Bisanthracene bis(dicarboxylic imide)s as soluble and stable NIR dyes

Article abstract of DOI:10.1002/chem.200901398

A series of soluble and stable near-infrared (NIR) dyes based on laterally expanded bisanthracene bis(dicarboxylic imide)s M1-M4, which can also be regarded as dibenzoperylene derivatives, was reported. The anthracene dicarboxylic imide dimer 5 was synthesized by [Ni(cod)₂]-mediated Yamamoto homocoupling of 4b and this was followed by tBuOK- and 1,5- diazabicyclo[4.3.0]non-5-ene (DBN)-mediated cyclization reaction to afford the desired cis-bisanthracene bis(dicarboxylic imide)s M2 in 85% yield. The transisomer M3 was obtained in 8% yield by heating the anthracene dicarboxylic imide 4a in melted KOH. Separation of the compound 7a from other isomers was successful after the attachment of the 2,6-diisopropylaniline groups. The dimer 8 was then prepared by a similar Yamamoto coupling and followed by tBuOK- and DBN-mediated cyclization reaction. M1-M4 have very good solubility in common organic solvents such as CH₂Cl₂ and THF and they showed intense absorptions in NIR regions with absorption maximum at 830nm.

Full text of DOI:10.1002/chem.200901398

COMMUNICATION
DOI: 10.1002/chem.200901398
Bisanthracene Bis(dicarboxylic imide)s as Soluble and Stable NIR Dyes
Jun Hong Yao, Chunyan Chi, Jishan Wu,* and Kian-Ping Loh*[a]
Dyes with near-infrared (NIR) absorption and/or emission
have shown promising applications for optical recording,
laser filter, thermal writing display, NIR photography, bio-
imaging, photodynamic therapy, and solar cells.[1,2] Most of
the commercially available NIR dyes such as polymethine
dyes (dominated by cyanine dyes), quinone dyes, azo dyes,
charge transfer salts, radical dyes, and phthalocyanine dyes
usually suffer from either poor thermal and photostability
or low solubility which limit their practical applications. Al-
ternatively, dyes based on polycyclic aromatic hydrocarbons
(PAHs) showed excellent stability and thus they can be used
as key building blocks for the design of stable and soluble
NIR dyes with tunable light absorption and emission wave-
length. One class of widely investigated dyes based on
PAHs are rylenes and their dicarboxylic imide derivatives,
which serve as key chromophores in dyestuff chemistry due
to their excellent chemical and photostability.[3] To achieve
intense absorption/emission in NIR region, a lot of work has
been done to extend the p-conjugation along the long axis
of rylene molecules and stable polyaromatic NIR dyes such
as quarterylene, pentarylene and hexarylene bis(dicarboxylic
imide)s have been reported.[4] Although the extension along
the long axis is an efficient way to approach NIR absorp-
tion, the syntheses of higher order soluble rylene dyes are
very cumbersome. Theoretical calculations also predicted
that lateral extension of rylene dyes by fusion of additional
benzene rings will provide an alternative method to extend
the p-conjugation and result in NIR absorption,[5] however,
until today, there are only a few reports on the effective ex-
pansion of p-conjugation along the short axis of rylene mol-
ecules mainly due to practical synthetic challenges. In this
communication, we report a series of soluble and stable
NIR dyes based on laterally expanded bisanthracene bis(di-
carboxylic imide)s M1–M4 (Scheme 1), which can also be
regarded as dibenzoperylene derivatives. Optical and elec-
Scheme 1. Structures of bisanthene bis(dicarboxylic imide) (M1) and par-
tially cyclized bisanthene bis(dicarboxylic imide)s (M2–M4).
trochemical measurements of these dyes indicated that all of
these dyes showed intense absorption between 600 and
1000 nm with significant lower band gaps in comparison
with the perylene tetracarboxylic diimide (PDI), suggesting
that lateral extension is indeed an efficient way to obtain
polyaromatic NIR dyes.
The fully fused anthracene dimer, the so-called bisan-
thene, has been synthesized with absorption maximum at
662 nm, indicating its great potential as an NIR dye building
block. However, it showed low solubility and very poor sta-
bility upon exposure to air due to its high lying HOMO
energy level.[7] Therefore, herein electron-withdrawing dicar-
boxylic imide groups were introduced to improve its stabili-
ty as well as solubility. The partially cyclized bisanthracene
bis(dicarboxylic imide)s, aceanthrene green,[7,8] has ever
been synthesized and used as textile dyes and pigments with
NIR absorption peak at 701 nm. One cis-isomer in which H
is attached to the N atoms was also prepared but it co-exist-
ed with the trans-isomer as a mixture. Nevertheless, the
most desired structure, that is, fully cyclized bisanthene bis-
(dicarboxylic imide)s M1 is still unknown although its at-
[a] Dr. J. H. Yao, Dr. C. Chi, Prof. J. Wu, Prof. K.-P. Loh
Department of Chemistry
National University of Singapore
3 Science Drive 3 (Singapore)
Fax : (+65)6779-1691
E-mail: chmwuj@nus.edu.sg
chmlohkp@nus.edu.sg
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901398.
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tractive properties have been theoretically predicted 14
years ago.[5] Herein we report the successful synthesis of this
interesting compound which showed remarkable NIR ab-
sorption with absorption maximum at 830 nm (i.e., 168 and
130 nm red-shift compared with the parent bisanthene and
aceanthrene green, respectively). In addition, the previously
reported synthesis of aceanthrene green dyes/pigments was
mainly done in melted KOH (ca. 2208C) followed by oxida-
tion in air or by H2O2, and the yields were usually low and
most functional groups can not survive under so harsh con-
ditions.[7,8] Therefore, improved synthetic method has to be
developed to obtain sufficient materials for practical appli-
cations. For this reason, the partially fused bisanthracene
bis(dicarboxylic imide)s M2–M4 with different linking
modes were also prepared in a more efficient way. The com-
pounds obtained can be used to understand the effect of
geometrical structure on physical properties by comparing
with the fully cyclized compound M1. For all molecules,
bulky 2,6-diisopropylaniline units were introduced to im-
prove their solubility.
Scheme 2 outlines the synthetic route for compounds M1–
M4. The synthesis started from Friedel–Crafts reactions of
anthracene (1a) or 9-bromoanthracene (1b) with oxalyl
chloride to provide the aceanthrylene 1,2-diones[9] (2a and
2b, respectively) which were subsequently oxidized to the
carboxylic acid anhydrides (3a and 3b, respectively) by
oxone in high yields. The carboxylic anhydride was usually
prepared first by oxidative ring-opening reaction by using
H2O2 in NaOH solution and followed by ring-closing reac-
tion in acetic anhydride.[9] Herein, we developed a simple,
one-pot synthesis of the anthracene carboxylic anhydride
from the respective aceanthrylene 1,2-diones by using a
modified procedure.[10] Reactions between 3a or 3b with
2,6-diisopropylaniline gave the corresponding imides (4a
and 4b). The anthracene dicarboxylic imide dimer 5 was
Scheme 2. Synthetic route for compounds M1–M4: i) oxalyl chloride,
AlCl3, CS2, 08C; ii) oxone, methanol, reflux; iii) 2,6-diisopropylaniline,
propionic acid, reflux; iv) [NiACTHUNTRGNE(NUG cod)2]/COD/BPy, DMF, toluene, 808C; v)
tBuOK, DBN, diglyme, 1308C; vi) FeCl3, CH3NO2, CH2Cl2; vii) melted
KOH, then air; viii) Br2, conc. H2SO4, RT. BPy = bipyridine.
then synthesized by [Ni
coupling of 4b and this was followed by tBuOK- and 1,5-
diazabicyclo[4.3.0]non-5-ene (DBN)-mediated cyclization
ACHTUNGERTN(NUNG cod)2]-mediated Yamamoto homo-
the bromination of 3a in concentrated sulfuric acid which
unavoidably gave a mixture of three isomers 6a–c consisting
of about 50% the desired isomer 6a. Separation of the com-
pound 7a from other isomers (7b and 7c) was successful
after the attachment of the 2,6-diisopropylaniline groups.
The dimer 8 was then prepared by a similar Yamamoto cou-
pling and followed by tBuOK- and DBN-mediated cycliza-
tion reaction. Both partially fused compound M4 and fully
fused M1 were separated in pure forms with reasonable
yields. The chemical structures and purity of compounds
M1–M4 were verified by 1H NMR, 1H,1H-COSY NMR,
13C NMR, mass spectroscopy and elemental analysis (Sup-
porting Information).
ACHTUNGTRENNUNG
reaction[11] to afford the desired cis-bisanthracene bis(dicar-
boxylic imide)s M2 in 85% yield. This is also the first effi-
cient synthesis of the pure cis-isomer of aceanthrene green
dyes. Unfortunately, compound M2 cannot be further cy-
clized into the fully fused bisanthene carboximides M1
under various cyclization conditions such as FeCl3-mediated
oxidative cyclodehydrogenation probably due to the strong
electron-withdrawing effect of the imide groups which deac-
tivated the reactive sites on the aromatic rings.[12] The trans-
isomer M3 was obtained in 8% yield by heating the anthra-
cene dicarboxylic imide 4a in melted KOH according to a
reported method.[7,8] It is interesting that treatment of 4a
using tBuOK and DBN in diglyme selectively gave the cis-
isomers M2 in 73% yield and the detailed mechanisms of
the selectivity are still under investigation. This simple
method provided an efficient synthesis of soluble acean-
threne green cis-isomer in large scale. To prepare the target
compound M1, the monobromo-substituted anthracene di-
carboxylic imide 7a was first synthesized. The key step was
M1–M4 have very good solubility in common organic sol-
vents such as CH2Cl2 and THF and they showed intense ab-
sorptions in NIR regions with absorption maximum at
830 nm (e=15000mÀ1 cmÀ1), 697 nm (e=47000mÀ1 cmÀ1),
697 nm
(e=38000mÀ1 cmÀ1),
and
685 nm
(e=
27000mÀ1 cmÀ1) for M1, M2, M3 and M4, respectively
(Figure 1). The absorption peak of 830 nm observed for M1
suggested that it could be used as laser absorbing dye which
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Soluble and Stable NIR Dyes
COMMUNICATION
Figure 1. UV/Vis-NIR absorption and photoluminescence spectra of com-
pounds M1–M4 in CH2Cl2.
Figure 2. Cyclic voltammograms of compounds M1–M4 in dry CH2Cl2,
Bu4NPF6 (0.1m) as supporting electrolyte, gold disk as working electrode,
Pt as counter electrode and AgCl/Ag as reference electrode, scan rate at
can absorb the light from the commercial semiconductor
laser that emits light at 830 nm. This wavelength is also
same to the absorption maximum of pentarylene tetracar-
boxylic diimide which contains five fused naphthalene units
and required multiple-step synthesis.[4] Compared with PDI
0.1 VsÀ1
.
(labs
(max)=526 nm), the laterally expanded molecule M1
M2–M4 displayed relatively larger band gaps (1.52–
1.57 eV). The reduction and oxidation onset potentials also
varied depending on the linking modes and more detailed
data of redox potentials, energy levels and band gaps was
listed in Table 1. The high electron affinity (the LUMO
exhibited a remarkable 305 nm red-shift, suggesting that the
lateral extension of p-conjugation is a more efficient way to
obtain NIR absorption than by extension along the long
axis. The 168 nm red-shift of M1 in comparison with the
parent bisanthene can be ascribed to the substitution by
electron-withdrawing dicarboxylic imide groups which lead
to a convergence of HOMO–LUMO energy gap as previ-
ously calculated.[5,13] The relatively lower e value of M1
compared with other rylene imides has been theoretically
explained by Feng et al.[13] The partially fused dimers M2–
M4 showed a dark green color in solution and exhibited
similar absorption bands. The absorption band of the fully
fused bisanthene carboximide M1 displayed a significant
red-shift by more than 130 nm due to extended p-conjuga-
tion. Compounds M2–M4 showed similar NIR photolumi-
nescence bands with emission maximum at 923 nm for M3
and M4 and 912 nm for M2. Compound M1 only exhibited
weak photoluminescence. The photoluminescence quantum
yields[14] of dyes M2–M4 are less than 0.5% which was nor-
mally observed for many NIR absorbing dyes. However, for
many applications such as laser filter and solar cell, photolu-
minescence is not necessary and even should be avoided.
Solutions of M1–M4 are stable upon exposure to air due
to the attachment of electron-withdrawing dicarboxylic
imide groups. Cyclic voltammetry (CV) was applied to ex-
amine their electrochemical behaviors as well as to deter-
mine their energy levels (Figure 2). All compounds showed
one reversible oxidation at potentials above 1.1 V and two
reversible one-electron reductions at potential below
À0.11 V (vs. AgCl/Ag). The two-reduction processes indi-
cate that the molecules can be reduced into stable anion
and dianion which are stabilized by the two imide groups.
At the same time, these large p-conjugated systems can also
stabilize the oxidized cationic species. The fully fused dimer
M1 has a narrow band gap (1.26 eV) and can undergo
charge transfer readily. The other partially fused molecules
Table 1. Summary of electrochemical properties of compounds M1–M4.
Eox, E1 and E2 are the half-wave potentials for respective redox
red
red
waves with E (Fc+/Fc) as reference. Fc+/Fc was used as internal refer-
ence and under our experimental conditions, E (Fc+/Fc)=0.42 V vs Ag/
AgCl.
Compound Eox
E1
[V]
E2
[V]
HOMO[a]
[eV]
LUMO[b]
[eV]
Eg
[eV]
red
red
[V]
M1
M2
M3
M4
0.74
0.94
0.95
0.84
À0.62 À0.84 À5.53
À0.74 À0.95 À5.67
À0.75 À0.96 À5.68
À0.91 À1.08 À5.57
À4.27
À4.15
À4.16
À4.00
1.26
1.52
1.52
1.57
[a] HOMO energy level was calculated from the onset of oxidation wave.
[b] LUMO energy level was calculated from the onset of the first reduc-
tion wave. The energy levels were calculated by using the following equa-
tions: HOMO = À(4.8 + Eox) (1), LUMO = À(4.8 + Ered) (2).
energy level) of these molecules also suggested that these
imide-substituted polycyclic aromatic compounds, and in
particular the fully planar bisanthene carboximide, could be
used as building blocks to construct n-type semiconductors
for electronic devices such n-channel field effect transistors
and solar cells. Of course, appropriate modifications on the
substituents are necessary to control their molecular order
in thin film for these purposes.
In summary, a series of soluble and stable bisanthracene
bis(dicarboxylic imide)s were synthesized and their optical
and electrochemical properties were studied. The fully
planar bisanthene carboximide M1 was synthesized as stable
compound for the first time and it showed interesting NIR
absorption properties. The cis-isomer of aceanthrene green
(M2) was also prepared in pure form by two simple methods
Chem. Eur. J. 2009, 15, 9299 – 9302
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www.chemeurj.org
9301

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of rylene molecules was an effective way to shift the absorp-
tion into near infrared region in a fast mode. The intense
NIR absorptions suggested that they could be used as useful
dyes for many applications such as NIR laser protection
glasses. In addition, the n-type character and the intense
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cases.
Acknowledgements
This work was financially supported by Singapore NRF Competitive Re-
search Program (“Graphene-related Materials and Devices”), DSTA
DIRP project (DSTA-NUS-DIRP/2008/03) and the NUS Young Investi-
gator Award (R-143-000-356-101).
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Keywords: anthracenes · hydrogenation · dyes/pigments ·
rylenes
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Received: May 26, 2009
Published online: August 5, 2009
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