Cyanines derived from heptamethines were investigated in combination with iodonium salts as initiators of the radical polymerization of tripropylene glycol diacrylate and epoxides derived from bisphenol‐A‐diglycidylether. A new near‐infrared (NIR) LED prototype emitting at 805 nm with an exposure intensity of 1.2 W cm−2 facilitated initiation of both radical and cationic polymerization using sensitizers derived from cyanines. This new light‐emitting device has brought new insight into the photochemistry of cyanines with the general structure 1 because a combination of photonic and thermal processes strongly influences reaction pathways. In particular, cationic cyanines comprising a cyclopentene moiety and diphenylamino group in the center initiated the cationic polymerization of epoxides. Selective oxidation of this unit explains why specifically these derivatives may function as initiators for cationic polymerization. In contrast, when the diphenylamino group was replaced by a barbital group at the meso‐position cationic polymerization of epoxides was not initiated.

Reactivity of new iodonium salts [A-I-B] + X À was studied with near infrared (NIR) initiated radical polymerization by photo-DSC using the polymethine dye S1 (5- (6-(2-(3-ethyl-1,1-dimethyl-1H-benzo[e] indol-2(3H)-ylidene)ethylidene)-2-(2-(3-ethyl-1,1-dimethyl-1H-benzo[e]indol-3-ium-2-yl)vinyl)cyclohex-1-en-1-yl)-1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydropyr-imidin-4-olate) as sensitizer. The iodonium salt [A-I-B] + X À functioned as a radical initiator bearing a different substitution pattern for the cation and the anion, respectively. Electron transfer of the excited state of S1 to [A-I-B] + X À (X À : benzilate, lactate, NO 3, (SO 2 -CF 3 ) 2 N À ) results in initiating radicals. The reactivity of S1/[A-I-B] + X À correlated with the conductivity of the salt in acrylate monomers such as hexane-1,6-diol diacrylate, tripropylene glycol diacrylate, poly(ethylene glycol) diacrylate and trimethylolpropane triacrylate. A high conductivity related always to a better reactivity in the monomer chosen. The solubility of [A-I-B] + X À determined ranged between several g L À1 up to well mixable systems (>2000 g L À1 ). Particular the bis(trifluoromethylsulfonyl) imide anion (N(SO 2 -CF 3 ) 2 À ) resulted in giant solubilities depending on the [A-I-B] + cation. A high solubility did not always lead to a high reactivity. Furthermore, iodonium salts comprising the bis(trifluoromethylsulfonyl) imide anion exhibited a lower cytotoxicity compared to those with the tetraphenyl borate anion as determined by the MTT-test using CHO-9-cells.

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.

The recombination of photolytically generated lophyl radicals has been investigated by UV/Vis spectroscopy in 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides (NTf2) in comparison with 1-butyl-3-methylimidazolium NTf2 , dimethyl sulfoxide, and triacetin. The 1-alkyl-1-methylpyrrolidinium-based ionic liquids contain an alkyl substituent varying between butyl and decyl groups. Optically pure ionic liquids are used in these studies. Temperature-dependent investigation of lophyl radical recombination shows an increase in the radical recombination rate with increasing temperature in each solvent, which is caused by decreasing viscosity with increasing temperature. Furthermore, the viscosity of the 1-alkyl-1-methylpyrrolidinium NTf2 increases nearly linearly within the row of these ionic liquids. In contrast, the recombination of the photolytically generated lophyl radicals is significantly faster in the ionic liquids than in the traditional organic solvents under investigation. Moreover, the recombination rate increases with the length of the alkyl chain bound at the cation of the ionic liquid at a given temperature. This may be caused by an increase in the extent of lophyl radical recombination within the solvent cage. Solvent cage effects dominate in the case of lophyl radical recombination in ionic liquids bearing a long alkyl chain or if the temperature is near the melting temperature of the ionic liquid. The positive value of the activation entropy supports this hypothesis. The results obtained are important for discussion of bimolecular radical reactions in ionic liquids.

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