Self-Contained Photoacid Generator Triggered by Photocyclization of Triangle Terarylene Backbone

Article abstract of DOI:10.1021/jacs.5b02826

We herein propose a new type of efficient neutral photoacid generator. A photoinduced 6π-electrocyclization reaction of photochromic triangle terarylenes triggers subsequent release of a Br?nsted acid, which took place from the photocyclized form. A H-atom and its conjugate base were introduced at both sides of a 6π-system to form the self-contained photoacid generator. UV irradiation to the 6π-system produces a cyclohexa-1,3-diene part with a H-atom and a conjugate base on the sp³ C-atoms at 5- and 6-positions, respectively, which spontaneously release an acid molecule quantitatively forming a polyaromatic compound. A net quantum yield of photoacid generation as high as 0.52 under ambient conditions and a photoinitiated cationic polymerization of an epoxy monomer are demonstrated.

Full text of DOI:10.1021/jacs.5b02826

Communication
pubs.acs.org/JACS
Self-Contained Photoacid Generator Triggered by Photocyclization
of Triangle Terarylene Backbone
Takuya Nakashima,*,† Kenta Tsuchie,† Rui Kanazawa,† Ruiji Li,† Shunsuke Iijima,† Olivier Galangau,‡
Hisako Nakagawa,§ Katsuya Mutoh,§ Yoichi Kobayashi,§ Jiro Abe,§ and Tsuyoshi Kawai*,†,‡
†Graduate School of Materials Science, Nara Institute of Science and Technology, NAIST, Ikoma, Nara 630-0192, Japan
‡NAIST-CEMES International Collaborative Laboratory for Supraphotoactive Systems, NAIST, CEMES-UPR 8011 CNRS, 29, rue
Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4, France
§Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa 252-5258, Japan
S
* Supporting Information
acids as high as 0.6,13 the acid release processes often require an
external proton source such as protic solvents.13 To our best
ABSTRACT: We herein propose a new type of efficient
neutral photoacid generator. A photoinduced 6π-electro-
cyclization reaction of photochromic triangle terarylenes
triggers subsequent release of a Brønsted acid, which took
place from the photocyclized form. A H-atom and its
conjugate base were introduced at both sides of a 6π-
system to form the self-contained photoacid generator. UV
irradiation to the 6π-system produces a cyclohexa-1,3-
diene part with a H-atom and a conjugate base on the sp3
C-atoms at 5- and 6-positions, respectively, which
spontaneously release an acid molecule quantitatively
forming a polyaromatic compound. A net quantum yield
of photoacid generation as high as 0.52 under ambient
conditions and a photoinitiated cationic polymerization of
an epoxy monomer are demonstrated.
knowledge, there has been no report on PAGs which exhibit a
photoacid release with high quantum efficiency over 0.5 based on
a self-contained unimolecular system.
We herein propose a PAG based on a self-contained
mechanism for acid generation. A chemical reaction for acid
generation is combined with a photochromic reaction. Although
there have been several examples of pKa change of acidic groups
combined with photochromism of diarylethenes16 and spiropyr-
ans,17 the intended release of acid molecules has never been
discussed. Among various photochromic systems, 6π-electro-
cyclization is known to proceed efficiently even in solid state for
diarylethenes and their aromatic analogues,18,19 which is coupled
with acid release as a subsequent reaction. A number of chemical
reactions20−25 were integrated with the 6π-based photoreactions.
The change in electron-delocalization state upon photo-
irradiation also induced the change in Lewis acidity of boron26
or imidazolium24 moiety. Meanwhile, the 6π-cyclization reaction
followed by the oxidative dehydrogenation was classically used
for the formation of aromatic (poly)cycles including helicenes.27
However, their use from the viewpoint of acid generation has so
far been ignored. Hydrogen and its conjugate base (X) are herein
introduced at both ends of the 6π-system (Scheme 1). The 6π-
hotoacid generators (PAGs) have been widely used for a
Pvast range of practical applications including microlitho-
graphic imaging with photoresists, photocuring, and rapid-
prototyping (3D printing), in which PAGs are employed as a
phototrigger of cationic polymerization.1−3 In recent works, use
was also found for photodynamic therapy4 and photocontrol of
enzymatic acitivity.5 Onium salt type PAGs such as diphenylio-
donium and triarylsulfonium salts have been at the forefront for
more than 30 years since they were developed by Crivello.6 More
recently, chemical modifications to the substituents of the onium
derivatives sensitized them to the visible range and two-photon
activation.7−10 Nonionic type PAGs represented by N-oxy-
imidesulfonate derivatives have also been in use for many
years.11,12 Both the onium salt type and nonionic type PAGs
undergo homolytic bond cleavage reaction after light irradi-
ation.2,12 Photogenerated radical species need to extract a H-
atom from solvent molecules or other H-sources to generate a
Brønsted acid, which gives rise to medium-dependent sensitivity
and complicated fragments remaining in the systems. A number
of caged compounds with photoremovable protecting groups
such as ortho-nitrobenzyl and benzoin derivatives release
Brønsted acids.13 The nitrobenzyl derivatives13,14 were also
used for a photolithographic application, while the quantum yield
of photoacid release was relatively small.15 Although the benzoin
derivatives show good quantum yields for release of carboxylic
Scheme 1. Photoacid Generation Reaction Proposed in the
Present Work
system undergoes pericylization reaction to form a cyclohexa-
1,3-diene moiety with a hydrogen (H) and conjugate base (X)
pair at adjacent positions upon photoirradiation. The photo-
produced cyclized compound is expected to show spontaneous
elimination of an acid molecule (HX) in dark conditions,
producing a chemically definable and stable fused polycycle with
four aromatic rings.
Received: March 23, 2015
© XXXX American Chemical Society
A
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1a (Scheme 2) was designed based on a photochromic
bis(thiazolyl)benzothiophene molecule with methyl groups on
absorption maximum at 314 nm (ε = 3.3 × 104 cm−1 M−1) gave a
colored solution of 2b with a new band at 608 nm (ε = 1.8 × 104
cm−1 M−1). The quantum yield of the ring-cyclization reaction
with irradiation at 313 nm was estimated to be 0.64 for 2a in dry
hexane. X-ray crystallographic analysis of 2a revealed that 2a
possesses a photochromic reactive conformation in crystal with
quasi-C2 symmetry around the central hexatriene backbone,
which is stabilized via S−N and CH−N interactions19 between
the central and both side units (Figure S1). Although the
photochromic reactive conformation in the single crystal was
stabilized in a similar manner to 3a, the ring-cyclization efficiency
was decreased from 3a of 0.98. The chemical structures of 2a and
3a differ only in substituents on the reactive C-atoms at both
ends of the 6π-system (Scheme 3). Therefore, the decrease in the
ring-cyclization efficiency could be attributed to the absence of
CH−π interactions between methyl groups and the opposing
thiazolyl rings in solution.
UV irradiation to the 2a solution in polar solvents such as
methanol gave irreversible photobleaching reaction. The blue-
colored isomer decomposed spontaneously in the dark (Figure
S2). The decomposed product was isolated by HPLC, and its
chemical structure was determined to be 1c by 1H NMR and X-
ray crystallography (Figures S3, S4). It should be noted that the
decomposition of 2b indeed took place very slowly in hexane too.
The polar solvent or the presence of acid could facilitate the
elimination reaction via the E1-mechanism, stabilizing the
carbocation intermediate.20
a
Scheme 2. Synthesis of PAG-1a
a
Reagents and conditions: (a) PPh3 (0.11 mol equiv), Pd(PPh3)4
(0.074 mol equiv), 2 M K3PO4 aq./1,4-dioxane; (b) Pd(PPh3)4 (0.070
mol equiv), CsF (2.3 mol equiv), toluene; (c) BBr3 (5.0 mol equiv),
dichloromethane; (d) MsCl (2.2 mol equiv), NEt3, dichloromethane.
Scheme 3. Photochromism of 1a−3a, and Photogated
Elimination Reaction of 1a and 2a
We then investigated the photoreaction of PAG 1a with a
mesylate (OMs) group at a reactive C-atom. Due to the limited
solubility of 1a in hexane, the photoreaction was investigated in
toluene. Unlike 2a, the UV irradiation to the 1a solution in
toluene gave a pale yellowish solution with an absorption
maximum at 375 nm and no absorption above 450 nm, which
never reverted to the colorless state with visible irradiation
(Figure 2a). Given the formation of closed-ring isomer 1b as a
both photoreactive carbons (3a in Scheme 3) which showed
ultimate sensitivity to UV light, giving a photon-quantitative
electrocyclization reaction.19 A H-atom and mesylate group were
introduced at both ends of the 6π-system in 1a as leaving
substituents. The sequence controlled Pd-catalyzed aryl−aryl
cross-coupling reactions between aromatic components using
the position dependent reactivity of benzothiophene led to the
synthesis of 2a. The methoxy group of 2a was deprotected to a
hydroxyl group by BBr3, and a subsequent esterification with
mesyl chloride (MsCl) gave 1a (Scheme 2). In principle, types of
acids can be incorporated in this esterification process.
2a showed a reversible photochromic reaction in dry hexane as
depicted in Scheme 3 with absorption spectral change
accompanying isosbestic points (at 340 and 265 nm), which
indicates a two-component photochromic reaction18 (Figure 1).
Upon UV irradiation, the colorless solution of 2a with an
Figure 2. (a) Absorption spectral change of PAG-1a by UV (313 nm)
irradiation in toluene (4.7 × 10−5 M) under the ambient conditions. (b)
Differential absorption spectrum upon UV irradiation for 3 min to PAG-
1a in 2-MeTHF at 77 K.
primary photoproduct, it should have given an absorption band
at around 600 nm, since 1b has a similar π-system to that of 2b
(Scheme 3). However, the spectral profile after UV irradiation
was identical with that of isolated 1c (Figure S5). Interestingly,
the spectral change of 1a to 1c accompanied an isosbestic point at
345 nm, indicating a quasi-two-components reaction system with
a quantitative and immediate conversion from 1b to 1c even in
the low-polar medium. The net photochemical quantum yield
from 1a to 1c was estimated to be 0.52 under ambient conditions,
which is among the highest values for neutral PAGs so far
reported8 but dropped to 0.40 and 0.32 in tetrahydrofran (THF)
and methanol, respectively. The decrease of the quantum yields
Figure 1. Absorption spectral change of 2a by UV (313 nm) irradiation
in dry hexane (2.3 × 10−5 M).
B
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Journal of the American Chemical Society
Communication
with decreasing the solvent polarity is consistent with the
previous result of 3a, where the hydrogen bonding like CH-N
interaction plays a certain role to support the photoreactive
conformation.19 However, it should be noted that the similar
quasi-two-components reaction was observed regardless of
solvents (Figure S6). Furthermore, since the photochemical
pericyclization reaction proceeds purely through the singlet state,
but neither triplet state nor radicals is involved, it is free from
oxygen-sensitivity.18,28
the UV irradiation is shown in Figure 2b. The differential
absorption profile highlights the formation of a colored species
with an absorption band at 600 nm, which was similar to that for
2b (Figure 1). Furthermore, the visible absorption band was in
good agreement with the calculated one based on the optimized
geometry of 1b (Figure S10), indicating transient formation of
1b.28 Interestingly, the slight increase in temperature to 90 K
diminished the trace of the formation of photoisomer 1b. The
photoirradiation to 1a at 90 K directly gave the absorption
spectrum of 1c (Figure S11). The significantly short lifetime of
1b is not ascribed to the backward reaction to 1a, of which rate
constant should be smaller than 10−6 s−1 with high thermal
stability of the ring-closed form of diarylethene derivatives.18
This kinetic consideration also supports the quantitative
conversion of photochemically formed 1b to 1c accompanying
the acid generation.
1
The photoreaction of 1a was also monitored by H NMR
spectral change in toluene-d8 (Figure 3; also see Figure S7 for
The elimination reaction from 1b to 1c is most likely to
proceed through the carbocation intermediate 1b+ (Figure 4).20b
Figure 4. Acid-releasing reaction of 1b together with optimized
structures of each state.
Figure 3. 1H NMR spectral change of 1a upon UV (365 nm) irradiation
in toluene-d8 (Inset: enlarged charts of aromatic region).
The free energy of each state was calculated by the DFT method
using the Gaussian 09 suite.30 For the calculations of geometry
optimization and subsequent frequency calculations, the CAM
B3LYP functional31 with the 6-31G(d,p) basis set was employed
(Figure S12). Solvent polarity was taken into account by using
the IEFPCM (solvent = methanol).32 The carbocation
intermediate 1b+ was estimated to have high stability with a
relative energy of −44 kcal mol−1 to the proton adduct of 1b (1b
+H+) which was assumed as an active substance. The arenium
cation 1b+ has a coplanar structure with an extended conjugation,
which is typically indicated by the sum of inner angles of the
central six-membered ring of 719°. The hyperconjugation of the
vicinal C−H group vertically bound to the polycyclic π-plane
may also stabilize 1b+ (Figure 4).33 The further elimination of the
proton should be driven by the formation of an aromatic
polycyclic 1c with a relative energy of −3 kcal mol−1 to that of the
1b+ state. The elimination reaction should be catalyzed efficiently
not only by free H+20b but also by any proton-donating substance
such as H2O and ROH, which may facilitate the elimination of
the acid group through H-bonding. Moreover, the photo-
generated acid at the early stage could catalyze the subsequent
elimination reaction, providing an autocatalytic photoacid
generation system.
more details including peak assignment). Owing to the low
solubility of the produced 1c, the measurements were performed
at 323 K. 1a gave a singlet peak at 1.87 ppm corresponding to the
methyl protons in the OMs group. The UV irradiation to the
toluene-d8 solution in an NMR tube diminished this peak and
three peaks appeared (indicated by arrows), which originated
from MsOH dispersed in toluene-d8. The peaks at 2.36 and 2.96
ppm may correspond to methyl protons, and the broad peak
above 9.5 ppm could be assigned to the OH proton in MsOH
(see Figure S7). Meanwhile, the water signal shifted downfield
with broadening, indicating the interaction between residual
water and the released MsOH. In the aromatic region, all peaks
showed downfield shifts and the chart of the final state was
identical with that of 1c with a characteristic doublet peak at 9.6
ppm. Thus, 1H NMR monitoring clearly suggested the
generation of MsOH and the polycycle 1c. Similarly, the
1
formation of 1b could not be recognized by H NMR. The
absence of spectral evidence for the formation of closed ring
isomer 1b indicated that the acid generation from 1b to 1c was
energetically favored and proceeded quantitatively.
To confirm the trace of the formation of 1b after UV
irradiation, transient absorbance was monitored at 630 nm in a
nanosecond laser flash photolysis measurement. The observed
transient absorbance at 630 nm was, however, attributed to the
decay of the triplet state of 1c (Figures S8, S9) based on an O-
quenching experiment, indicating the rapid conversion of 1b to
1c at least within 10 ns. The photoreaction of 1a was then
conducted at low temperature in a glass matrix of 2-
methyltetrahydrofran. Since the conversion rate of the photo-
reaction was low at 77 K, a differential absorption spectrum upon
Finally, 1a was demonstrated as a photoinitiator for the
cationic polymerization of cyclohexene oxide in a preliminary
photopolymerization experiment (see Scheme 4). UV irradiation
to the deaerated dry solution of cyclohexene oxide with 0.2 mol
% of 1a in a glass tube triggered the polymerization of monomers
with the formation of 1c. The mixture after the UV irradiation
was characterized with MALDI-MS. A periodic pattern of mass
peaks with the difference in m/z of 98 clearly demonstrated the
C
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Scheme 4. Reaction Scheme of Photoinduced Cationic
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In summary, we propose a new type of self-contained proton-
releasing PAG utilizing an efficient 6π-photocyclization reaction
and a subsequent quantitative acid releasing reaction. Opti-
mization of molecular design based on supramolecular
interaction to support a photoreactive conformation19 would
lead to a further increase in the quantum yield of acid generation.
Chemical modifications toward the two-photon photochrom-
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, crystallographic analysis of 2a, character-
ization of 1c, detailed study on photoreaction of 1a, photo-
polymerization of cyclohexene oxide, and complete ref 30. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/jacs.5b02826.
(19) Fukumoto, S.; Nakashima, T.; Kawai, T. Angew. Chem., Int. Ed.
2011, 50, 1565−1568.
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J. Org. Chem. 2012, 4493−4500.
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2007, 46, 8017−8019.
AUTHOR INFORMATION
■
(22) Wu, T.; Tang, H.; Bohne, C.; Branda, N. R. Angew. Chem., Int. Ed.
2012, 51, 2741−2744.
Corresponding Authors
*tkawai@ms.naist.jp
*ntaku@ms.naist.jp
(23) Asadirad, A. M.; Boutault, S.; Erno, Z.; Branda, N. R. J. Am. Chem.
Soc. 2014, 136, 3024−3027.
Notes
(24) Nakashima, T.; Goto, M.; Kawai, S.; Kawai, T. J. Am. Chem. Soc.
2008, 130, 14570−14575.
The authors declare no competing financial interest.
(25) Gostl, R.; Hecht, S. Angew. Chem., Int. Ed. 2014, 53, 8784−8787.
̈
ACKNOWLEDGMENTS
(26) Lemieux, V.; Spantulescu, M. D.; Baldridge, K. K.; Branda, N. R.
Angew. Chem., Int. Ed. 2008, 47, 5034−5037.
■
We thank Mr. F. Asanoma, Ms. Y. Nishikawa, and Mr. S. Katao at
NAIST for their help in the measurements and analyses with 1H
NMR, MS, and X-ray crystallography, respectively. The authors
also thank Mr. L. McDowell for proofreading the entire text in its
original form. This research was partly supported by the Green
Photonics Project at NAIST and the Grant-in-Aid for Scientific
Research on an Priority Area, “Photosynergetics”, sponsored by
the Ministry of Education, Culture, Sports, Science and
Technology, MEXT (Japan). T.N. is also grateful for financial
support from the Mazda Foundation.
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suggests formation of 1c even under the present conditions (see also
Figure S10 in the Supporting Information).
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