N-heteroquinones: Quadruple weak hydrogen bonds and n-channel transistors

Article abstract of DOI:10.1039/c001215g

This study demonstrates that the easily synthesized N-heteroquinones, having unusual quadruple weak hydrogen bonds of a DDAA-AADD pattern, can function as n-type organic semiconductors in OTFTs with high electron mobility. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c001215g

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N-heteroquinones: quadruple weak hydrogen bonds and n-channel
transistorsw
Qin Tang,a Zhixiong Liang,a Jing Liu,a Jianbin Xub and Qian Miao*ac
Received (in Cambridge, UK) 19th January 2010, Accepted 11th February 2010
First published as an Advance Article on the web 5th March 2010
DOI: 10.1039/c001215g
6,9b
This study demonstrates that the easily synthesized N-hetero-
quinones, having unusual quadruple weak hydrogen bonds of a
DDAA–AADD pattern, can function as n-type organic
semiconductors in OTFTs with high electron mobility.
condensation10 and subsequent oxidation by K2Cr2O7
as
shown in ESI.w Both TAPQ and DATQ are yellow crystalline
powders, which are stable to light irradiation. As monitored
by differential scanning calorimetry (DSC), TAPQ is stable up
to 450 1C and DATQ melts at 320 1C with decomposition.
While TAPQ is almost insoluble in common organic solvents,
DATQ is soluble in several organic solvents, such as CHCl3,
N-type organic semiconductors are the key materials that
accept and transport electrons in organic electronics, such as
organic phovoltaic solar cells and n-channel organic thin film
transistors (OTFTs).1,2 The development of n-type organic
semiconductors has lagged behind that of p-type ones primarily
due to the inherent instability of organic anions in the presence
of air and water.2a To develop high performance n-type
organic semiconductors, new designs need to be explored.
Here we report the easily synthesized N-heteroquinones, which
feature two types of electron-withdrawing moieties, quinonoid
carbonyl groups and pyrazine-type nitrogen atoms,3 function
as n-type semiconductors in OTFTs. Quinones are well known
as organic oxidizing reagents in organic synthesis and
biological systems, but their ability to accept electrons has
rarely been explored in connection with n-type organic semi-
conductors.4 Nitrogen-rich heteroacenes that feature pyrazine
rings are also promising n-type organic semiconductors,5,6 but
only a few of them have been tested in n-channel OTFTs.7
In fact the reported N-heteroacenes are mainly p-type
semiconductors.8 Because of the electron-withdrawing
quinone and pyrazine moieties, the N-heteroquinones have
low-lying and well delocalized lowest unoccupied molecular
orbitals (LUMOs), which are suitable for n-type semiconductors.
Although the two N-heteroquinones studied here have been
known for decades,9 this study is the first time that their
electronic structures, molecular packing and semiconductor
properties have been investigated. Another interesting finding
of this study is the unusual quadruple weak hydrogen bonds in
the crystal structures of N-heteroquinones.
toluene and DMF, with solubility larger than 2 mg mlꢀ1
.
The electronic structures of the two molecules were studied
using both experimental and computational methods. Cyclic
voltammograms of TAPQ and DATQ in DMF both exhibit
one reversible reduction wave. The half-wave reduction
potential vs. ferrocenium/ferrocene is ꢀ1.02 V for TAPQ
and ꢀ1.20 V for DATQ, from which the LUMO energy levels
of the two molecules are estimated as ꢀ3.78 eV and ꢀ3.60 eV,
respectively.11 The UV-vis absorption spectroscopy from
solutions in DMF reveals the HOMO–LUMO gap is 3.15 eV
for TAPQ and 3.33 eV for DATQ based on the absorption
edge. In comparison, the corresponding acenequinones have
higher LUMO energy levels (ꢀ3.29 eV for 6,13-pentacenequinone
and ꢀ3.39 eV for 5,12-tetracenequinone) and smaller
HOMO–LUMO gaps (2.92 eV for 6,13-pentacenequinone
and 2.82 eV for 5,12-tetracenequinone). The frontier molecular
orbitals of TAPQ and DATQ were calculated by using the
hybrid density functional method B3LYP with a 6-31+G*
basis set. As depicted graphically in Fig. 1, both TAPQ and
DATQ have
the LUMOs.
a
delocalized electron distribution in
Single crystals of TAPQ and DATQ were grown using the
physical vapor transport technique12 and from a solution in
DMSO, respectively. As shown in Fig. 2, X-ray crystallo-
graphic analysisz reveals the molecular packing of TAPQ,
which consists of characteristic p–p stacking and C–Hꢁ ꢁ ꢁN/O
hydrogen bonds. Molecules of TAPQ form infinite stacks in
two directions with a p–p distance of 3.32 A, which is slightly
shorter than the interplanar distance in graphite (3.35 A).
5,7,12,14-tetraaza-6,13-pentacenequinone (TAPQ) and
6,11-diaza-5,12-tetracenequinone (DATQ) were simply
synthesized from very cheap starting materials by solvent-free
a Department of Chemistry, The Chinese University of Hong Kong,
Shatin, New Territories, Hong Kong, China.
E-mail: miaoqian@cuhk.edu.hk
b Department of Electronic Engineering, The Chinese University of
Hong Kong, Shatin, New Territories, Hong Kong, China
c Center of Novel Functional Molecules, The Chinese University of
Hong Kong, Shatin, New Territories, Hong Kong, China
w Electronic supplementary information (ESI) available: Synthesis
and characterization of TAPQ and DATQ, CV and UV-vis for TAPQ,
DATQ and acenequinones, fabrication and characterization of thin
film transistors, crystal information file for TAPQ (CCDC 761663)
and DATQ (CCDC 761662). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c001215g
Fig. 1 Structures and calculated LUMOs of TAPQ and DATQ.
Chem. Commun., 2010, 46, 2977–2979 | 2977
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Three Cꢁ ꢁ ꢁC contacts shorter than the sum of van der Waals
radii (3.40 A) are found between two neighboring molecules in
a stack (Fig. 2a). Intermolecular contacts shorter than the
sum of van der Waals radii are also found between TAPQ
molecules of the neighboring stacks (Fig. 2b). These short
contacts are weak C–Hꢁ ꢁ ꢁN/O hydrogen bonds.13 Interestingly
quadruple hydrogen bonds, which have a self-complementary
DDAA–AADD pattern (D is hydrogen donor and A is
hydrogen acceptor), are found between the neighboring
stacks of the same direction. The close p–p stacking associated
with weak hydrogen bonds between p-stacks indicates
relatively strong intermolecular forces, which are consistent
with the low solubility of TAPQ. Similar p–p stacking and
quadruple weak hydrogen bonds are also found in the crystal
structure of DATQ as shown in Fig. 2c and d. Although
the C–Hꢁ ꢁ ꢁO hydrogen bonds are known for benzoquinones,13,14
the quadruple weak hydrogen bonds of a DDAA–AADD
pattern found here are very unusual.15 In these quadruple
weak hydrogen bonds, the Hꢁ ꢁ ꢁN and Hꢁ ꢁ ꢁO distances
are 2.68–2.69 A and 2.67 A respectively, and the C–Hꢁ ꢁ ꢁN
and C–Hꢁ ꢁ ꢁO angles are 168–1691 and 1351, respectively.
TAPQ has a denser packing than 6,13-pentacenequinone in
the crystals as indicated by the fact that the unit cell volume
of 6,13-pentacenequinone (718.193 A3) is 10.4% larger than
that of TAPQ (650.336 A3).16 Because the p–p distance between
two molecules of 6,13-pentacenequinone is 3.50 A, only 4.5%
larger than that of TAPQ, the denser packing of TAPQ
must arise from not only the shorter p–p distance but also
the closer contacts between the stacks, which are in good
agreement with the weak hydrogen bonds found between
TAPQ molecules of the neighboring stacks. Such dense
molecular packing with short p–p distance and close contacts
between p-stacks may enhance charge transport by increasing
orbital overlap.
To fabricate OTFTs, TAPQ was purified by the physical
vapor transport technique and then deposited by thermal
evaporation onto a silicon wafer. The devices had gold as
the top-contact source and drain electrodes, highly doped Si as
a gate electrode and a 300 nm thick layer of SiO2 as dielectrics.
When the SiO2 dielectrics were pretreated with a self-assembled
monolayer of octadecyltrimethoxysilane (OTMS)17 and the
substrate temperature was kept at 100 1C during deposition,
the resulting films of TAPQ were polycrystalline as indicated
by the X-ray diffraction (XRD)18 and atomic force micro-
graphs (shown in ESIw). These films performed as n-channel
transistors with field effect mobilities of 0.04–0.12 cm2 Vꢀ1 sꢀ1
,
which were measured under vacuum. Shown in Fig. 3 are the
typical transfer I–V curves for such devices, from which a
field-effect mobility of 0.09 cm2 Vꢀ1 sꢀ1 was measured in the
saturation regime using the equation: IDS = (mWCi/2L)(VG ꢀ VT)2
and Ci of 11 nF/cm2 for 300 nm SiO2.19 The on/off ratio of the
drain current obtained between 0 and 80 V gate bias was
greater than 1 ꢃ 106. The relatively high threshold voltage
(about 27 V) found from the transfer curve suggested a high
density of charge carrier traps in the transistor.19 When
OTFTs of TAPQ were tested in the ambient air, the measured
electron mobility decreased to 2 ꢃ 10ꢀ3 cm2 Vꢀ1
s
ꢀ1. On the
other hand, the n-channel OTFTs of DATQ fabricated by
thermal evaporation showed a much lower electron mobility
(10ꢀ6 cm2 Vꢀ1 ꢀ1), which should be mainly attributed to the
s
very low crystallinity of the films as found from the X-ray
diffraction as shown in ESI.w
In summary, the studies above demonstrate that the easily-
synthesized N-heteroquinones, which have low-lying and
delocalized LUMOs and form close p–p stacks, can function
as n-type organic semiconductors in OTFTs with high electron
mobility. The crystal structures of TAPQ and DATQ reveal
unusual quadruple C–Hꢁ ꢁ ꢁN/O hydrogen bonds of
a
DDAA–AADD pattern, which may be applied as a supra-
molecular synthon in crystal engineering.13 This study suggests
that p-extended quinones equipped with electron-withdrawing
groups would be a general design for n-type organic semi-
conductors. Novel molecules with such design are being
studied in our laboratory.
Fig.
2 Molecular packing of TAPQ and DATQ in crystals:
(a) p-stacks of TAPQ in two directions, showing short intermolecular
contacts within a stack; (b) weak hydrogen bonds between TAPQ
molecules of the neighboring p-stacks; (c) p-stacks of DATQ in two
directions; (d) quadruple weak hydrogen bonds between neighboring
DATQ molecules.
Fig. 3 Drain current (IDS) versus gate voltage (VGS) with VDS = 80 V
measured under vacuum from the thin film transistors of TAPQ
deposited on OTMS-treated SiO2 at substrate temperature of 100 1C
with channel dimension of W = 2 mm and L = 50 mm.
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This work is financially supported by the Research Grant
Council of Hong Kong, General Research Fund 402508 and
Collaborative Research Fund CUHK2/CRF/08. The authors
would like to thank Prof. Zhifeng Liu (The Chinese University
of Hong Kong) for the DFT calculations and Prof. Feng Yan
(The Hong Kong Polytechnic University) for XRD.
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Notes and references
z Crystal data for TAPQ: C18H8N4O2: M = 312.28, monoclinic, space
group P21/n, a = 3.8926(2) A, b = 9.0707(4) A, c = 18.4339(10) A,
a = 901, b = 92.332(4)1, g = 901, V = 650.34(6) A3, Z = 2,
3495 reflections collected, 1088 unique (Rint = 0.0220). The final R
was 0.0561 (all data) and wR was 0.1704 (all data). Crystal data for
DATQ: C16H8N2O2: M = 260.24, monoclinic, space group P21/c,
a = 12.1301(6) A, b = 3.8545(2) A, c = 24.3276(12) A, a = 901,
b = 94.5780(10)1, g = 901, V = 1133.82(10) A3, Z = 4, 12 534
reflections collected, 2067 unique (Rint = 0.0378). The final R was
0.0564 (all data) and wR was 0.1315 (all data).
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11 The commonly used HOMO energy level of ferrocene is ꢀ4.80 eV.
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derivatives, but were not recognized in that report. See: Y.-S. Kim,
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and 2y = 18.571 (d-spacing: 4.77 A). In comparison, the (002)
diffraction derived from the single crystal structure appears at
2y = 9.641 (d-spacing: 9.17 A). The fact that the diffractions from
thin films are different from those derived from the single crystal
struture indicates different crystalline polymorphs of TAPQ in thin
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