Coumarin-Based Oxime Esters: Photobleachable and Versatile

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Applications of Polymer, Composite, and Coating Materials

Coumarin-based Oxime-Esters: Photobleachable and Versatile Unimolecular Initiators for Acrylate and Thiol-based Click Photopolymerization under Visible LED Light Irradiation Zhiquan Li, Xiucheng Zou, Guigang Zhu, Xiaoya Liu, and Ren Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01767 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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ACS Applied Materials & Interfaces

Coumarin-based Oxime-Esters: Photobleachable and Versatile Unimolecular Initiators for Acrylate and Thiol-based Click Photopolymerization under Visible LED Light Irradiation Zhiquan Li1, 2, Xiucheng Zou2, Guigang Zhu2, Xiaoya Liu2 and Ren Liu1, 2*

1

International Research Center for Photoresponsive Molecules and Materials,

Jiangnan University, Wuxi, Jiangsu 214122, P. R. China. 2

Key Laboratory of Food Colloids and Biotechnology, School of Chemical and

Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, People’s Republic of China.

Abstract Developing

efficient

unimolecular

visible

LED

light

photoinitiators

with

photobleaching capability, which are essential for various biomedical applications and photopolymerization of thick materials, remains a great challenge. Herein, we demonstrate the synthesis of a series of novel photoinitiators, containing coumarin moieties as chromophores and oxime-ester groups as initiation functionalities, and explore their structure-activity relationship. The investigated oxime-esters can effectively induce acrylates and thiol-based click photopolymerization under 450 nm visible LED light irradiation. The initiator O-3 exhibited excellent photobleaching capability and enabled photopolymerization of thick materials (∼4.8 mm). The efficient unimolecular photobleachable initiators show great potential in dental materials and 3D printings.

Keywords oxime-ester;

photoinitiator;

photopolymerization;

polymerization; decarboxylation

1

ACS Paragon Plus Environment

photobleaching;

click

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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INTRODUCTION Photopolymerization has become a versatile strategy for polymer synthesis due to its high efficiency and spatiotemporally controllable characteristics.1-3 The thiol-based photopolymerization possesses unique features of click chemistry4-6 and provides added advantages, such as uniform network formation, oxygen insensitivity and low shrinkage.7-9 Photopolymerization employing soft irradiation sources, such as visible light emitting diodes (LEDs) instead of traditional mercury lamps,10-16 is desirable because visible LEDs are energy efficient and avoid ozone generation.17

Therefore,

applying visible LEDs, with low energy and stronger penetration ability, to trigger acrylates and thiol-based click photopolymerization would further extend its application rang, especially in biomedical fields and 3D printings.

The successful introduction of LED light into photopolymerization applications requires the optimization of photoinitiating systems, with optimal absorption of the photoinitiator/photosensitizer matching the emission of visible LED irradiation, for efficient photon absorption. Unlike traditional mercury lamps with broad emission spectra, the very narrow emission bandwidth of LED lamps (typically ∼20 nm) at relatively long wavelength (e.g. ultraviolet at 365 nm, near-ultraviolet at 385 and 395 nm, violet at 405 nm) has significantly limits the use of most commercial photoinitiators (PIs).1, 17-18 In recent years, a series of multi-component visible LED light sensitive initiation systems, containing non-cleavable PIs such as organometallic complex, polyoxometalates and modified dyes, combined with onium salt, amine, silanes and N-vinylcarbazole as co-initiators, have been reported.18-25 Such bimolecular or multicomponent approach is practical but decreased initiation activity cannot be avoided, particularly in the high viscous and rigid polymeric formulations, because the efficiency of electron transfer or energy transfer between the excited PIs and coinitiators is diffusion-controlled.26

The photoinduced direct cleavage is a more efficient mechanism to generate active free radicals. However, to date limited reports, such as acylphosphine oxides,27-28 2


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organometallic germanium29-31 and silyl glyoxylates32 as PIs sensitive to visible LED, are available. The greater challenge in developing unimolecular LED PIs is associated with two adverse effects when introducing auxochromic groups or long conjugation bridges onto the core structures for better spectra overlap.33-35 The extended conjugation length generally reduces LUMO-HOMO energy gap,35, 36 which leads to the lower absorbed energy insufficient for the breakage of the chemical bonds (i.e., C–C bond of carbonyl moiety) to produce initiation species. Furthermore, the introduction of conjugation system often causes a crossover between σ* and π* LUMO states.35 Consequently, the excited molecules would be quenched via fluorescence emission instead of undergoing desired cleavage reactions.

Besides the initiation activity, photobleaching is another important property for visible LED PIs (Here photobleaching can be defined as the decreased absorbance of PIs at a certain wavelength with the exposure time).37 Generally, visible LED PIs take on a yellow coloration due to the absorption of the blue end of the visible spectrum, which limits their application in clear coatings and dental materials.38-39 Moreover, the absorption of PIs in a photocuring resin ineluctably causes light-intensity gradient along the radiation path, leading to the heterogeneity of the cured materials in the vertical direction, as well as limits the curing depth.2,

38

The employment of the

photobleachable PIs can address these issues. Upon exposure, the photobleachable PIs lose their color and provide colorless cured materials due to the decreased or even completely disappeared absorbance in the visible light region.2 In addition, since the photolysis products do not absorb visible light, the incident visible LED light can penetrate further into the resin, to realize through curing with better homogeneity.2, 39 Until now, due to the still unclear structure-property relationship, the reports on photobleachable PIs are scarce39-42 and exploring efficient unimolecular LED PIs with photobleachable properties is gaining increased research attention.

Herein, we report a series of unimolecular photobleachable PIs containing coumarin moieties as chromophores sensitive to visible LED light and oxime-ester groups43-44 as 3



initiation functionalities. Various PIs O-4 and O-3 have been designed to investigate the effect of different substituted positions on the structure-reactivity relationship. O-3O and O-3F were prepared to study the impact of the electronic effect on the photophysical and photochemical behavior (Scheme 1). For comparison, the commercial oxime-ester 1-[4-(phenylthio)phenyl]-1,2-octandione-2-O-benzoyloxime (OXE-1, Figure 1) was also tested. The photophysical properties, initiation efficiency and thermal stability of the photoinitiators have been investigated in detail. Furthermore, the mechanism of photoinduced cleavage process has been proposed according to the photolysis study.

Figure 1. Structures of the novel PIs (O-4, O-3, O-3F, O-3O), reference PI (OXE-1) and monomers used in photopolymerization study.

EXPERIMENTAL SECTION Materials The 4-(diethylamino) salicylaldehyde, 7-diethylamino-4-methylcoumarin, sodium hydride, 4-(trifluoromethyl) benzoyl chloride, 4-methoxybenzoyl chloride and 4


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(3-mercaptopropionate) (PETMP) were purchased from Energy Chemical Co., Ltd. The 1,7-Octadiyne (ODY), triallyl isocyanurate (TAIC), triallyl cyanurate (TAC), pentaerythritol

triallyl

ether

(APE),

camphorquinon

(CQ)

and

ethyl

4-dimethylaminobenzoate (EDB) were obtained from Sigma-Aldrich Chemistry, China. Selenium dioxide and dry N, N-dimethylformamide were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd and the other reagents were supplied by Sinopharm Chemical Reagent Beijing Co., Ltd. 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 analysis. The trimethylolpropane triacrylate (TMPTA) was supplied by Jiangsu Sanmu Chemical Co.,

Ltd,

China.

1-[4-(phenylthio)phenyl]-1,2-octandione-2-O-benzoyloxime

(OXE-1) and Irgacure 784 was obtained from BASF (China) Co., Ltd. All the chemicals were used without any further purification unless otherwise stated. The solvents were dried and purified by standard laboratory methods.

Synthesis 7-diethylaminocoumarin

(3a),45

7-diethylaminocoumarin-3-aldehyde

(3b),45

7-diethylamino-4-formylcoumarin (4b)46 were prepared according to the literature. A detailed synthesis procedure was given in the Supporting Information.

Synthesis of 7-diethylamino-4-hydroxyiminocoumarin (4c) A mixture of 4b (1.30 g, 5 mmol), hydroxylamine hydro-chloride (0.52 g, 7.5 mmol) and anhydrous sodium acetate (0.62 g, 7.5 mmol) was refluxed in ethanol (50 ml) under N2 atmosphere for 2 h. The solvent was evaporated under reduced pressure and the residue was washed by water. The raw product was recrystallized from ethanol to give red needles with a yield of 76 %. 1H NMR (400 MHz, DMSO-d6) δ 12.37 (s, 1H), 8.45 (s, 1H), 8.19 (d, J = 9.1 Hz, 1H), 6.75 (d, J = 8.2 Hz, 1H), 6.62 (s, 1H), 6.30 (s, 1H), 3.52 (q, J = 7.1 Hz, 4H), 1.20 (t, J = 6.7 Hz, 6H).

Synthesis of 4-(7-diethylaminocoumarin)-O-benzoyl oxime (O-4) 5



NaH (0.6 g, 15 mmol, 60%) and 4c (2.60 g, 10 mmol) were dissolved in dry THF (100 mL) under N2 atmosphere. Then the mixture was cooled to 0 °C and kept stirring for 30 min. Then benzoyl chloride (1.7 mL, 15 mmol) was added dropwise and the mixture was stirred for 20 min. The solution was quenched with aqueous sodium bicarbonate solution (5.0 %, 200 mL) and extracted with dichloromethane (3 × 50 mL). Then the organic phase was dried over anhydrous sodium sulfate, filtered and evaporated under reduced pressure. The raw product was purified by column chromatography (petroleum ether/ethyl acetate = 5:1) to give red powder with a yield of 70 %. 1H NMR (400 MHz, CDCl3) δ 8.59 (s, 1H), 8.35 (d, J = 9.2 Hz, 1H), 8.22-8.13 (m, 2H), 7.68 (dd, J = 10.6, 4.3 Hz, 1H), 7.55 (t, J = 7.7 Hz, 2H), 6.71 (dd, J = 9.2, 2.6 Hz, 1H), 6.56 (d, J = 2.6 Hz, 1H), 6.34 (s, 1H), 3.46 (q, J = 7.1 Hz, 4H), 1.25 (t, J = 7.1 Hz, 6H).

13

C NMR (101 MHz, CDCl3) δ 163.26, 161.28, 156.90,

154.61, 151.00, 142.26, 133.85, 129.88, 128.72, 128.15, 127.95, 112.78, 109.44, 104.80, 97.73, 44.81, 12.46. Q-Tof-MS (m/z): calcd for C21H20N2O4 (M+Na), 387.1321. Found: 387.1273.

Synthesis of 7-diethylamino-3-hydroxyiminocoumarin (3c). A mixture of 3b (1.30 g, 5 mmol), hydroxylamine hydro-chloride (0.52 g, 7.5 mmol) and anhydrous sodium acetate (0.62 g, 7.5 mmol) was refluxed in ethanol (50 ml) under N2 atmosphere for 2 h. The solvent was evaporated under reduced pressure and the residue was washed by water. The raw product was recrystallized from ethanol to give orange needles with a yield of 74%. 1H NMR (400 MHz, DMSO-d6) δ 11.30 (s, 1H), 8.15 (s, 1H), 7.99 (s, 1H), 7.54 (d, J = 8.6 Hz, 1H), 6.73 (d, J = 8.3 Hz, 1H), 6.49 (d, J = 2.8 Hz, 1H), 3.43 (t, J = 13.8 Hz, 4H), 1.13 (t, J = 6.0 Hz, 6H).

Synthesis of 3-(7-diethylaminocoumarin)-benzoyl oxime ester (O-3). NaH (0.6 g, 15 mmol, 60%) and 3c (2.60 g, 10 mmol) were dissolved in dry THF (100 mL) under N2 atmosphere. Then the mixture was cooled to 0 °C and stirred for 30 min. Benzoyl chloride (1.7 mL，15 mmol) was added dropwise and the solution was stirred for 20 min. The mixture was quenched with aqueous sodium bicarbonate 6


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solution (5.0 %, 200 mL) and extracted with dichloromethane (3 × 50 mL). The organic phase was dried over anhydrous sodium sulfate, filtered and evaporated under reduced pressure. The raw product was purified by column chromatography (petroleum ether/ethyl acetate = 5:1) to give orange powder with a yield of 64 %. 1H NMR (400 MHz, CDCl3) δ 8.80 (s, 1H), 8.53 (s, 1H), 8.20-8.11 (m, 2H), 7.73-7.59 (m, 1H), 7.52 (t, J = 7.7 Hz, 2H), 7.40 (d, J = 8.9 Hz, 1H), 6.66 (dd, J = 8.9, 2.5 Hz, 1H), 6.52 (d, J = 2.3 Hz, 1H), 3.48 (q, J = 7.1 Hz, 4H), 1.27 (t, J = 7.1 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ 164.06, 161.08, 157.75, 152.34, 151.94, 141.80, 133.41, 130.85, 129.73, 128.56, 109.86, 108.93, 108.35, 97.19, 45.18, 29.68, 12.46. Q-Tof-MS (m/z): calcd for C21H20N2O4 (M+Na), 387.1321. Found: 387.1325.

Synthesis of 3-(7-diethylaminocoumarin)-4-trifluoromethyl benzoyl oxime ester (O-3F). A similar synthetic method as described for O-3 was used to prepare O-3F and a dark red solid was obtained. The raw product was purified by column chromatography (petroleum ether/ethyl acetate = 10:1) to give orange powder with a yield of 60 %. 1H NMR (400 MHz, CDCl3) δ 8.81 (s, 1H), 8.51 (s, 1H), 8.30-8.16 (m, 2H), 7.78 (d, J = 8.3 Hz, 2H), 7.40 (d, J = 9.0 Hz, 1H), 6.51 (d, J = 2.2 Hz, 1H), 3.48 (q, J = 7.1 Hz, 4H), 1.26 (t, J = 7.1 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ 162.89, 161.03, 157.84, 152.59, 152.48, 142.02, 131.91, 130.95, 130.12, 125.64, 125.60, 109.93, 108.48, 108.33, 97.20, 45.12, 30.89, 12.45. Q-Tof-MS (m/z): calcd for C22H19F3N2O4 (M+Na), 455.1195. Found: 455.0490.

Synthesis of 3-(7-diethylaminocoumarin)-4-methoxybenzoyl oxime ester (O-3O). A similar synthetic method as described for O-3 was used to prepare O-3O and dark red raw product was obtained. The raw product was purified by column chromatography (petroleum ether/ethyl acetate = 5:1) to give orange powder with a yield of 55 %. 1H NMR (400 MHz, CDCl3) δ 8.77 (s, 1H), 8.51 (s, 1H), 8.20-8.00 (m, 2H), 7.39 (d, J = 8.9 Hz, 1H), 7.04-6.95 (m, 2H), 6.64 (dd, J = 8.9, 2.5 Hz, 1H), 6.51 (d, J = 2.3 Hz, 1H), 3.90 (s, 3H), 3.47 (q, J = 7.1 Hz, 4H), 1.26 (t, J = 7.1 Hz, 6H). 13C 7



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NMR (101 MHz, CDCl3) δ 163.82, 161.16, 157.70, 152.27, 151.52, 141.72, 132.27, 131.85, 130.83, 120.74, 113.80, 109.83, 109.11, 108.39, 97.19, 55.49, 45.09, 12.47. Q-Tof-MS (m/z): calcd for C22H22N2O5 (M+Na), 417.1426. Found: 417.1365.

Characterization The 1H NMR (200 MHz) and

13

C NMR (50 MHz) were measured with a Bruker

AVANCE III HD 400MHz NMR-spectrometer. The chemical shift (s = singlet, bs = broad singlet, d = doublet, t = triplet, m = multiplet) is stated in ppm using the nondeuterated solvent as internal standard. The solvents with a grade of deuteration of at least 99.5% were used. The high-resolution mass spectrometer measurements were performed using Q-Tof-MS from Bruker Daltomics. UV-vis spectra were measured with a Beijing Purkinje TU-1901 UV-VIS spectrophotometer. The incident light intensity was monitored by radiometers (UV-A, Photoelectric Instrument Factory of Beijing Normal University). One-photon excited fluorescence (OPEF) spectra were recorded at the concentration of 1×10-6 mol/L in acetonitrile on an F-4500 (Hitachi High-Technologies Corporation) fluorescence spectrophotometer. The fluorescence quantum yields were obtained with quinine sulfate dehydrate (1 × 10-6 mol/L) in 0.1 M sulfuric acid aqueous solution as a reference standard (Фref = 0.54) by equation (1):47 Φ =

Aref n 2 F 2

An ref Fref

Φ ref

(1)

where Ф is the quantum yield, n is the refractive index, A is the absorbance of the solution at the exciting wavelength and F is the integrated area under the emission spectrum.

Photolysis study The photolysis study of the novel oxime-ester photoinitiators, O-3 and O-4, was carried out in acetonitrile at room temperature. The O-3 and O-4 (1.2 × 10-3 M solvent) was dissolved in acetonitrile solvent, in a cuvette, and irradiated by 450 nm 8


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LED light. The absorbance of exposed solvent was tested to obtain the UV-vis spectra for different times. The light intensity was adjusted to 200 mW/cm2 for the tests.

Theoretical calculation method The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of PIs were calculated at UB3LYP/6-31G* level (iso-value = 0.03).48 Graph of molecular orbitals has been draw by the GaussView. The bonding energy of the PIs was calculated at M06-2X/def2-TZVP. 48

Electron spin resonance (ESR) experiments The ESR experiments were carried out by using an EMXplus-10/12 X-band spectrometer, at 100 kHz, under magnetic field modulation. The power intensity was adjusted

to

20

mW.

The

investigated

photoinitiators

(1

mol

%)

and

phenyl-N-tert-butyl-nitrone (PBN, 2 mol %) were dissolved in benzene and deoxygenated with nitrogen, for 5 min, before irradiation. The radicals were generated through photolysis, at room temperature, when exposed to the 450 nm LED light source (300 mW cm-2) for 60 s. The ESR spectra simulations were performed using the Bruker Xenon software.

Photopolymerization under the visible LED light The photocurable formulations were prepared by mixing monomers and the investigated PIs (5.5 × 10-5 mol/g resin) via ultrasonic vibration until a homogeneous resin was obtained. A piece of thin silicone membrane (thickness = 0.5 mm), with a hole (radius = 7 mm) in the middle, was attached on a KBr tablet and the investigated resin was filled into the hole. Another KBr tablet was used to cover the hole for the subsequent tests. The polymerization experiments were carried out by using a Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific, 500-4000 cm-1 wavelength range, resolution 8 cm-1) to monitor the conversion of different groups as a function of exposure time. A visible LED-curing unit (SKI-803, Foshan Duoyimei Medical 9



Page 10 of 31

Instrument Co., Ltd) for dental application with an emission wavelength of 450 nm was used as an irradiation source. The light intensity was adjusted to 200 mW/cm2 for the tests. The polymerization profiles were recorded during 300 s irradiation at room temperature. The polymerization kinetics were measured by monitoring the disappearance of the double bond, thiol and alkynyl groups. For each sample, the curing process was repeated three times. The conversion of different groups was calculated by using formula (2):

% = − × %

(2)

Where At corresponds to the area of the double bond (about 1000-800 cm-1), thiol (peak at 2570 cm-1) or alkynyl (peak at 2100 cm-1) group characteristic absorbance peak at time t and A0 represents the initial area of the peak.

The photobleaching study of the novel oxime-ester photoinitiator O-3 was carried out in PETMP/APE resin. The prepared resin formulation was coated on the surface of a glass slide, with the thickness of about 100 µm. 450 nm LED light was employed to irradiate the center of the glass slide (the diameter of the light spot is about 1 cm) for 2 minutes. The remaining area was protected from light by tin foil. Meanwhile, the photobleaching in bulk sample was employed by a piece of 5 mm thick silicone

membrane, with a hole (radius = 7 mm) in the middle, attached to a glass slide. The investigated resin was filled into the hole and irradiated by 450 nm LED light for 10 minutes.

Thermal stability tests The thermal gravimetric analysis (TGA) experiments were performed by using a Mettler Toledo TGA 1/1100SF with about 5 mg sample under a nitrogen atmosphere. The temperature was ramped from 25 to 800 °C, at a ramping rate of 20 °C min−1. The differential scanning calorimetry (DSC) was carried out by using a PerkinElmer DSC-8000, with the sample of photoinitiators dissolved in TMPTA. The solution concentration was 2.8 × 10-5 mol/g resin and the ramping rate was 10 °C min−1. 10


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RESULTS AND DISCUSSION

Synthesis Coumarin is a versatile chromophore which has been used in various applications such as fluorescent probes49 and laser dyes.50 Some modified coumarin derivatives exhibits strong absorption and high sensitizing efficiency.51-52 Therefore, a proper molecular design to incorporate the oxime-ester into the functionalized coumarin as a chromophore would provide efficient PIs. O-4 and O-3 were designed to investigate the effect of different substituted positions on structure-reactivity relationships. O-3O and O-3F were prepared to study the impact of the incorporation of electron donating/withdrawing groups on the photoinitiation behavior.

Scheme 1. Synthetic routines for the novel PIs

All PIs were synthesized via a condensation reaction between coumarin aldoxime and corresponding benzoic acid chloride as depicted in Scheme 1. There are two key intermediates 3b and 4b based on coumarin aldehyde with different substation position on the coumarin ring. For the preparation of O-4, the key precursor 4a was obtained via oxidation of commercial methylcoumarin derivative 4a using selenium dioxide as an oxidizing agent. The methyl group was converted to aldehyde with a yield of 74%. Another important precursor 3b was synthesized via Vilsmeier-Haack reaction, a versatile method to introduce aldehyde group into the aromatic ring. After 11



the reaction of diethylaminocoumarin with POCl3 in dry DMF solution, the aldehyde was incorporated into the 3-position of the coumarin ring with a yield of 70%. In the presence of anhydrous sodium acetate, the formed coumarin aldehydes reacted with hydroxylamine hydrochloride to give corresponding aldoxime. The attempt to prepare O-4 as described by Dworak et al26 using triethylamine as a deacid reagent in methylene chloride solution did not give the desired product. The 1H NMR result (Figure S1) showed that both the characteristic proton peaks of N-OH at 12.5 ppm and -CH=N at 8.5 ppm disappeared. The oxime group might be converted to cyano group via dehydration reaction. Therefore, the reactions of oxime with corresponding benzoic acid chloride were performed as reported by Hwu et al53 using NaH as a strong base to deprotonated the hydroxyl group, which subsequently reacted with acid chloride to generate oxime-ester derivatives with yields of 55-70%.

Theoretical calculations The HOMO and LUMO of the synthetic photoinitiators were calculated and results are presented in Figure 2. The electronic clouds of all four PIs were mainly distributed in coumarin and oxime ester group instead of benzene, which indicates that the introduction of -CF3 and -OCH3 has a marginal effect on the maximum wavelength due to the similar π electron delocalization in the structures.54 Moreover, the energy gap (LUMO-HOMO) and N-O bond energy were also calculated (Table 1). The O-4 might be excited under weak irradiation due to the lowest energy gap of 3.39 eV. For O-3, the highest energy gap, combined with the lowest N-O bond energy, is beneficial for the N-O bond cleavage.

12


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Figure 2. The HOMO and LUMO orbits of the synthesized compounds at B3LYP/6-31G* level (iso-value = 0.03).

Table 1. The theoretical data of oxime-ester compounds N-O bond energy

Energy gap

(kJ mol-1)

(eV)

O-4

264.97

3.390

O-3

252.59

3.545

O-3F

265.85

3.436

O-3O

271.16

3.394

PIs

Spectroscopy Figure 3a depicts the UV-vis absorption spectra of the novel photoinitiators (O-4, O-3, O-3F and O-3O) in acetonitrile at room temperature. The samples have exhibited a broad absorption band, ranging from 400 nm to 480 nm. The maximum absorption of O-4 was located at 433 nm, with a molar absorption coefficient εmax of 7680 M-1 cm-1. By changing the substitution position of the oxime-ester group, as in the structure of O-3, the maximum absorption band have shown a marginal red-shift, however, the εmax has remarkably increased. Wu et al46 have reported that the coumarin derivatives, with a 4-substitution, show a twisted conformation, while the 3-position substituted analogs show planar conformation. Therefore, the enhanced absorption can be attributed to the increase in co-planar conformation and degree of 13



conjugation. The effect of the substitution position not only impacted on the absorption behavior, but also on the fluorescence emission. As shown in Figure 3b and Table 2, the maximum band in the fluorescence emission spectrum of O-4 has been significantly red-shifted compared to that of O-3. The larger Stokes shift of O-4 suggests a more significant change in geometry between the ground state and first excited singlet state. The fluorescence quantum yields Фf of O-3 has shown a 1.5 times increase as compared to O-4. Generally, the low fluorescence quantum yields are preferred for an efficient Type I photoinitiator, due to the presence of a higher population of the active states to produce active free radicals for the subsequent polymerization. The incorporation of an electron-withdrawing group (-CF3), as in O-3F, or an electron-donating group (-OCH3), as in O-3O, did not alter the maximum wavelength in the absorption and emission spectra, compared to O-3, but led to the reduction of the molar absorption coefficient (εmax) and enhancement of the fluorescence quantum yields (Фf). These observations are in good agreement with the theoretical calculation.

Figure 3. The UV-vis absorption (a) and fluorescence emission spectra (b) of O-4, O-3, O-3F and O-3O, in acetonitrile, at room temperature.

Table 2. The photophysical properties of as-synthesized PIs (O-4, O-3, O-3F and O-3O) 14


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ε450

Фf

λmax

λem

Stokes shift

(nm)

(nm)

(cm-1)

O-4

433

593

6231

7680

6790

0.1112

O-3

436

500

2935

41690

37450

0.1502

O-3F

436

502

3015

29930

26630

0.1722

O-3O

436

498

2855

29950

26620

0.1912

PIs

εmax

(L mol−1 cm−1) (L mol−1 cm−1)

Photolysis study The steady-state photolysis experiments are used in studying the photolysis behaviors after light absorption. The steady-state photolysis tests were carried out in acetonitrile, at room temperature, with different irradiation times. The changes in the shape and the intensity of the absorption bands indicate the occurrence of photoreactions. The photolysis of O-4 could be divided into two stages (Figure 4a). Within 1 min of irradiation, the wavelength of the maximum absorption peak slightly red-shifted and the absorbance increased. With prolonged irradiation, the absorption wavelength slightly blue-shifted and a weak reduction of the corresponding absorption was observed.

Figure 4. The steady-state photolysis of (a) O-4 and (b) O-3, in acetonitrile, under the irradiation of 450 nm LED light. The inset of O-3 photolysis shows the photobleaching of O-3 (1.2 × 10-3 M) in the solvent system.

15



The isomer O-3, with a different substitution position, exhibited a completely different photolysis behavior, compared to O-4 (Figure 4b). The maximum absorption peak of O-3, at 436 nm, is blue-shifted and decreased with longer irradiation time. The solution changed from light yellow to colorless (Figure 4b, inset) after 10 min irradiation. The photobleaching property would be beneficial for deep curing because the photolysis products do not absorb the visible light, allowing the incident light to penetrate deeper into the resin. This will be discussed in sub-section, “photobleaching study”, in detail. A similar trend of photolysis was observed for O-3O and O-3F (Figure S2).

The oxime-ester derivatives have been reported to undergo photoinduced decarboxylation in high quantum yields.55-56 In order to detect the generated CO2 during photolysis, a xylene solution of O-3 (2.4×10-2 M) was placed in a sealed bottle that was connected to another bottle, containing an aqueous solution of Na2CO3 (1.8×10-4 M) and phenolphthalein (2.1×10-4 M) (Figure 5a). If the decarboxylation occurred, the released CO2 would diffuse through the tube and neutralize K2CO3; the pink color would become lightened or even disappeared. When the irradiation time was increased to 10 minutes, the color of the solution, containing K2CO3 and phenolphthalein, altered from pink to colorless (Figure 5b). The change in color confirmed the release of CO2 during O-3 irradiation. Moreover, after the photolysis, the PI solution showed orange color. The non-photobleaching observation might be attributed to the generation of complicate photolysis products derived from the used higher concentration of O-3.

After irradiation, the temperature of the investigated sample increased up to about 80 °C. Considering that the oxime-ester derivatives can undergo decarboxylation at certain high temperature, a control experiment was performed, in an oven, with a set temperature of 80 °C in the absence of light for 10 minutes to evaluate the heat effect on decarboxylation during irradiation. Figure S3 shows that the color remained 16


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unchanged in the control sample, indicating that decarboxylation was induced by the light, rather than heat, during irradiation.

For the O-4, however, the pink color became slightly lightened under the same experimental conditions (Figure S4). This demonstrates that the rate of photoinduced decarboxylation of O-4 was much slower than that of O-3. Considering that the fluorescence quantum yield of O-4 was less than those of the analogs, the absorbed photon energy of O-4 might induce cis-trans isomerization, rather than the expected decarboxylation which can produce active free radicals to initiate polymerization. Similar trends in color change were observed for O-3F and O-3O, but with relatively slower photolysis rates. The photolysis rates, observed by the increased transmittance at 436 nm, decreased in the given order of the molecular structures O-3 > O-3F > O-3O (Figure 5c).

Figure 5. The detection of generated CO2 from the investigated PIs: The O-3 dissolved in xylene with a concentration of 2.4×10-2 M. (a) before and (b) after irradiation for 10 min under 450 nm LED light. (c) The comparison of photolysis rates based on the observation of the increased transmittance at 436 nm.

Electron spin resonance (ESR) experiment was conducted to further investigate the formed radicals after photodecarboxylation. Figure 6 exhibits that only one spin adduct was observed (aN = 14.37 G, aH = 2.21 G) after the photoinitiator O-3/PBN system was irradiated under 450 nm LED light for 60 s. The observed spin adduct 17



corresponds to the known phenyl radical (aN = 14.41 G, aH = 2.21 G).57 For the O-4/PBN system, no spin adduct was detected under the same experimental conditions. These results confirm that the rate of active free radicals generation of the O-4 was slower than that of O-3.

Figure 6. The ESR spectra of the radicals trapped by PBN in benzene: (a) experimental and (b) simulated spectra.

Based on the photolysis results and previous studies,58-59 a possible mechanism of photoinduced decomposition is proposed in Scheme 2. Under the light irradiation, the excited photoinitiator undergoes cleavage of the N-O bond, followed by a fast release of carbon dioxide. It should be noted that the irreversible decarboxylation efficiently suppresses the recombination of the initially formed radicals. This makes oxime-ester photoinitiator highly efficient in viscous formulations such as photoresists.58 After decarboxylation, the formed phenyl radical, known as an efficient initiating species, was trapped by PBN, generating a relatively stable radical. On the other hand, the formed primary iminyl was highly stable, which shows inefficient initiation ability58 and cannot be trapped by PBN.

18


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Scheme 2. The proposed photolysis mechanism of the radical generated from the O-3 under 450 nm LED light irradiation and trapped by PBN.

Photopolymerization under visible LED light The commercial oxime-ester (OXE-1) is commonly used as a UV photoinitiator due to its limited absorption in the visible light region.58 The excitation by a short-wave UV light, with a mercury lamp, raises problems such as ozone production, radiation safety and limited curing depth. The inefficient mercury lamps are being substituted by some visible light irradiation sources, such as LEDs, due to improved irradiation safety and efficient energy utilization.18 The broad absorption band, ranging from 400 nm to 480 nm, makes the novel oxime-ester attractive PIs for visible LED light photopolymerization. Particularly, the dental applications can use 450 nm LED light as a curing source. The photoinitiating efficiency of the PIs, irritated under a 450 nm LED light, was evaluated via the photoinduced crosslinking reaction of acrylate monomers and results are presented in Figure 7a. The structures of the monomers have illustrated in Figure 1. The real-time FT-IR was employed to study polymerization kinetics via detecting the conversion of characteristic functional groups. The kinetics study of the different photosensitive samples was carried out in the absence of air by using laminates.

19



Figure 7. The double-bond conversion with different (a) oxime-ester photoinitiators and (b) commercial visible light photoinitiators (system) (5.5 × 10-5 mol/g resin), irradiated by 450 nm visible LED light with an intensity of 200 mW/cm2.

For the OXE-1, the limited absorption, at 450 nm, led to an induction period of photopolymerization. Still, the final conversion of 45% was obtained due to the unique photolysis mechanism of the OXE-1. After photo-fragmentation of the N-O bonds, the iminyl radical undergoes further decomposition, producing benzoyl radical. Therefore, the OXE-1 can generate two initiating radicals after excitation. The O-3 exhibited a faster photopolymerization rate and an improved conversion, up to 61%, as compared to OXE-1. The enhanced photopolymerization rate and conversion can be attributed to the strong absorption at 450 nm. The longer induction period and lower conversion rate were observed for the O-4 due to the slower photo-decarboxylation rate for generating active free radicals.

The introduction of an electron-withdrawing group (-CF3), as in O-3F, and an electron-donating group (-OCH3), as in O-3O, led to the reduction of the absorption and photolysis rates compared to O-3. Therefore, both O-3F and O-3O have exhibited less initiation efficiency than O-3. Although, the O-3F and O-3O have displayed nearly the same εmax at 450 nm (Table 1), a faster polymerization rate and an improved conversion were obtained for the O-3F.

20


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Two commercially available visible light photoinitiators (system), namely Irgacure 784 and CQ/EDB were selected as references to further evaluate the photoinitiating efficiency of the novel oxime-ester. As shown in Figure 7b, Irgacure 784 exhibited the best initiation performance with highest double bond conversion up to 74%. The photoinitiating efficiency of O-3 was superior to that of CQ/EDB, a highly efficient photoinitiation system widely used in dental materials. Considering the complex synthesis of Irgacure 784 with relatively high price, the straightforward synthesis and relatively high photo activity make O-3 quite attractive for industrial applications.

Besides chain polymerization, based on acrylate resins, thiol-ene step-growth polymerization is another robust polymerization strategy.1,

6-8

The thiol-ene

polymerization well meets the criteria of a ‘click’ reaction and has become a versatile tool for polymer synthesis.2,

7, 38, 60

The excellent absorption and photoinitiation

properties of the O-3 make it an ideal candidate for thiol-based click photopolymerization under the visible LED light irradiation. The initiating efficiency of the thiol-based click photopolymerization was evaluated via photoinduced crosslinking reactions of thiol-ene and thiol-yne as typical examples. The results are presented in Figure 8 and the structures of monomers are illustrated in Figure 1..

Figure 8. Real-time FT-IR of the O-3 (5.5 × 10-5 mol/g resin) in different resin systems: (a) The conversion of thiol and (b) double bond and alkynyl. The systems were irradiated by 450 nm visible LED light with an intensity of 200 mW/cm2. 21



Excellent polymerization profiles are also obtained for three different thiol−ene photopolymerizations, using only O-3, under a 450 nm blue light irradiation. The reactions of three thiol−ene photopolymerizations are approximately completed within 20 sec and have shown high conversion. In the case of the thiol–ene binary systems (PETMP/TAIC and PETMP/TAC), the thiol conversions eventually reached up to 80% and the vinyl double bond conversions up to 90%. This is mainly due to the excellent initiation activity of the O-3. However, in the thiol-ene system (PETMP/APE), the vinyl double bond conversion (95%) was higher than S-H bond conversion (80%). The similar phenomenon can be found in the previous studies.61-62 The thiol-yne photopolymerization was also carried out and O-3 have shown promising results. The fast rate and high conversion of the thiol (70%) and alkynyl (80%) indicates that O-3 can be used for the efficient thiol-yne photopolymerization. Therefore, the novel oxime-ester photoinitiator O-3 can be used as visible LED light PIs with enhanced performance in various thiol-based click photopolymerization.

Photobleaching study The photobleaching properties are important in determining the curing depth and critical in a number of applications, particularly the applied materials emphasizing exterior. Figure 9 presents the photobleaching of the O-3 in the thiol-ene system. After 2 min of irradiation, the irradiation area practically became completely photo-bleached, similar to the background, and the area without irradiation remained unchanged (Figure 9a). The fading phenomenon can be attributed to colorless photolysis products of the O-3 without absorption at 450 nm (similar photobleaching was observed in acrylate system shown in Figure S5). Therefore, the deeper penetration of the light through the sample make the area, with enough irradiation intensity, colorless (Figure 9b). About 4.8 mm column sample, with 2.6 mm complete bleaching, was obtained under 10 min irradiation (Figure 9b insert). This impressive deep photocuring is due to the excellent photobleaching properties of the O-3. 22


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Figure 9. The photobleaching of O-3 (5.5× 10-5 mol/g resin) in thiol-ene resin system (a) as coating and (b) in bulk. The irradiation was carried out by using 450 nm visible LED light with an intensity of 200 mW/cm2.

Thermal Stability Besides high initiation efficiency, sufficient thermal stability in the dark is also essential for oxime-ester photoinitiators. The active N-O bonds in oxime-ester PIs make them vulnerable to the decomposition at elevated temperatures. The thermal gravimetric analysis (TGA) was carried out to evaluate the thermal stability of the photoinitiators in the absence of monomers (Table 3). The commercially available oxime-ester photoinitiator OXE-1 has high photosensitivity and excellent thermostability, consistent with previous studies.58, 63 The OXE-1 has shown the start of decomposition at 185 °C, while the O-3 started to decompose from 150 °C. The results indicate that the O-3 has inferior thermal stability than OXE-1, in the absence of monomers, but the thermal stability of O-3 is sufficient for usual storage.

Since the polymerization is an endothermic chemical process, the differential scanning calorimetry (DSC) is a useful tool to monitor the thermal stability of the photoinitiators in the presence of monomers. The higher onset temperature indicates better thermal stability of the formulation. For the control sample, comprising of TMPTA, the onset temperature appeared at 130 °C (Table 3). It is well-known that the acrylate monomer undergoes self-induced polymerization at elevated temperatures. 23



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The addition of initiators led to a reduction in the onset temperature. The onset temperature of the formulation containing OXE-1 was 70 °C, while the corresponding temperature for O-3 increased up to 105 °C. This indicates that the O-3 exhibits better thermal stability than OXE-1, in the presence of the TMPTA monomer.

Table 3. The thermal gravimetric analysis of PIs without monomers and the DSC measurements of pure TMPTA, O-3/TMPTA and OXE-1/TMPTA (concentration of 2.8 × 10-5 mol/g) Decomposition temperature Onset temperature Samples

(°C)

(°C)

O-3

150

106

OXE-1

185

70

TMPTA

-

132

CONCLUSIONS We demonstrate the successful synthesis of a series of novel coumarin-based oxime-ester

photoinitiators,

containing

diethylaminocoumarin

moieties

as

chromophores and oxime-ester groups as initiation functionalities, and evaluated their structure-property relationship. The substitution positions of the oxime-ester, on the coumarin ring, remarkably affect the absorption behavior and the fluorescence emission spectra. The O-3 exhibited a much stronger linear absorption and 1.5 times higher fluorescence quantum yield, compared to O-4, in accordance with our theoretical calculations. In comparison with the O-3，the incorporation of an electron donating/withdrawing group on the benzene ring, as in O-3F and O-3O, led to the weaker absorption and improved fluorescence quantum yields. A photolysis mechanism, including photoinduced cleavage

of N-O bond followed by

decarboxylation to generate active radicals, was proposed based on CO2 detection and ESR tests. Due to the efficient initiation mechanism combined with a broad absorption band, ranging from 400 nm up to 480 nm, the investigated oxime-ester could effectively induce acrylate and thiol-based click photopolymerization under 24


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visible LED irradiation. The O-3 exhibited excellent photobleaching properties both in resin system and solvent. Furthermore, we have shown the sufficient thermal stability of the photoinitiators, in the dark, by TGA and DSC.

ASSOCIATED CONTENT Supporting Information. Synthesis of the precursors, NMR of the byproduct and photolysis of the photoinitiator. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the financial support by the National Nature Science Foundation of China (21404048 and 51673086), the Fundamental Research Funds for the Central Universities (JUSRP 51719A) and the Open Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2016KF09). We would like to thank Prof. Robert Liska from Vienna University of Technology for useful discussions.

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Coumarin-Based Oxime Esters: Photobleachable and Versatile

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