Molecular packing and n-channel thin film transistors of chlorinated cyclobuta[1,2-b:3,4-b′]diquinoxalines

Article abstract of DOI:10.1039/c5cc00537j

Novel chlorinated cyclobuta[1,2-b:3,4-b′]diquinoxalines were synthesized and investigated in the solid state. It is found that their molecular packing can be tuned by varying the number and position of chlorine substituents, and 2,8-dichloro-cyclobuta[1,2-b:3,4-b′]diquinoxaline either in pure form or as a mixture with its regioisomer functions as n-type semiconductors in organic thin film transistors with a field effect mobility of up to 0.42 cm² V^-1 s^-1 and 0.20 cm² V^-1 s^-1, respectively. This journal is

Full text of DOI:10.1039/c5cc00537j

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Molecular packing and n-channel thin film transistors
of chlorinated cyclobuta[1,2-b:3,4-b0]diquinoxalines†
Cite this:DOI: 10.1039/c5cc00537j
Shuaijun Yang,a Danqing Liu,a Xiaomin Xua and Qian Miao*ab
Received 20th January 2015,
Accepted 3rd February 2015
DOI: 10.1039/c5cc00537j
www.rsc.org/chemcomm
Novel chlorinated cyclobuta[1,2-b:3,4-b0]diquinoxalines were synthe- However, the parent compound of 1a, 5,7,12,14-tetraazapentacene
sized and investigated in the solid state. It is found that their molecular (1b), has never been synthesized with unambiguous characterization
packing can be tuned by varying the number and position of chlorine likely because of its instability. Although Dutt claimed the synthesis
substituents, and 2,8-dichloro-cyclobuta[1,2-b:3,4-b0]diquinoxaline of 1b by condensation of 2,3-diaminophenazine with o-benzo-
either in pure form or as a mixture with its regioisomer functions as quinone,9 the compound obtained by Dutt was later found to be
n-type semiconductors in organic thin film transistors with a field effect 5,7,12,14-tetraaza-6,13-pentacenequinone instead.10 In contrast,
mobility of up to 0.42 cm2 Vꢀ1 sꢀ1 and 0.20 cm2 Vꢀ1 sꢀ1, respectively.
cyclobuta[1,2-b:3,4-b0]diquinoxaline (2a) is a stable molecule combin-
ing both motifs of N-heteropentacene and dibenzo[b,h]biphenylene.
Acenes, a family of polycyclic aromatic hydrocarbons (PAHs) con- Although 2a was first synthesized four decades ago,11 the potential
sisting of linearly annelated benzene rings as shown in Fig. 1a, are application of 2a and its derivatives for semiconductor materials was
one of the most important groups of organic semiconductors, not recognized until very recently with a computational study.12 Here
particularly, for organic thin film transistors (OTFTs).1 With similar we report new derivatives of 2a (Fig. 1b) with substitution of chlorine
linear geometry, linear [N]phenylenes (Fig. 1a) have benzene rings atoms, which is an effective design for n-type air-stable organic
fused together by four-membered rings, formally with alternating semiconductors,13 and demonstrate that the molecular packing
aromatic and antiaromatic characters.2 Combination of acene and of cyclobuta[1,2-b:3,4-b0]diquinoxalines can be tuned by varying
[N]phenylene leads to phenylene-containing oligoacenes, which were the number and position of chlorine substituents. It is found that
recently investigated as a new class of fully unsaturated ladder 2,8-dichloro-cyclobuta[1,2-b:3,4-b0] diquinoxaline (2e) either in pure
structures.3 In contrast to acenes, either [N]phenylenes or
phenylene-containing oligoacenes in the solid state have been rarely
explored as electronic materials. Although its structure is closely
related to pentacene, a benchmark p-type organic semiconductor for
applications in OTFTs, dibenzo[b,h]biphenylene (Fig. 1a) was docu-
mented only with two Japanese patents to function as a p-type
organic semiconductor in OTFTs to the best of our knowledge.4
Introducing N atoms to acenes leads to N-heteroacenes, which have
recently been developed into a class of organic semiconductors with
high performance in OTFTs.5 For example, silylethynylated tetraaza-
pentacene 1a (Fig. 1b)6 has been so far the best semiconductor based
on N-heteroacenes with a high field effect mobility for electrons.7,8
a Department of Chemistry, the Chinese University of Hong Kong, Shatin,
New Territories, Hong Kong, China
b Institute of Molecular Functional Materials (Areas of Excellence Scheme,
University Grants Committee), Hong Kong, China
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of 2a-f, fabrication and characterization of thin film transistors, crystal-
Fig. 1 Molecular structures of (a) acenes, [N]phenylenes and dibenzo[b,h]-
biphenylene; (b) tetraazapentacenes (1a-b) and cyclobuta[1,2-b:3,4-b0]-
diquinoxalines (2a-f).
lographic data and crystallographic information files for 2a, 2b, 2d/e, 2e, 2f.
CCDC 1043927–1043931. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5cc00537j
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form or as a mixture with its regioisomer 2d functions as an n-type
organic semiconductor in OTFTs with a field effect mobility of up to
0.42 cm2 Vꢀ1 sꢀ1 and 0.20 cm2 Vꢀ1
stability.
s
ꢀ1, respectively, and good air
As shown in Scheme 1 and detailed in the ESI,† substituted
cyclobuta[1,2-b:3,4-b0]diquinoxalines 2b-f were synthesized by modifying
the reported synthesis of 2a.11 In the synthesis of 2b-e, the intermediate
4 was a complicated mixture, which was directly used in the next step
without separation. Only in the synthesis of 2f, the intermediate 4 could
be isolated as a pure compound with full characterization. This method
yielded dichloro-derivatives 2d and 2e as a mixture (2d/e), which
exhibited two sets of peaks in the 13C NMR spectrum (Fig. S4 in the
ESI†). Chromatography or recrystallization from solutions did not allow
separation of the two regioisomers (2d and 2e). Through a physical
vapor transport14 process, 2e was isolated in pure form but only in a
small amount (Fig. S2–S4 in the ESI†). Compounds 2a-e are only
slightly soluble (1 mg mLꢀ1 or lower) in dichloromethane or chloro-
form, while 2f is almost insoluble. All compounds 2a-f are thermally
stable in air when heated at 250 1C or even higher temperature.
The energy level of frontier molecular orbitals and the molecular
packing of semiconductor molecules are two key factors that deter-
mine their performance in OTFTs. To determine the energy levels of
the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO), the cyclobuta[1,2-b:3,4-b0]-
diquinoxalines, except 2d, which could not be prepared in pure
form, were studied using UV-vis absorption spectroscopy and cyclic
voltammetry. Yellow solutions of 2a-f exhibited similar UV-vis
absorption with the longest-wavelength absorption maximum at
Fig. 2 UV-vis spectra for 0.01 mM solutions of 2a in hexane/CH2Cl2
(99 : 1, v/v) and quinoxaline in hexane.
Table 1 Absorption edge, reduction potentials and energy levels of
frontier molecular orbital for cyclobuta[1,2-b:3,4-b0]diquinoxalines
a
a
Absorption E1red
edge (nm) (V)
E2red
(V)
HOMO–UMO LUMOc HOMOd
gapb (eV)
(eV)
(eV)
2a 465
ꢀ1.18 ꢀ2.04 2.67
ꢀ1.14 ꢀ1.91 2.63
ꢀ1.08 ꢀ1.83 2.62
ꢀ1.06 ꢀ1.87 2.61
ꢀ3.92
ꢀ3.96
ꢀ4.02
ꢀ4.04
ꢀ4.14
ꢀ6.59
ꢀ6.59
ꢀ6.64
ꢀ6.65
ꢀ6.71
2b 471
2c 474
2e 475
2f
483
ꢀ0.96
—
2.57
a
Half-wave potential versus ferrocenium/ferrocene. b Estimated from the
absorption edge in the UV-vis absorption spectrum from a solution in
dicholromethane (1 ꢁ 10ꢀ5 M). c Estimated from LUMO = ꢀ5.10 ꢀ Ered (eV).15
d Calculated from the HOMO–LUMO gap and the LUMO energy level.
449 to 471 nm. Fig. 2 compares the UV-vis absorption spectrum of the number of chlorine substituents leads to lower energy levels of
2a with that of quinoxaline showing that 2a has a red-shifted and both HOMO and LUMO.
more intense absorption. This suggests considerable conjugation
Single crystals of 2a grown from a solution in CHCl3 and single
between the two quinoxaline subunits in 2a, in agreement with the crystals of 2b, 2d/e, 2e, 2f grown by physical vapor transport14 were
reported absorption spectra of phenylene-containing oligoacenes, qualified for X-ray crystallography. Particularly, when a mixture of 2d
which are red shifted relative to the corresponding oligoacenes.3 and 2e was used as the source for physical vapor transport, two types
Except compound 2f, all cyclobuta[1,2-b:3,4-b0]diquinoxaline exhib- of crystals, namely yellow crystals containing both 2d and 2e and
ited two reduction waves in the test window of cyclic voltammetry orange crystals containing only 2e, were collected as shown in Fig. S2
(Fig. S8 in the ESI†). All of these reduction waves were quasi- (the ESI†). The crystal structures of 2a, 2b, 2d/e, 2e and 2f show
reversible with the first half-wave potential in the range of ꢀ1.18 V almost the same backbone of dibenzo[b,h]biphenylene but different
to ꢀ0.96 V versus ferrocenium/ferrocene. Based on the reduction motifs of molecular packing.16 As a representative, the essentially flat
potentials15 and the absorption edges found from the UV-vis absorp- backbone of 2a is shown in Fig. 3a. Examination of the bond lengths
tion spectra, the HOMO and LUMO energy levels of 2a-f are of 2a reveals that its central four-membered ring contains four single
estimated and summarized in Table 1. It is found that increasing C–C bonds. Two of them (C5a–C5b and C11a–C11b) are 1.50 Å long,
which is even longer than that of the typical nonconjugated single
bond between two sp2-hybridized C atoms (1.47–1.48 Å),17 and the
other two bonds (C5a–C11b and C5b–C11a) are 1.44 Å long, which is
longer than typical C–C double bonds in alkenes and arenes but is
very close to the conjugated single bond between two sp2-hybridized
C atoms (1.45–1.46 Å).17 The four-membered ring is attached to four
N atoms with four short C–N bonds (1.29 Å for C5a–N5 and C11a–
N11; 1.30 Å for C5b–N6 and C11b–N12), which are not only shorter
than other C–N bonds in 2a but also shorter than the typical bond
length of C–N double bonds in imines (1.35 Å).17 These bond lengths
exhibit a radialene-like structure and suggest very poor conjugation
between the two quinoxaline subunits. This is however contrary to
the considerable conjugation between the two quinoxaline subunits
as found from UV-vis absorption spectra.
Scheme 1 Synthesis of 2b-f, where the yield of 2b-f is indicated as a total
yield from 3.
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Fig. 3 Crystal structure of 2a: (a) the backbone of 2a with some C atoms
labelled and some bond lengths highlighted (H atoms are removed for
clarification); (b) p–p stacking of 2a; (c) short contacts (shown as red
dashed lines) between molecules of 2a and the unit cell. (C and N atoms
are shown as ellipsoids at the 50% probability level.)
As shown in Fig. 3b, molecules of 2a form one-dimensional offset
face-to-face p-stacks with a distance of 3.33 Å between two adjacent
p-planes. The molecules of 2a are oriented in two directions, and the
p-planes in the two different directions are nearly parallel with a
small angle of 4.851 between the p-planes. As shown by the red
dashed lines in Fig. 3c, the neighboring p-stacks of 2a are linked by
C–HꢂꢂꢂN weak H-bonds, which involve a HꢂꢂꢂN contact (2.65 Å)
shorter than the sum of van der Waals radii of H and N atoms and a
C–HꢂꢂꢂN angle of 1461. With a disordered arrangement of its Cl
atom, 2b exhibits almost the same crystal structure as 2e. Fig. 4
shows the crystal structure of 2e, which exhibits one-dimensional
offset face-to-face p-stacks similar to those of 2a. However, unlike
those of 2a, molecules of 2e are oriented in two directions that
intersect with an angle of 531 as shown in Fig. 4a and form double
weak H-bonds between neighboring stacks as shown in Fig. 4b. The
double H-bonds are associated with a HꢂꢂꢂN short contact of 2.57 Å
and a C–HꢂꢂꢂN angle of 1641. The molecular packing of 2b and 2e
differs from that of 2a likely because the C–HꢂꢂꢂN contacts in the
crystal structure of 2a (shown as red dashed lines in Fig. 3c) become
not available to 2b and 2e, which have Cl substituents at the
corresponding positions. The co-crystal of 2d/e contains 2d and 2e
in a disordered arrangement leading to a molecular packing motif
Fig. 4 Molecular packing of 2e (a) as viewed along the c axis of the unit
cell showing p–p stacking and (b) as viewed along the a axis of the unit cell
showing weak H-bonds. (C, N and Cl atoms are shown as ellipsoids at the
50% probability level.)
Fig. 5 Crystal structure of 2d/e showing (a) weak H-bonds and (b) p–p
stacking. (The green atoms contain disordered Cl and H atoms, non-H
atoms are shown as ellipsoids at the 50% probability level.)
very similar to that of 2f. Similar to the crystal of 2e, the co-crystal of the CDPA-modified AlOy/TiOx is a general dielectric as recently
2d/e exhibits offset face-to-face p-stacks and double weak H-bonds developed by us for high-performance vacuum-deposited and
between neighboring stacks as shown in Fig. 5. The distance solution-processed OTFTs with a capacitance per unit area (Ci) of
between two adjacent p-planes of 2d/e is 3.38 Å, and the weak 210 nF cmꢀ2.8 The fabrication of OTFTs was completed by depositing
H-bond of C–HꢂꢂꢂN contains a HꢂꢂꢂN short contact of 2.65 Å and a a layer of gold on the organic films through a shadow mask to form
C–HꢂꢂꢂN angle of 1681. However, unlike 2e, molecules of 2d/e are all top-contact source and drain electrodes. As measured under vacuum,
oriented in the same direction.
the films of 2e and 2d/e functioned as n-type semiconductors with a
The low-lying LUMO and p–p stacking of 2a-f suggest that they field effect mobility of 0.23 ꢃ 0.07 and 0.11 ꢃ 0.05 cm2 Vꢀ1 sꢀ1
,
can in principle function as n-type semiconductors. To test semi- respectively. When measured in air, the average mobility of 2e and
conductor properties of 2a-f in OTFTs, their thin films were depos- 2d/e decreased to 0.06 ꢃ 0.02 and 0.07 ꢃ 0.03 cm2 Vꢀ1 sꢀ1
,
ited by thermal evaporation under a high vacuum onto a high-k respectively. Fig. 6b shows the transfer I–V curves of the best-
dielectric of aluminium oxide and titanium oxide (AlOy/TiOx),18 performing OTFT of 2e measured in the saturation regime under
which was pre-treated with a self-assembled monolayer (SAM) of vacuum exhibiting a field effect mobility of 0.42 cm2 Vꢀ1 ꢀ1; and
s
12-cyclohexyldodecylphosphonic acid (CDPA shown in Fig. 6a). Here, Fig. S12 (the ESI†)showsthetransferI–Vcurves of the best-performing
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charge transport in the thin films of 2d/e and 2e can be attributed
to the edge-on orientation of molecules in a continuous polycrystal-
line film. In contrast, the absence of field effect in the films of 2a, 2b,
2c and 2f is likely related to the morphology of isolated crystallites or
the face-on orientation, either of which prevents charge transport
parallel to the dielectric surface.
In conclusion, this study puts forth a new design of n-type
organic semiconductors by introducing electronegative nitrogen
and chlorine atoms into dibenzo[b,h]biphenylene. A series of new
chlorinated derivatives of cyclobuta[1,2-b:3,4-b0]diquinoxaline
were synthesized and investigated in the solid state. It was found
that 2e and 2d/e functioned as n-type semiconductors in OTFTs
exhibiting a field effect mobility of up to 0.42 cm2 Vꢀ1 sꢀ1 and
0.20 cm2 Vꢀ1 sꢀ1 and good air stability.
Fig. 6 (a) Phosphonic acid that is used to modify the AlOy/TiOx dielectric
surface in the OTFTs; (b) drain current (IDS) versus gate voltage (VG) with
drain voltage (VDS) at 3 V for OTFT of 2e with an active channel of 1 mm
wide and 150 mm long as measured under vacuum.
We thank Ms Hoi Shan Chan (the Chinese University of
Hong Kong) for the single crystal crystallography. This work was
OTFT of 2d/2e in the saturation regime measured under vacuum and supported by a grant from the Research Grants Council of Hong
in air exhibiting a field effect mobility of 0.20 and 0.16 cm2 Vꢀ1 sꢀ1
Kong (project number: GRF402011) and the University Grants
,
respectively. In contrast, the thin films of 2a, 2b, 2c and 2f all appeared Committee of Hong Kong (project number: AoE/P-03/08).
insulating in our experiments.
To understand the different performance of 2a-f, their thin films
were investigated using atomic force microscopy (AFM) and X-ray
Notes and references
diffraction (XRD). The AFM images (Fig. S15 in the ESI†) revealed
that the films of 2a, 2b and 2c contained isolated crystallites while
the films of 2d/e, 2e and 2f were continuous. Particularly, the films of
2e and 2d/e exhibited a terraced morphology that is typical for a
vacuum-deposited film of organic semiconductors. As measured
from the section analysis, each terrace step is about 16 Å high
(Fig. S16 in the ESI†), roughly equal to the length of the long
molecular axis in 2d and 2e (14 Å). This suggests that molecules of
2e and 2d/e stand on the substrate with its long molecular axis
roughly perpendicular to the substrate surface. XRD from a 100 nm-
thick film of 2a exhibited two weak peaks at 2y = 11.71 (d spacing =
7.56 Å) and 2y = 27.71 (d spacing = 3.22 Å), which correspond to the
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%
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%
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%
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structures (Fig. S20 in the ESI†), indicating thin film phases that
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Chem. Commun.
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