Radiation-induced reactions via the lowest excited states in cinnamic acid crystals

Article abstract of DOI:10.1039/b204848e

Radiation-induced reactions of cinnamic acid derivatives have been examined and compared with photoreactions in the crystalline state; all the reaction products were exactly the same as those of the photoreactions, indicating that the reactions proceed only via the lowest excited state to give [2 + 2] cycloadducts, E/Z isomerization products, or starting molecules.

Full text of DOI:10.1039/b204848e

Radiation-induced reactions via the lowest excited states in cinnamic acid
crystals
Yasunari Maekawa,*a Tomonori Inaba,b Hiroki Hobo,b Tadashi Narita,b Hiroshi Koshikawa,a
Seongyun Moon,a Jun Katoa and Masaru Yoshidaa
a Department of Material Development, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki,
Gunma 370-1292, Japan. E-mail: yasum@taka.jaeri.go.jp
b Department of Materials Science and Engineering, Graduate School of Engineering, Saitama Institute of
Technology, 1690 Fusaiji, Okabe, Saitama 369-0293, Japan
Received (in Cambridge, UK) 20th May 2002, Accepted 8th August 2002
First published as an Advance Article on the web 16th August 2002
Radiation-induced reactions of cinnamic acid derivatives
have been examined and compared with photoreactions in
the crystalline state; all the reaction products were exactly
the same as those of the photoreactions, indicating that the
reactions proceed only via the lowest excited state to give
[2 + 2] cycloadducts, E/Z isomerization products, or starting
molecules.
Radiation-induced reactions by ionizing beams such as electron
beams (EB) and ion beams, which can be focused on the
nanometer scale and scanned at high speed, have recently
attracted a great deal of attention as a promising technique for
fabricating next generation electronic nano-devices as well as
nano-lithography.1 Radiation-induced reactions of saturated
hydrocarbons have already been reported, and the mechanisms
of ionization and excitation, as well as the subsequent radical or
ionic reactions were described in detail.2 In contrast, there have
been few reports on rearrangement or C–C bond formation of
organic molecules via radiation-induced ionization or excitation
except for reports on simple compounds such as alkenes and
stilbenes.3 Chapiro et al. reported the mechanistic study for the
radiolysis of diene and acenaphthylene crystals; however, these
reactions proceeded with low conversion, along with a
considerable amount of polymeric products.4 Nevertheless,
cinnamic acid crystals are known to undergo clean photoreac-
tions such as [2 + 2] cycloaddition and E/Z isomerization
between two olefinic double bonds; these reactions proceed
without obvious side reactions by virtue of crystalline lattice
restrictions.5 Therefore, we applied this restricted matrix system
for EB-induced reactions, which in general lead to many
products in complex mechanisms.6 Three cinnamic acids,
which showed disparate photochemical behavior, were em-
ployed in order to reveal the reaction mechanism of radiation-
induced reactions by comparison with the corresponding
photochemical reactions.
Fig. 1 The relative concentration of E1 (1) and E1-dimer (5) plotted as a
function of EB dose irradiating an E1 crystal.
value of 58% with 5 MGy, then gradually decreased to 17%
with E1 conversion of 86% at 20 MGy. The change in the X-ray
diffraction patterns of the crystals during EB irradiation
indicates that crystallinity was maintained with 5 MGy, but
gradually deteriorated with further irradiation. Accordingly, EB
irradiation of E1 induces a [2 + 2] cycloaddition reaction under
crystal lattice control with a dose of up to 5 MGy to give E1-
dimer as the sole product in a yield of 58% (Scheme 1).
Z2 Crystals obtained by recrystallization from a mixture of
benzene and hexane also showed EB reactivity. With an
irradiation dose of 10 MGy, the olefinic peaks of Z2 at 5.9 and
7.0 ppm shifted to new olefinic peaks at 6.5 and 7.8 ppm,
respectively. Since these peaks can be assigned to the olefinic
protons of the E-isomer of Z2 (E2) by comparison with the
photoproduct of Z2, it is clear that EB irradiation induced Z/E
Crystals of E-cinnamic acid (E1) and Z-o-methoxycinnamic
acid (Z2) were obtained by recrystallization from ethanol. The
salt crystal of E1 and (±)-1-phenylethylamine with a 1+1 molar
ratio was prepared by recrystallization from an ethanol solution
of E1 with an equimolar amount of (±)-1-phenylethylamine.
Finely powdered crystals (ca. 0.07 g) were charged into a
hollow of a glass plate with 1 mm depth and 10 mm diameter.
The samples, which were covered with a Kapton™ film (12 mm
thickness) and placed on the water-cooled copper metal plates,
were irradiated with an EB of 1 MeV and 0.5 mA using a
Cascade Type Electron Accelerator (Dynamitron).
An a-type crystal of E1 was obtained by recrystallization
from ethanol and was irradiated with doses of 1–20 MGy
(0.56–11 mC cm22). The conversion and the products of the
EB-induced reactions were analyzed by 1H NMR spectroscopy.
With an irradiation dose of 5 MGy, the olefinic peaks of E1 at
6.6 and 7.6 ppm decreased with new increasing peaks at 3.8 and
4.3 ppm, which were attributed to the cyclobutane protons of
centrosymmetric a-truxillic acid (E1-dimer) by comparison
with the photoproduct of E1.7 As shown in Fig. 1, the relative
concentration of the product, E1-dimer, reached its maximum
Scheme 1
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isomerization of Z2 with a conversion of 61%.8 Although Z2
decreased with further irradiation and was quantitatively
consumed with a dose of 20 MGy, the relative concentration of
E2 remained essentially constant (Fig. 2). On the other hand, a
centrosymmetric dimer (E2-dimer), which was also charac-
terized from the cyclobutane proton appearing at 4.4 ppm in the
1H NMR spectra by comparison with the photoproducts of Z2
crystal, appeared with an irradiation dose of > 15 MGy and
reached a relative concentration of 28% at 20 MGy. A
continuous change of X-ray diffraction patterns was observed
with a slight deformation of the crystal during irradiation. Thus,
EB irradiation of Z2 induces Z/E isomerization to give E2,
which in subsequent [2 + 2] cycloaddition yields E2-dimer
under crystal lattice control. Note that the behavior observed
during EB-induced reaction of E2 crystals is exactly the same as
the corresponding photoreaction behavior, which was reported
by Schmidt et al.
a free electron, which subsequently recombine within pico-
seconds, resulting in the same CA**.2,11 (2) The CA molecules
in an upper excited state quickly relax to the lowest excited
states (CA*), which is the same state as that introduced by
photoexcitation. (3) From their lowest excited states, E-CA (E1
and E2) and Z-CA (Z2) undergo [2 + 2] cycloaddition and Z/E
isomerization, respectively; in contrast, CA Salt (Salt-E1)
reverts to a ground state by thermal relaxation. Both excited
singlet and triplet states of CA derivatives are assumed to be
involved in the course of the above EB-induced reactions since
it is well known that [2 + 2] photocycloaddition and Z/E
isomerization of CA derivatives proceed via the lowest singlet
and triplet states, respectively.12 Although we have not
examined other radiation sources such as g- and X-rays for the
above reactions, the proposed mechanism should also be
applicable because the energy diagram of the reactions include
excited states and radical cations that are common reactive
species in radiation-induced reactions.1,2
In conclusion, EB-induced reactions of cinnamic acid
crystals are confirmed to proceed via the lowest excited state to
give [2 + 2] cycloaddition and Z/E isomerization products. It
should be stressed that the series of EB-induced reactions of CA
derivatives is the first clear example to show exactly the same
behavior as those of the corresponding photoreactions. This
study has provided a preliminary view of the types of radiation-
induced excited state reactions for olefinic molecules. Further
studies are currently underway in the spectral analysis of these
excited states as well as in exploring other EB-induced
reactions.
Fig. 2 The relative concentration of Z2 (1), E2 (5) and E2-dimer (/),
plotted as a function of EB dose irradiating a Z2 crystal.
Notes and references
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K. Suzuki, S. Matsui and Y. Ochiai, Sub-Half-Micron Lithography for
ULSIs, Cambridge University Press, Cambridge, 2000; (d) R. Spohr, Ion
tracks and Microtechnology: Principles and Applications, Vieweg &
Sons Verlagsgesellschaft mbH, Braunschweig, 1990.
In contrast to the EB-induced reactions of E1 and Z2 crystals,
a salt of E1 and (±)-1-phenylethylamine with a 1+1 molar ratio
(Salt-E1) were stable upon EB-irradiation with doses of up to 10
MGy. This stability of the Salt-E1 with regard to EB is also the
same as found for Salt-E1 upon UV irradiation.9
These EB-induced reactions, which give rise to the same
products as those of the corresponding photoreactions, can be
formulated by a mechanism in which all EB-induced reactions
proceed via the lowest excited state, as is the case of
photoreactions of cinnamic acid derivatives upon UV irradia-
tion.10 Fig. 3 shows a schematic energy diagram of EB-induced
reactions of cinnamic acid crystals to compare with the
corresponding photoreactions. The EB induced reactions are as
follows: (1) radiolysis of molecules in the ground state (CA)
leads primarily to upper exited states of CA (CA**) by direct
excitation or to dissociation into a CA radical cation (CA+·) and
2 (a) X. X. Farhataziz and M. A. J. Rodgers, Radiation Chemistry;
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Carroll, In Radiation Effects on Organic Materials, ed. R. O. Bolt and
J. G. Carroll, Academic Press, 1963, ch. 4, pp. 63–125; (d) A. Hummel,
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Fig. 3 A schematic energy diagram of EB-induced reactions of cinnamic
acid crystals with the corresponding photoreactions. CA, CA*, CA** and
CA+· represent the ground state, the lowest excited state, an upper exited
state, and radical cation of cinnamic acid derivatives (E1, Z2, and Salt-E1),
respectively.
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