Revisiting zethrene: Synthesis, reactivity and semiconductor properties

Article abstract of DOI:10.1039/c3sc51158h

Zethrene, a unique polycyclic aromatic hydrocarbon with formally fixed C-C double bonds, is predicted to have interesting properties and potential applications as an optical and electronic material. Here we report a novel synthesis of zethrene with improved yield, which presumably involves dinaphtho[10]-annulene as an unstable intermediate. With this convenient access to zethrene, we used zethrene as a p-type semiconductor in thin film transistors for the first time. It is found that Diels-Alder addition to the bay region of zethrene leads to new derivatives of benzo[pqr]naphtho[8,1,2-bcd]perylene, which behave as n-type organic semiconductors.

Full text of DOI:10.1039/c3sc51158h

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Revisiting zethrene: synthesis, reactivity and
semiconductor properties†
Cite this: DOI: 10.1039/c3sc51158h
a
a
a
a
ab
*
Liang Shan, Zhixiong Liang, Xiaomin Xu, Qin Tang and Qian Miao
Zethrene, a unique polycyclic aromatic hydrocarbon with formally fixed C–C double bonds, is predicted to
have interesting properties and potential applications as an optical and electronic material. Here we report
a novel synthesis of zethrene with improved yield, which presumably involves dinaphtho[10]-annulene as
an unstable intermediate. With this convenient access to zethrene, we used zethrene as a p-type
semiconductor in thin film transistors for the first time. It is found that Diels–Alder addition to the bay
region of zethrene leads to new derivatives of benzo[pqr]naphtho[8,1,2-bcd]perylene, which behave as
n-type organic semiconductors.
Received 29th April 2013
Accepted 7th June 2013
DOI: 10.1039/c3sc51158h
www.rsc.org/chemicalscience
similarity of zethrene to tetracene and pentacene, two well-
known organic semiconductors, zethrene and its derivatives
Introduction
are also expected to function as organic semiconductors.5
These interesting applications were proposed mainly on the
basis of computational or theoretical studies,6 but have
remained experimentally unexplored, mainly because of the
difficulty involved in the synthesis of zethrene. Here we report
a novel synthesis of zethrene, which led to new ndings
regarding the reactivity of zethrene and fabrication of thin lm
transistors of zethrene for the rst time. It was found that
zethrene functions as a p-type semiconductor in organic thin
lm transistors (OTFTs) with a eld effect mobility of up to
Zethrene is a polycyclic aromatic hydrocarbon (PAH) that is
characterized by its formally xed C–C double bonds as shown
in Fig. 1a. Early interest in zethrene was mainly associated
with its unique structure in terms of aromaticity, while the
recently revived interest in zethrene and its derivatives has
arisen due to their potential applications as functional mate-
rials.1,2 As suggested by their theoretically predicted second-
order hyperpolarizability3 and singlet diradical character,4
zethrene and its derivatives may function as nonlinear optical
materials and near-infrared dyes.1,2 In view of the structural
0.05 cm2 Vꢀ1 sꢀ1
.
The rst synthesis of zethrene was reported by Clar and co-
workers from chrysene through an inefficient route in 1955.7
A more convenient route to zethrene via cross-coupling of
Cu(I) acetylides and iodoarenes was later reported by Staab8
and Sondheimer9 during their attempted syntheses of tetra-
dehydrodinaphtho[10]annulene (1 as shown in Fig. 1b), which
was suggested to form zethrene through spontaneous trans-
annular cyclization. However, this mechanism remains
unproven,1,10 and the overall yields of zethrene in these
syntheses were relatively low.11 In close relation to the trans-
formation of 1 to zethrene, a series of zethrene derivatives
were recently synthesized via Pd-catalyzed coupling reactions
of alkynes,5,12,13 which nevertheless did not improve the yield
of zethrene itself. Another documented synthetic precursor of
zethrene is dinaphtho[10]annulene (2 as shown in Fig. 1b),
which was reported to result in zethrene through 7,14-dihy-
drozethrene when oxidized in air.14 However, 2 was prepared
via a Wittig reaction in very low yield (2%),14 and the identity
of 2 appears controversial since a compound with the same
spectroscopic properties was later assigned a structure of
7,7a-dihydrozethrene.15
Fig. 1 Molecular structures of (a) zethrene with the fixed double bonds high-
lighted and (b) two reported synthetic precursors of zethrene.
aDepartment of Chemistry, The Chinese University of Hong Kong, Shatin, New
Territories, Hong Kong, China. E-mail: miaoqian@cuhk.edu.hk
bInstitute of Molecular Functional Materials, Areas of Excellence Scheme, University
Grants Committee, Hong Kong, China
† Electronic supplementary information (ESI) available: Details of synthesis and
characterization, crystallographic information le (CIF) for zethrene, fabrication
and characterization of organic thin lm transistors, NMR spectra. CCDC
924397. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3sc51158h
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double bonds, in agreement with the recent nding that cata-
lytic hydrogenation of zethrene occurs not on the xed double
bonds but on the naphthalene rings.13 As shown in Scheme 2,
heating zethrene with N-alkyl maleimide at reux in toluene led
to an adduct (7a or 7b), which was obtained as a mixture with
the corresponding oxidized product (8a or 8b, respectively). This
Diels–Alder reaction employs zethrene as the diene with one of
its xed double bonds and one of its naphthalene ring involved.
Oxidation of 7a/b with DDQ yielded 8a/b leading to p-expansion
of zethrene. Further reaction of 8a/b with an excess of the cor-
responding N-alkyl maleimide in reuxing diphenyl ether
directly led to diimide 9a/b, possibly because the Diels–Alder
adduct was oxidized by air. With two electron-withdrawing
imide moieties attached to the large p-face of benzo[pqr]naph-
tho[8,1,2-bcd]perylene, 9a and 9b can be considered as potential
n-type organic semiconductors.
Results and discussion
Inspired by the reported conversion of 2 to zethrene,14 we
designed a new synthesis of zethrene, which presumably
involves 2 as an unstable intermediate. As shown in Scheme 1a,
this synthesis involves a Wittig reaction followed by a Heck
reaction to link two naphthalene moieties together. The Wittig
reaction has bis(triphenylphosphonium) 316 reacting with two
aldehydes, subsequently leading to diene 5. It is particularly
worth noting that more than one equivalent of palladium
acetate is necessary in the reaction of 5 to achieve a good yield of
zethrene. When a catalytic amount (10%) of palladium acetate
was applied in this reaction, zethrene was isolated in a low yield
of 7%, possibly because annulene 2 either deactivated Pd(0) by
strong coordination or consumed Pd(II) during its conversion to
zethrene. In comparison, an alternative approach to zethrene
from 1,8-divinylnaphthalene17 and 1,8-diiodonaphthalene18 led
to zethrene in a much lower yield (10%) under the same
conditions as shown in Scheme 1b. When reacting under typical
Heck reaction conditions with a catalytic amount of palladium
acetate, the two starting materials yielded compound 6 instead
of zethrene.
Single crystals of zethrene suitable for X-ray crystallography
were grown from solutions in chloroform. The crystal structure
of zethrene is shown in Fig. 2, and it conrms the formally xed
C–C double bonds and reveals p-stacking. As shown in red in
Fig. 2a, the C7–C7a and C14–C14a bonds have a bond length of
˚
˚
1.35 A, which is very close to the typical length (1.31–1.34 A) for
a C(sp2)–C(sp2) double bond in alkenes, but shorter than the
typical length (1.38–1.40 A) for a C(sp )–C(sp ) aromatic bond in
arenes.19 The C7a–C14a bond shown in green has a bond length
of 1.46 A, which is typical for a single bond between two sp
carbon atoms (1.45–1.46 A). These bond lengths are in
The reactivity of zethrene was studied in terms of bromina-
tion and Diels–Alder reactions. Bromination of zethrene under
different conditions, such as Br2, H2O2/HBr and NBS, was found
to result in formation of a complex mixture of products, while
brominated products due to simple addition on the central
double bonds were not isolated. This indicates that the reac-
tivity of zethrene is not only governed by the formally xed
2
2
˚
2
˚
20
˚
agreement with the xed C–C double bonds in zethrene. Unlike
pentacene and tetracene, which exhibit well-known herring-
bone packing, zethrene forms offset p-stacks in two directions
as illustrated in Fig. 2b. The p-to-p distance between two
˚
parallel neighbours within a stack is 3.45 A, and the angle
between p-planes in neighboring stacks is 78.6ꢁ. Such molec-
ular packing of zethrene is also known as the g-type packing of
PAHs, which arises from the increased relative importance of
C/C interactions over C/H interactions according to Desiraju
and Gavezzotti's predictive model.20 Unlike the g-type packing,
Scheme 1 (a) Synthesis of zethrene; (b) attempted synthesis of zethrene from
1,8-divinylnaphthalene.
Scheme 2 Diels–Alder reactions of zethrene and p-expansion.
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estimated from the electrochemical potentials. The cyclic vol-
tammogram of 9a from a solution in dichloromethane (shown
in ESI†) exhibited two reversible reduction waves with half-wave
reduction potentials at ꢀ1.52 V and ꢀ1.73 V versus ferrocenium/
ferrocene. From the rst reduction wave, the LUMO energy level
of 9a is estimated as ꢀ3.28 eV. This value, to our surprise, is
much higher than the reported LUMO energy levels of zethrene
diimide (ꢀ3.96 eV),12 naphthalene diimides (about ꢀ3.8 eV) and
perylene diimides (about ꢀ3.8 eV).22 A solution of 9a in
dichloromethane exhibited an absorption edge at 561 nm cor-
responding to a HOMO–LUMO energy gap of 2.21 eV (shown in
ESI†). The relatively high HOMO energy level along with the g-
type p-packing suggests that zethrene is a promising candidate
as a p-type semiconductor for OTFTs.23 In contrast, the LUMO of
9a is not sufficiently low to promise a high performance in n-
channel OTFTs.
Fig. 2 (a) Structure of zethrene in the single crystal with carbon atom positions
shown as 50% probability ellipsoids; (b) p-stacking of zethrene in the single crystal.
simple herringbone packing has nearest neighbouring PAH
molecules nonparallel to each other leading to more C/H
To test the semiconductor properties of zethrene and dii-
mide 9a in OTFTs, thin lms were deposited by thermal evap-
oration under high vacuum onto silicon wafers, whose SiO2
dielectric surface was pre-treated with a self-assembled mono-
layer of octadecyltrimethoxysilane.24 When the substrate
temperature was kept at 60 ꢁC during deposition, the resulting
lms of zethrene were polycrystalline, exhibiting a polymorph
different from the bulk crystals as found from X-ray diffraction
(XRD).25 Atomic force microscopy (AFM) images of the lms of
zethrene exhibited a terraced morphology that is typical for
vacuum-deposited lms of organic semiconductors. The device
fabrication was completed by depositing a layer of gold onto the
zethrene lms 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. As measured from these devices, zethrene
functioned as a p-type organic semiconductor with eld effect
mobility in the range of 0.01 to 0.05 cm2 Vꢀ1 sꢀ1. Fig. 3 shows
typical transfer I–V curves for the best-performing OTFT of
zethrene as tested in air. When measured under vacuum, the
OTFTs of 9a exhibited an n-channel eld effect with electron
mobility of up to 2 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1 and a threshold voltage
larger than 40 V. In contrast, the lms of 9a appeared insulating
when measured in ambient air because of electron trapping by
oxygen and water. The low air stability and large threshold
voltage of 9a can be attributed to its high-lying LUMO. More-
over, the high-lying LUMO leads to high-energy mobile elec-
trons in the lm of 9a, which are easily trapped by defects. Such
electron trapping can in principle make the measured electron
mobility much lower than the intrinsic value.
interactions throughout the crystal lattice. Zethrene (C24H14
)
adopts g-type packing rather than herringbone packing because
it, in comparison to pentacene (C22H14), has an increased
number of carbon atoms relative to hydrogen atoms and
furthermore, the additional carbon atoms are core atoms that
promote face-to-face p-stacking.
Besides molecular packing in the solid state, the energy level
of frontier molecular orbital is another key factor governing
semiconductor properties of conjugated molecules. Therefore
zethrene and diimide 9a were investigated using cyclic vol-
tammetry and UV-vis absorption spectroscopy to determine the
energy levels of highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO). The cyclic
voltammogram of zethrene as measured from a solution in
dichloromethane exhibited a reversible reduction peak at ꢀ1.79
V and a quasi-reversible oxidation peak at 0.27 V versus ferro-
cenium/ferrocene as shown in ESI.† From these potentials, the
energy levels of HOMO and LUMO for zethrene are estimated as
ꢀ5.07 eV and ꢀ3.01 eV, respectively.21 The UV-vis absorption
spectrum of zethrene (shown in ESI†) was recorded using a
solution in chloroform and revealed a HOMO–LUMO energy
gap of 2.16 eV based on the absorption edge at 575 nm. This gap
is in agreement with the HOMO and LUMO energy levels
Conclusions
In summary, the above studies demonstrate not only a new
synthesis of zethrene with improved yield, but also for the rst
time that zethrene functions as a p-type semiconductor in thin
lm transistors. Moreover, Diels–Alder addition to the bay
region of zethrene is found to allow p-expansion leading
to new derivatives of benzo[pqr]naphtho[8,1,2-bcd]perylene,
which behave as n-type organic semiconductors in thin lm
transistors.
Fig. 3 Drain current (IDS) versus gate voltage (VG) with drain voltage (VDS
)
at ꢀ50 V for the best-performing OTFT of zethrene with an active channel of W ¼
1 mm and L ¼ 150 mm as measured in air.
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12 Z. Sun, K.-W. Huang and J. Wu, Org. Lett., 2010, 12,
4690–4693.
Acknowledgements
We thank Ms Hoi Shan Chan (The Chinese University of Hong
Kong) for the X-ray crystallography. This work was supported by
a grant from the University Grants Committee of the Hong Kong
Special Administrative Region, China (Project no. AoE/P-03/08).
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10 Tetradehydrodinaphtho[10]-annulene (1) was isolated
without any visible decomposition at room temperature in
2009 but direct transformation of 1 to zethrene was not
observed. See: ref. 5.
11 The yields of zethrene from cross-coupling of Cu(I) acetylides
and iodoarenes were 50ꢃ52% and 22% as reported by Staab
and Sondheimer, respectively. In our own experiments to
prepare zethrene following Staab's procedure, the best
yield of zethrene was 28% before crystallization, and lower
than 10% aer crystallization.
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23 As indicated by the study of Bao and coworkers on a series of
functionalized acene derivatives, the HOMO energy level of a
semiconductor molecule should be ꢀ5.60 eV or higher to
allow the p-channel eld effect, and the LUMO energy level
of a semiconductor molecule should be ꢀ3.15 eV or lower
to allow the n-channel eld effect when gold is used as the
metal for drain and source electrodes in organic thin lm
transistors. See: M. L. Tang, A. D. Reichardt, P. Wei and
Z. Bao, J. Am. Chem. Soc., 2009, 131, 5264–5273.
24 Y. Ito, A. A. Virkar, S. Mannsfeld, J. H. Oh, M. Toney,
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9404.
25 XRD of the lm of zethrene shows four peaks at 2q ¼ 7.08ꢁ
ꢁ
˚
˚
(d-spacing: 12.48 A), 2q ¼ 14.42 (d-spacing: 6.14 A), 2q ¼
21.76ꢁ (d-spacing: 4.08 A) and 2q ¼ 28.27 (d-spacing:
ꢁ
˚
˚
3.15 A). In comparison, the (100) diffraction derived from
the single crystal structure of zethrene appears at 2q ¼
ꢁ
˚
7.58 (d-spacing: 11.65 A).
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