Recent Advances in Photoredox Catalysts

Photoredox catalysis constitutes a flourishing and fascinating field of organic chemistry, enabling the efficient construction of a variety of non-traditional bonds [1]. By developing a series of new activation modes based on single electron transfers, innovative bond-forming protocols could be proposed, and these could be utilized in research fields such as medicinal chemistry, drug discovery or organic electronics [2,3,4,5,6,7,8,9,10]. The conversion of light into chemical energy enables us to activate small molecules and generate reactive species that can be used for various bond formation processes. By developing catalytic systems, photocatalyst contents could be drastically reduced, addressing the toxicity issue [11,12,13]. Although photoredox catalysis was initially developed for organic chemistry, its scope of applications has been extended to other research fields requiring low contents of catalysts to be used. Additionally, photopolymerization has notably benefited from these major advancements [14,15]. This Special Issue includes six articles in total, composed of four research articles and two reviews. The contribution of these different works is the following.

In their contributions, Bonardi et al. investigated a series of organocatalysts based on the 1,8-naphthalimide scaffold [16]. Without impacting the light absorption properties of photocatalysts, a series of co-initiators were covalently linked to the naphthalimide scaffold via an aliphatic spacer. The influence of these co-initiators on the polymerization profiles could be determined, with all dyes comprising the same chromophore. The free radical polymerization of acrylates could be achieved using three-component photoinitiating systems including an iodonium salt and a phosphine, upon irradiation at 405 nm and 470 nm with LEDs of low light intensity (LED@405 nm: I = 110 mW/cm²; LED@470 nm: I = 80 mW/cm²). In this strategy, the incorporation of phosphine in photoinitiating systems was crucial, enabling researchers to address the oxygen inhibition issue by converting the unreactive peroxyl radicals to alkoxyl radicals [17]. By optimizing the composition of the three-component systems, a photoinitiator content as low as 0.1 wt% could be used.

On their side, Tomal and al. designed and synthesized a series of biphenyl and terphenyl derivatives (2-amino-4-methyl-6-phenyl-benzene-1,3-dicarbonitrile derivatives) that could be used as photoredox catalysts upon excitation in the UV range, at 365 nm (I = 7.97 mW/cm²) but also at 405 nm (I = 11.89 mW/cm²) [18]. These two-component photoinitiating systems could be advantageously used for 3D printing experiments and this application was clearly the outcome of this research. The versatility of these systems was demonstrated by allowing access to the cationic polymerization of epoxides, the free radical polymerization of acrylates, the synthesis of interpenetrated polymer networks resulting from the concomitant polymerization of epoxides and acrylates, and finally thiol-ene polymerization. Noticeably, efficient photoinitiating systems could be prepared by combining these chromophores with amines or onium salts, evidencing that these photoredox catalysts could indifferently be used in photo-oxidative or photo-reductive mechanisms. The most performant bimolecular photoinitiating systems based on an iodonium salt enabled to use a photoinitiator content as low as 0.1 wt%.

In the same spirit, Fiedor et al. developed a series of photoredox catalysts analogous to the previous ones but based on the 2-amino-4,6-diphenylpyridine-3-carbonitrile scaffold [19]. Here, again, these structures could generate initiating species by mean of oxidative and reductive pathways. Moreover, the most efficient bimolecular systems could be obtained via the combination of 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives with an iodonium salt. Similar to the previous work conducted by Tomal et al. for which a special focus was devoted to the development of photoinitiating systems designed for 3D printing, photoinitiating systems based on 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives were all optimized for 3D printing applications [20]. Indeed, most of the commercial photosensitive resins that are commercially available are still based on free radical polymerization [21]. Only a few resins are based on the cationic polymerization of epoxides due to the lack of reactivity of the photoinitiating systems and slower polymerization kinetics [22]. This issue can be addressed by better fitting the absorption spectrum of the photosensitizer to that of the light source used in 3D printers but also by optimizing the concentration of the photosensitizer, allowing us to tune the reactivity of the photoinitiating systems as well as the depth of the cure. After optimization of the polymerization conditions, high resolution objects can be designed. In their contribution, the series of 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives showed absorption maxima in the 349–365 nm range, with absorption spectra tailing up to 450 nm. A photosensitizer concentration as low as 0.1 wt% could be used with the two-component dye/iodonium salt and dye/amine systems.

Thioxanthone is a historical photoinitiator that was studied at the early stage of photopolymerization [23,24,25,26,27,28]. In as early as 1967, Davidson and Lambeth proposed a generation of initiating radicals via the photoreduction of thioxanthone with aliphatic tertiary amines [29]. The key step of radical generation is directly related to electron transfer occurring from the ground state of the amine to the triplet excited state of the ketone, followed by rapid proton transfer, generating an aromatic ketyl radical and an α-aminoalkyl radical [30,31]. If the reactivity of these bimolecular systems cannot be denied, a lack of absorption of thioxanthone in the visible range can be mentioned. This point was notably addressed in the contribution of Hola et al. who designed a series of thioxanthone/carbazole conjugates displaying a significant absorption up to 450 nm [32]. By extending the π-conjugation of thioxanthones with carbazoles, a red shift of the absorption spectra could be obtained, compared to the parent thioxanthone. Photopolymerization experiments could thus be performed at 405 and 420 nm and the performance of two-component and three-component photoinitiating systems, including amines, iodonium or sulphonium salts, or alkyl halides, was examined for the free radical polymerization of acrylates and the cationic polymerization of epoxides. Here, the outcome of this research was dedicated to the development of photosensitive resins for 3D printing.

Although organocatalysts are popular structures for the design of photoinitiating systems due to reduced toxicity and the advantage of organic compounds compared to metal complexes, since 2014, a great deal of effort has been devoted to the development of metal-based photocatalysts with low toxicity. The contribution of Lalevée/Dumur and coworkers is relevant in this field [33]. Metal complexes are highly efficient visible light photocatalysts, and the proof of concept was given at the beginning of the 2000s with the different iridium complexes examined for this purpose [34,35,36,37]. However, the scarcity of iridium on Earth, its high cost, and its averred toxicity have rapidly discounted iridium complexes from being used as photoredox catalysis. Conversely, copper (I) complexes are cheaper and less toxic than iridium complexes while maintaining long-living excited states. Thus, copper complexes are promising alternatives to iridium complexes as photocatalysts of polymerization. In their review, Noirbent and coworkers reported that all copper complexes examined to date are photoinitiators of polymerization [38]. Interestingly, a series of copper complexes could be prepared using mechanochemistry, providing photocatalysts with safe synthetic conditions [39]. Numerous copper complexes could also be used as sunlight-activable photocatalysts [40], enabling researchers to develop green polymerization processes.

Recently, a family of photocatalysts has been extended to inorganic structures, and, in this field, quantum dots (QDs) are unique structures. QDs are notably characterized by their excellent photochemical and thermal stability, high molar extinction coefficients, and tunable optical properties [41,42,43,44,45,46]. In their review, Wang et al. summarized recent advances in QDs as photocatalysts for various organic transformations [47].

To conclude, this compilation of articles and reviews highlights the recent advances in photoredox catalysis with the development of organic organometallic or inorganic catalysts devoted to organic chemistry or photopolymerization.