New azaborine-thiophene heteroacenes

Article abstract of DOI:10.1039/c0cc01963a

A new class of heteroacenes containing B, N and S elements in the 22-electron aromatic nucleus has been synthesized by reaction of diaminoterthiophenes with dichlorophenylborane. Their structure was studied by X-ray crystallography and DFT calculations. UV-Vis absorption /emission spectroscopy shows high rigidity and deep-blue fluorescence of these compounds.

Full text of DOI:10.1039/c0cc01963a

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New azaborine-thiophene heteroacenesw
Marc Lepeltier, Olena Lukoyanova, Alex Jacobson, Shehzad Jeeva and
Dmitrii F. Perepichka*
Received 18th June 2010, Accepted 10th August 2010
DOI: 10.1039/c0cc01963a
A new class of heteroacenes containing B, N and S elements in
the 22-electron aromatic nucleus has been synthesized by reaction
of diaminoterthiophenes with dichlorophenylborane. Their structure
was studied by X-ray crystallography and DFT calculations.
UV-Vis absorption /emission spectroscopy shows high rigidity
and deep-blue fluorescence of these compounds.
better stabilization can in principle be achieved by incorporating
a BN fragment in an aromatic ring to form azaborines
which are isoelectronic to corresponding polyaromatic hydro-
carbons, but these are still few.13
For several decades conjugated oligo- and polythiophenes
have been extensively investigated as promising materials for
organic electronics. A great number of conjugated thiophene
derivatives has been synthesized and studied leading to materials
with high charge and exciton mobility, tunable band-gap, high
conductivity in the doped state, photochromic switching, etc.1
For most of these derivatives, however, a relatively low
luminescent efficiency limits their use in applications where
light-emitting properties are required (e.g., emissive diodes
and transistors, fluorescent sensors).2 From this perspective,
thiophene derivatives incorporating boron atom, featuring an
empty p-orbital are of particular interest due to strong electron-
acceptor ability of boron and highly emissive properties of
many of its derivatives. This offers a number of materials
design opportunities, including non-linear optical chromophores,3
low band-gap polymers,4 ambipolar semiconductors,5
white-color6 and tunable light-emitters.7 Boron-containing
conjugated materials have only started to be actively explored
in the last 5–10 years.8 One of the reasons is that strong Lewis
acidity (electrophilicity) of boron translates into chemical
instability of organoboron compounds, which is an obvious
drawback for materials application. The major approach to
suppress this reactivity was steric screening of the boron atom,
usually with crowded aryl substituents.8 A number of stable
boron-contained oligothiophenes have been prepared in this
way,3,5–10 although bulky substituents also suppress p-stacking in
these materials, which is detrimental for charge mobility.
Another possible approach to stabilize the boron center is
through orbital interaction of the boron’s empty orbital with a
lone pair of an adjacent atom (such as nitrogen). Diazaborole-
substituted11 and B,B-diaminodiborine-fused12 thiophenes
that are stabilized by B–N interactions rather than steric
substituents have been reported as stable compounds with
photoluminescence quantum yield FPL = 16–59%. Even
Herein we describe synthesis, crystallographic, spectral and
electrochemical properties of novel polycyclic 22-e aromatic
compounds 1 consisting from alternating azaborine and
thiophene rings. We note that related thienoazaborine14 and
dithienoazaborine15 have been synthesized earlier but their
properties were not reported.
Bis-azaborine 1a was obtained with 89% yield by heating
the known diaminoterthiophene with excess of PhBCl2 in the
presence of Et3N (Scheme 1). The reaction most likely starts
as an electrophilic borylation of the amino group to yield
chloroaminoborane intermediate. The following intramolecular
cyclization occurs with the neighboring thiophene ring to form
an azaborine cycle rather than with the second amino group
(which would give a diazaborole derivative). No diazaborole
was formed even when strictly 1 equiv. of PhBCl2 was used
and the only isolated product was the corresponding mono-
azaborine fused terthiophene (ESIw). As the strong expected
acidity of N–H protons (consider resonance form 10) could be
problematic for applications and further functionalization, we
decided to protect them with an ethylene bridge which fits well
between the two nitrogen centers and does not distort the
planarity of the ring system. The required ethylenediamino-
terthiophene 3 was prepared by reduction of the pyrazine 2
with NaBH4. Its borylation with excess of PhBCl2 yields
bis-azaborine 1b with 54% yield. Both 1a and 1b showed
thermal stability to B300 1C in TGA.
Department of Chemistry and Centre for Self-Assembled Chemical
Structures, McGill University, Montreal, Qc, Canada.
E-mail: dmitrii.perepichka@mcgill.ca; Fax: 1 514 3983797;
Tel: 1 514 3986233
w Electronic supplementary information (ESI) available: Experimental
part, quantitative fluoride binding experiments; TGA curves for 1a
and 1b; copies of NMR spectra. CCDC 782155 and 782156. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc01963a
Scheme 1 Synthesis of azaborines 1a and 1b.
Chem. Commun., 2010, 46, 7007–7009 7007
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The molecular structures of azaborines 1a and 1b were
studied by X-ray crystallography and density functional
theory (DFT) calculations at B3LYP/6-311G(d,p) level.
Azaborine 1a crystallizes with two inequivalent molecules
and includes the molecule of solvent (acetone) that is hydrogen-
bonded to two NH sites (Nꢀ ꢀ ꢀO distance 3.04 A). The B–N
length of the azaborine cycles in 1a and 1b (1.405–1.460 A
from X-ray, 1.43 A from DFT) is similar to that of other
azaborines13 and confirms a significant contribution of the
zwitter-ionic form 10. According to DFT calculations, and in
agreement with the structure of 10, there is a significant
HOMO density on the boron atom (but none on the nitrogen)
and significant LUMO density on the nitrogen (but none on
the boron) (Fig. 1).
Fig. 2 Solid state packing of azaborines 1a (left) and 1b (right).w The
solvating molecules of acetone are removed from the structure of 1a
and the Ph rings in 1b are dimmed for clarity.
The introduction of azaborine moieties rigidifies the
oligothiophene backbone: the molecules are almost planar
with an angle between the planes defined by the terminal
thiophene rings is 4.31 (twisting) and 71 (bending) for 1a and
1b, respectively. The two phenyl rings are out of the plane
(dihedral angle 26–331 for 1a and 51–561 for 1b) which limits
their conjugation with the polycyclic moiety. Both compounds
form slipped p-stacks with alternating head-to-tail orientation
(Fig. 2). No p–p interaction was observed between the phenyl
substituents, either within or across the stacks. In the stack,
the interplanar distances between the parallel terthiophene
ring system is 3.47 A and 3.62 A for two unequivalent
molecules of 1a. For 1b, the ethylene bridge causes some
expansion of the p-stack and the molecules appear dimerized
(interplanar distances of 3.58 A and 3.83 A).
than gas-phase calculated HOMO–LUMO gaps (3.55 and
3.53 eV, respectively).
The compounds 1a and 1b exhibit deep-blue photo-
luminescence at lmax = 407–410 nm, with FPL = 25–34%
(Fig. 3, Table 1). This is a moderately high value in thiophene
series. High rigidity of the azaborinethiophenes is further
manifested in a small Stockes shift of B0.1 eV which is much
smaller than that of non-fused terthiophene (0.64 eV) and
even sulfur-bridged pentathienoacene (0.28 eV).17 Together
with insignificant solvatochromism (lmax shift of o3 nm for
solvents range from CHCl3 to acetonitrile), this also suggests
that there is no significant charge transfer upon excitation.
Redox properties of azaborines 1a and 1b have been studied
by cyclic voltammetry (ESIw). Both compounds undergo
irreversible electrochemical oxidation with anodic peak potential
The rigidity and efficient conjugation in azaborines 1
explains a relatively low reorganization energy l for the
transfer of the hole, which, together, with p-stacking, in an
important attribute of an efficient semiconductor. At B3LYP/
6-311G(d,p) level of theory azaborine 1b shows l = 0.27 eV,
which is lower than that of linear and S-fused oligothiophene
(thienoacenes) of a similar size.16 While device fabrication with
1a and 1b was hampered by poor film-forming properties, this
should not be an issue for corresponding polymers.
E
pa at 0.48 V vs. Fc/Fc+. This is ca. 0.2 V more positive than
the oxidation of diaminoterthiophene,18 in line with the electron
withdrawing effect of boron. The HOMO energy, estimated
from the redox potential at ꢁ5.3 eV (assuming Fc/Fc+ at
ꢁ4.8 eV), is in a good agreement with DFT calculated values
(Table 1). In accord with the HOMO–LUMO gap, no reduction
processes were observed. The electron-donating ethylene
bridge in 1b does not significantly affect its oxidation potential,
which is consistent with the HOMO located on the boron
rather than on the nitrogen.
UV-Vis absorption spectra of azaborines 1a and 1b shows
vibronically splitted bands, yet another indication of their
structural rigidity. The spacing between the vibronic features
(B1470 cmꢁ1) corresponds well to the thiophene C–C vibrations.
The ethylene bridge in 1b does not significantly affect the
electronic structure and both compounds display almost identical
absorption spectra. The labs (395 and 397 nm for 0–0 transition
of 1a and 1b, respectively, Table 1) is red-shifted compared to
the non-fused terthiophene (labs = 354 nm) and an S-bridged
analogue pentathienoacene (labs = 357 nm).17 The optical
gaps deduced from the absorption maxima (3.14 and
3.12 eV for 1a and 1b, respectively) are somewhat smaller
The intramolecular Lewis acid–base interaction in 1a,b
(represented by the resonance structure 10) reduces the electro-
philicity of the boron stabilizing the structure. No complexation
was detected with weak or moderate Lewis bases, e.g. H2O,
THF, amines, Clꢁ, Brꢁ, Iꢁ, OHꢁ or OCH3ꢁ. A specific binding
of the fluoride ion, characteristic of boronic derivatives,19 was,
nevertheless, found in absorption and emission spectra
(Fig. 4). Adding Bu4NF to solutions 1b leads to appearance
of a long-wavelength absorption band at 470 nm and extenuates
of the original absorption at B400 nm. Two isosbestic points
at 408 nm and 353 nm observed throughout the titration, even
when a very large excess of Fꢁ is added, indicate a clear
transformation between the 1b and one single product.z
Expectedly, the binding constant log Ka = 3.3 ꢂ 0.1
(in CH2Cl2) is lower than that of triarylboranes without
electronic stabilization of the empty orbital (e.g, log Ka = 6.3
for dithienylmesitylborane in CH2Cl2).19 The fluoride adduct
is also characterized by a pronounced red-shifted emission
band at 540 nm. The fluorescence change from blue to green
Fig. 1 Calculated HOMO/LUMO topologies of 1b.
7008 Chem. Commun., 2010, 46, 7007–7009
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Table 1 Electronic properties of azaborines 1a and 1b
aromatic molecules and polymers. An interaction of the boron
empty orbital with the lone pair of the nitrogen suppresses the
Lewis acidity of the former, leading to increased stability of
these materials. This interaction is confirmed by X-ray crystallo-
graphy and DFT calculations. Planarity and rigidity of
azaborine-fused oligothiophenes as well as p-stacking
ordering in the solid state make them potential candidates
for semiconducting applications.
HOMO/eV
electrochem
(calc)
l
abs/nm
lem
nm
/
FPL
[%]
Eg/eV
opt (calc)
(log e)
1a
1b
395
407
410
34
25
ꢁ5.3
3.14
(4.58)
397
(4.74)
(ꢁ5.42)
–5.3
(–5.36)
(3.55)
3.12
(3.53)
We thank NSERC for supporting this work and CFI for the
infrastructure grant. OL is grateful to FQRNT for the PBEEE
fellowship. We thank Dr A. Moiseev and M. Morantz
(McGill) for the EPR and TGA measurements, respectively,
and F. Belanger (Universite de Montreal) for X-ray analysis.
´ ´ ´
Notes and references
z Prolonged (more than a few minutes) exposure to air leads to
oxidation of the fluoride addition product resulting in radical species.
It is, however, completely stable in the nitrogen atmosphere.
y A weaker red-shift upon Fꢁ addition was previously reported for
boryl-substituted dithienophospholes.9
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addition can also be monitored by 11B NMR causing chemical
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tetracoordinate boron center. Fluoride sensing by conjugated
boron derivatives is not rare, most of these undergo a blue shift
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oligothiophenes paving a way to a new class of p-functional
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7007–7009 7009