Site-selective bimodal absorption and emission of distonic radical cation

Article abstract of DOI:10.1021/jo1003949

Figure presented An acyclic 1,4-distonic dimer radical cation (DAE ₂⁺) was generated from the dimerization of 1,1-bis(4-methoxyphenyl)ethylene radical cation (DAE⁺) with the neutral molecule (DAE) in solution. The absorption spectrum of DAE ₂⁺ shows bimodal absorption bands with peaks at 350 and 500 nm corresponding to the 1,1-bis(4-methoxyphenyl)ethyl radical (An ₂CCH₃) and 1,1-bis(4-methoxyphenyl)ethyl cation (An₂C⁺CH₃), respectively. Therefore, DAE₂⁺ in the ground state has the spin and positive charge localized on the 1- and 4-positions, respectively. The bimodal characteristic emissions by the site-selective excitation of radical and cation sites of DAE₂⁺ were observed at 77 K, showing that the excitation energy is localized on the radical or cation site of DAE ₂⁺ in the excited state. The interaction between radical and cation sites of DAE₂⁺ in the ground and excited states are discussed on the basis of the steady-state spectroscopic and transient absorption measurements, as well as theoretical calculations.

Full text of DOI:10.1021/jo1003949

pubs.acs.org/joc
Site-Selective Bimodal Absorption and Emission of
Distonic Radical Cation
Sachiko Tojo, Mamoru Fujitsuka, and Tetsuro Majima*
The Institute of Scientific and Industrial Research (SANKEN), Osaka University Mihogaoka 8-1, Ibaraki,
Osaka 567-0047, Japan
majima@sanken.osaka-u.ac.jp
Received March 5, 2010
•þ
An acyclic 1,4-distonic dimer radical cation (DAE2 ) was generated from the dimerization of 1,1-
bis(4-methoxyphenyl)ethylene radical cation (DAE•þ) with the neutral molecule (DAE) in solution.
The absorption spectrum of DAE2•þ shows bimodal absorption bands with peaks at 350 and 500 nm
corresponding to the 1,1-bis(4-methoxyphenyl)ethyl radical (An2C•CH3) and 1,1-bis(4-methoxy-
phenyl)ethyl cation (An2CþCH3), respectively. Therefore, DAE2•þ in the ground state has the spin
and positive charge localized on the 1- and 4-positions, respectively. The bimodal characteristic
emissions by the site-selective excitation of radical and cation sites of DAE2•þ were observed at 77 K,
showing that the excitation energy is localized on the radical or cation site of DAE2•þ in the excited
state. The interaction between radical and cation sites of DAE2•þ in the ground and excited states are
discussed on the basis of the steady-state spectroscopic and transient absorption measurements, as
well as theoretical calculations.
Introduction
separately are suggested.2-4 Distonic radical cations have
been suggested and evidenced for many radical cations in the
gas phase by studies using mass spectroscopy and so on.5-8
In solution, although no direct evidence has been shown for
distonic radical cations, various distonic radical cations have
Radical cations have bimodal reactivities as radical and
cation, showing various unimolecular and bimolecular reac-
tions such as isomerization, dimerization, deprotonation,
decomposition, rearrangement, and addition to nucleo-
philes.1 The distributions of spin and charge are generally
overlapped in the radical cations, and the reactivities depend
on the structure and substituent. On the other hand, distonic
radical cations having radical and positive charge sites
(3) Tamai, T.; Ichinose, N.; Tanaka, T.; Sasuga, T.; Hashida, I.; Mizuno,
K. J. Org. Chem. 1998, 63, 3204.
(4) Mizuno, K.; Tamai, T.; Hashida, I.; Otsuji, Y. J. Org. Chem. 1995, 60,
2935.
(5) Stirk, K. M.; Kiminkinen, L. K. M.; Kenttamaa, H. I. Chem. Rev.
1992, 92, 1649.
(1) Schmittel, M.; Burghart, A. Angew. Chem., Int. Ed. Engl. 1997, 36,
2551.
(6) Gozzo, F. C.; Moraes, L. A. B.; Eberlin, M. N.; Laali, K. K. J. Am.
Chem. Soc. 2000, 122, 7776.
(2) Mizuno, K.; Tamai, T.; Hashida, I.; Otsuji, Y.; Kuriyama, Y.;
Tokumaru, K. J. Org. Chem. 1994, 59, 7329.
(7) Hammerum, S. Mass Spectrom. Rev. 1988, 7, 123.
(8) Gronert, S. Chem. Rev. 2001, 101, 329.
3618 J. Org. Chem. 2010, 75, 3618–3625
Published on Web 05/07/2010
DOI: 10.1021/jo1003949
r
2010 American Chemical Society

Tojo et al.
JOCArticle
been speculated to exist as intermediates in solution on the
basis of product analysis.9-11 For example, Newcomb et al.
reported the reversible cyclization of alkene radical cation
via a cyclic distonic radical cation in solution.12 Fewer
examples for the absorption and emission properties of the
distonic radical cation have been reported.
The formation of aromatic distonic radical cation in
solutions is unfavorable without a substituent stabilization
of radical and/or cation sites. For example, introduction of a
p-methoxyl group to an aromatic ring causes the stabiliza-
tion of the cation and radical sites of the distonic radical
cation. Therefore, it is expected that the distonic radical
cation will be formed efficiently when a substrate has ani-
sylmethyl sites even in solution.13,14 Recently, Metzger et al.
reported the formation of an acyclic distonic dimer radical
cation of trans-anethole in solution based on the EESI-MS/
MS measurement.15,16 Takamuku et al. reported that the
regioselective addition of O2 toward trans-4-methoxylstil-
bene radical cation (MOST•þ) occurs rapidly, because of the
charge-spin separation and localization of an unpaired
electron on the β-olefinic carbon, whereas trans-stilbene
radical cation (ST•þ) has little reactivity toward O2.17 Eluci-
dation of the properties of distonic radical cations is neces-
sary for understanding the site selective reactivity of radical
cation.
Fewer physical and chemical properties of excited radical
ions, such as fluorescence, have been revealed. Usually
excited radical ions have weak or nonfluorescent nature
and extremely short lifetimes, typically much shorter than
1 ns.18-20 Only a few radical cations have been reported to
show fluorescence in solution.21,22 Usually, assignment of
fluorescence from radical cations in the excited state is
difficult because of the formation of fluorescent byproduct,
as has been already pointed out by Fox.23
FIGURE 1. Steady-state absorption spectrum of An2C•CH3 and
An2CþCH3 observed after γ-irradiation of An2CHCH3 (10 mM) in
n-BuCl rigid glass at 77 K.
excited states have been extensively studied under various
conditions.24-28 Turro et al. reported the independent for-
mation of diphenylmethyl radical and cation in zeolites
based on spectroscopic measurement.29 No interaction was
observed between the free radical and cation in both the
ground and excited states inside the zeolites.
In particular, a little has been known on the absorption
and emission properties of the distonic radical cation, while
both cooperative and independent interactions have been
suggested on the basis of product analysis. Thus, it is
important to elucidate the interaction of these sites and to
differentiate from the corresponding free radical and cation.
This issue will be clarified by emission spectroscopy. If the
radical and cation sites have no interaction even in the
excited state, bimodal emissions from the radical and cation
sites in distonic radical cation will be observed upon the
site-selective excitation.
In this work, we report the bimodal absorptions and
the emissions from the site-selective excitation of the ra-
dical and cation site of an acyclic 1,4-dimer radical cation,
DAE2•þ, which was indicated as an intermediate to produce
the 1,2-dioxane via addition of molecular oxygen during the
photooxygenation of aromatic alkene radical cation.30 This
is the first example of bimodal emissions of distonic radical
cation.
Contrary to radical cations, the spectroscopic properties
of free-radical or carbon-centered cations in ground and
(9) Bouchoux, G.; Berruyer, F.; Hiberty, P. C.; Wu, W. Chem.;Eur. J.
2007, 13, 2912.
(10) Janovsky, I.; Knolle, W.; Naumov, S.; Williams, F. Chem.;Eur. J.
2004, 10, 5524.
(11) O’Neil, L. L.; Wiest, O. J. Org. Chem. 2006, 71, 8926.
(12) Horner, J. H.; Newcomb, M. J. Org. Chem. 2007, 72, 1609.
(13) Tojo, S.; Morishima, K.; Ishida, A.; Majima, T.; Takamuku, S.
J. Org. Chem. 1995, 60, 4684.
(14) Majima, T.; Tojo, S.; Ishida, A.; Takamuku, S. J. Org. Chem. 1996,
61, 7793.
(15) Marquez, C. A.; Wang, H.; Fabbretti, F.; Metzger, J. O. J. Am.
Chem. Soc. 2008, 130, 17208.
Results and discussion
Absorption and Emission of Free Radical and Cation at
77 K. To observe the absorption and emission of 1,1-bis(4-
methoxyphenyl)ethyl radical (An2C•CH3) and 1,1-bis(4-
methoxyphenyl)ethyl cation (An2CþCH3), we examined γ-
irradiation of n-butyl chloride (n-BuCl) rigid glass contain-
ing An2CHCH3 (1 ꢀ 10-2 M) at 77 K. Figure 1 shows the
absorption spectrum with two peaks (λmax) at 350 and 500
nm assigned to the D2-D0 absorption of An2C•CH3 and the
S1-S0 absorption of An2CþCH3, respectively.31
(16) Meyer, S.; Koch, R.; Metzger, J. O. Angew. Chem., Int. Ed. 2003, 42,
4700.
(17) Majima, T.; Tojo, S.; Ishida, A.; Takamuku, S. J. Phys. Chem. 1996,
100, 13615.
(18) Zimmer, K.; Godicke, B.; Hoppmeier, M.; Meyer, H.; Schweig, A.
Chem. Phys. 1999, 248, 263.
(19) Ishida, A.; Fukui, M.; Ogawa, H.; Tojo, S.; Majima, T.; Takamuku,
S. J. Phys. Chem. 1995, 99, 10808.
(20) Cai, X.; Sakamoto, M.; Fujitsuka, M.; Majima, T. J. Phys. Chem. A
2007, 111, 1788.
(21) Ichinose, N.; Tanaka, T.; Kawanishi, S.; Suzuki, T.; Endo, K.
J. Phys. Chem. A 1999, 103, 7923.
(22) Ichinose, N.; Tojo, S.; Majima, T. Chem. Lett. 2000, 1126.
(23) Breslin, D. T.; Fox, M. A. J. Phys. Chem. 1994, 98, 408.
(24) Das, P. K. Chem. Rev. 1993, 93, 119.
Figure 2a shows the D1-D0 fluorescence spectrum with a
peak at 533 nm and characteristic vibrational bands at the
(25) Boyd, M. K. Mol. Supramol. Photochem. 1997, 1, 147.
(26) Weir, D.; Johnston, L. J.; Scaiano, J. C. J. Phys. Chem. 1988, 92,
1742.
(29) Jockusch, S.; Hirano, T.; Liu, Z.; Turro, N. J. J. Phys. Chem. B 2000,
104, 1212.
(27) Sakamoto, M.; Cai, X.; Fujitsuka, M.; Majima, T. J. Phys. Chem. A
2006, 110, 9788.
(28) Hara, M.; Tojo, S.; Majima, T. J. Phys. Chem. A 2003, 107, 4778.
(30) Fujita, M.; Shindo, A.; Ishida, A.; Majima, T.; Takamuku, S.;
Fukuzumi, S. Bull. Chem. Soc. Jpn. 1996, 69, 743.
(31) Popielarz, R.; Arnold, D. R. J. Am. Chem. Soc. 1990, 112, 3068.
J. Org. Chem. Vol. 75, No. 11, 2010 3619

Tojo et al.
•þ 32 but a σ-dimer
JOCArticle
the naphthalene dimer radical cation (Np2
)
radical cation having σ-bonding character as discussed
below.
•þ
The two bands at 350 and 500 nm of DAE2 showed
different decay kinetics as shown in Figure 4. The absorption
band at 350 nm showed two decay processes. The fast and
slow decay constants were calculated to be 8.9 ꢀ 105 and
<2.0 ꢀ 104 s-1, respectively. On the other hand, a second-
order fit can be applied to the decay of absorption band at
500 nm, indicating that the absorption band at 500 nm
decays bimolecularly by the neutralization with BN•- (eq 6).
•þ
Therefore, it is suggested that DAE2 has two different
chromophores that are spatially separated in DAE2•þ. In
•þ
order to elucidate the site-selective reactivities of DAE2
,
the pulse radiolysis of DAE was performed in the presence of
O2 or methanol (MeOH).
In the presence of O2 under the same experimental condi-
•þ
tions, the absorption band at 350 nm of DAE2 decays
rapidly, accompanied by the concomitant appearance of a
new absorption band at 520 nm (Figure 5).33 The rate
•þ
constant for the reaction of DAE2 with O2 in BN was
determined from the decay at 350 nm and the formation at
520 nm to be kO > 109 M-1 s-1. The similar rate constant
2
(kO > 109 M-1 s-1) for the reaction of An2CH• with O2 in
2
•þ
BN was observed.34 The spectral change of DAE2 in the
presence of O2 clearly indicates the formation of the addition
product between O2 and DAE2•þ with the localized unpaired
electron. The similar phenomenon was observed for the
reaction of trimesitylphosphine radical cation with O2,
where the radical cation having a peak at 600 nm reacts with
O2 to give the peroxyl radical cation with a peak at 570 nm,
assigned to the P-centered cation.35 In addition, the second-
order rate constant of DAE2•þ with O2 was similar to that of
FIGURE 2. Steady-state fluorescence excitation (λem = 530 nm)
and emission (λex = 350 nm) spectra of An2C•CH3 (a) and fluore-
scence and phosphorescence excitation (λem = 490 nm) and emis-
sion (λex = 520 and 630 nm) spectra of An2CþCH3 (b) at 77 K.
trianisylphosphine radical cation with O2 (kO = 1.9 ꢀ 109
2
longerwavelength around550-600 nm (τ = 285ns, φ = 0.18)
by the excitation of An2C•CH3 at 350 nm. Figure 2b shows
the S1-S0 fluorescence spectrum with a peak at 515 nm (τ <
4 ns, φ = 0.27) and the T1-S0 phosphorescence spectrum
with a peak at 600 nm (τ = 2.5 ms) by the excitation
of An2CþCH3 at 490 nm. It was indicated that An2CþCH3
in the singlet excited state transforms partly to that in the
triplet excited state which emits the T1-S0 phosphorescence.
The excitation spectrum of each emission was in good agree-
ment with the absorption spectrum of the corresponding
species.
Formation and Reactivity of 1,4-Distonic Radical Cation.
The transient absorption spectra recorded at various times
after an 8-ns electron pulse during the pulse radiolysis of
DAE in benzonitrile (BN) at room temperature under Ar
atmosphere are shown in Figure 3. The transient absorption
bands of DAE•þ around 390, 590, and 1600 nm were
observed at 50 ns after the pulse. The observed spectrum
was the same as that reported for DAE in 1,2-dichloroethane
at room temperature.30 These bands of DAE•þ decayed with
simultaneous formation of the two bands at 350 and 500 nm
in the same time scale (Figure 3a). The formation rate of 500
nm increased with increasing the concentration of DAE,
indicating that DAE•þ reacts with a neutral DAE to give
DAE dimer radical cation (DAE2•þ) at the dimerization rate
constant of 1.2 ꢀ 109 M-1 s-1 in BN (eq 4). No CR band was
observed for DAE2•þ in the near-infrared region (Figure 3b),
suggesting that DAE2•þ is not a π-dimer radical cation like
M-1 s-1). The absorption spectrum with a peak at 520 nm
observed by the addition of O2 can be reasonably assigned to
the cation site of a peroxyl radical cation of the oxygen
adduct, since the peroxyl radical An2CHOO• generated from
the reaction of free An2CH• with O2 has no absorption in the
visible regions (eq 5).
In the presence of MeOH as a nucleophile under the same
experimental condition, the transient optical densities
(ΔOD) of DAE2•þ decreased because BN•þ is partly trapped
by MeOH. No effect of MeOH was observed on the decay
profile at 350 nm. The decay rate of DAE2•þ at 500 nm was
not accelerated by addition of MeOH. The bimolecular
rate constant was estimated to be kMeOH < 105 M-1 s-1. The
rate constant for the reaction of An2CHþ with MeOH in
BN was determined to be kMeOH = 2.9ꢀ105 M-1 s-1. In
•þ
addition, the decay rate of DAE2 at 500 nm did not
increase with increasing the concentration of DAE as a
nucleophile. The present observations indicated that the
(32) Cai, X.; Tojo, S.; Fujitsuka, M.; Majima, T. J. Phys. Chem. A 2006,
110, 9319.
(33) Pulse radiolysis of 1,1,2,2-tetraanisylethane (TAE) was carried out in
BN at room temperature. TAE•þ undergoes C-C bond cleavage to yield
An2CH• and An2CHþ. In the presence of O2, An2CH• was scavenged by O2,
but the shift of absorption band for free An2CþH was not observed.
(34) The rate constants with O2 could not determine because the transient
absorption bands at 350 nm of DAE2•þ and An2CH• rapidly decayed (<50 ns)
under even the air atmosphere.
(35) Tojo, S.; Yasui, S.; Fujitsuka, M.; Majima, T. J. Org. Chem. 2006, 71,
8227.
3620 J. Org. Chem. Vol. 75, No. 11, 2010

Tojo et al.
JOCArticle
FIGURE 3. Transient absorption spectra obtained at 50 (black circle), 100 (red triangle), and 500 ns (green square) after an electron pulse of
DAE 7.5 mM in BN in the visible region (a) and in the near-infrared region (b).
FIGURE 5. Transient absorption spectra obtained at 50 (black),
250 (red), and 500 ns (green) after electron pulse of DAE 7.5 mM in
BN in the presence of O2. The inset shows the second-order plots for
the decay kinetics at 520 nm (red).
FIGURE 4. Time profiles of the transient absorption at 350 (black)
and 500 nm (red) after an electron pulse of DAE 7.5 mM in BN. The
inset shows the second-order plots for the decay kinetics of DAE2
at 350 (black) and 500 nm (red).
•þ
transient of 500 nm has a rather low reactivity compared with
that of 350 nm.
BN ' BN•þ þ e-
e- þ BN f BN• -
BN•þ þ DAE f DAE•þðAn2CdCH2•þÞ
ð1Þ
ð2Þ
ð3Þ
DAE þ DAE•þ f DAE2•þðAn2C•CH2CH2CþAn2Þ ð4Þ
DAE2•þ þ O2
f 1, 6-peroxyl radical cation ðAn2CðOO•ÞCH2CH2CþAn2Þ
ð5Þ
•þ
DAE2 þ BN• - f neutralization productðsÞ
ð6Þ
FIGURE 6. Steady-state absorption spectra of DAE•þ and DAE2
•þ
observed after γ-irradiation of DAE (10 mM) in n-BuCl rigid glass
under vacuum at 77 K.
DAE•þ was generated by γ-radiolysis (4.5 ꢀ 106 Gy) in
n-BuCl rigid glass at 77 K (Figure 6, black line). The
absorption spectral change was observed upon warming
from 77 to 100 K. The absorption band of DAE•þ disap-
peared upon warming of the rigid glass, together with the
formation of sharp absorption bands at λmax 350 and 500 nm
(Figure 6, red line).
The absorption band at λmax 350 nm is similar to that of
An2C•CH3. On the other hand, the absorption band at λmax
500 nm is similar to that of An2CþCH3. The absorption
•þ
spectrum of DAE2 can be explained on the basis of the
superposition of the absorption of dianisylethyl radical
(An2C•CH3, 350 nm) and dianisylethyl cation (An2CþCH3,
500 nm) chromophores, indicating that the radical and
cation sites are distributed to subunits I and II, respectively
n-BuCl ' n-BuCl•þ þ e-
n-BuCl•þ þ DAE f DAE•þ
ð7Þ
ð8Þ
•þ
(Scheme 1). These results suggest that DAE2 generated
J. Org. Chem. Vol. 75, No. 11, 2010 3621

Tojo et al.
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FIGURE 7. Steady-state absorption spectrum of 1,6-peroxyl radi-
cal cation of DAE2•þ (An2C(OO•)CH2CH2CþAn2) observed after
γ-irradiation of DAE (10 mM) in n-BuCl rigid glass in the presence
of O2 at 77 K.
FIGURE 8. Steady-state fluorescence excitation (λem = 530 nm)
•þ
and emission (λex = 350 nm) spectra of radical site of DAE2
(An2C•H) at 77 K.
SCHEME 1. Generation of Distonic Radical Cations from the
Dimerization of DAE•þ
through C-C bond formation between β-carbons of DAE is
a 1,4-distonic radical cation with an acyclic linear structure
(Scheme 1).
In the presence of O2, the radical site with λmax at 350 nm
disappeared upon warming of the rigid glass. The radical site
of DAE2•þ was scavenged by O2 completely to form the new
absorption band with a peak at 520 nm and weak absorption
around 350 nm (Figure 7), which is consistent with the results
of the pulse radiolysis at room temperature. This result
suggests that the absorption band at λmax 520 nm is assigned
to the cation site of 1,6-distonic peroxyl radical cation with
an acyclic structure formed by the addition of O2 to subunit I
of radical site of DAE2•þ (Scheme 1).
FIGURE 9. Steady-state fluorescence (black) and phosphorescence
(red) excitation (λem = 500 nm) and emission (λex = 520 and 630 nm)
spectra of cation site of DAE2•þ (An2CþH) at 77 K.
φ = 0.18), showing that the selective excitation of the radical
site of DAE2•þ results in the emission from the radical site.
Therefore, the excitation energy is localized on the radical
site of DAE2•þ, and no excitation energy transfer occurs
•þ
from the radical site to the cation site in DAE2
.
Reactivity of 1,6-Peroxyl Distonic Radical Cation. As
shown in the inset of Figure 5, the second-order decay of a
1,6-peroxyl distonic radical cation was similar to that of the
cation site of DAE2•þ. In order to elucidate the reactivity of
1,6-peroxyl distonic radical cation, pulse radiolysis of DAE
was performed in the presence of MeOH. In the presence of
MeOH, decay of the cation site of the 1,6-peroxyl distonic
radical cation monitored at 520 nm was not accelerated by
The fluorescence with a λf at 520 nm (τf < 4 ns, φ = 0.12)
and the phosphorescence with a peak (λp) at 630 nm (τp = 2.5
ms) were observed by the selective excitation of the cation
site at 490 nm, tuned to the absorption of the cation site of
•þ
DAE2 (Figure 9). The properties of fluorescence and
phosphorescence generated by the selective excitation of
the cation site of DAE2•þ were similar to those of An2CþCH3
(λf = 515 nm, τf < 4 ns, φ = 0.27, λp = 600 nm, and τp = 2.5
ms). The excitation spectrum with peaks at 350 and 500 nm
was observed for the fluorescence at 520 nm and the phos-
phorescence at 600 nm. Ikeda et al. reported that diphenyl-
ethyl cation possessed absorption bands with two peaks at
314 and 424 nm.36 The results also suggest that the observed
weak excitation band at 350 nm is assigned to the cation site
rather than the radical site of DAE2•þ, which is also sup-
ported by the fact that the phosphorescence was not ob-
served by the selective excitation of the radical site of
DAE2•þ. It was indicated that no interaction exists between
the radical and cation sites of DAE2•þ in the excited state.
addition of MeOH (kMeOH < 105 M-1 s-1). This phenom-
•þ
enon was similar to that of the cation site of DAE2
.
Emission of 1,4-Distonic Radical Cation at 77 K. Generally
radical cations have no emission. Similarly, no emission was
observed with DAE•þ generated by the γ-irradiation in n-
BuCl rigid glass at 77 K. On the other hand, the emissions
dependent on the excitation wavelength were observed with
•þ
DAE2 at 77 K. The fluorescence spectrum displayed a
peak (λf) at 533 nm and characteristic vibrational bands
around 550-600 nm (τf = 189 ns, φ = 0.12) by the selective
•þ
excitation of the radical site of DAE2 (λmax = 350 nm)
(Figure 8). The fluorescence properties were similar to those
of An2C•CH3 (λf = 500 and 520-600 nm, τf = 285 ns,
(36) Ikeda, H.; Tanaka, F.; Kabuto, C. Tetrahedron Lett. 2005, 46, 2663.
3622 J. Org. Chem. Vol. 75, No. 11, 2010

Tojo et al.
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CHART 1
•þ
It is concluded that the radical and cation sites in DAE2
can be selectively excited by the excitation at 350 and 490 nm,
respectively (Chart 1). The excitation at 350 nm generates the
D1-D0 fluorescence after the internal conversion from the
initially generated the D2 state of radical site of DAE2•þ. The
excitation at 490 nm generates the S1-S0 fluorescence and
T1-S0 phosphorescence from the cation site of DAE2•þ, indi-
cating the contribution of the intersystem crossing from S1 to
T1 of cation site.
FIGURE 10. Steady-state fluorescence excitation (λem = 520 nm)
and emission (λex = 560 nm) spectra of cation site of 1,6- peroxyl
radical cation of DAE2•þ at 77 K.
derivative substituted with a p-methoxy group such as An2-
CHCH3•þ with MeOH.38 Similarly, the reactivity of MOST•þ
with MeOH was lower than that of ST•þ in BN.39 This result
suggests that the positive charge in the cation site of DAE2•þ is
delocalized on two 4-methoxyphenyl groups (subunit II in
Scheme 1). These results suggest that no interaction exists
Emission of 1,6-Distonic Peroxyl Radical Cation at 77 K. In
the presence of O2, the S1-S0 fluorescence with a peak at 560
nm for the cation site was clearly observed without inter-
ference of emission from the radical site of a 1,6-distonic
peroxyl radical cation (Figure 10) upon the selective excita-
tion at 520 nm, tuned to the absorption of the cation site of a
1,6-distonic peroxyl radical cation. The fluorescence peak
shifted to wavelength longer than that of An2CþCH3. The
excitation spectrum is in good agreement with the absorption
spectrum observed. The T1-S0 phosphorescence was not
observed because of the quenching of the triplet excited state
by O2.
•þ
between the radical and cation sites of extended acyclic DAE2
•þ
in the ground state. DAE2 exhibits the site-selective and
bimodal reactivities at the radical and cation sites, indepen-
dently. DAE2•þ reacts selectively as either radical or carboca-
tion, depending on the choice of the neutral reaction partner.
The spin is localized on the C1 of subunit I, and positive charge
is delocalized in subunit II in the ground state, respectively.
The φf values for the excited radical and cation sites of
DAE2•þ were slightly smaller than those of An2CH•CH3 and
An2CHþCH3, respectively. However, the fluorescence and
phosphorescence spectra obtained by the site-selective ex-
citation of DAE2•þ were almost equivalent with those of the
free radical and cation. These results suggest that no inter-
Properties of Distonic Radical Cations in the Ground and
Excited States. The present experimental results proved that
two chromophores of An2C• and An2Cþ linked by an ethylenyl
•þ
group were independent in DAE2 as a 1,4-distonic radical
cation. The absorption and emission positions of the radical
(An2C•) and cation (An2Cþ) chromophores are summarized in
Table 1.
•þ
action exists between the radical and cation sites of DAE2
in the excited states, too.
The absorption bands with λmax at 350 and 500 nm of
DAE2 are similar to those of the free radical and cation,
•þ
The optimized structure and spin distribution of 1,6-
distonic peroxyl radical cation calculated using the density
functional theory is shown in Figure 11. The C1-C2-C3-C4
dihedral angle of 173.4° indicates the extended acyclic struc-
ture.
The calculated spin (F) and positive charge (q) densities of
the 1,6-peroxyl radical cation of DAE2•þ are shown in Figure
12. The spin densities at the two oxygen atoms of peroxyl
group are þ0.654 and þ0.315, respectively, suggesting that
the unpaired electron is mainly localized on the peroxyl
moiety. In addition, the sum of the positive charge densities
for subunit II is þ0.768, indicating that positive charge is
mainly delocalized on C4 and two 4-methoxyphenyl groups
on subunit II.
respectively. Takamuku et al. observed an unusually long
wavelength absorption band at 505 nm when 4-methoxystyrene
and 1,2-bis(4-methoxyphenyl)cyclobutane were independently
subjected to pulse radiolysis in 1,2-dichloroethane.37 They
assigned the band to the 1,4-diarylbutane-1,4-diyl radical ca-
tion in which radical site (AnC•H) and cation site (AnCþH) do
not exist separately. However, the absorption spectra of
DAE2•þ have dual characteristics that correspond to both the
radical and the cation subunits. The bimodal reactivities of
DAE2•þ toward O2 and MeOH were similar to those of the free
An2CH• and An2CHþ. The reactivity toward O2 for DAE2•þ is
higher than that for the general aromatic radical cation. This
result suggests that an unpaired electron is localized on the sp3
carbon in the radical site (subunit I in Scheme 1). On the other
hand, the low electrophilic reactivity of the cation site of
In the case of the cation sites of the 1,6-distonic peroxyl
radical cation, the absorption and fluorescence peaks shifted
to a wavelength longer than those of An2CþCH3. This
shift was not observed for An2CþCH3 in the presence of
O2. The absorption and fluorescence peaks of the cation site
•þ
DAE2 is similar to that of an aromatic radical cation
(37) Tojo, S.; Toki, S.; Takamuku, S. J. Org. Chem. 1991, 56, 6240.
(38) Pulse radiolyses of An2CHCH3 was carried out in Ar-saturated B•Nþ
at room temperature. The transient absorption band of An2CHCH3
around 450 nm was observed. The heterolytic and homolytiþc cleavage of
the C-H bond of An2CHCH3•þ to form An2C•CH3 and An2C CH3 did not
occur. The decay of An2CHCH3•þ at 450 nm was not accelerated by addition
of MeOH (750 mM) (kMeOH < 105 M-1 s-1).
(39) Pulse radiolyses of MOST and ST were carried out in Ar-saturated
BN at room temperature. Recations of MOST•þ and ST•þ with MeOH occur
with rate constant of kr = 9.5 ꢀ 105, and 1.7 ꢀ 106 M-1 s-1, respectively.
J. Org. Chem. Vol. 75, No. 11, 2010 3623

Tojo et al.
JOCArticle
TABLE 1. Absorption and Emission Peaks (λabs and λem, Respectively) of Free Radical, Free Cation, and Distonic Radical Cation
radical chromophore (An2C•)
cation chromophore (An2Cþ)
λabs/nm
λem/nm
λabs/nm λem/nm
free radical and cation
350
533, 560
(τ = 285 ns, φ = 0.18)
500
515 (fluorescence)
(τ < 4 ns, φ = 0.27)
600 (phosphorescence)
535(fluorescence)
(τ < 4 ns, φ = 0.12)
630 (phosphorescence)
560 (fluorescence)
distonic 1,4-radical cation
distonic 1,6-peroxyl radical cation
350
nd
530, 560
(τ = 189 ns, φ = 0.12)
500
nd
350, 520
cations in the ground and excited states, which are classified
to bimodal reactive species in contrast to usual reactive
species, such as free radical and cation. This is the first
example to show the bimodal absorption and emissions of
acyclic distonic radical cation in which spin and positive
charge are separately localized. It is found that the electronic
interaction between bond-deficient groups such as radical
and cation linked by an ethylene chain is not strong in the
ground and excited states. Therefore, our results provide
meaningful information not only for a new perspective on
radical cation but also for the molecular design of functio-
nalized intermediates, such as a distonic radical cation with
unique electronic structure.
Experimental Section
Materials. 1,1-Bis(4-methoxyphenyl)ethylene (DAE) was
prepared by a Wittig reaction of (p-CH3OC6H4)2CO with
(C6H5)3PdCH2 and purified by column chromatography
(silica gel) and recrystallization from n-hexane. HPLC grade
BN was used as solvent without further purification. n-BuCl was
shaken with concentrated sulfuric acid, washed with water,
dried over calcium chloride, and fractionally distilled.
FIGURE 11. Optimized structure of 1,6-peroxyl radical cation of
DAE2•þ obtained by the density functional theory at the UB3LYP/
6-31G* level.
Pulse Radiolysis. Pulse radiolysis experiments were per-
formed using an electron pulse (28 MeV, 8 ns, 0.87 kGy per
pulse) from a linear accelerator at Osaka University. All experi-
ments were performed in PhCN saturated with argon for 20 min.
A 1-mL solution was placed in a quartz cell (10 mm ꢀ 5 mm ꢀ 40
mm) and sealed with a silicon rubber stopper. The kinetic
measurements were performed using a nanosecond photoreac-
tion analyzer system (Unisoku, TSP-1000). The monitor light
was obtained from a pulsed 450-W Xe arc lamp (Ushio, UXL-
451-0), which was operated by a large current pulsed power
supply that was synchronized with the electron pulse. The
monitor light was passed through an iris with a diameter of
0.2 cm and sent into the sample solution at a perpendicular
intersection to the electron pulse. The monitor light passing
through the sample was focused on the entrance slit of a
monochromator (Unisoku, MD200) and detected with a photo-
multiplier tube (Hamamatsu Photonics, R2949). The transient
absorption spectra were measured using a photodiode array
(Hamamatsu Photonics, S3904-1024F) with a gated image
intensifier (Hamamatsu Photonics, C2925-01) as a detector.
The kinetic traces in the near-infrared region were estimated
using a fast InGaAs PIN photodiode with a monochromator
(Horiba, TRIAX190) and the transient digitizer. The total
system was controlled with a personal computer via GPIB
interface.
FIGURE 12. Select spin and charge densities (F and q, respectively)
of 1,6-distonic peroxyl radical cation of DAE2 obtained by the
density functional theory at UB3LYP/6-31G* level.
•þ
of a 1,6-distonic peroxyl radical cation shifted to wave-
lengths longer than those of the 1,4-distonic radical cation.
These behaviors perhaps suggest that slight through-space
electronic interaction exists between an unpaired electron on
oxygen and the cation site of the 1,6-distonic peroxyl radical
cation in the ground and excited states. However, the spin
and positive charge are separated on the peroxyl oxygen of
subunit I and in subunit II, respectively.
Consequently, 1,4- and 1,6-acyclic distonic radical cations
display characteristic distonic structures with separated radi-
cal and charge sites in the ground and excited states.
γ-Radiolysis. The γ-radiolysis of the rigid glass of degassed n-
butyl chloride solutions was carried out in 2 mm thick Suprasil
cells in liquid nitrogen at 77 K by a 60Co γ source (dose, 4.5 ꢀ 106
Gy). The absorption spectra were measured in a Dewar vessel at
77 K and also during the annealing up to approximately 100 K
by a multichannel photodetector (Shimadzu, MultiSpec-1500).
Conclusions
Absorption and emission measurements enabled us to
confirm the properties of aromatic acyclic distonic radical
3624 J. Org. Chem. Vol. 75, No. 11, 2010

Tojo et al.
JOCArticle
The fluorescence and phosphorescence spectra were measured
at 77 K by a fluorescence spectrophotometer (Hitachi, 850). The
fluorescence lifetime was measured at 77 K by a single photon
counting (Horiba, NAES-1100).
of Education, Culture, Sports, Science and Technology
(MEXT) of Japanese Government. S.T. thanks the Special
Coordination Funds for Promoting Science and Technology
from MEXT for the Osaka University Program for the
Support of Networking among Present and Future Women
Researchers. T.M. thanks the WCU (World Class University)
program through the National Research Foundation of Korea
funded by the Ministry of Education, Science and Technology
(R31-10035) for support.
Theoretical Calculations. Optimized structure of 1,6-peroxy
radical cation of DAE2 was estimated by density functional
þ
theory at the UB3LYP/6-31G* level using the Gaussian03
package.
Acknowledgment. We thank the members of the Research
Laboratory for Quantum Beam Science of ISIR, Osaka Uni-
versity for assistance in pulse radiolysis and γ-ray radiolysis
experiments. This work has been partly supported by a Grant-
in-Aid for Scientific Research (Projects 17105005, 21350075,
22245022, Priority Research (477) and others) from the Ministry
Supporting Information Available: Optimized molecular
structure of 1,6-distonic peroxyl radical cation of DAE2•þ. This
material is available free of charge via the Internet at http://
pubs.acs.org.
J. Org. Chem. Vol. 75, No. 11, 2010 3625