Synthesis, characterization and optical properties of π-conjugated systems incorporating closo-dodecaborate clusters: New potential candidates for two-photon absorption processes

Article abstract of DOI:10.1039/b504414f

Non-centrosymmetric π-conjugated systems incorporating closo-dodecaborate clusters, [NC-C₆H₄-C(H)=N(H)-B ₁₂H₁₁]^- (2), [NC-C₆H ₄-C(H)=C(H)-C₆H₄-C(H)=N(H)-B

Full text of DOI:10.1039/b504414f

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F U L L P A P E R
Synthesis, characterization and optical properties of p-conjugated
systems incorporating closo-dodecaborate clusters: new potential
candidates for two-photon absorption processes†
a
a
b
c
a
a
ˇ
R. Bernard, D. Cornu,* P. L. Baldeck, J. Ca´slavsky´, J.-M. Le´toffe´, J.-P. Scharff and
P. M i e l e a
a Laboratoire des Multimate´riaux et Interfaces, UMR CNRS 5615-Universite´ Claude
Bernard-Lyon 1, 43 Bd du 11 Novembre 1918, F-69622, Villeurbanne cedex, France.
E-mail: David.Cornu@univ-lyon1.fr
b Laboratoire de Spectrome´trie Physique, UMR CNRS 5588-Universite´ Joseph Fourier,
140 Avenue de la Physique-BP 87, F-38402, Saint Martin d’He`res, France
c Institute of Analytical Chemistry, Czech Academy of Sciences, Veverˇ´ı 97, 611 42,
Brno, Czech Republic
Received 30th March 2005, Accepted 30th June 2005
First published as an Advance Article on the web 2nd August 2005
Non-centrosymmetric p-conjugated systems incorporating closo-dodecaborate clusters,
−
−
=
=
=
[NC–C6H4–C(H) N(H)–B12H11] (2), [NC–C6H4–C(H) C(H)–C6H4–C(H) N(H)–B12H11] (3), and
−
=
=
=
[NC–C6H4–C(H) C(H)–C6H4–C(H) C(H)–C6H4–C(H) N(H)–B12H11] (4) have been synthesized by reaction of
the monoamino derivative of B12, [B12H11NH3]− (1), with various arylaldehydes, R–C6H4–CHO. These Schiff
base-like compounds were fully characterized by multinuclear NMR spectroscopy and mass spectrometry. In order
to evaluate these boron rich p-systems as potential materials for two-photon absorption (TPA) processes, UV linear
absorption curves were recorded for 3 and 4, and comparatively studied with those of the boron-free p-systems
=
=
=
NC–C6H4–C(H) N–CH3 (5) and NC–C6H4–C(H) C(H)–C6H4–C(H) N–CH3 (6). The donor effect of the boron
cluster was evidenced by a shift to the lower energy of the absorption band in the spectra of systems incorporating
B12. The two photon absorption (TPA) spectrum of compound 3, obtained by the up-conversion method, shows a
resonance at 720 nm with a cross-section rTPA of 35 × 10−50 cm4 s photon−1 molecule−1. This value suggests the
potential of B12 clusters to be used as new donor groups for the synthesis of non-linear materials.
preparation of “push–pull” or centrosymmetric systems con-
Introduction
taining three-coordinate boron atoms,16,18,19 relatively little work
was directed towards the incorporation of 12-vertex clusters
into p-systems. Within this field, the most interesting results are
large first hyperpolarizability (b) values calculated or measured
for some derivatives of 12-vertex analogues of closo-[B12H12]2−,
namely the 1-carba-closo-dodecaborate anion, [CB11H12]−,20 and
ortho-, meta- or para-carborane, C2B10H12.21–24 Besides second-
order NLO, the two-photon absorption (TPA) process is a
three-order NLO process in which materials simultaneously
absorb two photons. Materials exhibiting a large two-photon
cross section and excellent up-conversion properties can find
applications in various domains, like optical limiting,25,26 three-
dimensional optical data storage,27 and microfabrication.28 To
our knowledge, the TPA properties of closo-[B12H12]2− derivatives
have never been studied. In this context, two kinds of p-systems
can be envisaged: (i) centrosymmetric systems incorporating
one B12 cluster at each end of the p-transmitter, and (ii) non-
centrosymmetric systems incorporating only one B12 cluster
as the donor group. By considering the complexity of B12
chemistry, we first focused our attention on the synthesis and
characterization of non-centrosymmetric p-conjugated systems
incorporating closo-B12 as potential candidates for the TPA
process. The results concerning the investigation of some of their
optical properties will also be discussed.
The dodecahydro-closo-dodecaborate dianion, [B12H12]2−, pos-
sesses the unique structure of a regular icosahedron and unique
properties, such as high thermal stability, remarkable chemical
and hydrolytical stability, and low toxicity. Since the isolation
of closo-[B12H12]2− was first reported in 1960,1 several routes to
substituted closo-dodecaborate anions were envisaged via the
formation of boron–nitrogen,2,3 –oxygen,4,–6 –sulfur,7 –halogen,8
–phosphorus9 or –carbon10,11 bonds. Their intrinsic properties
make B12-derivatives suitable for various possible applications,
ranging from biomedical ones like boron neutron capture
therapy (BNCT), a method for the treatment of cancer based
upon the interaction of 10B atoms and thermal neutrons,12 to the
selective extraction of radionuclides from nuclear wastes arising
from the PUREX process.13 The chemistry of closo-[B12H12]2−,
together with the potential applications of its derivatives, was
reviewed by Sivaev et al. in 2002.14
Despite the unique structure of closo-[B12H12]2−, only little
attention has been devoted to the optical properties of its deriva-
tives. In 1999, Kaszynski et al. investigated the potentialities of
closo-[B12H12]2− as building blocks for liquid crystal materials.15
Due to the variety of possible photonic applications, ranging
from organic light-emitter diodes (OLEDs) for electrolumines-
cence applications, to data storage, and optical switching and
communications, another area of interest concerns the fabrica-
tion of second-order nonlinear optical (NLO) materials. In 2002,
Marder et al. reviewed NLO and electrooptical (EO) materials
containing boron.16,17 While many efforts were devoted to the
Results and discussion
Synthesis and characterization of non-centrosymmetric
p-conjugated systems
† Electronic supplementary information (ESI) available: Synthesis of
5; fluorescence emission spectrum of MSB, 3 and 6; normalized
fluorescence emission spectrum of MSB, 3 and 6. See http://dx.doi.org/
10.1039/b504414f
The synthesis of the p-conjugated systems has been conducted
via the formation of Schiff bases. For that purpose, we have
used the useful general pathway proposed by Sivaev et al.29
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 3 0 6 5 – 3 0 7 1
3 0 6 5
©

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Indeed, these authors have reported that the monoamino
derivative of the closo-dodecaborate cluster, [B12H11NH3]− (1),
can react with various arylaldehydes, R–C6H4–CHO, to give the
pared from triethyl phosphite and 4-(bromomethyl)benzonitrile,
following a procedure described by Kagan et al.35 In a second
step and in the presence of potassium hydroxide, the reaction
of terephthaldehyde with diethyl 4-cyanobenzylphosphonate
yielded 4-cyano-4ꢀ-formylstilbene (7) (Scheme 3).
=
corresponding B12-containing Schiff bases, [B12H11N(H) C(H)–
C6H4–R]−. Therefore, we have prepared a series of p-systems
consisting of electron-rich B12-clusters bonded to electron-
This intermediate was fully characterized by 1H and 13C
NMR analysis, FTIR and elemental analysis. According to
DSC analysis, compound 7 exhibits a high thermal stability
with decomposition starting at 382 ◦C. In a third step, the
reaction of the monoamino derivative of B12 with 7, and in the
acceptor CN groups through p-bridges of various lengths, i.e.
−
=
=
[NC–C6H4–C(H) N(H)–B12H11] (2), [NC–C6H4–CH CH–
−
=
=
C6H4–C(H) N(H)–B12H11] (3), [NC–C6H4–CH CH–C6H4–
−
=
=
CH CH–C6H4–C(H) N(H)–B12H11] (4) (Scheme 1). These
expected donor–p-acceptor (D–p-A) systems should be clas-
sified as type II chromophores.30,31 In order to evaluate the
contribution of B12-clusters to the optical properties of such p-
presence of a catalytic amount of sodium hydroxide, yielded
−
=
=
[NC–C6H4–CH CH–C6H4–C(H) N(H)–B12H11] (3) in 60%
yield (Scheme 4).
systems, we have also prepared boron-free analogues of (2) and
32
=
=
(3), i.e. NC–C6H4–CH N–CH3 (5) and H3CN CH–C6H4–
CH CH–C6H4–CN (6) (Scheme 2).
=
Scheme 4
Compound 3 was fully characterized by multinuclear NMR
analysis. The 11B{ H} NMR spectrum of 3 shows only two
1
Scheme 1
signals, which are a singlet at −3.58 ppm and an unresolved
signal at −14.64 ppm with a relative intensity of 1 : 11. The
singlet at −3.58 ppm is the only one to be unaffected by the
proton coupling during 11B NMR analysis. It was thus attributed
to the boron atom bearing the imino substituent, whereas the
unresolved signal at −14.64 ppm is the result of the overlap
of the signals featuring the boron atoms in equatorial (10B)
and apical (1B) positions relative to the pendant group. The
complete identification of the substituent was conducted by 1H
NMR analysis. Beside the broad and unresolved signals (2.0
to 0.2 ppm) characteristic of the hydrogen atoms linked to
the cluster, and to the signals (0.87, 1.28, 1.51 and 3.02 ppm)
featuring the tetrabutylammonium cation, two doublets were
observed at 8.75 and 10.43 ppm. Consistent with the 1H NMR
data of 2, these signals were attributed to hydrogen atoms
bonded to the carbon and nitrogen atom of the imino bond,
respectively. The three signals at 7.77, 7.79 and 7.96 ppm were
attributed to aromatic hydrogen atoms of the stilbene backbone.
Consistent with the assumed structure for 3, the two doublets at
7.42 and 7.49 ppm were attributed to the two hydrogen atoms of
the ethenyl bridge connecting the twophenylrings ofthe stilbene.
One should also note that the value of the coupling constant of
the ethenyl hydrogen atoms (16.4 Hz) revealed that the stilbene
exhibits an (E)-configuration. The structure of 3 deduced from
the NMR data was undoubtedly confirmed by FAB-MS analysis
with the observation of the molecular peak at m/z = 372.
Behind its potentially interesting optical properties, compound
Scheme 2
=
The first element of the series, [NC–C6H4–C(H) N(H)–
B12H11]− (2), was prepared in 60% yield from [B12H11NH3]−
and 4-cyanobenzaldehyde, according to the procedure initially
reported by Sivaev et al.29 According to DSC analyses, 2 exhibits
◦
a thermal stability up to 248 C. In the same way, NC–C6H4–
=
C(H) N–CH3 (5), an organic analogue of 2 consisting of a
methyl group in place of the B12-vertex, was synthesized from
4-cyanobenzaldehyde and aqueous methylamine.32
The preparation of the other elements of the series was driven
by the idea of increasing the length of their p-bridges, which
is known to improve the two-photon absorption properties of
the organic D–p-A systems.33,34 The synthesis of [NC–C6H4–
−
=
=
CH CH–C6H4–C(H) N(H)–B12H11] (3) was conducted in
three steps. First, diethyl 4-cyanobenzylphosphonate was pre-
Scheme 3
3 0 6 6
D a l t o n T r a n s . , 2 0 0 5 , 3 0 6 5 – 3 0 7 1

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3 may also present some biological or biomedical activity, since
it combines a B12 cluster, active for BNCT as mentioned in
the introduction, and an (E)-stilbene, whose natural analogues
(hydroxylated (E)-stilbenes) are being evaluated as antitumor
agents.36
the reaction of 8 with 4-cyanobenzaldehyde and then tereph-
thalaldehyde gave, in the presence of NaOCH3, (E,E)-4-(2-
(4-(2-(4-cyanophenyl)ethenyl)phenyl)ethenyl)benzaldehyde (9)
(Scheme 6). In a third step, the reaction of 9 with 1 yielded
4 (Scheme 7).
The DSC analysis of 3 reveals an unusual feature with
the observation of a glass transition temperature at 89 ◦C
followed by a thermal degradation starting at 260 ◦C. This
result underlines the thermoplastic behavior of 3, and one
direct consequence is the ability of the latter to be shaped after
softening, allowing the formation of complex shapes of 3-based
materials such as films, lenses, coatin◦gs, fibers, foams, etc. As
an illustration, when heated at ∼160 C, pure 3-powder melts
and has sufficient viscosity to allow the formation of a lens in
a glass mold (Fig. 1). This result is of particular scientific and
technologic importance since one limitation for the integration
of molecular p-systems into usable devices is the difficulty of
their large-scale and homogeneous incorporation into polymer
or inorganic matrices, forming efficient hybrid materials. This
difficulty should be bypassed by the direct processing of 3
forming pure optically-active materials in complex shapes.
By comparing 3 and 4, the main feature is the lower yield
obtained for the synthesis of the latter (8% in comparison to
60% in the case of 3). This could be related to the extremely
low solubility of 9, and therefore of 4, in organic solvents
which is assumed to be the limiting step for the formation of 4.
Moreover, this low solubility, together with the small amount of
4 obtained, precluded many analyses of this compound and only
high-resolution 13C and 1H NMR analysis have been performed.
1
The attribution of the different signals was allowed by H–1H,
1H–13C and 13C–13C COSY NMR analyses. All the NMR data
are consistent with the structure suggested for compound 4. The
four hydrogen atoms of the two ethenyl groups appeared as two
doublets with a coupling constant of 16.2 Hz, emphasizing the
(E,E)-configuration of the styrylstilbene derivative. The small
amount of 4 obtained and its low solubility precluded also any
optical characterization for this compound, and therefore there
was no need for an organic analogue of 4.
UV-VIS linear absorption spectra of non-centrosymmetric
p-systems 2, 3, 5, and 6
In order to evaluate the two-photon absorption (TPA) properties
of our compounds, we had to determine first the potential of
the B12-cluster as a donor group in such p-systems. For that
purpose, the first results concerning the optical properties of our
systems are deduced from their linear absorption curves. Only
little information is reported in the literature concerning the
UV-VIS absorption properties of closo-boranes. As we found,
the UV-VIS absorption spectrum of the parent monoamino
derivative, [B12H11NH3][NBu4], [1][NBu4], did not show any
absorption band in the range 220–500 nm. Fig. 2 shows the
UV-VIS absorption curves in molar extinction of compounds 2,
3, 5, and 6 in the range 200–500 nm.
Fig. 1 Fabrication of a lens by softening a powder of 3 at ∼160 ◦C.
The glass transition temperature of 3 should be related to the
existence of weak intermolecular interactions in the bulk. The
exact nature of these interactions is unknown and further work
is clearly needed in order to understand this phenomenon, and
+
=
notably the role played by the counter cation, N(n-Bu)4 .
The spectrum of NC–C6H4–C(H) N–CH3 (5) exhibits a first
As we found, the reaction of 7 with aqueous methylamine
intense absorption band centered at 256 nm (emax = 23650 L
=
=
yielded H3C–N CH–C6H4–CH CH–C6H4–CN (6), the or-
ganic analogue of 3 exhibiting a methyl group in place of the B12
cluster (Scheme 5). This compound was fully characterized by
FTIR, EA, and 1H and 13C NMR analysis.
mol−1 cm−1) and a second weak one at 290 nm (emax
=
2000 L mol−1 cm−1). This pattern is in good agreement with
those reported for disubstituted benzene derivatives.38 Basically,
the linear absorption spectrum of benzene shows two intense
=
Following a similar strategy, the synthesis of [NC–C6H4–CH
absorption bands at 180 nm (the second primary band, emax
=
CH–C6H4–CH CH–C6H4–C(H) N(H)–B12H11][N(n-Bu)4], [4]
[N(n-Bu)4], was conducted in three steps. First, p-xylylene-
bis(triphenylphosphonium chloride) (8) was synthesized from
triphenylphosphine and p-xylylene dichloride.37 In a second step,
47000 L mol−1 cm−1) and at 203 nm (the first primary band, emax
=
=
=
7400 L mol−1 cm−1), together with a weak third one at 254 nm
(secondary band, emax = 204 L mol−1 cm−1). These three bands are
related to the p electrons and are shifted to higher wavelengths
Scheme 5
Scheme 6
D a l t o n T r a n s . , 2 0 0 5 , 3 0 6 5 – 3 0 7 1
3 0 6 7

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Scheme 7
who reported similar patterns and the systematic observation of
the second primary absorption band since Dk > 51.5 nm for the
first primary one.38
Therefore, the comparative study of the spectrum recorded
with 2 and 5 clearlydemonstrates the donor effectofthe electron-
rich B12-cluster. According to Doub et al., the influence of
the donor/acceptor couple (H)C N(H)–B12H11−/CN on the
=
optical properties of the benzene ring is in the same range as
O−/C(O)CH3.38
The investigation of the solvatochromism for 2 was conducted
by recording linear absorption curves into a series of solvent:
CH2Cl2, CH3CN and CH3OH (Fig. 3).
Fig. 2 Linear absorption spectra for compounds 2, 3, 5, and 6
(acetonitrile solution).
(red shift) in the presence of substituents. The magnitude of these
red-shifts depends of the nature of the substituents and on their
position on the ring.38,39
In the case of 5, red-shifts, Dk, of 53 nm and 36 nm
have been measured for the first primary band and secondary
band, respectively. These values are in the same range as
those reported for the reference compound, 4-aminobenzonitrile
(ABN).40 Moreover, the relative intensity of the first primary and
secondary bands of 5, estimated by the measurement of their
oscillator strengths (0.47 and 0.08, respectively), is close to that
reported for ABN.
=
The spectrum corresponding to [NC–C6H4–C(H) N(H)–
Fig. 3 Solvatochromic behavior of 2 (CH2Cl2, kmax = 338 nm; CH3CN,
kmax = 328 nm; CH3OH, kmax = 318 nm).
B12H11]− (2), analogue of 5 but containing a B12-cluster in place of
the methyl group, shows two intense absorption bands at 328 nm
and 255 nm. The evolution of the pattern from 2 to 5 corresponds
to that obtained with disubstituted benzene derivatives when
increasing the strength of the donor/acceptor substituents.38
On this basis, we assume that the intense absorption band at
328 nm corresponds to the first primary absorption band. We
assume that this band overlapped the secondary absorption one.
As we found, the red-shift for the first primary absorption band
is Dk = 125 nm compared to benzene. Moreover, we attribute the
intense band at 255 nm to the second primary absorption. This
assumption is in good agreement with the works of Doub et al.,
Fig. 3 shows a blue shift of the first primary absorption
band when increasing the polarity of the solvent. This result
clearly indicates that 2 has a negative solvatochromic behavior.
This effect should be related to a decrease in the solute dipole
moment upon excitation i.e. between ground state and excited
state.30 Since similar effects are usually observed for zwitterionic
compounds,30,41 this result should be correlated with the ionic
nature of 2 with a partial compensation of the (2−) charge of
the cluster by the nitrogen atom of the iminium group. Moreover,
the band at 255 nm, which we attribute to the second primary
3 0 6 8
D a l t o n T r a n s . , 2 0 0 5 , 3 0 6 5 – 3 0 7 1

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absorption band, is unaffected by the nature of the solvent. This
result confirms our assumption concerning the attribution of
the bands, since the second primary absorption band is known
to remain unaffected when varying the solvent.
=
The stilbene-like compound NC–C6H4–C(H) C(H)–C6H4–
=
C(H) N–CH3 (6) contains the same donor and acceptor groups
as 5. Its electronic spectrum (Fig. 2) shows two absorption band
at 334 nm (emax = 18540 L mol−1 cm−1) and 236 nm (emax = 9080 L
mol−1 cm−1), consistently with the spectra reported for other
stilbene derivatives.42,43 For instance, the parent trans-stilbene,
=
C6H5–C(H) C(H)–C6H5, exhibits two bands at 237 and 311 nm
corresponding to p → p* electronic transition. The red-shift of
the band located at the lowest energy is due to the presence of
donor and attractor groups bonded to the trans-stilbene core in
para position.44
The comparative study of 5 and 6 shows a strong red-shift
of the absorption bands. This observation is consistent with the
increase of the length of the p-bridge going from 5 to 6, which is
known to reduce the difference between HOMO and LUMO via
(i) an increase in the number of electrons in the p-systems and
(ii) a shift in the p–p* electronic transition to the lower energy.
The spectrum corresponding to compound 3 shows two
absorption bands at 362 nm (emax = 50601 L mol−1 cm−1) and
235 nm (emax = 16930 L mol−1 cm−1). It appears therefore that
the band located at the lowest energy exhibits a red-shift (Dk =
30 nm) compared to 6. Consistent with the result described
above, this red-shift confirms the donor effect of the B12-cluster.
Moreover, a negative solvatochromic behavior was observed for
3 with a shift in the intense band going from 360 nm in methanol,
to 363 nm in acetonitrile, and to 370 nm in dichloromethane.
Fig. 4 Nanosecond and femtosecond TPA spectrum in the range
550–800 nm for 3 (acetonitrile solution, 5 × 10−3 M).
molecule−1 for (E)-stilbene).47 These results clearly point out the
role played by the B12 cluster on the TPA properties of such
p-systems.
Conclusion
We report on the synthesis of several non-centrosymmetric
p-conjugated systems incorporating B12 clusters. In particu-
=
lar, a stilbene-like derivative [NC–C6H4–C(H) C(H)–C6H4–
−
=
C(H) N(H)–B12H11] (3) was prepared. Its characterization by
DSC reveals a glass transition temperature, allowing the possible
melt-shaping of 3 into various forms, as a lens for instance.
The comparative study of the linear absorption of these boron-
based systems compared to boron-free analogues emphasizes
the potential of the electron-rich B12 cluster as a donor group.
The two photon absorption (TPA) of compound 3 was also
investigated, and a cross section rTPA of 35 × 10−50 cm4 s photon−1
molecule−1 was measured. Further work is in progress in order
to incorporate B12 clusters in more efficient organic backbones,
and then to improve the 3rd order NLO properties of our
compounds.
Two-photon absorption (TPA) properties of 2, 3 and 6
=
The fluorescence quantum yields of [NC–C6H4–C(H) N(H)–
B12H11]− (2), [NC–C6H4–C(H) C(H)–C6H4–C(H) N(H)–
=
=
−
=
=
B12H11] (3), and NC–C6H4–C(H) C(H)–C6H4–C(H) N–CH3
(6) were determined from the fluorescence emission spectra
using p-bis(o-methylstyryl)benzene (MSB) as a reference.
The spectra were recorded using an excitation wavelength
of 340 nm, with an optical density of 0.050. For compound
2, we did not detect any fluorescence. For compound 6, the
measured fluorescence quantum yield is as low as 0.7%, which
precluded any interest in further optical characterization for
this compound. The intensity of fluorescence for compound
3 is 0.9% compared to MSB. This value is close to organic
stilbene-like compounds containing strong donor and acceptor
groups. Indeed, the quantum yield of fluorescence for 4-cyano-
4ꢀ-methoxystilbene varies between 1% and 1.7%, depending on
the solvent.45 The TPA spectrum for compound 3 was obtained
by up-conversion fluorescence measurments using a Nd:YAG
pumped optical paramagnetic oscillator that produce 2.6 ns (full
width at half maximum) in the range 550–650 nm and using a
Ti:sapphire femtosecond laser in the range 700–800 nm (Fig. 4).
The excitation beam is collimated over the cell length (5 mm).
The fluorescence is collected at 90◦ of the excitation beam and
focused into an optical fiber connected to a spectrometer. The
TPA cross section is determined at 720 nm using MSB as a
reference standard, for which rTPA = 1.6 × 10−50 cm4 s photon−1
molecule−1.46 One should note that the TPA spectrum obtained
with the nanosecond laser beam (550–600 nm) is expressed in
arbitrary units without any calibration. Indeed, this spectrum
was just recorded to verify the decrease in rTPA for the lower
wavelength.
Experimental
General considerations
All reactions were carried out under an atmosphere of pure
argon using vacuum-line and Schlenk techniques with solvents
purified by standard methods.48 (Et3NH)2[B12H12] was purchased
from KATCHEM Ltd., Prague, and used without further
purification. 1H-NMR spectra were recorded at 300 MHz
or 500 MHz on a Bruker AM 300 or a Bruker DRX 500
spectrometer, respectively. 13C-NMR spectra were recorded at
75 MHz or 125 MHz on a Bruker AM 300 or a Bruker
DRX 500 spectrometer, respectively. 11B-NMR spectra were
recorded on a Bruker AM 300 spectrometer at 96.29 MHz with
Et2O·BF3 as external reference (positive values downfield). The
infrared spectra were recorded on a FTIR Nicollet Magna 550
spectrometer. High resolution mass spectra of ionic species were
measured on an Esquire 3000 Ion Trap System, and alternatively
on a Bruker Esquire-LC Ion Trap Instrument using electrospray
ionization. Negative ions were detected. Samples dissolved in
acetonitrile (1 ng lL−1) were introduced to the ion source by
infusion of 3 lL min−1; the drying temperature was 300 ◦C,
drying gas flow 5 L min−1, nebulizing gas pressure 10 psi. UV-VIS
linear absorption curves were recorded in acetonitrile solution
(10 mm cell) on a Varian CARY 1E spectrometer. Differential
scanning calorimetry (DSC) analyses were performed on a
TA8000 Mettler-Toledo apparatus.
The TPA spectrum of 3 shows a strong resonance at 720 nm
with a cross section rTPA = 35 × 10−50 cm4 s photon−1 molecule−1.
This resonance is about twice that of the one-photon spectrum
(362 nm), consistent with previous results on “push–pull” dyes.33
Moreover, this value is red-shifted compared to the one of the
parent molecule (E)-stilbene (514 nm), and, at this wavelength,
the corresponding rTPA is also larger (12 × 10−50 cm4 s photon−1
D a l t o n T r a n s . , 2 0 0 5 , 3 0 6 5 – 3 0 7 1
3 0 6 9

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=
Synthesis of [NC–C6H4–C(H) N(H)–B12H11][N(n-Bu)4],
[2][N(n-Bu)4]
The suspension was stirred at r.t. overnight. The title compound
was extracted from the methanol solution using diethyl ether.
After evaporation of the solvent, the crude product was washed
with 100 mL of water. Further recrystallization in diethyl ether
The monoamino derivative [B12H11NH3]− (1) was prepared from
[B12H12]2− as described by Hertler and Raash.2 The synthesis of
=
yielded 0.123 g (0.5 mmol, 50%) of pure H3CN CHC6H4–
−
=
[NC–C6H4–C(H) N(H)–B12H11] (2) was conducted from 1 by
=
CH CH–C6H4–CN (6) (Found: C, 83.2; H, 6.3; N, 10.5%. 6
following the procedure reported by Sivaev et al.29 The DSC
analysis of 2 shows a melting point at 241 ◦C followed by a
strong exothermic effect starting at 248 ◦C.
requires C, 82.9; H, 5.7; N, 11.4%); dH (CD3CN, 300 MHz) 3.41
3
=
(s, 3H, N–CH3), 7.10 (d, 1H, HC CH, JH–H = 16.4),7.20 (d,
3
=
1H, HC CH, JH–H = 16.4), 7.74 (s, 4H, C6H4), 7.83 (d, 2H,
=
C6H4), 7.96 (d, 2H, C6H4), 8.19 (s, 1H, N CH–); dC (CD3CN,
=
=
Synthesis of [NC–C6H4–CH CH–C6H4–C(H) N(H)–B12H11]-
75 MHz) 48.46 (1C, N–CH3), 127.36 (4C, C6H4), 128.14 (1C,
[N(n-Bu)4], [3][N(n-Bu)4]
=
=
HC CH), 128.65 (2C, C6H4), 131.92 (1C, HC CH), 132.92
(2C, C6H4), 136.48 (1C, C6H4), 138.75 (1C, C6H4), 141.95 (1C,
In a first step, diethyl p-cyanobenzylphosphonate was synthe-
sized according to a procedure previously reported by Kagan
et al.35 In a second step, 4-cyano-4ꢀ-formylstilbene was prepared
by the procedure described below, consistent with a patent
deposited by Reinehr.49 1.87g (33 mmol) of potassium hydroxide
was dissolved in 25 mL of methanol at 45 ◦C. The solution
was cooled to 0 ◦C, and then 4.09g (31 mmol) of tereph-
thalaldehyde were added. Then 6.9g (25 mmol) of diethyl p-
cyanobenzylphosphonate were added dropwise during 2 hours.
The mixture was heated at 40 ◦C during 2 hours, and then
allowed to cool to r.t. 10 mL of methanol were added and the
solution was kept under stirring during 12 hours. A pale yellow
precipitate was obtained and subsequently isolated by filtration.
The ensuing solid was washed with 250 mL of methanol,
yielding 4.89g (21.2 mmol, 85%) of 4-cyano-4ꢀ-formylstilbene
(7). (Found: C, 82.8; N, 5.9; O, 6.5; H, 4.6%. 7 requires C,
C6H4), 151.12 (1C, C6H4), 161,83 (1C, HC N); tmax/cm−1 2215
=
◦
=
(CN), 1640 (C N) (KBr); DSC analysis: mp 146 C.
=
=
Synthesis of [NC–C6H4–CH CH–C6H4–CH CH–C6H4–
C(H) N(H)–B12H11][N(n-Bu)4], [4][N(n-Bu)4]
=
In a first step, p-xylylene-bis(triphenylphosphonium chloride)
(8) was synthesized from triphenylphosphine and p-xylylene
dichloride, as described in the literature.37 In a second step,
2.124 g (3.0 mmol) of p-xylylene-bis(triphenylphosphonium
chloride) and 0.219 g (1.6 mmol) of 4-cyanobenzaldehyde were
dissolved into 30 mL of freshly distilled ethanol. A solution of
0.187 g (3.5 mmol) of NaOCH3 into 50 mL of ethanol was added
dropwise. After 12 hours of stirring at r.t., 0.432 g (3.2 mmol) of
terephthalaldehyde was added to the reaction mixture, and then
a solution of 0.221 g (4.1 mmol) of NaOCH3 in 50 mL of ethanol
was added dropwise. The color of the solution turned to pale
yellow and the mixture was kept under stirring at r.t. during 12 h.
The reaction mixture was concentrated under vacuum, yielding
a yellow precipitate which was filtered off. The precipitate was
washed with a mixture water/ethanol (1 : 1). Further recrystal-
lization in DMF yielded 0.201 g (0.6 mmol, 20%) of (E,E)-4-(2-
(4-(2-(4-cyanophenyl)ethenyl)phenyl)ethenyl)benzaldehyde (9).
82.4; N, 6.0; O, 6.9; H 4.7%); dH (CD3CN, 300 MHz) 7.43 (s,
2H, HC CH), 7.74 (s, 4H, C6H4), 7.83 (d, 2H, C6H4, JH–H
3
=
=
3
8.29), 7.96 (d, 2H, C6H4, JH–H = 8.29), 10.01 (s, 1H, CHO);
dC (CD3CN, 75 MHz) 111.79 (1C, C6H4), 119.16 (1C, C6H4),
=
127.73 (4C, C6H4), 129.84 (1C, HC CH), 130.47 (2C, C6H4),
=
131.22 (1C, HC CH), 132.65 (2C, C6H4), 136.48 (1C, C6H4),
142.06 (1C, C6H4), 191 (1C, C O); tmax/cm−1 2215 (CN), 1687
=
(CHO) (KBr). DSC analysis: mp 210 ◦C; strong exothermic
=
dH (CD3CN, 300 MHz) 7.18 (m, 4H, HC CH), 7.50–7.60 (m,
effect starting at 382 ◦C.
8H, C6H4), 7.63 (d, 2H, C6H4), 7.81 (d, 2H, C6H4), 10 (s, 1H,
-CHO).
In a third step, 0.442g (1.0 mmol) of [B12H11NH3][NBu4]
and 0.183g (0.8 mmol) of 7 were dissolved in 30 mL of
dichloromethane. A few drops of an aqueous solution of sodium
hydroxide (5 wt%) were then added to the mixture. The reaction
mixture was stirred overnight at r.t. The solvent was removed
in vacuum and further purification was performed by chro-
matography on silicagel (NORMASIC 40 lm-60 lm, Aldrich)
using a CH3CN/CH2Cl2 (4 : 1) solution and yielded, after
evaporation of the solvent in vacuum, 0.31g (0.5 mmol, 60%)
In a third step, 0.107 g (0.32 mmol) of 9 and 0.262 g
(6.54 mmol) of 1 were dissolved in 20 mL of DMF. The
pH of the solution was adjusted to 9–10 by adding a few
drops of aqueous sodium hydroxide (5 wt%). The color of
the solution turned to orange. The mixture was kept un-
der stirring at r.t. during 24 hours. Then DMF was evacu-
ated under vacuum. Further purification was performed by
chromatography on silicagel (NORMASIC 40 lm-60 lm,
Aldrich) using CH2Cl2 then a CH3CN/CH2Cl2 (1 : 4) solution
and yielded, after evaporation of the solvent in vacuum,
=
=
of pure [NC–C6H4–CH CH–C6H4–C(H) N(H)–B12H11][N(n-
Bu)4], [3][N(n-Bu)4]. dH (CD3CN, 300 MHz) 0.2–2.0 (unresolved,
B–H), 0.87 (t, 12H, N–(CH2)3CH3), 1.28 (m, 8H, N–(CH2)2–
=
18 mg (0.025 mmol, 8%) of pure [NC–C6H4–CH CH–C6H4–
CH2–CH3), 1.51 (m, 8H, N–CH2–CH2–CH2–CH3), 3.02 (t, 8H,
=
=
CH CH–C6H4–C(H) N(H)–B12H11][N(n-Bu)4], [4][N(n-Bu)4].
dH (DMSO, 500 MHz) 0.2–2.0 (u, B–H), 0.98 (t, 12H, N–
(CH2)33), 1.38 (m, 8H, N–(CH2)2–CH2–CH3), 1.65 (m, 8H, N–
3
=
N–CH2–(CH2)2–CH3), 7.42 (d, 1H, HC CH, JH–H = 16.39 Hz),
=
7.49 (d, 1H, HC CH), 7.77 (s, 4H, NC–C6H4), 7.79 (d, 2H,
3
C6H4, JH–H = 8.47 Hz), 7.96 (d, 2H, C6H4), 8.75 (d, 1H,
CH2–CH2–CH2–CH3), 3.12 (t, 8H, N–CH2–(CH2)2–CH3), 7.32
3
=
=
HC NH, JH–H = 19.78 Hz), 10.43 (d, 1H, HC NH); dC
(CD3CN, 75 MHz) 12.23 (4C, N–(CH2)3CH3), 18.76 (4C, N–
(CH2)2–CH2–CH3), 23.73 (4C, N–CH2–CH2–CH2–CH3), 58.79
(4C, N–CH2–(CH2)2–CH3), 111.59 (1C, C6H4), 119.16 (1C, CN),
3
=
=
(d, 2H, HC CH, JH–H = 16.2 Hz), 7.59 (d, 2H, HC CH),
3
7.65 (s, 4H, C6H4), 7.83 (d, 4H, C6H4, JH–H = 8.28 Hz),
3
=
7.94 (d, 4H, C6H4) 8.71 (d, 2H, HC NH, JH–H = 19.59 Hz),
=
10.34(d, 2H, HC NH); dC (DMSO, 125 MHz) 13.59 (4C,
=
128.03 (4C, C6H4), 129.12 (1C, C6H4), 130.80 (1C, HC CH),
131.53 (2C, C6H4 together with 1C, HC CH), 131.64 (2C, C6H4
together with 1C, HC CH), 133.18 (2C, C6H4), 141.59 (1C,
N–(CH2)3CH3), 20.09 (4C, N–(CH2)2–CH2–CH3), 24.06 (4C,
N–CH2–CH2–CH2–CH3), 59.02 (4C, N–CH2–(CH2)2–CH3),
110.50 (1C, C6H4), 119.16 (1C, CN), 123.01 (2C, C6H4), 126.92
=
=
=
C6H4), 143.99 (1C, C6H4)169.31 (1C, HC NH); dB{H} (CD3CN)
=
(2C, C6H4), 128.48 (4C, HC CH), 129.18 (4C, C6H4), 132.50
(2C, C6H4), 134.75 (2C, C6H4), 134.88 (1C, C6H4), 140.01
(2C, C6H4), 144.42 (1C, C6H4), 145.41 (1C, C6H4), 154.19 (1C,
−3.58 (s, 1B), −14.64 (u, 11B); tmax/cm−1 3286, 3248 (NH), 2487
−
−
=
(B–H), 2223 (CN), 1640 (C N) (KBr); m/z (FAB ) 372 (M );
DSC analysis: Tg = 89 ◦C, strong exothermic effect starting at
260 ◦C.
=
HC NH).
=
=
Synthesis of NC–C6H4–CH CH–C6H4–C(H) N–CH3 (6)
Acknowledgements
In a typical experiment, 0.220 g (1.0 mmol) of 7 were added to
6 mL of an aqueous solution of monomethylamine (40 wt%).
The authors are grateful to B. Fenet (NMR analysis center,
UCBL) for his significant assistance in obtaining the NMR data.
3 0 7 0
D a l t o n T r a n s . , 2 0 0 5 , 3 0 6 5 – 3 0 7 1

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21 J. Taylor, J. Caruso, A. Newlon, U. English, K. Ruhlandt-Senge and
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D a l t o n T r a n s . , 2 0 0 5 , 3 0 6 5 – 3 0 7 1
3 0 7 1