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 φextended analogues

Article abstract of DOI:10.1246/cl.130190

Octafluorobiphenyl-2,2-diylbis(diarylmethylium) dye 2a²⁺ prepared from 1,2-dibromotetrafluorobenzene is interconvertible with colorless dihydrophenanthrene donor 1a. By the SNAr reactions of 1a with acetylides,φ-extended analogues 1b and 1c were obtained. Their electrochromic behavior is accompanied by a 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.

Full text of DOI:10.1246/cl.130190

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
1Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
2Division of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571
3Chemical 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 SNAr
reactions of 1a with acetylides, ³-extended analogues 1b and 1c
were obtained. Their electrochromic behavior is accompanied by
a 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.
prompted us to design a novel biaryl-based switching unit to
modify conjugation length using DHP/BD2+ pairs.
Electron donor 1a is an octafluoro derivative of 1A and
would 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 octafluoro-
biphenyl moiety5 is the SNAr reactivity toward nucleophiles
including acetylides. Hence, 1a could be used as a synthon to
prepare the 2,7-disubstituted derivatives, such as 1b and 1c,
whose switching phenomenon is more discernible due to a
linearly 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 was
converted 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 con-
Biaryls can adopt various conformation in terms of the
torsion angle (º) about the central C-C bond.1 Thus, the control
of º by external stimuli is a reliable protocol to modify
³-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 we
found that the biaryl geometry in the redox pairs of 9,9,10,10-
tetraaryl-9,10-dihydrophenanthrenes (DHP) and biphenyl-2,2¤-
diyl-type dications (BD2+) is drastically changed since their
redox interconversion is accompanied by C-C bond formation/
cleavage (Scheme 2):4 the biaryl unit in DHP is more or less
coplanar (º: ca. 20°) due to the C9-C10 ethano bridge whereas
BD2+ adopts a twisted conformation (º: ca. 70°). This finding
¹
7
ditions, into stable dication salt 2a2+(BF4
)
in 88% yield.
2
Upon treatment with Zn powder, this salt gave 1a,7 the switching
unit with the fluorobiphenyl skeleton, in 97% yield. By the
conjugation "ON"
conjugation "OFF"
π
π
π
π
¹
reaction of 1a with (4-BrC6H4)3N•+ SbCl6 (2 equiv), dication
¹
2a2+ was regenerated and isolated as (SbCl6 )2 salt7 in 87%
φ
switching unit
yield. Such a high-yield interconversion indicates that 1a/2a2+
can be considered as a “reversible” redox pair although
electrochemical reversibility is not maintained due to concom-
itant C-C bond formation/breaking (“dynamic redox pair”).3
According to X-ray crystallography,9 1a adopts a helical
geometry as in other DHP derivatives (Figures 1 and S18). The
torsion angle [º(C4-C4a-C4b-C5): 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
external stimuli
Scheme 1.
a)
Ar
X
Ar
X
Ar Ar
2e–
2e–
Ar
Ar
X
X
X
X
X
X
X
X
X
X
X
X
X
X
2A2+: X = H
Ar Ar
BD2+
1A: X = H
1a: X = F
2a2+: X =
DHP
F
Ar Ar
HO
CO2R F
F
F
F
b)
F
F
Ar Ar
Ar Ar
2e–
2e–
ArMgBr
THF
Ar
Ar
F
F
F
HBF4
TFAA
F
F
F
–
F
2a2+(BF4
)
2
F
F
F
Y
Y
Y
Y
F
F
F CO2R
F
F
OH F
Ar
Zn
1a
3: R = H
4: R = Me
5
Ar
F
F
Ar
F
F
F
F
2b2+: Y = Ph
2c2+: Y= i-Pr3Si
1b : Y = Ph
Ar
Me3SiCHN2
1c : Y = i-Pr3Si
[ Ar = 4-MeOC6H4
]
[ Ar = 4-MeOC6H4
]
Scheme 2.
Scheme 3.
Chem. Lett. 2013, 42, 703-705
© 2013 The Chemical Society of Japan
www.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/
ꢀ
3hexane solvate crystal (cubic, Ia3, Z = 24, T = 153 K). The
elongated C9-C10 bond [1.632(4) ¡] is characteristic of the
sterically congested DHP derivatives.3
Table 1. Redox potentials measured in MeCNa
Compd
Eox (1 ¼ 22+
)
Ered (22+ ¼ 1)
1a/2a2+
1b/2b2+
1c/2c2+
(X = F)
+1.56
+1.56
+1.61
+1.47
+0.37
+0.37
+0.38
+0.18
Figure 3. A continuous change in UV-vis spectrum upon
constant current electrochemical oxidation of 1a [4.2 © 10¹5 M]
in MeCN containing 0.05 M Et4NClO4 (60 ¯A, every 10 min).
(Y = Ph)
(Y = i-Pr3Si)
1A/2A2+ b (X = H)
aE/V vs. SCE, 0.1 M Et4NClO4, Pt electrode, scan rate
500 mV s¹1. All of the waves are irreversible, and the
oxidation 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
version between 1a and 2a2+, and thus a continuous change of
UV-vis 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 of
the 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 or
other DHPs as a reversible electrochromic system with bist-
ability.
b
+0.38 V under similar conditions. Ref. 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-conjugation
in 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 a
drastic structural change takes place during the conversion of 1a
We next turn our attention to use 1a as a synthon to produce
³-extended derivatives having a longer linear conjugation. As
shown in Scheme 4, the SNAr reaction of 1a with ethynylben-
zene or ethynyltriisopropylsilane under the presence of NaN-
(SiMe3)2 proceeded in an acceptable yield to give the corre-
sponding 2,7-diethynyl derivative 1b or 1c. Upon treatment with
to 2a2+
.
The electrochemical oxidation of 1a in MeCN occurs at
+1.56 V (vs. SCE), which is an irreversible process as in the
case 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 of
redox peak is characteristic of the redox pairs with dynamic
structural changes.3 Negligible steady-state concentration of the
intermediary cation radical is another feature of the intercon-
¹
(4-BrC6H4)3N•+ SbCl6 (2 equiv), 1b and 1c were transformed
¹
into stable dicationic salts 2b2+(SbCl6
) (84% yield) and
2
¹
2c2+(SbCl6 )2 (87% yield), respectively. The redox potentials
of the newly prepared redox pairs of 1b/2b2+ and 1c/2c2+
were virtually identical to that of 1a/2a2+ (Table 1), since
³-extension along the longer axis of the biphenyl skeleton
hardly affects the frontier orbitals of 1a/2a2+
.
Chem. Lett. 2013, 42, 703-705
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

705
Ar Ar
Ar Ar
335. c) J. Cioslowski, P. Piskorz, G. Liu, D. Moncrieff,
J. Phys. Chem. 1996, 100, 19333.
Ar
Ar
Ar
Ar
F
F
F
F
F
Y
H
F
C2
F
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.
T. Suzuki, E. Ohta, H. Kawai, K. Fujiwara, T. Fukushima,
Synlett 2007, 851.
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.
C7
Y
Y
NaN(SiMe3)2
THF
F
F
F
F
F
F
F
1a
1b: Y = Ph
1c: Y = i-Pr3Si
y. 40%
y. 55%
3
4
[ Ar = 4-MeOC6H4
]
Scheme 4.
5
6
K. Geramita, J. McBee, T. D. Tilley, J. Org. Chem. 2009, 74,
820.
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
8
Experimental details and physical data for new compounds are
given in Supporting Information (Ref. 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,
Figure 4. A continuous change in UV-vis spectrum upon
constant current electrochemical oxidation of 1b [1.7 © 10¹5 M]
in MeCN containing 0.05 M Et4NClO4 (60 ¯A, every 5 min).
3
ꢀ
cubic Ia3 (#206), a = 27.6408(9) ¡, V = 21118.0(12) ¡ ,
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 (all
data) for 4045 reflections and 252 parameters. Esds for 1a are
0.005-0.005 ¡ for bond lengths and 0.2-0.3° for bond angles,
respectively. CCDC 923999.
Meanwhile, the UV absorption profiles12 were significantly
different among 1a-1c (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 the
absorption band due to 4,4¤-bis(phenylethynyl)perfluorobiphen-
yl chromophore (the “ON” state). Upon electrochemical oxida-
tion of 1b to 2b2+, this band gradually disappeared with
concomitant 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+ is
accompanied by a drastic change also in the visible region by
reversible appearance/disappearance of triarylmethylium dye
units. The redox pair of 1c/2c2+ likewise exhibited electro-
chromism, 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
10 Two of the four aryl groups are located at the quasi equatorial
positions whereas other two at the quasi axial positions on the
central 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
¹1
easily with the ring-flip energy-barrier of 11.0 kcal mol
,
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 UV-vis spectral data [-max/nm (log ¾)] in MeCN are as
follows. 1a: 271 (4.15); 1b: 345 (4.58); 1c: 330 (4.47);
¹
2a2+(SbCl6 )2: 564sh (4.62), 533 (4.80), 380 (4.41), 275
¹
(4.55); 2b2+(SbCl6 )2: 570sh (4.62), 533 (4.87), 411 (4.45),
¹
318 (4.79), 279 (4.79); 2c2+(SbCl6 )2: 566sh (4.50), 534
(4.70), 409 (4.30), 274 (4.75). Close similarity in the wave-
length of the first-band among 2a2+-2c2+ can be accounted for
by assuming that the orbital coefficients in their frontier
orbitals are localized on the bis(4-methoxypheny)(polyfluoro-
phenyl)carbenium unit.
We are currently developing one-dimensional rod-like
molecules14 with multiple numbers of the switching units, using
bis(triisopropylsilylethynyl) derivative 1c as a key intermediate.
This work was performed under the Cooperative Research
Program of “Network Joint Research Center for Materials and
Devices.” 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.
13 The “OFF”-state absorption was estimated by measuring the
spectrum of 4,4¤-bis(phenylethynyl)-2,2¤-bis[bis(4-methoxy-
phenyl)methyl]-3,3¤,5,5¤,6,6¤-hexafluorobiphenyl H22b [-max
/
nm (log ¾): 313 (4.76), 293 (4.78)] in CH2Cl2, which was
¹
obtained by hydride addition on 2b2+(SbCl 6)2 using NaBH4.
References and Notes
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.
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, 703-705
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/