Polymerization profiles obtained during the FRP of TMPTA using IQBr, CQ and Irgacure 651 as the dyes and CN-NPG as the electron donor using a dental lamp as the light source.
In 2023, Jędrzejewska and coworkers revisited IQH in the context of a comparative study between indenono- and indoloquinoxaline derivatives IN1-IN5, IQH, IN7 and IN8 (See
Figure 10)
[219][220]. The different dyes were used as electron acceptors for (phenylthio)acetic acid (PTAA)
[221], and the resulting photoredox pairs were used as photo-initiating systems for dental applications. The mechanism of radical generation is presented in Scheme 2.
Figure 10. Chemical structures of indenono- and indoloquinoxaline derivatives.
Scheme 2. Mechanism of photoinitiation with dye/phenylthioacetic acid system.
From the absorption viewpoint, all dyes IN1-IN5, IQH, IN7 and IN8 showed an intense absorption band centered in the near UV range. Only IN8 exhibited an absorption peak located in the visible range, peaking at 417 nm and attributable to the presence of the phenanthrene moiety extending the aromaticity of this dye. In fact, IN4 and IN8 exhibited the most red-shifted absorption for the two series of dyes, namely the indenono- and indoloquinoxaline series. This redshift was beneficial for the FRP experiments. Indeed, when paired with PTAA, the highest polymerization rates were obtained with these two dyes due to a better match between the emission of the dental lamp emitting at 400 nm and the absorption maxima of these chromophores. In fact, the photo-initiating abilities of these photoredox pairs were comparable to that of camphorquinone (CQ), a benchmark photoinitiator commonly used in dentistry. Interestingly, in the context of dental fillings, an increase of the temperature lower than 5 °C was evidenced, which did not exceed the temperature tolerance threshold for the tooth pulp.
The same performances were also obtained with another series of photoredox pairs based on acenaphthoquinoxalines and 2-mercaptobenzoxazole (MBX) used as the co-initiator (See
Figure 11)
[222]. In this series of dyes (QNX5, AN1, AN3-AN-8) and during the FRP of TMPTA, the lowest heat increase was obtained with the AN6/MBX combination. In addition, the temperature increase with the other dyes remained in the tolerance threshold for dental applications. Due to their strong absorption located in the UV range, colorless dental fillings could be obtained with these different acenaphthoquinoxalines, making these dyes suitable candidates for dental applications.
Figure 11. Chemical structures of various acenaphthoquinoxalines AN1, AN3-AN8 and 2-mercaptobenzoxazole used as the hydrogen donor.
While NPG was extensively used as an electron donor for quinoxaline derivatives, other compounds were also examined, as exemplified with
N-methyldiethanolamine (MDEA) that was used as an electron donor for UV photopolymerization experiments (See
Figure 12)
[223]. 2,3-Diphenylquinoxaline (QNX-1), quinoxaline (QNX-3), 2,3-dimethylquinoxaline (QNX-4) and acenaphthoquinoxaline (QNX-5) were tested as UV photoinitiators.
Figure 12. Quinoxalines investigated in combination with MDEA as the electron donor.
Interestingly, by using a polychromatic light, very fast polymerization processes were evidenced since a full curing of TMPTA was obtained within 4 s of irradiation (See
Figure 13). The highest final monomer conversion was obtained for QNX-1, with a conversion of 60% after 4 s. Noticeably, the highest monomer conversions were obtained for the most polyaromatic structures (QNX-1, QNX-5), certainly attributable to higher molar extinction coefficients in the visible range. The TMPTA conversion could be monitored by Real-Time Fourier Transform Infrared (RT-FTIR) spectroscopy, giving direct access to the monomer conversion. To conduct this, modification of the IR peak at ca 6120 cm
−1 was monitored, corresponding to a characteristic peak of TMPTA. The polymerization profiles were established from the difference between the initial peak area before irradiation and the peak area after irradiation for a given time t
[224][225].
Figure 13. Polymerization profiles obtained during the FRP of TMPTA under air and using the two-component dyes/MDEA (1%/10% w/w).
This trend of reactivity was confirmed during the FRP of other monomers, such as a 75% P-3038 epoxyacrylate (EA) and 25% tripropyleneglycol diacrylate (TPGDA) (3/1
v/
v) blend (See
Figure 14). However, by elongating the irradiation time to 180 s, QNX-5 furnished a similar monomer conversion to QNX-1. Clearly, the difference in monomer conversions is directly related to differences in polymerization rates at early irradiation time.
Figure 14. Polymerization profiles obtained during the FRP of an EA/TPGDA (3/1) blend under air and using the two-component dyes/MDEA (1%/10% w/w).
In the case of 5,12-dihydroquinoxalino[2,3
b]quinoxalines (i.e., fluoflavins), these dyes were interesting candidates to induce the reductive decomposition of alkoxypyridinium salts (See
Figure 15)
[226]. The fluoflavin dyes/alkoxypyridinium salts combinations proved to be excellent two-component systems to initiate the FRP of TMPTA under visible light. It has to be noticed that previously to this work, cyanines
[227], ketocoumarins
[228] and acridinedione dyes
[229] were also used as sensitizers capable of inducing the decomposition of alkoxypyridinium salts by photoinduced electron transfer. From the mechanistic viewpoint, upon excitation of the dye, a photoinduced electron transfer from fluoflavins towards the alkoxypyridinium salt can occur, generating an alkoxypyridinium radical that immediately decomposes, generating initiating alkoxy radicals (RO•) (See Scheme 3).
Figure 15. Chemical structures of 5,12-dihydroquinoxalino[2,3b]quinoxalines and alkoxypyridinium salts.
Scheme 3. Mechanism of photoinduced decomposition of alkoxypyridinium salts by electron transfer.
Examination of their photo-initiating abilities during the FRP of TMPTA upon irradiation with a visible light revealed QNX-7 and QNX-8 furnished the best polymerization rates (See
Figure 16). More precisely, QNX-7 was determined as outperforming all the other dyes, irrespective of the alkoxypyridinium salt. Experiments carried out with Py2 also furnish higher monomer conversion than Py1, once again evidencing the necessity to screen both the electron donors and acceptors in order to obtain the best combination. Interestingly, an efficient photobleaching of the TMPTA resins was observed during polymerization, assigned to the addition of alkoxy radicals on fluoflavins, according to the mechanism depicted in Scheme 4. Precisely, a reaction between the fluoflavin radical cation and the ethoxyl radicals at the 9-position of the phenyl ring was proposed by analogy to previous works performed on anthracene
[230][231]. After proton release, an ethoxy derivative of the fluoflavin dyes can be formed, enabling the ability to obtain an efficient bleaching of the final polymers.
Figure 16. Photopolymerization profiles of TMPTA recorded for QNX-6 (▪); QNX-7 (O); QNX-8 (Δ); QNX-9 (•); QNX-10 (□) and (A) Py1 or (B) Py2.
Scheme 4. Mechanism of photobleaching occurring with the fluoflavin dyes/alkoxypyridinium salts combinations.
The possibility to initiate free-radical/cationic hybrid polymerizations (concomitant polymerization of TMPTA and 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate (EPOX)) with fluoflavin derivatives was also examined. Using QNX-6-QNX-8 as the photosensitizers for triarylsulfonium hexafluoroantimonates, epoxide conversions higher than that of acrylates were observed with all photo-initiating systems
[232]. The lower acrylate conversion determined during hybrid photopolymerization was assigned to oxygen inhibition favoring the cationic polymerization of epoxides. Indeed, contrarily to the cationic polymerization, which is insensitive to oxygen inhibition, free radicals can react with molecular oxygen, generating unreactive peroxyl radicals. In 2009, the same authors examined a series of dyes analogues to fluoflavins, namely QNX-11-QNX-19, that were used as electron donors for the reductive photosensitization of
N-methoxy-4-phenylpyridinium tetrafluoroborate (Py1) and
N-ethoxy-2-methylpyridinium hexafluorophosphate (Py2) examined in the previous work and for diphenyliodonium hexafluorophosphate (Ph
2I
+PF
6−) (See
Figure 17)
[233]. These photoinitiators were advantageously used for the FRP of acrylates under visible light and the cationic polymerization (CP) of cyclohexene oxide (CHO).
Figure 17. Chemical structures of QNX-11-QNX-19 examined as photosensitizers for iodonium salts.