doi:10.1246/cl.130190
Published on the web May 21, 2013

703

Redox Switching of Conjugation Length Using 9,9,10,10-Tetraaryl-9,10-dihydrophenanthrene
as an ON/OFF Unit: Preparation, X-ray Structure, and Redox Properties
of Perfluorobiphenyl Derivative and Its SNAr Reactions to ³-Extended Analogues
Takanori Suzuki,*1 Hitomi Tamaoki,1 Ryo Katoono,1 Kenshu Fujiwara,1 Junji Ichikawa,2 and Takanori Fukushima3
1
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
2
Division of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571
3
Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503(Received March 5, 2013; CL-130190; E-mail: tak@mail.sci.hokudai.ac.jp)

Octaﬂuorobiphenyl-2,2¤-diylbis(diarylmethylium) dye 2a2+
prepared from 1,2-dibromotetraﬂuorobenzene is interconvertible
with colorless dihydrophenanthrene donor 1a. By the SNArreactions of 1a with acetylides, ³-extended analogues 1b and 1c
were obtained. Their electrochromic behavior is accompanied bya drastic absorption change not only in the visible but also UV
region, because the torsion angle of the biaryl unit is modiﬁed
upon redox reactions.
Biaryls can adopt various conformation in terms of thetorsion angle (º) about the central C­C bond.1 Thus, the controlof º by external stimuli is a reliable protocol to modify³-conjugation between the two aryl groups in the biaryl unit2(Scheme 1). During the course of our studies on organic
electrochromic systems with bistability (e.g., 1A/2A2+),3 wefound that the biaryl geometry in the redox pairs of 9,9,10,10tetraaryl-9,10-dihydrophenanthrenes (DHP) and biphenyl-2,2¤diyl-type dications (BD2+) is drastically changed since theirredox interconversion is accompanied by C­C bond formation/cleavage (Scheme 2):4 the biaryl unit in DHP is more or lesscoplanar (º: ca. 20°) due to the C9­C10 ethano bridge whereas
BD2+ adopts a twisted conformation (º: ca. 70°). This ﬁnding
conjugation "ON"conjugation "OFF"

π

π

π

π
φ

switching unit
external stimuli

Scheme 1.
a)

Ar Ar
X Ar

Ar

Ar Ar
2e–

X

X

X
X

X

b)

X
X

–

2e

Ar

Ar

2e

F

F

F

F

F

Ar

–

F

Ar
F

F

F

2e–
1b : Y = Ph
1c : Y = i -Pr3Si

Y
F

F
Ar

F
Ar

CO2R FFF

F CO2R F

3: R = H

Ar
F

F

F
F

F

4: R = MeAr

2+

2b : Y = Ph
2c2+: Y= i -Pr3Si

F

ArMgBr
THF

OH F
5
Ar

HBF4
TFAA

–

2a 2+(BF4 )2Zn
1aMe3SiCHN2[ Ar = 4-MeOC6H4 ][ Ar = 4-MeOC6H4 ]Scheme 2.

Chem. Lett. 2013, 42, 703­705

Ar
F HO

F

F

Y

Y

Y

2A2+: X = H
2a2+: X = F

Ar

BD2+

Ar Ar
F

X

X
Ar

1A: X = H
1a: X = F

DHP

X

X

X

X

X

X

prompted us to design a novel biaryl-based switching unit tomodify conjugation length using DHP/BD2+ pairs.
Electron donor 1a is an octaﬂuoro derivative of 1A andwould be interconvertible with bond-dissociated dication 2a2+
upon two-electron transfer. Although the biphenyl skeleton has
electron-withdrawing F atoms, the methoxy group on each aryl
substituent would stabilize positive charges, generated upon
two-electron oxidation, to allow isolation of dication 2a2+. The
F atoms at the 6,6¤-positions may increase the º value in 2a2+,
thereby enhancing geometric contrast of the switching unit in the
ON and OFF states. The most important feature of octaﬂuorobiphenyl moiety5 is the SNAr reactivity toward nucleophiles
including acetylides. Hence, 1a could be used as a synthon toprepare the 2,7-disubstituted derivatives, such as 1b and 1c,
whose switching phenomenon is more discernible due to alinearly ³-extended chromophore with strong UV absorptions.
Here we report preparation, redox behavior, and spectroscopic
properties of 1/22+ with the ﬂuorobiphenyl skeleton.
Commercially available 1,2-dibromotetraﬂuorobenzene wasconverted to octaﬂuorobiphenyl-2,2¤-dicarboxylic acid 3,6 which
was transformed into methyl ester 47 in 91% yield (Scheme 3).
Reaction of 4 with excess ArMgBr gave diol 57 in 54% yield,
which was then transformed, under acidic dehydrating conditions, into stable dication salt 2a2+(BF4¹)27 in 88% yield.
Upon treatment with Zn powder, this salt gave 1a,7 the switchingunit with the ﬂuorobiphenyl skeleton, in 97% yield. By thereaction of 1a with (4-BrC6H4)3N•+ SbCl6¹ (2 equiv), dication
2a2+ was regenerated and isolated as (SbCl6¹)2 salt7 in 87%
yield. Such a high-yield interconversion indicates that 1a/2a2+
can be considered as a “reversible” redox pair althoughelectrochemical reversibility is not maintained due to concomitant C­C bond formation/breaking (“dynamic redox pair”).3
According to X-ray crystallography,9 1a adopts a helicalgeometry as in other DHP derivatives (Figures 1 and S18). The
torsion angle [º(C4­C4a­C4b­C5): 35.1(4)°] is slightly larger
than those of the related molecules.4b,4c Nevertheless, as
observed by VT-NMR spectroscopy, 1a undergoes an easy

Scheme 3.

© 2013 The Chemical Society of Japanwww.csj.jp/journals/chem-lett/704

Figure 2. Cyclic voltammogram of 1a measured in MeCN
(E/V vs. SCE, 0.1 M Et4NClO4, Pt electrode, scan rate 500
mV s¹1).

Figure 1. Two views of ORTEP drawing of 1a in 1a¢1/ Z = 24, T = 153 K). The
3hexane solvate crystal (cubic, Ia3,
elongated C9­C10 bond [1.632(4) ¡] is characteristic of thesterically congested DHP derivatives.3
Table 1. Redox potentials measured in MeCNa
Compd
1a/2a2+
1b/2b2+
1c/2c2+
1A/2A2+ b(X = F)(Y = Ph)(Y = i-Pr3Si)(X = H)Eox (1 ¼ 22+)
+1.56
+1.56
+1.61
+1.47

E red (22+ ¼ 1)
+0.37
+0.37
+0.38
+0.18

a
E/V vs. SCE, 0.1 M Et4NClO4, Pt electrode, scan rate
500 mV s¹1. All of the waves are irreversible, and theoxidation potentials (Eox) and reduction potentials (Ered) were
estimated as Eanodic peak ¹ 0.03 and Ecathodic peak + 0.03 V,
respectively. Ferrocene undergoes one-electron oxidation at+0.38 V under similar conditions. bRef. 4a.

ring-ﬂip at room temperature10 suggesting that the steric
repulsion caused by the bay-region substituents is not severe.
Considering the cos2 º-dependency11 of eﬀective ³-conjugationin biaryls, ca. 70% of conjugation is still working in this “ON”
state (Scheme 1). Although the precise geometry of dication
2a2+ (the “OFF” state) is not available at this moment, as
described below, the voltammetric analysis suggests that adrastic structural change takes place during the conversion of 1a
to 2a2+.
The electrochemical oxidation of 1a in MeCN occurs at+1.56 V (vs. SCE), which is an irreversible process as in thecase of 1A (Table 1). The corresponding cathodic peak is largely
shifted to a negative potential region and assigned as the
reduction process of 2a2+ (Figure 2).8 Such a separation ofredox peak is characteristic of the redox pairs with dynamic
structural changes.3 Negligible steady-state concentration of theintermediary cation radical is another feature of the interconChem. Lett. 2013, 42, 703­705

Figure 3. A continuous change in UV­vis spectrum upon
constant current electrochemical oxidation of 1a [4.2 © 10¹5 M]
in MeCN containing 0.05 M Et4NClO4 (60 ¯A, every 10 min).
version between 1a and 2a2+, and thus a continuous change ofUV­vis spectrum with several isosbestic points was observed
upon electrolysis of 1a (Figure 3). The strongest absorption in
the visible region of 2a2+ [­max 533 nm (log ¾ 4.80) in MeCN]12
is red-shifted compared with that of 2A2+ [514 nm (log ¾ 4.87)],
since the electron-withdrawing nature of F lowers the LUMO ofthe dication, as shown by the less negative Ered of 2a2+ by 0.2 V
than that of 2A2+. These observations clearly demonstrate that
octaﬂuoro derivative 1a inherits attractive features of 1A orother DHPs as a reversible electrochromic system with bistability.
We next turn our attention to use 1a as a synthon to produce³-extended derivatives having a longer linear conjugation. As
shown in Scheme 4, the SNAr reaction of 1a with ethynylbenzene or ethynyltriisopropylsilane under the presence of NaN(SiMe3)2 proceeded in an acceptable yield to give the corresponding 2,7-diethynyl derivative 1b or 1c. Upon treatment with
(4-BrC6H4)3N•+ SbCl6¹ (2 equiv), 1b and 1c were transformed
into stable dicationic salts 2b2+(SbCl6¹)2 (84% yield) and2c2+(SbCl6¹)2 (87% yield), respectively. The redox potentialsof the newly prepared redox pairs of 1b/2b2+ and 1c/2c2+
were virtually identical to that of 1a/2a2+ (Table 1), since³-extension along the longer axis of the biphenyl skeleton
hardly aﬀects the frontier orbitals of 1a/2a2+.

© 2013 The Chemical Society of Japanwww.csj.jp/journals/chem-lett/

705
Ar Ar
F
F

Ar

F
C2

C7
F

Ar Ar
Ar

F

F

F

F
F

Y

Ar

Ar

F

H

NaN(SiMe3)2
THF

Y

Y

1a

F

F

F

F

y. 40%
1b: Y = Ph
1c: Y = i -Pr3Si y. 55%[ Ar = 4-MeOC6H4 ]Scheme 4.

Figure 4. A continuous change in UV­vis spectrum upon
constant current electrochemical oxidation of 1b [1.7 © 10¹5 M]
in MeCN containing 0.05 M Et4NClO4 (60 ¯A, every 5 min).
Meanwhile, the UV absorption proﬁles12 were signiﬁcantly
diﬀerent among 1a­1c (Figure S2).13 Thus, the ³-extended
compound 1b has a strong band at 345 nm (log ¾ 4.58 in MeCN),
which is absent in 1a [271 nm (4.15)] and assignable as theabsorption band due to 4,4¤-bis(phenylethynyl)perﬂuorobiphenyl chromophore (the “ON” state). Upon electrochemical oxidation of 1b to 2b2+, this band gradually disappeared withconcomitant appearance of the blue-shifted band at 318 nm(4.79) (Figure 4). The observed behavior is best described as the
successful ON/OFF switching of ³-conjugation shown in
Scheme 1 based on the external control of the torsion angle.
It is noteworthy that interconversion between 1b and 2b2+ isaccompanied by a drastic change also in the visible region by
reversible appearance/disappearance of triarylmethylium dyeunits. The redox pair of 1c/2c2+ likewise exhibited electrochromism, while the UV-region absorption bands were slightly
blue-shifted in both states [1c: 330 nm (4.47); 2c2+ 275 nm(4.75) in MeCN]12 (Figure S3).8
We are currently developing one-dimensional rod-likemolecules14 with multiple numbers of the switching units, usingbis(triisopropylsilylethynyl) derivative 1c as a key intermediate.
This work was performed under the Cooperative Research
Program of “Network Joint Research Center for Materials andDevices.” This work was supported by Grant-in-Aid for
Scientiﬁc Research on Innovative Areas: “Organic Synthesis
Based on Reaction Integration” (No. 2105) from MEXT, Japan.
References and Notes
1 a) M. Gómez-Gallego, M. Martín-Ortiz, M. A. Sierra, Eur. J.
Org. Chem. 2011, 6502. b) D. J. Edwards, R. G. Pritchard,
T. W. Wallace, Acta Crystallogr., Sect. B: Struct. Sci. 2005, 61,

Chem. Lett. 2013, 42, 703­705

335. c) J. Cioslowski, P. Piskorz, G. Liu, D. Moncrieﬀ,
J. Phys. Chem. 1996, 100, 19333.
2 a) J.-i. Nishida, T. Miyagawa, Y. Yamashita, Org. Lett. 2004, 6,2523. b) T. Suzuki, R. Tamaki, E. Ohta, T. Takeda, H. Kawai,
K. Fujiwara, M. Kato, Tetrahedron Lett. 2007, 48, 3823.
3 T. Suzuki, E. Ohta, H. Kawai, K. Fujiwara, T. Fukushima,
Synlett 2007, 851.
4 a) T. Suzuki, J.-i. Nishida, T. Tsuji, Angew. Chem., Int. Ed.
Engl. 1997, 36, 1329; T. Suzuki, J.-i. Nishida, T. Tsuji, Angew.
Chem. 1997, 109, 1387. b) T. Suzuki, J.-i. Nishida, T. Tsuji,
Chem. Commun. 1998, 2193. c) T. Suzuki, A. Migita, H.
Higuchi, H. Kawai, K. Fujiwara, T. Tsuji, Tetrahedron Lett.
2003, 44, 6837.
5 K. Geramita, J. McBee, T. D. Tilley, J. Org. Chem. 2009, 74,820.
6 a) F. Leroux, M. Schlosser, Angew. Chem. Int. Ed. 2002, 41,4272; F. Leroux, M. Schlosser, Angew. Chem. 2002, 114, 4447.
b) R. Filler, A. E. Fiebig, M. Y. Pelister, J. Org. Chem. 1980,
45, 1290.
7 Experimental details and physical data for new compounds are
given in Supporting Information (Ref. 8).
8 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
9 Crystal data of 1a¢1/3hexane is as follows: C44H32.67F8O2,
Mr = 544.69, pale yellow prism, 0.10 © 0.10 © 0.10 mm3,
cubic Ia3 (#206), a = 27.6408(9) ¡, V = 21118.0(12) ¡3,μ(Z = 24) = 1.467 g cm¹1, T = 123 K, ® = 1.212 cm¹1. The
ﬁnal R1 and wR2 values are 0.076 (I > 2·I) and 0.215 (alldata) for 4045 reﬂections and 252 parameters. Esds for 1a are
0.005­0.005 ¡ for bond lengths and 0.2­0.3° for bond angles,
respectively. CCDC 923999.
10 Two of the four aryl groups are located at the quasi equatorial
positions whereas other two at the quasi axial positions on thecentral 6-membered ring. According to the VT-NMR analysis,
the two MeO resonances at 3.68 and 3.64 ppm are coaleased
at 213 K (300 MHz). Thus, (P)- and (M)-1a are interconverting
easily with the ring-ﬂip energy-barrier of 11.0 kcal mol¹1,
which is similar to that of DHP (12.6 kcal mol¹1) (Ref. 4).
11 a) L. Venkataraman, J. E. Klare, C. Nuckolls, M. S. Hybertsen,
M. L. Steigerwald, Nature 2006, 442, 904. b) T. Sakano, K.
Higashiguchi, K. Matsuda, Chem. Commun. 2011, 47, 8427.
12 UV­vis spectral data [­max/nm (log ¾)] in MeCN are as
follows. 1a: 271 (4.15); 1b: 345 (4.58); 1c: 330 (4.47);
2a2+(SbCl6¹)2: 564sh (4.62), 533 (4.80), 380 (4.41), 275(4.55); 2b2+(SbCl6¹)2: 570sh (4.62), 533 (4.87), 411 (4.45),
318 (4.79), 279 (4.79); 2c2+(SbCl6¹)2: 566sh (4.50), 534(4.70), 409 (4.30), 274 (4.75). Close similarity in the wavelength of the ﬁrst-band among 2a2+­2c2+ can be accounted forby assuming that the orbital coeﬃcients in their frontier
orbitals are localized on the bis(4-methoxypheny)(polyﬂuorophenyl)carbenium unit.
13 The “OFF”-state absorption was estimated by measuring thespectrum of 4,4¤-bis(phenylethynyl)-2,2¤-bis[bis(4-methoxyphenyl)methyl]-3,3¤,5,5¤,6,6¤-hexaﬂuorobiphenyl H22b [­max/
nm (log ¾): 313 (4.76), 293 (4.78)] in CH2Cl2, which was
obtained by hydride addition on 2b2+(SbCl¹6)2 using NaBH4.
14 B. Schlicke, P. Belser, L. De Cola, E. Sabbioni, V. Balzani,
J. Am. Chem. Soc. 1999, 121, 4207; E. A. Weiss, M. J. Ahrens,
L. E. Sinks, A. V. Gusev, M. A. Ratner, M. R. Wasielewski,
J. Am. Chem. Soc. 2004, 126, 5577.

© 2013 The Chemical Society of Japanwww.csj.jp/journals/chem-lett/