Molecular Wires with Controllable π-Delocalization Incorporating Redox-Triggered π-Conjugated Switching Units

Article abstract of DOI:10.1002/cplu.201800341

A perfluorobiphenyl-2,2′-diyl dication and its corresponding dihydrophenanthrene-type electron donor are interconvertible upon two-electron transfer. Redox-triggered C?C bond-formation/cleavage caused a drastic change in the torsion angle of the biphenyl

Full text of DOI:10.1002/cplu.201800341

Accepted Article
Title: Molecular Wires with Controllable pi-Delocalization Incorporating
Redox-triggered Switching Units of pi-Conjugation
Authors: Wataru Nojo, Hitomi Tamaoki, Yusuke Ishigaki, Ryo Katoono,
Kenshu Fujiwara, Takanori Fukushima, and Takanori Suzuki
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Molecular Wires with Controllable -Delocalization Incorporating
Redox-Triggered -Conjugated Switching Units
Wataru Nojo,[a] Hitomi Tamaoki,[a] Yusuke Ishigaki,[a] Ryo Katoono,[a] Kenshu Fujiwara,[b] Takanori
Fukushima,[c] Takanori Suzuki*[a]
In memory of Shô Itô (1924-2018)
electronic communication along the system.[6]
A biaryl-type molecular response system[7] is a promising
switching unit since its torsion angle () can be controlled by an
external stimulus. Thus, the larger degree of delocalization is
expected when the switching unit adopts a nearly planar
structure (“ON-state“), whereas a tightly twisted geometry drives
the molecular wire into the “OFF-state“ by disconnecting the
Abstract:
A
perfluorobiphenyl-2,2'-diyl
dication
and
its
corresponding dihydrophenanthrene-type electron donor are
interconvertible upon two-electron transfer. Redox-triggered C-C
bond-formation/cleavage caused a drastic change in the torsion
angle of the biphenyl unit. Thus, -delocalization ON/OFF switching
was observed as a change in the UV spectrum upon electrolysis of
the linearly extended analogue with two phenylethynyl groups. A
further extended system with a molecular length of ca. 3.5 nm,
effective conjugation (Scheme 1a).
Molecular wires with
multiple switching units have attracted further attention[8]
because a greater ON/OFF ratio regarding delocalization is
expected due to the simultaneous action of the switching units
(Scheme 1b), although only a few examples have been reported
so far.
which
has
two
switching
units,
was
synthesized.
Spectroelectrograms as well as voltammetric analyses showed that
the two units act nearly simultaneously because of the very small
inter-unit electrostatic repulsion in the tetracationic state. Thus, the
present pair is a promising candidate as a switching unit for the
"molecular wires" with controllable delocalization, in which a
higher ON/OFF ratio of delocalization could be realized by
incorporating multiple switching units.
The effectiveness of ON/OFF switching can be evaluated
through changes in the delocalization of electrons across the
switching units. Thus, ON/OFF efficiency can be deduced from
the stimulus-induced changes in UV-vis spectra.
Introduction
Linearly conjugated electron systems such as polyynes[1] or
polyarylenes[2] are often considered as models for the “molecular
wires“.[3] which can conduct electrons along the system.[4] On
the other hand, they can also "wire" two chromophores at the
termini.
The electronic communication between the
chromophores depends on the nature of the components,
which consist of repeating units in the main chain structure.[5]
When a conjugation-switching unit (“switching unit“) is inserted
into the linear system, the molecular wire is endowed with the
ON/OFF switching properties of delocalization to modify the
[a]
W. Nojo, Dr. H. Tamaoki, Dr. Y. Ishigaki, Dr. R. Katoono, Prof. T.
Suzuki
Department of Chemistry, Faculty of Science, Hokkaido University,
Sapporo 060-0810 (Japan)
E-mail: tak@sci.hokudai.ac.jp
Prof. K. Fujiwara
Department of Life Science, Graduate School of Engineering,
Akita University, Akita, 010-8052 (Japan)
Scheme 1 ON/OFF switching of -delocalization by
geometrical change of the switching unit(s)
[b]
[c]
Prof. T. Fukushima
Laboratory for Chemistry and Life Science, Institute of Innovative
Research, Tokyo Institute of Technology, Yokohama 226-8503
(Japan)
Supporting information for this article is given via a link at the end
of the document: The synthetic details and spectral data of 2-6, 8,
9 and 11. UV spectra of 4, 5, 6, 11, and salts of 22+,42+ and
54+;geometrical parameters of 22+ in EtOH solvated 22+(SbCl6–)2;
DFT calculations of syn-5, anti-5, and syn-6; comparisons of
calculated bond lengths in tolane units; cyclic voltammograms of
2,4, and 22+ salt; spectroelectrograms of 5 and 6, and plausible
redox scheme of 5,6 to 54+,64+
.
Scheme 2 Dynamic redox systems based on DHP/BP2+
skeleton that undergo drastic geometrical change
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Formula
[ Ar = 4-MeOC6H4 ]
ArAr
ArAr
Ar
Ar
Here, we designed
a
perfluorobiphenyl-based dyrex pair
Ar
Ar
F
F
F
F
F
(2/22+)[10] as a switching unit (Scheme 2), which would undergo
SNAr reactions at the F atom(s)[11] with acetylides to give
ethynylated derivatives such as 3 or molecular wire 4. A longer
wire with two switching units (5) would be synthesized by Pd-
catalyzed cross-coupling reactions of 3. By the further SNAr
reaction of 5, compound 6 would be obtained as the longest wire
in this work, with an approximate molecular length of 3.5 nm.
Here we report the preparation and redox-dependent
spectroscopic properties of 2 - 6, and discuss the validity of the
perfluorobiphenyl-electrophore (2/22+) in terms of redox-type
conjugation switching units.
C7
F
C2
F
C2
F
F
F F
F F
3'
3
ArAr
Ar
Ar
F
F
F
F
F F
4
Ar
Ar
ArAr
F F
Ar
Ar
Ar
Ar
F
F
F
F
F
F
F
F
F
F
Results and Discussion
F F
5
Molecular Design of a Switching Unit (2/22+) and Molecular
Wires 4 - 6.
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
DHP/BP2+-type dyrex pairs have several advantages as redox-
type switching units. First, the biaryl unit in DHP is more or less
coplanar (: ca. 20°)[12] due to the C9-C10 ethano bridge, whereas
BP2+ adopts a twisted conformation (: ca. 70°)[9b,13] since the
bridging C-C bond is cleaved upon oxidation. Second, in
contrast to ordinary organic redox pairs, the facile two-electron
transfer occurs nearly at the same potential, which favors high
reversibility of the redox processes by reducing the steady-state
concentration of the reactive open-shell species.[14] Third, thanks
to C-C bond formation/cleavage,[15,16] which causes a drastic
change in electronic structures, there is a large potential shift (ca.
1 V) between the oxidation potential (Eox) of DHP and the
reduction potential (Ered) of BP22+, so that an overpotential is
necessary to switch from one state to another.
F
F
F
F
F
F
F
F
F F
F F
6
Scheme 3.
a)
switching unit
delocalization
ON
OFF
ON
Effectiveness of conjugation of the biaryl unit can be estimated
as cos2,[17] that determines the degree of delocalization over
the unit. Thus, the introduction of substituents at the 6,6'-
positions of the biphenyl skeleton is desirable to increase the 
value for the disconnection of conjugation in the BP2+ state.
Dication 22+ is an octafluoro derivative of 12+ (Scheme 2) and
would be interconvertible with DHP-type donor 2 upon two-
electron transfer. Furthermore, in the dicationic state, the
positive charge would not be delocalized into the
perfluorobiphenyl skeleton, but rather localized on the bis(4-
methoxyphenyl)carbenium moieties. Thus, even when the
molecular wire contains multiple switching units, the through-
bond electrostatic repulsion could be minimized in the
polycationic species. Accordingly, it is highly likely that multiple
switching units can work nearly simultaneously in response to an
external stimulus (Scheme 1b) to realize a high ON/OFF ratio of
delocalization.
b)
OFF
ON
c)
OFF
Molecular wires (4, 5, and 6) were designed so that the
switching unit(s) and other system(s) (phenyl, phenylene) are
connected with ethynyl bonds in anticipation of their highly
coplanar geometries in the ON state. They contain different
numbers of benzene rings (4, 5, and 7, respectively) in the main
chain. In four-ring system 4, the redox reaction would cause a
drastic change in absorption in the UV region because of the
ON/OFF switching of conjugation between the two tolane (1,4-
diphenylacetylene) units (Scheme 3a). In five-ring system 5 with
Scheme 3 ON/OFF switching of -delocalization in the
molecular wires with DHP/BP2+-type switching unit(s)
We have shown that the dynamic redox (dyrex) pairs of
9,9,10,10-tetraaryl-9,10-dihydrophenanthrenes
(DHP)
and
biphenyl-2,2'-diyl-type dications (BP2+) such as 1/12+ undergo
reversible C-C bond formation/cleavage upon 2e-transfer.[9]
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two switching units connected by 1,4-diethynylbenzene, the
change in UV absorption would be marginal, since only the
tetrafluorobenzene termini are twisted (Scheme 3b). However, 5
would provide valuable information on the interaction between
the two switching units to confirm their nearly simultaneous
action. The longest wire 6 is a unique touchstone for evaluating
the proposal in this work by confirming that both switching units
actually contribute to ON/OFF switching of the main-chain
delocalization (Scheme 3c).
cationic state of molecular wires. Chemical reversibility was
confirmed by reducing 52+(SbCl6 )4 with Zn to give
5 in 93%
–
yield.
By the reaction of
under SNAr conditions, molecular wire
5
with ethynylbenzene (2.2 equiv.)
was obtained in
6
43% yield as an orange solid. We noticed that
decomposed, especially in solution.
6 slowly
Scheme 4.
[ Ar = 4-MeOC6H4 ]
MeO2C
Preparation of a Switching Unit (2/22+) and Molecular Wires
4 - 6.
HO2C
F
F
F
F
F
F
F
Me3SiCHN2
(y. 91%)
As shown in Scheme 4, octafluorodiphenoic acid (8)[18] derived
from 1,2-dibromo-3,4,5,6-tetrafluorobenzene (7) was converted
into methyl ester 9 in 91% yield. Reaction of 9 with excess 4-
MeOC6H4MgBr gave tetraarylated diol 10 in 57% yield.
Under acidic dehydrating conditions [HBF4 - (CF3CO)2O],
diol 10 was transformed into BP2+-type salt, which was
F
F
F
F
F
F
F
F
F
CO2H
CO2Me
8
9
Ar
HO
Ar
F
F
F
ArMgBr
(y. 57%)
HBF4
(CF3CO)2O
directly converted to DHP-type donor
2 as a yellow solid
Zn
2
F
F
upon treatment with Zn powder in 76% yield over two steps.
(y. 76%
2 steps)
with (4-BrC6H4)3N+•SbCl6 (2
F
–
F
OH
Through the reaction of
equiv.), dication salt
2
F
–
2+(SbCl6 )2 was isolated as a stable
Ar
2
Ar
10
entity in 93% yield. The dark red powder of the salt was
transformed back to yellow donor upon treatment with Zn
powder in 96% yield. This high-yield interconversion
indicates that and
2+ can be considered to be a kind of
reversible redox pair, similar to non-fluorinated derivatives
2
Scheme 4 Synthetic scheme for 2
2
2
Scheme 5.
[ Ar = 4-MeOC6H4 ]
1
and 12+ (Scheme 2), although microscopic electrochemical
reversibility is not maintained due to concomitant C–C
bond-formation/cleavage upon redox reactions (Scheme 5).
We continued our synthesis by taking advantage of the
SNAr-reactivity of perfluorobiphenyls that proceeds
selectively at the 4,4’-positions,[7a,11a,b] which is desirable for
extending the conjugation along the long-axis of the
ArAr
Ar
Ar
e-
F4
F4
fast
2+
Ar
e-
(DHP)
Ar
Ar
Ar
2
22+
(BP)
biphenyl skeleton. As shown in Scheme 6, the reaction of
2
(DHP)
with ethynyltriisopropylsilane (1.2 equiv.) in the presence of
NaN(SiMe3)2 proceeded at the 2-position of DHP as
expected, and detachment of the silyl group gave 2-ethynyl
e-
F4
F4
Ar
F4
fast
2+
(BP)
Ar
Ar
Ar
e-
derivative (
low yield can be accounted for if we consider the by-
production of 2,7-diethynyl derivat recovery of (ca. 20%).
The reaction of with ethynylbenzene (2.2 equiv.) under
similar SNAr conditions gave molecular wire in 40% yield
3) in 28% yield over two steps. The relatively
2
F4
2
22
(BP)
4
Scheme 5 Mechanism of dyrex interconversion between 2 and 22+
by selective reaction at the 2,7-positions of DHP. Upon
oxidation with (4-BrC6H4)3N+•SbCl6 (2 equiv.), dication salt
–
Scheme 6.
–
4
2+(SbCl6 )2 was isolated as a stable entity in 84% yield,
1) iPr3Si
NaN(SiMe3)2
Ph
showing that extension along the long-axis of the
biphenyl unit does not induce fatal instability of the cationic
NaN(SiMe3)2
(y. 40%)
(y. 28%)
2
6
4
3
2) Bu4NF
state of the switching unit. Chemical reversibility was
(4-BrC6N4)3N+
2+(SbCl6 )2 with Zn to give
4
in 99%
–
1,4-C6H4I2
Pd(PPh3)4
CuI
confirmed by reducing
yield.
4
-
SbCl6
Zn
(y. 99%)
(y. 84%)
Next, we sought to synthesize the longer wire
5
under
Ph
Sonogashira conditions. The reaction of
diiodobenzene proceeded smoothly to give
3
5
with 1,4-
(y. 81%),
NaN(SiMe3)2
42+
-
(y. 81%)
Zn
(y. 43%)
5
(SbCl6 )2
which was isolated as a stable orange solid.[19] Upon the
oxidation of
5
with (4-BrC6H4)3N+•SbCl6 (4 equiv.),
–
(4-BrC6N4)3N+
4+(SbCl6 )4 was isolated as a stable dark
–
-
tetracation salt
5
SbCl6
(y. 93%)
(y. 77%)
54+
-
(SbCl6 )4
red solid in 77% yield, showing that the incorporation of two
switching units does not induce fatal instability of the
Scheme 6 Synthetic schemes for 3 - 6 as well as
interconversion of 4/42+ and 5/54+.
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All of the molecular wires (4 - 6) exhibit strong absorptions
Table 1. UV-Vis spectral data measures in MeCN
absorption max/nm (log / M-1cm-1)]
in the UV region (Table 1, Figure S1). The absorption band
of four-ring system (345 nm) is absent in and is
compound
4
2
assigned to the 4,4'-bis(phenylethynyl)perfluorobiphenyl
skeleton.[20] This strong band is red-shifted to 351 nm in the
2
3
4
5
271 (4.15), 230 (4.65).
328 (4.62), 265 (4.65), 2441 (4.69)
345 (4.58), 287 (4.45)
five-ring system
6
5 and to 357 nm for the seven-ring system
due to extension along the main chain.[21]
X-ray Structures of the Switching Unit (2/22+)
According to an X-ray analysis, adopts a shallow helical
351 (4.80)
2
6
357 (4.90)
geometry, as in other DHP derivatives (Figure 1a). The
torsion angle [<(C4-C4a-C4b-C5)] corresponding to DHP is
35.1(4)°, which is slightly greater than those of the related
molecules[12] due to the F atoms at the bay regions. The
22+(SbCl6
42+(SbCl6
54+(SbCl6
)
536 (4.79), 392 (4.31), 2770 (4.53)
570sh (4.62), 533 (4.87), 411 (4.45), 318 (4.80), 279 (4.79)
536 (5.08), 400 (4.68), 3337 (4.90), 272 (4.89)
–
2
2
4
–
)
)
C9–C10 bond [1.632(4) Å] is longer than the standard Csp3
–
–
Csp3 bond (1.54 Å), which is characteristic of sterically
congested DHP derivatives.[22]
An X-ray analysis was also conducted for the dication salt
2+(SbCl6 )2 (Figures 1b and S2). When the C9–C10
–
of
2
bond of
2
is cleaved, the cationic centers are separated by
3.745(9) Å. The two molecular halves are tightly twisted:
the torsion angle of the biphenyl skeleton [<(C6-C1-C1'-
C6')] is 79.0(1)°, which corresponds to BP
biphenyl bond length is 1.504(7) Å, which is longer than that
in neutral [1.480(4) Å] due to disconnection of conjugation.
.
The central
2
In both triarylmethylium units, each of 4-methoxyphenyl
group is connected to the sp2 cationic plane with a small
dihedral angle [17.0(2), 17.5(2), 29.5(2), and 34.4(3)°],
whereas the both cationic planes form a larger dihedral
angle with the fluoroarene unit [52.6(3) and 56.8(3)°]. The
latter values indicate that the positive charges are not
delocalized on the perfluorobiphenyl skeleton. This was
further confirmed by comparing the bond lengths of C+ -
Cipso(C6H4OMe) [1.406(8), 1.424(8), 1.425(9), and 1.441 (8)]
with those of C+ - Cipso(C6F4) [1.471(9) and 1.498(7) Å], which
shows that there is a greater increase in double-bond
character in the former bonds.
Thus, X-ray analysis revealed that the dyrex reaction was
associated with a large difference between the torsion
angles
(DHP
andBP).
By
considering
the
cos2dependency of the delocalization along the system,
the expected OFF-state delocalization would be ca. 5% of
that in the ON-state (ON/OFF ratio of 18.4). When the two
simultaneously-acting switching units are incorporated, the
expected OFF-state delocalization would be ca. 0.3% of
that in the ON-state (ON/OFF ratio of 18.42 = 340), and
thus nearly complete ON/OFF switching would be realized.
DFT Calculations Regarding the Geometrical and
Electronic Structures of Molecular Wires 4, 5 and 6
The geometrical and electronic structures of
4 were
estimated by DFT calculations (B3LYP/6-31G*). The DHP
unit has a molecular geometry (C9-C10 bond length, 1.682
Å; biphenyl bond length, 1.470 Å; torsion angle of biphenyl
skeletons: 39.6º) similar to that determined by the X-ray
Figure 1. ORTEP drawings of (a)
2
in hexane solvate and (b) 22+ in
analysis of 2. The external ethynylbenzene unit is located
–
EtOH solvated
2
2+(SbCl6 )2 salt.
nearly on the same plane as the trifluoroaryl groups
(dihedral angle between the aromatic rings in the tolane
units: 3.0 and 0.3º).
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Scheme 7
Ar Ar
Ar
Ar
Ar
e-
F
F
F
F
cation radical is more easily oxidized than
dyrex reaction may proceed to give 42+
DFT calculations for with two switching units were
4, and thus the
.
5
F
F
F
carried out for both the syn- and anti-conformers (Figures
S3 and S4), in which the relative orientation of the two
switching units is different. The geometrical and electronic
structures are very close to each other, and thus only the
data for the syn-conformer are shown below. Again, each of
the DHP units has a slightly helical geometry (torsion angle
of biphenyl skeletons: 39.4 and 39.8º). The 1,4-phenylene
spacer and the connected trifluoroaryl units exist nearly on
the same plane (dihedral angle between the aromatic rings
in the tolane units: 1.1 and 0.2º). The HOMO and LUMO
Ar Ar
Ar
F
F
F
main-chain
cation radical
F
F
F
F
Ar Ar
Ar
Ar
diarylmethylene
cation radical
mainly show coefficients on this conjugated main-chain
・
system (Figure 2b). HOMO-1, HOMO-2, HOMO-3 and
HOMO-4 are nearly degenerated (–5.57 - –5.74 eV), and
bond cleavage
(dyrex reaction)
F
F
F
F
each
has
large
coefficients
on
the
bis(4-
Scheme 7 Two-types of cation radicals in the morecular wires.
methoxyphenyl)methylene groups of one of the two DHP
units (Figure S3). This shows that the main-chain-type
cation radical is the major contributor in the longer wire
compounds.
This holds true for the longest wire
6 with two additional
phenylethynyl units. As designed for the ON state, the
main chain is nearly coplanar (torsion angle of the biphenyl
skeletons: 39.3 and 39.3º, and the dihedral angle between
the aromatic rings in the tolane units: 1.3, 1.7, 3.9, and 4.9º)
(Figure S5). Effective delocalization of the main chain is
confirmed by comparing the bond lengths of the tolane units
to those in
and LUMO mainly show coefficients on the main-chain
system (Figure 2c), whereas each of the nearly
4 and 5 (Figure S6). Furthermore, the HOMO
・

degenerated HOMO-1, HOMO-2, HOMO-3 and HOMO-4 (–
5.58 - –5.65 eV) has large coefficients on the bis(4-
methoxyphenyl)methylene groups.
These results indicate that longer molecular wires with
1,4-diethynylbenzene and the present switching units would
have similar coplanar geometries.[23] The large coefficients
of HOMO and LUMO over the main-chain structure are in
accord with effective delocalization in the ON-state.
Table 2. Redox potentials measured in MMeCN[a]
compound
2 / 22+
Epa
Epc
+0.30
+1.51
3 / 32+
+1.51
+0.30
+0.30
+0.30
Figure 2. Molecular orbitals and their orbital energies obtained by DFT
4 / 42+
+1.53
calculations (B3LYP/6-31G*) on (a) 4, (b) syn-5 and (c) syn-6.
5 / 54+
+1.47[b]]
ca. +1.5[c]
The LUMO of
4
mainly shows coefficients on the main-
HOMO, HOMO-1 and
[c]
6 / 64+
-
chain system (Figure 2a).
・

HOMO-2 are nearly degenerated, and have large
coefficients on the main-chain as well as on the bis(4-
methoxyphenyl)methylene groups of the DHP skeleton.
[a] E / V vs SCE, 0.1 M Et4NClO4, Ptt electrode, scan rate 500 mV s–1
.
Ferrocene undergoes one-electron oxiddation at +0.38 V under the similar
conditions. All of the waves are irreverssible, and the anodic peak potential
(Epa) in the oxidation wave as well as tthe cathodic peak potentials (Epc) in
the reduction wave are reported. [b] Anoother peak appeared around ca. 1.6
V (See Figure 3d). [c] Due to less solubbility, the peaks are very broad (See
Figure 3e).
Thus, upon one-electron oxidation of
4, both the main-
chain-type and diarylmethylene-type cation radicals
contribute, and the latter is responsible for the dyrex
reaction (Scheme 7). Once the C-C bond of the DHP
skeleton is cleaved (Scheme 5), the resulting BP-type
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Redox Properties and Spectroelectrograms of 2 and 4 with
One Switching Unit
delocalization of positive charges into the main-chain
component was proven to be minimal.
The electrochemical oxidation of
2 in MeCN occurs at an
anodic peak potential (Epa) of +1.51 V vs SCE, which is an
irreversible process as in the case of non-fluorinated
derivative
1
(+1.50 V).[9b] As expected from the
interconversion mechanism shown in Scheme 5, the
corresponding cathodic peak is largely shifted to a negative
potential region (Epc = +0.30 V) (Figure 3a, Table 2), which
corresponds to the 2e-reduction process of 22+
as
confirmed by the measurement of the isolated salt of 22+
(Figure S7). This separation of redox peaks is characteristic
of dyrex pairs, and favors switching the delocalization in
molecular wires by demanding overpotentials for switching
from one state to another.
Upon electrochemical oxidation in MeCN, the pale
yellow solution of
2 turned to a deep purple. The clean
conversion of
2
to 22+ as well as a negligible steady-state
concentration of open-shell intermediates were confirmed
by observing a continuous change with several isosbestic
points in the spectroelectrogram (Figure 4a). The strongest
absorption appeared in the visible region [

max/nm (log
2+(SbCl6 )2 (Table 1, Figure S1)],
characteristic of the bis(4-
) in
–
MeCN: 536 (4.79) for
which is
2
methoxyphenyl)carbenium skeleton. A similar absorption
was observed for non-fluorinated dication 12+ [514 (4.87)].
On the other hand, only a minor change was observed in
the UV region upon redox interconversion of the switching
Figure 3. Cyclic voltammograms measured in MeCN: (a)
(d) , and (e) (E / V vs SCE, 0.1 M Et4NClO4, Pt electrode, scan rate
500 mV s–1.).
2, (b) 3, (c) 4,
5
6
unit (2/2
2+), which suggests that it would be favorable for
later studies on molecular wires to scrutinize the changes in
UV-region absorption.
Based on the voltammetric analysis, the redox behavior of
molecular wire
separation of redox peaks (Figure 3c), indicating that
extension along the long axis of the biphenyl skeleton has
4 is very close to that of 2 or 3, with a large
・

only marginal effects on the dyrex behavior of the switching
unit. On the other hand, the spectroelectrogram in MeCN
(Figure 4b) is dramatically different from that of
2 in the UV-
region. The strong band at 345 nm for disappeared upon
4
electrochemical oxidation with a concomitant increase in
the blue-shifted band at 318 nm. The latter absorption is
similar to that of 2,3,4,5,6-pentafluorotolane (296 nm).[20c]
Thus, the redox-induced change in the geometry of the
switching unit disconnects the conjugation along the main
chain system, to give ON-OFF switching of
delocalization in the four-ring wire
4 (Scheme 3a).
Redox Properties and Spectroelectrograms of 5 and 6
Containing Two Switching Units
When the two switching units in
5 and 6 act nearly
simultaneously, a sharp difference in delocalization would
be realized for the molecular wires. A major issue is inter-
unit interactions,[24] such as electrostatic repulsion, which
may interrupt the redox conversion of one unit after another
unit is oxidized to the corresponding dication. While both
the through-bond and the through-space interactions should
Figure 4. A UV-Vis spectral change upon electrochemical oxidation (60
A) of (a) (42 M, every 10 min) and (b) (17 M, every 5 min) in
MeCN containing 0.05 M Et4NClO4.
be considered, the former interaction is less important

2

4

–
because X-ray analysis of
2
2+(SbCl6 )2 revealed that the
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10.1002/cplu.201800341
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in 54+ indicates that there is no electrostatic repulsion
between the two dicationic switching sites.
Due to low solubility, the Epaa of
6 appeared less cleanly
compared to those of other compounds. According to the
DFT calculation (Figure 2c), the main contributor to 6+• is
the main-chain-type species (Scheme 7). However, the
diarylmethylene-type cation radical should also contribute
via the configuration interaction, so that
6 can also undergo
facile dyrex reactions as evidenced by the rapid rise in
bis(4-methoxyphenylmethylium) absorption upon the
electrolysis of
6. A continuous change was observed in
both MeCN (Figure 6) and in CH2Cl2 (Figure S8). The
spectroelectrograms contain two pseudo isosbestic points,
both of which show a gradual change in the wavelength as
electrolysis progresses (Figure S9). The observed spectral
change can be best accounted for by the sequential
Figure 5. A UV-Vis spectral change upon electrochemical oxidation (60
A) of 5 (12 M, every 2 min) in MeCN containing 0.05 M Et4NClO4.
oxidation of
6 6
to 62+ and then to 4+. The solution contains
the molecules at these three states, and the proportion of
64+ is increased by the progress of electrochemical
oxidation. The strong band of
6 at 361 nm disappeared
with a concomitant increase in a new band in the UV-region,
the wavelength of which converged to ca. 333 nm, which is
4++(SbCl6 )4 (337 nm) with the
–
close to a strong band of
5
same chromophore in the center of the molecule (Schemes
3b and 3c). Based on these observations, it is highly likely
that the new band can be assigned to the 1,4-
bis(arylethynyl)benzene-type
which was generated by changes in the geometries of both
switching units upon the conversion of to
4+. We
concluded that, in molecular wires with two switching units,
both switching units act nearly simultaneously.
conjugation
system,[26]
Figure 6. A UV-Vis spectral change upon electrochemical oxidation (20
A) of (10 M, every 4 min) in MeCN containing 0.05 M Et4NClO4.
6
6

6

To scrutinize the through-space interaction, we first
conducted the electrochemical oxidation of The
spectroelectrogram in MeCN exhibits a continuous change
(Figure 5) even after the generation of 54+
max/nm (log ) in
5.
Conclusions
[

In this work, a DHP-BP2+-based dyrex pair was designed
as a switching unit to control the delocalization along a
conjugated main chain for application in future molecular-
wire studies. By incorporation of a perfluorobiphenyl
skeleton in a dyrex pair, an effective ON/OFF switching
ratio was attained with an increased torsion angle
especially in the BP2+ state. A perfluorobiphenyl skeleton
also endows the dyrex pair with SNAr reactivity to connect
with another system, the selectivity of which is favorable
for obtaining linearly-extended molecular wires. According
to DFT calculations, the resulting large systems have a
nearly coplanar geometry for the main chain, on which are
located large coefficients of HOMO and LUMO.
MeCN: 536 (5.08) (Table 1)], as suggested by the vis-
region absorption,[25] which is much greater than that for the
monomeric switching unit in 22+ [536 (4.79)].
A quite
similar spectroelectrogram was obtained when electrolysis
was conducted in CH2Cl2, which does not as effectively
stabilize the polycationic state by solvation (Figure S8).
Thus,
seamlessly oxidized into dication 52+ and then tetracation
54+
suggesting that the through-space electrostatic
interaction is marginal, and the molecules of these three
redox states (
2+and 4+) could coexist in the middle of
the electrochemical oxidation.
In the voltammogram of , another peak is followed by the
5 with two switching units could be smoothly and
,
5
,
5
5
5
first 2e-oxidation peak. Since only poorly resolved
shoulders were observed, it is not clear if it could be
Due to redox-triggered C-C bond-formation/cleavage
(dyrex behavior), two-electron transfer of the switching units
occurs nearly simultaneously to assure the reversibility of
redox interconversion, and electrochemical bistability is also
favored to keep both the ON and OFF states intact, at least
in the absence of an overpotential. Notably, the two
switching units in the two-unit system can act nearly
simultaneously upon electrochemical oxidation, which could
realize complete switching of delocalization (ON/OFF
ratio = 340).
assigned to the oxidation of 5
2+ to 54+ (Figure 3d). Even in
that case, the potential difference of ca. 0.1 V does not
necessarily reflect the electronic structure of cationic
species due to the sequential electron-transfer
accompanied by multiple bond-cleavage processes
(Scheme S1). At any event, the small difference is in line
with the negligible electrostatic repulsion suggested for 54+
This is further supported by the fact that Epc of 4+ is nearly
the same as that of
2+. The lack of the positive shift of Epc
.
5
2
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Cyclic Voltammetry was
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Schemes 2 and 4. Details on their preparation and spectral data are
given in the supplementary material.
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X-ray Analyses
Crystal data of 2•1/3hexane: MF C44H32.67F8O4, MW = 777.40, pale
yellow prism, 0.10 x 0.10 x 0.10 mm3, cubic Ia3bar (#206), a =
27.6408(9) Å, V = 21118.0(12) Å3, (Z = 24) = 1.467 g cm-1, T = 153 K, 
= 1.212 cm-1. The final R1 and wR2 values are 0.076 (I > 2) and 0.215
(all data) for 4045 reflections and 252 parameters. Esds for 2 are 0.004-
0.005 Å for bond lengths and 0.2-0.3 ° for bond angles, respectively.
CCDC 923999.
-
Crystal data of 22+(SbCl6 )2•EtOH: MF C44H34Cl12F8O5Sb2, MW
=
1463.61, dark red plate, 0.20 x 0.20 x 0.05 mm3, monoclinic P 21/c (#14),
a = 18.0750(2), b = 16.05605(15), c = 20.5277(2)Å, β = 106.9256(12)°, V
= 5699.37(11) Å3, (Z = 4) = 1.706 g cm-1, T = 150 K,  = 13.290 cm-1.
The final R1 and wR2 values are 0.073 (I > 2I) and 0.230 (all data) for
11487 reflections and 618 parameters. Esds for 22+ are 0.007-0.015 Å
for bond lengths and 0.5-1.3° for bond angles, respectively. CCDC
1849047.
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Acknowledgements
We thank Grant-in-Aid for Scientific Research on Innovative
Areas: "Middle molecular strategy" (No. 2707) from MEXT and
Grant-in-Aid from JSPS (Nos. 15H03790, 16K13968, 18K05069)
Japan. This work was also supported by the “Five-star Alliance”
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.
Keywords: Dyrex System • Electrochromism • Dication •
Molecular Device • Molecular Switch
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Schäferling, I. F. Perepichka, D. Stalke, and B. Walfort,. Eur. J. Org.
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that of 22+, the solution at the later stagge of electrolysis contains 70% of 52+
and 30% of 54+, respectively.
[26] Due to fluoro-substitution, this band is red-shifted compared to that for the
non-fluorinated 4,4'-bis(phenylethynyl)beenzene (320 nm in dioxane, ref 20b)].
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[19] Although the shallow helical structure of DHP in
2 causes the
stereoisomerism of P and M in terms of helicity, inversion of the sense of
helicity is very fast due to facile ring flipping (G≠ = 11.0 kcal mol–1 for 2).
Thus,
5 can be treated as a single entity without considering their
diastereoisomerism among PP, PM, and MM.
[20] (a) Due to fluoro-substitution, this band is red-shifted compared to that for
the non-fluorinated 4,4'-bis(phenylethynyl)biphenyl (308 nm in dioxane, ref
20b)]. A similar red shift (from 279 nm to 296 nm in MeCN) was also observed
upon 2,3,4,5,6-pentafluoro substitution of tolane (ref 20c). (b) E. Birckner, U.-
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[21] To confirm the conjugation between the two switching units through 1,4-
diethynylbenzene in 5, its isomer 11 with 1,3-diethynylbenene was prepared
for spectral comparisons. By the cross coupling reaction of 3 and 1,3-
diiodebenzene, 11 was obtained in 88% yield. Its UV spectrum in MeCN
[max/nm (log ) 319 (4.79)] (Figure S1) is quite different from that of 5 with
strong band being blue-shifted. Such an observation can be rationalized by
the less conjugation of two switching units through 1,3-diethynylbenzene in 11,
which in turn indicates effective conjugation in 5.
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[23] (a) Besides the energy-minimized nearly coplanar conformation shown in
Figure 2, partially twisted conformers might coexist for the long chain
molecules such as 5 and 6. Since the twisted conformer has no energy-
minimized geometry for the tolane unit (ref. 23b), the non-planar structure for
the 1,4-bis(phenylethynyl)benzene unit should be concerned (ref 23c). (b) G.
Brizius, K. Billingsley, M. D. Smith, U. H. F. Bunz, Org. Lett. 2003, 5, 3951-
3954. (c) When the DFT calculation of 5 was conducted by using the twisted
conformation as an initial structure (dihedral angle between the aromatic rings
in the tolane units: 90.0 and 90.0º), the geometrical optimization again
converged to the geometry with the nearly planar main-chain structure (anti-5
shown in Figure S4), suggesting that the twisted conformer of 1,4-
bis(arylethynyl)benzene has no energy-minimized geometry in the DFT
method.
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Entry for the Table of Contents
FULL PAPER
Wataru Nojo, Hitomo Tamaoki, Yusuke
Ishigaki, Ryo Katoono, Kenshu Fujiwara,
Takanori Fukushima and Takanori
Suzuki*
Page No. – Page No.
Molecular Wires with Controllable -
Delocalization Incorporating Redox-
Triggered -Conjugated Switching
Units
Due to the drastic change in the torsion angle upon 2e-oxidation of tetraaryl-
dihydrophenanthrene to biphenyl-2,2'-bis(diarylmethylium), this redox pair can be
used as a unique switching unit in linearly extended conjugating systems, which
were synthesized via SNAr- and cross coupling-reactions. Multiple switching units act
nearly simultaneously, and thus promise a higher ON/OFF ratio in molecular wires,
the delocalization of which can be controlled by an external stimulus.
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