Soluble and stable heptazethrenebis(dicarboximide) with a singlet open-shell ground state

Article abstract of DOI:10.1021/ja204501m

A soluble and stable heptazethrene derivative was synthesized and characterized for the first time. This molecule exhibits a singlet biradical character in the ground state, which is the first case among zethrene homologue series. Exceptional stability of

Full text of DOI:10.1021/ja204501m

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Soluble and Stable Heptazethrenebis(dicarboximide) with a Singlet
Open-Shell Ground State
Zhe Sun,† Kuo-Wei Huang,*,‡ and Jishan Wu*,†
†Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore
‡KAUST Catalysis Center and Division of Chemical and Life Sciences and Engineering, King Abdullah University of Science and
Technology, 4700 King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
S Supporting Information
b
resonance to biradical state. Thus, heptazethrene is more likely to
ABSTRACT: A soluble and stable heptazethrene derivative
was synthesized and characterized for the first time. This
molecule exhibits a singlet biradical character in the ground
state, which is the first case among zethrene homologue
series. Exceptional stability of this heptazethrenebis-
(dicarboximide) raises the likelihood of its practical applica-
tions in materials science.
display biradical character while at the same time is less stable and
more difficult to make. Despite all theoretical studies, experi-
mental evidence has not been available so far. An attempt to
synthesize heptazethrene by Clar was not successful due to its
extremely high reactivity.13 In light of this background, we report
the first synthesis of a stable and soluble heptazethrene deriva-
tive, a heptazethrenebis(dicarboximide) (2, Figure 1), with
remarkable singlet biradical character in the ground state. Since
the investigations of teranthene6a and bisanthene14 raise the
question of the crossover from a closed-shell form to an open-
shell form in a homologue series of anthenes, herein we also
address the comparison between 1 and 2 for better under-
standing of zethrene series and their biradical properties.
The design consideration of 2 originates from our previous
experience that an electron-withdrawing dicarboximide with a
bulky diisopropylphenyl group can effectively decrease the
HOMO energy level of the zethrene core and suppress dye
aggregation, providing enhanced stability and solubility.12 In the
meantime, as a narrow band gap is believed to be a precondition
of the singlet biradical ground state, attachment of an imide on
the heptazethrene core can narrow the band gap by generating a
pushÀpull structure, thus making the singlet biradical form
accessible. The synthesis of 2 is shown in Scheme 1, and an
intramolecular coupling pathway was adopted toward the
octadehydronaphthoannulene key intermediate 6.15 Successive
Sonogashira coupling reactions between 312 with trimethylsily-
lacetylene (TMSA) and then triisopropylsilylacetylene (TIPSA)
gave 4, which underwent selective deprotection and oxidative
coupling to yield 5. The careful choice of the alkyne coupling
condition is essential at this stage because commonly used Glaser
conditions with oxygen as oxidant failed or gave 5 in extremely
low yield, indicating an oxygen-sensitive nature of the terminal
alkyne. Alternatively, the Pd-catalyzed coupling reaction with
p-benzoquinone as oxidant in the absence of oxygen led to
improved yield.16 Subsequent in situ deprotection and intramo-
lecular alkyne coupling afforded the key intermediate 6, which
underwent simultaneous transannular cyclization to give the
target compound 2 as a dark-green solid.
inglet biradical polycyclic aromatic hydrocarbons (PAHs)
S
bearing Kekulꢀe structures have attracted increasing attention
not only due to the passion for exploring their intrinsic electronic
properties,1 but also because of their promising applications in
materials science,2 molecular electronics, and information
processing.3 Some have been experimentally proven to be
excellent two-photon absorption materials and ambipolar
semiconductors.4 However, because of their open-shell charac-
ter, most singlet biradical species tend to be short-lived, thus
obscuring their identification and applications. Recent progress
on thermodynamically stable biradicals involve bis(phenalenyl)
hydrocarbons which are stabilized by delocalization of unpaired
5
electrons through a phenalenyl moiety and a teranthene
biradical whose stability stems from the formation of three
additional Clar sextets.6
Computational studies predict a prominent biradical character
for higher-order acenes7 and zethrenes.8 Zethrenes represent an
attractive class of hydrocarbons with a fixed double bond
embedded in the molecular skeleton, and its derivatives can
potentially serve as semiconductors, near-infrared dyes,9 and
nonlinear optical chromophores.10 However, challenges to ob-
tain zethrene derivatives lie in synthetic difficulties and their poor
stability. Recently, relatively stable 7,14-substituted zethrene
derivatives11 and a more stable zethrenebis(dicarboximide) (1)
with smaller band gap12 were synthesized and characterized,
providing the first experimental data for zethrene species
(Figure 1). Although calculations indicate a resonance between
a closed-shell quinoid form and an open-shell biradical form for
zethrene (Figure 1), none of those above-mentioned zethrene
derivatives displays biradical features. The heptazethrene, which
can be viewed as two phenalenyl moieties fused by one benzene
ring, is predicted to possess a larger biradical character, consisting
of energy balance between loss of the double bond and aromatic
stabilization by the central aromatic benzene ring (Figure 1). In
addition, heptazethrene will form one more Clar sextet in the
biradical form, while zethrene will lose one Clar sextet upon
The biradical character of 2 was revealed by low-temperature
1H NMR spectra (Figure 2). A solution of 2 in CD2Cl2 exhibited
no resonance signals for the heptazethrene core in the aromatic
region, while peaks began to emerge and sharpen upon cooling.
Received: May 17, 2011
Published: July 14, 2011
r
2011 American Chemical Society
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Figure 1. Resonance structures of zethrene and heptazethrene, and
molecular structures of 1 and 2.
Scheme 1a
1
Figure 2. Variable-temperature H NMR spectra of 2 in CD2Cl2 in
aromatic region and assignment of aromatic protons. The resonance
assignment referred to the structure shown in Figure 1.
achieve an unrestricted and symmetry-broken solution accordingly.
The resulting structure with spin contamination of ÆS2æ = 1.017 was
located to be 5.8 and 7.5 kcal/mol more stable than the closed-shell
and the triplet biradical forms, respectively. These observations
confirm that the ground state of 2 is open-shell singlet. The small
singletÀtriplet energy gap indicates that the radicals are bonded
very weakly, similar to other singlet open-shell hydrocarbons.5,6
Interestingly, the spin densities are homogeneously distributed
through the whole heptazethrene molecule, including the carbonyl
groups in 2 (Figure 3). This is slightly different from Kubo’s
benzoquinoid and naphthoquinoid bisphenalenyls,5 in which the
spins are mainly delocalized among the two phenalenyl units. In
addition, detailed bond length analysis on the central six-membered
ringrevealsaquinoidcharacterinclosed-shellformandanaromatic
benzenoid character in singlet and triplet biradical states (Figure S5
in the Supporting Information (SI)), indicating that the biradicals
are stabilized by aromatization. For comparison, the same calcula-
tions for 1 were also conducted, and it was confirmed that 1 favors a
singlet closed-shell groundstate, which is 10.8 kcal/mol more stable
than its triplet biradical state.
Compound 2 exhibited excellent solubility in common or-
ganic solvents due to the attachment of bulky 2,6-diisopropyl-
phenyl groups. The absorption spectra of 1 and 2 in chloroform
are shown in Figure 4. Compound 1 showed a well-resolved p-
band with maximum absorption at 648 nm along with a shoulder
band at 596 nm, corresponding to 0À0 and 0À1 electronic
transitions, which is also typical for other closed-shell PAHs such
as rylenes.20 For 2, the absorption spectrum displayed four bands
from the far-red to the near-infrared region with maxima at 827,
747, 701, and 641 nm, and a similar band structure was also
observed for teranthene derivative and quinoidal oligo-
thiophenes.6a,21 The weak, lowest-energy band most likely
originates from the presence of a low-lying excited singlet state
dominated by a doubly excited electronic configuration (H,
HfL, L).21 Variable-temperature absorption spectral measure-
ments were conducted in dichloromethane (Figure S4, SI), and
with decrease of temperature, absorbance of bands at lower energy
a Reagents and conditions: (a) TMSA, 1 equiv, PdCl2(PPh3)2/CuI,
THF/Et3N, rt, 70%; (b) TIPSA, 2 equiv, PdCl2(PPh3)2/CuI, THF/
Et3N, 70 ꢀC, 95%; (c) K2CO3, MeOH/THF, rt, 60%; (d) p-benzoqui-
none, PdCl2(PPh3)2/CuI, diisopropylamine, toluene, 60 ꢀC, 85%;
(e) TBAF/THF, rt, 30 min; (f) PdCl2(PPh3)2/CuI, toluene, rt, 1 h,
22% for two steps.
At À40 ꢀC, all peaks from heptazethrene backbone became clear
and could be reasonably assigned according to integration values
and the chemical environment of protons. This phenomenon
was also observed in phenalenyl-based biradicals and teranthene
biradical, resulting from a thermally excited triplet species which
is slightly higher in energy than the singlet biradical state.5,6
A
THF solution of 2 showed no ESR signal at room temperature,
presumably due to too many antiparallel couplings of unpaired
electrons and cancellations of magnetic moments. On the other
hand, the zethrenebis(dicarboximide) 1 exhibited sharp 1H
NMR signals at room temperature as a result of a larger
singletÀtriplet energy gap and smaller amount of triplet species,
which is common for most closed-shell Kekulꢀe hydrocarbons.
Theoretical calculations were conducted to further interrogate
the biradical character of molecule 2. The geometry was first
optimized by the CAM-B3LYP17 method using the Gaussian09
suite of programs.18 The HOMO and LUMO of 2 obtained from
the closed-shell solution can serve as a reference, which indicates
an extended delocalization through the heptazethrene core, and
shows that the central p-quinodimethane unit possesses the
largest HOMO and LUMO coefficients (Figure 3), suggesting
the most reactive site. As expected, further analysis performed
with the STABLE=OPT keyword19 revealed the instability of the
restricted wave function. The reoptimization was carried out to
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Figure 5. Cyclic voltammogram of 2 in dichloromethane with 0.1 M
Bu4NPF6 as supporting electrolyte, Ag/AgCl as reference electrode, Au
disk as working electrode, Pt wire as counter electrode, and scan rate at
50 mV/s. (Inset) Differential pulse voltammograms of 2.
Figure 3. (a) HOMO, (b) LUMO, and (c) spin densities of 2. The
calculations were performed at CAM-B3LYP level of theory. Blue and
green surfaces represent R- and β-spin densities, respectively.
a relatively low lying HOMO (À5.22 eV) was determined for 2 by
cyclic voltammetry, indicating a high oxidation resistance. The
impressive stability of the biradical form can be explained by (1)
recovery of aromaticity in the central six-membered ring, (2) large-
extent delocalization of the biradicals, and (3) stabilization of radicals
by the electron-withdrawing imide groups. Since the delocalization
and electron-withdrawing effects are similar in 1 and 2, but 1 barely
showed biradical character, we believe that the aromatic linker in the
middle is the major contribution to the biradical stability of heptaze-
threne. However, considering that the aromatic stabilization energy of
one more benzene ring is ∼90 kJ/mol, which can by no means fully
compensate the cleavage of CÀC π-bond energy of ∼270 kJ/mol,22
the delocalization and electron-withdrawing effects of imides will still
play key roles in stabilizing the biradical.
In conclusion, a stable heptazethrene derivative 2 was synthe-
sized and well-characterized for the first time. Attachment of
electron-withdrawing dicarboximide groups is proven to be an
efficient approach toward soluble and stable open-shell PAHs,
which is very important for further studies on their optical/
electronic properties and practical applications. Our research also
provided the first experimental proof on the singlet biradical
character of zethrenes, an intriguing class of well-defined gra-
phene fragments. Synthesis and investigation of higher-order
zethrenes are currently underway in our laboratories.
Figure 4. Absorption spectra of 1 and 2 in chloroform.
(827 and 747 nm) increased, while the optical density for the
bands at higher energy (701 and 641 nm) decreased, indicating a
slight shiftofthe equilibriumbetweendifferent species presumably
including singlet species together with a small amount of triplet
species. Moreover, 2 does not show any photoluminescence but 1
exhibits a moderate fluorescence quantum yield of 50%,12 further
’ ASSOCIATED CONTENT
indicating the biradical character of 2. The cyclic voltammogram
1
S
of 2 exhibited one reversible two-electron oxidation wave at Eox2
=
=
Supporting Information. Experimental details and char-
b
0.62, one quasi-reversible one-electron oxidation wave at Eox
acterization data of all new compounds, theoretical calculation
data, photostability test, variable-temperature UVÀvisÀNIR
spectral measurements, and complete ref 18. This material is
available free of charge via the Internet at http://pubs.acs.org.
0.93 and two very close reversible one-electron reduction waves at
Ered1 = À0.65 V and Ered2 = À0.69 V (vs Fc/Fc+) (Figure 5). A
low HOMOÀLUMO band gap was determined as 0.99 eV for 2,
0.55 eV smaller than that of 1. This observation provided further
evidencethata small bandgap isone of thecrucialpreconditions in
forming a singlet open-shell ground state.
Outstanding chemical and photostability were observed for 2,
making it one of the most stable singlet biradical PAHs known so
far. The half-life time of 2 in chloroform exposed to ambient air
and light conditions was found to be as long as 6 days (Figure S3
in SI). Such a high stability can be mainly ascribed to the attachment
of strong electron-withdrawing imide groups, which can lower the
high-lying HOMO energy level of the parent heptazethrene. In fact,
’ AUTHOR INFORMATION
Corresponding Author
hkw@kaust.edu.sa (K.H.); chmwuj@nus.edu.sg (J.W.)
’ ACKNOWLEDGMENT
J.W. acknowledges the financial support from NUS Young
Investigator Award (R-143-000-356-101), NRF Competitive
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Research Program (R-143-000-360-281), and A*Star BMRC-
NMRC Joint Grant (10/1/21/19/642). K.-W.H. acknowledges
the financial support from KAUST. We thank Professor Richard
D. Webster for the ESR measurement and Professor Yuan-
Chung Cheng, Dr. Bei Zhang, Professor Xixiang Zhang, and
Dr. D. J. Fox for useful discussions.
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