New chelating stilbazonium-like dyes from Michler's ketone

Article abstract of DOI:10.1021/ol702793j

(Chemical Equation Presented) A series of "push-pull" salts substituted with an electron-donating bis(N,N-dimethyl)aniline unit and different electron-withdrawing methyl or chelating pyridinium units have been designed and synthesized from Michler's ketone. The spectroscopic and electronic properties were investigated and compared to their DAST homologues. The studies revealed that a lower HOMO-LUMO gap is obtained in all cases, showing the ability of our donor to increase the "push-pull" effect. Two chromophores with a terpyridine as acceptor end group have also been prepared.

Full text of DOI:10.1021/ol702793j

ORGANIC
LETTERS
2008
Vol. 10, No. 2
321-324
New Chelating Stilbazonium-Like Dyes
from Michler’s Ketone
Fre´de´ric Dumur,*,† Ce´dric R. Mayer,*,† Eddy Dumas,† Fabien Miomandre,‡
Michel Frigoli,† and Francis Se´cheresse†
Institut LaVoisier de Versailles, UniVersite´ de Versailles, UMR 8180 CNRS,
45 aVenue des Etats-Unis, 78035 Versailles, France, and Laboratoire PPSM, Ecole
Normale Supe´rieure de Cachan, UMR 8531, 61 aVenue du Pre´sident Wilson,
94235 Cachan, France
cmayer@chimie.uVsq.fr
Received November 21, 2007
ABSTRACT
A series of “push
chelating pyridinium units have been designed and synthesized from Michler’s ketone. The spectroscopic and electronic properties were
investigated and compared to their DAST homologues. The studies revealed that a lower HOMO LUMO gap is obtained in all cases, showing
pull” effect. Two chromophores with a terpyridine as acceptor end group have also been
−pull” salts substituted with an electron-donating bis(N,N-dimethyl)aniline unit and different electron-withdrawing methyl or
−
the ability of our donor to increase the “push
prepared.
−
Electro-optic (EO) and nonlinear optical (NLO) properties
of organic dyes have attracted considerable attention because
of their potential applications in optical data processing
technologies.1 These chromophores are based on a push-
pull system, which consists of an electron-donating group
(D) and an electron-withdrawing group (A) coupled through
a π-conjugated spacer. The molecular properties of the
chromophores depend on the strength of the “push-pull”
effects which are functions of the ability of the donor to
provide electrons and the acceptor to withdraw electrons.
The efficiency of the dye can be enhanced by introducing a
better donor and/or a better acceptor. Concerning pure
organic chromophores exhibiting intense NLO response,
“push-pull” molecules bearing N,N-dimethylaniline as
donating group have been widely studied.2 In the solid state,
organic crystals of 4-(4-N,N-dimethylaminostyryl)-1-meth-
ylpyridinium tosylate (DAST) are of particular interest
because they have the largest NLO coefficient among organic
materials.3 In that case, the acceptor is the methyl pyridinium
† Institut Lavoisier de Versailles.
‡ Ecole Normale Supe´rieure de Cachan.
(2) (a) Roberto, D.; Ugo, R.; Bruni, S.; Cariati, E.; Cariati, F.; Fantucci,
P.; Invernizzi, I.; Quici, S.; Ledoux, I.; Zyss, J. Organometallics 2000, 19,
1775. (b) Coe, B. J.; Harris, J. A.; Asselberghs, I.; Wostyn, K.; Clays, K.;
Persoons, A.; Brunschwig, B. S.; Coles, S. J.; Gelbrich, T.; Light, M. E.;
Hursthouse, M. B.; Nakatani, K. AdV. Funct. Mater. 2003, 13, 347.
(1) (a) Zyss, J. Molecular Nonlinearoptics: Materials, Physics and
DeVices; Academic Press: Boston, 1994. (b) Nonlinear Optics of Organic
Molecules and Polymers; Nalwa, H. S., Miyara S., Eds.; CRC Press: Boca
Raton, FL, 1997.
10.1021/ol702793j CCC: $40.75
© 2008 American Chemical Society
Published on Web 12/23/2007

group. But recent research has also shown that metal
complexes offer many possibilities. Several systems where
the acceptor is a pyridine chelated to a metal have also been
reported.4 Our research group is involved in the design and
investigation of metal complexes for the conception of
nanocomposites exhibiting electrochemical properties and
photoluminescence at room temperature.5 In the course of
our studies, it was interesting to investigate new chro-
mophores bearing on one side two N,N-dimethylaniline
groups as donating moiety and on the other side ligands able
to coordinate metals as accepting moiety. Herein, we report
the preparation and properties of these chromophores linked
to ligands such as terpyridine and bipyridine via a meth-
ylpyridinium group. Two chromophores with a terpydine as
the acceptor end group have been prepared.
Chromophores 1a-d, 2a-d, and 3a,b have been prepared
according to two different strategies. The first one, used for
the synthesis of 1a-d, consisted of the preparation of
molecule 6, which is an interesting building block as it can
be further functionalized to obtain various donor-acceptor
systems that are amenable to further functionalization.
Molecule 6 was obtained in three steps starting from the
commercially available Michler’s ketone (Scheme 1).
using a procedure described in the literature.6 Molecule 6
was finally synthesized using a Horner-Wadsworth-Em-
mons olefination. The generation of a carbanion from 5
proceeded smoothly by treatment with the non-nucleophilic
base KOtBu, and the desired molecule was obtained in
moderate yield (63%). Reaction of 1 equiv of methyl iodide,
brominated terpyridine 7a,b, or bipyridine 7c with molecule
6 at reflux for one night in acetonitrile afforded the salts
1a-d with yields ranging from 72% to 85%. Terpyridine
7b, used in the previous reaction, has been obtained
differently from the literature,7 in 67% yield, by bromination
of 4′-(3,5-bis-methylphenyl[2,2′,6′,2′′]terpyridine) with NBS
and benzoyl peroxide in CCl4.
Pyridine derivatives 2a,b,d and 3a,b bearing extended
conjugated olefinic substituent in an all-trans configuration
were obtained by a second procedure as illustrated in
Scheme 2.
Scheme 2. Synthesis of Chromophores 2a,b,d and 3a,b
Scheme 1. Synthesis of Dyes 1a-d
In the first step, three different N-substituted pyridinium
8a-c were prepared from 4-picoline and the corresponding
halogenated derivative 7a,c or methyl iodide, respectively.
Then base-catalyzed condensations of aldehydes 9a,b with
salts 8a-c in the presence of piperidine afforded the
chromophores 2a,b,d and 3a,b with yields ranging from 38%
to 90%. The Knoevenagel reaction furnished each dye
as a mixture of cis/trans isomers, with the trans isomer as
the major product in all cases. Isomerization with diluted
iodine in a mixture of toluene and chloroform afforded
all-trans configuration dyes, except 3b, which was never fully
isomerized. To facilitate the purification of the salts and to
increase the solubility, bromide salts 1c and 1d were
converted to the corresponding hexafluorophosphate
ones.
Alcohol 4 was obtained in moderate yield by reduction
of the Michler’s ketone with sodium borohydride in boiling
THF and was then directly converted to the phosphonate 5
Due to the side products formed during the synthesis of
2b and 2d and the difficulties in separating them from the
target molecule, a third method was developed to obtain an
extended analogue of pyridine 6. Moreover, this molecule
can be easily used to functionalize various bromo derivatives.
(3) Lee, K. S.; Kim, O.-K. Photonics Sci. News 1999, 4, 9.
(4) (a) Lucenti, E.; Cariati, E.; Dragonetti, C.; Manassero, L.; Tessore,
F. Organometallics 2004, 23, 687. (b) Tessore, F.; Locatelli, D.; Righetto,
S.; Roberto, D.; Ugo, R. Inorg. Chem. 2005, 44, 2437. (c) Coe, B. J.; Jones,
L. A.; Harris, J. A.; Brunschwig, B. S.; Asselberghs, I.; Clays, K.; Persoons,
A.; Garin, J.; Orduna, J. J. Am. Chem. Soc. 2004, 126, 3880.
(5) (a) Mayer, C. R.; Dumas, E.; Miomandre, F.; Me´allet-Renault, R.;
Warmont, F.; Vigneron, J.; Etcheberry, A.; Se´cheresse, F. New J. Chem.
2006, 30, 1628. (b) Mayer, C. R.; Dumas, E.; Michel, A.; Se´cheresse, F.
Chem. Commun. 2006, 4183.
(6) Zheng, S.; Barlow, S.; Parker, T. C.; Marder, S. R. Tetrahedron Lett.
2003, 44, 7989.
(7) Galgliardo, M.; Perelear, J.; Hartl, F.; van Klink, G.P.M.; von Koten,
G. Eur. J. Inorg. Chem. 2007, 2111.
322
Org. Lett., Vol. 10, No. 2, 2008

Molecule 12 was obtained in three steps as outlined
in Scheme 3. Aldehyde 9a was reduced with sodium
borohydride in ethanol to give the alcohol 10 in 84%
Scheme 3. Synthesis of All-trans Pyridine 12 and 2b-d
Figure 1. UV-vis spectra of dyes 1a (solid line), 2a (short dashed
line), and 3a (dashed line) at 20 °C in acetonitrile (0.5 mM) and
emission (dotted line) spectrum of 2a in acetonitrile. The sample
was excited at 471 nm for the luminescence spectrum.
dyes have the same methylpyridinium iodide accepting
moiety but differ from the number of ethylenic conjugated
spacers (n ) 0-2). All of the dyes exhibit three transitions
and cover a broad range (250-750 nm) of the UV-vis
spectrum. Each dye shows an intense visible absorption due
to intramolecular charge transfer (ICT) excitations and less
intense bands ascribed to nondirectional π f π* transitions
at higher energy.
As expected, these ICT bands red shift as n increases. The
same phenomenon is observed with dyes 1b, 2b, and 3b.
Compared to DMF, the absorption maxima obtained for 1a,
2a and 3a in dichloromethane are strongly shifted. The
greatest shift is observed for the dye 3a with ∆λ ) 83 nm.
Examination of the solvatochromism for each dye in seven
solvents revealed a good correlation with the solvent
parameters π* determined by Kamlet and Taft (Table 1).10
yield. It was then converted to the phosphonium 11 using 1
equiv of phosphonium hydrobromide in methanol at
room temperature for 4 days. Finally, pyridine 12 was
prepared by a Wittig reaction with the phosphonium 11 and
pyridine-4-carboxaldehyde. All-trans isomerization was
achieved by treatment with iodine in toluene at reflux.
Pyridine 12 was obtained in 62% yield. Finally, salts 2b-d
were prepared according to the same procedure used for 1a-
d. All obtained dyes are orange or black in the solid state
and freely dissolve in polar solvents to produce orange or
violet solutions.
We also investigated the synthesis of chromophores with
a terpyridine as accepting end group (Scheme 4). The
Scheme 4. Preparation of Terpyridines 13a,b
Table 1. Absorption Maxima (nm) Obtained Experimentally
by UV-vis Spectroscopy and π* Values by Kamlet and Taft for
Dyes 1a, 2a, and 3a
solvent
π*
λmax (1a)
λmax (2a)
λmax (3a)
toluene
THF
0.54
0.58
0.60
0.71
0.75
0.82
0.88
1
475
484
484
480
478
523
478
468
527
524
521
521
519
589
518
520
545
550
535
533
530
613
530
535
MeOH
acetone
CH3CN
CH2Cl2
DMF
terpyridine 13a was synthesized using the Kro¨hnke meth-
odology.8 The terpyridine core was prepared by conden-
sation of the aldehyde 9a with 2-acetylpyridine in the
presence of KOtBu and NH4OAc. Surprisingly, when the
reaction was carried out in the same conditions with the
aldehyde 9b, the expected terpyridine 13b was not obtained
but the decomposition of the aldehyde 9b was observed
instead. 13b was finally obtained by a one-pot procedure
using a mixture of 2-acetylpyridine and the aldehyde 9b in
alkaline methanolic solution, as previously reported in the
literature.9
DMSO
The negative solvatochromism observed for 1a, 2a, and 3a
was expected12 and reflects a decrease of the ground state
dipole moment of the dye molecules upon excitation. The
(10) (a) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W.
J. Am. Chem. Soc. 1977, 99, 6077. (b) Kamlet, M. J.; Abboud, J.-L. M.;
Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877.
(11) Coe, B. J.; Harris, J. A.; Hall, J. J.; Brunschwig, B. S.; Hung, S.-
T.; Libaers, W.; Clays, K.; Coles, S. J.; Horton, P. N.; Light, M. E.;
Hursthouse, M. B.; Garin, J.; Orduna, J. Chem. Mater. 2006, 18, 5907.
(12) Jedrzejewska, B.; Kabatc, J.; Paczkowski, J. Dyes Pygments 2007,
74, 262.
The absorption spectra recorded for the acetonitrile solu-
tion of the dyes 1a, 2a, and 3a are shown in Figure 1. These
(8) Kro¨hnke, F. Synthesis 1976, 1.
(9) Vaduvescu, S.; Potvin, P. G. Eur. J. Inorg. Chem. 2004, 1763.
Org. Lett., Vol. 10, No. 2, 2008
323

HOMO-LUMO gap can be experimentally calculated from
the position of the ICT band. A stronger decrease of the gap
occurs from n ) 0 to n ) 1 (∆E ) 0.2 eV) compared with
the gap difference observed from n ) 1 to n ) 2 (∆E )
0.05 eV). The variation of the gap value from n ) 0 to n )
1 can be explained by the steric hindrance between the
aromatic groups in 1a, steric hindrance that indeed decreases
with the lengthening of the ethylenic spacer.
The HOMO-LUMO gap was also investigated by cyclic
voltammetry. The cathodic part corresponds to the reversible
reduction of the pyridinium moiety, which is shifted toward
more positive potentials as n increases.
at ca. +0.75 V (almost independently on n) in the previously
oxidized products, involving precipitation of the coupling
products on the electrode.
The lack of chemical reversibility for these oxidation
processes prevents us from measuring true standard poten-
tials, leading only to minimal values for the band gap
measured from the difference between oxidation potential
of the donor and reduction potential of the acceptor (Table
2). Nevertheless, the results obtained by this second technique
Table 2. HOMO-LUMO Gap Obtained Experimentally by
UV-vis Spectroscopy and Cyclic Voltammetry in Acetonitrile
for Dyes 1a, 2a, and 3a
The anodic part exhibits a much more complicated
behavior with three non reversible signals followed by a
reversible adsorption-desorption process (Figure 2).
DAST
analogues, n )
dyes
∆Ea (eV)
∆Eb (eV)
∆Ec (eV)
1a
2a
3a
> 2.18
> 1.75
> 1.56
2.59
2.39
2.34
0
1
2
2.64
2.55
2.48
a Results from cyclic voltammetry in CH2Cl2. b Results from UV-vis
spectroscopy in CH3CN. c Data taken from ref 11.
clearly exhibit the same trend for the variation of Eg
according to the value of n as those given by UV-vis
spectroscopy. Comparison of the HOMO-LUMO gap
determined for our dyes with reported dyes (DAST and
extended analogues) bearing only one N,N-dimethylaniline
group11,13 as donor shows that in all cases, the gap obtained
for Michler’s derivatives is smaller. Comparison of both
donors revealed that Michler’s donating group is the strongest
of the two groups (Table 2). Photoluminescence studies
showed that, compared to DAST, all three dyes 1a, 2a and
3a have rather weak fluorescence in acetonitrile, as shown
in Figure 1 for chromophore 2a (see the Supporting Informa-
tion). The largest Stokes shift of fluorescence emission is
obtained for 2a (∼250 nm). The emission intensity drops to
about 10% of the intensity observed for DAST. A stronger
decrease in luminescence intensity occurs upon replacement
of the methyl pyridinium by other groups (terpyridyl for 2b
and bipyridyl for 2d).
Figure 2. Cyclic voltammograms (CVs) of dyes 1a (solid line, n
) 0), 2a (short dashed line, n ) 1), and 3a (dashed line, n ) 2) at
20 °C in dichloromethane + TBAPF6 (ca. 2 mM) on platinum
electrodes at 50 mV/s: (a) cathodic part, (b) anodic part. For clarity,
the signal due to oxidation of the iodide counterion was substracted
by recording CV of TBAI.
In conclusion, we have succeeded in the development of
a family of new dyes bearing two N,N-dimethylaniline groups
as donor and ligands such as terpyridine and bipyridine as
acceptor. Such ligands can be used to prepare metal
complexes, currently studied in our laboratory. Thus, these
chromophores are potential candidates for advanced materials
applicable to optoelectronic fields, such as nonlinear optics.
In such conditions the first redox peak can be ascribed to
the irreversible oxidation of the vinyl moiety into cation
radical followed by dimerization:14 the corresponding oxida-
tion potential is strongly shifted to less positive values as n
increases, since the radical created is more stabilized by a
longer polyvinyl spacer. This redox step is followed by the
irreversible oxidation of the dimethylaniline end functions
Acknowledgment. We thank the C’NANO IdF, CNRS,
and the ANR agency (AURUS program) for financial
support.
Supporting Information Available: Experimental pro-
cedures and spectroscopic characterizations. This material
is available free of charge via the Internet at http://pubs.acs.org.
(13) The drawing corresponding to DAST and extended analogues is
presented in the Supporting Information.
(14) Suzuki, T.; Higushi, H.; Ohkita, M.; Tsuji, T. Chem. Commun. 2001
1574.
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