Production of three radical cations from a single photon using a photo acid generator

Article abstract of DOI:10.1016/j.tetlet.2014.01.092

Efficient generation of the organic radicals is a fundamental technology for preparing the spintronic materials. In this Letter, we present the chemical reaction of the three radical generation from a single photon. A photo acid generator which can releas

Full text of DOI:10.1016/j.tetlet.2014.01.092

Tetrahedron Letters 55 (2014) 1635–1639
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Production of three radical cations from a single photon using
a photo acid generator
Kazuo Tanaka a, Wataru Ohashi a, Hiroshi Okada a,b, Yoshiki Chujo a,
⇑
a Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
b Matsumoto Yushi-Seiyaku Co., Ltd, 2-1-3, Shibukawa-cho, Yao-City, Osaka 581-0075, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Efficient generation of the organic radicals is a fundamental technology for preparing the spintronic
materials. In this Letter, we present the chemical reaction of the three radical generation from a single
photon. A photo acid generator which can release the multiple acid molecules via the automatic ampli-
fication mechanism was synthesized. The synthesized acid generator immediately released methanesulf-
onic acid by UV irradiation. Due to the amplification system, a maximum of three acid molecules can be
produced from the single acid generator. In addition, the release of acid is induced by UV irradiation and
automatically proceeds until the release of three acid molecules is finished. Finally, by employing the
acid-catalyzed radical generation of tetrathiafulvalene, we also demonstrate the efficient radical genera-
tion triggered by UV irradiation in the polymer film.
Received 4 December 2013
Revised 15 January 2014
Accepted 22 January 2014
Available online 28 January 2014
Keywords:
Photo acid generator
Radical
Tetrathiafulvalene
Photoreaction
Amplification
Ó 2014 Elsevier Ltd. All rights reserved.
Introduction
The further elaboration for progress of the transformation of TTF to
the radical cation is necessary.
Corresponded to development in spintronics, the robust materi-
als involving the spin sources have been strongly desired to prove
the theoretical predictions and apply the material functions for the
practical devices. In particular, organic radicals based on the small
molecules and polymeric materials have attracted much attention
as a spin source because of the diversity of molecules and the tun-
able properties.1,2 To expand the applicability of the organic radical
species, the fundamental technologies for the facile generation and
the restrictedly control of the amount of the radical species are
required. To satisfy the demands for the control of the generation,
we have reported that the gradient materials with the radical
concentration can be obtained in the polymer matrices with the
photo-acid generator.3,4 Based on the acid-catalyzed radical gener-
ation via intermolecular electron transfer between tetrathiafulva-
lenes (TTF),5 variable radical concentrations can be realized in
the polymer films simply by changing the light-irradiation time.
Because of various advantages in the photoreaction such as the
minimum requirement to additives and time- and site-specificity,
chemically-active species can be localized based on the prepro-
grammed design in the material. However, because of the intrinsic
strong light-absorbing ability of TTF cation radical in the UV and
visible region, the efficiency of the photoreaction was suppressed.6
Photo acid generators are valid for the modification of the poly-
mer-based materials.7 As a practical example, on the fabrication of
the integrated circuits, photo acid generators are used to prepare
the patterning at the substrate by mixing to the resist materials.7
Triggered by light irradiation, photo acid generators can play a role
in the creation of the different polarity regions. For the improve-
ment of the reaction yield, conventional photo acid generators
can produce the acid species via the self-amplification system.8
After the initial release of acid via the photoreaction, another photo
acid generator can release acid via the acid-catalyzed reaction. As a
result, releasing rates should be drastically enhanced, and finally
all acid generators can be consumed. Such self-catalytic mecha-
nism can significantly contribute to the improvement of the reac-
tion yields. Whereas, the regulation of the reaction rate and
amounts of the acid species are difficult in the self-catalytic sys-
tem. It is expected that the restricted control of the acid generation
involving the amplification mechanism is promised to contribute
not only to the efficient radical generation with TTF in the materi-
als but also to the precise regulation of the amount of the radical
species in the materials.
Herein, we present the photo acid generator for the efficient
production of TTF radical cation. Our acid generator can absorb
UV light and subsequently release the acid molecule. In particular,
this acid generator can automatically produce a maximum of three
acid molecules without assistance of the released acids. Finally, we
demonstrate the acid generation can proceed not only in the
⇑
Corresponding author. Tel.: +81 75 383 2604; fax: +81 75 383 2605.
E-mail address: chujo@chujo.synchem.kyoto-u.ac.jp (Y. Chujo).
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.092

1636
K. Tanaka et al. / Tetrahedron Letters 55 (2014) 1635–1639
solution but also in the film states, resulting in the generation of
the TTF radical cation. This is the first example, to the best of our
knowledge, to offer the amplification of the radical production
from a single photon.
chlorination occurred instead of mesylation to aryl alcohol and
benzyl alcohol.14 The total value of the peak integration in the 1H
NMR spectrum of 5 was corresponded to the theoretical value. At
120 °C, the peak pattern was once simplified, and the same spec-
trum was reversibly detected after cooling to 25 °C. These data rep-
resent that in the synthesis of compound 5 the cis isomer was also
generated.15
Results and discussion
Initially, to check the photo reactivity of the compound 7, the
time-course of the spectrum change in UV–vis absorption of 7
was examined with variable time of UV irradiation (Fig. 1). By
increasing UV irradiation time, the absorption band around 350
nm assigned as the nitro group decreased.16–18 In addition, the
absorption band around 400 nm increased. These data mean the
generation of the nitroso compound which is the degradation
product from o-nitorobenzyl ether in the photoreaction. Within
30 min, the starting material was consumed. From these results,
it was indicated that 7 can be activated by UV irradiation.
Next, the production of acid from 7 was investigated. To detect
the existence of acid molecules, TTF was used as an indicator. It is
known that TTF derivatives can be immediately transformed to TTF
radical cation in the presence of acid via the intermolecular elec-
tron transfer,19,20 leading to characteristic absorption bands at
434 nm and 579 nm from the TTF radical cation.4,21 Based on these
changes in the optical properties, the amount of TTF radical cation
can be evaluated quantitatively.4 Significantly, by adding TTF to the
sample, the amount of the released acid can be estimated at this
moment. After UV irradiation, the TTF/acetonitrile solution was
added to the photoreacted mixtures and measured UV–vis absorp-
tion spectra (Fig. 2). Significant absorption bands with the peaks at
434 nm and 579 nm were observed from the UV-irradiated sample.
The photo acid generator 7 was designed to release three equiv-
alents of methanesulfonic acid from the single photon (Scheme 1).
By UV irradiation, the o-nitrobenzyl ether moiety is released, fol-
lowed by the generation of the amino group. Electron donation
of the amino group can tandemly facilitate the release of the mesyl
groups, resulting in the formation of the azaquinone methide skel-
eton. Subsequently, the mesyl groups at the side chains are re-
leased after the restoration of the amino group by employing
residual water. Finally, three mesyl groups can be produced from
a single molecule of 7. TTF can be protonated, followed by the for-
mation of the radical cation via the intermolecular electron trans-
fer.9 Based on this reaction sequence called ‘self-immolative
mechanism’, the multiple radical cations can be generated.10
Although the neutral radical could be decomposed immediately,
the TTF radical cation can exist in the solution and films and show
the characteristic absorption band in the visible regions. The syn-
thesis of 7 was performed as outlined in Scheme 2. The intermedi-
ates and compound 7 were prepared from commercially available
materials according to the previous reports.11,12 The deprotection
of the tert-butyldimethylsilyl (TBS) group was conducted in
1 wt % HCl/THF to prohibit the degradation of the carbamate group
by tetrabutylammonium fluoride (TBAF).13 When the reaction of 6
with mesyl chloride was conducted at room temperature or 0 °C,
MsOH
MsO
MsO
H2O
ν
h
NH
O
NH2
NH
O
OMs
OMs
OMe
OMs
OMs
OMe
OMs
OMs
Azaquinone methide
O
CO2
+
O2N
OMe
ON
Nitroso compound
HO
OMe
HO
MsOH
NH2
NH
MsOH
OMs
OMs
OMs
Azaquinone methide
S
S
S
S
S
H
S
S
S
S
S
S
S
S
S
S
S
+ MsOH
TTF
MsO-
TTF
MsO-
Radical cation
Scheme 1. Plausible mechanism for the production of TTF cation radical with photo acid generator 7.

K. Tanaka et al. / Tetrahedron Letters 55 (2014) 1635–1639
1637
OEt
OEt
HO
O
O
a
b
I
I
I
I
NH2
NH2
NH2
1
2
TBSO
c
d
O
B
TBSO
O
3
HO
R2
Figure 2. UV–vis absorption spectra of the reaction mixtures containing 100
lM 7
and 600 M TTF in acetonitrile with or without UV irradiation for 15 min. The
l
spectra represented by yellow and green lines were recorded after 1 h incubation at
room temperature.
e
NH2
NH
O
O
R1
R1
R1
R1
4
OMe
OMe
O2N
5
: R1 = OTBS, R2 = OH
6: R1 = R2 = OH
: R1 = R2 = OMs
f
g
7
Scheme 2. Synthesis of 7 Reagents and condition: (a) I2, Ag2SO4, ethanol, rt, 30 min,
69%; (b) diisobutylaluminium hydride, THF, 0 °C ? rt, 1 h, 70%; (c) pinacol borane,
Schwartz’s reagent, triethylamine, 65 °C, 16 h, 95%; (d) Pd(OAc)2, P(PH3)3, K3PO4, 2,
dioxane, water, 85 °C, 3 h, 67%; (e) 4,5-dimethoxy-2-nitrobenzylchloroformate,
dichloromethane, sat. NaHCO3 aq, water, rt, 2 h, 63%; (f) 1 wt % HCl/THF, rt, 30 min,
87%; (g) MsCl, triethylamine, DMF, À40 °C, 1 h, 86%. TBS = tert-butyldimethylsilyl,
Ms = methanesulfonyl.
Figure 3. Time-courses of the concentration change of TTF radical cation in the
samples containing 100 lM 7 and 600 lM TTF in acetonitrile. The samples were
treated with or without UV irradiation at room temperature. The absorption
changes at 434 nm are plotted.
of methanesulfonic acid and TTF radical cation in the sample are
300 lM. Obviously, by increasing the irradiation time, the amount
of acid in the sample increased. The reaction approximately
reached a plateau after 15 min irradiation. In contrast, without
UV irradiation, slight enhancement was observed. These data also
indicate that 7 can work as a photo acid generator. After UV irradi-
ation for 30 min, 67 lM TTF radical cation was produced in the
sample. This result represents that the same amount of acid was
generated. To determine the origin of the acid generation, further
analyses were performed.
The generation of TTF radical cation after UV irradiation was
examined. The changes of UVÀvis spectra were monitored after
the photoreaction (yellow and light green lines in Fig. 2). From
the sample with UV irradiation for 15 min, the increase of the
absorption assigned as the generation of TTF radical cation was ob-
served even under the dark conditions. On the other hand, the
enhancement of the absorption was hardly obtained from the sam-
ple without UV irradiation. These data indicate that automatic
amplification mechanism can work as programmed to produce
the multiple acid molecules. To evaluate the acid release quantita-
tively, with or without UV irradiation to the sample, the concentra-
tion of the TTF radical cation was monitored (Fig. 4). At the initial
state (t = 0), the generation of TTF radical cation with high concen-
tration was already detected from the UV-irradiated sample. It is
likely that UV irradiation for 15 min can induce the acid generation
Figure 1. UV–vis spectra of 100 lM 7 solution in acetonitrile by changing UV
irradiation time.
These data clearly indicate that 7 can work as a photo acid
generator.
According to the reported value of the distinct molar coefficient
of TTF radical cation at 434 nm, the concentrations of TTF radical
cation in the samples can be estimated.4 Figure 3 represents the
time-courses of the concentration changes of TTF radical cation
in the sample containing 100
lM 7 and 600 lM TTF in the acetoni-
trile by UV irradiation. In theoretical, the maximum concentrations

1638
K. Tanaka et al. / Tetrahedron Letters 55 (2014) 1635–1639
sample. This result means that the second and the third acid mol-
ecules should be gradually released. Since these intermediates
such as the azaquinone methide species as proposed in Scheme 1
are stabilized in the organic solvent, the elimination rate decreases,
22–24
comparing to that in water.
The concentration of TTF radical
M. This
cation at the initial state (just after UV irradiation) was 66
l
means that the same amount of acid should be generated from UV
irradiation. After 8 h incubation in the dark at room temperature,
the concentration increased to 200 lM. This value was about three
times larger than that at the initial state. These results strongly
suggest that all photo-reacted compounds can finally release three
equivalents of acid. From 1H NMR measurements, compound 7
(1 mM) can be maintained in acetonitrile containing 1 mM
methanesulfonic acid after 1 h incubation at room temperature.25
These data indicate that UV irradiation should be triggered for
the acid generation. In other words, the amount of acid produced
from 7 can be restrictedly controlled by UV irradiation. In sum-
mary, it was proved that 7 can work as a photo acid generator with
automatic amplification system.
Figure 4. Time-courses of the concentration change of TTF radical cation in the
samples containing 100 lM 7 and 600 lM TTF in acetonitrile. The samples were
treated with or without UV irradiation for 15 min and incubated at room
temperature. The absorption changes at 434 nm are plotted.
Influence of the reaction temperature on the acid generation
from 7 was investigated (Fig. 5). Under the same condition as
above except for the solution temperature, the time-courses of
the concentration changes of TTF radical cation were evaluated at
various temperatures. By decreasing the reaction temperature, the
rate of the concentration change was reduced. In particular, at 0 °C,
the generation of TTF radical cation was largely suppressed. These
data mean that the decrease of the reaction temperature can sup-
press the initial release of acid molecules. It is suggested that the
control of the acid generation can be accomplished by temperature
changes.
Finally, the acid generation with 7 followed by TTF radical cat-
ion was performed in the polymer matrix. The yellow-colored
transparent films of PVP with TTF and 7 were prepared, and UV
was irradiated at the left side of the film (Fig. 6). Apparently, the
film color turned dark. From the absorption spectra, correspond-
ingly the increase of the absorption band around 550 nm was ob-
served after 7 h irradiation. These data indicate that the acid
amplification system worked in the polymer matrix. The TTF radi-
cal cation can exist at least for two weeks in the films. Although the
reaction efficiency decreased comparing to the solution states, it is
likely that the accessibility between acid and TTF and TTF them-
selves should be restricted in the film state, and subsequently
the protonation of TTF and the intermolecular electron transfer be-
tween the neutral TTF and the protonated TTF should be sup-
pressed. The decrease of the absorption band around 450 nm was
observed. It is presumed that the UV irradiation could induce the
degradation of TTF radical cation.
Figure 5. Influence of temperature on the concentration changes of TTF radical
cation in the samples containing 100 lM 7 and 600 lM TTF in acetonitrile. The
samples were treated with or without UV irradiation for 15 min and incubated at
various temperatures. The absorption changes at 434 nm are plotted.
from 7. This result also suggests that the first molecule should be
immediately released after the photo-triggered elimination of the
photo-cleavable group. The increase of the concentration of TTF
radical cation was continuously observed from the UV-irradiated
Figure 6. Acid generation in PVP films. (a) Appearance of the film with (left) or without (right) UV irradiation for 7 h at room temperature. (b) Time-course of the absorption
change of the film by UV irradiation.

K. Tanaka et al. / Tetrahedron Letters 55 (2014) 1635–1639
1639
2. (a) Oyaizu, K.; Kawamoto, T.; Suga, T.; Nishide, H. Macromolecules 2010, 43,
10382; (b) Suga, T.; Ohshiro, H.; Sugita, S.; Oyaizu, K.; Nishide, H. Adv. Mater.
2009, 21, 1627; (c) Suga, T.; Sugita, S.; Ohshiro, H.; Oyaizu, K.; Nishide, H. Adv.
Mater. 2011, 23, 751; (d) Inagi, S.; Naka, K.; Chujo, Y. J. Mater. Chem. 2007, 17,
4122.
3. Tanaka, K.; Ishiguro, F.; Chujo, Y. Langmuir 2010, 26, 1152.
4. Tanaka, K.; Ishiguro, F.; Chujo, Y. Langmuir 2010, 26, 10254.
5. (a) Tanaka, K.; Ishiguro, F.; Kunita, T.; Chujo, Y. J. Polym. Sci. A: Polym. Chem.
2009, 47, 6441; (b) Tanaka, K.; Matsumoto, T.; Nakamura, K.; Kawabe, S.; Chujo,
Y. Synth. Met. 2013, 180, 49; (c) Tanaka, K.; Matsumoto, T.; Chujo, Y. Synth. Met.
2013, 163, 13; (d) Tanaka, K.; Matsumoto, T.; Ishiguro, F.; Chujo, Y. J. Mater.
Chem. 2011, 21, 9603.
6. Tanaka, K.; Kunita, T.; Ishiguro, F.; Naka, K.; Chujo, Y. Langmuir 2009, 25, 6929.
7. (a) Crivello, J. V. J. Polym. Sci. A: Polym. Chem. 1999, 37, 4241; (b) Jeon, S.;
Malyarchuk, V.; Rogers, J. A.; Wiederrecht, G. P. Opt. Express 2006, 14, 2300; (c)
Kim, E. K.; Ekerdt, J. G.; Willson, C. G. J. Vac. Sci. Technol., B 2005, 23, 15154; (d)
Moon, S.-Y.; Kim, J.-M. J. Photochem. Photobiol., C 2007, 8, 157.
8. (a) Wang, M.; Yueh, W.; Gonsalves, K. E. J. Photopolym. Sci. Technol. 2007, 5, 751;
(b) Ito, H. Adv. Polym. Sci. 2005, 172, 37; (c) Ito, H. J. Polym. Sci. A: Polym. Chem.
2003, 41, 3863; (d) Asakura, T.; Yamato, H.; Ohwa, M. J. Photopolym. Sci. Technol.
2006, 19, 335.
9. (a) Pohlers, G.; Scaiano, J. C.; Sinta, R.; Brainard, R.; Pai, D. Chem. Mater. 1997, 9,
1353; (b) Schlegel, L.; Ueno, T.; Shiraishi, H.; Hayashi, N.; Iwayanagi, T. Chem.
Mater. 1990, 2, 299.
10. (a) Sagi, A.; Weinstain, R.; Karton, N.; Shabat, D. J. Am. Chem. Soc. 2008, 130,
5434; (b) Meyer, Y.; Richard, J.-A.; Massoneau, M.; Renard, P. Y.; Romieu, A. Org.
Lett. 2008, 10, 1517; (c) Blencowe, C. A.; Russell, A. T.; Greco, F.; Hayes, W.;
Thornthwaite, D. W. Polym. Chem. 2011, 2, 773; (d) Grinda, M.; Clarhaut, J.;
Renoux, B.; Tranoy-Opalinski, I.; Papot, S. Med. Chem. Commun. 2012, 3, 68.
11. Lindsay, D. M.; Dohle, W.; Jensen, A. E.; Kopp, F.; Knoche, P. Org. Lett. 2002, 4,
1819.
12. Lee, H. M.; Oberhuber, C.-N.; Shair, M. D. J. Am. Chem. Soc. 2008, 130, 16864.
13. Jacqueemard, U.; Bénéteau, V.; Lefoix, M.; Routier, S.; Mérour, J.-Y.; Coudert, G.
Tetrahedron 2004, 60, 10039.
14. Mocanu, G.; Constantin, M.; Carpov, A. Angew. Makromol. Chem. 1996, 241,
4168.
Conclusion
We demonstrate the radical generation via the amplification
mechanism. As a result, from a single photon, three radicals can
be obtained. This system is promised to be valid for preparing
the stable materials with higher amount of spin sources. Further-
more, our photo acid generator involves many advantages in terms
of a molecular release: (1) The multiple molecules can be released
by the single photon. The maximum reaction quantum yield in our
system can be theoretically three because of the amplified system.
The effective molecular release is expected even with the weak
incident light. (2) In addition, light-driven system is versatile for
a wide variety of applications such as in biotechnology as well as
in the circuit fabrication because the photoreaction can proceed
with high time- and site-specificity without additives. (3) Molecu-
lar release can automatically proceed once the light is irradiated.
The releasing molecules can be changed from acid. (4) Different
from the conventional acid generators, our system can proceed
without assistance of acid. Thereby, the amount of the released
molecules can be regulated restrictedly. Moreover, by introducing
or changing substituents, the tuning of the releasing rate or the
light wavelength for the initiation can be tuned. Thus, our concept
is also valid for developing the scaffold for light-driven molecular
release system.
Acknowledgements
This work was partially supported by the Kansai Research Foun-
dation for the promotion of technology (for K.T.). This work was
partially supported by a Grant-in-Aid for Scientific Research on
Innovative Areas ‘New Polymeric Materials Based on Element-
Blocks (No. 2401)’ (25102521) of The Ministry of Education,
Culture, Sports, Science, and Technology, Japan.
15. Azumiya, I.; Kagechika, H.; Yamaguchi, K.; Shudo, K. Tetrahedron 1995, 51,
5277.
16. Kostianen, M. A.; Smith, D. K.; Ikkata, O. Angew. Chem., Int. Ed. 2007, 46, 7600.
17. Choi, S. K.; Thomas, T.; Li, M.-H.; Kotlyar, A.; Desai, A.; Baker, J. R., Jr. Chem.
Commun. 2010, 2632.
18. Griffin, D. R.; Patterson, J. T.; Kasko, A. M. Biotechnol. Bioeng. 2010, 107, 1012.
19. Kobayashi, Y.; Yoshioka, M.; Saigo, K.; Hashizume, D.; Ogura, T. J. Am. Chem. Soc.
2009, 131, 9995.
20. Giffard, M.; Alonso, P.; Garin, J.; Gorgues, A.; Nguyen, T. P.; Richomme, P.;
Robert, A.; Roncali, J.; Uriel, S. Adv. Mater. 1994, 6, 298.
21. Hiinig, S.; KieJlich, G.; Quast, H.; Scheutzow, D. Liebigs Ann. Chem. 1973, 2, 310.
22. Erez, R.; Shabat, D. Org. Biomol. Chem. 2008, 6, 2669.
23. Weinstain, R.; Sagi, A.; Karton, N.; Shabat, D. Chem. Eur. J. 2008, 14, 6857.
24. Sagi, A.; Segal, E.; Satchi-Fainaro, R.; Shabat, D. Bioorg. Med. Chem. 2007, 15,
3720.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.01.
092.
25. See Figure S1 in the Supporting information.
References and notes
1. Fahrenbach, A. C.; Bruns, C. J.; Cao, D.; Stoddart, J. F. Acc. Chem. Res. 2012, 45,
1581.