Synthesis and properties of merocyanine dyes derived from N-aryl-substituted pyridinium salts and cyanoacetic acids

Article abstract of DOI:10.1007/s11172-018-2300-7

A series of merocyanine dyes were synthesized by two-component condensation of N-aryl-4-picolinium salts and cyanoacetic acid derivatives. Physicochemical properties of the synthesized dyes were studied.

Full text of DOI:10.1007/s11172-018-2300-7

Russian Chemical Bulletin, International Edition, Vol. 67, No. 10, pp. 1868—1872, October, 2018
1868
Synthesis and properties of merocyanine dyes derived from
N-aryl-substituted pyridinium salts and cyanoacetic acids
I. A. Borisova, A. A. Zubarev, L. A. Rodinovskaya, and A. M. Shestopalov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
E-mail: irdbor@gmail.com
A series of merocyanine dyes were synthesized by two-component condensation of N-aryl-
4-picolinium salts and cyanoacetic acid derivatives. Physicochemical properties of the synthe-
sized dyes were studied.
Key words: 4-picolinium salts, heterocycles, merocyanine dyes, spectral properties,
protonation.
One of the key fundamental directions of the organic
densation of ethoxymethylidene derivatives of CH acids 7
and 8 with γ-picolinium salts 2—6. The starting salts 2—6
were obtained from γ-picoline 1 as earlier described.23
Synthesis of merocyanines 9a—h revealed that the ac-
ceptor substituents on the aromatic ring either lowered the
yields of the target dyes (synthesis of compounds 9g,h from
salt 2), or completely suppress the reaction (synthesis of
merocyanine dyes from salt 6 failed). Salts 3—5 bearing
the donor substituents on the aryl group gives dyes 9a—f
in good yields (42—62%). Note that the yields of the tar-
get dyes decrease as the donor ability of the substituent on
the aromatic ring increases.
chemistry is the design of new compounds with desired
properties that are capable of undergoing transformations
with the changes of the conditions (temperature, electric
current, light, pH, etc.).1 Among such compounds are
merocyanine dyes, the systems bearing simultaneously
donor and acceptor moieties linked with the polymethine
chain.2,3 As a rule, the donor fragments of the known
merocyanines are heterocycles, e.g., indoles, benzothi-
azoles, and benzoxazoles.2,4—11 Examples of merocyanines
comprising more basic heterocycles, for instance, pyridine
and quinoline, are scarce.12—19
It was assumed, that merocyanine dyes bearing the
dihydropyridine moiety are prospective for use in opto-
electronic devices, photovoltaic applications, laser filters,
and photodynamic therapy.20,21
Earlier, we have developed synthesis of merocyanine
dyes from N-alkyl-substituted azinium salts and cyano-
acetic acid derivatives.12 To date, only Bespalov and co-
workers22 published the synthesis of merocyanine systems
bearing N-aryl-substituted dihydropyridine cycles. It
should be noted that merocyanines have been synthe-
sized22 via modification of the γ-pyrone donor fragment
with amines and the ring-closed methine chain linking
donor and acceptor fragments was a structural part of
bis(dicyanomethylene)indane.
We recorded the UV spectra of the synthesized dyes
(Fig. 1). It was found that the absorption maxima of the
studied compounds 9a—h are at about 500 nm. It follows
from Fig. 1 that the absorption maxima of merocyanine
dyes 9a—h are shifted slightly to the long wavelength range
with increasing donor ability of the substituents on the
aromatic ring. The replacement of the donor substituent
with the acceptor one (compounds 9g,h) results in the
opposite changes in the UV spectra, i.e., the absorption
maximum is shifted towards shorter wavelengths. Analysis
of the UV spectra of merocyanines 9a—h reveals that in-
tensity of the absorption maximum noticeably decreases
when the acceptor substituent with two cyano groups is
replaced with the acceptor with one CN.
Therefore, the aim of the present work is to develop
a simple and versatile procedure to access merocyanine
dyes from N-aryl-substituted 4-picolinium salts and cyano-
acetic acid derivatives. The studies of physicochemical
properties of the synthesized dyes will allow uncovering
structure—property relationship.
Solvatochromic properties of the synthesized dyes were
studied using compound 9a as a model (Fig. 2). It follows
from Fig. 2 that an increase in the solvent polarity (relative
to medium polar dichloromethane) leads to a hypsochro-
mic shift of the absorption maximum. A decrease in the
solvent polarity (benzene is used as a solvent) results in
the broadening of the absorption maximum and its shift
to the long wavelength range.
Results and Discussion
Protonation of merocyanine 9a with trifluoroacetic
acid occurs at the γ position of the polymethine chain and is
accompanied with decolorization of its solution (Scheme 2).
In the present work, merocyanine dyes 9a—h (Scheme 1)
were synthesized by base-mediated two-component con-
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1868—1872, October, 2018.
1066-5285/18/6710-1868 © 2018 Springer Science+Business Media, Inc.

Merocyanine dyes: synthesis and properties
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
1869
Scheme 1
R = H (3), Me (4), OMe (5), Cl (6)
R3 = CN (7), COOEt (8)
Com-
pound 9
a
b
c
d
R1
R2
R3
Yield (%)
Com-
pound 9
e
f
g
h
R1
R2
R3
Yield (%)
H
H
Me
Me
H
H
H
H
СN
COOEt
CN
50
62
63
62
OMe
OMe
NO2
NO2
H
H
NO2
NO2
CN
COOEt
CN
42
44
30
38
COOEt
COOEt
This direction of the protonation of dye 9a is confirmed
by the presence of the doublet at δ 4.5 and a low-field shift
of the heterocyclic proton signals in the 1H NMR spectrum
(Fig. 3). UV spectrum of the protonated dye 9a lacks the
absorption at 504 nm. This is indicative of the disruption
of the conjugation between the heterocyclic moiety and
the side chain (Fig. 4).
In the IR spectra of dyes 9a,c,e,g bearing the acceptor
cyano groups, the absorption bands of the CN groups are
shifted to the 2200—2190 cm–1 range as compared to
benzylidenemalononitrile24 showing the CN absorption
bands in the 2220 cm–1 range. Since two cyano groups of
dyes 9a,c,e,g are nonequivalent, two IR absorption bands at
2200 and 2190 cm–1 are observed thus evidencing the greater
involvement of one of the CN groups in conjugation.
The absorption bands of the CN groups of dyes 9b,d,f,h
bearing the acceptor ethyl cyanoacetate moiety are also
appeared in the 2190 cm–1 range and their ester groups
absorb at about 1660 cm–1. The signal of the CO2Et group
is significantly shifted relative to the same signal of the
reference compound,25 consequently, the CO2Et group
generally contributes to the negative charge delocalization
over the dye molecule.
Scheme 2
In summary, we synthesized new merocyanine dyes
9a—h bearing the aryl substituents on the nitrogen atom
of 1,4-dihydropyridine moiety. The absorption maxima of

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Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
Borisova et al.
A
a
9a
2.5
2.0
1.5
1.0
0.5
9d
9e
9b
9f
3.12 6.26
1.00
1.00
9.0
8.0
7.0
6.0
5.0
4.0 δ
9g
9c
b
9h
400
450
500
550
600
650
λ/nm
2.17
2.17 6.32
2.00
Fig. 1. UV spectra of compounds 9a—h (CH2Cl2, CM
= 1•10–5 mol L–1).
=
9.0
8.0
7.0
6.0
5.0
4.0
δ
Fig. 3. 1H NMR spectra of unprotonated (a) and protonated (b)
forms of compound 9a. Spectrum of unprotonated form
was recorded in DMSO-d6, spectrum of protonated form, in
CF3COOH.
A
4
2
1
2.5
2.0
1.5
1.0
0.5
A
3
2.0
1
1.5
1.0
5
6
0.5
2
350
400 450 500 550
600
λ/nm
Fig. 4. UV spectra of unprotonated (1) and protonated (2) forms
of compound 9a (CH2Cl2, CM = 1•10–5 mol L–1).
400
450
500
550
600
λ/nm
pound 9a results in disruption of the π,π conjugation
between the heterocyclic fragment and the side chain;
therefore, the protonation occurs at the C(γ) atom of the
polymethine chain.
Fig. 2. UV spectra of compounds 9a recorded in acetonitrile (1),
acetone (2), dichloromethane (3), DMSO (4), ethanol (5), and
benzene (6) (CM = 1•10–5 mol L–1).
Experimental
the synthesized dyes appear at the long wavelength range
(500 nm). Using dye 9a as a model, it was shown that dyes
derived from N-aryl-substituted pyridinium salts and
cyanoacetic acid derivatives exhibit a small negative
solvatochromism. It was found that protonation of com-
1H and 13C NMR spectra were run on a Bruker AM300
spectrometer in DMSO-d6. IR spectra were recorded with
a Bruker ALPHA-T spectrometer in KBr pellets. UV spectra were
obtained on an Agilent 8453 UV-visible spectrophotometer

Merocyanine dyes: synthesis and properties
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
1871
equipped with the 1-cm path length quartz cell in CH2Cl2
5.79 (d, 1 H, C(3)H, J = 14.2 Hz); 7.01—7.12 (m, 3 H, 2 CHAr
at a concentration of the studied compounds of CM
=
and CHpyr); 7.59 (m, 3 H, 2 CHAr and CHpyr); 7.95 (d, 1 H,
C(2)H, J = 14.2 Hz); 8.12 (d, 2 H, CHpyr, J = 7.1 Hz). 13C NMR,
δ: 56.16, 103.23, 115.47, 118.70, 120.81, 124.85, 135.94, 138.99,
139.45, 139.90, 149.66, 152.33, 159.92. MS, found: m/z 276.1140
[M + H]+. C17H13N3O. Calculated: [M + H] = 276.1131.
Ethyl 2-cyano-4-(1-[4-methoxyphenyl]pyridin-4(1H)-ylid-
ene)but-2-enoate (9f). Yield 142 mg (44%), crimson crystals,
m.p. 191—193 °C. IR, ν/cm–1: 2187 (CN), 1665 (C=O). UV
(CH2Cl2), λmax/nm (lgε): 508 (4.46). 1H NMR, δ: 1.21 (t, 3 H,
CO2CH2CH3, J = 7.1 Hz); 3.82 (s, 3 H, OCH3); 4.10 (q, 2 H,
CO2CH2CH3, J = 7.1 Hz); 5.76 (d, 1 H, C(3)H, J = 14.2 Hz);
6.97 (br.s, 1 H, CHpyr); 7.12 (d, 2 H, 2 CHAr, J = 9.0 Hz); 7.36
(br.s, 1 H, CHpyr); 7.56 (d, 2 H, 2 CHAr, J = 9.0 Hz); 7.98—8.11
(m, 3 H, C(2)H and 2 CHpyr). 13C NMR, δ: 15.04, 56.13, 59.70,
102.50, 115.50, 118.94, 120.04, 124.63, 135.98, 138.63, 139.25,
147.21, 152.16, 159.76, 166.17. MS, found: m/z 323.1395 [M + H]+.
C19H18N2O3. Calculated: [M + H] = 323.1390.
= 1•10–5 mol L–1. High resolution mass spectrometry was per-
formed with a micrOTOF instrument. All solvents were purified
by standard procedures prior to use. The reaction courses were
monitored by TLC on Merck plates; the spots were visualized
using iodine vapors or UV light (λ = 254 nm). Salts 2—6 were
synthesized by known procedure.23
Synthesis of merocyanine dyes 9a—h (general procedure). To
a solution of the starting salts 2—6 (1.0 mmol) in DMF (3 mL),
triethylamine (279 μL, 2.0 mmol) was added dropwise followed
by addition of CH acid derivative 7, 8 (2.0 mmol). The stirred
mixture was heated at 70 °C for 3 h, cooled down, diluted
with water, the precipitate formed was collected by filtration,
washed successively with aqueous HCl (pH 4) and water, and
air dried.
2-(2-[1-Phenylpyridin-4(1H)-ylidene]ethylidene)malononitrile
(9a). Yield 123 mg (50%), orange crystals, m.p. 250—252 °C. IR,
ν/cm–1: 2190 (=C=N), 2206 (CN). UV (CH2Cl2), λmax/nm
(lgε): 504 (4.43). 1H NMR, δ: 5.81 (d, 1 H, C(3)H, J = 13.9 Hz);
7.05 (br.s, 1 H, CHpyr); 7.45—7.81 (m, 5 H, 5 CHAr); 8.00 (d, 1 H,
C(2)H, J = 13.9 Hz); 8.12—8.25 (m, 3 H, 3 CHpyr). 13C NMR,
δ: 103.43, 114.63, 119.10, 120.59, 123.34, 129.31, 130.47, 138.69,
139.64, 142.64, 150.13, 152.14. MS, found: m/z 246.1038
[M + H]+. C16H11N3. Calculated: [M + H] = 246.1026.
Ethyl 2-cyano-4-(1-phenylpyridin-4(1H)-ylidene)but-2-eno-
ate (9b). Yield 181 mg (62%), burgundy crystals, m.p. 200—202 °C.
IR, ν/cm–1: 2187 (CN), 1671 (C=O). UV (CH2Cl2), λmax/nm
(lgε): 508 (4.47). 1H NMR, δ: 1.21 (t, 3 H, CO2CH2CH3,
J = 7.1 Hz); 4.11 (q, 2 H, CO2CH2CH3, J = 7.1 Hz); 5.74 (s, 1 H,
C(3)H); 6.98 (br.s, 1 H, CHpyr); 7.30—7.77 (m, 6 H, 5 CHAr and
CHpyr); 7.99—8.20 (m, 3 H, 2 CHpyr and C(2)H). 13C NMR, δ:
15.01, 59.81, 102.75, 113.73, 118.96, 119.83, 123.08, 129.07, 130.50,
138.16, 138.84, 142.67, 147.81, 152.15. MS, found: m/z 293.1273
[M + H]+. C18H16N2O2. Calculated: [M + H] = 293.1285.
2-(2-[1-{p-Tolyl}pyridin-4(1H)-ylidene]ethylidene)malono-
nitrile (9c). Yield 163 mg (63%), orange crystals, m.p. 243—245 °C.
IR, ν/cm–1: 2191 (=C=N), 2208 (CN). UV (CH2Cl2), λmax/nm
(lgε): 505 (4.21). 1H NMR, δ: 2.38 (s, 3 H, CH3); 5.80 (d, 1 H,
C(3)H, J = 14.1 Hz); 7.10 (br.s, 1 H, CHpyr); 7.37—7.62 (m, 5 H,
4 CHAr and CHpyr); 7.97 (d, 1 H, C(2)H, J = 14.1 Hz); 8.14
(d, 2 H, 2 CHpyr, J = 7.4 Hz). 13C NMR, δ: 20.95, 103.35, 114.64,
118.57, 120.68, 123.12, 130.82, 138.67, 139.11, 140.36, 149.92,
152.40. MS, found: m/z 260.1181 [M + H]+. C17H13N3. Calculat-
ed: [M + H] = 260.1182.
2-(2-[1-{2,4-Dinitrophenyl}pyridin-4(1H)-ylidene]ethylid-
ene)malononitrile (9g). Yield 101 mg (30%), dark violet crystals,
m.p. 242—244 °C. IR, ν/cm–1: 2202 (CN). UV (CH2Cl2),
λmax/nm (lgε): 472 (4.37), 500 (4.37). 1H NMR, δ: 5.87 (d, 1 H,
C(3)H, J = 13.5 Hz); 6.99 (d, 1 H, CHpyr, J = 6.2 Hz); 7.93—8.20
(m, 4 H, 3 CHpyr and C(2)H); 8.76 (dd, 1 H, CHAr, J = 8.7 Hz,
J = 2.2 Hz); 8.99 (d, 2 H, 2 CHAr, J = 2.2 Hz). 13C NMR, δ:
104.75, 113.69, 117.51, 118.33, 119.60, 122.23, 130.04, 131.73,
138.80, 139.56, 140.00, 143.87, 147.71, 151.85. MS, found: m/z
336.0723 [M + H]+. C16H9N5O4. Calculated: [M + H] =
= 336.0727.
Ethyl 2-cyano-4-(1-[2,4-dinitrophenyl]pyridin-4(1H)-ylid-
ene)but-2-enoate (9h). Yield 146 mg (38%), dark violet crystals,
m.p. 219—221 °C. IR, ν/cm–1: 2202 (CN), 1689 (C=O). UV
(CH2Cl2), λmax/nm (lgε): 468 (4.27), 499 (4.24). 1H NMR, δ:
1.23 (t, 3 H, CO2CH2CH3, J = 7.0 Hz); 4.14 (q, 2 H, CO2CH2CH3,
J = 7.0 Hz); 5.84 (d, 1 H, C(3)H, J = 13.8 Hz); 6.90 (d, 1 H,
CHpyr, J = 7.1 Hz); 7.35 (d, 1 H, CHpyr, J = 7.1 Hz); 7.70—7.86
(m, 2 H, 2 CHpyr,); 8.08—8.26 (m, 2 H, CHAr and C(2)H); 8.74
(dd, 1 H, CHAr, J = 8.8 Hz, J = 2.4 Hz); 8.98 (d, 1 H, CHAr
,
J = 2.4 Hz). 13C NMR, δ: 14.90, 60.28, 104.43, 112.90, 118.15,
122.29, 129.96, 131.43, 137.87, 138.49, 140.15, 143.83, 147.40,
148.74, 151.03, 165.42. MS, found: m/z 383.0979 [M + H]+.
C18H14N4O6. Calculated: [M + H] = 383.0986.
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Ethyl 2-cyano-4-(1-[p-tolyl]pyridin-4(1H)-ylidene)but-2-enoate
(9d). Yield 190 mg (62%), crimson crystals, m.p. 204—206 °C.
IR, ν/cm–1: 2188 (CN), 1674 (C=O). UV (CH2Cl2), λmax/nm
(lgε): 507 (4.52). 1H NMR, δ: 1.21 (t, 3 H, CO2CH2CH3,
J = 7.1 Hz); 2.38 (s, 3 H, CH3); 4.10 (q, 2 H, CO2CH2CH3,
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Received June 8, 2018;
in revised form July 31, 2018;
accepted August 16, 2018