New bichromophoric-2,2′-bipyridines: synthesis and optical properties

Article abstract of DOI:10.1016/j.tetlet.2006.12.048

The synthesis and characterization of new unsymmetrically substituted bipyridyl-based chromophores featuring π-conjugated donor, acceptor or photoisomerizable backbones are reported. Their absorption and emission properties are discussed in comparison to

Full text of DOI:10.1016/j.tetlet.2006.12.048

Tetrahedron Letters 48 (2007) 1229–1232
New bichromophoric-2,20-bipyridines:
synthesis and optical properties
Olivier Lohio, Lydie Viau, Olivier Maury and Hubert Le Bozec*
‘Sciences Chimiques de Rennes’, UMR 6226-CNRS-Universite´ de Rennes 1, Campus de Beaulieu, 35042 Rennes, France
Received 1 December 2006; accepted 11 December 2006
Available online 4 January 2007
Abstract—The synthesis and characterization of new unsymmetrically substituted bipyridyl-based chromophores featuring p-con-
jugated donor, acceptor or photoisomerizable backbones are reported. Their absorption and emission properties are discussed in
comparison to those of the parent ligands.
Ó 2006 Elsevier Ltd. All rights reserved.
2,20-Bipyridines are ubiquitous chelating ligands in
coordination chemistry, and the resulting metal com-
plexes have been intensively studied for their chemical,
electrochemical and optical properties.1 Another interest
is their uses as building blocks for the construction of
efficient molecular and macromolecular nonlinear opti-
cal (NLO) chromophores.2–5 During the last decade,
our group has mainly been concerned with the NLO
properties of octupolar tris(4,40-p-donor substituted-
2,20-bipyridine) metal complexes in which intra-ligand
charge transfer transitions (ILCT) mainly contribute
to the NLO response.6 In this respect, we have devel-
oped several methods for the preparation of symmetrical
bipyridines featuring a variety of p-donor conjugated
substituents such as p-dibutylaminostyryl (1)7 and very
recently the photoisomerizable p-dibutylaminophenyl-
azostyryl group (2).8 In order to design octupolar
NLO-phores in which the NLO activity would be
associated with only metal-to-ligand charge transfer
transitions (MLCT), we also reported the synthesis of
bipyridine 3 bearing the strong electron-accepting
p-nitrostyryl substituent (Scheme 1).7 Unfortunately,
attempts to prepare the corresponding metal complexes
were unsuccessful due to the very poor solubility of 3 in
common organic solvents. Continuing our work toward
chromophores based on functionalized bipyridyl
ligands, we sought to prepare unsymmetrical bipyridines
incorporating electron donor (D) and acceptor (A) moi-
eties. With such type of ‘push–pull’ bichromophoric
molecule, it might be possible to design a novel class
of D3 octupolar fac-tris(bipyridine) metal complexes
with alternating D and A groups. In this Letter we
report the synthesis and optical properties of the first
example of such D-bpy-A chromophore (4), in which
p-dibutylaminostyryl and p-nitrostyryl substituents are
the donor and acceptor parts, respectively. By using
the same strategy, we also describe the preparation of
another interesting unsymmetrically disubstituted
bipyridine (5) containing on one side a fluorophore
and on the other side a photoisomerizable azobenzene
group.
Scheme 2 depicts the synthetic pathways we used to
prepare the key intermediate 9. This compound was
obtained in 64% overall yield by using a four-step
synthesis. The first step involved the deprotonation
of 4,40-dimethyl-2,20-bipyridine with 1 equiv of LDA
followed by addition of p-dibutylaminobenzaldehyde,
giving rise to 6 in quantitative yield. The trimethylsilyl
derivative 7 was prepared by addition of another equiv-
alent of LDA followed by trapping the resulting anion
with TMSCl. The bromomethyl derivative 8 was then
produced in high yield (92%) by using the procedure
already described to prepare 4,40-bis(bromomethyl)-
2,20-bipyridine.9 Finally the phosphonate derivative 9
was synthesized in good yield (80%) by means of an
Arbuzov reaction.6,10
Keywords: Bipyridines; Chromophores; Fluorophores.
*
Corresponding author. Tel.: +33 2 23236544; fax: +33 2 23236939;
e-mail: lebozec@univ-rennes1.fr
Present address: ‘Laboratoire de Chimie’ UMR 5182-CNRS-ENS
Lyon, 46 alle
´e d’Italie, 69007 Lyon.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.12.048

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O. Lohio et al. / Tetrahedron Letters 48 (2007) 1229–1232
NBu2
N
NBu2
NO2
N
N
N
N
N
N
N
1
3
2
Bu2N
O2N
N
N
NBu2
Bu2N
N
NO2
N
N
N
N
N
5
4
Bu2N
Bu2N
Scheme 1. Structures of bichromophoric bipyridines 4, 5 and of the parent ligands 1–3.
Bu2N
Bu2N
(i)-(iii)
HO
(iv)-(v)
N
N
HO
N
N
N
N
TMS
6
7
(vi)
Bu2N
Bu2N
N
(vii)
HO
N
N
N
P(O)(OEt)2
9
8
Br
Scheme 2. Reagents and conditions: (i) LDA (1.1 equiv), THF, ꢀ78 °C; (ii) p-dibutylaminobenzaldehyde, THF, ꢀ78 °C, 1 h; (iii) H2O, rt, 97%; (iv)
LDA (2.2 equiv), 20 min, ꢀ78 °C; (v) Me3SiCl (1 equiv), ꢀ78 °C, 2 min followed by EtOH (10 ml), 90%; (vi) C2Br2F4 (3.8 equiv), DMF, rt, 10 min
followed by CsF (4 equiv), 20 min, 92% and (vii) P(OEt)3 (15 equiv), CHCl3, reflux, 3 h; PPTS (0.1 equiv), reflux, 3 h, 80%.
The synthesis of unsymmetrically substituted ligands 4
and 5 is shown in Scheme 3. They were prepared in
excellent yield by means of a Wadsworth–Emmons reac-
tion between synthon 9 and the corresponding benzalde-
hyde derivatives. Their structures were unambiguously
1
confirmed by H NMR spectroscopy.11,12 In particular,

O. Lohio et al. / Tetrahedron Letters 48 (2007) 1229–1232
1231
H
(91%)
4
O2N
P(O)(OEt)2
O
tBuOK (1.2 equiv)
N
THF, rt
N
H
9
(82%)
5
N
N
O
Bu2N
Bu2N
Scheme 3. Synthesis of ligands 4 and 5.
Table 1. Photophysical data (absorption and emission)a
which is half. Similarly, the emission maximum of 4 is
observed at the same wavelength as that of 1, showing
that the emission occurs only from the p-dibutylamino-
styryl pyridine sub-chromophore. These observations
indicate that there is no electronic interaction between
the donor and acceptor moieties, as expected by their
meta-arrangements with respect to the 2,20-bipyridine
bridge. Bipyridine 5 exhibits an intense broad band with
Compound kabs (nm)
e (L molꢀ1 cmꢀ1
)
kem
Stokes
(nm) shift (nm)
1
3
4
5
401
480
400
65,000
76,500
30,200
497
—
495
502
97
—
95
94
b
408, 470 (sh) 43,000
a Performed at 298 K in diluted dichloromethane solution (ca. 10ꢀ5
10ꢀ6 mol Lꢀ1).
b Non emissive.
–
k
max at 408 nm and a shoulder at ca. 470 nm. A compari-
son with the optical data of 1 and 3 clearly shows that
the absorption spectrum of 5 consists in the overlap of
the absorptions of the two different sub-chromophoric
units. Like compounds 1 and 4, 5 exhibits a structureless
fluorescence band at ca. 500 nm, and the excitation spec-
trum (kexc = 395 nm) closely matches the absorption
spectrum of 1 (Fig. 2).
they display two different AB systems for the vinylic
protons with typical coupling constants (3JHH ꢁ 16 Hz)
for all-trans (E) isomers.
The UV–visible and emission spectra of ligands 4 and 5
have been measured in dichloromethane and the results
are presented in Table 1 and Figures 1 and 2. The
spectra feature intense p–p* transitions in the UV region
together with broad charge transfer bands in the visible
region. In the visible, the absorption spectrum of 4 is
almost similar to that of 1, except the molar absorptivity
In conclusion, we have prepared two new bipyridine
chromophores by using a stepwise functionalization of
4,40-dimethyl-2,20-bipyridine,
a methodology which
could provide access to a family of unsymmetrically
substituted bipyridines. The use of these ligands for
the construction of new octupolar metal NLO-phores
is under investigation.
80000
2
70000
1
60000
50000
5
40000
4
30000
20000
10000
0
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Figure 1. UV–visible absorption spectra of 1, 2, 4 and 5 in CH2Cl2 at room temperature.

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O. Lohio et al. / Tetrahedron Letters 48 (2007) 1229–1232
1.2
1
0.8
0.6
0.4
0.2
0
4
5
1
330
380
430
480
530
580
630
680
Wavelength (nm)
Figure 2. Normalized emission spectra of 1, 4 and 5 upon excitation at 400 nm in CH2Cl2 at room temperature (right); excitation spectrum of 4 (left).
9. Fraser, C. L.; Anastasi, N. R.; Lamba, J. J. S. J. Org.
Chem. 1997, 62, 9314–9317.
Acknowledgements
10. Selected data for 9: 1H NMR (500.13 MHz; CDCl3) d
(ppm): 8.64 (d, J = 5.0 Hz, 1H); 8.62 (d, J = 5.0 Hz, 1H);
8.45 (s, 1H); 8.32 (s, 1H); 7.50–7.25 (m, 4H); 7.19 (d,
J = 16.6 Hz, 1H); 6.86 (d, J = 16.6 Hz, 1H); 6.61 (d,
J = 8.8 Hz, 2H); 4.08 (qt, J = 7.9 Hz, 4H); 3.27 (t, J =
7.2 Hz, 4H); 3.23 (d, JPꢀH = 22.4 Hz, 2H); 1.61–1.46 (m,
4H); 1.43–1.22 (m, 4H); 1.07 (t, J = 7.2 Hz, 12H); 31P
NMR (81.02 MHz; CDCl3) d (ppm): 25.75; HRMS (EI)
[MꢀH2O]+ calcd for C31H42N3O3P. Found 535.2963
(535.2944).
The authors thank ANRT and Franc¸ois Charles Ober-
thur Fiduciaire Co. for a grant to O.L., and CNRS
´
and Region Bretagne for a grant to L.V.
References and notes
1. (a) Smith, A. P.; Fraser, C. L. In Comprehensive Coordi-
nation Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.;
Pergamon Press: Oxford, 2004; Vol. 1, pp 1–23; (b) Kaes,
C.; Katz, A.; Hosseini, M. W. Chem. Rev. 2000, 100, 3553–
3590.
2. Zyss, J.; Dhenaut, C.; Chau Van, T.; Ledoux, I. Chem.
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3. Maury, O.; Le Bozec, H. Acc. Chem. Res. 2005, 38, 691–704.
4. Vance, F. W.; Hupp, J. T. J. Am. Chem. Soc. 1999, 121,
4047–4053.
11. Selected data for 4. 1H NMR (500.13 MHz, CD2Cl2) d
(ppm): 8.73 (d, J = 5.0 Hz, 1H); 8.60 (d, J = 5.0 Hz, 1H);
8.59 (s, 1H); 8.49 (s, 1H); 8.27 (d, J = 8.7 Hz, 2H); 7.70 (d,
J = 8.7 Hz, 2H); 7.50 (d, J = 16.6 Hz, 1H); 7.46 (d,
J = 8.7 Hz, 2H); 7.42 (d, J = 16.6 Hz, 1H); 7.40–7.38 (m,
2H); 7.30 (d, J = 16.6 Hz, 1H); 6.92 (d, J = 16.6 Hz, 1H);
6.67 (d, J = 8.7 Hz, 1H); 3.34 (t, J = 7.7 Hz, 4H); 1.62 (qt,
J = 7.7 Hz, 4H); 1.40 (st, J = 7.5 Hz, 4H); 1.00 (t,
J = 7.5 Hz, 6H); HRMS (ESI) [M+H]+ calcd for
C34H37N4O2. Found 533.9165 (533.9130).
5. Coe, B. J.; Harris, J. A.; Brunschwig, B. S.; Asselberghs,
I.; Clays, K.; Garin, J.; Orduna, J. J. Am. Chem. Soc.
2005, 127, 13399–13410.
12. Selected data for 5. 1H NMR (500.13 MHz, CD2Cl2) d
ppm: 1H NMR (500.13 MHz, CD2Cl2) d ppm: 8.70 (d,
J = 5.0 Hz, 2H) 8.60 (s, 2H); 7.96 (d, J = 16.2 Hz, 1H);
7.91–7.68 (m, 3H); 7.57 (d, J = 16.4 Hz, 1H); 7.53–7.34
(m, 5H); 7.29 (d, J = 16.2 Hz, 1H); 6.95 (d, J = 16.4 Hz,
1H); 6.75 (d, J = 9.2 Hz, 2H); 6.72 (d, J = 8.7 Hz, 4H);
3.36 (t, J = 7.2 Hz, 8H); 1.77–1.53 (m, 8H); 1.52–1.27 (st,
J = 7.2 Hz, 8H); 1.01 (t, J = 7.2 Hz, 12H); HRMS (ESI)
[M+H]+ calcd for C34H37N4O2. Found 719.4801
(719.4802).
´ ´
´
6. Maury, O.; Viau, L.; Senechal, K.; Corre, B.; Guegan,
J.-P.; Renouard, T.; Ledoux, I.; Zyss, J.; Le Bozec, H.
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´
7. Maury, O.; Guegan, J.-P.; Renouard, T.; Hilton, A.;
Dupau, P.; Sandon, N.; Toupet, L.; Le Bozec, H. New J.
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