Chiral organic radical cation and dication. A reversible chiroptical redox switch based on stepwise transformation of optically active tetrakis(p-alkoxyphenyl)ethylenes to radical cations and dications

Article abstract of DOI:10.1021/jp044917m

Optically active tetrakis(p-alkoxyphenyl)ethylenes were found to function as reversible chiroptical switches upon redox transformations. Successive one-electron oxidations of chirally modified tetraarylethylene to the corresponding radical cation and then to the dication led to dramatic changes in the electronic absorption and circular dichroism (CD) spectra. The neutral species showed no color or CD in the visible region, while the radical ion was blue in color and exhibited a weak Cotton effect, with the dication green and giving an intense Cotton effect and a sign opposite that observed for the radical cation, at a longer wavelength. Molecular orbital calculations and X-ray crystallographic studies clearly indicate that the olefinic C=C bond is significantly twisted in the dication to minimize the electrostatic and steric repulsions. By lowering the temperature of the dication, the twist around the double bond is more firmly fixed in either P or M chirality to give a stronger Cotton effect and a larger anisotropy (g) factor. Since the spectral changes are completely reversible and reproducible for multiple redox cycles, this chiral redox system can be used in novel redox-driven chiroptical applications, such as molecular switches and memory devices, in which the information is written/read chiroptically in the ternary mode, giving zero CD signal in the neutral form, positive CD for the radical cation, and negative CD for the dication at a given wavelength.

Full text of DOI:10.1021/jp044917m

2728
J. Phys. Chem. A 2005, 109, 2728-2740
Chiral Organic Radical Cation and Dication. A Reversible Chiroptical Redox Switch Based
on Stepwise Transformation of Optically Active Tetrakis(p-alkoxyphenyl)ethylenes to
Radical Cations and Dications
Tadashi Mori*,† and Yoshihisa Inoue*,†,‡
Department of Molecular Chemistry, Osaka UniVersity, 2-1 Yamada-oka, Suita 565-0871,
Entropy Control Project, ICORP, JST, 4-6-3 Kamishinden, Toyonaka 560-0085, Japan
ReceiVed: NoVember 5, 2004; In Final Form: December 21, 2004
Optically active tetrakis(p-alkoxyphenyl)ethylenes were found to function as reversible chiroptical switches
upon redox transformations. Successive one-electron oxidations of chirally modified tetraarylethylene to the
corresponding radical cation and then to the dication led to dramatic changes in the electronic absorption and
circular dichroism (CD) spectra. The neutral species showed no color or CD in the visible region, while the
radical ion was blue in color and exhibited a weak Cotton effect, with the dication green and giving an
intense Cotton effect and a sign opposite that observed for the radical cation, at a longer wavelength. Molecular
orbital calculations and X-ray crystallographic studies clearly indicate that the olefinic CdC bond is significantly
twisted in the dication to minimize the electrostatic and steric repulsions. By lowering the temperature of the
dication, the twist around the double bond is more firmly fixed in either P or M chirality to give a stronger
Cotton effect and a larger anisotropy (g) factor. Since the spectral changes are completely reversible and
reproducible for multiple redox cycles, this chiral redox system can be used in novel redox-driven chiroptical
applications, such as molecular switches and memory devices, in which the information is written/read
chiroptically in the ternary mode, giving zero CD signal in the neutral form, positive CD for the radical
cation, and negative CD for the dication at a given wavelength.
Introduction
skeleton, consistently afford relatively low oxidation potentials
of 0.98-1.45 V (vs SCE) and are readily oxidized to radical
cations in quantitative yield.11 Tetraarylethylenes are of par-
ticular interest, possessing mutually interacting aromatic and
olefinic moieties as electron reservoirs in a single molecule.
As a consequence of the low oxidation potentials, most tetra-
arylethylenes undergo a variety of chemical transformations such
as reduction, oxidation, photocyclization, halogenation, and
oxygenation,12,13 precise control of which is, in general, fairly
difficult. In contrast, tetraanisylethylene exhibits a more control-
lable oxidation behavior, which allows us to successively oxidize
it to the radical cation and then to the dication by choosing the
appropriate oxidant. As a result, one can isolate two oxidized
species, i.e., the radical cation and dication, which are reasonably
stable and suited for conventional analysis by X-ray crystal-
lography.14 By replacing all or some of the methoxy groups of
tetraanisylethylene with chiral alkoxyl groups, a reversible chiral
redox molecular system is obtained without seriously altering
the original spectral and redox properties as well as the reactivity
and stability.
Chiroptical molecular devices for molecular switches and data
storage systems have recently attracted much attention.1-3 For
these purposes, a variety of photochromic1,4,5 and electochromic6-8
molecules, which show significant spectral changes upon photo-
and electrochemical transformations, have been proposed.
Chiroptical molecular redox switches require thermal stability
and photo/electrochemical sensitivity and reversibility. Apart
from the chiroptical property switching upon redox reaction of
binaphthyl boron dipyrromethane,6 cyclohexanediol bispyrene
ester,7 and dihydro[5]helicene8 reported recently, practically no
systematic study has hitherto been done to directly relate the
stereochemical and electronic structural changes with the
resulting chiroptical properties. Hence, it is highly desirable to
elucidate the relationship between structure and its chiroptical
consequences to develop a new strategy for systematically
modulating the chiroptical properties of molecular switches. In
the present study, we have chosen a series of chirally modified
tetraarylethylenes as redox-sensitive substrates for systematically
elucidating the structure-chiroptical property relationship, and
also as candidates for redox-driven chiroptical switches.
It is likely that the through-space interaction between a pair
of vicinal aromatic rings in di- and tetraarylethylenes is
responsible for the lowered oxidation potentials and the
enhanced electron-transfer reactivity.9 Consequently, the ioniza-
tion potentials of (Z)-stilbene derivatives are much lower than
those of the corresponding (E)-stilbenes.10 Similarly, 2,3-
diarylbicyclo[2.2.2]oct-2-enes, possessing a fixed (Z)-stilbene
The exciton chirality method is a versatile spectroscopic tool
for elucidating the absolute configuration and conformation of
chiral organic compounds.15 It is well documented that most
chirally modified benzenes exhibit only week Cotton effect
peaks of low molar circular dichroism (CD) intensities (∆ꢀ) on
the order of 10-2 16
. This is also the case with aromatic radical
cations; we have recently shown that the radical cations of
simple chiral benzene derivatives give only very weak CD
intensities.17 As the amplitude of the exciton couplet (A) is
proportional to the extinction coefficient (ꢀ) and inversely
proportional to the square of the distance between the coupling
chromophores, the use of chiral stilbene and tetraarylethylene
* To whom correspondence should be addressed. E-mail: tmori@
chem.eng.osaka-u.ac.jp.
† Osaka University.
‡ JST.
10.1021/jp044917m CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/09/2005

Tetraarylethylene Chiroelectrochemical Switch
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2729
SCHEME 1: Syntheses of Chiral 2,3-Diarylbicyclo[2.2.2]oct-2-ene 1 and Radical Cation 1•+
derivatives, possessing two or more chromophores in a single
molecule, is advantageous for sensitive detection of the chi-
roptical signals of radical cationic and dicationic species.
Taking into account all the foregoing observations and
discussion, we have employed a series of chirally modified
tetraarylethylenes (2-4) and stilbene 1 (as a reference) as
sources of chiral radical cations and dications and examined
their chiroptical properties for the first time. These tetraaryl-
ethylenes are readily oxidized successively to the corresponding
radical cations and dications in a controlled fashion by choosing
appropriate oxidants or applying a controlled electrostatic
potential. Since the chiral radical cations and dications produced
are stable and the redox processes reversible if these chiral
species display completely different UV-vis and particularly
CD spectra, this system is expected to function as a redox-driven
chiroelectrochemical ternary switch.
stilbenoid donor 1, since the methoxy analogue 2,3-di-p-
anisylbicyclo[2.2.2]oct-2-ene is known to give a stable radical
cation upon mild oxidation. Chiral stilbene 1 was synthesized
by the Grignard coupling of chiral (R)- or (S)-4-(2-methylpro-
pyloxy)phenylmagnesium bromide with 2,3-dibromobicy-
clooctene (Scheme 1).
The UV-vis and CD spectra of the neutral donor were
recorded at 25 °C for a 10-5 M dichloromethane solution. As
can be seen from Figure 1, the antipodal (R,R)- and (S,S)-1
afforded almost mirror-imaged CD spectra, exhibiting first
negative/positive and second positive/negative Cotton effects,
which coincide in wavelength with the 1Lb and 1La bands,
1
respectively. A weak molar CD (∆ꢀ ) (0.05) for the Lb
transition is typical of chiral benzene derivatives15,16 (Table 1),
1
1
with no excitation couplets observed for either the Lb or La
bands of donor 1, most likely due to the parallel orientation of
the two transition moments of the aromatic rings.
Results and Discussion
Stilbenoid 1 was readily oxidized by using a variety of
oxidants. Electrochemical oxidation of donor 1 in anhydrous
Redox and Chiroptical Properties of Stilbenoid Donor 1.
We first examined the redox and chiroptical behavior of chiral
dichloromethane occurred reversibly at a potential of E0
)
OX
Figure 1. (Left) UV and CD spectra and anisotropy (g) factors of neutral (a) (R,R)-1 and (b) (S,S)-1 in dichloromethane at 25 °C. (Right) UV-vis
and CD spectra and g factors of radical cations (a) (R,R)-1•+ and (b) (S,S)-1•+ in dichloromethane at 25 °C, obtained by treating 1 (10-4 M) with
1.5 equiv of Et3O+SbCl6
.
-

2730 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Mori and Inoue
TABLE 1: Chiroptical Properties of the Chiral Radical Cations and Dications of 1-4a
λ
max/nm
λext/nm
compd
1
config
oxidant
speciesb
N
(log ꢀ)c
(∆ꢀ)d
10-6 ge
R,R
294 (4.15)
241 (4.31)
302 (-0.048)
241 (+0.099)
700 (-0.002)f
560 (+0.002)
483 (-0.002)
402 (+0.006)
297 (+0.050)
239 (-0.098)
700 (+0.002)f
583 (-0.002)
482 (+0.001)
418 (-0.002)
337 (+0.315)
297 (-0.612)
263 (+0.852)
700 (+0.092)f
582 (+0.183)
371 (+0.290)
-9.7
+4.8
-100f
+7.0
-1.4
+1.6
+6.9
+4.9
+90f
-0.9
+4.3
-5.1
+15
Et3O+SbCl6
RC
>900f
-
-
554 (3.46)
485 (3.30)g
398 (3.56)
293 (4.14)
241 (4.28)
S,S
N
Et3O+SbCl6
RC
>900f
554 (3.45)
485 (3.29)g
398 (3.66)
327 (4.29)
2
3
4
R,R,R,R
N
-64
261 (4.55)
+25
Et3O+SbCl6
SbCl5
RC
>900f
+5.5f
+12
-
582 (4.17)
360 (4.35)
271 (4.52)
660 (4.50)g
577 (4.65)
471 (4.40)
422 (4.43)
326 (4.19)
300 (4.16)g
262 (4.51)
+14
DC
646 (-1.872)
592 (-0.923)
470 (+0.735)
412 (-0.619)
333 (-0.506)
300 (+1.706)
265 (-1.541)
700 (-0.245)f
∼580 (-0.047)
373 (-0.578)
305 (+0.683)
661 (+1.341)
579 (-1.109)
471 (-1.925)
404 (+0.475)
335 (+0.255)
256 (+0.547)
700 (-0.013)
576 (+0.052)
349 (+0.245)
+64
-22
+29
-25
-51
+118
-75
-26f
-7.4
-27
+56
+112
-64
-193
+60
+18
+20
-2.1
+5.2
+12
S,S,S,S
N
Et3O+SbCl6
SbCl5
RC
>900f
586 (3.77)
329 (4.35)
-
DC
670 (4.01)g
579 (4.24)
475 (3.96)
423 (3.98)
326 (4.21)
260 (4.47)
R,R
N
Et3O+SbCl6
SbCl5
RC
>900f
-
576 (4.00)
352 (4.27)
264 (4.60)
650g (4.51)
DC
659 (-1.173)
594 (-0.305)
572 (+0.123)
460 (+0.686)
403 (-0.094)
-42
-9.4
+2.7
+24
-8.7
572 (4.66)
465 (4.46)
417 (4.51)
a All spectra obtained in dichloromethane at 25 °C under an Ar atmosphere. b Oxidation state: N, neutral; RC, radical cation; DC, dication.
c Electronic absorption maxima λmax and molar extinction coefficient ꢀ in M-1 cm-1 d Circular dichroism extrema λext and molar circular dichroism
.
∆ꢀ in M-1 cm-1 e Anisotropy factor g ()∆ꢀ/ꢀ) at λext, unless noted otherwise. f Peak observed at >900 nm, and therefore, the ∆ꢀ and g values at
.
700 nm are reported. g Shoulder.
1.02 V (vs SCE). A stable magenta solution of radical cation
was obtained on a preparative scale by anodic oxidation of 1 in
anhydrous dichloromethane containing 0.2 M tetrabutylammo-
nium hexafluorophosphate as a supporting electrolyte, using a
1 × 1 cm2 platinum working electrode. Alternatively, the radical
cation was produced upon treatment with a mixture of chloranil
and methanesulfonic acid in dichloromethane.18 More conve-
niently, the dark red radical cation was obtained by mixing 1
(0.1 mM) with triethyloxonium hexachloroantimonate (1.5
equiv) in anhydrous dichloromethane at 0 °C.19 A UV-vis
analysis of the resulting solution revealed the quantitative
formation of the radical cation (Figure 1). Major new bands
attributable to 1•+ appeared at 398, 485 (sh), 554, and >900
nm. The last two bands are slightly red-shifted from those of
the dianisyl analogue (λ ) 547 and 880 nm) reported earlier,11
but the general profile is consistent with the absorption
characteristics of (Z)-stilbene radical cations.20
very weak in intensity, yet shows appreciable mirror-imaged
deviations from the baseline to the negative/positive direction
for (R,R)- and (S,S)-1•+ at a wavelength >700 nm with |∆ꢀ| <
0.005. The anisotropy factors (g ) ∆ꢀ/ꢀ), shown in Figure 1,
also exhibit mirror-imaged behavior although the data are
moderately scattered. The g factors for neutral donor 1 and the
corresponding radical cation 1•+ were on the order of 10-5
,
which is almost similar to or slightly smaller than the ordinary
values obtained for the allowed transitions of simple chirally
substituted arenes.15,16 It is interesting to note that the g factor
of the radical cation was twice as large as that of the
corresponding neutral donor. For the radical cations, the g factor
was only detectable for the d0-d1 band, and was almost
negligible at other transition wavelengths. Although we cannot
elucidate the exact reason(s) for this very weak CD intensity of
radical cation 1•+ at present, the twist angle around the original
olefinic bond would seem to be responsible at least in part.
Recent studies of radical cationic bicyclo[2.2.2]oct-2-ene by
EPR spectroscopy as well as by DFT calculations revealed that
As can be seen from Figure 1, the CD spectrum of the
dichloromethane solution of the dark-red radical cation 1•+ is

Tetraarylethylene Chiroelectrochemical Switch
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2731
CHART 1: Chiral Tetraarylethylene Donors 2-4 and
the Parent Tetraanisylethylene 5
the original double bond is twisted by 11-12°, while dimeth-
ylation at the 2,3-positions of bicyclo[2.2.2]oct-2-ene reduces
the twist angle of the radical cation significantly to 6°.21 2,3-
Diarylation of 1 may similarly reduce the twist angle, since it
has been demonstrated that the amplitude of exciton coupling
(A) is maximized at a projection angle of ∼70°, but is reduced
to almost zero if the angle is 0° or 180°.22,23
Figure 2. Cyclic voltammograms (left, scanning rate 0.2 V s-1, Pt
electrode) and Osteryoung square-wave voltammograms (right) of 1
mM chiral tetraarylethylenes (a) 2, (b) 3, and (c) 4 in dichloromethane
containing 0.2 M tetrabutylammonium hexafluorophosphate as an
electrolyte at 25 °C.
Oxidation of Tetraarylethylene Donors 2-4. Recently,
tetraarylethylenes, such as tetraanisylethylene and tetratolyleth-
ylene, were successfully isolated as crystalline salts in neutral,
radical cation, and dication forms.14 In this study, to explore
redox-active chiroptical switches, we prepared a series of
chirally modified tetraarylethylenes (2-4). By the Mitsunobu
coupling24 of (S)-(+)-2-butanol or (R)-(-)-2-octanol with di-
hydroxybenzophenone, we synthesized the corresponding enan-
tiopure dialkoxybenzophenones, which were subjected in turn
to the McMurry coupling using a conventional low-valent
titanium reagent25 (Chart 1). This allows us, by simple modi-
fications of the peripheral substituents, to directly compare the
steric effects on the chiroptical properties in different redox
states. These tetraarylethylenes 2-4 are expected to be readily
oxidized or reduced in a stepwise manner by conventional
chemical oxidants or reductants, as is the case with tetraanisyl
derivative 5.14 Hence, we first examined the electrochemical
behavior of 2-4 by means of cyclic voltammetry (CV) to gain
quantitative information for these redox systems.
TABLE 2: Disproportionation Constants for
Tetraarylethylene Cation Radicals (Kdisp) Determined by
Electrochemical Analysis of the Cyclic Voltammetric Data
compd solvent E0OX(I)a E0OX(II)a ∆Eb/mV
Kdispc/M-1
2
3
4
5
CH2Cl2 0.83
CH2Cl2 0.84
CH2Cl2 0.86
CH2Cl2 0.79
MeCNe 0.90
0.90
0.90
0.96
0.91
0.90
66
62
96
120
0
7.6 × 10-2
8.9 × 10-2
2.4 × 10-2
9.3 × 10-3 (2 × 10-3 d
)
8.4 × 10-1
a The first and second oxidation potentials shown in volts versus
SCE, obtained by Osteryoung square-wave voltammetry. See ref 28.
b ∆E ) E0OX(II) - E0OX(I). c Calculated from the cyclic voltammetric
data using the Nernstian expression Kdisp ) exp(-0.039∆E) at 25 °C.
d Value determined by the spectroscopic method; for details, see ref
14. e Reference 29.
The first and second oxidation potentials of 2-4 were
separated by only 62-96 mV (Table 2). A qualitative treatment
of these two oxidation potentials gave disproportionation
constants of the relevant radical cations, Kdisp ) [R2+][R]/[R•+]2
(R ) 2, 3, or 4), shown in Table 2. The small Kdisp values of
0.02-0.09 M-1 clearly indicate that the radical cation R•+ is
the dominant species in dichloromethane solutions of R•+. The
difference between the two oxidation potentials (∆E0OX) de-
creases as the size of the (chiral) alkyl substituents becomes
larger: from methyl (5, ∆E ) 0.12 V) to methyl/1-methylpropyl
(4, ∆E ) 0.10 V) to 1-methylpropyl (2, ∆E ) 0.07 V), and
then to 1-methylheptyl (3, ∆E ) 0.06V). This may be attributed
to the increased hydrophobicity caused by elongating the alkyl
chains, which disturbs in particular the first oxidation step, while
the E0OX(II) values are comparable for most tetraarylethylenes
except the unsymmetrically substituted 4.
Chemical Oxidation of 2-4 to Radical Cations and
Dications. Possessing a reduction potential of 1.11 V, the robust
radical cation salt of methanoanthracene derivative 6•+ (Scheme
2) functions as a selective organic oxidant.26 The treatment of
tetraarylethylene 2 with 6•+ under an argon atmosphere in
anhydrous dichloromethane at 25 °C immediately yielded a
bright blue solution; the UV-vis spectral profile is very close
Electrochemical Oxidation of Chiral Tetraarylethylenes.
Chirally modified tetraarylethylenes 2-4 (1 mM) were oxidized
electrochemically in anhydrous dichloromethane containing 0.2
M tetrabutylammonium hexafluorophosphate as a supporting
electrolyte by using a platinum electrode. For each of 2-4, a
reversible cyclic voltammogram with two closely coupled one-
electron oxidation waves was consistently obtained at a scan
rate of 0.2 V s-1 with an accompanying theoretical anodic/
cathodic peak current ratio (ia/ic) of unity, as shown in Figure
2. By calibration with ferrocene on Osteryoung square-wave
voltammograms (OSWV measurements), the reversible two-
step oxidation potentials (E0OX), shown in Table 2, were
determined for the production of radical cations via the one-
electron redox couple and of dications via the two-electron redox
couples (eq 1).

2732 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Mori and Inoue
SCHEME 2: Chemical Oxidation of Tetraarylethylenes
to Radical Cations and Dications by Electron Exchange
with Methanoanthrancene Radical Cation
22+ to a solution of 2. If dication 22+ was treated with an
equimolar amount of electron-rich octamethylbiphenylene (7)
(E0
) 0.80 V),30 the solution again turned dark blue,
OX
displaying a pair of major absorption maxima at 600 and >800
nm. Subtraction of the spectrum of 2•+ from the resulting spectra
afforded well-resolved absorption bands at 602 and 550(sh) nm,
indicating the quantitative formation of octamethylbiphenylene
radical cation (7•+). When a solution of dication 22+ was treated
with an excess amount of zinc dust, the solution immediately
turned colorless, and neutral donor 2 was recovered as indicated
by the UV-vis analysis.
X-ray Crystallography and Molecular Orbital Studies on
Conformational Changes upon Stepwise Oxidation. Tetra-
anisylethylenes are known to undergo considerable conforma-
tional changes around the ethylenic double bond upon successive
oxidations from the neutral state to radical cation and then to
the dication. AM1 calculations nicely simulated the conforma-
tional differences of neutral, radical cationic, and dicationic 2.
The calculated central C-C bond length is elongated upon
stepwise oxidations by more than 10% (overall) from 1.354 Å
for 2 to 1.430 Å for 2•+ and then to 1.493 Å for 22+, and the
dihedral, or twist, angle around the original ethylenic bond is
SCHEME 3: Oxidation of Chiral Tetraarylethylenes
with Antimony Oxidants
SCHEME 4: Electron Transfer from Tetraarylethylene
Dications to Neutral Tetraarylethylene and Biphenylene
Donors
increased from 4° to 35° and then to 79° for 2, 2•+, and 22+
,
respectively. DFT calculations at the B3LYP/6-31G(d) level for
AM1-optimized structures also afforded similarly optimized
structures, which are illustrated in Figure 3. The central C-C
bond length and the twist angle shifted in a similar manner,
from 1.370 Å and 13.7° for 2 to 1.421 Å and 27.9° for 2•+ and
then to 1.503 Å and 57.5° for 22+, as summarized in Table 3.
The calculated twist angles are in good agreement with those
determined by X-ray crystallography, i.e., 5°, 31°, and 62° for
5, 5•+, and 52+, respectively.14 A more dramatic twisting of the
ethylenic double bond has been proposed to occur in the
unsubstituted ethylene dication31 and radical cation.32 Thus, the
theoretical calculations predict a bisected structure for the
ethylene dication with a twist angle of up to 90° and a C-C
bond length of up to 1.46 Å. The sudden conformational change
from the planar neural form to the bisected radical cation and
dication forms may be interpreted in terms of the decreased
bond order and Coulombic repulsion, particularly in dications,
both of which are directly reflected in their structure in the
absence of conjugating (aromatic) chromophore(s). Such a twist
has also been observed with some push-pull substituted
ethylenes, and is attributed to their highly polar zwitterionic
natures.33
to that of tetraanisylethylene radical cation (5•+), showing the
characteristic absorption at 580 nm. A careful spectral analysis
of the blue solution indicates the essentially quantitative
formation of radical cation 2•+. Treatment of 2 with 2 equiv of
6•+ at 25 °C immediately gave a dark green solution of dication
22+ in a quantitative yield. Thus, the radical cation 2•+ and
dication 22+ could be generated cleanly in discrete one- and
two-electron oxidations by simply adjusting the amount of the
oxidant.
Antimony pentachloride is a strong electron-transfer oxidant,27
which was used in the preparation and isolation of pure
tetraarylethylene dications. Thus, the treatment of 2 (1.0 mmol)
with excess SbCl5 (5.0 mmol) in anhydrous dichloromethane
at -78 °C immediately gave a dark green solution, from which
the crystalline dication salt readily precipitated in a quantitative
yield upon slow addition of anhydrous ether or hexane (Scheme
3). The microcrystalline precipitate was filtered under an argon
atmosphere and dried in vacuo.
Crystals of the neutral donor 2 and of its dication salt
2
2+(SbCl6-)2were obtained, but no single crystal suitable for
X-ray crystallography was obtained for the radical cation 2•+,
despite extensive efforts. The dication crystals were prepared
by the slow diffusion of anhydrous hexane into a dichlo-
romethane solution of the dication salt at -20 °C. The X-ray
structures are shown in Figure 4 (also see the Supporting
Information for details). The neutral donor 2 shows the average
dihedral angle of 9° with a central C-C bond length of 1.35
Å. These values are in good agreement with those reported for
the corresponding anisyl derivative 5, i.e., 5° and 1.35 Å.14 The
dihedral angle and C-C bond length are dramatically increased
in dication 22+ up to 83° and 1.53 Å.
Triethyloxonium hexachloroantimonate was also used as a
selective oxidant to produce 2•+. A suspension of Et3O+SbCl6
-
(3.0 mmol) and 2 (2.0 mmol) in dichloromethane was stirred
at -10 °C for 3 h to give a dark blue solution. The UV-vis
spectral analysis of the highly colored solution indicated the
formation of the radical cation, showing the characteristic
spectrum at 580 nm, in an essentially quantitative yield.
When a dark green solution of 22+ was mixed with an
equimolar amount of neutral 2, the color immediately changed
to bright blue. UV-vis spectral analysis established the concur-
rent oxidation of neutral 2 and reduction of 22+, giving the
radical cation 2•+ (Scheme 4). In the absorption spectra, a pair
of isosbestic points were observed upon stepwise addition of
Although the quality of the crystals of the dication 22+
-
34
(SbCl6
)
2
was not accurate enough to discuss the detailed
structural changes, the X-ray analysis clearly revealed that the
(R,R,R,R)-substituents in 22+ induce S (M) chirality around the
central C-C bond. This assignment is consistent with that
elucidated by the CD exciton chirality method applied to the

Tetraarylethylene Chiroelectrochemical Switch
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2733
Figure 3. B3LYP/6-31G(d)-optimized structures of neutral, radical cationic, and dicationic forms of tetraarylethylenes 2 and relevant molecular
orbitals.
TABLE 3: Structural Features, Chiroptical Responses and Color of Neutral, Radical Cationic and Dicationic
Tetraarylethylenes 2 and 3
sign of the Cotton effectb
oxidation
state
dihedral
C-C bond
color
(λmax/nm)
anglea/deg
lengtha/Å
2
3
neutral
radical cation
dication
9 (14, 4)
- (28, 35)
83 (58, 79)
1.35 (1.37, 1.35)
- (1.42, 1.43)
0
+
-
0
-
+
colorless (∼330)
blue (∼580)
1.53 (1.50, 1.49)
green (∼660)
a Values obtained by X-ray crystallography; B3LYP/6-31G(d)- and AM1-calculated values in parentheses. b At 640 nm.
absorption around 700 nm for dication 22+, as discussed in the
following section.
CD Spectra of Neutral, Radical Cationic, and Dicationic
Tetraarylethylenes with Chiral Auxiliaries. Neutral 2, pos-
sessing (S)-(+)-2-methylpropyl auxiliaries, showed the first
positive, second negative, and third positive Cotton effect peaks,
as shown in Figure 5 and Table 1. By using Lorenzian
deconvolution, the observed CD spectrum of 2 was analyzed
as a sum of two coupled signals of A1 ) +0.65 at 312 nm and
A2 ) -1.80 at 264 nm, assignable to the 1Lb and 1La transitions.
As can been seen from Figure 3, the four aromatic rings of 2
are positioned like propeller blades attached to the central Cd

2734 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Mori and Inoue
Figure 4. PLUTO drawings of (a) the neutral donor 2 and (b) the corresponding dication 22+(SbCl6-)2. Some atoms are omitted for clarity.
Figure 5. UV-vis and CD spectra and anisotropy (g) factors of (a) neutral, (b) radical cation, and (c) dication forms of tetraarylethylenes 2, 3, and
4 (10-5 M) in dichloromethane at 25 °C. Note that the UV-vis spectra of 2-4 gave essentially the same shape; only those of 2 are presented here
for clarity.
C bond with a twist around the vinylic bond. In such a
conformation, each transition of the aromatic chromophores is
likely to couple with each other;15,22 this is also the case with
3. Possessing larger antipodal auxiliaries, neutral 3 displayed
stronger signals of opposite signs (A1 ) -1.44 at 291 nm,
A2 ) +3.70 at 267 nm). Interestingly, when two chiral groups
of 2 in the trans positions are replaced by achiral methyls, the
resulting hemichiral 4 never shows exciton coupling,35 although
the UV-vis spectra of 2-5 are quite similar to each other. This
may indicate that the geminal aryls are mostly responsible for
the exciton coupling. This tentative conclusion is also compatible
with the fact that stilbenoid 1 does not show any exciton
coupling.
donors 2-4 are in the range of 10-5-10-4, which are
comparable to that of stilbenoid 1. These small g factors are
typical of the allowed π-π* transitions.15,16 It is noted however
that the longer chiral substituents in 3 give larger ∆ꢀ and g
factor values, while the trans-disubstitution in 4 greatly reduces
these values, presumably due to the lack of exciton coupling.
Chiroptical Properties of Radical Cations 2•+-4•+. Oxida-
tions of 2-4 were performed by using triethyloxonium hexa-
chloroantimonate.19 The UV-vis spectra of the resulting radical
cations 2•+-4•+ were essentially superimposable, though with
slight differences in absorption coefficient, as exemplified in
Figure 5, for compound 2. The spectral changes upon successive
oxidations to radical cation and then to the dication were in
excellent agreement with those reported for tetraanisylethylene
(5).14 Although the radical cations 2•+-4•+ could be isolated
as crystalline salts, we were unable to obtain good single crystals
The spectral presentation of the anisotropy (g) factor at each
wavelength, shown in Figure 5 (bottom), resembles in shape
the related CD spectrum. The observed g factors of neutral

Tetraarylethylene Chiroelectrochemical Switch
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2735
suitable for X-ray crystallographic analysis (vide supra). CD
spectra of the radical cations were then measured by redissolving
the isolated salts into freshly prepared anhydrous dichloro-
methane under argon at 25 °C. Essentially no disproportionation
was observed under the conditions employed (vide supra). Only
weak, but appreciable, CD spectra were obtained for radical
cations of 2-4, the CD intensities of which were much smaller
than those of the neutral species (Figure 5). However, antipodal
2•+ and 3•+ gave the opposite CD signs at ca. 350 and >700
nm, indicating that these faint signals are not artificial or noise.
Although radical cations 2•+ and 3•+ appear to show a weak
exciton-coupled pattern at ca. 250 nm, an overlap with the
absorption of the counteranion prevents further analysis.
those of the neutral and radical cationic species 2 and 2•+, whose
HOMO or SOMO clearly shows central symmetry. The
structural feature is unique to the dicationic tetraarylethylenes,
differing from those of the neutral and radical cationic species,
where the twist around the central double bond is much smaller
and the four alkoxyphenyl groups have essentially the same
degree of quinoidal character. Thus, the negative first Cotton
effect of (R,R,R,R)-22+ can be related to the left-handed twist,
which is consistent with the X-ray crystallographic structure.
As was the case with the neutral and radical cationic species,
the ∆ꢀ value of the dication increases with increasing bulkiness
and the number of the appended chiral auxiliaries; i.e.,
∆ꢀ(22+) < ∆ꢀ(32+) and ∆ꢀ(42+) < ∆ꢀ(22+). The X-ray crys-
tallographic structure and the sign of the CD couplet of 22+
consistently indicate that (S,S,S,S)-32+ possesses P chirality,
while (R,R)-42+ has M chirality. It is interesting to compare the
CD spectrum of “hemichiral” 42+ with that of “fully” chiral
The neutral and oxidized species are considered to give the
same sign in the CD spectrum, since the sign of the Cotton
1
effect of the Lb band of the neutral species always coincides
with that of the d0-d1 transition. For instance, both of the first
absorption bands of neutral 2 (∼330 nm) and radical cation 2•+
(>900 nm) gave a positive Cotton effect, and this may indicate
that the transition moment of the arene ring(s) is essentially
oriented in the same direction, as indicated by molecular orbital
calculations shown in Figure 3. Thus, the shapes of frontier
molecular orbitals (127th-129th) are essentially unchanged
between the neutral and radical cationic species.
2
2+. Thus, the half number of chiral auxiliaries in 42+ does not
immediately lead to half-reduced CD intensities but causes only
slight decreases in intensity compared to that for 22+. This seems
reasonable, as the four aromatic rings in 22+/42+ no longer
behave as independent chromophores but rather form two
diarylcarbenium chromophores, the twist angle of which is
governed by the chiral group introduced at the para position(s).
Intriguingly, the CD intensity, as a measure of chiral induction,
is not proportional to the number of chiral auxiliaries at the
para position, but is more sensitive to the presence/absence (and
the bulkiness) of the chiral auxiliary. In other words, the first
chiral auxiliary introduced to the diarylcarbenium chromophore
is more effective than the second one in inducing the chiral
twist in tetraarylethylene dication.
As can be seen from Figure 5 and Table 1, the anisotropy
(g) factors observed for tetraarylethylene radical cations
2•+-4•+ are significantly smaller than those exhibited by the
corresponding neutral dicationic species. The largest g factors
are obtained mostly for the lowest energy transition of
2•+-4•+, as is the case with the stilbenoid 1•+. Interestingly,
the g factor is almost negligible for the other d-d transitions,
probably due to the random orientation of the relevant transition
moments.
The g factors of dications 22+-42+ are roughly comparable
to those observed for the corresponding neutral species, being
on the order of 10-4-10-5, which are slightly larger than the
values reported for the allowed transitions of monosubstituted
benzenes.15,16 In sharp contrast to the mutually resembling
profile and intensity of ∆ꢀ and g for 2-4 (cf. the middle and
bottom traces in Figure 5a,b), the relative intensity of the g
factors of dications 22+-42+ is very different from that of the
∆ꢀ values, being strongly dependent upon the size (and number)
of the introduced chiral substituents. Thus, the g factor of 32+
is much larger than those of the lower homologues 22+ and 42+
over all the wavelengths examined. Indeed, the UV spectra of
the neutral and radical cationic species are almost superimpos-
able for 2-4, while the dications show small substituent-
dependent UV spectra, giving smaller ꢀ values in particular for
Chiroptical Properties of Dications 22+-42+. Further
oxidation of radical cations 2•+-4•+ or direct two-electron
oxidation of neutral 2-4 with an excess amount of (stronger)
oxidant affords dicationic species 22+-42+. Such dications were
isolated as hexachloroantimonate salts by simply filtering the
precipitates formed upon addition of dry hexane or ether to the
solution under an argon atmosphere. The obtained salts of
dications 22+-42+ were sufficiently pure (>95%), as determined
by iodometry.14 Dichloromethane solutions of the isolated salts
exhibited essentially the same CD spectra as those of the
dications generated in situ in dichloromethane using a slight
excess of antimony pentachloride. In sharp contrast to the
relevant neutral and in particular radical cationic species,
dications 22+-42+ show much stronger Cotton effects. In all
cases, the first Cotton effect, observed at ∼650 nm, gives a
fairly strong CD signal with a sign opposite that observed for
the first Cotton effect peak (1Lb band) of the corresponding
neutral species. The CD spectra of 22+-42+ show an inverted
sign at wavelengths around 500-600 nm to give opposite Cotton
effect peaks at ∼480 nm. The opposite first Cotton effect signs
observed for neutral versus dicationic species can be attributed
simply to the twisted structure of the dications, in which the
first transition moment is perpendicular to that of the neutral
species. However, it is more reasonable to attribute the observed
CD to the exciton coupling between the two diarylcarbenium
chromophores in the bisected dication, where each moiety
consists of one quinoidal and one coplanar benzene ring (see
the DFT-calculated structure in Figure 3). Such an assumption
is further supported by molecular orbital calculations (Figure
3), in which the dication 22+ shows an axisymmetric HOMO.
The pattern of the MOs of the dication differ completely from
3
2+ (although radical cation 3•+ also gives slightly smaller values
compared with 2•+ and 4•+ which are reflected in its appreciably
enhanced g values). It is noted that the size of the alkyl
substituent that is introduced at a distal position and is seemingly
irrelevant to the electronic transition does affect the UV and
CD spectra (and hence g factor) of dicationic and, to a lesser
extent, radical cationic species, probably through the confor-
mational changes of the ethylenic moiety due to the steric bulk
of the introduced substituent.
Temperature Effects on the Chiroptical Properties of
Neutral 2 and Dication 22+. In the above discussion, the fairly
strong Cotton effect observed for dication 22+ was tentatively
assigned not to a simple CD induced by the chiral para-auxiliary
but to the exciton coupling of the two diarylcarbenium chro-
mophores. To further examine this hypothesis, the CD spectra
of neutral 2, radical cation 2•+, and dication 22+ were recorded
in 1,2-dichloroethane at temperatures ranging from -10 to +70
°C. As shown in Figure 6, the CD spectrum of neutral 2

2736 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Mori and Inoue
Figure 6. Variable-temperature CD spectra of (a) chiral tetraarylethylene 2 and (b) dication 22+ in 1,2-dichloroethane at -10, 10, 30, 50, and
70 °C.
exhibited only small bathochromic shifts with very small
accompanying intensity changes over the temperature range
employed. Similarly, the radical cation 2•+ showed only
negligible CD changes in the same temperature range (see the
Supporting Information). These results clearly indicate that
simple conformational changes in the chiral auxiliary are not
sufficient to cause any appreciable changes in the CD spectrum
within the temperature range examined, at least in the present
system. In contrast, the CD spectrum of dication 22+ turned
out to be more sensitive to temperature, as shown in Figure 6.
Although the spectra obtained are somewhat noisy due to the
instrumental limitations of the cryostat and the CD spectrometer,
the CD intensity dramatically decreases with increasing tem-
perature from -10 to +70 °C, without accompanying a peak
Figure 7. (a) On/off, or 0/-, switching of the CD signal of 2 at 640
shift. In contrast, the UV-vis spectra of neutral, radical cationic,
and dicationic species (2, 2•+, 22+) showed no significant
changes over the same temperature range. Hence, the observed
temperature dependence of the g factor is most probably
attributable to the gradual conformational fixation of distal chiral
groups at decreased temperatures, which affects the overall CD
signal to a different degree that depends on the oxidation state.
Recent theoretical calculations have shown that chiral 2-bu-
tanol can adopt nine conformations with different dihedral
angles.36,37 DFT calculations at the B3LYP/6-31G* level indicate
that 68% of 2-butanol molecules are populated to the three most
stable conformers at room temperature.38 Although the energy
of each conformer and the population ratio would be different
in the case of the 1-methylpropoxy group used in the present
study, the temperature-insensitive CD spectrum of neutral 2
experimentally reveals that no serious conformational changes
that affect the CD spectrum take place in chiral 1-methylpropoxy
auxiliaries over the temperature range employed. The temper-
ature-dependent CD spectrum of dication 22+ can be rationalized
by the conformational fixation of the chiral group, which
enhances the exciton coupling between the two diarylcarbenium
chromophores. This finding indicates that the chiroptical proper-
ties of chiral tetraarylethylenes can be modulated not only by
temperature but also by other entropy-related factors such as
pressure and solvent polarity, the fundamental idea of which
has recently been recognized in asymmetric photoreactions.39
Temperature-driven chiroptical inversion of flexible linear
polysilylenes40 and helical switching of polystyrene modified
with â-cyclodextrin41 have also been reported recently.
nm upon repeated oxidation-reduction cycles and (b) ternary 0/-/+
switching of the CD signal of 3 at 640 nm upon successive oxidation-
reduction cycles; see the text for the detailed conditions.
2 was oxidized with 4 equiv of antimony pentachloride in
dichloromethane to afford a dark green solution, the CD
spectrum of which indicated the complete formation of dication
2
2+. After the solution was left for 10 min at room temperature,
the dication formed was reduced by adding Zn powder to the
solution, quantitatively regenerating neutral 2. A clear solution
of 2, retrieved by removing the excess zinc dust by filtration
under Ar, was subjected to the same oxidation-reduction-
filtration cycle. This protocol was repeated several times without
any accompanying loss of CD intensity at 640 nm, as shown in
Figure 7, which indicates the completely reversible nature of
the process.
A stepwise oxidation-reduction cycle is also possible,
consisting of the first two successive one-electron oxidations
with subsequent two-electron reduction. For instance, neutral
donor 3 was selectively oxidized to radical cation 3•+ by using
1.5 equiv of triethyloxonium hexachloroantimonate, which was
further oxidized to dication 32+ with antimony pentachloride.
The resulting solution was treated with zinc powder to regenerate
neutral 3 in quantitative yield; thus, the colorless solution of 3
obtained after filtration of excess zinc power afforded a CD
spectrum essentially identical to the original one in shape and
intensity. This oxidation-reduction-filtration cycle was re-
peated to give the CD spectral changes at 640 nm shown in
Figure 7. It is noteworthy that this reversible three-stage redox
cycle may function as a ternary switching device that uses the
negative/null/positive chiroptical output and/or the color change
among colorless/blue/green. Table 3 summarizes the chiroptical
property changes of these tetraarylethylenes induced by the
multistep redox cycle. Thus, the alterations in dihedral angle
and C-C bond length are systematically converted to the well-
Redox-Driven Chiroptical Switching. Since the present
redox system is completely reversible, we examined the
chiroptical properties of chiral tetraarylethylene species in
different oxidation states produced upon successive oxidation-
reduction cycles. First investigated was the CD spectral behavior
of tetraarylethylene 2 upon complete redox cycles. Thus, neutral

Tetraarylethylene Chiroelectrochemical Switch
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2737
distinguishable chiroptical/colorimetric ternary digit information,
assigning the absence of a CD signal or color of the neutral
donor to “0”, the positive CD signal or blue color of the radical
cation to “+1”, and the negative CD or green color of the
dication to “-1”. Although we have not examined other donor
molecules or redox methods, this ternary chiroptical switch
involving neutral/radical cationic/dicationic states of a chiral
donor is potentially applicable to a wide variety of optically
active electron-rich molecules by using the appropriate chemical
and electrochemical redox processes.
(SUPELCO, SPB5, 30 m × 0.25 mm i.d.). Melting points were
measured with a Yanaco MP-21 apparatus and are uncorrected.
Mass spectra were obtained with a JEOL JMS-DX-303 instru-
ment. Elemental analysis was performed at the Center for
Chemical Analyses, Osaka University. CD spectra were mea-
sured in a conventional quartz cell (light path 1 cm) on a JASCO
J-720WI spectropolarimeter equipped with a PTC-348WI tem-
perature controller. An aqueous solution of (+)-ammonium
camphorsulfonate-d10 (0.06%) was used for calibration of the
spectrometer sensitivity and wavelength (θ ) 190.4 mdeg at
290.5 nm). ∆-Tris(ethylenediamine)cobalt(III) was further
employed for calibration in the visible region (∆ꢀ ) +1.89 M-1
cm-1 at 490 nm).42 The optical rotations were measured in a
thermostated conventional 10 cm cell with a JASCO DIP-1000
digital polarimeter at the sodium D-line (589.3 nm). Cyclic
voltammetry was performed on a BAS CV-100B electrochemi-
cal analyzer. Stilbenoid donor 1 and tetraarylethylene 2-4 (1
mM) and tetra-n-butylammonium hexafluorophosphate (0.2 M)
as an electrolyte were placed in a gastight Schlenk-type
separated cell under Ar. Anodic and cathodic peak potentials
were measured with a Pt electrode in anhydrous dichloro-
methane at a sweep rate of 0.2 V s-1. A Ag+/AgNO3 reference
electrode was used and calibrated with ferrocene (E1/2 ) 0.02
V), with the measured potential converted to the voltage vs SCE
by using the relationship E(SCE) ) E(Ag+/AgNO3) + 0.25 V.
All DFT calculations were performed using the GAUSSIAN
98 software package43 running on 16-CPU parallel Pentium IV
processors at the Handai FRC Computer Center. The optimiza-
tions were performed at the B3LYP/6-31G(d) level for an AM1-
optimized structure, with the molecular orbital calculation
followed by the POP)Full keyword. Semiempirical calculations
were performed with the MOPAC (version 6) program using
the PM3 and AM1 Hamiltonians implemented on a Sony
Tektronix CAChe system (version 3.5). Unrestricted Hartree-
Fock wave functions were employed, and the calculations were
fully optimized using an extra keyword, PRECISE.
Conclusions
We have investigated the chiroptical properties of chirally
modified tetraarylethylenes in different oxidation states. Step-
wise electrochemical or chemical oxidations of these tetraaryl-
ethylenes in anhydrous dichloromethane afford the correspond-
ing stable radical cations and dications in quantitative yields.
Judging from the disproportionation constant (Kdisp) determined
by the electrochemical analysis, the generated radical cation does
not disproportionate to neutral and dicationic species and exists
as the predominant species in dichloromethane solution, which
allowed us to determine their chiroptical properties and also to
use the radical cationic species as an element of the sequential
redox switch. Neutral, radical cationic, and dicationic tetraaryl-
ethylenes with chiral auxiliaries display distinctly different
chiroptical properties and visible color changes, reflecting the
alteration in twist angle and bond length of the bisected
chromophores. In the tetraarylethylene system, (R)-1-methyl-
alkyl groups introduced to the para positions induce a left-
handed twist, or M helicity, to the bisected chromophore of 22+
.
Taking into account the durability upon repeated redox cycles,
this system may be used in novel chiroptical devices, such as
switches and memory devices working on ternary logic.
Experimental Section
General Procedures. All starting materials, reagents, and
solvents were commercially available and used without further
purification, unless stated otherwise. Chiral alcohols (S)-(+)-
and (R)-(-)-2-butanol and (R)-(-)-2-octanol were used as
obtained from Aldrich. Dichloromethane (Nacalai) was repeat-
edly stirred with fresh aliquots of concentrated sulfuric acid
(∼10% v/v), until the acid layer remained clear. After separation,
it was washed with water, aqueous sodium bicarbonate, brine,
and water and then dried over calcium chloride. The dichloro-
methane was distilled twice from P2O5 under an argon atmo-
sphere and stored in a Schlenk tube equipped with a Teflon
valve fitted with Viton O-rings. Tetrahydrofuran (THF; Wako)
was freshly distilled from sodium/benzophenone. Anhydrous
toluene (Wako) and dry hexane (Wako) were distilled from
calcium hydride under Ar and stored in Schlenk flasks. Chemical
and electrochemical oxidations were performed by following
the literature procedure.14
Column chromatography was performed on silica gel (70-
230 mesh, Merck) and GPC on Jaigel 1-H and 2-H columns
(Japan Analytical Industry Co.) with chloroform as an eluent.
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
obtained with a JEOL GSX-400. All chemical shifts are reported
in parts per million relative to the peak for Me4Si (0 ppm) or
residual solvent (CDCl3 as 77.16 ppm for 13C) as a reference,
with coupling constants (J) reported in hertz. IR and UV spectra
were obtained on a JASCO FT/IR-230 and JASCO V-550
equipped with a temperature controller. Gas chromatographic
analyses were performed on a Shimadzu GC-14B instrument
fitted with a C-R6A integrator using a capillary column
Synthesis of (R,R)-2,3-Bis(p-(1-methylpropyloxy)phenyl)-
bicyclo[2.2.2]oct-2-ene (1). p-Bromophenol (5.19 g, 30 mmol),
triphenylphosphine (7.87 g, 30.0 mmol) and (S)-(-)-2-butanol
(2.76 mL, 30.1 mmol) were dissolved in THF (100 mL), to
which diisopropyl azodicarboxylate (DIAD; 6.50 mL, 33.0
mmol) was added dropwise under Ar at ambient temperature.
The solution was stirred for 2 days, and the solvent was
evaporated to give a residue which was chromatographed on
silica gel with a hexane eluent to afford 4.36 g (63%) of the
corresponding chiral (R)-ether; the (S)-isomer was obtained in
the corresponding manner. The (R,R)-isomer of 1 was obtained
according to the literature procedure.44,45 Thus, magnesium
turnings (0.48 g, 20 mmol) and the corresponding chiral aryl
bromide (2.86 g, 13 mmol) were dissolved in THF (25 mL),
and the solution was refluxed for 5 h. In a separate flask, 2,3-
dibromobicyclo[2.2.2]oct-2-ene44 was dissolved in THF (20
mL), the Grignard reagent added via a cannula, and the mixture
refluxed for 20 h under Ar. The resulting solution was quenched
with aqueous ammonium chloride, and the organic components
were extracted with ether. The ether extract was washed with
water and brine, and dried over anhydrous MgSO4. The mixture
was filtered and evaporated in vacuo to afford a residue, which
was purified by silica gel column chromatography with hexane/
ether (1:1) eluent to afford 4.58 g (91%) of the corresponding
chiral stilbenoids.
Data for (S)-p-bromo-1-methylpropyloxybenzene: color-
less oil; 1H NMR δ 0.88 (3H, t, J ) 7.2), 1.19 (3H, d, J ) 6.2),
1.53 (1H, sym m, J ) 6.2), 1.63 (1H, sym m, J ) 7.3), 4.15

2738 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Mori and Inoue
(1H, m, J ) 6.2), 6.68 (2H, d, J ) 9.1), 7.26 (2H, d, J ) 9.1);
13C NMR δ 9.85, 19.25, 29.20, 75.56, 112.58, 117.83, 132.37,
157.49; HRMS m/z found 228.0157, calcd for C10H13BrO
228.0150.
19.84, 22.72, 25.63, 29.42, 31.93, 36.64, 73.84, 115.08, 132.69,
136.87, 138.52, 156.51; HRMS m/z found 844.6368, calcd for
C58H84O4 844.6370.
Synthesis of (R,R)-(E)-Bis(p-(1-methylpropyloxy)phenyl)-
bis(p-methoxyphenyl)ethylene (4). 4,4′-Dihydroxybenzophe-
none (51.3 g, 0.24 mol) and potassium carbonate (35.4 g, 0.26
mol) were dissolved in acetone (1 L), to which iodomethane
(20.0 mL, 0.32 mol) was added all at once. The mixture was
refluxed for 12 h, and then the organic solvent was evaporated.
The crude mixture was chromatographed on silica gel with a
hexanes-ethyl acetate (2:1) eluent. The first fraction was the
dimethylated product (4.7 g, 8%), and the second fraction was
the desired monomethylated benzophenone 4-hydroxy-4′-meth-
oxybenzophenone,46 yield 13.6 g (25%), mp 150-151 °C. The
ketone (4.56 g, 20 mmol), (S)-(+)-2-butanol (1.92 g, 26 mmol),
and triphenylphosphine (5.24 g, 20 mmol) were dissolved in
THF (100 mL), to which DIAD (5.0 mL, 23 mmol) in THF
(30 mL) was added dropwise over 3 h under an Ar atmosphere.
The solution was warmed to room temperature and stirred for
an additional 30 h. After evaporation of the solvent the crude
product was chromatographed on silica gel with a hexane-
dichloromethane (5:1) eluent to afford 5.80 g (99%) of the
corresponding chiral alkylated ketone. The chiral monometh-
oxybenzophenone (5.82 g, 20 mmol) and zinc dust (5.69 g, 60
mmol) were vigorously stirred in THF (200 mL) under Ar, to
which titanium tetrachloride (3.3 mL, 30 mmol) was added
dropwise at 0 °C, with the resulting mixture refluxed for 10 h.
The obtained crude mixture was poured onto an aqueous sodium
carbonate solution, and the organic material was extracted with
ether (2 × 200 mL). The organic phase was washed with brine
and water and dried over sodium sulfate. The filtrate was
evaporated to give a residue, which was purified by silica gel
column chromatography with a hexanes-ethyl acetate (25:1)
eluent. The material obtained (4.12 g, 77%) was further purified
by recrystallization from ethanol to give 3.51 g (66%, E/Z >
98) of the gas chromatographically pure (E)-isomer.
Data for (R,R)-2,3-bis(p-(1-methylpropyloxy)phenyl)-
1
bicyclo[2.2.2]oct-2-ene (1): mp 74-75 °C; H NMR δ 0.96
(6H, t, J ) 7.3), 1.26 (6H, d, J ) 5.9), 1.50-1.78 (12H, m),
2.87 (2H, s), 4.22 (2H, sym m, J ) 5.9), 6.69 (4H, d, J ) 8.4),
7.00 (4H, d, J ) 8.4); 13C NMR δ 9.94, 19.44, 26.45, 29.35,
37.61, 75.06, 115.45, 129.63, 133.72, 138.77, 156.44; IR (CaF2,
ν/cm-1) 1240, 1502, 1604, 2860, 2937, 2969; HRMS m/z found
404.2713, calcd 404.2715; [R]25D(c 0.10, CHCl3) ) -22.0°.
Anal. Calcd for C28H36O2: C, 83.12; H, 8.97. Found: C, 82.98;
H, 8.93. The corresponding (S,S)-isomer was similarly obtained
in 86% yield, and the spectroscopic characteristics are essentially
the same. Anal. Found: C, 82.83; H, 9.00.
Synthesis of Tetrakis(p-(1-methylpropyloxy)phenyl)eth-
ylene (2). 4,4′-Dihydroxybenzophenone (10.28 g, 48 mmol),
(S)-(+)-2-butanol (7.94 g, 107 mmol), and triphenylphosphine
(28.09 g, 107 mmol) were dissolved in THF (175 mL), to which
DIAD (23.0 mL, 117 mmol) in THF (25 mL) was added
dropwise over 1 h under an Ar atmosphere at 0 °C. The resulting
solution was warmed to room temperature and stirred for an
additional 20 h. After evaporation of the solvent, the crude
product was chromatographed on silica gel with hexanes-ethyl
acetate (50:1) as an eluent to afford 15.0 g (96%) of the
corresponding ketone as the second fraction. The ketone (11.34
g, 35 mmol) and zinc dust (6.88 g, 105 mmol) were vigorously
stirred in THF (200 mL) under Ar, to which titanium tetrachlo-
ride (5.8 mL, 105 mmol) was added dropwise, with the resulting
black chalky mixture refluxed for 20 h. The resulting mixture
was poured into an aqueous sodium carbonate solution and the
organic material extracted with ether (3 × 500 mL). The
combined ether extract was washed with brine and water, and
then dried over sodium sulfate. After filtration, the solution was
evaporated to dryness and the residue purified by silica gel
column chromatography with hexanes-ethyl acetate (50:1)
eluent. The crude product obtained was further purified by
recrystallization from an ethanol/dichloromethane mixture to
give a colorless solid, yield 8.42 g (78%).
Data for (R)-4-methoxy-4′-(1-methylpropyloxy)benzophe-
none: colorless oil; 1H NMR δ 0.99 (3H, t, J ) 7.3), 1.34 (3H,
d, J ) 5.9), 1.67 (1H, m, J ) 6.2), 1.78 (1H, m, J ) 7.7), 3.88
(3H, s), 4.41 (1H, m, J ) 5.9), 6.93 (2H, d, J ) 8.8), 6.96 (2H,
d, J ) 8.8), 7.77 (2H, d, J ) 9.2), 7.79 (2H, d, J ) 9.5); 13C
NMR δ 9.87, 19.31, 29.25, 55.59, 75.30, 113.56, 115.03, 130.41,
130.99, 132.33, 132.43, 161.85, 162.90, 194.57; HRMS m/z
found 284.1414, calcd 284.1412. Anal. Calcd for C18H20O3: C,
76.03; H, 7.09. Found: C, 75.83; H, 7.06.
Data for (R,R)-bis(p-1-methylpropyloxy)benzophenone:
1
colorless oil; H NMR δ 0.99 (6H, t, J ) 7.3, 6.2), 1.33 (6H,
d, J ) 5.9), 1.66 (2H, m, J ) 7.3, 6.2), 1.78 (2H, m, J ) 7.3,
6.2), 4.41 (2H, m, J ) 5.9), 6.93 (4H, d, J ) 8.8), 7.77 (4H, d,
J ) 8.8); 13C NMR δ 9.84, 19.28, 29.23, 75.27, 115.00, 130.49,
132.24, 161.76, 194.49; IR (As2Se3, ν/cm-1) 2973, 2935, 1647,
1601, 1508, 1303, 1252, 925, 769, 618; HRMS m/z found
326.1879, calcd 326.1882. Anal. Calcd for C21H26O3: C, 77.27;
H, 8.03. Found: C, 76.98; H, 7.97.
Data for (R,R)-(E)-bis(p-(1-methylpropyloxy)phenyl)bis-
1
(p-methoxyphenyl)ethylene (4): mp 167-168 °C; H NMR
δ 0.95 (3H, t, J ) 7.5), 1.24 (3H, d, J ) 6.2), 1.57 (1H, m,
J ) 7.0), 1.72 (1H, m, J ) 6.2), 3.74 (3H, s), 4.21 (1H, m, J )
5.9), 6.60 (2H, d, J ) 8.8), 6.63 (2H, d, J ) 8.8), 6.89 (2H, d,
J ) 8.1), 6.94 (2H, d, J ) 8.4); 13C NMR δ 9.93, 19.39, 29.32,
55.22, 75.01, 113.10, 115.14, 132.66, 132.72, 136.81, 137.13,
138.51, 156.55, 157.87; IR (KBr, ν/cm-1) 2971, 2934, 1606,
1506, 1463, 1173, 1127, 1107,1034, 989, 924, 836; HRMS m/z
found 536.2922, calcd 536.2927; [R]25D(c 0.10, CHCl3) )
-25.4°. Anal. Calcd for C36H40O4: C, 80.56; H, 7.51. Found:
C, 80.34; H, 7.45.
Data for (R,R,R,R)-tetrakis(p-(1-methylpropyloxy)phenyl)-
1
ethylene (2): mp 174-175 °C; H NMR δ 0.87 (12H, t, J )
7.4), 1.17 (12H, d, J ) 5.8), 1.50 (4H, m, J ) 7.4, 6.3), 1.64
(4H, m, J ) 7.4, 6.2), 4.13 (4H, sym m, J ) 6.2, 5.8), 6.53
(8H, d, J ) 8.3), 6.83 (8H, d, J ) 8.3); 13C NMR δ 9.94, 19.38,
29.33, 75.03, 115.13, 132.72, 136.91, 138.59, 156.53; IR (KBr,
ν/cm-1) 2971, 2935, 1605, 1284, 1240, 1175, 1128, 1115, 989,
925, 834; HRMS m/z found 620.3862, calcd 620.3866; [R]25
-
D
(c 0.10, CHCl3) ) -53.2°. Anal. Calcd for C42H52O4: C, 81.25;
Preparative Oxidation of Tetraarylethylenes with Anti-
mony Pentachloride. General Procedure. A solution of
antimony pentachloride in dichloromethane (0.01 M, 5 mL) was
cooled at -78 °C under an argon atmosphere while tetraaryl-
ethylene 2 (62 mg, 0.1 mmol) in anhydrous dichloromethane
(10 mL) was slowly added. The reaction mixture immediately
took a green coloration, and the mixture was stirred for 10 min.
H, 8.44. Found: C, 81.01; H, 8.38.
Data for (S,S,S,S)-tetrakis(p-(1-methylheptyloxy)phenyl)-
1
ethylene (3): pale yellow oil; H NMR δ 0.80 (12H, t, J )
7.3), 1.15 (12H, d, J ) 6.2), 1.16-1.36 (36H, m), 1.35-1.49
(4H, m), 1.52-1.67 (4H, m), 4.17 (4H, sym m, J ) 6.2), 6.51
(8H, d, J ) 8.4), 6.82 (8H, d, J ) 8.4); 13C NMR δ 14.20,

Tetraarylethylene Chiroelectrochemical Switch
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2739
Addition of prechilled anhydrous hexane (50 mL) led to the
precipitation of the dication salt. The dark microcrystal-
line precipitate [22+(SbCl6-)2] (λmax ) 422, 471, 577, 660;
log ꢀ577 ) 4.65 M-1 cm-1) was filtered under an argon
atmosphere, washed with dry diethyl ether (3 × 10 mL), and
dried in vacuo to give dark microcrystals (71 mg, 55%). The
purity of the dication was determined by iodometric titration14
and found to be 96%.
data_request/cif, by e-mailing data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre, 12
Union Rd., Cambridge CB2 1EZ, U.K., fax +44 1223 336033.
Acknowledgment. Financial support of this work by a
Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan
(No. 16750034), the 21st Century COE for Integrated Eco-
Chemistry, and Handai FRC (to T.M.) is gratefully acknowl-
edged. We thank Dr. Asao Nakamura of the Kuroda Chiro-
morphorogy Project and Prof. Takanori Suzuki at Hokkaido
University for fruitful discussion. We thank Dr. Guy A.
Hembury for assistance in the preparation of this manuscript.
X-ray Crystallography of Neutral Tetraarylethylene 2. A
colorless prismatic crystal was cut into a cubic shape with
approximate dimensions of 0.30 × 0.30 × 0.30 mm. This was
mounted on a glass fiber. All X-ray crystallographic measure-
ments were made on a Rigaku AFC5R diffractometer with
graphite-monochromated Mo KR radiation (λ ) 16.3 cm-1) with
a rotating anode generator at 23 ( 1 °C (Brutto formula
C45H52O4, MW ) 620.87, monoclinic P21, a ) 9.481(5) Å,
b ) 14.163(5) Å, c ) 13.946(4) Å, â ) 98.51(3)°, V )
1852(1) Å3, Dc ) 1.11 g cm-3, Z ) 2). The total number of
reflections measured was 4700, 4436 of which were sym-
metrically nonequivalent. The structure was solved by direct
methods,47 expanded using Fourier techniques,48 and refined by
the full-matrix least-squares procedure with the teXsan49 crystal-
lographic software package of Molecular Structure Co. The final
residuals were R1 ) 0.065 and wR2 ) 0.165 for 1893 observed
reflections with I > 2σ(I) and 495 variable parameters. CCDC
258915 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif, by e-mailing data_request@
ccdc.cam.ac.uk, or by contacting The Cambridge Crystal-
lographic Data Centre, 12, Union Rd., Cambridge CB2 1EZ,
U.K., fax +44 1223 336033.
Supporting Information Available: Lorenzian deconvo-
luted curve fittings for the CD spectra of the neutral donors
2-4, CD spectra of the radical cation 2•+ at different temper-
atures, calculated conformations of neutral donor 2, radical
cation 2•+, and dication 22+, and ORTEP drawings of 2 and
22+ obtained by X-ray crystallographic analyses (PDF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
References and Notes
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X-ray Crystallography of Tetraarylethylene Dication 22+
.
Tetraarylethylene 2 (62 mg, 0.10 mmol) and triethyloxonium
hexachloroantimonate (66 mg, 0.15 mmol) were dissolved in
dichloromethane (10 mL) under Ar at 0 °C, and the solution
was stirred for 1 h to complete the oxidation. To the resulting
blue-purple solution was added anhydrous ether to form a
precipitate, which was then filtered under an argon atmosphere.
The filtrate was washed with prechilled dry hexane (2 × 10
mL), and then redissolved in anhydrous dichloromethane (20
mL). The organic phase was carefully layered with anhydrous
toluene (30 mL) and stored in a freezer at -20 °C for 5 days;
slow diffusion of toluene afforded the dication salt produced
through the disproportionation of the radical cationic species.
Note that crystals of the same dication salt (of lower quality)
were obtained by the reaction of 2 and antimony pentachloride
at -78 °C and the subsequent crystallization procedures using
the slow diffusion of toluene as mentioned above. A red prism
crystal with approximate dimensions of 0.30 × 0.30 × 0.10
mm was mounted on a glass fiber. All X-ray crystallographic
measurements were carried out on a Rigaku RAXIS-RAPID
imaging plate diffractometer with graphite-monochromated Mo
KR radiation (λ ) 16.3 cm-1) at 23 ( 1 °C (Brutto formula
C45H52O4Sb2Cl12, MW ) 1289.81, tetragonal I41, a ) 11.3809-
(3) Å, c ) 41.655(1) Å, V ) 5395.3(3) Å3, Dc ) 1.59 g cm-3
,
Z ) 4). The total number of reflections measured was 24868,
3092 of which were symmetrically nonequivalent. The structure
was solved by direct methods,47 expanded using Fourier
techniques,48 and refined by the full-matrix least-squares
procedure with teXsan.49 The final residuals were R1 ) 0.043
and wR2 ) 0.078 for 1603 observed reflections with I > 2σ(I)
and 272 variable parameters. CCDC 258916 contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
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