Molecular Engineering of UV/Vis Light-Emitting Diode (LED)-Sensitive Donor–π–Acceptor-Type Sulfonium Salt Photoacid Generators: Design, Synthesis, and Study of Photochemical and Photophysical Properties

Article abstract of DOI:10.1002/chem.201703414

A series of donor–π–acceptor-type sulfonium salt photoacid generators (PAGs) were designed and synthesized by systematically changing electron-donating groups, π-conjugated systems, electron-withdrawing groups, and the number of branches through molecular engineering. These PAGs can effectively decompose under UV/Vis irradiation from a light-emitting diode (LED) light source because of the matching absorption and emitting spectra of the LEDs. The absorption and acid-generation properties of these sulfonium salts were elucidated by UV/Vis spectroscopy and so forth. Results indicated that the PAG performance benefited from the introduction of strong electron-donating groups, specific π-conjugated structures, certain electron-withdrawing groups, or two-branched structures. Most sulfonium salts showed potential as photoinitiators under irradiation by a wide variety of UV and visible LEDs.

Full text of DOI:10.1002/chem.201703414

A Journal of
Accepted Article
Title: Molecular Engineering of Light-Emitting Diode-Sensitive D-π-A
Type Sulfonium Salt Photoacid Generators: Design, Synthesis,
and Study of Photochemical and Photophysical Properties
Authors: Xingyu Wu, Ming Jin, Jianchao Xie, Jean-Pierre Malval, and
Decheng Wan
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To be cited as: Chem. Eur. J. 10.1002/chem.201703414
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Molecular Engineering of UV–Vis Light-Emitting Diode-Sensitive
D-π-A Type Sulfonium Salt Photoacid Generators: Design,
Synthesis, and Study of Photochemical and Photophysical
Properties
Xingyu Wu,[a] Ming Jin,*[a] Jianchao Xie,[a] Jean-Pierre Malval,[b] and Decheng Wan[a]
Abstract:
A
series of D-π-A type sulfonium salt photoacid
with conventional light sources, LEDs offer numerous
advantages,[14] such as low costs, long life time, small size, and
less heat generation. LED bears significance and is useful for
achieving excitation of near UV or/and visible LEDs in sulfonium
salt PAGs.[15] Two approaches are mainly adopted in
photoinitiating systems (PISs).[16] The first one comprises
sensitizing methods, which typically include photosensitizers
generators (PAGs) were designed and synthesized by systematically
changing electron-pushing groups, π-conjugated systems, electron-
withdrawing groups, and the number of branches through molecular
engineering. These PAGs can effectively decompose under
irradiation of UV–visible light-emitting diode (LED) light source
because of their matching absorption spectra and emitting spectra of
LEDs. Absorption and acid generation properties of these sulfonium
salts were elucidated by UV–vis spectra, etc. Results indicated that
PAG performance benefited from introduction of strong electron-
donating groups, specific π-conjugated structures, certain electron-
withdrawing groups or two-branched structures. Most of sulfonium
salts showed potential as photoinitiators under irradiation of a wide
variety of UV and visible LEDs.
(e.g.,
perylene,[17]
fluoflavin,[18]
phenothiazine,[19]
and
thioxanthone[20]) and commercially available PAGs, in which
transfer of photoinduced electrons from excited sensitizers to
PAG results in formation of acid and other species.[21] In some
systems, improvement of reactivity requires one or more
additives.[22] Numerous multicomponent PISs with excellent
initiation properties under LED irradiation were proposed in
recent years.[16, 23] However, several problems still exist in these
systems; such problems include (i) slow initiation rates because
of global kinetics limitation by diffusion and (ii) toxicity and
mutagenicity of additives, inducing substrate corrosion and
Introduction
causing yellowing of cured films.[24] Therefore, an integrated
Sulfonium salt photoacid generators (PAGs)[1] can be used to
activate various reactions,[2] including cationic ring-opening
polymerization of epoxides and oxetanes,[3] addition reaction of
vinyl ethers,[4] and cleavage reactions of esters.[5] PAGs are
extensively developed in various research fields, such as
chemically amplified photoresists,[6] photopolymerization,[7] 3D
microfabrication,[8] and high-density optical data storage.[9]
25]
approach was alternatively proposed.[8,
Chromophores can
sensitize decomposition of sulfonium moiety through
intramolecular electron transfer. This integrated approach was
well verified by Saeva,[11a, 11b, 26] Marder and Perry,[8] Belfield,[25]
and Jin and Malval.[7b-j] The present study proved that
photochemical and photophysical properties of these sulfonium
salts change significantly with different donating groups, π-
conjugated structures, leaving groups (e.g., methyl, benzyl, and
Commercially
available
sulfonium
salt
PAGs
(e.g.,
triarylsulfonium salt) exhibit excellent photosensitivity and
outstanding thermal stability.[10] However, as these PAGs are
excited by deep UV,[1a, 6b, 10-11] bulk inner filter and high-energy
consumption are inevitable. These circumstances require
development of long-wavelength UV or visible light source-
sensitive sulfonium salt PAG systems.[3a, 8]
As ecofriendly light sources, light-emitting diodes (LEDs)
exhibit a wide range of applications, including illumination,
fluorescent detection,[12] and antique identification.[13] Compared
4-cyanobenzyl) and the number of branches.
This integrated approach provided several routes for
designing highly sensitive sulfonium salt PAGs under long-
wavelength UV and visible-light excitation. However, to our
knowledge, effects of such combination on PAG properties
remain to be rationalized. In this context, four series of D-π-A-
type sulfonium salt PAGs were designed by systematically
changing electron-pushing groups (D), π-conjugated systems
(π), electron-withdrawing groups (A), and the number of
branches and summarized (Scheme 1, changing the number of
branches is not included). In these PAGs, 1b (b′), 1c′, 1d (d′), 1e
(e′), 1g′, 1h′, 2a (a′), 2b (b′), 2j (j′), 3c′, 3d′, and 3e′ were newly
synthesized for good comparison. Discussion addresses
photochemical and photophysical properties of these PAGs
under LED irradiation and the relationship between structures
and properties. This method provides a clear route for designing
LED-sensitive sulfonium salts with good photoacid generation
performance.
[a]
[b]
X. Wu, M. Jin,* J. Xie and D. Wan
School of Materials Science & Engineering
Tongji University
4800 Caoan Road, Shanghai, China
E-mail: mingjin@tongji.edu.cn
J-P Malval
Institut de Science des Matériaux de Mulhouse
UMR CNRS 7361, Université de Haute-Alsace
15 rue Jean Starcky, 68057 Mulhouse, France
Supporting information for this article is given via a link at the end of
the document.
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Scheme 1. Screening of D-π-A type sulfonium salt PAGs with different groups.
Results and Discussion
Effects of different electron-pushing groups (D) on
photochemical and photophysical properties of PAGs
PAGs 1a–1h′ possess the same substituted stilbene as π-
conjugated systems (Scheme 2). Among these PAGs, different
substituents, e.g., ethoxy (1a and 1a′), carbazolyl (1b and 1b′),
N,N-diphenylamino (1c, 1c′, 1d, 1d′, and 1f′), N,N-
dialkylphenylamino
(1e
and
1e′),
and
N,N-
dihalogenophenylamino (1g′ and 1h′), are introduced as
electron-pushing groups (D). Then, photochemical and
photophysical properties of these PAGs were compared.
Scheme 2. Molecular structures of PAGs 1a–1h′ with different electron-
pushing groups.
Figure 1 shows absorption spectra of these PAGs in
acetonitrile; Table 1 summarizes corresponding spectroscopic
data. Maximum absorption peaks varied from 347 nm for ethoxy-
containing PAG (1a) to 400 nm for N,N-diphenylamino-
containing PAG (1c). Absorption edges reached 500 nm (Figure
1); this value could be used in a wide variety of UV and visible
LEDs. All PAGs exhibited intensive absorption in the range of
200–500 nm with two well-separated bands. Primary absorption
bands were mainly dominated by a singlet transition with π–π*
bond of sulfonium salts occurred, producing photoacid.[26-27]
Thus, aside from absorption peaks, ethoxy, carbazolyl, and N,N-
diphenylamino electron-pushing groups (D) posed significant
+
effects on quantum yields of photoacid generation. ΦH
(quantum yield for acid generation) for 1a reached 0.10 and 0.23
for 1c. This finding indicated that electron-pushing groups
positively influenced photoacid generation properties. When the
position of sulfonium substituent changed from para (1c) to meta
(1c′), maximum absorption wavelength (λmax) displayed a slight
characteristic, proving that all molar extinction coefficients (εmax
)
were higher than 20,000 M−1 cm−1 (Table 1). Interestingly, an
additional weak tail was observed in the low-energy side of
absorption spectrum when sulfonium was located at the meta-
position of π-conjugated systems. This condition indicated the
presence of another transition, which should belong to π–σ*
transition, with a σ* lowest unoccupied molecular orbital (LUMO)
located on the sulfur–carbon (S–C) bond. Given that the lowest
excited state is π–σ*, efficient photoinduced cleavage of S–C
+
blueshift. However, ΦH significantly increased (from 0.23 to
0.44, e.g., 1c to 1c′). Strong conjugate effect of para-isomers
caused the redshift from meta- to para-substitution. This
conjugated effect was consistent with delocalization of electron
clouds in frontier orbits of molecules. Wider distribution of
electron clouds resulted in stronger conjugation. Distribution of
electron clouds in frontier orbits influenced absorption properties
+
of molecules.[28] The dramatic increase in ΦH was ascribed to
high ΔE (π*−σ*) value of meta-isomers according to the theory
of Seave, who indicated that ΔE (π*−σ*) is proportional to ΦH+ in
sulfonium salt PAGs [11a]. When flexible chain (1e and 1e′) or
halogen atoms (1g′ and 1h′) were modified on N,N-
diphenylamino groups, slight influence was observed on
+
a b s o r p t i o n P A G p r o p e r t i e s , i n c l u d i n g Φ H
.
To compare performances of PAGs under irradiation of
+
different LEDs, products of max  ΦH were also calculated, and
results are shown in Table 1 and Figure 2. In this series, all
PAGs exhibited good comprehensive performances. However,
PAG properties with diphenylamino groups [1c (c′), 1d (d′), and
1f′] were better than those with carbazole [1b (b′)] and ethoxy
groups [1a (a′)], wherein diphenylamino PAG (1d′) performed the
best. Nevertheless, these molecules exhibit potential as
photoinitiators at UV and visible LED irradiation.
Figure 1. UV–vis absorption spectra of PAGs 1a–1h′ in acetonitrile.
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+
Figure 2. Bar graph of max and ΦH for PAGs with different electron-pushing
groups.
Effects of different conjugate structures (π) on
photochemical and photophysical properties of PAGs
Scheme 3 presents another series of PAGs containing
different π-conjugated structures. Molecular engineering of π-
conjugated systems is based on permutations and combinations
of aryl groups (e.g., styryl, phenyl, and fluorenyl) with different
numbers (e.g., n=1–4) and positions (e.g., para- or meta-
positions). π-Conjugated systems exerted dominating influence
on photochemical and photophysical properties, as summarized
in Table 1 and Figure 3. In general, the following rules could be
deduced.
First, styryl groups benefited redshifting absorption peaks but
posed slightly negative influence on photoacid generation
properties. For example, compared with 1c and 1c′, PAGs 2a
and 2a′ possess two styryl groups composed of the conjugated
system (Scheme 3). Maximum absorption peak of 2a redshifted
at 3 nm than 1c, and εmax reached at 50,000 M−1 cm−1 (32,000
M−1 cm−1 for 1c). This result was ascribed to good π orbital
electron delocalization between donor and acceptor groups, as
promoted by planar conformation of PAGs with styryl structures.
Scheme 3. Molecular structures of PAGs 2a–2k′ with different π-conjugated
systems.
+
On the contrary, ΦH of PAGs with styryl groups was lower than
+
those of corresponding PAGs, such as PAG 1c (ΦH ~ 0.23),
Finally, a compromise molecular design strategy could solve
the problem, that is, introduction of fluorenyl group in PAGs, as
triphenyl-containing PAG 2c (ΦH+ ~ 0.58), and fluorenyl-phenyl-
containing PAG 2d (ΦH+ ~ 0.43). Meta-substituted PAGs (1c′, 2c′,
and 2d′) showed the same trend. Second, phenyl groups
showed blueshift absorption peaks compared with styryl group
but played significant roles in increasing photoacid generation
properties. For example, PAGs 2c, 2c′, 2g′, 2h′, and 2i feature
one to four phenyl rings as conjugated systems, and maximum
absorption peaks was observed in the range of 292–360 nm.
Molar extinction coefficients were measured between 13,400
and 23,500 M−1 cm−1. These parameter values were notably
lower than those of corresponding PAGs 1c and 1c′, which
contained double bonds in their conjugated systems. However,
shown in PAGs 2d, 2d′, 2j, and 2j′. As depicted in reference[28d]
,
the highest energy occupied molecular orbital (HOMO) orbits of
2d and 2d’ were located in diphenylamine and fluorene moieties,
which ensured long absorption wavelength. For LUMO orbits,
existence of S–C σ* possibly led to acid production. This effect
was verified by experiments. Through experiments, we deduced
the following. Existence of five-membered ring in fluorenyl group
maintained good planar configuration. On the other hand, no
double bonds were observed in conjugated systems, ensuring
+
good photoacid generation abilities, e.g., ΦH of both meta-
positioned PAGs 2d′ and 2j′ reached 0.6 ± 0.03, and para-
positioned PAGs 2d and 2j totaled 0.37 ± 0.05. In addition, λmax
was detected at 368–378 nm. Absorption peaks also extended
to 450 nm, and thus, they could serve as good candidates for
application under LED irradiation at 365–425 nm, which is the
best wavelength range for commercial LED light sources. Ability
of 2d (d′) to initiate cationic polymerizations was described in
+
their ΦH (0.42–0.73) was excellent in the entire series. Such
results were ascribed to twisted molecular configuration, as
proven in the reference[7f]. Dihedral angles among phenyl rings
determined existence of nonplanar molecular configuration and
corresponding properties.
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+
Products of max  ΦH of all PAGs were also calculated, and
results are shown in Figure 4 and Table 1. All PAGs yielded
remarkable values. Regardless of synthetic routes and
preparation costs, prepared PAGs were of quality. Overall, the
reasonable choice of π-conjugated system by these strategies
can match requirements in different kinds of application fields.
Effects of different electron-withdrawing groups (A) on
photochemical and photophysical properties of PAGs
Different sulfonium salts possess electron-withdrawing groups
in PAGs 3a–3e′ (Scheme 4). Their effects on PAGs lie in two
aspects. One is substituents, e.g., methyl, benzyl, and 4-
cyanobenzyl; another is substitution positions, e.g., para and
meta-position on conjugated systems. Saeva et al. studied
effects of leaving groups (R) on aryl methyl sulfonium salts. R is
attached to sulfur through a saturated carbon, preventing
electronic resonance interaction between R and sulfur. Thus, the
π system of aryl methyl sulfonium salts was expected to be less
affected by R. As shown in Figure 5 and Table 1, electron-
withdrawing groups displayed minimal effects on λmax. However,
R mainly affected acid-generating properties. When alkyl
substituent on sulfonium salt changed from methyl [3a (1f′)] to
Figure 3. UV–vis absorption spectra of PAGs 2a–2k′ in acetonitrile.
reference.[28d]
triethyleneglycol divinyl ether (DVE-3), cyclohexene oxide (CHO),
and (3,4-epoxycyclohexyl)methyl 3,4-
High
conversions
were
observed
for
epoxycyclohexylcarboxylate (EPOX) (nearly 100% for DVE-3,
79%–85% for CHO, and 60%–70% for EPOX). Introduction of
long alkyl chains 9,9-dihexyl [9,9-dihexyl-fluorenyl-phenyl PAG
2j (2j′)] into fluorenyl-phenyl PAG 2d (2d′) (with excellent
performance) posed minimal effects on absorption wavelength.
Another compromise molecular design strategy combined styryl
and phenyl groups, e.g., PAGs 2e, 2f, and 2k′. Two kinds of
arrangement methods exist. One involves connection of styryl
groups with sulfonium (2e), and another connects the phenyl
ring with sulfonium (2f and 2k′). As shown in Table 1, their
+
absorption peaks were noted at 380–390 nm, and ΦH was in
the range of ca. 0.44–0.5. All PAGs performed well in cationic
polymerization. These PAGs could also initiate free radical
polymerization and thiol-ene polymerization (i.e., 2d and 2d′)
during LED irradiation because of the generation of free radicals
during cleavage of S–C bond. As fluorescent byproducts
(precursors and precursor analogs) were generated under
irradiation, not only PAGs could produce acid, but resulting
fluorescent byproducts could also sensitize iodonium or
sulfonium salts in cationic polymerization systems. Thus,
conversions were significantly improved (i.e., 2k′).
Scheme 4. Molecular structures of PAGs 3a–3e′ with different electron-
withdrawing groups.
Figure 5. UV–vis absorption spectra of PAGs 3a–3e′ in acetonitrile.
+
Figure 4. Bar graph of max and ΦH for PAGs with different -conjugated
structures.
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+
benzyl [3b (3b′)) or 4-cyanobenzyl [1c (1c′)], H significantly
increased. This result was ascribed to poor stability of methyl
radical compared with benzyl and 4-cyanobenzyl radicals.[11a]
Table 1. Summary of optical data for all PAGs.
+[b]
+
PAG
1a
1a′
1b
1b′
1c
λabs (nm)
347
324
352
342
400
380
403
381
390
371
381
378
378
403
359
360
378
380
390
397
364
349
368
292
344
351
377
371
381
395
400
380
348
349
350
404
εmax (M-1 cm-1)[a]
36100
26000
27200
22600
32000
23000
32200
23100
32100
22800
23700
26100
26500
50000
18800
23500
39400
24500
26800
48000
24600
23500
36900
13400
19300
26700
32000
23200
23200
34300
34400
25200
16800
18300
18300
40600
ΦH
εmax * ΦH
+
Meta-substituted sulfonium salts showed higher H than para-
0.10
0.24
0.17
0.37
0.23
0.44
0.34
0.61
0.21
0.44
0.44
0.33
0.33
0.37
0.63
0.58
0.43
0.50
0.48
0.56
0.46
0.73
0.63
0.60
0.69
0.42
0.32
0.57
0.44
0.05
0.31
0.50
0.62
0.48
0.60
0.002
3610
isomers, and this result was discussed in the text and in
previous studies.[7b, 7c, 7j] As for 3c′, 3d′, and 3e′, given the strong
electron-withdrawing ability of trifluomethyl (one or two) or nitro,
these PAGs all showed good overall performance (Figure 6).
6240
4620
8360
7360
1c′
1d
1d′
1e
1e′
1f'
10120
10950
14090
6740
10030
10428
8610
1g′
1h′
2a
2b
2c
8750
18500
11844
13630
16942
12250
12864
26880
11310
17150
23250
8040
+
Figure 6. Bar graph of max and ΦH for PAGs with different electron-
withdrawing groups.
Effects of different numbers of branches on photochemical
and photophysical properties of PAGs
2d
2e
2f
Our previous study discussed the relationship between
structures and properties of PAGs with different numbers of
branches. Compared with properties of corresponding mono-
branched PAGs 4a and 4a′, two-branched structures (Scheme 5,
4b and 4b′) presented extended absorption wavelength and a
twofold increase in H+. Thus, these PAGs exhibit potential as
two-photon cationic photoinitiators.
2a′
2b′
2c′
2d′
2g′
2h′
2i′
13320
11210
10240
13220
10430
1715
2j
2j′
2k′
3a
3b
3b′
3c′
3d′
3e′
4a
10660
12600
10420
8780
Scheme 5. Molecular structures of PAGs 4a–4b′ with different numbers of
branches.
10980
81
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Synthesis: PAG 1a-1h’ (Scheme 2), 2a-2k’ (Scheme 3), 3a-3e’ (Scheme
4) and 4a-4b’ (Scheme 5) were synthesized. The synthetic route and
detailed synthetic procedure of PAGs are shown in ESI.
4b
392
413
399
32000
37000
30800
0.20
0.004
0.40
6400
148
4a′
4b′
12320
Acknowledgements
[a] In acetonitrile, and the error is approximately 10%.
[b] the error is approximately 15%.
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.
Conclusions
Molecular engineering of a series of sulfonium salt PAGs was
performed for a wide range of UV and visible LED irradiation.
Based on results for photophysical and photochemical
properties of these molecules, we derived the following. (i) The
N,N-diphenylamino group displays excellent capability as
electron-pushing group. (ii) Styryl in π-conjugated structures
aids λmax extension, and phenyl introduction benefits acid
production. Feasible options include compromise molecular
design strategies, fluorenyl introduction, or combination of styryl
and phenyl groups into π systems. (iii) In these UV–visible LED-
sensitive PAGs, strong electron-withdrawing groups, e.g., 4-
cyanobenzyl, are also suitable. (iv) Two-branched structures can
extend absorption peak and increase H+. (v) λmax of meta-
substituted sulfonium salts is shorter than that of para-
Keywords: molecular engineering • LED • photoacid generator •
photoinitiator • photopolymerization
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2016, 132, 128-135.
Experimental Section
Materials: 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.
General instrumentations: 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. Quantum yields for acid generation were measured
under irradiation using LED point curing (Uvata, Shanghai). 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
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Entry for the Table of Contents
Layout 1:
FULL PAPER
By systematically changing electron-
Xingyu Wu, Ming Jin,* Jianchao Xie,
Jean-Pierre Malval, and Decheng Wan
pushing
groups,
π-conjugated
systems, electron-withdrawing groups,
and the number of branches through
molecular engineering, a series of D-
π-A type sulfonium salt photoacid
generators (PAGs) were designed and
synthesized. Each part has different
Page No. – Page No.
Molecular Engineering of UV–Vis
Light-Emitting Diode-Sensitive D-π-A
Type Sulfonium Salt Photoacid
Generators: Design, Synthesis, and
effects
photophysical properties of PAGs.
These molecules exhibit great
on
photochemical
and
Study
of
Photochemical
and
Photophysical Properties
potential as photoinitiators in a wide
variety of UV and visible LEDs
irradiations.
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