Thioxanthone acetic acid ammonium salts: highly efficient photobase generators based on photodecarboxylation

Abstract

A series of photobase generators (PBGs), which contain thioxanthone as the chromophore and different quaternary ammonium salts as latent active species, were straightforwardly synthesized in two steps and their structures were characterized. Investigation on the photophysical properties of the PBGs indicated that the PBGs were photosensitive to long wavelengths due to the characteristic absorption of thioxanthone at 380 nm. A study on photolysis confirmed the photoinduced decarboxylation mechanism, through which the PBGs released active basic species efficiently. All the novel PBGs showed high activity in catalysing thiol–epoxy polymerization without postexposure baking. This is especially true for the PBG containing 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), which exhibited a faster polymerization rate and higher epoxy conversion than the previously described highly active PBG system.

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Introduction

The base-mediated thiol–epoxy reaction is a versatile tool and meets the criteria of a ‘click’ reaction: benign reaction conditions, fast reaction rate, high yields and outstanding orthogonal reactivity.^1–3 Polymerization based on the thiol–epoxy reaction has long been employed in commercial applications including adhesives and high performance coatings.⁴ Photoinduced thiol–epoxy polymerization provides additional advantages of spatial and temporal control of the process.⁵ The degree of the polymerization can be adjusted by simply tuning the exposure dose. The significant features of the photo-control strategy have expanded the application range of the thiol–epoxy reaction to many important biosynthetic and biomedical applications.³

A highly active photobase generator (PBG) is essential to obtain an efficient photoinduced thiol–epoxy polymerization.^6,7 An ideal PBG requires a suitable chromophore to ensure strong absorption at certain wavelengths, as well as an efficient mechanism, through which the excited molecule can produce active species in high yields.⁸ Moreover, the initiating ability of the formed active species has great influence on the initiation efficiency.

A variety of PBGs, such as acyloxyimines, oxime–urethanes, and α-keto carbamates, have been reported.^9,10 Since only a relatively weak amine base is formed after irradiation, long exposure time and high postbaking temperature are needed.¹¹ Recently developed PBGs containing powerful organic nitrogen bases like DBU (1,5-diazabicyclo[5.4.0]undec-5-ene) and TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) can provide efficient polymerization without postbaking.¹² However, most of the reported PBGs are limited to wavelengths in the UV-C and UV-B range (<320 nm). Excitation by short UV light suffers from some issues including ozone generation, radiation safety and limited curing depth.¹³ With a rapid development of the long wavelength irradiation sources, such as light emitting diode (LED) which have advantages in irradiation safety and energy utilization efficiency,¹⁴ it is highly desired to develop efficient PBGs sensitive to UV-A and visible light.

Sensitization by a photosensitizer can increase the photosensitivity of PBGs to the long wavelength irradiation.¹⁵ Thioxanthones are well-known Norrish type II photoinitiators with absorptions between 360–400 nm.^14,16–18 The long wave absorption combined with higher triplet energy makes thioxanthones attractive as visible light photosensitizers.¹⁹ Bowman and co-workers reported a visible-light base generating system based on mixture of 2-isopropylthioxanthone (ITX) and triazabicyclodecene tetraphenylborate (TBD·HBPh₄).⁸ The PBG system was able to induce thiol–epoxy/thiol–acrylate hybrid polymerizations efficiently.³ The limitation of such PBG system derives from the intrinsic drawback of bimolecular system that the back electron transfer occurs between the excited photosensitizer and the base generator, which would decrease the initiation efficiency.^20,21

Photoinduced decarboxylation is an effective strategy to prevent back electron transfer.²² Arsu has proved that thioxanthone acetic acid is a highly efficient free radical photoinitiator, which undergoes photoinduced decarboxylation to produce active free radicals.²³ With a proper molecular design, such concept should be transferred to produce PBGs releasing active base in high quantum yields after irradiation. In this paper, a straightforward synthesis of a series of novel PBGs based on thioxanthone were presented and photochemical behaviours of the PBGs in thiol–epoxy reaction were investigated. For comparison, the efficient PBG system, based on physical mixture of ITX and TBD·HBPh₄ from the literature,²⁴ was also tested. Investigation on the photophysical properties of the PBGs was conducted via UV-vis absorption and fluorescence emission. Finally, photopolymerization kinetics was studied via real time FTIR (RT-FTIR) and the initiation mechanism of the PBGs was proposed according to the photolysis study.

Experimental

Materials and instruments

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), and thiosalicylic acid were purchased from Aladdin Industrial Corporation. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) was obtained from J&K Scientific Ltd. Pentaerythritol tetra(3-mercaptopropionate) was supplied by TCI Chemicals (Shanghai) Pvt. Ltd. Cyclohexylamine, phenylacetic acid, tetramethylammonium hydroxide, sodium tetraphenylboron and the other solvents were bought from Sinopharm Chemical Reagent Co., Ltd. Epoxy monomer (Epalloy 4100) was a gift from Jiangsu Sanmu Group. Column chromatography was performed with conventional techniques on silica gel (200–300 mesh, Qingdao Haiyang Chemical Co., Ltd), and silica gel plates were used for TLC analyses. All the chemicals were used as received without further purification. Fig. 1 lists the structures, abbreviations and their code names employed in this study.

Fig. 1 Structures of the investigated PBGs and monomers.

Synthetic procedures

The following preparation were performed in the dark.

Synthesis of thioxanthone acetic acid

Thioxanthone-acetic acid was synthesized according to Yilmaz et al.²⁵ Thiosalicylic acid (1.54 g, 10 mmol) was slowly added to 20 mL of concentrated sulfuric acid and the mixture was stirred for 30 min to ensure through mixing. Phenylacetic acid (4.05 g, 30 mmol) was added slowly to this stirred mixture. After the addition, this mixture was fiercely stirred for 2 hours at room temperature and then for 4 hours at 75 °C. Then the mixture was cooled to room temperature and stirred overnight. Afterward, the mixture was slowly poured into boiling water (200 mL) with stirring, and precipitation was collected and washed with water to afford a crude blue solid, which was recrystallized from dioxane/water mixture to give a pure blue solid (yield, 68%). ¹H NMR (400 MHz, d6-DMSO) δ 12.53 (s, 1H), 8.52–8.34 (m, 2H), 7.92–7.49 (m, 5H), 3.79 (s, 2H).

Synthesis of compound (9-oxo-9H-thioxanthen-2-yl)-acetate-1,2,3,4,6,7,8,9-octahydro-pyrimido[1,2-a]pyrimidin-5-ylium (TX-TBD)

A solution of thioxanthone-carboxylic acid (0.270 g, 1 mmol) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, 0.14 g, 1 mmol) in THF (5 mL) was stirred for 2 h at room temperature (Scheme 1). The resulting precipitate was filtered off and dried in vacuum to obtain 0.21 g (yield, 51%) colourless crystals. ¹H NMR (400 MHz, d6-DMSO) δ 8.52–8.25 (m, 2H), 7.65 (m, 5H), 3.42 (s, 2H), 3.29 (s, 2H), 3.23 (s, 4H), 3.11 (s, 4H), 1.84 (s, 4H). ¹³C NMR (101 MHz, d6-DMSO) δ 178.86, 175.30, 151.13, 138.24, 136.66, 134.75, 133.30, 132.91, 132.80, 129.07, 128.35, 127.88, 126.77, 126.59, 126.56, 125.89, 125.81, 46.02, 45.09, 42.77, 40.08, 39.87, 39.04, 37.08, 20.41. UltrflexTOF/TOF (m/z): calcd for C₇H₁₃N₃⁺, 139.1104. Found: 140.1183 [M]⁺.

Scheme 1 Synthetic route of PBGs.

Synthesis of compound (9-oxo-9H-thioxanthen-2-yl)-acetatecyclohexyl-ammonium (TX-CyA)

With a similar synthetic scheme to TX-TBD, cyclohexylamine (CyA) was treated with thioxanthone-carboxylic acid to obtain the carboxylate in a 46% yield as orange viscous solid. ¹H NMR (400 MHz, d6-DMSO) δ 8.51–8.31 (m, 2H), 7.87–7.47 (m, 5H), 3.54 (d, J = 47.6 Hz, 5H), 2.81–2.72 (m, 1H), 1.86–1.72 (m, 2H), 1.66 (d, J = 11.8 Hz, 2H), 1.54 (d, J = 12.5 Hz, 1H), 1.26–0.98 (m, 5H). ¹³C NMR (101 MHz, d6-DMSO) δ 178.82, 136.64, 134.69, 133.45, 132.87, 132.79, 129.16, 129.07, 128.83, 128.35, 127.90, 126.69, 126.59, 126.54, 125.88, 125.73, 66.99, 49.09, 31.59, 24.76, 23.96. UltrflexTOF/TOF (m/z): calcd for C₆H₁₄N⁺, 100.1121. Found: 100.1126 [M]⁺.

Synthesis of compound (9-oxo-9H-thioxanthen-2-yl)-acetate-2,3,4,6,7,8,9,10-octahydro-1H-pyrimido[1,2-a]azepin-5-ylium (TX-DBU)

0.152 g of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was added to a solution of thioxanthone-carboxylic acid (0.270 g, 1 mmol) in THF. The solution was stirred at room temperature overnight. The resulting yellowish viscous oil was obtained and washed with toluene. This viscous oil was dried in vacuum and 0.19 g (yield, 72%) yellowish liquid was obtained. ¹H NMR (400 MHz, d6-DMSO) δ 8.53–7.44 (m, 7H), 3.70–3.30 (m, 6H), 3.19 (d, J = 14.8 Hz, 3H), 2.72 (d, J = 7.5 Hz, 2H), 1.84 (s, 2H), 1.60 (d, J = 25.8 Hz, 6H). ¹³C NMR (101 MHz, d6-DMSO) δ 179.98, 165.94, 137.44, 134.48, 134.39, 132.06, 132.03, 129.93, 129.66, 129.16, 128.71, 127.52, 126.34, 126.06, 126.01, 125.73, 54.10, 48.50, 38.00, 32.09, 29.01, 26.82, 23.99, 19.52. UltrflexTOF/TOF (m/z): calcd for C₉H₁₇N₂⁺, 153.1386. Found: 153.1390 [M]⁺.

Characterization

¹H NMR (400 MHz) and ¹³C (101 MHz) spectra were measured in dimethyl sulfoxide-d6 using a Bruker AVANCE III HD 400 MHz. High-resolution mass spectrometer measurements were performed using Ultraflex TOF/TOF from Bruker Daltomics. UV-vis spectra were measured with a Beijing Purkinje TU-1901 UV-VIS spectrophotometer. The pH of the PBG solutions, which was irradiated by IWATA UV-LED at 365 nm, was measured with a Shanghai REX PHS-25 pH meter in water. The polymerization experiments were carried out using real-time FTIR with a Nicolet 6700 FT-IR spectrometer worked with an OmnicCure Series 1000 UV spot curing system. The incident light intensity was monitored by radiometers (Photoelectric Instrument Factory of Beijing Normal University, or UV Power Puck® II from EIT Inc).

Photopolymerization study

The UV-curable formulations were prepared by mixing monomers and 2 mol% of the PBGs, then 0.1 mL of dichloromethane was added for a homogenous mixture and the mixture were well-mixed by ultrasonic vibration for 10 minutes. Before the photopolymerization, the mixture were dried under vacuum for 30 minutes at room temperature in order to remove the dichloromethane. The thickness of the mixture film was adjusted by sandwiching the samples between two KBr salt plates.

Real-time FTIR

In this work, real-time FTIR spectroscopy was used to monitor the thiol group and epoxy group conversion as a function of exposure of irradiation in the resins. An Hg lamp adapted to the FTIR spectrometer by means of a light guide, was adjusted the light intensity to 20.3 mW cm⁻² and used to irradiate the resins. Polymerization profiles were recorded during 1000 seconds irradiation at room temperature. The polymerization kinetics were measured by monitoring the disappearance of the thiol and epoxy group. For each sample, the real-time FTIR runs were repeated three times.

The epoxy and thiol bond conversion was calculated by formula:²⁶

where A_t is the area of the epoxy and thiol characteristic absorbance peak at 915 cm⁻¹ and 2568 cm⁻¹ at time t, A₀ stands for the initial area of this peak.

Results and discussion

Synthesis

The novel PBGs contain thioxanthone acetic acid as the chromophore which is able to undergo photoinduced decarboxylation with a high quantum yield, and different quaternary ammonium salts as latent active species (Fig. 1). The PBGs can be synthesized straightforwardly in two steps. The synthesis of the precursor thioxanthone acetic acid has been well developed via the coupling of thiosalicylic acid and phenylacetic acid in the presence of concentrated sulfuric acid.²⁵ The desired PBGs can be obtained by simply neutralizing the thioxanthone acetic acid with corresponding amines. Besides strong bases (TBD and DBU), relatively weaker base cyclohexanamine (CyA) was chosen to study the structure–property relationship, especially the influence of alkaline on the initiation efficiency.

Photophysical properties

To obtain the information about the photophysical properties, the UV-vis absorption spectra were recorded in acetonitrile (Fig. 2). The shape and the wavelength of the absorption peaks of the PBGs are quite similar due to the same chromophore of thioxanthone derivatives. The absorption peaks of TX-DBU at 278 nm and 382 nm are characteristic peaks of thioxanthone derivatives, which are attributed to the π–π* transition and n–π* transition, respectively. The absorption at 382 nm makes thioxanthone as an attractive chromophore sensitive to UV-A light.

Fig. 2 Absorption spectra of PBGs in acetonitrile solution.

Although the PBGs possess the same thioxanthone chromophore, the maximum absorption peaks (λ_max) and the molar extinction coefficient (ε) varied due to the different types of quaternary ammonium cations (Table 1). Compared to TX-CyA with relatively weak base absorbing at 380 nm, a slight red-shift absorption of TX-DBU and TX-TBD at 382 nm was observed. The different might be derived from the protonation.²⁷

Table 1 Photophysical properties of PBGs^a

	λmax (nm)	ε365 (L mol^−1 cm^−1)	εmax (L mol^−1 cm^−1)	λex (nm)	λem (nm)
a Where ε365 and εmax stand for the molar extinction coefficient at 365 nm and λmax, respectively. λex and λem are the maximum fluorescence excitation and emission of PBGs in acetonitrile (10^−5 M).
TX-TBD	382	2101	3081	383	436
TX-DBU	382	1946	2489	390	432
TX-CyA	380	1379	1682	382	440

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The emissions of the PBGs were studied with the terms of fluorescence measurements (Fig. 3). The fluorescence spectra of PBGs were investigated in acetonitrile at room temperature. The maximum of the emission wavelengths of the PBGs range from 432 nm to 440 nm.

Fig. 3 Fluorescence excitation and emission spectra of PBGs in acetonitrile at room temperature.

Photolysis study

Photolysis of the PBGs was investigated in acetonitrile. The solution of TX-DBU was irradiated with increased photo energy. Only marginal difference of UV-vis spectra was observed under various irradiation energy (Fig. 4a). The observation indicated that the absorption behaviour of the photolysis products was very similar to that of TX-DBU. Nearly identified results were also reported by Arimitsu, et al.¹²

Fig. 4 UV-vis absorption spectra changes of TX-DBU solution (10⁻⁴ M) without (a) and with (b) the addition of phenol red solution irradiated with an Hg lamp at different light doses.

In order to confirm the photobase-generating ability of the PBGs, phenol red, a general pH indicator was added into the solution.¹³ Different acetonitrile solutions of TX-DBU (4.5 × 10⁻³ M) were irradiated with increasing doses and the absorption spectra were shown in Fig. 4b.

Increasing the dose of light led to the increased absorption at 578 nm, which is the characteristic absorption of phenol red in deprotonated form. The result indicated that neutralization occurred between phenol red and the base, which was generated from the PBG under irradiation. The increased long-wavelength absorption of deprotonated phenol red altered the colour of the solution from yellow to red. The absorption of 578 nm levelled off when the irradiation dose over 2000 mJ cm⁻², demonstrating that most of the caged bases had been already released.²⁸

The change of the basicity of the irradiated solution of TX-DBU was probed by pH monitoring. Upon exposure, the pH of the medium increased immediately, changing from 8.0 to 11.5 and changed very little due to most of the basic species released after 2000 mJ cm⁻² (Fig. 5).

Fig. 5 pH monitoring of TX-DBU in acetonitrile upon UV irradiation at several doses: a 0.021 M solution irradiated by IWATA UV-100 LED at 365 nm.

Arsu has proved that thioxanthone acetic acid derivatives were able to undergo photoinduced decarboxylation in high quantum yields.²³ In order to detect the generated CO₂ of the PBGs, a methanol solution of TX-TBD (2.4 × 10⁻² M) was placed in a sealed bottle which was connected to anther bottle containing an aqueous solution of Na₂CO₃ (1.8 × 10⁻⁴ M) and phenolphthalein (2.1 × 10⁻⁴ M) (Fig. 6a). If the decarboxylation occurred, the released CO₂ would diffuse through the tube and neutralize Na₂CO₃; the pink would become lightened or even disappeared. Upon irradiation, bubbles were observed in the bottle containing the PBG solution. When the irradiation time prolonged to 50 min, the colour of the solution with Na₂CO₃ and phenolphthalein altered from pink to colourless (Fig. 6b). The disappeared colour confirmed the release of CO₂ when TX-DBU solution was irradiated.¹

Fig. 6 Photos of generated CO₂ detecting: (a) before and (b) after UV irradiation.

Based on the photolysis results combined with the previous report by other groups,^12,29 a possible mechanism of photoinduced decomposition was proposed (Scheme 2). After irradiation, the excited PBG underwent decarboxylation, releasing CO₂ and basic species which were responsible for the subsequent thiol–epoxy polymerization. Since the decarboxylation is irreversible, back electron transfer process would be suppressed and therefore higher initiation efficiency can be obtained.

Scheme 2 Proposed photodecomposition mechanism.

Photopolymerization

Owing to the formation of strong nucleophilic thiolate anions able to deficiently initiate the epoxide polymerization, the development of a thiol–epoxy provides the possibility to produce materials with desirable properties such as low shrinkage, high strength and large glass transition temperature.³ RT-FTIR measurements were performed to study the initiation efficiency of the PBGs. Tetra (3-mercaptopropionate) (PETMP), contained four thiol groups, was chosen and used to enhance the epoxide crosslinking reaction. A mixture of 50 mol% of BADGE and 50 mol% of PETMP was mixed in the presence of different PBGs (2 mol%). The conversion was recorded by monitoring the disappearance of epoxy band at 915 cm⁻¹ (Fig. 7) and thiol band at 2568 cm⁻¹ (Table 2).

Fig. 7 Epoxy conversion curves for thiol/epoxide polymerization catalyzed by the different PBGs without heat treatment.

Table 2 Functional group conversions after irradiation (20.3 mW cm⁻², 1000 s irradiation)

Conversion (%)	TX-TBD	TX-DBU	TX-CyA	ITX/TBD·HBPh4
Thiol	82 ± 9	76 ± 5	63 ± 7	72 ± 8
Epoxy	82 ± 3	71 ± 3	50 ± 5	69 ± 5

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The degree of conversion was calculated using the area of the monitored absorption peak according to the literature.²⁶ All the investigated PBGs can induce the thiol–epoxy polymerization under irradiation. The basicity of the released amines has great influence on the polymerization rate as well as final epoxy conversion. TX-CyA containing relatively weaker base cyclohexanamine exhibited the slowest polymerization rate and lowest final epoxy conversion among the investigated PBGs. When changing cyclohexanamine with stronger base as in TX-DBU, faster polymerization rate as well as higher final epoxy conversion was obtained. TBD has been reported with a pK_a of 26.03 in acetonitrile, 100 times more basic than DBU.²⁴ The increased basicity of the released base with higher nucleophilicity can provide faster consumption of the thiols to initiate the polymerization. Therefore, TX-TBD exhibited the best performance on both polymerization rate and epoxy conversion of about 82%.

Although generated the same basic specie TBD, the performance of the reported efficient bimolecular PBG system, based on physical mixture of ITX and TBD·HBPh₄, was inferior to the unimolecular TX-TBD. The enhancement of TX-TBD might originate from the efficient initiation mechanism involving photoinduced decarboxylation, which can prevent the back electron transfer and therefore provide higher initiation efficiency.

It should be noted that the final thiol conversion was slightly higher than the corresponding epoxy conversion. The higher conversion might be caused by the coupling of thiyl radicals.³⁰ As shown in the mechanism in Scheme 2, after irradiation, the PBG underwent decarboxylation releasing CO₂, base species and thioxanthone-methylene free radical. The methylene free radical was able to abstract hydrogen from the thiol, leaving thiyl radicals. The highly active thiyl radicals might couple with each other to provide higher thiol conversion compared to the corresponding epoxy conversion. The results demonstrated that the PBGs could induce not only the thiol–epoxy reaction, but also the free radical reaction such as acrylated polymerization and thiol–ene polymerization. The dual initiation ability makes the PBGs quite attractive for hybrid polymerization, which usually provides excellent properties of the materials.

Conclusions

A straightforward synthesis of novel PBGs containing thioxanthone as the chromophore and different quaternary ammonium salts as latent active species were presented. The structures of the PBGs were characterized by ¹H NMR, ¹³C NMR and high-resolution mass spectrometer. Study on the photophysical properties of the PBGs showed that the absorption of the thioxanthone derivatives lied at 380 nm, making the PBGs photosensitive to UV-A or even visible light region. Investigation on photolysis indicated that under irradiation, the PBGs underwent decarboxylation to release active basic species, which were confirmed by pH monitoring. Since the irreversible photoinduced decarboxylation can prevent the back electron transfer and therefore provide higher initiation efficiency, all the novel PBGs showed high activity in catalysing thiol–epoxy polymerization without postexposure baking. Specially, the PBG containing strong base TBD exhibited faster polymerization rate and higher epoxy conversion than the counterparts with other amines and previously described highly active PBG system. Straightforward synthesis combined with high initiation efficiency makes these novel PBGs promising candidates for commercialization.

Acknowledgements

We thank the National Nature Science Foundation of China (21404048 and 21307002) and the Fundamental Research Funds for the Central Universities (JUSRP11513) for financial support.

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