3D-VAT printing of nanocomposites by photopolymerisation processes using amino-meta-terphenyls as visible light-absorbing photoinitiators

ABSTRACT In this article, the application of 10 new amino-m-terphenyls in 3D-VAT printing was described. New compounds have specially designed D-π-A structure, where the central phenyl ring with nitrile and amino groups is the acceptor and the modifiable amino group is donor. Such design eliminates problem with acid scavenging and guaranteed desire properties and photoactivity as well as it allows further development of such system for 3D-VAT printing. Efficient excitation with intramolecular charge transfer provides excellent absorption and electrochemical properties, which can be tuned by modification of the amino group. The design allows photoinitiation of free radical, hybrid and especially cationic polymerisation even at 455 nm with more than 70% of monomer conversion. Such properties allow to use the developed compounds as efficient visible light photoinitiators for 3D printing of nanocomposite materials. The terphenyls can efficiently cure resins containing CuO and Al2O3 nano additives leading to high-resolution 3D prints. GRAPHICAL ABSTRACT


Introduction
In recent years, photopolymerisation has attracted increasing attention.Not only for its energy efficiency [1] and low environmental impact (elimination of Volatile Organic Compounds -VOCs) [2], but especially for its high productivity [3].Utilisation of light as a precise energy deliverer enables to efficiently obtain optoelectronic circuits [4], composites [5,6] or dental fillings [7,8].Photoinitiation finds application in such modern polymer synthesis methods as 3D-VAT printing [9][10][11], photoinduced frontal polymerisation [12,13], or multi-photon lithography [14,15].In particular, the rapidly growing area of using 3D printing to obtain macro functional materials is important [16,17].However, despite its many advantages, photopolymerisation has the significant drawback of low absorption of one-component photoinitiating systems at longer wavelengths, especially above 400 nm [18].This causes great difficulties in using this method in more advanced applications.Therefore, shifting absorption towards longer wavelengths is desirable.Photoinitiating systems that absorb in the visible range allow thicker coatings to be cured [19] and are compatible with most common 3D printers that use sources emitting at 405 nm [20].In addition, visible light sources are less expensive and do not emit harmful UV radiation [21].
The first approach uses photoinitiators with better absorption characteristics in the near-UV range and reaching visible light as co-initiators.Photoinitiators such as 2,2dimethoxy-2-phenylacetophenone (DMPA) [43], trimethylbenzoyl diphenylphosphine oxide (TPO) [44], dibenzoyldiethylgermane [45] or dimanganese decacarbonyl [46,47] are used to sensitise iodonium salts in a process called free radical promoted cationic photopolymerisation (FRPCP).In this process, under light illumination, the radical co-initiator photodecomposes with the generation of free radicals.Such radicals can be oxidised to carbocations by iodonium salts, which decompose.In the result, an acid is released that initiates cationic polymerisation.However, in the case of FRPCP, the cationic active centres are generated not by a proton but by a carbocation formed by oxidation of the radical.Such a carbocation can easily polymerise some monomers, such as oxetanes, which are difficult to polymerise by protic acids [48].Process may proceed at longer wavelengths than indicated by absorption of diaryliodonium salts [38].
However, much more progress has been made in the development of photosensitisers based on coloured aromatic compounds [40].This approach in the photosensitisation of photoinitiators involves the transfer of energy or an electron from the light-excited photosensitiser to the initiator, causing its decomposition with the release of active chemical entities (e.g. in the case of iodonium salts, this would be protic acid).Onium salts can be successfully photosensitised by energy transfer with aromatic compounds such as anthracene [49,50], pentacene [51], pirene [52] or perylene derivatives [53].The most spectacular effect has been observe for violanthrone derivatives which activate simple diaryliodonium salts under sunlight exposure [54].However, many more efficient photoinitiating systems have been developed using aromatic heterocyclic compounds or those containing electron-donating groups that are easily oxidised during electron transfer process [18,55,56].Examples of compounds often used in this type of photosensitisation are phenothiazine [57], carbazole [58,59], thioxanthone [60], or squarylium dyes [61].The last group of compounds is particularly interesting because it is also used in initiating polymerisation in the NIR range (so-called photothermal polymerisation) [62].Recently, natural dyes have also been gaining attention in photosensitising, with applications in photoredox photoinitiating systems [63].In addition, metal complexes such as ruthenium [64] and iridium [65], known for their high photoactivity, are also used in the photosensitisation of onium salts.The use of carbon dots as photosensitisers is also an intriguing discovery [66].
The use of photosensitisers equipped with amine groups leads to particularly promising results.Amines are efficient electron donors due to their redox properties [18].This finds application, for example, in CTC-based photoinitiating systems where, in combination with a onium salt [13,67] (most commonly iodonium salts but there are also systems with sulfonium [68] and pyridyl salts [69]), they form a complex with an absorption band that reaches far beyond the absorption properties of each component separately.Unfortunately, the basic properties of the amines make such systems used mainly for initiating radical polymerisation [70,71].Although there are reports of their use in cationic polymerisation as well [72,18].
Amine groups are also used in typical photoredox systems, where a photosensitiser that absorbs at the longer wavelengths reduces the onium salt during irradiation, thereby initiating the polymerisation process [73,74].Amino groups work particularly well in systems with thioxanthone chromophores, photoinitiating not only radical polymerisation, but also cationic polymerisation (leading to 50% conversion of epoxy monomers) at visible light [75].Appropriate design of the photosensitiser makes it possible to provide the desired donor properties of amines while limiting their ability to be protonated.An example of such a design is the developed terphenyl core [76,77].Due to the arrangement of electronacceptor groups in the vicinity of the amine group, a photosensitiser capable of efficiently photoinitiating cationic polymerisation in a photoinitiating system with iodonium salt was obtained [78].Another advantage of the terphenyl core is that such photosensitisers can be used as internal fluorescent probes to on-line monitoring the photopolymerisation process [79,80].
In this work, the further development of terphenylbased two-component photoinitiating system is presented.The new compounds were modified with amine groups that strongly activate the frontier phenyl ring of the terphenyl core.This modification improves the absorption intensity of terphenyls under visible light and lowers their oxidation potentials providing the desired conversion of both cationic and radical monomers in polymerisation initiated by multi-component photoinitiating systems.These new compounds were applied to high-resolution 3D-VAT printing.

Materials
Ter-ASMe) were synthesised and studied for the role of photosensitisers of iodonium salt in bimolecular photoinitiating systems for photopolymerisation processes.Structures of studied compounds are depicted in Figure 2. Synthesis details are described in the Supporting Information, NMR and LCMS data are shown in Table S1. 1 H NMR spectra of obtained terphenyl derivative are shown in Figures S1-S10 in the Supporting Information.

UV-Vis absorption spectroscopy
All presented absorption spectra were recorded using a SilverNova TEC-X2 spectrometer (StellarNet Inc., Tampa, FL).The broadband tungsten-deuterium UV-vis light source SL5 (StellarNet Inc., Tampa, FL) was used as a light source.Spectra were measured in a quartz cuvette with a 1.0 cm optical path with acetonitrile as a solvent.The concentration of the samples was 20 mg•dm −3 .Raw absorption spectra were converted into molar extinction coefficients, expressed in [mol - 1 •dm 3 •cm -1 ] units according to Equation (1): where

Determination of electrochemical properties
The cyclic voltammetry was used to determine the reduction and oxidation potentials of m-terphenyl derivatives.Cyclic voltammogram were recorded using an Electrochemical Analyzer M161 and an Electrode Stand M164 (MTM-ANKO, Poland).The following components were used for the measurements: platinum disk as the working electrode, silver chloride (Ag/AgCl) as the reference electrode and tetrabutylammonium hexafluorophosphate in acetonitrile (0.1 M, Sigma-Aldrich) as a supporting electrolyte.The determined potentials were recalculated relative to the saturated calomel electrode (SCE).
The E 00 values were determined from the crossing point of normalised excitation and fluorescence spectra.Both spectra were recorded using a Fluoromax-4P spectrofluorometer (Horiba, Kyoto, Japan).Acetonitrile (spectral grade) was used as a solvent (the concentration of terphenyls solutions were the same as for absorption measurements).The E 00 values were used to calculate the free energy change values (ΔG et ) based on the Rehm-Weller equation (which is described in the supporting information) [81].

Photopolymerisation processes monitoring
Real-time FT-IR (Nicolet TM iS10, from Thermo Scientific U.S) was applied for monitoring photopolymerisation processes for free radical and cationic polymerisation.Light-emitting diodes (LED) from ThorLabs powered by by a DC2200 regulated power supply were used as a light sources.Irradiation were provided to sample through an optical fibre located 2.1 cm above sample.Light was switch on 10 s after the start of the IR spectral measurement.The conversion of monomers functional groups during photopolymerisation measurements was calculated according to below Equation (2): where A before polymerisation is the absorbance peak area characteristic for tested monomer functional groups before the photopolymerisation process; A after polymerisation is the absorbance peak area characteristic for tested monomer functional groups after the photopolymerisation process.
Examples of CADE and TMPTA monomer spectra before and after photopolymerisation with monitored bands are shown in the supporting information (Figures S32 and S33).

Photopolymerisation processes monitoring of samples for 3D-VAT printing
The M405L3 LED from ThorLabs (I 0 = 1.99 mW•cm −2 ) was used for measurement of photopolymerisation kinetics of 3D-VAT printing compositions.This LED (with emission maximum at 405 nm) was used to mimic the conditions provided by the used 3D printer.Resin samples were prepared in orange glass vials in a room with limited access to light.The ring with the sample was placed on the measurement holder, the light source in the form of a VIS-LED@405 nm was turned on at the 10th second of the measurement duration.Each measurement lasted 900 s.The measurements were carried out under anaerobic conditions on a ring of 100 µm thickness.

Free radical photopolymerisation of samples for 3D-VAT printing
The radical photopolymerisation process kinetics was measured for the following resin formulations: B-Ter-ACN/IOD/EDB (0.1/1.0/1.5% w/w/w) or B-Ter-ASMe/IOD (0.1/1.0% w/w) photoinitiating systems with TMPTA/ BEDA monomers (in 3/7 ratio) with 3.0% CuO, 3.0% Al 2 O 3 and without nanoparticles.The measurements were carried on a ring of 100 µm thickness.With samples of this thickness, the standard bands are saturated (absorbance should be below 1).Therefore, the acrylate group conversion was determined by the disappearance of the band, which had a maximum at a wavenumber of 6165 cm −1 (=C-H bond oscillation) [82].This band has lower intensity and is not saturated for samples of this thickness.The same procedure was applied to other monomers in thick samples.

Hybrid photopolymerisation of samples for 3D-VAT printing
To investigate the development of hybrid photopolymerisation, the following photocurable compositions were prepared: B-Ter-ASMe/IOD (0.1/1.0% w/w) and TMPTA/BEDA/S130/CADE monomers (in 4/4/1/1 ratio) with 3.0% CuO, 3.0% Al 2 O 3 or without nanoparticles.The acrylate group conversion was determined by the disappearance of the band located between wavenumbers 6110 cm -1 and 6250 cm -1 , while the epoxy groups were determined by the band whose maximum occurs at wavenumber 3750 cm −1 .

Determination of 3D printing parameters -Jacob's basic working curves
Jacob's basic working curves were applied to calculate the optimal 3D printing parameters such as critical energy (E cenergy necessary to initiate photopolymerisation) and light penetration depth (D p ): where Based on the resulting data, a graph C d = f(E 0 ) was determined, from which the critical energy (the point of intersection of the graph with the X-axis) and the depth of light penetration (the slope of the resulting curve) were calculated.Knowing D p and E c allows choosing the appropriate light exposure settings (for example printer power) to optimise printing conditions to achieve the best resolution.

Optical microscopy
OLYMPUS DSX1000 Optical Microscope was employed to visualise the obtained three-dimensional objects.

Design concept
The main concept behind the design of the new terphenyl derivatives was to use the D-π-A structure in such a way that the push-pull effectand thus the CT transition energycould be controlled (Figure S11).
As it was demonstrated earlier, the frontier orbital LUMO is located within the central terphenyl ring, thus it can be treated as an acceptor in the D-π-A structure [76].This is because the strongly electron-donor character of the diethylamine group is weakened by the direct courtship of two nitrile groups with relatively strong electron-acceptor character [85].In order to obtain the desire structure, at one of the border phenyl rings, the functional groups from the amine family were placed.Amines, as groups with a particularly strong donor character, made it possible to obtain the push-pull effect (group I of amino-m-terphenyls) [86].
At the same time, appropriate control of the electron density within the amine donor groups will help to ensure the desired efficiency during the photoinitiation of cationic polymerisation (removing the problem of scavenging the generated acid by the amine groups).The diethylamino group at central phenyl ring has limited susceptibility to protonation because of two nitrile groups [87].However, a high electron density is further localised on the border amine group (the donor in the structure), which can undergo easy protonation competing with monomer molecules in the process.In order to investigate how the strength of the donor affects photoinitiating activity, the group II of amino-m-terphenyls was prepared.In this group, acceptor and donor functional groups were placed at the phenyl ring of the N-phenylamine group to properly control the donor properties of this group.

Absorption properties
UV-Vis absorption spectra of studied compounds are presented in Figure 3.In order to show the effect of particular amine functional groups, the unmodified compound B-Ter-H was used as a reference.It was described in our previous work [78].
It was proved that functionalisation of the amine group by the introduction of two ethyl substituents has a positive impact on absorption properties of m-terphenyl derivatives.The absorption properties of m-terphenyl compounds were extended by introduction amine moiety to 2-(diethylamino)-4,6-diphenylbenzene-1,3-carbonitrile core (B-Ter-H) -Figure 2. It can be seen that absorption spectra of compounds after modification are shifted towards longer wavelengths in comparison to basic compound B-Ter-H (for group I, Figure 3A).Modified compounds possess much higher values of molar extinction coefficients than compound B-Ter-H, because of the extension of its structure and thus increase of absorption crosssection.Absorption spectrum of compound B-Ter-DPA, with diphenylamine substituent extends furthest, up to 470 nm.Spectroscopic properties of studied compounds (values of molar extinction coefficients and absorption maximum) are summarised in Table S3.
In the case of group II, the absorption properties change differently (Figure 3B).Red or blue shift is obtained depend on character of functional group placed at amine phenyl ring.Deactivation of the amine group achieved by the use of electron-acceptor groups leads to a visible blue-shift of the absorption bands, and thus to a worsening of the absorption properties.
On the other hand, the use of electron-donor groups increases the donority of the amine group thereby increasing the polarity of the hole molecule.This visibly leads to a red-shift of the absorption bands.However, the effect for two of the best derivatives B-Ter-AMe and B-Ter-ASMe is similar to that for the diphenylamine substituent in the terms of red-shifting but intensity of absorption is still better for B-Ter-DPA.

Electrochemical properties
In order to scrutinise redox behaviour, electrochemical properties of 2-(diethylamino)-4,6-diphenyl-benzene-1,3-carbonitrile derivatives were determined using cyclic voltammetry technique.Values of oxidation and reduction potentials of terphenyl derivatives are listed in Table 1 (cyclic voltammograms for terphenyl derivatives are presented in Figures S12-S31 in Supporting Information).
Modification of the compound B-Ter-H with amino groups resulted in a significant reduction in the oxidation potential of terphenyl derivatives.The observed effect is due to the increased electron density within the terphenyl provided by the amino groups used.All substituents used have electron-donor properties and thus increase the susceptibility of the obtained compounds to oxidation.All amino groups from group I resulted in a lowering of the oxidation potential by about 50%.The only exception is B-Ter-Car for which the lower reduction in oxidation potential is most likely due to its rigid structure.In the case of group II derivatives, the effect of electron density tuning within the amine group on oxidation potential values is evident.All derivatives in this group have a lower oxidation potential than the reference B-Ter-H.However, within the group, significant differences can be seen in the values obtained.Thus, with the decrease in electron density with electron-acceptor groups such as CF 3 and CN values as high as about 1350 mV were observed (for B-Ter-ACF 3 and B-Ter-ACN).Conversely, with the use of electron-donor groups such as Me and SMe significantly lower values were observed, falling in the range of 930-1050 mV.Thus, these results demonstrate that it is possible to rationally control the oxidation potential of m-terphenyl derivatives using modification of the electron density localised within the amine group.This approach leads to even better E ox than use of simple amine groups (the lowest E ox for B-Ter-ASMe).On the other hand, presented modifications have no significant impact on reduction potential of such compounds.All E red values are about -1650 mV.
The direct photosensitising of iodonium salts ability was investigated by determination of free energy change values (ΔG et ).This value determines how easily electron transfer from the photosensitiser to the iodonium salt can occur and whether the process is thermodynamically favoured.Electron transfer process between investigated compounds and iodonium salt is favourable if ΔG et is negative.The values of ΔG et are negative for all investigated compounds, the results are shown in Table 1.It was observed, that ΔG et values are lower for all modified derivatives in comparison with reference B-Ter-H.Thus, amine groups placed at border phenyl ring in m-terphenyl structure leading to better photosensitisers.For most of the compounds in Group I, it was observed that the lower the values of E ox , the lower the values of ΔG et .This relationship is also preserved in the case of group II compounds, where it is clear that the ΔG et value can be tuned by functional groups in the amine phenyl ring.Summarising this section, all tested compounds should be effective photosensitisers of the iodonium salt, what is more, they should perform better than the basic compound B-Ter-H.

Cationic photopolymerisation
Studied 2-(diethylamino)-4,6-diphenyl-benzene-1,3-carbonitrile derivatives were investigated in the role of photosensitisers of iodonium salt for cationic polymerisation.Ring opening photopolymerisation of epoxy monomer was performed with irradiation of LED 405, 420 and 455 nm during 800 s.Photopolymerisation profiles for epoxy monomer CADE under air at 455 nm are presented in Figure 4A,B (profiles at 405 and 420 nm are gathered in Figures S34-S37).The CADE monomer was chosen because of its relatively low reactivity.After protonation of oxirane ring (and secondary oxonium ion formation) the hydrogen bond is formed between protonated oxirane ring and carbonyl bond in monomer structure [20].Such stabilisation effect make it easier to compare the activity of photoinitiating systems.Final conversions for all photosensitisers and wavelengths are summarised in Table 2.
There are several factors that affect the efficiency of two-component photoinitiating systems, they are related to the properties of the photosensitiser and the interaction of the sensitiser with the other component of the initiating system.First, the photosensitiser must absorb the radiation emitted by the light source.In addition, photosensitiser must have appropriate oxidation potential for the electron transfer process between the iodonium salt and terphenyl to be thermodynamically allowed (negative ΔG et values).
According to Figure 4A,B, it can be seen, that all studied molecules in combination with an iodonium salt form efficient systems that photoinitiate cationic polymerisation upon exposure to LED 455 nm.The described compounds show activity in a wider range of visible light, actively initiating cationic polymerisation using also LED 405 and 420 nm (Figures S34-S37).Their performance is higher than commercial THX in most cases (especially at 455 nm where activity of THX is negligible).This is due to worse absorption properties (3-6 times lower absorption intensity) and a lower ΔG et value (0.69 eV) for THX compared to the terphenyls presented [73].Even assuming that the electron transfer efficiency will be similar, the significantly weaker absorption results in a significant reduction in the photoactivity of the photosensitiser.In this case, the value of ΔGet is  [81].E ox (D/D •+ )the electrochemically determined oxidation potential of the sensitiser (vs.Ag/AgCl).E red (A • -/A) the electrochemically determined reduction potential of the electron acceptor, −0.64 V for the diaryliodonium salt (vs.Ag/AgCl) [42].E 00singlet state energy of the sensitiser determined based on excitation and emission spectra.
also the least negative, that is, the electron transfer for THX is less favourable than for the developed terphenyls.
The noticeably better properties of the presented compounds are mainly due to the increased electron density within the aromatic rings provided by the applied modifications in the form of amine groups.The functional groups used not only provide higher absorption intensity, but also increase the susceptibility of terphenyls to oxidation.It can be seen that reference compound, m-terphenyl core -B-Ter-H and compound with piperidine substituent B-Ter-Pip exhibit the weakest initiating performance.Especially upon exposure to LED 420 nm and LED 455 nm, long induction time is observed.The reference B-Ter-H has the highest ΔG et value so its low performance is justified.This value is still higher than THX thus basic terphenyl exhibit low but much better activity.Surprisingly, the activity of B-Ter-Pip exceptionally low.Piperidine substituent has enough donating properties ensuring desire ΔG et value and absorption properties.However, this substituent exhibits relatively high basicity.The obtained results show that piperidine moiety can efficiently scavenge the generated acid, decreasing photoinitiating activity of its terphenyl derivative.
The investigation showed that tuned amine group of group II derivatives exhibits better photosensitising properties than most of compounds from group I.The best photoinitiating activity was observed for conjunction of iodonium salt with B-Ter-ASMe.This compound possesses the lowest values of oxidation potential (E ox = 934 mV) and the most negative value of ΔG et parameter (ΔG et = −1.32eV).Which, along with some of the better absorption properties provided by a substituent with the high electron donation, ensures high photoinitiating activity for cationic polymerisation.
Obtained results shows that used design of m-terphenyl derivatives ensure their high photosensitising ability and do not interfere in the cationic polymerisation process despite two amine groups in their structure (expected B-Ter-Pip).Additionally, proper tuning of electron-donating character of aminophenyl group can provide high photoactivity of such kind of compound in cationic polymerisation.
The presented compounds show similar photoinitiating activity for radical polymerisation of acrylates to THX using 405 nm LED.However, due to significantly better absorption properties in the form of up to six times more intense absorption, the compounds show significantly higher activity at 420 and 455 nm.In the case of radical polymerisation, the acid scavenging effect does not occur.The photoinitiating activity of investigated compounds depends on the absorption properties and ΔG et .Therefore, most of observed effects originate from the donority of amine substituents.The photoinitiation ability upon exposure to LED@405 nm is similar for all studied systems, the final conversion values are in the range 45-60%.The best initiating efficiency show B-Ter-DPA and B-Ter-PMA (FC ∼ 60%).In this case, tertiary aromatic amines show the best performance in two-component initiation systems.The weakest performance during irradiation with a 420 nm diode was observed for iodonium salt in the presence of reference compound -B-Ter-H and B-Ter-Pip.The other molecules work similarly, the final conversion values obtained after the process carried out with the use of LED@420 nm are in the range 45-52%.B-Ter-H/IOD system was also showed the weakest performance under irradiation with LED 455 nm, long induction time was observed and low value of final conversion 19%.Initiating system based on B-Ter-DPA showed the best performance during 455 nm LED irradiation due to the best absorption properties in this range.These results clearly show that for photoinitiating of radical polymerisation, the efficient D-π-A structure brings a significant improvement in photosensitising properties.Commercial photosensitiser THX exhibits significantly lower activity at 420 nm and neglected activity at 455 nm (where terphenyls are still active).

Trimolecular photoinitiating systems
The photoactivity of 2-(diethylamino)-4,6-diphenylbenzene-1,3-carbonitrile derivatives were also investigated as components in three-component photoinitiating systems with iodonium salt and an amine.The studies were performed for systems consisting of iodonium Table 2. Final conversions of CADE and TMPTA in the presence of terphenyl sensitisers (∼0.1%) and iodonium salt IOD (∼1.0%) under LED@405 nm, LED@420 nm and LED@455 nm irradiation.The investigation showed that all m-terphenyl derivatives are able to photoinitiate free-radical photopolymerisation of TMPTA monomer at 405 and 420 nm with final conversion in range from 8% to even 69% in all hybrid systems (Table 3).However, it can be noted that these compounds show high activity mainly in the presence of iodonium salt.For systems containing only terphenyl and EDB, the conversions obtained are lower.This effect is particularly evident for compounds having stronger donors at the border phenyl ring (B-Ter-DPA or B-Ter-ASMe).The addition of the tertiary amine ensures the generation of more radicals which positively affects the photopolymerisation of acrylate monomers such as TMPTA.
The example of compound B-Ter-ACN shows well the differences in photopolymerisation in the presence and absence of iodonium salt (Figure 5).Nevertheless, the significant activity of the terphenyl/EDB system shows that the developed m-terphenyl derivatives are able to efficiently generate radicals also without the use of an oxidant in the form of iodonium salt.This is the evidence for proton abstracting properties of prepared terphenyl derivatives, because in the terphenyl/ EDB system only terphenyl is able to absorb and excite.Therefore, the appropriate energy for proton abstraction can be accumulated only in terphenyl derivatives.The photoactivity comparison of other terphenyl derivatives was shown in the Figures S42-S49 in the Supporting Information.

Photopolymerisation kinetics of formulations for composites 3D-VAT printing
Real-time FT-IR kinetic tests were performed for resin formulations, which were employed for 3D printing experiments in the next stage of the study.Measurements were carried out on 100 µm thick rings, which corresponds to the thickness of one layer of 3D printed objects during the printing process.The concentration of nanoparticles was established as 3.0% because the photoactivity of the presented compounds was studied.Such concentration of heterogenous additive does not cause disturbances on the FT-IR spectrum during measurements of the photopolymerisation process kinetics and so that it does not cause problems during 3D printing associated with low light penetration.The most photoactive terphenyl derivatives B-Ter-ACN and B-Ter-ASMe have been chosen for printing tests.As references, initiator systems were used, where a commercially available THX compound was applied to the role of photosensitiser.The intensity of light on the sample during the photopolymerisation process was 1.99 mW•cm −2 , a value compatible with the power of the used printer (at a power setting of 20%).Table 4 shows the degrees of monomer conversion during the photopolymerisation processes and the conducted process conditions.The photopolymerisation profiles were presented in Figures S50-S53.

Free radical photopolymerisation (acrylate groups)
For the radical photopolymerisation, two simultaneous experiments were conducted.One was to verify the suitability of the investigated compounds for initiating radical photopolymerisation in a ternary initiator system (terphenyl/IOD/EDB 0.1/1/1.5% w/w/w) (resins 1a-c).The second experiment was to check the sensitising properties of the tested terphenyl derivatives in a two-component initiator system (terphenyl/IOD 0.1/1% w/w) (resin 2a-c).The resulting BEDA/TMPTA (7/3) acrylate monomer conversions are shown in Table 4.
As can be seen, the basic resins without the addition of nanoparticles (resins 1a and 2a) show very similar conversions of acrylate groups (about 80%).Both photoinitiating systems shows excellent curing ability of '3D printed layers'.They are also able to efficiently cure resins containing nanoparticles.However, especially in the case of efficiently absorbing nanoparticles like CuO (resins 1b and 2b), their presence lowers the final conversion of the monomer (even to 20%).This is due to competition for light between CuO nanoparticles and molecules of the photoinitiating system.The photoinitiating system is able to convert fewer monomer molecules because less light reaches it.Two-component system exhibit noticeable higher activity of nanocomposites curing.

Catonic photopolymerisation (epoxy and vinyl groups)
For cationic photopolymerisation, it was noticed that for resins 3a and 3b, much higher epoxy and vinyl monomer conversions were obtained than for the reference composition (Reference 3).For resin 3b it was not possible to determine the conversion rates of cationic monomers, which was related to the absorption of light by CuO nanoparticles, which prevented the photoinitiation of cationic polymerisation process.Much higher conversions of epoxy monomer (CADE) were observed than in standard photoinitiating activity testes (described above, Table 2).This phenomenon was caused by dilution of epoxy monomer in vinyl ethers.The lower concentration of CADE makes the intermolecular hydrogen bonds formation (between two monomer molecules) less efficient.Such approach allows successful 3D printing.

Hybrid photopolymerisation (acrylate and epoxy groups)
The final step of the kinetic research was to examine the sensitising properties of B-Ter-ASMe for initiating the photopolymerisation of hybrid photocurable polymer nanocomposites.It was observed that both the conversions of epoxy groups and acrylate groups are very similar (resin 4a-c) and oscillate from 57 to 79%.The advantage of 4a-c resins over the reference resin with a commercially available photosensitiser (THX/IOD (0.1/ 1.0% w/w) and TMPTA/BEDA/S130/CADE (4/4/1/1) is the much higher conversion of oxirane groups, which for Reference 4 is only 31%.Additionally, no significant decrease of final conversion of both monomer was observe.Thus, photoinitiating system based on m-terphenyl derivative is efficient not only in hybrid polymerisation but also in hybrid composite synthesis.

3D-VAT Printing of nanocomposites
As the results presented above indicate, the developed m-terphenyl derivatives are suitable for the preparation of nanocomposites in free-radical, cationic and hybrid photopolymerisation.Therefore, the developed compounds were used to obtain 3D printed nanocomposites.

Setting printing parameters and 3D-VAT printing experiments
Jacob's basic working curves were employed to set printing parameters.The experiments were based on printing several slices (6-9 pieces) of 1 × 1 cm for each of the investigated resins, then measuring the thickness of every printed slice using DLP technology (example slices are shown in the Figure S54).According to the thicknesses obtained, as well as the intensity of light applied to receive the print, it was possible to determine the critical energy and depth of light penetration.
Table 5 shows the calculated E c and D p values for all the resins that were prepared for the 3D printing experiments, as well as for the reference resins.
For radical photopolymerisation with both threecomponent and two-component initiator systems, it can be seen that the addition of 3.0% CuO significantly increases the critical energy value, and the depth of light penetration is reduced compared to an analogous resin without nanoparticles, this phenomenon is unfavourable from an application point of view.Such photocurable resins (such as 1b or 2b) require a higher energy input to achieve high-resolution printing.Concerning photocured resins polymerising according to the radical mechanism (resin 1c and 2c) with the addition of 3.0% Al 2 O 3 , it can be observed that the values of critical energy E c are significantly reduced with reference to 1a and 2a, which is very promising in terms of obtaining photocurable polymer nanocomposites in light-initiated technologies.In the case of cationic resins (3), 3D printing parameters were not determined for resin 3b, which contained 3.0% CuO, which in this case caused excessive light absorption, leading to a lack of photopolymerisation and making it impossible to obtain slices.Meanwhile, the addition of nanoparticles in the form of Al 2 O 3 (resin 3c) caused a worsening of parameters: increased E c and decreased D p (compared to 3a).Regarding the hybrid resin polymerising according to the radical and cationic mechanism (resin 4), the highest value of the D p parameter was obtained for 4c, while 4b showed the lowest value of the E c parameter.Figure 6 shows a comparison of the curves providing the basis for determining the optimal 3D printing parameters for radical resins with a three-component photoinitiating system (1a-c, Reference 1), other graphs showing the determination of E c and D p for resins 2-4 are presented in the Supporting Information (Figures S55-S57).

Comparison of a three-component and a twocomponent photoinitiating systems in freeradical 3D-VAT printing
The performed 3D-VAT printing experiments were aimed at verifying whether terphenyl derivatives can initiate the radical photopolymerisation process as iodonium salt photosensitisers in a two-component initiating system or it is necessary to use a three-component photoinitiating system (terphenyl/IOD/EDB).In accordance with the results obtained, it can be concluded that the addition of amine to the initiating system allows the application of shorter exposure times for individual print layers, as well as lower light output (comparison with an analogous resin without the addition of amine: resin 2).Prints obtained from the analogous resin, where commercially available thioxanthone (THX) was used as a photosensitiser, were employed as reference prints.Table S4 shows a comparison of 3D printing process parameters for resin 1 and 2.
Although slightly worse results were obtained for resin 2 (longer layer exposure time compared to resin 1), the experiments conducted showed that it is possible to obtain prints from radical resin with a two-component photoinitiating system (terphenyl/IOD).The resolution of these prints is as high as with resin 1.This fact is very promising, because it enables the elimination of amine from the initiator system, which is very toxic [88] and often affects the initiation of the photopolymerisation process too quickly, so that the resulting in lack of resolution [89].The comparison of 3D-VAT printing experiment results is presented in In Figure 7A and B  (1a-c, Reference 1 and 2a-c, Reference 2).
The resulting print quality is very high compared to the reference resins.Prints achieved with reference resins have significantly lower resolution, despite the optimal printing parameters employed for these resins.

Photocurable polymer nanocomposites obtained from cationic polymerising resin
A subsequent research step was the preparation of polymer nanocomposites from resins that polymerise according to the cationic mechanism.The suitability of CuO and Al 2 O 3 nanoparticles for obtaining DLP prints was examined.The addition of 3.0% CuO to the epoxyvinyl photocurable resin resulted in the inability to obtain a print, despite applying the highest power (9.95 mW•cm −2 ) and the longest layer curing time (initial layers: 150 s, subsequent layers: 30 s).Table S5 shows the printing parameters for the cationic resins (3).
In view of the living nature of the cationic photopolymerisation process, it can be seen on the resulting prints that the final product layers are slightly overpolymerised (which is particularly noticeable for resin 3b).Preparation of a nanocomposite with 3.0% CuO addition was not achievable, due to too much radiation absorption by the nanoparticles.Obtaining a three-dimensional object from a reference resin (Reference 3) was also not possible, as the print damaged during the printing process, it broke apart, while the initial layers are significantly overpolymerised despite the application of adequate power and exposure time.Obtaining a good quality print from a cationic resin with the addition of 3.0% Al 2 O 3 nanoparticles is a promising application potential.The printed structures are shown in Figure 7C.

3D-VAT Printed hybrid radical-cationic nanocomposites
Lastly, the research involved 3D-VAT printing experiments from hybrid radical-cationic formulations.The applied printing parameters for the hybrid resin without nanoparticles (4a) are identical to those for the resin with the addition of 3.0% Al 2 O 3 (3c)the light intensity incident on the sample was 2.98 mW/ cm 2 , layer thickness was 100 µmas shown in Table S6.However, the well-absorbing black CuO nanoparticles strongly affected the efficiency of photoinitiation using the presented terphenyls.This is due to a reduction in the amount of light reaching the photoinitiating system.In this case, it was necessary to increase the amount of light which effectively reach the molecules of the photoinitiating system.Accordingly, the light intensity was increased to 9.95 mW/cm 2 , and the layer thickness was reduced to 50 µm, making it possible to generate a high-resolution print.The corresponding values were determined using Jacob's curves (the procedure is described in the experimental section) The two-component photoinitiating systems (terphenyl/IOD) demonstrate a much higher suitability for printing photocurable hybrid polymer nanocomposites than the commercial system (THX/IOD), as can be seen in the Figure 7D, where a comparison of the resulting resin printouts is shown: 4a-c, Reference 4.During the experiments, the possibility of producing prints from hybrid resins with the addition of nanoparticles was confirmed.For cationic-radical resins, the effect of overpolymerisation was not noticed, as it is the case for living cationic photopolymerisation.

Conclusions
The presented amino-m-terphenyl derivatives through the use of D-π-A structure show desirable absorption characteristics, especially in visible light.Due to the appropriate structure design, these compounds have very favourable redox properties which allows them to be used in two-component photoinitiating systems with iodonium salt for free-radical, cationic and hybrid photopolymerisation using light even from 455 nm VIS-LED.They are able to efficiently photoinitiate cationic polymerisation despite numerous amine groups in their structure.Moreover, the use of appropriate modifications allows tuning the photochemical and electrochemical properties of these compounds by changing the electron density within the donor amine group.High photoactivity of the described compounds allowed to use them as efficient photoinitiators for 3D-VAT printing of nanocomposites (especially containing Al 2 O 3 nanoparticles but prints for resins with much more demanding CuO nanofiller were also obtained) with outstanding resolution under clearly defined conditions on DLP-type 3D printer.

Figure 2 .
Figure 2. Structures of described amino-m-terphenyl derivatives.Group I with simple amino group donor, group II with amino group tuned by functional groups at phenyl ring.

Figure 3 .
Figure 3. UV-Vis absorption spectra of amino-m-terphenyl derivatives in acetonitrile, A -Group I derivatives, B -Group II derivatives.

Figure 5 .
Figure 5. Polymerisation profiles of TMPTA monomer, in laminate, upon exposure to an LED at 405 nm (A), 420 nm (B), in the presence of two-and three-component photoinitiating systems based on m-terphenyl derivatives.

Figure 7 .
Figure 7. Digital 3D projects and resulted printed 3D nanocomposites patterns of resins contain terphenyl based and referential photoinitiating systems: Aradical resin with three-component photoinitiating system, Bradical resin with two-component photoinitiating system, Ccationic resin with two-component photoinitiating system, Dhydride radical/cationic resin with two-component photoinitiating system.

Table 3 .
The TMPTA polymerisation photoinitiating efficiency of m-terphenyl derivatives in two-and three-component photoinitiating system consisting of iodonium salt and EDB amine.

Table 4 .
Photopolymerisation kinetics and process parameters of formulation based on m-terphenyls for nanocomposites 3D-VAT printing.

Table 5 .
Determined printing properties for photocurable resins for 3D-VAT printing.