Biphenyl-type electron acceptors exhibiting dynamic redox properties: a novel
electrochromic system with ‘write protect’ option
Takanori Suzuki,*† Hyou Takahashi, Jun-ichi Nishida and Takashi Tsuji
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
A novel redox pair undergoing reversible C–C bond making/
breaking has been constructed based on
bis(dicyanovinyl)biphenyl derivative and a dianion with the
dihydrophenanthrene skeleton; further cyclization of the
latter to an enaminonitrile endows the ‘write protect’ option
to its electrochromic response.
Voltammetric analyses have revealed quite different behav-
ior of 1a compared with the m,m-isomer 4.§ The redox
behaviour of 4 is nearly identical to that of reference compound
a
Ar
C(CN)2
Recently much attention has been focused on molecules whose
geometry and properties can be controlled by the external
stimuli.1 From this point of view redox systems undergoing
reversible C–C bond making/breaking upon electron transfer
(ET) are interesting2 and might be applicable to the construction
of electrochemical switches or molecular devices3 based on
their optical response and bistability. We have designed the
novel redox pair shown in Scheme 1 which has the following
interesting features: (i) 2,2A-bis(dicyanovinyl)biphenyls 1 are
expected to undergo facile ring closure to dihydrophenanthrene-
type dianions 222 upon two-electron reduction; (ii) the resulting
dianions are sterically congested molecules and will regenerate
the starting material 1 by C–C bond cleavage upon oxidation;
(iii) in the case of p-dimethylaminophenyl derivatives 1a and
2a22, a sharp change in color is expected during ET because
only the former shows strong absorption in the visible region
due to the p-(dicyanovinyl)aniline skeleton; (iv) besides the
reversible interconversion between 1 and 222, further cycliza-
tion to 3 induced by protonation of 222 endows the system with
the ‘write protect’ option in its response (Scheme 2). Here, we
report the preparation and unique redox properties of the title
acceptors and their reduction products.
(NC)2C
(NC)2C
Ar
( Ar = p-Me2NC6H4
)
Ar
4
5
5,6 which undergoes reversible one-electron reduction to 5·2
(Ered = 21.28 V) and oxidation to 5·+ (Eox = +1.08 V). By
contrast the reduction process of 1a (Ered = 21.29 V) is
irreversible in the sense that the corresponding anodic peak is
absent in its cyclic voltammogram (Fig. 1). Instead, a new peak
appeared in the anodic region (+0.04 V). This was assigned to
the oxidation peak of 2a22 by independent measurement, and a
new cathodic peak corresponding to the reduction of 1a was
observed after the oxidation of 2a22. Such hysteresis in redox
waves is characteristic of ‘dynamic’ redox systems that undergo
reversible and drastic structural change upon ET.1b,2 Electro-
chromic behaviour was shown by spectrophotometric monitor-
ing of the electrochemical reduction of 1a, and the isosbestic
point at 380 nm is indicative of the quantitative conversion to
2a22 (Fig. 2).
The stereospecific nature of the ring closure was evidenced
by product analysis on the mixture obtained by the reaction of
1a with SmI2 then with acid.¶ Thus, trans-H22a‡ (mp 221–223
°C) was formed free from the cis-isomer and isolated in 64%
yield as the sole product. Only by heating in EtOH, does this
material isomerize quantitatively to trans-3a‡ (mp 198–200 °C)
by Thorpe condensation8 which no longer regenerates 1a upon
oxidation, suggesting that ‘write protection’ can be performed
very easily.
X
NH2
2e-
–
–
C(CN)2
(NC)2C
C(CN)2
CN
CN
X
NC
X
X
H+
X
heat
2e-
(NC)2C
X
1
22-
3
Thorpe condensation occurred more rapidly in the absence of
dimethylaminophenyl groups. Thus, the enaminonitrile trans-
3b‡ (mp 276–278 °C; 3JHH 13.2 Hz) was obtained as the major
( a: X = p-Me2NC6H4; b: X = H )
Scheme 1
erase
write
intensely
colored
faintly
colored
faintly
colored
write
protect
10 µA
+0.6
0.0
–1.5
Scheme 2
Condensation reaction of 2,2A-diformylbiphenyl with malo-
nonitrile in the presence of TiCl4 and pyridine4 gave 1b‡ (mp
224–226 °C) as colorless crystals in 54% yield. Dye 1a‡ (mp
333–334 °C) was prepared from 2,2A-diiodobiphenyl via its
2,2A-dilithio derivative5 by successive reactions with
p-dimethylaminobenzonitrile and malononitrile,6 and obtained
as orange plates [lmax (MeCN): 455 nm (log e 4.24), 279 (4.02)]
in 7% yield.
E / V vs. SCE
Fig. 1 Cyclic voltammogram of dye 1a in MeCN (E/V vs. standard calomel
electrode, 0.1 mol dm23 NEt4ClO4, Pt electrode, scan rate 500 mV s21).
The oxidation peak at +0.04 V is absent when the voltammogram was first
scanned anodically. Another oxidation peak at +1.07 V was also observed
but not shown, which corresponds to the oxidation of dimethylaniline
moieties as in 4 and 5.
Chem. Commun., 1998
1331
Suzuki, Takanori
