Synthesis, characterization, and optical properties of 4 H-pyran-4-ylidene donor-based chromophores: The relevance of the location of a thiophene ring in the spacer

Article abstract of DOI:10.1021/jo300373m

A series of new 4H-pyran-4-ylidene donor-based chromophores with a thiophene ring in the spacer has been synthesized. The linear and nonlinear optical (NLO) properties of these compounds have been determined and compared with the results of computational calculations. The position of the thiophene ring proved essential to optimize the figure of merit μβ, with the best results obtained when the heterocyclic system was closer to the donor moiety.

Full text of DOI:10.1021/jo300373m

Article
pubs.acs.org/joc
Synthesis, Characterization, and Optical Properties of 4H-Pyran-4-
ylidene Donor-Based Chromophores: The Relevance of the Location
of a Thiophene Ring in the Spacer
Natalia Martínez de Baroja,† Javier Garín,† Jesus Orduna,† Raquel Andreu,† María Jesus Blesa,†
́
́
Belen Villacampa,‡ Raquel Alicante,‡ and Santiago Franco*,†
́
†Departamento de Química Organ
́
ica and Departamento de Física de la Materia Condensada, ICMA, Universidad de
‡
Zaragoza-CSIC, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: A series of new 4H-pyran-4-ylidene donor-based
chromophores with a thiophene ring in the spacer has been
synthesized. The linear and nonlinear optical (NLO) properties
of these compounds have been determined and compared with
the results of computational calculations. The position of the
thiophene ring proved essential to optimize the figure of merit
μβ, with the best results obtained when the heterocyclic system
was closer to the donor moiety.
the spacer can determine the second-order NLO properties of
the molecule because the heterocycle can act as an auxiliary
donor or acceptor group.32 For instance, significantly enhanced
NLO responses have been obtained with a pyrrole attached to
the donor33−35 and a thiazole ring in the acceptor side.36−38
The thiophene ring is certainly one of the most widely
employed heteroaromatic spacers in push−pull systems, and as
a π-excessive heterocycle,39 it should behave as an auxiliary
donor when it is closer to the donor.34,40,41 However, several
authors have found greater nonlinearities when thiophene is
next to the acceptor group, and therefore, the general rule is not
always followed.36,42−45
In this paper, we describe the synthesis and the second-order
NLO properties of 12 efficient new chromophores. All of the
molecules include a 4H-pyran-4-ylidene as the donor moiety, a
thiophene ring as part of the conjugated π-system, and four
common strong organic acceptors. Despite the fact that the
thiophene ring is a very common heteroaromatic unit in push−
pull systems, to our knowledge, there are only two papers in
which the synthesis of a 4H-pyran-4-ylidene derivative linked to
a thiophene ring46,47 is described. Although one of these papers
concerns a study of the third-order nonlinearities,47 the second
order NLO properties of such molecules remain unexplored. In
order to investigate the role of the thiophene position within
the spacer, we prepared two isomers for each acceptor,
changing the location of the heterocycle toward the donor
and the acceptor units (series b and c). The second-order NLO
properties of the chromophores were evaluated and compared
to the results of theoretical calculations.
INTRODUCTION
■
In recent years, a large number of push−pull organic molecules
have been synthesized and used for the preparation of materials
with electronic and optical applications.1,2 Generally, the main
effort has been focused on the design of new chromophores
with large first hyperpolarizability values (β). While enormous
progress has been made in this field, interest in organic electro-
optic materials has reemerged, and it is still desirable to develop
highly efficient nonlinear optical (NLO) chromophores.3
A
typical strategy used in the design of new NLO chromophores
involves the careful optimization of the ground-state polar-
ization by choosing the appropriate pair of donor and acceptor
moieties linked through a π-conjugated spacer.4−8 In this sense,
the proaromatic 4H-pyran-4-ylidene unit has been scarcely
employed as a donor in NLO and constitutes an interesting
option to prepare new push−pull systems.9−12 The donor
character of this moiety is comparable to and even higher than
those of other common donors used in the field of NLO,
including triarylamines,13−16 N,N-dialkylaryl/heteroaryl-
amines,17−21 pyrid-4-ylidenes,22,23 and 1,3-dithiol-2-yli-
denes,24−27 as demonstrated by the increased ground-state
polarization shown by push−pull compounds bearing a
pyranylidene moiety.11,28
Polyenes are often used as π-conjugated spacers, and they
lead to large nonlinear responses. However, these systems have
the drawback of lower chemical and thermal stability, an
important aspect to consider in the manufacture of a
device.29−31 A typical way to optimize the stability is to include
in the spacer an aromatic and, even better, a heteroaromatic
ring with low resonance energy. The aromaticity of the ring
lowers the HOMO energy, thus reducing the ease with which
the material is oxidized or chemically modified. On the other
hand, the nature of the heterocyclic system and its position in
Received: February 22, 2012
Published: April 23, 2012
© 2012 American Chemical Society
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Scheme 1
Scheme 2
These precursor aldehydes were obtained according to the
synthetic strategy depicted in Scheme 1. Thus, the bromo
derivatives 7a and 7b were prepared by a Horner reaction of
pyranylphosphine oxide 548 with commercially available 5-
RESULTS AND DISCUSSION
■
Synthesis. The twelve new chromophores [1−4(a−c)]
described in this paper were prepared from aldehydes 8(a−c).
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bromo-2-thiophenecarbaldehyde 6a and with the bromoalde-
hyde 6b,49 respectively. Compound 7a is unstable on exposure
to light and chlorinated solvents, and it was necessary to use it
immediately in the next step. Derivatives 7a and 7b were then
treated with butyllithium and anhydrous N,N-dimethylforma-
mide to obtain, after aqueous acidic workup, compounds 8a
and 8b, respectively. Moreover, aldehyde 8a can be prepared in
one step and with a better yield by an alternative synthetic
route, coupling pyranylphosphine oxide 5 with 2,5-thiophene-
dicarbaldehyde. In this case, only one 4H-pyran-4-ylidene ring
was incorporated, probably because of the lower reactivity of
the formyl group in aldehyde 8a after introduction of the donor
group. Aldehyde 8c was obtained in moderate yield from
bromothiophene 7a followed by lithiation and reaction with
N,N-dimethylaminoacrolein.
Figure 1. Representative canonical forms for compounds (1−4)a.
Furthermore, if we compare the largest isomers [1−4(b,c)]
(Figure 2), in which the thiophene ring is closer to the donor,
compounds (1−4)c, the pyranylidene ring protons (Ha, Hb)
are shifted downfield in most cases, suggesting that the 4H-
pyran-4-ylidene ring has a higher aromatic character (greater
contribution of canonical form ZW1c) than in the correspond-
ing compounds (1−4)b. It can therefore be concluded that the
pyranylidene ring increases its donor character when the
thiophene is closer, with the latter acting as an auxiliary donor.
On the other hand, the arrangement in which the thiophene
ring is closer to the acceptor unit [compounds (1−4)b] is
better suited for transferring part of its own electron density (as
in canonical form ZW2b, Figure 2) to the acceptor, as
Finally, chromophores [1−4(a−c)] were obtained by
Knoevenagel condensation of aldehydes 8(a−c) and the
corresponding acceptors50−52 (1−4) in yields ranging from
11 to 89% (Scheme 2). Typically, it was necessary to purify the
final products by flash chromatography (see the Experimental
Section).
1H NMR Studies. The study of the chemical shifts and
coupling constants of some representative protons in the NMR
spectra can afford valuable information about both the
geometry and the distribution of the electronic density along
1
evidenced by H NMR spectroscopy because, in all cases, the
hydrogens Hc and Hd are more deshielded in compounds (1−
4)b than in (1−4)c.
3
the molecule. Concerning compounds 1−4, JHH coupling
Calculated Geometries and Charges. The optimization
of the molecular geometries of the studied compounds by DFT
methods (B3P86/6-31G* model chemistry) resulted, in every
case, in a planar arrangement of the pyranylidene-thiophene-
acceptor π system, with the two phenyl groups linked to the
4H-pyran-4-ylidene moiety twisted ca. 20−22° with respect to
the molecular plane. The phenyl and dicyanovinyl groups, in
the acceptor moiety of compounds 2 and 9, arrange also out of
the molecular plane because of steric hindrance in agreement
with previously reported X-ray studies on related derivatives.53
The Bird Index, despite its limitations, is a simple way to
estimate the aromaticity of a molecule.54,55 This index can be
obtained from experimentally determined or calculated bond
lengths and it has proven to be of general applicability in the
case of heterocycles. It has been established that a lower value
signifies that the molecule is less aromatic. The Bird Index
(I6)55 values for the 4H-pyran-4-ylidene moiety of all
synthesized compounds are shown in Table 2, and as can be
seen, these are in the range of 41 to 42, which is consistent with
previously reported results11 and confirms a significant
contribution of the zwitterionic limiting forms (values of ca.
50 are described for pyrylium cations). It can also be seen from
the results in Table 2 that the I6 index (and thus the
aromaticity) decreases on increasing the length of the polyenic
chain. In all cases reported here, the 4H-pyran-4-ylidene ring in
compounds (1−4)c is more aromatic (and therefore more
strongly donating) than in compounds (1−4)b, thus
confirming the auxiliary donor effect of thiophene in the
former compounds. The aromaticity of the thiophene ring can
also be estimated from its Bird Index (I5) (66 for thiophene
itself).54 In this case, the differences are more important when
comparing the longest isomers [1−4(b,c)], with higher values
of I5 obtained for compounds (1−4)c, in which the thiophene
ring is closer to the 4H-pyran-4-ylidene unit. These results are
constant analysis of the spacer shows that in all cases the CH
3
CH bonds have an (E)-configuration (15.4 Hz > JHH > 14.3
Hz), while for the CH−CH bonds, an s-trans conformation is
proposed (a nuclear Overhauser effect was not observed
between the CHCH protons).
The chemical shifts of some representative hydrogen atoms
(Ha, Hb, Hc and Hd) of the NLO chromophores are shown in
Table 1. In particular, analysis of the protons in positions 3 and
1
Table 1. H NMR Chemical Shifts of Some Representative
a
Hydrogen Atoms
compd
δ Ha (ppm)
δ Hb (ppm)
δ Hc (ppm)
δ Hd (ppm)
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
6.74
6.49
6.57
6.67
6.51
6.59
6.62
6.54
6.52
6.71
6.60
6.60
7.70
6.99
7.23
7.16
7.11
6.98
7.00
7.00
6.96
7.01
7.01
6.95
7.05
7.07
6.99
7.72
7.41
6.92
7.23
7.48
7.31
7.43
7.39
7.22
7.52
7.48
7.32
6.95
7.19
7.32
7.01
7.23
a
In some cases, it was not possible to specify a chemical shift because
of the overlap of the signals with those of other protons.
5 of the 4H-pyran-4-ylidene ring may serve to evaluate the
donor ability and the ground state polarization of the donor
fragment. For the compounds reported here, we observed a
decrease in the chemical shifts of the hydrogens Ha and Hb
(Figure 1) on lengthening the spacer, a finding that suggests an
increased contribution of the zwitterionic canonical form ZWa
for the shorter compounds, in agreement with previously
described D−π−A systems with a 4H-pyran-4-ylidene moi-
ety.11,12
1
in agreement with the H NMR data described above.
The polarization degree of a molecule can also be estimated
by the bond-length alternation (BLA).7 In this paper, BLA is
calculated as the difference between the average carbon−carbon
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Figure 2. Representative canonical forms for compounds (1−4)b and (1−4)c.
a
aromatic) in the ground state than compounds (1−4)b, a
Table 2. Calculated Values of Bird Index, Mulliken Charge,
1
finding consistent with the H NMR data (see above).
and BLA for Compounds [1−4(a−c)]
Electrochemistry. The redox properties of the push−pull
systems were studied by cyclic voltammetry (CV) in CH2Cl2
(Table 3). All compounds show two irreversible waves,
corresponding to the oxidation of the 4H-pyran-4-ylidene
fragment and to the reduction of the acceptor moieties,
respectively.
b
c
d
e
compd
I6
I5
q(D) [e]
BLA
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
42.0
41.1
41.4
42.2
41.4
41.8
42.2
41.2
41.6
42.7
42.0
42.2
56.8
58.6
62.0
57.8
57.8
61.9
58.2
58.4
61.8
58.4
58.5
62.0
+0.317
+0.279
+0.293
+0.325
+0.288
+0.307
+0.316
+0.285
+0.302
+0.336
+0.300
+0.318
+0.012
+0.022
+0.024
+0.013
+0.022
+0.020
+0.022
+0.028
+0.027
+0.017
+0.024
+0.023
a
Table 3. Electrochemical Data and EHOMO and ELUMO
b
Values
compd
Eox (V)
Ered (V)
EHOMO (eV)
ELUMO (eV)
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
9
+0.67
+0.55
+0.65
+0.82
+0.62
+0.72
+0.67
+0.50
+0.57
+0.82
+0.63
+0.71
+0.64
+0.51
−0.89
−0.87
−0.78
−0.76
−0.71
−0.63
−0.69
−0.64
−-0.59
−0.59
−0.54
−0.49
−0.71
−0.69
−5.80
−5.61
−5.66
−5.93
−5.74
−5.79
−5.91
−5.75
−5.80
−5.98
−5.81
−5.86
−6.06
−6.03
−3.43
−3.48
−3.53
−3.71
−3.74
−3.79
−3.85
−3.87
−3.89
−3.99
−4.00
−4.02
−3.82
−3.92
a
b
B3P86/6-31G* level. Bird Index for the 4H-pyran-4-ylidene moiety.
Bird Index for the thiophene ring. Mulliken charge in the
pyranylidene moiety. Along the bonds shown in Figure 3.
c
d
e
single and double bond lengths in the conjugated bridge,
including the thiophene ring (see Figure 3). BLA values are
likely to be lower than expected because the formally double
CC bonds of the thiophene ring are slightly longer than the
formally single C−C bond.
10
a
10−3 M in CH2Cl2 versus Ag/AgCl (3 M KCl), glassy carbon working
electrode, Pt counter electrode, 20 °C, 0.1 M NBu4PF6, 100 mV s−1
b
scan rate. Ferrocene internal reference E1/2 = +0.43 V. Calculated at
the B3P86/6-31G* level in the gas phase.
Figure 3. Bond lengths involved in calculated BLA values.
It must be remarked that, as expected, the introduction of a
thiophene ring in the spacer renders the oxidation more
difficult and, to a lesser extent, the reduction processes. For
Mulliken charge analysis estimates partial atomic charges in a
molecule from calculations carried out by computational
chemistry, and because of its simplicity, it is a very useful
method to estimate the polarization of a molecule.56 The
Mulliken charges q(D) supported by the 4H-pyran-4-ylidene
moiety are shown in Table 2, and as expected, q(D) decreases
on lengthening the chain of the spacer,11 probably because of a
weaker donor−acceptor interaction. Moreover, Mulliken charge
analysis shows a good correlation between I6 and q(D), even
when comparing chromophores b and c, indicating that
derivatives (1−4)c are more zwitterionic (and therefore more
instance, when comparing the redox properties of 2a (Eox =
+0.82 V; Ered = −0.76 V) and 3a (Eox = +0.67 V; Ered = −0.69
V) with those of the previously reported 9 (Eox = +0.64 V; Ered
= −0.71 V) and 10 (Eox = +0.51 V; Ered = −0.69 V) (Figure
4),11 we observe an increase in the electrochemical bandgap for
the former compounds. This behavior can be explained by the
loss of aromaticity of the thiophene ring in the intramolecular
charge transfer (ICT), which is a disadvantageous process from
an energetic point of view.
Otherwise, the lengthening of the polyenic chain of the NLO
chromophores facilitates the oxidation and reduction processes
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Figure 4. Structures of compounds 9 and 10.
and gives rise to a decrease in the Eox and Ered values, a change
that indicates a weaker interaction between the donor and
acceptor groups (Table 3). The observed trends are also
confirmed by computational calculations, which show that the
EHOMO (ELUMO) values increase (decrease) with the length of
the spacer. Concerning the electrochemical bandgap, major
differences were not found between isomers b and c, except for
a slight increase for the latter systems. The Eox data in Table 3
also allow a qualitative comparison of the electron-withdrawing
ability of the corresponding acceptors. Thus, on comparing
chromophores with the same spacer length, the results obtained
show that the strongest acceptor is 4 (CF3−TCF), while
thiobarbiturate 1 is the weakest.
order to confirm whether this solvatochromic behavior is
general, we also determined absorption maxima (λmax) in more
polar solvents like acetone, DMF, and DMSO. As shown in
Figure 5, the transition energy plot as a function of solvent
polarity (ETN) based on an empirical relationship59 declines
with increasing solvent polarity, thus confirming the afore-
mentioned positive solvatochromism.60,61 This observation
indicates an increase in the dipole moment upon excitation
(μg < μe) (where μg and μe are the ground- and excited-state
dipole moments, respectively) and can be interpreted in terms
of the predominance of the neutral form in the ground-state
structure, a situation in accordance with theoretical calculations
(DFT). Likewise, as can be seen in the graph, for compounds
with the strong acceptor CF3−TFC 4(a−c), the slope is higher,
and therefore these compounds show a marked positive
solvatochromism relative to the other molecules, a result that
could indicate a higher second-order NLO response (μβ). The
λmax TD-DFT calculated data (in gas phase, CH2Cl2, and
DMF) are shown in Table 4, and it can be seen that the
theoretical data in CH2Cl2 fit much better to the experimental
data. In any case, TD-DFT calculations predict the positive
solvatochromism observed in this series of compounds.
Linear Optical Properties. The UV−vis absorption
maxima and the extinction coefficients (log ε) of the studied
chromophores in 1,4-dioxane and CH2Cl2 are summarized in
Table 4. All compounds show the typical strong and broad
Table 4. UV−Vis Data
a
a
b
bc
,
bc
,
λmax (log ε)
λmax (log ε) λmax (gas
λmax
λmax
Compd
1,4-dioxane
CH2Cl2
phase)
(CH2Cl2) (DMF)
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
611 (4.76)
643 (4.76)
657 (4.62)
647 (4.66)
672 (4.66)
677 (4.34)
665 (4.55)
697 (4.48)
673 (4.65)
724 (4.80)
770 (4.69)
751 (4.60)
644 (4.64)
668 (4.59)
688 (4.70)
676 (4.70)
715 (4.61)
731 (4.44)
708 (4.70)
739 (4.74)
733 (4.71)
825 (4.96)
845 (4.74)
533
584
583
563
621
610
594
654
636
607
669
650
636
730
721
687
791
768
665
836
809
756
863
832
655
760
748
710
825
797
761
873
842
782
900
865
NLO Properties. The second-order NLO properties of
derivatives 1−4 were measured by electric field-induced second
harmonic generation (EFISH) in dichloromethane at 1907 nm,
and the static μβ0 values were calculated by using the two-level
model62 (for the sake of comparison, disperse red 1 gives a μβ0
value of ca. 480 × 10−48 esu under the same experimental
conditions). Both the experimental and theoretical results
(CPHF, coupled perturbed Hartree−Fock) are collected in
Table 5. As mentioned above, in some cases the UV−vis
spectra show vibronic bands and/or relevant absorption at
harmonic wavelength [especially for compounds 4(a−c)]; thus,
the estimation of their μβ0 values should be treated with
caution. For the sake of comparison, the experimental μβ values
of the previously reported derivatives 9 and 1011 are shown in
Table 5, and these data allow some interesting conclusions to
be drawn.
In all cases positive μβ values were obtained and the second-
order NLO response increased on lengthening the spacer, with
the best results obtained with the strongest acceptors 3 and 4.
Comparison of the μβ values of derivatives 2a and 3a with
those of 9 and 10, respectively, reveals that the incorporation of
a thiophene ring gives rise to a lower NLO response.57,63,64
However, compounds 9 and 10 have the drawback of
considerably lower thermal and photochemical stability (see
below) and a narrower transparency range than 2a and 3a,
respectively, which make them less suitable for practical
purposes. Moreover, with the exception of derivatives 1b and
1c (which display the same response within the limits of the
experimental error) a general behavior is observed for the
molecules in this study: the second-order NLO response is, in
all cases, higher for compounds in series c. Since for a given
acceptor the length of conjugation in both cases is the same, it
is reasonable to consider that the location of thiophene is the
key factor. On considering the results, it can be stated that for
827 (4.80)
903 (sh)
9
719 (4.92)
787
10
753 (4.92)
841 (4.94)
b
a
All λmax data are in nm. TD-DFT calculations at the B3P86/6-31G*
level. PCM solvation model
c
band in the visible region of the spectrum, corresponding to an
ICT, and in some cases, the UV−vis spectra show vibronic
bands (2a, 3a, 4a−c). In all series, for a given acceptor, a
bathochromic shift is observed on lengthening the spacer, and
on increasing the acceptor strength, significant red-shifts in
absorption occur in the order 4 ≫ 3 > 2 > 1. It is important to
highlight that compounds with the strong acceptor TCF−CF3
(4a−c) show pronounced bathochromic shifts (>100 nm as
compared to TCF systems 3a−c), and in some cases, the
absorption bands are extended into the near-infrared.
As expected, when comparing compounds 2a with 9 and 3a
with 10, the replacement of a diene by a thiophene ring leads to
a moderate hypsochromic shift of the absorption band.21,57,58
The data in Table 4 indicate in all cases a positive
solvatochromism on passing from 1,4-dioxane to CH2Cl2. In
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Figure 5. Solvatochromism study in 1,4-dioxane, CH2Cl2, acetone, DMF, and DMSO for compounds [1−4(a−c)].
Table 5. Experimental and CPHF-Calculated NLO Properties and TD-DFT-Calculated Parameters
a
b
c
c
d
d
d
d
compd
μβ (10−48 esu)
μβ0 (10−48 esu)
μβ0 (10−48 esu)
β0 (10−30 esu)
μg (D)
Δμge (D)
f
E
(eV)
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
9
1380
3130
665
1400
1420
1260
2600
2990
2630
4100
5900
2740
6000
8000
2000
3110
1162
3218
90
239
260
142
320
347
306
561
584
381
721
742
12.1
13.1
13.9
18.8
20.1
20.7
22.8
24.2
24.8
23.1
24.6
25.3
0.9
3.6
1.6
3.1
5.3
4.1
2.4
5.0
4.2
2.3
4.3
3.7
1.66
1.98
1.99
1.59
1.77
2.03
2.00
2.10
2.38
2.04
2.14
2.42
2.33
2. 12
2.13
2.20
2.00
2.03
2.09
1.90
1.95
2.04
1.85
1.91
e
3400
3626
2900
6900
2753
6443
8500
7187
6790
7165
12000
17000
13600
13404
14326
8970
f
35000
17445
18493
f
40000
7500
10
17400
a
b
μβ values determined in CH2Cl2 at 1907 nm (experimental accuracy 15%). Experimental μβ0 values calculated using the two-level model.
c
d
e
Calculated at the CPHF/6- 31G* level. TD-DFT calculations at the B3P86/6-31G* level. The μβ value was determined from a freshly prepared
f
sample because of its relative instability in CH2Cl2. The proximity to harmonic resonance makes this extrapolated value questionable.
this type of molecule, the thiophene ring exerts an auxiliary
donor effect when it is closer to the 4H-pyran-4-ylidene moiety.
The μβ0 values calculated using the CPHF approach are
gathered in Table 5. It can be seen that while calculated values
are overestimated, they reproduce quite well the experimental
trends because both μg and β0 (and therefore μβ0) are larger for
the NLO chromophores in series c.
We also calculated (see Table 5) the parameters involved in
the two level model62 (β0 ∝ (Δμge f/E3)) using the time-
dependent density functional theory (TD-DFT) method. In
this approach, Δμge represents the difference in dipole moment
between the first excited state and the ground state, f is the
oscillator strength, and E is the first excitation energy. However,
a comparison of the second-order NLO properties of
compounds b and c is not easy on the basis of this simple
approach: while oscillator strengths (f) are larger for
compounds c, the dipole moment changes upon excitation
(Δμge) are larger for b. As a consequence of these opposing
effects, the small differences in the NLO behavior of b and c are
probably found in higher excited states that are not considered
in the two level approach.
Positive BLA and hyperpolarizability β values suggest that
molecules in this study are located in the A/B region of the
Marder plot,7 i.e., very close and on the left-hand side of the
peak (higher values of β on increasing the polarity). In order to
validate this hypothesis, EFISH measurements were carried out
on compounds 3a−c in DMSO [μβ (10−48 esu) 3a, 1400; 3b,
7900; 3c, 9100]. In all three cases, a significant decrease in the
μβ values was observed, but they still remain positive, indicating
that the cyanine limit has not been exceeded.
Thermal Stability. The thermal stabilities of the
chromophores were studied by thermogravimetric analysis
(TGA) under nitrogen at a heating rate of 10 °C/min. The
decomposition temperatures (Td) were estimated as the
temperature at the intercept of the leading edge of the weight
loss with the baseline of the TGA scans.
The results show that all compounds (1−4) are thermally
stable, with decomposition temperatures above 220 °C [Td
(°C): 1a, 272; 1b, 229; 1c, 268; 2a, 294; 2b, 233; 2c, 231; 3a,
334; 3b, 268; 3c, 264; 4a, 281; 4b, 251; 4c, 220; 9, 184; 10,
205]. The shortest derivatives (1−4)a have the highest Td
values, and the thermal stability decreases on lengthening the π-
4639
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The Journal of Organic Chemistry
Article
( × 2), 129.1, 128.8, 125.5, 125.0, 108.9, 108.5, 107.4, 102.7; HRMS
(ESI+) Calcd for C22H16BrOS [M + H]+ 407.0100, found 407.0087.
5-[(2,6-Diphenyl-4H-pyran-4-ylidene)methyl]thiophene-2-
carbaldehyde, 8a.
conjugated system. As expected, inclusion of a thiophene ring
in the spacer leads to an increase in Td by more than 100 °C
compared to those of their polyene analogues (compounds 9
and 10 versus 2a and 3a). Systems with the TCF acceptors
have the highest thermal stability (slightly lower for the
fluorinated compounds 4 in agreement with previously
reported data);27 they are soluble in common solvents, and
consequently, they are very promising candidates for use as
active materials in electro-optic devices (their properties
compare favorably, for instance, to those described for the
benchmark chromophore CLD-1: μβ = 14065 × 10−48 esu; Td
= 275 °C).65
In summary, a new series of efficient 4H-pyran-4-ylidene
NLO chromophores with a thiophene ring in the π-spacer and
four strong acceptors have been synthesized and studied. The
chromophores show high hyperpolarizability values and
adequate thermal stability for potential EO applications,
obtaining the best nonlinear responses with strong TCF-type
acceptors. Both experimental and theoretical data show that
higher μβ values are obtained when the thiophene ring is closer
to the pyranylidene ring, and therefore, the thiophene acts as an
auxiliary donor. In all cases, EFISH measurements reveal that
the chromophores are located at the A/B region of the Marder
plot, probably slightly to the left of the positive maximum on
the BLA graph.
Method 1. A solution of 4-[(5-bromothiophen-2-yl)methylene]-
2,6-diphenyl-4H-pyran 7a (104 mg; 0.25 mmol) in anhydrous THF (3
mL) was prepared, purged with argon, and cooled to −78 °C. To the
solution was added dropwise n-BuLi (0.17 mL; 0.27 mmol; 1.6 M in
hexanes), and the resulting mixture was stirred for 3 min. Anhydrous
N,N-dimethylformamide (0.13 mL; 1.61 mmol) was added, and the
mixture was stirred for one additional hour. Saturated NH4Cl solution
was added to quench the reaction, and the mixture was then warmed
to room temperature. After the solvent was evaporated, the organic
layer was extracted with ethyl acetate (2 × 25 mL). The combined
organic layers were washed with water and dried over anhydrous
MgSO4. After removal of the solvent, the aldehyde was purified by
silica gel column chromatography (20% ethyl acetate in hexanes).
Yield: red solid (45 mg; 0.13 mmol; 50%).
Method 2.
A solution of 2,6-diphenyl(4H-pyran-4-yl)-
diphenylphosphine oxide 5 (1.20 g; 2.76 mmol) in anhydrous THF
(35 mL) was prepared, purged with argon, and cooled to −78 °C. n-
BuLi (1.7 mL; 2.72 mmol; 1.6 M in hexanes) was added dropwise to
the solution, and the resulting mixture was stirred for 15 min. 2,5-
Thiophenedicarbaldehyde (0.39 g; 2.76 mmol) was added dropwise,
and the mixture was stirred for an additional 20 min. Saturated NH4Cl
solution was added to quench the reaction, and the mixture was then
warmed to room temperature. After the solvent was evaporated, the
organic layer was extracted with ethyl acetate (2 × 50 mL). The
combined organic layers were washed with water and dried over
anhydrous MgSO4. After removal of the solvent, the aldehyde was
purified by silica gel column chromatography (10% ethyl acetate in
hexanes).
EXPERIMENTAL SECTION
■
Starting materials: 2,5-thiophenedicarbaldehyde, 5-bromo-2-thiophe-
necarbaldehyde 6a and N,N-dimethylaminoacrolein are commercially
available and were used without further purification. Compounds 548
and 6b49 were prepared as previously reported.
Yield: red solid (429 mg; 1.20 mmol; 44%); mp 141−148 °C; IR
1
(Nujol/NaCl, cm−1) 1645 (CO), 1578 (CC); H NMR (400
MHz, CD2Cl2) δ 9.81 (s, 1H), 7.94−7.78 (m, 4H), 7.67 (d, J = 4.0 Hz,
1H), 7.56−7.42 (m, 6H), 7.24 (d, J = 2.0 Hz, 1H), 7.02 (d, J = 4.0 Hz,
1H), 6.57 (d, J = 2.0 Hz, 1H), 6.20 (s, 1H); 13C NMR (100 MHz,
CDCl3) δ 182.0, 155.0, 152.5, 152.4, 139.0, 137.4, 132.7, 132.5, 132.4,
130.1, 129.6, 128.8, 128.7, 126.2, 125.3, 124.7, 108.6, 107.0, 102.7;
HRMS (ESI+) Calcd for C23H17O2S [M + H]+ 357.0944, found
357.0946; Calcd for C23H16NaO2S [M + Na]+ 379.0763, found
379.0758.
(4H-Pyran-4-yl)diphenylphosphine Oxide, 5. A solution of 2,6-
diphenylpyrylium fluoroborate66 (0.81 g; 2.54 mmol) in anhydrous
acetonitrile (5 mL) was prepared and purged with argon at room
temperature. To the solution were added both sodium iodide (0.40 g;
2.67 mmol) and methyl diphenylphosphinite (0.6 mL; 2.99 mmol).
The mixture was stirred for 1 h and then diluted five times with water.
The resulting solid was filtered off, washed with water, and dried. Pure
compound 5 was obtained by recrystallization from ethyl acetate.
Yield: white solid (6.97 g; 16 mmol; 94%); mp 185−189 °C; IR
(E)-4-[3-(5-Bromothiophen-2-yl)allylidene]-2,6-diphenyl-4H-
1
(Nujol/NaCl, cm−1) 1175 (PO), 1674 (CO); H NMR (300
pyran, 7b.
A solution of 2,6-diphenyl(4H-pyran-4-yl)-
diphenylphosphine oxide 5 (515 mg; 1.19 mmol) in anhydrous
THF (8 mL) was prepared, purged with argon, and cooled to −78 °C.
n-BuLi (0.8 mL; 1.28 mmol; 1.6 M in hexanes) was added dropwise to
the solution, and the resulting mixture was stirred for 15 min. A
solution of (E)-3-(5-bromothiophen-2-yl)acrylaldehyde 6b (302 mg;
1.31 mmol) in anhydrous THF (4 mL) was added dropwise. The
mixture was stirred for an additional 5 h (TLC monitoring using 30%
ethyl acetate in hexanes), and then saturated NH4Cl solution was
added to quench the reaction. After removing the THF, the crude
product was extracted with ethyl ether (2 × 40 mL), washed with
water (2 × 40 mL), and dried over anhydrous MgSO4. Pure
compound 7b was obtained by silica gel column chromatography
(30% ethyl acetate in hexanes).
MHz, CDCl3) δ 7.92−7.81 (m, 4H), 7.59−7.42 (m, 10H), 7.41−7.28
(m, 6H), 5.40 (t, J = 4.2 Hz, 2H), 4.32 (dt, J = 21.0 Hz, J′ = 4.2 Hz,
1H); 13C NMR (100 MHz, CDCl3) δ 151.3, 151.2, 133.6 ( × 2), 132.0
( × 2), 131.9, 130.9, 129.9, 128.8, 128.5, 128.4, 128.2, 124.6, 92.4, 38.5,
37.8; MS (ESI+) C29H23NaO2P [M + Na] 457, C29H23KO2P [M + K]
473.
4-[(5-Bromothiophen-2-yl)methylene]-2,6-diphenyl-4H-
pyran, 7a.
A solution of 2,6-diphenyl(4H-pyran-4-yl)-
diphenylphosphine oxide 5 (350 mg; 0.81 mmol) in anhydrous
THF (8 mL) was prepared, purged with argon, and cooled to −78 °C.
To the solution was added dropwise n-BuLi (0.55 mL; 0.88 mmol; 1.6
M in hexanes), and the resulting mixture was stirred for 15 min. 5-
Bromo-2-thiophenecarbaldehyde 6a (0.2 mL; 1.60 mmol) was added
dropwise, and the mixture was stirred for 1.5 h and then quenched
with saturated NH4Cl solution. After removal of the THF, the organic
layer was extracted with ethyl acetate (2 × 30 mL). The combined
organic layers were washed with water (2 × 30 mL) and dried over
anhydrous MgSO4. Pure compound 7a was obtained by silica gel
column chromatography (20% ethyl acetate in hexanes).
Yield: orange solid (145 mg; 0.33 mmol; 28%); mp 116−120 °C;
1
IR (KBr, cm−1) 1581 (CC); H NMR (300 MHz, CD3COCD3) δ
8.03−7.82 (m, 4H), 7.57−7.40 (m, 6H), 7.24 (dd, J = 15.0 Hz, J′ =
11.7 Hz, 1H), 7.17 (d, J = 1.8 Hz, 1H), 7.01 (d, J = 3.8 Hz, 1H), 6.83
(d, J = 3.8 Hz, 1H), 6.65 (d, J = 1.8 Hz, 1H), 6.56 (d, J = 15.0 Hz,
1H), 5.80 (d, J = 11.7 Hz, 1H); 13C NMR not registered because of
rapid decomposition in solution; HRMS (ESI+) Calcd for C24H17BrOS
[M]+ 432.0178, found 432.0173.
Yield: orange solid (231 mg; 0.57 mmol; 70%) (observations:
unstable product in the presence of chlorinated solvents and/or light);
1
mp 213-217 °C; IR (Nujol/NaCl, cm−1) 1580 (CC); H NMR
(E)-5-[3-(2,6-Diphenyl-4H-pyran-4-ylidene)prop-1-enyl]-
thiophene-2-carbaldehyde, 8b. A solution of (E)-4-[3-(5-
bromothiophen-2-yl)allylidene]-2,6-diphenyl-4H-pyran 7b (462 mg;
1.07 mmol) in anhydrous THF (13 mL) was prepared, purged with
argon, and cooled to −78 °C. n-BuLi (0.73 mL; 1.17 mmol; 1.6 M in
(400 MHz, CD3COCD3) δ 7.98−7.86 (m, 4H), 7.59−7.42 (m, 6H),
7.08 (d, J = 3.9 Hz, 1H), 7.04 (d, J = 1.9 Hz, 1H), 6.88 (d, J = 3.9 Hz,
1H), 6.72 (d, J = 1.9 Hz, 1H), 6.15 (s, 1H); 13C NMR (100 MHz,
CD2Cl2) δ 153.9, 151.5, 144.2, 133.6, 133.5, 130.5, 130.2, 129.7, 129.2
4640
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Article
hexanes) was added dropwise to the solution, and the resulting
mixture was stirred for 1 h, and then anhydrous N,N-dimethylforma-
mide (0.55 mL; 7.09 mmol) was added. The reaction was stirred for
one additional hour, and then saturated NH4Cl solution was added to
quench the reaction. After removal of the THF, the organic layer was
extracted with ethyl acetate (2 × 30 mL), and the combined organic
layers were washed with water (2 × 40 mL) and dried over anhydrous
MgSO4. Pure compound 8b was obtained by silica gel column
chromatography (20% ethyl acetate in hexanes).
7.96−7.74 (m, 4H), 7.72 (d, J = 4.0 Hz, 1H), 7.63−7.42 (m, 7H), 7.11
(d, J = 4.0 Hz, 1H), 6.99 (s, 1H), 6.61 (d, J = 14.7 Hz, 1H), 6.49 (s,
1H), 5.83 (d, J = 11.9 Hz, 1H), 4.63 (q, J = 6.9 Hz, 2H), 4.59 (q, J =
6.9 Hz, 2H), 1.38 (t, J = 6.9 Hz, 3H), 1.32 (t, J = 6.9 Hz, 3H); 13C
NMR (100 MHz, CDCl3) δ 178.5, 163.2, 161.3, 160.2, 154.2, 153.4,
148.2, 148.0, 136.0, 135.6, 132.6, 132.3, 130.1, 129.8, 128.8, 126.7,
125.3, 124.8, 120.2, 119.4, 115.4, 108.9, 108.8, 107.5, 102.8, 100.0,
43.9, 43.0, 12.6, 12.4; HRMS (ESI+) Calcd for C33H28N2O3S2 [M]+.
564.1536, found 564.1532.
(E)-5-(3-{5-[(2,6-Diphenyl-4H-pyran-4-ylidene)methyl]-
thiophen-2-yl}allylidene)-1,3-diethyl-2-thioxodihydropyrimi-
dine-4,6(1H,5H)-dione, 1c. The crude product was purified by silica
gel column chromatography (10% hexane in dichloromethane).
Yield: dark red solid (232 mg; 0.41 mmol; 82%); mp 292−292 °C;
Yield: red solid (386 mg; 1.01 mmol; 95%); mp 150−154 °C; IR
1
(Nujol/NaCl, cm−1) 1655 (CO), 1569 (CC); H NMR (400
MHz, CDCl3) δ 9.79 (s, 1H), 7.88−7.71 (m, 4H), 7.59 (d, J = 3.9 Hz,
1H), 7.53−7.38 (m, 7H), 7.29 (dd, J = 14.9 Hz, J′ = 11.9 Hz, 1H), 6.97
(d, J = 3.9 Hz, 1H), 6.80 (d, J = 1.6 Hz, 1H), 6.55 (d, J = 14.9 Hz,
1H), 6.41 (d, J = 1.6 Hz, 1H), 5.75 (d, J = 11.9 Hz, 1H); 13C NMR
(100 MHz, CDCl3) δ 182.1, 154.6, 153.1, 139.8, 137.7, 133.4, 132.8,
132.7, 129.8, 129.5, 129.4, 128.8, 128.7, 124.9, 124.8, 124.6, 119.4,
113.8, 108.3, 102.1; HRMS (ESI+) Calcd for C25H18O2S [M]+.
382.1022, found 382.1019; Calcd for C25H18NaO2S [M + Na]+
405.0920, found 405.0909.
1
IR (KBr, cm−1) 1655 (CO), 1536 (CC); H NMR (400 MHz,
CDCl3) δ 8.31 (dd, J = 14.3 Hz, J′ = 12.5 Hz, 1H), 8.14 (d, J = 12.5
Hz, 1H), 7.95−7.76 (m, 4H), 7.60−7.44 (m, 7H), 7.42 (d, J = 4.2 Hz,
1H), 7.23 (d, J = 1.9 Hz, 1H), 6.99 (d, J = 4.2 Hz, 1H), 6.57 (d, J = 1.9
Hz, 1H), 6.20 (s, 1H), 4.59 (q, J = 7.0 Hz, 2H), 4.56 (q, J = 7.0 Hz,
2H), 1.35 (t, J = 7.0 Hz, 3H), 1.31 (t, J = 7.0 Hz, 3H); 13C NMR (100
MHz, CDCl3) δ 178.7, 161.1, 160.2, 157.7, 155.9, 153.3, 152.3, 147.7,
138.3, 136.2, 133.7, 132.5, 132.3, 130.4, 129.9, 129.0, 128.9, 128.3,
125.6, 124.8, 123.1, 111.1, 109.2, 108.1, 103.4, 43.6, 43.0, 29.7, 12.6,
12.5; HRMS (ESI+) Calcd for C33H29N2O3S2 [M + H]+ 565.1614,
found 565.1596.
(E)-3-{5-[(2,6-Diphenyl-4H-pyran-4-ylidene)methyl]-
thiophen-2-yl}acrylaldehyde, 8c. A solution of 4-[(5-bromothio-
phen-2-yl)methylene]-2,6-diphenyl-4H-pyran 7a (724 mg; 1.78
mmol) in anhydrous THF (15 mL) was prepared, purged with
argon, and cooled to −78 °C. To the solution was added n-BuLi (1.2
mL; 1.95 mmol; 1.6 M in hexanes), the resulting mixture was stirred
for 30 min, and then N,N-dimethylaminoacrolein (1.0 mL; 9.0 mmol)
was added. The reaction mixture was stirred for 2.5 h (TLC using 30%
ethyl acetate in hexanes) and then allowed to cool to room
temperature. The reaction was quenched by addition of HCl (1 N)
(36 mL; 36 mmol). The solvent was evaporated, and the organic layer
was extracted with ethyl acetate and dried over anhydrous MgSO4.
After removal of the solvent, the aldehyde was purified by silica gel
column chromatography (30% ethyl acetate in hexanes).
Yield: red solid (327 mg; 0.86 mmol; 48%); mp 140−143 °C; IR
(KBr, cm−1) 1667 (CO), 1556 (CC); 1H NMR (300 MHz,
CDCl3) δ 9.60 (d, J = 8.0 Hz, 1H), 7.89−7.74 (m, 4H), 7.59−7.40 (m,
7H), 7.26 (d, J = 3.9 Hz, 1H), 7.18 (d, J = 2.0 Hz, 1H), 6.92 (d, J = 3.9
Hz, 1H), 6.49 (d, J = 2.0 Hz, 1H), 6.48 (dd, J = 15.4 Hz, J′ = 8.0 Hz,
1H), 6.12 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 192.7, 154.5,
152.1, 148.0, 144.4, 135.7, 133.5, 132.8, 132.7, 131.1, 130.0, 129.5,
128.9, 128.7, 126.7, 125.1, 125.1, 124.6, 108.6, 107.3, 102.8; HRMS
(ESI+) Calcd for C25H19O2S [M + H]+ 383.1100, found 383.1117;
Calcd for C25H18NaO2S [M + Na]+ 405.0920, found 405.0926; Calcd
for C25H18KO2S [M + K]+ 421.0659, found 421.0673.
Compounds 2a−c: General Procedure. Equimolar quantities of
2-phenyl-1-propene-1,1,3-tricarbonitrile (97 mg; 0.5 mmol) and the
corresponding aldehyde 8a (178 mg; 0.5 mmol), 8b (191 mg; 0.5
mmol), or 8c (191 mg; 0.5 mmol) were dissolved in acetic anhydride
under an argon atmosphere. The mixture was heated to 100 °C for 3 h
(8a), 3.5 h (8b), or 1 h (8c). After cooling to room temperature, a
saturated solution of NaHCO3 was added and stirred for 15 min, and
the organic phase was extracted with AcOEt. The combined organic
layers were dried over anhydrous MgSO4, and the crude product was
purified by filtration or silica gel column chromatography.
(Z)-4-{5-[(2,6-Diphenyl-4H-pyran-4-ylidene)methyl]-
thiophen-2-yl}-2-phenylbuta-1,3-diene-1,1,3-tricarbonitrile,
2a. The crude product was isolated by filtration, washed with cold
hexane, and dried.
Yield: green solid (144 mg; 0.27 mmol; 54%); mp 285−288 °C; IR
(KBr, cm−1) 2202 (CN), 1519 (CC); 1H NMR (400 MHz,
CDCl3) δ 8.01−7.79 (m, 4H), 7.68−7.42 (m, 9H), 7.02 (d, J = 4.5 Hz,
1H), 6.69 (d, J = 1.8 Hz, 1H), 6.30 (s, 1H). The 13C NMR spectrum
was not registered because of the low solubility of 2a; HRMS (ESI+)
Calcd for C35H21N3NaOS [M + Na]+ 554.1298, found 554.1291;
Calcd for C35H21KN3OS [M + K]+ 570.1037, found 570.1029.
(Z)-4-{5-[(E)-3-(2,6-Diphenyl-4H-pyran-4-ylidene)prop-1-
enyl]thiophen-2-yl}-2-phenylbuta-1,3-diene-1,1,3-tricarboni-
trile, 2b. The crude product was purified by silica gel column
chromatography (first 30% ethyl acetate in hexanes, and then the
polarity was gradually increased to 50% ethyl acetate in hexanes).
Yield: red solid (31 mg; 0.06 mmol; 11%); mp 285−288 °C; IR
(KBr, cm−1) 2212 (CN), 1538 (CC); 1H NMR (400 MHz,
CDCl3) δ 7.92−7.73 (m, 5H), 7.65−7.37 (m, 13H), 7.00 (d, J = 4.2
Hz, 1H), 6.92 (br s, 1H), 6.59 (d, J = 14.6 Hz, 1H), 6.51 (br s, 1H),
5.83 (d, J = 12.1 Hz, 1H); 13C NMR was not registered because of low
solubility; HRMS (ESI+) Calcd for C37H23N3OS [M]+. 557.1556,
found 557.1534; Calcd for C37H24N3OS [M + H]+ 558.1635, found
558.1598; Calcd for C37H23N3NaOS [M + Na]+ 580.1454, found
580.1450.
Compounds 1a−c: General Procedure. Equimolar quantities of
1,3-diethyl-2-thioxodihydropyrimidine-4,6-dione (100 mg; 0.5 mmol)
and the corresponding aldehyde 8a (178 mg; 0.5 mmol), 8b (191 mg;
0.5 mmol), or 8c (191 mg; 0.5 mmol) were dissolved into absolute
ethanol under an argon atmosphere. The mixture was heated under
reflux for 30 min (8a) and (8b) or 5 h (8c) (TLC monitoring) and
then cooled to 0 °C. The resulting solid was filtered off, washed with
ethanol/dichloromethane mixtures, and dried.
5-{[5-(2,6-Diphenyl-4H-pyran-4-ylidene)methyl]thiophen-2-
yl}methylene-1,3-diethyl-2-thioxodihydropyrimidine-4,6-
(1H,5H)-dione, 1a. Yield: green solid (197 mg; 0.37 mmol; 73%);
mp 258−261 °C; IR (KBr, cm−1) 1655 (CO), 1377 (C−N), 1253
(C−O), 1144 (CS); 1H NMR (400 MHz, CD2Cl2) δ 8.53 (s, 1H),
8.03 (d, J = 3.9 Hz, 1H), 7.93−7.80 (m, 4H), 7.70 (s, 1H), 7.62−7.43
(m, 6H), 7.16 (d, J = 3.9 Hz, 1H), 6.74 (s, 1H), 6.31 (s, 1H), 4.64 (q, J
= 6.9 Hz, 2H), 4.56 (q, J = 6.9 Hz, 2H), 1.39 (t, J = 6.9 Hz, 2H), 1.29
(t, J = 6.9 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 179.0, 161.7, 156.6,
150.7, 147.2, 146.5, 136.5, 135.2, 132.5, 130.7, 130.2, 129.0, 127.8,
125.6, 125.1, 109.5, 108.4, 104.6, 43.8, 43.1, 12.6, 12.5; HRMS (ESI+)
Calcd for C31H27N2O3S2 [M + H]+ 539.1457, found 539.1400.
(E)-5-({5-[3-(2,6-Diphenyl-4H-pyran-4-ylidene)prop-1-enyl]-
thiophen-2-yl}methylene)-1,3-diethyl-2-thioxodihydropyrimi-
dine-4,6(1H,5H)-dione, 1b. Yield: green solid (155 mg; 0.28 mmol;
55%); mp 263−265 °C; IR (Nujol/NaCl, cm−1) 1651 (CO), 1562
(3Z,5E)-6-{5-[(2,6-Diphenyl-4H-pyran-4-ylidene)methyl]-
thiophen-2-yl}-2-phenylhexa-1,3,5-triene-1,1,3-tricarbonitrile,
2c. The crude product was purified by silica gel column
chromatography (30% ethyl acetate in hexanes).
Yield: dark green solid (153 mg; 0.28 mmol; 55%); mp 245−249
1
°C; IR (KBr, cm−1) 2213 (CN), 1574 (CC); H NMR (400
MHz, CDCl3) δ 7.91−7.76 (m, 4H), 7.64−7.39 (m, 11H), 7.31 (d, J =
4.0 Hz, 1H), 7.29 (d, J = 14.4 Hz, 1H), 7.26 (d, J = 11.8 Hz, 1H), 7.23
(d, J = 1.9 Hz, 1H), 7.07 (dd, J = 14.4 Hz, J′ = 11.8 Hz, 1H), 6.96 (d, J
= 4.0 Hz, 1H), 6.59 (d, J = 1.9 Hz, 1H), 6.20 (s, 1H); 13C NMR (100
MHz, CDCl3) δ 166.8, 157.7, 156.7, 154.1, 153.1, 143.9, 138.2, 137.0,
1
(CS), 1529 (CC); H NMR (400 MHz, CDCl3) δ 8.53 (s, 1H),
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134.9, 134.8, 132.9, 132.8, 132.5, 131.2, 130.6, 130.3, 129.7, 129.6,
129.4, 129.1, 126.1, 125.4, 121.3, 115.1, 115.0, 113.6, 109.7, 108.6,
104.3, 104.2, 104.0; HRMS (ESI+) Calcd for C37H23N3OS [M]+
557.1556, found 557.1552.
washed with CH2Cl2. The combined organic layers were dried over
anhydrous MgSO4 and evaporated. The resulting crude product was
purified as described below.
(E)-2-[3-Cyano-4-(2-{5-[(2,6-diphenyl-4H-pyran-4-ylidene)-
methyl]thiophen-2-yl}vinyl)-5-methyl-5-(trifluoromethyl)-
furan-2(5H)-ylidene]malononitrile, 4a. The pure compound was
obtained by silica gel column chromatography [first using 100%
dichloromethane, and then the polarity was gradually increased to
CH2Cl2/AcOEt (98/2)].
Compounds 3a−c: General Procedure. Equimolar quantities of
2-dicyanomethylen-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (100 mg;
0.5 mmol) and the corresponding aldehyde 8a (178 mg; 0.5 mmol),
8b (191 mg; 0.5 mmol), or 8c (191 mg; 0.5 mmol) were dissolved in
absolute ethanol. To this mixture was added a pyridine/acetic acid (10
equiv/7 equiv) solution, and the resulting mixture was heated under
reflux for 24 h (8a and 8c) or 48 h (8b) (TLC monitoring) and then
cooled. The solvent was evaporated, and the crude product was
dissolved in dichloromethane. Finally, the solution was washed with
saturated NaHCO3 solution, water and dried over anhydrous MgSO4,
and the resulting solid was purified as described below.
Yield: dark green solid (177 mg; 0.30 mmol; 60%); mp 259−264
1
°C; IR (KBr, cm−1) 2220 (CN), 1527 (CC); H NMR (400
MHz, CDCl3) δ 8.14 (d, J = 15.0 Hz, 1H), 7.97−7.74 (m, 4H), 7.60−
7.48 (m, 7H), 7.32 (d, J = 1.9 Hz, 1H), 7.05 (d, J = 4.4 Hz, 1H), 6.71
(d, J = 1.9 Hz, 1H), 6.47 (d, J = 15.0 Hz, 1H), 6.31 (s, 1H), 1.92 (s,
3H); 13C NMR (125.8 MHz, CDCl3) δ 175.6, 160.9, 157.7, 156.2,
155.2, 152.4, 140.1, 137.7, 137.4, 134.4, 132.0, 131.8, 131.2, 130.6,
129.7, 129.2, 129.1, 104.1, 56.2, 29.7, 19.5, 10.0; HRMS (ESI+) Calcd
for C34H20F3N3O2S [M]+ 591.1222, found 591.1200.
(E)-2-[3-Cyano-4(2-{5-[(2,6-diphenyl-4H-pyran-4-yliedene)-
methyl]thiophen-2-yl}vinyl)-5,5-dimethylfuran-2(5H)-ylidene]-
malononitrile, 3a. The resulting solid was filtered off and purified by
recrystallization from dichloromethane.
2-[3-Cyano-4-((E)-2-{5-[(E)-3-(2,6-diphenyl-4H-pyran-4-
ylidene)prop-1-enyl]thiophen-2-yl}vinyl)-5-methyl-5-
(trifluoromethyl)furan-2(5H)-ylidene] malononitrile, 4b. The
resulting solid was purified by reverse C18 column chromatography
(first using a mixture of 70% acetonitrile in water, and then the polarity
was gradually changed to 100% acetonitrile).
Yield: copper solid (110 mg; 0.21 mmol; 41%); mp 336−340 °C;
1
IR (KBr, cm−1) 2220 (CN), 1524 (CC); H NMR (400 MHz,
CDCl3) δ 7.94−7.75 (m, 5H), 7.60−7.46 (m, 7H), 7.43 (d, J = 4.2 Hz,
1H), 7.01 (d, J = 4.2 Hz, 1H), 6.62 (d, J = 1.8 Hz, 1H), 6.56 (d, J =
15.4 Hz, 1H), 6.23 (s, 1H), 1.76 (s, 6H). The 13C NMR spectrum was
not registered because of the low solubility of 3a; HRMS (ESI+) Calcd
for C34H23N3O2S [M]+. 537.1501, found 537.1505; Calcd for
C34H23N3NaO2S [M + Na]+ 560.1403, found 560.1398; Calcd for
C34H23KN3O2S [M + K]+ 576.1143, found 576.1125.
Yield: dark red solid (154 mg; 0.25 mmol; 50%); mp 266−269 °C;
1
IR (KBr, cm−1) 2224 (CN), 1520 (CC); H NMR (300 MHz,
CD2Cl2) δ 8.09 (d, J = 15.1 Hz, 1H), 7.98−7.76 (m, 4H), 7.59−7.38
(m, 8H), 7.07 (d, J = 4.4 Hz, 1H), 7.01 (d, J = 1.8 Hz, 1H), 6.66 (d, J
= 14.5 Hz, 1H), 6.60 (d, J = 1.8 Hz, 1H), 6.51 (d, J = 15.1 Hz, 1H),
5.93 (d, J = 12.2 Hz, 1H), 1.93 (s, 3H). 19F NMR (282 MHz, CD2Cl2)
δ −78.80; 13C NMR (100 MHz, CD2Cl2) δ 176.1, 161.9, 158.5, 155.3,
154.6, 141.0, 139.1, 137.6, 133.2, 133.0, 132.8, 130.9, 130.6, 129.5 ( ×
2), 129.4, 128.7, 125.9, 125.8, 125.4, 120.3, 115.9, 112.1 ( × 2), 111.7,
110.8, 109.5, 103.3, 97.6, 96.2, 19.8; HRMS (ESI+) Calcd for
C36H22F3N3O2S [M]+. 617.1379, found 617.1385; Calcd for
C36H22F3N3NaO2S [M + Na]+ 640.1277, found 640.1274.
2-[3-Cyano-4-((E)-2-{5-[(E)-3-(2,6-diphenyl-4H-pyran-4-
ylidene)prop-1-enyl]thiophen-2-yl}vinyl)-5,5-dimethylfuran-
2(5H)-ylidene]malononitrile, 3b. The resulting solid was filtered off
and sequentially washed with hexane, hexane/CH2Cl2 (9/1), hexane/
CH2Cl2 (8/2), cold ethanol, and dried.
Yield: dark red solid (251 mg; 0.45 mmol; 89%); mp 305−308 °C;
IR (Nujol/NaCl, cm−1) 2219 (CN), 1541 (CC); 1H NMR (300
MHz, CD2Cl2) δ 7.94−7.77 (m, 4H), 7.72 (d, J = 15.4 Hz, 1H), 7.60−
7.29 (m, 8H), 7.01 (d, J = 4.1 Hz, 1H), 6.95 (br s, 1H), 6.63 (d, J =
14.5 Hz, 1H), 6.61 (d, J = 15.4 Hz, 1H), 6.54 (br s, 1H), 5.87 (d, J =
12.2 Hz, 1H), 1.76 (s, 6H). The 13C NMR spectrum was not
registered because of the low solubility of 3b; HRMS (ESI+) Calcd for
C36H25N3O2S [M]+. 563.1662, found 563.1655; Calcd for
C36H25N3NaO2S [M + Na]+ 586.1560, found 586.1546.
2-[3-Cyano-4-((1E,3E)-4-{5-[(2,6-diphenyl-4H-pyran-4-
ylidene)methyl]thiophen-2-yl}buta-1,3-dienyl)-5-methyl-5-
(trifluoromethyl)furan-2(5H)-ylidene]malononitrile, 4c. The
resulting solid was filtered off and washed with hexane/dichloro-
methane mixtures and dried.
Yield: red/copper solid (161 mg; 0.26 mmol; 52%); mp 227−232
1
°C; IR (KBr, cm−1) 2222 (CN), 1558 (CC); H NMR (400
2-[3-Cyano-4-((1E,3E)-4-{5-[(2,6-diphenyl-4H-pyran-4-
ylidene)methyl]thiophen-2-yl}buta-1,3-dienyl)-5,5-dimethyl-
furan-2(5H)-ylidene]malononitrile, 3c. The resulting solid was
filtered off and washed with cold hexane.
MHz, CDCl3) δ 7.98 (dd, J = 14.3 Hz, J = 11.8 Hz, 1H), 7.94−7.75
(m, 4H), 7.62−7.45 (m, 6H), 7.41 (d, J = 14.3 Hz, 1H), 7.32 (d, J =
4.2 Hz, 1H), 7.23 (d, J = 1.6 Hz, 1H), 6.99 (d, J = 4.2 Hz, 1H), 6.77
(dd, J = 14.3 Hz, J′ = 11.8 Hz, 1H), 6.35 (d, J = 14.3 Hz, 1H), 6.60 (d,
J = 1.6 Hz, 1H), 6.23 (s, 1H), 1.87 (s, 3H), 6.23 (s, 1H); 13C NMR
(75 MHz, CDCl3) δ 175.5, 161.2, 166.2, 153.8, 152.1, 150.3, 141.5,
138.5, 136.1, 134.4, 132.4, 132.1, 130.6, 130.1, 129.1, 128.9, 125.5,
125.3, 124.8, 114.3, 111.7, 111.5, 111.0, 109.4, 108.5, 103.6, 19.3;
HRMS (ESI+) Calcd for C36H22F3N3O2S [M]+. 617.1379, found
Yield: dark green solid (242 mg; 0.43 mmol; 86%); mp 261−264
1
°C; IR (KBr, cm−1) 2216 (CN), 1547 (CC); H NMR (400
MHz, CDCl3) δ 7.93−7.72 (m, 4H), 7.61 (dd, J = 15.2 Hz, J′ = 11.4
Hz, 1H), 7.55−7.41 (m, 5H), 7.28 (d, J = 14.5 Hz, 1H), 7.22 (d, J =
4.0 Hz, 1H), 7.19 (d, J = 1.5 Hz, 1H), 6.95 (d, J = 4.0 Hz, 1H), 6.75
(dd, J = 14.5 Hz, J′ = 11.4 Hz, 1H), 6.52 (d, J = 1.5 Hz, 1H), 6.42 (d, J
= 15.2 Hz, 1H), 6.16 (s, 1H), 1.71 (s, 6H); 13C NMR (100 MHz,
CDCl3) δ 172.6, 155.4, 153.0, 149.4, 148.1, 148.0 ( × 2), 139.0, 138.1,
133.9, 132.8, 132.5, 130.3, 129.8, 129.0, 128.8, 127.8, 125.4, 124.8,
115.6, 112.3, 111.5, 111.3, 109.0, 107.9, 103.3, 96.7, 26.5; HRMS
(ESI+) Calcd for C36H25N3O2S [M]+. 563.1662, found 563.1665;
Calcd for C36H25N3NaO2S [M + Na]+ 586.1560, found 586.1550;
Calcd for C36H25N3KO2S [M + K]+ 602.1299, found 602.1299.
Compounds 4a−c: General Procedure. Equimolar quantities of
2-(3-cyano-5-methyl-5-(trifluoromethyl)furan-2(5H)-ylidene]-
malononitrile (127 mg; 0.5 mmol) and the corresponding aldehyde 8a
(178 mg; 0.5 mmol), 8b (191 mg; 0.5 mmol), or 8c (191 mg; 0.5
mmmol) were dissolved in absolute ethanol. To this mixture was
added a pyridine/acetic acid (10 equiv/7 equiv) solution, and the
resulting mixture was heated under reflux for 4 h (8a), 24 h (8b), or 2
h 30 min (8c) (TLC monitoring) and then cooled. The solvent was
evaporated, and the crude product was dissolved in dichloromethane.
A saturated NaHCO3 solution was added, and the mixture was stirred
for 15 min. The phases were separated, and the aqueous layer was
+
617.1370; Calcd for C36H22F3N3NaO2S [M + Na] 640.1277, found
640.1283.
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental methods, NMR and UV−vis spectra of
new compounds, NLO measurements, computed energies and
Cartesian coordinates of optimized geometries. This material is
available free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sfranco@unizar.es.
Notes
The authors declare no competing financial interest.
4642
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Article
(26) Alías, S.; Andreu, R.; Blesa, M. J.; Cerdan
Garín, J.; Lop
Allain, M. J. Org. Chem. 2008, 73, 5890.
(27) Andreu, R.; Cerdan, M. A.; Franco, S.; Garín, J.; Marco, A. B.;
Orduna, J.; Palomas, D.; Villacampa, B.; Alicante, R.; Allain, M. Org.
Lett. 2008, 10, 4963.
́
, M. A.; Franco, S.;
ez, C.; Orduna, J.; Sanz, J.; Alicante, R.; Villacampa, B.;
ACKNOWLEDGMENTS
■
́
We thank the Spanish Ministry of Science and Innovation,
MICINN-FEDER (Projects CTQ2008-02942, CTQ2011-
22727, MAT2008-06522-C02-02, and MAT-2011-27978-C02-
́
02) and the Gobierno de Aragon
for financial support. N.M.d.B. thanks the Departamento
Educacion, Gobierno de Navarra, for a predoctoral grant.
́
-Fondo Social Europeo (E39)
́
(28) Andreu, R.; Galan, E.; Orduna, J.; Villacampa, B.; Alicante, R.;
Navarrete, J. T. L.; Casado, J.; Garín, J. Chem.Eur. J. 2011, 17, 826.
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Chem. 2009, 19, 7466.
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