(23−25) This photosensitization can proceed in two ways: using charge-transfer complexes or through aromatic photosensitizers. One of them is the utilization of photosensitizers, compounds that absorb in the near-UV or visible-light region and can efficiently decompose iodonium salts with superacid generation. (15) However, there are more successful approaches to iodonium salt photosensitization. There have been attempts to improve spectral properties by adding single-electron-rich substituents in position 4 of aryl rings (such as alkyl or methoxy groups), but it red shifts the absorption only slightly (10–20 nm in most cases). (22) This raises the need to improve the absorption properties of the initiator to make them active at longer wavelengths. (20) In addition, new applications, especially three-dimensional (3D) printing, require the absorption of visible light. (21) No efficient light sources emit in this region of the electromagnetic spectrum. Currently, commercially available salts do not absorb above 300 nm. However, diaryliodonium salts show poor absorption properties at longer wavelengths. However, the use of asymmetrical salts is desirable when substrates for their synthesis are expensive, but in more cases, symmetrical salts are preferred. (4) In the enolate reaction, more electron-deficient aryl moieties can be selectively transferred, and in cross-coupling reactions, the electron-rich ring is preferably transferred. The difference in structure leads to unique behavior during the course of the reaction. (14) They can be used as symmetrical salts with both aryl substituents with the same structure or as asymmetrical salts where the structure of each aryl substituent is different, especially as one ring has an electron-rich substituent and another has an electron-deficient substituent. (12) Despite the arylation reaction, they can be used in the dearomatization of phenols (13) and as benzyne precursors. (3) They found applications in reactions such as α-arylation of carbonyl compounds (7,8) and arylation of arenes, (9) alkynes, (10) and alkenes (11) in the presence of metallic catalysts as well as arylation of heteroatom nucleophiles. In organic synthesis, they are most often used as arylation agents. The research described represents a significant advance in the development of iodonium salts. In addition, it was possible to study in detail substituent effects on the properties of the new iodonium salts due to the easily modifiable structure of the benzylidene chromophore. They can be used in advanced applications such as three-dimensional (3D) VAT printing using an LCD printer. Thus, they can act as one-component photoinitiators even in visible light, where common diaryliodonium salts remain inactive. New symmetric iodonium derivatives effectively photoinitiate cationic polymerization using light-emitting diodes (LEDs) with no additives. Such an improvement ensures excellent absorbing properties in the UV-A and visible range, representing a significant advance over the commonly used diaryliodonium salts. This is the first example of the use of a double bond in the structure of a symmetric iodonium salt. We present a group of seven new advanced symmetric iodonium salts bearing two benzylidene-based chromophores.
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