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.

Photochemical oxidation of near infrared (NIR) photosensitizers in the presence of diaryl iodonium salts bearing either bis(trifluoromethylsulfonyl)imide or hexafluorophosphate was investigated by exposure with NIR LEDs emitting either at 790 nm, 830 nm, 850 nm or 870 nm. Four different cyanines with barbituryl group at the meso position exhibit similar absorption in the NIR. These photosensitizers initiate in combination with diaryliodonium salts radical photopolymerization of dental composites with the focus to cure large thicknesses. Furthermore, the mixture comprising the cyanine and the iodonium salt was used to generate brown color in dental composites on demand. This required to understand the mechanism of dye decomposition in more detail applying exposure kinetics and a coupling of Ultra Performance Liquid Chromatography (UPLC) with mass spectrometry (MS) to analyze the photoproducts formed. Data showed cleavage of the polymethine chain at typical positions in case of the oxidized species. These were formed as result of electron transfer between the excited state of the photosensitizer and the iodonium salt. UPLC‐MS experiments additionally indicated a certain sensitivity of the system upon adding of acids and radicals generated by thermal treatment of azobisisobutyronitrile (AIBN). Thus, treatment of the photoinitiator composition led almost to the same products no matter the system was either exposed with NIR light or treated with acids or radicals generated by thermal decomposition of AIBN. These findings helped to understand the large curing depth of 14 mm upon NIR exposure at 850 nm and the brown color formed.

Photo-initiated cross-linking of multifunctional acrylic esters in polymeric binders was investigated based on digital imaging using the Computer-to-Plate (CtP) technology applying laser exposure in the near-infrared (NIR). Generation of initiating radicals occurs by electron transfer from the excited state of the NIR-sensitizer to the radical generator, an onium salt. Iodonium salts derived from several borates and those with the bis(trifluoromethylsulfonyl)imide anion resulted in lithographic materials with high sensitivity. Photo-induced electron transfer plays a major function to generate initiating radicals by a sensitized mechanism but thermal events also influence sensitivity of the coating. Internal conversion was the major deactivation pathway while a certain fraction of NIR-dye fluorescence was also available. A line shape focused laser system with emission in the NIR was successfully used to bake the materials.

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Photoinitiated Polymerization of TPGDA was initiated with a NIR LED emitting at 790 nm using a redox based photoinitiator system comprising a cyanine as photosensitizer and a diaryl iodonium salt.