Photochemical Oxidation of NIR Photosensitizers in the Presence of Radical Initiators and Their Prospective Use in Dental Applications

Abstract

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.

Comparison between NIR and UV-Sensitized Radical and Cationic Reactivity of Iodonium Salts Comprising Anions with Different Coordination Behavior

“…9. The fast decomposition of this species forms also the boundary conditions to reduce the efficiency of electron back transfer in NIR systems as previously shown for Sens1 [21].…”

“…The differences obtained for the iodonium salts were not so significant in comparison with the NIR system. From this point of view, the NIR system needs to exhibit a lower yield because the photoproducts formed possess a higher nucleophilicity and possess therefore a high affinity to add the cations formed [8,21]. This explains the lower yield on cations formed.…”

Advances of Near Infrared Sensitized Radical and Cationic Photopolymerization: from Graphic Industry to Traditional Coatings

“…Highly light sensitive materials are needed for such procedures because the time frame to expose one pixel covers only several s/pixel. Nowadays, some feasibility studies successfully showed the working principle of NIR initiated photopolymerization for coatings [3][4][5][6][7]. The majority of radiation curing relies on the well established UV-curing [8].…”

“…One reason can be seen in insufficient solubility of the initiator components Sens and RI in the high viscous resins. Many NIR absorbers based on cyanines and radical initiators based on iodonium salts possess an insufficient solubility in common industrial coatings [3][4][5][6]. There is a need for materials exhibiting an improved solubility in such surroundings either.…”

“…This reacts in a consecutive step to photoproducts and protons. Further details about the reaction mechanism can be found elsewhere [5,6,12].…”