Benzenoid and quinonoid nitrogen-containing heteropentacenes

Article abstract of DOI:10.1002/chem.200900160

A study was conducted to demonstrate the isolation and investigation of benzenoid and quinonoid nitrogen-containing heteropentacenes. The complete characterization of benzenoids and heteropentacenes revealed π-electron delocalization in these polynuclear

Full text of DOI:10.1002/chem.200900160

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DOI: 10.1002/chem.200900160
Benzenoid and Quinonoid Nitrogen-Containing Heteropentacenes
Qin Tang,[a] Jing Liu,[a] Hoi Shan Chan,[a] and Qian Miao*[a, b]
Nitrogen-containing heteroacenes are an interesting class
of p-functional materials,[1] but they have been much less
studied than oligoacenes, which are very important building
blocks for organic electronics.[2,3] The recent interest in ni-
trogen-containing heteropentacenes has arisen from the op-
portunities of tuning the electronic structure, stability, solu-
bility, and molecular packing by introducing nitrogen atoms
into the backbone of pentacene.[4] Although some nitrogen-
containing heteropentacenes have been known for over a
century,[5] few of them were fully characterized structurally.
Dihydro-5,7,12,14-tetraazapentacene (DHTAP), also known
as fluorindine, is not only regarded as a small-molecule
model for ladder polymers,[6] but also functions as an organ-
ic semiconductor in organic field effect transistors
(OFETs).[7] The true structure of DHTAP is benzenoid (1a
in Scheme 1) as determined by 1H NMR spectroscopy.[8]
However in most of the reports on DHTAP until recently, it
Scheme 1. Structures of dihydro-5,7,12,14-tetraazapentacene (DHTAP)
and N,N’-dimethyldihydro-5,7,12,14-tetraazapentacenes (DMTAPs).
of both benzenoid and quinonoid nitrogen-containing hetero-
pentacenes as detailed below.
was erroneously given
a
quinonoid structure (1b in
The synthesis of 2a and 2b is shown in Scheme 2a. A new
method of condensing o-phenylenediamine and 2,5-dihy-
droxy-1,4-benzoquinone without solvents and acidic cata-
lysts produced DHTAP in a good yield. Deprotonation of
DHTAP with nBuLi followed by treatment with iodome-
thane yielded three products (2a–c), which were easily iso-
lated by chromatography on silica gel.[11] Unlike the parent
molecule DHTAP, 2a–c are soluble in organic solvents, pre-
sumably due to the lack of hydrogen bonds in the solid
state, and were completely characterized. The formation of
2c involves both N- and C-methylation, indicating that the
dianion of DHTAP has negative charges delocalized not
only on nitrogen atoms but also on the neighboring carbon
atoms through a series of resonance forms. The resonance
forms of the DHTAP dianion that lead to 2a–c are shown in
Scheme 2b.[12] The intermediate 4, which has a relatively
acidic proton, is further deprotonated and methylated to
yield 2c.
Scheme 1).[9,10] This quinonoid structure was first proposed
based on a belief that the absorption of DHTAP near
600 nm is not compatible with the benzenoid structure 1a,[10]
which has two nitrogen atoms connecting a phenazine ring
and a benzene ring.[4a,8a] In connection with the debate on
the structure of DHTAP, we have recently found that meth-
ylation of DHTAP allowed not only N-alkylation but also
C-alkylation, and yielded both benzenoid and quinonoid
heteropentacenes (2a and 2b in Scheme 1). This finding led
to an exploratory study on the molecular and electronic
structures, molecular packing, and semiconductor properties
[a] Q. Tang, J. Liu, H. S. Chan, Prof. Dr. Q. Miao
Department of Chemistry, The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong (China)
Fax : (+852)26035057
1
E-mail: miaoqian@cuhk.edu.hk
In the H NMR spectra of 2a (see the Supporting Infor-
[b] Prof. Dr. Q. Miao
mation) the protons of the two terminal benzene rings give
Center of Novel Functional Molecules
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong (China)
rise to two groups of characteristic AA’XXꢀ patterns,[13]
1
which are also found in the H NMR spectrum of DHTAP
and can be regarded as direct evidence for the benzenoid
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900160.
structure of 1a.[8] In contrast, the protons of two terminal
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Figure 1. Crystal structures of 2a–c showing: a) a single molecule of 2a
with bond lengths highlighted; b) p-stacks and the bending angle of 2a;
c) a single molecule of 2b with H-bonds to two water molecules; d) a H-
bonded chain of water molecules between two p-stacks of 2b (H-bonds
between water molecules are shown as dashed lines); e) top view of a
single molecule of 2c; f) side view of a stacked dimmer of 2c.
Scheme 2. a) Synthesis of 2a–c; b) proposed mechanism for the forma-
tion of 2a–c.
1
benzene rings of 2b give rise to four peaks in the H NMR
spectra (see the Supporting Information) corresponding to a
disubstituted benzene ring with two different substituents
ortho to each other. The protons of the central ring of 2a
and 2b show single peaks at d=6.67 and 5.97 ppm, respec-
tively, in agreement with the deshielding effect by the ring
current of the benzenoid central ring of 2a.
Crystals of 2a–c were grown by slow evaporation from
solutions in acetone. X-ray crystallography of these crystals
reveals the structures and assemblies of the three molecules
as shown in Figure 1.[14] The backbone of 2a bends at the
methylated nitrogen atoms with a bending angle of 1608 as
À
illustrated in Figure 1b. The N5 C5a bond (1.38 ꢂ) is short-
À
er than the N5 C4a bond (1.41 ꢂ) and the corresponding
[15]
À
N C bond of N,N’-dimethyldihydrophenazine (1.41 ꢂ),
revealing the partial double-bond character of the N5 C5a
bond. The N12 C12a bond (1.34 ꢂ) is shorter than the
N12 C11a bond (1.37 ꢂ), indicating the former has more
À
Scheme 3. Resonance structures of 2a and 2b.
À
À
double-bond character than the latter.[16] Therefore 2a
should be represented by a series of resonance contributors
shown in Scheme 3 rather than a single Kekulꢃ structure.
Molecules of 2a form infinite stacks in two directions with
the phenazine planes being 3.51 ꢂ apart from each other.
Unlike 2a, 2b is essentially flat as found in the crystal struc-
(1.39 ꢂ) in agreement with a double bond between the N14
À
and C13a atoms. The N5 C5a bond (1.38 ꢂ) is shorter than
À
the N5 C4a bond (1.40 ꢂ), indicating the former has more
double-bond character than the latter. This can be explained
by the minor resonance contributors of 2b shown in
Scheme 3. Interestingly, water molecules, which come from
the solvent of wet acetone, are found in the crystal lattice of
2b. As shown in Figure 1c, one molecule of 2b forms hydro-
À
tures shown in Figure 1c and 1d. The N14 C13a bond
À
(1.33 ꢂ) is significantly shorter than the N14 C14a bond
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Nitrogen-Containing Heteropentacenes
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gen bonds with two molecules of water. Molecules of 2b as-
semble into stacks with a p–p distance of 3.42 ꢂ, and water
molecules are linked by hydrogen bonds to form a chain be-
tween the neighboring stacks of 2b. Similar to 2a, 2c is also
a bent molecule, and the two adjacent methyl groups are
distorted to avoid steric repulsions. A stacked dimer of 2c
(Figure 1 f) is the unit of molecular packing in the crystal.
The quinonoid structure was misassigned to DHTAP be-
cause of a lack of knowledge on the electronic structure of
benzenoid and quinonoid nitrogen-containing heteroacenes.
Therefore the electronic structure of 2a and 2b was studied
by using both experimental and computational methods.
Both 2a and 2b are red in solution. A comparison of the ab-
sorption spectra of 2a and 2b is shown in Figure 2a. In the
moiety. Cyclic voltammograms (CVs) of 2a and 2b (see the
Supporting Information) in DMF exhibit one reversible oxi-
dation wave and one reversible reduction wave. The half-
wave oxidation potential versus ferrocenium/ferrocene is
0.21 V for 2a and À0.01 V for 2b, from which the HOMO
energy levels of 2a and 2b are estimated as À5.01 eV and
À4.79 eV, respectively.[18] The half-wave reduction potentials
versus ferrocenium/ferrocene is À1.91 V for 2a and À1.84 V
for 2b, from which the LUMO energy levels of 2a and 2b
are estimated to be À2.89 eV and À2.96 eV, respectively.
The HOMO and LUMO energy levels of 2a obtained from
CVs are in good agreement with the optical gap calculated
from the absorption edge. However the electrochemical
HOMO–LUMO gap of 2b is about 0.3 eV smaller than its
optical gap. The HOMO energy levels of 2a and 2b ob-
tained from CVs correspond with the computed ones,
whereas the LUMO energy levels obtained from CVs are
lower than the computed ones. The above studies on the
electronic structure of 2a indicate that the N5 and N14
atoms have the lone pairs of electrons in effective conjuga-
tion with the adjacent phenazine ring although the molecule
is bent at the two N atoms. Such conjugation leads to the
HOMO and LUMO energy levels that are close to those of
2b, and is consistent with bond lengths found in crystal
structures. This finding appears an amendment to the previ-
ous understanding that the two NH units in nitrogen-con-
taining heteropentacenes break the delocalization of the p
system in the pentacene skeleton.[4a,8a,19]
The studies on the crystal and electronic structure of 2a
and 2b indicate that both the molecules self-assemble into
stacks with intermolecular p-orbital overlap, and have delo-
calized HOMOs with energy levels accessible for charge in-
jection, suggesting that 2a and 2b can function as p-type or-
ganic semiconductors.[20] Therefore, the two molecules were
tested in organic field effect transistors (OFETs).[21] Flake-
or needle-shaped thin crystals of 2a and 2b were grown by
physical vapor transport (PVT),[22] and were placed on a
SiO2/Si substrate with pre-deposited drain and source elec-
trodes of gold.[23] The resulting devices were bottom-contact
OFETs with highly n-doped silicon as a gate electrode, a
300 nm thick layer of SiO2 as dielectrics, and the semicon-
ductor channel length in a range of 50 mm to 150 mm. A mi-
crograph of such a type of device is shown in Figure 3a. It is
Figure 2. a) Absorption of 2a and 2b (3ꢄ10À6 m in THF); b) frontier mo-
lecular orbitals of 2a and 2b.
visible light range, 2a exhibits a peak at 498 nm and a
shoulder at 520 nm, whereas 2b exhibits three peaks at 551,
512, and 479 nm. Although the longest-wavelength absorp-
tion peak of 2b is red-shifted by about 50 nm in comparison
with that of 2a, the absorption edges of the two molecules
are close to each other (ca. 565 nm for 2a and ca. 575 nm
for 2b). From the absorption edge, the HOMO–LUMO gap
is calculated to be 2.19 eV for 2a and 2.16 eV for 2b. The
frontier molecular orbitals of 2a and 2b were calculated by
using the hybrid density functional method B3LYP with a 6-
31+G* basis set,[17] and is depicted graphically in Figure 2b.
Like 2b, 2a has a delocalized highest-occupied molecular or-
bital (HOMO). However, the lowest unoccupied molecular
orbital (LUMO) of 2a mainly resides on the phenazine
Figure 3. a) A micrograph of an OFET fabricated from a very thin crystal
of 2b; b) drain current versus drain voltage for the OFET of 2a with an
active channel of L=150 mm, W=365 mm.
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Q. Miao et al.
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See: Ref. [6c].
[12] The DHTAP dianion has more possible resonance forms, which are
not shown in Scheme 2b. The real structure of the DHTAP dianion
should be a hybrid of all of the possible resonance forms.
[13] Benzene with two identical substituents ortho to each other is well
known to give an AA’XXꢀ (or AA’BB’) spin system. See: U. Weber,
H, Thiele, NMR Spectroscopy: Modern Spectral Analysis, Wiley-
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drain current, m is the field effect mobility, W is the channel
width, L is the channel length, VG is the gate voltage, VT is
the threshold voltage, and Ci is the capacitance per unit area
(11nFcmÀ2 for 300 nm SiO2). In comparison, 2b shows a
lower field effect mobility of 1ꢄ10À4 cm2 VÀ1 sÀ1. The mobili-
ties of 2a and 2b measured in this study appear lower than
the one reported for DHTAP.[24] The low mobilities of 2a
and 2b might be not intrinsic of the materials but limited by
the poor contacts in the devices.
In summary, benzenoid and quinonoid nitrogen-contain-
ing heteropentacenes were successfully isolated and investi-
gated. The complete characterization of 2a and 2b revealed
p-electron delocalization in these polynuclear heterocycles
and p-stacks in their molecular assemblies. These findings
led to a better understanding of the electronic structures
and clearly supported the benzenoid structure of DHTAP,
which has been debated. It is found that both benzenoid
and quinonoid nitrogen-containing heteropentacenes can
function as p-type organic semiconductors. Furthermore, be-
cause the parent molecule DHTAP was regarded as a small-
molecule model for a series of ladder polymers,[6] the find-
ings presented in this report may also have implications for
interesting ladder polymers.
[14] CCDC-714520, CCDC-714521, and CCDC-714522 contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif
[15] M. Holzapfel, C. Lambert, C. Selinka, D. Stalke, J. Chem. Soc.
Perkin Trans. 1 2002, 2, 1553–1561.
À
[16] In comparison, the four N C bonds of phenazine are averaged with
the same bond length of 1.34 ꢂ. See: K. Wozniak, B. Kariuki, W.
Jones, Acta Crystallogr. Sect. A 1991, 47, 1113.
[17] Density function theory (DFT) calculations were performed by
using: Gaussian 03, Revision D.01, M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Mont-
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N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
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[18] The commonly used HOMO energy level of ferrocene is À4.80 eV.
See: a) J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H.
Bꢅssler, M. Porsch, J. Daub, Adv. Mater. 1995, 7, 551–554; b) B. W.
D’Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov,
M. E. Thompson, Org. Electron. 2005, 6, 11–20.
[19] Our recent study on the electronic structure of 6,13-dihydro-6,13-di-
azapentacene indicates that it has a delocalized HOMO with the
energy level essentially the same as that of pentacene although its
HOMO–LOMO gap is significantly larger than that of pentacene.
This study will be published elsewhere.
Acknowledgements
We thank Prof. Zhifeng Liu (Department of Chemistry, CUHK) for help
with the DFT calculations. We acknowledge the financial support from
Research Grant Council of Hong Kong, General Research Funding 2008-
09 (project number 402508).
Keywords: acenes · electronic structure · heterocycles · self-
assembly · semiconductors
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Received: January 20, 2009
Published online: March 4, 2009
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