Approaching a stable, green twisted heteroacene through clean reaction strategy

Article abstract of DOI:10.1039/c2cc32048g

A new, longest, stable, green twisted heteroacene 2-methyl-1,4,6,13- tetraphenyl-7:8,11:12-bisbenzo-anthro[g]isoquinolin-3(2H)-one (3) was synthesized by employing a clean reaction strategy based on thermally eliminating lactam bridges. Calculation shows that the HOMO-LUMO bandgap is in good agreement with experimental data.

Full text of DOI:10.1039/c2cc32048g

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Showcasing research from Nanyang Technological
University, Singapore and Department of
Chemistry and Biochemistry and Center for Polymers
and Organic solids, University of California,
Santa Barbara, USA
Approaching a stable, green twisted heteroacene through
“clean reaction” strategy
A new, longest, stable, green twisted heteroacene was synthesized
by employing a “clean reaction”strategy based on thermally
eliminating lactam bridges. Calculation shows that the
HOMO−LUMO bandgap is in good agreement with
experimental data.
See Fred Wudl and Qichun Zhang et al.,
Chem. Commun., 2012, 48, 5974.
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COMMUNICATION
Approaching a stable, green twisted heteroacene through
‘‘clean reaction’’ strategyw
Gang Li,a Hieu M. Duong,b Zhonghan Zhang,a Jinchong Xiao,a Lei Liu,c Yanli Zhao,c
Hua Zhang,a Fengwei Huo,a Shuzhou Li,a Jan Ma,a Fred Wudl*d and Qichun Zhang*a
Received 21st March 2012, Accepted 24th April 2012
DOI: 10.1039/c2cc32048g
A new, longest, stable, green twisted heteroacene 2-methyl-1,4,6,13-
tetraphenyl-7:8,11:12-bisbenzo-anthro[g]isoquinolin-3(2H)-one (3)
was synthesized by employing a ‘‘clean reaction’’ strategy based
on thermally eliminating lactam bridges. Calculation shows
that the HOMO–LUMO bandgap is in good agreement with
experimental data.
remain unknown. This gap in knowledge strongly encourages
us to explore the larger isoquinolinones.
Recently, Pascal, Wudl, and our group have already proved
that the twisted topology can serve as a protective motif to aid
synthesis of the larger stable polyacenes.7,8 Surprisingly, the
moderately twisted structures did not affect the electronic
properties. Employing this concept, we anticipated that we could
prepare the longer, highly conjugated, twisted isoquinolinones.
Furthermore, 1,4-cycloaddition involving isoquinolinones
(n = 1, 2 and 3), demonstrated by our group and others,7,9
proved that they were valuable diene intermediates in the
construction of larger oligoacene frameworks. The soluble
1,4-cycloaddition lactam-bridged adducts (oligocene precursors)
were key elements in the ‘‘clean reaction’’ strategy to produce the
corresponding acenes through thermally eliminating lactam bridges
(a retro-Diels–Alder reaction, Scheme 2). The amazing advantage
for this strategy was that the target products can be readily
obtained in pure state with nearly 100% yield and tedious separa-
tion will be avoided. Employing this method, we have successfully
synthesized several conjugated compounds such as 5,7,14,16-tetra-
phenyl-8:9,12:13-bisbenzo-hexatwistacene and 3-(2H)-isoquinoli-
none.7 It was our continuous interest in this field to employ the
‘‘clean reaction’’ strategy to prepare larger conjugated isoquino-
linones. Herein, we report the synthesis, physical properties and
theoretical study of a novel, stable, green twisted compound
2-methyl-1,4,6,13-tetraphenyl-7:8,11:12-bisbenzo-anthro[g]-
isoquinolin-3(2H)-one (3).
Heteroacenes, analogs of oligoacenes 1 (Scheme 1), have at
least one heteroatom such as N, P, O and S in their conjugated
frameworks.1 Recently, heteroacenes have attracted much atten-
tion both in fundamental research (theoretical studies and synthetic
challenges) and potential technological applications.2,3 The replace-
ment of carbon atoms in oligoacenes 1 by heteroatoms not only
leads to very interesting electronic properties (e.g. position changes
for the HOMO and LUMO, HOMO–LUMO gap, UV-Vis
absorption) but also affects the arrangement of conjugated frame-
works (e.g. p–p stacking, hydrogen bonding, SÁÁÁS interaction).4
One member of the heteroacene family, isoquinolinones 2
(Scheme 1) have been widely studied in biological systems.5
However, as potential candidates for electronics, their remark-
able electronic structures have not caught scientists’ attention.
Although higher conjugated isoquinolinones might have
similar ground states (open shell singlet or singlet polyradical
character) as predicted in larger acenes,6 experimental data
related to these predictions are rare. In addition, we have
already demonstrated that the HOMO–LUMO gaps of
isoquinolinones 2 (n = 2, and 3) are close to those of tetracene
and pentacene,7 but the longer isoquinolinones 2 (n 4 3)
Although many routes have been reported in the literature
for preparing various isoquinolinone derivatives,5 the practical
methodology to achieve larger isoquinolinones (n 4 2,
Scheme 1) was recently established by employing a ‘‘clean reaction’’
strategy based on thermally eliminating lactam bridges from
soluble precursors through a retro-Diels–Alder reaction.
In this report, the soluble precursor 6 was obtained through
a School of Materials Science Engineering, Nanyang Technological
University, 50 Nanyang Avenue, Singapore. E-mail: qczhang@ntu.edu.sg;
Fax: (+65)67909081; Tel: (+65)67909081
b Department of Chemistry and Biochemistry, University of California,
Los Angeles, CA 90095, USA
c School of Physical and Mathematical Sciences,
Nanyang Technological University, 50 Nanyang Avenue,
Singapore 639798, Singapore
d Mitsubishi, Chemical Center for Advanced Materials,
Department of Chemistry and Biochemistry and Center for Polymers
and Organic solids, University of California, Santa Barbara, USA.
E-mail: wudl@chem.ucsb.edu
w Electronic supplementary information (ESI) available: Experimental
section, all characterization data for compounds 3 and 6 and CIF file
of compound 6. CCDC 858853. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2cc32048g
Scheme 1 The structure of oligoacenes (1), isoquinolinones (2) and
2-methyl-1,4,6,13-tetraphenyl-7:8,11:12-bisbenzo-anthro[g]isoquinolin-
3(2H)-one (3).
c
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Scheme 2 ‘‘Clean reaction’’ strategy to approach higher acenes.
a [4 + 2] cycloaddition involving in situ arynes as dienophiles
(from precursor 4 reacting with isoamyl nitrite)8a and meso-
ionic pyrimidine 59 as a diene (Scheme 3). Compound 3 was
obtained in quantitative yield from precursor 6 through
thermal elimination of lactam bridges at 220 1C in the solid
state or in tetrahydronaphthalene solvent.
Besides full characterizations of spectral data of precursor 6,
good crystals suitable for single-crystal X-ray diffraction
analysis were obtained in a mixed solution of CH2Cl2–methanol
(1: 1) through slow evaporation. The solved structure of
precursor 6 with atomic labeling is shown in Fig. 1. Although
we were able to grow single crystals of compound 3 upon slow
cooling of a tetrahydronaphthalene solution of 3 in a sealed
tube from 250 1C to room temperature, unfortunately, we could
not solve the crystal structure due to the small size and poor
quality. It is noted that these crystals were very stable in air for
more than 15 days without any protection. Thermogravimetric
analysis (TGA) analysis of compound 3 shows that it is stable
up to 390 1C (Fig. S10, ESIw). The as-prepared compound 3
Fig. 1 The crystal structure of precursor 6.
Fig. 2 UV-vis spectrum of 3 in DCM (a) and cyclic voltammogram
of 3 in 0.1 M TBAP–ODCB (b).
1
was further characterized by H NMR, 13C NMR, MS-ESI,
HiResMALDI-TOF and FT-IR, differential scanning calorimetry
(DSC) and elemental analysis.
The electronic states of compound 3 were also strongly
governed by the pyridone end unit. The two possible major
contributing resonance structures in compound 3 are the
neutral polyene state and the charge-separated state
(meso-ion state).7b Either electronic state can be readily
favoured through the appropriate choice of solvents together
with Lewis acid or strong acid. When strong Lewis acids such
as anhydrous ZnCl2 were added into aprotic solvents
(for example dichloromethane as shown in Fig. 3), the longest
absorption maxima centre was blue-shifted to 510 nm with a
603 nm shoulder (similar phenomena have been reported
previously10). The same phenomenon was also observed in
strong acidic solvents (for example trifluoroacetic acid (TFA),
shown in Fig. 3) and the absorption spectrum was drastically
blue-shifted to 509 nm. It is believed that strong Lewis acids or
strong organic acids can shorten the N–CO bond through
protonation.7b As to a relatively weaker acid (e.g. acetic acid
(AcOH) shown in Fig. 3), the absorption peak (lmax = 610 nm)
fell between that in dichloromethane and that in trifluoroacetic
The absorption spectrum of 3 in dichloromethane (DCM)
displayed an absorption lmax at 665 nm with a shoulder at
710 nm (Fig. 2a). We were very surprised by the fact that
compound 3 with its relatively small p-framework had an optical
band gap at 1.86 eV, which is a similar HOMO–LUMO gap to
that of hexacene, but the twisted heteroacene 3 was more stable.
We observed no obvious change in the UV-Vis absorption of 3 in
o-dichlorobenzene (ODCB) solution in 6 h even without degassing
and shielding of light. Cyclic voltammetry (CV) of 3, measured in
0.1 M tetrabutylammonium perchlorate (TBAP)–ODCB, showed
that the oxidation and reduction processes were chemically and
electrochemically reversible (Fig. 2b). The HOMO–LUMO gap
calculated from the difference between the half-wave redox
ox
red
potentials (E1/2 = +0.40 eV and E1/2 = À1.50 eV) is
1.90 eV. This value was in good agreement with the optical
band gap, 1.86 eV, calculated from UV-Vis data (665 nm).
acid, which indicates that compound 3 is partially protonated.7b
11
Density function theory (DFT) at B3LYP/6-31G(d,p)
was
applied to optimize the geometry of heterotwistacene 3, which was
fully relaxed using the Gaussian 09 code12 with a convergence
criterion of 10À3 a.u. on the gradient and displacement and
10À6 a.u. on energy and electron density. The structure of 3 was
a local minimum since there were no imaginary frequencies in
harmonic vibrational analyses carried on 6-311+G(d,p) basis set.
The HOMO–LUMO band gap was 2.1 eV, corresponding to the
p–p* type transition as shown in Fig. 4, which was in good
agreement with the experimental data (1.9 eV). In addition, the
calculation demonstrated that CQO and the pyrene unit showed a
genuine contribution to the HOMO–LUMO gap.
Scheme 3 Synthetic route to 2-methyl-1,4,6,13-tetraphenyl-7:8,11:12-
bisbenzo-anthro[g]isoquinolin-3(2H)-one (3).
c
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Fig. 4 Wave functions for the HOMO and LUMO of 3.
In conclusion, we developed a ‘‘clean reaction’’ strategy to
approach a novel, longest, stable, green heterotwistacene
2-methyl-1,4,6,13-tetraphenyl-7:8,11:12-bisbenzo-anthro[g]-
isoquinolin-3(2H)-one (3). The fact that it had a low HOMO–
LUMO gap, which could be compared with that of hexacene,
makes it a potential candidate for electronics. In addition, we
believed that the as-prepared heterotwistacene (3) could act as
a scaffold towards the construction of larger twisted polycyclic
compounds. Further work to form highly-conjugated twisted
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Q. Z. acknowledges the finincial support from start-up grant
(Nanyang Technological University), AcRF Tier 1 (RG 18/09)
from MOE, CREATE program (Nanomaterials for Energy
and Water Management) from NRF, and New Initiative Fund
from NTU, Singapore. F. W. acknowledges support from the
NSF while at UCLA.
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