Synthesis and characterization of symmetric cyclooctatetraindoles: Exploring the potential as electron-rich skeletons with extended π-systems

Article abstract of DOI:10.1021/ol501083d

A facile one-pot tetramerization of indolin-2-one with phosphoryl chloride was applied for the first convenient direct synthesis of C_2v- symmetric cyclooctatetraindole with an 8π annulene as the center. Tetra- and octa-arylated cyclooctatetraindole derivatives functionalized with fluorescent fluorene and pyrene units were thus facilely synthesized and characterized.

Full text of DOI:10.1021/ol501083d

Letter
pubs.acs.org/OrgLett
Synthesis and Characterization of Symmetric Cyclooctatetraindoles:
Exploring the Potential as Electron-Rich Skeletons with Extended
π‑Systems
Fang Wang,†,§ Xiang-Chun Li,†,§ Wen-Yong Lai,*,†,‡ Yao Chen,† Wei Huang,*,† and Fred Wudl‡
†Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of Advanced Materials (IAM), Nanjing
University of Posts and Telecommunications, Nanjing 210023, China
‡California NanoSystems Institute (CNSI), University of California, Santa Barbara, California 93106-6105, United States
S
* Supporting Information
ABSTRACT: A facile one-pot tetramerization of indolin-2-one with
phosphoryl chloride was applied for the first convenient direct synthesis of
C2v-symmetric cyclooctatetraindole with an 8π annulene as the center. Tetra-
and octa-arylated cyclooctatetraindole derivatives functionalized with
fluorescent fluorene and pyrene units were thus facilely synthesized and
characterized.
ndole related molecules are important heteroaromatic
Icompounds in diverse natural products and compounds
displaying significant pharmacological and biological activities.1
They are also being extensively investigated in well-known hole-
transporting, light-emitting, and light-harvesting materials and,
therefore, are considered as potential building blocks for
functional materials due to their excellent electrical and thermal
properties.2−4 Many indole-based polymers and oligomers have
been widely explored. Among them, new indole-related building
blocks such as indolo[3,2-b]carbazole,3 isoindigo,4 and 10,15-
dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (triindole or triaza-
truxene, TAT) (Figure 1)5−8 have attracted much recent interest.
This kind of compound constitutes two or three indole units
linked together or fused via an extra six-membered benzene ring
Figure 1. Chemical structures of (a) indole, (b) fused indolo[3,2-
b]carbazole, (c) isoindigo, (d) triazatruxene or cyclic triindole (TAT),
(e) cyclic tetraindole (TTI), and (f) the optimized geometry structure
for TTI.
with extended π-systems. Such novel structures have proven to
be very promising platforms and are being widely investigated in
the field of organic electronics, i.e. organic light-emitting diodes,5
organic solar cells,4,6 field-effect transistors,4 liquid crystals,7
biosensors,8 etc.
Polycyclic hydrocarbons with special topologies have been the
subject of particular attention.9 Cyclooctatetraene (COT) is one
of the typical flexible π-conjugated skeletons,10 which has an 8π
annulene possessing an inherently nonplanar saddle-shaped
geometry with D2d symmetry. Various sophisticated COT
structures fused with phenyl, thiophene, furan, pyrimidine,
thiazole, and benzothiophene have been designed and
synthesized;11 these COT-based materials have found interesting
applications as cavity-size-controlled cage molecules,12 electro-
mechanical actuators,13 buckycatchers,14 and molecular tweez-
ers.15 To the best of our knowledge, no such COT fused with
indole units has been known until recently.16
Cyclic indole derivatives, more specifically triindole or
triazatruxene (TAT), recently emerged as strong electron-
donating and hole-transporting motifs. Recent works by our
group and others have established their intriguing potentials as
high-performance organic semiconductors for optoelectronics.5,6
In contrast, the symmetric cyclic tetramer COT analogue
5,10,15,20-tetrahydrotriindolo[2′,3′:3,4:2′,3′:5,6:2′,3′:7,8]
cycloocta[1,2-b]indole (tetraindole, TTI) has been very rarely
studied, probably due to difficulties in its synthesis.17,18
A
Received: April 15, 2014
© XXXX American Chemical Society
A
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Organic Letters
Letter
theoretical simulation indicates that the cyclic tetraindole rings
form a saddle-shaped structure (Figure 1f), with an 8π COT
annulene as the center, that could be transformed into an
aromatic “planar” form by reduction to a 10π dianion or by
Suzuki coupling reactions,19 affording the tetra- and octa-arylated
tetraindoles efficiently (21−48%). The resultant starbursts are
readily soluble in common solvents, such as CHCl3, THF, and
toluene.
oxidation to a 6π dication according to Huckel’s rule.
The structures of the resultant cyclooctatetraindoles were well
̈
1
Furthermore, owing to its cyclic electron-rich tetraindole
structure and its twisted saddle-like structure, this C2v-symmetric
motif may also be interesting as a central skeleton for the
construction of novel molecules with extended π-systems.
As part of our continuing efforts and interest in cyclic indole
derivatives,5 we report herein a facile one-pot procedure for the
first direct synthesis of symmetric cyclic tetraindole. Its potential
as electron-rich building blocks for constructing extended π-
systems has also been investigated.
identified by MALDI-TOF mass spectra, H and 13C NMR
spectra, and elemental analysis (Figures S1−S22). The results
unequivocally confirmed the symmetrically cyclic nature of TTI.
MALDI-TOF spectra revealed the molecular ion peak of 461,
consistent with that of the indole tetramer. The 1H NMR of TTI
shows three signals of aromatic protons (7.56 (d, 4H, J = 7.6 Hz),
7.41 (d, 4H, J = 7.7 Hz), and 7.09 (dt, 8H, J = 14.5, 7.0 Hz) ppm)
in CD3SOCD3. These signals indicate a highly symmetrical
structure. The successful cyclotetramerization is also evident
when comparing the 1H NMR of TTI with those of the starting
material indolin-2-one and TAT as shown in Figure 2. The NH-
Synthesis of cyclic tetraindole was carried out in neat POCl3
under dry conditions using a one-pot process (Scheme 1).
Scheme 1. One-Pot Synthesis toward Symmetric Cyclic
Indoles
Unexpectedly, treatment of indolin-2-one in POCl3 provided
triindole and tetraindole mixtures. The isolated yields of the
products were varied with different reaction conditions that were
found to be significantly dependent upon the added amount of
POCl3 (Table S1). When indolin-2-one (3 g, 14.5 mmol) was
treated with phosporyl chloride (POCl3, 20.0 mL) at 100 °C for
8 h, TTI (16%) and TAT (18%) were afforded. Following
alkylation with 1-bromohexene in basic conditions, C6TTI was
afforded in high yield (80%). N-Bromosuccinimide (NBS) was
found to be successful for the direct bromination of C6TTI.
Tetra- and octabrominated tetraindoles were then achieved
easily by adjusting the molar ratios of NBS.
To investigate the impact of the cyclooctatetraindole core
structure on functions of the resulting molecules, the bromo
substituents were then replaced by fluorescent fluorene and
pyrene units. The synthetic routes to tetra- and octa-arylated
tetraindoles, 4FTTI, 8FTTI, 4PyTTI, and 8PyTTI, are shown in
Scheme 2. A well-developed microwave-enhanced multiple
coupling strategy was adopted to accelerate the 4- and 8-fold
Figure 2. 1H NMR of TTI, TAT, and indolin-2-one.
protons for TTI appeared at 11.43 (s, 4H) ppm, while the
1
chemical shift of the N−H bond in the H NMR spectra of
indolin-2-one and TAT appeared at 11.35 and 11.85 ppm,
respectively. The existence of the N−H bond in the tetramer
according to the 1H NMR spectra proved the symmetric
cyclocondensation of the tetraindole occurred at the 2,3-position
of indolin-2-one (Figure 2). Further evidence was also given in
the 13C NMR spectrum, in which eight carbon signals were
presented at 137.8, 135.2, 127.8, 121.7, 120.0, 119.2, 111.8, and
105.0 ppm. For the reaction with 2,3-couplings of monomeric
indole units, only a symmetric cyclotrimer and -tetramer were
obtained. Based on the above experimental investigations, the
formation of cyclic triindole and tetraindole could probably be
elucidated by chlorination of indolin-2-one in the presence of
POCl3 and a further stepwise cyclocondensation mechanism
(Scheme S3).
Scheme 2. Synthetic Routes to Arylated Cyclooctatetraindoles
Thermogravimetric analysis revealed that the resultant
cyclooctatetraindoles are thermally stable with 5% weight loss
temperature up to 360 °C for C6TTI and over 400 °C for the
arylated ones (Figure S23, Table 1). No weight loss below 300
°C was detected for all the samples, indicative of good thermal
stability. Reversible DSC curves were recorded for C6TTI with a
sharp endothermic peak at 129 °C during heating and a sharp
exothermic peak at 65 °C during cooling (Figure S24), which was
ascribed to the melting and crystallization process, respectively.
In contrast, no distinctive melting or crystallization transitions
were observed during the DSC scans from rt to 250 °C for all
arylated tetraindoles, suggesting that the resultant starbursts
could be amorphous. The amorphous characteristics of the
samples were further confirmed by wide-angle X-ray diffraction
B
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Letter
Table 1. Thermal and Photophysical Properties of the Cyclooctatetraindoles
a
b
c
Td (°C)
λabs,solution (nm)
εmax (104 L·mol−1·cm−1) [log ε]
λabs,film (nm)
λPL,DCM (nm)
λPL,film (nm)
λonset (nm)
Egopt (eV)
C6TTI
4FTTI
8FTTI
4PyTTI
8PyTTI
360
412
406
423
433
312
340
301
325
324
2.89 [4.46]
5.10 [4.71]
3.14 [4.50]
4.37 [4.64]
321
365
339
348
346
352
382
368
508
558
494
466
471
479
548
410
430
438
458
550
3.02
2.88
2.76
2.71
2.25
3.49 [4.54]
a
b
c
Temperature corresponding to 5% weight loss. λonset was the onset for the film absorption spectra. Eg (optical energy gap) calculated from the
absorption onset of film spectra.
(WAXD) patterns (Figure S25). The good thermal and
morphological stability of these materials enables the preparation
of homogeneous and stable amorphous thin films by solution
processing, which is crucial for the applications for organic
optoelectronics.
The optimized geometries show that the saddle-shaped
tetraindole core is significantly twisted, rendering the resultant
molecules with a twisted nonplanar structure (Figure 1f).
Furthermore, the resultant tetraindoles take a tub conformation
with an average bend angle θ of 51.57°, 51.55°, 51.46°, and
51.41°, for 4FTTI, 4PyTTI, 8PyTTI, and 8FTTI, respectively
(Scheme S4). These values are a bit smaller than that of TTI (θ =
53.12°). The steric repulsion upon the increase in ortho-
substituted chromophores may play a role in increasing the
planarity of these analogues. These geometrical characteristics
are believed to be beneficial to the formation of the amorphous
shape for these compounds and, thus, inhibit the molecular
crystallization process. This has been evidenced as revealed in
thermal and morphology analysis. For 4FTTI, and 8FTTI, an
obvious overlap exists between the molecular HOMO and
LUMO orbitals (Scheme S6). This overlap indicates that
delocalization occurs over the tetraindole core structure.
However, for 4PyTTI and 8PyTTI, the LUMO is dominated
by the pyrene units for both compounds, while the HOMO is
mainly on the tetraindole unit, with little extension to the pyrene
segment in 8PyTTI. The HOMO−LUMO energy gaps of the
tetraindoles were found to be in the range of 2.83−3.77 eV
(B3LYP/6-31G(d), Scheme S6).
The electrochemical characteristics were studied by cyclic
voltammetry (CV) to further reveal the electronic properties.
The data are collected in Table S2, in comparison with those
calculated with B3LYP/6-31G(d). C6TTI exhibited two-
electron oxidation processes with a rather low onset oxidation
potential (0.32 V), which corresponds to a HOMO value of −5.3
eV. The low onset oxidation potential suggests good electron-
donating properties of the resulting tetraindole core, consistent
with its electron-rich nature considering the symmetric cyclic
tetraindole core structure with four nitrogen bridges. The
oxidation process of the C6TTI film was pseudo reversible,
whereas the reduction process was essentially irreversible (Figure
3a). Such a redox behavior is reasonable considering the good
electron-donating ability for the cyclic tetraindole core structure,
making the system more susceptible to oxidation (p-doping)
rather than reduction (n-doping).
The photophysical data of the compounds are collected in
Table 1 and Figure S26. For C6TTI, the absorption spectra in
dilute DCM solution show an intense π−π* absorption peak at
312 nm, whereas the absorption spectrum in film (Figure S27)
exhibits a strong π−π* transition at 321 nm with a tail at low
energy that might be related to excimers or aggregates of the
tetraindole molecules. The emission spectrum of C6TTI is
located in the near-ultraviolet region with peak emission at 352
nm, while its thin film displays a large red shift at 494 nm. The
arylated tetraindoles exhibit strong π−π* electron absorption
bands, with peaks at 340, 301, 325, and 324 nm, respectively, in
dilute DCM solution, whereas their films showed absorption
peaks at 365, 339, 348, and 346 nm, respectively. Increasing the
ortho-substituted fluorene units alongside the tetraindole core
structure resulted in significant blue shifts (of 39 and 26 nm)
from 340 nm for 4FTTI to 301 nm for 8FTTI in dilute solution
and from 365 nm for 4FTTI to 339 nm for 8FTTI in thin films.
The results suggest a nonplanar structurally hindered geometry
induced by the ortho substituents that limit π-delocalization
through the tetraindole core. The optical band gaps derived from
the absorption edge of film spectra of arylated cyclooctatetra-
indoles gave values ranging between 2.25 and 2.88 eV, while
C6TTI showed a large optical gap of 3.02 eV.
The emission maxima of 4FTTI, 8FTTI, 4PyTTI, and
8PyTTI were recorded at 382, 368, 508, and 558 nm in solution,
respectively, whereas they are at 466, 471, 479, and 548 nm,
respectively, in films. Significant red shifts in emission spectra
were observed for 4FTTI (84 nm) and 8FTTI (103 nm) upon
moving from a dilute solution to thin films. To understand the
origin of the red-shifted emission, PL spectra with increasing
concentration were recorded (Figure S28). Clearly, the resultant
arylated tetraindoles exhibited distinct concentration dependent
luminescence. With increasing concentration, the long-wave-
length emission in the range of 500−600 nm increased
progressively. Considering the PL emission peak at 494 nm for
C6TTI, such a red-shifted emission could be ascribed to the
combination of the radiative decay from the tetraindole core
and/or from the molecular aggregates or excimers. According to
the absorption spectra of 8PyTTI films in Figure S26d, the
aggregate formation was also evident by the appearance of a long-
wavelength broad tail ranging from 430 to 550 nm. This red-
shifted emission became dominant especially for the pyrene-
functionalized tetraindoles at high concentrations probably
because of the rigid pyrene structure that tends to form
aggregates or excimers in such a crowded structure. Theoretical
calculations were performed to further interpret this phenom-
enon.
The anodic CVs of arylated cyclooctatetraindoles are depicted
in Figure 3b. All these compounds exhibited overlapped
Figure 3. CV curves of cyclooctatetraindoles in films.
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Letter
multireversible oxidation waves, attributed to the oxidation of the
cyclic tetraindole core and the aryl substituents. The onset of the
oxidation potentials was found to be 0.50, 0.70, 0.32, and 0.47 V,
respectively, corresponding to HOMO values in the range of
−5.03 to −5.41 eV. The LUMO levels were deduced from the
HOMO and optical band gaps ranging from −2.32 to −2.93 eV
(Table S2).
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In summary, we have applied a facile and simple one-pot
synthetic procedure for the first direct synthesis of C2v-symmetric
cyclic tetraindole with an 8π annulene as the center. A
cyclocondensation mechanism has been proposed for the
formation of cyclic tri- and tetraindole via treatment of indolin-
2-one with POCl3. Based on the core structure, a novel series of
tetra- and octa-arylated tetraindole derivatives with fluorene and
pyrene subsituents have thus been facilely constructed. The
thermal, optical, and electrochemical properties were inves-
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functional properties of the resultant molecules. The results
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AUTHOR INFORMATION
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Corresponding Authors
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Author Contributions
§These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We acknowledge financial support from the National Key Basic
Research Program of China (973 Program, 2014CB648300,
2009CB930601), the National Natural Science Foundation of
China (20904024, 51173081, 61136003, 61106036), the Natural
Science Foundation of Jiangsu Province (BK20130037,
BK2011760), Program for New Century Excellent Talents in
University (NCET-13-0872), Specialized Research Fund for the
Doctoral Program of Higher Education (20133223110008), the
Ministry of Education of China (IRT1148), the NUPT Scientific
Foundation (NY213119, NY210016), the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD), the Six Talent Plan (2012XCL035), and the Qing Lan
Project of Jiangsu Province. We are grateful to Weidong Xu and
Xiaochun Fan at IAM for the experimental and characterization
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