Dibenzo[2,3:5,6]pyrrolizino[1,7-bc]indolo[1,2,3-lm]carbazole: A new electron donor

Article abstract of DOI:10.1039/c0nj00100g

The new heterocyclic system, dibenzo[2,3:5,6]pyrrolizino[1,7-bc]indolo[1,2, 3-lm]carbazole 3, which can be considered as a planar hybrid of carbazole and p-phenylenediamine, can readily be prepared from commercial precursors and possesses high thermal stability and strong electron donor properties comparable to tetra- and pentacene.

Full text of DOI:10.1039/c0nj00100g

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www.rsc.org/njc | New Journal of Chemistry
Dibenzo[2,3:5,6]pyrrolizino[1,7-bc]indolo[1,2,3-lm]carbazole:
a new electron donorw
Claude Niebel,a Vladimir Lokshin,a Amos Ben-Asuly,b Wladimir Marine,a
Artak Karapetyanac and Vladimir Khodorkovsky*a
Received (in Montpellier, France) 7th February 2010, Accepted 8th April 2010
First published as an Advance Article on the web 14th May 2010
DOI: 10.1039/c0nj00100g
The new heterocyclic system, dibenzo[2,3:5,6]pyrrolizino-
[1,7-bc]indolo[1,2,3-lm]carbazole 3, which can be considered as
a planar hybrid of carbazole and p-phenylenediamine, can readily
be prepared from commercial precursors and possesses high
thermal stability and strong electron donor properties comparable
to tetra- and pentacene.
oligomerization.7 Further structural variations of carbazoles
have demonstrated the potential of indolocarbazoles 18 and,
very recently, bisindoloquinolines 29 for applications involving
charge transfer.
Here we report on the preparation and properties of hitherto
unknown parent heterocyclic system, dibenzo [2,3:5,6]pyrrolizino-
[1,7-bc]indolo[1,2,3-lm]carbazole 3, which can be considered as
a planar hybrid of carbazole and p-phenylenediamine. This
compound can be prepared both via C–C (Scheme 1, route A)
and C–N (route B) cyclization. Route A employs arylation of
5,11-dihydroindolo[3,2-b]carbazole with o-iodonitrobenzene
(59% yield), reduction of the resulting o-nitrophenyl derivative
by SnCl2 to the o-diaminophenyl derivative (61% yield).
Diazotization of the latter in sulfuric acid provides bis-diazonium
Organic p-conjugated compounds are the focus of much
current research owing to their potential as components of
charge transporting, nonlinear optical and other advanced
materials. In particular, derivatives of triaryl amines and
carbazole are considered to be the most promising classes of
organic electron donors and are the subjects of intensive
investigations. Applications related to photoconductivity
require high stability of both electron donors and their oxidized
forms and, therefore, several structural modification patterns
have been exploited.1 For instance, the known instability of
the triaryl amine cation radicals, which undergo dimerization
to form tetraarylbenzidines2 has led to the design of fully
reversible redox systems3 involving the p-phenylenediamine
moiety.4 Carbazoles, involving a larger planar aromatic system,
exhibit also strong fluorescence5 and enhanced thermal
stability. These derivatives can also serve as the efficient
electron donating moieties in two-photon absorbing (TPA)
chromophores.6 However, their oxidation gives rise to
a Centre Interdisciplinaire de Nanoscience de Marseille CINaM (UPR
´
´
´
CNRS 3118), Universite de la Mediterranee, 13288 Marseille Cedex
9, France. E-mail: khodor@cinam.univ-mrs.fr;
Fax: +33 4 9182 9301
b Department of Chemistry, Ben Gurion University of the Negev,
Beer Sheva, 84105, Israel
c Institute for Physical Research, NAS of Armenia, Ashtarak-2,
Armenia
w Electronic supplementary information (ESI) available: Experimental
part, spectra fitting details, TD B3LYP calculation results, CV of 3
and EPR spectrum of the cation radical. CCDC 706871. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0nj00100g
Scheme 1
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salt 4, which can be isolated quantitatively as the purple
perchlorate.
Refluxing the perchlorate in acetic acid with or without
copper powder afforded a mixture of 3 along with the
symmetrical and asymmetrical isomers 6 and 7, from which
pure 3 can be isolated in 5–9% yield. The same reaction occurs
when solid 4 or its DMSO solution are exposed to light. It is
noteworthy that upon light exposure solid 4 forms 3 in
10–12% yield without admixture of the isomers, probably
owing to the fixed favorable conformation within the solid.
Route B is more suitable for the large-scale preparation of 3.
Derivative 5 can be prepared in two steps from indole and
o-chlorobenzaldehyde.10 It undergoes cyclization to 3 upon
heating in DMF in the presence of tetrabutylammonium
hydroxide and catalytic amount of CuI in 74% yield. The
second approach is more flexible and allows substituent
variations. Thus, starting from 2-chloro-4,5-dimethoxy-
benzaldehyde, 2,3,11,12-tetramethoxy derivative of 3 was
obtained in 75% yield on the cyclization step.
The new heterocycle 3 possesses high thermal stability: it
does not melt and sublimes above 300 1C. The X-ray structure
determination showed that the molecule is almost planar (the
terminal benzenic rings are twisted by 51 from the molecular
plane). The molecules in crystal form slipped p-stacks with the
distance between each molecular plane of 3.380 A (Fig. 1).
In solution 3 exhibits a number of unique features. It shows
strong fluorescence (the quantum yield is close to 100% in
toluene) and features unprecedentedly small Stokes shift of 3 nm
(Fig. 2). However, both visible absorption and fluorescence
maxima are only parts of the broader vibronically split bands.
Surprisingly, the same is true for TPA. Whereas the TPA cross
section expectedly is not large (about 20 GM at 850 nm), the
narrow peaks that cover all the investigated range are apparently
vibronically split as well. Indeed, both the one-photon and
TPA spectra can easily be fitted using the Pekarian functions11
with the high precision (see ESIw for the details).
Deconvolution showed that minimum two Pekarian functions
are needed to reproduce the shape of the fluorescence and TPA
spectra, whereas four such functions were needed for the
one-photon absorption (OPA) spectra. The determined from
fitting absorption maxima are given in Table S1 (ESIw). These
are in good agreement with the electronic absorption spectra
calculated using the TD B3LYP/6-31G(d,p) method (Table S1,
ESIw).12 The individual deconvoluted band shapes of OPA,
TPA and fluorescence are strikingly similar. In particular, the
fluorescence band shape indicates that fluorescence occurs
from two almost degenerate excited states.
Fig. 1 Molecular and crystal structure of 3 (thermal ellipsoids are
presented at 50% of probability). Selected bond lengths in A: C1–C2
1.366(14); C1–C6 1.411(12); C2–N1 1.384(12); C13–N1 1.437(10);
C13–C18 1.438(14); C6–C30 1.476(13).
10ꢁ4 M. Only poorly discernible oxidation peaks were
observed in warm DMSO at about 1 V (vs. Ag/AgCl), which
is very close to the peak oxidation potential of N,N0-diethyl-
5,11-dihydroindolo[3,2-b]carbazole prepared by alkylation of
the N-unsubstituted derivative. The immediate deposition of a
black shining product on the working electrode at this potential
confirmed that oxidation indeed occurs, but prevented
verification of its reversibility. However, the green cation
radical of 3 can easily be prepared by chemical13 oxidation.
It is also poorly soluble and features a broad vibronically split
absorption band centered at about 730 nm and the EPR signal
can be seen in benzonitrile and propylene carbonate. In the
presence of trace amounts of water, a part of the radical cation
species undergo reduction to 3 and the neutral species interact
with the radical cations producing p-stacked aggregates, which
can exist in the solution as dimers and/or oligomers. This
process can be monitored by the UV-Vis-NIR spectral
The electron donating properties of 3 could be evaluated
using cyclic voltammetry; however, the solubility of this
compound in most of the suitable solvents does not exceed
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c 10.982(5) A, a 901, b 99.368(5)1, g 901, U = 948.0(10) A3,
T = 293(2) K, Z = 2, 7689 reflections measured, 1298 unique,
1018 with I > 2s(I) were used in all calculations. The final
R(F2) was 0.0850 (observed data). CCDC 706871.
Dibenzo[2,3:5,6]pyrrolizino[1,7-bc]indolo[1,2,3-lm]carbazole (3)
(a) Route A: 2-[11-(2-aminophenyl)indolo[3,2-b]carbazol-
5(11H)-yl]aniline (0.25 g, 0.57 mmol) was dissolved in 6 ml
of acetic acid. A solution of 7.2 ml of concentrated sulfuric
acid and 18 ml of water was added dropwise to the mixture.
The mixture was cooled to 10 1C and a solution of sodium
nitrite (0.10 g, 1.49 mmol) was added in one portion. After
2 min of stirring the mixture was diluted with 30 ml of water.
Activated copper (0.20 g, 3.15 mmol) was added and the
mixture was refluxed for 1 h. A green precipitate was filtered
and dried. The crude product was dissolved in hot toluene,
filtered and the concentrated filtrate was filtered through a
20 cm silica gel column. The first yellow fraction was collected
and evaporated. The resulting orange solid was dissolved in
carbon disulfide filtered through a 10 cm silica gel column. The
first yellow fraction afforded 0.023 g of the crude product.
Crystallization from benzonitrile gave 0.02 g of 3 (9%) as
yellow micro-needles, which sublimed above 300 1C without
Fig. 2 One-photon absorption (black trace), two-photon excitation
(blue trace) and fluorescence (red trace) spectra of 3 in toluene.
changes: the band at 730 nm loses the fine structure and a
broad band characteristic of p-stacked aggregates appears
above 1500 nm. No further changes occur even in the presence
of excess of water, but warming the solution at 80 1C leads to
the quantitative recovery of 3. The dication of 3 prepared by
chemical oxidation absorbs at 500 nm.
1
melting. H NMR (in CS2, referenced to acetone d6) d, ppm:
We conclude that the new heterocyclic system 3 is a
thermally stable compound possessing strong electron donor
ability close to those of tetra- and pentacene. This compound
or its properly substituted derivatives can serve as a promising
component for applications involving charge transporting and
as the efficient electron donating moiety for fluorescent
chromophores including those with strong TPA absorption.
We thank the French Ministry of Education and Ecole
Doctorale des Sciences Chimiques (ED 250, Marseille) for a
fellowship to C.N.
6.78 (t, J = 7.4 Hz, 4H), 6.93 (t, J = 7.4 Hz, 4H), 7.33
(d, J = 8.1 Hz, 4H), 7.80 J = 8.1 Hz, 4H). HRMS calcd. for
C30H16N2: 404.1313; found: 404.1320.
(b) Route A—hn: compound 4 (0.15 g, 0.22 mmol) was
exposed to sunlight under glass cover for 2 h with frequent
scratching of the top layer. The dark green solid obtained was
extracted with boiling toluene. The workup analogous to (a)
gave 0.011 g (12%) of 3.
(c) Route B: a suspension of compound 5 in freshly distilled
DMF (B3 ml) was purged with argon for 15 min. A solution
of 40% of tetrabutylammonium hydroxide in methanol
(0.15 g, 0.23 mmol), previously degassed with argon, was
added at room temperature to the reaction mixture. After all
5 went into solution, CuI (0.01 g, 0.05 mmol) was added and
the mixture was then warmed to 120 1C and stirred at this
temperature for 20 h. The reaction mixture was then cooled
down to room temperature. The precipitate was isolated by
filtration, washed with acetonitrile and dried. It was purified
by recrystallization in benzonitrile to afford pure 3 (0.03 g) as
fine yellow needles in 74% yield.
Experimental
Unless otherwise noted, starting materials, reagents and
solvents were obtained from commercial suppliers and were
used without further purification. UV-VIS and fluorescence
measurements were, respectively, carried out on a Varian Cary
50 Scan UV-Visible and Fluorescence spectrophotometers.
1H and 13C NMR spectra were recorded on a Bruker
250 MHz spectrometer. Two photon excitation spectra were
obtained using broad band (690–1020 nm) femtosecond laser
(Mai Tai, Spectra Physics). The laser beam was focalised by
fast optical objective (f = 15 mm, NA = 0.7). Constant mean
power excitation (1 W) at 80 MHz repetition rate was used.
Pulse duration was 100 fs. Photoluminescence spectra were
monitored at 901 detection geometry on the Acton 500
monochromator coupled with gated ICCD camera (p-Star,
Prinston Instrument). The quantum yield of fluorescence was
determined using 9,10-diphenylanthracene as a reference in
toluene at 20 1C.
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monoclinic, space group P21,
a 5.795(5), b 15.097(5),
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