Photoinitiation Mechanisms of Novel Phenothiazine-Based Oxime and Oxime Esters Acting as Visible Light Sensitive Type I and Multicomponent Photoinitiators

In this work, three new photoinitiators, based on the phenothiazine scaﬀold as a chromophore and potentially bearing the oxime ester functionality as an initiating group are designed and synthesized for the free radical polymerization of acrylates, the cationic polymerization of epoxides, and the formation of interpenetrated polymer networks upon irradiation with a light emitting diode emitting at 405 nm. These phenothiazine-based oxime and oxime esters revealed impressive photoinitiation ability manifested by excellent polymerization rates and high ﬁnal reactive function conversions. Signiﬁcantly, they can be used as both; one-component (Type I) and two-component photoinitiating systems. Photoinitiation mechanisms through which reactive species are produced are investigated by means of diﬀerent complementary techniques including real-time Fourier transform infrared spectroscopy, UV–visible absorption spectroscopy, electron spin resonance spectroscopy, ﬂuorescence (steady state and time resolved), cyclic voltammetry, and molecular modeling calculations. Thermal initiation behavior of the diﬀerent oxime esters is also studied by using diﬀerential scanning calorimetry, highlighting their dual thermal/photochemical initiation ability. Finally, 3D printed objects are successfully fabricated by conducting both direct laser writing and 3D printing experiments.


Introduction
In recent years, light-induced polymerization processes have gained an accelerative attention due to their environmental advantages over other industrial processes.It proceeds with low or no volatile organic compounds emissions, low energy consumption, and excellent time and spatial control. 1,2Consequently, this polymerization technique has been extensively used in numerous applications including adhesives, coatings, dental materials, biomaterials, 3D printing, optics, microelectronics, and nanotechnology etc. [3][4][5] Nowadays, light-emitting diodes (LEDs) appeared to be significant alternatives to the traditional UV irradiation sources (e.g.Hg lamps).7][8] The initiation step for free radical polymerization (FRP) of acrylate function or cationic polymerization (CP) of epoxy function depends on the utilized photoinitiator (PI) which constitutes the key-component of the resin in charge to generate active species under light irradiation.Therefore, one of the most important challenges is the development of new PIs for both FRP and CP with absorption properties that are in complement with the emission wavelength of the LEDs. 9,10 order to overcome the drawbacks of two-component photoinitiating systems (or even more generally for multicomponent systems) which are easily influenced by the electron transfer efficiency, the viscosity and the polarity of the resin, [11][12][13] it is advantageous to develop unimolecular photoinitiating systems (Type I PIs) where these diffusion controlled influences do not exist. 14wever, most of the Type I PIs absorb the irradiation only in the UV range and only few PIs such as amino acetophenones (e.g.Irgacure 369) and phosphine oxides (e.g.TPO, TPO-L and BAPO) can be activated upon excitation at 405 nm.In this context, oxime esters (OXEs) are considered as highly-efficient Type I PIs 15,16 as OXEs can undergo a homolytic cleavage of the N-O bond under light irradiation, enabling to generate iminyl and acyloxy free radicals.The latter can decarboxylate and produce active species along with the generation of CO2 17 .Especially, in these structures, the decarboxylation step is a key parameter avoiding radical recombination.Indeed, due to the decarboxylation reaction, alkyl radicals are less prone to recombine.As a result of this, availability of radicals for initiating radical polymerization can be greatly improved.Two commercially available OXEs O-benzoyl-α-oxooxime (Irgacure OXE-01) and O-acetyloxime (Irgacure OXE-02) are well-known to exhibit outstanding curing performances in highly pigmented resist formulations even at low concentrations.These Type I PIs are also useful for the radical polymerization of acrylic monomers. 18,19 evertheless, these OXEs only absorb for rather short wavelengths and their performances decline with increasing wavelength using e.g.near UV LEDs (385 nm, 395 nm) or visible LEDs (405 nm, 415 nm, 450 nm).Hence, recent research studies have been focused on shifting the absorption wavelength of OXEs to the near UV or visible range by introducing different chromophores into their structures (triphenylamine 20 , coumarin 21 , nitrocarbazole 22 , anthracene 23 , pyrene 24 , phenothiazine 25 etc).
Phenothiazines are nitrogen-and sulfur-containing heterocyclic compounds which have been widely used in the chemical industry because of their pharmacological properties and their applications in solar energy conversion. 26,27Phenothiazine chromophores have also been employed as photosensitizers along with onium salts in cationic and free radical polymerization reactions 28 , but they are only able to initiate polymerization process when excited with broad-band UV irradiation (315-400 nm).Recently, many attempts have been made to shift the absorption of phenothiazine-based photosensitizers into the visible region.Sulfonium salt-phenothiazine initiation systems have notably been used for the cationic photopolymerization of tetrahydrofuran. 29Thiophene-substituted phenothiazines were used as photosensitizers for bis(4methylphenyl)iodonium hexafluorophosphate (ION) and utilized for both CP and FRP processes 30 .
Conjugated phenothiazine oxime esters were also used for the sensitization of ION salt, enabling the FRP of tripropylene glycol diacrylate (TPGDA). 31Also, phenothiazine derivatives were combined with bis-(4-tert-butylphenyl)iodonium hexafluorophosphate (Iod) or with (Iod/N-Phenylglycine) for the FRP of TMPTA and the CP of EPOX. 32,25 ll these polymerization reactions were performed under the effect of visible light irradiations.However, the efficiency of these different photoinitiating systems was directly related to the formation of multicomponent photoinitiating systems, complicating the elaboration of photosensitive formulations.Photoinitiation ability of the above proposed systems were examined upon exposure to a LED@405 nm.Mechanisms governing the photoinitiation process of the mono and twocomponent systems were investigated by employing different techniques and characterization methods.Moreover, the thermal initiating behavior was assessed using differential scanning calorimetry (DSC), and their potential use in laser write and 3D printing applications is also presented as a proof of their high reactivity.
Scheme 2. Chemical structures and abbreviations of the synthesized phenothiazine-based PIs (this work); the hexyl group in PTZ3 has a linear chain structure.

Synthesis of the investigated phenothiazine-based OXEs
The synthesis of the newly proposed photoinitiators (PTZ1, PTZ2 and PTZ3) is described in detail in supporting information.

Other chemical compounds
The chemical compounds used for the preparation of the resin mixtures were selected with the highest purity available and used as received, and their chemical structures are represented in Scheme 3. Chemical structures and abbreviations of the reference initiator (TPO), the additive (Iod) and the benchmark monomers TA and EPOX.

Irradiation Sources
Different light-emitting diodes (LEDs) were used as irradiation sources: (1) LED@375 nm with an incident light intensity at the sample surface I0 = 40 mW•cm -2 , and (2) LED@405 nm with an incident light intensity at the sample surface I0 = 110 mW•cm -2 .

UV-Visible absorption and photolysis experiments
The investigation of the UV-visible absorption properties of PIs in acetonitrile was done by using a JASCO V730 spectrometer.Steady-state photolysis experiments carried out for the different prepared formulations (PIs alone; PIs with Iod (10 -2 M)) were performed upon exposure to a LED@375 nm, and their associated UV-vis spectra at different irradiation times were measured by means of the JASCO V730 spectrometer.The concentration of PIs used throughout the experiments was 4 × 10 -5 M.

Computational procedure
Molecular orbital calculations were carried out using the Gaussian 03 suite of programs 33,34 .
The electronic absorption spectra for the different compounds were calculated with time-dependent density functional theory at the MPW1PW91/6-31g* level of theory on the relaxed geometries optimized at the UB3LYP/6-31G* level of theory.The triplet state energy levels were also calculated at this level of theory.

Steady state fluorescence
The Investigation of the fluorescence properties of PIs in acetonitrile was done by using a JASCO FP-6200 spectrofluorimeter.Fluorescence quenching experiments of PIs by the cumulative addition of Iod (concentrations mentioned in Figure 13) were done by means of the JASCO FP-6200 spectrofluorimeter.The concentration of PIs used throughout the experiments was 4 × 10 -5 M.

Time correlated single photon counting (TCSPC)
Fluorescence excited state lifetimes were determined using a time correlated single-photon counting system named HORIBA DeltaFlex with a HORIBA PPD-850 as detector.The excitation source was a HORIBA nanoLED-370 with an excitation wavelength of 367 nm and a pulse duration inferior to 1.4 ns.Fluorescence intensity decay profiles were recorded in DCM in a quartz cell.A silica colloidal solution LUDOX AS 30, 30 wt % suspension in H2O was used to evaluate the impulse response function (IRF) of the apparatus.

Oxidation Potentials
Oxidation potentials for PTZ compounds (Eox) were measured by cyclic voltammetry using tetrabutylammonium hexafluorophosphate dissolved in acetonitrile as the electrolyte (potentials vs. Saturated Calomel Electrode -SCE).The free energy change (ΔGet) for an electron transfer reaction was calculated from eqn (1) 35 , where Eox, Ered, E*, and C stand for the oxidation potential of the electron donor, the reduction potential of the electron acceptor, the considered excited state energy level and the coulombic term for the initially formed ion pair, respectively.Here, the reduction potential of Iod is Ered(Iod) = -0.7 V, and C was neglected as usually done for polar solvents. 25et = Eox -Ered -E * + C (1)

Photopolymerization kinetics (RT-FTIR)
The photopolymerization kinetics of TA and EPOX were obtained by using real-time Fourier transform infrared spectroscopy (JASCO FTIR 6600).For the FRP of TA, the formulations were put in laminate between two propylene films (thickness ~ 25 μm) to reduce O2 inhibition whereas they were placed on polypropylene films (under air) for the CP of EPOX.Decrease of the C=C double bond peak or the epoxide group band were continuously observed from 1581 to 1662 cm -1 or from 768 to 825 cm -1 respectively.The final acrylate function conversion of TA and the final epoxy function conversion of EPOX were obtained by using the following equation: Where FC is the final function conversion, A0 is the proportion of the peak area at 0 sec, and At is the portion of the peak area at t s.The prepared formulations (PIs with monomers) were stirred in the dark for 24 h.All the polymerization experiments were performed by using the LED@405 nm at room temperature and the irradiation was initiated after t = 10 s.The weight of the system was calculated from the monomer content.More detailed experimental conditions for each formulation have been noted in the caption of the different figures.The procedure has been already described in Refs.12 and 36.

Differential scanning calorimetry (DSC)
About 10 mg of TA containing 1% wt PI was inserted into a 100 μL aluminum crucible.
Thermal polymerization was performed from 25°C to 300°C at a heating rate of 10 °C min -1 under nitrogen flow (100 mL.min -1 ).A Mettler Toledo DSC 1 differential scanning calorimeter was used for this purpose. 21

Direct laser write and 3D printing experiment
For direct laser writing experiments, a laser diode @405 nm was used for the spatially controlled irradiation.The intensity of laser was 110 mW, and the spot size was 50 μm.The photopolymerization process was done under air and the generated 3D patterns were analyzed using a numerical optical microscope (DSX-HRSU from Olympus Corporation) as presented in the literature 36,37 .For 3D printing experiments, a SLA 3D Printer (PeoPoly MOAI 130 Printer) was used (λ = 450 nm; Intensity = 150 mW•cm -2 ).

UV-visible absorption properties
UV-visible absorption spectra of the newly presented PIs recorded in acetonitrile are shown in Figure 1.Their maximum absorption wavelengths (max), molar extinction coefficients (max) at max and at the emission wavelength at 405 nm are assembled in Table 1.
The maximum absorption wavelength (max) of the oxime PTZ1 is located at 325 nm.This value is slightly redshifted upon introduction of the ester functional group in PTZ2 and PTZ3 (Figure 1, Table 1).The highest molar extinction coefficient was found for PTZ3 carrying a hexyl group as a substituent.This can be confirmed by the HOMO orbital of this molecule (Figure 2), where the first sigma C-C bond of the hexyl group is involved in the enhanced delocalization of electrons.All the investigated PIs revealed high molar extinction coefficients and good absorption properties in line with the emission spectra of the visible LED used in this work.
From the molecular modelling data, the optimized geometries for the different structures and their frontier orbitals (Highest Occupied Molecular Orbital -HOMO and Lowest Unoccupied Molecular Orbital -LUMO) involved in the lowest energy transition are represented in Figure 2.
It can be observed that both the HOMO and LUMO are delocalized all over the π-system apparently manifesting a π→π * lowest energy transition.Moreover, it can be noted that the HOMO's are wrapped on the phenothiazine unit and the LUMO's are also delocalized on the oxime and oxime ester units.

Type I photoinitiator features
Initiation abilities of the investigated compounds (1% phr) working as mono-component photoinitiating systems for the polymerization of the benchmark TA monomer, were studied using RT-FTIR in thin samples (25 μm, in laminate) under a LED@405 nm.Typical acrylate conversion

PTZ1 PTZ2 PTZ3
versus irradiation time profiles are shown in Figure 3 and the associated final acrylate conversions (FCs) are recapped in Table 2.
PTZ1, the compound which has no oxime-ester functional group, showed a long inhibition period followed by a small polymerization rate in the FRP of TA.
PTZ2 and PTZ3 i.e. two compounds with a methyl substituent on the carboxyl side showed fast polymerization rates as well as high final conversions, which is not only related to their good absorption properties but also to other factors including the cleavage process, the decarboxylation reaction, and the reactivity of the generated radicals 21 .This accentuate the significant role of the oxime-ester function to act as a Type I PI.PTZ3 (with a hexyl substituent attached to the phenothiazine chromophore) reached the highest FC (81%) which renders it as an efficient PI by comparing it with the commercial benchmark PI (TPO; FC = 83%) (See Fig. 3 and Table 2), in agreement with the good absorption properties and high molar extinction coefficient related to

PTZ3.
Other phenothiazine-based OXEs bearing a hexyl chain for solubility denoted as (Hex1→Hex10) (Scheme S1) or bromine substituent to modify the absorption maxima denoted as (1A→13A) (Scheme S2) achieved also high final conversions of the acrylate functional group of the TA monomer (See Figures S1 and S2).In this series of photocleavable groups, it can be noticed that all OXEs with an acetyl group on the oxime ester moiety (like PTZ2 and PTZ3) exhibited the best photoinitiation ability compared to the other OXEs having different substituents (Hex1→Hex10 and 1A→13A).This higher photoinitiating ability can be assigned to the favorable enthalpy of decarboxylation reaction in the case of the methyl substituent (CH3-C(=O)O • → CH3 • + CO2; ΔHdecarboxylation = -4.94Kcal mol -1 ) 21 .The detection of CO2 release consequent to the polymerization of TA monomer confirms the occurrence of a decarboxylation reaction in the photoinitiation mechanism.The infrared spectra obtained by real time Fourier transformed infrared spectroscopy (RT-FTIR) before and after polymerization and presented in Figures 4 (A) and (C) show a peak appearance at 2337 cm -1 for both PTZ2 and PTZ3 which proves the release of CO2.The positive correlation between CO2 release and the efficiency of the polymerization process for both PTZ2 and PTZ3 illustrated in initiating radicals for the polymerization process.These results are in agreement with the favorable enthalpy of decarboxylation reaction observed in the case of methyl substituent.In order to study the photochemical properties of PIs, steady-state photolysis was performed in acetonitrile upon exposure to a LED@375 nm.The results presented in Figure 6 3.
We can notice that the BDEN-O bond for all compounds is higher than their triplet excited state energies ET1, and thus the cleavage processes from T1 are not energetically favorable (ΔHcleavageT1 = BDE -ET1 > 0) whereas it is lower than their singlet excited state energies ES1, suggesting a cleavage occurring from S1 (ΔHcleavageS1 = BDE -ES1 < 0).The fluorescence lifetime of the phenothiazine-based oxime esters was also measured and the values are gathered in Table 3 and

Use in 3D printing
Because of its considerable photoinitiation ability during the free radical polymerization process of TA, PTZ3 (0.1% w/w) was selected as an appropriate system to perform the direct laser write (DLW) and 3D printing experiments.The denotation "PTZ" shown in Figure 7(A) was obtained through DLW by using a laser diode @405 nm and characterized by numerical optical microscopy.Appealingly, the obtained 3D pattern has a great thickness (≈ 1570 µm), high spatial resolution, and it requires a very short irradiation time to be generated (

Thermal initiator features
In the purpose of studying the thermal initiator features of our phenothiazine-based PIs after manifesting their photoinitiating abilities during the FRP of TA under LED @ 405 nm, two-cycle DSC experiments of PTZ1, PTZ2, and PTZ3 in TA have been performed and their associated curves (temperature vs heat flow) are presented in Figure 8.The onset temperature of thermal polymerziation (TOnset), the temperature of maximum polymerization rate (Tmax), and the final acrylate FCs of TA are given in Table 4. Markedly, the importance of the oxime-ester functionality for thermal polymerization initiator behavior is noticed with a decline in Tmax from 241 °C in PTZ1 (without OXE function) to nearly 170 °C in PTZ2 and PTZ3.Moreover, it can be seen that oxime PTZ1 needs a temperature of 206 °C to start the polymerization, which decreases significantly after the introduction of the oxime ester moiety in PTZ2 and PTZ3.

Multi-component photoinitiating system features
The photoinitiation abilities of the different derivatives in the presence of an iodonium salt as an additive (1% w/w) for the FRP of TA acrylate monomer were also tested upon irradiation with a LED@405 nm.Typical acrylate function conversion vs. irradiation time profiles are presented in Figure 9 and the associated final acrylate function conversions (FCs) are summed up in Table 5.
The experimental results show that the addition of an iodonium salt has a favorable impact on the final acrylate function conversion compared to those obtained with PTZ derivative alone.
This impact is clearly observed for PTZ1 and PTZ2 where the FC increases from 14% to 68% for PTZ1 and from 71% to 82% for PTZ2 (See Figure 9 and Figure 3).It should be noted that the iodonium salt alone was unable to initiate the FRP of TA under the same conditions, clearly showing the importance of the PTZ derivative for an efficient process.The efficiency trend for the PTZ/Iod couples determined during the FRP of TA respects the following order: PTZ3≈PTZ2>PTZ1 which is not only related to the absorption properties of the investigated PIs, but also to their photochemical reactivity with the iodonium salt and the yield of electron transfer to generate the initiating aryl radicals Ar • (Equations (r1) and (r2) below).The CP of EPOX upon LED irradiation @405 nm was also investigated by using our PIs as photosensitizers for iodonium salt (Iod).Epoxy function conversion versus irradiation time profiles are shown in Figure 10 and the associated final epoxy conversions (FCs) are gathered in Table 6.
The obtained results showed that the CP of EPOX in the presence of a bi-component system (PI/Iod) is very efficient in terms of polymerization rate (Rp) and final epoxy function conversion (FCs) where it reaches 83% for the PTZ2/Iod system.Iod alone in EPOX showed a poor polymerization behavior in agreement with its lack of absorption at 405 nm which highlights the importance of PTZ derivatives for designing an efficient photoinitiating system.The efficiency trend for the PI/Iod couples determined during the CP of EPOX respects the following order: PTZ2>PTZ1>PTZ3 which is not only linked to the absorption properties of the investigated PIs, but also to their photochemical reactivities with the iodonium salt and the yield of electron transfer to generate the initiating radical cations PTZ •+ (Equations (r1) and (r2) below).The synthesis of Interpenetrated Polymer Networks (IPN) by combining both free radical and cationic polymerization processes is interesting because it combines the advantages of both processes without their respective disadvantages.Acrylate and Epoxy function conversion versus irradiation time profiles for PTZ2/Iod system and Iod alone are shown in Figure 11, and those for PTZ1/Iod and PTZ3/Iod systems are shown in supporting information (Figure S3 and Figure S4 respectively).The associated final acrylate and epoxy conversions (FCs) for all systems are summarized in Table 7.
The final acrylate and epoxy functional groups conversions (FCs) showed that the different PI/Iod systems can initiate the hybrid polymerization of TA/EPOX blend (50%/50% wt/wt).PTZ2 and PTZ3 achieved the highest TA monomer conversions with an acrylate function reaching a FC = 99%, whereas PTZ1 achieved the higher polymerization of the EPOX function reaching a FC = 91%.Iod alone without the presence of dyes showed its inability to form IPNs.
Reactivity of PTZs is clearly influenced by different effects.First, for PTZ2 and PTZ3 that are very reactive in FRP, it can be assumed that the solidification due to the FRP is very fast preventing an efficient cationic polymerization (epoxy conversion < 53% ; Table 7) as vitrified state is rapidely reached.For PTZ1 system, the FRP is not very efficient (see Figure 3 above) and therefore the CP pathway can be more favourable (epoxy conversion =80% ; Table 7).Due to the same effect, the final double bond conversions of the PTZ2 and PTZ3 systems are so exceptionally high.The initiator and monomer dissolved in the liquid EPOX phase have significant mobility, so that the acrylic esters are almost completely consumed.Steady-state photolysis experiments of the PI/Iod (10 -2 M) systems were performed in acetonitrile under light irradiation (LED@375 nm).We can observe a fast photolysis occurring in the presence of Iod which evidently reveals the favorable interaction between the investigated PIs in their excited singlet states with the iodonium salt besides the N-O bond cleavage process, in agreement with the results obtained in the photopolymerization experiments.Isobestic points were also observed suggesting photochemical processes without side reactions.
In order to better understand the interaction between the new PIs and the iodonium salt (Iod), fluorescence quenching experiments were carried out in acetonitrile.The concentration of Iod was increased consecutively and each time a fluorescence spectrum was obtained.Figure 13 shows a   In  Due to its high photoreactivity during the FRP of TA, the two-component system PTZ3/Iod (0.1%/1% w/w) was selected for 3D printing experiments upon laser diode irradiation at 405 nm.

Conclusion
In this article, three new photoinitiators based on the phenothiazine chromophore were based on the phenothiazine scaffold that can be activated at longer wavelengths will be proposed in upcoming studies.
photoinitiating ability, presence of the chromophore also enabled the different phenothiazine derivatives to be used as photosensitizers for the sensitization of an iodonium salt and the resulting two-component photoinitiating systems could be used for the FRP of di(trimethylolpropane)tetraacrylate (TA), the CP of epoxides, and in turn the formation of interpenetrated polymer networks (IPNs) by the concomitant polymerization of epoxides and acrylates.
X-Band spectrometer (Bruker EMX-plus) was used in order to realize the ESR-ST experiments.The experiments were performed out at room temperature (RT) under N2 with 405 nm LED illumination within the cavity of the ESR spectrometer.Radicals were trapped by XM phenyl-N-tert-butylnitrone (PBN) obtained from TCI-Europe in tert-butylbenzene according to a procedure described in refs.36 and 37.The PEST WINSIM program was used to obtain the ESR spectrum simulations.

Figures 4 (
Figures 4 (B) and (D) emphasize the prominent role of the decarboxylation step in yielding

Figure 5 .Scheme 4 .
Figure 5. ESR spectra for PTZ3 recorded in the presence of PBN and tert-butylbenzene with a LED@405nm: (A) before and after irradiation; (B) experimental and simulated spectra observed after irradiation (at t = 60 s).
photoreaction occurrence.These photolysis results are in agreement with the poor Type I behavior of PTZ1 and the affluent Type I behavior of PTZ2 and PTZ3 observed during the photopolymerization experiments.

Figure 6 (
Figure 6 (E).PTZ1 showed the longest lifetime (7.65 ns), which decreased upon introduction of the oxime-ester function up to 7.1 ns in PTZ2 and 7.06 ns in PTZ3, demonstrating a cleavage occurring from the S1 state in agreement with the negative favorable enthalpy for the singlet excited state obtained above.
the aim of confirming the quenching process of PTZ1 by Iod and the detection of the type of initiating radicals released, ESR spin trapping experiments have been performed.Aryl radicals were trapped by PBN in PTZ1 solution after 30 s of irradiation (Figure 14).Values of the hyperfine coupling constants for the radical adduct obtained were αN = 14.3 G and αH = 2.1 G which could be assigned to the aryl radical (Ar • ), thus the fluorescence quenching of PTZ1 in its first excited singlet state by Iod is confirmed, in agreement with the results obtained in the photopolymerization experiments.

Figure 14 .
Figure 14.ESR spectra for PTZ1/Iod solution recorded in the presence of PBN and tert-butylbenzene with a LED@405nm: (A) before and after irradiation; (B) experimental and simulated spectra observed after irradiation (at t = 30 s).
The 3D patterns denoted by "IS2M" shown in Figure15characterized by numerical optical microscopy have a great thickness (≈ 1400 µm), high spatial resolution, and require a very short irradiation time to be generated (15 s) which confirms the high photosensitivity of the system used (PTZ3/Iod).

Figure 15 .
Figure 15.3D patterns obtained upon exposure to a laser diode @405 nm characterized by numerical microscopy for compound PTZ3 with Iod (0.1%/1% w/w) in TA.
successfully synthesized and used as Type I and two-component visible light photoinitiating systems.These compounds achieved high final conversions during the FRP of acrylate, the CP of epoxides, and during IPN synthesis upon irradiation using a LED@405 nm.Photoinitiation mechanism for the mono-component systems was established by mean of the CO2 detection tests, and during the ESR spin-trapping experiments.Conversely, photoinitiating mechanism occurring for two-component systems could be determined on the basis of fluorescence quenching experiments.Notably, photoinitiation ability of PTZ3 allowed the fabrication of 3D objects by laser write and 3D printing technologies.In addition, the dual photo/thermal behavior of these compounds have been also shown.Future developments of other Type I and Type II photoinitiators

Table 1 .
Light absorption properties of the investigated compounds: maximum absorption wavelength (λmax), and molecular extinction coefficients at λmax, and at 405 nm.

Table 2 .
FCs of acrylate function using one component photoinitiating system (1% w/w) after 150 s of irradiation with a LED (λ = 405 nm).