Azobenzenes are among the most extensively used molecular switches for many different applications. The need to tailor them to the required task often requires further functionalization. Cross-coupling reactions are ideally suited for late-stage modifications. This review provides an overview of recent developments in the modification of azobenzene and its derivatives by cross-coupling reactions.1 Introduction2 Azobenzenes as Formally Electrophilic Components2.1 Palladium Catalysis2.2 Nickel Catalysis2.3 Copper Catalysis2.4 Cobalt Catalysis3 Azobenzenes as Formally Nucleophilic Components3.1 Palladium Catalysis3.2 Copper Catalysis3.3 C–H Activation Reactions4 Azobenzenes as Ligands in Catalysts5 Diazocines5.1 Synthesis5.2 Cross-Coupling Reactions6 Conclusion
Stimuli-responsive polymers processed into microfibers bear the potential to be fabricated as systems detecting stress-induced failures. Microfiber processing of spiropyran (SP)-functionalized polymer poly(methyl acrylate) (PMA), in pristine form and as a blend with poly(methyl methacrylate) (PMMA), was studied. Material inhomogeneities such as bead formation and fiber–fiber fusion affected the durability of the microfibers which were aimed to be applied as self-reporting materials. The present study investigates the effect of polymer blending to improve the structural stability, i.e., the shape of the fibers compared to the nonblended PMA fibers (NBF). Blended microfibers (BF) with <4 wt % PMMA could overcome the structural malfunctions observed in NBF. Rheological analyses confirmed that there were no drastic alterations in the mechanical properties due to blending with a low amount of PMMA. We report the mechanoactivation in the structurally stable and durable BF processed via centrifugal force spinning (CFS). These fibers detected high strain deformations and retained the mechanochromic response after unloading. The color change in the BF confirms strain-induced isomerization of SP which will further assist in the visual inspection of damage before failure. The microfibers also showed reversible photochromism upon irradiation with ultraviolet (UV) light (360–370 nm) and green light (520–530 nm). The outstanding benefit of processing microfibers from polymer blends is the ease of obtaining structurally stable, self-reporting material with longer durability.
Azobenzenes are important molecular switches that can still be difficult to functionalize selectively. A high yielding Pd-catalyzed cross-coupling method under mild conditions for the introduction of NHS esters to azobenzenes and diazocines has been established. Yields were consistently high with very few exceptions. The NHS functionalized azobenzenes react with primary amines quantitatively. These amines are ubiquitous in biological systems and in material science.
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