Triphenylsulfonium topophotochemistry

Article abstract of DOI:10.1039/c7pp00324b

The products from the 193 nm irradiation of triphenylsulfonium nonaflate (TPS) embedded in a poly(methyl methacrylate) (PMMA) film have been characterized. The analysis of the photoproduct formation was performed using chromatographic techniques including HPLC, GPC and GC-MS as well as UV-vis and NMR spectroscopic methods. Two previously unreported TPS photoproducts, triphenylene and dibenzothiophene, were detected; additionally, GPC and DOSY-NMR spectroscopic analyses after irradiation suggested that TPS fragments had been incorporated into the polymer film. The irradiation of acetonitrile solutions containing 10% w/v PMMA and 1% w/v TPS in a 1 cm-path-length cuvette showed only a trace amount of triphenylene or dibenzothiophene, indicating that topochemical factors were important for the formation of these molecules. The accumulated evidence indicates that both products were formed by in-cage, secondary photochemical reactions: 2-(phenylthio)biphenyl to triphenylene, and diphenylsulfide to dibenzothiophene.

Full text of DOI:10.1039/c7pp00324b

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Ayoub, W. W. Kramer, W. Sattler, A. Sattler, P. Labeaume, J. W. Thackeray, J. Cameron, T. Cardolaccia, A.
A. Rachford, J. Winkler and H. B. Gray, Photochem. Photobiol. Sci., 2017, DOI: 10.1039/C7PP00324B.
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Triphenylsulfonium topophotochemistry
E. Despagnet-Ayoub,*a W. W. Kramer,a W. Sattler,b A. Sattler,a P. J. LaBeaume,c J. W. Thackeray,c J.
F. Cameron,c T. Cardolaccia,c A. A. Rachford,c J. R. Winkler,*a and H. B. Gray*a
Received 00th January 20xx,
Accepted 00th January 20xx
Products from 193-nm irradiation of triphenylsulfonium nonaflate (TPS) embedded in a poly(methyl methacrylate)
(PMMA) film have been characterized. Analysis of photoproduct formation was performed using chromatographic
techniques including HPLC, GPC and GC-MS as well as UV-vis and NMR spectroscopic methods. Two previously unreported
TPS photoproducts, triphenylene and dibenzothiophene, were detected; additionally, GPC and DOSY-NMR spectroscopic
analyses after irradiation suggested that TPS fragments had been incorporated into the polymer film. Irradiation of
acetonitrile solutions containing 10% w/v PMMA and 1% w/v TPS in a 1-cm-path-length cuvette showed only a trace
amount of triphenylene or dibenzothiophene, indicating that topochemical factors were important for the formation of
these molecules. The accumulated evidence indicates that both products were formed by in-cage, secondary
photochemical reactions: 2-(phenylthio)biphenyl to triphenylene; and diphenylsulfide to dibenzothiophene.
DOI: 10.1039/x0xx00000x
www.rsc.org/
photochemistry revealed decomposition pathways similar to
those found in solution, but with increased amounts of in-cage
products, likely owing to greatly reduced fragment diffusion.8,9
Introduction
Rigid media restrict the motions of embedded molecules,
greatly reducing the number of allowed reaction pathways.
Importantly, investigations of related solution and solid state
photoreactions have established that differences in product
distributions can be exploited in stereo- and regioselective
syntheses.1
There have been few prior reports of the photochemistry of
TPS in polymeric matrices,10 only one of which with results
from experiments involving 193-nm irradiation.11
As TPS is normally embedded in polymers in the
photolithographic process, it is of importance to elucidate the
photodecomposition pathways of this photoacid generator in
confined environments, with the goal of establishing whether
or not these pathways are qualitatively different from those
observed in solution.
We are studying the photochemistry of triphenylsulfonium
(TPS) cation,
a
photoacid generator employed in
photolithography,2 in both solution and rigid media employing
193-nm as well as other excitation sources. In earlier work on
TPS reactions in solution, it was established that both in-cage
and cage-escape products3-5 are formed, most likely by
heterolysis or homolysis of sulfur-carbon bonds. The
Notably, in our work on the 193-nm photochemistry of
triphenylsulfonium nonaflate in multiple polymeric matrices,
we found two products that were not observed in liquid phase
experiments: the new topochemistry12 product is triphenylene,
a desulfurized TPS derivative. Such discovery will also have a
great impact on organic electronics; introduction of TPS in
polymer light emitting devices allowed the improvement of
conductivity by charge injection. Therefore, formation of
polycyclic aromatic compounds during the irradiation process
is an attractive characteristic as it will certainly favor even
more the conductivity of these polymer films.13
heterolysis pathway produces
a
phenyl cation and
diphenylsulfide, whereas a diphenylsulfinyl radical cation6 and
phenyl radical are the expected products from homolysis.3,7
Recombination reactions of the different fragments yielded 2-,
3- and 4-(phenylthio)biphenyl with one equivalent of acid.
After cage-escape, the fragments reacted with solvent or with
fragments from another TPS molecule or other contaminants,
generating diphenylsulfide, biphenyl, and acid (Scheme 1). An
investigation of crystalline or finely ground solid onium salt
Experimental
General Methods
Poly(methyl methacrylate) (PMMA, average Mw ~ 15 kDa by
GPC, cas # 9011-14-7), poly(ethylene glycol) methyl ether
(PGME, Mw = 2,000), 4-(phenylthio)biphenyl and tris(4-tert-
butylphenyl)sulfonium nonaflate (tBu-TPS) were used as
received from Aldrich. 2- and 3-(phenylthio)biphenyl were
synthesized according to a reported procedure.14 Triphen-
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ylsulfonium nonaflate (TPS) and poly((tert-butylmethacrylate)- ESI). Coated discs were placed in front of the light source for 3
co-(2-hydroxyethylmethacrylate)-co-(gamma- h (Xe-lamp) or 3 days (3 d) (D2-lamp). Pulsed laser excitation
butyrolactonemethacrylate)) poly(tBMA-co-HEMA-co-GBLMA)) (266-nm, 8-ns) was from the fourth harmonic of a Q-switched
Nd:YAG laser (Spectra-Physics Quanta-Ray PRO-Series)
operating at 10 Hz. A mechanical shutter was used to select
individual pulses at a reduced repetition rate (1 Hz).
Crossover experiment in a tBu-TPS/TPS-PMMA film
A solution of 10% w/v PMMA, 0.5% w/v TPS, and 0.5% w/v
tBu-TPS in THF was spin-coated on a 1.5-inch diameter fused-
silica disc using the conditions described above. The disc was
irradiated at 193 nm for 3 d after which the film was dissolved
in acetone and evaporated to dryness. 1H NMR (400 MHz,
3
8.74 (m, triphenylene), 8.66 (d, JHH = 5.6 Hz, tBu-
MeOH-d4):
δ
triphenylene), 8.63 (s, tBu-triphenylene); other signals were
hidden under TPS peaks.
Irradiation of solution samples
Solution phase experiments were conducted with 193- and
254-nm irradiation under the same conditions described for
the TPS-PMMA films. Experiments were performed in a 1-cm
fused-silica cuvette; solutions of 10% w/v PMMA and 1% w/v
TPS (17.8 mM TPS concentration) in CH3CN were first purged
with an inert gas and cooled in an ice bath to limit
evaporation. The samples were then irradiated with constant
stirring. After irradiation, 100 µL of the reaction mixture was
diluted with approximately 2 mL DMSO. The resulting solution
was filtered through a celite plug and further diluted to a final
volume of 10 mL with DMSO to obtain samples for HPLC
analysis.
Scheme 1. 193-nm photochemistry of triphenylsulfonium cation.
with different weight average molecular weights (Mw = 4,600;
14,000; 5,300 and 8,600) were provided by The Dow Chemical
Company. Reported weight-average molecular weights (Mw)
were determined using narrow poly(styrene) calibration
standards. Organic solvents (acetonitrile, DMSO, pentane and
THF) were used as received from VWR. UV-vis absorption
spectra were measured on an HP 8453 spectrometer. In situ
UV-vis spectra were collected using an HP spectrometer (254
nm) and a B&W Tek spectrometer (193 nm). NMR spectra
were recorded on a Bruker 400 with a Prodigy broadband
cryoprobe and a broadband Smart probe (103Rh-31P plus 19F/1H)
using Shigemi NMR tubes in MeOH-d4.
Irradiation of wafers in a semiconductor fabrication facility at Dow
Electronic Materials
A solution of poly(methyl methacrylate) (PMMA, 5 g) and
triphenylsulfonium nonaflate (TPS, 0.5 g) in propylene glycol
monomethyl ether acetate (PGMEA, 50 g) was prepared by
mixing at room temperature and then rolling overnight. Six
different 200 mm silicon wafers were first primed with
hexamethyldisilazane (HMDS) at 120 °C for 30 seconds. AF40A
bottom antireflective coating (BARC) was then spin-coated and
baked at 205 °C for 60 seconds to give a thickness of 80 nm on
each wafer. The PMMA/TPS/PGEMA formulation was then
spin coated onto each wafer at 1500 rpm, and baked at 95 °C
for 60 seconds to give film thicknesses of approximately 320
nm. An exposure ladder was accomplished using an ASM-
L/1100 lithography tool. The center doses were set to 0, 10,
20, 30, 40 and 50 mJ/cm2. A center portion of each wafer (ca.
1.5 x 3 inches) was then cleaved, which was extracted by
either THF (GPC analysis) or DMSO (HPLC analysis).
General film preparation and irradiation of TPS-PMMA
A solution of 10% w/v PMMA (1g), 1% w/v triphenylsulfonium
nonaflate (TPS) (0.1g) in tetrahydrofuran (10 mL) was spin-
coated on a 1.5 in diameter fused-silica disc at a rotation rate
of 3,000 rpm for 30 s. Film thickness was determined using a
Dektak XT Stylus profilometer and found to be 0.8 ꢀm.
Irradiations were performed using one of three different
light sources. 193-nm irradiation employed a home-built
nitrogen-purged exposure chamber equipped with a D2-lamp
and a 193-nm interference filter (20 ± 5 nm FWHM, Tmax =
15%) and a 1-cm-water filter (emission spectrum: See Figure
The absorbance k-value were determined using 320 nm
thick film, soft-baked at 95 °C for 60 s on 200 mm silicon
wafers, and a J.A. Woollam WVASE32 vacuum ultraviolet
variable angle spectroscopic ellipsometer using polarized light.
Changes to the polarized light phase and amplitude after it
passed through the coated film were collected to cover 180-
900 nm using a range of 1.4 ev to 6.875 ev in increment of
0.0375 ev and three incident angles of 65, 70, and 75 degrees.
These raw data were fit to a model using J.A. Woollam
S1 in ESI, dose < 10 ꢀW as measured by the high-sensitivity
thermal power sensor (ThorLabs S401C)). 254-nm radiation
was produced using a krypton fluoride excimer mirror to direct
the output from a Xe arc lamp through two 1-cm-path-length
water infrared filters, and a 254-nm interference filter. The
irradiation power measured with a photodiode (ThorLabs
Standard Photodiode Power Sensor (S120C), Si, 200-1100 nm)
was 360 ꢀW (360 ꢀJ/s) (emission spectrum: See Figure S2 in
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software to determine the real & imaginary optical constants photoproducts (Scheme 1). In situ UV-vis spectroscopy showed
of the film at each wavelength.
that formation of this photoproduct increased over the course
Chromatography
After irradiation, the film was dissolved in 1 mL DMSO and
filtered through celite. The resulting solution was then
analyzed by High Performance Liquid Chromatography (Agilent
1100 series preparative HPLC) and eluted through an
InertSustain® Phenyl column (GL Sciences, 5
ꢀm, 4.6 x 250
mm) at a flow rate of 0.7 mL/min using a UV detector at 254
nm (Agilent 1100 Variable Wavelength Detector). A gradient
elution was used starting from 70:30 (v/v) H2O/CH3CN up to
30:70 (v/v) H2O/CH3CN over a period of 60 min, then held at
30:70 (v/v) H2O/CH3CN for 10 min. 50
ꢀL samples were
injected by auto-sampler. Due to the poor separation of the
three (phenylthio)biphenyl isomers, a MATLAB program was
written to fit the HPLC data and quantify each isomer based on
reference solutions. Each product was identified by retention
time and spiking with pure compounds. Calibration curves
were generated for quantification of all photoproducts.
Detection of the remaining TPS in the reaction mixture
employed a Zorbax SB-C18 column (5 ꢀm, 4.6 x 150 mm) with
Figure 1. UV-vis spectra of PMMA/TPS films: before and after (3 d) 193-
UV-detection at 214 nm. The eluent used was a 50:50 (v/v)
CH3CN/(H2O + 0.1% TFA) during 15 min, then held at 100%
CH3CN during 10 min to wash the column.
nm irradiation.
Gas Chromatography - Mass Spectrometry (GC-MS) was
conducted on a MSD5972/GC5890 with HP6890 autosampler
and an electron ionization (EI) method. The column was a DB-5
30 m x 0.25 mm x 0.25 µm film. The injector was set at 250 °C
and the detector at 280 °C. The procedure: 50 °C for 2 min; 15
°C for 15 min; then 270 °C for 3 min.
Gel Permeation Chromatography (GPC) employed a Waters
Alliance system with an e2695 separations module using
differential refractive index and photodiode array detection
and THF (1.2 mL/min) as the elution solvent (30 °C).
DOSY-NMR performed in DMSO-d6 was a DgcsteSL (DOSY
gradient compensated stimulated echo Spin Lock)
experiment15 with a diffusion delay of 50 ms, a difference in
gradient pulse of 5 ms and an array of 15 gradient values.
Figure 2. HPLC chromatogram of PMMA/TPS film after 193-nm irradiation for 3d.
of 3-d irradiation (Figure S3 in ESI).
HPLC was employed to separate and identify the different
photoproducts present in the irradiated TPS-PMMA films. The
HPLC trace (Figure 2) shows the presence of several
compounds in the reaction mixture. Many of these peaks
were assigned by comparing their retention times in HPLC and
GC-MS to those of previously reported photoproducts 2-, 3-
and 4-(phenylthio)biphenyl (2-BiPhSPh, 3-BiPhSPh, and 4-
BiPhSPh), biphenyl (PhPh), and diphenylsulfide (PhSPh).3,5,18
However, two peaks with retention times of ~43 and ~51 min
did not match any known photoproducts. The product with the
retention time of 43 min was identified as dibenzothiophene
by GC-MS and by spiking an HPLC sample with a small amount
of the pure substance. The one with the retention time of 51
min was isolated by prep-HPLC and characterized by NMR
Results and Discussion
Detection of secondary photoproducts
Fused-silica discs spin-coated with a solution of 10% w/v
PMMA and 1% w/v TPS in tetrahydrofuran afford 0.8 ꢀm films
(measured by profilometry). UV-vis spectroscopy of the as-
prepared films revealed absorption bands at 268 and 276 nm
that are characteristic of the TPS starting material (Figure 1).16
PMMA films containing 1% w/v TPS were then irradiated at
193 nm for 3 d under an N2 atmosphere. UV-vis spectra of
irradiated films revealed the presence of a new feature at 260
nm that displayed prominent vibronic structure (Figure 1). This
feature indicates that there is minor structural distortion in the
electronic excited state, consistent with a molecule containing
fused/multiple aromatic rings.17 This signal did not match the
UV-vis spectrum of the starting PMMA-TPS film, or of spin-
coated PMMA films containing previously reported
1
spectroscopy. The H NMR spectrum of the product showed
only two downfield signals (8.87 and 7.70 ppm), consistent
with
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Table 1. Photoproduct distributions (%) after 3-d (193 nm) and 3-h (254 nm) irradiation. (a) TPS unreacted + photoproducts.
hν
(nm)
193
254
TPS conv.
Triphen
ylene
5.5
PhPh
PhSPh
Dibenzo
thiophene
2.1
2-BiPhSPh
3-BiPhSPh
4-BiPhSPh
Products
recovereda
47
90
1.4
2.1
4.8
2.8
18.6
12.4
0.3
1.4
4.1
1.4
100
5.5
2.1
27
a highly symmetric aromatic ring system. GC-MS of the irradiated film showing aromatic signals matching the diffusion
product gave a molecular weight of 228 m/z. Based on 1H rate of the PMMA polymer (Figure S7 in ESI).
NMR and GC-MS analyses, we concluded that the
TPS solution photoproducts
photoproduct was triphenylene. We further confirmed this
Solution phase experiments were conducted in order to
assignment by comparing the UV-vis spectrum, HPLC retention
time, and 1H NMR spectrum to those of pure triphenylene.
As neither dibenzothiophene nor triphenylene has been
previously identified as a product of TPS photolysis, additional
control experiments were conducted in order to validate the
assignment. The influence of coating solvent as well as the
identity of the polymer and irradiation wavelength were
investigated by monitoring the appearance of triphenylene by
UV-vis spectroscopy of the irradiated films. Triphenylene
formation was not affected by the use of propylene glycol
monomethyl ether acetate (PGMEA) or cyclohexanone as
coating solvent in place of THF; and the use of polymers other
than PMMA had no effect on triphenylene formation. Indeed,
UV-vis spectra of irradiated films of 1% w/v TPS in PGME and
poly(tBMA-co-HEMA-co-GBLMA) with different molecular
weight (Mw = 4,600; 14,000; 5,300 and 8,600) (Figures S4 and
S5 in ESI) clearly demonstrated the presence of triphenylene,
as evidenced by the sharp vibronic feature at 260 nm.
Additionally, we investigated the effect of irradiation
wavelength by testing the TPS-PMMA film after exposure to
254 nm light from a Xe-lamp. In situ UV-vis spectra of these
films at 254-nm irradiation (Figure S6 in ESI) show triphenylene
generation (260-nm signal), reaching a maximum at 3-h
exposure followed by a small decrease in band intensity.
Triphenylene was observed in all cases.
evaluate whether formation of triphenylene as
a TPS
photoproduct was dependent on sulfonium salt immobilization
in the polymer. Acetonitrile solutions containing 10% w/v
PMMA and 1% TPS w/v were irradiated for 3 d (193 nm) and 3
h (254 nm). UV-vis spectra of these solutions after irradiation
showed no evidence for formation of triphenylene (Figure S8
in ESI). Irrespective of irradiation wavelength, only
4
photoproducts were detected by HPLC: acetanilide, PhSPh, 2-
BiPhSPh, and 4-BiPhSPh (Figures S9, S10 and Table S1 in ESI).
Neither triphenylene nor dibenzothiophene could be detected.
The variation in irradiation wavelength had no effect on the
distribution of these photoproducts. The ratio of in-cage
products 2-BiPhSPh and 4-BiPhSPh to the cage-escape product
PhSPh was 1.5:1 for both 193- and 254-nm irradiated samples.
Photoproduct concentrations were much greater in samples
irradiated at 254 nm, in good agreement with TPS
consumption data. Note that there was 25% loss of initial TPS
concentration after 3 h of 254-nm irradiation; but there was
much less TPS loss (7%) after 3 d of 193-nm irradiation, a
difference attributable to the lower power of the 193-nm
source.
Solution experiments showed lower TPS conversion com-
pared to in-film experiments (3 d at 193-nm irradiation: 7% vs.
90%; 3 h at 254-nm irradiation: 25% vs. 100%). To confirm that
detectable quantities of triphenylene or dibenzothiophene are
not produced in solution at higher TPS conversion,
longer
The photoproducts from 193- and 254-nm irradiation of TPS-
PMMA films were quantified by HPLC. Irradiation for 3 h with
254-nm radiation led to quantitative conversion of TPS,
whereas irradiation for 3 d with 193-nm radiation resulted in
90% conversion (Table 1). In each case the main product was
the in-cage molecule, 2-(phenylthio)biphenyl (2-BiPhSPh), in
addition to smaller amounts of triphenylene; substantial
amounts of cage-escape products (diphenylsulfide (PhSPh),
dibenzothiophene) are observed. At both irradiation
wavelengths, the photoproduct distribution favored in-cage
products over cage-escape ones (77% in both cases), as
expected due to limited diffusion through the polymer film.9
The material recovered only accounts for 47% (or 27%) of
initial TPS after 193 (or 254)-nm exposure despite ≥ 90%
consumption of the starting material. This finding suggests
that many TPS fragments reacted with PMMA. Indeed, GPC
analysis of the PMMA-TPS film after irradiation showed
increased near-UV absorption (260 nm) for irradiated samples
compared to PMMA itself (Figure 3), indicating that aromatic
TPS fragments were incorporated in the polymer matrix.
Furthermore, a DOSY-NMR experiment was performed on an
irradiations were conducted at 254 nm. TPS consumption
Figure 3. UV-vis spectra of high MW GPC fractions of films before and after
irradiation with and without TPS.
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was monitored by HPLC and the reaction was stopped after 48
h as TPS consumption had reached 88%. Shown in Figure 4 are
time traces of the consumption of TPS and yields of the various
photoproducts over the course of the reaction. Again,
acetanilide, PhSPh, 2-BiPhSPh, and 4-BiPhSPh, but biphenyl
also appeared at longer irradiation times. The ratio of in-cage
to cage-escape products remained constant throughout the
reaction. After 24 hours of irradiation (72% TPS consumption),
benzene, dibenzothiophene, and triphenylene could be
detected, but only in trace amounts (< 1% yields). The total
material recovered after 48 hours of irradiation accounts for
73% of the starting TPS, suggesting that reactions between TPS
fragments and PMMA occur in solution as well.
TPS consumption in addition to production of the same
photoproducts observed with conventional irradiation sources.
Figure 5 shows the GPC traces obtained for each of the films.
Due to small differences in areas of cleaved wafers, the GPC
traces were normalized at a retention time of 12 minutes
corresponding to the PMMA fraction. As expected, TPS (ca.
retention time = 14.8 minutes) decreases with increasing
exposure, whereas the photoproducts increase (retention
times between 15 and 16 minutes) with increasing exposure
dose. The use of a photodiode array detector on the GPC
allowed for definite determination of photoproducts. Figure 6
shows UV-vis spectra obtained at a retention time of 15.8
minutes, corresponding to the elution of triphenylene (Figure
S11: UV-vis spectra obtained at a retention time of 15.1
TPS photoproducts determined in a Semiconductor Fabrication
Facility
Triphenylsulfonium cation embedded in polymer film is a key minutes, corresponding to the elution of 2-BiPhSPh, see ESI).
molecule in photolithigraphy for generation of acid. To As was observed in the studies described above, triphenylene
determine the relevance of the film studies with the industrial is produced with increasing exposure dose. Notably,
process, additional experiments were performed in
semiconductor fabrication facility. Specifically, we used an Significant production of triphenylene occurs with increasing
ASM-L/1100 lithographic scanner that produced exposure as clearly demonstrated by the UV-vis spectra. In
a
triphenylene is produced at doses as low as 20 mJ/cm2.
monochromatic 193 nm radiation. 200 mm silicon wafers order to quantify the photoproducts, we applied the same
were first primed with hexamethyldisilazane (HMDS) after HPLC method as described above. The relative amounts of
which a bottom antireflective coating (BARC) was applied photoproducts are plotted in Figure 7, demonstrating the rapid
(coated and cured) in order to prevent reflection from the production of 2-BiPhSPh with later production of secondary
silicon wafer back into the films. Under these conditions, photoproducts such as triphenylene. As these data were
precise measurement of exposure dose can be obtained. The obtained in a DOW semiconductor fabrication facility, with
absorbance k-value at 193 nm for the resist film unexposed is quantified exposure doses, they demonstrate the relevance of
0.0496 which primarily is attributed to the TPS cation. A run this topophotochemistry to the lithographic world. At doses in
sequence was developed that resulted in complete wafer the range of common exposure regimes, triphenylene is
exposure. A ladder study was run which had doses of 0, 10, 20, produced with loss of sulfur.
30, 40 and 50 mJ/cm2. Following exposure, the wafers were
The Dill C-parameter is a commonly used term to quantify
cleaved and extracted with solvent in order to perform GPC the rate of photochemical reaction of a photoacid generator
and HPLC analyses. Both chromatography
showed definitive evidence for
experiments (PAG) in a photoresist, and is defined as the first-order
photodecomposition decay constant (i.e., M = M0 e-CE, where
M = PAG concentration, M0 = initial PAG concentration, C = Dill
C-parameter and E = exposure energy).19 Assuming that the
destruction of TPS is first order and that
each
photochemical event leads to one photoacid produced,
then the C-parameter
Figure 4. Consumption of TPS and formation of photoproducts during 48 h
irradiation 1% TPS, 10% PMMA acetonitrile solutions at 254 nm. Concentration
has been normalized versus TPS, which is set to 1 at an exposure dose of 0
mJ/cm2.
Figure 5.
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Scheme 2. Products of the crossover experiment between TPS and tBu-TPS.
butyltriphenylene, establishing the connectivity of three
aromatic rings (Scheme 2).
Dose dependence of TPS photoproduct formation
In order to have a better control of the exposure of the
PMMA/TPS discs and to control the amount of
secondary/tertiary photoproducts, the discs were irradiated
Figure 6.
with
a 266-nm, 8-ns pulsed laser at a dose of 2.2
mJ/cm2/pulse. Different exposures were used with each disc,
and the photoproducts were analyzed by HPLC to quantify the
amount of TPS consumed. Results for the dose dependence of
TPS consumption and photoproduct formation are shown in
Figure 8.
The consumption of TPS plateaued after exposure to 658
mJ/cm2 (300 shots) at 83% conversion.21 The major
photoproduct at all doses was the ortho isomer, 2-
(phenylthio)biphenyl (19.6%), as expected given diffusion
constraints in the polymer matrix. A small amount of the para
isomer 4-(phenylthio)biphenyl (1.8%) also was formed; but
very little of the meta-isomer (0.6%) was observed. These are
the primary products of TPS photochemistry, arising from
cleavage of the S-aryl bond followed by in-cage fragment
recombination that strongly favors formation of the ortho
isomer over para, both being electronically preferred
compared to the meta isomer (Scheme 3).
Figure 7. Photoproduct quantification by HPLC analysis. Concentrations have
been normalized to initial TPS consumption, which is set to 1 at an exposure
dose of 0 mJ/cm2.
As
expected,
formation
of
the
cage-escape
photoproduct,
can be determined from fitting the equation, [H+] = 1 – e-CE
,
where [H+] is the normalized acid concentration (which is
assumed to be equal to 1- [TPS]/[TPS]0), C is the C-parameter
and E is the exposure energy. From the data shown in Figure 7,
from exposure energies of 0 to 20 mJ/cm2, C is determined to
be approximately 0.06 cm2/mJ, which is consistent with
previous literature values.20
tBu-TPS/TPS-PMMA crossover experiment
A crossover experiment was conducted using TPS and (tris(4-
tert-butylphenyl)sulfonium nonaflate: tBu-TPS) to probe the
pathway for formation of triphenylene in the polymer film. The
UV-vis spectrum taken after irradiation of a 1:1 mixture of TPS
and tBu-TPS in PMMA at 193 nm exhibited the characteristic
vibronic absorption signature of triphenylene (Figure S12 in
ESI). NMR analysis of the photoproducts (Figures S13 and S14
in ESI) revealed only non-substituted triphenylene and tris-
tert-butyltriphenylene. No cross-products were observed,
indicating that triphenylene was formed by an in-cage process,
likely owing to sluggish fragment diffusion through the
polymer matrix. Moreover, the main isomer of tris-tert-
butyltriphenylene (by NMR) was C3 symmetric 2,6,11-tris-tert-
Figure 8.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
CONCLUSION
S
The photochemistry of TPS-PMMA films was studied at
different irradiation wavelengths (193, 254 and 266 nm). In all
cases, two previously unreported TPS photoproducts,
triphenylene and dibenzothiophene, were detected. The
formation of both molecules occurred by secondary photolysis
of two primary photoproducts, 2-BiPhSPh and PhSPh. Such
reactivity is attributable to molecular confinement in the
polymer matrix. Further mechanistic investigations are in
progress in our laboratory, especially regarding triphenylene
formation; this photochemical transformation is particularly
interesting as it involves the loss of one sulfur and two
hydrogen atoms.
O3S(CF2)3CF3
h
S
S
h
h
S
1%
20%
7%
3%
S
Most importantly, our findings highlight the role of the
polymer in the microlithography process. Before our work in
films, neither triphenylene nor dibenzothiophene had been
observed in TPS photodecomposition, the former
photoproduct resulting from complete loss of sulfur.3,5,18
Moreover, although these photoproducts are generated via
successive photolysis processes, our work demonstrates that
2%
1%
Polymer
S
1%
In-cage products
this sequence can occur with low exposure under
lithographically relevant conditions.
a
Cage-escape products
Primary photoproducts
Secondary photoproducts
Scheme 3. Photoproducts obtained upon 266-nm irradiation (2631 mJ/cm2) of a
PMMA/TPS film.
Conflicts of interest
There are no conflicts to declare.
diphenylsulfide (maximum of 7.1%), was less favored
compared to in-cage products. Diphenylsulfide was
produced by loss of phenyl radical (homolysis pathway) or
phenyl cation (heterolysis pathway, Scheme 1). Biphenyl can
be formed by recombination of two phenyl radicals or more
likely by secondary photolysis of one or more
(phenylthio)biphenyl isomers.3 A small induction period was
observed in the formation of biphenyl consistent with
secondary photolysis. Reaction with the polymer matrix is
possible as only 42% of the TPS consumed led to molecular
photoproducts. Longer exposure of the film allowed the
formation of dibenzothiophene (1%) and triphenylene (3%).
The long induction periods before formation of these products
(especially for dibenzothiophene) suggest that secondary
photolysis was in play (Scheme 3).
Acknowledgements
We thank Dr. VanderVelde for his help on the DOSY-NMR
experiment. This work was supported by the National Science
Foundation Center for Chemical Innovation in Solar Fuels (CHE-
1305124) and in part by The Dow Chemical Company through
a university partnership program (Agreement # 227027AK).
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Triphenylsulfonium topophotochemistry
E. Despagnet-Ayoub, W. W. Kramer, W. Sattler, A. Sattler, P. J. LaBeaume, J. W. Thackeray, J. F.
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Light on new photoproducts of triphenylsulfonium salt embedded in polymer matrices, showing the
importance of limited diffusion in rigid media.
193 nm
193 nm
S
S
X
-HX
PMMA film
PMMA film
PMMA film
primary photoproduct
in-cage mechanism
secondary photoproduct