Photobase generators liberating two bases by absorbing one photon and their application to photosensitive materials

Article abstract of DOI:10.1246/cl.150405

We propose a photobase generator (PBG) that liberates two bases by absorbing one photon. Phototriggered decarboxylation proceeds first to generate one base, and subsequently the unstable reaction intermediate is thermally decarboxylated to generate another base. It was found that the photosensitivity of poly(glycidyl methacrylate) films containing the PBG is much higher than that of films containing a conventional PBG that generates one base by absorbing one photon.

Full text of DOI:10.1246/cl.150405

CL-150405
Received: April 28, 2015 | Accepted: May 25, 2015 | Web Released: June 4, 2015
Photobase Generators Liberating Two Bases by Absorbing One Photon
and Their Application to Photosensitive Materials
Koji Arimitsu,* Yuya Maruyama, and Masahiro Furutani
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,
2641 Yamazaki, Noda, Chiba 278-8510
(E-mail: arimitsu@rs.noda.tus.ac.jp)
We propose a photobase generator (PBG) that liberates two
such as TBD, DBU, and 1,5-diazabicyclo[4.3.0]non-5-ene
(DBN).14,15 However, these PBGs generate only one base
molecule with one photon, where the maximum achievable
quantum yield is Φmax,ach = 1.
bases by absorbing one photon. Phototriggered decarboxylation
proceeds first to generate one base, and subsequently the
unstable reaction intermediate is thermally decarboxylated to
generate another base. It was found that the photosensitivity of
poly(glycidyl methacrylate) films containing the PBG is much
higher than that of films containing a conventional PBG that
generates one base by absorbing one photon.
We have designed PBGs using a new concept, wherein one
PBG could release two base molecules with one photon. In this
case, Φmax,ach = 2, which would be realized by combining a
photochemical process with a thermochemical process in the
photodecomposition reaction (Scheme 1). First, the o-nitro-
benzyloxycarbonyl group of the PBG is photodecomposed to
generate an amine and an unstable β-keto acid salt in a
decarboxylation reaction. Subsequently, the β-keto acid salt is
thermally decomposed by another decarboxylation reaction to
liberate another base molecule at room temperature. Not only
weak aliphatic amines but also strong organic bases could be
chosen to liberate base in the thermochemical process. Whereas
PBG 1a generates two cyclohexylamines, PBG 1b generates one
cyclohexylamine and one TBD.
For the shared acidic part, 3-cyclohexylcarbamoyloxy-3-
(4,5-dimethoxy-2-nitrophenyl)propionic acid was synthesized
in a 24% total yield via three reaction steps from tert-butyl
acetate and 6-nitroveratraldehyde as the starting compounds (see
Supporting Information). Cyclohexylamine or TBD was simply
mixed with the carboxylic acid in THF to precipitate each
product, 1a or 1b, in 70% or 48% yield, respectively. The one-
to-one salt formation was confirmed by 1H NMR and high-
resolution mass spectroscopy (HR-MS) measurements.
Photosensitive materials have been studied extensively
because they are widely used in industrial processes. They are
utilized in the fields of printing, resists, coatings, adhesion,
and electronic materials.1-3 In a UV curing system, radical or
cationic initiators are often used, because such species have
relatively high photosensitivity. However, they also have crucial
problems such as oxygen inhibition and volume shrinkage in a
radical process, or corrosion of metallic substrates in a cationic
process. Now, an anionic UV curing system using photobase
generators (PBGs) is in the limelight because it could circum-
vent these problems. However, in general, basic species from
PBGs are weak bases, and their quantum yields are very low,4-11
which leads to low photosensitivity of the system.
As PBGs generate strong bases, Sun et al. developed a
tetraphenylborate that generates a superbase, 1,5,7-triazabicy-
clo[4.4.0]dec-5-ene (TBD), although its quantum yield is not
high (Φ254 = 0.18).12 Suyama et al. reported the use of a benzo-
formate-type PBG to generate 1,5-diazabicyclo[5.4.0]undec-5-
ene (DBU).13 Our group has focused on photodecarboxylation
reactions of ketoprofen and xanthone acetic acid, which could
be photodecomposed with high quantum yields (Φ313 = 0.75
for ketoprofen and Φ350 = 0.64 for xanthone acetic acid), and
developed their carboxylate-type PBGs generating superbases
Carboxylate-type PBGs 1a and 1b were dissolved in
methanol and in chloroform. The photodecomposition behaviors
during 365 nm light irradiation were examined using their
methanol solutions (Figure 1). The maximum absorption
wavelength, -max, for each PBG was 339 nm. Before UV
irradiation, the molar extinction coefficients, ¾339, were 5.0 ©
photochemical process
O
O
NO2
O
H
O
O
NO
H:B
hν
- CO2
N
H2N
O
+
O
O
O
O
H:B
O
unstable
H2N
N
O
NO
:B =
+
N
N
:B
- CO2
1bH
O
1a
thermalOchemical process
Scheme 1. Mechanism of generation of two base molecules by absorbing one photon from PBGs 1a and 1b.
1194 | Chem. Lett. 2015, 44, 1194–1196 | doi:10.1246/cl.150405
© 2015 The Chemical Society of Japan

(a)
(b)
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
mJ cm−2
mJ cm−2
0
320
0
320
640
1280
2560
5120
10240
640
1280
2560
5120
10240
200 250 300 350 400 450 500
Wavelength/nm
200 250 300 350 400 450 500
Wavelength/nm
Figure 1. UV-vis spectral changes of 1a (a) and 1b (b) in methanol during 365 nm light irradiation. The concentration of PBG was
6.0 © 10¹5 mol L¹1, in both cases.
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
mJ cm−2
mJ cm−2
0
2560
5120
0
2560
5120
10240
10240
350 400 450 500 550 600 650 700
Wavelength/nm
350 400 450 500 550 600 650 700
Wavelength/nm
Figure 2. UV-vis spectral changes of 1a (a) and 1b (b) with phenol red in methanol during 365 nm light irradiation. The
¹3
concentrations of PBG and phenol red were 4.4 © 10 and 2.4 © 10¹5 mol L¹1, respectively.
¹1
¹1
103 L mol¹1 cm for 1a and 5.7 © 103 L mol¹1 cm for 1b. In
each case, a spectral change was clearly observed during UV
irradiation. The absorption peak at 339 nm decreased, indicating
a smooth photodecomposition. The isosbestic points were
observed at 310 and 390 nm. These spectral changes would
result from the decomposition of the carboxylate part of the
PBGs.
about the same rate (Figures 3b and 3c). This is probably
because the decarboxylation of β-keto acid salts in the
thermochemical process follows the photochemical process.
On the other hand, the peak appeared at 1480 cm¹1 was assigned
to the nitroso groups of the corresponding by-products.
PBG 1a was applied to an anionic UV curing system of
poly(glycidyl methacrylate) (PGMA, Mw = 12000, Mw/Mn =
1.5). The generated primary amines reacted with the epoxy
groups of PGMA, resulting in the formation of crosslinking sites
and causing insolubilization. The PGMA film containing
5 mol % of 1a (toward epoxy groups of PGMA) was insolubi-
Generation of basic species from PBGs 1a and 1b was
examined using their methanol solutions containing phenol red
as an indicator.13 In both cases, with 365 nm light irradiation, a
new absorption peak appeared at 562 nm, indicating an increase
in the basicity (Figure 2). The peak intensity with 1b was much
larger than with 1a. This is probably because superbases (TBDs)
are generated during the thermal decomposition of 1b, while
only weak bases (cyclohexylamines) are generated in the case of
1a. It is noteworthy that, in both cases, no basicity was detected
without UV irradiation.
¹2
lized with at least 400 mJ cm of irradiation (Figure 4). On the
other hand, in the case of a conventional PBG 2 (10 mol %
toward the epoxy groups of PGMA), the PGMA films were not
¹2
insolubilized, even after 10000 mJ cm of irradiation. Indeed,
the generation of two bases by absorbing one photon would
contribute to a much higher photosensitivity of the PGMA films
containing 1a than of films containing 2 that photodecomposes
into one base by absorbing one photon.
In conclusion, carboxylate-type PBGs 1a and 1b were
designed and synthesized: they could liberate two base mole-
cules with one photon by combining a photochemical process
with a thermochemical process. Their smooth photodecomposi-
The photodecomposition behavior of PBG 1a in a polysty-
rene (PSt) film was also examined with FT-IR (Figure 3a).
During 365 nm light irradiation, the intensities of the peaks at
¹1
¹1
1520 cm (assigned to nitro groups), 1720 cm (carbamates),
¹1
and 1390 cm (carboxylates) were decreased, indicating the
photodecomposition of 1a. Each of the peaks was decreased at
Chem. Lett. 2015, 44, 1194–1196 | doi:10.1246/cl.150405
© 2015 The Chemical Society of Japan | 1195

(a)
1520 cm-1
NO2
1720 cm-1
carbamate
1390 cm-1
COO-
1480 cm-1
NO
[mJ cm−2
]
0
640
2560
10240
40960
1750
1700
1600
1500
1400
1350
Wavenumber/cm-1
(b)
(c)
1.7
1.0
0.8
0.6
0.4
0.2
0
1.0
carbamate
1.5
1.3
1.1
0.9
0.7
0.5
0.8
0.6
0.4
0.2
0
NO
NO2
COO-
30
0
10
20
40
0
10
20
30
40
41
41
Irradiation energy/Jcm−2
Irradiation energy/Jcm−2
Figure 3. (a) FT-IR spectral changes of 1a in PSt during 365 nm light irradiation. (b) Consumption of nitro groups (1520 cm¹1) and
generation of nitroso groups (1480 cm¹1) during UV irradiation. (c) Consumption of carbamates (1720 cm¹1) and carboxylates
(1390 cm¹1) during UV irradiation.
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1196 | Chem. Lett. 2015, 44, 1194–1196 | doi:10.1246/cl.150405
© 2015 The Chemical Society of Japan