Indolocarbazole-based ligands for ladder-type four-coordinate boron complexes

Article abstract of DOI:10.1021/ol301339y

A novel class of π-conjugated systems, which combine the indolo[3,2-b]carbazole unit with the formation of four-coordinate boron complexes, is presented. The resulting conjugated compounds have a double-laddered structure that provides interesting optical and electrochemical properties. The wide absorption range, covering most of the visible spectrum, along with the narrowing of the HOMO-LUMO energy gap, due to the presence of diphenylboryl centers, reinforces the potential of these molecules within the area of organic electronics.

Full text of DOI:10.1021/ol301339y

ORGANIC
LETTERS
2012
Vol. 14, No. 13
3360–3363
Indolocarbazole-Based Ligands
for Ladder-Type Four-Coordinate
Boron Complexes
ꢀ
ꢀ
David Curiel,* Miriam Mas-Montoya, Laura Usea, Arturo Espinosa, Raul A. Orenes,
and Pedro Molina
Department of Organic Chemistry, Faculty of Chemistry, University of Murcia,
Campus of Espinardo, Murcia 30100, Spain
davidcc@um.es
Received May 15, 2012
ABSTRACT
A novel class of π-conjugated systems, which combine the indolo[3,2-b]carbazole unit with the formation of four-coordinate boron complexes, is
presented. The resulting conjugated compounds have a double-laddered structure that provides interesting optical and electrochemical
properties. The wide absorption range, covering most of the visible spectrum, along with the narrowing of the HOMOꢀLUMO energy gap, due to
the presence of diphenylboryl centers, reinforces the potential of these molecules within the area of organic electronics.
Organoboron complexes have attracted much attention
because of their outstanding optical properties. Regarding
four-coordinate complexes, Bodipy derivatives are by far
the most studied systems.1 The versatility of the reported
synthetic approaches has enabled a relatively easy adapta-
tion of the Bodipy core which can be modified by attaching
substituents to different positions (meso, β-pyrrolic, and
boron center)2 or by expanding the aromatic surface.3 This
has resulted in compounds that can be tuned to absorb
and/or emit over all the visible spectrum and even in the
near-infrared.4 Consequently, the different areas where
these compounds have been applied cover a wide range
of possibilities, such as molecular sensors,5 fluorescent
sensitizers,6 imaging probes,7 lasers,8 electroluminescent
devices,9 or absorbing materials in organic solar cells.10
This great potential has motivated the search for new
π-conjugated ligands which can provide promising electronic
and optical properties to the resulting four-coordinate
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r
10.1021/ol301339y
Published on Web 06/20/2012
2012 American Chemical Society

boron complexes. In this regard, several NꢀN,11 NꢀO,12
NꢀC,13 OꢀO,14 or CꢀC15 bidentate ligands have been
reportedintheliterature. Sincethe fabrication ofelectronic
devices requires the use of π-conjugated molecules that
show good charge transport ability, the integration of
boron complexes into ladder-type systems represents a
useful strategy.16 A very elegant approach consists in the
rigidification of aromatic or heteroaromatic oligomers by
virtue of boron coordination.17 In our case, we present the
integration of four-coordinate boron atoms into the struc-
ture of a laddered system such as indolo[3,2-b]carbazole,
which has revealed itself as a good hole-transporting
material in organic electronics.18 Interestingly, the pre-
sence of boron centers will contribute to modify the
electronic properties of the fused polyheteroaromatic unit.
The synthesis of the 6,12-disubstituted indolocarbazole
was carried out in a three-step route (Scheme 1) that
involved an initial condensation of pyridine-2-carboxalde-
hyde with indole to yield the corresponding 3,30-bis-
(indolyl)methane, 1.19 This compound, when heated in
the presence of iodine, has been proposed to evolve
through a 3-arylideneindole, which dimerizes to yield
6,12-diarylindolo[3,2-b]carbazole, 2.20 The reaction of
2 with triphenylborane led to the isolation of two four-
coordinate boron complexes, namely the mononuclear
(9%), 3, and the dinuclear (5%), 4, compounds.
analysis could be grown for the mononuclear complex 3
(Figure 1). It is worth highlighting the distortion of the
indolocarbazole planarity as a result of the crystal packing
and the structural stretching introduced by the chelation of
the boron atom. The concave arrangement of the fused
heteroaromatic system defines a 157.7° angle between the
planes corresponding to each of the outer benzene rings.
The pyridyl substituents are twisted out of the hypothetical
plane containing the aromatic core. In addition, the boron
atom forms a six-membered ring with a twisted-boat con-
formation. Different BꢀN bond lengths are observed for
˚
˚
the pyridine (1.65 A) and the indolocarbazole (1.54 A) as a
result of their dative and covalent nature, respectively. Con-
˚
versely, BꢀC distances are almost identical (1.62 A) for each
phenyl ring. Despite the presence of some crystallization
solvent molecules and the bulky diphenylboryl group, some
intermolecular contacts are observed in the crystal packing.
In this regard, molecules stack along the a axis following an
ABC columnar pattern in which intra- and intercolumnar
van der Waals interactions are detected between different
indolocarbazole units (see the Supporting Information)
The thermal stability of both 3 and 4 was checked by
thermogravimetric analysis (see the Supporting Information).
Regarding the mononuclear complex 3, an initial weight loss
was detected slightly over 100 °C, presumably due to the
release of solvent molecules trapped in the solid. After this, a
decomposition curve was registered that showed a 5% weight
loss at 400 °C. Similarly, compound 4 displayed a thermal
profile whose decomposition temperature was 380 °C.
Scheme 1. Synthetic Route for the Ligand and Boron Complexes
Figure 1. Two views of the crystal structure of compound 3.
Hydrogen atoms and crystallization solvent have been removed
for the sake of clarity.
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Y.; Chi, S.; Qian, X. Org. Lett. 2008, 10, 633.
All of the products were fully characterized by NMR
spectroscopy (1H, 13C, and 11B) and mass spectrometry.
Additionally, single crystals suitable for X-ray diffraction
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H. H. Y.; Williams, I. D.; Kwok, H. S.; Zhang, Y.; Tang, B. Z. Chem.
Commun. 2011, 47, 11216.
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E.; Leriche, P.; Rihn, S.; De Nicola, A.; Ziessel, R.; Roncali, J. Chem.
Commun. 2010, 46, 5082. (c) Kumaresan, D.; Thummel, R. P.; Bura, T.;
Ulrich, G.; Ziessel, R. Chem.;Eur. J. 2009, 15, 6335.
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2009, 127.
Org. Lett., Vol. 14, No. 13, 2012
3361

Absorption spectra of the ligand 2 and the two com-
plexes 3 and 4 were recorded in several solvents. In all
cases, four well-resolved absorption bands were detected
(Figure 2). According to the aromatic structure of the
compounds, these bands could be ascribed to different
πꢀπ* electronic transitions. In general, all three com-
pounds displayed a hypsochromic effect upon increasing
solvent polarity which, according to the BaylissꢀMcRae
model, is interpreted as the ground state being more polar
than the excited state.21 The complexation of boron causes
a gradual red shift of the absorption spectra on going from
the plain indolocarbazole ligand, 2, to the dinuclear com-
plex 4 which is evident to the naked eye since the color of
the complexes changes from yellow, 2, to red, 3, to dark
green, 4. In agreement with this result, a narrowing of the
HOMOꢀLUMO gap (2.48, 2.05, and 1.76 eV for 2, 3, and
4 respectively) could be determined from the absorption
spectra onset.
atom, which is interpreted as an evidence of the higher
stability of the intermediate radical cations.
Figure 3. Cyclic voltammetry of compounds 2ꢀ4 in CH2Cl2
(10ꢀ3 M) at room temperature, TBAPF6 (0.1 M), scan speed:
0.1 V/s, Ag/AgCl reference electrode, Pt working electrode.
The energy of the HOMOs can be estimated from the
onset of the first oxidation wave.22 The subtle differences,
detected on the first oxidation potential, have a negligible
influence on the HOMO energy. Thus, the presence of
boron does not affect the ionization potential of the
indolocarbazole system. Consequently, the gradual de-
crease in the HOMOꢀLUMO gap upon boron chelation
results in an electronic structure of compounds 3 and 4,
where LUMOs are stabilized. The energy of these orbitals
can be indirectly determined by adding the HOMO energy
to the HOMOꢀLUMO optical gap (Table 1).
The significant stabilization of the LUMO results in one
of the lowest values reported for a noncopolymeric indolo-
[3,2-b]carbazole derivative. In this regard, the HOMO and
LUMO energies determined for compound 4 get close to
the range of an ambipolar semiconductor which validates
the strategy of boron chelation to modify the transporting
properties of a heteroacenic semiconductor.23
Figure 2. Absorption spectra of compounds 2 (yellow), 3 (red),
and 4 (green) in CH2Cl2 (2 10ꢀ5 M).
3
The emission spectra recorded for 2, 3, and 4 displayed
very weak fluorescence (see the Supporting Information).
Although the emission efficiency increased upon boron
chelation, the quantum yields of the complexes remained
very low (Table 1). The general solvent effect shows again a
hypsochromic shift. As previously discussed, the decrease
in the HOMOꢀLUMO gap (2 > 3 > 4) also becomes
evident in the significant red shift of the emission spectra.
The electrochemical properties were studied by cyclic
voltammetry. All of the compounds 2ꢀ4 displayed two
oxidation waves typical of the indolo[3,2-b]carbazole sys-
tem (Figure 3). Interestingly, the coordination of the
diphenylboryl group slightly widens the difference between
the electrochemical processes by decreasing the potential
for the first oxidation and increasing the potential for the
second one. Moreover, the reversibility of the oxidation
processesisenhanced bythe presence ofthe chelatedboron
Table 1. Optical and Electrochemical Characterization
2
3
4
λabsa (nm)
441
545
643
ε (Mꢀ1 cmꢀ1
)
8300
13375
608
19290
695
λ
em (nm)
498
Φb (%)
0.4
6.4
2.8
E1/2c (mV)
Eonset (mV)
847; 1076
720
784; 1120
710
731; 1270
640
LUMOexp (eV)
[LUMOtheor
HOMOexp (eV)d
[HOMOtheor
ꢀ2.58
[ꢀ1.94]
ꢀ5.06
[ꢀ5.01]
ꢀ3.00
[ꢀ2.40]
ꢀ5.05
[ꢀ5.01]
ꢀ3.22
[ꢀ2.80]
ꢀ4.98
[ꢀ5.02]
]
]
a Recorded in dichloromethane (2 ꢁ 10ꢀ5 M) at room temperature.
b Determined in ethanol at room temperature using anthracene as
reference (λexc(2) = 410 nm; λexc(3) = 480 nm; λexc(4) = 500 nm).
c Calculated from the anodic and cathodic peaks in cyclic voltammetry
(Epa þ Epc)/2. d Calculated from EHOMO = ꢀ(4.34 þ Eox onset).
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Zhang, H. Inorg. Chem. 2011, 50, 4825. (b) Fukazawa, A.; Yamaguchi, E.;
Ito, E.; Yamada, H.; Wang, J.; Irle, S.; Yamaguchi, S. Organometallics
2011, 30, 3870.
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Electron. 2010, 11, 1649. (b) Li, Y.; Wu, Y.; Gardner, S.; Ong, B. S. Adv.
Mater. 2005, 17, 849.
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3362
Org. Lett., Vol. 14, No. 13, 2012

Theoretical calculations were carried out to get a better
understanding of the electronic structure of the boron
complexes. It is worth highlighting that whereas the
HOMO is predominantly distributed along the fused poly-
heteroaromatic system defined by the indolocarbazole
unit, the LUMO spreads along the axis described by the
attached pyridines (Figure 4). Thus, the location of HOMO
and LUMO depicts a perpendicular arrangement which
outlines the double ladder structure of these indolocarbazole
derivatives. The theoretical values determined for the fron-
tier orbital energies show an excellent agreement for the
HOMO and an approximately regular deviation for the
LUMO when compared to the experimental values.
higher energy bands within the same spectra, which are
less solvent sensitive and would denote a more π f π*
character.24
Considering the potential application of these boron
complexes in the area of molecular electronics, absorption
spectra were also registered for thin films prepared by spin
coating from chloroform solutions (see the Supporting
Information). A very good matching was detected between
the solid-state spectra and those registered in solution,
which means that the same energy levels are involved in the
electronic transitions. As was also observed for the spectra
registered in solution, when the thin film absorption spectra
of 3 and 4 are superimposed, an alternation of the absorp-
tion maxima can be observed, resulting in a combined
coverage which extends from the ultraviolet (200 nm) to
the red region of the visible spectrum (750 nm). This feature
might be interesting for the preparation of a dye blend for
organic solar cells or even for tandem solar cells.25
To summarize, we have synthesized 6,12-di(pyridin-2-
yl)indolo[3,2-b]carbazole as well as its mononuclear and
dinuclear diphenylboryl complexes. The electronic struc-
ture of these complexes has been determined by spectro-
scopic, electrochemical, and computational methods, re-
vealing a narrowing effect of the HOMOꢀLUMO energy
gap upon boron complexation. This narrowing is almost
exclusively due to a decrease in the LUMO energy, which
approaches the ambipolar materials energy range. The
four-coordinatecomplexesshowawide absorption spectra
which, when combined, cover most of the visible spectrum.
These properties confer them the potential to be used in the
area of organic electronics.
Figure 4. Computed KohnꢀSham isosurfaces (0.04 isovalue)
for the frontier molecular orbitals.
Acknowledgment. We are grateful to the Consejerıa de
Time-dependent DFT calculations were also performed
for the interpretation of the UVꢀvis spectra (see the
Supporting Information). According to these results, the
lower energybands forall three compoundsare assigned to
the S0fS1 and S0fS2 states which show a preferential
correspondence with the HOMOfLUMO and HOMO-
1 f LUMO transitions, respectively. Higher energy bands
involved contributions from several molecular orbitals.
Regarding the distribution of molecular orbitals and the
theoretical assignment of the absorption bands, the two
lower energy bands denote a charge transfer identity. This
could also be inferred from the larger solvatochromic
effect detected for these two bands when compared to
ꢀ
ꢀ
Universidades, Empresa e Investigacion de la Region de
Murcia, for financial support received through the Regional
Plan of Science and Technology. We also acknowledge
financial support from MICINN (project CTQ2008-01402
and M.M.-M. FPU fellowship).
Supporting Information Available. Experimental de-
tails, NMR spectra, thermogravimetric analysis, UVꢀvis
spectra, fluorescence spectra, cyclic voltammetry, single-
crystal X-ray diffraction data, and computational details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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Energy Environ. Sci. 2009, 2, 347. (b) Rousseau, T.; Cravino, A.; Bura,
T.; Ulrich, G.; Ziessel, R.; Roncali, J. J. Mater.Chem. 2009, 19, 2298.
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2012, 112, 2208. (b) Welch, G. C.; Coffing, R.; Peet, J.; Bazan, G. C.
J. Am. Chem. Soc. 2009, 131, 10802.
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The authors declare no competing financial interest.
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