Amphiphilic polyenic push-pull chromophores for nonlinear optical applications

Article abstract of DOI:10.1039/a908717f

Amphiphilic cationic polyenic push-pull chromophores which offer interesting supramolecular possibilities for second harmonic generation have been synthesised and their optical non-linearities studied for the first time by electric field induced second ha

Full text of DOI:10.1039/a908717f

Amphiphilic polyenic push–pull chromophores for nonlinear optical
applications
Valérie Alain,a† Mireille Blanchard-Desce,*ab Isabelle Ledoux-Rakc and Joseph Zyssc
a Ecole Normale Supérieure, Département de Chimie (CNRS, UMR 8640), 24 rue Lhomond, 75231 Paris Cedex 05,
France
b UMR 6510, Université Rennes 1, Campus de Beaulieu, Bât. 10A, 35042 Rennes cedex, France.
E-mail: mireille.blanchard-desce@univ-rennes1.fr
c
Laboratoire de Photonique Quantique et Moléculaire (UMR 8537), ENS Cachan, 61 avenue du Président Wilson,
94235 Cachan, France
Received (in Liverpool, UK) 28th October 1999, Accepted 24th January 2000
Amphiphilic cationic polyenic push–pull chromophores
which offer interesting supramolecular possibilities for
second harmonic generation have been synthesised and their
optical non-linearities studied for the first time by electric
field induced second harmonic (EFISH) generation in
solution.
the pyridinium acceptor end group by a quinolinium end group,
larger nonlinearities are expected.
Stilbazolium dyes and analogous chromophores are usually
prepared using Konevenagel condensation (Scheme 1).6 How-
ever, this simple strategy proved ineffective in the case of
polyenic analogues (n > 1) bearing long alkyl chains. It leads to
both poor yields and purification problems due to the side
formation of shorter homologues. To ensure satisfactory
preparation yields, we have implemented an alternative syn-
thetic scheme based on the Wittig–Horner condensation of a
pyridine moiety10 on a polyenal bearing the donating end group
(Scheme 1). This allows for the preparation in medium to good
yield of compounds of series 4 from polyenals of series 1 using
solid–liquid phase transfer conditions. Polyenals of series 1 can
be obtained in high yields from the generic molecule 1[0] using
a sequential vinylic homologation.9 Molecules of series 4 were
obtained as pure all-E isomers (as shown by NMR spectra and
elemental analyses) after catalytic isomerisation and column
chromatography n = 1, 2) or recrystallisation (n > 2).
Alkylation of molecules of series 4 in neat MeI readily afforded
pure amphiphilic push–pull polyenic chromophores of series
2.
The electric field induced second harmonic (EFISH) genera-
tion technique11 is a useful method to derive the quadratic
nonlinearity (and more precisely the projection of the dipolar
part of the b tensor on the dipole moment m) of dipolar
chromophores. It is in principle precluded in the case of ionic
species. Yet, we have been able to implement the EFISH
technique for the determination of the mb values of ionic
amphiphilic chromophores 2 and 3 by operating in a solvent of
low polarity thanks to the formation of close ion pairs. The
EFISH experiments were carried out in chloroform (er = 4.7)
and at 1.907 mm in order to avoid absorption of the second
harmonic. The mb values and the wavelength of the maximum
absorption (lmax) are collected in Table 1. The static mb(0)
values extrapolated at zero frequency by using the two-level
dispersion model for b11 (valid in the case of 1-D intramolecular
charge transfer only) are also included.
Molecular nonlinear optics (NLO) has attracted increasing
interest over the past ten years.1 NLO materials are highly
promising for applications in various fields including tele-
communications, optical data storage and processing, optical
power limitation etc. NLO phenomena also present interesting
opportunities for probing and imaging purposes. For instance,
second harmonic generation (SHG) has been successfully used
for probing asymmetrical systems (such as interfaces)2 or high
resolution imaging of biological cells.3
Within this context, we have investigated a strategy based on
the design of amphiphilic push–pull chromophores [i.e. mole-
cules combining an electron-donating group (D) and an
electron-withdrawing group (A) connected by a conjugated
system] with enhanced quadratic molecular response (i.e.
hyperpolarizability b) and prone to interact in an asymmetrical
way with a lipid membrane. Such characteristics provide an
interesting way to induce the asymmetry required for quadratic
NLO phenomena both at the molecular and macroscopic levels,
by taking advantage of specific interactions with hydrophilic–
hydrophobic interfaces. Hereafter, we present the design and
synthesis of amphiphilic cationic polyenic chromophores
showing large nonlinearities. Their quadratic nonlinearities
have been studied for the first time by electric field induced
second harmonic (EFISH) generation in solution, a technique
usually precluded for ionic species.
Stilbazolium dyes have been the topic of many studies in the
field of NLO. They have led to highly SHG efficient crystals.4
In addition, they are interesting candidates for designing NLO
micro- and nanostructures. For examples, derivatives bearing an
elongated alkyl chain have been deposited into Langmuir–
Blodgett films showing SHG activity5 and amphiphilic deriva-
tives have been shown to be of interest for the design of optical
probes for membrane potentials.6 Our choice has been to
investigate amphiphilic vinylogous derivatives of stilbazolium
dyes (Fig. 1). The presence of two butyl hydrophobic tails
grafted on the donor end group and of the positively-charged
hydrophilic acceptor moiety confers an amphiphilic character to
these push–pull chromophores and should facilitate their
interaction with a lipidic membrane while avoiding deleterious
detergent effects. Increasing the polyenic chain length is
expected to lead to a marked increase of b as already known for
other push–pull polyenic systems.7–9 Similarly, by substituting
As clearly observed from Table 1, amphiphilic salts of series
2[n] and 3[1] show large mb values. Lengthening the polyenic
chain induces both a bathrochromic shift and a significant
increase of the quadratic nonlinearity, although the effect seems
to slow down for the longest derivatives. This leads to a mb(0)
† Present address: Laboratoire de Photophysique et de Photochimie
Supramoléculaires et Macromoléculaires, ENS Cachan, 61 avenue du
Président Wilson, 94235 Cachan, France.
Fig. 1 Molecular engineering of amphiphilic push–pull chromophores.
DOI: 10.1039/a908717f
Chem. Commun., 2000, 353–354
This journal is © The Royal Society of Chemistry 2000
353

transparency. Compound 2[3] exhibits a mb(0) value twice as
large as that for compound 3[1] for a similar molecular weight
while remaining significantly blue-shifted. Elongation of the
polyenic chain of derivatives of series 2 is thus preferable—in
terms of nonlinearity–transparency trade-off—to turning to a
stronger acceptor moiety.
Finally, cationic polyenic push–pull chromophores have been
synthesised and their optical non-linearities studied for the first
time by electric field induced second harmonic generation in
solution, owing to the formation of ion pairs in a solvent of low
relative permittivity. Their quadratic nonlinearities can be
enhanced by increasing the polyenic chain length and/or
adjusting the charged heterocyclic acceptor. Recent second
harmonic generation and two-photon excited fluorescence
experiments reported in ref. 12 provide evidence that chromo-
phores of type 2 are incorporated in the outer leaflet of model
bilayer lipid membranes and orientated perpendicular to the
membrane surface. These amphiphilic chromophores hold
promise as sensitive probes for SHG imaging of membrane
potentials.3,13 In addition, incorporation of the chromophores in
the lipid bilayer is expected to significantly influence the
molecular nonlinear responses, as shown recently for the
inclusion of a stilbazolium dye in a supramolecular com-
plex.14
The Délégation Générale pour l’Armement (DGA) is ac-
knowledged for a fellowship to V.A. We thank Idrissa Njabira
for his participation in EFISH measurements.
Notes and references
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2 K. B. Eisenthal, Chem. Rev., 1996, 96, 1343.
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Scheme 1 Reagents and conditions: i, N-methyl-4-picolinium iodide (0.9
equiv.), cat. piperidine, EtOH, reflux, 16 h; ii, N-methyllepidinium iodide
(0.9 equiv.), cat. piperidine, EtOH, reflux, 16 h; iii, (1,3-dioxolan-
2-ylmethyl)tributylphosphonium bromide (1.1 equiv.), NaH (1.5 equiv.),
cat. 18-C-6, THF, room temp., 20 h; iv, HCl (10%), THF, room temp., 1 h;
v, diphenyl(4-pyridyl)methylphosphine oxide (1.1 equiv.), THF, NaH (1.5
equiv.), cat. 18-C-6, room temp. 15 h; vi, MeI, room temp., 1 h.
Table 1 Linear and nonlinear optical properties of amphiphilic push–pull
chromophores 2[1–5] and 3[1] in chloroform; mb values were derived from
EFISH measurements at 1.907 mm
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Compound
mb(2w)a/10248 esu lmax/nm mb(0)/10248 esu
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Desce, J.W. Perry and J. Skindhøj, Science, 1994, 263, 511.
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C. Runser, M. Barzoukas, S. Lebus and R. Wortmann, Chem. Eur. J.,
1997, 3, 1091.
2[1]
2[2]
2[3]
2[4]
2[5]
3[1]
840
2230
2700
3380
3690
1380
517
558
581
589
598
603
550
1340
1540
1890
2020
745
10 M. Blanchard-Desce, T. S. Arrhenius and J.-M. Lehn, Bull. Soc. Chim.
Fr., 1993, 130, 266.
11 J.-L. Oudar, J. Chem. Phys., 1977, 67, 446.
12 L. Moreaux, O. Sandre, M. Blanchard-Desce and J. Mertz, Opt. Lett.,
2000, 25, 3220.
a The b values are defined using the X convention (ref. 15).
13 O. Bouevitch, A. Lewis, I. Pinevsky, J. P. Wuskell and L. Loew, Biophy.
J., 1993, 65, 672.
14 K. Clays, G. Olbrechts, T. Munters, A. Persoons, O.-K. Kim and L.-S.
Choi, Chem. Phys. Lett., 1998, 293, 337.
15 A. Willets, J. E. Rice, D. M. Burland and D. P. Shelton, J. Chem. Phys.,
1992, 97, 7590.
value as high as 2000 3 10248 esu for the 2[5] ion pair, which
is more than four times larger than the corresponding experi-
mental value (450 3 10248 esu) determined in similar
conditions for the benchmark dipolar chromophore Disperse
Red 1. Compound 3[1], which bears the stronger heterocyclic
acceptor, shows a mb(0) value about 50% larger than that of
compound 2[1], but at the expense of a much reduced
Communication a908717f
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Chem. Commun., 2000, 353–354