Aryl tosylates as non-ionic photoacid generators (PAGs): Photochemistry and applications in cationic photopolymerizations

Article abstract of DOI:10.1039/c5ra03522h

The irradiation of various substituted aryl tosylates was investigated in a solution and homolysis of the ArO-SO₂C₆H₄CH₃ bond was the path exclusively observed. The corresponding phenols and photo-Fries adducts were obtained and p-toluenesulfinic and p-toluenesulfonic acids were liberated. The nature and amount of the acid photoreleased were tuned by changing the reaction conditions and the nature and position of the aromatic substituents. In deaerated solutions p-toluenesulfinic acid was formed exclusively, whereas under oxygenated conditions the stronger p-toluenesulfonic acid was released, as shown by HPLC ion chromatography analyses. ArO-S bond photocleavage takes place from the singlet state, as confirmed by laser flash photolysis experiments, and competitive intersystem crossing can make the aryl tosylate unreactive when a nitro group is present. The application of these aryl tosylates as non-ionic photoacid generators (PAGs) in hybrid organic/inorganic sol-gel photoresists has been explored.

Full text of DOI:10.1039/c5ra03522h

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Aryl tosylates as non-ionic photoacid generators
(PAGs): photochemistry and applications in cationic
photopolymerizations†
Cite this: RSC Adv., 2015, 5, 33239
a
b
a
Edoardo Torti, Gioia Della Giustina, Stefano Protti, Daniele Merli,c
*
b
a
*
Giovanna Brusatin and Maurizio Fagnoni
The irradiation of various substituted aryl tosylates was investigated in a solution and homolysis of the ArO–
SO2C6H4CH3 bond was the path exclusively observed. The corresponding phenols and photo-Fries adducts
were obtained and p-toluenesulfinic and p-toluenesulfonic acids were liberated. The nature and amount of
the acid photoreleased were tuned by changing the reaction conditions and the nature and position of the
aromatic substituents. In deaerated solutions p-toluenesulfinic acid was formed exclusively, whereas under
oxygenated conditions the stronger p-toluenesulfonic acid was released, as shown by HPLC ion
chromatography analyses. ArO–S bond photocleavage takes place from the singlet state, as confirmed
by laser flash photolysis experiments, and competitive intersystem crossing can make the aryl tosylate
unreactive when a nitro group is present. The application of these aryl tosylates as non-ionic photoacid
generators (PAGs) in hybrid organic/inorganic sol–gel photoresists has been explored.
Received 26th February 2015
Accepted 17th March 2015
DOI: 10.1039/c5ra03522h
www.rsc.org/advances
two mechanisms compete, namely, heterolysis of the Ar–OS
bond (path a) and homolysis of the ArO–S bond (path b),
Introduction
depending on the substituent X present on the aromatic ring
and on the sulfonic moiety.7,8 In the former case, a sulfonic acid
is released directly (path a0), whereas in the latter case trapping
of the generated RSO2c radical by oxygen leads to the formation
The development of new compounds able to release acid upon
light absorption (so-called photoacid generators, PAGs) is of
crucial importance in the eld of materials chemistry and in
microelectronics, photonics, nanofabrications and sensors
development.1 Aromatic onium salts (either iodonium or
sulfonium) are widely used for this purpose as suitable “caged
protons”.2 Despite the thermal stability of the latter
compounds, their poor solubility in polymer matrices3 has led
to an increasing demand for more soluble non-ionic PAGs for
photo(nano)lithography
applications.
The
compounds
proposed for this role usually release a sulfonic acid upon
photocleavage of a weak N–O bond in iminosulfonates,4,5 imi-
dosulfonates5 (Scheme 1a) or related compounds.6 Such
cleavage generates a RSO3c radical, which upon reaction with
the medium, is able to form a strong sulfonic acid.
We recently reported, however, that sulfonic acids can be
generated just as efficiently (>90%) by the irradiation of aryl
mesylates and triates, as shown in Scheme 1b.7 In this case,
aPhotogreen Lab, Department of Chemistry, University of Pavia, V. Le Taramelli 12,
27100 Pavia, Italy. E-mail: fagnoni@unipv.it; prottistefano@gmail.com; Fax: +39
0382 987323; Tel: +39 0382 987198
bDepartment of Industrial Engineering, University of Padova and INSTM, Via Marzolo
9, 35131 Padova, Italy
cDepartment of Chemistry, University of Pavia, V. Le Taramelli 12, 27100 Pavia, Italy
† Electronic supplementary information (ESI) available: Experimental procedures,
copies of 1H and 13C NMR spectra of compounds 1a–g, 3a–c, 3g, potentiometric
titration of photolysed solutions of 1a–g in methanol. See DOI:
10.1039/c5ra03522h
Scheme 1 (a) Photorelease of sulfonic acid upon the photocleavage
of a N–O bond according to ref. 5c. (b) Competitive pathways in the
photorelease of acids from (substituted) phenyl mesylates and triflates.
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of RSO3H (path c) provided that fragmentation of the sulfonyl (1a–f in Chart 1). The silylated derivative 1g was also tested
radical intermediate is avoided. Fragmentation occurs with aryl because we previously demonstrated that the presence of
triates, with the liberation of weak acids, such as HF and silicon-based substituents was able to inuence the photo-
H2SO3, in N2-equilibrated methanol (path b0, Scheme 1b), rather reactivity of aryl sulfonates.16,17 These were synthesized from the
than sulfonic acids.7
corresponding phenols.18
Release of strong acids thus does not depend on initial
The photophysical and photochemical properties of tosy-
homolytic or heterolytic cleavage, and other aryl sulfonates lates 1a–g are presented in Table S1 and Fig. S1–S7 (see ESI†).
appeared to be worth evaluating as photoacid generators. Aryl Two absorption maxima in the UV region around 260–290 nm
tosylates constitute a worthwhile target, because they are crystal- (3 > 5000 L cmꢀ1 molꢀ1) and 220–240 nm (3 > 104 L cmꢀ1 molꢀ1
)
line compounds, readily prepared from inexpensive reagents and were observed for compounds 1a–c and 1e–g, whereas
more stable toward hydrolysis than other commonly used sulfo- compound 1d exhibited a single signicant absorption at 239
nates (e.g. triates). In addition, aryl tosylates have exhibited nm (3 ¼ 17 800 L cmꢀ1 molꢀ1). All the tosylates investigated
higher thermal stability than the corresponding mesylates when exhibited low (1a,b: FF < 0.01) or negligible (1c–g) uorescence
heated in a poly(4-hydroxystyrene) matrix.9 In fact, nitrobenzyl emission (Table S1†).
tosylates10,11 and N-oxysuccinimidoarylsulfonates12 have previ-
The photochemistry of 4-methoxyaryl tosylate 1b was inves-
ously been used for the photogeneration of p-toluenesulfonic acid tigated initially (Table 1) and its disappearance quantum yield
(PTSA) and some iminotosylates are commercially available for (Fꢀ1) was measured. This turned out to be modest in all the
such an application (e.g. the iminotosylate Irgacure PAG 121).
solvents tested (Fꢀ1 ¼ 0.07–0.14). Photolysis of 1b under UV
The photochemistry of aryl tosylates, however, has rarely irradiation (l ¼ 254 nm) caused ArO–S bond cleavage exclu-
been investigated13 and, to the best of our knowledge, the use of sively and 4-methoxyphenol 2b and o-tosylphenol 3b (a photo-
aryl tosylates as PAGs has only been tested with o-arenesulfo- Fries adduct) were formed in variable amounts depending on
nyloxyanilide derivatives, where photorelease of PTSA was the solvent employed. Compound 3b was the predominant
found to be almost quantitative, albeit with very low efficiency product formed in cyclohexane, whereas the amount of 2b
(Fꢀ1 ca. 0.05 13b).
increased with solvent proticity (methanol, 2-methoxyethanol)
The aim of this work is exploration of the photochemistry of or in the presence of labile C–H bonds (e.g. in the case of THF,
some substituted phenyl tosylates, with an investigation of their see Tables 1 and S2†). No products arising from Ar–OS bond
product distribution in solution, as well as a test of their use as cleavage, such as anisole or 1,4-dimethoxybenzene, were
photoinitiators for acid-induced polymerization in innovative detected, in contrast to what was observed for other methoxy-
epoxy-based hybrid organic–inorganic systems obtained via sol– aryl sulfonates.7 Irradiation in neat acetone (a triplet sensitizer)
gel processes.14 These systems exhibit several attractive prop-
erties such as mechanical robustness, high thermal and
chemical stability and transparency, and have found increasing
interest within the eld of photo(nano)lithography.15 Their
Table 1 Irradiation of aryl tosylate 1b in neat solventsa
preparation proceeds by hydrolysis and condensation reactions
of organically modied alkoxides with formulas RnSi(OR0)4ꢀn
with the addition of an acidic or basic catalyst. The presence of a
non-hydrolyzable Si–C bond provides a stable linkage between
the organic unit and the oxide matrix, resulting in an inorganic
three-dimensional network with pendant moieties that can be
derivatized (e.g. double bonds or, as in our case, acid-sensitive
epoxy rings) and lead to crosslinking.
Solvent
Fꢀ1
2b (%)
3b (%)
Results
Cyclohexane
CH2Cl2
CH3COOCH2CH3
THF
CH3COCH3
CH3CN
CH3OCH2CH2OH
CH3OH
—
4
68
33
36
24
10
30
29
28
We have examined a series of phenyl sulfonates substituted
with both electron-donating and electron-withdrawing groups
0.08b
—
12
26
39
30
24
34
41
—
—
0.13b
0.07b
0.14b
a
A nitrogen-saturated solution of 1b (10ꢀ2 M) in the chosen solvent
irradiated at 254 nm (4
consumption of 1b was achieved. Product yields calculated on the
ꢁ
15 W Hg lamps, 2 h) until >80%
b
basis of the amount of 1b consumed. Disappearance quantum yield
(Fꢀ1) measured for a 10ꢀ2 M 1b solution in the chosen solvent
(l ¼ 254 nm, 1 ꢁ 15 W Hg lamp) using potassium ferrioxalate as
actinometer.
Chart 1
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Table 3 Amount and type of acid released upon the irradiation of
1a–g in oxygen-saturated methanola
likewise caused complete consumption of 1b, giving 2b as the
main product. The same experiment carried out in oxygenated
acetone led to no 1b consumption.
ArOTs
H+b (% yield)
Anionsc (% yield)
The photolyzed solution in methanol was subjected to
potentiometric titration and ionic chromatography analyses,
revealing that 4-tolylsulnic acid was liberated in modest yield
(ca. 40%, Fig. S10, ESI†).
1a
1b
1c
1d
1e
1f
46
82
96
CH3C6H4SO3H (53)
CH3C6H4SO3H (82)d
CH3C6H4SO3H (90)
100
CH3C6H4SO3H (100)
With these results in hand, we proceeded to investigate the
photorelease of sulnic and sulfonic acids from tosylates 1a–g
in methanol under both nitrogen- and oxygen-saturated
conditions (Tables 2 and 3, Fig. S8–S19†). Under deaerated
conditions, the Fꢀ1 measured was in the 0.1–0.15 range except
for 1d, in which the MeCO-group provides higher photo-
reactivity (Fꢀ1 ¼ 0.29), and 1e which is virtually photostable
under these conditions (Table 2). It is notable that the presence
of a trimethylsilyl group makes the tosylate more photoreactive
(compare the Fꢀ1 value of 1b with 1g, Table 2).
e
e
99
84
CH3C6H4SO3H (95)
CH3C6H4SO3H (77)
1g
a
A 10ꢀ2 M solution of tosylates 1a–g in oxygen-saturated methanol
b
irradiated for 2 h at 254 nm (4 ꢁ 15 W Hg lamps). Determined by
c
potentiometric titration.
chromatography analyses.
Determined by means of HPLC ion
CH3C6H4SO2H (<4%) was likewise
d
formed. e No consumption of 1e was observed.
Cleavage of the sulfonyl group upon UV light absorption led
either to phenols 2 or to photo-Fries adducts 3, in analogy with
what had previously been observed for 1b. Formation of 3 was
exclusive when a strong electron-donating group (FG ¼ NMe2)
was present, negligible with electron-withdrawing groups (e.g.
MeCO, Table 2) or of the same magnitude as 2 with less strongly
donating groups (FG ¼ OMe, tBu). Photocleavage of the ArO–S
bond occurred independently of the nature and position of the
substituents present on the aromatic ring (except for the case of
unreactive 1e). Formation of photo-Fries products, on the
contrary, depended on the position, as shown by the different
results for isomeric 1b and 1f (no Fries adduct in the latter case).
In some cases, the reaction was very clean as demonstrated by
the UV-monitored conversion of 1d to p-acetylphenol 2d (see
Fig. S20 and S21†). With respect to the release of acid, p-tolue-
nesulnic acid was formed in variable amounts in nitrogen-
saturated methanol, with the single exception of tosylate 1d,
for which a mixture of sulnic acid and PTSA was observed. As
expected, the higher the amount of phenol 3 formed, the
smaller was the amount of acid released (Table 2). Notably, the
disappearance quantum yield (Fꢀ1) values for 1b and 1d were
not affected by the presence of oxygen (Table 2).
Table 2 Irradiation experiments on aryl tosylates 1a–g in neat nitrogen-purged methanol
lmaxa (nm), 3/(L molꢀ1 cmꢀ1
)
Fꢀ1
Photoproductc, (%)
H+d (%)
Acidse (% yield)
b
1
f
f
1a
231, 8241
264, 10 287
317, 1171
227, 21 190
274, 2423
227, 15 060
263, 1230
232, 28 320
0.11
3a, 100
1b
1c
1d
1e
1f
0.14g
0.14
2b, 41
3b, 28
2c, 52
3c, 18
2d, 73
40
50
CH3C6H4SO2H, 38
CH3C6H4SO3H, < 1h
CH3C6H4SO2H, 48
0.29g
<0.01
0.11
67
CH3C6H4SO2H, 45h
CH3C6H4SO3H, 29
i
f
f
227, 7980
264, 6350
224, 20 940
273, 3057
228, 17 100
285, 2010
2f, 52
49
32
CH3C6H4SO2H, 47
CH3C6H4SO2H, 22h
1g
0.21
2g, 21
3g, 18
a
b
Maximum absorption wavelength and molar absorption coefficient. Disappearance quantum yield measured for a 10ꢀ2 M 1a–g solution in
c
MeOH (l ¼ 254 nm, 1 ꢁ 15 W Hg lamp) using potassium ferrioxalate as actinometer. Isolated yields. 2 h irradiation time (complete
d
e
f
g
consumption of 1a–g). Determined by potentiometric titration. Determined by HPLC ion chromatography analyses. No acid released.
A
similar Fꢀ1 value was measured under oxygen-saturated conditions. h Formic acid (<1%) detected. i No photoproducts detected by GC analysis.
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On the other hand, under oxygenated conditions, a large remaining absorption bands remained almost unchanged
amount of PTSA was released depending on the tosylate used (Fig. 1a). A residual absorption around 300–350 nm, along with
(up to quantitative yield for 1d) and the corresponding sulnic the broad band at 580–700 nm, was observed 150 ms aer the
acid was found only as a minor product in the case of 1b. The laser pulse (see Fig. S22†).
lowest yield of released PTSA was found in the photolysis of 1a
(Table 3).
With acetyl derivative 1d (ca. 10ꢀ4 M in methanol, see
Fig. 2a–c), two absorption bands (lmax ¼ 330 nm and 410 nm),
Some mechanistic studies were carried out with tosylates 1b along with a broad signal (580–700 nm, Fig. 2a), were again
and 1d as well as the photostable 1e. Laser ash photolysis at detected under argon-saturated conditions. Kinetic analysis
266 nm of an argon-saturated solution of 1b (ca. 10ꢀ4 M) in revealed the presence of three different transient species, for
methanol revealed the rapid formation (within a few ns) of two which the longest-lived (s ¼ 33 ms) represented the main
intense absorption bands with lmax ¼ 330 nm and 410 nm, contributor to both maxima, whereas two short-lived bands
respectively, with a weaker, broad absorption located at longer were observed at 340 (s ¼ 2.9 ms) and 420 nm (s ¼ 0.7 ms),
wavelengths (580–700 nm, Fig. 1a). The time evolution of these respectively.
absorption bands indicated the presence of different transient
species, with the predominant contribution from a long-lived
intermediate (s ¼ 24 ms, k ¼ 1.5 ꢁ 1010 Mꢀ1 sꢀ1) in both
bands (see also Fig. S10†). Kinetic analysis revealed the
presence of a second short-lived (s ¼ 4.1 ms) species absorbing
at 330 nm.
Shiing to an oxygen-saturated solution resulted in a slight
modication of the transient absorption prole, with quench-
ing of that portion of the 330 nm absorption attributable to the
short-lived species (see Fig. 1a and b), while the kinetics of the
Fig. 1 Transient spectra of a solution of 1b (10ꢀ4 M) in (a) argon- Fig. 2 Transient spectra of a solution of 1d (10ꢀ4 M) in (a) argon-
saturated methanol and O2-saturated methanol recorded 0.1 ms after a saturated methanol (black) and O2-saturated methanol (red) recorded
20 ns laser pulse (l ¼ 266 nm); (b) profile of the absorbance change 0.1 ms after a 20 ns laser pulse (l ¼ 266 nm); Profile of the absorbance
observed at 330 nm for a solution of 1b in argon-saturated (black) and change observed at (b) 340 nm and (c) 420 nm for a solution of 1d in
O2-saturated (red) methanol.
argon-saturated (black) and O2-saturated (red) methanol.
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The latter was the main contributor to the broad absorption
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band at 580–700 nm. Shiing to an oxygen-saturated solution
resulted in the quenching of the two short-lived intermediates
(see Fig. 2b and c), whereas no signicant effects were observed
for the longest-lived species. In contrast, no signicant tran-
sient species were observed with the nitro derivative 1e.
The results obtained in solution encouraged us to determine
the efficiency of the photoactive aryl tosylates 1a–d and 1f–g in
photoinduced cationic polymerization processes. With this
aim, their inclusion in a hybrid organic–inorganic photoresist
(G8Ge2), obtained via acid-catalyzed condensation of 3-glyci-
dyloxypropyltrimethoxysilane (GPTMS) and germanium tet-
raethoxide (TEOG) in 2-methoxyethanol, was planned.19
Preliminary experiments to determine the effect of the photo-
resist components were rst carried out on 1d, for which the
generation of PTSA was most efficient in methanol. Irradiation
of 1d was carried out in oxygen-saturated 2-methoxyethanol and
PTSA was photoreleased in 71% yield, analogous to what was
found in neat oxygen-saturated methanol. The presence of
GPTMS (0.73 M), germanium tetraethoxide (TEOG, 0.18 M) and
H2O (2.55 M) produced comparable results (PTSA yield ¼
57%).20
At this point, tests were carried out on the assembled G8Ge2
containing acid-sensitive epoxy functionalities obtained via sol–
gel techniques from GPTMS and TEOG (molar ratio: 80 : 20) in
2-methoxyethanol.21 The synthesized aryl tosylates were then
incorporated in the G8Ge2 system and the resulting sol was
spin-coated on silicon (100) substrates. The lms obtained were
then exposed to UV light (Hg–Xe UV spot light source) at
increasing doses and the structural modications induced by
aryl tosylates were investigated by means of both UV and FT-IR
analysis.
Fig. 3a illustrates the UV-vis spectra of the hybrid samples
before and aer addition of 1d to the sample. An absorption
band at 240 nm, due to the presence of aryl tosylate 1d (1%), was
apparent and was markedly reduced within the rst minute of
UV irradiation. Among the vibrational modes that characterize
the epoxy moiety in G8Ge2, the signals assigned to C–H
stretching at 3060 and 3000 cmꢀ1 were chosen to determine the
degree of polymerization (see Fig. 3b).19
As shown in Fig. 3b for the G8Ge2/1d system, UV irradiation
caused a decrease in absorbance in the bands at 3060 and 3000
cmꢀ1, indicating gradual ring-opening of the epoxy moiety with
subsequent formation of an interpenetrating organic network
and densication of the exposed area. The degree of photo-
polymerization induced by phenyl tosylates 1a–d and 1f–g was
then measured by monitoring the evolution of epoxy ring-
opening as a function of the UV dose. The degree of polymeri-
zation (at least within the rst three minutes of UV exposure,
Fig. 3c) roughly followed the amount of PTSA photoreleased
under oxygenated conditions (with 1d the best and 1a the worst
of the series, see Table 3).
Fig. 3 (a) UV-vis spectra of the photoresist G8Ge2 in the 200–800 nm
region in the absence and presence of 1% tosylate 1d before and after
UV irradiation; (b) FTIR spectra of G8Ge2 films in the 4000–2600 cmꢀ1
region with 1% molar concentration of 1d before and after UV irradi-
ation. Samples were spin-coated on fused silica slides and silicon,
respectively; and (c) degree of photopolymerization of epoxy groups
versus UV curing time, achieved by 1% molar addition of aryl tosylates
1a–d, 1f–g to G8Ge2 sol–gel matrix.
It is noteworthy that a degree of polymerization above 60%
was achieved in all cases (except 1g) aer 18 min UV exposure.
Photodegradation of the PAG (see Fig. 3a) was essentially
complete in 1–2 min, while epoxy polymerization required
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several minutes (no polymerization took place when UV curing 350–900 ps range in EtOH28), thus preventing any photo-Fries
was carried out in the absence of 1a–g).
reaction, in analogy with other nitroaryl esters (Scheme 2,
paths b and b0).29 On the other hand, acetone-sensitized
experiments on 1b revealed that in some cases, when triplets
were populated by sensitization, a photo-Fries reaction also
took place (paths b%, b00), but was completely inhibited in
oxygen-saturated solution.
Finally, in contrast to what was reported for other aryl
sulfonates such as mesylates and triates, heterolysis of the Ar–
OS bond7,8,13a,16 plays no role in the photochemical behavior of
these substrates, even in the presence of acetone as a triplet
sensitizer.
When generated in N2-saturated methanol, the resulting
phenoxy and sulfonyl radical intermediates can follow two
competing pathways. The rst is escape from the solvent cage
followed by hydrogen abstraction from the medium to give
phenol 2 and paratoluenesulnic acid, respectively (paths c and
c0). On the other hand, direct recombination of the two radicals
results in formation of the Fries rearrangement product 3, via
the cyclohexadienone intermediate III (paths d and d0). In the
case of 1b, the residual, persistent absorption at 300–350 nm
and 600–700 nm (see Fig. S22 in ESI† and Fig. 1a) found in LFP
experiments could be assigned to III, in analogy with what has
already been reported about the photo-Fries rearrangement of
naphthyl30a and phenyl acetates.30b,c
Discussion
The photochemistry of phenyl 4-methylbenzenesulfonate was
investigated in 1966 by Havinga et al., who observed homolytic
cleavage of the ArO–S bond.22 More recently, our research group
noticed competition between ArO–S bond homolytic cleavage
and Ar–OS heterolysis taking place in aryl triates, mesylates,7
nonaates23 and imidazylates.24 For the substrates examined in
this paper, the homolytic pathway is exclusive, with subsequent
formation of a phenoxy (I)/paratoluenesulfonyl (II) radical pair
(see Scheme 2 below, path a).
Laser ash photolysis analyses of 1b in methanol (Fig. 1a)
showed two signicant absorption bands at 330 and 410 nm. The
oxygen-sensitive signal dominating at 330 nm was recognized as
due to the 4-methylbenzenesulfonyl radical II, as previously
suggested,12,25 whereas the long-lived intermediate with
a
maximum at 410 nm (and the portion of the 330 nm absorption
not quenched by oxygen) can be condently attributed to the 4-
methoxyphenoxy radical Ib.26 In the case of 1d, however, LFP
revealed the presence of an additional transient signal, assigned
to triplet 31d (absorption at 420 and 580–700 nm), accompanying
the two radicals absorbing at 330 and 410 nm.
With the single exception of the nitroaryl tosylate 1e, all the
sulfonates examined displayed comparable photoreactivity in
solution (Fꢀ1 ¼ 0.11–0.29, the highest value measured for
tosylate 1d), but different product distributions. We reasoned
that the stability of the phenoxy radical I and accordingly the
O–H bond dissociation energy (BDE) of 2 could play a role in
determining the fate of the reaction. As is apparent from
Table 4, the lower the O–H BDE (e.g. 321.3 kJ molꢀ1 in the case
of amino tosylate 1a), the higher is the amount of the Fries
adduct 3, whereas formal hydrolysis to 2 is the sole reaction
promoted for tosylates with electron-withdrawing substituents
and stronger O–H bonds (e.g. BDE ¼ 380.3 kJ molꢀ1 for 1d).
Thus, competition between paths c, c0 and d, d0 (Scheme 2)
depends on the nature and stability of the radical I. Recombi-
nation to III (paths d and d0) is favored for electron-rich tosy-
lates such as the amino derivative 1a, making the photoinduced
release of PTSA inaccessible. On the other hand, although the
Photo-Fries reactions are known to arise mainly from excited
singlets, although triplets have occasionally been suggested.27
Our data show that cleavage of the ArO–S bond in the excited
singlet is the pathway exclusively observed upon direct irradi-
ation. A triplet is not involved, as conrmed by the lack of
oxygen quenching of photocleavage for 1b and 1d (Table 2) and
as further supported by LFP experiments. As for compound 1d,
none of the photoreactivity normally observed in triplet
aromatic ketones (e.g. hydrogen abstraction) was found, even in
an excellent hydrogen-donating solvent such as methanol.
Triplets play a role only when cleavage from singlets does not
take place, as in the case of 1e where efficient intersystem
crossing leads to a very short-lived triplet (lifetime in the s ¼
Table 4 Comparison of the product distributions observed in the
irradiation of tosylates 1a–f in methanol with the O–H BDE values of
phenols 2a–f
O–H BDE fora 2 (kJ molꢀ1
)
Ar–OTs
Photoproducts 2 : 3
321.3
349.3
364.3
371.3
380.3
396.3
371.3
1a
1b
1c
1f
1d
1e
PhOTs
0 : 100
60 : 40
74 : 26
100 : 0
100 : 0
b
—
—
a
b
Measured values from ref. 31. No signicant consumption of 1e
observed.
Scheme 2 Photoreactivity of aryl tosylates 1a–g.
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presence of electron-withdrawing groups is desirable to maxi- downeld from TMS. The photochemical reactions were per-
mize the yield of acid generated, moving to a stronger EWG (e.g. formed using nitrogen- or oxygen-saturated solutions in quartz
in the nitro derivative 1e) completely inhibits the overall pho- tubes in a multi-lamp reactor tted with 4 ꢁ 15 W Hg lamps
toreactivity. A peculiar case is represented by 1g. As previously (emission centered at 254 nm) for irradiation. The reaction
observed in related sulfonate esters (mesylates, triates),16 the course was followed by GC analyses and the products formed
presence of a silicon-based substituent ortho to the sulfonate were identied and quantied by comparison with authentic
moiety signicantly increased the overall photoreactivity.16 The samples. Work-up of the photolytes involved concentration in
same behaviour was found here, although the cleavage shied vacuo and chromatographic separation using silica gel. Solvents
from heterolysis of an Ar–OS bond (in the cases of mesylate and of HPLC purity were employed in the photochemical reactions.
triate)16 to ArO–S homolysis (in the case of tosylate). The Quantum yields were measured at 254 nm (1 Hg lamp, 15 W).
presence of oxygen promoted efficient trapping of the ArSO2c
The acidity released was determined from 5 mL of the pho-
radical II (Scheme 2, path e), prior to recombination with the tolysed solutions by dilution with water (25 mL) and titration
phenoxy radical, and PTSA was obtained via the corresponding with aqueous 0.1 M NaOH by means of a potentiometer
arylperoxysulfonyl radical IV.25 Both experimental and spectro- equipped with a pH glass combined electrode module. para-
scopic data exclude SO2 loss from II to afford the phenyl radical Toluenesulfonic (PTSA), para-toluenesulnic and formic acid
V (path f), in contrast to results previously reported for aryl were determined via HPLC ion chromatography and quantied
sulfonyl radicals in acetonitrile.25
Most of the aryl tosylates tested were found to be suitable available samples.
by means of a calibration curve obtained with commercially
PAGs for the release of PTSA to be used for cationic ring-opening
Nano-to-microsecond transient absorption experiments
of epoxy groups in sol–gel systems. When included in photore- were performed using a nanosecond laser ash photolysis
sists, compounds 1a–g were the only species responsible for the apparatus equipped with a 20 Hz Nd:YAG laser (20 ns, 1 mJ at
absorption of light (see Fig. 3a). Acid release preceded polymer- 266 nm) and a 150 W Xe ash lamp as the probe light. Samples
ization (see the different rates observed for 1d conversion and were placed in a quartz cell (10 ꢁ 10 mm section) at a concen-
induced polymerization, see Fig. 3a and c), which is consistent tration adjusted to obtain an OD value of 1.0 at 266 nm.
with the key role of the substrates examined as PAGs. In this
Phenols 2b–f, 3-glycidyloxypropyltrimethoxysilane (GPTMS)
study, the efficiency of organic crosslinking of G8Ge2 acid-labile and germanium tetraethoxide (TEOG) were commercially
moieties depended on the tosylate used, the most favourable case available and used as received. 4-N,N-Dimethylaminophenol
being the acetyl derivative 1d. Furthermore, in contrast to what 2a13a and 4-methoxy-2-trimethylsilylphenol 2g16 were prepared
was observed for other aryl sulfonates,7 where competing path- by known procedures.
ways affected the yield and quality of the generated acids,
photodecomposition of aryl tosylates proceeds via an unambig-
uous mechanism, and strong PTSA was the only acid obtained
Synthesis of aryl tosylates 1a–g
during irradiation in oxygenated solutions.
Compounds 1a–g were synthesized by adapting a known
procedure.18 To a solution of the chosen phenol (20 mmol) in
dichloromethane (80 mL) and triethylamine (15 mL) at room
temperature, p-toluenesulfonyl chloride (24 mmol) was added
Conclusion
Summing up, the photochemistry of substituted phenyl tosylates portionwise and the resulting mixture was stirred overnight.
proceeds exclusively via ArO–S bond homolytic cleavage to Water (25 mL) was then added and the resulting mixture was
generate a phenoxy/sulfonyl radical pair. Depending on the stirred for an additional 3 h. The mixture was then extracted
reaction conditions, either paratoluenesulnic or para- with ethyl acetate (4 ꢁ 50 mL) and the organic layers were
toluenesulfonic acid can be released, the latter being obtained reunited and washed with water (3 ꢁ 50 mL), 10% aqueous HCl
exclusively under oxygenated conditions. It can be noted that (3 ꢁ 150 mL, except in the case of 1a), water (2 ꢁ 150 mL),
acids are photoreleased even in the solid state. The results saturated aqueous NaHCO3 (2 ꢁ 150 mL) and brine (2 ꢁ 100
highlight the potential of aryl tosylates as a promising class of mL), and dried over Na2SO4. The solvent was evaporated under
non-ionic PAGs, able to promote cationic polymerization of epoxy vacuum. The resulting residue was puried by column chro-
groups in hybrid sol–gel photoresists. Furthermore, structural matography or recrystallization.
changes can result in a decrease in the solubility of exposed areas
4-N,N-Dimethylaminophenyl p-toluenesulfonate (1a). Obtained
in organic or suitable solvents. This peculiarity can be exploited in 72% yield from 4-N,N-dimethylaminophenol (2a),13a purica-
for photo(nano)lithography on sol–gel materials, which plays an tion by column chromatography (eluant: cyclohexane/ethꢂyl acetate
important role in the realization of many devices.32
9/1). Colorless solid, mp ¼ 123–125 C, lit.13a 117–120 C. Spec-
troscopic data for 1a are in accordance with the literature.18 Anal.
calcd for C15H17NO3S: C, 61.83; H, 5.88; N, 4.81. Found: C, 61.8; H,
5.9; N, 4.8.
ꢂ
Experimental
General information
4-Methoxyphenyl p-toluenesulfonate (1b). Obtained in 89%
NMR spectra were recorded on a 300 MHz spectrometer. Attri- yield from 4-methoxyphenol 2b. Purication by column chro-
butions were made on the basis of 1H and 13C NMR, as well as matography (eluant: cyclohexane/ethyl acetate 9/1). Colorless
DEPT-135 experiments; chemical shis are reported in ppm solid, mp ¼ 67–70 ꢂC, lit.18 69–70 ꢂC. Spectroscopic data for 1b
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are in accordance with the literature.18 Anal. calcd for 32% yield based on consumed 1b) and unreacted 1b (3 mg). 3b:
14H14O4S: C, 60.42; H, 5.07. Found: C, 60.4; H, 5.1.
1H NMR (CDCl3, from the mixture, d) 7.85–7.30 (AA0BB0, 4H),
C
4-tert-Butylphenyl p-toluenesulfonate (1c). Obtained in 85% 7.15 (d, 1H, J ¼ 3 Hz), 7.05 (dd, 1H, J ¼ 3 and 9 Hz), 6.95 (d, 1H, J
yield from 4-tertbutylphenol (2c), colorless solid, mp ¼ 112–114 ꢂC ¼ 9 Hz), 6.80 (s, 1H), 3.70 (s, 3H), 2.40 (s, 3H). 13C NMR (CDCl3,
(from MeOH/H2O), lit.33 113–115 ꢂC. Spectroscopic data for 1c are from the mixture) d: 153.0, 149.7, 144.8, 138.4, 130.0 (CH), 126.8
in accordance with the literature.33 Anal. calcd for C17H20O3S: C, (CH), 123.7 (CH), 123.4, 120.1 (CH), 111.2 (CH), 55.7 (CH3), 21.5
67.08; H, 6.62. Found: C, 67.1; H, 6.6.
(CH3).
Irradiation of 4-tert-butylphenyl p-toluenesulfonate (1c) in
4-Acetylphenyl p-toluenesulfonate (1d). Obtained in 71%
yield from 4-hydroxyacetophenone (2d). Colorless solid, mp ¼ MeOH. A nitrogen-saturated solution of 1c (340 mg, 0.022 M) in
71–73 C (from MeOH/H2O), lit.34 69–70 C. Spectroscopic data methanol (50 mL) was irradiated for 6 h. The photolysed solu-
for 1d are in accordance with the literature.18 Anal. calcd for tion was then evaporated and the resulting residue was puried
ꢂ
ꢂ
C
15H14O4S: C, 62.05; H, 4.86. Found: C, 62.0; H, 4.8.
by column chromatography (eluant: cyclohexane/ethyl acetate
4-Nitrophenyl p-toluenesulfonate (1e). Obtained in 62% yield 8 : 2) to afford 143 mg of a mixture of 4-tert-butylphenol (2c, 79
from 4-nitrophenol (2e). Pale yellow solid, mp ¼ 102–104 ꢂC (from mg, 23%) and 4-tert-butyl-2-(p-toluenesulfonyl)phenol (3c, 64
MeOH/H2O), lit.35 98 C. Spectroscopic data for 1e are in accor- mg, 19% yield). 3c: 1H NMR (from the mixture, CDCl3) d: 7.80–
ꢂ
dance with the literature.35 Anal. calcd for C13H11NO5S: C, 53.24; 7.35 (AA0BB0, 4H), 7.65 (d, 1H, J ¼ 3 Hz), 7.48 (dd, 1H, J ¼ 3 and 9
H, 3.78; N, 4.78. Found: C, 53.2; H, 3.8; N, 4.9.
Hz), 7.35 (s, 1H), 6.90 (d, 1H, J ¼ 9 Hz), 3.80 (s, 3H), 2.45 (s, 3H).
3-Methoxyphenyl p-toluenesulfonate (1f). Obtained in 68% 13C NMR (from the mixture, CDCl3) d: 153.4, 143.9, 138.4, 133.5
yield from 3-methoxyphenol (2f). Colorless solid, mp ¼ 49–51 ꢂC (CH), 130.0 (CH), 129.7, 126.7 (CH), 125.0 (CH), 123.4, 118.6
(from MeOH/H2O), lit.36 59–60 ꢂC. Spectroscopic data are in (CH), 34.2, 31.1 (CH3). IR of the mixture (neat, n/cmꢀ1): 3392,
accordance with the literature.36 Anal. calcd for C14H14O4S: C, 2963, 1516, 1227, 1141, 831.
60.42; H, 5.07. Found: C, 60.6; H, 5.2.
Irradiation of 4-methoxy-2-trimethylsilylphenyl p-toluene-
4-Methoxy-2-trimethylsilylphenyl p-toluenesulfonate (1g). sulfonate (1g) in MeOH. A nitrogen-saturated solution of 1g
Obtained in 55% yield from 4-methoxy-2-trimethylsilylphenol (180 mg, 0.01 M) in methanol (50 mL) was irradiated at 254 nm
(2g).16 Purication by column chromatography (eluant: for 5 hours (90% consumption of 1g). The photolysed solution
cyclohexane/ethyl acetate 9 : 1). Colorless solid, mp ¼ 60–62 ꢂC. was then evaporated and the resulting residue was puried by
1
1g: H NMR (CDCl3, d) 7.85–7.35 (AA0BB0 system, 4H), 6.95 (m, column chromatography (eluant: cyclohexane/ethyl acetate
2H), 6.75 (dd, 1H, J ¼ 3 Hz and 9 Hz), 3.80 (s, 3H), 2.45 (s, 3H), 9 : 1), to afford
a mixture of 22 mg 4-methoxy-2-(p-
0.30 (s, 9H). 13C NMR (CDCl3, d) 157.0, 148.4, 145.0, 134.2, toluenesulfonyl)-5-trimethylsilylphenol (3g, 14% yield based on
134.0, 129.7 (CH), 128.2 (CH), 121.0 (CH), 120.5 (CH), 114.5 consumed 1g) along with 6 mg unreacted 1g. 3g: 1H NMR (from
(CH), 55.4 (CH3), 21.6 (CH3), ꢀ0.7 (CH3). IR (neat, n/cmꢀ1): 2956, the mixture, CDCl3, d) 9.05 (s, 1H), 7.85–7.30 (AA0BB0, 4H), 7.15
1467, 1370, 1194, 1179, 1157, 1093, 1037. Anal. calcd for (d, 1H, J ¼ 3 Hz), 7.10 (d, 1H, J ¼ 3 Hz), 3.80 (s, 3H), 2.40 (s, 3H),
C
17H22O4SSi: C, 58.25; H, 6.33. Found: C, 58.2; H, 6.2.
0.30 (s, 9H). 13C NMR (from the mixture, CDCl3, d) 153.9, 152.6,
144.6, 136.4, 132.3, 129.9 (CH), 126.7 (CH), 126.4 (CH), 122.1,
111.6 (CH), 55.8 (CH3), 21.5 (CH3), ꢀ1.3 (CH3).
Preparative irradiations
Irradiation of 4-N,N-dimethylaminophenyl p-toluenesulfo-
nate (1a) in MeOH. A nitrogen-saturated solution of 1a (440 mg,
Use of aryl tosylates 1a–g as PAGs in polymerization processes
1.5 mmol, 0.03 M) in MeOH was irradiated for 4 h at 254 nm. The detailed protocol for the synthesis of the G8Ge2 system has
The photolysed solution was then evaporated and the resulting already been described.21 As indicated above, GPTMS and TEOG
residue was puried by column chromatography (eluant: are the main precursors of the sol and react in a molar ratio of
cyclohexane/ethyl acetate 9 : 1) to afford 330 mg 4-(N,N-dime- 80 : 20 under acidic conditions to generate the G8Ge2 system
thylamino)-2-(p-toluenesulfonyl)phenol (3a, colorless solid, with a nal concentration of 150 (SiO2 + GeO2) g Lꢀ1. Aryl
1
ꢂ
75% yield, mp ¼ 133–135 C). 3a: H NMR (CDCl3, d): 8.55 (s, tosylates 1a–d, 1f–g (1% mol with respect to the concentration
1H), 7.80–7.30 (AA0BB0 system, 4H), 6.90–6.80 (m, 3H), 2.80 (s, of the starting material GPTMS) were incorporated in the sol.
6H), 2.40 (s, 3H). 13C NMR (CDCl3, d) 146.0, 145.0, 144.4, 138.9, Films were deposited by spin-coating (spinning rate: 2000 rpm,
129.9 (CH), 126.7 (CH), 123.5, 121.8 (CH), 119.6 (CH), 110.9 spinning time: 30 s) on silicon (100) substrates and fused silica
(CH), 41.0 (CH3), 21.5 (CH3). IR (neat, n/cmꢀ1): 3377, 2924, 1503, slides to give a thickness of about 1 mm.
1139, 1090, 963. Anal. calcd for C15H17NO3S: C, 61.83; H, 5.88;
N, 4.81. Found: C, 61.9; H, 6.1; N, 4.9.
The samples obtained were pre-baked for 30 min at 80 ꢂC to
remove residual solvent and irradiated with UV light for an
Irradiation of 4-methoxyphenyl p-toluenesulfonate (1b) in increasing curing time using a Hg–Xe UV spot light source,
MeOH. A nitrogen-saturated solution of 1b (150 mg, 0.01 M) in enhanced in the UV region (254–365 nm range), with a power
MeOH (55 mL) was irradiated at 254 nm for 12 h (90% density of around 150 mW cmꢀ2 on the sample surface. The
consumption of 1b). The photolysed solution was then evapo- progress of the photoinduced epoxy polymerization was fol-
rated and the resulting residue was puried by column chro- lowed by means of Fourier transform infrared spectroscopy (FT-
matography (eluant: cyclohexane/ethyl acetate 9 : 1) to give a IR) analyses. The analyses were performed in transmission
mixture of 4-methoxy-2-(p-toluenesulfonyl)phenol (3b, 44 mg, mode within the 400–4000 cmꢀ1 range with 4 cmꢀ1 resolution
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for a total of 32 scans. To evaluate and compare the efficiency of 11 K. E. Uhrich, E. Reichmanis and F. A. Baiocchi, Chem. Mater.,
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Acknowledgements
S.P. acknowledges MIUR, Rome (FIRB-Futuro in Ricerca 2008 15 G. Brusatin and G. Della Giustina, J. Sol-Gel Sci. Technol.,
project RBFR08J78Q) for nancial support and Prof. A. Albini 2011, 60, 299–314.
(University of Pavia) and Prof. P. Hoggard (Santa Clara Univer- 16 E. Abitelli, S. Protti, M. Fagnoni and A. Albini, J. Org. Chem.,
sity) for fruitful discussions. This work has been supported by
the Fondazione Cariplo (grant no. 2012-0186).
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