Aromatic/proaromatic donors in 2-dicyanomethylenethiazole merocyanines: From neutral to strongly zwitterionic nonlinear optical chromophores

Article abstract of DOI:10.1002/chem.201002158

Push-pull compounds, in which a proaromatic electron donor is conjugated to a 2-dicyanomethylenethiazole acceptor, have been prepared, and their properties compared to those of model compounds featuring an aromatic donor. A combined experimental (X-ray di

Full text of DOI:10.1002/chem.201002158

DOI: 10.1002/chem.201002158
Aromatic/Proaromatic Donors in 2-Dicyanomethylenethiazole
Merocyanines: From Neutral to Strongly Zwitterionic Nonlinear Optical
Chromophores
Raquel Andreu,[a] Elena Galꢀn,[a] Jesffls Orduna,[a] BelØn Villacampa,[b]
Raquel Alicante,[b] Juan T. López Navarrete,[c] Juan Casado,[c] and Javier Garín*[a]
Abstract: Push–pull compounds, in
which a proaromatic electron donor is
conjugated to a 2-dicyanomethylene-
thiazole acceptor, have been prepared,
and their properties compared to those
of model compounds featuring an aro-
matic donor. A combined experimental
(X-ray diffraction, 1H NMR, IR,
Raman, UV/Vis, nonlinear optical
(NLO) measurements) and theoretical
study reveals that structural and sol-
vent effects determine the ground-state
polarisation of these merocyanines:
whereas 4H-pyran-4-ylidene- and 4-
pyridylidene-containing compounds are
zwitterionic and 1,3-dithiol-2-ylidene
derivatives are close to the cyanine
limit, anilino-derived merocyanines are
essentially neutral. This very large
range of intramolecular charge transfer
(ICT) gives rise to efficient second-
order NLO chromophores with mb
values ranging from strongly negative
to strongly positive. In particular, pyra-
nylidene derivatives are unusual in that
they show an increase in the degree of
ICT on lengthening the p-spacer, a fea-
ture that lies behind the very large neg-
ative mb values they display. The link-
ing of the formally quinoidal 2-dicya-
nomethylenethiazole moiety to proaro-
matic donors seems a promising ap-
proach towards the optimisation of
zwitterionic NLO chromophores.
Keywords: charge transfer · chro-
mophores · heterocycles · nonlinear
optics · zwitterions
Introduction
language (Scheme 1), they can be represented by two ex-
treme limiting forms, “neutral” (N) and zwitterionic (ZW),
although it is also advantageous to depict the intermediate
situation corresponding to the ideal polymethine state, also
called the “cyanine limit” (CL).
Merocyanines are continuing to attract considerable atten-
tion both in industry and in academia.[1] Their very basic
structure comprises an electron donor (D) and an electron
acceptor (A) linked by a fully conjugated chain with an odd
number (n) of p-centres, and featuring (n+3) p-electrons
delocalised over the whole system. In simple valence bond
Scheme 1. Canonical forms of merocyanines.
[a] Dr. R. Andreu, E. Galꢀn, Dr. J. Orduna, Prof. J. Garꢁn
Departamento de Quꢁmica Orgꢀnica, ICMA
Universidad de Zaragoza-CSIC
The contribution of each of these canonical structures de-
pends not only on the nature of the subunits, but also on the
surrounding medium (as exemplified by their solvatochrom-
ism),[2] thus allowing the fine tuning of their structural and
optical properties. The interplay between these factors was
soon rationalised,[3] and later on Marder disclosed correla-
tions between a structural parameter (the bond-length alter-
nation (BLA), defined as the average difference in length
Pedro Cerbuna 12, 50009 Zaragoza (Spain)
Fax : (+34)976-761194
E-mail: jgarin@unizar.es
[b] Dr. B. Villacampa, R. Alicante
Departamento de Fꢁsica de la Materia Condensada
ICMA, Universidad de Zaragoza-CSIC
Pedro Cerbuna 12, 50009 Zaragoza (Spain)
[c] Prof. J. T. L. Navarrete, Dr. J. Casado
Departamento de Quꢁmica Fꢁsica, Universidad de Mꢀlaga
Campus de Teatinos s/n, 29071 Mꢀlaga (Spain)
ꢀ
between adjacent C C bonds in the polymethine chain),
and the polarisability (a), and first (b) and second (g) hyper-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002158.
polarisabilities of merocyanines, thus establishing very
826
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FULL PAPER
useful guidelines for the design of nonlinear optical (NLO)
chromophores.[4]
compounds 3 and 5 have also been prepared. The former
allows a direct comparison between pyranylidene and pyri-
dylidene donors, whereas the latter is designed to disclose
the effect of the introduction of an aromatic thiophene ring
on the properties of these chromophores. For the sake of
comparison, merocyanines 6a and b with an aromatic donor
are also included. The detailed investigation of compounds
2–6 reveals the important effects that the donor, the spacer,
and the surrounding medium have on the structure and opti-
cal properties of these chromophores, and shows that mero-
cyanines 2–6 display either negative or positive mb values
spanning several orders of magnitude.
In the field of second-order nonlinear optics,[5] most ef-
forts have focused on chromophores showing high positive b
values, and, because this condition is met at the left-hand
side of Marderꢃs plot,[6] such molecules with ground states
lying halfway between the N and the CL forms have been
named “left-hand side” (LHS) chromophores. Nevertheless,
the central region and the right-hand side (RHS) of the plot
offer many possibilities. Thus, although b vanishes at (or
close to)[7] the cyanine limit (at which BLA=0), the a pa-
rameter peaks at that coordinate, with the result that, for in-
stance, photorefractive effects[8] dominated by the orienta-
tional birefringence contribution are maximised close to[9] or
somewhat beyond[10] the cyanine limit. Moreover, strongly
polar molecules, with ground states adequately described by
a balanced contribution of the CL and the ZW forms can
display high negative b values, but comparatively minor ef-
forts have been devoted to these RHS chromophores. This
is partly due to synthetic considerations, as to achieve the
required degree of charge transfer (CT) in the ground state,
one[11] or two[12] “proaromatic” fragments (those that gain
aromaticity upon charge separation, thereby stabilising the
ZW contribution) are usually needed. Whereas p-benzo-
Results and Discussion
Synthesis: The synthetic procedures for compounds 2 and 3
are shown in Scheme 3.
ACHTUNGTRENNUNGquinACHTUNGTRENNUNGoid and thienoquinoid moieties have been widely used
to that end, merocyanines with a formally quinoidal thiazole
ring have been scantly studied, and mainly in connection
with their photorefractive properties.[13]
In this work we focus our attention on new merocyanines
2–5 (Scheme 2), which are expected to be efficient RHS
NLO chromophores, due to the simultaneous presence of a
proaromatic donor and a quinoidal thiazole in their struc-
tures.
Besides 2a–d and 4a–c, endowed with 4H-pyran-4-ylidene
and 1,3-dithiol-2-ylidene electron donors,[14] respectively,
Scheme 3. Synthetic routes to compounds 2 and 3.
Compounds 2a–c were prepared by the acetic anhydride
mediated reaction of 2-dicyanomethyl-4-phenylthiazole (1,
known to exist predominantly as its tautomer 1’)[15] with the
commercially available pyranone 7 or the 4H-pyranylidene
aldehydes 8b and c.[14c,16] A different approach was adopted
for the longer pyran derivative 2d. Thus, the reaction of 1
with glutaconaldehyde dianilide hydrochloride afforded the
previously unreported acceptor 9, which was reacted with
the pyrylium salt 10[17] in the presence of triethylamine to
give 2d. The reaction of 2a with butylamine in ethanol af-
forded the pyridinium derivative 3. This heteroatom ex-
change reaction has been widely applied to pyrylium salts,[18]
and, in some cases, to push–pull 4H-pyranylidene deriva-
tives.[19] Compounds 4a–c were synthesised in an analogous
Scheme 2. Structures of compounds 2–6.
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827

J. Garꢁn et al.
ꢀ
Figure 1. Molecular structure of 6a; selected bond lengths [ꢄ]: C8 C11
ꢀ
ꢀ
ꢀ
ꢀ
1.419(4), C11 C12 1.375(3), C14 C15 1.396(4), N2 C14 1.357(3), N1 C5
1.343(4).
ꢀ
C14 C15 distance (1.396 ꢄ) is considerably longer than the
analogous bond in tetracyanoquinodimethane (TCNQ;
1.373 ꢄ),[25] thus confirming a certain zwitterionic (dicyano-
methanide) contribution to the ground state of 6a. More-
over, Figure 1 reveals that the C11=C12 double bond has Z
geometry, and that there is a large twist between the thi-
azole and the phenyl rings (dihedral angle N2-C13-C18-C23:
62.68). The steric hindrance between the substituent at 5-po-
sition of the thiazole ring and the phenyl group might be the
origin of such a large dihedral angle, because in 5-unsubsti-
tuted 4-phenylthiazoles the rings are essentially coplanar,[26]
whereas this is not the case for their 5-substituted ana-
logues.[27] Notably, the packing in the crystal is polar (in
keeping with the space group, P21), meaning that this mate-
rial will show bulk NLO activity. For the asymmetric unit
and absolute structure reported, the molecules are aligned
in such a way that all of the acceptor groups point to the
positive direction of the b axis, and the donor fragments all
point to negative b.
Scheme 4. Synthetic routes to compounds 4, 5, and 6.
way (Scheme 4) from 1 and salt 11[20] or 1,3-dithiole alde-
hydes 12b and c,[21] respectively, and gave good yields (81–
94%). For the synthesis of the thiophene derivative 5, the
new aldehyde 15 was first prepared from 13 and the tributyl-
phosphonium salt 14,[22] using a procedure already described
for related aldehydes.[23] The Knoevenagel reaction of 15
with 1 yielded the desired compound 5. Compounds 6a and
b (Scheme 4) were prepared using the same experimental
protocol, starting from commercially available aldehydes
16a and b (compound 6b had been already reported,[13b] but
not fully characterised).
Structural characterisation by X-ray crystallography: Crys-
tals of 6a suitable for crystallographic analysis were grown
by slow crystallisation from dichloromethane. The molecular
structure of 6a (Figure 1) shows that the p-system is nearly
coplanar (with a twisting angle between the mean planes of
the diethylaminophenyl and the thiazole rings of 6.38), and
Single crystals of compound 2c grown from acetone were
of more limited quality, resulting in weaker data. Neverthe-
less, some of the main features of its structure (Figure 2) are
similar to those of 6a.
ꢀ
ꢀ
that the C8 C11 bond is longer than the C11 C12 bond by
only ꢁ0.04 ꢄ.
Thus, the D-p-A system is essentially planar (angle of 9.58
between the mean planes of the pyran and the thiazole
rings), whereas the phenyl substituent is not coplanar with
the thiazole ring (dihedral angle N3-C23-C24-C29: ꢀ44.88).
The polyenic chain has an all-trans geometry (except for the
C17=C18 formal double bond of Z configuration), and its
This relative bond length equalisation implies a noticeable
intramolecular charge transfer (ICT) from the donor to the
acceptor end, which is confirmed on the donor side by the
2
ꢀ
short N1 C5 distance (1.34 ꢄ), by the sp hybridisation of
ꢀ
the diethylamino nitrogen (dihedral angles of ꢀ8.68 for C2-
N1-C5-C10 and ꢀ3.38 for C4-N1-C5-C6), and by the notice-
C C bonds are of the same length within experimental
error. This clearly points to a strong charge transfer from
the pyran to the acceptor end, and, therefore, to an impor-
tant contribution of the zwitterionic limiting form to the de-
scription of 2c.
ACHTUNGTRENNUNGable bond-length alternation of the amino-substituted
phenyl ring, which can be expressed through the dr parame-
ter (which is 0 for benzene and ꢁ0.10 for fully quinoid
rings).[12d,24] Thus, the dr value of 0.04 for the donor-substi-
tuted phenyl ring of 6a reveals its partly quinoid character
and supports the ICT process. The acceptor end of the mole-
cule also reveals the intramolecular charge transfer, as the
1H NMR studies: NMR spectra afford valuable information
about both the geometry and the ground-state polarisation
of the studied systems. Concerning their stereochemistry,
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Aromatic/Proaromatic Donors
FULL PAPER
an oscillatory behaviour, reflecting the alternation in the
electron densities of the carbon atoms to which they are
bonded, and characteristic of strongly polarised merocya-
3
N
ꢀ
protons of the =CH CH= spacer (2b: 13.4 Hz, 4b: 12.8 Hz
in CDCl3) reveal a relative equalisation of bond orders as a
result of this polarisation, and point to a decreasing contri-
bution of the zwitterionic form in the order 2>4. For the
longer derivatives this equalisation is confirmed by their low
DJ values (defined as the difference between the averaged
3
ꢀ
JHH values of the formal double and single C C bonds
along the polymethine chain).[31] Thus, whereas DJ ranges
from ꢁ0 Hz in cyanines to 6 Hz in polyenes, the observed
DJ values for 6b and 2c in CDCl3 are 2.6 and ꢀ0.5 Hz, re-
ꢀ
Figure 2. Molecular structure of 2c; selected bond lengths [ꢄ]: C13 C14
ꢀ
ꢀ
ꢀ
ꢀ
1.390(4), C14 C15 1.383(4), C15 C16 1.383(3), C16 C17 1.382(4), C17
C18 1.389(4).
3
spectively (the negative sign indicating larger JHH values for
the single bonds, as depicted in the neutral limiting form).
Compound 2c is therefore much more polarised than 6b,
and is, in fact, close to the cyanine limit in CDCl3. In the
much more polar solvent [D6]DMSO, 2c is further polarised
and becomes essentially zwitterionic, as shown by its DJ
value (ꢀ1.8 Hz).
3
analysis of JHH coupling constants of the polyenic deriva-
tives (2b–d, 4b,c, and 6a,b) indicates an all-trans geometry
along the spacer. Moreover, selective ge-1D NOESY experi-
ments (CDCl3, room temperature) carried out on model
compound 4b show that the thiazole exocyclic C=C bond
formed in the reaction has Z geometry, as there is a NOE
between Hortho of the phenyl group and its nearest methine
H atom. This stereochemistry mimics that observed by X-
ray diffraction in compounds 2c and 6a. On the other hand,
the complete configurational and conformational assignment
of compound 5 cannot be ascertained with the available ex-
perimental data. Thus, although selective ge-1D NOESY ex-
periments (CD2Cl2, room temperature) confirm the s-trans
conformation of the bond linking the thiophene to the meth-
ylenethiazole moiety, they were unsuccessful in determining
the stereochemistry of the C=C bond exocyclic to the thia-
zole ring, which we assume to be Z by analogy with the
structures of the remaining compounds. This configuration
not only avoids the steric hindrance caused by the phenyl
group in the alternative E isomer, but also gives rise to a
stabilising 1,5-S···S interaction between the sulfur atoms of
the thiazole and the thiophene rings. By the same token, the
conformation of the bond linking the thiophene and the di-
thiafulvene moieties can confidently be assumed to be s-
trans, given the well-established preference for such a geom-
etry in dithiafulvenylthiophenes.[23c,28] In fact, the calculated
distances of the intramolecular
Concerning the donor end, compounds 2 show two main
features. The first one is exemplified by solvent-dependent
1H NMR studies on model compound 2c. Given that the hy-
drogen atoms at the 3- and 5-positions of a pyranylidene
ring undergo downfield shifts as the pyrylium character of
the ring increases,[32] the increase in chemical shifts for such
protons on passing from CDCl3 (6.63 and 6.30 ppm) to
[D6]acetone (7.00 ppm, broad singlet) and [D6]DMSO
(7.35 ppm, singlet) reveals a higher contribution of the zwit-
terionic form as the polarity of the solvent increases. The
second, unexpected feature is that the chemical shifts (in
CDCl3) of these protons in series 2a–c increase on lengthen-
ing the spacer, which suggests that the contribution of the
zwitterionic form is higher for the longer derivatives, a con-
clusion also supported by IR studies, as discussed below
(overlapping of signals prevents conclusions being drawn for
2d). Moreover, an important zwitterionic character should
give rise to a single signal for the nonequivalent H3 and H5
protons of the pyran ring, due to the rapid rotation around
the essentially single bond linking the pyran moiety to the
rest of the molecule, whereas such hydrogen atoms would
be anisochronous in a compound with an essentially neutral
ground state.[19b,33] To study this effect, we recorded temper-
ature-dependent spectra of 2c in different solvents. In
[D6]DMSO at room temperature, H3 and H5 appear as a
singlet, which becomes narrower on heating to 343 K. On
the other hand, these protons are anisochronous at room
temperature in the much less polar solvent CDCl3. In
[D6]acetone, a solvent of intermediate polarity, anisochrony
is observed at 223 K, with the two well-resolved signals
giving rise to one broad signal at room temperature. These
results reveal: 1) restricted rotation around the pyranylidene
spacer bond, and, therefore, a balanced contribution of the
N and ZW forms to the description of compounds 2 in low-
polarity solvents, and 2) a zwitterionic ground state for these
push–pull systems in strongly polar solvents. The chemical
S···S contacts (Figure 3) are
nearly identical to those found
in related derivatives, and are
well below the sum of the van
der Waals radii of two sulfur
atoms (3.62 ꢄ).[29]
Both chemical shifts and JHH
3
coupling constants are known
to be sensitive to ground-state
polarisation. For compounds
ꢀ
Figure 3. Calculated C C bond
lengths of the thiophene ring
and intramolecular S···S distan-
ces [ꢄ] in compound 5.
with a polyenic fragment (2, 4
and 6), the chemical shifts of
the polymethine protons show
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829

J. Garꢁn et al.
shifts of H3 and H5 of the pyridine ring of 3 show a similar
dependence on the solvent, increasing from 6.76 (in CDCl3)
and 6.85 ppm (in CD2Cl2) to 7.13 (in [D6]acetone) and
7.16 ppm (in [D6]DMSO). Moreover, these hydrogen atoms
are isochronous in all these solvents at room temperature
(and at 223 K in CD2Cl2). These facts point to a free rota-
tion around the pyridine–thiazole bond, and suggest a pre-
dominant contribution of the pyridinium (zwitterionic)
form[34] to the description of 3.
On the other hand, compounds 4 are expected to lie
much closer to the cyanine limit, but still in the LHS. (The
very low BLA value calculated for 5 is mainly due to the
fact that the formally double C=C bonds of the thiophene
ꢀ
ring are longer than the formally single C C bond
(Figure 3). A similar pattern has been found both experi-
mentally[23c,39] and theoretically[40] for closely related deriva-
tives, which lends confidence to the results of our PCM cal-
culations). Compounds 2 show small but negative BLA
values, pointing to a zwitterionic (RHS) character, in good
1
Theoretical calculations: To gain a deeper understanding of
the structural features of the herein reported compounds,
we optimised their geometries using DFT at the B3P86/6-
31G* level in the gas phase. Given the marked effects that
solvents have on the structure and optical properties of
agreement with the structural and H NMR data.
Mulliken analysis of the ground and excited states of the
present compounds is in good agreement with the conclu-
sions drawn from geometric data. Thus, an increase of
charge separation upon excitation (calculated as the varia-
tion of the Mulliken charge on the donor, Dq(D)) occurs for
compounds 5 and 6, whereas the reverse trend is observed
for compounds 2 and, especially, 3 (Table 1). In turn, com-
pounds 4 show an intermediate behaviour, with only small
differences between the Mulliken charges of their ground
and excited states. Taken together, these data indicate that
the zwitterionic character of the ground state increases in
the series: 6<5<4<2<3. It is worth noting that most of
the charge is concentrated in the donor and acceptor ends
of these molecules, with the contribution of the polyenic
linker being very limited, although more appreciable on
lengthening the spacer. (This kind of charge distribution is
similar to that calculated for other strongly polarised chro-
mophores.[12c]) In particular, the negative charge spans the
dicyanomethylene group and the N atom of the thiazole,[13a]
although the total charge borne by the thiazole ring is very
small. This observation confirms both the strong electron-
withdrawing effect of this heterocyclic ring and the inability
of its sulfur atom to stabilise the negative charge in 2-thiazo-
lylmethanide anions.[41] In summary, experimental (crystal
structures, 1H NMR data) and theoretical evidence shows
that all these chromophores are strongly polarised, with pre-
dominantly neutral (zwitterionic) ground states for com-
pounds 5, 6 (2, 3), and a balanced contribution of both limit-
ing forms for compounds 4.
meroACHTUNGTRENNUNG
cyanines,[6,35] and in order to correlate the calculations
with most of the experimental measurements in this work,
we also carried out these optimisations at the same level in
dichloromethane using the polarised continuum model
(PCM).[36] Gas-phase and PCM calculations afford qualita-
tively similar trends for these compounds, although the in-
clusion of the solvent gives rise to more polarised and less
alternated structures, in agreement with previous re-
ports.[35b,37] Therefore, the following discussion will focus on
the PCM-optimised structures. Compounds 2–4 and 6 were
calculated as having the geometries depicted in Scheme 2,
which are supported by crystallographic and spectral data.
BLA has been widely used as a parameter reflecting the
ground-state polarisation of merocyanines. Because its defi-
nition is not univocal for the present compounds, in this
ꢀ
work BLA encompasses all the C C bonds of the acyclic
polyenic spacer linking the donor and the thiazole rings (in
ꢀ
the case of 5 the C C bonds of the thiophene ring are also
included). Moreover, positive (negative) BLA values indi-
cate a major contribution of the neutral (zwitterionic) form
to the ground state. Inspection of Table 1 shows that com-
pounds 6 are calculated as LHS chromophores with BLA
values close to the optimal one[38] (and in excellent agree-
ment with the results of X-ray diffraction data in the case of
6a).
Table 1. PCM-B3P86/6-31G* calculated parameters[a] and experimental
Vibrational spectroscopy: Infrared (IR) spectroscopy can
afford useful information concerning the degree of intramo-
lecular charge transfer of the merocyanines reported herein.
It is known that the frequency of the stretching vibrations of
ꢂ
n(C N) stretching bands.
BLA[b] [ꢄ]
q(D)[c] [e]
Dq(D)[d] [e]
n(CN)[e] [cmꢀ1
]
2a
2b
2c
2d
3
4a
4b
4c
5
–
+0.59
+0.54
+0.54
+0.52
+0.76
+0.37
+0.35
+0.31
+0.26
+0.42
+0.38
ꢀ0.10
ꢀ0.06
ꢀ0.06
ꢀ0.04
ꢀ0.19
0.00
+0.01
+0.02
+0.07
+0.11
+0.11
2198, 2178
2196, 2177
2188, 2174
2180, 2153
2187, 2162
2192, 2174
2196
2203
2196
2202
2200
ꢂ
the C N bonds in dicyanomethylene derivatives is highly
ꢀ0.010
ꢀ0.009
ꢀ0.007
–
sensitive to the electron density borne by the nitrile groups,
because such bonds are weakened upon reduction,[42] upon
complexation with electron donors,[43] or as a consequence
of ICT in the case of push–pull compounds.[44] For instance,
–
+0.007
+0.013
+0.004
+0.040
+0.032
ꢂ
phenylmalononitrile shows its C N stretching band at
2254 cmꢀ1, whereas its anion displays two bands at 2163 and
6a
6b
2117 cmꢀ1.[45] The values of the C N stretching vibrations of
ꢂ
compounds 2–6, measured by attenuated total reflectance
(Table 1), are generally lower than those of 1 (2212 and
2175 cmꢀ1), pointing to a noticeable degree of ICT in the
[a] In dichloromethane. [b] Along the polyenic spacer. [c] Mulliken
charge in the donor unit. [d] Change in the donor Mulliken charge on ex-
citation. [e] Measured by attenuated total reflection infrared spectrosco-
py of pure solids.
ꢂ
studied compounds. [The comparison between the C N
830
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Chem. Eur. J. 2011, 17, 826 – 838

Aromatic/Proaromatic Donors
FULL PAPER
ꢂ
stretching bands of 2–6 and those of 1 is, nevertheless, com-
plicated by the fact that in tautomer 1’ a different contribu-
tion to the charge transfer takes place, that is, from the thia-
zole N atom to the dicyanomethylene group. This process,
which cannot occur in 2–6, could eventually modify the two
ꢂ
the coupling between the two C N stretching modes is high,
resulting in the appearance of two well-separated vibrational
bands. On the other hand, for compounds such as 5 and 6
the lower degree of ICT should lead to more planar dicya-
nomethylenethiazole moieties and to a smaller coupling be-
ꢂ
n
(C N) vibrations of 1, one of them resulting in a low
tween the nACTHNGUTERNNU(G C N) modes (mediated by the lesser amount of
energy (2175 cmꢀ1).] Moreover, the lower frequency of the
negative charge delocalised over the C(CN)2 group). The
ꢂ
n
N
local planar symmetry of the latter, together with the small
ꢂ
stronger electron-donating ability of the pyridylidene frag-
coupling force constant between the two n
N
ment relative to the pyranylidene one. Table 1 also reveals
two different trends concerning the dependence of n
on chain lengthening. Thus, for compounds 4, increasing the
length of the polyenic spacer gives rise to a frequency up-
gives rise to the appearance of a single IR-active band. The
fact that the X-ray-determined angle between the best-fit
planes of the thiazole and the dicyanomethylene groups is
larger for 2c (11.38) than for 6a (3.78) is in line with this ex-
planation.
shift of n
zwitterionic character) for the longer derivatives, as is usual-
ly the case for merocyanines given the growing difficulty of
separating charges over larger distances.[46] On the other
hand, lengthening the spacer in the 2 series leads to a fre-
ꢂ
Figure 4 displays the Raman spectra of the compounds of
the 2 series. The Raman band at 1382 cmꢀ1 in 2a, marked
with red arrows in Figure 4, is typical of an aromatic thia-
zole (this Raman band appears at 1381 cmꢀ1 in thiazole
itself),[49] and experiences a Raman frequency up-shift on
going to 2b, 2c, and 2d due to
quency downshift of the nACTHNUTRGENU(GN C N) vibration (Figure 4), which
the progressive conjugation of
the thiazole ring in larger p-
conjugated systems (this is re-
lated to the red-shift of the ab-
sorption band in the electronic
spectra with increasing spacer
length). Simultaneously, in 2b,
the band at 1360 cmꢀ1 is the
marker of the vinylene spacer
denoted with blue arrows (note
that this is absent in 2a, but ap-
pears in 2b), and multiplies its
bands from 2b to 2c and 2d.
Figure 4. Spectra of compounds 2 in the solid state as pure samples: ATR spectra on the left and 532 excitation
Raman spectra on the right. The methyl band is shaded in grey.
The multiplet of bands associat-
ed with the spacer is in agree-
ment with the lowering of the
BLA for the larger members
suggests an increased contribution of the zwitterionic form
for the longer derivatives. (IR spectra recorded in solution
show the same trend, thus ruling out intermolecular interac-
tions in the solid as a possible cause for this phenomenon).
This behaviour may be related to the greater extension of
the conjugation path, which facilitates polarisation of the p
system towards the acceptor end. Although this trend is un-
common, some examples are known in quinoid A-p-A com-
pounds,[42,47] and in strongly polarised and zwitterionic D-p-
A derivatives.[12d,f,48]
(i.e., length equalisation might promote stronger vibrational
coupling and band splitting). In symmetric polyconjugated
oligoenes, the increment of vinylene units (i.e., BLA lower-
ing) produces a simplification of the Raman spectrum due
to the symmetry of the molecules (ECC model).[50] Howev-
er, in asymmetrically substituted oligoenes, like those con-
sidered here, the absence of molecular symmetry gives rise
to a multiplication of Raman bands with the increasing
number of vinylene units.[51] In all cases, the spacer bands
are the strongest of the spectra, in line with these modes
It is also worth noting that some of the studied com-
bringing the larger polarisability changes along the nACHTUNGTRENNUNG(C=C/
ꢂ
ꢀ
pounds show two C N stretching bands, whereas others dis-
C C) modes to which the Raman intensity is particularly
play only one. This result is somewhat surprising, given that
an excess of electron density on the dicyanomethylene
moiety (as a result of CT or reduction) is known to cause a
ꢂ
sensitive. The bands around 1640 cmꢀ1 are due to the C C
ꢀ
pyran stretches, which are also downshifted in frequency
with the elongation of the spacer, possibly due to the same
effect of conjugation occurring through the thiazole ring.[52]
Figure S1 in the Supporting Information compares the
Raman spectra of compounds 2a, 4a, and 6a (i.e., without
vinylenic spacers). The bands of the thiazole moiety appear
at 1382 and 1375 cmꢀ1 for 2a and 4a, respectively, revealing
splitting of the nACHTUNGTRENNUNG(C N) vibrations. A tentative explanation is
as follows: the high extent of the ICT process in compounds
such as 2 and 3 gives rise to an easy rotation around the
bond linking the thiazole and the C(CN)2 groups, with the
result that the local planar symmetry is partially broken and
Chem. Eur. J. 2011, 17, 826 – 838
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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831

J. Garꢁn et al.
an aromatic thiazole character, whereas for 6a this band ap-
pears at over 1400 cmꢀ1, indicating a more quinoidal thia-
zole ring for this compound.
and acceptor end groups. These observed trends in Eox (Ered)
are confirmed by theoretical calculations (PCM, CH2Cl2),
which show that the EHOMO (ELUMO) values increase (de-
crease) with the length of the spacer. For any given donor,
the decrease in Eox is more noticeable than that in jEred j, in
agreement with the trends shown by the energy levels of the
frontier orbitals. The behaviour of compound 5 is somewhat
different because of the presence of a thiophene moiety in
the spacer. Thus, 5 not only shows an Eox value similar to
that of 4c, despite the longer conjugated path in the former,
but also exhibits a slightly increased jEred j value. These
facts indicate that the incorporation of the thiophene moiety
hinders the oxidation and reduction processes, a behaviour
that can be related to the delocalisation of the positive and
negative charges in the electrogenerated ion radicals. These
charges are expected to be better stabilised in 4c than in 5,
because, for the latter, such delocalisation implies disruption
of the aromaticity of the thiophene ring, leading to a higher
energetic cost. Similar trends have been observed for ex-
tended tetrathiafulvalenes containing polyene and thiophene
spacers.[54] On passing from 2a to 3 there is a shift of the Eox
value towards a less anodic potential, and a shift of the Ered
value towards a more cathodic potential, pointing to the su-
perior donor ability of the nitrogen heterocycle compared to
its oxygen analogue. This fact is again in good agreement
with the calculated trends for the corresponding EHOMO and
ELUMO values.
We observe in the Raman spectra the symmetric stretch-
ing modes of the n(CN) that always appear at higher fre-
quencies than the infrared bands arising from the antisym-
metric stretching modes. The Raman spectrum of the tetra-
cyanoethylene (TCNE) anion (Figure S2 in the Supporting
Information), in which each dicyanomethylene moiety bears
around ꢀ0.5 e, displays an intense band at 2196 cmꢀ1, which
can be taken as a reference or frontier value to determine if
the charge density over the dicyano group is greater or
lower than ꢀ0.5 e.[44,53] In fact, although the n(CN) Raman
band of 2a could not be recorded, the corresponding bands
for 2b, 2c, and 2d are measured at 2200, 2195, and
2186 cmꢀ1, respectively, indicating the increasing accumula-
tion of electron density on the dicyano group in that series.
This is in contrast to the almost constant value of the calcu-
lated Mulliken atomic charges on the donor group (+0.54 e
in 2b, +0.54 e in 2c, and +0.52 e in 2d), thus revealing the
electron-releasing effect of the spacer that increases its elec-
tron donor ability with the number of vinylene groups, in
nice agreement with the measured oxidation potentials,
which also decrease from 2a to 2d (see below). Also, the
calculated charges on the donor for compounds 4, 5, and 6
are below +0.50 e (lying between +0.3 and +0.4 e,
Table 1), and the corresponding Raman n(CN) bands are
higher than or equal to 2200 cmꢀ1. Thus, there is a reason-
A
Optical properties and solvatochromism: The UV/Vis ab-
sorption data of compounds 2–6 are collected in Table 2; in-
spection of this data reveals some common trends. All com-
pounds show strong intramolecular charge-transfer transi-
tions in the visible region, extending into the near-infrared
in some cases, and in every series the lmax values increase on
lengthening the spacer, showing large vinylene shifts (de-
fined as the shifts in lmax which result from increasing the
length of the conjugated chain by adding a vinylene group).
Thus, for compounds 2 and 4 in CH2Cl2 these shifts are
ꢃ100 nm and increase with the spacer length, suggesting
sition and the degree of zwitterionic character of the studied
compounds.[44] Compounds with a n(CN) Raman frequency
over ꢁ2200 cmꢀ1 are dominated by the neutral limiting
form (their ground states, to which Raman frequencies are
sensitive, being less polarised than the first excited states),
whereas the reverse holds for those samples with n(CN)
Raman frequencies below 2196 cmꢀ1. This correlation is sup-
ported by the experimentally determined sign of the mb
values of the compounds studied herein (see below).
Electrochemistry: The redox
properties of the NLO chromo-
Table 2. Electrochemical data,[a] EHOMO and ELUMO values,[b] and UV/Vis data.
phores
were
investigated
through cyclic voltammetry in
CH2Cl2 (Table 2).
[d]
Eox
[V]
Ered
[V]
EHOMO/ELUMO
[eV]
lmax (loge)[c]
[nm]
lmax
[nm]
Most of the compounds show
two irreversible waves, corre-
sponding to the oxidation of
the donor and the reduction of
the acceptor moieties, respec-
tively. For compounds 2, 4, and
6, lengthening the p-spacer
gives rise to a decrease of the
Eox and jEred j values, the easier
oxidation and reduction pro-
cesses on chain lengthening
pointing to a decrease in the in-
2a
2b
2c
2d
3
4a
4b
4c
5
+1.14
+0.93
+0.71
+0.54
+0.79
+1.21
+0.92
+0.73
+0.72
+1.05[f]
+0.85[f]
ꢀ0.94
ꢀ0.74
ꢀ0.65
ꢀ0.53
ꢀ1.33[f]
ꢀ0.87
ꢀ0.61
ꢀ0.51
ꢀ0.58
ꢀ0.72
ꢀ0.61
ꢀ6.18/ꢀ3.59
ꢀ5.97/ꢀ3.66
ꢀ5.77/ꢀ3.71
ꢀ5.63/ꢀ3.78
ꢀ5.80/ꢀ3.12
ꢀ6.28/ꢀ3.68
ꢀ6.04/ꢀ3.77
ꢀ5.89/ꢀ3.85
ꢀ5.84/ꢀ3.87
ꢀ6.09/ꢀ3.69
ꢀ5.92/ꢀ3.78
550 (4.52), 583 (4.51)
634 (4.71), 683 (4.85)
726 (4.80), 790 (5.00)
734 (sh), 821 (4.90), 900 (5.12)
533 (4.68)
561 (sh), 596 (4.72)
648 (4.63), 699 (4.82)
734 (4.74), 803 (4.88)
725 (sh), 795 (4.76)
598 (sh), 638 (4.86)
679 (4.76), 732 (4.83)
564
666
756
[e]
494
587
695
795
822
657
754
6a
6b
[a] 10ꢀ3 m in CH2Cl2 versus Ag/AgCl (KCl 3m), glassy carbon working electrode, Pt counter electrode, 208C,
0.1m NBu4 PF6, 100 mVsꢀ1 scan rate. Ferrocene internal reference Eox =+0.51 V. [b] Calculated at the PCM-
B3P86/6-31G*//B3P86/6-31G* level in CH2Cl2. [c] In CH2Cl2. [d] In DMSO. [e] Decomposes. [f] Reversible
teraction between the donor wave.
832
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Chem. Eur. J. 2011, 17, 826 – 838

Aromatic/Proaromatic Donors
FULL PAPER
ꢀ
that the ground states of these merocyanines are close to
the cyanine limit in this solvent. Concerning compounds 6,
their vinylene shift in CH2Cl2 is lower (94 nm), indicating a
somewhat higher electronic asymmetry for their ground
states. A more precise characterisation of such states for the
studied compounds follows from solvatochromic studies (see
below). Compound 5 is the only exception to the classical
trend (the longer the p-system the more bathochromic), be-
cause, despite having an effective conjugation length greater
than that of 4c by one double bond, it has a blue-shifted ab-
sorption (in dichloromethane) relative to the latter, a behav-
iour that has been noted previously in related push–pull
compounds.[55] Again with the exception of 5, TDDFT calcu-
lations correctly reproduce the experimentally observed
bathochromic shifts on increasing the conjugation length, as
shown in Table 3. Comparison of the lmax values of 2a and 3
reveals that the latter is more blue-shifted than the former.
This fact, together with the calculated lower oscillator
strength (f) of 3 and its much larger negative change in
dipole moment upon excitation (see below), confirms[5a] that
3 is more zwitterionic (more RHS) than 2a, in agreement
with the results of the Mulliken analysis.
metric C C valence vibration of the polymethine chain in
the electronic excited state (S1),[56] and on the variation of
the spectral shape in different solvents, that is, an increase
in the lower energy band at the expense of the higher
energy band on increasing the polarity of the solvent[57] (see
Figure S3 in the Supporting Information). Moreover, the
concentration-dependent UV/Vis spectra of 2b in the low-
polar solvent dioxane (Figure S4 in the Supporting Informa-
tion) allow us to rule out the formation of H-aggregates as
the origin of the higher energy band.[39,58] On the contrary,
compounds 6 show positive solvatochromism and a similar
behaviour of their vibronic bands, in agreement with a pre-
vious study on a closely related derivative investigated as a
solvatochromic probe.[59] Therefore, the ground states of 6
are expected to be well represented by a mixed contribution
of the neutral and the cyanine limit forms. The solvato-
chromism of compounds 4 is less clear-cut because: 1) their
solvatochromic shifts are slightly negative, as judged from
their lmax values, but the use of such values to analyse sol-
vent effects can be incorrect when the absorption bands are
structured,[57b] and 2) their more limited solubility has pre-
cluded a full comparative study. In any case, the calculated
change in dipole moment upon excitation is negative for 4a,
only slightly negative for 4b, and positive for 4c and 5; 5
displays a clear positive solvatochromism, thus rendering it
more red-shifted than 4c in DMSO.
Table 3. Experimental and CPHF-calculated NLO properties, and
TDDFT-calculated[a] parameters.
mb
mb0
mb0
mg
E
[eV]
f
Dmge(z)
[D]
[10ꢀ48 esu][b] [10ꢀ48 esu][c] [10ꢀ48 esu][d] [D]
Nonlinear optical properties: The second-order NLO prop-
erties of 2–6 have been measured by electric-field-induced
second harmonic generation (EFISH) in CH2Cl2 at 1907 nm.
The measured values of the figures of merit, mb, together
with the corresponding static mb0 values calculated using the
two-level model (TLM),[60] are gathered in Table 3 (for the
sake of comparison Disperse Red 1, a common benchmark
for NLO chromophores, gives a mb0 value of ꢁ480ꢅ
10ꢀ48 esu under the same experimental conditions). TLM is
widely used to estimate zero-frequency mb0 values from
single-frequency measurements, but, strictly speaking, this
model can only be applied to one-dimensional systems in
which one excited state dominates the nonlinearity. As dis-
cussed previously, most compounds studied herein exhibit
two bands in their UV/Vis absorption spectra, thus making
the calculation of mb0 values problematic. Therefore, the
TLM-derived mb0 values in Table 3, calculated using the
lowest energy absorption maxima, should be treated with
caution, although they can be useful for comparison purpos-
es. The mb0 values calculated using coupled perturbed Har-
tree–Fock (CPHF) theory (PCM in CH2Cl2) are also collect-
ed in Table 3. Although such values are clearly overestimat-
ed due to the inclusion of solvent effects,[61] they correctly
reproduce the experimental trends (except for compound
4b).
2a
2b ꢀ1070
2c
ꢀ3900
ꢀ360
ꢀ200
ꢀ450
ꢀ3881
ꢀ9654
ꢀ24443
ꢀ54058
ꢀ4729
ꢀ1659
ꢀ1859
+2628
+4788
+2477
+7601
23.63 2.32 0.89 ꢀ4.78
28.97 2.07 1.43 ꢀ3.84
34.88 1.83 1.92 ꢀ3.74
40.40 1.64 2.44 ꢀ3.46
31.92 2.32 0.81 ꢀ7.89
22.19 2.28 0.83 ꢀ1.02
26.95 2.00 1.32 ꢀ0.38
30.68 1.78 1.82 +0.53
21.15 1.69 1.20 +2.32
23.97 2.13 1.36 +2.08
28.19 1.88 1.83 +2.56
ꢀ1010
ꢀ3200[e]
ꢀ730
2d ꢀ38000
3
4a
4b
ꢀ1150
ꢀ210
+300
ꢀ115
+120
+460
+1370
+790
+2080
4c +1900
+5440
5
6a +1610
6b +5940
[a] Calculated at the PCM-B3P86/6-31G*//B3P86/6-31G* level in CH2Cl2.
[b] mb values determined in dichloromethane at 1907 nm (experimental
accuracy ꢄ10%). [c] Experimental mb0 values calculated using the two-
level model. [d] Calculated at the PCM-CPHF/6-31G*//6-31G* level in
CH2Cl2. [e] The proximity to harmonic resonance makes this extrapolat-
ed value questionable.
Compounds 2 and 3 show negative solvatochromism in
the solvents studied. The blue shift in lmax values on increas-
ing the solvent polarity reveals a decrease in the dipole
moment upon excitation, and is in agreement with the re-
sults of TDDFT calculations (Table 3), which predict that
the ground states should be more polar than the excited
states for these derivatives. Except for 3, all the studied
compounds present structured absorption bands in their
spectra in CH2Cl2, their absorption curves displaying a defi-
nite shoulder or a second band on the high-frequency side,
which can be attributed to a vibronic transition. This assign-
ment is based on the energy spacing between these sub-
bands (1100ꢄ100 cmꢀ1), corresponding to the totally sym-
In the following discussion, the experimental measure-
ments will be compared to the results of TDDFT calcula-
tions, which give a useful insight into the factors responsible
for the NLO response by providing the parameters involved
in the two-level model. Within this approximation, b0 /
Chem. Eur. J. 2011, 17, 826 – 838
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833

J. Garꢁn et al.
Dmgef/E3, in which Dmge is the difference between the excit-
ed-state and ground-state dipole moments (me and mg, re-
spectively), f is the oscillator strength, and E is the excita-
tion energy. Due to the lack of colinearity of the me and mg
vectors for some of these compounds, we have included in
Table 3 the Dmge(z) parameter, defined as the projection of
Dmge onto the ground-state dipole moment direction, arbitra-
rily named z.
with the same conjugation length[22] show much higher
second-order NLO responses. This fact reveals that the elec-
tron-withdrawing effect of thiazole 1, pushing the ground
states of 4 closer to the cyanine limit, is stronger than that
of TCF. This conclusion is also supported through compari-
son of compounds 2 (with negative mb values) to their less
polarised pyran/TCF analogues, which show positive figures
of merit.[14c] Thus, whereas TCF excels at achieving very
high NLO responses in LHS chromophores, the combination
of the doubly proaromatic character of the pyranylidene
and the dicyanomethylenethiazole fragments seems to be a
promising approach towards the design of RHS chromo-
phores. Moreover, our results on compounds 2–6 confirm
the important effect that the different aromatic stabilisation
energies of the rings along the conjugated bridge have on b,
as suggested by the recent proposal of modulation of conju-
gation.[63]
It can be seen that compounds 2 and 3 show negative mb
values, and that lengthening the spacer gives rise to a steep
increase in the absolute values of the corresponding NLO
responses. These experimental trends are supported by cal-
culations, as Dmge(z) values are negative and, on passing
from 2a to 2d, there is a marked increase in f together with
an important decrease of E. These two factors strongly en-
hance b (and therefore mb), and more than compensate for
the decrease in the absolute values of the Dmge(z) parameter.
In fact, it should be noted that most of the mb enhancement
on lengthening the spacer is due to the hyperpolarisability
contribution, given the comparatively smooth increase in mg
values (also calculated by TDDFT). The larger negative
values measured in DMSO for 2b (in 10ꢀ48 esu: mb=ꢀ1200,
mb0 =ꢀ540) and 2c (in 10ꢀ48 esu: mb=ꢀ5960, mb0 =ꢀ1865)
clearly show that compounds 2 are located in region C of
Marderꢃs plot, approaching region D as the solvent polarity
increases. Moreover, compound 3, with much more negative
Dmge(z) and mb values than 2a, is more RHS than the latter,
thus confirming the higher donor ability of the pyridylidene
compared to that of the pyranylidene moiety.
On the other hand, the behaviour of compounds 6 is typi-
cal of predominantly neutral chromophores, with increasing-
ly positive mb0 values on lengthening the spacer, due to the
favourable trend of every parameter involved in the two-
level model. Their LHS character (A/B region) is confirmed
by EFISH measurements of 6b in DMSO, which show a de-
creased NLO response (in 10ꢀ48 esu: mb=+3040, mb0 =+
960) as the chromophore approaches the cyanine limit. Con-
cerning the dithiole derivatives, on passing from 4a (with a
small negative mb0 value) to 4b (with a low positive NLO re-
sponse) the cyanine limit is reached. In the case of 4c,
which shows a high positive mb0 value, this limit is clearly
surpassed to the left. Given that the mb values of 4b and c
become negative in DMSO (e.g., for 4b, mb=ꢀ900ꢅ
10ꢀ48 esu; mb0 =ꢀ370ꢅ10ꢀ48 esu), it is clear that these com-
pounds are located in regions B/C of Marderꢃs plot. Eventu-
ally, there is a sharp increase in the mb value on passing
from compound 4c to 5. The latter has a lower oscillator
strength and a much larger Dmge(z) than 4c, thus pointing to
a more neutral (less zwitterionic) chromophore lying in
region B. It is also worth noting that 5, with a higher mb
value, is also more transparent (in CH2Cl2) than its polyenic
analogue 4c.[55]
Conclusion
Merocyanines 2–5, featuring a quinoidal thiazole moiety and
a proaromatic donor, have been synthesised and studied,
and their properties compared to those of analogous systems
bearing aromatic donors (6). Whereas compounds 6 show
1
the typical features of neutral chromophores, H NMR and
UV/Vis spectroscopy, solvatochromism and nonlinear opti-
cal measurements, and theoretical calculations reveal that
pyranylidene- and pyridylidene-containing merocyanines (2
and 3, respectively) are essentially zwitterionic, and that di-
thiolylidene derivatives (4) constitute a borderline case,
lying closer to the cyanine limit. An intriguing feature of
compounds 2 is that, unlike most D-p-A systems, the degree
of ICT increases on lengthening the spacer, as disclosed by
NMR, IR, and Raman spectroscopy. The ground states of
the studied compounds are strongly solvent-dependent, and
EFISH measurements reveal that 2–6 can be located at dif-
ferent regions of Marderꢃs plot, from the left-hand side A/B
region (6,5) to B/C (4), and eventually to the right-hand side
C/D (2,3).
In short, the linking of the strong electron-withdrawing 2-
dicyanomethylenethiazole quinoidal moiety to a proaromat-
ic donor gives rise to cyanine-like or strongly dipolar com-
pounds displaying mb values as high as ꢀ38000ꢅ10ꢀ48 esu,
whereas the use of aromatic donors affords compounds with
highly positive mb values. Thus, the 2-dicyanomethylenethia-
zole fragment looks promising for modulating the sign of b
and for optimising zwitterionic, RHS NLO chromophores.
Experimental Section
Finally, it is instructive to compare the dicyanomethylene-
thiazole moiety studied herein and the widely used 2-dicya-
nomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran
(TCF) acceptor.[62] Thus, while compounds 4b and c have
low or moderate positive mb values, their TCF analogues
Starting materials: Compounds 1,[15] 8b,[16] 8c,[14c] 10,[17] 11,[20] 12b,[21a]
12c,[21b] and 14[22] were prepared as described previously.
X-ray analysis: X-ray data collection was carried out on an Xcalibur,
Sapphire 3 diffractometer equipped with a graphite monochromator
(MoKa radiation, l=0.71073 ꢄ). Cell dimensions were obtained by least-
834
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Chem. Eur. J. 2011, 17, 826 – 838

Aromatic/Proaromatic Donors
FULL PAPER
squares refinement of all measured reflections (HKL, Scalepack[64a]),
qmax =28.78. All structures were solved by direct methods (SHELXS-
97[64b]). All non-hydrogen atoms were refined anisotropically, and H-
atoms isotropically by full matrix least-squares with SHELXL-97[64c]
using experimental weights (1/[s2(Fo2)+(0.0115P)2] for which P=(F2o +
2F2c)/3.
ysis calcd (%) for C29H29N3OS: C 74.48, H 6.25, N 8.99; found: C 74.58,
H 6.02, N 8.76.
N-[(1E,3E,5Z)-5-(2-Dicyanomethylene-4-phenylthiazol-2-ylidene)penta-
1,3-dienyl]-N-phenylacetamide (9): A mixture of glutaconaldehyde diani-
lide hydrochloride (279 mg, 0.98 mmol), 1 (225 mg; 1 mmol) and anhy-
drous sodium acetate (82 mg, 1 mmol) in acetic anhydride (2 mL) was
heated at reflux under argon for 5–10 min. After cooling, the oily residue
formed was washed several times with hexane, and finally precipitated
with a mixture of hexane/ethanol 8:2. The resulting solid was filtered,
washed with cold hexane, and dried. Yield: violet solid (287 mg; 68%);
m.p. 227–2298C; 1H NMR (400 MHz, CDCl3): d=8.11 (d, J=13.6 Hz,
1H), 7.75–7.72 (m, 2H), 7.64–7.53 (m, 6H), 7.29 (d, J=12.0 Hz, 1H),
7.22–7.20 (m, 2H), 6.98 (dd, J=13.8 Hz, J’=11.5 Hz, 1H), 6.16 (dd, J=
13.8 Hz, J’=12.0 Hz, 1H), 5.30 (dd, J=13.6 Hz, J’=11.5 Hz, 1H),
1.96 ppm (s, 3H); 13C NMR (100 MHz, CD2Cl2): 181.6, 179.3, 169.7,
150.8, 141.9, 141.3, 138.8, 136.3, 132.8, 132.1, 131.1, 130.6, 130.2, 129.6,
128.9, 127.7, 124.5, 120.2, 115.6, 113.8, 113.7, 65.7, 23.7 ppm; IR (Nujol):
X-ray crystal structure of compound 6a: Single crystals were obtained by
slow crystallisation of 6a from dichloromethane at room temperature:
C23H20N4S, 0.5 (CH2Cl2), Mr =426.95; crystal dimensions 0.48ꢅ0.11ꢅ
0.04 mm; monoclinic space group P21 (no. 4); 1calcd =1.220 gcmꢀ3; Z=2;
a=5.6204(2), b=22.4214(11), c=9.2384(4) ꢄ; b=93.565(4)8; V=
1161.94(9) ꢄ3 at 293(2) K. Number of measured and unique reflections
23372 and 5219, respectively (Rint =0.0759). Final R(F)=0.0553,
wR(F2)=0.0614 for 282 parameters and 2464 reflections with I>2s(I)
(corresponding
0.0696).[65]
R values based on all 5219 reflections 0.1504 and
X-ray crystal structure of compound 2c: Single crystals were obtained by
slow crystallisation of 2c from acetone at room temperature:
C29H29N3OS. Mr =467.61; crystal dimensions 0.10ꢅ0.09ꢅ0.03 mm; triclin-
n˜ =2202, 1691, 1609, 1581 cmꢀ1
;
HRMS (ESI+): m/z calcd for
C25H19N4OS: 423.1274; found: 423.1283 [M++H]; elemental analysis
calcd (%) for C25H18N4OS: C 71.07, H 4.29, N 13.26; found: C 71.16, H
4.01, N 13.32.
ic space group P1 (no. 2); 1calcd =1.197 gcmꢀ3; Z=2; a=9.7569(8), b=
¯
10.3079(8),
c=14.6035(11) ꢄ;
a=96.296(6),
b=99.587(7),
g=
114.013(8)8; V=1296.94(18) ꢄ3 at 293(2) K. Number of measured and
unique reflections 22310 and 4640, respectively (Rint =0.1406). Final
R(F)=0.0497, wR(F2)=0.0420 for 307 parameters and 1463 reflections
with I>2s(I) (corresponding R values based on all 4640 reflections
0.2269 and 0.0594).[65]
2-{(Z)-5-[(2E,4E)-6-(2,6-Di-tert-butyl-4H-pyran-4-ylidene)hexa-2,4-dien-
ACHUTNGRENNUyG lACTHUNGTRENNUiG dene]-4-phenylthiazol-2-ylidene}malononitrile (2d): A solution of salt
10 (81 mg, 0.23 mmol), acceptor 9 (96 mg, 0.23 mmol), and triethylamine
(32 mL) in ethanol (2 mL) was heated at reflux under an argon atmos-
phere for 30 min (thin-layer chromatography monitoring). After cooling,
the resulting solid was isolated by filtration, washed with cold ethanol
and hexane, and dried. The crude product was purified by column chro-
matography on silica gel (eluent: CH2Cl2, then CH2Cl2/AcOEt 20:1, final-
ly CH2Cl2/AcOEt 20:1.5) to afford a green solid (27 mg, 24%). M.p. 202–
2068C; 1H NMR (400 MHz, CD2Cl2): d=7.71–7.69 (m, 2H), 7.56–7.50
(m, 3H), 7.42 (t, J=13.0 Hz, 1H), 7.37 (d, J=12.8 Hz, 1H), 7.16 (t, J=
12.6 Hz, 1H), 6.54–6.35 (m, 4H), 5.96 (d, J=13.2 Hz, 1H), 1.30 ppm (s,
18H); 13C NMR (75 MHz, CD2Cl2): d=173.0, 172.3, 154.2, 148.9, 146.9,
141.6, 133.7, 131.3, 131.2, 130.4, 129.3, 128.7, 127.9, 126.4, 117.0, 37.1,
30.3, 28.2 ppm; IR (Nujol): n˜ =2181, 2154, 1633, 1596, 1571 cmꢀ1; HRMS
(ESI+): m/z calcd for C31H32N3OS: 494.2261; found: 494.2269 [M++H];
elemental analysis calcd (%) for C31H31N3OS: C 75.42, H 6.33, N 8.51;
found: C 75.59, H 6.12, N 8.62.
2-[5-(2,6-Dimethyl-4H-pyran-4-ylidene)-4-phenylthiazol-2-ylidene]malo-
ACHTUNGTRENNUNGnoACHTUNGTRENNUNGnitrile (2a): A mixture of 1 (225 mg; 1 mmol) and 7 (143 mg;
1.15 mmol) in acetic anhydride (1.5 mL) was heated at reflux under an
argon atmosphere for 5 min, then stirred overnight at room temperature.
Hexane was added, and the resulting solid was filtered, washed with cold
hexane, and dried. Yield: dark violet solid (66 mg; 20%); m.p. 216–
2208C (decomp); 1H NMR (400 MHz, CDCl3): d=7.57–7.45 (m, 5H),
6.32 (s, 1H), 6.23 (s, 1H), 2.40 (s, 3H), 2.10 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=179.3, 173.0, 166.7, 163.6, 145.3, 134.6, 130.8,
129.1, 128.8, 119.9, 117.7, 115.3, 112.4, 108.7, 55.4, 20.5, 20.3 ppm; IR
(Nujol): n˜ =2197, 1647, 1554 cmꢀ1
;
HRMS (ESI+): m/z calcd for
C19H14N3OS: 332.0864; found: 332.0844 [M++H]; m/z calcd for
C19H13N3NaOS: 354.0672; found: 354.0659 [M++Na]; elemental analysis
calcd (%) for C19H13N3OS: C 68.86, H 3.95, N 12.68; found: C 69.11, H
3.67, N 12.47.
2-[5-(1-Butyl-2,6-dimethylpyridin-4-ylidene)-4-phenylthiazol-2-ylidene]-
malononitrile (3): Butylamine (0.5 mL; 5.06 mmol) was added to
a
warmed solution of compound 2a (50 mg; 0.15 mmol) in EtOH (1 mL) .
The mixture was heated at reflux under argon for 30 min, then cooled.
The resulting solid was filtered, washed with diethyl ether, and dried.
Yield: red solid (47 mg; 80%); m.p. 292–2938C; 1H NMR (300 MHz,
CD2Cl2, 08C): d=7.51–7.45 (m, 5H), 6.84 (s, 2H), 4.13–4.07 (m, 2H),
1.77–1.69 (m, 2H), 1.53–1.43 (m, 2H), 0.99 ppm (t, J=7.2 Hz, 3H);
13C NMR of 3 was not registered due to its low solubility; IR (Nujol): n˜ =
2189, 2163, 1631, 1550 cmꢀ1; HRMS (ESI+): m/z calcd for C23H23N4S:
387.1638; found: 387.1641 [M++H]; m/z calcd for C23H22N4NaS:
409.1457; found: 409.1434 [M++Na]; elemental analysis calcd (%) for
C23H22N4S: C 71.47, H 5.74, N 14.50; found: C 71.70, H 5.56, N 14.76.
General procedure for compounds 2b and c: A mixture of 1 (157 mg;
0.7 mmol) and the corresponding aldehyde (8b,c) (0.8 mmol) in acetic
anhydride (1 mL) was heated at reflux under an argon atmosphere for
5 min, then stirred overnight at room temperature. Hexane was added,
and the resulting solid was filtered, washed with cold hexane, and dried.
(Z)-2-{5-[2-(2,6-Di-tert-butyl-4H-pyran-4-yliden)ethylidene]-4-phenyl
ACHTUNGTNERtNUNG hi-
ACHTUNGTRENNUNGaACHTUNGTRENNUNGzol-2-ylidene}malononitrile (2b): Yield: green–blue microcrystalline
solid (145 mg; 47%); m.p. 296–2998C; 1H NMR (400 MHz, CDCl3): d=
7.75–7.71 (m, 3H), 7.58–7.49 (m, 3H), 6.47 (s, 1H), 6.36 (s, 1H), 1.32 (s,
9H), 1.31 ppm (s, 9H); 13C NMR (100 MHz, CDCl3): d=180.0, 174.4,
173.4, 172.9, 151.9, 137.4, 132.9, 131.2, 130.1, 129.4, 128.7, 117.6, 115.4,
113.8, 109.2, 102.3, 58.8, 36.8, 36.7, 27.9, 27.8 ppm; IR (Nujol): n˜ =2197,
2-[5-(4,5-Dimethyl-1,3-dithiol-2-ylidene)-4-phenylthiazol-2-ylidene]malo-
nonitrile (4a): A solution of salt 11 (227 mg; 0.86 mmol) and 1 (170 mg;
0.75 mmol) in a mixture of acetic acid (0.86 mL) and pyridine (0.083 mL)
was heated at reflux under argon for 30 min. After cooling, the resulting
precipitate was filtered, washed with water, then EtOH, and finally Et2O,
to afford a maroon solid (250 mg; 94%). M.p. 340–3458C (decomp);
1H NMR (400 MHz, CD2Cl2): d=7.63–7.51 (m, 5H), 2.16 ppm (brs, 6H);
13C NMR of 4a was not registered due to its low solubility; IR (Nujol):
n˜ =2191 cmꢀ1; HRMS (ESI+): m/z calcd for C17H12N3S3: 354.0188; found:
354.0181 [M++H]; m/z calcd for C17H11N3NaS3: 376.0007; found:
376.0000 [M++Na]; m/z calcd for C17H11N3KS3: 391.9747; found:
391.9740 [M++K]; elemental analysis calcd (%) for C17H11N3S3: C 57.76,
H 3.14, N 11.89; found: C 57.88, H 3.02, N 12.11.
2176, 1645, 1542 cmꢀ1
;
HRMS (ESI+): m/z calcd for C27H28N3OS:
442.1948; found: 442.1957 [M++H]; m/z calcd for C27H27N3NaOS:
464.1767; found: 464.1761 [M++Na]; elemental analysis calcd (%) for
C27H27N3OS: C 73.44, H 6.16, N 9.52; found: C 73.66, H 5.94, N 9.38.
2-{(Z)-5-[(E)-4-(2,6-Di-tert-butyl-4H-pyran-4-ylidene)but-2-enylidene]-4-
phenylthiazol-2-ylidene}malononitrile (2c): Yield: green microcrystalline
solid (173 mg; 53%); m.p. 282–2898C; 1H NMR (300 MHz, CDCl3,
ꢀ158C): d=7.65–7.62 (m, 2H), 7.55–7.41 (m, 5H), 6.72 (s, 1H), 6.40 (s,
1H), 6.22 (t, J=12.6 Hz, 1H), 6.03 (t, J=13.4 Hz, 1H), 1.32 (s, 9H),
1.31 ppm (s, 9H); 13C NMR (100 MHz, CDCl3): d=179.1, 172.9, 149.8,
148.8, 142.5, 133.0, 130.8, 129.9, 128.7, 124.4, 118.1, 116.6, 115.8, 36.7,
27.9 ppm; IR (Nujol): n˜ =2191, 1640, 1558, 1542 cmꢀ1; HRMS (ESI+):
m/z calcd for C29H30N3OS: 468.2104; found: 468.2106 [M++H]; m/z calcd
for C29H29N3NaOS: 490.1924; found: 490.1904 [M++Na]; elemental anal-
General procedure for compounds 4b and c: A mixture of 1 (86 mg;
0.38 mmol) and the corresponding aldehyde (12b,c) (0.43 mmol) in acetic
anhydride (1 mL) was heated at reflux under an argon atmosphere for 5–
Chem. Eur. J. 2011, 17, 826 – 838
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
835

J. Garꢁn et al.
10 min, then stirred overnight at room temperature. The resulting solid
was filtered, washed with cold hexane, and dried.
2-{(Z)-5-[(E)-3-{4-(diethylamino)phenyl}allylidene]-4-phenylthiazol-2-yli-
dene}malononitrile (6b): Yield: dark green microcrystalline solid
(190 mg; 46%); m.p. 189–1928C (decomp); 1H NMR (300 MHz, CDCl3):
d=7.80–7.75 (m, 2H), 7.60–7.45 (m, 6H), 7.71 (d, J=14.5 Hz, 1H), 6.79
(dd, J=14.5, 11.9 Hz, 1H), 6.70–6.66 (m, 2H), 3.48 (q, J=7.1 Hz, 4H),
1.25 ppm (t, J=7.1 Hz, 6H); 13C NMR (75 MHz, [D6]acetone): d=182.5,
178.5, 156.3, 153.9, 146.8, 135.2, 134.3, 134.2, 133.5, 132.0, 130.8, 125.5,
123.0, 118.1, 115.9, 114.1, 46.6, 13.9 ppm; IR (Nujol): n˜ =2204, 1609, 1573,
1540 cmꢀ1; HRMS (ESI+): m/z calcd for C25H23N4S: 411.1638; found:
411.1624 [M++H]; elemental analysis calcd (%) for C25H22N4S: C 73.14,
H 5.40, N 13.65; found: C 72.93, H 5.26, N 13.82.
(Z)-2-{5-[2-(4,5-Dimethyl-1,3-dithiol-2-ylidene)ethylidene]-4-phenylthi-
azol-2-ylidene}malononitrile (4b): Yield: green microcrystalline solid
(120 mg; 83%); m.p. 273–2758C; 1H NMR (400 MHz, CDCl3): d=7.73–
7.71 (m, 2H), 7.60–7.50 (m, 3H), 7.33 (d, J=12.8 Hz, 1H), 6.39 (d, J=
12.8 Hz, 1H), 2.20 ppm (s, 6H); 13C NMR of 4b was not registered due
to its low solubility; IR (Nujol): n˜ =2200, 1538 cmꢀ1; HRMS (ESI+): m/z
calcd for C19H14N3S3: 380.0344; found: 380.0332 [M++H+]; m/z calcd for
C19H13N3NaS3: 402.0164; found: 402.0149 [M++Na]; m/z calcd for
C19H13KN3S3: 417.9903; found: 417.9983 [M++K]; elemental analysis
calcd (%) for C19H13N3S3: C 60.13, H 3.45, N 11.07; found: C 60.29, H
3.66, N 10.83.
2-{(Z)-5-[(E)-4-(4,5-Dimethyl-1,3-dithiol-2-ylidene)but-2-enylidene]-4-
phenACHTUNGTRENNUNGylthiazol-2-ylidene}malononitrile (4c): Yield: green solid (125 mg;
81%); m.p. 238–2398C; 1H NMR (300 MHz, [D6]acetone): d=7.30–7.11
(m, 6H), 7.00–6.89 (m, 1H), 6.67 (d, J=12.9 Hz, 1H), 6.02 (t, J=12.5 Hz,
1H), 1.87 ppm (s, 6H); 13C NMR of 4c was not registered due to its low
solubility; IR (Nujol): n˜ =2206, 1560 cmꢀ1; HRMS (ESI+): m/z calcd for
C21H16N3S3: 406.0501; found: 406.0497 [M++H+]; elemental analysis
calcd (%) for C21H15N3S3: C 62.19, H 3.73, N 10.36; found: C 62.37, H
3.86, N 10.19.
Acknowledgements
We thank Prof. Larry Falvello (University of Zaragoza) for helpful dis-
cussions. Financial support from MICINN-FEDER (CTQ2008-02942,
MAT2008-06522C02-02 and CTQ2009-10098), Gobierno de Aragꢆn-
Fondo Social Europeo (E39) and Junta de Andalucꢁa (Proyecto de Exce-
lencia P09-4708) is gratefully acknowledged. The predoctoral fellowships
for E. Galꢀn (CSIC, JAE 2008) and R. Alicante (FPI BES2006-12104)
are also acknowledged.
5-[(4,5-Dimethyl-1,3-dithiol-2-ylidene)methyl]thiophene-2-carbaldehyde
(15): Triethylamine (1 mL; 7.17 mmol) was added to a solution of 13
(210 mg; 1.5 mmol) and 14 (717 mg; 1.5 mmol) in anhydrous THF
(50 mL). The mixture was stirred under an argon atmosphere at room
temperature for 24 h. The solvent was evaporated and CH2Cl2 (50 mL)
was added. The resulting organic layer was washed with water (3ꢅ
150 mL), dried (MgSO4), and evaporated. The crude product was purified
by flash chromatography (silica gel) using hexane/AcOEt (4:1) to afford
a maroon solid (289 mg; 76%). M.p. 127–1328C; 1H NMR (400 MHz,
CDCl3): d=9.82 (s, 1H), 7.65 (d, J=4.1 Hz, 1H), 6.87 (dd, J=4.1 Hz,
J’=0.5 Hz, 1H), 6.76 (s, 1H), 2.10–2.08 (m, 3H), 2.04–2.02 ppm (m, 3H);
13C NMR (100 MHz, CD2Cl2): d=182.3, 151.5, 142.7, 140.3, 137.7, 124.5,
124.0, 123.9, 104.9, 14.1, 13.3 ppm; IR (KBr): n˜ =1632 cmꢀ1; ESI-MS:
m/z: 255 [M+H+]; elemental analysis calcd (%) for C11H10OS3: C 51.93,
H 3.96; found: C 52.16, H 3.83.
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(s, 1H), 7.78–7.76 (m, 2H), 7.64–7.57 (m, 3H), 7.52 (d, J=4.5 Hz, 1H),
6.99–6.98 (m, 2H), 2.24 (s, 3H), 2.13 ppm (s, 3H); 13C NMR of 5 was not
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registered due to its low solubility; IR (Nujol): n˜ =2201, 1561 cmꢀ1
;
HRMS (ESI+): m/z calcd for C23H16N3S4: 462.0222; found: 462.0225
[M++H]; elemental analysis calcd (%) for C23H15N3S4: C 59.84, H 3.28,
N 9.10; found: C 59.97, H 3.16, N 9.22.
General procedure for compounds 6a and b: A mixture of 1 (226 mg;
1 mmol) and the corresponding aldehyde (16a,b; 1.77 mmol) in acetic an-
hydride (1.5 mL) was heated at reflux under an argon atmosphere for 5–
10 min, then stirred overnight at room temperature. The resulting solid
was filtered, washed with cold methanol, and dried.
(Z)-2-[5-(4-diethylaminobenzylidene)-4-phenylthiazol-2-ylidene]malono-
nitrile (6a): Yield: green microcrystalline solid (290 mg; 75%); m.p. 191–
1958C; 1H NMR (400 MHz, [D6]acetone): d=7.86–7.84 (m, 3H), 7.78–
7.73 (m, 2H), 7.71–7.61 (m, 3H), 7.05–7.01 (m, 2H), 3.66 (q, J=7.1 Hz,
4H), 1.27 ppm (t, J=7.1 Hz, 6H); 13C NMR (75 MHz, CDCl3): d=182.3,
179.4, 151.8, 143.0, 135.4, 132.2, 131.7, 130.4, 128.9, 120.8, 116.1, 114.0,
112.4, 45.2, 12.6 ppm; IR (Nujol): n˜ =2203, 1609, 1561 cmꢀ1; HRMS
(ESI+): m/z calcd for C23H21N4S: 385.1481; found: 385.1483 [M++H];
m/z calcd for C23H20N4NaS: 407.1301; found: 407.1277 [M++Na]; ele-
mental analysis calcd (%) for C23H20N4S: C 71.85, H 5.24, N 14.57;
found: C 71.70, H 5.11, N 14.71.
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ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 826 – 838

Aromatic/Proaromatic Donors
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Received: July 28, 2010
Published online: November 29, 2010
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