Synthesis And Optical Properties Of Azobenzene-substituted Polydiacetylenes

The diacetylene (DA) derivatives have been intensely investigated due to their topochemical polymerization, the ability to form well-defined nano/ microstructures as well as their subsequent elegant properties and promising applications in nonlinear optical materials, photoresists, thermal, mechanical, chemical, and biosensors. The adjustable molecular structure of diacetylenic monomers allows molecular architecture controlling of the supramolecular assembly, resulting in modification of morphology, stacking structure, and physical–chemical properties of polydiacetylene (PDA).Three accessible azobenzene mesogen-substituted diacetylene monomers (DA1, DA2, and DA3) were synthesized and the effect of the molecular structure on the photopolymerization behavior and self-assembled structures of the resulting azobenzene-substituted polydiacetylene (PDA) films had been investigated in detail. The films derived from DA1 were substantially polymerized into blue phase. However, the films derived from DA2 and DA3 were polymerized into red phase directly. The morphology, packing structure, and thermal stability of the resulting azobenzen -substituted PDA films were characterized in detail by SEM, XRD as well as POM measurements. PDA1 formed micrometer-sized lamellar crystal structures, while PDA2 and PDA3 both formed spherulitic crystal structures. The strongπ–πstacking interaction among side chains was essential for maintaining high stability upon thermal stimulus.Langmuir-Blodgett (LB) films of three azobenzene-substituted diacetylene monomers (DA1, DA2 and DA3) were fabricated and their optical and chiroptical properties were investigated in detail by ultraviolet-visible (UV-vis) spectra and circular dichroism (CD) spectra. Achiral DA1 molecules could form chiral LB films through overcrowdedpacking of the azobenzene moiety, while achiral DA2 and DA3 molecules not. When exposure to left- or right-handed circular polarized UV light (CPUL), striking left- or right-handed (opposite) CD signals for azobenzene chromophores and polydiacetylene chains were observed for the polymerized DA1 (PDA1) and DA2 (PDA2) LB films. However, DA3 LB films could hardly be polymerized in this case, and only striking opposite CD signals for azobenzene chromophores could be observed. It was demonstrated that the intermolecular steric hindrance and irregular arrangement of azobenzene chromophores were not favorable for the topopolymerization and chirality formation of polydiacetylenes backbone. Further, the effects of thermal treatment on the supramolecular chirality of above three LB films were studied. Strong collective noncovalent interactions (π-πstacking) were believed to be responsible for the thermal stability of chiral supramolecular assemblies.