A soluble and stable quinoidal bisanthene with nir absorption and amphoteric redox behavior

Article abstract of DOI:10.1021/ol902241u

A soluble and stable quinoidal bisanthene (3) was synthesized, and it exhibited high molar absorptivity in the near-IR spectral region. In addition, compound 3 also showed good electrochemical amphotericity with four reversible one-electron transfer steps

Full text of DOI:10.1021/ol902241u

ORGANIC
LETTERS
2009
Vol. 11, No. 21
4854-4857
A Soluble and Stable Quinoidal
Bisanthene with NIR Absorption and
Amphoteric Redox Behavior
Kai Zhang, Kuo-Wei Huang, Jinling Li, Jing Luo, Chunyan Chi, and Jishan Wu*
Department of Chemistry, National UniVersity of Singapore,
3 Science DriVe 3, Singapore, 117543
chmwuj@nus.edu.sg
Received August 18, 2009
ABSTRACT
A soluble and stable quinoidal bisanthene (3) was synthesized, and it exhibited high molar absorptivity in the near-IR spectral region. In
addition, compound 3 also showed good electrochemical amphotericity with four reversible one-electron transfer steps. Compound 3 represents
a rare example of soluble and stable π-conjugated polycyclic hydrocarbons with quinoidal character.
Quinoidal structure plays an important role in determining
the electronic and optical properties of π-conjugated sys-
tems.1 For example, oxidative doping of conjugated polymers
such as polypyrrole resulted in formation of radical cations
(polarons) along the polymer chain with quinoidal character,
which account for the long wavelength absorption in the
doped state.2 A neutral poly(isothianaphthene) as a prototype
of small band gap polymers also contains a quinoidal
structure in the main chain.3 Several linear π-conjugated
quinones or dicyanoquinodimethanes extended by phenyl4
or five-membered heteroaromatics5 (i.e., thienylene, furylene,
etc.) have been reported and showed intense near-infrared
(NIR) absorption. Moreover, a quinoidal porphyrin displays
an absorption spectrum at the far-red spectral region, which
is significantly different from normal aromatic porphyrins.6
Usually, the quinoidal π-conjugated systems exhibit a lower
band gap in comparison with their aromatic analogues, and
thus, materials with broad absorption in the visible-to-NIR
spectral range are normally obtained. In addition, quinoidal
compounds such as extended benzoquinones have frequently
shown electrochemical amphoteric redox behavior; i.e., the
materials can be easily oxidized and reduced by a multistage
electron transfer with a small span of the oxidation and
reduction potentials.4,5,7 The multistage redox properties as
well as the long-wavelength absorption make the quinoidal
compounds useful for many applications such as NIR dyes
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10.1021/ol902241u CCC: $40.75
Published on Web 10/09/2009
 2009 American Chemical Society

and pigments,8 electrical conductors,9 ambipolar field effect
transistors,10 solar cells,11 and nonlinear optics.12
To prepare materials with high electrochemical amphot-
ericity and long wavelength absorption, one strategy is to
construct large π-conjugated systems by (i) increasing the
quinoidal character, (ii) increasing the rigidification of the
π-system into a nearly planar conformation, and (iii)
introducing electron-donating and electron-accepting moieties
along the conjugated chain.13 Polycyclic aromatic hydrocar-
bons (PAHs) with large π-conjugation and rigid planar
structure have proven to be not only exciting objects for
structural chemists but also important opto-electronic materi-
als for material scientists due to their many potential
applications for organic electronic devices.14 Quinoidal PAHs
are expected to be another type of interesting material with
intriguing properties. However, to the best of our knowledge,
there are few reports about design and synthesis of soluble
and stable quinoidal PAHs with electrochemical amphot-
ericity. Some quinones of active PAHs such as oligoacenes
are known, but the quinones of larger π-conjugated PAHs
are usually insoluble.15
Our recent goal is to prepare soluble and stable NIR dyes
by using zigzag edged nanographenes such as periacenes as
building block which are supposed to have small band gaps.16
Among them, the bisanthene (1, Figure 1) represents an
interesting object with absorption maximum at 662 nm.17
However, bisanthene 1 is a very unstable compound due to
its high-lying HOMO energy level, and thus, attachment of
electron withdrawing groups such as carboximides is neces-
sary to prepare stable bisanthene derivatives with NIR
absorption.18 On the other hand, the bisanthene quinone (2)
Figure 1. Molecular structures of bisanthene (1), bisanthene
quinones (2 and 3), and bisanthracenequinone (4).
was found to be very stable, but it is virtually insoluble in
most organic solvents and difficult to process. Thus, our
objective is to prepare the di-tert-butyl-substituted extended
bisanthenequinone 3 which is supposed to possess larger
π-conjugation along the long axis and better solubility than
the bisanthenequinone 2. Consequently, intense NIR absorp-
tion and electrochemical amphotericity are expected from
compound 3 due to its quinoidal character and large
delocalized structure. For the sake of comparison, an
extended bisanthracenequinone 4 with a twisted structure was
also prepared.
The synthesis of quinones 3 and 4 is outlined in Scheme
1. The bisanthracenequinone 5 and bisanthenequinone 2 were
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Scheme 1. Synthetic Route of Compounds 3 and 4
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first prepared according to the literature.17 Reaction of 5 with
the lithium reagent of (4-bromo-2,6-di-tert-butylphenoxy)t-
(18) Yao, J.; Chi, C.; Wu, J.; Loh, K. Chem.sEur. J. 2009, 15, 9299–
9302.
Org. Lett., Vol. 11, No. 21, 2009
4855

rimethylsilane19 generated by lithium-halogen exchange
reaction gave the alcohol 6 in 74% yield. Subsequent
desilylation of 6 with tetrabutylammonium fluoride (TBAF)
and dehydration of the as-formed phenol by POCl3 in
pyridine afforded the desired bisanthracenequinone 4 in 56%
yield for two steps. The product 4 precipitated from the
solution as a light yellow powder, and simple purification
by filtration and washing with ethanol is sufficient to give
the pure compound. However the synthesis of compound 3
by photocyclization reaction of 4 in benzene failed due to
the decomposition of starting materials. Alternatively, the
compound 3 was prepared by a similar synthetic route
starting from the bisanthenequinone 2. We also found that
reaction of 2 with the same lithium reagent gave a compli-
cated mixture but reaction with the Grignard reagent of (4-
bromo-2,6-di-tert-butylphenoxy)trimethylsilane worked well,
and the target compound 7 was obtained in 66% yield.
Subsequent desilylation and dehydration of 7 under similar
conditions provided the final product 3 as a dark green solid
in 20% yield. The chemical structures and purity of all new
compounds were verified by 1H NMR, 13C NMR, elemental
analysis, mass spectrometry (EI, HR-EI or MALDI-TOF),
and FT-IR spectroscopy (see the Supporting Information).
Both 3 and 4 exhibited sharp NMR signals in organic
solvents even at elevated temperature (e.g., 100 °C in 1,2-
dichlorobenzene-d4), indicating that both molecules exist in
the form of quinone instead of biradicals. Their quinoidal
structure was further confirmed by their FT-IR spectra
(Figure S1, Supporting Information). The intense bands at
1650 cm-1 for 3 and 1613 cm-1 for 4 are assigned to the
CdO stretching vibration. Both compounds also showed
quite good thermal stability as determined by thermogravi-
metric analysis (TGA), and the initial thermal decomposition
temperatures (Td) are 355 and 340 °C for 3 and 4,
respectively (Figure S2, Supporting Information).
Figure 2. (A) UV-vis-NIR absorption spectra of compounds 3-
5 in DCM (1.0 × 10-5 M). (Inset) photos for dilute solutions of 3
and 4 in DCM. (B) Calculated molecular structures and frontier
molecular orbital profiles for 3 and 4.
For comparison, solution of 3 in concentrated sulfuric
acid displays long-wavelength absorption bands with a
maximum at 700 nm (log ε ) 4.5, see Figure S3,
Supporting Information). These data proved that the
further extension of quinoidal structure in the bisanthene-
quinone resulted in extended conjugation along the long
axis and led to NIR absorption. Furthermore, the molecule
3 can be also regarded as an analogue of the very unstable
p-quarterphenoquinone,5e,f with the two center quinoidal
benzene rings replaced by a quinoidal bisanthene unit. The
solution of quinone 3 is very stable at ambient conditions
and even upon exposure to 4 W UV lamp for several days.
In comparison with a stable thiophene-containing hetero-
quarterphenoquinone5 and the bisanthene itself, the absorp-
tion maximum of 3 exhibited 12 and 28 nm red-shifts,
respectively. The molecular structures and the frontier
molecular orbit profiles of 3 and 4 were calculated using
DFT methods (Figure 2B; see the Supporting Information
for details).
Compounds 3 and 4 are soluble in chlorinated solvents
such as dichloromethane (DCM), chloroform, and chloroben-
zene, and the UV-vis-NIR absorption spectra of 3-5 are
shown in Figure 2A. Solution of 3 in DCM displays a dark
green color, and well-resolved absorption bands between 500
and 800 nm with the peak at 690 nm (log ε ) 4.7; ε: molar
extinction coefficient in M-1 cm-1) are observed. Additional
bands in the UV range are also found. The quinone 4,
however, mainly exhibits absorption from 300 to 500 nm
with the peak at 428 nm (log ε ) 5.2), and the solution of
4 in DCM shows a yellow color. There is a 34 nm red-shift
compared with the absorption spectra of quinone 5, indicating
that π-conjugation is further extended in 4 by incorporating
di-tert-butyl benzoquinone moieties. The significant red-shift
(262 nm) in the absorption of 3 in comparison with 4
indicates largely enhanced π-conjugation in the rigid bisan-
thene-based quinone. In addition, although the bisanthene-
quinone 2 is insoluble in organic solvents, it can be dissolved
in concentrated sulfuric acid, showing a low energy absorp-
tion peak at 572 nm (log ε ) 4.2).17a
Molecule 3 has a distorted bisanthene unit with an out-
of-plane distortion angle of approximately 4.5° relative to
the central quinoidal benzene rings. The dihedral angle
between the bisanthene unit and the terminal benzoquinone
is about 42°, presumably due to steric congestion. Even with
such a distorted structure, 3 exhibits efficient π-conjugation
along the bisanthene and the benzoquinone units as can be
(19) Takahashi, K.; Suzuki, T.; Akiyama, K.; Ikegami, Y.; Fukazawa,
Y. J. Am. Chem. Soc. 1991, 113, 4576–4583.
(20) Bre´das, J. L.; Chance, R. R.; Baughman, R. H.; Silbey, R. J. Chem.
Phys. 1982, 76, 3673–3678.
4856
Org. Lett., Vol. 11, No. 21, 2009

predicted by their electron cloud distribution in its LUMO
and HOMO profiles.20 In contrast, molecule 4 has a central-
symmetric, double-saddle-like structure due to the strong
steric congestion at the anthracene/anthracene and benzo-
quinone-anthracene interfaces. Nevertheless, electron de-
localization can still be observed along the long axis, with
the bond length alternation showing a typical quinoidal
character. Such a difference in conformation and π-conjuga-
tion explains their big difference in electronic properties. In
agreement with the absorption spectra, fluorescence spectra
of 3 and 4 in DCM show emission peaks at 726 and 455
nm, respectively (Figure S4, Supporting Information).
Cyclic voltammetry was employed to investigate the redox
behavior of 3 and 4. As shown in Figure 3A, 3 in
directly into a dianion state. Thus, compound 3 is character-
ized as a new quinone material exhibiting a high amphoteric
red
redox behavior with a small span (Esum ) E1/2ox + (-E1/2
)
) 1.64 V) of the oxidation and reduction potentials. In
contrast, compound 4 in DCM shows one irreversible
reduction wave and one quasi-reversible oxidation wave.
The large peak-peak separation (∆Ep ) 450 mV) in the
oxidative wave of 4 indicates possible large conformation
change during the transition between the neutral and radical
cation, a phenomenon usually observed in highly distorted
π-systems.7b,21 Compared with 4, the first reduction peak
of 3 is shifted to a more negative value by 0.4 V due to low
electron affinity of the bisanthene core. The large π-conju-
gated system in 3 also stabilizes the charged species created
during the amphoteric redox processes. The very different
redox behavior between 3 and 4 indicates the importance of
rigid planar structure on the electrochemical amphotericity.
The HOMO and LUMO energy levels of 3 and 4 were also
determined from the onset potentials of the oxidation and
reduction waves,22 and it turns out that compound 3 has a
higher lying HOMO (- 4.52 eV for 3 vs -5.74 eV for 4)
and LUMO energy levels (-2.95 eV for 3 vs -3.32 eV for
4) due to the electron donating character of the bisanthene
unit. The compound 3 also has a smaller band gap (1.57
eV) than 4 (2.42 eV), and as a result, long wavelength
absorption was observed.
In summary, we have successfully synthesized a soluble
and stable bisanthenequinone which shows intense NIR
absorption and amphoteric redox behavior. Compound 3
represents a rare example of quinoidal large polycyclic
aromatic hydrocarbons which are supposed to show interest-
ing opto-electronic properties. The observed material proper-
ties for 3 such as intense NIR absorption, electrochemical
amphotericity and good thermal and photostability qualify
it as a useful NIR dye for many potential applications such
as NIR laser filter, optical data recording and ambipolar
charge transporting materials. The synthetic strategy reported
in this work also provides clues to develop higher order
soluble and stable quinoidal polycyclic hydrocarbons such
as periacenes in the future.
Figure 3. (A) Cyclic voltammograms of compounds 3 (in chlo-
robenzene) and 4 (in dichloromethane) with 0.1 M Bu4NPF6 as
supporting electrolyte, AgCl/Ag as reference electrode, gold disk
as working electrode, and Pt wire as counter electrode. (B) Four
stable redox states of 3 through the amphoteric redox processes.
chlorobenzene exhibits clear amphoteric multistage redox
behavior, consisting of two reversible one-electron oxidation
waves (E1/2ox ) 0.22, 0.71 V vs AgCl/Ag) and two reversible
Acknowledgment. This work was financially supported
by a NRF Competitive Research Program (R-143-000-360-
281) and a NUS Young Investigator Award (R-143-000-356-
101).
red
one-electron reduction waves (E1/2 ) -1.42, -1.86 V vs
AgCl/Ag). The multistage reversible redox waves provide
evidence for the formation of stabilized singly and doubly
charged species of quinone 3, i.e., the dication, radical cation,
radical anion, and dianion (Figure 3B). The charged states
of 3 were also studied by chemical oxidation and reduction
titrations followed by absorption spectroscopic measurements
(Figure S5, Supporting Information). Compound 3 was
oxidized into stable radical cation by iodine and further to
stable dication by stronger oxidant such as SbCl5. The
oxidized species can be reversibly reduced into a neutral state
by Zn dust. Meanwhile, 3 can also be reduced by lithium
Supporting Information Available: Experimental details
and characterization data of all new compounds and details
for DFT calculations. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL902241U
(21) Bryce, M. R.; Coffin, M. A.; Hursthouse, M. B.; Karaulov, A. I.;
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H.; Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551–554.
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