Synthesis and physicochemical properties of strong electron acceptor 14,14,15,15-tetracyano-6,13-pentacenequinodimethane (TCPQ) diimide

Article abstract of DOI:10.1002/ejoc.201201065

14,14,15,15-Tetracyano-6,13-pentacenequinodimethane (TCPQ) diimide, a new tetracyanoquinodimethane analogue with extended conjugation and imide substituents, was synthesized by double Diels-Alder reactions and a Knoevenagel condensation reaction. Experimental results showed that TCPQ diimide has a low LUMO energy level (-4.03 eV) and good solubility in organic solvents.

Full text of DOI:10.1002/ejoc.201201065

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201065
Synthesis and Physicochemical Properties of Strong Electron Acceptor
14,14,15,15-Tetracyano-6,13-pentacenequinodimethane (TCPQ) Diimide
Jie Li,[a] Yu Xiong,[a] Qinghe Wu,[a] Shitao Wang,[a] Xike Gao,[a] and Hongxiang Li*[a]
Keywords: Cycloaddition / Conjugation / Diimide / Electrochemistry / UV/Vis spectroscopy
14,14,15,15-Tetracyano-6,13-pentacenequinodimethane condensation reaction. Experimental results showed that
(TCPQ) diimide, a new tetracyanoquinodimethane analogue
with extended conjugation and imide substituents, was syn-
thesized by double Diels–Alder reactions and a Knoevenagel
TCPQ diimide has a low LUMO energy level (–4.03 eV) and
good solubility in organic solvents.
analogues increases dramatically with the increase in conju-
gation length, which eliminates the unique properties of
TCNQ. Till now, only a few examples of TCNQ analogues
with LUMO energy levels lower than –4.0 eV have been ex-
plored.
Introduction
Organic semiconductors with electron-withdrawing
properties (organic electron acceptors) have attracted great
attention because of their wide applications in organic elec-
tronic devices such as organic thin-film transistors
(OTFTs),[1] organic photovoltaics (OPVs),[2] and organic
light-emitting diodes (OLEDs).[3] Though great effort has
been devoted to the field of organic electron acceptors, the
development of organic electron acceptors still largely lags
behind that of organic electron donors (organic semi-
conductors with electron donating properties). Moreover,
there is a great need to design and synthesize new types
of organic electron acceptors, especially those with lowest
unoccupied molecular orbital (LUMO) energy levels lower
than –4.0 eV.[4]
Tetracyanoquinodimethane (TCNQ, Scheme 1) is a well-
known organic electron acceptor with a very low LUMO
energy level (–4.6 eV). It has been used in charge-transfer
(CT) complexes,[5] electrical and optical memory devices,[6]
switching devices,[6a,7] and sensors.[8] However, the short
conjugation length and low solubility of TCNQ hinders its
further applications in organic electronics. Hence, TCNQ
analogues such as tetracyanoanthraquinodimethane
(TCAQ)[9] and 15,15,16,16-tetracyano-6,13-pentacenequin-
odimethane (TCPQ[10], Scheme 1) were synthesized with the
aim to increase the conjugation length. Very recently, Chi
et al.[11] prepared a seven-ring fused TCNQ analogue
19,19,20,20-tetracyano-7,16-tetrakis(5Ј-dodecylthieno)-
[a,c,p,r]1). Unfortunately, the LUMO energy level of TCNQ
Scheme 1. The chemical structures of TCNQ, TCAQ, TCPQ,
TCTTHQ, TPAFO, and PDPTP.
Arylene diimides generally have low LUMO energy levels
due to the electron-withdrawing effects of the diimide
groups. The most representative compounds of arylene di-
imides are naphthalene diimides (NDI)[12] and perylene di-
imides (PDI),[13] which have a LUMO energy level lying at
about –3.80 and –3.85 eV, respectively. In addition, the sol-
ubility of the arylene diimides can be easily improved by
changing the substituents on the N atoms. Considering the
[a] Laboratory of Material Sciences, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences,
Shanghai, 200032, China
Fax: +86-21-54925024
E-mail: lhx@sioc.ac.cn
Homepage: http://sourcedb.sioc.cas.cn/cn/expert/br/200906/
t20090621_1750754.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201065.
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J. Li, Y. Xiong, Q. Wu, S. Wang, X. Gao, H. Li
SHORT COMMUNICATION
merits of diimide substituents, herein TCPQ diimide (1), a cloaddition product 3, and 3 was then treated with 2-ethyl-
new TCNQ analogue with extended conjugation and di- hexylamine to give 4. The total yield of the second pro-
imide substituents, was synthesized. The physicochemical cedure was ≈73%. N-(2-Ethylhexyl)-4,5-dimethylphthalic
properties of 1 were also investigated.
imide (5) was synthesized in 78% yield by sulfur-mediated
oxidation of 4. Imide 5 was brominated with NBS in the
presence of benzoyl peroxide (BPO) to give 6. Key interme-
diate 7 was prepared by treating 6 with 1,4-benzoquinone
Results and Discussion
The synthetic route to 1 is outlined in Scheme 2. Di- in dry DMF.
imide-substituted pentacenedione compound 7 is the key
A new synthetic method to further simplify the synthesis
intermediate in this pathway. The 4,8,12,16-substituted de- of 7 was also tried (Scheme 3). In this procedure, pyro-
rivative of compound 7, 4,8,12,16-tetraphenyl-2,10-dimeth- mellitic dianhydride was firstly treated with 2-ethylhexyl-
ylpentaceno[2,3-c:9,10-cЈ]dipyrrole-1,3,6,9,11,14(2H,10H)- amine (1 equiv.) to afford monoimide compound 8, which
hexone (PDPTP,[14] Scheme 1), was synthesized by treating was then reduced to afford 9 on the basis of the different
1,3,7,9-tetrakisphenylanthra[2,3-c:6,7-c]difuran-5,11-dione reactivities of the imide and anhydride functionalities. Oxi-
(TPAFO, Scheme 1) with N-alkylmaleimide followed by dization of 9 afforded dialdehyde 10, which was further
aromatization in the presence of concentrated sulfuric acid. treated with cyclohexane-1,4-dione to give 7. However, the
In this procedure, the synthesis of TPAFO requires nine reduction potentials of the imide and anhydride functionali-
tedious synthesis steps from commercially available fumaryl ties in 8 are close; thus, a very low yield was obtained,
chloride. In this paper, a new synthetic route to synthesize which hindered the application of this method.
compound 7 was developed.
Target product 1 was synthesized by Knoevenagel con-
Starting from maleic anhydride and 3,4-dimethylbutadi- densation of 7 with the Lehnert reagent[16] (TiCl4, malonon-
ene, N-(2-ethylhexyl)-4,5-dimethyl-cis-4-tetrahydrophthal- itrile, pyridine) and obtained as a yellow solid. Compound
imide (4) was synthesized by two procedures. In the first 1 had good solubility in organic solvents such as CH2Cl2,
method, maleic anhydride was treated with 2-ethylhex- CHCl3, and THF and was characterized by NMR spec-
ylamine, followed by Ac2O-mediated dehydration in the troscopy and mass spectrometry. Figure 1 illustrates the
presence of AcONa to give N-2-ethylhexylmaleimide (2) in UV/Vis spectrum of compound 1 in chloroform solution.
50% yield. Then, Diels–Alder cycloaddition between 2 and Three peaks are observed in the absorption spectra, which
3-dimethylbutadiene afforded 4 in nearly quantitative yield. are located at 416, 330, and 295 nm. The maximum absorp-
The low yield of the first step limited the applications of tion wavelength of 1 was the same as that of TCPQ[10]
this method.[15] In the second procedure, 2,3-dimethylbuta- (415 nm in chloroform) and about 69 nm redshifted com-
diene was first treated with maleic anhydride to afford cy- pared to that of TCAQ[17] (347 nm in dichloromethane),
Scheme 2. Synthetic route to compound 1.
2
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Synthesis and Physicochemical Properties of a Strong Electron Acceptor
Table 1. Cyclic voltammetry data for 1, TCNQ, TCAQ, and TCPQ.
(1)
(2)
Compound
Solvent
CH3CN
E1/2 (V) E1/2 (V) LUMO[a] [eV]
TCNQ[19]
TCAQ[20]
TCAQ[9a]
TCPQ[10]
TCTTHQ[11]
1
0.08
–0.48
CH3CN –0.285
CH3CN –0.46
CH3CN –0.57
CH2Cl2 –0.82
CH3CN –0.38
–0.65
–0.91
–3.67
–4.03
–1.10
+
[a] The LUMO was calculated based on ELUMO = –(4.8 – Fc/Fc
E1/2).
+
Scheme 3. Another possible route to key intermediate 7.
which can be ascribed to the enlarged conjugation length
of 1. The HOMO–LUMO energy band gap estimated from
the initial absorption was 2.79 eV.
Figure 2. Cyclic voltammetry of 1 in CH3CN solution: Bu4NF as
electrolyte, Pt as working electrode, Pt wire as counter electrode,
and SCE as reference electrode. Ferrocene was used as internal
standard.
Conclusions
In summary, a new type of TCNQ analogue, compound
1, with extended conjugation and diimide substituents was
successfully synthesized. In the preparation of 1, a new
method that can simplify the synthesis of the key intermedi-
ate, diimide-substituted pentacenedione 7, was developed.
Compound 1 has a low LUMO energy level (–4.03 eV) and
displays good solubility in organic solvents, suggesting the
potential use of 1 in organic electronics. The application
of compound 1 in devices such as thin-film transistors is
underway in our laboratory.
Figure 1. UV/Vis spectrum of 1 in chloroform solution (1ϫ10–5 m).
The electrochemical properties of compound 1 were in-
vestigated by cyclic voltammetry (CV). To compare with
other TCNQ analogues, CV was carried out in a CH3CN
solution by using SCE as the reference electrode and ferro-
cene as the internal standard reference. The redox potential
of 1 is listed in Table 1 together with those of TCNQ,
TCAQ, TCPQ, and TCTTHQ. Compound 1 exhibited two
Supporting Information (see footnote on the first page of this arti-
1
cle): Experimental details and copies of the H NMR, 13C NMR,
and mass spectra of all key intermediates and final products.
Acknowledgments
reversible reduction waves (Figure 2). The first half-wave re- This work was supported by the National Natural Sciences Foun-
dation of China (21190031) and the National Basic Research Pro-
gram of China (2011CB808405)
duction potential was –0.38 V, which is lower than that of
TCPQ and close to that of TCAQ. In view of its good solu-
bility and a LUMO energy level that is similar to that of
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Eur. J. Org. Chem. 0000, 0–0
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Received: August 7, 2012
Published Online: ᭿
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Synthesis and Physicochemical Properties of a Strong Electron Acceptor
Electron Acceptors
14,14,15,15-Tetracyano-6,13-pentacene-
quinodimethane (TCPQ) diimide, a new
tetracyanoquinodimethane analogue with
extended conjugation and imide substitu-
ents, was synthesized by double Diels–
Alder reactions and a Knoevenagel con-
densation reaction. Experimental results
showed that TCPQ diimide has a LUMO
energy level (–4.03 eV) and good solubility
in organic solvents.
J. Li, Y. Xiong, Q. Wu, S. Wang, X. Gao,
H. Li* ............................................... 1–5
Synthesis and Physicochemical Properties
of Strong Electron Acceptor 14,14,15,15-
Tetracyano-6,13-pentacenequinodimethane
(TCPQ) Diimide
Keywords: Cycloaddition / Conjugation /
Diimide / Electrochemistry / UV/Vis spec-
troscopy
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