Synthesis and structure characterization of a stable nonatwistacene

Article abstract of DOI:10.1002/anie.201200949

A clean reaction strategy based on thermally eliminating lactam bridges from a soluble acene precursor through a retro-Diels-Alder reaction, led to the synthesis and characterization of a novel, stable, green crystalline 6,8,10,17,19,21-hexaphenyl-1.22,4.5,11.12,15.16-tetrabenzononatwistacene. An analysis of the single crystal structure of the nonatwistacene indicates that this molecule is twisted at an angle of 25.44°. Copyright

Full text of DOI:10.1002/anie.201200949

Angewandte
Chemie
DOI: 10.1002/anie.201200949
Oligotwistacene
Synthesis and Structure Characterization of a Stable Nonatwistacene**
Jinchong Xiao, Hieu M. Duong, Yi Liu, Wenxiong Shi, Li Ji, Gang Li, Shuzhou Li, Xue-Wei Liu,
Jan Ma, Fred Wudl,* and Qichun Zhang*
As an important class of organic electronic materials, acenes
have already received much attention in both fundamental
investigations and practical applications.[1–5] Although theo-
retical modelling and calculations suggest that larger acenes
can have a singlet disjointed biradical character or an
antiferromagnetic character in the ground state,[4] there are
few successful studies on the synthesis and experimental
Figure 1. Twistacene structures.
characterization of larger acenes (> 7 repeating benzo units).
Owing to difficult multistep synthetic approaches, tedious
separation, poor solubility, and extreme instability (sensitivity
to light, oxygen, and polymerization),[1–3,6] linear polyacenes
remain challenging systems. In order to avoid these draw-
backs and approach the larger stable acenes, several groups
have reported successful results by selectively adding protect-
ing groups, such as phenyl, fluoride, arylthio, or silylethyne, on
the periphery of the conjugated acene frameworks to increase
solubility and enhance stability.[1–3,5,7]
Our groups are particularly interested in acenes with rigid
terminal pyrene units (Figure 1). The acene family containing
rigid terminal pyrenes (or phenanthrenes) can be divided into
two types:[6,8,9] double-terminal pyrenes (or phenanthrenes;
type I, Figure 1) and single-terminal pyrene (or phenan-
threne, type II, Figure 1). The non-bonding interactions
between substituents (or even H atoms) on the twistacene
backbone and the pyrene can cause the framework of the
acenes to twist.[5,7a,8–10] We believe that this twist, together
with selectively appended phenyl substitutents, offers
enhanced protection of the larger acene molecules in this
family from dimerization and oxidation. In fact, we have
already reported several remarkably stable twisted acenes
and heteroacenes containing single/double-terminating
pyrene units.[5,7a,9,11]
Recently, we developed a clean reaction method to
prepare the larger acenes and their derivatives in a pure
state and in near quantitative yield from the corresponding
precursors.[9,11] The key step in this strategy is a retro-Diels–
Alder process involving the thermal elimination of lactam
bridges from soluble acene precursors (Scheme 1). This
method, together with the twist concept, led us to believe
that a higher twistacene could be prepared. Herein we report
the synthesis and characterization of 6,8,10,17,19,21-hexa-
[*] Dr. J. Xiao,[+] Dr. Y. Liu, Dr. W. Shi, Dr. G. Li, Prof. Dr. S. Li,
Prof. Dr. Q. Zhang
School of Materials Science and Engineering
Nanyang Technological University
Singapore 639798 (Singapore)
E-mail: qczhang@ntu.edu.sg
Homepage: http://www.ntu.edu.sg/home/qczhang/
phenyl-1.22,4.5,11.12,15.16-tetrabenzononatwistacene
Scheme 2).
(1;
Dr. H. M. Duong[+]
Department of Chemistry, University of California
Los Angeles, CA, 90095 (USA)
L. Ji, Prof. X.-W. Liu
School of Physical and Mathematical Sciences
Nanyang Technological University
1 Nanyang Walk, Singapore, 637616 (Singapore)
Scheme 1. Clean reaction approach to higher acenes.
Prof. F. Wudl
Department of Chemistry and biochemistry
University of California
Santa Barbara, CA, 93106 (USA)
The desired precursor 2, was obtained in 22% yield
through a cycloaddition reaction between 3 and the aryne
generated from 4 (Scheme 2; see the Experimental Section
for full chemical names).[5a,11] A reaction to eliminate the
bridge, performed in either diphenyl ether (sealed tube at
3308C; Scheme 2) or the solid state, proved to be an efficient
way to form compound 1. Slowly cooling the diphenyl ether
solution afforded cubic, dark-green crystals of 1. These
crystals are stable in air for more than five days without any
protection.
E-mail: Wudl@chem.ucsb.edu
[+] These two authors made equal contribution to this paper.
[**] Q.Z. acknowledges financial support from a start-up grant
(Nanyang Technological University), an AcRF Tier 1 (RG 18/09)
from MOE, the CREATE program (Nanomaterials for Energy and
Water Management) from NRF, and the New Initiative Fund from
NTU, Singapore. F.W. acknowledges support from the NSF while at
UCLA.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200949.
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Scheme 2. Synthesis of 6,8,10,17,19,21-hexaphenyl-
1.22,4.5,11.12,15.16-tetrabenzononatwistacene (1).
DCE=1,2-dichloroethane.
The structure of nonatwistacene 1 was clearly confirmed
by X-ray crystallography (Figure 2).[17] Immediately apparent
from the side-on view of the crystal packing of 1 (Figure 2b) is
the presence of some twist to the nonacene p system. The
twist angle is about 25.648, which is larger than that of the
heptacene analogue.[7a] The most prominent intermolecular
interactions stabilizing the crystal packing are the p–p
stacking (3.44 ꢀ) interactions of the pyrene units (Figure 2b).
The pentacene units, which are the main contributors to the
electronic properties, do not exhibit strong p–p interactions.
This arrangement makes compound 1 more stable.
Figure 2. Single crystal analysis of 1. a) One molecule unit. b) Molec-
ular packing.
The UV–Vis spectrum of 1 (measured in CH2Cl2), with the
longest absorption lmax at 739 nm (1.68 eV), is shown in
Figure 3a. The bands in the finger region (580–800 nm) are
characteristic of the acene family (inset in Figure 3a). Cyclic
voltammetry of 1, measured in 0.1m tetrabutylammonium
perchlorate/1,2-dichlorobenzene (TBAP/ODCB; recorded at
1508C), showed that the oxidation and reduction processes
are chemically and electrochemically reversible (Figure 3b).
The HOMO–LUMO gap calculated from the difference
between the half-wave redox potentials (E1/2ox =+ 0.30 eV
and E1/2red = À1.42 eV) is 1.72 eV. The DEHOMO-LUMO corrob-
orates the value of 1.68 eV derived from the UV–Vis data.
Furthermore, these results place the HOMO–LUMO gap of
Figure 3. a) The UV–Vis absorption spectra of 1 in CH2Cl2. The inset
shows the magnified spectrum. b) Cyclic voltammogram of 2 in 1,2-
dichlorbenzene using tetrabutylammonium perchlorate as an electro-
lyte.
1
between that of heptacene (1.36 eV) and hexacene
(1.84 eV),[1c,2a] which is much lower than that of pentacene
a local minimum. The dipole-allowed vertical excitation in the
gas phase was calculated by time-dependant DFT using the
same basis set.[16] The first absorption peak, which has an
excitation energy of 1.64 eV, is the electron excitation from
HOMO to LUMO. The peak corresponds to the p–p* type
transition, as shown in Figure 4, which is in good agreement
with the experimental data (1.68 eV). Furthermore, the
calculation demonstrates that the pyrene unit does contribute
to the HOMO–LUMO gap.[9]
(2.1 eV).[12]
Density functional theory (DFT) calculations were per-
formed at the B3LYP/6-31G(d,p) level of theory[13,14] using
the Gaussian09 program.[15] The geometry of 1 is fully
optimized with a convergence criterion of 10À3 a.u. on the
gradient and displacement and 10À6 a.u. on energy and
electron density. Harmonic vibrational analyses show no
imaginary frequency, indicating that the structure of 1 is
2
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Chemie
X-ray crystal structure of 1 (CCDC 857476).[17] A single-crystal
data set was collected at 103 K on a Bruker SMART APEX II CCD
with graphite-monochromatized Mo Ka radiation (l = 0.71073 ꢀ).
Data processing (APEXII and SMART) and absorption correction
(SADABS) were accomplished by standard methods. The structure
was solved by direct-methods using SHELXS-97 and refinement, with
anisotropic displacement parameters, hydrogen atoms in the riding
model approximation, and a weighting scheme of the form w = 1/
[s2(Fo ) + (0.064P)2 + 0.376P] for P = (Fo2 + 2Fc2)/3, was on F2 by
means of SHELXL-97.
2
Figure 4. Wave function plot for the HOMO and LUMO of 2.
Received: February 3, 2012
Published online: && &&, &&&&
In conclusion, we have successfully synthesized and fully
characterized the novel, stable, green crystalline
6,8,10,17,19,21-hexaphenyl-1.22,4.5,11.12,15.16-tetrabenzo-
nonatwistacene 1 through a clean reaction strategy. Currently,
nonatwistacene 1 is the longest acene type I system and
possesses a low HOMO–LUMO gap, which is close to that
reported for hexacene. Crystallographic analysis revealed the
presence of the conjugated nonatwistacene chromophore
with a twisted topology. We are currently exploring the
potential electronic applications of this new nonatwistacene.
Keywords: density functional calculations ·
.
organic electronic materials · retro-Diels–Alder reaction ·
synthesis design · twistacenes
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Oligotwistacene
A clean reaction strategy based on ther-
mally eliminating lactam bridges from
a soluble acene precursor through a retro-
Diels–Alder reaction, led to the synthesis
and characterization of a novel, stable,
green crystalline 6,8,10,17,19,21-hexa-
phenyl-1.22,4.5,11.12,15.16-tetrabenzo-
nonatwistacene. An analysis of the single
crystal structure of the nonatwistacene
indicates that this molecule is twisted at
an angle of 25.448.
J. Xiao, H. M. Duong, Y. Liu, W. Shi, L. Ji,
G. Li, S. Li, X.-W. Liu, J. Ma, F. Wudl,*
Q. Zhang*
&&&& — &&&&
Synthesis and Structure Characterization
of a Stable Nonatwistacene
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!