A self-contained photoacid generator for super acid based on photochromic terarylene

Article abstract of DOI:10.1039/C7CC01635B

A self-contained photoacid generator for super acid along with the photoinduced 6π-electrocyclization reaction of the terarylene backbone is demonstrated. The photoinitiated cationic polymerization of epoxy-monomers is achieved with an efficient photochem

Full text of DOI:10.1039/C7CC01635B

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Self-Contained Photoacid Generator for Super Acid Based on
Photochromic Teraylene
Ruiji Li,a Takuya Nakashima*a and Tsuyoshi Kawai*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Self-contained photoacid generator for super acid along with the cyclohexadiene structure17-19. Since some of terarylenes are
photoinduced 6π-electrocyclization reaction of terarylene possible to show
a photon-quantitative reactivity with
backbone is demonstrated. Photoinitiated cationic polymerization photochemical quantum yield close to unity,20,22 it is facile to
of epoxy-monomers is achieved with efficient photochemical expect improvement in the photochemical quantum yield for
quantum yield, Фacid = 0.47.
photo-acid releasing reaction. Their highly photosensitive acid-
releasing capability is partly because of their “self-contained”
nature. That is, the compound readily release an anion and,
then, H+ ion after photochemical ring-cyclization reaction,
quantitatively. Although the alkyl sulfonic acids is sufficiently
strong acid for polymerization of epoxy monomers, release of
super acids, such as triflic acid, CF3SO3H, is still challenging.
Super acids are crucially demanded for some practical photo-
polymers such those based on the mono-substituted epoxy,
e.g. SU-8, an epoxy-based negative resist material based a
resin with pending epoxy groups used for crosslinking. Some of
the monomers further throw a critical demand for non-ionic
PAGs and still limited number of PAGs are compatible for
them. In the present manuscript, we report on a new self-
contained PAG, which releases triflic acid as a super acid with
high photo-chemical quantum yield.
Photoacid generators (PAGs) which release strong acids with
light exposure have gained enormous interest over the past
few decades.1 The acid generated from the PAG is a key
element for the bio-stimulation systems and the photopolymer
systems such as those in photoresists for very large scale
integration (VLSI)-fabrication2,3 and photo-curable resins
having acid-polymerizable epoxy units1,2,4,5. In the practical
photo-curable polymer systems, for example, the photo-
generated acid induces either polymerization of monomers or
cross-linking between polymer chains leading to the less-
soluble and robust resins. The acid-induced hydrolysis reaction
is also practically used in the chemically amplified photoresist
systems, in which the polymers in the exposed part become
soluble to the developing solvents for making the positive-
mode photo-patterning. New PAG backbones and their
chemical modifications are still extensively studied for
improved sensitivity to the deep- and near-UV light, visible
light, and near-IR with two photon excitation.8-14 Whereas
previous ionic and non-ionic PAGs release the anionic part
upon photoexcitation, H+ ions and/or H radicals are extracted
from coexisting hydrogen-sources such as moisture,
monomers and/or polymers. This causes in the environmental
dependent sensitivity of usual PAGs, in unpredictable and less
reproducible by-product and also in partly incomplete acid
In our former PAGs, shown in Scheme 1a, the RSO3-and H-
units were introduced at the two reactive carbon atoms of
photochromic terarylene scaffold, which convert their
configuration from sp2 to sp3 upon photocyclization and
release
(a)
S
PAG-1:
R = SO2Me;
S
S
UV
PAG-2: R = COMe
Acid
H
PAG-3:
R = SO2-p-tol(Me);
N
N
Vis.
H
O
N
PAG-4: R = SO2-p-tol(CF3
)
Ph
Ph
S
S
S
S
Ph
Ph
O
R
S
S
Ph
Ph
R
generation, which restrict their applications8,15,16
.
a-form
b-form
PT
Recently, the authors have proposed new class of PAGs, which
is triggered by photochemical cyclization of hexatriene to
(b)
S
S
Ph
N
N
S
H
O
H
O
Ph
Ph
S
S
S
Ph
Scheme. 1 Previous self-contained PAGs (a) and formation of a dimer (b).
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suggesting formation of 1c as the major photoproduct in
methanol. These results are similar to those of the terarylene
derivatives based on the five-membered aromatic rings with
MeO- and H- units, indicating minor effect
S
S
S
N
O
S
N
UV
N
N
F3C
OH
H
N
N
Vis.
O
O
S
S
S
O
S
O
S
O
O
O
1c
TfOH
CF3
PAGQ-CF3
CF3
PAGQ-CF3 (CF)
Scheme 2. Photocyclization reaction of PAGQ-CF3 to PAGQ-CF3(CF) and the release of
triflic acid forming the annulated compound 1c.
the acids forming condensed aromatic structure.18 That is, these
PAGs are based on the photo-activation of the ionic bond-
cleavage at the reaction center carbon atoms. It is associated
with the relative stability of carbocation formed after releasing
the RSO3- anions. This hypothesis is supported by the
expanded π-conjugation in the ring-closed form and the
adjacent electro- donating sulfur atom. In the present study
for the super-acid generator, unfortunately, all attempts for
introducing triflateunit, CF3SO3-, to the terarylene backbone
were failed, but we obtained a dimeric derivative (Scheme 1b,
Scheme S2 and Figure S3). These results strongly suggested us
the instability of CF3SO3- binding even in the ring-open form
and motivated us to alter the aromatic unit of lower reactivity
instead of thiophene, thiazole and benzo-thiophene. We here
designed PAGQ-CF3 (Scheme 2), in which a less reactive
quinoline unit was introduced for stable binding of CF3SO3- in
the open-ring form and facile release of super acid (CF3SO3H)
from the ring-closed form, PAGQ-CF3(CF). We expected the
CH/N and S/N tethering interactions between the quinoline
and the benzothiophene units and the thiazole and the
benzothiophene units, respectively, for stabilizing the reactive
conformation and higher photochemical reactivity. DFT
calculations with ɷB97XD/ 6-31G(d) functional predicted the
reactive C2-symmetric conformation as a most stable structure
associated with the CH/N and the S/N interactions. The
calculated distance between CH/N atoms and S/N atoms were
2.70 Å and 3.21 Å, respectively, which are shorter than the
sum of the van der Waals radii of H, N, and S atoms (Figure
S13). The synthetic procedure is summarized in the supporting
information.
Figure 1. a) Absorption spectral change of PAGQ-OMe upon UV (313 nm) irradiation in
hexane at r.t. (c = 5.1 × 10-5 M. Inset: before irradiation and after 30s irradiation
followed by storing in dark for 3 min); b) in methanol at r.t. (c = 4.9 ×10-5M) upon UV
(313 nm) irradiation; c) Absorption spectral change of PAGQ-CF3 in hexane at r.t. upon
UV irradiation (λirra. = 313 nm, c = 1.98 × 10-5 M, Inset: change of absorbance at 564
nm); d) Absorption spectral change of PAGQ-CF3 in toluene (c = 2.45 × 10-5 M) upon UV
irradiation (λ = 313 nm) at 60 °C.
of quinoline unit on their photochemical behaviour.
Critical impact of quinoline units was observed upon
preparation of PAGQ-CF3. That is, we could successfully
introduce a CF3SO3- group at the reacting center carbon atom.
PAGQ-CF3 showed similar photo-induced coloration in pale
yellow under UV light irradiation in hexane accompanying with
weak absorption band at 564 nm (Figure 1c). Interestingly, this
absorption band became diminishing with further UV-light
irradiation. The final absorption band after the prolonged UV-
light irradiation was similar to that of Figure 1b. 1c was also
isolated from the UV-irradiated solution of PAGQ-CF3 with
HPLC. The spectral feature suggested that the PAGQ-CF3(CF)
formed as the intermediate before the final products 1c and
CF3SO3H. The acid elimination process was conveniently
monitored by the pH paper with clear color change upon light
A reference compound, PAGQ-OMe, showed reversible
photochromic coloration and bleaching upon UV and visible
light irradiation in hexane solution (Figure 1a). Clear isosbestic
point at 335 nm suggested two component photochromic
nature and the absorption band at 560 nm is typical for the
ring-closed form isomers of diarylethene and terarylene
derivatives. The reaction was quasi-reversible with alternative
UV and visible light irradiation along with gradual decrease in
the maximum absorption peak at 560 nm. UV light irradiation of a
methanol solution displayed significantly different behaviour, with
increasing absorption bands between 250nm-350nm and an
isosbestic point at 270 nm but no absorption band in the
visible range. The irreversible two component reaction in
methanol is assigned to the formation of annulated
compound, 1c, after spontaneous release of methanol. The
annulated compound 1c was isolated from the irradiated
solution and characterized with X-ray crystallography (Figure
S13). Absorption spectrum of the 1c (Figure S10) coincided
well with the steady state absorption spectrum of Figure 1b,
1
irradiation (see Figure S6), and also by H NMR measurement
of PAGQ-CF3 in DMSO-d6 (Figure S15). Comparing the acid-
releasing capabilities of PAGQ-CF3 and PAGQ-OMe in hexane,
-
stable CF3SO3 anion seems to dissociate easily in comparison
with MeO- anion. The rate-determining step should thus
involve the dissociation of these anions. The CF3SO3- unit
seems to dissociate easier than the MeO- unit, resulting in
gradual formation of 1c. Similar gradual release of CF3SO3H
was also observed in toluene and the spontaneous reaction of
PAGQ-CF3(CF) was studied in dark condition after UV-light
irradiation for 60 sec. The absorption band at 564 nm
indicated typical first-order exponential decay with a rate
constant of 1.50 × 10-3 s-1 at room temperature (Figures S11,
S12). As shown in Figure 1d, PAGQ-CF3 immediately releases
CF3SO3H at the elevated temperature even in toluene forming
1c under UV light irradiation. This temperature dependent acid
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releasing capability is not a practical disadvantage, since most
In summary, newly designed self-contained photoacid
of cationic photo-curing process need heating the sample after generator with quinoline group was synthesized, which
photo-irradiation for
releases CF3SO3H as super acid. PAGQ-CF3 showed apparent
photochemical quantum yield from 30% to 47 %, highest
among the PAGs for super acids. Cationic polymerization of
epoxy monomers and photo-polymerization patterning of SU-8
were successfully induced by UV-light irradiation for PAGQ-
CF3
.
This work was partly supported by JSPS KAKENHI Grant
Number JP26107006 in the Scientific Research on Innovative
Areas “Photosynergetics”. The SU-8 monomer and its standard
polymerization procedure were kindly supplied by NIHON-
KAYAKU Co. Ltd. The authors thank Mr. S. Katao and Ms Y.
Nishikawa, technical staffs in NAIST for their assistance with X-
ray crystallography analysis and mass measurements,
respectively.
Scheme 3. Reaction scheme of photo-induced cationic polymerization of propylene
oxide
facile progression of polymerization. Since the acid releasing
reaction from PAGQ-CF3(CF) proceeds quantitatively, the
apparent photochemical quantum yield of acid releasing
process is determined by the initial photo-chemical cyclization
reaction. The apparent photochemical quantum yield from
PAGQ-CF3 to 1c was 0.47 in toluene, which is among the
highest values for PAGs for super acids so far reported. Those
in chloroform and methanol were 0.40 and 0.3, respectively,
Notes and references
1
2
3
4
J. V. Crivello, Photoinitiators for Free Radical, Cationic and
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8,16,23
G. M. Wallraff and W. D. Hinsberg, Chem. Rev., 1999, 99
1801-1822.
,
super acids.
Although the acid releasing reactivity
depends on the environment and may be less at lower
temperature, the self-contained elimination reaction
guarantees for a high overall efficiency of acid formation that
exceeds that of other PAGs. The decrease of the quantum
yield with increasing the solvent polarity is consistent with our
previous reported,17,18 where the hydrogen binding like CH-N
interaction and CH/π interaction play a certain role to support
the photoreactive conformation.19,20 Photochemical and
photo-physical data of PAGQ-CF3 and 1c were summarized in
ESI (Table S3).
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the cationic polymerization of propylene oxide. After UV
irradiation to a mixture of propylene oxide and 0.2 mol% of
PAGQ-CF3, formation of polymerized products was confirmed
as a series of mass peaks with the interval of 58 (Scheme 3,
Figure S16). This result displayed significant contrast with our
previous PAGs releasing CH3SO3H, which served as a photo-
initiator of polymerization for cyclohexane oxide but not for
the mono-substituted epoxy such as propylene oxide.17,18 We
further demonstrated capability of PAGQ-CF3 for photo-
polymerization patterning of SU-8 (Figure S17). In this
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calculation of 1c (Figures S13 and S14). The distance between
H and N atoms is about 0.22 nm, which is markedly shorter
1
than the sum of van der Waals radii (0.27nm). Significant H-
NMR shift was observed (Figure S15) due to the strong non-
covalent interaction. These results suggest tightly binding CH/N
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quinoline unit.
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Super acid is efficiently released from self-contained photoacid
generator based on triangle terarylene backbone upon
photoinduced 6π-electrocyclization reaction.
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