New 4,4′-oligophenylenevinylene functionalized-[2,2′] -bipyridyl chromophores: Synthesis, optical and thermal properties

Article abstract of DOI:10.1016/j.tetlet.2003.10.099

The synthesis and characterization of new bipyridyl-based chromophores featuring extended oligophenylenevinylene π-conjugated backbones are reported. Their absorption and emission properties as well as their thermal stabilities are discussed in comparison

Full text of DOI:10.1016/j.tetlet.2003.10.099

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 125–128
New 4,40-oligophenylenevinylene functionalized-[2,20]-bipyridyl
chromophores: synthesis, optical and thermal properties
Lydie Viau, Olivier Maury and Hubert Le Bozec*
‘‘Organomeꢀtalliques et Catalyse: Chimie et Electrochimie Moleꢀculaires’’, UMR 6509 CNRS-Universiteꢀ de Rennes 1,
Institut de Chimie de Rennes, Campus de Beaulieu, 35042 Rennes, France
Received 17 September 2003; revised 13 October 2003; accepted 20 October 2003
Abstract—The synthesis and characterization of new bipyridyl-based chromophores featuring extended oligophenylenevinylene
p-conjugated backbones are reported. Their absorption and emission properties as well as their thermal stabilities are discussed in
comparison to those of the parent ligand.
Ó 2003Elsevier Ltd. All rights reserved.
Organic molecules bearing extended p-conjugated sys-
tems are attracting much attention for their properties in
the field of molecular electronics1 and photonics.2 In
particular, oligophenylenevinylene derivatives (OPV)
have been extensively investigated in regard to applica-
tions in the fields of photo- and electroluminescence,3
photovoltaism,4 or nonlinear optics (NLO).5 In this
context, our group has been involved in the design of
4,40-disubstituted-[2,20]-bipyridines as fluorophores and
precursors to molecular,6a;b macromolecular6c and
supramolecular6d metallo-octupoles for second order
nonlinear optics. From the wide range of 4,40-p-con-
jugated bipyridyl ligands already prepared,7 such as
4,40-bis(dibutylaminostyryl)-[2,20]-bipyridine 1, we have
previously shown how simple modification of the p
linker enables the generation of tunable chromophores
and fluorophores. Continuing our effort to probe the
structural factors that could improve the optical and
nonlinear optical properties of p-conjugated bipyridyl
ligands and complexes, we report herein the synthesis of
new bipyridines featuring extended oligophenylenevin-
ylene p-conjugated backbones (OPV-bpy) 2–3 which are
the styrene-based homologues of 1 (Chart 1). Their
optical properties (absorption and emission) as well as
their thermal stability will be discussed with regard to
those of the parent ligand 1.
Several methodologies were tested for the preparation of
OPV-bpy 2–3. The first attempts, involving a classical
Knoevenagel condensation between 4,40-dimethyl-[2,20]-
bipyridine and the corresponding aldehydes, in the
presence of lithium diisopropylamide or potassium-tert-
butoxide, failed.7 A Heck cross-coupling reaction
between 4,40-bis-vinyl-[2,20]-bipyridine8 and 4-dibutyl-
amino-40-bromostilbene was also carried out by analogy
with the preparation of analogous push–pull OPV-
pyridines,9 but in our case no reaction occurred. Further
investigations involving Heck cross-coupling between
4,40-bis(bromostyryl)-[2,20]-bipyridine10 and p-dibutyl-
aminostyrene under various conditions always gave
mixtures of 2 and the corresponding monofunctional-
ized derivative which could not be separated by crys-
tallization or chromatography. Finally, the synthesis of
OPV-bpyꢀs 2–3 was achieved by means of a double
Wadsworth–Emmons reaction, as depicted in Scheme 1.
The preparation of the aldehyde precursors 711 and 11
was carried out using different routes. Reaction of 4-
bromobenzyl bromide 4 in refluxing P(OEt)3 gave the
corresponding phosphonate 5; subsequent Wittig-type
reaction with p-dibutylaminobenzaldehyde resulted in
the formation of the stilbene 6. Further halogen–lithium
exchange, followed by quenching of the lithiated inter-
mediate with dimethylformamide led to the formation of
the desired aldehyde 7 in 36% overall yield after purifi-
cation by column chromatography. The use of a similar
methodology failed for the preparation of the ‘‘styrylo-
gous’’ compound 11. However, this could be prepared in
21% overall yield by using an alternative four-step
Keywords: bipyridines; chromophores; fluorophores.
* Corresponding author. Tel.: +33-2-23236544; fax: +33-2-23236939;
e-mail: lebozec@univ-rennes1.fr
0040-4039/$ - see front matter Ó 2003Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.099

126
L. Viau et al. / Tetrahedron Letters 45 (2004) 125–128
Bu2N
Bu2N
Bu2N
N
1
N
N
2
N
N
NBu2
3
N
NBu2
NBu2
Chart 1.
NBu2
obtained selectively as their E-isomer as shown by the
3JH–H coupling constant of ca. 16 Hz between all vinylic
protons.
NBu2
Br
Br
O
(i)
(ii)
(iii)
4,40-Bis(phosphonate)-[2,20]-bipyridine 13 was synthe-
sized quantitatively by means of an Arbuzov reaction
from the 4,40-bis(bromomethyl)-[2,20]-bipyridine pre-
cursor 12.13 Finally, treatment of 13 with aldehydes 7
and 11 under normal Wittig conditions afforded the
desired OPV-bpyꢀs 2–3 in excellent yields after purifi-
cation by recrystallization (76% and 72%, respec-
tively).14 Bipyridine 2 is soluble in common organic
solvents (CH2Cl2, THF, . . .) and was characterized by
1H, 13C NMR, absorption and emission spectroscopies,
high resolution mass spectrometry and gave satisfactory
elemental analysis. The NMR data were in agreement
with the proposed structure; in particular the E config-
uration of all double bonds was established unambig-
O
Br
P
O
7
6
4
Br
5
O
NBu2
H
I
I
I
(v)
(vi), (vii)
(iv)
O
O
P
O
Br
8
10
11
9
O
O
3
uously on the basis of the JH–H vinylic coupling
constant of ca. 16 Hz. Conversely, the high insolubility
of 3 bearing the most extended conjugated chain (up to
six styryl moieties),15 prevented its complete spectro-
scopic characterization.
O
H
O
P
EtO
EtO
Br
N
N
(viii)
(ix)
2, 3
N
N
OEt
OEt
Br
P
O
12
13
Bipyridines 1–3 exhibit intense structureless charge
transfer (ICT) transitions in the visible range (Table 1
and Fig. 1). The extension of conjugation results in a
moderate bathochromic shift in the case of the first
homologation (Dkmax (2 vs 1) ¼ 19 nm, but an additional
red shift (Dkmax (3 vs 2) ¼ 1 nm) is not observed for the
second homologation. Such saturation in kmax has
already been observed for other push–pull OPV deriv-
atives and can be explained by a decrease of the charge
transfer character with an increase of the conjugation
length.5;16 Interestingly, the kmax saturation limit occurs
at ca. 421 nm (msat ¼ 23;753cm À1), a value in good
cmÀ1) reported by Meier and co-workers in the case of a
four push–pull OPV series bearing a dialkylamino donor
group and various acceptor fragments (NO2, CHO, CN,
H).16a In addition the absorption half bandwidth
(m1=2 ¼ 4280, 5048, 6039 cmÀ1 for 1, 2, 3, respectively)
Scheme 1. Reagents and conditions: (i) P(OEt)3, D, 17 h (85%); (ii)
p-dibutylaminobenzaldehyde, NaH, THF, D, 3h (88%); (iii) nBuLi,
Et2O, )10 °C followed by DMF, )10 °C, 1 h (48%); (iv) P(OEt)3, D,
17 h (83%); (v) terephthalaldehyde monodiacetal, NaH, THF, D, 3 h
(98%); (vi) p-dibutylaminostyrene, P(o-tolyl)3, Pd(OAc)2 cat.,
NEt3:DMF (2:1), D, 17 h (48%); (vii) HCl (6 N), EtOH, D, 12 h (54%);
t
(viii) P(OEt)3, CHCl3, D, 3h (93%); (ix) 7, 11, BuOK, THF, rt, 3h
(76–72%).
synthesis.12 The key step of this synthesis was the Heck
cross-coupling reaction between the iodo-stilbene
monodiethylacetal derivative 10 and p-dibutylamino-
styrene in the presence of a catalytic amount of palla-
dium acetate and tris-o-tolylphosphine in NEt3–DMF.
It is worth noting that both aldehydes 7 and 11 were
agreement with the convergence limit (m ¼ 23;231
1

L. Viau et al. / Tetrahedron Letters 45 (2004) 125–128
Table 1. Absorption, emission properties and thermal stability of OPV-bpy 1–3
127
a
a
b
Compound
kmax (nm)
e (L molÀ1 cmÀ1
)
kem (nm)
Stokes shift (nm)
/
Td
(°C)
F
5–10
1
2
3
401
420
421
65,000
78,000
––
497
598
630
96
178
209
0.23
0.70
––
355–380
330–360
330–385
a Fluorescence experiments were performed at 298 K in diluted dichloromethane solution (ca. 10À5–10À6 mol LÀ1).
b 5% and 10% weight loss temperatures.
100000
80000
60000
40000
20000
0
Thermal stability is one of the key requirements for
practical application of organic chromophores as
dopants in a polymeric or inorganic matrix. The de-
1
3
(a.u)
composition temperatures Td
corresponding to 5%
5–10
and 10% weight loss, respectively, were determined by
thermogravimetric analysis under nitrogen and are
summarized in Table 1. In the case of the stryrylogous
series 1–3, the thermal stability remains very high,
around 350 °C, showing that the homologation does not
induce any significant decrease of the thermal stability.
In a previous study,7 we have already observed that the
nature of the conjugated backbone has a dramatic in-
fluence on the Td values, the styryl-bpy derivatives being
much more stable thermally than the corresponding azo
and imino compounds. Here we also show that the
introduction of styryl groups in the conjugated back-
bone of 1 allows conservation of the molecular thermal
stability. Thus, these OPV-bpyꢀs can be considered as
thermally robust chromophores with among the highest
thermal stabilities reported in the literature for organic
chromophores and fluorophores.17
2
1
0
600
300
400
500
wavelength (nm)
1
2
3
1
To summarize, we have synthesized new bipyridine
chromophores featuring oligophenylenevinylene conju-
gated groups. Work towards the use of these ligands for
the design of octupolar bipyridyl metal complexes
combining high thermal stability and large first hyper-
polarizability is in progress.
0
400
500
600
700
800
wavelength (nm)
Acknowledgements
Figure 1. Absorption (top) and emission (bottom) spectra of OPV-
bpyꢀs 1–3 in dichloromethane solution at 298 K.
ꢀ
The authors thank the CNRS and the Region Bretagne
(PRIR A2CA16) for financial support and a grant for
L.V.
increases with the conjugation length, in agreement with
an increased number of one-photon allowed excited
states.5
References and Notes
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128
L. Viau et al. / Tetrahedron Letters 45 (2004) 125–128
1198–1203; (c) Wong, M. S.; Li, Z. H.; Shek, M. F.;
(d, J ¼ 7:9 Hz, 2H); 7.42 (d, J ¼ 8:6 Hz, 2H); 7.28
(d, J ¼ 16:2 Hz, 1H); 7.15 (d, J ¼ 16:2 Hz, 1H); 7.11
(d, J ¼ 16:2 Hz, 1H); 6.90 (d, J ¼ 16:2 Hz, 1H); 6.67
(d, J ¼ 8:6 Hz, 2H); 3.32 (t, J ¼ 7:3Hz, 4H); 1.62
(qt, J ¼ 7:3Hz, 4H); 1.42 (st, J ¼ 7:3Hz, 4H); 0.99
(t, J ¼ 7:3Hz, 6H); 13C NMR (75.47 MHz; CDCl3) d
(ppm): 191.60; 147.99; 143.69; 138.82; 135.13; 134.65;
132.06; 130.26; 129.56; 127.92; 127.24; 126.78; 126.30;
124.26; 122.90; 111.62; 50.79; 29.49; 20.36; 14.02; FTIR
(KBr): 1696 cmÀ1 (mC@O); HRMS (EI) [M]þ Calcd for
C31H35NO (found) 437.2719, (437.2751); Anal. Calcd
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3.20 (3.13).
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(ppm): 8.69 (d, J ¼ 4:4 Hz, 2H); 8.63(s, 2H); 7.61
(d, J ¼ 8:2 Hz, 4H); 7.55 (d, J ¼ 8:2 Hz, 4H); 7.51
(d, J ¼ 16:3Hz, 2H); 7.47 (d, J ¼ 4:4 Hz, 2H); 7.42
(d, J ¼ 8:8 Hz, 4H); 7.21(d, J ¼ 16:3Hz, 2H); 7.14
(d, J ¼ 16:2 Hz, 2H); 6.94 (d, J ¼ 16:2 Hz, 2H); 6.68
(d, J ¼ 8:8 Hz, 4H); 3.35 (t, J ¼ 7:5 Hz, 8H); 3.36
(q, J ¼ 7:5 Hz, 8H); 1.41 (st, J ¼ 7:5 Hz, 8H); 1.01
(t, J ¼ 7:5 Hz, 12H); 13C–1H NMR (125.77 MHz, CD2Cl2)
d (ppm): 156.50; 149.47; 148.11; 145.74; 138.94; 134.56;
132.85; 129.59, 127.83; 127.40; 126.18; 125.30; 124.07;
122.48; 120.83; 117.98; 111.59; 50.68; 29.43; 20.30; 13.77;
HRMS (FAB) [M+H]þ Calcd for C58H67N4 (found)
819.5366, (819.5364); Anal. Calcd (found) for
C58H66N4ÆCH2Cl2: C, 78.38 (79.09); H, 7.58 (7.76); N,
6.20 (6.08).
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(d, J ¼ 8:3Hz, 2H); 7.53 (d, J ¼ 7:9 Hz, 2H); 7.51