"clean reaction" strategy to approach a stable, green heptatwistacene containing a single terminal pyrene unit

Article abstract of DOI:10.1002/asia.201100910

Let's twist again: A "clean reaction" strategy based on thermally eliminating lactam bridges from a soluble acene precursor through a retro-Diels-Alder reaction gives a new, stable, green heptatwistacene (see structure). The molecule has a twist angle of

Full text of DOI:10.1002/asia.201100910

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DOI: 10.1002/asia.201100910
“Clean Reaction” Strategy to Approach a Stable, Green Heptatwistacene
Containing a Single Terminal Pyrene Unit
Jinchong Xiao,[a] Christos D. Malliakas,[b] Yi Liu,[a] Feng Zhou,[a] Gang Li,[a] Haibin Su,[a]
Mercouri G. Kanatzidis,[b] Fred Wudl,[c] and Qichun Zhang*[a]
Acenes, first introduced by Clar to describe the family of
hydrocarbons with linear annellation of benzene rings
(Scheme 1),[1] can be considered
as one piece of carbon nano-
tube or graphene, and have re-
reaction (Scheme 2). In fact, a similar method has been re-
ported by our group and other groups.[7h,9]
Because of the poor solubility and extreme instability of
higher acenes, several types of protecting groups (e.g.
phenyl, fluoride, aryl thio, silylethyne) have been successful-
ly employed to enhance their solubility and stabili-
ty.[7e,f,8b,c,d,e] Interestingly, the nonbonded interactions be-
tween the phenyl substituents and the benzo unit of pyrene
can cause PAHs to twist.[4e,f,10] Such a twist together with
strategically located phenyl substituents nearly perpendicu-
lar to the acene plane might substantially inhibit the dimeri-
zation and oxidation observed in larger acenes. Previously,
ceived a lot of attention as
active elements in organic semi-
Scheme 1. The structure of oli-
conducting
devices.[2–4]
Al- goacene.
though polycyclic aromatic hy-
drocarbons (PAHs) can be found in heavy oil or be generat-
ed from petroleum asphaltenes,[5] higher conjugated acenes
are not observed owing to their high reactivity, such as easy
oxidation and photodimerization. Natural rarity or non-exis-
tence of higher conjugated oligoacenes together with the
theoretical interest[6] (e.g. in the gap between the highest oc-
cupied and lowest unoccupied molecular orbitals (HOMO–
LUMO gap) and electronic properties) gave scientists more
reasons to synthesize them. Unfortunately, the extreme in-
stability, poor solubility, and tedious purification of higher
conjugated oligoacenes make the synthetic work more chal-
lenging. Although several groups have reported the photo-
spectroscopic characterization and the study of electronic
we reported
a
novel, twisted 6,8,15,17-tetraphenyl-
1:18,4:5,9:19,13:14-tetrabenzoheptacene containing two ter-
minating pyrene units, which was shown to be remarkably
stable.[8b] The stability is attributed to the protection gained
from the rigid terminal pyrenes as well as the strategically
installed phenyl substituents.[8b] This exciting result, together
with the promise of increasing device performance in thin-
film transistors (TFTs) using a narrow HOMO–LUMO gap
acene as observed when going from tetracene to penta-
cene,[11] prompted us to investigate larger conjugated PAHs.
The system containing rigid terminal pyrenes (or phenan-
threnes) can be divided into two types:[1,8a,10] double-termi-
nal pyrenes (or phenanthrenes, type I) and single-terminal
pyrene (or phenanthrene, type II, Scheme 3). In type II, the
longest member which has been reported in the literature is
1:16,4:5-bisbenzohexacene 1 (Scheme 3), whose absorption
is similar to pentacene.[12] The result leads us to synthesize
the next benzolog, 1,2,3,4,6,8,15,17-octaphenyl-9:10,13:14-
bisbenzoheptacene (2, Scheme 4), whose HOMO–LUMO
gap is similar to that of hexacene.[13]
structure of higher acenes ACHTGNUTRENNUNG
ples of higher acenes (nꢀ7) characterized by X-ray single-
crystal structures.[8]
By facing above-mentioned problems, we aim at a “clean
reaction” to prepare the higher acenes from their precursors
in pure state and in quantitative yield. The “clean reaction”
strategy is based on thermally eliminating lactam bridges
from soluble acene precursors through a retro-Diels–Alder
[a] Dr. J. Xiao, Y. Liu, F. Zhou, G. Li, Prof. H. Su, Prof. Q. Zhang
School of Materials Science and Engineering
Nanyang Technological University
The desired precursor 6,17-dihydro-1,2,3,4,6,815,17-octa-
phenyl-9:10,13:14-bisbenzo-20-methyl-6,17-(iminomethano)-
heptacene-19-one (3) was obtained in 40% yield through
the cycloaddition reaction between 2-methyl-1,4,6,7,8,9-hex-
Singapore 639798 (Singapore)
Fax : (+65)67909081
E-mail: qczhang@ntu.edu.sg
Homepage: http://www.ntu.edu.sg/home/qczhang/
[b] Dr. C. D. Malliakas, Prof. M. G. Kanatzidis
Department of Chemistry, Northwestern University
Evanston, Illinois 60208 (United State)
[c] Prof. F. Wudl
Department of Chemistry and Biochemistry, University of California
Santa Barbara, CA, 90095 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100910.
Scheme 2. “Clean reaction” strategy to approach higher acenes.
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 672 – 675

10H), and 6.86 ppm (m, 10H). The remaining small impuri-
ties at 7.00 and 7.10 ppm are the result of residual diphenyl
ether. The compound is very stable, and the CH2Cl2 solution
of it in open air did not show obvious bleaching in three
days (Figure S2 in the Supporting Information).
High-quality single crystals (Figure 1b, CCDC number:
846054) suitable for X-ray structural analysis are stable in
air for more than 10 days without any protection. As pre-
dicted, compound 2 displays a twisted structure with a twist
angle of 23.498, and p–p stacking (3.24 ꢁ) only happens by
the pyrene units (Figure 2). The pentacene unit, which has a
main contribution to electronic properties, is involved in no
p–p interaction at all. Such an arrangement makes com-
pound 2 more stable.
Scheme 3. Types of twistacenes.
The UV/Vis spectrum of 2
(1.0ꢂ10À6 m,
measured
CH2Cl2), with the longest ab-
sorption lmax at 683 nm
in
(1.82 eV), is shown in Fig-
ure 3a. The “finger” peaks in
the range of 550–720 nm are
characteristic of the acene
family (Figure 3a). Cyclic vol-
tammetry (CV) of 2, measured
in 0.1m tetrabutylammonium
perchlorate (TBAP)/1,2-dichlor-
obenzene (ODCB), showed
that the oxidation and reduc-
tion processes are chemically
and electrochemically reversi-
ble (Figure 3b). The HOMO–
Scheme 4. Synthesis of 1,2,3,4,6,8,15,17-octaphenyl-9:10,13:14-bisbenzoheptatwistacene (2).
aphenyl-benz(g)-isoquinolin-3
N
LUMO gap calculated from the difference between half-
red
generated from 3-amino-5,12-diphenyl-6:7,10:11-bisbenzote-
wave redox potentials (E1/2ox =+0.37 eV and E1/2
=
tracene-2-carboxylic acid (5, Scheme 4).[4e]
À1.55 eV) is 1.92 eV. DEHOMOÀLUMO corroborates the value
of 1.82 eV derived from the UV/Vis data. Furthermore,
these results placed the HOMO–LUMO gap of 2 right in
Thermogravimetric analysis (TGA) of 3 showed about a
5% weight loss starting at approximately 3308C (Figure S1
in the Supporting Information). The observed weight loss is
closely in line with the calculated weight percent for a
methyl isocyanate unit (5.1%). Obvious decomposition of 2
is observed in the thermogram if the sample temperature is
higher than 3758C. Therefore, the temperature to remove
the lactam bridges should carefully be controlled. Eliminat-
ing the bridge in diphenyl ether (sealed tube at 3208C,
Scheme 4) proved to be an efficient, better way to form
compound 2 (which can moderately dissolve in hot 1,3,5-tri-
chlorobenzene to form a green solution; Figure 1a). Slow
cooling of the diphenyl ether solution afforded plate-like
green crystals of 2 (Figure 1b). The structure of compound 2
1
was confirmed by H NMR spectroscopy, elemental analysis,
1
and mass spectrometry. A clean H NMR spectrum, record-
ed in CD2Cl2 at 500 MHz, is shown in Figure 1c. The signals
observed at d=8.42 (s, 2H), 7.76 (d, 2H), 7.66 (d, 2H), and
7.23 ppm (t, 2H) are from protons belonging to the pyrene
system. The singlets at d=8.07 (2H) and 7.80 ppm (2H) are
assigned to the protons residing on the pentacene frame-
work. Proton signals corresponding to the phenyl substitu-
ents observed at d=7.42 (m, 4H), 7.34 (m, 16H), 7.11 (m,
Figure 1. a) Solution of
2 in ODCB. b) Single-crystal image of 2.
c) 1H NMR spectrum in CD2Cl2 (the peaks with * are from solvent di-
phenyl ether).
Chem. Asian J. 2012, 7, 672 – 675
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www.chemasianj.org
673

Q. Zhang et al.
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mize the geometry of 2. The geometry of 2 is fully relaxed
using Gaussian 09 code[17] 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, thus indicating that the struc-
ture of 2 is a local minimum. The dipole-allowed vertical ex-
citation in the gas phase is calculated by the time-dependent
density functional theory (TD-DFT) approach[18] using the
same basis set. The first absorption peak with an excitation
energy of 1.64 eV is the electron excitation from HOMO to
LUMO and corresponds to the p–p* type transition as
shown in Figure 4, which is in good agreement with the ex-
Figure 2. Single-crystal analysis of 2. a) One molecule unit. b) Molecular
stacking.
Figure 4. Plots of wave functions for HOMO (left) and LUMO (right) of
2.
perimental data (1.8 eV). In addition, the calculation dem-
onstrates that the pyrene unit does contribute to the
HOMO–LUMO gap.
In conclusion, we developed a “clean reaction” method to
approach a novel heptatwistacene. The novel, stable, green
twistacene 2 possesses a low HOMO–LUMO gap, which is
close to that of the reported hexacene. Crystallographic
analysis revealed the presence of the conjugated heptacene
chromophores with a twisted shape. Using our strategy, we
are synthesizing longer oligotwistacenes.
Acknowledgements
Q.Z. acknowledges the financial support from start-up grant (Nanyang
Technological University), AcRF Tier 1 (RG 18/09) from MOE, the Sin-
gapore National Research Foundation under its CREATE program:
Nanomaterials for Energy and Water Management, and New Initiative
Fund from NTU, Singapore. H.S. thanks the support from A*STAR
SERC grant (Grant No. M47070020). X-ray work was performed at the
Integrated Molecular Structure Education and Research Center
(IMSERC) at Northwestern University supported by the State of Illinois.
Figure 3. a) The UV/Vis absorption of 2 (1.0ꢂ10À6 m) in CH2Cl2. b) Cyclic
voltammogram of 2 in ODCB using tetrabutylammonium perchlorate as
electrolyte.
Keywords: density functional theory · physical properties ·
polycycles · structure elucidation · twistacenes
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A Stable Heptatwistacene Containing a Single Terminal Pyrene Unit
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