Novel chromophores from alternated pyridine-ethylenedioxythiophene unit oligomers: Dramatic enhancement of photoluminescence properties in elongated derivatives

Article abstract of DOI:10.1039/b815656e

Novel chromophores based on the alternation of electron-poor (pyridyl) and electron-rich (ethylenedioxythienyl) heterocycles were synthesized for the first time and shown to exhibit attractive optical properties in relation with their specific electronic and geometrical (coiled structure) features. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b815656e

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Novel chromophores from alternated pyridine–ethylenedioxythiophene
unit oligomers: dramatic enhancement of photoluminescence properties in
elongated derivativesw
Floris Chevallier, Marina Charlot, Claudine Katan, Florence Mongin*
and Mireille Blanchard-Desce*
Received (in Cambridge, UK) 8th September 2008, Accepted 7th November 2008
First published as an Advance Article on the web 12th December 2008
DOI: 10.1039/b815656e
1
0
Novel chromophores based on the alternation of electron-poor
pyridyl) and electron-rich (ethylenedioxythienyl) heterocycles
(Bu MgLi) in THF at room temperature. Subsequent cross-
3
(
coupling of the lithium tri(aryl)magnesate intermediate with
2-bromopyridine afforded the DA dimeric species 2a in 87% yield
were synthesized for the first time and shown to exhibit attrac-
tive optical properties in relation with their specific electronic
and geometrical (coiled structure) features.
0
using a combination of PdCl and 1,1 -bis(diphenylphosphino)-
2
ferrocene (dppf) ligand (3 mol% each) as the catalyst. Repeating
the same deprotonation/cross-coupling sequence starting from
heterodimer 2a readily afforded heterotrimer 3 in 73% yield.
The same procedure starting from 1 and using 2,6-dibromo-
pyridine led to coupling product 2b with only low yield. The
synthetic strategy was accordingly modified and switched to
1
In the last decade, oligothiophenes have received considerable
attention in relation with their optical and electronic properties.
3,4-Ethylenedioxythiophene (EDOT, 1) has been used as a
building block in several conjugated systems due to its strong
electron-donating ability which allows the tuning of the electronic
9,11
Negishi-like cross-couplings.
the corresponding thienylzinc chloride by deprotometalation with
butyllithium at 0 1C followed by transmetalation with ZnCl
TMEDA. Subsequent reaction with 2,6-dichloropyridine, catalytic
amounts of PdCl and dppf in refluxing THF afforded the mono-
Compound 1 was converted to
2
properties of p-conjugated systems. More generally oligomeric
and polymeric conjugated derivatives can present conductive and
3
ꢀ
2
electronic properties of major interest for the elaboration of
optoelectronics devices such as organic field-effect transistors
2
(
OFETs), organic light-emitting diodes (OLEDs) or photovoltaic
halogenated derivative 4, which was isolated in 75% yield. The key
building block 4 was then used as the starting material for
the synthesis of heterotrimer 5 using the modified Negishi
cross-coupling reaction after deprotonation/transmetalation of
1 (73% overall yield). This strategy can be extended to the
preparation of elongated EDOT–pyridine alternating oligomers.
Using the same cross-coupling method in an iterative synthetic
approach and making use of selective deprotonation of the EDOT
4
devices. Combining electron-rich (D) and electron-poor (A)
hetero)aromatic units in alternated (DA)n polymeric systems
i.e. charge transfer (CT)-type polymers) has recently emerged
as a popular strategy in the field of molecular-based electronics
(
(
5
n
and photonics. Whereas oligomeric (DA) derivatives made of
alternated thiophene and pyridine heterocyclic units have been
6
reported, oligomers built from alternated electron-rich EDOT
and electron-poor pyridine moieties have not been described up to
now although electrochemical (including electrochromic) pro-
7
perties of poly(EDOTpyridine) have been reported. Only the
moiety, heterotetramer (DA) 6 and heterohexamer (DA) 7
2 3
were synthesized in high yields by coupling 4 with thiophene-
deprotonated 2a and 6 respectively (Scheme 1).
generic compound DA has been synthesized, with somewhat low
8
yields (25–34%).
The X-ray diffraction analysis of single crystals of com-
2
pounds 3 (ADA), 5 (DAD) and 6 ((DA) ) reveals that
With this aim in mind, we have implemented an efficient and
stepwise route towards alternated ethylenedioxythiophene-
pyridine oligomers of defined and increasing length. Firstly, we
proceeded via deprotonation of the thiophene moiety followed by
alternated (EDOT–pyridine) oligomers tend to adopt a coiled
structure in the solid state (see ESIw). Interestingly HF/6-31G
optimized geometry calculations (see ESIw) indicate that this
structural conformation is also favoured in the case of isolated
molecules (Table 1). This particular structural feature can be
related to the repulsion between N and O atoms of successive
rings in linear conformation while coiled conformations
favour intramolecular hydrogen bonds.
9
palladium-catalyzed cross-coupling with pyridine halides in a
‘one-pot’ procedure. We carried out the deprotonation of EDOT
at the 2 position using 1/3 equivalent of lithium tributylmagnesate
All oligomeric compounds are readily soluble in chloroform
and dichloromethane, and soluble in medium to high polarity
solvents such as ethyl acetate, THF, acetone, DMSO etc.
Although the coiled structure demonstrated in the solid state
and supported by isolated molecules calculations cannot be
directly transposed to molecules in solution, the common
observation for boundary conditions provides a strong hint
that the molecules adopt similar conformations in solution.
This should be the case as long as strong interactions (such as
H-bonds, p-interactions) with solvent molecules can be ruled
Chimie et Photonique Mole´culaires, UMR 6510 CNRS-Universite´ de
Rennes 1, Baˆtiment 10A, Case 1003, Campus Scientifique de Beaulieu,
35042 Rennes Cedex, France. E-mail: florence.mongin@univ-rennes1.fr.
E-mail: mireille.blanchard-desce@univ-rennes1.fr;
Fax: +33(0)2 2323 6955; Tel: +33(0)2 2323 6931/6277
w Electronic supplementary information (ESI) available: General
methods, procedures and characterization of products, H and C
NMR spectra, crystal data, solvent effect on absorption and emission
spectra, computational methods, optimized geometries. CCDC 699627,
699628 and 707007 for 3, 5 and 6. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b815656e
6
92 | Chem. Commun., 2009, 692–694
This journal is ꢁc The Royal Society of Chemistry 2009

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energy which gain relative intensity in comparison to the
lower energy absorption band whose intensity levels off with
increasing length.wz This low energy band bathochromically
shifts towards the visible with increasing length indicating that
electronic conjugation extends beyond the DA pair. Indeed,
1
2
natural transition orbitals (NTO) calculations indicate that
significant electronic redistribution involving the whole struc-
ture takes place upon excitation (Table 1). Dimeric chromo-
phore DA (2a) clearly shows a directional intramolecular CT
from the EDOT unit to the pyridine moiety with an increase in
dipole moment upon excitation (from 4.5 to 7 D based on
TD-B3LYP calculations). In contrast, trimers 3 (ADA) and
5
(DAD) show a quadrupolar charge redistribution from the
core to the periphery (3) or from the periphery to the core (5)
with only small dipole moment changes upon excitation. The
charge redistribution in tetramer 6 and hexamer 7 is somewhat
of a combination of both processes and results in lower
electronic density on the peripheral EDOT moiety as com-
pared to the inner EDOT moieties (Table 1). In both cases,
excitation leads to an increase of the dipole moment (from 5 to
Scheme 1 Reagents and conditions: [a] Bu
h; [b] 2-bromopyridine (R = H) or 2,6-dibromopyridine (R = Br)
1 eq.), PdCl (3 mol%), dppf (3 mol%), reflux, 18 h; [c] BuLi (1 eq.)
then ZnCl TMEDA (1 eq.), THF, 0 1C, 1 h; [d] 2,6-dichloropyridine
1 eq.), PdCl (2 mol%), dppf (2 mol%), reflux, 48 h; [e] 3,4-ethylene-
dioxythienylzinc chloride (1 eq.), PdCl (2 mol%), dppf (2 mol%),
reflux, 48 h; [f] 4 (1 eq.), PdCl (2 mol%), dppf (2 mol%), reflux, 48 h.
3
MgLi (1/3 eq), THF, rt,
2
(
2
2
(
2
9
D for 6, and 2 to 6 D for 7). These calculations are supported
by the solvatochromic behaviour of compounds 2a, 3, 5, 6 and
in solution: whereas the lowest energy absorption band of
2
2
7
out via suitable choice of solvents. The photophysical pro-
perties of the series of derivatives 2a (DA), 3 (ADA), 5 (DAD),
chromophores 2a, 6 and 7 shows a slight red-shift with
increasing polarity, chromophores 3 and 5 absorption spectra
are almost insensitive to solvent polarity.w
6
2 3 3
((DA) ) and 7 ((DA) ) in CHCl are collected in Table 2. All
chromophores present an intense absorption band in the near
UV region as well as additional absorption bands at higher
All chromophores show photoluminescence (PL) properties
whose characteristics strongly depend on their length. The
Table 1 ORTEP diagrams of ADA 3, DAD 5 heterotrimers and (DA)
optimized geometries for DA 2a and (DA) 7, and TD-B3LYP/6-31G//HF/6-31G natural transition orbitals for all derivatives
3
2
6 heterotetramer (thermal ellipsoids at 50% probabilities), and HF/6-31G
12
Table 2 Photophysical data of alternated oligomeric EDOT-pyridine derivatives in chloroform
abs a
ꢂ4
ꢂ1 ꢂ1
b
abs a
max2
ꢂ4
ꢂ1 ꢂ1
b
abs a
max1
ꢂ4
ꢂ1 ꢂ1
b
em c
max
d
e
ꢂ9
f
r
ꢂ9
f
nr
g
l
max3
Compound (nm)
10
(cm
e
max3
l
(nm)
10
(cm
e
max2
l
(nm)
10
(cm
e
max1
l
(nm)
F
t
(%) (ns) (s
10
k
10
(s
k
E
0–0
(eV)
ꢂ1
ꢂ1
M
)
M
)
M
)
)
)
2
3
a (DA)
(ADA)
—
—
—
—
286 (sh) 1.22
—
315.5
1.71
372
1.3 0.36 0.04
2.74
5.31
3.64
3.25
—
339 (sh), 356, 3.26
73 (sh)
348
373, 389 (sh) 3.12
391 (sh), 411, 4.4 0.18 0.24
437 (sh)
3
5
6
(DAD)
—
299
—
2.49
287
297
3.65
2.23
2.07
399
408, 429 (sh), 15
59 (sh)
426, 450 (sh), 41
85 (sh)
19
1.39 0.14
0.51 0.29
0.58
1.67
3.33
3.11
((DA)
2
)
4
7
((DA)
3
)
310
3.47
353
3.62
390
3.11
1.95 0.21
0.30
2.97
4
a
b
c
d
Experimental absorption maximum. Molar extinction coefficient. Experimental emission maximum. Fluorescence quantum yield deter-
e
f
Experimental fluorescence lifetime. Radiative (k
mined in chloroform relative to quinine in 0.5 M H .
g
rates. Electronic gap (0–0 transition energy) estimated from the intersection of normalized absorption and emission spectra.
2
SO
4
r
) and non-radiative (knr) decay
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gap and broader (multibands) absorption as well as batho-
chromic shift of the lowest energy absorption band. A
dramatic enhancement of their PL properties is also obtained
as a consequence of the increased rigidity in the excited state
which reduces non-radiative processes. The present work has
thus opened a new route towards coiled DA oligomers with
intense absorption in the UV region and whose PL charac-
teristics can be tuned from near UV to visible blue by playing
on the length or symmetry.
The authors thank T. Roisnel for the X-ray structure analysis.
Calculations were funded by the ‘‘Centre Informatique National
de l’Enseignement Superieur’’ (CINES).
´
Notes and references
Fig. 1 Normalized emission spectra of chromophores 2a, 3, 5, 6 and
in chloroform.
z With the exception of heterotrimer 3 (ADA) which shows only a
weak higher energy absorption band.
y Chromophore 2a (DA) shows a much lower radiative decay rate
7
n
than elongated oligomers (DA) although its extinction coefficient is
emission band progressively shifts from near UV to visible
blue region with increasing length (Fig. 1). Dimeric chromo-
phore 2a shows an unstructured emission band with a signifi-
roughly twice smaller. This suggests that emission originates from a
different excited state with lower transition dipole than the vertical
excited state.
ꢂ1
cant Stokes shift (4800 cm ) indicative of a CT excited state.
1
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(
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n
the fluorescence quantum yield of (DA) oligomers although the
2
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shorter compound 2a shows only weak fluorescence,y the longest
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with lower transition dipole) and non-radiative decay rates.
This suggests that odd-numbered elongated derivatives of the
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(
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1
2
(
to even-numbered (DA)
DA)
n
D type might be complementary emitters as compared
derivatives. Such derivatives would
8
9
n
however require excitation at shorter wavelengths thus taking
advantage of the intense higher energy band (Table 2) which is
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´
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n
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chromophores exhibit intense and broad absorption in the UV
region. Increasing their length results in decreasing electronic
1
1 A. O. King, N. Okukado and E.-I. Negishi, J. Chem. Soc., Chem.
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6
94 | Chem. Commun., 2009, 692–694
This journal is ꢁc The Royal Society of Chemistry 2009