N-Phenylated N-Heteroacenes: Synthesis, Structures, and Properties

Article abstract of DOI:10.1002/cplu.201600465

Herein, two novel N-phenylated N-heteroacenes (1 and 2) are reported, along with details of their synthesis, structures, and properties. Compound 1 is a N-hetero analogue of rubrene, but differs from rubrene by having a bent backbone and behaving as an insulator in the solid state owing to the lack of π–π interactions. Compound 2 is a N-hetero analogue of 6,13-diphenylpentacene (DPP) with essentially the same molecular geometry and crystal packing as DPP, but exhibiting a field effect mobility higher than that of DPP by two orders of magnitude.

Full text of DOI:10.1002/cplu.201600465

Accepted Article
Title: N-Phenylated N-Heteroacenes: Synthesis, Structures and Properties
Authors: Xiao Gu; Bowen Shan; Zikai He; Qian Miao
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N-Phenylated N-Heteroacenes: Synthesis, Structures and
Properties
Xiao Gu, Bowen Shan, Zikai He§ and Qian Miao*
Abstract: Herein we report two novel N-phenylated N-
heteroacenes (1 and 2) detailing their synthesis, structures and
properties. 1 is a N-hetero analogue of rubrene, but differs from
rubrene by having a bent backbone and behaving as an
insulator in the solid state due to lack of π–π interactions. 2 is a
N-hetero analogue of 6,13-diphenylpentacene (DPP) with
essentially the same molecular geometry and crystal packing as
DPP, but exhibiting field effect mobility higher than that of DPP
by two orders of magnitude.
with phenylboronic acid resulted in 6,11-diphenyl-5,12-
diazatetracene (4). Nucleophilic addition of phenyl lithium to 4
yielded 5, which was used in the next step without purification.
[
17 ]
The Buchwald–Hartwig coupling reaction
of 5 with
iodobenzene using (t-Bu)3P as ligand yielded 1 as a yellow solid.
Similarly, the Buchwald–Hartwig coupling reaction of 6,13-
[18]
dihydro-6,13-diazapentacene (6)
with iodobenzene using 2-
dicyclohexylphosphino-2’,6’-diisopropoxybiphenyl (RuPhos) as
the ligand yielded 2 in a moderate yield.
Functionalization of acenes [1] and N-heteroacenes [2, 3, 4] with
various substituting groups has led to a family of organic
semiconductors. The most successful functional groups for this
strategy are (trialkylsilyl)ethynyl groups, which were first
[ 5 ]
introduced by Anthony to acenes
and later applied to N-
heteroacenes [6, 7] resulting in a few solution-processed organic
semiconductors with high field effect mobility in orgnaic thin film
transistors (OTFTs).[8] Unlike acenes, N-heteroacenes can be
functionalized by attaching substituents to not only C atoms but
also N atoms. However, N-functionalization has been less
explored than C-functionalization for N-heteroacenes. The
known N-functionalized N-heteroacenes were synthesized by
[9, 10]
Figure 1 Structures of N-phenylated N-heteroacenes 1 and 2 as well as their
hydrocarbon analogues.
direct N-alkylation of N-heteropentacenes
or by attaching
phenyl groups to N atoms in the synthetic precursors of N-
phenylated N-heteroacenes. [11] Here, we report synthesis of two
new members of N-phenylated N-heteroacenes, 1 and 2, by N-
phenylation of N-heteroacenes. As shown in Figure 1, 1 is a N-
hetero analogue of rubrene,
a
leading p-type organic
semiconductor with high field effect mobility of up to 40 cm2 V–1
s–1 as measured from single crystal transistors,[12, 13, 14] and 2 is a
N-hetero analogue of 6,13-diphenylpentacene (DPP), a p-type
organic semiconductor with low field mobility (10−5 cm2 V–1 s–1) in
thin films. [15] In the following study, we compare 1 and 2 with
their hydrocarbon analogues in terms of structures and
properties to better understand the structure-property
relationship.
Scheme 1 shows the synthesis of 1 starting from 6,11-
dibromo-5,12-diazatetracene (3).[16] The Suzuki coupling of 3
X. Gu, B. Shan, Z. He, Prof. Q. Miao
Department of Chemistry
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong, China
Fax: (+852)26035057
Scheme 1 Synthesis of 1 and 2.
E-mail: miaoqian@cuhk.edu.hk
Single crystals of 1 qualified for X-ray crystallography were
grown slow evaporation of n-hexane solution, while those of 2
§ Current address: School of Science, Harbin Institute of
Technology Shenzhen, HIT Campus of University Town of
Shenzhen, Shenzhen, China, 518055
were grown by sublimation in a physical vapor transport (PVT)
[20]
system. [19] Figure 2 compares the crystal structures of 1
and
rubrene. [21] As shown in Figure 2a and b, the crystal structure of
Supporting information for this article is given via a link at the end of
the document.
1 exhibits a significantly bent backbone with an angle of 142.7°
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between the naphthalene plane and the benzene plane (Figure
2b), in agreement with sp3 hybridized N atoms. The four C-N
bonds (shown in red in Figure 2a) in the dihydropyrazine ring
have bond lengths of 1.42 to 1.44 Å and are longer than the
corresponding C-N bonds in 2 (shown in red in Figure 3b) by 0.2
to 0.4 Å, suggesting poorer conjugation between the benzene
ring and the N atom in the tetracene backbone of 1. In contrast,
rubrene in the crystal has an essentially flat tetracene backbone,
which is surrounded with four substituting phenyl groups roughly
parallel to each other and roughly perpendicular to the tetracene
plane as shown in Figure 2c. Unlike the phenyl substituents in
rubrene, the two substituting phenyl rings on N atoms in 1 are
roughly perpendicular to each other. The different molecular
geometries of 1 and rubrene suggest that 1 has a more flexible
backbone than rubrene, in agreement with poor conjugation
between the benzene rings in the backbone. The observed
molecular geometry of 1 is a result of bending the backbone to
avoid repulsions between the neighboring phenyl substituents.
With the bent backbone and unsymmetrical arrangement of
phenyl substituents, 1 is chiral and its crystal lattice consists of a
pair of enantiomers, which pack in an alternate arrangement
without π–π interactions between the tetracene backbone.
Instead, as shown in Figure 2b, edge-to-face interactions are
found between a phenyl substituent and the naphthalene moiety
with a H-to-C contact (2.86 Å) shorter than the sum of van der
Waals radii of H and C atoms as shown in Figure 2b. In contrast,
rubrene exhibits a slipped π–π stacking between neighboring
tetracene backbones with a π-to-π distance of 3.72 Å as shown
in Figure 2d.
Figure 3a and b show the crystal structure of 2,[20] which has
an essentially flat pentacene backbone with slight bending along
its long axis, suggesting the two N atoms are sp2 hybridized.
Unlike 1, molecule 2 does not need to bend its backbone
because its phenyl substituents do not experience repulsions
from crowdedness. The four C-N bonds (shown in red in Figure
3b) in the central ring are 1.40 Å long. The shorter bond length
of these C-N bonds in comparison to those in 1 suggests
stronger conjugation between C and N atoms in the pentacene
backbone of 2. The two phenyl substituents of 2 are roughly
perpendicular to the pentacene plane with an angle of 85°
between the ideal pentacene plane and either of the phenyl
planes, and an angle of 10° between two phenyl rings. As shown
in Figure 3a, the pentacene backbone of 2 is arranged co-
facially but the long axes of the nearest neighboring acenes are
orthogonal, resulting in a columnar cage-like supramolecular
structure. The edge-to-face interactions between the phenyl
substituents and the pentacene backbone are responsible for
this type of molecular packing. It is found that 2 has a large π-to-
π distance of 4.9 Å as measured from the distance between the
two central rings of neighboring molecules. In comparison to this,
the (phenyl)edge-to-(pentacene)face interactions involve shorter
intermolecular distances of 3.7 to 3.8 Å as measured between C
atoms. The adjacent supramolecular columns of 2 contact with
each other with a C-to-C distance (3.36 Å) shorter than the sum
of van der Waals radii of two C atoms as shown in Figure 3b.
The molecular geometry and packing motif of 2 as described
above are essentially the same as those of DPP in the crystal,[15]
which are shown in Figure 3c for comparison. The cage-like
supramolecular structure of DPP has the same π-to-π distance
of 4.9 Å and similar phenyl-to-pentacene interactions with edge-
to-face contacts of about 3.8 Å as measured from C to C atoms.
Similar short C-to-C contacts of 3.34 Å are also found between
the supramolecular columns of DPP.
Figure 2 (a) Structure and (b) molecular packing of 1 in the single crystal; (c)
structure and (d) molecular packing of rubrene in the single crystal. [22] (C, N
atoms are shown as grey and blue ellipsoids, respectively, at 50% probability
level.)
Figure
3 (a) Crystal structure of 2 showing the columnar cage-like
supramolecular structure; (b) crystal structure of 2 showing the short C-to-C
contacts; (c) crystal structure of DPP.[23] (C and N atoms are shown as
ellipsoids at 50% probability level.)
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To contrast the isolated molecular structure with that
observed in the crystal, we conducted density functional theory
(DFT) calculation to optimize the molecular structures of 1 and 2
at the B3LYP level with the 6-31G(d,p) basis set. As shown in
Figure 4a, energy-minimized model of 1 is very similar to its
structure in the crystal having a distorted backbone bent at the N
atoms with a bending angle of 139.2° between the benzene and
naphthalene planes. The two phenyl groups attached to the N
atoms are roughly perpendicular to each other. As shown in
Figure 4b, the energy-minimized model of 2 is essentially the
same as its structure in the crystal except that the model has a
completely flat backbone with two phenyl groups on N atoms
perpendicular to the pentacene plane. In agreement with the
structures observed in the crystals, the four red C-N bonds in
the model of 1 (Figure 4a) are longer than those in the model of
2 (Figure 4b) by about 0.2 Å. As calculated at the B3LYP level of
DFT with the 6-311++G(d,p) basis set, the highest occupied
molecular orbital (HOMO) energy levels of 1 and 2 are –5.39 eV
and –4.85 eV, respectively, while the lowest unoccupied
molecular orbital (LUMO) energy levels of 1 and 2 are –1.51 eV
and –1.27 eV, respectively.
Stokes shift of 102 nm, while 2 exhibits a Stokes shift of 12 nm.
The much larger Stokes shift of 1 as well as its broad absorption
band without fine structures can be attributed to its flexible
structure, which allows the energy of the excited state to be
consumed by bending the tetracene backbone and rotating the
phenyl substituents on the N atoms. In contrast, the small
Stocks shift of 2 suggests its pentacene backbone is rigid.
Figure 5 UV-vis absorption and fluorescence spectra of 1 and 2 in solution
(1 × 10−5 M in CH2Cl2). (The fluorescence spectra of 1 and 2 were recorded
with excitation at 371 nm and 397 nm, respectively.)
Table 1 Absorption edge, oxidation potentials and energy levels of frontier
molecular orbital for 1, 2, and their hydrocarbon analogues.
Experimental
Calculated e
1
Optical
Eox
HOMO
LUMO
(eV) d
HOMO
(eV)
LUMO
(eV)
Gap (eV) a
(V) b
(eV) c
1
2
2.87
2.83
2.18
1.97
0.17
0.14
0.37
0.25
−5.27
−5.24
−5.47
−5.35
−2.40
−2.41
-3.29
-3.38
−5.39
−4.85
−5.03
−4.84
−1.51
−1.27
−2.46
−2.71
rubrene
DPP
a
Estimated from the absorption edge of the UV-vis absorption spectrum from
a solution in CH2Cl2 (1 × 10–5 M). Half-wave potential versus ferrocenium/
ferrocene. Estimated from HOMO = –5.10 – Eox (eV). Calculated from the
b
c
1
d
e
optical gap and the HOMO energy level. calculated at the B3LYP level of
Figure 4 Calculated structures of 1 (a) and 2 (b).
DFT with the 6-311++G(d,p) basis set.
The redox behaviours of
1 and 2 in solution were
Both 1 and 2 formed yellow solutions in CH2Cl2 exhibiting
strong blue fluorescence when excited with UV light. Figure 5
shows the absorption and emission spectra of 1 and 2 as
measured from their solutions in CH2Cl2 at the same
concentration. 1 exhibits the longest-wavelength absorption
maxima at 371 nm, which is blue shifted by 156 nm relative to
that of rubrene. 2 exhibits the longest-wavelength absorption
maxima at 421 nm, which is blue shifted by 177 nm relative to
that of DPP. The blue-shifted absorption of 1 and 2 relative to
their hydrocarbon analogues (rubrene and DPP, respectively)
are similar to the absorption of 6,13-dihydro-6,13-
diazapentacene, which is blue shifted relative to that of
pentacene by 157 nm.[24] As found from Figure 5, 1 exhibits a
investigated with cyclic voltammetry and compared with rubrene
and DPP, respectively. The cyclic voltammograms of 1, 2 and
DPP (Figure S2 in the Supporting Information) all exhibited two
quasi-reversible oxidation waves, while that of rubrene exhibited
one quasi-reversible oxidation wave and one irreversible
oxidation wave. Based on the first oxidation potentials
the absorption edges found from the UV-vis absorption spectra,
the HOMO and LUMO energy levels of 1, 2, rubrene and DPP
are estimated and summarized in Table 1. It is found that 1 and
2 have slightly higher HOMO energy levels and significantly
higher LUMO energy levels than their hydrocarbon analogues
(rubrene and DPP, respectively), in agreement with the fact that
1 and 2 are more electron-rich than rubrene and DPP,
[25]
and
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respectively. The DFT-calculated HOMO energy levels of 1 and
2 are in good agreement with the corresponding experimental
values, while the calculated LUMO energy levels are higher than
the corresponding experimental values by a larger degree
similar to the reported N-hetero acenes.[7]
Since 1 and 2 are N-hetero analogues of rubrene and DPP,
respectively, it is interesting to test whether 1 and 2 can also
function as semiconductors in thin films. Therefore, thin films of
1 and 2 were deposited by thermal evaporation under a high
vacuum onto silicon wafers, whose SiO2 dielectric surface was
lattice plane parallel to the surface. Analysis of the crystal
structure reveals that molecules of
2 adopt an edge-on
orientation with the pentacene plane perpendicular to the
substrate surface when the (110) plane is parallel to the
substrate surface. The AFM image for the film of 2 exhibits
columnar crystallites in agreement with the crystalline nature of
the films. Therefore the higher field effect mobility of 2 in
comparison to DPP can be attributed to the higher ordering of
molecules in the polycrystalline films of 2. The pathways for
charge transport in the films of 2 presumably include the π-π
stacks within the column of 2 despite the large π-to-π distance
(Figure 3a) and the short C-to-C contacts between the columns
(Figure 3b). However, it remains an unanswered question why 2
easily crystallizes in the vacuum-deposited films but its
hydrocarbon analogue, DPP, with essentially the same
molecular geometry and crystal packing cannot form crystalline
films by vacuum deposition.
In summary, two novel N-phenylated N-heteroacenes 1 and
2 were synthesized and fully characterized. 1 differs from its
hydrocarbon analogue, rubrene, by having a bent backbone and
lacking π–π interactions in the solid state. In agreement with this
structure, 1 behaves as an insulator in thin films. In contrast, 2
has essentially the same molecular geometry and crystal
packing as its hydrocarbon analogue, DPP. But unlike DPP, 2
crystallizes in vacuum-deposited films functioning as a p-type
semiconductor with hole mobility higher than that of DPP by two
orders of magnitude.
pre-treated with
octadecyltrimethoxysilane (OTMS).
a
self-assembled monolayer (SAM) of
[ 26 ]
The device fabrication
was completed by depositing a layer of gold on the organic films
through a shadow mask to form top-contact source and drain
electrodes. The resulting devices had highly doped silicon as the
gate electrode and a 300 nm-thick layer of SiO2 as dielectrics.
Not surprisingly, the films of 1 behaved as an insulator in
agreement with the lack of π–π interactions in the solid state. In
contrast, 2 functioned as p-type semiconductors with a field
effect mobility of 0.012 ± 0.003 cm2 V–1 s–1 as measured in
ambient air from at least 20 channels. The highest mobility of 2
is 0.02 cm2 V–1 s–1 as extracted from the transfer I-V curve shown
in Figure 6, using the equation: IDS = (µWCi/2L)(VGS – Vth)2,
where IDS is the drain current, µ is the field-effect mobility, Ci is
the capacitance per unit area for the OTMS-modified SiO2
dielectric, W is the channel width, L is the channel length, VGS
and Vth are the gate and threshold voltage, respectively. The
mobility of 2 is higher than the reported mobility of DPP (8×10–5
[15]
cm2 V–1 s–1) in amorphous vacuum-deposited films
orders of magnitude.
by two
Acknowledgements
We thank Ms. Hoi Shan Chan (the Chinese University of Hong
Kong) for the single crystal crystallography. This work was
supported by the Research Grants Council of Hong Kong
(GRF402613).
Keywords: arenes • polycycles • N-heterocycles • organic
semiconductors
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Herein we report synthesis of two
novel N-phenylated N-heteroacenes,
and compare them with their
Xiao Gu, Bowen Shan, Zikai He and
Qian Miao*
Page No. – Page No.
hydrocarbon analogues, rubrene and
6,13-diphenylpentacene, in terms of
structures and properties.
N-Phenylated N-Heteroacenes:
Synthesis, Structures and Properties
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