Thioxanthone dicarboxamide derivatives as one-component photoinitiators for near-UV and visible LED (365–405 nm) induced photopolymerizations

Abstract

A series of thioxanthone dicarboxamide derivatives (TX-DCAs) containing hydrogen donors as one-component photoinitiators (PIs) have been prepared. In particular, the photoreactivity of these PIs was remarkably improved due to the covalent binding of the N-phthalimido derivative and type II chromophore thioxanthone. These derivatives exhibit interesting shifted absorption so that they can be utilized as versatile PIs upon exposure to various violet and visible LEDs (365 nm, 385 nm, 395 nm and 405 nm), and can efficiently actuate the free radical photopolymerization of acrylates.

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Introduction

Photopolymerizations using light-emitting diodes (LEDs) are receiving a great deal of attention, and demonstrate an enormous potential as substitutes of traditional Hg lamp induced photopolymerizations due to their advantages including lower cost, better environment-friendliness and larger application fields.¹ However, the classical/commercial photoinitiators (PIs) usually suffer from poor light absorption properties for λ > 365 nm due to their initial development for UV lamps and their good matching with the emission spectra of Hg lamps.^2,3 Hence, to successfully use LEDs in photopolymerization reactions, the design and development of high-performance PIs with adapted absorption wavelengths and excellent photochemical properties is one of the most important points.

Among these recently developed PIs with novel structures,^2,4–9 a lot of aromatic ketones are renowned for their excellent optical characteristics. Of particular interest is thioxanthone (TX) due to its adaptability for bearing different functionalities and its applications in various modes of photopolymerization, in which it accomplishes photoinitiation in conjunction with other co-initiator compounds; a behavior that is referred to as bi-molecular photoinitiation.^10–13 Interestingly, if there is a hydrogen-donating site on these introduced groups, a one-component initiating system can be achieved by the intramolecular or intermolecular hydrogen donating reaction.^14–20 It is worth noting that these one-component initiating systems usually have a higher initiating efficiency than those two-component initiating systems.²¹

Very recently, we have reported a thioxanthone-based N-phthalimidoamino acid ammonium salt (thioxanthen-DBU, Scheme 1) used as the highly effective photocaged superbase.²² It exhibits good absorption characteristics with a maximum at 285 nm and 418 nm and surprisingly a tail over 480 nm. A clear red shift was observed compared with TX due to the amide substituents on the TX skeleton. Furthermore, the amine-mediated redox photopolymerization of acrylate was able to rapidly actuate by thioxanthen-DBU in combination with a benzoyl peroxide (BPO) initiator.

Scheme 1 Chemical structures of the studied thioxanthone dicarboxamide derivatives (TX-DCAs) and previously investigated thioxanthen-DBU and BPO.

These encouraging results prompted us to explore further the possibility to prepare novel thioxanthone dicarboxamide derivatives (TX-DCAs) use as LED PIs with adapted absorption wavelengths and excellent photochemical properties. In the present paper, we synthesized three new TX-DCAs in conjunction with hydrogen donors such as carboxyl, thiol and hydroxyl groups (Scheme 1), which had taken the following points into consideration. First, TX with maxima at 383 nm possesses good absorption characteristics and high photoinitiation efficiency in the near-UV and visible regions. Second, the interactions between N-substituted maleimides and type II PIs will lead to enhanced photoefficiency. Finally, one-component molecular structures were designed by introducing the hydrogen bond donors such as carboxyl, thiol and hydroxyl groups. The photophysical and photochemical properties were investigated. In particular, in order to demonstrate the potential of TX-DCAs as LED PIs, the free radical polymerization (FRP) of acrylates were monitored (by RT-FTIR) in the ultraviolet-to-visible wavelength range using selected LEDs at 365 nm, 385 nm, 395 nm and 405 nm.

Experiments

Materials

1H-Thioxantheno[4,3-c]furan-1,3,6-trione was synthesized according to the literature procedure.²² Trans-4-(aminomethyl)cyclohexanecarboxylic acid (98%), 2-isopropylthioxanthone (ITX, Tianjin Jiuri Chemical) and 2-aminoethanol (99%) were purchased from Aladdin-reagent (China). Mercapto-ethylamine (95%, Xiya Reagent) and tripropylene glycol diacrylate (TPGDA, Sartomer Company) were used as received. All other chemicals used were analytical grade and used without further purification.

Characterization

The NMR spectra were obtained on a Varian 300 MHz spectrometer with DMSO-d₆ and TMS as the solvent and internal standard, respectively. FTIR spectra were obtained on a Bruker/Tensor 27 spectrophotometer and recorded from 32 scans with a resolution of 4 cm⁻¹. Elemental analysis was obtained on an Elementar Vario EL analyzer. Thermogravimetric (TG) tests were performed in the 40–750 °C range, using a TG-209 Netzsch thermogravimetric analyzer at a heating speed of 20 °C min under N₂ atmosphere. UV-vis absorption spectra were obtained on a Perkin Elmer Lambda 750 UV-visible spectrophotometer. Acrylate conversions were monitored by real-time Fourier transform infrared (RTIR) spectroscopy using a modified Nicolet 5700 spectrometer. Photopolymerizations were conducted in a mold from two glass plates and spacers with 15 ± 1 mm in diameter and 1.2 ± 0.1 mm in thickness, changes in the peak area from 6104 to 6222 cm⁻¹ attributed to the stretching vibration were used to monitor acrylate polymerization kinetics.

2-(2-Hydroxyethyl)thiochromeno[2,3-e]isoindole-1,3,6(2H)-trione (TX-DCA-OH)

A solution of 1H-thioxantheno[4,3-c]furan-1,3,6-trione (1.410 g, 5 mmol) and 2-aminoethanol (0.305 g, 5 mmol) in 1,4-dioxane (70 mL) was refluxed for 3 h. The reaction mixture was cooled to room temperature, dried in vacuo, and then washed with ether to give a yellow product. Yield: 94.0%. ¹H NMR (300 MHz, DMSO-d₆, δ, ppm): 8.79 (1H, d), 8.44 (1H, d), 7.95 (2H, t), 7.82 (1H, t), 7.62 (1H, t), 3.66 (4H, m). IR (KBr, cm⁻¹): 735 (ν_C–S), 1589 (ν_C=C), 1640, 1765 (ν_C=O), 1463 (ν_O=H). Anal. found: C, 62.59; H, 3.49; N, 4.33; O, 19.68; S, 9.91. Calcd for C₁₇H₁₁NO₄S: C, 62.76; H, 3.41; N, 4.31; O, 19.67; S, 9.85%.

4-((1,3,6-Trioxothiochromeno[2,3-e]isoindol-2(1H,3H,6H)-yl)methyl)cyclohexanecarboxylic acid (TX-DCA-COOH)

A solution of 1H-thioxantheno[4,3-c]furan-1,3,6-trione (1.410 g, 5 mmol) and trans-4-(aminomethyl)-cyclohexanecarboxylic acid (0.785 g, 5 mmol) in acetic acid (500 mL) was refluxed for 3 h. The reaction mixture was cooled to room temperature and stood overnight. The obtained yellow crystalline product was filtered, washed with dimethylbenzene, and then dried in vacuo. Yield: 66.1%. ¹H NMR (300 MHz, DMSO-d₆, δ, ppm): 8.80 (1H, d), 8.44 (1H, d), 7.95 (2H, t), 7.83 (1H, t), 7.63 (1H, t), 3.44 (2H, d), 2.14 (1H, m), 1.90 (2H, m), 1.72 (3H, m), 1.22 (2H, m), 1.05 (2H, m). IR (KBr, cm⁻¹): 734 (ν_C–S), 927, 1400 (ν_O=H), 1590 (ν_C=C), 1640, 1773 (ν_C=O). Anal. found: C, 65.29; H, 4.56; N, 3.24; O, 19.21; S, 7.70. Calcd for C₂₃H₁₉NO₅S: C, 65.54; H, 4.54; N, 3.32; O, 18.98; S, 7.61%.

2-(2-Mercaptoethyl)thiochromeno[2,3-e]isoindole-1,3,6(2H)-trione (TX-DCA-SH)

A solution of 1H-thioxantheno[4,3-c]furan-1,3,6-trione (1.410 g, 5 mmol) and mercapto-ethylamine (0.385 g, 5 mmol) in acetic acid (20 mL) was refluxed for 3 h. The reaction mixture was cooled to room temperature and stood overnight. The obtained yellow crystalline product was filtered, washed with dimethylbenzene, and then dried in vacuo. Yield: 76.2%. ¹H NMR (300 MHz, DMSO-d₆, δ, ppm): 8.85 (1H, d), 8.48 (1H, d), 7.99 (2H, t), 7.85 (1H, t), 7.66 (1H, t), 3.79 (2H, m), 2.78 (2H, m). IR (KBr, cm⁻¹): 736 (ν_C–S), 1583 (ν_C=C), 1640, 1767 (ν_C=O), 2537 (ν_S–H). Anal. found: C, 59.72; H, 3.26; N, 4.01; O, 14.61; S, 18.40. Calcd for C₁₇H₁₁NO₃S₂: C, 59.81; H, 3.25; N, 4.10; O, 14.06; S, 18.78%.

Photopolymerization procedure

Typical procedure used as follows: photoinitiator TX-DCAs (1 × 10⁻⁵ mol) was dissolved in 1,4-dioxane (0.5 mL) under ultrasonication, and then monomer TPGDA (0.5 g) was added to this solution. At last, the mixture was injected into a mold. The photopolymerization was initiated by optical cable-directed LED light source (UVEC-4II, Lamplic Technology China) possessing 365 nm, 385 nm, 395 nm and 405 nm were as used as the irradiation source. The light intensity at the surface level of the cured samples was measured to be 20 mW cm⁻².

Results and discussions

Synthesis and characterization of TX-DCAs

In this paper, we synthesize three new and one-component TX-DCAs in conjunction with hydrogen donors such as carboxyl, thiol and hydroxyl groups. First, type II TX-based PIs have become a preferable class of PIs over other similar structures such as benzophenones, primarily because of their spectral characteristics. Their absorption maxima appear in the range of 380–420 nm, laying in the near UV and visible ranges, which reduces the required energy for photoexcitations and subsequent formation of initiating radicals. Next, chemical incorporation of various co-initiators into the structure of PIs makes one-component PIs exhibit double functionality. One-component Type II PIs form initiating species through intramolecular and/or intermolecular interactions between the triplet state chromophore core and co-initiator part of the PI, and overcome the drawbacks such as odor, toxicity and migration resulted from the use of low molecular-weight coinitiators for most of the conventional hydrogen abstraction PIs.^9,17 In this regard, carboxyl-, thiol- and hydroxyl-like hydrogen donors have been incorporated into PI structures to make one-component photoinitiating systems. Finally, there are significant interactions between N-substituted maleimides and type II PIs, which lead to enhanced photoefficiency. Thus, we would design the new thioxanthone dicarboxamide derivatives (TX-DCAs) in conjunction with hydrogen donors. The obtained TX-DCA-COOH, TX-DCA-OH and TX-DCA-SH were characterized by ¹HNMR (Fig. S1–S3†), IR spectra (Fig. S4–S6†) and elemental analysis.

Thermal stability is one of the important parameters of PIs. As shown in Fig. 1 and Table 1, both TG₀ (the initial decomposition temperature) and TG_5% (the decomposition temperature at the point of 5% weight loss) were above 125 °C and 210 °C, respectively, exhibiting good thermal stability.

Fig. 1 Thermogravimetric profiles of TX-DCAs PIs with heating rate at 20 °C min⁻¹ under N².

Table 1 Thermal stability of TX-DCAs

PIs	TG0^a (°C)	TG5%^b (°C)
a The initial decomposition temperature.b The decomposition temperature at the point of 5% weight loss.
TX-DCA-COOH	309.8	335.5
TX-DCA-OH	128.3	214.1
TX-DCA-SH	170.0	278.3

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Light absorption of the studied photoinitiators

The light absorption spectra of TX-DCA-COOH, TX-DCA-OH and TX-DCA-SH in dimethyl sulfoxide are depicted in Fig. 2 and Table 2. All TX-DCAs exhibit similar absorption characteristics with maxima at 280 nm and 410 nm, and a tail over 460 nm, exhibiting a red shift compared with 2-isopropylthioxanthone (ITX), due to the presence of the carboxamide structures on the TX skeleton. In particular, for TX-DCA-COOH and TX-DCA-SH, the maxima are located in the UV light range (i.e., λ_max = 284 nm, ε_{284 nm} ∼ 132 250 M⁻¹ cm⁻¹ and λ_max = 283 nm, ε_{283 nm} ∼ 134 800 M⁻¹ cm⁻¹ for TX-DCA-COOH and TX-DCA-SH, respectively) and visible light region (i.e., λ_max = 414 nm, ε_{414 nm} ∼ 29 080 M⁻¹ cm⁻¹ and λ_max = 417 nm, ε_{417 nm} ∼ 27 940 M⁻¹ cm⁻¹ for TX-DCA-COOH and TX-DCA-SH, respectively). As to the spectrum of TX-DCA-OH, it presents a much better light absorption (λ_max = 280 nm, ε_{280 nm} ∼ 141 340 M⁻¹ cm⁻¹ and λ_max = 404 nm, ε_{404 nm} ∼ 41 020 M⁻¹ cm⁻¹). However, ITX with the shortest maximum absorption wavelength has the poorest light absorption (λ_max = 280 nm, ε_{280 nm} ∼ 124 338 M⁻¹ cm⁻¹ and λ_max = 404 nm, ε_{404 nm} ∼ 16 096 M⁻¹ cm⁻¹).

Fig. 2 UV-vis absorption spectra of TX-DCAs (1 × 10⁻⁵ M) in DMSO.

Table 2 Light absorption properties of the studied ITX and TX-DCAs: maximum absorption wavelengths λ_max and extinction coefficients at λ_max and at the maximum emission wavelengths of the different irradiation devices

PIs	λmax (nm)	εmax^a (M^−1 cm^−1)	ε365 nm^b (M^−1 cm^−1)	ε385 nm^b (M^−1 cm^−1)	ε395 nm^b (M^−1 cm^−1)	ε405 nm^b (M^−1 cm^−1)
a Maximum absorption wavelength in the near visible range.b For different UV or visible LEDs.
ITX	257, 383	124 338, 16 096	11 804	16 006	8098	234
TX-DCA-COOH	284, 414	132 250, 29 080	185	12 640	21 000	26 800
TX-DCA-OH	280, 404	141 340, 41 020	15 560	30 490	38 160	40 890
TX-DCA-SH	283, 417	134 800, 27 940	711	11 690	19 960	25 690

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The extended light absorption range of these three compounds makes them adapted to a wider range of LED devices (i.e. from near UV to visible LED light sources with a dominant emission wavelength range between 365 nm and 405 nm). For TX-DCAs under different LED (365 nm, 385 nm, 395 nm and 405 nm), the observed order of extinction coefficients is ε_{405 nm} > ε_{395 nm} > ε_{385 nm} > ε_{365 nm}. TX-DCA-OH presents a much better light absorption than other TX-DCAs and ITX. ITX with the shortest maximum absorption wavelength has the poorest light absorption in the near-UV and visible regions.

Photoinitiating ability of the investigated PIs

Initially the catalytic behavior of TX-DCAs and commercial PI ITX, were compared upon the near-UV and visible LED (365 nm, 385 nm, 395 nm and 405 nm) at room temperature by real-time Fourier transform infrared (FTIR) spectroscopy. As depicted in Fig. 3, these photopolymerizations without obvious induction periods could be rapidly actuated under photo-irradiation. Almost 100% final conversions of double bonds were achieved, and all PIs exhibited the fastest photopolymerization rates under the LED 365 nm. The maximum initiation rate (R_pmax) and reaction time at 90% conversion (T_C90%) are two more meaningful parameters for assessing photopolymerization efficiency of PIs. As listed in Table 3, TX-DCA-COOH had a relatively low efficiency under the LED 385 nm and 405 nm, 90% conversions were obtained in 1.4 min, while the highest efficiency (R_pmax = 328.1 min⁻¹, T_C90% = 0.63 min) was reached when the FRP was carried out under the LED 365 nm (Fig. 3a, Table 3). After LED 365 nm, the next fastest polymerization was observed under the LED 395 nm (R_pmax = 149.2 min⁻¹, T_C90% = 1.06 min). TX-DCA-OH also exhibited an excellent efficiency (Fig. 3b, Table 3), the observed order of polymerization rate was 365 nm (R_pmax = 148.4 min⁻¹, T_C90% = 1.46 min) > 405 nm (R_pmax = 133.4 min⁻¹, T_C90% = 1.70 min) > 395 nm (R_pmax = 109.1 min⁻¹, T_C90% = 2.03 min) > 385 nm (R_pmax = 65.9 min⁻¹, T_C90% = 2.36 min). As to the photopolymerizations of TX-DCA-OH and ITX (Fig. 3c and d, Table 3), the LED 365 nm and 395 nm exhibited similar order of polymerization rate, and the order of polymerization rate was 365 nm ≈ 395 nm > 405 nm > 385 nm.

Fig. 3 Photopolymerization profiles of TPGDA upon the near-UV and visible LED (365 nm, 385 nm, 395 nm and 405 nm) in the presence of (a) TX-DCA-COOH, (b) TX-DCA-OH, (c) TX-DCA-SH and (c) ITX. Experimental conditions: [TX-DCAs] = 1 × 10⁻⁵ mol, dioxane = 0.5 g, TPGDA = 0.5 g.

Table 3 Photopolymerization properties of the studied PIs: maximum initiation rate R_pmax and reaction time at 90% conversion T_C90%

PIs	365 nm			385 nm			395 nm			405 nm
	Rpmax^a (min^−1)	TC90%^b (min)		Rpmax (min^−1)	TC90% (min)		Rpmax (min^−1)	TC90% (min)		Rpmax (min^−1)	TC90% (min)
a The maximum initiation rate.b TC90% can be expressed by the reaction time at 90% conversion.
TX-COOH	328.1		0.63	112.1		1.40	149.2		1.06	145.5		1.40
TX-OH	148.4		1.46	65.9		2.36	109.1		2.03	133.4		1.70
TX-SH	88.1		2.40	40.9		4.38	69		2.50	44.9		3.07
ITX	73.7		2.80	34.0		5.06	66.7		3.15	45.9		3.12

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For the purpose of obtaining highly effective photopolymerization, the effect of the PI structures on the rate profiles of photopolymerization initiated by the same LED light source were investigated (Fig. 4). Generally, compared to TX-DCA-OH initiated photopolymerization, TX-DCA-COOH remarkably accelerated the photopolymerization, showing enhanced polymerization rate R_pmax and T_C90%. After TX-DCA-OH, the next faster polymerization was observed with TX-DCA-SH, followed by ITX. Indeed, under the same LED light source, the observed order of polymerization rate was TX-DCA-COOH > TX-DCA-OH > TX-DCA-SH > ITX. This can be explained by the fact that these one-component PIs were designed by chemical incorporation of various co-initiators into the structure of PIs, and significant interactions between N-substituted maleimides and type II PIs enhanced the photoefficiency.

Fig. 4 Photopolymerization profiles of TPGDA in the presence of different PIs (TX-DCA-COOH, TX-DCA-OH, TX-DCA-SH and ITX) upon the near-UV and visible LED (a) 365 nm, (b) 385 nm, (c) 395 nm and (c) 405 nm. Experimental conditions: [TX-DCAs] = 1 × 10⁻⁵ mol, dioxane = 0.5 g, TPGDA = 0.5 g.

The effect of a variation of power intensity (5, 10 and 20 mW cm⁻²) on the initiation capability was investigated, as exemplified by TX-DCA-COOH upon the 365 nm LED. TX-DCA-COOH possessing the fastest photosensitive rate was chosen for more in-depth study. As shown in Fig. 5, the polymerization rate and final conversion showed a consistent dependence on the power intensity, which also could be used to further shorten the polymerization duration.

Fig. 5 Photopolymerization profiles of TPGDA upon the 365 nm LED with a variation of power intensity (5, 10 and 20 mW cm⁻²).

Photopolymerization mechanism of TX-DCAs

Early mechanistic studies on one-component systems of thioxanthone acetic acid derivative suggested an aromatic carbonyl sensitized decarboxylation mechanism.^15,23,24 The decarboxylation process during the photolysis of N-phthalimidoamino acid derivative was also described in the literature.^25,26 Excited aromatic carbonyl compounds can undergo the abstraction of the acidic hydrogen by the triplet excited state TX core. Subsequent decarboxylation process evolves carbon dioxide, and yields initiating alkyl radicals. Taking the foregoing points into consideration, the principal photopolymerization mechanism of TX-DCA-COOH can be laid out in Scheme 2.

Scheme 2 Photopolymerization mechanism of TX-DCA-COOH through intramolecular hydrogen abstraction.

Thiol or hydroxyl substituted TXs, as one-component TX PIs, has been extensively utilized in preliminary works.^27–29 As revealed by the above laser flash photolysis studies, an intermolecular interaction between triplet ³TX-SH* and ground state TX-SH molecules results in the formation of thiyl radicals through consecutive electron transfer and hydrogen atom abstraction processes. Intramolecular interaction is unlikely to happen due to the rigidity of the spacer group between the carbonyl and thiol functionalities and therefore the dominant reaction is through an intermolecular hydrogen abstraction process. As the example of TX-DCA-SH, photopolymerization mechanism of TX-DCA-SH through hydrogen abstraction can be described in Scheme 3.

Scheme 3 Photoinitiated radical polymerization of TX-DCA-SH.

Conclusions

In this paper, three novel photoinitiators TX-DCA-COOH, TX-DCA-OH and TX-DCA-SH are proposed for the radical polymerization of acrylates upon exposure to near UV or visible LED lights. These one-component and odourless new photoinitiators are very attractive, since these do not require additional hydrogen donors. The interactions between N-substituted maleimides and type II PIs led to enhanced photoefficiency. For TX-DCA-based PIs, the excellent efficiency was found for the initiation of the polymerization of acrylates. The present paper opens a new direction for the design of new scaffolds and further novel derivatives usable as high-performance PIs under various UV or visible LEDs.

Acknowledgements

This research was financially supported by National Natural Science Foundation of China (21404042), Natural Science Foundation of Guangdong Province (2014A030310166), International Postdoctoral Exchange Fellowship Program ([2014] 29) and Fundamental Research Funds for the Central Universities (2016ZM060).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13512a

‡ These authors contributed equally to this work.

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