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.

Cyanines comprising either a benzo[e]‐ or benzo[c,d]indolium core facilitate initiation of radical photopolymerization combined with high power NIR‐LED prototypes emitting at 805 nm, 860 nm, or 870 nm, while different oxime esters function as radical coinitiators. Radical photopolymerization followed an initiation mechanism based on the participation of excited states, requiring additional thermal energy to overcome an existing intrinsic activation barrier. Heat released by nonradiative deactivation of the sensitizer favored the system, even under conditions where a thermally activated photoinduced electron transfer controls the reaction protocol. The heat generated internally by the NIR sensitizer promotes generation of the initiating reactive radicals. Sensitizers with a barbiturate group at the meso‐position preferred to bleach directly, while sensitizers carrying a cyclopentene moiety unexpectedly initiated the photosensitized mechanism.

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.

Upconversion nanoparticles (UCNPs) sensitised photolytic cleavage of the blue/UV photoinitiator bis(4‐methoxybenzoyl) diethyl germanium (1). 1 initiated radical photopolymerisation in combination with NaYF4:TmYb@NaYF4 UCNPs. A λ=974 nm near‐infrared (NIR) laser served as the excitation source to generate blue and UV light. In addition, a metal‐free photo‐ATRP system (ATRP=atom transfer radical polymerisation), composed of iso‐propyl thioxanthone (ITX), N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (PMDETA) and α‐bromo(iso‐butyl) ethylester (α‐BrBuEt) was used instead of 1. This resulted in a significant smaller polydispersity compared to the UCNP/1 system. Control of polymerization was successfully demonstrated by means of a chain extension experiment showing control of chain termination. For the first time, we demonstrated photolytic formation of acidic cations in such NIR UCNP systems composed of an ITX/iodonium photoinitiator for the sensitized generation of acidic cations. In addition, 1 also resulted in crosslinking of 1,6‐hexanediol diacrylate (HDDA) in combination with UCNPs and NIR laser excitation. UV filter materials such as TiO2 do not significantly interfere with crosslinking.

NIR light represents an interesting alternative for initiating both radical and cationic polymerization. It also offers the opportunity to provide non-radiative heat generated by the NIR sensitizer, resulting in initiation of physical processes, such as melting of powder coatings or decreasing the viscosity of liquids, which is needed to improve the spreading on certain substrates. This was concluded from the photophysical data of the NIR absorbers/sensitizers (cyanines). Generation of initiation species such as radicals and acidic cations follows both a phototonic and a thermally activated route, indicating the hybrid function of such photoinitiator systems. Iodonium salts with low coordinating anions function well as co-initiators/acceptors. Alternatively, upconverting nanoparticles (UCNPs) and upconversion phosphors represent an interesting alternative since these materials possess the capability to convert absorbed NIR laser light either into visible or UV light, resulting in excitation of the respective photoinitiator systems. Computer-to-plate (CtP) technology applying digital imaging of lithographic plates has mostly benefited from development of NIR-sensitized photopolymerization. Pioneering work was additionally pursued to cure both radical- and cationic-polymerizable multifunctional monomers. Formation of acidic cations was probed with Rhodamine B lactone. The curing of powder coatings applying NIR lasers instead of ovens can be seen as a breakthrough in the entire field of coatings. In addition, NIR photopolymerization can be applied for tailor-made synthesis of copolymers exhibiting a small polydispersity following the route of photo ATRP.