Anthraceno-perylene bisimides: The precursor of a new acene

Article abstract of DOI:10.1021/ol202417u

A controlled synthesis strategy for a anthracene-fused perylene bisimide was developed from the cyclization of an anthracene unit pendant to a perylene diimide scaffold. The direct cyclization led to a zigzag molecule, while a Diels-Alder strategy influen

Full text of DOI:10.1021/ol202417u

ORGANIC
LETTERS
2011
Vol. 13, No. 20
5692–5695
Anthraceno-Perylene Bisimides: The
Precursor of a New Acene
Yongjun Li,* Liang Xu, Taifeng Liu, Yanwen Yu, Huibiao Liu, Yuliang Li,* and
Daoben Zhu
Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory
of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
P.R. China, and Graduate University of Chinese Academy of Sciences, Beijing 100080,
P.R. China
ylli@iccas.ac.cn; liyj@iccas.ac.cn
Received September 6, 2011
ABSTRACT
A controlled synthesis strategy for a anthracene-fused perylene bisimide was developed from the cyclization of an anthracene unit pendant to a
perylene diimide scaffold. The direct cyclization led to a zigzag molecule, while a DielsꢀAlder strategy influenced the regiochemistry of
cyclization to afford the linear precursor of a new acene.
During the past decade, scientists have developed
morphologies for synthesizing and characterizing many
new conjugated hydrocarbons with extended polycyclic
frameworks.1 Acenes such as anthracene and pentacene
have received much attention as optical and electronic
materials.2 Interest in the synthesis of acenes larger than
pentacene, such as hexacene,3 heptacene,3b,4 octacene5 and
nonacene,6 is growing which is expected to increase the
conjugated length for tuning electronic structure, aroma-
ticity, and HOMOꢀLUMO gaps for molecular electronic
application.1d Understanding the structureꢀproperty re-
lationships that relate specifically to these molecules could
lead to new design rules for producing high-performance
conjugated molecules. Anthony’s group observed that
heptacene is a stable and synthetically accessible molecule
if it is protected by significantly bulky groups.3b Wudl et al.
have explored different synthetic methods to functionalize
heptacenes.4c Bettinger’s group reported the synthesis
and spectroscopic detection of unsubstituted octacene
and nonacene under conditions of matrix isolation.5
Later, Miller’s group reported a persistent substituted
nonacene with a very small HOMOꢀLUMO gap of
1.12 eV.6 Still, the design and synthesis of novel struc-
ture acenes with a controlled electronic structure is
a significant and ongoing challenge within molecular
electronics.7
(1) (a) Hopf, H.; Classics in Hydrocarbon Chemistry; Wiley-VCH:
Weinheim, 2000. (b) Anthony, J. E. Chem. Rev. 2006, 106, 5028.
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T. J. J., Bunz, U. H. F., Eds.; Wiley-VCH: Weinheim, 2007.
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Cheng, Y.; Wudl, F. Angew. Chem., Int. Ed. 2008, 47, 8380. (d) Kaur, I.;
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r
10.1021/ol202417u
Published on Web 09/26/2011
2011 American Chemical Society

Perylene tetracarboxdiimide derivatives are relatively
easy to functionalize at main or bay positions, leading
to soluble dyes for applications in organic photovoltaic
devices,8 artificial light-harvesting complexes,9 organic
electronic devices,10 light-emitting diodes,11 and fluorescent12
or NIR dyes.13 Benzannulation of the bay region of pery-
lene bisimide expanded π-systems along the equatorial
axis enabled new chromophores with significant hypso-
chromic absorption shifts,14 while the excellent photosta-
bilities and high fluorescence quantum yields are retained.
The benzannulation can be realized through the palla-
dium-catalyzed ring annulation reaction14b,15or phototrig-
gered intramolecuar cyclization.16 Core-extended perylene
chromophores, naphthoperylene tetracarboxdiimide (NP),16
dibenzocoronene tetracarboxdiimide analogues (BC),16,17
and nitrogen heterocoronene tetracarboxdiimide analogues14c
are obtained by phototriggered intramolecuar cyclization
(Figure 1). We reasoned that by further extending the pery-
lene bisimide along the equatorial axis and then removing
the diimide unit, we could produce a new analogue of acene-
like structure: perylene superimposed acenes (red core of 4
in Figure 1).
The high reactivity of anthracene rendered fruitful
chemistry for anthracene derivatives.18 Here we reported
the synthesis strategies to enlarge the aromatic π-system
of perylene bisimide along the equatorial axis to give a
precursor of a pentacene or nonacene analogue by the
phototriggered intramolecuar cyclization of 2-anthracene
substituted perylene bisimide. In addition to the synthesis
strategies, also discussed are the optoelectronic profiles of
these molecules.
Figure 1. Molecular structures of the perylene bisimide super-
imposed acenes.
Coupling 2-anthracen-2-yl-4,4,5,5-tetramethyl-[1,3,2]
dioxaborolane19 with 1-bromoperylene bisimide20 or 1,7-
dibromoperylene bisimide21 through Suzuki reaction gave
donorꢀacceptor molecules PDI-AN and PDI-2AN in
which photoinduced electron transfer (PET) between the
perylene bisimide unit and the anthracene unit could occur
(Scheme 1). The initial attempts to access the perylene
bisimide superimposed acenes with the photocyclization
reaction led to the zigzag constitutional compounds 1 and
2 in high yield, which is induced by the higher reactivity of
the 1-position of the anthracene unit than that of the
3-position. Instead, we introduced a directing group gen-
erated at the 9,10-position of the anthracene moiety by
DielsꢀAlder cycloaddition with N-3-hydroxypropyl mal-
eimide, which tuned the reactivity of the phenyl ring
connected with perylene bisimide by breaking the conjuga-
tion of anthracene. Positions 1 and 3 should have similar
reactivity, but the steric effect induced by the directing
group will enhance the possibility of position 3 to couple
with perylene bisimide (Scheme S2). For instance, photo-
cyclization of the endo- and exo-adduct PDI-AN-DA
would lead to endo/exo-PDI-AN-C, totally cyclized
at position 3. Followed by a re-DielsꢀAlder reaction,
the linear constitutional compound 3 can be generated
(Scheme 1). Perylene bisimide superimposed nonacene 4
can be obtained in the same way, but detailed character-
ization of 4 cannot be performed owing to the relatively
rapid degradation in solution; only MS spectra can be
obtained. In the synthesis procedure, the endo/exo-cy-
cloaddition intermediates would lead to the same target.
Spectral analysis of the mono- or dicyclized product by
HRFAB mass spectrometry indicated the expected mass
ion withthe lossoftwo or four hydrogen atoms. Thezigzag
compounds 1 and 2 showed two characteristic doublet
peaks of protons 3/4 of the anthracene unit, and the linear
compound 3 exhibited two singlet peaks for protons 1/4,
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Pugliesi, I. J. Am. Chem. Soc. 2010, 132, 16777. (d) Wasielewski, M. R.
Acc. Chem. Res. 2009, 42, 1910.
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M. A.; Wasielewski, M. R.; Marder, S. R. Adv. Mater. 2011, 23, 268.
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(11) Weil, T.; Vosch, T.; Hofkens, J.; Peneva, K.; Mullen, K. Angew.
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(12) Langhals, H.; Karolinb, J.; Johansson, L. B. J. Chem. Soc.,
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Org. Lett., Vol. 13, No. 20, 2011
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9/10 of anthracene. All of these compounds except 2 and 3
were soluble in various organic solvents, such as CH2Cl2,
CHCl3, toluene, and acetone. 13C NMR spectra of 2 and 3
cannot be well resolved due to poor solubility. The cyclized
compounds 1, 2, and 3 showed high thermal stability, with
their decomposition temperature at up to 450 °C.
PDI-AN emitted at 535 nm (φ = 0.212 in CH2Cl2, with a
lifetime of 4.17 ns), and a dimeric band at 575 nm appeared
with increasing concentration (Figure S11). PDI-2AN
emitted at 520 nm weakly (φ = 0.086), which was induced
by the PET from the anthracene unit to PDI. After
cyclization, the anthracene units and perylene plane in 1
and 2 are twisted, which decreased the conjugation be-
tween them. 1 showed two emission bands at 520 and
645 nm in cyclohexane. The short wavelength emission is
due to the perylene core which has a lifetime of 4.10 ns in
CH2Cl2. The emission band at a longer wavelength showed
a solvato-kinetic effect (Figure S10b), which is due to the
charge transfer from an anthracene unit to a perylene unit,
with a lifetime of 7.61 ns in CH2Cl2 (φ = 0.532). With
increasing solvent polarity, this band red-shifted and the
emission intensity decreased; it disappeared in acetonitrile,
DMF, and DMSO. Compound 2 emitted strongly at
650 nm (φ= 0.97), with a lifetime of 7.71 ns in CH2Cl2.
However, the linear compounds 3 did not emit, which was
due to the strong intramolecular charge transfer.
Scheme 1. Synthesis of the Anthracene Substituted Perylene
Diimides and Their Photocyclization
The UVꢀvis absorption spectra and fluorescent spectra
of PDI-AN, 1, 3, PDI-2AN, and 2 are presented in Figure
2. The UVꢀvis spectrum of PDI-AN shows a combination
of an anthracene unit (325ꢀ400 nm) and red-shifted
perylene bisimide absorption (530 nm). Dianthracene
substituted PDI-2AN shows similar but broader absor-
bance in the long wavelength region (450ꢀ650 nm). After
cyclization, the zigzag compound 1 showed a multipeak
band with defined structures at a shorter wavelength
(250ꢀ410 nm). A typical perylene vibronic structure with
λmax at 495 nm (high intensity band) and a shoulder
absorbance band at λmax = 570 nm (low intensity band)
was observed in the longer wavelength region, which is
similar to the UVꢀvis absorption of the coronenebis-
(dicarboximide)s.14a Interestingly, the low intensity
band at 570 nm is solvent-dependent, which has a
defined structure in cyclohexane (Figure S10a). For 2,
the absorption in the shorter wavelength region (320ꢀ
430 nm) is increased, while in the region of longer
wavelengths, the absorbances at 475 and 598 nm show
similar intensity. For linear compound 3, a broad band
between 550 and 720 nm, a perylene like band at 475
nm, a sharp band at 380 nm, and an intensive band at
290ꢀ350 nm were observed.
Figure 2. UVꢀvis absorption and fluorescence emission (inset)
spectra of PDI-AN, 1, 3, PDI-2AN, and 2 in CH2Cl2.
To obtain the theoretical aspect of the spectroscopic
properties of these compounds, the B3LYP/6-31þG* level
using the Gaussian 03program22 functional calculationsof
the molecular orbits of 1, 2, 3, and 4 was studied. The
HOMO and LUMO orbitals from the calculation are
depicted in Figure 3 (Figure S2). For 1 and 2, the density
in the HOMO concentrated in the anthracene ring, while
the density in the LUMO did not simplyconcentrate on the
perylene bisimide ring; it was on the plane of perylene
bisimide fused with one or two ethylenes in the bay region.
For linear compounds 3 and 4, the molecular orbits
showed different behavior. In the case of 3, the density in
the HOMO still concentrated in the anthracene ring and
the density in the LUMO concentrated on the perylene
(22) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Pittsburgh, PA, 2003.
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Org. Lett., Vol. 13, No. 20, 2011

predicted value (2.43 (1), 2.39 (2), 2.02 (3)). The band gap
widths were further confirmed by cyclic voltammetry
(Table S5). Upon cyclization, there existed two one-elec-
tron reduction waves for zigzag compounds 1 (ꢀ1.14
and ꢀ1.35 V) and 2 (ꢀ1.26 and ꢀ1.44 V). Compared with
that of PDI-AN (ꢀ0.98 V) and PDI-2AN (ꢀ1.00 V), the
first reduction potential of 1 was negatively shifted by
about 160 mV and that of 2 was negatively shifted about
260 mV. The oxidation process was irreversible, which was
attributed to oxidation processes that involved the anthra-
cene unit (1.04 V for 1, 0.96 and 1.10 V for 2). The band
gaps based on cyclic voltammetry were 2.18 and 2.22 eV
for 1 and 2. For linear compounds 3, the reduction and
oxidation processes are all irreversible, the electrodonation
ability is enhanced (Eox =0.58 V, negatively shifted about
400 mV compared with that of 1). The band gap of linear 3
(1.80 eV) is smaller than that of the zigzag 1, further
supporting the claim that the twisting of the aromatic
cores driven by steric congestion between 9-H on anthra-
cene and the neighboring hydrogen atoms on the perylene
core (Figure S1) greatly reduced the electronic commu-
nication between the anthracene and perylene units in the
zigzag constitutional compounds.
In conclusion, we demonstrated a facile method for the
synthesis of an anthracene-fused perylene bisimide from
the cyclization of an anthracene unit pendant to a perylene
diimide scaffold. The direct cyclization led to a zigzag
molecule, while a DielsꢀAlder strategy influenced the
regiochemistry of cyclization to thus afford ultimately the
linear molecule. Although we have not yet succeeded in
the isolation of 4, we expect that the stable analogue can
be obtained by protection of the anthracene unit. These
acene-like compounds showed mainly the properties of
the perylene bisimide core; the low energy absorbance for
the central acene-like core was not evident. However,
success in the synthesis ofthiskind offunctionalized acene-
like structures will help to stimulate significant new syn-
thetic targets; for example, after removal of the diimide
unit, structures with more acene-like properties should be
expected.
Figure 3. HOMO and LUMO of 1, 2, 3, and 4 calculated by the
B3LYP/6-31þG* method.
bisimide ring, but the gradual delocalization between these
two moietiesindicated the stronger conjugation in 3 than1.
For 4, the surfaces of the HOMO has a clear node in the
center. Theoretical calculation was also carried out by the
TD-DFT method at the B3LYP/6-31þG* level to inves-
tigate the electronic transitions (Figure S3ꢀS6, Table
S1ꢀS4). The electronic transition analysis of compound
1 suggests that the band at 570 nm is caused by a transi-
tion from an anthracene-like HOMO to a perylene-like
LUMO.17b The second band at 495 nm shows the typical
vibronic fine structure of perylene bisimide, as this band
is related to the transition between the perylene-like
HOMOꢀ1 and the perylene-like LUMO. The 320ꢀ430
nm region absorbances are due to a HOMOfLUMOþ1
transition. The transition of 3 is similar to 1, except that the
HOMOfLUMO transition is red-shifted for 80 nm and
the HOMOꢀ1fLUMO transition is blue-shifted for 18
nm. For 2, the absorbance at about 600 nm is caused by the
transition from HOMO to LUMO and the band at 550 nm
is due to the transition of HOMOꢀ2fLUMO.
Acknowledgment. We are grateful for financial support
from the National Nature Science Foundation of China
(21031006, 20971127, 90922017, 20831160507), NSFC-
DFG joint fund (TRR 61), and the National Basic Re-
search 973 Program of China.
Supporting Information Available. Detailed experimen-
tal procedures, characterization data, UVꢀvis absorp-
tion spectra, electrochemistry data, and results of theo-
retical calculation, complete ref 22. This material is
available free of charge via the Internet at http://pubs.
acs.org.
On the basis of the onset of the longest-wavelength
absorption, the optical HOMOꢀLUMO gaps for perylene
bisimide superimposed acenes were calculated to be 2.04
(1), 1.98 (2), 1.75(3), which are 0.2ꢀ0.4 eV smaller than the
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