Synthesis and spectral properties of malononitrile-based merocyanine dyes

Article abstract of DOI:10.1007/s11172-006-0196-0

Di-, tetra-, and hexamethine merocyanines derived from malononitrile and heterocycles with moderate (dyes 1-6), strong (7-9), and weak (10 and 11) electron-releasing ability were synthesized. The electronic structures of merocyanines 10 and 11 are similar

Full text of DOI:10.1007/s11172-006-0196-0

2820
Russian Chemical Bulletin, International Edition, Vol. 54, No. 12, pp. 2820—2830, December, 2005
Synthesis and spectral properties of
malononitrileꢀbased merocyanine dyes
A. V. Kulinich,a N. A. Derevyanko,b and A. A. Ishchenkob
ꢀ
aTaras Shevchenko Kiev State University,
64 ul. Vladimirskaya, 01033 Kiev, Ukraine,
Eꢀmail: kulinich@univ.kiev.ua
bInstitute of Organic Chemistry, National Academy of Sciences of Ukraine,
5 ul. Murmanskaya, 02094 Kiev, Ukraine.
Fax: +380 (44) 573 2643. Eꢀmail: alexish@i.com.ua
Diꢀ, tetraꢀ, and hexamethine merocyanines derived from malononitrile and heterocycles
with moderate (dyes 1—6), strong (7—9), and weak (10 and 11) electronꢀreleasing ability were
synthesized. The electronic structures of merocyanines 10 and 11 are similar to the neutral
polyene state, whereas those of 7—9 are similar to the ideal polymethine state. These tendenꢀ
cies become more pronounced with increasing length of the polymethine chain. The
merocyanines derived from heterocyclic residues with weak or moderate electronꢀreleasing
ability exhibit a positive solvatochromism, whereas those with strong electronꢀreleasing ability
show a negative solvatochromism. An increase in the polarity of the solvent makes the former
compounds more similar to polymethines, whereas the latter become more similar to polyenes
bearing opposite charges on the end groups. The nature of the factors (nonspecific solvation,
specific nucleophilic and electrophilic solvation, and vibronic interactions) responsible for the
observed characteristic features was analyzed.
Key words: merocyanines, polyene and polymethine states, positive and negative solvatoꢀ
chromism, vibronic and intermolecular interactions.
Merocyanine dyes possess a broad spectrum of practiꢀ
marily, the band widths, provide information on the
changes in the bond orders and the electron density in the
dye molecules upon excitation, which enables one to estiꢀ
mate the direction of the changes in the vibronic and
intermolecular interactions, respectively.14 Hence, these
characteristics beneficially supplement the absorption
maxima λmax, which primarily characterize the relative
arrangement of the electron levels of the ground (S0) and
first excited (S1) states.
With the aim of revealing general relationships beꢀ
tween the electronic absorption spectra of merocyanines,
on the one hand, and their structures and the nature of
the solvent, on the other hand, in the present study we
synthesized and characterized dyes 1—11 derived from
malononitrile and containing heterocyclic residues havꢀ
ing different electronꢀreleasing properties.
The mathematical processing of the longꢀwavelength
absorption bands of the dyes was performed by the method
of moments.15 This allowed us to reliably and quantitaꢀ
tively characterize not only the positions (M–1) and inꢀ
tensities ( f ) of the bands but also their shape (σ, γ1, γ2,
and F ). The parameter M–1 averaged over all vibronic
transitions reflects the average position of the band (on
the wavenumber scale, this parameter is the center of
cally important properties, such as a pronounced solvatoꢀ
chromism and the ability to change substantially the diꢀ
pole moment upon electronic excitation and sensibilize
various physicochemical processes, due to which they have
found increasing use in optoelectronics,1,2 nonlinear opꢀ
tics,3 data recording and processing systems,4,5 medicine,
and biology.6
To construct merocyanines with desired properties, it
is necessary to reveal relationships between the absorpꢀ
tion spectra, on the one hand, and the structures of the
dyes and the nature of the solvent, on the other hand.
Attempts have been made to study these relationships for
incomplete series of merocyanines7—12 with the use of
absorption maxima λmax as the reference. However, λmax
do not always adequately reflect the spectroscopic feaꢀ
tures, because they can be associated with various vibronic
transitions.13 In addition, the absorption maxima are deꢀ
termined with insufficient accuracy for broad, diffuse,
and highly structured bands. However, these changes in
the band shape are more characteristic of merocyanines
than of the corresponding ionic dyes. The relationship
between the absorption band shape and the structure of
merocyanines remained unknown. The band shapes, priꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2726—2735, December, 2005.
1066ꢀ5285/05/5412ꢀ2820 © 2005 Springer Science+Business Media, Inc.

Merocyanines based on malononitrile
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2821
Intermediate hemicyanines were prepared according
to known general procedures.18
Since this approach proved to be inefficient in the
synthesis of dyes 7—10 because of the formation of the
corresponding symmetrical both cationic and anionic
polymethines, these dyes were prepared with the use of
another synthetic sequence (Scheme 2).
1—3: X = C(Me)2, R = Me; 4—6: X = S, R = Et;
7—9: X = NPh, R = Ph
1, 4, 7, 10: n = 1; 2, 5, 8, 11: n = 2; 3, 6, 9: n = 3
Scheme 2
–
gravity of the band: ν = 107/M–1) and, unlike λmax, enꢀ
ables one to make a reliable comparison of the curves of
different shape. The integrated intensity (the oscillator
strength f ) has an analogous advantage over the peak
intensity (the extinction ε). The parameter σ characterꢀ
izes the deviation (dispersion) of the points belonging to
–
the spectral band from the center of gravity ν. Hence, this
parameter allows one to make a reliable quantitative comꢀ
parison of the band widths regardless of the band shape,
which is an advantage over the half width (the width at
half height of the band) traditionally used for this purꢀ
pose. The asymmetry coefficient γ1, the excess coeffiꢀ
cient γ2, and the fine structure coefficient F provide addiꢀ
tional information on the band shape. These coefficients
give quantitative estimates of, respectively, the symmetry,
the steepness (peakedness), and structurization of the
bands. The parameters M–1, f, σ, γ1, γ2, and F are listed in
Table 1 along with λmax and ε. This table also gives the
radiation lifetimes (τr) of the S1 state of the dyes calcuꢀ
lated according to the equation14
The synthesis of intermediates has been docuꢀ
mented.16,19
Merocyanines can be considered as hybrids of two
symmetrical dyes, viz., cationic and anionic. In one of
them, the polymethine chain is longer than that in
merocyanine, whereas the chain in another dye is shorter.
In the present study, the deviations were evaluated with
the use of 12—22 as cationic dyes14 and 23—25 as anionic
dyes.16,19
–
2
τ = 1.5•109/(ν f ).
(1)
r
The synthesis of merocyanines 1—3 has been described
earlier.16,17 Dyes 4—6 and 11 were synthesized according
to Scheme 1 (m = 0—2, n = m + 1).
Scheme 1
The spectroscopic characteristics of cationic cyanines
12—22 were given in the monograph.14 The correspondꢀ
ing characteristics of anionic cyanines 23—25, as well as
of dyes 1—11, were determined by the mathematical proꢀ
cessing of their spectra by the method of moments
(Table 2).
Let us consider merocyanines 1—3 containing the
indolylidene moiety with moderate electronꢀreleasing
ability. First and foremost, let us determine the sign of the
solvatochromism because it reflects the direction of the

2822
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005
Kulinich et al.
Table 1. Characteristics of the longꢀwavelength absorption bands of merocyanines 1—11 in CH2Cl2, ethanol, and DMF
Comꢀ
pound
Solvent
λ
Dλ
ε•10–4
/L mol–1 cm–1
M–1
DM
f
τ
/ns
σ
γ
γ
2
F•102
max
r
1
/cm–1
nm
nm
1
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
437
435
438
527
525
530
602
611
624
453
451
453
545
543
548
640
644
650
433
428
430
532
524
526
638
628
630
556
520
555
519
558
522
592
591
598
12.5
10.0
10.3
22.0
15.5
12.0
51.5
30.5
20.5
1.5
0.5
1.3
10.5
6.5
3.0
19.5
9.0
4.0
4.5
4.5
3.3
3.5
4.5
5.0
7.25
7.37
8.18
8.29
8.88
9.96
7.77
8.60
9.47
7.82
8.37
7.39
12.89
15.44
14.41
10.50
13.19
12.70
7.54
419.5
419.2
421.9
509.5
504.9
513.5
573.3
574.3
585.8
436.0
434.7
436.9
525.3
526.7
530.4
602.3
609.6
618.2
421.6
416.0
417.7
520.6
510.7
512.6
625.2
609.8
611.4
515.7
14.1
11.8
10.6
24.1
21.0
13.0
62.0
44.9
34.0
7.2
4.7
4.7
16.0
7.2
4.8
40.7
18.5
9.0
5.2
6.7
7.0
–0.3
4.1
2.5
–0.5
2.7
0.89
0.93
0.99
0.95
0.97
1.00
1.15
1.30
1.26
0.73
0.78
0.68
1.11
1.20
1.09
1.19
1.23
1.07
0.71
0.80
0.73
1.08
1.13
1.02
1.31
1.25
1.15
0.57
3.0
2.9
2.7
4.1
4.0
4.0
4.3
3.8
4.1
3.9
3.7
4.2
3.8
3.5
3.9
4.6
4.6
5.4
3.8
3.3
3.6
3.8
3.5
3.9
4.5
4.5
4.9
7.1
1295
1311
1287
1244
1223
1175
1411
1464
1408
1112
1092
1088
1083
1023
1014
1263
1227
1192
1070
1104
1104
894
1.06
1.10
1.08
0.98
1.11
1.15
0.93
0.98
1.05
1.08
1.07
1.08
1.19
1.26
1.36
1.07
1.25
1.38
0.98
0.99
1.00
1.33
1.34
1.29
1.43
1.50
1.48
1.05
1.8
2.1
1.9
1.6
2.0
2.2
1.4
1.4
1.6
1.6
1.5
1.6
2.0
2.4
3.0
1.6
2.1
2.7
1.6
1.6
1.7
3.3
3.2
3.0
3.8
3.9
3.8
1.9
4.5
4.6
4.4
3.3
4.4
4.6
3.3
3.8
4.6
5.3
5.1
5.0
5.6
6.1
6.4
4.9
6.8
7.6
3.5
3.5
3.5
5.8
5.7
5.4
6.0
6.8
6.8
4.7
2
3
4
5
6
7
8
9
8.05
7.37
15.83
14.84
15.20
21.99
16.94
15.44
3.87
4.10
3.74
3.98
979
999
822
1034
1023
1446
CH2Cl2
EtOH
DMF
1.5
2.5
–1.0
–0.4
16.4
10
CH2Cl2
38
EtOH
DMF
514.6
518.7
15.7
15.1
0.56
0.53
7.1
7.7
1462
1426
1.07
0.99
2.0
1.6
4.8
4.3
34.5
3.58
3.77
5.98
5.77
35.5
69
62
11
CH2Cl2
EtOH
DMF
577.9
577.5
586.4
57.8
48.9
43.4
0.82
0.83
0.85
6.1
6.1
6.1
1425
1493
1430
0.89
0.99
0.89
1.4
1.9
1.4
3.1
3.8
3.3
60
6.03
Table 2. Characteristics of the longꢀwavelength absorption bands of anionic symmetrical dyes in CH2Cl2, ethanol, and DMF
a
Comꢀ
pound
Solvent
λ
ε•10–4
M–1
f
τ
/ns
σ
γ
γ
2
F•102
max
r
1
/nm /L mol–1 cm–1 /nm
/cm–1
23
24
25
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
CH2Cl2
EtOH
DMF
349
345
346.5
446
441
442
549
542
543
3.24
3.11
3.09
9.53
9.66
341.2
337.9
339.4
434.3
429.7
430.9
536.8
529.1
532.4
0.48
0.47
0.46
0.84
0.87
0.86
1.13
1.25
1.20
3.7
3.7
3.8
3.2
3.2
3.3
3.8
3.4
3.6
1557
1595
1541
1052
1060
1072
886
1.11
1.19
1.10
1.19
1.13
1.18
1.37
1.39
1.35
2.5
2.8
2.4
2.3
2.1
2.4
3.5
3.6
3.4
3.9
4.4
3.8
4.8
4.5
4.6
5.6
5.7
5.4
9.61
16.73
17.81
17.74
918
879

Merocyanines based on malononitrile
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2823
changes in the electronic structure, in particular, in the
dipole moment, upon excitation.
potentially prone to strong electrophilic solvation up to
hydrogen bonding. At the same time, the spectra of
merocyanines 1—3 illustrate the aboveꢀmentioned comꢀ
monly occurring problem of the incorrect determination
of the sign of the solvatochromism. For example, in the
case of merocyanine 1, the use of more polar ethanol
instead of CH2Cl2 causes a hypsochromic shift of λmax
and M–1 rather than a bathochromic shift, which is obꢀ
served upon the replacement of the solvent with DMF. It
could be concluded that these dyes show a negative
solvatochromism. However, this shift is due to the smaller
value of nD of ethanol compared to that of CH2Cl2. The
hypsochromic shift indicates that this factor dominates
over the effects of an increase in εD, B, and E on going to
ethanol.
The influence of the solvent increases with increasing
length of the polymethine chain. For example, λmax and
M–1 for merocyanine 1 in CH2Cl2 differ from those in
DMF by 1 nm (52 cm–1) and 2.4 nm (136 cm–1), reꢀ
spectively, whereas these differences for 3 are 22 nm
(586 cm–1) and 12.5 nm (372 cm–1), respectively. The
solvatochromism of merocyanines 1—3 has a sign oppoꢀ
site to that of the corresponding symmetrical both catꢀ
ionic (12—22)14 and anionic cyanines (23—25, see
Table 2) and asymmetrical cationic dyes constructed on
the basis of various combinations of heterocycles involved
in compounds 1—11.13
The solvatochromism of organic dyes occurs due to
nonspecific (universal) and specific interactions between
the dye and solvent molecules.14 The former are deterꢀ
mined by macroscopic parameters of the solvent (the reꢀ
fractive index nD and the permittivity εD), whereas speꢀ
cific interactions are determined by microscopic paramꢀ
eters (the nucleophilicity B and the electrophilicity E).14
The parameters nD and εD characterize the polarizability
and polarity of the solvent; the parameters B and E, speꢀ
cific electrostatic (polar) interactions between the distribꢀ
uted positive and negative charges in the dye molecules,
on the one hand, and the negatively and positively charged
ends of the dipoles of the solvent, on the other hand.
Dispersion interactions14 proportional to nD (see Ref. 20)
make the major contribution to nonspecific solvation
of dyes.
A strengthening of dispersion interactions with the
solvent (an increase in nD of the solvent) causes a
bathochromic shift of absorption bands. This is associated
with the fact that the S1 state is more polarized than S0,
due to which the energy level of S1 decreases with inꢀ
creasing nD of the solvent to a greater extent than the
energy level of S0.
A strengthening of specific electrostatic interactions
(nucleophilic and electrophilic solvation), unlike that of
dispersion interactions, can lead to both bathochromic
and hypsochromic shifts. This is associated with the fact
that the solvation is more substantial in the state that is
characterized by a larger dipole moment. In the case of
merocyanines, this is possible both in the ground and
excited states.20 Consequently, the parameters of nonspeꢀ
cific and specific solvation can influence the absorption
band shift both in the same and opposite directions deꢀ
pending on the electronic structures of merocyanines.
Hence, to unambiguously determine the type of solvatoꢀ
chromism of merocyanines, it is necessary to compare the
spectral shifts of the bands in pairs of solvents, which have
similar refractive indices nD and differ substantially in the
macroscopic (εD) and microscopic (B and E) polarity
parameters. In the present study, we used methylene chloꢀ
ride (nD = 1.4242, εD = 8.9, B = 23 cm–1, E = 2.7) and
DMF (nD = 1.4303, εD = 36.7, B = 291 cm–1, E = 2.6)14
as such solvents. The latter solvent is much more polar
and nucleophilic than the former solvent, the values of
their nD and E being similar. Consequently, the observed
batochromic shifts of the bands of merocyanines 1—3 on
going from CH2Cl2 to DMF are directly related to an
increase in the polarity of the solvent and are indicative of
a positive solvatochromism of these dyes.
The electronic structures of merocyanine dyes are
clearly described by a superposition of three main canoniꢀ
cal structures A1—A3 (Scheme 3)21 corresponding to three
ideal states, viz., neutral polyene (A1), polymethine (A2),
and charged polyene (A3).
Scheme 3
D and A are the donor and acceptor ends of the chromophore,
respectively
The positive solvatochromism of merocyanines 1—3
indicates that the dipole moment (the contribution of the
bipolar canonical structure) in the S1 state is larger than
that in the S0 state.
The vinylene shifts gradually decrease with increasing
length of the polymethine chain. This effect is more proꢀ
nounced in less polar CH2Cl2 than in DMF. For exꢀ
ample, the vinylene shifts in CH2Cl2 on going from
merocyanine 1 to 2 and 3 are, respectively, 90 and 75 nm
In addition to the aprotic solvents CH2Cl2 and DMF,
we also used ethanol as a typical protonꢀdonor solvent
(nD = 1.3611, εD = 24.3, B = 235 cm–1, E = 11.6).14 First
and foremost, ethanol is considered as a solvent, which is
as related to λmax and 90 and 63.8 nm as related to M–1
,
whereas these shifts in DMF are 92 and 94 nm for λmax
and 91.6 and 74.3 nm for M–1. These data show that the

2824
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005
Kulinich et al.
electronic asymmetry of the chromophore for dyes 1—3
in CH2Cl2 and, consequently, the bond order alternation
are more pronounced compared to those in DMF. This
situation should be observed only when the S0 state of
merocyanines 1—3 is similar to the structure A1. The fact
is that the strengthening of polar interactions facilitates
the charge separation in this structure, resulting in an
increase in the single bond order and a decrease in the
double bond order, i.e., in their equalization. In other
words, an increase in the polarity of the solvent leads to a
decrease in the contribution of the neutral polyene strucꢀ
ture A1 and an increase in the contribution of the polyꢀ
methine structure A2 to the S0 state, which is responsible
for an increase in the vinylene shifts. Their values in DMF
for merocyanines 1—3 are similar to those for the correꢀ
sponding symmetrical dyes (∼100 nm) (see Table 2 and
Ref. 14).
The deviations provide even stronger evidence that
the S0 state of merocyanines 1—3 is similar to the strucꢀ
ture A1. It is known that the deviations serve as a quantitaꢀ
tive measure of the electronic asymmetry of dyes, i.e., of
the bond order alternation in chromophore.9 As can be
seen from Table 1, the use of both strongly nucleophilic
DMF and strongly electrophilic ethanol instead of less
polar CH2Cl2 leads to a decrease in Dλ and DM. This
indicates that the electronic asymmetry becomes weaker
(the bond orders are equalized) with increasing polarity of
the solvent. Like the vinylene shifts, this effect is most
pronounced in DMF. It should be noted that DM are
more sensitive to the changes in the electronic asymmetry
of dyes than Dλ (see Table 1) due to the difference in the
band shape of merocyanines 1—3.
The deviations show that the electronic asymmetry of
dyes 1—3 increases in all solvents with increasing length
of the polymethine chain. This means that the contribuꢀ
tion of the polyene structure A1 increases in this series of
dyes. For example, the parameters Dλ and DM for comꢀ
pound 3, which exhibits the largest solvatochromic shifts
in the series of merocyanines 1—3, in DMF are 20.5 nm
(510 cm–1) and 34 nm (936 cm–1), respectively, whereas
these parameters in CH2Cl2 are 51.5 nm (1309 cm–1) and
62 nm (1702 cm–1).
A considerable contribution of the polyene state to the
structure of dye 3 is evidenced by the fact that the spinꢀ
spin coupling constants of the protons of the polymethine
chain in CDCl3 are different, whereas all these constants
in DMSOꢀd6 are 12.8 Hz. The constants J12, J23, J34, J45,
and J56 (12.7, 13.3, 12.0, 13.4, and 12.4 Hz, respectively)
completely reflect the character of this alternation in the
canonical structure A1. The conformation of dye 3 in
DMSOꢀd6 differs only slightly from that in CDCl3. The
numerical values of the spinꢀspin coupling constants of
the protons of its polymethine chain in both solvents, like
those for all other merocyanines, are no smaller than
12.8 Hz, which indicates that the molecules adopt an
allꢀtrans configuration.
The aboveꢀconsidered directions of the changes in the
electronic structures of merocyanines 1—3 should be
manifested also in a weakening of vibronic interactions as
the polarity of the solvent increases, because a strengthꢀ
ening of the polar interactions with the solvent leads to an
increase in the charge separation and equalization of
the bond orders in the S0 state of the polyene canonical
structure A1 (or a nearly resonance structure) due to which
it becomes similar to the polymethine structure A2 (or
a nearly resonance structure) observed in the S1 state.
The similarity between the electronic structures of dyes
1—3 in the S0 and S1 states will lead to a decrease in
the changes in the internuclear equilibrium distances
upon excitation and, consequently, to a weakening of
vibronic interactions.22 This should be reflected in the
narrowing of the absorption bands with increasing polarꢀ
ity of the solvent. Actually, the use of DMF instead of
CH2Cl2 as the solvent leads to a decrease in σ for
merocyanines 1—3.
However, it should be noted that the bands are less
narrowed than would be expected from the changes in the
deviations. In addition, the bands of dyes 1 and 3 in
ethanol are even slightly broader than those in CH2Cl2.
These effects can be attributed to the fact that the influꢀ
ence of the nature of the solvent on the absorption band
width is associated with the polar interactions between
the solvent molecules and the charges of the dye molꢀ
ecules only in the S0 state, because, according to the
Frank—Condon principle, light absorption is not accomꢀ
panied by changes in the geometry of this state. Since
merocyanines 1—3 are structurally similar to the unꢀ
charged polyene state (structure A1), such interactions in
the S0 state are substantially weakened. Since the contriꢀ
bution of the structure A1 increases with increasing n, the
weakest interactions are present in dye 3, which is evident
from the fact that the difference in σ in DMF is smaller
than that in CH2Cl2 (see Table 1). In DMF, nucleophilic
solvation makes the major contribution to polar interacꢀ
tions. The nucleophilicity of ethanol is smaller, though is
rather high, whereas the electrophilicity is substantially
larger compared to DMF. Hence, ethanol, unlike DMF,
can efficiently strengthen polar interactions with the dye
due both to nucleophilic and electrophilic solvation. In
merocyanines, whose electronic structures are similar to
the neutral structure A1, the donor fragment D in ethanol
will, with higher probability, undergo electrophilic rather
than nucleophilic solvation, because a lone pair is located
on this fragment. This solvation increases the degree of
localization, thus decreasing the electronꢀreleasing abilꢀ
ity of D with the resulting increase in the electronic asymꢀ
metry and, consequently, in the bond order alternation
and vibronic interactions. This is responsible for an

Merocyanines based on malononitrile
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2825
ε•10–5/L mol–1 cm–1
anomalous broadening of the absorption bands of dyes 1
and 3 in ethanol compared to CH2Cl2.
The deviations reflect not only the changes in the
bond order alternation and solvation of the charges in the
S0 state but also the relative arrangement of the energy
levels in the ground and excited states. Since the excited
state is more bipolar than the ground state, polar interacꢀ
tion play a substantial role in the excited state by decreasꢀ
ing its energy. Hence, the solvatochromic shifts and, conꢀ
sequently, the deviations are very sensitive to changes in
the polarity of the solvent even in the case of merocyanines
1—3, which are structurally similar to neutral polyene.
An increase in the length of the polymethine chain by
one vinylene group on going from merocyanine 1 to 2
leads to a narrowing of the absorption bands in all solꢀ
vents. This situation is analogous to that observed for the
corresponding cationic14 and anionic dyes (see Table 2).
A further elongation of the chain (the increase in n from 2
to 3) is accompanied by a substantial broadening of the
bands. However, the bands continue to narrow in the
spectra of the corresponding symmetrical cationic dyes in
CH2Cl2 and in the spectra of the anionic dyes in CH2Cl2,
ethanol, and DMF. This is associated with a weakening of
vibronic interactions in the symmetrical dyes of both types
with increasing n (a broadening of the bands of their
higher vinylogs is associated with a strengthening of speꢀ
cific intermolecular interactions). To the contrary, an inꢀ
crease in the length of the polymethine chain in meroꢀ
cyanines 1—3 should strengthen vibronic interactions beꢀ
cause it is accompanied by an increase in the contribution
of the polyene resonance structure A1, which is responꢀ
sible for an increase in the bond order alternation. Analyꢀ
sis of the changes in the vinylene shifts and the deviations
shows that the bond alternation in merocyanine 3 is much
more substantial than that in 2. This is indicative of a
strengthening of vibronic interactions, which play the
major role in a substantial broadening of the bands in
weakly nucleophilic and electrophilic CH2Cl2, strongly
nucleophilic DMF, and strongly electrophilic ethanol.
The fact that the polyene state tends to be transformed
into the polymethine state should be reflected also in the
band shape. A decrease in the internuclear equilibrium
distances upon excitation, which results in a weakenꢀ
ing of vibronic interactions, reduces, according to the
Frank—Condon principle, the probability of transitions
to the higher vibrational sublevels of the S1 state (shortꢀ
wavelength vibronic transitions) and, on the contrary,
increases this probability in the 0—0 transition region.14
Hence, the curve should become not only narrower but
also more asymmetrical (due to the presence of the shortꢀ
wavelength edge), more peaky (sharper), more structured,
and more intense. This situation is observed when CH2Cl2
is replaced with ethanol and DMF, as evidenced by an
increase in γ1, γ2, F, ε, and f (see Table 1). The positive
2.0
1
2
1.5
3
5
1.0
0.5
4
6
7
450
500
550
600
650
λ/nm
Fig. 1. Electronic absorption spectra of merocyanines 9 (1—3),
3 (4, 5), and 11 (6, 7) in CH2Cl2 (1, 4, 6), DMF (2, 5, 7), and
toluene (3).
sign of γ1 indicates that the asymmetry of the spectral
curve is due to the shortꢀwavelength edge (Fig. 1).
A weakening of vibronic interactions with increasing
polarity of the solvent is most pronounced (the largest
changes are observed for γ1, γ2, F, ε, and f ) in dye 2
because, apparently, the effects of shortꢀchain dyes (dye 1)
associated with the closelyꢀspaced end groups23 are abꢀ
sent in 2, whereas the partial charges are more pronounced
than those in the higher vinylog, viz., merocyanine 3.
In all solvents, the radiation lifetimes of the S1 state
increase with increasing length of the polymethine chain
in the series of dyes 1—3. On the one hand, an increase in
τr is retarded by an increase in the oscillator strength, but,
on the other hand, it is assisted by a decrease in the
wavenumber of the center of gravity of the band. Since
the latter parameter is included in Eq. (1) as a square, it
has a decisive effect on the change in τr. However, the
decrease in τr in polar solvents compared to CH2Cl2 is
determined primarily by f, because the solvatochromic
shifts –ν are substantially smaller than the vinylene shifts.
The electronꢀreleasing ability of the benzothiazolylꢀ
idene system is higher than that of the indolylidene sysꢀ
tem. Hence, the canonical structure A2 bearing the partial
positive and negative charges on the fragments D and A,
respectively, would be expected to make a larger contriꢀ
bution to the S0 state of merocyanines 4—6 than to the S0
state of dyes 1—3. Hence, the replacement of indolylidene
with benzothiazolylidene should lead to a decrease in the
electronic asymmetry of these merocyanines due to which
they become structurally similar to the polymethine state.
This is evidenced by the changes in virtually all main
parameters. In merocyanines 4—6, the larger values are
observed for the first and second vinylene shifts as related
to λmax and M–1 (by 92—95 and 91.6—93.5 nm for the

2826
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005
Kulinich et al.
first shifts and by 95—102 and 77—87.8 nm for the second
shifts).
vinylene shifts as related to λa
and M–1, respectively,
max
are not only larger than the analogous shifts for meroꢀ
cyanines 1—6 but also do not decrease with increasing
chain length; instead, these shifts are increased and beꢀ
come similar to the value typical of symmetrical cyanine
dyes (100 nm). A substantial symmetrization of the elecꢀ
tronic states of dyes 7—9 is also evidenced by small Dλ
and DM. The deviations show that these dyes become
more symmetrical with increasing length of the polyꢀ
methine chain. For 8 and 9, the parameters Dλ and DM
are almost equal to zero. It is also important that the
deviations calculated from λmax are virtually equal to those
calculated from M–1. This indicates that the band shapes
of merocyanines 7—9 are similar to those of the correꢀ
sponding symmetrical ionic cyanines (see Fig. 1).
These changes in the vinylene shifts and the deviaꢀ
tions for dyes 7—9 suggest that their electronic structures
in the S0 state are virtually identical to the ideal polyꢀ
methine structure, i.e., to the canonical structure A2. This
phenomenon is most pronounced for merocyanine 9 in
CH2Cl2, as evidenced by the maximum intensity of the
color characteristic of this state.21 Absorption of meroꢀ
cyanine 9 is observed at longerꢀwavelengths than that
of its analog 3, whereas the inverse situation is observed
for the corresponding pairs of lower vinylog, 1 and 7 (in
all solvents) and 2 and 8 (in ethanol and DMF) (see
Table 1).
The spectra of compounds 7—9, like the spectra of
typical symmetrical dyes, but in contrast to the spectra of
indolylidene and benzothiazolylidene 1—6, show a subꢀ
stantial narrowing of the bands. This narrowing is due to a
weakening of vibronic interactions, which typically ocꢀ
curs with increasing chain lengths of symmetrical polyꢀ
methine dyes.24 The band width for 9 in CH2Cl2 is only
822 cm–1, which is smaller even than that observed for
the corresponding symmetrical cationic (1080 cm–1) and
anionic (886 cm–1) polymethines. This is possible only
for the ideal polymethine state in which the bonds are
completely equalized. Actually, the spinꢀspin coupling
constants of the protons of the polymethine chain of
merocyanine 9 in CDCl3 are virtually equal to each other
(J12 = 13.1 Hz and J23 = J34 = J45 = J56 = 12.7 Hz). It
should be noted that this state is difficult to achieve even
in the case of symmetrical dyes because deviations of the
electronꢀreleasing (electronꢀwithdrawing) ability of the
end groups from the average value in either direction lead
to the bond order alternation from the heterocyclic moiꢀ
eties to the center of the chain.
There is also a tendency toward a decrease in the
solvatochromic shifts of dyes 4—6 compared to 1—3, but
the sign of the solvatochromism remains unchanged. The
parameters Dλ and DM for these dyes are 1.5—2 times
smaller than those for the indolylidene derivatives (see
Table 1). However, the deviations are also substantially
increased with increasing chain length, which is indicaꢀ
tive of an increase in the electronic asymmetry.
The fact that the bond orders in dyes 4—6 are more
equalized compared to those in 1—3 leads to a weakening
of vibronic interactions, which is reflected in the narrowꢀ
ing of the spectral curves of the former compared to the
latter. In all cases, their widths are smaller than those for
the corresponding indolylidene derivatives, and the difꢀ
ference increases with increasing n. For compounds 4—6,
the band width regularly decreases with increasing polarꢀ
ity of the solvent in the series of CH2Cl2, ethanol, and
DMF. This agrees well with a weakening of vibronic inꢀ
teractions in polar solvents. The influence of the polarity
of the solvent on σ of merocyanines 4—6 is associated
with a higher degree of bipolarity of their ground state
compared to compounds 1—3, with the resulting increase
in nucleophilic and electrophilic solvation of their molꢀ
ecules, which is responsible for a weakening of vibronic
interactions. As in the case of 1—3, the narrowest curves
are observed for tetramethine merocyanine 5. The changes
in σ for dyes 4—6 in different solvents show that ethanol
acts predominantly as a nucleophilic solvating agent with
respect to the fragment D of these dyes. This inverse
solvation effect compared to 1—3 is attributed to the higher
electronꢀreleasing ability of benzothiazolylidene and, conꢀ
sequently, the larger positive charge on this fragment due
to larger delocalization of the lone pair.
The tendency toward a change in the band shape is
less pronounced but also noticeable. For dyes 5 and 6, the
coefficients γ1, γ2, and F are larger than those for 2 and 3,
which is indicative of an increase in the asymmetry and
structurization of their spectra due to a weakening of
vibronic interactions.
The fact that the electronic structures of benzoꢀ
thiazolylidene merocyanines are similar to the polyꢀ
methine state is also evidenced by an increase in the moꢀ
lar extinction coefficients. For example, the peak intenꢀ
sity for merocyanine 5 is, on the average, oneꢀandꢀaꢀhalf
times higher than that for 2, and the oscillator strength is
larger by 7—10%.
The electronꢀreleasing ability of the benzimidazolylꢀ
idene moiety is much higher than that of the benzoꢀ
thiazolylidene fragment. Consequently, the electronic
symmetry of merocyanines 7—9 would be expected to be
even higher and their structures would be similar to the
ideal polymethine state. Actually, the first (96—99 and
94.7—99 nm) and second (104—108 and 98.8—104.6 nm)
In dyes 7—9, the sign of the solvatochromism is oppoꢀ
site to that in merocyanines 1—6. These compounds, like
the symmetrical parent dyes (see Ref. 14 and Table 2),
exhibit a weak negative solvatochromism, i.e., a hypsochꢀ
romic shift with increasing polarity of the solvent (see
Table 1). This is unambiguously confirmed by the results
of the replacement of CH2Cl2 with DMF. It is noteworꢀ

Merocyanines based on malononitrile
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2827
thy that the hypsochromic shifts in ethanol are larger than
those in DMF (see Table 1). This is associated with the
fact that the macroscopic and microscopic parameters of
the solvent have a correlated effect on the shift of the
absorption bands of dyes 7—9, as opposed to 1—6. The
negative solvatochromism signifies that the dipole moꢀ
ments in the S0 state of 7—9 are larger than those in the S1
state. Hence, an increase in the polarity parameters in the
series of CH2Cl2, ethanol, and DMF is accompanied by a
decrease in the energy of the ground state. The smaller
value of nD of ethanol compared to those of CH2Cl2 and
DMF is responsible for weaker dispersion interactions
with the solvent molecules in the excited state of the dye.
As a result, the level of the S1 state in ethanol decreases to
a lesser extent than that in CH2Cl2 and DMF. As a conseꢀ
quence, the hypsochromic shift in the CH2Cl2—ethanol
pair is larger than that in the CH2Cl2—DMF pair.
Based on the negative solvatochromism of meroꢀ
cyanines 7—9 and the fact that the dipole moment of the
ideal polymethine structure A2 is larger than that of A1
and is smaller than that of A3, it can be concluded that the
electronic structure of the S1 state of dyes 7—9 is intermeꢀ
diate between the structures A2 and A1. Hence, an inꢀ
crease in the polarity of the solvent is favorable for an
increase in the charge separation in the S0 state, due to
which it is intermediate between A2 and A3. This would
lead to the reverse bond order alternation in the polyꢀ
methine chain compared to that observed in meroꢀ
cyanines 1—3. Actually, the constants J12, J23, J34, J45,
and J56 (the atomic numbering scheme is given in the
Experimental section) in the 1H NMR spectrum of dye 9
in DMSOꢀd6 are 14.4, 12.0, 14.2, 12.2, and 13.2 Hz,
respectively. Consequently, the observed bond alternaꢀ
tion is characteristic of the structure of the charged polyꢀ
ene A3 (double—single, etc., as opposed to merocyanine 3
in CDCl3 characterized by the following alternation:
single—double, etc.).
In the case intermediate between A2 and A3, an inꢀ
crease in the polarity of the solvent should increase the
bond order alternation and the related effects of a deꢀ
crease in the vinylene shifts, an increase in the deviations,
and the band broadening. Actually, the replacement of
CH2Cl2 with ethanol and DMF leads to a decrease in the
first and second vinylene shifts and an increase in σ for
merocyanines 7—9, as opposed to 4—6 (see Table 1). The
bands become broader in this series of solvents with inꢀ
creasing chain length, which is indicative of an increase
in the role of polar interactions with the solvent. The
latter, in turn, causes a strengthening of vibronic interacꢀ
tions. The strengthening of intermolecular interactions is
confirmed by an increase in the negative solvatochromism
with increasing n. For example, in the series of dyes 7, 8,
and 9, the hypsochromic shifts of the absorption bands as
related to M–1 on going from CH2Cl2 to DMF are 221,
300, and 361 cm–1, respectively.
It is noteworthy that the replacement of CH2Cl2 with
less polar toluene, in spite of the fact that it is characterꢀ
ized by the substantially larger value of nD (nD = 1.4969,
εD = 2.38, B = 58 cm–1, E = 1.3),14 as well as the use of
more polar ethanol and DMF, are accompanied by a
hypsochromic shift, a broadening of the band of meroꢀ
cyanine 9, and a decrease in its intensity (see Fig. 1). The
fact that both an increase and a decrease in the polarity of
the solvent lead to changes in the spectral effects in the
same direction is additional evidence for the ideal polyꢀ
methine state of dye 9 in CH2Cl2 because deviations from
this state toward either neutral or charged polyene are
accompanied by an increase in the bond order alternation
and, consequently, by an enhancement of the aboveꢀdeꢀ
scribed spectral effects.
The maximum similarity between merocyanine 9 and
the ideal polymethine state is also evident from the largest
increase in the asymmetry and excess coefficients of the
bands in the series of the dyes under consideration.
Merocyanine 9, like compounds 7 and 8, gives a typical
absorption band shape with a pronounced vibration maxiꢀ
mum near the shortꢀwavelength edge, which is characterꢀ
istic of symmetrical polymethine dyes (see Ref. 14
and Fig. 1). In addition, the spectra of 9 are narrow due
to which this compound is characterized by the largest
molar extinction coefficient and the largest oscillaꢀ
tor strength. For merocyanine 9 in CH2Cl2, ε•10–4
=
21.19 L mol–1 cm–1 and f = 1.31, which are larger than
those for all corresponding symmetrical dyes.
The electronꢀreleasing ability of the benzo[c,d]inꢀ
dolylidene moiety, as opposed to the benzothiazolylidene
and benzimidazolylidene moieties, is smaller than that of
the indolylidene moiety.14 Hence, the electronic asymꢀ
metry of merocyanines 10 and 11 should be larger than
that of 1 and 2, 4 and 5, and 7 and 8. Based on this fact,
the electronic structures of dyes 10 and 11 would be exꢀ
pected to be even more similar to the polyene state. The
larger values of Dλ and DM for these dyes in CH2Cl2
(16.4 nm (598 cm–1) for 10 and 57.8 nm (1573 cm–1
)
for 11) compared to those observed for 1 and 2 suggest
that this is actually true. The smaller vinylene shifts
(70—72 nm for λmax and 62.2—67.7 nm for M–1) addiꢀ
tionally confirm this conclusion.
The high electronic asymmetry of these dyes is reꢀ
flected also in the solvatochromic shifts (∆λmax and ∆M–1
for 11 in CH2Cl2 and DMF are 6 nm (169 cm–1) and
8.5 nm (251 cm–1), respectively, whereas the correspondꢀ
ing values for indolylidene analog 2 are as small as 3 nm
(107 cm–1) and 2 nm (77 cm–1)). The band widths for 10
and 11 are also larger than those for 1 and 2, the decrease
in this value on going to tetramethine merocyanine 11
being much smaller than that observed in the analogous
series of indolylidene and benzothiazolylidene dyes.
It should be noted that the more pronounced polyene
character of merocyanines 10 and 11 than that of 1—3

2828
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005
Kulinich et al.
The 1H NMR spectra were measured on a Varian Merꢀ
curyꢀ400 spectrometer (400.39 MHz) with Me4Si as the internal
standard. Since it was impossible to make unambiguous assignꢀ
ment of the methine protons for compounds 3, 6, and 9 based on
the usual spectra, the COSY spectra were recorded. The protons
in the chains were numbered starting from the heterocyclic
moiety.
leads to a substantial band broadening on going from
CH2Cl2 to ethanol for the former compounds containing
the shorter polymethine chain compared the latter comꢀ
pounds (see Table 1). The parameter σ for tetramethine
merocyanine 11 increases by 68 cm–1, whereas this paꢀ
rameter for its analog 2 decreases by 21 cm–1, and it is
increased (by 53 cm–1) for hexamethine merocyanine 3.
These data can be interpreted as a result of larger electroꢀ
philic solvation of the benzo[c,d]indolylidene fragment,
which is caused by a decrease in the electronꢀreleasing
ability, resulting in an increase in the localization of the
lone pair.
In addition, the similarity to polyene causes a subꢀ
stantial decrease in the peak and integrated intensities of
the bands, a decrease in their asymmetry (a decrease in γ1)
and steepness (a decrease in γ2), and the appearance of
the bellꢀshaped character (a decrease in F ). Besides, the
spectra of the compounds, like those of carotinoids, show
a vibrational structure both at short and long wavelengths,
which is virtually independent of the nature of the solvent
(see Fig. 1).
Dyes 10 and 11 are characterized by substantially larger
τr than merocyanines 1—6. This is associated primarily
with their substantially deeper absorption region. Due to
the difference in ν– of dyes 1—11, it is difficult to reveal
relationships between their molecular structures and τr. It
can only be stated that these parameters are of the same
order of magnitude as the relaxation times of typical
polymethine dyes.14,25
Therefore, the electronic structures of merocyanines
1—11 can be smoothly varied from neutral polyene to the
ideal polymethine state by changing the electronꢀreleasꢀ
ing ability of heteroresidues (from weak to moderate and
strong), the polymethine chain length, and the polarity of
the solvent. This allows one to invert the spectral effects,
i.e., to change the sign of the solvatochromism, to inꢀ
crease or decrease the vinylene shifts and the deviations,
and to achieve a narrowing or broadening of absorption
bands, an increase or decrease in the asymmetry, steepꢀ
ness, and structurization of the bands, and a strengthenꢀ
ing or weakening of vibronic and intermolecular interꢀ
actions.
The melting points were measured in an open capillary tube
and are uncorrected.
2ꢀ[(2E,4E)ꢀ6ꢀ(1,3,3ꢀTrimethylꢀ2,3ꢀdihydroꢀ1Hꢀ2ꢀindolylꢀ
idene)ꢀ2,4ꢀhexadienylidene]malononitrile (3). 1H NMR
(DMSOꢀd6), δ: 1.565 (s, 6 H, CMe2); 3.350 (s, 3 H, NMe);
5.824 (d, 1 H, H(1), J = 12.8 Hz); 6.326 (t, 1 H, H(3), J =
12.8 Hz); 6.334 (t, 1 H, H(5), J = 12.8 Hz); 7.034 (t, 1 H, J =
7.8 Hz); 7.104 (d, 1 H, J = 7.8 Hz); 7.275 (t, 1 H, J = 7.8 Hz);
7.417 (d, 1 H, J = 7.8 Hz); 7.475 (t, 1 H, H(4), J = 12.8 Hz);
7.668 (t, 1 H, H(2), J = 12.8 Hz); 7.716 (d, 1 H, H(6), J =
12.8 Hz). 1H NMR (CDCl3), δ: 1.618 (s, 6 H, CMe2); 3.290 (s,
3 H, NMe); 5.598 (d, 1 H, H(1), J = 12.7 Hz); 6.210 (dd, 1 H,
H(3), J = 13.3 Hz, J = 12.0 Hz); 6.510 (dd, 1 H, H(5), J =
13.4 Hz, J = 12.4 Hz); 6.814 (d, 1 H, J = 8.0 Hz); 7.025 (t, 1 H,
J = 7.8 Hz); 7.058 (dd, 1 H, H(4), J = 13.4 Hz, J = 12.0 Hz);
7.228 (d, 1 H, J = 7.8 Hz); 7.268 (t, 1 H, J = 7.9 Hz); 7.344 (d,
1 H, H(6), J = 12.4 Hz); 7.367 (dd, 1 H, H(2), J = 13.3 Hz, J =
12.7 Hz).
2ꢀ[2ꢀ(3ꢀEthylꢀ2,3ꢀdihydroꢀ2ꢀbenzothiazolylidene)ethylꢀ
idene]malononitrile (4). A solution of 3ꢀethylꢀ2ꢀ[(E)ꢀ2ꢀmeꢀ
thyl(phenyl)carboxamidoꢀ1ꢀethenyl)]ꢀ1,3ꢀbenzothiazolium ioꢀ
dide18 (450 mg, 1 mmol) and CH2(CN)2 (130 mg, 2 mmol) in
pyridine (5 mL) was refluxed for 4 min. The product was preꢀ
cipitated with water (20 mL). After 2 h, the precipitate was
filtered off, washed, and dried. According to the results of
TLC and 1H NMR spectroscopy, no additional purification
of the product was required. The yield was 230 mg (90%),
m.p. 197—198 °C. Found (%): C, 66.44; H, 4.47; N, 16.31.
C14H11N3S. Calculated (%): C, 66.38; H, 4.38; N, 16.59.
1H NMR (DMSOꢀd6), δ: 1.365 (t, 3 H, CH2CH3, J = 7.1 Hz);
4.303 (q, 2 H, CH2CH3, J = 7.1 Hz); 5.949 (d, 1 H, H(1), J =
13.0 Hz); 7.296 (t, 1 H J = 7.6 Hz); 7.464 (t, 1 H, J = 7.7 Hz);
7.581 (d, 1 H, J = 8.1 Hz); 7.684 (d, 1 H, H(2), J = 13.0 Hz);
7.814 (d, 1 H, J = 7.9 Hz).
2ꢀ[(E)ꢀ4ꢀ(3ꢀEthylꢀ2,3ꢀdihydroꢀ2ꢀbenzothiazolylidene)ꢀ2ꢀ
butenylidene]malononitrile (5). A mixture of 3ꢀethylꢀ2ꢀ[(1E,3E)ꢀ
4ꢀmethyl(phenyl)carboxamidobutaꢀ1,3ꢀdienyl)]ꢀ1,3ꢀbenzoꢀ
thiazolium iodide18 (240 mg, 0.5 mmol) and CH2(CN)2 (66 mg,
1 mmol) was heated in pyridine for 5 min. The product was
filtered off and twice chromatographed on aluminum oxide
(chloroform as the eluent). The yield was 70 mg (40%),
m.p. 232—233 °C. Found (%): C, 68.55; H, 4.65; N, 15.09.
Experimental
C
16H13N3S. Calculated (%): C, 68.79; H, 4.69; N, 15.04.
1H NMR (DMSOꢀd6), δ: 1.322 (t, 3 H, CH2CH3, J = 7.1 Hz);
4.228 (q, 2 H, CH2CH3, J = 7.1 Hz); 6.221 (d, 1 H, H(1), J =
12.5 Hz); 6.276 (t, 1 H, H(3), J = 12.5 Hz); 7.232 (t, 1 H, J =
7.6 Hz); 7.414 (t, 1 H, J = 7.7 Hz); 7.425 (t, 1 H, H(2), J =
12.5 Hz); 7.467 (d, 1 H, J = 8.0 Hz); 7.585 (d, 1 H, H(4), J =
12.5 Hz); 7.741 (d, 1 H, J = 7.8 Hz).
2ꢀ[(2E,4E)ꢀ6ꢀ(3ꢀEthylꢀ2,3ꢀdihydroꢀ2ꢀbenzothiazolylꢀ
idene)hexaꢀ2,4ꢀdienylidene]malononitrile (6). A mixture of
3ꢀethylꢀ2ꢀ[(1E,3E,5E)ꢀ6ꢀmethyl(phenyl)carboxamidohexaꢀ
1,3,5ꢀtrienyl)]ꢀ1,3ꢀbenzothiazolium iodide18 (0.5 mmol) and
CH2(CN)2 (1.5 mmol) was heated over a short period of time
The absorption spectra were recorded on a Shimadzu
UVꢀ3100 spectrophotometer in 1ꢀcm cells. The solvents were
purified according to known procedures;26 CH2Cl2 was stabiꢀ
lized by adding 1% of anhydrous ethanol. The addition of ethaꢀ
nol has virtually no effect on the spectroscopic characteristics,
as exemplified by strongly solvatochromic dye 3. The moments
of absorption bands were determined by a method described
earlier.15 Chromatography was carried out on silica gel 60 and
aluminum oxide 80 (Merck). The purity of merocyanines was
checked by TLC (Silufol UVꢀ254, CH2Cl2 as the eluent).

Merocyanines based on malononitrile
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2829
(3—5 min). The product was purified by chromatography (once
on silica gel and twice on aluminum oxide) using CHCl3 as
the eluent. The yield was 30 mg (20%), m.p. 195—196 °C.
Found (%): C, 70.67; H, 4.97; N, 13.93. C18H15N3S. Calcuꢀ
lated (%): C, 70.79; H, 4.95; N, 13.76. 1H NMR (DMSOꢀd6), δ:
1.316 (t, 3 H, CH2CH3, J = 7.1 Hz); 4.167 (q, 2 H, CH2CH3,
J = 7.1 Hz); 6.039 (d, 1 H, H(1), J = 12.5 Hz); 6.15—6.32 (m,
2 H, H(3) + H(5)); 7.118 (t, 1 H, H(4), J = 12.6 Hz); 7.164 (d,
1 H, J = 7.7 Hz); 7.234 (t, 1 H, H(2), J = 12.6 Hz); 7.357 (m,
2 H); 7.529 (d, 1 H, H(6), J = 12.6 Hz); 7.652 (d, 1 H,
J = 7.8 Hz).
2ꢀ[2ꢀ(1,3ꢀDiphenylꢀ2,3ꢀdihydroꢀ2ꢀbenzimidazolylꢀ
idene)ethylidene]malononitrile (7). A mixture of 2ꢀmethylꢀ1,3ꢀ
diphenylbenzimidazolium chloride (26)13 (150 mg, 0.47 mmol)
and Nꢀ(2,2ꢀdicyanovinyl)ꢀNꢀphenylacetamide (27)16 (105 mg,
0.5 mmol) was heated in anhydrous ethanol in the presence of
Et3N for a few minutes. The precipitate was filtered off and
washed with ethanol. The yield was 90 mg (50%). The product
was chromatographed on Al2O3 (CH2Cl2 as the eluent). Brightꢀ
yellow crystals were obtained, m.p. > 260 °C. Found (%):
C, 80.15; H, 4.54; N, 15.70. C24H16N4. Calculated (%): C, 79.98;
H, 4.47; N, 15.54. 1H NMR (DMSOꢀd6), δ: 5.078 (d, 1 H,
H(1), J = 14.6 Hz); 6.286 (d, 1 H, H(2), J = 14.6 Hz); 6.98—7.08
(m, 2 H, half of an AA´BB´ system), 7.28—7.37 (m, 2 H, half of
an AA´BB´ system); 7.66—7.95 (m, 10 H).
H(6), J = 12.7 Hz); 6.89—6.94 (m, 2 H, half of an AA´BB´
system); 7.17—7.22 (m, 2 H, half of an AA´BB´ system); 7.53
(d, 4 H, J = 7.7 Hz); 7.67—7.76 (m, 6 H).
2ꢀ[2ꢀ(1ꢀBenzylꢀ1,2ꢀdihydrobenzo[c,d]indolꢀ2ꢀylidene)ethylꢀ
idene]malononitrile (10). A mixture of 1ꢀbenzylꢀ2ꢀmethylꢀ
benzo[c,d]indolium tetrafluoroborate (28)27 (104 mg) and comꢀ
pound 27 (63 mg) was heated in anhydrous ethanol in the presꢀ
ence of Et3N for a few minutes. After cooling, the precipitate
was filtered off and washed with ethanol and acetone. The prodꢀ
uct was purified by successive heating with different solvents
(acetone, MeCN, and AcOH) and then filtered off. The yield
was 32 mg (32%), m.p. > 260 °C. Found (%): C, 82.66; H, 4.58;
N, 12.44. C23H15N3. Calculated (%): C, 82.86; H, 4.54; N, 12.6.
1H NMR (DMSOꢀd6), δ: 5.392 (s, 2 H, NCH2); 6.229 (d, 1 H,
H(1), J = 13.3 Hz); 7.26—7.37 (m, 5 H, Ph); 7.389 (d, 1 H, J =
7.1 Hz); 7.557 (t, 1 H, J = 7.7 Hz); 7.618 (d, 1 H, J = 8.2 Hz);
7.788 (t, 1 H, J = 7.7 Hz); 8.105 (d, 1 H, J = 8.1 Hz); 8.591 (d,
1 H, H(2), J = 13.3 Hz); 8.616 (d, 1 H, J = 7.1 Hz).
1ꢀBenzylꢀ2ꢀ[(1E,3E)ꢀ4ꢀmethyl(phenyl)carboxamidoꢀ
1,3ꢀbutadienyl]benzo[c,d]indolium tetrafluoroborate (29).
Equimolar amounts of salt 28 and Nꢀ[(1E,2E)ꢀ3ꢀanilinoꢀ2ꢀ
propenylidene]aniline hydrochloride were heated (bath, 120 °C)
in a 1 : 1 Ac2O—AcOH mixture for 30 min. The product was
precipitated with diethyl ether and filtered off. In the next step,
the resulting hemicyanine was used without additional purifiꢀ
cation.
2ꢀ[(E)ꢀ4ꢀ(1,3ꢀDiphenylꢀ2,3ꢀdihydroꢀ2ꢀbenzimidazolylidene)ꢀ
2ꢀbutenylidene]malononitrile (8). Triethylamine was added to a
mixture of salt 26 (150 mg, 0.47 mmol) and Nꢀ[(1E)ꢀ4,4ꢀdiꢀ
2ꢀ[(E)ꢀ4ꢀ(1ꢀBenzylꢀ1,2ꢀdihydrobenzo[c,d]indolꢀ2ꢀylidene)ꢀ
2ꢀbutenylidene]malononitrile (11). A mixture of salt 29 (100 mg,
cyanobutaꢀ1,3ꢀdienyl]ꢀNꢀphenylacetamide16
(118
mg,
0.2 mmol) and malononitrile (25 mg, 0.4 mmol) was heated in
Ac2O in the presence of NaOAc for 5 min. The precipitate was
filtered off and washed with a minimum amount of Ac2O and
diethyl ether. The product was twice chromatographed on silica
gel (CH2Cl2 as the eluent). The yield was 11 mg (15%),
m.p. > 260 °C. Found (%): C, 83.67; H, 4.82; N, 11.57.
0.5 mmol) in anhydrous ethanol (2 mL) and the mixture was
heated for 2 min. The precipitate was filtered off and washed
with ethanol. The yield was 58 mg (32%). The product was twice
chromatographed on aluminum oxide (CH2Cl2 as the eluent).
Darkꢀblue crystals with a metallic luster were obtained in a yield
of 44 mg (24%), m.p. 250—251 °C. Found (%): C, 80.76; H, 4.75;
N, 14.71. C26H18N4. Calculated (%): C, 80.81; H, 4.69; N, 14.50.
1H NMR (DMSOꢀd6), δ: 5.274 (d, 1 H, H(1), J = 14.4 Hz);
5.583 (dd, 1 H, H(3), J = 13.4 Hz, J = 12.1 Hz); 6.212 (dd,
1 H, H(2), J = 14.4 Hz, J = 12.1 Hz); 6.210 (d, 1 H, H(4),
J = 13.4 Hz); 7.03—7.10 (m, 2 H, half of an AA´BB´ sysꢀ
tem); 7.32—7.38 (m, 2 H, half of an AA´BB´ system);
7.68—7.82 (m, 10 H).
C
25H17N3. Calculated (%): C, 83.54; H, 4.77; N, 11.69.
1H NMR (DMSOꢀd6), δ: 5.328 (s, 2 H, NCH2); 6.455 (d, 1 H,
H(1), J = 12.8 Hz); 6.595 (t, 1 H, H(3), J = 12.8 Hz); 7.08 (m,
1 H); 7.22—7.37 (m, 5 H, Ph); 7.46 (m, 2 H); 7.748 (t, 1 H, J =
7.7 Hz); 7.974 (d, 1 H, J = 8.0 Hz); 8.118 (d, 1 H, H(4), J =
12.8 Hz); 8.344 (t, 1 H, H(2), J = 12.8 Hz); 8.483 (d, 1 H,
J = 7.1 Hz).
2ꢀ[(2E,4E)ꢀ6ꢀ(1,3ꢀDiphenylꢀ2,3ꢀdihydroꢀ2ꢀbenzimidazolylꢀ
idene)ꢀ2,4ꢀhexadienylidene]malononitrile (9). A mixture of chloꢀ
ride 26 (160 mg, 0.5 mmol) and Nꢀ[(1E,3E)ꢀ6,6ꢀdicyanoꢀ1,3,5ꢀ
hexatrienyl]ꢀNꢀphenylacetamide16 (132 mg, 0.5 mmol) was
heated in anhydrous ethanol in the presence of Et3N for 5 min.
The product was twice chromatographed on aluminum oxide
(CH2Cl2 as the eluent). The yield was 37 mg (18%), m.p.
208—210 °C. Found (%): C, 81.52; H, 4.97; N, 13.55. C28H20N4.
Calculated (%): C, 81.53; H, 4.89; N, 13.58. 1H NMR
(DMSOꢀd6), δ: 5.367 (d, 1 H, H(1), J = 14.4 Hz); 5.639 (dd,
1 H, H(5), J = 13.2 Hz, J = 12.2 Hz); 5.744 (dd, 1 H, H(3), J =
14.2 Hz, J = 12.0 Hz); 6.198 (dd, 1 H, H(2), J = 14.4 Hz, J =
12.0 Hz); 6.282 (dd, 1 H, H(4), J = 14.2 Hz, J = 12.2 Hz); 6.892
(d, 1 H, H(6), J = 13.2 Hz); 7.05—7.12 (m, 2 H, half of an
AA´BB´ system); 7.33—7.40 (m, 2 H, half of an AA´BB´ sysꢀ
tem); 7.70—7.88 (m, 10 H). 1H NMR (CDCl3), δ: 5.100 (d,
1 H, H(1), J = 13.1 Hz); 5.649 (t, 1 H, H(5), J = 12.7 Hz); 6.024
(t, 1 H, H(3), J = 12.7 Hz); 6.196 (t, 1 H, H(4), J = 12.7 Hz);
6.260 (dd, 1 H, H(2), J = 13.1 Hz, J = 12.7 Hz); 6.922 (d, 1 H,
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Received March 9, 2005;
in revised form August 2, 2005