Indolo[2,3-b]carbazoles with tunable ground states: How Clar's aromatic sextet determines the singlet biradical character

Article abstract of DOI:10.1039/c4sc01843e

Polycyclic hydrocarbons (PHs) with a singlet biradical ground state have recently attracted extensive interest in physical organic chemistry and materials science. Replacing the carbon radical center in the open-shell PHs with a more electronegative nitro

Full text of DOI:10.1039/c4sc01843e

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Indolo[2,3-b]carbazoles with tunable ground
states: how Clar's aromatic sextet determines the
Cite this: Chem. Sci., 2014, 5, 4944
singlet biradical character†
a
b
c
a
a
c
Ding Luo, Sangsu Lee, Bin Zheng, Zhe Sun, Wangdong Zeng, Kuo-Wei Huang,
d
b
e
af
Ko Furukawa,* Dongho Kim,* Richard D. Webster* and Jishan Wu*
Polycyclic hydrocarbons (PHs) with a singlet biradical ground state have recently attracted extensive interest
in physical organic chemistry and materials science. Replacing the carbon radical center in the open-shell
PHs with a more electronegative nitrogen atom is expected to result in the more stable aminyl radical. In this
work, two kinetically blocked stable/persistent derivatives (1 and 2) of indolo[2,3-b]carbazole, an
isoelectronic structure of the known indeno[2,1-b]fluorene, were synthesized and showed different
ground states. Based on variable-temperature NMR/ESR measurements and density functional theory
calculations, it was found that the indolo[2,3-b]carbazole derivative 1 is a persistent singlet biradical in
the ground state with a moderate biradical character (y
0
¼ 0.269) and a small singlet–triplet energy gap
ꢀ1
(
DES–T y ꢀ1.78 kcal mol ), while the more extended dibenzo-indolo[2,3-b]carbazole 2 exhibits a
quinoidal closed-shell ground state. The difference can be explained by considering the number of
aromatic sextet rings gained from the closed-shell to the open-shell biradical resonance form, that is to
say, two for compound 1 and one for compound 2, which determines their different biradical characters.
The optical and electronic properties of 2 and the corresponding aromatic precursors were investigated
by one-photon absorption, transient absorption and two-photon absorption (TPA) spectroscopies and
electrochemistry. Amphoteric redox behaviour, a short excited lifetime and a moderate TPA cross
section were observed for 2, which can be correlated to its antiaromaticity and small biradical character.
Compound 2 showed high reactivity to protic solvents due to its extremely low-lying LUMO energy
level. Unusual oxidative dimerization was also observed for the unblocked dihydro-indolo[2,3-b]-
carbazole precursors 6 and 11. Our studies shed light on the rational design of persistent aminyl
biradicals with tunable properties in the future.
Received 21st June 2014
Accepted 14th August 2014
DOI: 10.1039/c4sc01843e
www.rsc.org/chemicalscience
Introduction
To date, most p-conjugated benzenoid polycyclic hydrocarbons
a
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
(
PHs) in the neutral form have shown a typical closed-shell
1
b
17543, Singapore. E-mail: chmwuj@nus.edu.sg
1
electronic conguration in the ground state. However, recent
Spectroscopy Laboratory for Functional p-Electronic Systems and Department of
studies revealed that certain types of PHs with special structures
Chemistry, Yonsei University, Seoul 120-749, Korea. E-mail: dongho@yonsei.ac.kr
c
could possess a singlet biradical ground state and exhibit
Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi unique optical, electronic and magnetic properties. Represen-
KAUST Catalysis Center and Division of Physical Sciences and Engineering, King
2
3
Arabia
d
tative examples include Tobe and Haley’s indenouorenes,
Center for Instrumental Analysis, Institute for Research Promotion, Niigata
4
5
6
Kubo’s bisphenalenyls and anthenes, and our zethrenes and
extended p-quinodimethanes. In most cases, the fundamental
University, 8050 Ikarashi 2-no-sho, Nishi-ku, Niigta 950-2181, Japan. E-mail:
kou-f@chem.sc.niigata-u.ac.jp
7
driving force for the appearance of a singlet biradical ground
state is the gain of additional aromatic sextet rings in the bir-
adical resonance form in comparison to the closed-shell reso-
e
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical
Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore.
E-mail: Webster@ntu.edu.sg
f
7a–d
6d,e
Institute of Materials Research and Engineering, A*STAR, 3 Research Link, 117602, nance form. In addition, steric repulsion
and substituents
Singapore
sometimes also play important roles in stabilizing the biradical
form. Aer appropriate stabilization, this new type of molecular
materials have promising applications for organic electro-
†
Electronic supplementary information (ESI) available: Synthetic procedures and
characterization data of all new compounds, additional spectroscopic data, X-ray
crystallographic data, and DFT calculation details. CCDC 1009378–1009380. For
ESI and crystallographic data in CIF or other electronic format see DOI:
3
c,4d
6–8
9,10
nics,
non-linear optics,
organic photovoltaics,
spin-
1
1
12
1
0.1039/c4sc01843e
tronics, and energy storage devices.
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Although the carbon centered singlet biradicals of pure PHs resonance form. We recently demonstrated that Clar's aromatic
17
have been intensively studied in recent years, the nitrogen sextet rule can be further extended to PH-based singlet bir-
centered singlet biradicals have rarely been investigated due to adicaloids, that is, for the benzenoid PHs with the same chemical
the synthetic challenges. Replacing the carbon radical center composition, the molecule with more aromatic sextet rings in the
with a more electronegative nitrogen atom is expected to result biradical resonance form exhibits greater singlet biradical char-
6g
in improved stability compared with the unsubstituted carbon acter. In the indenouorene cases, all of the isomers can be
centered radicals, which has been demonstrated in phenalenyl drawn with three Clar's aromatic sextet rings (highlighted in blue
13,14
monoradicals.
In this context, we are interested in devel- color) in the biradical form (Fig. 1). In the closed-shell resonance
oping new nitrogen centered biradicaloids based on a PH forms, the rst two isomers can be regarded as dibenzannulated
framework, and indenouorene analogues, i.e., indolocarba- antiaromatic 12-pe s-indacene and as-indacene, respectively,
zoles, were chosen for this study.
with two aromatic sextet rings; the third isomer can be drawn as a
Indenouorenes have ve structural isomers based on the benzannulated 16-pe antiaromatic system with only one
fusion mode, and three representative isomers are shown in aromatic sextet. Therefore, from the closed-shell to the open-
15
Fig. 1. The parent indenouorenes are all highly reactive due to shell biradical resonance form, only one additional aromatic
the signicant contribution of the biradical resonance form to sextet ring is obtained for the rst two isomers, while two addi-
the ground state, therefore, kinetic blocking of the reactive sites tional aromatic sextet rings are gained for the third isomer,
(the carbon centers with high spin density, Fig. 1) by aryl or which can explain its relatively larger singlet biradical character.
alkyne groups is necessary to obtain stable materials. It was also Based on this analysis, we rationalize that the singlet biradical
3a
found that the bulky mesityl blocked indeno[1,2-b]uorene and character is mainly determined by the number of aromatic sextet
3b
indeno[2,1-a]uorene both show a closed-shell quinoidal rings gained from the closed-shell to the singlet biradical form,
structure in the ground state, while the mesityl derivative of and an addition to Clar's aromatic sextet rule for singlet bir-
3f
indeno[2,1-b]uorene has a singlet biradical ground state with a adicaloids can be tentatively described as: for PHs with the same
moderate biradical character (y
0
¼ 0.68 for the parent compound chemical composition, the molecule which can gain more
calculated by the Yamaguchi scheme using the occupation aromatic sextet rings from the closed-shell to the biradical form
numbers of the spin-unrestricted Hartree–Fock natural exhibits the greater singlet biradical character. Similar to the
16
orbitals ). Such a difference can be simply elucidated by the indenouorenes, the nitrogen centered indolocarbazoles also
aromaticity change from the closed-shell to open-shell biradical have ve isomers and three of them are shown in Fig. 1. The indo
[
3,2-b]carbazole was demonstrated to have a quinoidal structure
18
similar to the corresponding indeno[1,2-b]uorene. To the best
of our knowledge, the indolo[2,3-a]carbazole and indolo[2,3-b]-
19
20
carbazole have never been reported. Given the structural simi-
larity with the indeno[2,1-b]uorene, the indolo[2,3-b]carbazole is
expected to show signicant singlet biradical character and thus
is the focus of the current research. This type of aminyl biradical
should show high reactivity, thus kinetic blocking at the neigh-
bouring sites is necessary for obtaining stable/persistent
compounds. For this reason, the bulky tert-butyl or 4-tert-butyl-
phenyl groups are introduced to protect the otherwise highly
reactive aminyl radicals in our target compounds such as 1
(
Fig. 1). Such a space shielding effect has been demonstrated to
21
be effective at stabilizing triplet aminyl diradicals by Rajca et al.
A kinetically blocked dibenzannulated indolo[2,3-b]carbazole
derivative 2 with more extended p-conjugation was also
prepared. Our detailed studies demonstrated that compounds 1
and 2 showed different ground states which could be correlated
to the number of aromatic sextet rings gained from the closed-
shell to the biradical resonance form. Their detailed physical
properties were also investigated by various experiments and
assisted by DFT calculations, and compared with the corre-
sponding aromatic precursors. A clear structure-biradical char-
acter-physical properties relationship was drawn at the end.
Results and discussion
Synthesis and characterization of compound 1
Fig. 1 Chemical structures of three isomers of indenofluorene and
indolocarbazole, and the two new indolo[2,3-b]carbazole derivatives
reported in this work.
Although the dihydro-indolo[2,3-b]carbazole derivatives are
20
known, the dehydrogenated products were not reported, likely
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due to their high reactivity arising from their biradical char-
acter. To block the reactive aminyl radicals, the bulky tert-butyl
and 4-tert-butylphenyl groups were introduced into the neigh-
bouring sites by regio-selective chemistry (Scheme 1). The
synthesis commenced from the nitration of 1,3-dibromo-
benzene (3) to give the 1,3-dibromo-4,6-dinitrobenzene (4) in
58% yield. A Suzuki coupling reaction between 4 and 2-(3,5-di-
tert-butylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane affor-
ded compound 5 in 83% yield, which underwent a ring closing
reaction with excess triphenylphosphine in o-dichlorobenzene
to give the tert-butyl substituted dihydro-indolo[2,3-b]carbazole
6
in 58% yield. The bromination of 6 with N-bromosuccinimide
(NBS) selectively introduced a bromo-functional group at the
carbon center between the two amine sites (7) due to its high
electron density. A subsequent Suzuki coupling reaction gave
the precursor 8 in 77% yield and the structure of compound 8 Fig. 2 ORTEP drawings and selected bond length analyses of (a) 8 and
was conrmed by NMR spectroscopy as well as X-ray crystallo- (b) 11.
22
graphic analysis (Fig. 2). The bond length analysis of the
dihydro-indolo[2,3-b]carbazole core revealed a small bond
length alternation for the benzenoid rings and pyrrole rings, was treated with excess t-BuOK (20 equiv.) in anhydrous
which is a typical characteristic for aromatic compounds.
2-MeTHF at room temperature to give the corresponding dia-
The dehydrogenation of the precursor 8 was rst conducted mino-dianions, with an orange colour. The reaction was con-
ꢁ
by treatment with PbO
2
in dry ethyl acetate at 70 C. However, ducted in a dry ESR tube under nitrogen atmosphere.
ꢁ
this gave a complicated mixture, which is likely due to the high Subsequent oxidation (2.5 equiv.) with iodine at ꢀ78 C gave the
21
reactivity of the biradical product. Alternatively, compound 8 desired compound 1, which was a dark brown color. The VT
ꢁ
Scheme 1 Synthesis of indolo[2,3-b]carbazole derivatives 1 and 2. Reagents and conditions: (a) KNO
3
, conc. H
2
SO
4
, 130 C, 2 h; (b) 2-(3,5-di-
ꢁ
tert-butylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, K
2
CO
3
, Pd(PPh
3
)
4
, 1,2-dimethoxyethane, 90 C, overnight; (c) PPh
benzene, 170 C, 2 days for 6, 1 day for 11; (d) 1 equiv. NBS, dichloromethane, rt, 1 h for 7, 2 h for 14; (e) 4-tert-butyl-phenylboronic acid,
3
, o-dichloro-
ꢁ
ꢁ
ꢁ
Pd(PPh
3
)
4
, K
2
CO
3
, toluene, EtOH, 90 C, overnight; (f) 5 equiv. t-BuOK, 2-Me-THF, rt, 30 min; (g) I
2
, 2-Me-THF, ꢀ78 C; (h) 3,7-di-tert-butyl-1-
ꢁ
ꢁ
naphthylboronic acid, Pd(PPh
3
)
, K
4 2
CO
3
2
, 1,2-dimethoxyethane, 90 C, overnight; (i) PbO , ethyl acetate, 70 C, 2 days.
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ESR spectra of this freshly prepared solution were recorded However, only one isomer 11 was isolated in 14% yield from the
from 150 K to room temperature and a well-resolved ESR complicated products. The structure of 11 was unambiguously
spectrum was observed (Fig. 3). There were no obvious changes conrmed by X-ray crystallography, showing aromatic character
25
to the spectrum shape upon slow warming up to room for all of the rings based on the bond length analysis (Fig. 2).
temperature, indicating the persistence of the obtained radical The regio-selective bromination of 11 using NBS followed by a
species under nitrogen protection. So far, the parent indeno- Suzuki coupling with 4-tert-butyl-phenylboronic acid afforded
[2,1-b]uorene has not been synthesized and it is not possible to the dihydro-dibenzoindolo[2,3-b]carbazole 15 in high yield,
compare its stability with 1. The simulated ESR spectrum of the which was conrmed by both NMR spectroscopy and X-ray
26
open-shell biradical 1 was in good agreement with the observed crystallographic analysis (ESI†).
spectrum (Fig. 3). Therefore, we can assign the observed ESR The oxidative dehydrogenation was successfully conducted
spectrum to the triplet diradical of 1. The ESR intensity was by the treatment of precursor 15 with excess PbO in dry ethyl
2
ꢁ
found to decrease when lowering the temperature (Fig. S1 in acetate at 70 C for 2 days. Aer ltration and removal of the
ESI†), indicating that the compound has a singlet ground state, solvent, a deep blue solid was obtained in nearly quantitative
which is in equilibrium with a higher energy triplet state. Fitting yield, which has good solubility in chloroform, hexane, and
the temperature dependent ESR intensity data (I–T curve) to the ethyl acetate, moderate solubility in acetonitrile and methanol,
2
3
1
Bleaney–Bowers equation gave an antiferromagnetic exchange and poor solubility in ethanol. The H NMR spectrum of the
ꢀ1
J/k
B
¼ ꢀ895 K (ꢀ1.78 kcal mol ), which can be correlated to the obtained compound can be clearly assigned to compound 2
singlet–triplet energy gap (DES–T). Such a small DES–T value (Fig. 4). Compared with the aromatic precursor 15, the reso-
indicates that the population of the thermally excited triplet nance for the NH disappeared and all of the other peaks in the
state is not negligible at room temperature, which results in an aromatic region showed signicant shis to the high eld
intense ESR signal. So, it is clear that compound 1 has a singlet region. In particular, the resonance for proton “a” was shied
biradical ground state with a roughly estimated singlet–triplet from 9.67 ppm in 15 to 6.31 ppm in 2. Such a dramatic change
ꢀ1
gap of ꢀ1.78 kcal mol . The freshly generated biradical sample can be explained by the switching from the aromatic 15 to an
showed slow decay upon standing at room temperature and fast antiaromatic conjugated quinoidal compound 2. In fact, a 16 pe
decay upon contact with air and moisture, giving an unidenti- antiaromatic moiety can by drawn for 2 (Fig. 1 and 4, high-

ed mixture.
lighted in bold form), and the paratropic ring current resulted
in a strong shielding effect for the surrounding protons. The
reaction was also conducted in other anhydrous solvents such
as chloroform, but only incomplete conversion was observed.
A dramatic change of the electronic absorption spectrum
was also observed (Fig. 5). Compound 15 displayed an intense
absorption band in the UV-visible region with an absorption
maximum at 346 nm (log 3 ¼ 4.53; 3: molar extinction coeffi-
Synthesis and characterizations of compound 2
The synthesis of the precursor 15 followed a similar strategy to
the synthesis of the precursor to 1 (Scheme 1). A Suzuki
coupling between 4 and 3,7-di-tert-butyl-1-naphthylboronic
acid gave the 1,5-dinitro-2,4-bis(3,7-di-tert-butylnaphthalenyl)
benzene (10) in 95% yield. A PPh -mediated intramolecular
24
ꢀ
1
ꢀ1
cient in M cm ), along with intense absorptions at 372 and
90 nm. However, compound 2 showed a broad absorption
3
3
cyclization could in principle form three regio-isomers 11–13,
with 5/5, 5/6, and 6/6 membered ring skeletons, respectively.
from 450 nm to 760 nm with a maximum at 600 nm (log 3 ¼
4.06), which can be correlated to a combination of electronic
Fig. 3 Observed ESR spectrum of 1 (triplet diradical) in 2-MeTHF at
3
A
00 K and the simulated ESR spectrum with parameters g
¼ 3.7 MHz (ꢂ2), A ¼ 14.8 MHz (ꢂ2), A ¼ 2.90 MHz (ꢂ2). Since the
exchange J/k
e
¼ 2.0039,
N
H
H
1
B
is much larger than the room temperature and the Fig. 4 A comparison of the H NMR spectra of compounds 15 (top)
hyperfine coupling, it is not included in the simulation.
3
and 2 (bottom) in the aromatic region (in CDCl ).
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transitions from HOMOꢀ1 / LUMO (543.6 nm, f ¼ 0.3358), comparison, the CV measurements of the aromatic precursors 8
HOMOꢀ2 / LUMO (465.8 nm, f ¼ 0.3247), and HOMOꢀ3 / and 15 were also conducted (Fig. 6 and Table 1). Compound 8
LUMO (429.5 nm, f ¼ 0.1177) based on time dependent (TD) only showed three oxidation waves with half-wave potentials
1
/2
+
DFT calculations (ESI†). The singlet excited state lifetime of (Eox ) at 0.55, 1.10, and 1.47 V (vs. Fc /Fc) due to its electron-
compound 15 was measured as 4.17 ns by the time correlated rich character. Similarly, compound 15 also exhibited three
1
/2
+
single photon counting (TCSPC) technique (Table 1 and Fig. S2 oxidations with Eox at 0.51, 1.09, and 1.49 V (vs. Fc /Fc) which
in ESI†), but compound 2 exhibited ultrafast excited state are close to those of 8. The most positive oxidation potentials for
dynamics with a singlet excited state lifetime of 4.7 ps according compounds 8 and 15 are only approximate as they appear
to femtosecond transient absorption measurements (Table 1 chemically irreversible, showing no reverse reductive peaks
and Fig. S3 in ESI†). TPA measurements were also conducted by when the scan direction was reversed following the initial
the Z-scan technique and compound 2 showed a TPA cross oxidation. Furthermore, the HOMOs of 8 and 15 were also
(
2)
section (s ) of 600 GM when excited at 800 nm. Under the same found to be similar, which were determined as ꢀ5.29 and ꢀ5.26
(
2)
conditions, 15 exhibited a s of 290 GM (Table 1 and Fig. S4 in eV, respectively, from the rst oxidation onset. In contrast, the
2
7
1/2
ESI†). The dramatic singlet excited state lifetime changes can dehydrogenated product 2 displayed two oxidation waves Eox
1
/2
be correlated to the change of the electronic structure from an at 0.82 and 1.29 V and two reductive waves with Ered at ꢀ0.42
28
+
aromatic to an antiaromatic framework.
and ꢀ1.03 V (vs. Fc /Fc). The HOMO of 2 was determined to be
1
The VT H NMR spectra of 2 in CDCl
2
CDCl
2
did not show ꢀ5.56 eV, which is ca. 0.3 eV lower than those of 8 and 15. The
ꢁ
signicant broadening upon heating up to 100 C, indicating LUMO of 2 was found to be as low as ꢀ4.48 eV, and such a low-
that 2 has a closed-shell electronic conguration in the ground lying LUMO can explain the vulnerability of 2 towards reductive
state (Fig. S5 in ESI†). Compound 2, however, was not stable on hydrogenation. On the basis of the HOMO and LUMO energies,
EC
g
silica gel or aluminium oxide columns and the dihydro- a low electrochemical energy band gap (E ) was calculated as
precursor 15 was obtained aer column chromatography with 1.08 eV for 2. In addition, the analyses of the UV-vis spectra
Opt
dichloromethane (DCM)/hexane as an eluent. Further studies (Fig. 5) gave their optical energy gaps (E
g
) as 3.22 eV for 8,
on the stability of 2 in different solvents were also conducted 2.99 eV for 15, and 1.58 eV for 2. Compared with 8, the two
and monitored by UV-vis spectroscopy (Fig. S6 in ESI†). It was additional fused benzene rings in 15 decreased the energy
found that in aprotic solvents such as chloroform and THF, the gap slightly. However, the energy gap in the quinoidal structure
optical intensity at 600 nm (I600) showed minimal changes over of 2 was found to be much lower than those in the other two
4.5 h. However, when mixing a small amount of water into the dihydro-compounds. This is further evidence for the loss of
THF (H
2
O–THF ¼ 1 : 20), the optical intensity I600 decreased aromaticity in 2.
slowly to 84% aer 4.5 h. In a protic solvent such as methanol,
almost all of the samples of 2 were converted into 15 within 1.5
h. Reductive hydrogenation also occurred when conducting
MALDI-TOF mass spectroscopic measurements, which gave a
peak for the dihydro-precursor 15. In the solid state, it could be Theoretical calculations (UCAM-B3LYP/6-31G*) were con-
stored under an inert atmosphere at room temperature for at ducted to further investigate the ground states of 1 and 2. For 1,
DFT calculations and discussion on the ground states of
and 2
1
30
least 1 week without obvious decomposition.
the singlet biradical solution with a spin contamination of
ꢀ
1
The high reactivity of 2 towards protons indicates that it can hS**2i ¼ 0.8912 is calculated to be 4.4 and 3.6 kcal mol more
be easily reduced and hydrogenated to give the more stable stable than the closed-shell and the triplet diradical forms,
aromatic compound 15. To further understand the origin of respectively, indicating that the ground state of 1 is a singlet
this, cyclic voltammetry (CV) was performed to investigate its biradical. The calculated small singlet–triplet energy gap
ꢀ
1
redox behavior and to determine its energy levels. For (DES–T ¼ ꢀ3.6 kcal mol ) and moderate singlet biradical
character y of 0.269 imply its intermediate biradical nature
0
between the typical closed-shell state and a pure diradical state.
The singly occupied molecular orbital (SOMO) proles of the a
and b spins of 1 show disjoint features and the spins are evenly
distributed along the whole p-conjugated framework (Fig. 7a).
On the other hand, the ground state of 2 was calculated to be a
ꢀ1
closed-shell state that is 4.1 kcal mol more stable than its
open-shell triplet state. The HOMO and LUMO from the
closed-shell solution (B3LYP/6-31G*) exhibit an extended
delocalization through the dibenzo-indolo[2,3-b]carbazole core,
representing the most reactive sites (Fig. 7b). These calculated
results are in good agreement with the experimental results.
31
Nucleus-independent chemical shi (NICS) calculations
were then conducted for the ground state of 1 (singlet biradical)
and 2 (closed-shell quinoid) (Fig. 8). Calculations show that
both the central diaza-as-indacene units have large positive
3
Fig. 5 UV-vis absorption spectra of compounds 15 and 2 in CHCl .
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a
Table 1 Electrochemical and optical data for 8, 15, and 2
(2)
s
1
/2
1/2
Comp.
E
red (V)
E
ox (V)
HOMO (eV)
LUMO (eV)
s (ps)
(GM)
<50
8
—
0.55
.10
.47
ꢀ5.29
—
4040
1
1
1
5
—
0.51
ꢀ5.26
ꢀ5.56
—
4170
4.8
290
600
1
1
.09
.49
2
ꢀ0.42
0.82
1.29
ꢀ4.48
ꢀ1.03
a
1/2
1/2
+
E
red and Eox are half-wave potentials of the reductive and oxidative waves (vs. Fc /Fc), respectively. The HOMO and LUMO are determined by
onset onset onset onset
the equations: HOMO ¼ ꢀ(4.8 + Eox ) and LUMO ¼ ꢀ(4.8 + Ered ), where Eox and Ered are the onset potentials of the rst oxidative and
29
reductive waves, respectively.
NICS(1)zz values, indicating that both have a large antiaromatic
character. 2 showed more positive values than 1, suggesting that
it is less aromatic. The outmost benzene rings in 1 exhibit a
nearly zero NICS(1)zz value with a non-aromatic character. The
two outmost naphthalene rings in 2 display negative NICS(1)zz
values and thus are more aromatic. The calculated bond lengths
also showed much larger alternation compared with the
aromatic precursors. The difference of their ground states can
be elucidated by the number of aromatic sextet rings gained
from the closed-shell to the open-shell biradical form. As shown
in Fig. 1, two more aromatic sextet rings are gained for 1 but
only one aromatic sextet ring is obtained for 2, thus 1 has a
greater driving force favouring the biradical form that results in
Fig. 6 Cyclic voltammograms of 8, 15, and 2 in dry DCM with 0.1 M
Bu NPF
as the supporting electrolyte, Ag/AgCl as a reference elec- a larger biradical character.
trode, Au disk as a working electrode, Pt wire as a counter electrode,
4
6
ꢀ1
and a scan rate of 50 mV s
.
Unexpected oxidative dimerization of 6 and 11
During the synthesis of 1 and 2, we also attempted the oxidative
dehydrogenation of the unblocked diamine intermediates 6
and 11 by using an excess of PbO in chloroform. Interestingly,
2
the homo-coupled dimers 16 and 18 were obtained in 65% and
63% yields, respectively (Scheme 2) aer overnight reuxing.
Extension of reaction time up to 3 days in chloroform or ethyl
acetate resulted in partial decomposition of the as-formed
products 16 and 18 and did not generate the predicted qui-
noidal products 17 and 19 or dihydrazine products (via intra-
molecular N–N bond formation). Compounds 16 and 18 showed
Fig. 7 (a) Calculated (UCAM-B3LYP/6-31G*) SOMOs for the a and b
electrons and spin densities distribution of the singlet biradical 1. (b)
Calculated (B3LYP/6-31G*) HOMO and LUMO of the closed-shell 2.
Fig. 8 Calculated bond lengths and NICS(1)zz values of 1 (singlet
biradical) and 2 (closed-shell quinoid).
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long singlet excited state lifetimes (3.13 and 3.72 ns, respec-
tively, Fig. S3 in ESI†), similar to those of the corresponding
Acknowledgements
aromatic monomers 8 and 15. The unusual oxidative dimer- J. W. acknowledges the nancial support from MOE Tier 2
ization provided evidence of the high reactivity of the central grants (MOE2011-T2-2-130, MOE2014-T2-1-080), MINDEF-NUS
benzene moiety in the dihyro-indolo[2,3-b]carbazole molecules JPP Grant (12-02-05), and A*STAR JCO grant (1431AFG100). The
and the effectiveness of our regio-selective chemistry.
work at Yonsei Univ. was supported by Mid-career Researcher
Program (2010-0029668) and Global Research Laboratory
(
2013K1A1A2A02050183) through the National Research Foun-
Conclusions
dation of Korea (NRF) funded by the Ministry of Science, ICT
In summary, the current study provided the rst example of (Information and Communication Technologies) and Future
systematically investigating the open-shell features of qui- Planning. K.-W. H. acknowledges the nancial support from
noidal indolo[2,3-b]carbazole derivatives. Bulky protecting KAUST. We thank Dr Tan Geok-Kheng for crystallographic
groups were successfully introduced to kinetically stabilize the analysis.
two target compounds 1 and 2. It is interesting to observe that
compound 1 is a persistent singlet biradical with a moderate
Notes and references
biradical character (y
0
¼ 0.269) and a small singlet–triplet
ꢀ1
energy gap (DES–T y ꢀ1.78 kcal mol ) while the more p-
extended compound 2 is a stable closed-shell quinoid in the
ground state, which was proved by various experimental
measurements and DFT calculations. The origin of this
difference can be simply correlated to the number of aromatic
sextet rings gained from the closed-shell to the open-shell
biradical form, which is an addition to Clar's aromatic sextet
rule for singlet biradicaloids. Compared with the aromatic
precursors, both compounds 1 and 2 have large non-aromatic
character and compound 2 showed ultrafast excited dynamics
and a moderate TPA cross section. Compound 2 also showed
high reactivity to protic solvents due to its extremely low-lying
LUMO energy level. An unexpected dimerization of the
unblocked dihydro-indolo[2,3-b]carbazoles was observed
due to their unique electronic structure, which also allowed
regio-selective introduction of a bulky protecting group.
These studies disclose how the aromatic sextet determines
the ground-state electronic structures and also provide guid-
ance to design nitrogen centered diradicals and diradical
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comes from absorption of 8.
7
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d) Z. Zeng, S. Lee, J. L. Zafra, M. Ishida, N. Bao,
H
58
C
l6
N
2
. M
w
: 851.64;
˚
(
monoclinic; space group P2(1)/c; a ¼ 13.6043(18) A, b ¼
˚
˚
ꢁ
ꢁ
R. D. Webster, J. T. L ´o pez Navarrete, J. Ding, J. Casado,
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Am. Chem. Soc., 2011, 133, 16342.
29.340(4) A, c ¼ 11.7173(15) A, a ¼ 90 , b ¼ 101.006(3) , g
ꢁ
˚
3
ꢀ3
¼ 90 ; V ¼ 4590.9(10) A ; Z ¼ 4; rcalcd ¼ 1.232 Mg m ; R
¼ 0.0489, wR ¼ 0.1210 (I > 2s(I)); R ¼ 0.0632, wR
1
¼
2
1
2
0.1279 (all data). CCDC number: 1009379 (see ESI†).
8
9
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42 48 2 w
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˚
J. Lee, P. Jadhav, P. D. Reusswig, S. R. Yost, N. J. Thompson,
monoclinic; space group C2/c; a ¼ 18.183(5) A, b ¼
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˚
˚
ꢁ
ꢁ
ꢁ
1
7.083(5) A, c ¼ 10.329(3) A, a ¼ 90 , b ¼ 97.513(5) , g ¼ 90 ; 28 (a) T. K. Ahn, J. H. Kwon, D. Y. Kim, D. W. Cho, D. H. Jeong,
3
ꢀ3
˚
V ¼ 3181.0(15) A ; Z ¼ 4; r
¼ 1.213 Mg m ; R ¼ 0.0488,
S. K. Kim, M. Suzuki, S. Shimizu, A. Osuka and D. Kim, J. Am.
Chem. Soc., 2005, 127, 12856; (b) M. Ishida, J.-Y. Shin,
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calcd
1
wR ¼ 0.1163 (I > 2s(I)); R ¼ 0.0676, wR ¼ 0.1260 (all data).
2
1
2
CCDC number: 1009378 (see ESI†).
66 76 2 w
6 Crystallographic data for 15: C H N . M : 897.29;
2
˚
monoclinic; space group C2/c; a ¼ 25.799(6) A, b ¼ 29 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt,
˚
˚
ꢁ
ꢁ
1
9
0
8.171(4) A, c ¼ 11.441(3) A, a ¼ 90 , b ¼ 103.053(4) , g ¼
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ꢁ
˚
3
ꢀ3
0 ; V ¼ 5225(2) A ; Z ¼ 4; rcalcd ¼ 1.141 Mg m ; R
1
¼
.0839, wR ¼ 0.1196, wR ¼ 0.2628
2
¼ 0.2355 (I > 2s(I)); R
1
2
(
all data). CCDC number: 1009380. The 4-tert-butylphenyl
is disordered and had to be rened under PART-1. The 31 H. Fallah-Bagher-Shaidaei, C. S. Wannere, C. Corminboeuf,
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2
7 Our TPA set-up is 1200–2400 nm and 800 nm and we cannot
measure at OPA wavelengths shorter than 600 nm except 400
nm. Therefore, herein only one excitation wavelength (800
nm) was used. This makes it difficult to compare the TPA
spectra of the aromatic 15 and antiaromatic 2.
4952 | Chem. Sci., 2014, 5, 4944–4952
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