Quinoidal Oligo(9,10-anthryl)s with Chain-Length-Dependent Ground States: A Balance between Aromatic Stabilization and Steric Strain Release

Article abstract of DOI:10.1002/chem.201503033

Quinoidal π-conjugated polycyclic hydrocarbons have attracted intensive research interest due to their unique optical/electronic properties and possible magnetic activity, which arises from a thermally excited triplet state. However, there is still lack o

Full text of DOI:10.1002/chem.201503033

DOI: 10.1002/chem.201503033
Full Paper
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Radicals |Hot Paper|
Quinoidal Oligo(9,10-anthryl)s with Chain-Length-Dependent
Ground States: A Balance between Aromatic Stabilization and
Steric Strain Release
Zhenglong Lim,[a] Bin Zheng,[b] Kuo-Wei Huang,*[b] Ye Liu,*[c] and Jishan Wu*[a, c]
Abstract: Quinoidal p-conjugated polycyclic hydrocarbons
have attracted intensive research interest due to their
unique optical/electronic properties and possible magnetic
activity, which arises from a thermally excited triplet state.
However, there is still lack of fundamental understanding on
the factors that determine the electronic ground states.
Herein, by using quinoidal oligo(9,10-anthryl)s, it is demon-
strated that both aromatic stabilisation and steric strain re-
lease play balanced roles in determining the ground states.
Oligomers with up to four anthryl units were synthesised
and their ground states were investigated by electronic ab-
sorption and electron spin resonance (ESR) spectroscopy, as-
sisted by density functional theory (DFT) calculations. The
quinoidal 9,10-anthryl dimer 1 has a closed-shell ground
state, whereas the tri- (2) and tetramers (3) both have an
open-shell diradical ground state with a small singlet–triplet
gap. Such a difference results from competition between
two driving forces: the large steric repulsion between the
anthryl/phenyl units in the closed-shell quinoidal form that
drives the molecule to a flexible open-shell diradical struc-
ture, and aromatic stabilisation due to the gain of more aro-
matic sextet rings in the closed-shell form, which drives the
molecule towards a contorted quinoidal structure. The
ground states of these oligomers thus depend on the overall
balance between these two driving forces and show chain-
length dependence.
Introduction
Polycyclic aromatic hydrocarbons (PAHs) have, over the past
few decades, garnered increasing interest, especially in the
field of organic materials science and structural organic
chemistry.[1] Although most PAHs exist in the closed-shell elec-
tronic configuration, recent research has revealed that certain
types of quinoidal polycyclic hydrocarbons (PHs) could show
an open-shell singlet diradical ground state and display unique
optical, electronic and magnetic properties.[2] These quinoidal
PHs can be regarded as pro-aromatic systems due to the re-
covery of one or more aromatic sextet rings (highlighted in
blue in Scheme 1) in the open-shell diradical form, and typical
examples are the Thiele hydrocarbon[3] and the Tschitschibabin
Scheme 1. Resonance structures of the Thiele hydrocarbon, the Tschitschiba-
bin hydrocarbon, and the quinoidal oligo(9,10-anthryl)s 1–3.
[a] Z. Lim, Prof. J. Wu
hydrocarbon,[4] which were first prepared more than 100 years
ago and many recent studies are inspired therefrom. Whereas
the Thiele hydrocarbon is sufficiently stable, the Tschitschiba-
bin hydrocarbon is highly reactive and can only be isolated in
an inert atmosphere.[5] The instability of the latter comes from
its large diradical character because two more aromatic sextets
are generated from the closed-shell quinoidal form to the
open-shell diradical form. Thus, the synthesis of more extend-
ed quinoidal PHs is a big challenge. One way is to embed pro-
aromatic p-quinodimethane (p-QDM), 2,6-naphthoquinodime-
thane or 2,6-anthraquinodimethane moieties into a p-conju-
gated PAH framework to form relatively stable quinoidal PHs.
Examples of this type include bis(phenalenyl)s,[6] zethrenes[7]
Department of Chemistry, National University of Singapore
3 Science Drive 3, 117543 (Singapore)
Fax: (+65)6779-1691
E-mail: chmwuj@nus.edu.sg
[b] Dr. B. Zheng, Prof. K.-W. Huang
Division of Physical Sciences & Engineering and
KAUST Catalysis Center, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955-6900, (Saudi Arabia)
E-mail: kuowei.huang@kaust.edu.sa
[c] Dr. Y. Liu, Prof. J. Wu
Synthesis & Integration, Institute of Materials Research
and Engineering, 3 Research Link, 117602 (Singapore)
E-mail: ye-liu@imre.a-star.edu.sg
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503033.
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and indenofluorenes,[8] and many of them display very interest-
ing open-shell singlet diradical characteristics in the ground
state. Another approach is to stabilise the extended p-QDMs
by benzannulation, for example, very stable tetrabenzo-Tschit-
schibabin hydrocarbon 1[9] (Scheme 1) was successfully syn-
thesised by us, and this concept has been successfully extend-
ed to a series of quinoidal rylene based diradicaloids.[10] Mole-
cule 1 has a closed-shell ground state with a butterfly geome-
try in the single crystals to minimise steric repulsion between
the anthryl/phenyl units, and it is believed that aromatic stabi-
lisation by two more aromatic sextet rings in the closed-shell
form is the origin of a contorted quinoidal structure rather
than a flexible open-shell diradical form. It seems that a balance
between aromatic stabilisation and steric repulsion plays a criti-
cal role in determining the ground state. This raises the ques-
tion of the ground state of the more extended analogues, qui-
noidal 9,10-anthryl trimer 2 and tetramer 3 (Scheme 1). With
extension of the chain length, both driving forces are enforced
and it is interesting to determine the changes to the ground
states and physical properties with increasing chain length. In
this context, herein, we report the synthesis of quinoidal
oligo(9,10-anthryl)s 2 and 3, and detailed studies on their
ground states, physical properties and chemical reactivity. In-
sights from this study would enhance our understanding of
the reaction mechanisms involved in the synthesis of atomical-
ly precise graphene nanoribbons.[11] Our studies demonstrate
that a balance between aromatic stabilisation and steric strain
release finally determine the ground states of these oligomers.
benzophenone (4)[12] with Lawesson’s reagent gave the thioke-
tone 5, which reacted with the 10-diazo-9-anthrone (6)[12] to
generate the thioexpoxide 7. Reaction of 7 with PPh3 afforded
the desired ketone 8 in 89% yield over three steps. The 9,10-
dibromoanthracene and 10,10’-dibromo-9,9’-bianthryl[13] com-
pounds were separately lithiated with nBuLi to give intermedi-
ates 9a/9b, which reacted with ketone 8 to afford precursors
diols 10a/10b in 12 and 13% yield, respectively.
Subsequent reduction of 10a/10b with SnCl2 in CH2Cl2
under an inert atmosphere gave target compounds 2 and 3 in
nearly quantitative conversion (based on MS and TLC analysis).
However, both products were unstable on silica gel or alumini-
um oxide columns and thus they were isolated simply by filtra-
tion of the resulting solution through a 0.45 mm polytetrafluo-
roethylene (PTFE) syringe cartridge followed by removal of the
solvent under vacuum.
The MALDI-TOF mass spectra of 2 and 3 both agree well
with their respective molecular weights, with loss of two hy-
droxyl groups (see the Supporting Information). The absorp-
tion spectra of freshly synthesised 2 and 3 are shown in
Figure 1. Both compounds display two sharp bands (2: l=499
and 533 nm, 3: l=500 and 533 nm) and a broad absorption
band between l=650 and 1050 nm, with bands at lꢀ813
and 801 nm, respectively. The long-wavelength broad absorp-
tion bands are very similar to the in situ generated diradical of
dimer 1,[9] which indicates that both 2 and 3 are open-shell di-
Results and Discussion
Synthesis, optical properties and stability
The synthetic route to 2 and 3 is shown in Scheme 2. Key in-
termediates are diols 10a/10b. Treatment of 4,4’-di-tert-butyl-
Scheme 2. Synthetic routes to 2 and 3. Reaction conditions: a) Lawesson’s
reagent, toluene, 808C, overnight; b) THF, RT, overnight; c) PPh3, toluene,
24 h, 89% yield over three steps; d) THF, À788C, 1 h, 12–13% yield; e) SnCl2,
CH2Cl2, quantitative yield.
Figure 1. UV/Vis/near-infrared (NIR) absorption spectra of in in situ genera-
tion of a) 2 and b) 3 in dry toluene from their precursors, 10a and 10b re-
spectively (t=0 h). Insets: plots showing the changes of the absorbance at
l=533 nm with time.
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radicals rather than a closed-shell quinoidal compound. A
small optical energy gap of about 1.23 eV was estimated for
both compounds. Strong electron spin resonance (ESR) signals
showing hyperfine coupling were detected for the freshly pre-
pared samples in CH2Cl2 (below), which suggested the para-
magnetic character of both 2 and 3.
2.7 kJmolÀ1 was calculated for both 2 and 3. The closed-shell
configuration of 2 and 3 with a butterfly geometry was calcu-
lated to be 33.5 and 78.1 kJmolÀ1, respectively, higher in
energy than the TB ground states, which indicated that the
steric strain increased as the number of anthryl units increased.
One more singlet diradical state for 2 and two more singlet
diradical states for 3 were also calculated when mixing both
the double- and single-bond linkages along the long axis, and
all showed very high energies (Figure 2).
UV/Vis/NIR absorption spectroscopic measurements were
used to follow the generation and decay processes of 2 and 3
in dry toluene under a nitrogen atmosphere (Figure 1). Both 2
and 3 show similar absorption patterns to that of compound
1[9] (sharp band at l=507 nm and a broad absorption band
between l=680 and 1050 nm) in the open-shell electronic
conformation, which indicates that these compounds undergo
formation of a biradical electronic state before decomposition
proceeds. The plot of absorbance (at l=533 nm for both 2
and 3) versus time revealed that the initial generation of 2 and
3 reached a maximum in about 4 h and then gradually de-
cayed over a span of days. The ESR signals of the freshly pre-
pared samples showed similar degradation over time, and the
colour of the sample solutions gradually changed from red to
yellowish brown in one week, even when placed under an
argon atmosphere throughout. The slow decay is believed to
be due to some intermolecular radical–radical coupling reac-
tions and reaction with residual oxygen in the solution. Gel
permeation chromatography (GPC) measurements were per-
formed on the fresh and one-week-old samples of both 2 and
3 (Figures S1–S4 in the Supporting Information) and a new
low-intensity broad band was observed in the high-molecular-
weight region after one week of storage, which suggested the
formation of oligo- or polymers in both cases. The 13C NMR
spectra of 2 and 3 after storage for one week revealed the ap-
pearance of signals for a carbonyl group (d=156.02 and
168.14 ppm for compounds 2 and 3, respectively; Figures S7
and S8 in the Supporting Information), as further supported by
FTIR measurements. New vibrational bands at n˜ =1671 and
1658 cmÀ1 correlated to nC were observed (Figure S14 and
The ground-state electronic structures of 2 and 3 were fur-
ther investigated by ESR measurements both in solution and
in the solid state. The measured ESR signals of the freshly pre-
pared samples of 2 and 3 in CH2Cl2 both showed a hyperfine
structure with ge =2.0026 (Figure 3a and b). The spin concen-
tration at the peak (after reaction for 4 h), with 2,2-diphenyl-1-
picrylhydrazyl (DPPH) as a standard, was determined to be
about 39% for 2 and 125% for 3, relative to the DPPH radical
concentration (Figure S16 in the Supporting Information). The
ESR spectrum was simulated based on the anticipated proton
coupling strength obtained from calculated spin density values
(Figure 3c), with the parameters A(H1)=3.26 G (”4), A(H2)=
1.30 G (”2) and A(H3)=1.26 G (”2) for 2 and A(H1)=3.54 G (”
4), A(H2)=1.40 G (”2) and A(H3)=1.40 G (”2) for 3, both are
in good agreement with the experimental data (Figure 3a and
b). The solvent was removed by bubbling argon through the
solution and the residual solid was submitted to temperature-
dependent ESR measurements. The ESR intensity (I) increased
with deceasing temperature (T), and the I versus 1/T curve
showed a linear relationship (Figure S17 in Supporting Informa-
tion), which further supporting a TB (or two isolated radicals)
ground state for both. Due to the simultaneous degradation of
the diradicals, the exact singlet–triplet gap was not deter-
mined.
=
O
Conclusion
S15 in the Supporting Information). The presence of carbonyl
groups may be due to the termination of the radical sites by
oxygen, which was also observed in other reactive diradica-
loids.[14] Nevertheless, both 2 and 3 still showed certain persis-
tence under an inert atmosphere, which could be ascribed to
steric protection of the radicals by anthryl units and spin de-
localisation at the terminal units (see below).[9,15]
We successfully synthesised quinoidal 9,10-anthryl trimer 2 and
tetramer 3, which both showed an open-shell diradical ground
state and were in contrast to the closed-shell quinoidal dimer
1. This difference could be explained by increased steric strain
upon elongation of the chain length, which surpassed the aro-
matic stabilisation energy in the closed-shell form, and thus,
led to the more flexible diradical form in the ground state. Due
to the orthogonal character of the anthryl units, both 2 and 3
could be regarded as two isolated radicals, as supported by re-
sults from DFT calculations and ESR measurements. Both 2 and
3 were fairly persistent under an inert atmosphere due to ki-
netic blocking and spin delocalisation; however, they still un-
derwent slow decay due to radical–radical combination and
oxidation. Our research demonstrates that balancing the aro-
matic stabilisation and steric strain release will allow us to tune
the ground state and physical properties of quinoidal PHs.
Ground-state geometry and electronic structure
To understand the different ground-state electronic and geo-
metric structures of 2 and 3 from those of 1, broken-symmetry
DFT calculations (UCAM-B3LYP/6-31G*) were performed
(Figure 2). The energies of different possible geometries were
calculated, and it was found that, in both cases, the triplet bi-
radical (TB) had the lowest energy (ground state). In this form,
all anthryl units are nearly orthogonal to each other and the
spins are mainly delocalised along the two terminal diphenyl-
anthrylmethene units (Figure 2). The two spins are nearly iso-
lated, and thus, they are better described as two individual
radicals. Accordingly, a small singlet–triplet energy gap of
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Figure 2. Calculated energies of 2 (a) and 3 (b) with different configurations. Insets: spin density distribution mappings of the triplet biradicals (TBs).
Synthesis of 10b
Experimental Section
The above procedure for the synthesis was followed to prepare
10b by using 10,10’-dibromo-9,9’-bianthryl instead of 9,10-dibro-
Synthesis of 10a
moanthracene. Compound 10b was obtained as a yellow solid in
13% yield. HRMS (APCI): m/z calcd for C98H87O2: 1295.6701; found:
1295.6706.
9,10-Dibromoanthracene (100 mg, 0.30 mmol) was added to an
oven-dried Schlenk flask. The flask was then degassed and purged
with argon three times before dry THF (10 mL) was added by
means of a syringe. The solution was cooled to À788C and a solu-
tion of nBuLi (1.6m in hexane, 0.37 mL, 0.60 mmol) was then
slowly added. The mixture was stirred for 1 h at À788C and then
a solution of 8 (294 mg, 0.62 mmol) in THF (5 mL) was added by
syringe dropwise at À788C. The solution was slowly warmed to
room temperature and stirred for 3 days at RT and a clear yellow
solution was obtained. Upon completion of the reaction, as moni-
tored by TLC, the solution was quenched with water and extracted
by CHCl3. The organic layer was washed with water and dried over
anhydrous Na2SO4. The solvent was removed under vacuum and
the residue was purified by column chromatography (silica gel,
hexane/CH2Cl2/ethyl acetate=10/1/1, v/v/v, Rf =0.4) to give
a yellow solid in 12% yield. Due to the presence of isomers that
Synthesis of 2 for UV measurements
In an oven-dried cuvette, anhydrous tin(II) chloride (10 mg) was
added, and the cuvette was capped with a rubber septum. The
cuvette was then purged with nitrogen for 15 min and a balloon
of inert gas was attached to the cuvette by a syringe needle
through the rubber septa. In a separate flame-dried 25 mL Schlenk
tube, compound 10a (10 mg, 8.93”10À6 mmol) was added. The
flask was then degassed and purged with argon three times
before dry toluene was added to give a yellow solution of known
concentration. The resulting solution (3 mL) was then transferred
to the cuvette by means of a syringe and the reaction was moni-
tored by UV/Vis spectroscopy.
1
could not be separated, H and 13C NMR spectra were not record-
ed. HRMS (APCI): m/z calcd for C35H35O2: 1119.6075; found:
1119.6080.
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Acknowledgements
We acknowledge financial support from the MOE Tier 3 Pro-
gramme (MOE2014-T3-1-004), Tier 2 grant (MOE2014-T2-1-080)
and A*STAR JCO grant (1431AFG100). K.-W.H. thanks financial
support from KAUST.
Keywords: aromaticity · electronic structure · hydrocarbons ·
radicals · strained molecules
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2 (a) and 3 (b) in CH2Cl2 recorded at 193 K. c) Calculated spin densities of 2
and 3.
Synthesis of 2 for ESR measurements
In a flame-dried 25 mL Schlenk tube, anhydrous tin(II) chloride
(10 mg) was added, and the Schlenk tube was degassed and
purged with argon three times. In a separate flame-dried 25 mL
Schlenk tube, compound 10a (10 mg, 8.93”10À6 mmol) was
added. The flask was also degassed and purged with argon three
times before dry dichloromethane was added to give a yellow so-
lution. The resulting solution was then added to the Schlenk tube
containing the anhydrous tin(II) chloride and the reaction was
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dish solid that slowly turned brown. HRMS (APCI): m/z calcd for
C35H35O [M+1]: 1085.6020; found: 1085.6014.
Synthesis of 3 for the respective measurements
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analysis. Compound 3 was obtained quantitatively, as ascertained
by TLC, as a red solid that slowly turned brown. HRMS (APCI): m/z
calcd for C35H35O [M+1]: 1261.6646; found: 1261.6631.
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Received: August 2, 2015
Published online on November 12, 2015
Chem. Eur. J. 2015, 21, 18724 – 18729
www.chemeurj.org
18729
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