Structure - Second-order polarizability relationship in chromophores incorporating a spacer: A joint experimental and theoretical study

Article abstract of DOI:10.1002/(sici)1521-3765(19990104)5:1<369::aid-chem369>3.0.co;2-5

A joint theoretical and experimental study is reported on the structure- polarizability relationship in a novel type of push-pull conjugated system which contains a spacer within the conjugated backbone. The chromophores are based on a dithienyl conjugati

Full text of DOI:10.1002/(sici)1521-3765(19990104)5:1<369::aid-chem369>3.0.co;2-5

FULL PAPER
Structure ± Second-Order Polarizability Relationship in Chromophores
Incorporating a Spacer: A Joint Experimental and Theoretical Study
C. Maertens,[a] C. Detrembleur,[a] P. Dubois,[a, d] R. JeÂroÃme,*[a] C. Boutton,[b]
A. Persoons,*[b] T. Kogej,[c] and J. L. BreÂdas*[c]
Abstract: A joint theoretical and exper-
imental study is reported on the struc-
of the acceptor group and the length of
the conjugated path were varied. The
electronic properties of two model com-
pounds were studied theoretically by
means of correlated quantum-chemical
calculations. The degree of ground-state
polarization was varied in the calcula-
tions by the application of an external
electric field and varied experimentally
by the modification of the dielectric
constant of a binary mixture of solvents.
The linear and nonlinear optical proper-
ties of the compounds (measured in
solution by hyper-Rayleigh scattering)
are discussed in detail.
ture ± polarizability relationship in
a
novel type of push ± pull conjugated
system which contains a spacer within
the conjugated backbone. The chromo-
phores are based on a dithienyl conju-
gation pathway, spaced by a keto group,
and selectively end-capped by a donor
and an acceptor. Four chromophores
were synthesized, in which the strength
Keywords: donor± acceptor systems
´
ketones
´
nonlinear optics
calculations
´
´
semiempirical
solvent effects
by a p-conjugated segment.[1, 2] Although polyenes are the
most effective p-conjugated systems, their thermal and
photochemical stability is not high enough for practical
applications; on the other hand, the large aromaticity of the
benzene ring[2, 3] has a detrimental effect on b. Since the
ground-state aromaticity of thiophene is lower than that of
benzene and the solubility of thiophene derivatives is usually
higher than that of the parent benzene compounds, much
attention has recently been paid to chromophores that contain
thiophene[4±6] (this has led to the appearance of the first
commercial organic-based electro-optical device[7]).
Introduction
A key objective in the development of materials for electro-
optical applications is to find highly active chromophores with
large second-order polarizabilities b. The classical molecules
that have large nonlinear optical (NLO) activities usually
contain an electron donor and an electron acceptor connected
[d]
 Ã
[a] Prof. R. Jerome, C. Maertens, C. Detrembleur, Prof. P. Dubois
Center for Education and Research on Macromolecules (C.E.R.M)
Á
University of Liege
For many applications, compounds are required that
combine high nonlinear optical activities with a wide trans-
parency window.[8] One route that has been followed in order
to improve on the transparency ± efficiency trade-off was the
introduction of a silane or disilane group as a spacer in the
center of the conjugated bridge;[9, 10] the role of this spacer is
to modulate the conjugation between the donor and acceptor
termini. Such compounds were found to be transparent above
n ˆ 360 nm; however, their second-order polarizabilities are
rather low. Quantum-chemical calculations have indicated
that push ± pull polyenes containing a (di)methylene spacer
group that interrupts the conjugation along the molecule
could induce large hyperpolarizabilities;[11] these are, how-
ever, correlated to very low first optical transition energies.
More recent calculations suggest that the use of a carbonyl
(ketone) function as a spacer could produce a compromise:
high polarizabilities without an overly large decrease in the
bandgap energy.[12]
Á
Sart Tilman B6, B-4000 Liege (Belgium)
Fax : (‡ 32)4-3663497
E-mail: rjerome@ulg.ac.be
[b] Prof. A. Persoons, C. Boutton
Center for Research on Molecular Electronics and Photonics
Laboratory for Chemical and Biological Dynamics, University of
Leuven
Celestijnenlaan 200D, B-3001 Heverlee (Belgium)
Fax : (‡ 32)16-327982
E-mail: andre@lcbdiris.fys.kuleuven.ac.be
Â
[c] Prof. J. L. Bredas, T. Kogej
Â
Centre de Recherche en Electronique et Photonique Moleculaires
Service de Chimie des Materiaux Nouveaux, Universite de Mons-
Â
Â
Hainaut
Place du Parc 20, B-7000 Mons (Belgium)
Fax : (‡ 32)65-37-33-66
E-mail: jeanluc@averell.umh.ac.be
[d] New permanent address
Â
Á
Service des Materiaux Polymeres et Composites
Universite de Mons-Hainaut
Place du Parc 20, B-7000 Mons (Belgium)
Â
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369

 Ã
Â
R. Jerome, A. Persoons, J. L. Bredas et al.
FULL PAPER
Here, we report on a theoretical and experimental study of
the effect of the solvating medium on the linear and nonlinear
optical properties of push ± pull systems in which the p-
conjugated core (composed of two thiophene rings) is
modulated by a carbonyl spacer (Figure 1). This choice of
chemical structure potentially combines the advantages of:
i) the incorporation of the promising carbonyl spacer; ii) the
experimental study of the compounds shown in Figure 1b, that
is, 5-(5-piperidino-2-thienylcarbonyl)-2-thiophenecarbalde-
hyde (Pi-CO-CHO), ethyl (E)-2-cyano-3-[5-(5-piperidino-2-
thienylcarbonyl)-2-thienyl]-2-propenoate (Pi-CO-CN), 1,3-di-
methyl-5-[5-(5-piperidino-2-thienylcarbonyl)-2-thienylmeth-
ylene]hexahydro-2,4,6-pyrimidinetrione (Pi-CO-Ba), ethyl
(E)-2-cyanomethyl-3-{5-2-oxo-3-[(E)-1-(5-piperidino-2-thien-
ylmethylidene)cyclohexylide-
nemethyl]-2-thienyl}-2-prope-
noate (Pi-CHex-CN); a de-
tailed synthesis of these
carbonyl-spaced
chromo-
phores is given. In the case of
Pi-CO-CN, we describe the
experimental development of
the charge-transfer absorption
band in solution (solvato-
chromism was determined
with UV/Vis spectroscopy)
and the molecular hyperpolar-
izability (determined with hy-
per-Rayleigh scattering) as a
function of the polarity of the
solvent; the polarity was con-
trolled by mixing toluene and
DMSO, two solvents with al-
most identical refraction indi-
ces but very different dielectric
constants. We investigated the
effect of the acceptor strength
and p-conjugation pathlength
on the molecular hyperpolar-
izability for the carbonyl-
spaced chromophores in solu-
tion. Finally, a discussion of the
theoretical results is presented
in correlation with the exper-
imental data.
Figure 1. Molecular structures of the compounds studied theoretically.
well-known properties of the thiophene ring in terms of
thermal stability and decreased aromaticity, as discussed
above, and iii) their suitability as guests in polymer host
matrices for electro-optical applications. Note that the
solvatochromic[13] and second-harmonic generation (SHG)
properties of similar chromophores based on bis(benzylide-
ne)cycloalkanones[14a, b] have already been described; how-
ever, since the latter compounds present a nearly centrosym-
metrical structure, the SHG signal is weak. The nonlinear
properties of some thienylchalcone molecules have also been
investigated;[14c, d] for example, 1-(2-thienyl)-3-(4-methylphe-
nyl)propene-1-one displays a SHG signal 15 times as large as
that of urea.
This paper is organized as follows. Firstly, a theoretical
evaluation is presented of the influence of a reaction field on
the model chromophores 5-(5-N,N-dimethylamino-2-thienyl-
carbonyl)-2-nitrothiophene (DM-CO-NO2) and 5-(5-N,N-di-
methylamino-2-thienylcarbonyl)-2-thiophenecarbaldehyde
(DM-CO-CHO) (Figure 1a). The second part focuses on the
Results and Discussion
Theoretical study:
Methodology: The compounds DM-CO-NO2 and DM-CO-
CHO (Figure 1a) provide adequate and simple models for Pi-
CO-CN and Pi-CO-CHO, respectively (Figure 1b). The N,N-
dimethylamino and piperidino groups have comparable
electrodonating strengths, while the nitro and vinyl cyanoa-
cetate groups can be considered as having comparable
electroacceptor strengths.[2a]
The methodology used here is the same as that used in a
number of previous reports;[15, 11, 12] it allows us to compare the
results obtained here on compounds which contain a spacer
with those obtained earlier on fully conjugated systems. The
procedure consists of three steps:
1) Geometry optimizations are carried out with the help of
the semiempirical Hartree ± Fock INDO (intermediate
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Chromophores with Spacers
369±380
neglect of differential overlap) method.[16, 17] A homoge-
1
neous static electric field (F) in the range 107 ± 108 Vcm
is applied to tune the degree of ground-state polariza-
tion[18a] (we note that this range in F values is comparable
to the reaction fields associated with common dipolar
solvents; for example, the reaction field in chloroform is
1
0.9 Â 107 Vcm
and that in nitrobenzene is 4.3 Â
107 Vcm ).[18b] Note that the results we present were
obtained assuming a fully planar structure for all systems:
since the optimal torsion occurring around the ketone
bridge is calculated to be small (ꢀ188) and to be largely
independent of the applied electric field, it is more
convenient to use a planar conformation in order to
provide electronic properties, such as p-bond orders, more
easily.
1
Figure 2. Evolution, as a function of the applied electric field, of the
INDO-SDCI total formal charge in the ground state per moiety of the DM-
CO-NO2 compound (the donor/acceptor segments contain the adjacent
thiophene ring).
2) The electronic structure (state dipole moments, transition
energies, and transition dipole moments between states) is
evaluated with the INDO method coupled to a config-
uration interaction scheme that takes account of single and
double excitations[16] (single excitations between all p
orbitals; double excitations between the five highest
occupied p MOꢁs and the five lowest unoccupied p*
MOꢁs).
3) On the basis of the state characteristics, the static and
dynamic values of the first- and second-order polarizabil-
ities, a and b, are calculated within the sum-over-states
method (SOS) derived from the perturbation theory.[19]
The summations are performed over 50 states, which
provides convergent values for the molecular polarizabil-
ities of the compounds investigated here. For the dynamic
calculations, the damping factor associated with the
excited states is set at 0.2 eV.
It should be borne in mind that the application of a
homogenous external electric field (rather than the applica-
tion of the self-consistent reaction field technique[20]) to the
molecules allows us to modulate the evolution of the degree
of ground-state polarization in the molecule from a neutral
structure to a fully charge-separated zwitterionic structure.
We stress that only a part of this evolution is expected to be
observed experimentally, since the actual reaction fields in
solution are smaller than the largest external fields we
apply.[20, 21]
molecular segments (donor, spacer, acceptor) within DM-
CO-NO2. The external electric field is oriented in such a way
as to promote charge transfer from the donor segment to the
acceptor segment. We observe that the charge flow increases
continuously as a function of F along the whole molecule; the
three segments of the molecule actively participate in the
charge transfer for any value of the electric field. This
confirms that the carbonyl spacer does not strongly interrupt
the charge delocalization between the terminal donor and
acceptor segments.[12] However, two regimes can be distin-
guished in the charge transfer evolution depicted in Figure 2.
1
The first regime, from 0 to ꢀ6 Â 107 Vcm , is mainly
associated with a flow of charge from the donor segment to
the carbonyl spacer group; the spacer is acting as the main
acceptor group and only a small amount of charge is
transferred through the spacer towards the NO2 acceptor
1
segment. In the second regime (beyond 6 Â 107 Vcm ) the
acceptor role of the spacer saturates and a substantial charge
transfer occurs through the spacer from the donor to the NO2
acceptor segment.
In this semiconjugated system (considered to be planar for
all values of F), it is convenient to depict the change in
geometry induced by the charge redistribution by considering
the evolution of the p-bond orders (p-BO) with respect to F
(note that the value of p-BO ˆ 0 corresponds to a purely s-
bond, while p-BO ˆ 1 is related to a purely double p-bond). In
Table 1, we present the p-BOs in the DM-CO-NO2 molecule
Both theoretical and experimental polarizabilities are
calculated within a power series expansion of the dipole
moment.[18c] We recall that the average value of the first-order
1
polarizability a is defined as hai ˆ =3(axx ‡ ayy ‡ azz). In order
1
for values of F ranging from 0 to 108 Vcm . Charge transfer
to obtain a better comparison between the theoretical and
experimental values of the hyper-Rayleigh scattering (HRS),
we have calculated the second-order polarizability b with the
HRS expression in Equation (1),[22] where hbH2 RSi is defined by
Equation (2) with i, j, k ˆ x, y, z.
leads to a reduction in the p-BOs for the initially double
bonds and an increase for the single bonds. When comparing
the p-BOs of the two bonds connecting the donor and
acceptor groups to the conjugated pathway (bonds 1 and 11,
respectively, see labeling in Table 1), bond 1 (donor segment)
is affected twice as much as bond 11 (acceptor counterpart)
when F increases from 0 to 6 Â 107 Vcm 1; on the other hand,
s
35
bzzz
ˆ
hb2HRS
i
(1)
6
X
X
X
X
6
16
38
16
20
35
1
for F values beyond 6 Â 107 Vcm , the two bonds present the
hb2HRSiˆ
b2iii
‡
biii bijj‡
b2ijj
‡
biij bjkk
‡
bij2k (2)
35
105
105
105ijk; cycl:ˆj
i
iˆj
iˆj
same change in p-BO value (the same statement can be made
for the other corresponding bonds of the molecule). This
evolution of the p-BOs is thus fully consistent with the
analysis of the charge redistribution in the ground state which
Results: Figure 2 illustrates the evolution of the formal total
charge in the ground state as F increases for the individual
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R. Jerome, A. Persoons, J. L. Bredas et al.
FULL PAPER
Table 1. p-Bond orders in the ground state of the DM-CO-NO2 calculated with INDO-SDCI.
ˆ
Donor segment
C O
Acceptor segment
F
1
[107 Vcm
]
1
2
3
4
5
6
7
8
9
10
11
0
2
4
6
8
10
0.446
0.495
0.555
0.627
0.707
0.781
0.785
0.745
0.692
0.620
0.535
0.452
0.457
0.504
0.566
0.644
0.727
0.792
0.798
0.754
0.695
0.618
0.534
0.471
0.406
0.455
0.515
0.583
0.643
0.667
0.746
0.718
0.680
0.627
0.564
0.506
0.292
0.285
0.284
0.295
0.329
0.394
0.853
0.852
0.845
0.827
0.788
0.724
0.414
0.421
0.435
0.462
0.511
0.584
0.842
0.829
0.809
0.777
0.727
0.655
0.312
0.336
0.368
0.409
0.469
0.549
occur from the donor segment to the acceptor segment. A
schematic representation of the evolution of the molecule
upon application of the external electric field is presented in
Scheme 1 and Figure 3.
Scheme 1. Evolution of the geometric structure as a function of ground-
state polarization in keto-spaced chromophores: from the neutral form A
to zwitterion-like forms B and C.
Figure 3. INDO-SDCI/SOS calculated evolution of: a) the ground-state
dipole moment, b) the average first-order polarizability (hai), and c) the
second-order polarizability (bHRS) as a function of the applied electric field
!
*
in DM-CO-NO2 ( ) and DM-CO-CHO ( ).
The charge transfer in the ground state can also be
characterized by the evolution of the ground-state dipole
1
1
moment (Figure 3). Beyond a field of 6 Â 107 Vcm , a more
beyond 6 Â 107 Vcm , the evolution is more rapid since a
abrupt increase in the dipole moment occurs, in agreement
with a redistribution of the charge over the whole molecule
from the donor end to the acceptor end. The evolutions of the
first- and second-order polarizabilities with respect to F are
also given in Figure 3. The derivative relationships among the
polarizabilities are correctly reproduced by our calculations;
for instance, it can be seen that the a curve peaks and the bHRS
curve vanishes in DM-CO-NO2 at the same value of F
substantial amount of charge is transmitted through the
spacer. The peak value of bHRS, obtained at ꢀ6.5 Â 107 Vcm ,
1
is ꢀ1 Â 103 Â 10 30 esu; we note that this value is not as high as
the those previously calculated for a push ± pull polyene
containing a methylene or dimethylene spacer[11, 12] (ꢀ105 Â
10 30 esu); however, in these compounds the first optical
transition is of the order of a few tenths of an eV. In the case of
DM-CO-NO2, the peak value for b is related to a transition
energy at ꢀ1.25 eV.
1
(ꢀ8.5 Â 107 Vcm ). In agreement with the evolution behav-
ior of the ground-state dipole moment discussed above, a
smooth increase was found in the first-order polarizability for
The DM-CO-CHO molecule displays a lower hyperpolar-
izability b compared to DM-CO-NO2 (Figure 3). This illus-
trates the influence of the donor/acceptor strength on the
1
the weaker field values (ranging from 0 to ꢀ6 Â 107 Vcm );
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Chromophores with Spacers
369±380
Scheme 2. Reaction sequences used for the synthesis of Pi-CO-CHO (5a), Pi-CO-CN (6a), and Pi-CO-Ba (6b). Reagents and conditions: i) AlCl3, CH2Cl2;
ii) piperidine, 1208C, overnight; iii) a) LDA, THF, 408C, 30 min, b) DMF, rt, 90 min, c) HCl, H2O; iv) ethanol, CH2CN(CO2Et), piperidine, 608C;
v) ethanol, barbituric acid, triethylamine.
polarizabilities. Accordingly, the peak value of b is not
reached within the range of electric fields that are applied.
required a different reaction pathway (Scheme 3). 2-Thio-
phenecarboxaldehyde (7) was easily transformed to 5-dime-
thoxymethyl-2-thiophene carboxaldehyde (8) by protection of
the aldehyde group and further formylation with the nBuLi/
DMF sequence, as reported previously.[24] This aldehyde 8 was
then condensed onto cyclohexanone by the reaction described
by Tsukerman and al.[25] to yield 2-{(E)-1-[(5-dimethoxy-
methyl)-2-thienyl]methylidene}-1-cyclohexanone (9). 5-Pi-
peridino-2-thiophenecarboxaldehyde (11), which is readily
obtained by the reaction of 5-bromo-2-thiophenecarboxalde-
hyde (10) with piperidine in the presence of triethylamine,[26]
was then condensed with the cyclohexanone derivative 9 to
yield 5-{2-oxo-3-[(E)-1-(5-piperidino-2-thienyl)methylidene]-
cyclohexylidenemethyl}-2-thiophenecarboxaldehyde (12) af-
ter deprotection of the aldehyde group under acidic con-
ditions. A final Knoevenagel condensation of 12 with ethyl
cyanoacetate led to 13 (Pi-CHx-CN).
Experimental study:
Synthesis of carbonyl-spaced chromophores: In a previous
paper,[23] we reported on the electro-optical properties of the
keto-spaced ethyl (E)-2-cyano-3-[5-(5-piperidino-2-thienyl-
carbonyl)-2]-2-propenoate chromophore 6a (Pi-CO-CN).
This chromophore was obtained by the synthetic pathway
described in Scheme 2. The parent chromophores described in
the present study were obtained by a related procedure with
5-(5-piperidino-2-thienylcarbonyl)-2-thiophenecarbaldehyde
5a (Pi-CO-CHO) as the key intermediate. A Knoevenagel
condensation of 5a with ethyl cyanoacetate led to 6a (Pi-CO-
CN), while the condensation of the N,N-dimethyl barbituric
acid led to the parent chromophore 1,3-dimethyl-5-[5-(5-
piperidino-2-thienylcarbonyl)-2-thienylmethylene]hexahy-
dro-2,4,6-pyrimidinetrione 6b (Pi-CO-Ba) under the same
conditions. The synthesis of the elongated chromophore 13
(Pi-CHex-CN), based on a cyclohexanone central spacer,
Solvent effect on the linear and nonlinear optical properties of
Pi-CO-CN: In any solution, the solute molecules are located
Scheme 3. Reaction sequences used for the synthesis of Pi-CHx-CN (13). Reagents and conditions: i) LDA, THF, DMF; ii) cyclohexanone, NaOH, H2O;
iii) Br2, CHCl3; iv) piperidine, triethylamine, toluene; v) NaOH, H2O; vi) ethanol, CH2CN(CO2Et), piperidine, 608C.
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 Ã
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R. Jerome, A. Persoons, J. L. Bredas et al.
FULL PAPER
in the reaction field of the solvent. This field basically acts on
the solute in the same way as an external electric field, that is,
it is capable of modifying the ground-state polarization of the
solutes, thus causing a shift of the optical band and a change in
the transition moment.[21, 27] The accurate determination of the
mean reaction field is difficult since it depends on the nature
and arrangement of the surrounding solvent molecules. In the
elegant approximation of Onsager,[28] the reaction field is
directly related to the dielectric constant of the solvating
medium.
In the present study, we decided to use a mixture of two
solvents in different ratios in order to control the dielectric
constant of the solvating medium in a continuous manner.
Toluene and dimethylsulfoxide (DMSO) were selected for:
i) their very different dielectric constants (etoluene ˆ 2.38;
eDMSO ˆ 46.45), ii) their almost equal refraction indices
(ntoluene ˆ 1.4969; nDMSO ˆ 1.4793)Ðthis feature provides an
additional advantage: the use of Lorentz local field correction
factors can be avoided, iii) their high miscibility, and iv) the
high solubility of the chromophore in their mixtures. The large
difference in their dielectric constants allows a continuous
variation of e over a wide range. Figure 4 depicts the evolution
of the dielectric constant and the DMSO:toluene ratio.
experimentally by a UV/Vis analysis of Reichardtꢁs dye.[30]
A
strong preferential solvation of the betaine dye by the more
polar DMSO solvent can be deduced from the sharp effect of
DMSO on the ENT values, even at very low volume ratio. This
Â
phenomenon has already been described by Bosch and Roses
in binary solvent mixtures.[31] Figure 4 also represents the
fitted curve of the ENT -polarity scale by means of the model
[31]
Â
developed by Bosch and Roses, in which the preferential
solvation parameters of the benzene ± DMSO mixture instead
of the toluene ± DMSO mixture are used. The following
discussion and figures will refer mostly to the e-polarity scale,
which is linearly related to the DMSO content.
The evolution of the lowest transition energy versus the
dielectric constant, e, of the binary mixture is indicative of the
influence of the electric field on the charge transfer between
the piperidino donor and the vinyl cyanoacetate acceptor in
Pi-CO-CN. The solvatochromic behavior of Pi-CO-CN is
shown in Figure 5. A substantial red shift or positive
*
Figure 4. Evolution of the dielectric constant e ( , as measured by a
capacitance technique) and the ENT polarity scale ( ˆ experimental,
ˆ
~
&
theoretical[30]) versus the DMSO volume ratio in the mixture of toluene/
DMSO.
Figure 5. a) Evolution of the energy of the lowest optical band peak of Pi-
CO-CN versus the polarity of the solvent; b) absorption spectra of Pi-CO-
CN in cyclohexane, toluene, and DMSO.
The dielectric constant, determined experimentally by a
capacitance technique, was found to vary linearly with the
DMSO content, as has already been described by McRae in
the case of diethyl ether and acetonitrile mixtures.[29] The
electrostatic model for the description of the solvation
phenomena based on the dielectric constant considers the
solvent as a macroscopic nonstructured continuum. However,
specific solute/solvent interactions can take place on a
molecular level within a structured discontinuum which
consists of individual solvent molecules organized around
the dipolar solute. A useful solvent polarity scale, based on a
negative solvatochromic pyridinium N-phenolate betaine dye,
is able to account for solute ± solvent interactions[30] on a
microscopic level, in contrast to the physical parameter e. The
evolution of the corresponding ENT scale with DMSO content
is also depicted in Figure 4. The ENT values were determined
solvatochromism of 23 nm (0.14 eV) is observed as the solvent
polarity is increased from pure toluene to pure DMSO. This is
consistent with an increase in the dipole moment from the
ground to the excited state. This red shift is even more
pronounced when the solvent is changed from cyclohexane
(e ˆ 2.02) to DMSO, a difference of l ˆ 35 nm (0.22 eV)
between the lowest energy excitation values then measured.
The UV/Vis spectra of Pi-CO-CN in the three pure solvents
are displayed in Figure 5b. Two charge-transfer bands are
present in the visible region of the spectrum; the second band,
located around l ˆ 350 nm, is also influenced by the solvent
polarity since a red shift of 21 nm (0.21 eV) occurs on
changing from cyclohexane to DMSO.
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Chromophores with Spacers
369±380
The evolution of the (resonant) second-order polarizability
for Pi-CO-CN as a function of the dielectric constant was
measured by HRS at a fundamental wavelength of l ˆ
1064 nm. The b value of p-nitroaniline (PNA) in DMSO
(b ˆ 29  10 30 esu) was used as an external reference. All the
values were corrected for the difference in the refractive
indices by means of the local field factors. No multiphoton-
induced fluorescence could be observed. The results are
shown in Figure 6. After a small decrease in the bHRS value
electronic structure of model chromophores is to study the
evolution of the 1H NMR spectra with solvent polarity, as has
been investigated previously by several groups.[33±38] Laszlo
1
et al. focused on the effect of the solvent on the H chemical
shifts of several organic compounds,[33] while B. Nagy et al.
modeled specific solvation phenomena[34] by means of NMR
spectroscopy. More recently, 1H NMR studies on merocyanine
dyes have evidenced the proton shift induced by the
solvent;[35±38] from the J3 (1H NMR) coupling constants of
the olefinic protons, it was shown that simple merocyanine
dyes exhibit a polyene-like structure in nonpolar solvents and
a more zwitterionic structure in polar solvents.
In order to analyze the solvent effect on the electronic
structure of the carbonyl-spaced chromophores, 1H NMR
spectra of 1 wt% solutions of ethyl (E)-2-cyano-3[5-(5-
piperidino-2-thienylcarbonyl)-2]-2-propenoate Pi-CO-CN in
[D8]toluene/[D6]DMSO mixtures were studied. Table 2 lists
the 1H chemical shifts of this chromophore as a function of the
solvent polarity and the corresponding ENT values. The assign-
ment of the chemical shifts in the aromatic region was
checked by 1H ± 1H COSY experiments. It is also important to
note that the chemical shift of tetramethylsilane (TMS), used
as the internal reference for toluene/DMSO mixtures, is
hardly influenced by the solvent polarity, as described in the
Experimental Section. Although one cannot expect any
simple relationship between chemical shift and p-electron
densities in this type of complex molecules, a general trend
can be deduced. The very sharp increase in chemical shift of
the thiophene protons (A, B, C, D) and the methylene protons
E, F, G at a low [D6]DMSO content is indicative of a
preferential solvation of the Pi-CO-CN solute by the more
polar DMSO solvent molecules (see labeling in Table 2). A
rather strong acid ± base interaction between DMSO (or more
Figure 6. Evolution with solvent polarity of the experimental HRS b of Pi-
CO-CN (fundamental wavelength l ˆ 1064 nm).
between e ˆ 2.3 and 15, we observe a large peak with a
maximum value of 600 Â 10 30 esu for a dielectric constant of
40. The b value in pure DMSO (e ˆ 46) corresponds approx-
imately to the initial and minimum value in toluene. We did
not try to evaluate the static hyperpolarizability by means of
the two-state model,[32] since it is clear from Figure 5 that two
charge transfer transitions are present in the visible region of
the spectrum; the two-state model might thus break down. We
have observed the same absorption intensities at the resonant
wavelength of 532 nm (corresponding to a half of the incident
wavelength (1064 nm) for two toluene/DMSO ratios: 0/100
and 20/80; this suggests that the significant difference in the b
values between the two solutions (257 Æ 13 Â 10 30 esu and
611 Æ 31 Â 10 30 esu, respectively) is not simply due to a
resonance phenomenon corresponding to the absorption of
the second harmonic of the laser beam (provided that the
lifetimes in the excited states are similar).
‡
likely its charged mesomeric form, Me2S ± O ) and the
(partly) positively charged amino nitrogen in the delocalized
quinoid B and C forms of Scheme 1 can be expected to occur.
Actually, the linear relationship observed between the proton
shifts and the ENT -polarity scale (Figure 7) is consistent with
the presence of such specific solvation phenomena. The
occurrence of a solvent polarity-induced evolution from a
neutral aromatic structure to a zwitterionic quinoid structure
can be also deduced from an analysis of chemical shifts and J3
coupling constants. A net downfield solvent-induced chemical
shift of up to 0.56 ppm for the nonconjugated methylene F
Solvent effect on 1H NMR spectra of Pi-CO-CN: Another way
to investigate the solvent effect on the geometric and
Table 2. Chemical shifts and coupling constants for Pi-CO-CN in CDCl3, [D8]toluene, [D6]DMSO, and a 50/50 mixture of [D8]toluene/[D6]DMSO.
dA
dB
J(HA,HB) dC
[Hz]
dD
J(HC,HD) dE
[Hz]
dF
dG
dH
e
ETN
Solvent
CDCl3
[D8]toluene
[D8]toluene/[D6]DMSO (50/50)
[D6]DMSO
D(toluene/DMSO)
6.08
7.73
4.8
4.5
4.6
4.8
7.73
7.87
4.0
4.0
4.0
4.0
8.30
3.40
4.39
1.39
4.67
1.81
20.05
46.45
0.259
0.099
0.385
0.444
5.60
6.22
6.37
0.75
7.58
7.84
7.89
0.31
7.38
7.83
7.98
0.6
7.23
8.00
8.09
0.78
7.93
8.58
8.64
0.7
2.75
3.23
3.31
0.56
3.94
4.23
4.32
0.38
1.00
1.24
1.31
0.31
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375

 Ã
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R. Jerome, A. Persoons, J. L. Bredas et al.
FULL PAPER
to those of Pi-CO-CN. On the scale of acceptor strength, the
aldehyde group is the weakest of the three while the barbituric
group is known to be one of the strongest acceptor groups, the
vinyl cyanoacetate group in Pi-CO-CN is of intermediate
strength. Substitution of the vinyl cyanoacetate group by the
aldehyde group leads to a significant blue shift of the lowest
transition energy; moreover, only one transition is observable
in the visible part of the Pi-CO-CHO spectrum. Solvatochro-
mic analysis of the latter shows a red shift of ꢀ19 nm (0.13 eV)
on changing from toluene to DMSO and 30 nm (0.21 eV) for
the change from cyclohexane to DMSO (Figure 8a). On the
Figure 7. Evolution of the 1H chemical shifts in Pi-CO-CN with respect to:
a) the dielectric constant and b) the ENT polarity scale (see labeling in
Table 2).
protons attached to the N-piperidino donor group is observed
on changing from pure toluene to pure DMSO (see Table 2).
This can be attributed to the electron deficiency on these
protons due to the positive charge induced on the amino
nitrogen in the delocalized quinoid
B and C forms
Figure 8. Absorption spectra of: a) Pi-CO-CHO and b) Pi-CO-Ba in
cyclohexane, toluene, and DMSO.
(Scheme 1). This solvent effect is significantly larger than
that induced by the polar solvent on the nonconjugated
methylene G and H protons. The very low chemical shift
induced by the solvent on proton B, compared to adjacent
protons A, C, and D, is also indicative of a net increase in
electron density on this position in the delocalized quinoid B
and C forms. A slight increase in the J(HA,HB) coupling
constant in the donor segment of the chromophore
[J(HA,HB) ˆ 4.5 Hz (pure toluene) to 4.8 Hz (pure DMSO)]
can be related to an increase in p-conjugation between
carbons bearing these A and B protons in the quinoid form,
which is in agreement with the results of the calculations. This
solvent-induced variation in the J(HA,HB) coupling constant
is, however, lower than in polymethine dyes,[36] probably
because of the aromatic character of the thiophene ring. The
analysis of J(HC,HD) coupling constants in the acceptor
thiophene leads to the conclusion that the electronic structure
in the acceptor segment is less affected by the increase in
polarity.
other hand, increasing the strength of the acceptor by
substituting the vinyl cyanoacetate group by the barbituric
acid terminal group induces a rather complex solvatochromic
behavior, as illustrated in Figure 8b; the increase in the
solvent polarity from cyclohexane to toluene induces a red
shift while a higher solvent polarity leads to a blue shift of the
lowest transition and a red shift of the second transition; in
pure DMSO, the two transitions are no longer distinguishable.
Figure 9a represents the evolution of the second-order
polarizability b for the Pi-CO-CHO chromophore as a
function of the dielectric constant of the binary mixture
toluene/DMSO. No significant variation of the resonant
hyperpolarizability is observed when the polarity is progres-
sively increased from e ˆ 2.38 to e ˆ 46.45. The small fluctua-
tions observed are in the range of experimental error.
More interesting is Figure 9b, which describes the evolution
of b for the Pi-CO-Ba chromophore. A peak in the resonant
hyperpolarizability is observed for e ˆ 32, which corresponds
to a maximum value of 1000 Â 10 30 esu. Because of the
presence of two charge-transfer transitions, the static hyper-
polarizability has not been evaluated by the two-level
Influence of the electron-acceptor strength: The solvent effects
on the optical spectra and molecular (hyper)polarizabilities of
Pi-CO-CHO and Pi-CO-Ba have been studied and compared
376
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Chem. Eur. J. 1999, 5, No. 1

Chromophores with Spacers
369±380
Figure 10. a) Absorption spectra of Pi-CHex-CN in cyclohexane, toluene,
and DMSO; b) evolution of the experimental HRS b of Pi-CHex-CN with
solvent polarity (fundamental wavelength l ˆ 1064 nm).
Figure 9. Evolution of the experimental HRS b of: a) Pi-CO-CHO and
b) Pi-CO-Ba with solvent polarity (fundamental wavelength l ˆ 1064 nm).
model.[32] As expected, a comparison between the b curves for
chromophores Pi-CO-CHO (Figure 9a), Pi-CO-CN (Fig-
ure 6), and Pi-CO-Ba (Figure 9b) indicates that a higher
solvent polarity or dielectric constant is needed in order to
reach the maximum value of b for a weaker acceptor strength;
in the case of the weakest acceptor strength (Pi-CO-CHO), no
peak is actually reached (a higher polarity than that of DMSO
appears to be necessary, but still might not be sufficient in
order to observe a maximum).
Comparison of theory and experiment: General trends in the
relationship between theoretical and experimental results can
be deduced from the observations reported above. The
experimental evolution of the energy of the first optical
transition with respect to e for Pi-CO-CN (Figure 5a) has been
compared to the corresponding theoretical quantity, that is,
the evolution of the transition energy between the ground
state and the first excited state versus F for DM-CO-CHO. An
overall decrease of 3 eV is determined theoretically for DM-
1
CO-NO2 for static fields going from 0 to 108 Vcm . Exper-
imentally, changing the solvent from cyclohexane (e ˆ 2.02) to
toluene (e ˆ 2.38) induces a bathochromic shift of 0.1 eV,
which confirms a very rapid decrease of the first transition
energy at low solvent polarities. However, a very small
variation of 0.14 eV is observed when e changes from 2.38
(toluene) to 46 (DMSO). The reason for this slow evolution is
probably related to a preferential solvation phenomenon of
the dipolar chromophore by the more polar solvent that
occurs as the content of DMSO increases in the toluene/
DMSO binary mixture. This preferential solvation phenom-
enon induces an inhomogeneous electric reaction field around
the chromophore in solution and the theoretical and exper-
imental transitions cannot be compared in any simple way.
A large influence of the electric field or reaction field in
solution on the electronic cloud in the donor segment of the
chromophore has been deduced from both theory and
experiment. The neutral and zwitterionic forms described in
Scheme 1 participate in the actual structure of chromophores,
as shown by the theoretical analysis of the p-BOs on DM-CO-
NO2 and the 1H NMR data in the case of Pi-CO-CN. Note that
a similar participation of the neutral and zwitterionic struc-
tures as the solvent polarity was increased has been reported
Effect of the p-conjugated path length: The chromophore Pi-
CHex-CN (see Figure 1b) is composed of a piperidino donor
group and a vinyl cyanoacetate acceptor group connected
through a 2-(2-thienylidene)cyclohexanone semiconjugated
spacer; it is thus longer than Pi-CO-CN by two double bonds.
The UV/Vis spectra of Pi-CHex-CN in cyclohexane, toluene,
and DMSO are given in Figure 10a. As was the case for Pi-
CO-Ba, no clear absorption maximum can be determined and
several transitions are present in the visible part of the
spectrum. Although no significant red shift is observed on
changing the solvent from cyclohexane to toluene, a sharp
positive solvatochromism of ꢀ30 nm of the lowest transition
is measured on changing to the more polar DMSO. The
molecular hyperpolarizability values determined by HRS (at
l ˆ 1064 nm) are displayed in Figure 10b. The maximum of b
is located here around a dielectric constant e ˆ 26 correspond-
ing to a 50/50 (v/v) toluene/DMSO mixture. The position of
the peak is shifted to a lower polarity compared to Pi-CO-CN,
which is consistent with the fact that a longer conjugated path
reduces the electric field required to observe the maximum of
b.[26]
Chem. Eur. J. 1999, 5, No. 1
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377

 Ã
Â
R. Jerome, A. Persoons, J. L. Bredas et al.
FULL PAPER
by Kessler and Wolfbeis[13] in related keto-spaced chromo-
phores.
Experimental Section
An interesting parallel can be made in terms of the
molecular hyperpolarizabilities. The evolution of the calcu-
lated hyperpolarizability bHRS versus F for DM-CO-NO2 can
be compared qualitatively with the evolution of the resonant
experimental second-order polarizability (at l ˆ 1064 nm)
versus e. They both present a maximum, the former at an
electric field of 6.7 Â 107 Vcm 1 and the latter at e ꢀ 40. There
is also a qualitative agreement between the theoretical b curve
(Figure 3c) and the experimental b evolution (Figure 9a) for
the aldehyde-substituted chromophores DM-CO-CHO and
Pi-CO-CHO, respectively; no peak can be observed exper-
imentally in the range of polarities of the toluene/DMSO
mixtures and only a very small increase in b versus F is
obtained theoretically.
To summarize, despite qualitatively parallel trends between
the evolution of the experimental data as a function of e and
that of the theoretical results versus F, there are major
quantitative disagreements: i) the experimental evolution in
transition energy is much smaller; for instance, in the case of
Pi-CO-CN, as the value of e is changed from 2.02 to the e value
leading to the b peak, the transition energy decreases by
ꢀ0.25 eV. If we take into consideration Onsagerꢁs theory
(since the evolution in the reaction field from e ˆ 1 to 2 is half
that up to the largest e values), this would mean a global
evolution from the gas phase to e ꢀ 40 of the order of 0.5 eV;
the theoretical evolution is about 6 times as large. ii) The
measured peak b value is only about twice as large as the b
values measured at small e; the corresponding calculated
increase is over one order of magnitude. These discrepancies
can be ascribed to the occurrence of specific interactions due
to DMSO in solution, which have not been taken into account
in the calculations.
General: 2-Thiophenecarboxaldehyde (Acros), trimethyl orthoformiate
(Janssen), cyclohexanone (Baker), triethylamine (Janssen), piperidine
(Janssen), lithium diisopropylamide (LDA; Aldrich, 2m solution in
heptane, THF, ethylbenzene), N,N-dimethylbarbituric acid (Fluka), and
ethyl cyanoacetate (Fluka) were used as received. N,N-dimethylformamide
(DMF) was dried and distilled over phosphorus pentoxide prior to use.
Tetrahydrofuran (THF) was dried and distilled over sodium/benzophe-
none. Dimethylsulfoxide (DMSO; Merck) and toluene (JP Baker) for
spectroscopic analyses were of analytical grades and used as received.
[D8]Toluene (Aldrich) and [D6]DMSO (Aldrich) were used as received.
The 1H spectra were recorded on a Bruker AM 400 spectrometer; the
chemical shifts in the experimental section are given relative to tetrame-
thylsilane (TMS) and recorded in CDCl3. J values are given in Hertz. Since
some authors have reported a large solvent effect on the 1H and 13C
chemical shifts of TMS,[33] mainly in aromatic solvents where diamagnetic
anisotropy is important, the validity of TMS as the internal reference was
checked by the use of cyclohexane as a second internal reference in the
toluene/DMSO mixtures. The chemical shift of cyclohexane is known to be
independent of solvent polarity;[39] its chemical shift in various toluene/
DMSO mixtures was found to be constant (d ˆ 1.40 relative to TMS).
Therefore, all chemical shifts are expressed relative to TMS, d ˆ 0.0. The IR
spectra were recorded on a Perkin ± Elmer 1600FT spectrometer. UV/Vis
analyses were performed on a Hitachi U-3300 spectrometer with concen-
trations ꢀ5 Â 10 5 m. Hyper-Rayleigh scattering was performed at
a
fundamental wavelength of n ˆ 1064 nm (Nd-YAG laser). The setup and
conditions have been described elsewhere.[40]
Synthetic procedures: The synthesis of ethyl (E)-2-cyano-3-[5-(5-piperidi-
no-2-thienylcarbonyl)-2]-2-propenoate (6a, Pi-CO-CN) and the intermedi-
ate compounds have been described elsewhere.[23]
Synthesis of 1,3-dimethyl-5-[5-(5-piperidino-2-thienylcarbonyl)-2-thienyl-
methylene]hexahydro-2,4,6-pyrimidinetrione (6b, Pi-CO-Ba): In a single-
necked flask (50 mL), 5-(5-piperidino-2-thienylcarbonyl)-2-thiophenecar-
baldehyde[23] (0.5 g, 0.001.6 mol) was dissolved in dry ethanol (40 mL). N,N-
dimethyl barbituric acid (0.312 g, 0.002 mol) and 2 drops of piperidine were
then added to the solution. The mixture was heated at 808C for 0.5 h and
then allowed to cool to room temperature. The mixture was hydrolyzed
with HCl (0.1n, 5 mL) and the aqueous phase was extracted with CHCl3
(2 Â 50 mL). The organic phases were washed thoroughly with HCl (0.1n)
and water and then dried over MgSO4. The solvent was evaporated under
reduced pressure and the red solid purified by chromatography (silica gel;
n-hexane/ethyl acetate (1:1, v/v)) to give 6b. Yield: 0.4 g (56%); 1H NMR
(CDCl3): d ˆ 8.69 (s, 1H), 7.80 (d, J ˆ 4.0 Hz, 1H), 7.73 ± 7.71 (m, 2H,), 6.09
(d, J ˆ 4.4 Hz, 1H), 3.37 (s, 6H), 3.35 (t, 4H, J ), 1.75 ± 1.60 (m, 6H); IR
Conclusions
(KBr): 2935, 2853, 1688, 1570, 1474, 1440, 1379, 1322, 1238, 1124, 1087, 788,
1
753, 725 cm
.
The synthesis of novel carbonyl-spaced NLO chromophores
has been carried out and the linear and nonlinear optical
properties in solution have been determined. Semiempirical
INDO/SDCI calculations of two related model compounds
have been correlated with the experimental results. Qualita-
tively, a parallel is found between the dependence of the HRS
second-order polarizability with respect to solvent polarity
and the theoretical evolution of b as a function of the ground-
state polarization, which was tuned by the application of a
static homogeneous electric field. The quantitative discrep-
ancies between the experimental data and the calculated
transition energies and b values are attributed to a prefer-
ential solvation phenomenon that induces an inhomogeneous
and very different reaction field around the dissolved
chromophore. This work illustrates that a joint theoretical
and experimental investigation can lead to a better perception
of, on one hand, the influence of environmental factors on the
polarizabilities and, on the other hand, the role of a carbonyl
spacer in tuning the charge transfer along a push ± pull-
conjugated chromophore.
Synthesis of 2-{(E)-1-[(5-dimethoxymethyl)-2-thienyl]methylidene}-1-cy-
clohexanone (9): In 250 mL flask, 5-dimethoxymethyl-2-thiophene
a
carboxaldehyde (8, 1.29 g, 0.0069 mol) was dissolved in cyclohexanone
(5 mL) and water (100 mL). A solution of sodium hydroxide (0.27 g,
0.0069 mol) in water (5 mL) was added dropwise and the reaction mixture
was stirred overnight. The crude mixture was then extracted with diethyl
ether (2 Â 50 mL) and the organic phases washed successively with sodium
bicarbonate and water and then dried over MgSO4. The solvent was
evaporated under reduced pressure, and the yellow oil obtained was
purified by silica gel chromatography (n-hexane/ethyl acetate (1:1, v/v)) to
give 9. Yield: 1.56 g (85%); 1H NMR (CDCl3): d ˆ 7.77 (d, J ˆ 2.0 Hz, 1H),
7.25 (d, J ˆ 3,6 Hz, 1H), 7.09 (d, J ˆ 3,6 Hz, 1H), 5.66 (s, 1H), 3.35 (s, 6H),
2.83 (m, 2H) 2.51 (t, 2H, J ˆ 6.0), 1.88 (m, 4H); IR (neat): 2936, 2828, 1710,
1
1673, 1578, 1348, 1317, 1250, 1193, 1142, 1084, 1063, 974, 818, 789 cm
.
Synthesis of 5-piperidino-2-thiophenecarboxaldehyde (11): In a 250 mL
flask equipped with a condenser and under a nitrogen atmosphere,
2-bromo-5-thiophene carboxaldehyde (10, 8.4 g, 0.051 mol), triethylamine
(3.7 mL, 0.051 mol), and piperidine (4.36 g, 0.051 mol) were dissolved in
dry toluene (100 mL) and refluxed at 1208C over a period of 3 days. The
reaction mixture was then hydrolyzed with HCl (0.1n, 5 mL) and washed
vigorously with HCl (0.1n), NaHCO3, and H2O. The organic phase was
dried over MgSO4, the solvent removed under reduced pressure, and the
brown solid obtained was purified by silica gel chromatography (n-hexane/
378
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Chem. Eur. J. 1999, 5, No. 1

Chromophores with Spacers
369±380
ethyl acetate (8:2, v/v)) to give 11. Yield 4.28 g (50%); 1H NMR (CDCl3):
d ˆ 9.50 (s, 1H), 7.47 (d, J ˆ 4.5 Hz, 1H), 6.06 (d, J ˆ 4.5 Hz, 1H), 3.34 (t,
[4] a) V. P. Rao, A. K.-Y. Jen, Y. Cai, Chem. Commun. 1996, 1237;
b) K. Y. Wong, A. K.-Y. Jen, V. P. Rao, K. Drost, R. M. Mininni, Proc.
SPIE 1992, 1775, 74 and references therein.
[5] a) S. Gilmour, S. R. Marder, J. W. Perry, L.-T. Cheng, Adv. Mater.
1994, 6, 494; b) A. K.-Y. Jen, Y. Cai, P. V. Bedworth, S. R. Marder,
Adv. Mater. 1997, 9, 132.
[6] M. Blenkle, P. Boldt, C. Bräuchle, W. Grahn, I. Ledoux, H. Nerenz, S.
Stadler, J. Wichern, J. Zyss, J. Chem. Soc. Perkin Trans. 2 1996, 1377.
[7] Chem. Eng. News 1996, 4 March, 22.
J ˆ 4.4 Hz, 4H), 1.69 (m, 6H); IR (KBr): 2974, 2882, 1672, 1596, 1440, 1415,
1
1333, 1054, 963, 792 cm
.
Synthesis of 5-{2-oxo-3-[(E)-1-(5-piperidino-2-thienyl)methylidene]cyclo-
hexylidenemethyl}-2-thiophenecarboxaldehyde (12): In 50 mL flask
a
equipped with a condenser, 5-piperidino-2-thiophenecarboxaldehyde (11,
0.5 g, 0.0026 mol) and 2-{(E)-1-[(5-dimethoxymethyl)-2-thienyl]methyli-
dene}-1-cyclohexanone (9, 0.7 g, 0.0026 mol) were dissolved in ethanol
(20 mL). A solution of sodium hydroxide (0.104 g, 0.0026 mol) in water
(5 mL), was slowly added to the reaction mixture, which was then heated at
1008C over a period of 4 days. Subsequently, the reaction mixture was
hydrolyzed with HCl (0.1n, 5 mL) and extracted twice with CHCl3. The
organic phases were washed with HCl (0.1n) and NaHCO3 and dried over
MgSO4, and the solvent was removed under reduced pressure. The crude
product was purified by silica gel chromatography (n-hexane/ethyl acetate
[8] J. Zyss, in Conjugated Polymeric Materials: Opportunities in Elec-
Â
tronics, Optoelectronics, and Molecular Electronics (Eds.: J. L. Bredas,
R. R. Chance), NATO ASI Ser., Ser. E: Applied Sciences, Vol. 182,
1990, 451.
[9] a) G. Mignani, M. Barzoukas, J. Zyss, G. Soula, F. Balegroune, D.
Grandjean, D. Josse, Organometallics 1991, 10, 3660; b) D. Hissink,
P. F. Van Hutten, G. Hadziioannou, F. Van Bolhuis, J. Organomet.
Chem. 1993, 454, 25.
1
(1:1, v/v)) to give 12. Yield: 0.25 g (28%); H NMR (CDCl3): d ˆ 9.83 (s,
 Ã
[10] J.-X. Zhang, P. Dubois, R. Jerome, J. Chem. Soc. Perkin Trans. 2 1997,
1H), 7.93 (m, 1H), 7.89 (m, 1H), 7.74 (d, J ˆ 4.0 Hz, 1H), 7.36 (d, J ˆ 4.0 Hz,
1H), 7.19 (d, J ˆ 4.3 Hz, 1H), 6.10 (d, J ˆ 4.3 Hz, 1H), 3.30 (t, J ˆ 5.0 Hz,
4H), 2.93 (t, J ˆ 4.0 Hz, 2H), 2.86 (t, J ˆ 4.0 Hz, 2H), 1.95 (m, 2H), 1.73 to
1209.
Â
[11] T. Kogej, F. Meyers, S. R. Marder, R. Silbey, J. L. Bredas, Synth. Met.
1997, 85, 1141.
1.69 (m, 6H); IR (neat): 2937, 2854, 1622, 1537, 1497, 1445, 1381, 1247, 1065,
1
Â
[12] T. Kogej, R. Silbey, S. R. Marder, J. L. Bredas, unpublished re-
890, 858, 763, 754 cm
.
sults.
Synthesis of ethyl (E)-2-2-cyanomethyl-3-{5-2 oxo-3-[(E)-1-(5-piperidino-
2-thienylmethylidene)cyclohexylidenemethyl]-2-thienyl}-2-propenoate
(13, Pi-CHex-CN): Following the previously described procedure for the
synthesis of 6a, 5-{2-oxo-3-[(E)-1-(5-piperidino-2-thienyl)methylidene]cy-
clohexylidenemethyl}-2-thiophenecarboxaldehyde (12, 0.09 g, 0.00027 mol),
and ethyl cyanoacetate (0.03 mL, 0.00027 mol) were dissolved in toluene
(10 mL). After the addition of 2 drops of piperidine, the reaction mixture
was heated overnight at 608C. The reaction mixture was then hydrolyzed
with HCl (0.1 n, 2 mL) and extracted twice with CHCl3. The organic phases
were washed with HCl (0.1n) and NaHCO3, dried over MgSO4 and the
solvent was removed under reduced pressure. The crude product was
purified by silica gel chromatography (n-hexane/ethyl acetate (1:1, v/v)) to
give 13. Yield: 0.05 g (43%); 1H NMR (CDCl3): d ˆ 8.29 (s, 1H), 7.93 (s,
1H), 7.75 (s, 1H), 7.76 (d, J ˆ 4.0 Hz, 1H), 7.36 (d, J ˆ 4.0 Hz, 1H), 7.20 (d,
J ˆ 4.6 Hz, 1H), 6.11 (d, J ˆ 4.3 Hz, 1H), 4.39 (q, J ˆ 7,2 Hz, 2H), 3.32 (t,
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