Push-pull type of diphenoquinoid chromophores as novel near-infrared dyes

Article abstract of DOI:10.1039/a701626c

Novel push-pull diphenoquinoid chromophores, in which one of the terminal positions carries a strong donor group, benzo-1,3-dithiol-2-ylidene, and the other a strong acceptor group, dicyanomethylene, are synthesized and show a strong absorption in the nea

Full text of DOI:10.1039/a701626c

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Push-pull type of diphenoquinoid chromophores as novel near-infrared dyes
Shinobu Inoue, Yoshio Aso and Tetsuo Otsubo*
Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739,
Japan
Novel push-pull diphenoquinoid chromophores, in which
one of the terminal positions carries a strong donor group,
benzo-1,3-dithiol-2-ylidene, and the other a strong acceptor
group, dicyanomethylene, are synthesized and show a strong
absorption in the near-infrared region.
the synthesis and spectroscopic properties of the benzo
derivatives 6a–c.
Although the Gompper method, involving reaction of
dicyanomethylbenzene 7 with 2-methylthio-1,3-dithiolium me-
thanesulfonate 8, is very convenient and useful for the
preparation of the benzoquinoid system,4 attempted syntheses
of the diphenoquinoid compounds 5 and 6 by the same method
using 4-dicyanomethylbiphenyl 9 were unsuccessful. Alterna-
tively, we synthesized 6a–c by another route starting with
4,4A-dibromobiphenyl 10 as shown in Scheme 1: one of the
functional groups of 10 was converted via lithiation to a formyl
group, and then the other one to a dicyanomethyl group by the
Takahashi reaction.6 The intermediate 13 thus obtained was
reacted with benzenedithiol 14 in the presence of a catalytic
amount of toluene-p-sulfonic acid to give the dithioacetal 15,
which was then dehydrogenated via the carbonium salt 16 to the
desired compound 6.7 All the reactions proceeded in reasonable
to excellent yields.†
The search for highly-coloured organic compounds has at-
tracted current attention in terms of advanced optoelectronic
materials, such as near-infrared dyes and nonlinear optics.1,2
Push-pull types of conjugated p-electronic systems provide an
entry to highly-coloured chromophores.3 Such compounds
containing a quinoid ring are especially interesting, because the
push-pull stabilization can be promoted by the aromatization of
the ring. Some time ago, Gompper et al. reported the quinoid
compound 1 and benzo-fused derivative 2, in which one of the
CN
CN
CN
CN
S
S
S
S
The benzoquinoid compounds 1 and 2 are blue in CH2Cl2
solution, and their electronic absorption spectra show a strong
1
2
i, ii
CN
CN
S
S
Br
Br
OHC
Br
69%
CN
CN
S
S
10
11
iii
quant.
O
O
Br
3
4
12
CN
CN
S
S
iv
60%
OHC
CH(CN)2
5
R1
R2
13
14
CN
CN
S
R1
R2
SH
SH
v
S
6a R1, R2 = H
b R1 = Me, R2 = H
c R1, R2 = C8H17
R1
H
S
S
S
S
CH(CN)2
+
CH(CN)2
SMe
R2
–
15a R1, R2 = H (74%)
b R1 = Me, R2 = H (quant.)
c R1, R2 = C8H17 (46%)
MeSO4
7
8
CH(CN)2
vi
9
R1
R2
S
+
terminal positions carries a strong donor group, 1,3-dithiol-
2-ylidene, and the other a strong acceptor group, dicyano-
methylene.4 These compounds showed a strong intramolecular
charge-transfer absorption band in the visible region (vide
infra). The 1,4-naphthoquinoid analogue 3 and the 9,10-anthra-
quinoid one 4 were also prepared, but their absorption bands
showed rather hypsochromic shifts.4,5 The extensively conju-
gated diphenoquinoid systems 5 and 6, so far unknown, are
expected to show a bathochromic shift of the absorption band
and serve as a receptor for near-infrared light. Here we report
CH(CN)2
S
–
vii
BF4
6a (50%)
16a R1, R2 = H (43%)
b R1 = Me, R2 = H (57%)
b (90%)
c (90% from 15c)
Scheme 1 Reagents and conditions: i, BunLi, Et2O, 215 °C, 0.5 h; ii, DMF,
278 °C to room temp., 1 h; iii, HOCH2CH2OH, TsOH, benzene, reflux, 1
h; iv, NaCH(CN)2, Pd(PPh3)4, THF, reflux, 4 h; v, TsOH, benzene, reflux,
1 h; vi, Ph3CBF4, CH2Cl2–MeCN, 60 °C, 3 h; vii, Et3N, MeCN, room temp.,
2 h
Chem. Commun., 1997
1105

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Table 1 lmax and nCN of quinoid compounds 1, 2 and 6
5
4
3
2
Compound
l
max/nma
n
CN/cm21 b
1
2
6a
6b
6c
551(4.46), 592(4.78), 644(4.89)
548(4.68), 589(4.93), 641(4.97)
957(3.71), 1045(3.91)
962(4.41), 1042(4.42)
963(4.46), 1031(sh, 4.25)
2191
2195
2174
2180
2180
a Measured in CH2Cl2. b Measured on a KBr disk.
200
400
600
800
λ / nm
1000
1200
further shifted (2174–2180 cm21) and comparable to those of
lithium phenyldicyanomethanide (2178 cm21) and sodium
phenyldicyanomethanide (2188 cm21). This indicates that the
diphenoquinoid system 6 predominantly exists as the ionic form
18, consistent compatible with the large negative solvatochro-
mism. The predominance of the ionic form 18 must presumably
be promoted by the benefit of gaining double aromaticity.
The present results clearly demonstrate that the dipheno-
quinoid compounds 6a–c are an interesting push-pull system
with highly polar character and a new type of dyes strongly
absorbing near-infrared light.
Fig. 1 Electronic spectra of 2 (- - -) and 6a (——) in CH2Cl2
colour-determining band with fine structure in the region of
550–650 nm. The band shows very little solvent shift. On the
other hand, the diphenoquinoids 6a–c look green–black in the
solid state but are faint yellow in solution, and their electronic
spectra have a characteristic broad band in the near-infrared
region as well as a second band at around 450 nm. Fig. 1
demonstrates the specific spectral features of 2 and 6a, and
Table 1 summarizes the electronic absorption maxima and
nitrile vibrational absorptions of 1, 2 and 6a–c. Interestingly, the
absorption bands of 6, unlike those of 1 and 2, show large
negative solvatochromism with increasing solvent polarity: for
example, lmax of 6a is 957 and 1045 nm in CH2Cl2, 934 nm in
PhCN, and 790 nm in Me2SO.
Footnotes
* E-mail: otsubo@ipc.hiroshima-u.ac.jp
† All the new compounds were characterized by spectroscopic measure-
ments and elemental analyses.
It is reasonable to assume that the push-pull quinoid systems
2 and 6 exist as a resonance hybrid with the zwitterionic forms
17 and 18, respectively. Infrared spectra provide definite
References
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–
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absorption of 2 appears at a much lower frequency (2195 cm21
)
than that of TCNQ (2226 cm21). This indicates a considerable
Received in Cambridge, UK, 7th March 1997; Com.
7/01626C
contribution by the ionic form 17. The absorptions of 6a–c are
1106
Chem. Commun., 1997