Dibenzoheptazethrene isomers with different biradical characters: An exercise of clar's aromatic sextet rule in singlet biradicaloids

Article abstract of DOI:10.1021/ja410279j

Clar's aromatic sextet rule has been widely used for the prediction of the reactivity and stability of polycyclic aromatic hydrocarbons with a closed-shell electronic configuration. Recent advances in open-shell biradicaloids have shown that the number of aromatic sextet rings plays an important role in determination of their ground states. In order to test the validity of this rule in singlet biradicaloids, the two soluble and stable dibenzoheptazethrene isomers DBHZ1 and DBHZ2 were prepared by different synthetic approaches and isolated in crystalline form. These two molecules have different numbers of aromatic sextet rings in their respective biradical resonance forms and thus are expected to exhibit varied singlet biradical character. This assumption was verified by different experimental methods, including nuclear magnetic resonance (NMR), electron spin resonance (ESR), superconducting quantum interference device (SQUID), steady-state and transient absorption spectroscopy (TA), and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. DBHZ2, with more aromatic sextet rings in the biradical form, was demonstrated to possess greater biradical character than DBHZ1; as a result, DBHZ2 exhibited an intense one-photon absorption (OPA) in the near-infrared region (λ_abs ^max = 804 nm) and a large two-photon absorption (TPA) cross-section (σ⁽²⁾_max = 2800 GM at 1600 nm). This investigation together with previous studies indicates that Clar's aromatic sextet rule can be further extended to the singlet biradicaloids to predict their ground states and singlet biradical characters.

Full text of DOI:10.1021/ja410279j

Article
pubs.acs.org/JACS
Dibenzoheptazethrene Isomers with Different Biradical Characters:
An Exercise of Clar’s Aromatic Sextet Rule in Singlet Biradicaloids
Zhe Sun,† Sangsu Lee,‡ Kyu Hyung Park,‡ Xiaojian Zhu,§ Wenhua Zhang,∥ Bin Zheng,⊥ Pan Hu,†
Zebing Zeng,† Soumyajit Das,† Yuan Li,† Chunyan Chi,† Run-Wei Li,§ Kuo-Wei Huang,*,⊥ Jun Ding,*,#
Dongho Kim,*,‡ and Jishan Wu*,†,∥
†Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore
‡Department of Chemistry, Yonsei University, Seoul 120-749, Korea
§Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of
Sciences, Ningbo 315201, China
∥Institute of Materials Research and Engineering, A*STAR, 3 Research Link, 117602 Singapore
⊥Division of Physical Science and Engineering and KAUST Catalysis Center, King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia
#Department of Materials Science & Engineering, National University of Singapore, 119260 Singapore
S
* Supporting Information
ABSTRACT: Clar’s aromatic sextet rule has been widely used
for the prediction of the reactivity and stability of polycyclic
aromatic hydrocarbons with a closed-shell electronic config-
uration. Recent advances in open-shell biradicaloids have shown
that the number of aromatic sextet rings plays an important
role in determination of their ground states. In order to test
the validity of this rule in singlet biradicaloids, the two soluble
and stable dibenzoheptazethrene isomers DBHZ1 and
DBHZ2 were prepared by different synthetic approaches and
isolated in crystalline form. These two molecules have different
numbers of aromatic sextet rings in their respective biradical
resonance forms and thus are expected to exhibit varied singlet
biradical character. This assumption was verified by different experimental methods, including nuclear magnetic resonance
(NMR), electron spin resonance (ESR), superconducting quantum interference device (SQUID), steady-state and transient
absorption spectroscopy (TA), and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional
theory (DFT) calculations. DBHZ2, with more aromatic sextet rings in the biradical form, was demonstrated to possess greater
biradical character than DBHZ1; as a result, DBHZ2 exhibited an intense one-photon absorption (OPA) in the near-infrared
region (λabs = 804 nm) and a large two-photon absorption (TPA) cross-section (σ(2) = 2800 GM at 1600 nm). This
max
max
investigation together with previous studies indicates that Clar’s aromatic sextet rule can be further extended to the singlet
biradicaloids to predict their ground states and singlet biradical characters.
quinodimethanes,9 and zethrenes,10 which allow a glimpse of
I. INTRODUCTION
their peculiar properties arising from singlet biradical character.
The study of open-shell polycyclic hydrocarbons (PHs) with
Benzenoid PHs usually can be drawn as two types of
ground state singlet biradical (SB) character is of fundamental
́
resonance structures, a closed-shell Kekule form and an open-
importance in understanding the nature of chemical bonding
and many chemical/physical phenomena in π-conjugated
systems.1 In addition, their unique optical, electronic, and
magnetic properties make them promising candidates for
organic electronics,2 nonlinear optics,3 spintronics,4 and energy
storage devices.5 A major challenge in the synthesis and
isolation of open-shell PHs is their intrinsic instability, which
can be partially overcome by introducing bulky or electron-
withdrawing substituents at strategic positions. So far, several
types of stable open-shell PHs have been reported, including
bisphenalenyls,6 indenofluorenes,7 anthenes,8 extended p-
shell biradical form. A straightforward evaluation of the
contribution of the biradical resonance form to the ground
state is the biradical character y (0 < y < 1, 0 representing
closed-shell state and 1 representing pure diradical state), which
can be obtained either by theoretical calculation on the basis of
the natural orbital occupation number (NOON)11 of the
LUMO or by experimental measurements.12,10c It was also
Received: October 7, 2013
© XXXX American Chemical Society
A
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demonstrated that the physical properties of open-shell singlet
biradicaloids are closely related to their biradical characters.
Therefore, it is of importance to understand the fundamental
structure−biradical character relationship.
1c) can be good test beds for our investigations. Heptazethrene
(HZ) was theoretically predicted to be a ground-state singlet
biradical,15 but our recent studies showed that the ground state
of HZ derivatives was dependent on the substituents and
HOMO−LUMO gap.10 Lateral fusion of two benzenoid rings
to the HZ core in different modes can lead to two isomeric
structures, 1,2:9,10-dibenzoheptazethrene (isomer 1) and
5,6:13,14-dibenzoheptazethrene (isomer 2) (Figure 1c). For
isomer 1, there are two aromatic sextets in the closed-shell
quinoidal form and a maximum of three in the biradical form,
regardless of the positions of the radicals. However, for isomer
2, two aromatic sextets are present in the quinoidal form while
three can be depicted when the two radicals are resident at the
bay region and a maximum of five can be drawn when the two
radicals are located at the terminal anthracene units. Since the
two isomers have the same carbon/hydrogen composition, the
discussion of the biradical character can be concentrated on the
numbers of Clar aromatic sextet rings. We anticipate that more
aromatic sextets for isomer 2 will help to stabilize the biradical
resonance form and thus it is expected to show greater biradical
character than isomer 1.
Clar’s aromatic sextet rule is known as a classic rule to evaluate
the stability and reactivity of closed-shell polycyclic aromatic
hydrocarbons (PAHs): that is to say, for benzenoid PAHs with
the same chemical composition, the molecule with more aromatic
sextet rings shows higher stability and lower reactivity.13 This is
because the benzene ring containing six π electrons exhibits a
large resonance energy, and thus a PAH with more aromatic
sextet rings will show higher stability. For example,
triphenylene (C18H12) is more stable than tetracene (C18H12)
because two more aromatic sextet rings can be drawn for the
former (Figure 1a). Recent studies by Kubo et al.8,14 disclosed
On the basis of the resonance structures shown in Figure 1c
and our previous investigations on zethrene-based molecules,10
a reasonable assumption is that the most reactive sites locate at
bay regions and terminal zigzag edges. In order to obtain stable
and soluble materials, our design is to block the reactive sites
with aryl (such as mesityl, tert-butylphenyl) or bulky alkyne
groups (triisopropylsilylethynyl group). In this work, the two
stable dibenzoheptazethrene derivatives DBHZ1 and DBHZ2
(Scheme 1) were synthesized and their biradical characters
were systematically investigated by various spectroscopic
measurements, X-ray crystallographic analysis, and DFT
calculations. Our studies provide a proof of concept on how
Clar’s aromatic sextet rule can be applied to the singlet
biradicaloid systems.
II. RESULTS AND DISCUSSION
Synthesis. Two facile synthetic approaches were developed
to construct these molecules utilizing either an intramolecular
Friedel−Crafts alkylation reaction or an intramolecular Heck
coupling reaction as a key step (Scheme 1). For the synthesis of
DBHZ1, we started from anthracene derivative 1 with the tert-
butylphenyl group to ensure sufficient solubility for inter-
mediates and the final product. Suzuki coupling of 1 with 1,4-
dimethyl-2,5-bis(4,4,5,5-tetramethyl[1.3.2]dioxaborolan-2-yl)-
phenyl gave intermediate 2, the dimethyl groups of which were
then transformed into dialdehyde to afford 6 via a
bromination−esterification−hydrolysis−oxidation sequence.
Then, 6 was treated with mesitylmagnesium bromide to give
diol 7, which was subjected to a Friedel−Crafts alkylation
reaction promoted by BF3·OEt2 to afford compound 8.16
DBHZ1 was obtained as a blue solid by oxidation of 8 with
DDQ. On the other hand, DBHZ2 was prepared starting from
anthracene derivative 9 with mesityl substitution.17 The as-
formed lithium reagent of 9 reacted with 2,5-dibromobenzene-
1,4-dicarbaldehyde to afford the diol 10, which then underwent
Swern oxidation18 to yield 11. Subsequently, the intramolecular
Heck reaction19 of 11 catalyzed by Pd(OAc)2 gave the diketone
precursor 12, which was treated with triisopropylsilyl
ethynylene lithium reagent followed by reduction with SnCl2
to give DBHZ2 as a green solid. The structures of DBHZ1 and
Figure 1. (a) Chemical structures of triphenylene and tetracene. (b)
Resonance structures of anthenes. (c) Chemical structure of
heptazethrene and the resonance structures of two DBHZ isomers.
The benzene rings with bold lines and gray color denote Clar’s
aromatic sextet rings.
the importance of aromatic stabilization for the ground state of
the anthene series: that is, the singlet biradical character y
increases from 0.07 to 0.91 from bisanthene to quarteranthene
as more aromatic sextet rings form in the biradical species
(Figure 1b).8 Our recent studies on a homologous series of
zethrenes also highlighted the importance of additional
aromatic sextet rings on stabilizing the biradical structure.10
In order to examine the validity of Clar’s aromatic sextet rule in
the open-shell singlet biradical PH systems, a better insight can
be provided by systems with the same chemical composition. In
this regard, homologous anthenes and zethrenes are not
suitable, considering other variables such as conjugation length.
However, two dibenzoheptazethrene (DBHZ) isomers (Figure
1
DBHZ2 have been identified unambiguously by 1D H/13C
NMR and 2D COSY NMR spectroscopy, high-resolution mass
B
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a
Scheme 1. Synthesis of DBHZ1 and DBHZ2
a
Reagents and conditions: (a) 1,4-dimethyl-2,5-bis(4,4,5,5-tetramethyl[1,3,2]dioxaborolan-2-yl)phenyl, Pd2(dba)3, DPEPhos, K2CO3 (aq), toluene/
ethanol, reflux, 43%; (b) NBS, BPO, CCl4, reflux; (c) KOAc, Bu4NBr, DMF, 100 °C, 57% in two steps from 2; (d) KOH (aq), THF/ethanol, reflux;
(e) PCC, DCM, room temperature, 50% in two steps from 4; (f) mesitylmagnesium bromide, THF, room temperature; (g) BF3·OEt2, CH2Cl2,
room temperature; (h) DDQ, toluene, room temperature, 45% in three steps from 6; (i) (1) n-BuLi, THF, −78 °C, (2) 2,5-dibromobenzene-1,4-
dicarbaldehyde, THF, −78 °C to room temperature, 35%; (j) (CF3CO2)O/DMSO, CH2Cl2, −78 °C, 76%; (k) Pd(OAc)2, KOAc, TBAB, DMAc,
160 °C, 27%; (l) (i) LiCCSi(i-Pr)3, THF; (ii) SnCl2, 75%. Abbreviations: NBS, N-bromosuccinimide; BPO, benzoyl peroxide; PCC, pyridinium
chlorochromate; DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; TBAB, tetrabutylammonium bromide.
spectrometry (see the Supporting Information), and crystallo-
graphic analysis (vide infra).
Magnetic Properties. DBHZ1 showed well-resolved sharp
1
peaks in the H NMR spectrum in C6D6 solution at room
temperature, and it is ESR silent. In contrast, no NMR signals
in the aromatic region were observed for DBHZ2 at room
temperature and a progressive sharpening of peaks was found
when the temperature was lowered in THF-d8 solution (Figure
2). The NMR peaks can be assigned to the structure of
DBHZ2 with the help of 2D-COSY NMR spectroscopy
(Figure S1 in the Supporting Information). This behavior is
common for many other singlet biradical PHs,8,10 which can be
explained by the presence of thermally excited triplet species
caused by the small singlet−triplet energy gap ΔESB‑TB. A broad
Figure 2. Variable-temperature 1H NMR spectra (aromatic region) of
ESR signal with g tensor of ge = 2.0027 was observed from a
DBHZ2 in THF-d8 and assignments of aromatic protons. Assignments
toluene solution of DBHZ2 (Figure 3a), and the signal
refer to the structure shown in Scheme 1. The peak labeled with an
asterisk is from the impurity in THF-d8.
intensity decreased with a decrease in temperature, indicating a
singlet biradical ground state for DBHZ2. The temperature-
dependent magnetic susceptibility behavior of DBHZ2 (5−380
K) obtained by SQUID measurements revealed that it indeed
has a singlet biradical ground state (Figure 3b). An exchange
interaction energy (2J/kB, i.e., ΔESB‑TB) of −1859.6 K (−3.7
kcal/mol) was estimated by a careful fitting of the data using
the Bleaney−Bowers equation:20
−1
Nβ2g2
3kT
⎡
⎤
⎥
⎦
⎛
⎞
J
1
3
s−t
χ =
⎢1 + exp
⎜
⎟
⎠
⎝
kT
⎣
where χ is the magnetic susceptibility, N is Avogadro’s number,
β is the Bohr magneton, g is the magnetic field splitting factor, k
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Figure 3. (a) ESR spectrum of DBHZ2 in toluene solution measured
at room temperature. (b) χT−T plot for the solid DBHZ2. The
measured data were plotted as open circles, and the fitting curve was
drawn using the Bleaney−Bowers equation with g = 2.00.
is Boltzmann’s constant, T is the temperature, and J is the
exchange integral. This small value again explains the presence
of an ESR signal in toluene solution and the line broadening of
the NMR at room temperature for DBHZ2.
Figure 4. ORTEP drawings of (a) DBHZ1 and (b) DBHZ2 measured
at 123 K. The hydrogen atoms are omitted for clarity. (c) Mean values
of bond lengths (Å) and calculated NICS(1)zz values in the DBHZ
core for DBHZ1 and DBHZ2. The numbers given in pink denote the
spin densities of the specific carbon atoms.
X-ray Crystallographic Analysis. Single crystals suitable
for X-ray crystallographic analysis were obtained for both
DBHZ1 and DBHZ2 by slow diffusion of methanol into a
chloroform solution, and the structures are shown in Figure 4.21
For both molecules, the DBHZ core slightly deviates from
planarity due to steric congestion at the cove region (DHBZ1)
or between the CC bond and the anthracene unit (DBHZ2),
with a torsional angles of 33.9 and 17.6°, respectively (Figure
4a,b). For DBHZ1, there is obviously a bond length alternation
in the central p-quinodimethane subunit, indicating that the
quinoidal resonance form has a significant contribution to the
ground state electronic structure (Figure 4c). However, the
bond a (1.429 Å) is much shorter than a typical C(sp2)−C(sp2)
bond (∼1.47 Å) and the bond b (1.376 Å) is much longer than
the double bond in olefins (1.33−1.34 Å), which means the
quinoidal character is diminished by the emergence of the
biradical contribution. Therefore, DBHZ1 is better described as
a borderline molecule with a ground state between a closed-
shell quinoid and a singlet biradical, which is in accordance with
the observed ESR silence. For DBHZ2, the central benzene
ring is almost aromatic, as shown from the uniform bond
lengths (1.399, 1.409, 1.411 Å). The bonds c (1.457 Å) and d
(1.454 Å) are more like single bonds, presumably due to the
large contribution of the second resonance form (Figure 1c) to
the ground state and steric hindrance induced bond length
elongation. The calculated nucleus independent chemical shift
(NICS(1)zz)22 values of DBHZ1 showed less aromatic
character for the central benzene ring (−11.6 ppm) (Figure
4c), indicating a large contribution from the quinoidal
structure. In contrast, the NICS(1)zz values of DBHZ2
displayed greater aromatic benzenoid character for the central
benzene ring (−19.0 ppm) and two terminal rings in the
anthracene unit (−15.0 and −17.6 ppm), implying a significant
contribution from the third resonance form (Figure 1c, the
form with five sextet rings). The calculated spin densities at the
specific carbon atoms (Figure 4c) also indicate that the third
resonance form contributes the most to the ground state of
DBHZ2. For DBHZ1, however, the second resonance
contributes more than the third resonance.
DFT Calculations. DFT calculations (UCAM-B3LYP/6-
31G(d,p)) were conducted to understand the ground states of
both molecules. It was found that the energy of the SB state is
6.9 and 7.6 kcal/mol lower than the triplet biradical (TB) and
closed-shell (CS) states for DBHZ1, respectively, and the
energy of the SB state is 3.3 and 13.4 kcal/mol lower than those
of the TB and CS states for DBHZ2, respectively. Therefore,
both molecules feature a singlet biradical ground state. The
calculated singlet−triplet gap of DBHZ2 is in line with that
determined by SQUID measurements (−3.7 kcal/mol), which
further explains the observed NMR line broadening and ESR
signal. On the other hand, the relatively larger singlet−triplet
gap of DBHZ1 (−6.9 kcal/mol) agrees well with the sharp
NMR peaks and ESR silence. On the basis of the UCAM-
B3LYP method, the biradical characters y for DBHZ1 and
DBHZ2 were calculated as 0.309 and 0.576, respectively, which
are in agreement with magnetic measurements and X-ray
analysis. The molecular orbital profiles exhibit a typical
disjointed character for the ground-state singlet biradicals
D
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(Figure 5a,b) and the spin densities are evenly delocalized over
the whole π-conjugated framework for both molecules (Figure
Figure 5. Calculated (UCAM-B3LYP) SOMOs of α and β electrons of
(a) DBHZ1 and (b) DBHZ2. (c) Calculated spin density distributions
of DBHZ1 and DBHZ2. The blue and green surfaces represent α and
β spin densities, respectively.
Figure 6. OPA spectra (solid line and left vertical axis) and TPA
spectra (blue symbols and right vertical axis) of (a) DBHZ1 and (b)
DBHZ2. TPA spectra are plotted at λex/2. Insets give photographs of
the solutions in chloroform.
5c), which are similar to other higher order zethrene biradicals
such as heptazethrene dimide and octazethrene.10 All of the
calculation results clearly point out a larger biradical character
for DBHZ2 than for DBHZ1, consistent with our anticipation
from numbers of Clar aromatic sextet rings in the biradical
resonance forms.
visible region for DBHZ2 make it almost transparent at lower
concentrations in solution, which meets the criteria for many
technological applications for near-IR dyes.
The excited-state dynamics of both isomers were probed by
femtosecond transient absorption (TA) measurements (Figure
7 and Figure S8 in the Supporting Information). The TA
spectrum of DBHZ1 exhibited a ground-state bleaching (GSB)
signal around 680 nm together with weak excited-state
absorption (ESA) bands in the 450−600 and 720−800 nm
spectral regions. For DBHZ2, a GSB signal was observed
around 800 nm and an intense ESA band appeared in the 450−
750 nm spectral region. The singlet excited-state lifetimes of
DBHZ1 and DBHZ2 were estimated to be 670 and 330 ps,
respectively, and the short singlet excited-state lifetimes explain
the nonfluorescent behavior well. In comparison to DBHZ1,
the smaller singlet excited-state lifetime of DBHZ2 could be
correlated to a faster nonradiative internal conversion process
arising from a smaller energy gap between the lowest excited
state and the singlet biradical ground state.
Optical Properties. DBHZ1 is blue in chloroform solution,
and its absorption spectrum shows a p band with a maximum
peak at 687 nm (ε = 7.2 × 104 M−1 cm−1) and a shoulder peak
at 628 nm, which is common for many closed-shell PHs such as
rylenes and acenes (Figure 6a and Table 1).23 However, a weak
absorption tail can be also observed at longer wavelengths up to
850 nm, which is usually related to the open-shell singlet
biradicals.8,10 This phenomenon again supports the borderline
character of DBHZ1 between a quinoidal structure and a
biradical structure, as revealed by X-ray analysis and
calculations. In sharp contrast, the lime-colored solution of
DBHZ2 in chloroform displays a typical absorption feature for
singlet biradicaloid molecules with an intense absorption at 804
nm (ε = 1.5 × 105 M−1 cm−1) and several small bands in the
lower energy region (Figure 6b). The weak absorption bands in
the low-energy region likely originate from admixing the doubly
excited electronic configuration (H,H → L,L) into the ground
state.24 No fluorescence is observed for both compounds,
which is a common behavior of biradicaloids. Despite the large
π conjugation and the biradical character, both DBHZ1 and
Recent theoretical and experimental studies have indicated
that a moderate biradical character could enhance TPA activity
for PH systems.3,9,15 Therefore, two-photon absorption
measurements for DBHZ1 and DBHZ2 were conducted by
the Z-scan technique in toluene in the near-IR region from
1200 to 1700 nm, where one-photon absorption contribution is
negligible (Figure 6 and Figure S9 in the Supporting
DBHZ2 exhibit a good photostability with a half-life time (t1/2
)
of more than 1 week in solution upon exposure to ambient air
and light conditions (Figures S2−S7 in the Supporting
Information), which can be explained by the proper blocking
of the most reactive sites and the large spin delocalization
shown in the spin density map. It is also worth noting that the
intense and sharp near-IR absorption and weak absorption at
Information). DBHZ1 exhibited the cross-section σ(2)
=
max
530 GM at 1400 nm, which is much smaller than for DBHZ2
(σ(2)max = 2800 GM at 1600 nm). Such differences in nonlinear
optical properties must be related to the greater biradical
character of the latter. Notably, in comparison to the TPA
cross-sections of a heptazethrene derivative (HZ-DI; 920 GM
E
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a
Table 1. Photophysical and electrochemical data of DBHZ1 and DBHZ2
εmax (M−1 cm−1
)
τ (ps)
σ(2) (GM)
E1/2 (V) E1/2 (V) HOMO (eV) LUMO (eV) EgEC (eV) Egopt (eV)
ox
red
λabs (nm)
max
DBHZ1
DBHZ2
819, 687, 628
72000
670
530 (1400 nm)
−0.20
0.26
−1.72
−2.04
−1.34
−1.68
−4.54
−3.20
1.34
1.46
989, 919
148900
330
2800 (1600 nm)
−0.18
0.19
−4.57
−3.55
1.02
1.20
879, 804, 727
a
Definitions: λabs, absorption maxima measured in CHCl3; εmax, molar extinction coefficient measured at absorption maximum; τ, the singlet excited-
state lifetime obtained from TA; σ(2)max, the maximum TPA cross section. E1/2ox and E1/2red are the half-wave potentials for respective oxidation and
onset
reduction waves with Fc/Fc+ as reference. HOMO and LUMO energy levels were calculated according to the equations HOMO = −(4.8 + Eox
)
and LUMO = −(4.8 +Eredonset), where Eoxonset and Eredonset are the onset potentials of the first oxidative and reductive redox wave, respectively. EgEC is
the electrochemical energy gap derived from LUMO−HOMO, and Egopt is the optical energy gap derived from the lowest energy absorption onset in
the absorption spectra.
Figure 8. Cyclic voltammograms of DBHZ1 and DBHZ2 in CH2Cl2
with 0.1 M Bu4NPF6 as supporting electrolyte, Ag/AgCl as the
reference electrode, Au disk as the working electrode, Pt wire as the
counter electrode, and the scan rate at 50 mV/s.
singly and doubly charged species by a chemical approach.
Indeed, the stable radical cations and dications of DBHZ1 and
DBHZ2 were prepared by a stepwise oxidative titration with
SbCl5 in CH2Cl2 at room temperature and the obtained UV−
Figure 7. Transient absorption spectra of (a) DBHZ1 and (b)
vis−near-IR absorption spectra for the cationic species are
DBHZ2 recorded in toluene.
shown in Figure 9a,c. In the course of chemical oxidation,
sequential oxidation to the radical cation and then the dication
were observed when 9 and 20 equiv of SbCl5 were added for
DBHZ1 and when 5 and 11 equiv of SbCl5 were added for
DBHZ2 (Figure S11 in the Supporting Information). More-
over, both of the obtained dications can be reduced to the
neutral states by Zn powder, in accordance with the reversible
oxidative waves observed in CV measurements (Figure S11 in
the Supporting Information). Notably, all the cationic species
exhibited reasonable stability in solution, which allowed further
investigation into their physical properties. As shown in Figure
9b,d, ESR spectra were recorded for the radical cation species
of DBHZ1 and DBHZ2 in CH2Cl2 solution at room
temperature, and the radical cation of DBHZ1 exhibited an
observable fine structure with ge = 2.0026, while the radical
cation of DBHZ2 showed a single-line signal with ge = 2.0027.
No ESR signals were observed for the dications. The good
stability of the radical cations and dications for both DBHZ
isomers can be explained by the recovery of aromaticity of the
central benzenoid ring in both cases (Figure 9e).
at 1250 nm, y = 0.159) and an octazethrene derivative (OZ-
TIPS; 1200 GM at 1250 nm, y = 0.56),10c DBHZ2 also
exhibited a large enhancement. Therefore, a TPA cross-
section−singlet biradical character relationship can be estab-
lished for zethrene-based compounds.
Electrochemical Properties. Cyclic voltammertry (CV,
Figure 8) and differential pulse voltammetry (DPV) (Figure
S10 in the Supporting Information) were employed to
investigate the electrochemical properties of DBHZ1 and
DBHZ2, and both compounds display well-resolved four-stage
amphoteric redox behavior. DBHZ1 gave two oxidation waves
ox
red
at E1/2 = −0.20, 0.26 V and two reduction waves at E1/2
=
−1.72, −2.04 V, while DBHZ2 showed two oxidation waves at
ox
red
E1/2 = −0.18, 0.19 V and two reduction waves at E1/2
=
−1.34, −1.68 V (vs Fc/Fc+, Fc = ferrocene). The electro-
chemical energy gaps were determined as 1.34 and 1.02 eV for
DBHZ1 and DBHZ2, respectively, from the onset potentials of
the first oxidation and reduction wave, which are consistent
with those obtained from the optical onset (Table 1). Such low-
energy gaps are believed to be a crucial prerequisite for the
formation of a singlet biradical ground state.
III. CONCLUSION
In summary, the two soluble and stable DBHZ isomers
DBHZ1 and DBHZ2 were prepared by two different synthetic
strategies. The substitutions of bulky aryl and alkyne groups
The multistage reversible redox waves and the large
separation between the redox waves allow us to attain the
F
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Figure 9. Absorption spectra of neutral compounds, radical cations, and dications of (a) DBHZ1 and (c) DBHZ2 recorded in CH2Cl2. ESR spectra
of (b) DBHZ1 radical cation (ge = 2.0026) and (d) DBHZ2 radical cation (ge = 2.0027) in CH2Cl2 (10−3 M) solution at 298 K. (e) Schematic
representation of the one-electron-oxidation process in DBHZ isomers.
guarantee the stability and crystallinity, which facilitated the
investigation. Their electronic structures in the ground state
were systematically studied by various experimental methods
and DFT calculations, and the results clearly demonstrated a
much greater biradical character of DBHZ2 in comparison to
DBHZ1. This difference in biradical character between two
isomeric structures can be explained by the different numbers
of aromatic sextet rings in the biradical resonance forms. With
this study, Clar’s aromatic sextet rule can be extended from
closed-shell benzenoid PAHs to open-shell singlet biradicaloids
(at least applicable to our systems): that is to say, for benzenoid
polycyclic hydrocarbons with the same chemical composition, the
molecule with more aromatic sextet rings in the biradical resonance
form exhibits greater singlet biradical character. We believe that
our findings provide useful guidance for the design and study of
PHs with open-shell singlet biradical ground states in the
future.
AUTHOR INFORMATION
Corresponding Author
chmwuj@nus.edu.sg; dongho@yonsei.ac.kr; msedingj@nus.
edu.sg; kuowei.huang@kaust.edu.sa
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.W. acknowledges financial support from a MOE Tier 2 grant
(MOE2011-T2-2-130) and IMRE Core funding (IMRE/13-
1C0205). The work at Yonsei University was supported by the
Midcareer Researcher Program (2010-0029668) and the Global
Research Laboratory (2013K1A1A2A02050183) through the
National Research Foundation of Korea (NRF) funded by the
Ministry of Science, ICT (Information and Communication
Technologies) and Future Planning. K.-W.H. acknowledges
financial support from KAUST.
From another perspective, the obtained DBHZ derivatives
exhibited interesting physical properties associated with the
biradical character. For example, DBHZ2 showed intense OPA
absorption in the near-IR region and a large TPA cross-section,
and stable cationic species for both DBHZ1 and DBHZ2 were
chemically accessible due to their pro-aromatic nature.
Together with their good stability, the DBHZ derivatives
reported in this study represent promising candidates in
materials science.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures and characterization data for all new
compounds, general experimental methods, additional spectro-
scopic data, DFT calculation details, and crystallographic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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H
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