Isomeric anthracene diimide polymers

Article abstract of DOI:10.1039/d0sc06164f

N-type semiconducting polymers are attractive for organic electronics, but desirable electron-deficient units for synthesizing such polymers are still lacking. As a cousin of rylene diimides such as naphthalene diimide (NDI) and perylene diimide (PDI), anthracene diimide (ADI) is a promising candidate; its polymers, however, have not been achieved yet because of synthetic challenges for its polymerizable monomers. Herein, we present ingenious synthesis of two dibromide ADI monomers with dibromination at differently symmetrical positions of the ADI core, which are further employed to construct ADI polymers. More interestingly, the two obtained ADI polymers possess the same main-chain and alkyl-chain structures but different backbone conformations owing to varied linking positions between repeating units. This feature enables their different optoelectronic properties and film-state packing behavior. The ADI polymers offer first examples of conjugated polymer conformational isomers and are highly promising as a new class of n-type semiconductors for various organic electronics applications.

Full text of DOI:10.1039/d0sc06164f

Chemical
Science
View Article Online
View Journal | View Issue
EDGE ARTICLE
Isomeric anthracene diimide polymers†
Dandan Tu, Qing Yang, Shuwen Yu, Xin Guo *a and Can Li *a
ab
ab
a
Cite this: Chem. Sci., 2021, 12, 2848
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
N-type semiconducting polymers are attractive for organic electronics, but desirable electron-deficient
units for synthesizing such polymers are still lacking. As a cousin of rylene diimides such as naphthalene
diimide (NDI) and perylene diimide (PDI), anthracene diimide (ADI) is a promising candidate; its polymers,
however, have not been achieved yet because of synthetic challenges for its polymerizable monomers.
Herein, we present ingenious synthesis of two dibromide ADI monomers with dibromination at
differently symmetrical positions of the ADI core, which are further employed to construct ADI polymers.
More interestingly, the two obtained ADI polymers possess the same main-chain and alkyl-chain
structures but different backbone conformations owing to varied linking positions between repeating
units. This feature enables their different optoelectronic properties and film-state packing behavior. The
ADI polymers offer first examples of conjugated polymer conformational isomers and are highly
promising as a new class of n-type semiconductors for various organic electronics applications.
Received 9th November 2020
Accepted 19th December 2020
DOI: 10.1039/d0sc06164f
rsc.li/chemical-science
have performed pioneering studies on the synthesis of acene
diimides such as anthracene-, tetracene-, pentacene- and
Introduction
N-type organic semiconductors, in particular conjugated poly- hexacene-diimides. Among them, the anthracene diimide (ADI)
mers, are very crucial for optoelectronic devices, but their with one ring extension relative to the NDI should possess
development lags far behind that of p-type counterparts due to similar electron affinity and potentially provide interesting
26
the lack of electron-decient building blocks. Among a handful optoelectronic properties. Moreover, its polymers have been
1
–13
of electron-decient units,
aromatic diimides, typically naphthalene diimide (NDI) and tics for organic electronics.
perylene diimide (PDI), attract enormous attention because of
However, access to the ADI unit and especially to its poly-
their high electron affinity and mobility. A plenty of NDI- and mers is rather challenging. Although Wudl et al. and Yamada
six-membered tetracarboxylic theoretically predicted to present excellent n-type characteris-
27
28
25
PDI-based polymers have been developed as n-type semi- et al. separately reported short alkyl chain-substituted ADIs
conductors for various organic electronics applications.
3,14–16
synthesized from different routes, ADI-based polymers were
The aromatic diimides can be categorized into rylene dii- never materialized since synthetic chemists were plagued by
mides and acene diimides according to the p-conjugation challenges in synthesizing polymerizable monomers of ADI.
17
core. The NDI and PDI are just representatives of rylene dii- Long alkyl chains attached to amide N atoms are, on the one
mides having extended conjugations along the normal axis of hand, indispensable for ensuring the solubility of resultant
the NDI scaffold (Fig. 1). More derivatives e.g. terrylene and
1
8–20
quaterrylene diimides were investigated by M u¨ llen,
Langh-
21
22
als and Adachi et al. However, they are not suitable for
constructing polymers because of high steric hindrance at the
longitudinal direction which is adverse to the charge transport
and molecular crystallinity, as claimed for PDI polymers. This
issue is absent in another class of aromatic diimides, namely
acene diimides with conjugation expansion along the equato-
rial axis of the NDI, yet their polymers cannot be achieved so far
23
24
25
owing to synthetic difficulties. Wang et al. and Yamada et al.
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian National Laboratory for Clean Energy, Zhongshan
Road 457, Dalian, 116023, China. E-mail: guoxin@dicp.ac.cn; canli@dicp.ac.cn
b
University of Chinese Academy of Sciences, Beijing 100049, China
Fig. 1 Rylene diimides and acene diimides derived from the NDI
scaffold by different ways to extend the conjugation of the aromatic
core, and two dibromination ways of ADI.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0sc06164f
1
2848 | Chem. Sci., 2021, 12, 2848–2852
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Edge Article
Chemical Science
polymers; on the other hand, functionalized groups on the
anthracene such as halogens, boronic esters, or tin salts
(bromines usually preferred) are needed for the cross-coupling
polymerization. Unfortunately, precursor compounds with
long alkyl chains are not easily accessible, and symmetrical
dibromination of ADI obviously suffers from great complexity
owing to the presence of several pairs of substitutable sites (2,6-,
3,7-, and 4,8-positions, Fig. 1). Establishing new synthetic
protocols to tackle these issues is thus of critical signicance.
Herein, we report the synthesis and properties of two 2-
octyldodecyl-substituted dibromide ADI monomers and their
derived polymers. The two monomers are dibrominated at 2,6-
and 3,7-positions (2,6-2Br-ADI and 3,7-2Br-ADI, Fig. 1), respec-
tively, by different strategies, making the two obtained polymers
(PADI-2,6-2T and PADI-3,7-2T) possess isomeric backbone
conformations. The two polymer conformers, thereby, present
noticeably different molecular congurations, optoelectronic
properties, as well as packing and oriented behavior in the lm
state. To the best of our knowledge, this is the rst report on
ADI-based polymers with backbone isomerism showing poten-
tial as a new class of promising n-type semiconducting
materials.
Results and discussion
The two monomers are synthesized with pre- and post-
bromination approaches, respectively. The 3,7-2Br-ADI is
synthesized by a post-bromination route (Scheme 1a), where the
3,7-dibromination is conducted aer obtaining the 2-
octyldodecyl-substituted ADI (6a). The ADI 6 is acquired via
a Lewis acid-mediated reaction between acyl chloride 3 and
alkyl isocyanate 5 (for their synthesis see the ESI†). This
approach was initially reported by Yamada et al. using bismuth
25
triate as a Lewis acid; however, the procedure in the literature
did not work for our reaction. By replacing the bismuth triate
3
with a classic Lewis acid of AlCl and swapping the addition
sequence of Lewis acid and isocyanate, the reaction is
Scheme 1 Synthetic routes to (a and b) monomers and (c) polymers.
successfully realized to give the ADI 6. When the substituent is
2
-octyldodecyl, 6a is produced with a low yield (<10%, two steps
higher. Meanwhile, long linear alkyl chains can provide better
solubility of intermediates 13 and 14, allowing for efficient
second-step intramolecular Friedel–Cras acylation, by which
the 6c containing the n-dodecyl is synthesized in a higher yield
than the n-hexyl-substituted 6b. The proposed mechanism
suggests that the ADI and its various derivatives could be ob-
tained in a desirable yield via tuning side chains. In the present
work, in order to ensure the solubility of the resulting polymers
and to compare with classic NDI and PDI analogs, the 2-
octyldodecyl-substituted 6a is used to prepare its dibromide
monomer. Aer careful optimization (Table S1†), it is selectively
dibrominated at 3,7-positions with a brominating agent, i.e. 5,5-
from 2 to 6a). Instead of branched alkyl chains, linear n-hexyl
and n-dodecyl are employed to synthesize ADI 6b and 6c with
improved yields.
It is worthy of note that the yield of 6c with longer alkyl
chains exceeds 50%, higher than that (ꢀ20%) of 6b. Based on
3
these observations and the fact of adding AlCl prior to isocy-
anate, we propose a mechanism of Lewis acid-mediated
successive two-step Friedel–Cras-type reaction (Scheme 2).
First, the AlCl coordinates with the acyl chloride 3 to generate
3
an electrophile carbocation 12. Then, with the addition of
isocyanate, nucleophilic attack to 12, namely the intermolecular
Friedel–Cras-type amidation, occurs to form intermediate 13,
which undergoes the intramolecular Friedel–Cras acylation to
offer the double-cyclized ADI. For the rst-step Friedel–Cras
reaction, the nucleophilic attack can be considerably impacted
by the alkyl chain of isocyanate. Therefore, the steric hindrance
of branched 2-octyldodecyl results in a low yield of 6a, and the
yields of linear alkyl chain-substituted 6b and 6c are much
dimethyl-1,3-dibromohydantoin (DBH), in conc. H
2
SO
4
at
ꢁ
a mild temperature of 65 C to produce the 3,7-2Br-ADI. On the
other hand, the 2,6-2Br-ADI is synthesized by a pre-bromination
route (Scheme 1b), where the 2,6-dibromination prior to
annulation of diimides is performed for the starting material (1)
to get tetrabromide anthracene 7 that follows similar acidation,
©
2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci., 2021, 12, 2848–2852 | 2849

View Article Online
Chemical Science
Edge Article
Scheme 2 Proposed mechanism of the Lewis acid-mediated two-
step Friedel–Crafts-type reaction.
acylation, and Friedel–Cras-type reactions to yield the target
monomer. The molecular structures of two isomeric monomers
1
are unambiguously conrmed by H NMR spectroscopy (Fig. 2)
and high-resolution mass spectrometry (see the ESI†).
Fig. 3 Optimized geometries of (a) ADI, (b) 3,7-2Br-ADI, and (c) 2,6-
Indicated by density functional theory (DFT) calculations
2
2
Br-ADI; backbone conformations of (d) PADI-3,7-2T and (e) PADI-
,6-2T trimers, obtained from DFT calculations (RB3LYP/6-31G (d,p)).
(Fig. 3a–c), the ADI skeleton is highly coplanar and the 3,7-
dibromination does not vary such a molecular conguration.
Interestingly, the 2,6-dibromination causes a small distortion of
the ADI plane, most likely owing to the steric hindrance
between the adjacent bromine and the carbonyl group. UV-Vis
absorption spectra (Fig. 4a) display that the absorption bands
of the two monomers locate in between those of dibromide NDI
and PDI. Notably, relative to the absorption of the ADI 6a
(
Fig. S1a†), the 3,7-2Br-ADI exhibits an evident red shi, while
the 2,6-2Br-ADI shows a blue shi. Such observations can be
ascribed to the effect of different extensions of frontier molec-
ular orbitals (Fig. S2†), induced by bromine substitutions at
different positions. These results suggest that dibromination
positions impact on the molecular geometry, the frontier
Fig. 4 UV-Vis absorption spectra of (a) monomers in toluene solution,
ꢂ5
and (b) PADI-3,7-2T and (c) PADI-2,6-2T in toluene (10 M) and in
film.
1
Fig. 2 H NMR spectra of ADI 6a and two dibrominated isomers.
2850 | Chem. Sci., 2021, 12, 2848–2852
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Edge Article
Chemical Science
Table 1 Molecular weights, thermal stability, optical and electrochemical properties of PADI-3,7-2T and PADI-2,6-2T
a
b
d
ꢁ
sol
c
ꢂ1
lmd
e
g
f
g
Polymer
M
n
(kDa)
PDI
T
( C)
l
max (nm)
3 (cm
)
l
max (nm)
E
(eV)
E
LUMO (eV)
EHOMO (eV)
4
4
PADI-3,7-2T
PADI-2,6-2T
17.8
14.2
2.8
2.3
434
446
444, 740
411, 477, 700
5.76 ꢃ 10
5.50 ꢃ 10
445, 742
415, 509, 719
1.44
1.40
ꢂ4.02
ꢂ4.06
ꢂ5.44
ꢂ5.43
a
b
c
Measured by GPC with 1,2,4-trichlorobenzene as the eluent. Decomposition temperature measured by TGA. Absorption in toluene with
a concentration of 10 M. Absorption of lms spin-coated from a 3 mg mL solution on a glass substrate. Optical bandgap estimated from
the onset of lm absorption. Energy levels measured using the CV method. Calculated from the LUMO and E .
g
ꢂ5
d
ꢂ1
g
e
f
orbitals, and the optical properties of the two bromide mono- repeating units and can induce diversity in various properties of
mers. The LUMOs of ADI 6a, 2,6-2Br-ADI and 3,7-2Br-ADI esti- the two polymers. In toluene solution, the absorption maximum
mated from their rst reduction wave of cyclic voltammogram (lmax, 740 nm) of PADI-3,7-2T manifests a bathochromic shi of
curves (Fig. S1b†) are ꢂ3.87, ꢂ3.95 and ꢂ3.97 eV, respectively, 40 nm compared with that (700 nm) of PADI-2,6-2T (Fig. 4b, c).
indicative of strong electron affinity of the ADI unit.
In the lm state, the PADI-3,7-2T presents a similar absorption
The two monomers were then copolymerized with distannyl spectrum to that in solution (Fig. 4b), while the PADI-2,6-2T
bithiophene to synthesize two polymers, PADI-3,7-2T and PADI- exhibits a red-shied one compared to its solution absorption
2,6-2T (Scheme 1c). Both polymers exhibit good solubility in (Fig. 4c). These results reect that both polymers with the same
chloroform, toluene, chlorobenzene, etc. at room temperature. repeating units but different linkage modes afford varied
Their molecular weights/polydispersity index determined by optical properties and aggregation behavior. The LUMO/HOMO
GPC are 17.8 kDa/2.8 and 14.2 kDa/2.3 (Table 1). Thermogra- levels (Fig. S5†) measured from CV are ꢂ4.02/ꢂ5.43 and ꢂ4.06/
vimetric analysis reveals their excellent thermal stability and the ꢂ5.44 eV for PADI-3,7-2T and PADI-2,6-2T, respectively. The
differential scanning calorimetry plot shows no detectable deep LUMO levels similar to those of NDI and PDI polymers
thermal transitions (Fig. S3†). The DFT simulations demon- suggest strong electron affinity of ADI polymers with potential
23,29
strate that the backbone conformations of the two polymers as n-type semiconductors.
vary profoundly. The PADI-3,7-2T shows a rigid and planar The effect of the different backbone linkage modes on
polymer backbone, with small dihedral angles between ADI and molecular packing and orientation in lms was investigated by
bithiophene units (Fig. 3d), whereas the PADI-2,6-2T main grazing incidence X-ray diffraction (GIXD). Both pristine lms
chains are more distorted (Fig. 3e). In view of their identical show a (010) diffraction ring (Fig. 5), indicating their mixed
polymer structures, the isomeric backbone conformations are face-on and edge-on orientation; however, the PADI-3,7-2T lm
bound to stem from the different linking positions between prefers the face-on one. This packing structure indicates the
coexistence of parallel and vertical charge transportation
30
channels in its lms. Aer thermal annealing (TA), the peak
intensities become signicantly stronger. It is clear that the
PADI-2,6-2T lm manifests distinct (h00) lamellar diffraction
ꢂ1
ꢀ
peaks (q ¼ h ꢃ 0.27 A , h ¼ 1–4) with large crystal coherence
z
lengths (CCLs) (Table S2†) in the out-of-plane direction, signi-
fying a high degree of molecular ordering. In contrast, no
signals implying ordered lamellar stacking are found for the
PADI-3,7-2T lm. Since the two polymers have the same back-
bone structures and alkyl chains, their difference in lamellar
stacking can be attributed to isomeric backbone conformation-
induced disparity of side-chain ordering.
Conclusions
In conclusion, we have presented the synthesis of two dibro-
mide 2-octyldodecyl-substituted ADI with elaborate pre- and
post-bromination methods, allowing for the exploration of ADI
polymers. The dibromination at different positions of the ADI
unit endows two monomers with variable coplanarity which
further leads to isomeric backbone conformations of the
resultant polymers even though they have the same molecular
structures. With the unique conformational isomerism, the
PADI-2,6-2T and PADI-3,7-2T exhibit distinct optical properties
and packing behavior. The advent of ADI polymers with deep
LUMO levels enriches the family of aromatic diimide polymers,
Fig. 5 (a) GIXD patterns of PADI-3,7-2T and PADI-2,6-2T thin films (as
cast and thermal annealing (TA) at 70 C); (b) line-cut profiles of in-
plane and out-of-plane.
ꢁ
©
2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci., 2021, 12, 2848–2852 | 2851

View Article Online
Chemical Science
Edge Article
which are promising for electron-carrying optoelectronic
devices.
Y. Li, T.-S. Kim, L. Hou, C. Yang, H. Yan, D. Yu and
E. Wang, Joule, 2020, 4, 658–672.
1
2 A. S. Molinari, H. Alves, Z. Chen, A. Facchetti and
A. F. Morpurgo, J. Am. Chem. Soc., 2009, 131, 2462–2463.
3 T. Pappenfus, R. Chestereld, C. Frisbie, K. Mann, J. Casado,
J. Raff and L. Miller, J. Am. Chem. Soc., 2002, 124, 4184–4185.
4 X. Guo, D. Tu and X. Liu, J. Energy Chem., 2015, 24, 675–685.
5 B. Fan, L. Ying, P. Zhu, F. Pan, F. Liu, J. Chen, F. Huang and
Y. Cao, Adv. Mater., 2017, 29, 1703906.
Conflicts of interest
1
There are no conicts to declare.
1
1
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (No. 51773204). The authors thank
the beamline BL14B1 at Shanghai Synchrotron Radiation
Facility (SSRF) for providing the beam time.
1
1
1
6 Y. Guo, Y. Li, O. Awartani, H. Han, J. Zhao, H. Ade, H. Yan
and D. Zhao, Adv. Mater., 2017, 29, 1700309.
7 C. Li, Z. Lin, Y. Li and Z. Wang, Chem. Rec., 2016, 16, 873–
885.
8 F. O. Holtrup, G. R. J. Muller, H. Quante, S. Defeyter,
References
F. C. DeSchryver and K. M u¨ llen, Chem.–Eur. J., 1997, 3,
219–225.
1
2
3
4
C. Dou, Z. Ding, Z. Zhang, Z. Xie, J. Liu and L. Wang, Angew.
Chem. Int. Ed., 2015, 54, 3648–3652.
1
2
2
9 S. K. Lee, Y. B. Zu, A. Herrmann, Y. Geerts, K. M u¨ llen and
A. J. Bard, J. Am. Chem. Soc., 1999, 121, 3513–3520.
0 H. Quante and K. M u¨ llen, Angew. Chem. Int. Ed., 1995, 34,
D. Muehlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller,
R. Gaudiana and C. Brabec, Adv. Mater., 2006, 18, 2884–2889.
H. Yan, Z. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dotz,
M. Kastler and A. Facchetti, Nature, 2009, 457, 679–687.
X. Zhan, Z. a. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow,
Y. Li, D. Zhu, B. Kippelen and S. R. Marder, J. Am. Chem.
Soc., 2007, 129, 7246–7247.
1
323–1325.
1 H. Langhals, D. Zgela and R. Lueling, J. Org. Chem., 2015, 80,
2146–12150.
1
2
2
2 M. Adachi and Y. Nagao, Chem. Mater., 2001, 13, 662–669.
3 Y. Guo, Y. Li, O. Awartani, J. Zhao, H. Han, H. Ade, D. Zhao
and H. Yan, Adv. Mater., 2016, 28, 8483–8489.
5
6
R. Stalder, J. Mei, J. Subbiah, C. Grand, L. A. Estrada, F. So
and J. R. Reynolds, Macromolecules, 2011, 44, 6303–6310.
M.-F. Falzon, A. P. Zoombelt, M. M. Wienk and
R. A. J. Janssen, Phys. Chem. Chem. Phys., 2011, 13, 8931–
2
4 X. Cui, C. Xiao, T. Winands, T. Koch, Y. Li, L. Zhang,
N. L. Doltsinis and Z. Wang, J. Am. Chem. Soc., 2018, 140,
12175–12180.
2
5 S. Katsuta, K. Tanaka, Y. Maruya, S. Mori, S. Masuo,
T. Okujima, H. Uno, K.-i. Nakayama and H. Yamada,
Chem. Commun., 2011, 47, 10112–10114.
6 X.-K. Chen, L.-Y. Zou, J.-F. Guo and A.-M. Ren, J. Mater.
Chem., 2012, 22, 6471–6484.
7 Z. Fu, W. Shen, X. Tang, M. He, R. He and M. Li, J. Phys.
Chem. A, 2015, 119, 6884–6896.
8 A. R. Mohebbi, C. Munoz and F. Wudl, Org. Lett., 2011, 13,
8939.
7
8
9
C. Dou, X. Long, Z. Ding, Z. Xie, J. Liu and L. Wang, Angew.
Chem., Int. Ed., 2016, 55, 1436–1440.
X. Long, Z. Ding, C. Dou, J. Zhang, J. Liu and L. Wang, Adv.
Mater., 2016, 28, 6504–6508.
F. Peng, K. An, W. Zhong, Z. Li, L. Ying, N. Li, Z. Huang,
C. Zhu, B. Fan, F. Huang and Y. Cao, ACS Energy Lett.,
2
2
2
2
3
2020, 5, 3702–3707.
2560–2563.
1
1
0 T. Jia, J. Zhang, W. Zhong, Y. Liang, K. Zhang, S. Dong,
L. Ying, F. Liu, X. Wang, F. Huang and Y. Cao, Nano
Energy, 2020, 72, 104718.
1 Q. Fan, W. Su, S. Chen, W. Kim, X. Chen, B. Lee, T. Liu,
U. A. M ´e ndez-Romero, R. Ma, T. Yang, W. Zhuang, Y. Li,
9 J. W. Jung, J. W. Jo, C.-C. Chueh, F. Liu, W. H. Jo, T. P. Russell
and A. K. Y. Jen, Adv. Mater., 2015, 27, 3310–3317.
0 T. Kumari, S. M. Lee, S.-H. Kang, S. Chen and C. Yang, Energy
Environ. Sci., 2017, 10, 258–265.
2852 | Chem. Sci., 2021, 12, 2848–2852
© 2021 The Author(s). Published by the Royal Society of Chemistry