Photochemical generation of superbases from carboxylates consisting of phthalimidoacetic acid derivatives and superbases

Article abstract of DOI:10.1246/cl.140073

We have developed simple and powerful photobase generators capable of highly efficient production of superbases. These photobase generators were prepared by simply mixing phthalimidoacetic acid derivatives with the corresponding superbases. The use of photobase generators enabled anionic UV curing at lower temperatures, in contrast to previous anionic UV curing materials that required heat treatment above 150 °C after UV irradiation. The cured films showed no volume shrinkage.

Full text of DOI:10.1246/cl.140073

Received: January 31, 2014 | Accepted: March 2, 2014 | Web Released: March 11, 2014
CL-140073
Photochemical Generation of Superbases from Carboxylates Consisting
of Phthalimidoacetic Acid Derivatives and Superbases
1
1
2
Koji Arimitsu,* Ken Fukuda, and Nobuhiko Sakai
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,
641 Yamazaki, Noda, Chiba 278-8510
Specialty Chemicals Division, Wako Pure Chemical Industries, Ltd., 1633 Matoba, Kawagoe, Saitama 350-1101
1
2
2
(
E-mail: arimitsu@rs.noda.tus.ac.jp)
We have developed simple and powerful photobase
generators capable of highly efficient production of superbases.
These photobase generators were prepared by simply mixing
phthalimidoacetic acid derivatives with the corresponding
superbases. The use of photobase generators enabled anionic
UV curing at lower temperatures, in contrast to previous anionic
UV curing materials that required heat treatment above 150 °C
after UV irradiation. The cured films showed no volume
shrinkage.
baking.14c On the other hand, it is well-known that N-
phthaloyl-α-amino acids such as phthalimidoacetic acid (1) also
undergo photodecarboxylation reactions to form N-methyl-
1
7,18
phthalimide (5) with high efficiency.
Therefore, we designed
novel photobase generators 3 that can be prepared by simply
mixing 1 with the corresponding base molecule, imino-tris(di-
methylamino)phosphorane (P1), TBD, or cyclohexylamine
(CyA) (Scheme 1). According to this concept, we can prepare
a variety of photobase generators 3a, 3b, and 3c that photo-
chemically generate superbases, as well as aliphatic amines.
Although 1 offers many advantages, it has one limitation, which
stems from the poor absorption above 300 nm. We have designed
substituted phthalimidoacetic acid 2 by nitration, which shows
much better absorption above 300 nm. Therefore, we designed
novel photobase generators 4 as well as 3. Our primary concern
in this paper is to demonstrate the photochemical generation of
superbases from carboxylates 3 and 4 and to show that a novel
anionic UV-curing system is realized by the combination of
A large number of investigations concerning photoreactive
materials utilizing acid-catalyzed reactions, such as chemically
1
2
amplified photoresists and cationic UV curing materials, have
been reported. On the other hand, only a few articles have
described analogous systems. This is probably due to the
relatively low quantum yields for photobase generation and the
weaker basicity of photogenerated bases, leading to low photo-
sensitivity of photoreactive materials doped with photobase
generators. Moreover, many of the photobase generators
1
9,20
4a with an epoxy­thiol formulation
comprising epoxy resin
jER828 and thiol monomer 6 (Scheme 2).
3­12
reported are generally prepared via several synthetic steps.
Recently, Sun et al. reported bicyclic guanidinium tetraphenyl-
borate as a novel photobase generator to generate a strong base,
1
O
O
:
B
h
ν
,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD).13 The quantum yield
N
CH2COOH
N
CH2COO H:B
:B
+
5
CO2
R
R
of the tetraphenylborate (Φ254 = 0.18) is not very high, although
the tetraphenylborate is an attractive photobase generator.
Lately, we have proposed novel photobase generators based
on photodecarboxylation reactions as more powerful and simple
photobase generators for the first time.14 Concretely, carbox-
ylates comprising a superbase and ketoprofen or xanthoneacetic
acid, which undergoes photodecarboxylation reactions with high
quantum yields, Φ313 = 0.7515 or Φ350 = 0.64, respectively,
upon UV irradiation. Furthermore, we have applied the photo-
base generators to anionic UV curing materials, leading to
highly sensitive UV curing systems without postexposure
O
O
1: R=H
2
3: R=H
: R=NO2
4: R=NO2
H
NH
h
ν CO2
N
N
N
P
N
N
a:
b:
c:
NH2
O
:B
N
P1
TBD
CyA
N
CH3
R
16
O
5
Scheme 1. Photochemical generation of organic bases from
photobase generators 3 and 4.
O
O
NH2
NH
P
h
ν
N
CH2COO
N
P
N
N
N
CH3
+
N
N
CO2
O2N
O2N
N
O
O
4
a
P1
SH
O
O
O
O
CH3
C
OH
CH3
O
HS
O
Crosslinked
networks
CH2 CH CH2
O
O
CH2 CH CH2
O
n
C
O CH2 CH CH2
CH3
CH3
O
SH
O
O
jER828
O
6
HS
Scheme 2. Application of 4a to anionic UV-curing systems.
Chem. Lett. 2014, 43, 831–833 | doi:10.1246/cl.140073
© 2014 The Chemical Society of Japan | 831

mJ cm–2
0
00
00
00
00
1
2
4
8
1
3
6
600
200
400
1
2800
2
54 nm light
Irradiation energy/mJ cm–2
Wavelength/nm
Figure 3. Power dependence for the consumption of the
carboxylate of 4a in a polystyrene film during 365-nm-light
irradiation.
Figure 1. UV spectral changes of 3a (4.0 © 10¹5 mol L¹1) in
methanol following 254-nm-light irradiation.
reactions of 3 and 4 proceeded in solution, leading to the
generation of free bases.
mJ cm–2
0
200
2800
3
A 2.0-¯m-thick film of polystyrene containing 4a was spin-
coated on a silicon wafer and irradiated with 365-nm light. The
1
¹
1
absorption band arising from the carboxylate of 4a at 1370 cm
in the FT-IR spectrum decreased after UV irradiation, as shown
in Figure 3. This means that the photodecarboxylation of 4a
proceeded smoothly in a polystyrene film. Other carboxylates
behaved in a similar manner. In addition, the quantum yield for
the photobase generation of 3b (Φ254 = 0.20) in a poly(methyl
2
1
methacrylate) film was determined. The quantum yield was
higher than that of conventional photobase generators generating
organic strong bases. These results show that 3 and 4 underwent
photodecarboxylation reactions in solution and a polymer matrix
upon UV irradiation, leading to the formation of free superbases.
A mixture of jER828 and 6 is expected to be cured in the
presence of 4a upon UV irradiation, owing to the ring-opening
Wavelength/nm
1
9,20
addition of thiolate anions to epoxy groups.
Thiolate anions
Figure 2. UV­vis spectral changes of phenol red solution
upon addition of a solution of 3a being irradiated as a function
of irradiation dose.
should be generated by photobase-induced deprotonation reac-
tions of 6. This anionic UV curing was achieved by exposing the
2
2
coating films consisting of 4a, jER828, and 6 to 365-nm light.
Salts 3 (or 4) were prepared by mixing 1 (or 2) and a strong
base (P1 and TBD) in organic solvents at room temperature. The
photodecomposition of 3 in methanol and 4a in CHCl3 was
investigated by UV­vis absorption spectral measurements when
Figure 4 shows the UV-curing behavior of coating films
consisting of jER828, 6, and a catalytic amount of 4a. The
hardness of the coatings was monitored by the pencil-scratch
2
3
method based on JIS K5400. The pencil hardness was
evaluated by scratching the UV-cured coatings with pencils,
and the hardness showed the following trend: 6B (softest), 5B,
4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, and 9H
(hardest). The coating film showed a level of 3H after 365-nm-
3
and 4a were irradiated with 254- and 365-nm light from a Hg­
Xe lamp, respectively. The absorption band at ­max decreased
slightly when 3a in methanol was irradiated with 254-nm light
(Figure 1). This result suggests that the photodecarboxylation of
¹
2
3
3
a proceeded in methanol. The photobase-generating ability of
light irradiation with an exposure dose of 10000 mJ cm ,
a was confirmed by using phenol red indicator.13 Upon the
followed by heating at 80 °C for 5 min, which is sufficient for
practical use. This means that the anionic UV curing material
sensitized with 4a is highly sensitive, taking into account the
fact that conventional anionic UV curing materials were cured
addition of the irradiated methanol solution of 3a to a methanol
solution of phenol red, a new band at 562 nm appeared, which
was assigned to the deprotonated phenol red after its reaction
with a released base, and the intensity of the band increased
with an increase in irradiation energy (Figure 2). These results
show that the photoinduced decomposition of 3a leads to the
generation of free bases. A similar behavior was seen for 3b, 4a,
and 4b. These results indicate that the photodecarboxylation
2
4
by heating at 150­200 °C for ca. 1 h after UV irradiation. By
contrast, the coating films showed a level lower than 6B when
we used 4c, which generates CyA. This is because the basicity
of CyA is too weak to induce deprotonation reactions of thiol
monomer 6.
832 | Chem. Lett. 2014, 43, 831–833 | doi:10.1246/cl.140073
© 2014 The Chemical Society of Japan

6
5
4
3
2
H
H
H
H
H
5
6
K.-i. Ito, M. Nishimura, M. Sashio, M. Tsunooka, J. Polym. Sci.,
Part A: Polym. Chem. 1994, 32, 1793.
J. F. Cameron, C. G. Willson, J. M. J. Fréchet, J. Am. Chem. Soc.
1996, 118, 12925.
T. Nishikubo, A. Kameyama, Y. Toya, Polym. J. 1997, 29, 450.
Y. Kaneko, A. M. Sarkar, D. C. Neckers, Chem. Mater. 1999, 11,
…
…
…
…
without 4a
r.t
60 °C
80 °C
7
8
H
F
HB
B
2
3
4
5
6
1
70.
9
1
K. H. Jensen, J. E. Hanson, Chem. Mater. 2002, 14, 918.
0 A. Blanc, C. G. Bochet, J. Am. Chem. Soc. 2004, 126, 7174.
B
B
B
B
B
11 K. Dietliker, K. Misteli, K. Studer, C. Lordelot, A. Carroy, T.
Jung, J. Benkhoff, E. Sitzmann, Proceedings of Radtech Inter-
national UVand EB Curing Technology Expo & Conference 2008,
2008, p. 583.
<
6B
12 N. Ishikawa, K. Arimitsu, T. Gunji, Y. Abe, Chem. Lett. 2014, 43,
Liquid
612.
0
100
1000
5000
10000
1
3 X. Sun, J. P. Gao, Z. Y. Wang, J. Am. Chem. Soc. 2008, 130, 8130.
14 a) K. Arimitsu, A. Kushima, R. Endo, J. Photopolym. Sci. Technol.
009, 22, 663. b) K. Arimitsu, R. Endo, J. Photopolym. Sci.
Irradiation energy/mJ cm–2
2
Figure 4. Pencil hardness of coating films comprising jER828
and 6 sensitized with 20 wt % of 4a (relative to jER828) as a
function of irradiation energy from 365-nm light without
postexposure baking.
Technol. 2010, 23, 135. c) K. Arimitsu, R. Endo, Chem. Mater.
2013, 25, 4461.
15 L. L. Costanzo, G. De Guidi, G. Condorelli, A. Cambria, M.
Fama, Photochem. Photobiol. 1989, 50, 359.
1
1
1
6 J. A. Blake, E. Gagnon, M. Lukeman, J. C. Scaiano, Org. Lett.
2
006, 8, 1057.
Radical UV curing
Anionic UV curing
7 Y. Sato, H. Nakai, T. Mizoguchi, M. Kawanishi, Y. Hatanaka, Y.
Kanaoka, Chem. Pharm. Bull. 1982, 30, 1263.
8 A. G. Griesbeck, A. Henz, W. Kramer, J. Lex, F. Nerowski, M.
Oelgemöller, K. Peters, E.-M. Peters, Helv. Chim. Acta 1997, 80,
9
12.
1
9 M. E. Nichols, C. M. Seubert, Proc. Am. Chem. Soc. Polym.
Mater. Sci. Eng. 2009, 101, 1141.
O
20 C. M. Seubert, M. E. Nichols, J. Coat. Technol. Res. 2010, 7, 615.
2
2
1 Φ was determined according to the literature.3
O
O
P
O
2 A UV-curable resin solution was prepared by dissolving 0.10 g of
jER828 (Weight per epoxy equivalent is 184­194), 0.060 g of 6,
and 0.020 g of 4a (20 wt % relative to jER828) in CHCl3. The
solution was coated on glass plates and prebaked at 80 °C on a hot
stage for 30 s to give a liquid film. UV curing was achieved by
exposing the films to 365 nm light. The pencil hardness of the
films was evaluated by scratching UV-cured coatings with pencils
according to JIS K5400. Thickness of the cured film was 1.8 ¯m.
O
O
O
O
O
O
O
Irgacure 819
PETA
Figure 5. Comparison of volume shrinkage between anionic
UV curing and radical UV curing systems.
23 JIS K5400 defined by Japanese Industrial Standards is a simple
method to test the scratch hardness of coatings. In this test, pencils
in a range of 6B to 9H hardness grade are used. The pencil is
moved scratching over the surface of the coating at a 45° angle
with a constant pressure.
Radical UV curing materials that are well established in
the marketplace have drawbacks because of their high volume
shrinkage and oxygen inhibition. Our anionically cured film
showed no volume shrinkage, in contrast to a conventional
2
4 On comparison, a coating film consisting of jER828, 6, and 4c
which generates a weak base, which is a model of conventional
anionic UV curing materials, showed a level of H after 365-nm-
light irradiation with an exposure dose of 10000 mJ cm followed
by heating at 150 °C for 1 h.
radical UV curing system, which showed large volume
shrinkage, as illustrated in Figure 5.25
¹2
In conclusion, we present novel photobase generators 3 and
based on photodecarboxylation reactions. These photobase
generators can be easily prepared by mixing 1 or 2 with the
corresponding superbases. We have also developed a novel
anionic UV curing system, which consists of an epoxy resin,
a thiol monomer, and the photosuperbase generator.26
4
2
5 An anionic UV-curable resin solution was prepared by dissolving
0
.10 g of jER828, 0.060 g of 6, and 4a (20 wt % relative to
jER828) in acetone. The solution was coated on an OHP sheet and
prebaked at 80 °C on a hot stage for 30 s to give 7 ¯m-thick film.
UV curing was achieved by exposing the film to 365-nm-light
¹2
(
1 J cm ). A radical UV-curable resin solution was prepared by
References and Notes
dissolving 0.1 g of PETA and 5 wt % of Irgacure 819 in acetone.
UV curing was achieved in a similar manner. Thickness of the
cured film was 2.2 ¯m.
1
2
3
H. Ito, Adv. Polym. Sci. 2005, 172, 37.
J. V. Crivello, J. Coat. Technol. 1991, 63, 35.
J. F. Cameron, J. M. J. Fréchet, J. Am. Chem. Soc. 1991, 113,
26 Experimental methods and preparation of photobase generators.
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
4
303.
4
J. M. J. Fréchet, Pure Appl. Chem. 1992, 64, 1239.
Chem. Lett. 2014, 43, 831–833 | doi:10.1246/cl.140073
© 2014 The Chemical Society of Japan | 833