Chemical modification of AlQ3 to a potential electron acceptor for solution-processed organic solar cells

Article abstract of DOI:10.1016/j.tetlet.2016.05.049

In this Letter, a new AlQ₃ derivative is designed and synthesized through introducing 2-ethylhexyl cyanoacrylate at C-4 of the quinoline ring. The obtained Al(4CAQ)₃ shows excellent solubility (>50 mg/ml) in common solvents. Furtherm

Full text of DOI:10.1016/j.tetlet.2016.05.049

Accepted Manuscript
Chemical modification of AlQ3 to a potential electron acceptor for solution-
processed organic solar cells
Shasha Wu, Shuixing Li, Chang-Zhi Li, Minmin Shi, Hongzheng Chen
PII:
S0040-4039(16)30572-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.05.049
TETL 47668
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
19 March 2016
8 May 2016
13 May 2016
Please cite this article as: Wu, S., Li, S., Li, C-Z., Shi, M., Chen, H., Chemical modification of AlQ3 to a potential
electron acceptor for solution-processed organic solar cells, Tetrahedron Letters (2016), doi: http://dx.doi.org/
10.1016/j.tetlet.2016.05.049
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Chemical modification of AlQ3 to a potential
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Shasha Wu, Shuixing Li, Chang-Zhi Li, Minmin Shi, and Hongzheng Chen

1
Tetrahedron Letters
Chemical modification of AlQ3 to a potential electron acceptor for solution-processed
organic solar cells
Shasha Wu, Shuixing Li, Chang-Zhi Li, Minmin Shi, and Hongzheng Chen
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon Materials, & Department of Polymer Science and
Engineering, Zhejiang University, Hangzhou 310027, P. R. China.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
In this letter, a new AlQ3 derivative is designed and synthesized through introducing 2-
ethylhexyl cyanoacrylate at C-4 of quinoline ring. The obtained Al(4CAQ)3 shows excellent
solubility (> 50 mg/ml) in common solvents. Furthermore, the LUMO energy level of
Al(4CAQ)3 is greatly lowered to -3.70 eV, which matches those of donors used in organic solar
cells (OSCs). Together with its octahedral molecular geometry and good UV-visible
absorptions, Al(4CAQ)3 would be a promising solution-processed electron acceptor for OSCs.
Available online
Keywords:
AlQ3
2016 Elsevier Ltd. All rights reserved.
Chemical modification
Non-fullerene acceptors
Organic solar cells
Introduction
Solution-processed bulk heterojunction (BHJ) organic solar
cells (OSCs) have attracted much attention for their merits of
light weight, flexibility, and low-cost fabrication.1 In the past
years, OSCs have achieved power conversion efficiencies (PCEs)
over 10%.2,3 In OSCs, fullerenes and their derivatives (e.g., [6,
6]-phenyl-C61-butyric acid methyl ester, abbreviated as
PC61BM) are the dominant electron acceptors owing to their high
electron mobilities and isotropic charge transport properties.4
Nevertheless, fullerene acceptors still suffer from the drawbacks,
like tedious purification, poor light absorption, and limited
chemical and energetic tunabilities.5
a tetraphenylethylene (TPE) core-based 3D SM acceptor, TPE-
PDI4, giving a PCE of 5.53% for the resulting OSCs.6 Later,
Chen and Yan reported a series of tetraphenyl carbon-group core-
based 3D SM acceptors, both achieving OSCs with decent
PCEs.4,10 Sauve et al. developed a series of azadipyrromethene
complexes with similar distorted tetrahedral geometry as SM
acceptors and got a best PCE of 4.1% for the non-fullerene
OSCs.11,12
Many metal chelates own a twisted octahedral 3D structure.
Among these, tris-(8-hydroxyquinoline) aluminum (AlQ3) is a
representative example. AlQ3 has a central metal Al(III) cation
coordinated by three bidentate 8-hydroxyquinoline anions, and it
can be vapor-deposited as isolated molecules with the roughly
ball-shaped geometry,13 showing excellent thermal stability, high
fluorescent efficiency, and relatively good electron mobility,
which lead to the wide uses of AlQ3 as the emissive and electron-
transporting material in organic light-emitting diodes (OLEDs).14
The above characteristics also make AlQ3 an attractive building
block for the design of new type non-fullerene acceptors for
solution-processed OSCs. However, there is no relevant study
reported to date. Two key factors hinder the application of AlQ3
in OSCs: one is its poor solubility in common solvents, the other
is its high the lowest unoccupied molecular orbital (LUMO)
energy level of -3.0 eV,15 which mismatches with those of donors
in OSCs.
The reason why fullerenes and their derivatives work
especially well as electron acceptors is that they have spherical
structures, thus, when blended with donors to build BHJs, they
can form nanoscale phase separation domains and three-
dimensional (3D) charge-transporting networks, which are
favorable for efficient exciton dissociation and charge transport
in OSCs.6 To work out the superior electron acceptors, a number
of small molecular (SM) acceptors have been developed to avoid
the defects of fullerenes while reserving their advantages.4,6-10
Among those materials, 3D or quasi-3D SM acceptors mimicking
the fullerene’s spherical shape are promising candidates. For
example,
Yan
and
co-workers
reported
———
 Corresponding author. Tel.: +0086-571-8795-2557; fax: +0086-571-8795-3733; e-mail: minminshi@zju.edu.cn
 Corresponding author. Tel.: +0086-571-8795-2557; fax: +0086-571-8795-3733; e-mail: hzchen@zju.edu.cn

2
Tetrahedron Letters
1
Figure 1. H NMR spectrum of Al(4CAQ)3 in CDCl3 solution. The inset is the steric configuration of mer-Al(4CAQ)3 (2-ethylhexyl is
replaced by methyl for clarity), in which three non-equivalent hydroxyquinoline rings are labeled by the numerals 1, 2, and 3, respectively;
and the letters a, b, . . ., f represent the positions of H-atoms on the hydroxyquinoline ring.
Result and discussion
The target molecule tris-(E)-[4-(2-ethylhexyl cyanoacrylate)-
8- hydroxyquinoline] aluminum (Al(4CAQ)3) is synthesized in
6-step reactions as shown in Scheme 1. First, 4-methyl-8-
hydroxyquinoline (1) is prepared through Michael addition of
methyl vinyl ketone to 2-aminophenyl, then the hydroxyl group
of compound 1 is protected by benzyl (2) prior to the oxidation of
the methyl at C-4 into the aldehyde group (3). After the
protecting group of benzyl is removed (4), 2-ethylhexyl
cyanoacrylate is introduced via Knoevenagel condensation with
the aldehyde group, giving the free ligand (E)-4-(2-ethylhexyl
cyanoacrylate)-8-hydroxyquinoline (5). Finally, the target
compound Al(4CAQ)3 is obtained by complexing the ligand with
aluminum isopropoxide. It should be pointed out that other
synthetic routes are also tried to synthesize the target molecule
Al(4CAQ)3, unfortunately, all attempts have failed. For example,
before
Knoevenagel
condensation
with
2-ethylhexyl
cyanoacetate, we have reacted compound 4 with Al cation to get
the AlQ3 derivative with the aldehyde group at C-4, in order to
avoid that the hydroxyl of compound 4 might be destroyed
during Knoevenagel reaction. But 4-aldehyde substituted AlQ3
derivative has very poor solubility and is difficult to purify, so we
fail to obtain the target molecule. Therefore, we synthesize
compound 5 firstly before its complexation with Al cation. And
in order to protect the hydroxyl group on quinoline ring, we
choose a solid-phase Knoevenagel condensation using cadmium
iodide (CdI2) as the catalyst.18
Scheme 1. Synthetic Route of Al(4CAQ)3.
The chemical structure of the obtained Al(4CAQ)3 is fully
characterized by 1H NMR, 13C NMR, MS, and elemental analysis
1
(Supplementary data). From H NMR spectrum (Fig. 1), it is
Taking the above considerations into account, we modify
AlQ3 by introducing 2-ethylhexyl cyanoacrylate at the C-4 of
quinoline ring. 2-Ethylhexyl cyanoacrylate is selected as the
substituent because long branched alkyl (2-ethylhsexyl) is very
soluble in organic solvents. Meanwhile, two electron-
withdrawing groups (cyano and ester) can significantly reduce
the LUMO energy level of AlQ3. In addition, their conjugation
effects with quinoline ring through a double-bond bridge would
further lower the LUMO energy level of AlQ3. And C-4 position
is chosen as the substitution site, because the LUMO of AlQ3 is
localized mainly on the pyridyl ring, and C-4 has the highest
electron cloud density judging from the molecular simulation of
electronic structure for AlQ3.16,17 Thereby, an AlQ3 derivative
with the enhanced solubility and appropriate energy levels for the
applications in OSCs is expected.
obviously observed that Al(4CAQ)3 exists as meridional isomer,
in which three hydroquinoline ligands are inequivalent by
geometric symmetry, i.e. each ligand is distinguishable, leading
to the splitting of the NMR peaks for each hydrogen atom.17,19
And as predicted, Al(4CAQ)3 can be well dissolved in
chloroform, dichloromethane, chlorobenzene et.al., and the
solubilities in the above solvents are all more than 50 mg/ml. It
even can be dissolved in hexane and ethanol. The thermal
properties of Al(4CAQ)3 are investigated by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) in a
nitrogen atmosphere (Supplementary data). As shown in Fig. S1,
Al(4CAQ)3 exhibits a thermal decomposition temperature (Td) of
256 °C with a 5% weight loss and no melting or crystallization
peaks during the temperature range of 50-200 °C are observed,
indicating that Al(4CAQ)3 is an amorphous solid.

3
Conclusion
20
10
0
In summary, we design and synthesize a new AlQ3 derivative,
Al(4CAQ)3, which has a quasi-spherical octahedral 3D structure
and exhibits good thermal stability. The introduction of electron-
withdrawing 2-ethylhexyl cyanoacrylate to the C-4 of quinoline
ring enables Al(4CAQ)3 with excellent solubility and appropriate
LUMO energy level to pair with donors in OSCs. In addition,
Al(4CAQ)3 has better UV-visible absorptions than PC61BM.
Therefore, Al(4CAQ)3 may be a promising non-fullerene
acceptor for solution-processed OSCs.
-10
Acknowledgments
The authors would like to gratefully acknowledge the
financial support from the National Natural Science Foundation
of China (No. 21474088) and the Zhejiang Province Natural
Science Foundation (No. LR13E030001). This work is also
supported by the Joint NSFC-ISF Research Program, jointly
funded by the National Natural Science Foundation of China and
the Israel Science Foundation (No. 51561145001).
-20
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (V, vs. SCE)
Figure 2. Cyclic voltammogram of Al(4CAQ)3 in CH2Cl2 soution.
Supplementary Material
Al(4CAQ) -solution
3
1.0
0.8
0.6
0.4
0.2
0.0
Al(4CAQ) -thin film
3
PC BM-solution
61
PC BM-thin film
61
Supplementary data (the experimental section, TGA and DSC
curves of Al(4CAQ)3, scanned NMR spectra and mass spectra)
associated with this article can be found, in the online version.
References and notes
1
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Wavelength (nm)
Figure 3. UV-vis absorption spectra of Al(4CAQ)3 and PC61BM
in CH2Cl2 solutions and thin films. Insets are the photographs of
Al(4CAQ)3 (left) and PC61BM (right) in CH2Cl2 solutions (upper)
and thin films (down).
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Cyclic voltammetry (CV) measurements are performed to
characterize the energy levels of Al(4CAQ)3. From the onset
oxidation potential (1.01 V versus SCE) and the onset reduction
potential (-0.70 V versus SCE) presented in Fig. 2, the highest
occupied molecular orbital (HOMO) and LUMO energy levels of
Al(4CAQ)3 are obtained as -5.41 and -3.70 eV, respectively.
Compared to the parent molecule AlQ3, the LUMO energy level
of Al(4CAQ)3 is lowered tremulously by 0.70 eV. It is ascribed
mainly to the inductive effect of the electronegative cyano and
ester groups, thus it can match donors used in OSCs.
Additionally, the HOMO energy level of Al(4CAQ)3 is 0.29 eV
higher than that of AlQ3 (-5.7 eV),15 which can be attributed to
the conjugation effect of 2-ethylhexyl cyanoacrylate with
quinoline ring. The narrowed band gap of Al(4CAQ)3 would
help improve the light-harvesting capability if it is applied as
electron acceptor in OSCs. Fig. 3 depicts UV-vis absorption
spectra of Al(4CAQ)3 in solution and film. It is interesting to see
that, the absorptions of Al(4CAQ)3 has been extended to visible
range when compared to those of AlQ3.16 And the absorptions of
Al(4CAQ)3 are also much stronger and broader than PC61BM.
From the absorption band-edge (λonset) at 686 nm, the optical band
gap (Egopt) of Al(4CAQ)3 is calculated as 1.81 eV, which is in
agreement with the value gotten from CV measurements. These
results demonstrate the rationality of our molecular design.
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5
Highlights:
 An AlQ3 derivative Al(4CAQ)3 is designed and synthesized in 6-step reactions.
 Al(4CAQ)3 have three 2-ethylhexyl cyanoacrylate groups at C-4 of quinoline ring.
 Al(4CAQ)3 owns an appropriate LUMO of -3.7 eV to pair with donors in OSCs.
 Al(4CAQ)3 shows excellent solubility and good UV-visible absorptions.
 Al(4CAQ)3 would be a promising non-fullerene acceptor for solution-processed OSCs.