Acenaphthoquinoxaline Derivatives as Dental Photoinitiators of Acrylates Polymerization

Abstract

A series of dyes based on the acenaphthoquinoxaline skeleton was synthesized. Their structure was modified by introducing electron-withdrawing and electron-donating groups, increasing the number of conjugated double bonds and the number and position of nitrogen atoms, as well as the arrangement of aromatic rings (linear or angular). The dyes were investigated as a component in the photoinitiating systems of radical polymerization for a potential application in dentistry. They acted as the primary absorber of visible light and the acceptor of an electron, which was generated from a second component being an electron donor. Thus, the radicals were generated by the photoinduced intermolecular electron transfer (PET) process. Electron donors used differed in the type of heteroatom, i.e., O, S and N and the number and position of methoxy substituents. To test the ability to initiate the polymerization reaction by photoinduced hydrogen atom transfer, we used 2-mercaptobenzoxazole as a co-initiator. The effectiveness of the photoinitiating systems clearly depends on both the modified acenaphthoquinocaline structure and the type of co-initiator. The lower amount of heat released during the chain reaction and the polymerization rate comparable to this achieved for the photoinitiator traditionally used in dentistry (camphorquinone) indicates that the studied dyes may be valuable in this field.

Recent Advances on Quinoxaline-Based Photoinitiators of Polymerization

“…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 coinitiator (See Figure 11) [223]. 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.…”

“…Due to their strong absorption located in the UV range (See Table 4), colorless dental fillings could be obtained with these different acenaphthoquinoxalines, making these dyes suitable candidates for dental applications. 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) [223]. 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.…”

“…Acenaphthoquinoxalines AN1-AN8 proved to be suitable candidates in combination with 2-mercaptobenzoxazole as the co-initiator (See Figure 11) [223]. Among the most interesting finding, the heat released during photopolymerization was comparable to that observed for photoinitiators traditionally used in dentistry, such as camphorquinone.…”

Highly Efficient Photoinitiation Systems Based on Dibenzo[a,c]phenazine Sensitivity to Visible Light for Dentistry

“…This is related to the type of reactive species formed in secondary reactions following the electron transfer process and their activity in initiating the polymerization reaction. [34,82,84]. However, these radicals may undergo secondary reactions producing various products.…”

“…Based on data in the literature [1] and our previous works [34,37], in which MBO was used as the hydrogen-atom donor, a general mechanism for the HAT process can be proposed as shown in Figure 12. Based on data in the literature [1] and our previous works [34,37], in which MBO was used as the hydrogen-atom donor, a general mechanism for the HAT process can be proposed as shown in Figure 12. The absorption of a quantum of electromagnetic radiation leads to the excitation of the dye molecule to a singlet excited state.…”