Recent Advances on Glyoxylates and Related Structures as Photoinitiators of Polymerization

: The design of photoinitiators activable under low-light intensity is an active research ﬁeld, supported by the recent energetic sobriety plans imposed by numerous countries in Europe. With an aim to simplify the composition of the photocurable resins, Type I photoinitiators are actively researched as these structures can act as monocomponent systems. In this ﬁeld, a family of structures has been under-investigated at present, namely, glyoxylates. Besides, the different works carried out in three years have evidenced that glyoxylates and related structures can be versatile for the design of Type I photoinitiators. In this review, an overview of the different glyoxylates and related structures reported to date is provided.


Introduction
During the past decade, photopolymerization has witnessed intense research efforts, supported by the development of more and more applications making use of photopolymerization, but also by the gradual abandonment of conventional UV curing using mercury lamps in favor of more energy-efficient LED-triggered polymerization processes [1]. Notably, the recent development of light-emitting diodes (LEDs) that are cheap, compact, lightweight, and energy-saving devices has discarded the historical UV irradiation setups that are expensive and energy-consuming devices [2][3][4][5][6][7][8][9]. Parallel to this, UV photopolymerization is facing numerous criticisms such as safety concerns (eye and skin damage) or the production of ozone during photopolymerization [10,11]. Intense efforts existing at present to develop photoinitiating systems absorbing visible light are also supported by the different applications using photopolymerization and 3D and 4D printing, dentistry, adhesives, solvent-free paints, microelectronics, coatings and varnishes can be cited as relevant examples [12][13][14][15][16][17][18][19][20][21][22][23][24][25].
Another point of interest concerns one of the most components of photocurable resins, namely the chromophore that interacts with light and can generate radicals in the presence of co-initiators and additives [10,11,26,27]. Indeed, photoinitiators can be divided into two different categories. The first one concerns Type II photoinitiators. In this case, photoinitiators are not capable to initiate a polymerization alone and additives have to be used. Notably, Type II photoinitiators are commonly combined with hydrogen/electron donors so that after a photoinduced electron transfer followed by a hydrogen abstraction reaction, initiating species can be generated. Parallel to this first mechanism, Type II photoinitiators can also be combined with onium salts (sulfonium or iodonium salts) so that aryl radicals can be formed after a photoinduced electron transfer. To render the system catalytic, a sacrificial amine can be used, enabling to introduction of the photosensitizer in a catalytic amount. Parallel to this first category, Type I photoinitiators can act as monocomponent systems, greatly simplifying the composition of the photocurable resins. The generation of initiating radicals is based on the homolytic cleavage of a selected bond (See Scheme 1). As the main drawback of this approach, the photodecomposition of Type I photoinitiators The reactivity of photoinitiators and the light penetration that can be achieved within the photocurable resin is also strongly related to the wavelength used for photoinitiation. Indeed, as shown in Figure 1, light penetration can vary from a few hundreds of micrometers up to a few centimeters, depending on the fact that photopolymerization is mostly carried out in the wavelength range between 350 nm and 800 nm [33]. By polymerizing at longer wavelengths, a higher light penetration can be obtained within the photocurable resin. Access to filled samples is also possible [34]. However, by polymerizing at long wavelengths, photons are also less energetic than UV photons so this issue can only be addressed by developing photoinitiating systems facilely producing initiating species in unfavorable low energetic conditions. This is the reason why after approximately two decades, a wide range of structures have been The reactivity of photoinitiators and the light penetration that can be achieved within the photocurable resin is also strongly related to the wavelength used for photoinitiation. Indeed, as shown in Figure 1, light penetration can vary from a few hundreds of micrometers up to a few centimeters, depending on the fact that photopolymerization is mostly carried out in the wavelength range between 350 nm and 800 nm [33]. By polymerizing at longer wavelengths, a higher light penetration can be obtained within the photocurable resin. Access to filled samples is also possible [34].
In this review, an overview of the different glyoxylates and related structures reported to date is provided. This family of dyes is of crucial interest for the future development of photoinitiators of photopolymerization.

Glyoxylate Derivatives
In 2021, a series of glyoxylate derivatives have been proposed by Sun and coworkers, bearing electron-donating or electron-accepting groups (See Figure 2) [229]. By means of this specific substitution, photopolymerization experiments could be carried out at 405 nm. In this series of dyes, dimethyl 1,4-dibenzoylformate (DM-BD-F) proved to be the most efficient photoinitiator during the free radical polymerization (FRP) of acrylates (tri (propylene glycol)diacrylate (TPGDA) or trimethylolpropane triacrylate (TMPTA)), resulting from its unique ability to produce twice more radicals than the nine other structures. To determine the real performance of the different glyoxylate derivatives, phenylbis (2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), and dibenzoyl (DB) were used as reference photoinitiators.
Due to the weak absorption of the different dyes at 405 nm, deep layer photocuring could also be obtained and a polymer thickness of 6.5 cm could be polymerized within 30 s. Parallel to this, due to the weak absorption of glyoxylate derivatives at 385, 395, and 405 nm, almost colorless coatings could be produced. From the absorption viewpoint, major differences could be found between the different dyes in acetonitrile (See Table 1 and Figure 3).
Interestingly, compared to the parent methyl benzoyl formate (MBF), all derivatives exhibited a redshifted absorption, except for TF-MBF exhibiting the strong electronwithdrawing group. Logically, the most redshifted absorptions were found for all dyes comprising an electron-donating group inducing an efficient intramolecular charge transfer (ICT) through a push-pull effect. Thus, N-MBF and S-MBF both exhibited the most redshifted absorptions located at 356 and 326 nm respectively, together with the highest molar extinction coefficients (43,800 M −1 ·cm −1 and 29,660 M −1 ·cm −1 respectively). Compared to BAPO, N-MBF exhibited higher molar extinction coefficients at all wavelengths later used for photopolymerization. Photolysis experiments revealed the occurrence of a de-carboxylation reaction using bromocresol green as the pH indicator. Consistent with the mechanism established in the literature, a decarboxylation reaction occurring subsequent to the photocleavage was proposed, as shown in Scheme 2. Due to the weak absorption of the different dyes at 405 nm, deep layer photocuring could also be obtained and a polymer thickness of 6.5 cm could be polymerized within 30 s. Parallel to this, due to the weak absorption of glyoxylate derivatives at 385, 395, and 405 nm, almost colorless coatings could be produced. From the absorption viewpoint, major differences could be found between the different dyes in acetonitrile (See Table 1   Reproduced with permission of Ref. [229]. Copyright 2021. The American Chemical Society. Due to the weak absorption of the different dyes at 405 nm, deep layer photocuring could also be obtained and a polymer thickness of 6.5 cm could be polymerized within 30 s. Parallel to this, due to the weak absorption of glyoxylate derivatives at 385, 395, and 405 nm, almost colorless coatings could be produced. From the absorption viewpoint, major differences could be found between the different dyes in acetonitrile (See Table 1   Reproduced with permission of Ref. [229]. Copyright 2021. The American Chemical Society.   Interestingly, compared to the parent methyl benzoyl formate (MBF), all derivatives exhibited a redshifted absorption, except for TF-MBF exhibiting the strong electron-withdrawing group. Logically, the most redshifted absorptions were found for all dyes comprising an electron-donating group inducing an efficient intramolecular charge transfer (ICT) through a push-pull effect. Thus, N-MBF and S-MBF both exhibited the most redshifted absorptions located at 356 and 326 nm respectively, together with the highest molar extinction coefficients (43,800 M −1 ·cm −1 and 29,660 M −1 ·cm −1 respectively). Compared to BAPO, N-MBF exhibited higher molar extinction coefficients at all wavelengths later used for photopolymerization. Photolysis experiments revealed the occurrence of a decarboxylation reaction using bromocresol green as the pH indicator. Consistent with the mechanism established in the literature, a decarboxylation reaction occurring subsequent to the photocleavage was proposed, as shown in Scheme 2.

Scheme 2.
Mechanism of radical generation with glyoxylates. Reproduced with permission of Ref. [229]. Copyright 2021. The American Chemical Society.
By theoretical calculations, the bond dissociation energy (BDE) of the different derivatives could be determined, and values ranging between 108.40 kJ/mol for TF-MBF and 150.94 kJ/mol for N-MBF were calculated (See Table 2). Parallel to this, the ΔH of all MBFs was determined as being negative, meaning that the cleavage reaction was energetically favorable [230]. By theoretical calculations, the bond dissociation energy (BDE) of the different derivatives could be determined, and values ranging between 108.40 kJ/mol for TF-MBF and 150.94 kJ/mol for N-MBF were calculated (See Table 2). Parallel to this, the ∆H of all MBFs was determined as being negative, meaning that the cleavage reaction was energetically favorable [230]. Examination of their photoinitiating abilities during the FRP of TPGDA revealed F-MBF to furnish a higher monomer conversion than BAPO (See Figure 4 and Table 3). Excellent monomer conversions could also be obtained with the other MBFs, except S-MBF and O-MBF for which conversions lower than 40% could be determined. The low reactivity of these derivatives was confirmed during the FRP of TMPTA. However, contrary to what was observed in TPGDA, none of the MBFs could outperform BAPO. Thus, if a TMPTA conversion of 59.4% could be obtained with BAPO, the best conversion with MBFs was obtained with O-MBF, peaking at 49.6%. The lower monomer conversion obtained with TMPTA compared to TPGDA was assigned to the higher viscosity of TMPTA and its trifunctionality. Examination of their photoinitiating abilities during the FRP of TPGDA revealed F-MBF to furnish a higher monomer conversion than BAPO (See Figure 4 and Table 3). Excellent monomer conversions could also be obtained with the other MBFs, except S-MBF and O-MBF for which conversions lower than 40% could be determined. The low reactivity of these derivatives was confirmed during the FRP of TMPTA. However, contrary to what was observed in TPGDA, none of the MBFs could outperform BAPO. Thus, if a TMPTA conversion of 59.4% could be obtained with BAPO, the best conversion with MBFs was obtained with O-MBF, peaking at 49.6%. The lower monomer conversion obtained with TMPTA compared to TPGDA was assigned to the higher viscosity of TMPTA and its trifunctionality. Noticeably, DM-BD-F could maintain an excellent monomer conversion with the two monomers, resulting from its unique ability to produce double radicals compared to the other MBFs. Overall, the following trend could be established: if the presence of electronaccepting groups could improve the monomer conversion, the opposite situation was found for the electron-donating groups. Indeed, in this series of dyes, N-MBF and S-MBF exhibiting the highest molar extinction coefficients also demonstrated the lowest photoinitiating abilities, evidencing that absorption was not the only parameter governing the photoreactivity. Determination of the enthalpy of the reaction revealed ΔH to be negative for all MBFs. Besides, if the cleavage reaction was determined as being energetically favorable, the photoinitiating capability is also strongly related to the values of ΔH. Thus, if DM-BD-F and TF-MBF exhibited ΔH values of −127.89 and −111.46 kJ/mol respectively, these values were only reduced to −68.87 and −72.57 kJ/mol for S-MBF and N-MBF respectively, explaining their lower photoinitiating abilities.   Noticeably, DM-BD-F could maintain an excellent monomer conversion with the two monomers, resulting from its unique ability to produce double radicals compared to the other MBFs. Overall, the following trend could be established: if the presence of electron-accepting groups could improve the monomer conversion, the opposite situation was found for the electron-donating groups. Indeed, in this series of dyes, N-MBF and S-MBF exhibiting the highest molar extinction coefficients also demonstrated the lowest photoinitiating abilities, evidencing that absorption was not the only parameter governing the photoreactivity. Determination of the enthalpy of the reaction revealed ∆H to be negative for all MBFs. Besides, if the cleavage reaction was determined as being energetically favorable, the photoinitiating capability is also strongly related to the values of ∆H. Thus, if DM-BD-F and TF-MBF exhibited ∆H values of −127.89 and −111.46 kJ/mol respectively, these values were only reduced to −68.87 and −72.57 kJ/mol for S-MBF and N-MBF respectively, explaining their lower photoinitiating abilities.
Finally, examination of the depth of cure for TPGDA after 30 s of irradiation at 405 nm with the different systems revealed F-MBF TF-MBF, and DM-BD-F to furnish a curing depth of 5.0, 6.3, and 6.5 cm respectively, greatly higher than that of BAPO (1.0 cm) (See Figure 5). Noticeably, good photobleaching could be obtained during photopolymerization so that colorless polymers could be obtained with all MBFs. Finally, examination of the depth of cure for TPGDA after 30 s of irradiation at 405 nm with the different systems revealed F-MBF TF-MBF, and DM-BD-F to furnish a curing depth of 5.0, 6.3, and 6.5 cm respectively, greatly higher than that of BAPO (1.0 cm) (See Figure 5). Noticeably, good photobleaching could be obtained during photopolymerization so that colorless polymers could be obtained with all MBFs.

Cinnamoyl Formate Derivatives
In 2022, the same group examined a new family of dyes derived from methyl benzoylformate (MBF), namely ethyl cinnamoyl formates (ECFs) [231]. Four structures were investigated, two of them bearing an electron-donating group (S-ECF and O-ECF) and one structure with an electron-accepting group (F-ECF) (See Scheme 3). The different dyes could be prepared by a two-step synthesis consisting first of a Claisen Schmidt condensation followed in the second step by an esterification reaction. F-ECF, S-ECF, and O-ECF could be prepared with reaction yields of 60, 55, and 59% for the two steps respectively. Examination of their absorption properties in acetonitrile revealed the shift of the absorptions to be comparable to that observed for the previous MBFs. Thus, the introduction of electron-donating groups redshifted the absorption (S-ECF and O-ECF) whereas the opposite effect was found in the presence of electron-accepting groups (F-ECF) (See

Cinnamoyl Formate Derivatives
In 2022, the same group examined a new family of dyes derived from methyl benzoylformate (MBF), namely ethyl cinnamoyl formates (ECFs) [231]. Four structures were investigated, two of them bearing an electron-donating group (S-ECF and O-ECF) and one structure with an electron-accepting group (F-ECF) (See Scheme 3). The different dyes could be prepared by a two-step synthesis consisting first of a Claisen Schmidt condensation followed in the second step by an esterification reaction. F-ECF, S-ECF, and O-ECF could be prepared with reaction yields of 60, 55, and 59% for the two steps respectively. Finally, examination of the depth of cure for TPGDA after 30 s of irradiation at 405 nm with the different systems revealed F-MBF TF-MBF, and DM-BD-F to furnish a curing depth of 5.0, 6.3, and 6.5 cm respectively, greatly higher than that of BAPO (1.0 cm) (See Figure 5). Noticeably, good photobleaching could be obtained during photopolymerization so that colorless polymers could be obtained with all MBFs.

Cinnamoyl Formate Derivatives
In 2022, the same group examined a new family of dyes derived from methyl benzoylformate (MBF), namely ethyl cinnamoyl formates (ECFs) [231]. Four structures were investigated, two of them bearing an electron-donating group (S-ECF and O-ECF) and one structure with an electron-accepting group (F-ECF) (See Scheme 3). The different dyes could be prepared by a two-step synthesis consisting first of a Claisen Schmidt condensation followed in the second step by an esterification reaction. F-ECF, S-ECF, and O-ECF could be prepared with reaction yields of 60, 55, and 59% for the two steps respectively.
Examination of their absorption properties in acetonitrile revealed the shift of the absorptions to be comparable to that observed for the previous MBFs. Thus, the introduction of electron-donating groups redshifted the absorption (S-ECF and O-ECF) whereas the opposite effect was found in the presence of electron-accepting groups (F-ECF) (See Figure 6 and Table 4). The most redshifted absorption was found for S-ECF, peaking at 362 nm. Irrespective of the substitution pattern, almost similar molar extinction coefficients could be found for the different dyes.
Macromol 2023, 3, FOR PEER REVIEW 8 Figure 6 and Table 4). The most redshifted absorption was found for S-ECF, peaking at 362 nm. Irrespective of the substitution pattern, almost similar molar extinction coefficients could be found for the different dyes.   Photolysis experiments carried out in acetonitrile revealed the different ECFs to be unable to generate radicals alone. Upon addition of ethyl dimethylaminobenzoate (EDB), a fast photolysis process could be evidenced and the formation of α-aminoalkyl radicals was confirmed by electron spin resonance (EPR) experiments. Overall, the mechanism of radical generation proposed in Scheme 4 was suggested. The initiation mechanism is that of a type II photoinitiator. Thus, upon photoexcitation, a photoinduced electron transfer between EDB and ECFs can occur, generating EDB radical cations and ECF radical anions. In the second step, a hydrogen abstraction reaction can occur, generating α-aminoalkyl radicals on EDB and constituting the initiating species. It has to be noticed that the different radicals formed during photolysis have been identified by electron spin resonance (ESR) experiments.   Photolysis experiments carried out in acetonitrile revealed the different ECFs to be unable to generate radicals alone. Upon addition of ethyl dimethylaminobenzoate (EDB), a fast photolysis process could be evidenced and the formation of α-aminoalkyl radicals was confirmed by electron spin resonance (EPR) experiments. Overall, the mechanism of radical generation proposed in Scheme 4 was suggested. The initiation mechanism is that of a type II photoinitiator. Thus, upon photoexcitation, a photoinduced electron transfer between EDB and ECFs can occur, generating EDB radical cations and ECF radical anions. In the second step, a hydrogen abstraction reaction can occur, generating α-aminoalkyl radicals on EDB and constituting the initiating species. It has to be noticed that the different radicals formed during photolysis have been identified by electron spin resonance (ESR) experiments.  Figure 7 and Table 5). After 240 s of irradiation, all ECFs could furnish monomer conversions comparable to that of ITX at 405 nm. Noticeably, no significant difference in monomer conversions could be observed between Polymerization tests carried out at 405 nm and 455 nm during the FRP of TPGDA revealed O-ECF to outperform the reference photoinitiator 2-isopropylthioxanthone (ITX) during the 20 first seconds of irradiation (See Figure 7 and Table 5). After 240 s of irradiation, all ECFs could furnish monomer conversions comparable to that of ITX at 405 nm. Noticeably, no significant difference in monomer conversions could be observed between ECFs substituted with electron-donating or electron-accepting groups. At 455 nm, a higher variation of the monomer conversion was found, attributable to differences in absorption at this specific wavelength. Scheme 4. Initiation mechanism evidenced for all ECFs. Reproduced with the permission of Ref. [231]. Copyright 2022, Elsevier.
Polymerization tests carried out at 405 nm and 455 nm during the FRP of TPGDA revealed O-ECF to outperform the reference photoinitiator 2-isopropylthioxanthone (ITX) during the 20 first seconds of irradiation (See Figure 7 and Table 5). After 240 s of irradiation, all ECFs could furnish monomer conversions comparable to that of ITX at 405 nm. Noticeably, no significant difference in monomer conversions could be observed between ECFs substituted with electron-donating or electron-accepting groups. At 455 nm, a higher variation of the monomer conversion was found, attributable to differences in absorption at this specific wavelength.  Here again, a good photobleaching of the resins could be evidenced, especially with S-ECF which is the dye exhibiting the most redshifted absorption of the series (See Figure  8). This result is remarkable considering that an opposite situation was found for ITX. Indeed, as shown in Figure 8, yellowing of the sample could be demonstrated after polymerization, despites the lack of color for the initial solution. By nuclear magnetic resonance (NMR), the authors demonstrated the photobleaching to originate from the suppression of the π-conjugated system, with the disappearance of the central double bond, therefore suppressing the electronic delocalization. Notably, the addition of EDB radicals on the central double bond was confirmed by mass spectrometry.  Here again, a good photobleaching of the resins could be evidenced, especially with S-ECF which is the dye exhibiting the most redshifted absorption of the series (See Figure 8). This result is remarkable considering that an opposite situation was found for ITX. Indeed, as shown in Figure 8, yellowing of the sample could be demonstrated after polymerization, despites the lack of color for the initial solution. By nuclear magnetic resonance (NMR), the authors demonstrated the photobleaching to originate from the suppression of the π-conjugated system, with the disappearance of the central double bond, therefore suppressing the electronic delocalization. Notably, the addition of EDB radicals on the central double bond was confirmed by mass spectrometry.
Considering the excellent photobleaching, the authors also investigated deep-layer polymerization. Using the two-component S-ECF/EDB system, a depth of cure of 7 cm could be determined upon irradiation at 455 nm for 20 min. A low extractability of 0.086% of S-ECF was determined, lower than that of ITX (0.97%). The low extractability of S-ECF is directly related to the photobleaching mechanism, demonstrating that EDB radicals could add on the cinnamoyl system, enabling covalently linking the photoinitiator to the polymer network. However, it could be as well any radicals on the growing polymer chain that can add to the cinnamoyl system, enabling covalently linking the photoinitiator to the polymer network. Low cytotoxicity was also determined for S-ECF. Notably, a cell viability of 98% could be determined for the samples prepared with 20 µg/mL of S-ECF. By increasing the photoinitiator content up to 20 µg/mL, the cell viability was only reduced to 90%, evidencing the good cytocompatibility of S-ECF. Considering the excellent photobleaching, the authors also investigated deep-layer polymerization. Using the two-component S-ECF/EDB system, a depth of cure of 7 cm could be determined upon irradiation at 455 nm for 20 min. A low extractability of 0.086% of S-ECF was determined, lower than that of ITX (0.97%). The low extractability of S-ECF is directly related to the photobleaching mechanism, demonstrating that EDB radicals could add on the cinnamoyl system, enabling covalently linking the photoinitiator to the polymer network. However, it could be as well any radicals on the growing polymer chain that can add to the cinnamoyl system, enabling covalently linking the photoinitiator to the polymer network. Low cytotoxicity was also determined for S-ECF. Notably, a cell viability of 98% could be determined for the samples prepared with 20 µg/mL of S-ECF. By increasing the photoinitiator content up to 20 µg/mL, the cell viability was only reduced to 90%, evidencing the good cytocompatibility of S-ECF.

Figure 8. Photobleaching experiments were carried out with the two-component S-ECF/EDB combination. (a) initial solution (b) sample before irradiation (c) after 8 s (d) after 15 s of irradiation.
Reproduced with the permission of Ref. [207]. Copyright 2007, Elsevier.
Considering the excellent photobleaching, the authors also investigated deep-layer polymerization. Using the two-component S-ECF/EDB system, a depth of cure of 7 cm could be determined upon irradiation at 455 nm for 20 min. A low extractability of 0.086% of S-ECF was determined, lower than that of ITX (0.97%). The low extractability of S-ECF is directly related to the photobleaching mechanism, demonstrating that EDB radicals could add on the cinnamoyl system, enabling covalently linking the photoinitiator to the polymer network. However, it could be as well any radicals on the growing polymer chain that can add to the cinnamoyl system, enabling covalently linking the photoinitiator to the polymer network. Low cytotoxicity was also determined for S-ECF. Notably, a cell viability of 98% could be determined for the samples prepared with 20 µg/mL of S-ECF. By increasing the photoinitiator content up to 20 µg/mL, the cell viability was only reduced to 90%, evidencing the good cytocompatibility of S-ECF.

Silyl Glyoxylates
In 2017, Lalevée and coworkers proposed a new family of glyoxylate, namely silyl glyoxylates (See Figure 9) [232,233]. Tert-butyl (tert-butyldimethylsilyl)glyoxylate (DKSi), ethyl(tert-butyldimethyl)silyl glyoxylate (Et-DKSi), and benzyl (tert-butyldimethyl)silyl glyoxylate (Bn-DKSi) were examined as monocomponent photoinitiating systems or in combination with additives (See Figure 10) for the FRP of a dental resin, namely a Bis-GMA/TEGDMA (70/30 w/w) blend (where BisGMA and TEGDMA stand for bisphenol Aglycidyl methacrylate and triethylene glycol dimethacrylate respectively) or urethane dimethacrylate (UDMA).  Examination of the absorption properties of DKSi in toluene revealed the absorption maximum to be located at 425 nm, therefore blueshifted compared to that of camphorquinone (CQ) (465 nm). Besides, compared to the previous MBF, a significant enhancement of the molar extinction coefficient could be evidenced at 405 nm (See Figure 11). By theoretical calculations, the redshift of the absorption maximum was determined as originating from a strong participation of the d orbital of the Si atom to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), decreasing the HOMO-LUMO gap compared to that of the previous MBF (4.19 eV for DKSi vs 4.71 eV for MBF). A good overlap between the emission of the LED emitting at 477 nm and DKSi and camphorquinone was thus found. Reproduced with permission of Ref. [232].
Examination of the absorption properties of DKSi in toluene revealed the absorption maximum to be located at 425 nm, therefore blueshifted compared to that of camphorquinone (CQ) (465 nm). Besides, compared to the previous MBF, a significant enhancement of the molar extinction coefficient could be evidenced at 405 nm (See Figure 11). By theoretical calculations, the redshift of the absorption maximum was determined as originating from a strong participation of the d orbital of the Si atom to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), decreasing the HOMO-LUMO gap compared to that of the previous MBF (4.19 eV for DKSi vs 4.71 eV for MBF). A good overlap between the emission of the LED emitting at 477 nm and DKSi and camphorquinone was thus found.  Reproduced with permission of Ref. [232].
Examination of the absorption properties of DKSi in toluene revealed the absorption maximum to be located at 425 nm, therefore blueshifted compared to that of camphorquinone (CQ) (465 nm). Besides, compared to the previous MBF, a significant enhancement of the molar extinction coefficient could be evidenced at 405 nm (See Figure 11). By theoretical calculations, the redshift of the absorption maximum was determined as originating from a strong participation of the d orbital of the Si atom to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), decreasing the HOMO-LUMO gap compared to that of the previous MBF (4.19 eV for DKSi vs 4.71 eV for MBF). A good overlap between the emission of the LED emitting at 477 nm and DKSi and camphorquinone was thus found. During the FRP of the BisGMA/TEGDMA blend, the ability of DKSi to act as a monocomponent system was demonstrated in laminate. After 80 s of irradiation, a conversion of 40% could be determined. Upon the addition of EDB, the conversion drastically increased and a conversion of 68% was obtained. Under air and due to strong oxygen inhibition in thin films, DKSi alone was almost unable to initiate the FRP of the resin. Conversely, the three-component DKSi/EDB/DPI (2/1.4/1.6% w/w/w) system could furnish a monomer conversion of 33%, which could be improved by using the four-component DKSi/EDB/DPI/CQ (2/1.4/1.6/1% w/w/w/w) system (38%) (See Figure 12). Improvement of the monomer conversion obtained with the four-component system compared to the three-component system can be assigned to improved light absorption properties due to the concomitant presence of DKSi and CQ, both contributing to light absorption.
Reproduced with permission of Ref. [232]. Copyright 2017. The American Chemical Society.
During the FRP of the BisGMA/TEGDMA blend, the ability of DKSi to act as a monocomponent system was demonstrated in laminate. After 80 s of irradiation, a conversion of 40% could be determined. Upon the addition of EDB, the conversion drastically increased and a conversion of 68% was obtained. Under air and due to strong oxygen inhibition in thin films, DKSi alone was almost unable to initiate the FRP of the resin. Conversely, the three-component DKSi/EDB/DPI (2/1.4/1.6% w/w/w) system could furnish a monomer conversion of 33%, which could be improved by using the four-component DKSi/EDB/DPI/CQ (2/1.4/1.6/1% w/w/w/w) system (38%) (See Figure 12). Improvement of the monomer conversion obtained with the four-component system compared to the three-component system can be assigned to improved light absorption properties due to the concomitant presence of DKSi and CQ, both contributing to light absorption. Investigation of the FRP of UDMA revealed the monomer conversion to increase with the photoinitiator content. Besides, by varying the content from 0.5 to 5 wt%, an optimum concentration at 2 wt% could be determined (See Figure 13). For dental applications, photobleaching is an important property. Good bleaching ability could be demonstrated with the two-component DKSi/EDB combination (see Investigation of the FRP of UDMA revealed the monomer conversion to increase with the photoinitiator content. Besides, by varying the content from 0.5 to 5 wt%, an optimum concentration at 2 wt% could be determined (See Figure 13). During the FRP of the BisGMA/TEGDMA blend, the ability of DKSi to act as a monocomponent system was demonstrated in laminate. After 80 s of irradiation, a conversion of 40% could be determined. Upon the addition of EDB, the conversion drastically increased and a conversion of 68% was obtained. Under air and due to strong oxygen inhibition in thin films, DKSi alone was almost unable to initiate the FRP of the resin. Conversely, the three-component DKSi/EDB/DPI (2/1.4/1.6% w/w/w) system could furnish a monomer conversion of 33%, which could be improved by using the four-component DKSi/EDB/DPI/CQ (2/1.4/1.6/1% w/w/w/w) system (38%) (See Figure 12). Improvement of the monomer conversion obtained with the four-component system compared to the three-component system can be assigned to improved light absorption properties due to the concomitant presence of DKSi and CQ, both contributing to light absorption. Investigation of the FRP of UDMA revealed the monomer conversion to increase with the photoinitiator content. Besides, by varying the content from 0.5 to 5 wt%, an optimum concentration at 2 wt% could be determined (See Figure 13). For dental applications, photobleaching is an important property. Good bleaching ability could be demonstrated with the two-component DKSi/EDB combination (see For dental applications, photobleaching is an important property. Good bleaching ability could be demonstrated with the two-component DKSi/EDB combination (see Figure 14). Interestingly, after nine months of storage, no modification of the color of the polymer film was detected for the samples prepared with the DKSI/EDB system. A different situation was found for the reference CQ/EDB combination for which a yellowing of the sample could be observed. Figure 14). Interestingly, after nine months of storage, no modification of the color of the polymer film was detected for the samples prepared with the DKSI/EDB system. A different situation was found for the reference CQ/EDB combination for which a yellowing of the sample could be observed. Figure 14. UDMA samples polymerized with different photoinitiating systems (just after irradiation with the LED at 455 nm and after 9 months of storage in the dark); DKSi/EDB (0.5/2% w/w) and CQ/EDB (0.5/2% w/w). Reproduced with the permission of Ref. [232]. Copyright 2017. The American Chemical Society.
Excellent monomer conversions could also be obtained using 4-diphenylphosphinobenzoic aid acid as the additive. The choice of this additive was notably motivated by its ability to efficiently overcome oxygen inhibition by converting the non-reactive peroxyl ROO• radicals as initiating species RO•. [234] Investigation of the substituent effects with Et-DKSi and Bn-DKSi revealed the absorption spectra not to be modified except for the molar extinction coefficients (see Figure 15). When tested as monocomponent systems for the FRP of UDMA (1 wt%), the order of monomer conversions perfectly fit with the order of the molar extinction coefficients, evidencing that the reactivity was governed by the molar extinction coefficients and not by the substitution pattern of silyl glyoxylates. By ESR, the formation of radicals in the close vicinity of Si was detected under an inert atmosphere (radical A). A different situation was found under air. No silyl radicals were detected anymore due to the fast reaction with oxygen, producing peroxyl radicals. Formation of t-BuOO• was also detected under air, resulting from a decarboxylation reaction of radical B and subsequent reaction with oxygen (See Scheme 5). EPR experiments enabled confirming the chemical structures of the radicals formed upon irradiation (See Scheme 5). In the case of radical B, the occurrence of a decarboxylation reaction was also demonstrated, enabling generating carbon-centered radicals. Figure 14. UDMA samples polymerized with different photoinitiating systems (just after irradiation with the LED at 455 nm and after 9 months of storage in the dark); DKSi/EDB (0.5/2% w/w) and CQ/EDB (0.5/2% w/w). Reproduced with the permission of Ref. [232]. Copyright 2017. The American Chemical Society.
Excellent monomer conversions could also be obtained using 4-diphenylphosphinobenzoic aid acid as the additive. The choice of this additive was notably motivated by its ability to efficiently overcome oxygen inhibition by converting the non-reactive peroxyl ROO• radicals as initiating species RO• [234]. Investigation of the substituent effects with Et-DKSi and Bn-DKSi revealed the absorption spectra not to be modified except for the molar extinction coefficients (see Figure 15). When tested as monocomponent systems for the FRP of UDMA (1 wt%), the order of monomer conversions perfectly fit with the order of the molar extinction coefficients, evidencing that the reactivity was governed by the molar extinction coefficients and not by the substitution pattern of silyl glyoxylates. By ESR, the formation of radicals in the close vicinity of Si was detected under an inert atmosphere (radical A). A different situation was found under air. No silyl radicals were detected anymore due to the fast reaction with oxygen, producing peroxyl radicals. Formation of t-BuOO• was also detected under air, resulting from a decarboxylation reaction of radical B and subsequent reaction with oxygen (See Scheme 5).
Macromol 2023, 3, FOR PEER REVIEW 13 Figure 14). Interestingly, after nine months of storage, no modification of the color of the polymer film was detected for the samples prepared with the DKSI/EDB system. A different situation was found for the reference CQ/EDB combination for which a yellowing of the sample could be observed. Excellent monomer conversions could also be obtained using 4-diphenylphosphinobenzoic aid acid as the additive. The choice of this additive was notably motivated by its ability to efficiently overcome oxygen inhibition by converting the non-reactive peroxyl ROO• radicals as initiating species RO•. [234] Investigation of the substituent effects with Et-DKSi and Bn-DKSi revealed the absorption spectra not to be modified except for the molar extinction coefficients (see Figure 15). When tested as monocomponent systems for the FRP of UDMA (1 wt%), the order of monomer conversions perfectly fit with the order of the molar extinction coefficients, evidencing that the reactivity was governed by the molar extinction coefficients and not by the substitution pattern of silyl glyoxylates. By ESR, the formation of radicals in the close vicinity of Si was detected under an inert atmosphere (radical A). A different situation was found under air. No silyl radicals were detected anymore due to the fast reaction with oxygen, producing peroxyl radicals. Formation of t-BuOO• was also detected under air, resulting from a decarboxylation reaction of radical B and subsequent reaction with oxygen (See Scheme 5). EPR experiments enabled confirming the chemical structures of the radicals formed upon irradiation (See Scheme 5). In the case of radical B, the occurrence of a decarboxylation reaction was also demonstrated, enabling generating carbon-centered radicals.

Water-Soluble Benzoylformic Acid Derivatives
The water solubility of photoinitiators is a property that is actively researched with the aim of developing greener polymerization processes [57,96,225,[235][236][237][238][239]. Indeed, polymerization in water becomes possible. This point was examined with a series of benzoylformic acid derivatives by the group of Sun and coworkers (See Figure 16) [240]. EPR experiments enabled confirming the chemical structures of the radicals formed upon irradiation (See Scheme 5). In the case of radical B, the occurrence of a decarboxylation reaction was also demonstrated, enabling generating carbon-centered radicals.

Water-Soluble Benzoylformic Acid Derivatives
The water solubility of photoinitiators is a property that is actively researched with the aim of developing greener polymerization processes [57,96,225,[235][236][237][238][239]. Indeed, polymerization in water becomes possible. This point was examined with a series of benzoylformic acid derivatives by the group of Sun and coworkers (See Figure 16) [240].

Scheme 5.
Radicals formed upon photocleavage of silyl glyoxylates. Reproduced with the permission of Ref. [232]. Copyright 2017. The American Chemical Society.

Water-Soluble Benzoylformic Acid Derivatives
The water solubility of photoinitiators is a property that is actively researched with the aim of developing greener polymerization processes [57,96,225,[235][236][237][238][239]. Indeed, polymerization in water becomes possible. This point was examined with a series of benzoylformic acid derivatives by the group of Sun and coworkers (See Figure 16) [240]. From the synthetic viewpoint, TF-BFA and CC-TFA could be prepared in one step, by oxidation of the acetyl groups with selenium oxide, and obtained reaction yields of 80 and 62%, respectively. As observed for MBFs and ECFs, the presence of the electron-accepting CF3 group blueshifted the absorption compared with the parent structure BFA (244 nm for TF-CFA vs. 253 nm for BFA). Conversely, a redshift of the absorption was found for CC-BFA at 262 nm. Interestingly, a significant increase of the molar extinction coefficient was found, peaking at 18,480 M −1 ·cm −1 contrarily to 8640 M −1 ·cm −1 for BFA and TF-BFA (See Figure 17). From the synthetic viewpoint, TF-BFA and CC-TFA could be prepared in one step, by oxidation of the acetyl groups with selenium oxide, and obtained reaction yields of 80 and 62%, respectively. As observed for MBFs and ECFs, the presence of the electron-accepting CF 3 group blueshifted the absorption compared with the parent structure BFA (244 nm for TF-CFA vs. 253 nm for BFA). Conversely, a redshift of the absorption was found for CC-BFA at 262 nm. Interestingly, a significant increase of the molar extinction coefficient was found, peaking at 18,480 M −1 ·cm −1 contrarily to 8640 M −1 ·cm −1 for BFA and TF-BFA (See Figure 17). By theoretical calculations, the BDE of the different dyes could be determined and values of 154.8, 158.6, and 151.9 kJ/mol could be determined for BFA, TF-BFA, and CC-BFA, evidencing that the BDE was only slightly modified by the substitution pattern of benzoylformic acids. Polymerization experiments done at 405 nm for TPGDA and TMPTA revealed CC-TFA to outperform BFA and TF-BFA during the FRP of TPGDA. A conversion of 83.4% could be obtained after 120 s contrarily to 64.6 and 66.6% for BFA and TF-BFA (See Figure 18). This is directly related to the ability of CC-TFA to produce twice more radicals. Noticeably, during the FRP of TMPTA, similar conversions could be obtained with the three derivatives (around 53%) and this result was assigned to the higher viscosity of TMPTA and the trifunctional character of the monomer speeding up the gelation process and adversely the double bond conversion. However, these monomer conversions remain lower than those previously obtained with DM-BD-F, with conversions of 79.1 and 46.8 being respectively obtained during the FRP of TPGDA and TMPTA. By theoretical calculations, the BDE of the different dyes could be determined and values of 154.8, 158.6, and 151.9 kJ/mol could be determined for BFA, TF-BFA, and CC-BFA, evidencing that the BDE was only slightly modified by the substitution pattern of benzoylformic acids. Polymerization experiments done at 405 nm for TPGDA and TMPTA revealed CC-TFA to outperform BFA and TF-BFA during the FRP of TPGDA. A conversion of 83.4% could be obtained after 120 s contrarily to 64.6 and 66.6% for BFA and TF-BFA (See Figure 18). This is directly related to the ability of CC-TFA to produce twice more radicals. Noticeably, during the FRP of TMPTA, similar conversions could be obtained with the three derivatives (around 53%) and this result was assigned to the higher viscosity of TMPTA and the trifunctional character of the monomer speeding up the gelation process and adversely the double bond conversion. However, these monomer conversions remain lower than those previously obtained with DM-BD-F, with conversions of 79.1 and 46.8 being respectively obtained during the FRP of TPGDA and TMPTA. A similar trend was determined during the FRP of a water-soluble monomer, namely PEG diacrylate (PEGDA). Upon irradiation at 405 nm and by performing the polymerization experiments in water, a conversion of ca. 80% could be obtained within 180 s (see Figure 19). Besides, a slower polymerization rate could be evidenced for CC-BFA, resulting from its poor water solubility. A similar trend was determined during the FRP of a water-soluble monomer, namely PEG diacrylate (PEGDA). Upon irradiation at 405 nm and by performing the polymerization experiments in water, a conversion of ca. 80% could be obtained within 180 s (see Figure 19). Besides, a slower polymerization rate could be evidenced for CC-BFA, resulting from its poor water solubility. Water solubility tests revealed the water solubility of BFA and TF-BFA to be between 10 and 5 wt%. Conversely, this value was reduced to only 0.5 wt% for CC-TFA, despites the present of two carboxylic acid groups. This counter-intuitive result was assigned to the absence of dipole moment in CC-TFA, affecting its solubility in high polar media. Finally, an investigation of the curing depth in PEGDA revealed BFA and TF-BFA to give a Figure 19. Photopolymerization profiles of PEGDA in laminate using BFAs (1 × 10 −4 mol/g monomer) upon irradiation at 405 nm with a LED. Reproduced with the permission of Ref. [240]. Copyright 2022. Elsevier.
Water solubility tests revealed the water solubility of BFA and TF-BFA to be between 10 and 5 wt%. Conversely, this value was reduced to only 0.5 wt% for CC-TFA, despites the present of two carboxylic acid groups. This counter-intuitive result was assigned to the absence of dipole moment in CC-TFA, affecting its solubility in high polar media. Finally, an investigation of the curing depth in PEGDA revealed BFA and TF-BFA to give a similar curing depth (6.3 cm and 6.7 cm respectively). This value is higher than that obtained with BAPO (only 1 cm). Additionally, colorless polymers could be obtained, which is highly worthwhile for future applications of these structures.

Cytotoxicity of Glyoxylates
If polymerization efficiency is an important parameter governing the choice of photoinitiators, their toxicity is another major issue as it drastically impacts the scope of applications of polymers. Indeed, for biomedical applications or food packaging, the use of photoinitiators exhibiting low toxicity is required. This point was examined with a series of seven benchmark photoinitiators including methyl benzoylformate (MBF) (see Figure 20) [241]. Water solubility tests revealed the water solubility of BFA and TF-BFA to be between 10 and 5 wt%. Conversely, this value was reduced to only 0.5 wt% for CC-TFA, despites the present of two carboxylic acid groups. This counter-intuitive result was assigned to the absence of dipole moment in CC-TFA, affecting its solubility in high polar media. Finally, an investigation of the curing depth in PEGDA revealed BFA and TF-BFA to give a similar curing depth (6.3 cm and 6.7 cm respectively). This value is higher than that obtained with BAPO (only 1 cm). Additionally, colorless polymers could be obtained, which is highly worthwhile for future applications of these structures.

Cytotoxicity of Glyoxylates
If polymerization efficiency is an important parameter governing the choice of photoinitiators, their toxicity is another major issue as it drastically impacts the scope of applications of polymers. Indeed, for biomedical applications or food packaging, the use of photoinitiators exhibiting low toxicity is required. This point was examined with a series of seven benchmark photoinitiators including methyl benzoylformate (MBF) (see Figure  20) [241]. Cytotoxicity tests carried out on four different tissue types of cells at concentrations ranging between 1 and 50 µM revealed phenylbis(acyl)phosphine oxide (BAPO), 2-benzyl-2-(dimethylamino)-4 -morpholinobutyrophenone (369), 4,4 -bis(diethylamino)benzophenone (EMK), diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (TPO), and 2-isopropylthioxanthone (ITX) to be more toxic than ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate (TPOL) and methyl benzoylformate (MBF). In this series of photoinitiators, the most toxic structure was identified as BAPO, which is extensively used in industry. In the case of TPOL and MBF, the less toxic structure was identified as being TPOL. These different results can help for future developments of new photoinitiators in light of the low cytotoxicity of MBF.

Conclusions
To conclude, glyoxylates and related structures have only been scarcely investigated in the literature. The different results obtained with these structures are promising. As the first point, low cytotoxicity should be highlighted, which constitutes a clear advantage for future applications of polymers. Water-soluble dyes could also be prepared, enabling the polymerization in water. Furthermore, to keep a good solubility in water, the molecule should exhibit a dipole moment to facilitate its dissolution. Glyoxylates and related structures can also operate as mono-component systems, greatly simplifying the composition of the photocurable resins. Excellent depths of cure and colorless coatings could also be obtained, evidencing the interest in these structures. At present, absorption of these structures remains strongly UV-centered. Future works will certainly consist of redshifting their absorption towards the visible range to further improve the depth of cure as well as the polymerization kinetics.

Conflicts of Interest:
The authors declare no conflict of interest.