Z-Shaped Pentaleno-Acene Dimers with High Stability and Small Band Gap

Article abstract of DOI:10.1002/anie.201508919

Acene-based materials have promising applications for organic electronics but the major constrain comes from their poor stability. Herein a new strategy to stabilize reactive acenes, by fusion of an anti-aromatic pentalene unit onto the zigzag edges of two acene units to form a Z-shaped acene dimer, is introduced. The Z-shaped acene dimers are extremely stable and show a small energy gap resulting from intramolecular donor-acceptor interactions. X-ray crystallographic analysis revealed their unique geometry and one-dimensional slip-stack columnar structure. Besides optical and electrochemical characterizations, solution-processed field-effect transistors were also fabricated.

Full text of DOI:10.1002/anie.201508919

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201508919
German Edition: DOI: 10.1002/ange.201508919
Acenes
Z-Shaped Pentaleno-Acene Dimers with High Stability and Small Band
Gap
Gaole Dai, Jingjing Chang, Jie Luo, Shaoqiang Dong, Naoki Aratani, Bin Zheng,
Kuo-Wei Huang, Hiroko Yamada, and Chunyan Chi*
Abstract: Acene-based materials have promising applications
for organic electronics but the major constrain comes from
their poor stability. Herein a new strategy to stabilize reactive
acenes, by fusion of an anti-aromatic pentalene unit onto the
zigzag edges of two acene units to form a Z-shaped acene
dimer, is introduced. The Z-shaped acene dimers are extremely
stable and show a small energy gap resulting from intra-
molecular donor–acceptor interactions. X-ray crystallographic
analysis revealed their unique geometry and one-dimensional
slip-stack columnar structure. Besides optical and electro-
chemical characterizations, solution-processed field-effect
transistors were also fabricated.
A
cenes (A, Figure 1) have attracted a long-term interest
because of their unique electronic structure and physical
properties, and promising applications for organic electron-
[
1]
ics. However, the intrinsic high reactivity arising from their
high-lying HOMO energy levels limited their practical
Figure 1. Three strategies towards stable five-membered ring fused
acenes.
[
2]
applications. In recent years, different strategies have been
developed to decrease the HOMO energy level and stable
acenes, up to nonacene, are now accesible.
[5b–c,6]
transistors (OFETs).
However, we believe that a more
[
2–3]
Incorporation
efficient way for stabilization is to fuse the pentalene unit to
the zigzag edges of acene molecules because the HOMO
coefficients of acenes mainly localize at the zigzag edges, and
thus we hypothesized that Z-shaped pentaleno-acene dimers
(D) would be stable and show dramatically different proper-
ties from their respective acene monomers. In addition, the
dimers could have a “fixed” double bond between the five-
membered rings, and would thus be different from any other
reported p-conjugated acene dimers. In this context, herein
we report our synthetic efforts towards these types of
interesting but challenging molecules, their unique electronic
and optical properties, their ground-state geometry, and their
potential applications for OFETs.
of a five-membered ring into a polycyclic aromatic hydro-
carbon framework has been demonstrated to be an efficient
[4]
way to approach more-stable materials, and this concept was
[5]
recently extended to acene systems by us. For example, we
found that the fusion of two five-membered rings onto the
zigzag edges of acene molecules resulted in very stable acenes
having a small band gap (B) as a result of an intramolecular
donor–acceptor interaction, as well as kinetic blocking of the
[7]
[5a]
reactive zigzag edges.
Alternatively, fusion of an anti-
aromatic pentalene unit along the long axis of an acene (C) is
another way to approach making stable acene-based materi-
als for applications such as ambipolar organic field effect
The synthesis of the parent pentaleno-anthracene dimer
D, (n = 1; Figure 1) was attempted by Clar but failed because
[
*] Dr. G. Dai, Dr. J. Chang, Dr. S. Dong, Prof. C. Chi
Department of Chemistry, National University of Singapore
[
8]
of the poor stability and solubility of the product. Therefore,
in our design, bulky trialkylsilylethynyl substituents are
strategically attached to outer zigzag edges to ensure suffi-
cient solubility and stability (Scheme 1). The key intermedi-
ates towards the target pentaleno-anthracene 1 and tetracene
dimer 2a,b are the corresponding quinones 7 and 11. Our
main synthetic strategy involved the synthesis of a dibenzo-
pentalene intermediate, carrying two aryl ester substituents,
3
Science Drive 3, 117543 (Singapore)
E-mail: chmcc@nus.edu.sg
Dr. J. Luo
Institute of Materials Research and Engineering, A*STAR
3
Research Link, 117602 (Singapore)
Prof. N. Aratani, Prof. H. Yamada
Graduate School of Materials Science, Nara Institute of Science and
Technology (NAIST), Ikoma 630-0192 (Japan)
[
9]
by a palladium-catalyzed cyclodimerization reaction fol-
lowed by hydrolysis and a regioselective Friedel–Crafts
acylation reaction. Sonogashira coupling between 1-bromo-
2-ethynylbenzene (3) and methyl 2-iodobenzoate (4) gave the
Dr. B. Zheng, Prof. K.-W. Huang
KAUST Catalysis Center and Division of Physical Sciences &
Engineering, King Abdullah University of Science and Technology
(
KAUST), Thuwal 23955-6900 (Saudi Arabia)
ortho-bromo tolane derivative. Then [Pd (dba) ]-catalyzed
cyclodimerization of 5 provided the dibenzopentalene diester
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508919.
2
3
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Scheme 1. Reagents and conditions: a) [Pd(PPh ) Cl ], CuI, Et N/THF (or DMF), RT; b) [Pd (dba) ], P(2-furyl) , Cs CO , CsF, hydroquinone, 1,4-
3
2
2
3
2
3
3
2
3
dioxane, 1358C; c) 1. LiOH, dioxane/H O, reflux; 2. SOCl , DCM; 3. AlCl , DCM, RT; d) 1. R SiCCLi, THF; 2. SnCl , HCl (aq.) dba=dibenzylidene-
2
2
3
3
2
acetone, DCM=dichloromethane, DMF=N,N-dimethylformamide, THF=tetrahydrofuran.
6
. The ester groups in 6 were hydrolyzed to give the diacid and
longest absorption maximum wavelength l = 556 nm, Fig-
max
[5b]
then converted into the acyl chloride by reaction with thionyl
chloride. Subsequent AlCl -catalyzed intramolecular Friedel–
Crafts acylation selectively afforded the desired pentaleno-
bis(anthracenequinone) 7 rather than an isomer containing
a seven-membered ring. Nucleophilic addition of 7 with
triisopropylsilylethynyl (TIPS) lithium and subsequent reduc-
tion of the resulting diol with SnCl gave the target 1. Following
a similar strategy, the pentaleno-bis(tetracenequinone) 11 was
obtained from 8
dimer 2a was obtained but it has poor solubility, thus more
bulky triisobutylsilylethynyl (TIBS) groups were introduced.
The obtained dimer 2b has good solubility in common organic
solvents such as THF, chloroform, and toluene. Similarly,
starting from an anthracene analogue of 5 and 9, a pentaleno-
bis(pentacenequinone) 19 was also synthesized (see Scheme S1
in the Supporting Information). However, the subsequent
nucleophilic addition reactions with various trialkylsilylethynyl
lithium (or magnesium bromide) and reduction all failed to
give any desired pentacene dimers because of the poor
solubility of the quinone. Anyway, the soluble and stable
ure S1a). An intense absorption band (band III) at short
wavelength (250–400 nm, lmax = 252 nm) was also observed.
Time-dependent density functional theory (TDDFT) calcu-
lations (B3LYP/6-31G*; see the Supporting Information for
details) indicate that band I originated from the HOMO!
LUMO transition (l=816.5 nm, oscillator strength f =0.2554),
while band II mainly comes from the HOMOÀ1!LUMO
transition (l=567.8 nm, f=0.4741). Band III is a combination
of multiple HOMOÀn!LUMO+m transitions. Interestingly,
calculations also show that both the HOMO and LUMO of
1 are delocalized along the Z-shaped p-conjugated framework
while the distribution is different with some disjointed features
(Figure 3), thus indicating an intramolecular donor–acceptor
interaction which can be correlated to the broad absorption at
long-wavelength (Figure 2a).
3
2
[10]
and 3. The TIPS-substituted tetracene
The compound 2b displays a green color in THF and its
absorption spectrum is different from that of the 5,12-bis-
TIPS-substituted tetracene monomer (TIPS-Tetracene) but
similar to that of 1, with a significant red-shift of each band
because of the extended p-conjugation (Figure 2; see Fig-
ure S1b). In particular, the shoulder of the longest-wave-
length absorption band (band I) is red-shifted by 155 nm, and
bands II and III are red-shifted by 98 and 33 nm, respectively.
The characteristics of these bands are similar to those for 1,
HOMO!LUMO transition (l = 1009.7 nm, f = 0.3197) for
band I and HOMOÀ2!LUMO transition (l = 655.7 nm, f =
1
and 2b allowed full characterizations and comparisons with
the respective monomers and other types of dimers.
The compound 1 displays a purple-pink color in THF and
shows a distinctively different UV-vis-NIR absorption spec-
trum (Figure 2a) from that of the 9,10-bis-TIPS-substituted
anthracene monomer (TIPS-Anthracene; see Figure S1a). In
addition to a well-resolved band with three peaks at l = 490,
opt
0.2354) for band II. The optical energy gap (Eg ) was
5
26, and 567 nm (band II), similar to the p-band of acenes
determined as 1.41 and 1.17 eV for 1 and 2b, respectively.
In addition, UV-vis-NIR absorption spectral measurements in
various solvents with different polarity did not show obvious
changes (see Figure S2), thus indicating a negligible intra-
molecular charger-transfer character.
(
Figure 2a), a broad band with three peaks at l = 694, 750,
and 831 nm (band I) is observed, and it is significantly red-
shifted compared to that of the pentalene-bridged anthracene
dimer analogue (C, n = 3, see derivative DAP1 with the
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ox
at 0.01, 0.29, and 0.67 V, and two reversible reduction waves
2
red
with E1/2 at À1.13 and À1.61 V. The HOMO/LUMO energy
levels are determined to be À4.95/À3.62 and À4.69/À3.75 eV
for 1 and 2b, respectively, from the onset of the first oxidation/
reduction wave. The corresponding electrochemical energy
EC
gaps (Eg ) were then estimated to be 1.33 eV for 1 and 0.94 eV
for 2b, and are consistent with their optical energy band gaps.
The photo-oxidative decay of 1 and 2b, and their
monomer counter parts TIPS-Anthracene and TIPS-Tetra-
cene, was monitored by UV-vis absorption spectroscopy in
À5
THF solutions (2 ” 10 m) under the same ambient light and
air conditions (see Figure S8). TIPS-Anthracene and TIPS-
Tetracene solutions are stable with a half-life time (t ) of 344
1/2
and 195 h, respectively. The compounds 1 and 2b both
exhibited high photostability, with a t1/2 of 221 h for 1 and
5
4 days for 2b, which are also much more stable than the
[7a]
perifused anthracene dimer [bis(anthene)] and tetracene
dimer [bis(tetracene)] analogues.
[7d]
The largely enhanced
photostability of 1 and 2b can be ascribed to: 1) the
introduction of an electron-deficient pentalene unit dramat-
ically lowers the HOMO energy levels in comparison to the
perifused bis(anthene) and peritetracene (see Figure S3); and
[11]
2
) kinetic stabilization by the TIPS groups and the five-
membered rings at the zigzag edge.
Single crystals of 1 and 2b suitable for X-ray crystallo-
graphic analysis were obtained by slow diffusion of either
[12]
methanol or acetonitrile into a THF solution. Both 1 and
b have a rigid planar p-conjugated skeleton (Figure 4). The
2
central carbon–carbon bond between the two five-membered
rings in 1 has a bond length of 1.384 Š, thus indicating
a significant double-bond character as predicted. The size and
quality of the single crystals of 2b were not good enough to
allow us to do an accurate bond length analysis. Interestingly,
both molecules show a slip-stack columnar structure through
close p–p interactions [3.294(6) Š in 1 and 3.271(4) Š in 2b]
and the neighboring molecules in the column adopt an
alternating orthogonal arrangement to suppress steric repul-
sion between the bulky substituents.
Figure 2. a) UV-vis-NIR absorption spectra of 1 and 2b recorded in
THF (1”10 m). Insert: photos of the solutions. b) Cyclic voltammo-
À5
grams of 1 and 2b in DCM with 0.1m Bu NPF as the supporting
4
6
electrolyte, Ag/AgCl as the reference electrode, Pt wire as the counter
À1
electrode, and a scan rate at 50 mVs . The electrode potential was
+
externally calibrated by Fc /Fc couple.
Considering the ordered packing, OFETs of 1 and 2b
were fabricated by solution processed thin films using
a bottom-gate top-contact device structure, and all devices
measured in N exhibited p-type behavior (see Figure S9).
2
Under optimal reaction conditions, a hole mobility (m ) of
h
À3
2
À1 À1
6
.0 ” 10 cm V s , threshold voltage (V ) of À4 V, and
T
5
current on/off ratio (I /I ) of 6 ” 10 were achieved for 1. The
on off
relatively low performance can be ascribed to the weak
crystalline nature and much grain bourdaries exist for the
solution-processed thin films as revealed by X-ray diffraction
and atomic force microscope measurements (see Figures S10
and S11). The devices of 2b gave comparable performance
Figure 3. Calculated (B3LYP/6-31G*) frontier molecular orbital profiles
and energy levels of 1 and 2a.
À3
2
À1 À1
3
(
mh: 1.7 ” 10 cm V s , V : À2 V, I /I : 10 ) for similar
T
on off
Cyclic voltammetry (CV) and differential pulse voltamme-
try (DPV) were used to study the electrochemical properties of
reasons. The device performance of both molecules could be
further improved by using vapour-deposited thin films or
single crystals in the future.
1
and 2b in dry dichloromethane (DCM; Figure 2b; see
Figure S7). Three quasi-reversible oxidation waves with half-
In summary, Z-shaped, the pentaleno-acene dimers 1 and
2a,b were designed and synthesized. It was found that the
fusion of an anti-aromatic pentalene moiety to the zigzag
edges of acenes dramatically changed their ground-state
electronic structure and physical properties. As a result of the
ox
wave potentials E1/2 at 0.22, 0.44, and 1.00 V and two
red
reversible reduction waves with half-wave potentials E1/2 at
+
À1.27 and À1.78 V (vs. Fc /Fc, Fc: ferrocene) were observed
for 1. 2b showed three quasi-reversible oxidation waves with E1/
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Acknowledgments
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Keywords: acenes · dimerization · materials science ·
polycycles · structure elucidation
[
12] CCDC 1055934 (1) and 1055935 (2b) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
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Received: September 23, 2015
Revised: November 30, 2015
Published online: January 25, 2016
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