Dicyanopyrazine-containing fused aromatic molecules: Potential n-type materials for use in optoelectronic devices

Article abstract of DOI:10.1080/00397911.2010.517893

Two dicyanopyrazine-containing aromatic compounds, pyrazino[2′, 3′:9,10]phenanthro[4,5-fgh]quinoxaline-5,6,12,13-tetracarbonitrile (PPQD) and pyrazino[2′,3′:9,10]phenanthro[4,5-fgh]quinoxaline-5,6,12,13- tetracarbonitrile,2,9-bis(1,1-dimethylethyl) (PPQB), which have remarkably high electron affinity and ionization potential, were synthesized and qualified as potential n-type materials for use in solar cells and organic light-emitting diodes.

Full text of DOI:10.1080/00397911.2010.517893

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Dicyanopyrazine-Containing Fused
Aromatic Molecules: Potential n-Type
Materials for Use in Optoelectronic
Devices
Qing Li a , Jiuyan Li b , Huicai Ren a , Zhanxian Gao a & Di Liu a
a School of Chemistry, Dalian University of Technology, Dalian, China
b State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology, Dalian, China
To cite this article: Qing Li , Jiuyan Li , Huicai Ren , Zhanxian Gao & Di Liu (2011) Dicyanopyrazine-
Containing Fused Aromatic Molecules: Potential n-Type Materials for Use in Optoelectronic Devices,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 41:22, 3325-3333, DOI: 10.1080/00397911.2010.517893
To link to this article: http://dx.doi.org/10.1080/00397911.2010.517893
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Synthetic Communications1, 41: 3325–3333, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.517893
DICYANOPYRAZINE-CONTAINING FUSED AROMATIC
MOLECULES: POTENTIAL n-TYPE MATERIALS FOR
USE IN OPTOELECTRONIC DEVICES
Qing Li,1 Jiuyan Li,2 Huicai Ren,1 Zhanxian Gao,1 and
Di Liu1
1School of Chemistry, Dalian University of Technology, Dalian, China
2State Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, Dalian, China
GRAPHICAL ABSTRACT
Abstract Two dicyanopyrazine-containing aromatic compounds, pyrazino[20,30:9,10]
phenanthro[4,5-fgh]quinoxaline-5,6,12,13-tetracarbonitrile (PPQD) and pyrazino[20,30:9,10]
phenanthro[4,5-fgh]quinoxaline-5,6,12,13-tetracarbonitrile,2,9-bis(1,1-dimethylethyl) (PPQB),
which have remarkably high electron affinity and ionization potential, were synthesized and
qualified as potential n-type materials for use in solar cells and organic light-emitting diodes.
Keywords Blue; high electron affinity; n-type; optoelectronic devices
INTRODUCTION
Organic conjugated compounds are widely used in optoelectronic devices, such
as organic light-emitting diodes (OLEDs),[1–3] solar cells,[4,5] and organic thin film
transistors.[6] n-Type organic materials, which can function as electron acceptors
in organic photovoltaic (OPV) devices and=or electron-transporting materials in
OLEDs, have attracted extra research attention in recent years.[7–9]
In an OPV device typically composed of electron donor (D) and acceptor (A)
components, with either bilayer or dispersed heterojunction configuration, the exci-
ton dissociation by charge transfer (CT) into free charges at the D–A interface is one
Received April 21, 2010.
Address correspondence to Di Liu, School of Chemistry, Dalian University of Technology,
2 Linggong Road, Dalian 116023, China. E-mail: liudi@dlut.edu.cn
3325

3326
Q. LI ET AL.
Scheme 1. Chemical structures of PPQD and PPQB.
key step to determine the overall photoelectrical conversion efficiency. An appropri-
ate energy level alignment is necessary for efficient and fast exciton dissociation.[10,11]
The famous C60 derivatives have been successfully used in OPV devices mainly
because their remarkably high electron affinity (EA, 4.3 eV) leads to ultrafast charge
transfer (40 fs) and thus rapid and efficient exciton dissociation at the D–A
junctions.[4,5] As there is no competing decay process of the optically excitd hole–
electron pairs in this time regime, an optimized mixture with C60 converts absorbed
photons to electrons with an efficiency close to 100%. It is generally accepted that
therefore a higher EA of the acceptor would be more favorable to efficient CT exci-
ton dissociation, although it seems not to result in a high open circuit voltage.[5]
Except for the C60 derivatives, however, not many other organic molecules have been
discovered to possess an EA of 4.3 eV and to be alternatives to C60 derivatives as
electron acceptors in solar cells. In this article, we report the synthesis and properties
of two dicyanopyrazine-containing p-conjugated aromatic compounds, pyrazino
Scheme 2. Synthetic route to compounds PPQD and PPQB: (i) AlCl3, t-butylchloride, dry CH2Cl2, 5 ꢀC
for 30 min, rt for 4 h; (ii) sodium periodate, ruthenium trichloride, acetonitrile, CH2Cl2, H2O, 30–40 ꢀC,
overnight; and (iii) diaminomaleonitrile, acetic acid, reflux, 8 h.

DICYANOPYRAZINE-CONTAINING COMPOUNDS
3327
[20,30:9,10]phenanthro[4,5-fgh]quinoxaline-5,6,12,13-tetracarbonitrile (PPQD) and
pyrazino[20,30:9,10]phenanthro[4,5-fgh]quinoxaline-5,6,12,13-tetracarbonitrile,2,9-
bis(1,1-dimethylethyl) (PPQB). The chemical structures are shown in Scheme 1.
The most important feature of these molecules is that they possess remarkably
high EA and ionization potential (IP) values, suggesting that they may find poten-
tial applications as excellent electron acceptors in solar cells or as hole=
exciton-blocking materials in organic light-emitting diodes (OLEDs).
EXPERIMENTAL
Materials and Instruments
All the chemicals for the synthesis are analytical grade and were used as
1
received without further purification. H NMR spectra was recorded on a Varian
Inova spectrometer (400 MHz), and chemical shifts were referenced to tetramethyl-
silane (TMS). Mass spectra were recorded on a GC-Tof MS (Micromass, UK) mass
spectrometer for turnover frequency–mass spectrometry–electron impact (TOF-MS-
EI). Absorption and fluorescence spectra were recorded with an ultraviolet–visible
(UV-vis) spectrophotometer (Perkin-Elmer Lambda 35) and a fluorospectrophot-
ometer (Perkin-Elmer Luminescence LS50B), respectively.
Synthesis of Compounds
2,7-Di-t-butyl-pyrene (2).[12] To a mixture of 10 g (49.5 mmol) of pyrene (1)
and 100 ml of dichloromethane in a 250-ml, three-necked flask were added 650 mg
(4.5 mmol) of aluminum chloride and a solution of 10 g (109 mmol) of t-butylchlor-
ide in 15 ml of dichloromethane under a nitrogen atmosphere at 5 ꢀC. Then the mix-
ture was stirred for 4 h at room temperature. Removing the solvent by evaporation
under reduced pressure followed by column chromatography over silica gel yielded
14.3 g of white crystals (2), yield 92%, MS (EI): m=z 314.20 ([M]þ).
General procedure for the preparation of pyrene-4,5,9,10-tetraones (3
and 4).[13] NaIO4 (17.5 g, 81.8 mmol), H2O (50 mL), and RuCl3 ꢁ 3H2O (0.25 g,
1.2 mmol) were added to a solution of 1 or 2 (10 mmol) in CH2Cl2(40.0 mL) and
CH3CN (40 mL). The dark brown suspension was stirred at 30–40 ꢀC overnight.
The reaction mixture was poured into 200 mL of H2O and then filtered. Both the
solid and the filtrate were extracted with a sufficient amount of CH2Cl2. The com-
bined CH2Cl2 solution was washed with H2O, dried with sodium sulfate, and con-
centrated by rotary evaporation. The crude product was purified by column
chromatography (CH2Cl2 as eluent) to yield the bright orange crystals. Compound
3: yield 30%, mp > 350 ꢀC, MS (EI): m=þz 262.03 ([M]þ). Compound 4: yield 33%, mp
339–340 ꢀC, MS (EI): m=z 374.15 ([M] ).
General procedure for the preparation of pyrazino[2’,3’:9,10]phenanthro
[4,5-fgh]quinoxaline-5,6,12,13-tetracarbonitrile (PPQD) and pyrazino[2’,3’:9,10]
phenanthro[4,5-fgh]quinoxaline-5,6,12,13-tetracarbonitrile-2,9-bis(1,1-dimethylethyl)
(PPQB). A suspension of 3 or 4 (0.22 mmol) and diaminomaleonitrile (0.44 mmol) in
acetic acid (10 ml) was refluxed for 8 h. The brown precipitate was filtered and

3328
Q. LI ET AL.
recrystallized in toluene to yield bright yellow product. PPQD: yield 84%. MS (EI):
1
m=z 406.07 ([M]þ). H NMR (400 MHz, CDCl3) d ¼ 9.74–9.72 (t, 4H, J ¼ 5.6 Hz),
1
8.43 (s, 2H, J ¼ 5.6 Hz). PPQB: yield 63%. MS (EI): m=z 518.2 ([M]þ). H NMR
(400 MHz, CDCl3): d ¼ 9.70 (s, 4H), 1.70 (s, 18H).
Electrochemical Measurements
Cyclic voltammetric (CV) measurements were carried out in a conventional
three-electrode cell using a glassy carbon working electrode, a platinum wire counter
electrode, and a Ag=AgCl reference electrode on a computer-controlled BAS 100-W
electrochemical analyzer at room temperature. The CV of PPQD and PPQB was
performed in CH2Cl2 containing Bu4NPF6 (0.1 M) as a supporting electrolyte. The
energy levels were calculated according to the following equations:[14]
red
onset
EA ¼ 4:4 þ E
IP ¼ EA þ Eg:
RESULTS AND DISCUSSION
Design and Synthesis
As a typical nitrogen-containing heterocyclic compound, pyrazine has been
proven to be a valuable building block for electron-accepting p-conjugated molecules
because of its electron deficient characteristics.[15,16] It can be expected that the
dicyanopyrazine-containing molecules would be more electron deficient than the cor-
responding pyrazine-based ones because of the combined electron-withdrawing
effect of pyrazine and cyano- moieties. PPQD and PPQB were designed in such a
way that the dicyanopyrazine rings in the target molecules were formed through
the condensation reaction between the corresponding pyrene-4,5,9,10-tetraone and
diaminomaleonitrile. To increase the solubility of the intermediates and the target
compound, two t-butyl groups were introduced into the 2,7-sites of pyrene ring in
the PPQB molecule. PPQD and PPQB can be easily synthesized through sequential
oxidation of pyrene and 2,7-di-t-butyl-pyrene with sodium periodate[15] and conden-
sation of the corresponding pyrene-4,5,9,10-tetraones with diaminomaleonitrile.
They were purified by recrystallization in toluene.
Photophysical Properties
The electronic absorption spectra of PPQD and PPQB in dilute CH2Cl2
solution are shown in Fig. 1. They both exhibited three absorption bands at the
wavelength range of 320–350, 385–395, and 405–420 nm. The absorption spectrum
of PPQB had a red shift of 7 nm in comparison with that of PPQD, which should
be ascribed to the introduction of the t-butyl substituents.
Figure 2 illustrates the photoluminescence (PL) emission of PPQD and PPQB
in CH2Cl2 solution. They are both blue fluorescent with the emission peaks at 445
and 458 nm, respectively. Similar to the electronic absorption, the emission of PPQB

DICYANOPYRAZINE-CONTAINING COMPOUNDS
3329
Figure 1. Normalized electronic absorption spectra of PPQD and PPQB in CH2Cl2.
also had a red shift of 13 nm in comparison with that of PPQD. The fluorescent
quantum yields of these two compounds were measured using quinine sulfate as a
standard (U ¼ 55%).[17] High fluorescent quantum yields of 0.34 and 0.15 were
obtained for PPQD and PPQB respectively. It would be reasonable that PPQD is
more efficiently luminescent than its t-butyl-substituted derivative, PPQB, if we take
into account the effect of the nonrigid t-butyl substituents to increase the molecular
vibration and non irradiative transition chance. The data of absorption and emission
of PPQD and PPQB are summarized in Table 1.
Electrochemical Properties
The electrochemical properties of PPQD and PPQB were investigated by
means of cyclic voltametry. Figure 3 shows the cyclic voltammograms of these
Figure 2. PL emission of compounds PPQD and PPQB in CH2Cl2 (10ꢂ5 mol Lꢂ1), excited at 345 nm.

3330
Q. LI ET AL.
Table 1. Photophysical data of PPQD and PPQB
Compound
kabs (nm)a
kem (nm)
Ub
PPQD
PPQB
332, 387, 411
346, 395, 418
445
458
0.34
0.15
aMeasured in CH2Cl2 at a concentration of 1.0 ꢃ 10ꢂ5 mol Lꢂ1
.
bRelated to quinine sulfate as a standard.
compounds. No oxidation signals were detected for these compounds even up to a
high positive potential. Both them exhibited three excellent reversible cathodic
reduction waves in CH2Cl2, indicative of strong electron-withdrawing and n-type
doping nature of these molecules. The absence of any waves at around ꢂ0.4 V in
blank solvent confirmed that the first reduction waves at ꢂ0.24 and ꢂ0.42 V
(Eorendset) observed for PPQD and PPQB indeed resulted from the reduction of these
molecules. The EA value was calculated from the onset potential of the first
red
onset
reduction wave by the equation EA ¼ E
þ 4:4. EAs of 4.16 and 3.98 eV were
obtained for PPQD and PPQB respectively. These EA values are quite comparable
with that of the famous fullerene C60 derivative PCBM (EA ¼ 4.3 eV).[17] Such a high
EA value should be ascribed to the combined contribution of strong electron-
deficient cyano and pyrazine groups. The difference of 0.18 eV in EA value between
PPQD and PPQB should be caused by the t-butyl substituents, in contrast to the
Figure 3. Cyclic voltammograms of PPQD and PPQB.

DICYANOPYRAZINE-CONTAINING COMPOUNDS
3331
Figure 4. Absorption spectrum (up) and cyclic voltamogram (down) of TPBI.
observation by Wang et al.[16] A significantly high EA value suggests that PPQD and
PPQB are potential electron-acceptor materials for use in solar cells. At the same
time, high EA values of PPQD and PPQB indicated that an efficient electron injec-
tion from the metal cathode or other organic layer can be expected when they are
used as electron-injecting or electron-transporting layer in OLEDs. The band gap
(Eg) of PPQD and PPQB were determined by the absorption edge technique. The
electronic absorption spectrum of the evaporated film of PPQD and PPQB has fea-
tures similar to those in solution as shown in Fig. 1 but with a red shift. The absorp-
tion edges were determined as 456 and 448 nm for PPQD and PPQB, respectively.
Thus, the band gaps were calculated to be 2.72 and 2.77 eV for PPQD and PPQB,
respectively. Therefore, the ionization potentials (IP) were calculated by the equation
[14]
IP ¼ EA þ Eg to be 6.88 and 6.75 eV.
For a comparison, the IP and EA values of the widely used hole=exciton block-
ing materials 2,20,200-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole (TPBI) and
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) were investigated by identical
experimental methods. As an example, the absorption spectrum and the cyclic vol-
tammogram of TPBI were illustrated in Fig. 4. All electrochemical and electronic

3332
Q. LI ET AL.
Table 2. Electrochemical and electronic data of PPQD, PPQB, TPBI, and BCP
Eg (eV)a
E
(V)b
EA (eV)c
IP (eV)d
red
onset
Copmpound
PPQD
PPQB
TPBI
BCP
2.72
2.77
3.46
3.56
ꢂ0.24
ꢂ0.42
ꢂ1.09
ꢂ0.94
4.16
3.98
3.31
3.46
6.88
6.75
6.77
7.02
aEnergy band gap, determined from solid films by absorption edge technique.
bOnset potential of the first reduction wave.
c
red
onset
EA ¼ 4:4 þ E
.
dIP ¼ EA þ Eg.
data of PPQD, PPQB, TPBI, and BCP are listed in Table 2. Evidently the IP values
of PPQD and PPQB are comparable with those of TPBI but less than that of BCP
by around 0.2 eV. Such high IP values of PPQD and PPQB indicate that these com-
pounds may find potential applications as excellent hole and exciton-blocking mate-
rials in OLEDs.
CONCLUSION
In summary, two dicyanopyrazine-containging p-conjugated aromatic com-
pounds, PPQD and PPQB, have been easily synthesized with good yields. They
exhibited bright blue fluorescence and three-step reversible reductive processes.
The remarkably high EA and IP values indicated that these compounds would be
excellent electron-acceptor materials in solar cells and hole-blocking layer in
OLEDs. Further studies on the applications of these two compounds in optoelectro-
nic devices are presently under way in our laboratory.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China (20704002 and
21072026) and the Fundamental Research Funds for the Central Universities
(DUT10LK16) for financial support of this work.
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