D-π-A-type aryl dialkylsulfonium salts as one-component versatile photoinitiators under UV/visible LEDs irradiation

Article abstract of DOI:10.1016/j.dyepig.2016.04.004

Two D-π-A-type aryl dialkylsulfonium salts with substituted-fluorene as a conjugated system were developed as one-component, versatile photoinitiators under soft irradiation conditions (365-450 nm Light-emitting diodes). The mechanisms for photochemical g

Full text of DOI:10.1016/j.dyepig.2016.04.004

Accepted Manuscript
D-π-A-type aryl dialkylsulfonium salts as one-component versatile photoinitiators
under UV/visible LEDs irradiation
Xingyu Wu, Jean-pierre Malval, Decheng Wan, Ming Jin
PII:
S0143-7208(16)30125-5
10.1016/j.dyepig.2016.04.004
DYPI 5182
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 24 February 2016
Revised Date: 25 March 2016
Accepted Date: 1 April 2016
Please cite this article as: Wu X, Malval J-p, Wan D, Jin M, D-π-A-type aryl dialkylsulfonium salts as
one-component versatile photoinitiators under UV/visible LEDs irradiation, Dyes and Pigments (2016),
doi: 10.1016/j.dyepig.2016.04.004.
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Graphical Abstract
One-component photoinitiators can initiate different photopolymerizations under soft
irradiation conditions (365–450 nm LEDs)

ACCEPTED MANUSCRIPT
D-π-A-type aryl dialkylsulfonium salts as one-component
versatile photoinitiators under UV/visible LEDs irradiation
Xingyu Wu,a Jean-pierre Malval,b and Decheng Wan,a Ming Jin*a
aSchool of Materials Science and Engineering, Tongji Univsersity, 4800 Caoan Road, Shanghai,
201804, P.R. China.
bInstitute de Science des Matériaux de Mulhouse, UMR CNRS 7361, Université de Haute-Alsace, 15
Rue Jean Starcky, Mulhouse, 68057, France.
* Corresponding author. Tel.:+86-21-65980143
E-mail address: mingjin@tongji.edu.cn
Abstract
Two D-π-A-type aryl dialkylsulfonium salts with substituted-fluorene as a conjugated system
were developed as one-component, versatile photoinitiators under soft irradiation conditions (365–
450 nm Light-emitting diodes). The mechanisms for photochemical generation of reactive species
(cations and radicals) by the photoinitiators were studied by UV-Vis spectroscopy, theoretical
calculations, fluorescence spectroscopy and cyclic voltammetry techniques. The high quantum yields
+
for acid generation (ΦH = ~0.43 to 0.63) of the compounds enable the cationic polymerizations of
epoxides and vinyl ether to proceed effectively. Meanwhile, the photocleavage products of
sulfoniums (i.e., radicals) initiate the free radical polymerization of acrylate. Such versatile
properties allowed them to serve as effective photoinitiators for the synthesis of interpenetrated
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polymer networks and thiol-ene polymerization without any co-initiators or additives. Based on
their beneficial features, these photoinitiators hold great promise for use in photocuring applications.
Keywords: Photopolymerization; Sulfonium Salts; Photoinitiating Systems; One-component;
Light-emitting diodes.
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1. Introduction
Photopolymerization has aroused great interest due to its wide applicability in the
manufacturing of coatings, adhesives, inks, and microelectronics; in 3D microfabrication; and
so on.[1-6] Free radical polymerizations (FRP),[7] cationic polymerizations (CP),[8]
cationic/radical copolymerizations for the syntheses of interpenetrated polymer networks
(IPN)[9] and thiol-ene polymerizations (TEP)[10] are typical and well-known
photopolymerization reactions for the formation of highly cross-linked networks that are
utilized in different photocuring fields.[6] The search for efficient, versatile photoinitiator
systems (PISs) which perform a concomitant radical/cationic dual curing in a one-step
procedure upon excitation in the UV-Visible range should be highly useful for the
development of new photopolymerization reactions.[11] Many multi-component mixtures
(photoinitiators/coinitiators) are good versatile PISs and have been described in recent
years.[12-16] Although the presence of co-initiators has always increased the efficiency of
the initiating process, many additional problems also emerged. Most co-initiators are toxic
and mutagenic, they tend to induce substrate corrosion, and cause cured films to yellow, all of
which greatly limit their application, and one-component photoinitiators show special
advantages.[17] Recently, Lalevee et al. reported iodonium-polyoxometalate and
thianthrenium-polyoxometalate as one-component UV photoinitiators for radical and cationic
polymerization through an intramolecular electron transfer from the counter anions to onium
salts and exhibited higher reactivity.[18] Meanwhile, inspiring from the technological
advances of light-emitting diodes (LEDs), PISs have been adapted to operate under LED
irradiations, especially Near UV/visible LEDs.[19, 20] These eco-friendly PISs exhibit many
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advantages including low cost,[21] a lack of harmful UV, and a higher operating efficiency.
Therefore, the use of one-component and versatile photoinitiators with long wavelength
absorption would be of significant benefit to industrial applications that require photocuring.
Sulfonium salts have generated interest in the microelectronics field for its use as a
photoacid generator (PAG)—a species that produces H+ upon irradiation—in both positive-
and negative-tone photoresist formulations.[2] In particular, sulfonium salts with
–
–
–
non-nucleophilic anions such as PF6 , AsF6 , or SbF6 have found widespread use as
photoinitiators for the cationic photopolymerizations of epoxides and vinyl ethers.[22, 23]
The development of new sulfonium salts, with a suitable absorption wavelength and high
performance, has been extensively explored.[24-27] The versatility of PAG photoinitiators is
greatly limited because sulfonium salts are mainly confined to cationic polymerizations by H+
production. However, as presumed from the photocleavage mechanism, the homolytic
cleavage of the S-C bond leads to the formation of a cation-radical, on the sulfur-containing
fragment, and a neutral radical; because this action occurs prior to the generation of H+, it
offers the possibility of initiating a radical polymerization.[6]
Herein, two novel D-π-A-type aryl dialkylsulfonium salts (Flu-PS and Flu-MS in Scheme
1) with substituted fluorenyl group as the π-conjugated system are designed and synthesized.
In these two compounds, the fluorene unit exhibited a planar geometry, where a strong
electron delocalization was expected,[28] thus, extending the absorption of sulfonium salts to
match UV-Visible LEDs irradiation. Both sulfonium salts showed high efficiency and
+
excellent photoacid generation (ΦH = 0.43 to ~0.63) and decomposed very rapidly under
irradiation of 365–450 nm LEDs. The mechanism and production of reactive species (i.e.,
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radicals and cations) were studied using UV-Vis spectroscopy, theoretical calculations,
steady state photolysis, NMR, fluorescence, and cyclic voltammetry and electron spin
resonance spin trapping techniques. Their roles as one-component (without any additives)
versatile photoinitiators for FRP, CP, IPN, and TER were confirmed by real-time FT-IR upon
exposure of Near UV or blue LEDs (365 – 450 nm) at low light intensities (1~4 mW cm-2).
The good wavelength tunability, high sensitivities, and economy, including high efficiency
(1.0% additive), low excitation energy (1–4 mW cm2, LEDs@365–405 nm) and
one-component, make them useful and advantage for versatile photopolymerizations.
2. Experimental section
2.1 Materials
The PAGs and monomers investigated here are presented in Scheme 1 and used with the best
purity available. N-vinylcarbazole (NVK), cyclohexene oxide (CHO), triethyleneglycol divinyl ether
(DVE-3), methyl diethanolamine (MDEA), trimethylolpropane tris(3-mercaptopropionate) (Trithiol),
trimethylolpropane
triacrylate
(TMPTA)
and
(3,4-Epoxycyclohexane)methyl
3,4-epoxycyclohexylcarboxylate (EPOX) were purchased from Sigma-Aldrich or J&K Chemical and
used as received without further purification. All chemicals for synthesis were purchased from
Sinopharm Chemical Reagent Co., Ltd.; TCI; or J&K Chemical, and they were used without further
purification unless otherwise specified. All solvents employed for photophysical measurements were
J&K Chemical spectroscopic grade.
2.2 General instrumentations
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1H NMR and 13C NMR measurements were recorded with a Bruker 400M NMR spectrometer and
chemical shifts were reported in parts per million (ppm) downfield from the Me4Si resonance, which
was used as the internal standard when recording NMR spectra. Elemental analysis was performed
by Elementar Vario El III (Germany). Mass spectra were recorded on a Micromass GCTTM.
UV-Vis spectra were recorded on a Mapada UV-6300 spectrophotometer. The photodecomposition
processes in a solution of acetonitrile and CHO were studied by UV-Vis spectra under an LED
irradiation of 365 nm. Steady-state fluorescence spectra were collected using a Hitachi-2700
spectrofluorometer. Emission spectra were spectrally corrected.
Quantum yields for acid generation were measured under irradiation at 365 nm using LED point
curing (Uvata, Shanghai). Both irradiated PAGs dissolved in acetonitrile were previously
N2-degassed. The progress of the photoreaction was monitored via UV-Vis absorption spectra. The
absorbance at the excitation wavelength was greater than 2.5 to assume a total absorption of the
incident photons. The dose rates were maintained sufficiently small so that the changes of Aλ were
lower than 10%. Rhodamine B (RhB) was used as a sensor of photoacid generation.[29] The acid
generation in acetonitrile was also evaluated from a calibration curve of RhB, which was gradually
protonated by the addition of p-toluenesulfonic acid. The incident light intensity was measured by
ferrioxalate actinometry.
Molecular orbital calculations were performed using the GAUSSIAN 09 package.[30] The
electronic absorption spectra for different compounds were determined by time-dependent density
functional theory (DFT) at the B3LYP/6-31G*. The molecular orbitals involved in these transitions
could be extracted, and the geometries were frequency checked.
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The cyclic voltammetry experiments (using a computer-controlled Radiometer Voltalab 6
potentiostat with a three-electrode single-compartment cell. The working electrode was a platinum
disk, and a saturated calomel reference electrode (SCE) was placed in a separate compartment
containing the supporting electrolyte and connected by a salt bridge) were performed at 300 K, in
N2-degassed acetonitrile with a constant concentration (0.1 M) of (n-Bu)4NBF4. Ferrocene (Fc) was
used as an internal reference (considering EFc/Fc+ = 0.53 V in ACN vs. aqueous SCE).[24]
The photopolymerization was monitored in situ by Fourier transform real-time infrared
spectroscopy (FT-RTIR) with a Nicolet iS5 spectrometer. The evolution of the epoxy group content
in EPOX and CHO, the double bond content of TMPTA and DVE-3, and the thiol (S-H) content of
Trithiol were continuously followed by real time FTIR spectroscopy at approximately 790, 780, 1630,
1620 and 2580 cm−1, respectively.[14, 31]
2.3 Synthesis
The synthetic route of PAGs are shown in Scheme 2, and the details of characterization are
provided in the electronic supporting information (ESI†).
2.3.1 Preparation of 2-bromo-7-N,N’-diphenylamino-9,9’-dimethyl-9H-fluorene (1)
2,7-dibromo-9,9’-dimethyl-9H-fluorene (25.5 g, 72.31 mmol) was added to a mixture of
diphenylamine (12.2 g, 72.31 mmol), Cu powder (2.0 g, 31.25 mmol), potassium carbonate (20.0 g,
0.14 mol) and 18-crown-6 (2.0 g, 8.33 mmol) in 1,2-dichlorobenzene (30 mL) under nitrogen
atmosphere at room temperature. The resultant mixture was stirred for 10 min and then refluxed at
200 °C for 24 h. Then, the reaction mixture was cooled to room temperature, the
1,2-dichlorobenzene was distilled at low pressure and the mixture was extracted three times with
CH2Cl2 (3 × 150 mL). The product was purified by column chromatography with use of 2% ethyl
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acetate in hexanes as eluent to give 25.2 g of colorless solid (79.0%). 1H NMR (400 MHz, CDCl3): δ
7.53–7.41 (m, 4H), 7.28–7.24 (m, 4H), 7.16–7.12 (m, 5H), 7.04–6.99 (m, 3H), 1.39 (s, 6H).
2.3.2 Preparation of 9,9’-dimethyl-2-(3-methylthio)phenyl-7-N,N’-diphenylamino-9H-fluorene
(Pre-M) and 9,9’-dimethyl-2-(4-methylthio)phenyl-7-N,N’-diphenylamino-9H-fluorene (Pre-P)
A
mixture of
1
(5.0 g, 11.4 mmol), 3-(methylthio)phenylboronic acid or
4-(methylthio)phenylboronic acid (2.1 g, 11.4 mmol), potassium carbonate (4.7 g, 34.2 mmol),
Pd(PPh3)4 (0.7 g, 0.6 mmol) and methyl trioctyl ammonium chloride (2.0 mL) in toluene (40 mL),
ethanol (24 mL) and water (8 mL) was stirred under N2 atmosphere at 100 °C for 24 h. The resulting
solution was cooled to room temperature, extracted with dichloromethane (3 × 100 mL), washed
with water and dried over anhydrous MgSO4. The crude product was purified by column
chromatography on silica gel using dichloromethane/petroleum ether (1:5, v/v) as eluent to afford the
desired product as a white solid.
Pre-M: 4.5 g, 82.3%. 1H NMR (400 MHz, CDCl3): δ 7.68 (d, J = 7.8 Hz, 1H), 7.59–7.52 (m, 4H),
7.43–7.35 (m, 2H), 7.28–7.23 (m, 5H), 7.20 (s, 1H), 7.15–7.13 (m, 4H), 7.04–7.01 (m, 3H), 2.55 (s,
13
3H), 1.45 (s, 6H). C NMR (101 MHz, CDCl3): δ 155.4, 154.3, 148.0, 147.5, 142.4, 139.0, 138.6,
133.6, 129.3, 126.3, 125.6, 125.2, 124.2, 123.3, 122.8, 121.3, 120.8, 119.8, 118.5, 47.0, 27.2, 16.1.
MS: m/z 484.20 [M + H+] (Calcd. for C34H29NS: 483.20). Anal. Calcd. (%) for C34H29NS: C, 84.43;
H, 6.04; N, 2.90; Found: C, 84.59; H, 6.07; N, 2.68.
1
Pre-P: 4.3 g, 80.2%. H NMR (400 MHz, CDCl3): δ 7.65 (s, 1H), 7.58 (m, 4H), 7.52 (d, J = 7.9
Hz, 1H), 7.36–7.33 (m, 2H), 7.29–7.25 (m, 5H), 7.14–7.13 (m, 5H), 7.02 (m, 2H), 2.53 (s, 3H), 1.45
(s, 6H). 13C NMR (101 MHz, CDCl3): δ 155.3, 154.3, 148.0, 147.4, 138.8, 138.5, 138.3, 137.3, 133.7,
129.3, 127.5, 127.1, 125.9, 124.2, 123.3, 122.7, 121.0, 120.7, 119.8, 118.5, 47.0, 27.2, 16.1. MS: m/z
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484.20 [M + H+] (Calcd. for C34H29NS: 483.20). Anal. Calcd. (%) for C34H29NS: C, 84.43; H, 6.04;
N, 2.90; Found: C, 84.49; H, 5.97; N, 2.78.
2.3.3 Preparation of Flu-MS and Flu-PS
In a dark room, precursors (500.0 mg, 1.03 mmol), silver trifluoromethanesulfonate (529.0 mg,
2.06 mmol) and Cs2CO3 (672.0 mg, 2.06 mmol) were added to anhydrous CH2Cl2 (30 mL) under N2
atmosphere at room temperature. Then, 4-cyanobenzyl bromide (403.8 mg, 2.06 mmol) dissolved in
5 mL of anhydrous CH2Cl2 was syringed in half hour. The mixture was stirred for 48 h in the dark.
Then, the crude products were directly purified by column chromatography on silica gel using
chloroform/methanol (10:1, v/v) as eluent to afford the desired product as a yellow solid. The
product was used in the ion-metathesis described below without further purification. A fresh 50 mL
portion of aqueous NaSbF6 (4.60 g, 25.0 mmol) was added to a solution of prepared PAGs in 20 mL
of acetone. The mixture was stirred for 2 h at room temperature in the dark. The solid was collected
by filtration and re-dissolved in 10 mL of acetone. The above anion-exchange was repeated three
times and gave the final products.
Flu-MS: 575.3 mg, 75.0%. 1H NMR (400 MHz, CD3CN): δ 8.06 (s, 1H), 8.00 (s, 1H), 7.76–7.58
(m, 9H), 7.43–7.41 (m, 2H), 7.28–7.22 (m, 4H), 7.05–7.00 (m, 6H), 6.94 (d, J = 8.2 Hz, 1H), 5.06 (d,
J = 12.7 Hz, 1H), 4.88 (d, J = 12.7 Hz, 1H), 3.32 (s, 3H), 1.42 (s, 6H). 13C NMR (101 MHz, CD3CN)
δ: 155.4, 154.3, 147.5, 143.6, 139.3, 135.8, 132.9, 132.7, 132.3, 131.3, 131.0, 129.1, 128.7, 128.3,
126.1, 123.9, 122.8, 122.6, 122.2, 121.2, 121.0, 119.8, 117.7, 117.5, 117.0, 113.3, 49.7, 46.5, 25.9,
24.1. MS: m/z 599.25 [sulfonium ion] (Calcd. for C42H35N2S+: 599.25). Anal. Calcd. (%) for
C42H35F6N2SSb: C, 60.37; H, 4.22; N, 3.35; Found: C, 60.57; H, 4.07; N, 3.68. Decomposition
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before melting and the thermal decomposition temperature (Td, temperature at 5% weight loss in
TGA): 147.5 oC.
Flu-PS: 590.0 mg, 78.0%. 1H NMR (400 MHz, CD3CN): δ 7.99–7.97 (m, 2H), 7.83–7.66 (m, 9H),
7.38–7.20 (m, 7H), 7.12–7.03 (m, 6H), 4.92 (d, J = 12.8 Hz, 1H), 4.74 (d, J = 12.8 Hz, 1H), 3.23 (s,
3H), 1.43 (s, 6H). 13C NMR (101 MHz, CD3CN) δ: 155.5, 154.4, 147.8, 147.5, 147.3, 139.7, 135.8,
132.8, 132.4, 132.2, 131.2, 131.0, 129.1, 128.6, 126.3, 123.9, 122.8, 122.5, 121.4, 121.0, 119.9,
119.1, 117.7, 117.6, 117.0, 113.3, 49.9, 46.6, 25.8, 24.4. MS: m/z 599.25 [sulfonium ion] (Calcd. for
C42H35N2S+: 599.25). Anal. Calcd. (%) for C42H35F6N2SSb: C, 60.37; H, 4.22; N, 3.35; Found: C,
60.61; H, 4.37; N, 3.57. Thermal decomposition temperature (Td): 184.0 oC.
3. Results and discussion
3.1 Synthesis
Two sulfonium salts, Flu-PS and Flu-MS, were prepared from 2,7-dibromofluorene in four steps
(Scheme 2). Cu-catalyzed coupling with diphenylamine produced the triphenylamine-containing
conjugated system because the triphenylamine group has been proven as a good substituent in
sulfonium salts.[32] Then, Suzuki reactions were performed with para/meta-(methylthio)-
phenylboronic acid to produce the precursors in high yield (over 80%). Finally, alkylation of the
precursors with 4-cyanobenzyl bromide and AgCF3SO3 gave fluorene-containing sulfonium triflate
–
salts; then, non-nucleophilic anions, such as SbF6 , were exchanged to benefit the polymerization
reactions.[32]
3.2 Photochemical and photophysical properties of the sulfonium salts
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The absorption spectra of the two sulfonium salts (Flu-MS and Flu-PS) in acetonitrile are shown
in Fig. 1. The maxima for Flu-MS and Flu-PS are located in the UV-Vis at λmax = 368 nm (ε368 nm
~
36,900 M−1 cm−1) and λmax = 378 nm (ε378 nm ~ 39,400 M−1 cm−1), respectively (see Table 1). As to
the spectrum of Flu-PS, it presents a slight redshift and a higher ε value due to its stronger
conjugated effect.[24] The excellent absorption properties of PAGs are associated with the large
delocalization of the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO), which are involved in the lowest energy transitions.[24, 33] This result is
further confirmed by DFT calculations, as shown in Fig. 2. The HOMOs of sulfoniums are mainly
localized on the triphenylamine (donor) and fluorene (conjugated system) moieties; however, the
LUMOs are chiefly distributed throughout the sulfonium moieties. Saeva has proved that for
arylsulfonium salts, the photoreaction can happen either by direct excitation of a π-σ* (sulfur-carbon
bond) transition or by excitation of a π-π* transition, which is followed by intramolecular electron
transfer to the σ* on sulfur-carbon bond.[34-36] In both cases, subsequent homolytic cleavage of the
S-C bond leads to the formation of a radical cation on the sulfur-containing fragment and a neutral
radical on the carbon fragment. The results obtained from the theoretical calculations confirm that
the S-C σ* bond (face to face) does exist in the LUMO orbitals of both PAGs, as shown in Fig. 2.
The typical evolution of the absorption spectra of Flu-MS in acetonitrile, upon irradiation by 365
nm LED, is shown in Fig. 3(a). While the band located at approximately 280 nm increases slightly,
the maximum-wavelength absorption band dramatically collapses with the appearance of a less
intensive band located at 356 nm. Moreover, upon photolysis, a highly fluorescent byproduct is
progressively generated (Fig. S1, ESI†). The excitation spectrum recorded at the maximum
fluorescence wavelength matches perfectly with the absorption spectrum of the precursor (Pre-M),
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which presents a distinctive band at 360 nm (Fig. S2, ESI†). In addition, this effect is corroborated
by the slight increase of the absorption band of Pre-M (λmax at 365 nm) when the photolysis of
Flu-MS is performed as the monomer, i.e., CHO, as shown in Fig. 3(b).
1
The photolysis of Flu-MS is also confirmed by H NMR spectra upon irradiation in CDCl3 (Fig.
S3(a), ESI†). The evolution of the proton signals proves the decomposition of Flu-MS. At the same
time, the single peaks located from 3 to ~5 ppm indicate the coupling of the 4-cyanobenzyl radical
with the molecular framework.[37] When 10 eqv. of CHO was co-photolyzed with Flu-MS, the
1
produced H+ can initiate the polymerization of CHO and the byproducts are being compared. H
1
NMR shows signals of the polymerization of CHO (Fig. S3b and 3c, ESI†). Interestingly, the H
NMR analyses also indicate that the yield for generation of Pre-M is strongly increased when the
photolysis is performed in CHO (Fig. S3(b), ESI†) due to fewer and weaker single peaks appearing
at approximately 4–5 ppm. Note that the same effects are observed for Flu-PS, as displayed in Fig.
S1 and Fig. S4 (ESI†). Similar to the mechanism established for aryl sulfonium salts,[34] we
proposed that the photogeneration of acids mainly stems from an intramolecular rearrangement
initiated by a photoinduced electron transfer (PET) from the amino group to sulfonium; and the
possible hydrolysis mechanism are shown in Scheme 3. As previously demonstrated,[32] this PET
process is supported by the estimation of the corresponding free energy change (ꢀGeT) using the
Rehm−Weller equation [eq (1)]. In this formula, Eox and Ered correspond to the oxidation and
reduction potentials of the donor and acceptor, respectively. E00 is the energy of the singlet excited
state, which is calculated by Berlman’s method.[38] C is the Coulombic energy term characterizing
the interaction of the radical-ion pairs. In acetonitrile, this term usually makes a small contribution to
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the overall energy change for PET and is neglected in the calculation, as was previously assumed for
other sulfonium-based derivatives.[34, 37]
ꢀGeT = Eox − Ered − E00 – C
(1)
The values of Eox and Ered are determined from cyclic voltammetry (Fig. 4 and Fig. S5, ESI†) The
sulfonium salts exhibit an irreversible electrochemical reduction wave with a single peak at -1.1 V
and -1.2 V vs. SCE for Flu-MS and Flu-PS, respectively. These cathodic potentials should be
assigned to the one-electron reduction of the sulfonium group.[36] The energy of the excited singlet
state, E00, was obtained from the wavelength at the intersection of the normalized absorption and
emission spectra; the values were estimated to be 3.02 eV and 2.88 eV for Flu-MS and Flu-PS,
respectively. Table 1 summarizes the free energy changes associated with the electron transfer (ꢀGeT)
in the PAGs, which are thermodynamically favorable. This indicates that there is a large driving
force for electron transfer (ꢀGeT < -100 kJ mol-1 for Flu-MS) from the first excited singlet state.
Then, the quantum yields of H+ generation were monitored at 365 nm irradiation (Table 1). It can
+
+
be observed that the ΦH of Flu-PS is 0.43. Interestingly, Flu-MS shows a higher ΦH of 0.63 at 365
nm irradiation. The high acid generation quantum yields of the fluorene-containing sulfonium salts
reported here show that dissociation is energetically favorable, i.e., ꢀGeT < - 100 kJ mol-1, which is
higher than the triphenylamine-based sulfonium salts without the conjugated systems as reported by
+
Perry and Marder (ꢀGeT = -85.3 kJ/mol, ΦH = 0.48) [32]. These results suggest that the electron
donating ability of the diphenylamine strongly activates the dissociation of the S-C bond by
stabilizing the triphenylamine-fluorene methyl sulfide cation species, as shown in Scheme 3, such
that the total product energy now favors the dissociation from the π-σ* excited state. In addition, the
stability of a 4-cyanobenzyl radical also favors the dissociation. [24]
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3.3 Photoinitiating ability of the sulfonium salts
a. The cationic photopolymerizations of epoxides and vinyl ether. The photopolymerizations of
EPOX, CHO and DVE-3 in the presence of the two sulfonium salts (1 wt%) and in air, were carried
out using 365 nm to 450 nm LEDs exposure because the two initiators showed considerable
extinction coefficients in this range (Table 2). Both of the sulfonium salts can initiate the
polymerizations of three monomers and the conversion profiles for these reactions are depicted in
Fig. 5, Fig. S6 and S7 (ESI), and their conversions at t = 3 min are summarized in Table 3 and Table
S1 (ESI). At 365 nm, Flu-MS shows faster conversions than Flu-PS (initial parts of curves) for all
+
monomers (Fig. 5) ascribing to the higher ΦH . The conversions of DVE-3 are nearly 100% (Table 3)
for both sulfonium salts and the conversions of CHO reach 79~85%. The conversions of EPOX are
relatively lower than DVE-3 and CHO but also can arrive at 70% for Flu-MS. Upon 385 to 450 nm
irradiation, the conversions are similar at same light intensity (Fig. S6, S7 and Table S1, ESI). In line
with the behaviour of other sulfonium salts cationic photoinitiators, reactions r1-r3 account for the
generation of H+ by the two sulfoniums [26, 27] (Scheme 3) and initiate the polymerizations.
b. The free radical photopolymerization of acrylates. Flu-MS and Flu-PS enable the free radical
polymerization (FRP) of TMPTA in laminate upon exposure to 365, 385 and 405 nm LEDs at low
intensity (4 mW cm-2) (Fig. 6, Table 3, and Table S1). Curves for Flu-MS exhibit conversions from
58% to 43% for 365 nm to 405 nm, while curves for Flu-PS give nearly stable conversions over the
same spectral range (~60%). It is concluded that Flu-PS is better than Flu-MS at initiating FRP. A
comparison of the Flu-MS/MDEA system with Flu-MS itself under soft irradiation (1 mW cm-2, Fig.
6) shows that Flu-MS has an induction time, whereas MDEA can reduce the induction time
efficiently.[1]
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c. On the IPN synthesis: photopolymerization of EPOX/TMPTA blends. A simultaneous
cationic/radical photopolymerization of an EPOX/TMPTA blend (50%/50% w/w) using the Flu-PS
or Flu-MS is feasible, as well (formation of an interpenetrated polymer network, Fig. 7). The final
conversions of TMPTA and EPOX are both higher in laminate than in air (Table 3 and Table S1), and
Flu-PS exhibits higher conversions than Flu-MS. It is interesting to find that conversions of TMPTA
is greater when blended (70%) than in single monomer systems (56%), which suggests a synergistic
effect in the IPN synthesis.
d. In Thiol-Ene photopolymerization. Flu-MS and Flu-PS are also capable of initiating a fast
thiol-ene (Trithiol/TMPTA) polymerization (r4-r6) with a 365 nm LED exposure at low light intensity,
i.e., 2 mW cm-2 (Fig. 8). After ~10 s of light irradiation, the vinyl double bond of TMPTA and the
thiol (−SH) group of Trithiol arrive at their maximum conversions (80% and 42%, respectively). The
difference in ultimate conversion efficiency between the vinyl double bond and −SH (82.4% and
49.7% after 60 s of light exposure, Table 3) suggests a significant contribution to the free radical
polymerization through the self-initiation of TMPTA.
M• + RS-H → M-H + RS•
(r4)
(r5)
(r6)
RS• + R’-CH=CH2 → R’-CH•-CH2SR
R’-CH•-CH2SR + RSH → R’-CH2-CH2SR + RS•
4. Conclusions
In conclusion, two new fluorene-containing D-π-A type sulfonium salts bearing a non-nucleophilic
–
counterion, SbF6 , have been designed to be used as versatile photoinitiators. Key features of these
sulfonium salts are that (i) they produce H+ very efficiently upon exposure to 365–405 nm radiation
and even to visible light, for example, λ > 400 nm for Flu-PS at very low intensity and (ii) the
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resulting Brønsted acid/radicals from the photolysis reactions initiate the versatile
photopolymerizations without any additives, i.e., they can be used as one-component photoinitiators.
Photochemically generated radicals and cations should be useful for photocuring highly viscous
media or coatings, as well as for acid-activated material processing and fabrication in the solid phase.
Finally, this approach suggests a new design strategy for the synthesis of very efficient sulfonium
salts that could be excited over a broad spectral range by either single-photon or multiphoton
excitation.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21273020,
51173134, 51573139); Fundamental Research Funds for the Central Universities and the Open
Measuring Fund for Large Instrument and Equipment, Tongji University.
Appendix A. Supplementary data
1
Evolution of the UV-Vis spectra, emission spectra, On-line H NMR spectra of sulfonium salts,
cyclic voltammetry of Flu-PS, photopolymerization profiles of CHO and DVE-3 under different
LED irradiations, and 1H NMR and 13C NMR of precursors and sulfonium salts.
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Figure Captions.
Scheme 1 Chemical Structures of PAGs, monomers and additives.
Scheme 2 Synthetic route of the precursors and PAGs. a) Cu, K2CO3, 18-Crown-6, N2, 200 °C, 24 h;
b) K2CO3, Pd(PPh3)4, toluene, EtOH, water, 100 °C, 24 h; c) Cs2CO3, CH2Cl2, RT; d) NaSbF6, water,
acetone.
Scheme 3 Proposed photolysis mechanism of sulfoniums.
Fig. 1 The UV-Vis spectra of Flu-PS and Flu-MS in acetonitrile.
Fig. 2 Highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals
(LUMO) of Flu-M(P)S at B3LYP/6-31G(d) level.
Fig. 3 Evolution of the absorption spectra of Flu-MS in ACN (a) and in CHO (b) during irradiation at
365 nm (1 mW cm-2).
Fig. 4 Cyclic voltammogram of Flu-MS in acetonitrile + (n-Bu)4NBF4 (0.1 M) on platinum electrode
at 100 mV s−1.
Fig. 5 Photopolymerization profiles of EPOX, CHO and DVE-3 under air in the presence of
Flu-P(M)S (1 wt%) at 365 nm LED irradiation.
Fig. 6 Photopolymerization profiles of TMPTA in laminate in the presence of Flu-PS and Flu-MS (1
wt%) with LED illumination at 365, 385, and 405 nm (4 mW cm-2); Flu-MS and Flu-MS/MDEA
(1%/3%, w/w) with LED illumination at 365 nm (1 mW cm-2); and Pre-P (1 wt%) with LED
illumination at 365 nm (4 mW cm-2).
Fig. 7 Photopolymerization profiles of a TMPTA/EPOX blend (50%/50%, w/w) in the presence of
Flu-MS (a) and Flu-PS (b) (1 wt%) in laminate and under air, and with LED illumination at 365 nm
(2 mW cm-2).
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Fig. 8 Photopolymerization profiles of Trithiol/TMPTA blend (50%/50%, w/w) in laminate in the
presence of Flu-MS and Flu-PS (1 wt%) with LED illumination at 365 nm (2 mW cm-2).
Table 1 Summary of optical data and electrochemical parameters for Flu-PS and Flu-MS.
Table 2 Light absorption properties of sulfoniums and their molar extinction coefficients (ε in M−1
cm−1) at the maximum emission wavelengths of different LEDs.
Table 3 DVE-3, CHO, EPOX, TMPTA, TMPTA/EPOX and TMPTA/Trithiol conversions obtained
with 365 nm LED illumination in the presence of Flu-MS or Flu-PS (1 wt%).
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Scheme 1 By Ming JIN et al.
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Scheme 2 By Ming JIN et al.
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Scheme 3 By Ming JIN et al.
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Fig. 1 By Ming JIN et al.
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Fig. 2 By Ming JIN et al.
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Fig. 3 By Ming JIN et al.
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Fig. 4 By Ming JIN et al.
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Fig. 5 By Ming JIN et al.
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