Organic Electronics: An El Dorado in the Quest of New Photocatalysts for Polymerization Reactions

Abstract

Photoinitiated polymerization has been the subject of continued research efforts due to the numerous applications in which this polymerization technique is involved (coatings, inks, adhesives, optoelectronic, laser imaging, stereolithography, nanotechnology, etc.). More recently, photopolymerization has received renewed interest due to the emergence of 3D-printing technologies. However, despite current academic and industrial interest in photopolymerization methodologies, a major limitation lies in the slow rates of photopolymerization. The development of new photoinitiating systems aimed at addressing this limitation is an active area of research. Photopolymerization occurs through the exposure of a curable formulation to light, generating radical and/or cationic species to initiate polymerization. At present, photopolymerization is facing numerous challenges related to safety, economic and ecological concerns. Furthermore, practical considerations such as the curing depth and the competition for light absorption between the chromophores and other species in the formulation are key parameters drastically affecting the photopolymerization process. To address these issues, photoinitiating systems operating under low intensity visible light irradiation, in the absence of solvents are highly sought after. In this context, the use of photoredox catalysis can be highly advantageous; that is, photoredox catalysts can provide high reactivities with low catalyst loading, permitting access to high performance photoinitiating systems. However, to act as efficient photoredox catalysts, specific criteria have to be fulfilled. A strong absorption over the visible range, an ability to easily oxidize or reduce as well as sufficient photochemical stability are basic prerequisites to make these molecules desirable candidates for photoredox catalysis. Considering the similarity of requirements between organic electronics and photopolymerization, numerous materials initially designed for applications in organic electronics have been revisited in the context of photopolymerization. Organic electronics is a branch of electronics and materials science focusing on the development of semiconductors devoted to three main research fields; organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells (OSCs). The contribution of organic electronics to the field of electronics is important as it paves the way toward cheaper, lighter, and more energy efficient devices. In the present context of photopolymerization, materials that were investigated as photocatalysts were indifferently organic semiconductors used for transistors, charge-transport materials, and light-emitting materials used in electroluminescent devices or conjugated polymers and small molecule dyes for solar cells. In this Account, we summarize our latest developments in elaborating on photocatalytic systems based on these new classes of compounds. Through an in-depth understanding of the parameters governing their reactivities and ou...

Structure‐Catalytic Activity in a Series of Push‐Pull Dicyanopyrazine/Dicyanoimidazole Photoredox Catalysts

“…They possess unique properties such as distinct color, dipolar arrangement, (non)linear optical properties, donor/acceptor centered HOMO/LUMO, and tunable HOMO‐LUMO gap. Beside organic electronics, they seem to be also well‐suited for photoredox catalysis . In this respect, several synthetic photoredox catalysts such as dicarbonitriles (e. g. benzene/naphthalene/anthracene dicarbonitriles), ketones (benzophenone, Michler's ketone, fluorenone or xanthone), quinones (e. g. DDQ, chloranil or anthraquinone), flavins, and heteroaromatic salts (e. g. acridinium, quinolinium or pyrylium) were developed to date.…”

Cationic Polymerization: From Photoinitiation to Photocontrol

“…Many multicomponent photoinitiating systems based on av ariety of photoactive and radical species have been developed. Recently,L alevØe, Fouassier,a nd co-workers [23,[32][33][34][35][36][37][38][39] pioneered the use of photoredox catalysis to promote cationic polymerizations in which the photosensitizer is regenerated and can therefore be used at very low loading. In this vein, an elegant three-component system comprising ap hotocatalyst, as ilane co-initiator,a nd diphenyliodonium was designed for cationic polymerization (Figure 6).…”

“…Numerous transition-metal or organic photocatalysts have demonstrated competencyi nt his photoinitiating system ( Figure 7), and this wealth of options has fostered the emergence of systems with specific photophysical and chemical properties. [36] Specifically,s ome ruthenium [32] and iridium [33] complexes have high molar extinction coefficients in the visible region that allow activation with inexpensive fluorescent light bulbs.Additionally, N-vinylcarbazole can be substituted for silanes as the co-initiator. [37] Thed evelopment of many ingenious multicomponent photoinitiating systems offers safer,greener,and more energy efficient alternatives to the early photoinitiated cationic Angewandte Chemie Minireviews polymerizations.Moreover,because both radical and cationic intermediates are generated during these processes,r adical and cationic polymerization can be performed concurrently in one vessel.…”

“…Adjusting the CTA-to-monomer ratio predictively tuned M n while maintaining low catalyst loading. Several other vinyl ethers (33)(34)(35)(36)w ere successfully polymerized using CTA 32 (Figure 16 c), which allowed for higher rates of polymerization while broadening the chain-length distribution only slightly. Theh igh chain-end fidelity of this chain growth process was further substantiated by chain-extending poly(ethyl vinyl ether) with 9 to yield ap oly(EVE-b-IBVE) diblock polymer ( Figure 17).…”

Kationische Polymerisation: von der Photoinitiierung zur Steuerung durch Licht

“…Viele aus mehreren Komponenten bestehende Photoinitiierungssysteme, die auf unterschiedlichen photoaktiven und radikalischen Spezies basieren, wurden entwickelt. In letzter Zeit bahnten Lalevée, Fouassier und Mitarbeiter den Weg für die Anwendung der Photoredoxkatalyse, um kationische Polymerisationen zu realisieren, in denen der Photosensibilisator zurückgewonnen wird und daher in einer sehr geringen Menge eingesetzt werden kann. In dieser Art wurde ein elegantes Dreikomponentensystem aus einem Photokatalysator, einem Silan als Coinitiator und Diphenyliodonium für die kationische Polymerisation entwickelt (Abbildung ).…”