Characterization of novel sulfonium photoacid generators and their microwave-assisted synthesis

Article abstract of DOI:10.1039/b815831b

Microwave-assisted synthesis of triarylsulfonium salt photoacid generators (PAGs) afforded reaction times 90 to 420 times faster than conventional thermal conditions, with photoacid quantum yields of new sulfonium PAGs ranging from 0.01 to 0.4. The Royal Society of Chemistry.

Full text of DOI:10.1039/b815831b

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Characterization of novel sulfonium photoacid generators and their
microwave-assisted synthesisw
Ciceron O. Yanez,a Carolina D. Andradea and Kevin D. Belfield*ab
Received (in Berkeley, CA, USA) 10th September 2008, Accepted 4th December 2008
First published as an Advance Article on the web 7th January 2009
DOI: 10.1039/b815831b
Microwave-assisted synthesis of triarylsulfonium salt photoacid
generators (PAGs) afforded reaction times 90 to 420 times faster
than conventional thermal conditions, with photoacid quantum
yields of new sulfonium PAGs ranging from 0.01 to 0.4.
Since their discovery, the use of photoacid generators, PAGs,
have been widely adopted by the polymer industry in coatings,
paints, anticorrosives, photoresists, and microelectronics,1 and
2D and 3D lithographic patterning.2,3
Two-photon absorption (2PA) has been reported for a
number of applications, exploiting the fact that the 2PA
probability is directly proportional to the square of the
incident light intensity (dw/dt p I2)4 (while one-photon
absorption bears a linear relation to the incident light inten-
sity, dw/dt p I). This intrinsic property of 2PA leads to 3D
spatial localization, important in fields such as optical data
storage,5–7 fluorescence microscopy,8,9 and 3D microfabrica-
Scheme 1 Synthesis of PAG 7. aReactions were carried out both by
tion.10,11 There have been demonstrations of successful
conventional methods, D, and microwave heating, MW.
two-photon microfabrication using commercial PAGs,11,12
even though the 2PA cross-section of these initiators is
low.13 Two-photon 3D microfabrication with a novel PAG
presence diphenyliodonium salts. Finally, photoacid quantum
was reported to fabricate MEMs structures.14 To further
yields were determined for several of the novel PAGs.
The new PAGs were designed to exhibit high 2PA cross-
sections. Because of its high thermal and photochemical
stability, fluorene was chosen as the core structure of the
PAGs.20 Quite advantageously, fluorene lends itself to ready
substitution in its 2-, 7-, and 9-positions. Stilbene or thiophene
motifs were introduced (2- and 7-positions) in order to extend
the p-conjugation. Ultimately, two acceptor groups (triaryl-
sulfonium and nitro) were introduced.
advance a number of emerging technologies, there is a great
need for PAGs with higher 2PA cross-sections.
Microwave-facilitated synthesis has been the subject of
substantial interest over the last decade.15,16 Seipel et al.
recently reported microwave-assisted reaction times 80 times
faster than conventional heating reaction times,17 and that
these reactions are potentially more energy-efficient than those
that use conventional heating. To our knowledge, there are no
reports of the microwave-assisted synthesis of sulfonium salts.
The synthesis of triarylsulfonium salts by thermolysis of the
diphenyliodonium counterpart in the presence of a diphenyl-
sulfide was originally reported by Crivello and Lam18,19
Herein, we report the microwave-assisted synthesis of triaryl-
sulfonium salt PAGs. Several of the PAGs are novel and
have potential for use as 2PA photoinitiators in negative
resists for 3D microfabrication or in optical data storage.
Furthermore, the efficiency of forming these sulfonium salts
by the microwave-assisted process was evaluated relative to
the conventional thermal reaction of diphenylsulfides in the
In order to enhance the photoacid quantum yield per
molecule, the first approach was to incorporate two sulfonium
salt motifs onto the fluorenyl scaffold, such as 7 in Scheme 1.
However, this molecule exhibited high fluorescence quantum
yield (0.80), limiting the photoacid quantum yield to (0.03)
when excited at 400 nm. The direct photolysis of triaryl-
sulfonium salts has been reported to occur primarily from
the first excited singlet state. However, sensitization studies
have shown that triplet triarylsulfonium salts are also labile.21
Consequently, we proceeded to incorporate a nitro group in
the structures to induce intersystem crossing.
As a result of this strategy, the fluorescence quantum yield
of precursors 10 and 12, and sulfonium salts 11 and 13, were
significantly decreased (Table 1), thereby reducing the radia-
tive decay pathway. Nitro-containing sulfonium salt 11
exhibited an increased photoacid quantum yield (Table 1).
The photoacid quantum yields were determined by a steady-
state method in which solutions of the PAGs in acetonitrile
were selectively irradiated at the desired wavelength with
a Department of Chemistry, University of Central Florida, 4000
Central Florida Blvd, 32816, USA Orlando, FL.
E-mail: belfield@mail.ucf.edu
b CREOL, The College of Optics and Photonics University of Central
Florida, 4000 Central Florida Blvd, 32816, USA Orlando, FL
w Electronic supplementary information (ESI) available: Detailed
synthetic procedures, microwave conditions, and characterization of
the compounds. See DOI: 10.1039/b815831b
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Table 1 Fluorescence quantum yields and photoacid quantum yields
of sulfide precursors and PAGs
Compound
FF
FH+
5
7
1.0
0.8
0.0
0.0
0.0
0.0
—
—
0.03a
—
10
11
12
13
20
0.16a; 0.30b
—
0.01b; 0.04d
0.53c
a
b
c
400 nm irradiation. 350 nm irradiation. Literature value.22
270 nm irradiation.
d
an excitation source and monochromator of a spectro-
fluorimeter. Rhodamine B was used as a sensor for photoacid
generation, as reported by Scaiano et al.23 Special care was
taken to record the photodecomposition conversion no greater
than 5% in order keep secondary photoproduct generation
to a minimum. Previously, we determined a number of
photochemical quantum yields under one- and two-photon
excitation, and found no significant difference relative to the
excitation mode. Hence, the values of Table 1 are good
estimates under 2PA.24
Scheme 2 Synthesis of PAG 11, aD: 125 1C; MW: 100 W, closed
vessel, standard mode; 40 psi; 125 1C.
Precursor 5 was prepared by a convergent synthesis
(Scheme 1) of benzyl bromide 2 and bis-diformylfluorene 4.
The phosphonate was obtained from 4-(phenylthio)benzalde-
hyde, which was first reduced with NaBH4 (10% in silica)
aided by conventional, multimode microwave radiation.25
Since hydrogen is extruded during the reaction when carried
out on scales larger than 0.500 g, safety concerns lead us to run
the reaction at room temperature for 24 h.
Scheme 3 Synthesis of PAG 13, a150 W, closed vessel, standard
mode; 100 psi; 190 1C, bD, 125 1C; MW: 100 W, closed vessel, standard
mode; 40 psi; 125 1C.
The resulting alcohol was refluxed in HBr (33% in AcOH)
to obtain the benzylic bromide intermediate, subsequently
used to obtain the bisphosphonate. The second branch of
the synthesis started from fluorene, alkylation of the
9-position with bromodecane imparted the desired solubility
to the molecule (3).26 Bromination with NBS at the 2- and
7-positions of the alkylated fluorene was followed by intro-
duction of the formyl group. Bisformyl fluorene 4 and the
bisphosphonate were coupled via a Wadsworth–Horner–
Emmons reaction, yielding exclusively the trans-isomer 5.
Bis-sulfide 5 was the precursor for bis-sulfonium salt 7 under
both conventional and microwave-assisted conditions.
The versatile 2-iodo-7-nitro-9,9-didecylfluorene 9 was used
as a precursor in the synthesis sulfonium salts 11 and 13
(Schemes 2 and 3). It was prepared as reported by Belfield
et al.26 Interestingly, stilbenyl sulfide 10 was obtained quanti-
tatively via a microwave-assisted palladium acetate-catalyzed
Heck reaction, coupling vinyl sulfide 8 and precursor 9. The
yields for both conventional and microwave-assisted methods
were the highest for PAG 11 of the entire series. Fluorenyl-
phenylsulfide 12 was prepared from Cu-catalyzed S-arylation
of 9 and thiophenol.
Scheme 4 Synthesis of PAG 16, aD: 125 1C; MW: 100 W, closed
vessel, standard mode; 40 psi; 125 1C.
14 and formylphenylsulfide 1 (Scheme 4). Both functionalities
were intact after the microwave-assisted reaction was
carried out.
Diphenyl sulfide 15 was prepared to evaluate how the
thiophene and chalcone functionalities would withstand the
rapid heating conditions of the microwave-assisted method,
further demonstrating the versatility of the microwave-
assisted-method. Precursor 15 resulted from a straight-
forward Claisen condensation of 5-bromo-2-acetylthiophene
Furthermore, we sought to compare the effectiveness of this
method when performed on less conjugated sulfides that may
be more relevant in UV or deep UV lithographic applications
(Scheme 5). The yields of 18 and 20 were comparable to those
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This journal is The Royal Society of Chemistry 2009
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The National Science Foundation (ECS-0621715 and
ECS-0524533) is acknowledged for support of this work.
Notes and references
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Scheme 5 Synthesis of PAGs 18, 20 and 22, aD: 125 1C; MW: 100 W,
closed vessel, standard mode; 40 psi; 125 1C.
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Table 2 Microwave-assisted vs. conventional heating reaction times
for sulfonium salt PAG formation
Reagent
Product
MW Time (yield)
D Time (yield)
5
7
11
13
16
18
20
22
6 min (75%)
8 min (97%)
10 min (51%)
14 min (83%)
120 s (72%)
120 s (98%)
90 s (88%)
29 h (42%)
8 h (87%)
70 h (10%)
24 h (71%)
3 h (100%)a
3 h (97%)b
3 h (83%)
10
12
15
17
19
21
a
Literature value.27 b Literature value.19
14 T. Y. Yu, C. K. Ober, S. M. Kuebler, W. H. Zhou, S. R. Marder
and J. W. Perry, Adv. Mater., 2003, 15, 517–521.
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2001, 57, 9225–9283.
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Metal-catalyzed Coupling Reactions’’ in Microwave Methods in
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Berlin, 2006, vol. 266, pp. 103–144.
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reported in the literature. As in the previous precursors,
microwave heating significantly reduced reactions times for
the formation of these sulfonium salts when compared to
conventional heating times performed by us, 21, or reported
in the literature, 20 and 22 (Table 2).
The reactions that were carried out under conventional
conditions were performed in an oil bath heated to 125 1C
at atmospheric pressure; whereas the microwave-assisted reac-
tions were run at the same temperature in closed vessel mode,
reaching pressures no higher than 30 psi.
In every case, the reactions that were heated conventionally
took significantly longer than the analogous microwave reac-
tions, and microwave-assisted reaction yields were higher.
In conclusion, we have developed a new, more favorable
methodology for the formation of sulfonium salts via micro-
wave-assisted decomposition of diphenyliodonium salts in the
presence of diphenylsulfides. Microwave-assisted reaction
times were 90 to 420 times faster, resulting in generally higher
yields. We have also shown that the introduction of groups
that favor intersystem crossing is a viable means for increasing
the photoacid quantum yield of novel triarylsulfonium
salt PAGs.
25 R. S. Varma and R. K. Saini, Tetrahedron Lett., 1997, 38,
4337–4338.
26 K. D. Belfield, K. J. Schafer, W. Mourad and B. A. Reinhardt,
J. Org. Chem., 2000, 65, 4475–4481.
27 P. Beak and T. A. Sullivan, J. Am. Chem. Soc., 1982, 104,
4450–4457.
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Chem. Commun., 2009, 827–829 | 829