7,7,8,8-tetraaryl-o-quinodimethane stabilized by dibenzo annulation: A helical π-electron system that exhibits electrochromic and unique chiroptical Properties

Article abstract of DOI:10.1002/chem.201203092

When two benzene rings are fused to a tetraaryl-o-quinodimethane skeleton, sterically hindered helical molecules 1 acquire a high thermodynamic stability. Because the tetraarylbutadiene subunit contains electron-donating alkoxy groups, 1 undergo reversibl

Full text of DOI:10.1002/chem.201203092

FULL PAPER
DOI: 10.1002/chem.201203092
7,7,8,8-Tetraaryl-o-quinodimethane Stabilized by Dibenzo Annulation: A
Helical p-Electron System That Exhibits Electrochromic and Unique
Chiroptical Properties
Takanori Suzuki,*[a] Yuto Sakano,[a] Tomohiro Iwai,[a] Shinichi Iwashita,[a]
Youhei Miura,[a] Ryo Katoono,[a] Hidetoshi Kawai,[a, b] Kenshu Fujiwara,[a]
Yasushi Tsuji,[c] and Takanori Fukushima[d]
Dedicated to Prof. James Michael McBride on his retirement
Abstract: When two benzene rings are
fused to a tetraaryl-o-quinodimethane
skeleton, sterically hindered helical
molecules 1 acquire a high thermo-
the point chirality (e.g., sec-butyl) on
the aryl groups to helicity induces a di-
astereomeric preference in dications
2b2+ and 2c2+, which represents an ef-
ficient method for enhancing circular-
dichroism signals. Thus, those redox
pairs can serve as new electrochiropti-
cal response systems. X-ray analysis of
dication 22+ revealed p–p stacking in-
dynamic stability. Because the tetraar-
teraction of the diarylmethylium moi
ACHTUNGTRENNUNGe-
ylbutadiene subunit contains electron-
ACHTUNGTRENNtUNG ies, which is also present in solution.
donating alkoxy groups, 1 undergo re-
The stacking geometry is the key con-
tributor to the chirosolvatochromic re-
sponse.
Keywords: chirality · circular di-
chroism · electrochromism · helical
structures · redox chemistry
versible two-electron oxidation to 22+
,
which can be isolated as deeply colored
stable salts. Intramolecular transfer of
Introduction
case of sterically hindered 7,7,8,8-tetraaryl-o-quinodimeth-
AHCTUNGTERGaNNUN nes (Ar4oQDs), in which four benzene rings are attached
o-Quinodimethane (oQD) is a unique molecule that con-
tains a cross-conjugating p-electron system. This hydro-
ACHTUNGTRENNUNG
to exocyclic carbons, such intermolecular reactivities are
suppressed due to steric congestion, which also causes the
unique helical arrangement of p electrons, similar to the
cases of helicenes consisting of ortho-condensed benzene
rings. However, unlike many helicenes, Ar4oQDs are un-
ACHTUNGTRENNUNG
AHCTUNGTREGsNNUN table. Spectroscopic characterization can be conducted only
at a low temperature due to a facile isomerization to 1,1,2,2-
tetraarylbenzocyclobutene (BCB) or 9,9a-dihydro-9,9,10-tri-
[a] Prof. T. Suzuki, Y. Sakano, Dr. T. Iwai, Dr. S. Iwashita, Dr. Y. Miura,
Dr. R. Katoono, Prof. H. Kawai, Prof. K. Fujiwara
Department of Chemistry
arylanthracene
(DHA)
through
electrocyclization
(Scheme 1).[2]
Faculty of Science, Hokkaido University
Kita 10, Nishi 8, Kita-ku, Sapporo, 060-0810 (Japan)
Fax : (+81)11-706-2714
If spontaneous transformation into cyclized isomers could
be suppressed, Ar4oQDs would become available as thermo-
E-mail: tak@sci.hokudai.ac.jp
[b] Prof. H. Kawai
Present address: Department of Chemistry
Faculty of Science, Tokyo University of Science
1–3 kagurazaka, Shinjuku-ku, Tokyo, 162-8601 (Japan)
[c] Prof. Y. Tsuji
Department of Energy and Hydrocarbon Chemistry
Graduate School of Engineering
Kyoto University, Kyoto 615-8510 (Japan)
[d] Prof. T. Fukushima
Chemical Resources Laboratory
Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku, Yokohama, 226-8503 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203092. CCDC-237042 (1a),
ACTHNUTRGNEUNG
CCDC-895519 (1c), and CCDC-895520 (2c2+[SbCl6À]2) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Scheme 1. Suppression of electrocyclization by dibenzo annulation on
Ar4oQD.
Chem. Eur. J. 2013, 19, 117 – 123
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117

dynamically stable species.[3] Keeping this in mind, we de-
signed 9,10-bis(diarylmethylene)phenanthrene (Ar4DMP,
1),[4] because dibenzo annulation[5] would destabilize the cor-
responding BCB and DHA isomers, but not Ar4DMP itself,
by an additional steric repulsion.[6] This idea was supported
by the PM3 calculation for Ph4DMP, which has a much
lower heat of formation than BCB- and DHA-type isomers
(Figure S1 in the Supporting Information).
Herein, the successful generation and isolation of a series
of (4-ROC6H4)4DMPs 1a–1d [RO=CH3O, C8H17O, (R)-
C2H5CHACHTUNGRTNENUG(CH3)O, (R)-C6H13CHACHTUGNTREN(NUNG CH3)O] as new isolable ex-
amples of Ar4oQD is reported.[4] The helically deformed
structure in 1 as well as the twisted geometry in the dication
22+ were demonstrated by low-temperature X-ray analyses.
As was designed, quinodimethane donors 1 and phenan-
threne-9,10-diyl dications 22+ constitute reversible electro-
chromic pairs that exhibit a vivid color change from yellow
to violet. Notably, the point chirality of the chiral alkoxy
group attached on the aryl rings is transmitted to a helicity
preference[15] to bias the diastereomeric ratio of 2c2+ and
2d2+, which enables us to newly construct electrochiroptical
systems that give two kinds of spectral output [UV/Vis and
circular dichroism (CD)] in response to electrochemical
input (Scheme 3).[16]
Because 1,1,4,4-tetraarylbutadienes[7] are important mem-
bers of violene/cyanine-hybrid-type[7,8] electrochromic sys-
tems,[9] stabilized Ar4DMPs 1 may also serve as new chromic
materials. If a suitable electron-donating group, such as an
alkoxy group, is attached to each of the aryl rings of 1, the
corresponding dications 22+ should become stable enough
for isolation of their salts that exhibit strong absorption in
the visible region, which is characteristic of triarylmethylium
dyes.[10]
Because the redox-active chromophore is framed in the
helically deformed skeleton of Ar4DMPs 1, a drastic change
in geometry is expected upon electron transfer. Two of the
four aryl rings in 1 are forced to overlap in proximity to
cause a large steric repulsion,[11] whereas two-electron oxida-
Scheme 3. Electrochiropitical response system with two kinds of spectro-
scopic output.
tion would induce twisting of the diarylmethylium units
+
around the exocyclic bonds [C9(10) C ] against the planar-
À
ized phenanthrene core in 22+ so as to reduce the steric re-
pulsion. Such a structural change is favorable for the con-
struction of molecular response systems in terms of revers-
Results and Discussion
A
E
Preparation and helical geometry of 9,10-bis(diarylmethyle-
ne)phenanthrenes (Ar4DMPs, 1):
ies on “dynamic redox systems”.[12] Another interesting fea-
ture of the redox pair of 1/22+ is the chiral element[13] of the
he
lic
E
Synthetic strategy for 1: Sterically hindered molecules are
generally difficult to obtain, and successful syntheses often
adopt a common approach, in which they are generated
from a less-hindered precursor by intramolecular reactions
to minimize the disadvantage associated with entropy.[17]
Against this background, we pursued the formation of
Ar4DMPs 1 from less-hindered dications 22+, which in turn
would be obtained from 2,2’-bis(diarylethenyl)biphenyls 3
(Scheme 4). The oxidative cyclization of 3 to 22+ consists of
ACHTUNGTRENNUNGisomers of the dications cannot be separated due to a rapid
inversion of their helical sense by rotation around C9(10)
bonds [(P)-22+ < = >(M)-22+]. It was found here that con-
figuration of the helical sense in Ar4DMPs 1 is also unstable
[(P)-1< = >(M)-1] (Scheme 2), similar to other DMP com-
pounds.[14]
À
several steps: 1) formation of a C C bond between the ben-
zylidene carbons upon the two-electron oxidation of 3; 2)
Scheme 4. Preparation of Ar4DMP 1 and interconversion with dicationic
Scheme 2. Helicity inversion in Ar4DMP 1 and dicationic dye 22+
.
dye 22+
.
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Chem. Eur. J. 2013, 19, 117 – 123

Helical p-Electron System that Exhibits Unique Properties
FULL PAPER
double deprotonation from the resulting butane-1,4-diyl di-
I4 bar, Z=8). The most striking feature is the large torsion
cation[18] to give 1; and 3) the further two-electron oxidation
angle of 63.4(6)8 for the Ar2C=C C=CAr2 unit, although
À
of 1 to furnish 22+
.
the twisting angle of the biphenyl skeleton is only 19.6(7)8,
as was determined by the torsion angle for the bay-region
carbons. The two exocyclic double bonds also deviate from
planarity [twisting angles: 7(1) and 3(1)8]. This deformation
is surely induced to avoid the anomalous proximity of the
two inner aryl groups, which are still close enough for p–p
interaction (Figure 1a and b). They overlap nearly in paral-
As shown in Scheme 5, the reaction of 2,2’-dimethylbi-
phenyl with BuLi in the presence of TMEDA gave 2,2’-bis-
ACHTUNGTRENNUNG
(lithiomethyl)biphenyl,[19] which was then treated with 4,4’-
Scheme 5. Preparation of 2,2’-bisACTHNUGRTENUNG[2,2-bis(4-alkoxyphenyl)ethenyl]biphenyl
3.
dimethoxybenzophenone 5a in THF to give bisACHTNUTRGNE[UNG bis(4-me-
thoxyphenyl)ethanol] derivative 4a. Treatment with a cata-
lytic amount of TsOH in benzene at reflux gave bis[bis(4-
AHCTUNGTRENNUNG
methoxyphenyl)ethenyl]biphenyl 3a in 50% yield over two
steps. Next, oxidative cyclization[11a] was performed by the
treatment of 3a with four equivalents of NOBF4 in CH2Cl2.
À
The dark purple powder of dicationic salt 2a2+
N
obtained in 94% yield. When a smaller amount of oxidant
was used, the same salt was obtained, and the starting mate-
rial 3a was recovered, because 1a generated in situ is more
easily oxidized than the starting material 3a (Scheme 4).
The dication exhibits a characteristic strong absorption
Figure 1. a) Top view and b) side view of X-ray structures of (P)-1a in
the racemic crystal [1a·ACTHNUTRGNEUNG(CHCl3)2] determined at 123 K. Thermal ellip-
soids are shown at the 50% probability level. c) Top view and d) side
view of X-ray structures of (P)-1c in the crystal of 1c containing both di-
astereomers [(P)- and (M)-1c] determined by X-ray analysis at 123 K.
Thermal ellipsoids are shown at the 30% probability level.
band in the visible region [lmax =502 nm (loge=4.76) in
À
MeCN]. When 2a2+
N
powder in dry DME, the deep purple color disappeared rap-
idly, and Ar4DMPs 1a [lmax =325 nm (sh; log e=4.04) in
MeCN] was isolated as stable yellow cubes in 92% yield
after recrystallization. Its thermodynamic stability was dem-
onstrated by quantitative recovery after heating at reflux for
24 h in toluene with no signs of electrocyclization to its iso-
mers.
When other 4,4’-dialkoxybenzophenones 5b–d were used
in the reactions of 2,2’-bis(lithiomethyl)biphenyl, bis(diaryl-
ACHTUNGTRENNUNGethenyl)biphenyls 3b–d with different alkoxy groups were
lel in a face-to-face manner and the closest contact between
the facing aromatic carbons is 3.19(1) ꢁ, which is much
shorter than the sum of the van der Waals radii (3.40 ꢁ).
1
In the H NMR spectrum of 1a (300 MHz), methoxy pro-
tons on the aryl groups appeared as two sharp singlets (d=
3.50 and 3.30 ppm in [D5]bromobenzene) at 258C. The
former resonance is assigned as that for the CH3O in the
inner aryl groups, which is shifted downfield due to de-
shielding effects. When the temperature increases, the two
peaks gradually coalesce (Tc =1008C), and a sharp single
resonance was then observed at higher temperature (Fig-
ure S2 in the Supporting Information), which shows that the
inner and outer aryl groups are exchanged with an energy
barrier of 18.4 kcalmolÀ1. This value also corresponds to the
inversion of helicity [(P)-1a< = >(M)-1a]. Such a small
barrier makes it impossible to perform the optical resolution
of (P)- and (M)-1a. This should also be the case for other
Ar4DMPs 1b–d. Although 1c and 1d with four chiral alkoxy
similarly obtained via diols 4b–d in respective yields of 56,
58, and 55% over two steps. The oxidative cyclization of
3b–d was conducted by using four equivalents of (4-
À
BrC6H4)3N+ SbCl6 . Without purification, the resulting dica-
C
tion salts of 2b2+–d2+ were subjected to reduction with Zn
to give Ar4DMPs as yellow crystals (1c, 89% yield) or
yellow oils (1b, 81%; 1d, 83%). Again, they are thermo-
ACHTUNGTRENNUNGdynamically stable and can be kept at room temperature
under air, which validates our design concept for stabilizing
Ar4oQD by dibenzo annulation.
Structural features of 1: To investigate the detailed structural
features of the isolable derivative of Ar4oQD, 1a was sub-
jected to a low-temperature X-ray analysis (CHCl3 solvate,
groups [(R)-C2H5CH
ACHUTTNGERN(NUG CH3)O or (R)-C6H13CHACHTUGNTREN(NUNG CH3)O, re-
spectively] exist as mixtures of diastereomers with an oppo-
Chem. Eur. J. 2013, 19, 117 – 123
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119

T. Suzuki et al.
À
C
site sense of helicity, they were treated as single entities due
to facile interconversion.
equivalents of (4-BrC6H4)3N+ SbCl6 , the corresponding di-
À
cation salts 2a2+–d2+
[SbCl6 ]2 were isolated as deep purple
Upon recrystallization, a single-crystal specimen of 1c
(P212121, Z=8, two independent molecules) was obtained.
Interestingly, the crystal is composed of an equal amount of
diastereomers [(P)-1c for molecule-1, (M)-1c for molecule-
2], which is not a common crystallization phenomenon.[15a,b]
X-ray analysis provided structural information on both dia-
stereomers with the same point chiralities on the aryl rings,
but with a different helical sense (Figures 1c, d, and S3 in
the Supporting Information). Despite the similar helical ge-
ometries of the (P)- and (M)-isomers, their structural pa-
rameters differ slightly and indicate larger deformation in
crystals or amorphous compounds in high yields (93, 95, 98,
and 97%, respectively).[21] The salt of 2a2+ (see above) and
those of 2b2+–d2+ reproduced Ar4DMP 1a–d upon reduc-
tion with Zn dust in high yields (92, 95, 100, and 97%, re-
spectively), as was confirmed by the reversible nature of the
present redox pairs.
Interestingly, the reduction potentials of 2a2+–d2+ were
observed in the far cathodic region (Ered = +0.11, +0.07,
+0.03 and +0.03 V for 2a2+–d2+, respectively; two-electron
process). Such a large shift of redox peaks as well as a one-
wave two-electron oxidation process are commonly ob-
served in dynamic redox pairs[12] undergoing drastic structur-
À
the (M)-isomer. The torsion angle for the Ar2C=C C=CAr2
À
unit, the twisting angle of the biphenyl skeleton, and the
twisting angles of the two exocyclic bonds are 59.7(8),
23.1(9), 4(1), and 2(1)8 in the (P)-isomer and 69.4(6),
25.2(8), 10(1), and 8(1)8 in the (M)-isomer, respectively.
al changes and/or reversible formation/breaking of C C
bonds upon electron transfer.[7,8,14,22] The separation by ap-
proximately 0.6 V indicates a high electrochemical bistability
of the redox couples of 1/22+, which is favorable for re-
1
In the H NMR spectrum of 1c at room temperature, the
ACHTUNGTRENaNUGN lizing switching phenomena of redox active molecules.
À
only one set of resonances that correspond to a single C2-
symmetric species was observed, which shows that the two
diastereomers [(P)- and (M)-1c] exhibit indistinguishable
chemical shifts. The slightly different steric interactions for
the diastereomeric pair may or may not bias the equilibrium
ratio in favor of one of the two diastereomers,[20] however,
this issue cannot be detailed experimentally in either 1c or
1d for the same reason.
Upon crystallization of 2c2+
ACHTNGUTERUNN[G SbCl6 ]2 from CH2Cl2/ether,
a high-quality, single-crystal specimen was obtained, X-ray
analysis of which (P21, Z=2) revealed that the phenan-
threne core in 2c2+ is nearly planar (Figure 3). The two di-
AHCTUNGTREGaNNUN rylmethylium units are attached at the 9,10-positions with
large twisting angles [68(1) and 63(1)8] to give a helical ar-
rangement of the p system, the helical sense of which is sim-
ilar to that in (P)-1c. Although the helical inversion of (P)-
2c2+ can readily occur in solution through rotation about
+
the C9(10)
bonds, there are no diastereomeric dications
À
C
Redox properties and interconversion between Ar4DMPs
(1) and phenanthrene-9,10-diylbis(diarylmethylium)s (2):
Due to not only by the electron-donating alkoxy groups, but
also the observed skeletal deformation and intramolecular
p–p interaction, Ar4DMP 1a has an increased HOMO level,
so that it undergoes a facile electrochemical oxidation
(Eox = +0.61 V vs. Ag/Ag+ in CH2Cl2; two-electron pro-
with (M)-helicity in the crystal. Thus, the point chirality of
the alkoxy group [(R)-C2H5CHACHTNUGRTENUNG(CH3)O] is transmitted to the
(P)-helicity preference of the dication, in which the two
chromophores are stacked nearly in parallel with a shortest
À
C C contact of 3.05(1) ꢁ. Such an asymmetric geometry is
suitable for realizing exciton coupling of the two chromo-
À
G
phores.[23] Accordingly, the CD spectrum of 2c2+
N
A
salt, taken as a KBr tablet, showed the negative bisignated
Cotton effects in the l=500–600 nm region (Figure S4 in
the Supporting Information).
d, respectively; Figure 2). Upon treatment of 1a–d with two
Chiroptical properties and a multi-input/multi-output re-
sponse of 1c/2c2+ and 1d/2d2+: A large negative couplet
À
was observed in the CD spectrum of 2c2+
N
corded in CH2Cl2 [lext =591 nm (De=À23), 520 (+17)],
showing that the transmission of point chirality to helicity[15]
also occurs in solution to induce a preference for (P)-helici-
1
À
ty in 2c2+. In the H NMR spectrum of 2c2+
ACHTUNTRGENUNG[SbCl6 ]2, only
one set of resonances that corresponded to a single C2-sym-
metric species was observed, indicating that the two dia-
AHCTUNGTRENNUNG
stereomers [(P)- and (M)-2c2+] interconvert so rapidly that
we cannot determine the diastereomeric excess in terms of
the helicity preference. The observed ellipticity in 2c2+ is
much larger than that in the reference monocation [4-(R)-
À
C2H5CH
(CH3)O-C6H4]2CPh+BF4
[lext =508 (De=À1.3),
411 (À0.83), 270 nm (À1.4)],[15a] which can be explained by
the effective amplification of CD signals through exciton
coupling of the two cationic chromophores in the preferred
Figure 2. Cyclic voltammogram of 1c recorded in CH2Cl2 (0.5 mm) con-
taining Bu4NBF4 (0.1m) as a supporting electrolyte (E/V vs Ag/Ag+,
scan rate 100 mVsÀ1, Pt electrode).
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Chem. Eur. J. 2013, 19, 117 – 123

Helical p-Electron System that Exhibits Unique Properties
FULL PAPER
Figure 4. Changes in a) UV/Vis and b) CD spectra upon constant current
(27 mA) electrolysis of 1c (3.5 mL, 1.8ꢂ10À5 m) in CH2Cl2 containing
Bu4NBF4 (0.05m) as a supporting electrolyte (every 5 min).
solved in a less-polar solvent without enough stabilization
by solvation.[25] Thus, the UV/Vis and/or CD spectra for
some helical dicationic dyes often exhibit spectra with inter-
esting solvent-dependent feature.[15a,16i] This falls true for the
case of the present dications 22+. The solvatochromic behav-
ior can be demonstrated more easily by using 2b2+ with
long alkyl chains due to their higher solubility in a variety of
solvents including less-polar ones, such as benzene. As
shown in Table 1 and Figure S5 in the Supporting Informa-
tion, the strongest absorption band of 2b2+ in MeCN ap-
peared at l=504 nm, whereas when recorded in benzene, it
is at 523 nm. Although the shift is not so spectacular, contin-
uous changes were observed in THF, CH2Cl2, and CHCl3
with intermediate polarity. Such solvent dependency is
likely originated from the stacking geometry allowing p–p
À
Figure 3. Molecular structure of dication 2c2+ in the SbCl6 salt deter-
mined by X-ray analysis at 153 K. All of the molecules in the crystal
have the same configuration of (P)-helicity in terms of the twisting
+
around C9(10)
bility level.
bonds. Thermal ellipsoids are shown at the 50% proba-
À
C
(P)-diastereomer. This is also the case for 2d2+ having (R)-
C6H13CH(CH3)O chiral auxiliaries on the aryl group [lext
589 (De=À18), 519 nm (+15) in CH2Cl2].
A
=
Strong CD signaling in 2c2+ and 2d2+ is advantageous for
their use as an electrochiroptical material, because an elec-
trochemical input can be transduced into two kinds of out-
puts, namely, UV/Vis and CD spectral changes. Thus, upon
electrochemical oxidation of 1c to 2c2+, both UV/Vis and
CD spectra changed drastically, as shown in Figure 4. The
presence of several isosbestic points indicates a clean con-
version between 1c and 2c2+ as well as a negligible steady-
state concentration of the intermediary cation radical spe-
cies. A two-way-output response was similarly observed in
the case of 1d/2d2+ (Figure S5 in the Supporting Informa-
tion). The observation validates our molecular design con-
cept for constructing novel electrochiroptical systems[16]
based on Ar4DMPs by attaching chiral auxiliaries on the
aryl groups.
À
Table 1. Solvent-dependent UV/Vis spectra[a] of [SbCl6
]
salts of 2b2+
2
and 2d2+
.
Solvent
Dielectric constant
l [nm] (loge)
2d2+
2b2+
504 (4.92)
MeCN
CH2Cl2
THF
37.5
9.1
7.6
4.9
2.3
508 (4.94)
560 (sh; 4.60)
517 (4.93)
565 (sh; 4.62)
518 (4.96)
575 (sh; 4.62)
513 (4.86)
580 (sh; 4.62)
514 (4.92)
567 (sh; 4.62)
519 (4.93)
565 (sh; 4.62)
520 (4.98)
CHCl3
benzene
573 (sh; 4.63)
523 (4.83)
580 (sh; 4.59)
523 (4.91)
Through overlap of the two cationic chromophores, the
electrostatic destabilization of 22+ can be reduced by p–p in-
teraction,[24] which is more important when the salt is dis-
572 (sh; 4.58)
576 (sh; 4.59)
[a] Data of absorption maxima (lmax and loge) and shoulders for the first
band.
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121

T. Suzuki et al.
interaction between two cationic chromophores, because the
Davydov splitting of the band is evident in all cases. Similar
solvatochromism was observed for 2d2+ with chiral auxilia-
ries, in which not only UV/Vis, but also CD spectra change
according to the solvent polarity (Tables 1, 2, and Figure 5).
Conclusion
The present work demonstrates that the molecular frame-
work of Ar4DMP 1 could be used as a unique platform for
constructing a new series of molecular-response systems.
The effective interconversion between the stabilized quino-
dimethanes 1 and dicationic dyes 22+ upon electron transfer
is the key feature for establishing these new electrochromic
systems to which the helical molecular framework in both
redox states adds chiroptical properties. The mobile helicity
in dications 22+ with a p–p stacking geometry can be biased
to prefer one handedness by the transmission of point chi-
Table 2. Solvent-dependent CD spectra[a] of 2d2+[SbCl6À]2 in various sol-
vents.
Solvent
Dielectric constant
lext [nm] (De)
MeCN
CH2Cl2
THF
CHCl3
benzene
37.5
9.1
7.6
4.9
2.3
581 (À27), 508 (+18)
589 (À18), 519 (+15)
586 (À16), 514 (+13)
589 (À14), 520 (+11)
587 (À6), 527 (+8)
AHCTUNGTREGrNNNU ality on the aryl groups. Due to the solvent dependency of
the association of triarylmethylium units, as well as solvo-
phobic effects, dication 2d2+ exhibits solvato- and chirosol-
vatochromic behavior, latter of which can be detected by
both UV/Vis and CD spectroscopy, and thus 1d/2d2+ may
present a new multi-input/multi-output response system.
[a] Data of extrema (lext and De) for the first couplet.
Acknowledgements
This work was performed under the Cooperative Research Program of
“Network Joint Research Center for Materials and Devices”. The Grant-
in-Aid for Scientific Research on Innovative Areas “Organic Synthesis
based on Reaction Integration. Development of New Methods and Crea-
tion of New Substances” (No. 2105) is gratefully acknowledged. We
thank Prof. Tamotsu Inabe (Department of Chemistry, Faculty of Science,
Hokkaido University) for the use of facilities to analyze the X-ray struc-
tures. Elemental analyses were done at the Center for Instrumental Anal-
ysis of Hokkaido University. Mass spectra were measured by Dr. Eri Fu-
kushi at the GC-MS & NMR Laboratory (Faculty of Agriculture, Hok-
kaido University).
[1] For reviews and highlight, see: a) J. L. Charlton, M. M. Alauddin,
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ACTHNUTRGNEUNG
Figure 5. CD spectra of 2d2+[SbCl6À]2 salt recorded in various solvents.
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À
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the solvent-dependent alternation in geometry, because the efficien-
cy of exciton coupling is angle dependent.
À
[27] The A values of 2c2+
[SbCl6
]
measured in several solvents are as
2
follows: À22 in MeCN, À35 in CH2Cl2, and À29 in CHCl3. The
value in benzene cannot be determined due to low solubility of the
salt.
Received: August 31, 2012
Published online: November 21, 2012
Please note: Minor changes have been made to this manuscript since its
publication in Chemistry—A European Journal Early View. The Editor.
Chem. Eur. J. 2013, 19, 117 – 123
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www.chemeurj.org
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