Hexaazatriphenylene derivatives with tunable lowest unoccupied molecular orbital levels

Article abstract of DOI:10.1021/ol201717d

A series of n-type hexaazatriphenylene derivatives were synthesized by condensation coupling of 1,2-diamines and 1,2-diketones. The study of their photophysical and electrochemical properties showed that their lowest unoccupied molecular orbital (LUMO) en

Full text of DOI:10.1021/ol201717d

ORGANIC
LETTERS
2011
Vol. 13, No. 16
4378–4381
Hexaazatriphenylene Derivatives with
Tunable Lowest Unoccupied Molecular
Orbital Levels
Ming Wang,†,‡ Ying Li,†,‡ Hui Tong,*,† Yanxiang Cheng,† Lixiang Wang,*,† Xiabin Jing,†
and Fosong Wang†
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and
Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China
chemtonghui@ciac.jl.cn; lixiang@ciac.jl.cn
Received June 26, 2011
ABSTRACT
A series of n-type hexaazatriphenylene derivatives were synthesized by condensation coupling of 1,2-diamines and 1,2-diketones. The study of
their photophysical and electrochemical properties showed that their lowest unoccupied molecular orbital (LUMO) energy levels could be
effectively tuned from ꢀ3.54 to ꢀ4.02 eV simply by increasing the number of pyrazine units in their molecular structures.
Organic conjugated compounds are of wide current
interest because of their potential applications in light-
emitting diodes,1,2 organic photovoltaic cells,3,4 and thin
film transistors.5,6 In the past two decades, the vast ma-
jority of research in the field has been devoted to p-type
organic semiconductors.7,8 Only a few n-type organic
molecules have been synthesized and studied, although
n-type organic semiconductors are also very important for
developing the light-emitting diodes, organic photovoltaic
cells, thin film transistors, and so on.9ꢀ15
Recently, n-type organic semiconductors have been
reported, include heteroaromatic compounds,16 fullerene
derivatives,17 naphthalene, and perylene diimides, etc.18
Among them, heterocyclic aromatic compounds including
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† Changchun Institute of Applied Chemistry.
‡ Graduate School of Chinese Academy of Sciences.
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r
10.1021/ol201717d
Published on Web 07/27/2011
2011 American Chemical Society

imine nitrogen atoms (dNꢀ) were extensively investigated
because they have a less negative reduction potential
compared to hydrocarbon analogues and heterocyclics
containing oxygen or sulfur atoms.19 As a typical nitrogen-
containing heterocyclic compound, hexaazatriphenylene
has proved to be a kind of n-type organic semiconductor
because it contains three strong π-deficient pyrazine units
at the center and can been used to construct novel electron
acceptor materials.20ꢀ22 For example, Tsutomu Ishi-I’s
group have reported a series of hexaazatriphenylene-based
electron-deficient heteroaromatics with an expanded
πꢀelectron system. The first reduction potentials were eval-
uated at around ꢀ1.7 V with Fc/Fcþ as reference electrode.22
The electron affinity was enhanced by the expansion of the
electron-deficient aromatic system.
Scheme 1. Synthetic Routes of the Four Hexaazatriphenylene
Compounds
Our group has reported a series of novel acene-type
conjugated molecules containing n-type pyrazine units in a
previous paper.23 We studied the electrochemical and
photophysical properties and found that the LUMO en-
ergy levels significantly decrease from ꢀ3.24 to ꢀ3.78 eV
with increasing the number of pyrazine rings and the con-
jugation length of the acene-type molecules. At the same
time, their absorption and emission spectra were red-shifted
too. These results suggested that increasing the number of
pyrazine units and the conjugation length of molecules are
effective ways to obtain n-type materials.
In this contribution, we reported the synthesis and
characterization of a series of hexaazatriphenylene deriva-
tives containing a different number of pyrazine units
(Figure 1). The different number of pyrazine units in their
molecular structures will effectively tune their photophy-
sical and electrochemical properties. From 1 to 4, as the
number of pyrazine units increased, our investigation dem-
onstrated not only that both the absorption and emission
spectra were red-shifted but also that their electron affinity
was enhanced significantly step by step.
The synthetic routes are outlined in Scheme 1. As most
of the literature methods for the preparation of the hex-
aazatriphenylene derivates,20,24 all the target molecules
were obtained by a typical high yield condensation-cou-
pling reaction between 1,2-diketones and corresponding
1,2-diamines in refluxing acetic acid. The four starting
materials (5, 6, 7, 9) are either commercial available or
have been prepared previously in our lab.20,23 The synth-
esis of the key intermediate 8 and 10 is also rather simple
and straightforward. Although their synthetic procedures
are similar, it should be noted that the starting material of 8
and 10 must be 7 and 5, otherwise the yield will be very low
or even no product can be obtained. The easy preparation
of these hexaazatriphenylene derivates promises our
further investigation.
Compound 1 is obtained asa yellow solid, while 2, 3, and
4 are deep-red solids. They are all soluble in common
organic solvents, such as dichloromethane, chloroform,
and tetrahydrofuran (THF). Their structures and purity
were characterized by 1H NMR, 13C NMR, MALDI-TOF,
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J. Am. Chem. Soc. 2003, 125, 13548–12558.
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Org. Lett. 2006, 8, 585–588.
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Wang, F. S. J. Am. Chem. Soc. 2008, 130, 8297–8306.
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Figure 1. The structures of the hexaazatriphenylene derivatives.
Org. Lett., Vol. 13, No. 16, 2011
4379

and elemental analysis (see Supporting Information). The
thermal properties of these four molecules were evaluated
by thermogravimetric analysis (TGA), which revealed
their good thermal stability with decomposition tempera-
ture higher than 400 °C.
Information), the chemical shifts of their aromatic protons
are shifted lowfield as the temperature increase from 25 to
120 °C. For both concentration- and temperature-depen-
1
dent H NMR spectra, it is clear that the tendency in
molecular aggregation is raised with increasing the number
of pyrazine rings,23 which is consistent with their PL
behaviors (Table 1).
Table 1. Physical Properties of Four Hexaazatriphenylene
Molecules
λmax
absorptiona
λmax
emissiona
Φb
(%)
T
(°C)c
compd
1
2
3
4
471
508
530
543
552
593
605
615
76
32
26
21
405
410
414
438
Figure 2. (a) Normalized absorption and (b) normalized PL
spectra of 1, 2, 3, and 4 in chloroform (10ꢀ5 M).
a In chloroform solution (1 ꢁ 10ꢀ5 M). b Fluorescence quantum yield
of 1 relative to Coumarin 6 in acetonitile (63%). Fluorescence quantum
yields of 2, 3, 4 relative to Rhodamine B in water (31%). c Onset
decomposition temperature measured by TGA under nitrogen.
Normalized absorption and PL emission spectra of
compounds 1ꢀ4 in chloroform solutions (10ꢀ5 M) are
shown in Figure 2. All spectra are well-resolved with clear
vibronic structures, which was consistent with the rigid
structural feature of the compounds. The colors of these
compounds in chloroform solution are changed from
yellow in 1 to deep-red in 4. The absorption peaks were
red-shifted from 471 nm in 1to 543nm in 4with the increase
of pyrazine number in their molecular structures. The
optical band gaps of the compounds 1ꢀ4, calculated from
these absorption onsets, were 2.43, 2.23, 2.13, and 2.08 eV,
respectively, indicating that the longer effective conjugation
length and the lowered energy gap can be achieved by the
introduction of more pyrazine units. The emission max-
imum was also red-shifted from 552 nm in 1 (yellow
emission) to 615 nm in 4 (red emission). Meanwhile, the
PL quantum yield decreased from 76% for 1 to 21% for 4,
perhaps due to the increasing aggregation tendency from 1
to 4 with the increasing of the number of pyrazine rings.23,25
Absorption and PL emission spectra of compounds 1ꢀ4
in thin films were also examined (Figure S1 in Supporting
Information). Their absorption spectra were similar to
those in chloroform solutions. The main absorption band
was also progressively red-shifted from 481 to 543 nm.
However, the PL spectra of hexaazatriphenylene deriva-
tives in thin films were different from those in solutions.
Only the emission of 1 at 563 nm can be observed. The
emission spectra of 2ꢀ4 were too weak to be detected,
possibly also due to their enhanced aggregation tendency
in the film state. The aggregation tendency of 1ꢀ4 was also
The electrochemicalpropertiesofthese fourmoleculesin
dichloromethane were studied in a three-electrode electro-
chemical cell with Bu4NClO4 (0.1 M) as electrolyte and
Ag/AgCl as reference electrode (Figure 3). The data of
cyclic voltammograms are shown in Table S1 (Supporting
Information). 1 and 2 exhibited two reversible redox
processes in the negative potential region. Compounds 3
and 4 exhibited three reversible redox processes in the
negative potential region. The LUMO energy level was
determined according to the empirical formula ELUMO
=
ꢀ[4.8 ꢀ EFOC þ Eredonset] eV.26 The EFOC is the half-wave
potential of ferrocene as the standard. The LUMO energy
levels of four molecules were estimated from the reduction
onset. The LUMO level (ꢀ3.84 eV) of 2 was 0.3 eV lower
than that of 1 (ꢀ3.54 eV), and the LUMO levels of 3 and 4
were further decreased to ꢀ3.95 eV of 3 and ꢀ4.02 eV of 4,
respectively. These facts indicated the enhanced electron
affinity clearly correlated with the introduction of more
1
studied by means of H NMR. According to their con-
1
centration-dependent H NMR spectra in chloroform-d
(Figure S2 in Supporting Information), the chemical shifts
of their aromatic protons are shifted upfield as the con-
centration is increased from 3 ꢁ 10ꢀ4 to 6 ꢁ 10ꢀ3 mol/L,
1
while for their temperature-dependent H NMR spectra
in 1,2-dichlorobenzene-d4 (Figure S3 in Supporting
Figure 3. Cyclic voltammograms of 1, 2, 3, and 4 in dichloro-
methane at a scan rate of 100 mV/s.
(25) Wasserfallen, D.; Kastler, M.; Pisula, W.; Hofer, W. A.; Fogel,
€
Y.; Wang, Z.; Mullen, K. J. Am. Chem. Soc. 2006, 128, 1334–1339.
4380
Org. Lett., Vol. 13, No. 16, 2011

optical results, the calculated LUMO energy levels and band
gaps were decreased effectively with increasing the pyrazine
unit number from 1 to 4, while the changes of the calculated
HOMO energy levels were rather small. It was also clear that
their conjugation length were increased as well.
In summary, we have synthesized a series of hexa-
azatriphenylene derivatives by a typical condensation-
coupling reaction between 1,2-diketones and correspond-
ing 1,2-diamines. These materials were investigated by
photophysical and electrochemical methods. The absorp-
tionand emissionspectrawerered-shifted withthe increase
of the pyrazine unit number. Especially, the LUMO energy
levels were drastically decreased from ꢀ3.54 eV (1) to
ꢀ4.02 eV (4), even lower than a well-known n-type materi-
al PCBM (ꢀ3.8 eV),27 indicating that the electron affinity
of these compounds could be significantly enhanced. The
low LUMO energy levels and ease of structural modifica-
tion clearly demonstrated that these hexaazatriphenylene
derivatives with different number of pyrazine units are
excellent n-type materials.
Figure 4. LUMO molecular orbitals of compounds 1ꢀ4 calcu-
lated by density functional theory calculations at the B3LYP/
6-31G* level.
Acknowledgment. This work was supported by the
Changchun Institute of Applied Chemistry, Chinese Acad-
emy of Sciences (CX07QZJC-24), the National Natural
Science Foundation of China (nos. 21074130, 50633040,
20904055), the Science Fund for Creative Research Groups
(no. 20921061), and the 973 Project (2009CB623601).
pyrazine units. Their HOMO energy levels were calculated
from the corresponding LUMO energy levels and the
optical band gap. The HOMO energy levels of 2, 3, and
4 were very close, suggesting that the influence of the
pyrazine units on the HOMO levels are trivial.
Furthermore, the LUMO molecular orbitals were cal-
culated by density functional theory calculations at the
B3LYP/6-31G* level (Figure 4). The LUMO was located
mainly on the pyrazine units. As for the electrochemical and
1
Supporting Information Available. HNMR, 13CNMR,
MALDI-TOF, UVꢀvis spectra, fluorescence spectra, and
experimental procedures. This material is available free of
charge via the Internet at http://pubs.acs.org.
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J. G.; Yu, G.; Liu, Y. Q. J. Polym. Sci., Part A: Polym. Chem. 2009, 47,
3651–3661.
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Y. H.; Wang, F. S. J. Mater. Chem. 2010, 20, 3092–3097.
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