main chain helices is restored. The change in the molecular arrangement of the side groups is controlled by the ordering tendencies and the dynamic properties of the combined liquid crystalline main and side chain polymers.

Dynamic and photochemical behavior of amorphous comb-like copolymers with photochromic azobenzene side groups

“…This is in contrast to the behavior of other polymers 24,28,29) . During the UV irradiation (365 nm) the azobenzene moiety undergoes the E/Z photoisomerization process as shown in Fig.…”

Athermal photofluidization of glasses

“…The corner for thermal 2D XY orientational relaxation of comparably anisotropic but untethered molecules in freely suspended smectic C liquid crystal films is t th B10 À 11 s (Supplementary Note 4) [28][29][30][31][32][33] , suggesting, along with Fig. 1c-e, that two principal factors produce such slow orientational dynamics: (i) The high in-plane packing density, nearly that of the neat solid dye, promotes the tendency for the azo cores to associate by dipole and VDW interaction of their nearly planar chromophore cores, leading to strong in-plane structural correlations 34 that greatly enhance the energetic barriers preventing molecules from moving past one another. (ii) Local (a) dMR molecular structure as synthesized and bonded to the glass, showing the VDW surface of the azo chromophore core, the molecular dipole moment d (pink), and the azo core long axis and chromophore transition moment t (blue).…”

Synthesis of New Epoxy Liquid-Crystalline Monomers with Azo Groups in the Central Mesogenic Core. Crosslinking with Amines

“…[7] However, the alignment depends on the enthalpic stability of the mesophase and on the spacer length because both of these factors influence the mobility of the mesogens at different temperatures. [8] Side-chain liquid-crystalline azo polymers have one main drawback: when the spacer is long enough to allow mobility, the glass transition temperature decreases and the rate of relaxation at room temperature increases. So, the higher the T g of the polymer, the greater the stability of the reorientation.…”