Multi-Input/Multi-Output Molecular Response System
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much greater than the standard value of 1.54 ꢂ, but is typi-
cal for sterically congested hexaphenylethane derivatives.[23]
Two methoxyphenyl groups at the pseudo-equatorial posi-
tions are located in proximity and face each other over the
Both peaks correspond to the two-electron transfer pro-
cess, and there are no other peaks in the voltammograms.
The negligible steady-state concentration of the intermedi-
+
+
+
ary cation radicals 1+ /2 (and 3 /4 ) can be accounted for
C
C
C
C
long C C bond (the shortest C C contact: 3.14(1) ꢂ). The
other two at the pseudo-axial positions face outward.
Figure 1b shows the ORTEP drawing of (R)-2a2+ in the
by assuming the reaction mechanism shown in Scheme 5.[21b]
À
À
À
SbCl6 salt obtained by low-temperature X-ray analysis. The
absolute configuration of the dication was confirmed by the
anomalous dispersion of X-ray (Flack parameter: x=
0.000(17)). The dihedral angle of the two naphthalene units
is 68.5(1)8 with a separation of 3.54(1) ꢂ between the two
À
carbenium centers. A much closer C C contact (3.29(1) ꢂ)
was observed between the two methoxyphenyl groups facing
each other (dihedral angle: 10.8(2)8), which reflects the ef-
fective p–p overlap of two diarylcarbenium units. The
broadness of the first band in the electronic absorption
(labs =534 nm in CH2Cl2) in 2a2+ may be related to Davy-
dov splitting through p–p interaction, although this band
may represent several transitions.
Scheme 5. Mechanism of interconversion.
In the oxidation process of the donors, the as-prepared
+
C
C
Redox behavior and mechanism of interconversion: The
redox potentials of newly prepared electron donors 1a–c
were measured by cyclic voltammetry in CH2Cl2 (238C, scan
rate 100 mVsÀ1) and the oxidation potentials (Eox) are sum-
marized in Table 1 along with those of reference compounds
cation radicals 1+ (and 3 ) would undergo rapid long-bond
fission[24] to give more stable species 2+ (and 4 ), in which
+
C
C
the unpaired electron and positive charge can be delocalized
over each of the triarylmethane units. The lifetimes of 2+
C
(and 4+ ) would also be very short during the oxidation pro-
cess because their oxidation potentials (ca. +0.2 V) must be
far less positive than those of the neutral donors 1 (and 3).
In the reduction process, electron capture at the two cationic
sites in 22+ (and 42+) would proceed in a stepwise manner
with marginal separation between the two reduction poten-
tials (Er1ed, Er2ed) due to electronic interaction of the two chro-
C
Table 1. Redox potentials[a] of dihydro[5]helicene-donors 1a–c and bi-
naphthylic dications 2a2+–2c2+ measured in CH2Cl2 along with those of
related compounds.
n
Eox [V]
Ered [V]
1a
1b
1c
3a
3b
3c
1
+1.29
+1.32
+1.32
+1.48
+1.50
+1.53
–
–
–
–
8
16
1
8
16
1
mophores through the face-to-face overlap.[6h,21a,25] Due to
the facile disproportionation of 2+ into 2 /2 (and 4+ into
2+
C
2C
C
2+
2C
2C
4 /4 ) as well as the rapid bond formation of 2 to 1 (and
to 3), the steady-state concentration of the intermediary
species would also be negligible upon reduction.
2C
4
2a2+
2b2+
2c2+
4a2+
4b2+
4c2+
+0.23
+0.21
+0.21
+0.21
+0.18
+0.17
8
16
–
–
The above idea regarding the mechanism shown in
Scheme 5 was supported by the following experimental re-
sults. For the reduction process, voltammetric analyses were
carried out at low temperature (À788C) with a fast scan
rate (5 VsÀ1) in CH2Cl2, which supported the sequential
[a] E versus SCE, scan rate 100 mVsÀ1, 0.1m Bu4NBF4 as a supporting
electrolyte, Pt electrode, 298 K.
two-step one-electron reduction (Er1ed =+0.09 V; E2red
=
À0.05 V for 2a2+) by retarding the disproportionation pro-
cess (Figure S1c in the Supporting Information). In those
voltammograms, the oxidation waves remained unchanged,
suggesting that a much faster method is necessary to eluci-
date the mechanism of the oxidation process. Therefore, for
this purpose, photoinduced electron-transfer (PET) reac-
tions of 3a,b were conducted with nanosecond-absorption
3a–c. The electrochemical oxidation of 1 occurs at approxi-
mately +1.3 V versus SCE, which is slightly less positive
than Eox of 3 (ca. +1.5 V), probably due to the electron-do-
nating properties of the binaphthyl core. The oxidation pro-
cess is irreversible since the corresponding reduction peak
was observed in the far cathodic region (ca. +0.2 V), which
was assigned to the reduction process of the bond-dissociat-
ed dication, as confirmed by the independent measurements
of 22+ (and 42+) under similar conditions (Figures S1a and b
in the Supporting Information). Such separation of redox
peaks in the voltammogram is a characteristic feature of dy-
namic redox systems,[7] and provides electrochemical bista-
bility for the redox pairs of 1/22+ (and 3/42+).
spectroscopy on laser flash photolysis.
system that used an N-methylquinolinium tetrafluoroborate
([NMQ][BF4], sensitizer, 1.0 mm)–toluene (cosensitizer,
A cosensitizing
AHCTUNGTRENNUNG
1.0m) couple in aerated MeCN at ambient temperature was
used, since this system allows us to directly detect the inter-
mediary cation radicals in a one-electron oxidation process
(Scheme S1 in the Supporting Information).[26]
Chem. Eur. J. 2009, 15, 9434 – 9441
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9437