Neue Hochleistungs‐LEDs ermöglichen Photochemie für die Nahinfrarot‐sensibilisierte radikalische und kationische Photopolymerisation

Abstract

Cyaninderivate von Heptamethinen wurden mit Iodoniumsalzen auf ihre Fähigkeit zur Initiierung der radikalischen und kationischen Polymerisation von Tripropylenglykoldiacrylat und Epoxiden des Bisphenol‐A‐diglycidylether untersucht. Ein neuer Nahinfrarot(NIR)‐LED‐Prototyp, der bei 805 nm mit einer Strahlungsintensität von 1.2 W cm−2 emittiert, ermöglicht die Initiierung beider Polymerisationsarten. Dies liefert neue Einblicke in die Photochemie von Cyaninverbindungen des Strukturtyps 1, da die Kombination von photonischen und thermischen Prozessen den Reaktionsweg maßgeblich beeinflusst. Zum ersten Mal wird die Initiierung der kationischen Photopolymerisation von Epoxiden mit solchen NIR‐Absorbern beschrieben, wobei insbesondere Cyanine mit Cyclopentenyl‐ und Diphenylaminogruppen im Zentrum effizient sind. Die selektive Oxidation dieser Verbindung erklärt, weshalb speziell diese Derivate die kationische Polymerisation initiieren. Bei Austausch der Diphenylaminogruppe durch eine Barbituratgruppe in meso‐Position erfolgt hingegen keine Initiierung der kationischen Polymerisation.

Rational Selection of Cyanines to Generate Conjugate Acid and Free Radicals for Photopolymerization upon Exposure at 860 nm

“…The cation of 5 served as oxidizing substrate exhibiting E r ed of −0.69 V [10] . Solvent effects, internal reaction coordinates, [60] and internal activation barriers [16, 38, 60] are not included here. Nevertheless, results obtained demonstrate huge differences regarding the reactivity.…”

“…In recent years, fundamental progress has been made in this field particularly to dry coatings [14–18, 20, 22, 23, 32] . This can physically occur to remove water as solvent resulting in formation of a solidified film [14, 32] .…”

“…Cyanines [1–8] comprising a heptamethine chain have received increasing attention for industrial applications based on digital recording, [9] imaging, [10–13] physical drying, [14] chemical drying, [5, 6, 15–28] laser marking for plastics, [29–32] or laser welding [33–36] . These cyanines work well as absorbers in combination with NIR lasers in industrial applications [5, 6, 13, 18] requesting the necessity that absorption of the cyanine and emission of the NIR‐source complement each other.…”

“…This can physically occur to remove water as solvent resulting in formation of a solidified film [14, 32] . In addition, chemical drying results in crosslinked films applying either liquid monomers [15–18, 20, 22, 23] or reactive powders [15, 17, 19, 21] . Application of the latter belongs to green technologies as well.…”

“…Application of the latter belongs to green technologies as well. Consequently, NIR‐lasers with line‐shaped focus [18] or recently introduced high‐power NIR‐LED devices [16, 32, 37, 38] have moved into the focus of the aforementioned applications. In addition, chemical drying based on activated photoinduced electron transfer ( PET ) mostly results in colored products, [5, 22, 37, 38] which can be beneficial to design materials with readout function at certain wavelengths.…”