Extension of N-Heteroacenes through a Four-Membered Ring

Article abstract of DOI:10.1002/chem.201600918

The synthesis of novel π-extended N-heteroacenes, which have a large tetraazaacene subunit and a quinoxaline subunit connected through a four-membered ring, is reported. They were studied with experimental and computational methods in comparison to the corresponding tetraazaacenes. As found from the DFT calculation, the four-membered ring is a better linker than a five-membered ring or a C-C single bond to extend N-heteroacenes for a new design of n-type semiconductors in terms of the spatial delocalization and energy level of LUMO as well as the reorganization energy. In solution-processed thin film transistors, the π-extended N-heteroacenes are found to function as n-type semiconductors with field effect mobility of up to 0.02?cm² V^-1 s^-1 under ambient conditions. Gang of four: This study demonstrates that N-heteroacenes can be extended through a four-membered ring into linear polycyclic frameworks with up to eight fused rings. It was found that the four-membered ring is a useful linker to extend electron delocalization for n-type organic semiconductors.

Full text of DOI:10.1002/chem.201600918

DOI: 10.1002/chem.201600918
Full Paper
&
Organic Semiconductors
Extension of N-Heteroacenes through a Four-Membered Ring
[
a]
[a]
[a]
[a, b]
Shuaijun Yang, Bowen Shan, Xiaomin Xu, and Qian Miao*
Abstract: The synthesis of novel p-extended N-hetero-
acenes, which have a large tetraazaacene subunit and a qui-
noxaline subunit connected through a four-membered ring,
is reported. They were studied with experimental and com-
putational methods in comparison to the corresponding tet-
raazaacenes. As found from the DFT calculation, the four-
membered ring is a better linker than a five-membered ring
or a CÀC single bond to extend N-heteroacenes for a new
design of n-type semiconductors in terms of the spatial de-
localization and energy level of LUMO as well as the reor-
ganization energy. In solution-processed thin film transistors,
the p-extended N-heteroacenes are found to function as n-
type semiconductors with field effect mobility of up to
2
À1 À1
0.02 cm V
s
under ambient conditions.
Introduction
N-heteroacenes have recently received considerable attention
with respect to synthesis, property evaluation, and applica-
[
1,2]
tions.
Most of the known N-heteroacenes are N-heteropen-
[
3]
tacenes, which are an important class of organic semiconduc-
tors exhibiting high performance in organic thin film transis-
[
4,5]
[6]
tors (OTFTs).
Similar to acenes, N-heteroacenes become
less stable and less soluble as their length increases. As
a result, N-heteroacenes larger than N-heteropentacenes are
rare, and the largest fully characterized N-heteroacene so far is
a diazaheptacene that was protected by four bulky tri(sec-bu-
[2]
tyl)silyl groups from fast dimerization. This becomes an ob-
stacle to development of organic semiconductors based on
larger N-heteroacenes, which can in principle have more exten-
sive electron delocalization and greater intermolecular p–p
overlap in the solid state. Here we demonstrate that N-hetero-
acenes can be extended through a four-membered ring into
linear polycyclic frameworks with up to eight fused rings. As
shown in Figure 1a, novel p-extended N-heteroacenes 1a and
Figure 1. a) Structures of p-extended N-heteroacenes (1a,b) and the corre-
sponding N-heteroacenes (2a,b); b) structures of related molecules.
1
b have a four-membered ring connecting a large tetraaza-
acene (tetraazatetracene 2a and tetraazapentacene 2b, re-
spectively) with quinoxaline, one of the smallest N-hetero-
acenes. Similar to the recently reported biphenylene-contain-
[N]phenylene, the latter of which formally has benzene rings
[7]
[8]
ing diazaacene 3 (Figure 1b), molecules 1a and 1b can be
fused to cyclobutadiene moieties in an alternating manner.
regarded as a result from combining N-heteroacene and
But unlike 3, molecules 1a,b contain a moiety of cyclobu-
[
9]
ta[1,2-b:3,4-b’]diquinoxaline (4 in Figure 1b) rather than bi-
phenylene. To better understand the effect of the four-mem-
bered ring in p-extension of N-heteroacenes, 2a,b, two unpre-
cedented tetraazaacenes were also synthesized as reference
compounds in this study. Detailed below is an exploratory
study on 1a,b with experimental and computational methods
in comparison to tetraazaacenes 2a,b with a focus on synthe-
sis, structures, and solution-processed OTFTs.
[
a] Dr. S. Yang, B. Shan, Dr. X. Xu, Prof. Dr. Q. Miao
Department of Chemistry
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong (P. R. China)
E-mail: miaoqian@cuhk.edu.hk
[
b] Prof. Dr. Q. Miao
Institute of Molecular Functional Materials
The University of Hong Kong
Pokfulam Road, Hong Kong (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600918.
Chem. Eur. J. 2016, 22, 6637 – 6642
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To better understand p-extension through the four-mem-
bered ring, the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) of 1a,b and
2a,b were studied in a comparative manner with both experi-
mental and computational methods. Figure 2a compares the
Results and Discussion
As shown in Scheme 1, compound 1a was synthesized by Pd-
[10]
catalyzed coupling of phenylenediamine 6a with 2,3-dibro-
mo-cyclobuta[1,2-b:3,4-b’]diquinoxaline (5), which was pre-
pared by modifying the recently reported synthesis of chlori-
[11]
nated cyclobuta[1,2-b:3,4-b’]diquinoxaline. Using 1,1’-bis(di-
phenylphosphino)ferrocene (dppf) as the ligand and Cs CO as
2
3
Figure 2. UV/Vis spectra for 0.01 mm solutions of 1a and 2a (a) and 1b and
2
b (b) in hexane.
Scheme 1. Synthesis of 1a,b. Reagents and conditions: a) Pd(PPh
Cs CO , toluene, 1208C; b) MnO , CH Cl
3 4
) , dppf,
2
3
2
2
2
.
UV/Vis absorption spectra of 1a and 2a, showing that 1a has
redshifted and more intense absorption relative to 2a in the
visible light region. Similarly, Figure 2b compares the UV/Vis
absorption spectra of 1b and 2b showing that the longest-
wavelength absorption of 1b shifts to red by 52 nm relative to
that of 2b. Such redshifted absorption of 1a and 1b relative
to the corresponding N-heteroacenes (2a and 2b, respectively)
is similar to the reported absorption spectra of phenylene-con-
taining oligoacenes, which are redshifted relative to the corre-
[
12]
the base, this Pd-catalyzed coupling reaction yielded 1a as
a mixture with its dihydro derivative 7a, which could be sepa-
rated into 1a and 7a in pure form. Oxidation of the mixture of
1
a and 7a with MnO resulted in pure 1a in a yield of 26%
2
from 5. In a similar manner, compound 1b was synthesized by
[
13]
Pd-catalyzed coupling of 5 with naphthalenediamine 6b,
which was prepared from 4,9-dibromonaphtho[2,3-c][1,2,5]thia-
[
14]
[15]
diazole, as detailed in the Supporting Information. However,
the Pd-catalyzed coupling reaction only yielded the dihydro
derivative 7b, which was then oxidized by MnO2 to 1b in
a high yield. As detailed in the Supporting Information, 2a and
sponding oligoacenes. In the test window of cyclic voltam-
metry, 1a,b and 2a,b all exhibited two quasi-reversible reduc-
tion waves (Figure S14 in the Supporting Information). Based
[
16]
on the first reduction potentials as well as the absorption
edges found from the UV/Vis absorption spectra, the LUMO
and HOMO energy levels of 1a,b and 2a,b are estimated and
summarized in Table 1. It is found that, from 2a to 1a, the
LUMO is lowered by 0.30 eV, whereas the HOMO is lowered by
only 0.13 eV. Similarly, from 2b to 1b, the LUMO is lowered by
0.20 eV, whereas the HOMO is lowered by only 0.08 eV.
2
b were synthesized by Pd-catalyzed coupling of the corre-
sponding diamines (6a and 6b, respectively) with 6,7-dibro-
moquinoxaline and subsequent oxidation with MnO2.
It was found that compounds 1a and 1b are stable toward
air and heating. After exposure to ambient light and air at
room temperature for 10 days, solutions of 1a and 1b in
CDCl did not exhibit any apparent change, as monitored with
The frontier molecular orbitals of 1a,b and 2a,b were calcu-
lated using simplified model molecules (1a’,b’ and 2a’,b’ in
Figure S15 in the Supporting Information), which have smaller
3
1
H NMR (Figures S1 and S2 in the Supporting Information).
Heating the solids of 1a and 1b at 2008C in air for 2 h did not
1
[17]
result in any chemical changes, as found from the H NMR
trimethylsilyl groups replacing the triisopropylsilyl groups.
spectra of the solutions of the heated solids (Figures S5 and S6
in the Supporting Information). In contrast, the reference com-
pounds 2a and 2b appeared to be unstable when their solids
were heated at 2008C in air for 2 h, as monitored with H NMR
in the same way, although they exhibited similar stability as 1a
and 1b in solution under ambient conditions.
The geometries of these model molecules were optimized at
the B3LYP level of density functional theory (DFT) with the 6-
31G(d, p) basis set, and the HOMO and LUMO were then calcu-
lated with the 6-311+ +G(d, p) basis set. As shown in Fig-
ure 3a, an interesting finding from the calculated orbitals is
that the LUMOs of 1a,b are fully delocalized in the polycyclic
1
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backbone while the HOMOs of 1a,b are localized on the tet-
raazaacene moieties. In agreement with this, the LUMO energy
levels of 1a and 1b are lower than 2a and 2b, respectively,
whereas the HOMO energy levels of 1a and 1b are essentially
the same as those of 2a and 2b, respectively. Moreover, the
calculated HOMO and LUMO energy levels of 1a,b are consis-
tent with the finding from the experimental values that from
Table 1. Absorption, reduction potentials, and frontier molecular orbital
energy levels of 1a,b and 2a,b.
[f]
Experimental
Calculated
LUMO HOMO Gap
[eV] [eV] [eV]
1
l
max
Gap
[eV]
E
red
[c]
LUMO
HOMO
[eV]
[a]
[b]
[d]
[e]
[nm]
[V]
[eV]
1
1
2
2
a
570
2.11
1.63
2.28
1.75
À0.77 À4.33
À0.68 À4.42
À1.07 À4.03
À0.88 À4.22
À6.44
À6.05
À6.31
À5.97
À3.89 À6.09 2.20
À3.97 À5.64 1.67
À3.61 À6.06 2.45
À3.79 À5.62 1.83
b 737
525
b 685
2
a to 1a and from 2b to 1b, the LUMO energy level is low-
a
ered by a larger degree than the HOMO energy level, leading
to the observed redshift of longest-wavelength absorption of
1a,b relative to 2a,b.
[
a] The maximum of the longest-wavelength absorption band (lmax) from
a 0.01 mm solution in hexane. [b] HOMO–LUMO gap as estimated from
the absorption edge from the same solution. [c] Half-wave potential
Similar to 1a,b, biphenylene-containing diazaacenes (e.g., 3)
were also reported to have the LUMO delocalized in the whole
polycyclic backbone but the HOMO localized on the diaza-
1
versus ferrocenium/ferrocene. [d] Estimated from LUMO=À5.10ÀEred
(
eV). [e] Calculated from the HOMO–LUMO gap and the LUMO energy
level. [f] Calculated at the B3LYP level of DFT with 6-311 + +G(d,p)//6-
1G(d,p) basis sets using simplified model molecules (1a’,b’ and 2a’,b’).
[
7]
3
acene moiety. To better understand the unequal distribution
of HOMO and LUMO of 1a,b, we calculated molecules 8a, 8b,
and 8c (Figure 3b), which have tetraazapentacene and qui-
noxaline linked together by a four-membered ring, a five-mem-
bered ring, and a CÀC single bond, respectively. As found from
the energy-minimized models (Figure S16 of the Supporting In-
formation), 8a is a planar molecule having two CÀC single
bonds of 1.495 Š long in the four-membered ring. The tetra-
azapentacene and quinoxaline subunits in 8b and 8c are in
the same plane but are linked by shorter CÀC bonds, as shown
with bold lines in Figure 3b. These calculated bond lengths
suggest that 8a has poorer conjugation between the tetraaza-
pentacene and quinoxaline subunits than 8b and 8c. It was
found that 8a–c all have their HOMOs localized on the tetra-
azapentacene moiety, as shown in Figure 3c, indicating that
the localized HOMO is due to the unsymmetrical molecular
structures rather than the linker. As shown in the same figure,
the LUMO of 8a is delocalized over the whole molecule and
evenly distributed on the tetraazapentacene and quinoxaline
moieties, whereas those of 8b and 8c are more localized on
the tetraazapentacene moiety. In agreement with the even dis-
tribution, the LUMO of 8a has a lower energy level than those
of 8b,c, as summarized in Figure 3c.
With greater spatial delocalization and lower energy level of
LUMO, 8a is a better candidate for n-type semiconductors
than 8b and 8c. With this idea in mind, we further calculated
the inner reorganization energy for electron transport (le),
which is a key parameter determining electron transfer rate in
[
18]
n-type organic semiconductors.
As the reorganization
energy for electron transport is due to geometric relaxation ac-
companying a self-exchange electron-transfer reaction be-
tween a neutral molecule and a neighboring radical anion,
a smaller reorganization energy leads to a larger electron mo-
[
19,20]
bility.
It is found that the reorganization energy of 8a
(
(
0.132 eV) is lower than those of 8b (0.148 eV), and 8c
0.173 eV). Moreover, the reorganization energy of 1b’ for elec-
tron transport (0.153 eV) is lower than that of 2b’ by 0.041 eV.
These reorganization energies for electron transport suggest
that the four-member ring is a promising linker to extend N-
heteroacenes for a new design of n-type organic semiconduc-
tors.
Single crystals of 1b selected for X-ray crystallography were
grown by slow diffusion of acetone vapor into a solution of
Figure 3. a) Calculated frontier molecular orbitals of 1a’ and 1b’; b) struc-
tures of 8a–c; c) calculated frontier molecular orbitals of 8a–c.
Chem. Eur. J. 2016, 22, 6637 – 6642
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direction molecules of 1b are arranged in a one-dimensional
face-to-face p-stack with head-to-tail overlap, which is similar
to the reported molecular packing of 1,4-silylethynyl-substitut-
[
24]
ed dihydrohexaazapentacene. It is found that neighboring
molecules of 1b stack with each other with two slightly differ-
ent arrangements alternating in one stack. One arrangement
has the two p-faces separated by 3.31 Š and the two silicon
atoms of neighboring molecules separated by 11.47 Š. In com-
parison, the other arrangement has two p-faces separated by
3
.34 Š and the two silicon atoms of neighboring molecules
separated by 7.34 Š, which is accompanied with a shift along
the long axis of the polycyclic backbone. The distances be-
tween the p-planes of 1b in the crystal are slightly smaller
than double the van der Waals radius (1.70 Š) of carbon.
To test semiconductor properties of 1a,b in OTFTs, their thin
films were dip-coated onto a silicon substrate, which had suc-
cessive layers of silica, alumina, and 12-cyclohexyldodecylphos-
phonic acid (CDPA, as shown in Figure 5a) as a composite di-
electric material. Here, CDPA formed a self-assembled monolay-
er (SAM) on alumina to passivate surface hydroxyl groups and
Figure 4. Crystal structure of 1b: a) the polycyclic backbone of 1b with
some atoms labeled and some bond lengths highlighted; b) p–p interac-
tions between molecules of 1b. (C and N atoms in the polycyclic backbone
are shown as ellipsoids at 50% probability and the substituents are shown
as sticks. H atoms are removed for clarification.)
[
21]
1
b in toluene. The crystal structure of 1b exhibits an essen-
tially flat backbone as shown in Figure 4a. Examination of the
bond lengths of 1b reveals that its central four-membered
ring contains four single CÀC bonds. Two of them (C14aÀC14b
and C8aÀC8b) are 1.50 Š long, which is even longer than that
2
of the typical nonconjugated single bond between two sp -hy-
[
22]
bridized C atoms (1.47–1.48 Š). Such long CÀC bonds are in
agreement with the calculated bond length of 8a, indicating
that these two bonds do not involve any p characters. This is
in agreement with the earlier conclusion that the p bonds in
Figure 5. a) Phosphonic acid that is used to modify the Al
surface in the OTFTs; b) drain current (IDS) versus gate voltage (V
voltage (VDS) at 30 V for OTFT of 1b with an active channel of W=1 mm and
L=150 mm as measured in air.
2
O
3
/SiO
2
dielectric
) with drain
G
[
N]phenylenes are localized within the six-membered
[
15,23]
rings.
The bond of C14bÀC8a, having a bond length of
to provide a highly ordered dielectric surface wettable by
[
25]
1
.46 Š, is longer than typical C=C double bonds in alkenes
1.31–1.34 Š) and typical CÀC 1.5 bonds in arenes (1.38–
.40 Š), but represents a conjugated single bond between two
common organic solvents and the resulting CDPA-Al O /SiO
2 3 2
À2 [26]
(
had a capacitance per unit area (Ci) of 26Æ1 nFcm . Thin
films of 1a and 1b were formed by immersing the CDPA-
Al O /SiO substrate into a solution (CH Cl /acetone, 3:1 by
1
2
sp -hybridized C atoms with a typical bond length of 1.45–
2
3
2
2
2
[
22]
1
.46 Š. Although slightly shorter than C14bÀC8a, the bond
volume for 1a and CH Cl for 1b) and then pulling it up with
2 2
À1
of C8bÀC14a still has a bond length (1.43 Š) close to that of
a constant speed of 1.7 mms . The dip-coated films of 1a and
1b consisted of aligned crystal fibers as shown in the polar-
ized-light micrograph in Figure S23 (see the Supporting Infor-
mation). The X-ray diffraction (XRD) from the film of 1a exhibit-
ed a diffraction at 2q=5.198 (d spacing=17.0 Š) accompanied
with two higher-order peaks with d spacing of 8.5 Š and 5.7 Š,
suggesting a lamellar structure. The dip-coated film of 1b ex-
hibited one peak at 2q=5.358 (d spacing=16.5 Š), which does
not correspond to any diffraction derived from the single crys-
tal structure, indicating a polymorph different from the single-
crystal phase. The fabrication of OTFTs was completed by
vacuum-deposition of gold on the dip-coated organic films
through a shadow mask to form top-contact source and drain
2
a typical conjugated single bond between two sp -hybridized
C atoms. The four-membered ring is attached to four N atoms
with four short CÀN bonds (1.28 Š for C8aÀN8 and C14bÀN15;
1
.30 Š for C8bÀN9 and C14aÀN14), which are shorter than typ-
[22]
ical C=N double bonds in imines (1.35 Š), as well as other
CÀN bonds in the same molecule. These bond lengths, similar
[11]
to the corresponding bond lengths in molecule 4, present
a radialene-like structure, presumably because 1b needs to
avoid an antiaromatic cyclobutadiene structure.
The crystal structure of 1b exhibits infinite p-stacks in two
directions that intersect with an angle of 68.08 (Figure S18 in
the Supporting Information). As shown in Figure 4b, in each
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electrodes. As measured from 30 channels in air, 1b functioned
as an n-type semiconductor with field effect mobility of
Acknowledgements
2
À1 À1
0
.015Æ0.004 cm V s . Shown in Figure 5b are the transfer I–
We thank Ms. Hoi Shan Chan (the Chinese University of Hong
Kong) for the single-crystal crystallography. This work was sup-
ported by the Research Grants Council of Hong Kong (GRF
V curves measured from a typical OTFT of 1b in its saturation
regime. By contrast, the films of 1a exhibited field-effect mobi-
lity of (1.5Æ0.7)”10À3 cm V s , which was one order of mag-
nitude lower than that of 1b. For comparison, thin films of 2a
and 2b were fabricated with the same method. It was found
that the film of 2b functioned as an n-type semiconductor
with field-effect mobility of (4.1Æ2.0)”10À3 cm V s as mea-
sured in air, whereas the film of 2a behaved as an insulator,
which can be attributed to the discontinuous crystal grains in
the film (Figure S23 in the Supporting Information).
2
À1 À1
4
02613) and the Areas of Excellence Scheme, University Grants
Committee of Hong Kong (project number: AoE/P-03/08).
Keywords: arenes · conjugation · organic thin film transistors ·
polycycles · semiconductors
2
À1 À1
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Conclusions
In summary, this study puts forth a new family of p-extended
N-heteroacenes by connecting a large tetraazaacene and qui-
noxaline through a four-membered ring. On the basis of the
crystal structure and DFT calculation, it is concluded that the p
bonds in 1a and 1b are localized within the six-membered
rings, in agreement with the earlier conclusion on [N]phenyl-
enes. Interestingly, 1a and 1b exhibit unequal distribution of
HOMO and LUMO, having their LUMOs delocalized in the
whole polycyclic backbone but their HOMOs localized on the
tetraazaacene subunit, owing to their unsymmetrical molecular
structures. As found from the DFT calculation, the four-mem-
bered ring is a better linker than a five-membered ring or a CÀ
C single bond to extend N-heteroacenes for a new design of
n-type semiconductors in terms of the spatial delocalization
and energy level of LUMO and the reorganization energy. In
agreement with this, 1a and 1b are found to function as n-
type organic semiconductors in solution-processed thin-film
transistors.
[
[
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[
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[
+
um/ferrocene (Fc /Fc) in the Fermi scale is À5.1 eV, which is calculated
on the basis of an approximation neglecting solvent effects using
a work function of 4.46 eV for the normal hydrogen electrode (NHE)
+
and an electrochemical potential of 0.64 V for (Fc /Fc) versus NHE, see:
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[
17] The same simplification was also applied to other silylethynylated N-
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Chem. Eur. J. 2016, 22, 6637 – 6642
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6641
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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[
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2 3 2
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from 100 Hz to 100 kHz from a metal–insulator–metal structure, where
vacuum-deposited gold (0.2 mm”1 mm) was the top electrode and
the highly doped silicon substrate was the bottom electrode. As taken
[
[
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at the lowest frequency (100 Hz), the Ci of CDPA-Al
2
O
3
/SiO
2
is 26Æ
1
8191.
À2
1
nFcm . For examples of measuring the capacitance of phosphonic
21] CCDC 1442380 and 1442381 contain the supplementary crystallograph-
ic data for 1b and 2a, respectively. These data are provided free of
charge by The Cambridge Crystallographic Data Centre.
acid-modified metal oxide dielectrics with the same method, see: a) O.
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[
[
[
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2, 7329–7332.
Received: February 26, 2016
Published online on March 30, 2016
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Chem. Eur. J. 2016, 22, 6637 – 6642
www.chemeurj.org
6642
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim