Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes

Abstract

Iodonium salts are well established photoacid generators, cationic photoinitiators, as well as additives commonly used in photoredox catalytic cycles. However, as a strong limitation, iodonium salts are characterized by low light absorption properties for λ > 300 nm so that these latter cannot be activated with cheap, safe, and eco-friendly near UV or even visible light emitting diodes (LEDs). To overcome this drawback, the covalent linkage of an iodonium salt to a chromophore absorbing at longer wavelength is actively researched. With aim at red-shifting the absorption spectrum of the iodonium salt, the synthesis of new compounds combining within a unique chemical structure both the chromophore (here the naphthalimide scaffold) and the iodonium salt is presented. By mean of this strategy, a polymerization could be initiated at 365 nm with the modified iodonium salts whereas no polymerization could be induced with the benchmark iodonium salt i.e., Speedcure 938 at this specific wavelength. To examine the effect of the counter-anion on the photoinitiating ability of these different salts, five different counter-anions were used. Comparison between the different anions revealed the bis(trifluoromethane)sulfonimide salt to exhibit the best photoinitiating ability in both the free radical polymerization of acrylates and the cationic polymerization of epoxides. To support the experimental results, molecular orbital calculations have been carried out. By theoretical calculations, the initiating species resulting from the photocleavage of the iodonium salts could be determined. The cleavage selectivity and the photochemical reactivity of the new iodoniums are also discussed.

Recent advances on ferrocene-based photoinitiating systems

“…Over the years, a wide range of families have been designed and synthesized as efficient photoinitiators of polymerization, activable under visible light and even under low light intensity. Among these structures, naphthalimides, [40,[52][53][54][55][56][57][58][59][60][61][62] copper complexes, [63][64][65][66][67][68][69][70][71][72][73] flavones, [20,74] iron complexes, [75][76][77][78][79] iodonium salts, [80][81][82] coumarins, [83][84][85][86][87][88][89], acridine-1,8dione, [90][91] iridium complexes, [92][93][94][95][96][97]…”

Leaving Groups in Metal‐Free Arylations: Make Your Choice!

“…Diaryliodonium (III) salts, in which the iodine atom adopts a T-shaped form with two bound aryl moieties and a ligand (in most cases a sulfonate anion), found application in organic synthesis as versatile arylating agents and benzyne precursors [40] as well as photoacid generators (PAGs) in cationic polymerizations. [41] Such air-and moisture-stable compounds are smoothly prepared from unfunctionalized aromatics and hypervalent iodine reagents or from simple iodoarenes in combination with appropriate oxidants. [42] Ackermann and co-workers described the thermal activation (100°C in DMF) of diaryl-Scheme 9.…”

Newly Synthesized Chromophore‐linked Iodonium Salts as Photoinitiators of Free Radical Photopolymerization

“…In addition, it was pointed out that the replacement with naphthalene or a biphenyl group was not efficient in drastically red‐shifting the absorption. In contrast, a replacement with naphthalimide chromophore could shift the light absorption to near‐UV region, [ 25 ] and the modified iodonium salt could initiate polymerization at 365 nm. In another research, both chromophore (erythrosine B) and an electron donor (ethyl 4‐dimethylaminobenzoate) were linked to the iodonium salts, and the modified iodonium salts were able to initiate polymerization at 540 nm.…”

“…[20] In the polymer field, increasing photo-reactivity of iodonium salts became a major concern; researchers covalently linked chromophores to iodonium salts whereby allowing the modified products to work as mono-component photoinitiators under near UV or even visible light irradiation. [21][22][23][24][25][26] Instead of diaryl, it has been reported that one of the phenyl groups was replaced by naphthalene [24] to expand its π-conjugation system. However, the absorption is still limited to 300 nm.…”