Propeller-shaped molecules with giant off-resonance optical nonlinearities

Article abstract of DOI:10.1039/b101425k

Propeller-shaped molecules based on a triphenylbenzene crux bearing three oligomeric phenylenevinylene branches have been designed; very large first-order hyperpolaris-abilities (up to ∥β∥ = 800 10^-30 esu) were obtained while maintaining wide t

Full text of DOI:10.1039/b101425k

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Propeller-shaped molecules with giant off-resonance optical
nonlinearities
Jérémie Brunel,a Isabelle Ledoux,b Joseph Zyssb and Mireille Blanchard-Desce*c
a Département de Chimie, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
b Laboratoire de Photonique Quantique et Moléculaire, ENS Cachan, 61 avenue du Président Wilson,
94235 Cachan, France
c
Synthèse et Electrosynthèse organiques (CNRS, UMR 6510), Université de Rennes 1, Campus de
Beaulieu, Bât. 10A, 35042 Rennes cedex, France. E-mail: Mireille.Blanchard-Desce@univ-rennes1.fr
Received (in Cambridge, UK) 13th February 2001, Accepted 17th April 2001
First published as an Advance Article on the web 2nd May 2001
Propeller-shaped molecules based on a triphenylbenzene
crux bearing three oligomeric phenylenevinylene branches
have been designed; very large first-order hyperpolaris-
abilities (up to ∑b∑ = 800 10230 esu) were obtained while
maintaining wide transparency in the visible region by
taking advantage and boosting of intramolecular charge
transfer between the centre and the periphery.
(Table 1). As expected, a red shift of the absorption band is
observed with increasing electron-withdrawing or electron-
releasing strength of the end groups. Interestingly, a bath-
ochromic shift of the absorption band is obtained with
increasing solvent polarity (Fig. 1a) and a noticeably more
pronounced positive solvatochromism is observed in emission
(Fig. 1b), indicative of a polar excited state. Such characteristic
behaviour hints to significant charge redistribution taking place
upon excitation, consistent with a multidimensional intra-
molecular charge transfer (MDICT) occurring between the core
and the peripheral groups. The large Stokes shift values (4050
Molecular nonlinear optics (NLO) has attracted increasing
interest over the past twenty years,1 owing to numerous
applications in various fields such as telecommunications,
optical data storage and information processing.2 A substantial
effort has been devoted to the design of molecules with
enhanced NLO responses. Molecular engineering of conjugated
one-dimensional (1D) compounds has been particularly suc-
cessful, leading to push–pull compounds displaying giant off-
resonance first-order (b)3 and/or second-order (g)4 hyper-
polarisabilities. Yet it became increasingly clear over recent
years that multidimensional and multipolar structures offer
challenging possibilities.5 Lehn, Zyss and coworkers have
opened a pioneering route towards octupolar molecules.6 Such
2D or 3D structures take benefit from the tensorial nature of b,
and potentially lead to very large b7 and g8 values but most often
at the expense of significantly reduced transparency in the
visible region.9 In order to achieve an improved nonlinearity–
transparency trade-off, we have prepared nanoscale propeller-
shaped molecules whose design is based on the grafting of three
extended blades bearing an electron-withdrawing or an elec-
tron-releasing tip on a triphenylbenzene crux (Scheme 1).
Oligomeric phenylenevinylene rods were selected to ensure
effective electronic conjugation and preserve transparency. The
triphenylbenzene core maintains a large distance between the
conjugated branches, thus preventing sterical hindrances.
Based on this strategy, we have prepared homologous
molecules with radius varying between 1.4 and 2.5 nm. The first
generation (n
= 1) was prepared from 1,3,5-tris(4-for-
mylphenyl)benzene (C010) via either a triple Wittig or Wittig–
Horner–Emmons reaction whereas the second generation (n =
2) was obtained from C1 via a triple Wittig–Horner–Emmons
condensation with extended conjugated aldehydes X–Ph–
CHNCH–Ph–CHO. The key reagent C1 was synthesized from
C0 in a three-step procedure (Scheme 1). The conjugated
aldehydes were primarily prepared in nearly quantitative yield
by reacting terephthalaldehyde mono(diethyl acetal) with one
equivalent of phosphonium salt X–Ph–CH2–P(Ph)3I (X =
N(nC6H13)2,4b or X = OnC8H17) or of diethyl 4-(methylsulfo-
nyl)benzyl phosphonate (X = SO2CH3) followed by acid-
catalysed hydrolysis. Molecules 1–6 were characterised by
NMR spectroscopy, elemental analysis and/or high-resolution
mass spectra, in agreement with the assigned structures.
Scheme 1 Reagents and conditions: i: KBH4 (1 equiv.), EtOH, 4 h, rt; ii:
HBr conc. (10 equiv.), 5 h, reflux; iii: P(OEt)3 (5 equiv.), 24 h, reflux; iv:
compounds 1 and 2: X–Ph–CH2–P(Ph)3I (3.3 equiv.), KOtBu (3.5 equiv.),
CH2Cl2, 4 h, rt; compounds 3 and 4: X–Ph–CH2–P(O)(OEt)2 (3.3 equiv.),
NaH (3.5 equiv.), THF, 4 h, rt; v: compounds 5 and 6: X–Ph–CHNCH–Ph–
CHO (3.3 equiv.), KOtBu (3.5 equiv.), CH2Cl2, 5 h, rt.
All molecules show good transparency in a wide range of the
visible region and an intense absorption in the near UV–blue
visible region whose position is dependent on both the nature of
the peripheral groups and the length of the transmitter system
DOI: 10.1039/b101425k
Chem. Commun., 2001, 923–924
This journal is © The Royal Society of Chemistry 2001
923

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Table 1 Linear and nonlinear optical properties of nanoscale propeller-
region of the chromophores and to avoid contamination by two-
photon fluorescence.12 Comparison of homologous molecules
bearing electron-releasing (1,2) or electron-withdrawing (3,4)
end groups clearly shows that increasing the donating or
accepting strength of the three terminal substituents results in a
significant enhancement of b (Table 1), providing evidence that
MDICT strongly influences the nonlinear responses. Following
this observation, increasing the distance between the core and
the periphery appeared as a rational way to boost the nonlinear
responses. This strategy proved particularly successful: mole-
cule 5 was found to exhibit a ∑b(0)∑ value about six times larger
than molecule 2, whereas molecule 6 exhibits a first-order
hyperpolarisability one order of magnitude larger than its
shorter analogue 3, while increasing by no more than 36% in
weight and maintaining suitable transparency in the visible
region. This is particularly advantageous in terms of efficiency-
transparency trade-off: molecule 6 exhibits a ∑b(0)∑ value about
forty times larger than the prototypical push–pull compound p-
nitroaniline (pNA), with a molecular weight no more than eight
times larger.
Finally, by grafting either electron-donating or electron-
withdrawing groups on the edges of conjugated blades branched
on a triphenylbenzene crux, propeller-shaped molecules ex-
hibiting high NLO properties and wide transparency in the
visible range have been designed. Optimization leads to
compounds presenting an improved transparency–nonlinearity
trade-off (∑b(0)∑ = 500 10230 esu, lmax = 377 nm) as
compared to tris-donor tris-acceptor octupolar 2D molecules. In
addition, the superlinear dependence of b on size and their
particular concave shape makes elongated analogues attractive
candidates for incorporation in polymeric materials.
shaped molecules derived from triphenylbenzene
a
lmax(abs)a
/nm
bHLS
/
∑b∑c/
∑b(0)∑d/
Dn˜b/cm21 lHLS/nm 10230 esu 10230 esu 10230 esu
1
344
387
342
385e
407
377
170
790
510
1060
—
1064
1064
1064
1340
1340
1340
1064
5
29
30
16
94
97
227
486
810
32
8
50
51
140
278
510
12
2
3
4e
5
70
150
250
10
6
1830
—
pNA 348
a In chloroform. b Absorption solvatochromic shift
=
1/lmax(toluene)
bi2jk
21/l max(DMSO). c The modulus ∑b∑ =
Â
is derived from bHLS
ijk
21
according to ∑b∑ =
bHLS for molecules with C3h symmetry, and to ∑b∑
2
35
= bzzz
=
bHLS for 1D dipolar chromophores such as p-nitroaniline
6
(pNA). d Static values are calculated using a two-level dispersion factor.6b
e In DMSO.
cm21 for molecule 2 in chloroform, and 8500 cm21 for
molecule 4 in DMSO) corroborate that important nuclear
reorganisation is taking place after excitation, prior to emission
as a result of significant electronic redistribution.
The first hyperpolarisabilities b have been determined by
performing harmonic light scattering (HLS) experiments which
yield the HLS molecular averaged hyperpolarisability A(b2) =
bHLS.
11 HLS experiments were performed at 1.064 or 1.34 mm
in order to locate the second harmonic signal in the transparency
Notes and references
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Fig. 1 Solvatochromic absorption (a) and emission (b) behaviour of
molecule 2.
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Chem. Commun., 2001, 923–924