Synthesis of new 2-(4-vinyl-2H-chromen-2-ylidene)malononitrile-based chromophores

Article abstract of DOI:10.1007/s11172-019-2613-1

An approach towards new 2-(4-vinyl-2H-chromen-2-ylidene)malononitriles-derived chromophores was developed. The optical, solvatochromic, and electrochemical properties of the synthesized compounds were studied. The first hyperpolarizability (β) was calculated using M06-2x functional with 6–31+G^* basis set. The synthesized compounds absorb at a wide spectral range, from 350 to 770 nm. Divinylthiophene-bridged compound showing the narrowest band gap (1.78 eV), strong solvatochromic effect, and the highest first hyperpolarizability value (β = 47805) was found to be the most promising of the synthesized chromophores.

Full text of DOI:10.1007/s11172-019-2613-1

Russian Chemical Bulletin, International Edition, Vol. 68, No. 9, pp. 1691—1701, September, 2019
1691
Synthesis of new
2-(4-vinyl-2H-chromen-2-ylidene)malononitrile-based chromophores
K. S. Levchenko,a D. Yu. Demin,a P. A.Chicheva,a K. A. Chudov,a E. V. Zinov´ev,a
K. A. Lyssenko,b A. N. Fakhrutdinov,c G. E. Adamov,a P. S. Shmelin,a and E. P. Grebennikova,d
aCentral Scientifical Research Technological Institute "Tekhnomash" (JSC "Technomash"),
4 ul. Ivana Franko, 121108 Moscow, Russian Federation.
E-mail: k.s.levchenko@gmail.com
bDepartment of Chemistry, M. V. Lomonosov Moscow State University,
1 Build. 3, Leninskie Gory, 119991 Moscow, Russian Federation
cN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation
dRussian Technological University (MIREA),
78 prosp. Vernadskogo, 119454 Moscow, Russian Federation
An approach towards new 2-(4-vinyl-2H-chromen-2-ylidene)malononitriles-derived
chromophores was developed. The optical, solvatochromic, and electrochemical properties of
the synthesized compounds were studied. The first hyperpolarizability () was calculated using
M06-2x functional with 6-31+G* basis set. The synthesized compounds absorb at a wide spec-
tral range, from 350 to 770 nm. Divinylthiophene-bridged compound showing the narrowest
band gap (1.78 eV), strong solvatochromic effect, and the highest first hyperpolarizability value
( = 47805) was found to be the most promising of the synthesized chromophores.
Key words: chromophores, dyes, coumarin, 2-(4-methylchromen-2-ylidene)malononitrile.
Non-symmetrical donor-acceptor organic dyes (chromo-
phores) exhibiting nonlinear optical (NLO) properties and
composites derived from them are prospective electro-
optical materials for radio-photonic applications. These
materials can be used for construction of high-speed
electro-optical modulators (Mach—Zehnder modulators)
operating at the frequencies above 100 GHz.
The characteristics of modern commercial modulators
fabricated from organic chromophores are an of order of
magnitude higher than those of the inorganic solid-state
electrochromic devices based on the lithium niobate-like
structures. For instance, electro-optical coefficient r33 for
LiNbO3 is 30 pm V–1, while this value for the organic
materials is up to and above 300 pm V–1.1 Besides, the
structure of organic chromophores can be easily modified
to ensure the desired properties such as high hyperpolar-
izability, solubility in polymers, thermal stability, manu-
facturability, scalability of production, etc.2,3
The chromophores used for the production of electro-
optic polymers contain in their structure electron donor
and electron acceptor moieties conjugated via -electron
bridge. These systems can align in the electric field produc-
ing nonzero dipole moment.
To date, the most common chromophores combine
the 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-
dihydrofuran (TCF) group4 and the malononitrile de-
rivatives as an electron acceptor and N,N-dialkylanilines,
julolidines, indolizines, aminothiophenes, and tetra-
hydroquinolines5 as an electron donor. Despite a wide
variety of the structures suitable for application as NLO
chromophores, research in the field of their design and
synthesis shows rapid increase over past 30 years that is evid-
enced by the large number of publications.1—14 The present
work is continuation of our studies on the chemistry of
benzopyrans and is aimed at the synthesis of donor-accep-
tor chromophores bearing the 2-(2H-chromen-2-ylidene)-
malononitrile acceptor and evaluation of their properties.
Earlier,6—8 we have shown the efficiency of such benzo-
pyran acceptors as 2-substituted 4-oxo-4H-chromene-3-
carbonitriles, 4-substituted 2-oxo-2H- chromene-3-carbo-
nitriles, and 2-substituted 2-(2H-chromen-4-ylidene)
malononitriles in the synthesis of chromophores A—C.
We also demonstrated the applicability of the synthesized
compounds for the designing optical modulators and other
electro-optical devices. Compounds B and C were found
to be the most promising due to their high hyperpolariz-
ability values and optical properties comparable to those
of the TCF derivatives.8 Sensitivity of compounds C to
solvent polarity are two-times higher than that of com-
pound A which indicates their great potential.
To the best of our knowledge, dyes with structure D,
2-(4-vinyl-2H-chromen-2-ylidene)malononitriles, was
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1691—1701, September, 2019.
1066-5285/19/6809-1691 © 2019 Springer Science+Business Media, Inc.

1692
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 9, September, 2019
Levchenko et al.
Scheme 1
not synthesized and studied. Therefore, the research in this
field is undoubtedly relevant. In the present work, we stud-
ied the synthesis of structures D by the Knoevenagel reaction
of 2-(4-methyl-2H-chromen-2-ylidene)malononitrile
with aldehydes. The reaction conditions were optimized
varying the solvent, the catalyst, and the reaction time.
Results and Discussion
Synthesis and structure elucidation of chromones. The
target 2-(4-vinyl-2H-chromen-2-ylidene)malononitriles
were synthesized in four steps (Scheme 1). The first step
involves the reaction of phenol (1) with ethyl acetoacetate
ester to give 4-methylcoumarin in ~30% yield. An at-
tempted direct synthesis of the malonodinitrile derivative
4 by refluxing 4-methylcoumarin (2) with malononitrile
failed. Therefore, we synthesized thiocoumarin 3 and
subjected it to the reaction with malononitrile. 4-Methyl-
thiocoumarin (3) was synthesized by thionation of com-
pound 2 with phosphorus pentasulfide in benzene under
reflux for 5 h. Then 4-methylthiocoumarin (3) was refluxed
with malononitrile for 5 h to give 2-(4-methyl-2H-chrom-
en-2-ylidene)malononitrile (4) in ~40% yield.9 The
synthesis was finalized by the introduction of the donor
and bridged substituents via the Knoevenagel reaction.
We studied the effects of different solvents and catalysts
on the Knoevenagel reaction of compound 4 with a series
of aromatic and heteroaromatic aldehydes bearing the
acceptor and donor substituents 6a—i. The synthesis of com-
pound 5g from 2-(4-methylchromen-2-ylidene)malono-
nitrile (4) and 4-(diethylamino)benzaldehyde (6g) was used
as the model reaction. Taking into account the experience
on successful synthesis of the related structures from
4-methyl-2-oxo-2H-chromene-3-carbonitriles, 2-methyl-
4-oxo-4H-chromene-3-carbonitriles, and 2-(2-methyl-
2H-chromene-4-ylidene)malononitriles, the studied reac-
tion was carried out in three different solvents, i.e., ethanol,
acetonitrile, and acetic anhydride, using piperidine (pip),
L-proline, ammonium acetate (in Ac2O), and diisopro-
pylethylamine (DIPEA) as the catalysts (Table 1). In
contrast to the reaction of 4-methyl-2-oxo-2H-chrom-
ene-3-carbonitriles with aldehydes that gives relatively
good results under catalysis with ammonium acetate in
Table 1. Effects of the Knoevenagel reaction conditions on the yields of compound 5g
Solvent
EtOH
Catalyst
pip
L-proline
DIPEA
pip
Reaction time/h
12
Yield (%)
33
EtOH
EtOH
MeCN
MeCN
MeCN
Ac2O
12
12
16
16
12
12
12
Trace
—
30
L-proline
DIPEA
—
26
—
Trace
Trace
Ac2O
NH4OAc

Chromene-based chromophores
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 9, September, 2019
1693
Table 2. Reaction conditions for the synthesis of chromophores 5a—i and the
melting points of the synthesized compounds
Compound
Reaction time/h
Yield (%)
M.p./C
5a
5b
5c
5d
5e
5f
5g
5h
5i
3
3
80
68
218—220
210—212
220—222
233—235
213—215
237—240
212—215
244—246
197—199
3
50
3
81
51
73.5
33
23
32
3
3
12
14.5
4
Note. Piperidine as a catalyst, EtOH as a solvent, reflux.
acetic anhydride, these conditions were found unsuitable
for the studied reaction. Thus, in acetic anhydride this
reaction was very slow despite the catalyst used. DIPEA
failed to give the target product in both MeCN and EtOH.
L-Proline catalyzes this reaction only in MeCN, piperidine
shows catalytic effect in both MeCN and EtOH. The reac-
tion in EtOH was faster than in MeCN but in EtOH the
amount of the side products was larger.
Therefore, we choose ethanol as a solvent and piper-
idine as a catalyst (Table 2). Table 2 indicates that com-
pound 4 reacts with aldehydes bearing either the five-mem-
bered heterocycle or the acceptor substituent relatively
fast (3 h). In general, the refluxing causes precipitation of the
product, which apparently exertes an essential effect on
the product yield. Aldehydes bearing the donor substituents
react much slower. In the case of compounds with the
aliphatic moieties enhancing the overall solubility of the
molecules, the product yields dramatically decrease and
the reactions produce the large amounts of the side prod-
ucts. For poorly soluble compounds 5a—f, the yields reach
50—80%. The structures of the synthesized compounds
1
were evaluated by H and 13C NMR spectroscopy, high
resolution mass spectrometry (HRMS); for the compound
5a HSQC experiment was performed (Fig. 1, Table 3).

J
H
G
D
E
{7.19, 108.92}{A}
{7.31, 116.56}{C}
A
E
110
115
120
125
130
135
140
145
150
{6.56, 113.12}{E}
M,N
C
D
G
{6.72, 115.73}{D}
{7.51, 117.69}{G}
{7.31, 126.29}{B}
{7.89, 124.75}{J}
J
B,J
{7.44, 126.26}{I}
{7.64, 133.36}{H}
H
F
{7.54, 144.80}{F}
8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6
Fig. 1. HSQC spectrum of compound 5a.


1694 Russ. Chem. Bull., Int. Ed., Vol. 68, No. 9, September, 2019
Levchenko et al.
Table 3. Assignment of the HSQC signals of compound 5a
Table 4. The selected bond lengths (d) and dihedral angles ()
for compound 5g based on the X-ray diffraction (XRD) analysis
data and quantum chemical calculations (M-062X/6-31+G*)
Position
C
H
Position
C
H
A
B
C
D
E
F
108.92
126.29
116.56
115.73
113.12
144.80
117.89
7.19
7.31
7.31
6.72
6.56
7.57
7.51
H
I
133.36
126.26
124.75
171.20
57.64
119.40
114.49
7.64
7.44
7.89
—
Parameter
XRD
M062x/6-31+G*
d/Å
J
Bond length
K
L
M
N
C(3)—C(13)
С(13)—С(14)
C(14)—C(15)
С(15)—С(16)
С(18)—С(19)
С(19)—С(20)
N(3)—C(18)
1.450(2)
1.352(2)
1.448(2)
1.407(2)
1.412(2)
1.374(2)
1.374(2)
1.454
1.350
1.455
1.403
1.414
1.385
1.372
—
—
—
G
The structure of compound 5g was analyzed by X-ray
diffraction (Fig. 2) and computed at M062x/6-31+G*
level of theory assuming isolated molecules in gas phase.
The main geometric parameters of compound 5g in crys-
tal and gas phase are practically the same except for the
dihedral angles describing the planarity of the molecule
(Table 4). Thus, the phenyl ring shows the distinct quino-
noid bond length alternation due to the pronounced
conjugation of the aromatic moiety with the C(13)=C(14)
double bond and the NEt2 group. It is of note that in the
crystal molecule 5g was found more planar than in the gas
phase. Namely, in the crystal the dihedral angle between
the (diethylamino)styryl-2H-chromen-2-ylidene fragment
and the chromene backbone is smaller (cf. dihedral angles
C(2)C(3)C(13)C(14) in Table 4) that results in the short-
ening of the intramolecular contacts C(13)H…HC(9) and
C(13)H…HC(16) up to 1.98—2.11 Å. In contrast, the
deviation of the NEt2 and C(CN)2 groups from the mean
plane of the molecule is larger in the crystal (see Table 4).
The distortion of planarity of 2H-chromene skeleton
in the crystal should be noted. Thus, in the gas phase this
fragment is perfectly planar; while, in the crystal two fused
rings of the chromene backbone are noticeably inclined
Dihedral angle
/deg
C(2)—C(3)—C(13)—C(14)
C(13)—C(14)—C(15)—C(16) –1.7
O(1)—C(1)—C(10)—C(11)
C(23)—N(3)—C(18)—C(19)
6.3
29.1
7.6
0.0
5.7
–3.4
18.0
to one another along the C(4)—C(5) bond at angle of 7.6.
The observed differences are apparently caused by the
specifics of the crystal packing of compound 5g. In the
crystal, the molecules of 5g interact via stacking to form
two types of centrosymmetric dimers (symmetry codes 1
– x, 1 – y, 1 – z and 1 – x, 1 – y, 1 – z) that are associated
into infinite chains. The dimers differ by both the mode
of the molecule overlapping (Fig. 3) and the distance
between the interacting atoms. Both dimers are formed
by the stacking interaction between the 2H-chromene
fragment with either the C(3)—C(13)=C(14) moiety or
the substituted benzene ring. In the first case (see Fig. 3,
a), the shortest C…C distance (~3.2 Å) is shorter by 0.2 Å
than in the second case (see Fig. 3, b). Apparently, the
chromene backbone distortion is caused by these interac-
C(8)
C(7)
C(9)
C(6)
C(22)
C(4)
C(21)
C(17)
C(18)
C(16)
C(15)
C(5)
C(3)
C(13)
C(14)
N(3)
C(23)
O(1)
C(2)
C(19)
C(1)
C(20)
C(11)
C(10)
C(12)
N(2)
Fig. 2. General view of compound 5g. The thermal ellipsoids are drawn at 50% probability level.
C(24)
N(1)

Chromene-based chromophores
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 9, September, 2019
1695
N(2A)
a
C(8)
C(7)
C(5)
N(1A)
C(9)
C(6)
C(2A)
C(1A)
C(4)
C(13)
O(1A)
C(3A)
N(3)
C(3)
C(4A)
C(5A)
N(3A)
O(1)
C(14)
C(6A)
C(2)
C(1)
C(9A)
C(7A)
N(1)
C(8A)
N(2)
b
N(2)
C(8A)
C(7A)
N(1)
C(9A)
C(19)
C(1)
C(6A)
C(18)
N(3)
C(2)
C(20)
C(15)
C(14)
C(5A)
C(4A)
C(16A)
C(3)
O(1)
C(13A)
C(13)
C(17A)
C(5)
N(3A)
C(3A)
O(1A)
C(18A)
C(17)
C(15A)
C(4)
C(14A)
C(1A)
C(6)
C(2A)
C(20A)
C(9)
C(19A)
C(7)
N(1A)
C(8)
N(2A)
Fig. 3. Overlapping of two cyclic systems of compound 5g linked into two centrosymmetric dimers (a, b) by stacking interactions in
the crystal.
tions. Interactions between the stack of the molecules are
much weaker and are realized by weak van der Waals in-
teractions mainly via the C—(CN)2 group.
All the products are trans isomers as indicated by the
high spin-spin coupling constants for the double bond
protons.
Spectral properties. Optical properties of compounds
5 were studied in chlorobenzene. Compounds 5f—i were
additionally examined in the solvents of different polarity,
from nonpolar toluene to strongly polar ethanol and
aprotic DMF.10 It should be highlighted that compounds
5a—d bearing the five-membered heterocycle bonded to
the vinyl moiety and compound 5e absorb at a wide wave-
length range, from 350 to 550 nm. UV-Vis spectra of these
compounds exhibit several maxima in the 404—450 range.
Compounds 5f—i are the dyes absorbing in the near-in-
frared (IR) border of the visible region or in the near IR
range (Table 5, Fig. 4). In contrast to compounds 5a—d,

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Russ. Chem. Bull., Int. Ed., Vol. 68, No. 9, September, 2019
Levchenko et al.
the UV-Vis absorption spectra of compounds 5f—i show
the intense absorption bands in the long-wavelength range
along the less intense absorptions in the short-wavelength
range (350—470 nm) similar to those of compounds
5a—e. It is of note that the ratios of the molar extinction
coefficients at the long-wavelength and short-wavelength
maxima vary by 2—3 times with an increase in the solvent
polarity (on going from toluene to ethanol). This effect is
less pronounced for compound 5i. The aggregation of the
dye molecules could be an explanation for this phenom-
enon but the strong dilution of the solutions leads to an
increase in the absorption intensities but not to the change
in the overall spectral pattern. In general, compounds 5f—i
are characterized by positive solvatochromism, i.e., on
going from less polar toluene to more polar ethanol the
absorption maximum shifts bathochromically (red shift).
However, all the studied dyes show the reverse solvato-
chromic behavior when changing a medium of intermediate
polarity (chlorobenzene) to a strong polar one (acetone).
Reverse solvatochromism is typical of compounds A—D.
The largest solvatochromic effect was found for compound
5h bearing the julolidine fragment. Besides, this compound
absorbs at the longest wavelength (595 nm) of the studied
compounds. The absorption maximum of compound 5h
is shifted to longer wavelengths by 10 nm as compared to
that of compound 5i with the longest conjugation chain
and by 80 and 43 nm than the absorption maxima of com-
pounds 5f and 5g bearing the N,N´-diphenylaniline and
N,N´-diethylaniline moieties, respectively.
Taking into account the pronounced solvatochromism
of the synthesized compounds, it was logical to assume
that they also have the relatively large first polarizability
() values. To estimate these values, we performed a full
optimization of the geometry of compounds 5f—i at
M06-2x level with 6-31+G* basis set. It was shown ear-
lier15—18 that this level is optimal for estimating the mo-
lecular hyperpolarizabilities of compounds of this type.
The  values were computed using Gaussian 09 software19
as the third derivative of the molecule energy with respect
to the applied field taken with the opposite sign. It is no-
table that irrespective to the values of the solvatochromic
shift and the positions of the absorption maximum the
highest  value (47805 a.u.) was computed for compound
5i with the longest conjugation chain; while, for the other
compounds this value falls in the range from 18700 to
21151 a.u. (cf.: the  value for p-nitroaniline computed at
the same level of theory is 1404 a.u.). A comparison of
these results with the results obtained earlier for the same
derivatives of chromene C shows that in the both cases the
highest  value was found for the thiophene derivative.
However, the  value of compound 5i is slightly inferior
to that of the related derivative of chromene C ( = 54453
a.u.); while, for the other derivatives of compound 5 the
somewhat higher  values were computed.8
This effect is unique and is not typical of the related
and isomeric derivatives of chromene B.6—8
To estimate the HOMO and LUMO energies, we used
cyclic voltammetry (CV) of compounds 5f—i. Electro-
Table 5. Solvatochromic properties of chromophores 5a—i and the first hyperpolarizabilities () calculated by M052X|6-31+ G(d)
method
Com-
pound

abs/nm (•10–3/ L mol–1 cm–1
)
abs/nm


toluene
—
1,4-dioxane chlorobenzene
acetone
—
DMF
—
EtOH
—
5a
5b
5c
5d
5e
5f
—
—
—
—
—
404 (18.2)
447 (15.5)
402 (16.9)
447 (15.4)
410 (21.3)
449 (18.7)
423 (21.8)
449 (18.8)
412 (20.5)
437 (17.2)
305 (34.3)
508 (24.4)
423 (15.2)
540 (19.2)
458 (21.0)
576 (17.2)
390 (19.6)
581 (27.7)
—
—
—
—
—
23
41
50
22
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
302 (34.5)
492 (27.8)
446 (16.8)
514 (21.5)
446 (17.7)
549 (20.2)
384 (20.8)
553 (26.9)
302 (30.1)
512 (26.9)
422 (12.2)
555 (20.7)
463 (10.9)
589 (23.4)
386 (15.5)
575 (25.1)
300 (26.5)
515 (25.0)
453 (10.9)
552 (22.2)
461 (9.7)
595 (22.5)
384 (16.7)
585 (27.3)
21045
18700
21151
47805
502 (28.6)
418 (14.8)
539 (22.5)
462 (9.4)
5g
5h
5i
453 (16.2)
516 (20.6)
449 (19.9)
557 (20.0)
385 (16.8)
560 (25.6)
573 (19.0)
382 (17.3)
561 (28.6)
Designations: abs is the absorption maximum; abs is the maximal shift of the absorption maxima of the compounds in different
solvents;  is the molar extinction coefficient;  is the first hyperpolarizability.

Chromene-based chromophores
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 9, September, 2019
a
1697
A
0.05
5a
5f
5g
5h
5i
0.04
0.03
0.02
0.01
400
450
500
500
500
550
600
650
700
750
/nm
A
b
0.06
0.05
0.04
0.03
0.02
0.01
Toluene
Chlorobenzene
Acetone
DMF
1,4-Dioxane
CH2Cl2
Ethanol
400
450
550
600
650
700
750
/nm
A
с
0.06
0.05
0.04
0.03
0.02
0.01
Chlorobenzene
1,4-Dioxane
Acetone
DMF
CH2Cl2
Ethanol
400
450
550
600
650
700
750
/nm
A
d
0.06
0.04
0.02
Toluene
1,4-Dioxane
Chlorobenzene
CH2Cl2
Ethanol
Acetone
DMF
400
450
500
550
600
650
700
750
/nm
A
0.06
0.05
0.04
0.03
0.02
0.01
e
Toluene
1,4-Dioxane
CH2Cl2
Ethanol
Chlorobenzene
Acetone
DMF
400
450
500
550
600
650
700
750
/nm
Fig. 4. Electronic absorption spectra of chromophores 5a—i in chlorobenzene (a). Effects of the solvent on the spectra of compounds 5f (b),
5g (c), 5h (d), and 5i (e). Figure 4 is available in full color on the web page of the journal (https://link.springer.com/journal/11172).

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Levchenko et al.
chemical measurements indicated the presence of two
quasi-reversible oxidation peaks for compounds 5h and 5i
(0.58 and 0.95 V for 5h and 0.63 and 1.03 V for 5i) and
one quasi-reversible oxidation peak for compounds 5f and
5g (1.10 V for 5f and 0.76 for 5g). For compound 5f, an
additional oxidation peak of low intensity was observed at
about 1.2 V. Potential of the first oxidation peak character-
izing the HOMO energy level was used in the calculations
(Fig. 5).
equal to 1.78—1.95 eV. The LUMO energies were calcu-
lated as
ELUMO = EHOMO + Egap
.
The experimental and intermediate estimates are sum-
marized in Table 6 and Fig. 6.
The obtained results show that the band gap for com-
pounds 5f—h (2.02—1.81 eV) significantly decreases with
an increase in the donor ability of the substituent. The
lowest band gap (1.78 eV) was found for compound 5i
bearing 1,5-divinylthiophene bridge.
In summary, we succeeded to develop a new approach
to novel chromophores by the Knoevenagel reaction be-
tween 2-(4-methyl-2H-chromen-2-ylidene)malononitrile
and the corresponding aldehydes. The reaction conditions
were optimized and the effects of different bases and sol-
vents used on the reaction course were analyzed. The
Using the potentials Eox1onset calibrated with ferrocene,
the energies of HOMO (EHOMO) of compounds 5f—i were
calculated as follows:
EHOMO = –(Eox1onset [vsFc+/Fc] + 4.4) (eV).
To determine the LUMO energies (ELUMO), the energy
band gap values (Egap) were calculated from the electronic
absorption of compounds 5g—i spectra in MeCN to be
I/A
I/A
b
a
30
Cycle 1
40
30
1195.3
Cycle 2
765.61
1103.1
Cycle 3
20
10
0
457.04
20
10
0
948.54
–10
–20
697.61
5f
5f (+ Ferrocene)
384.99
–10
500
750
1000 U/mV
0
500
1000
U/mV
I/A
c
d
I/A
10
15
10
6
Cycle 1
Cycle 2
Cycle 3
Cycle 1
Cycle 2
Cycle 3
1033.9
631.63
955.79
582.3
955.8
0
902.6
0
576.1
550.5
–250
0
250
500 750 1000
U/mV
500
750
1000
U/mV
Fig. 5. CV curves of compounds 5f (a), 5g (b), 5h (c), and 5i (d).

Chromene-based chromophores
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 9, September, 2019
1699
Table 6. Electrochemical properties of chromophores 5f—i
onset
onset
Compound
Eox(vs Ag/AgCl)
Eox1
Eox1
EHOMO
Egap
ELUMO
max/nm
(vsFc+/Fc)
mV
eV
5f
1103
910.0
671.3
469.4
327.4
0.49
–4.89
–4.64
–4.44
–4.30
2.02
1.95
1.81
1.78
–2.87
–2.69
–2.63
–2.52
509
541
577
560
5g
5h
5i
765.6
582.3
631.6
0.24
0.04
–0.1
E/eV
thesized compounds were performed on a PerkinElmer Lambda
35 UV/VIS spectrophotometer using a quartz cell (optical path
–2.52
–2.4
–2.8
–3.2
–3.6
–4.0
–4.4
–4.8
–5.2
–2.63
of 0.2 cm). The concentration of the samples was 0.003 mg mL–1
.
–2.69
LUMO
–2.87
Cyclic voltammetry was performed under argon on a P-8nano
potentiostat (Elins) at 25 C in 0.1 M solution of Bu4NPF6 in
acetonitrile using a glassy carbon electrode as a working electrode;
the counter electrode was Pt wire, the reference electrode was
a silver chloride electrode (Ag/AgCl/KCl).
LUMO
LUMO
LUMO
5f
5g
5h
5i
Commercially available aldehydes 6a—e were used. Aldehydes
6f—i were synthesized by either the Vilsmeier reaction from the
corresponding substituted anilines or formylation of the corre-
sponding lithium salt with DMF.6,12
Electrochemical studies of the chromophores were performed
using a three-electrode cell (working electrode was Pt; reference
electrode was AgCl; counter electrode was glassy carbon). For
the measurements, P-8nano potentiostat (Elins) was used. The
solvent was anhydrous acetonitrile. The potentials were measured
against ferrocene, scan rate was 30 mV s–1, supporting electrolyte
was NBu4PF6.
–4.3
–4.44
HOMO
–4.64
HOMO
–4.89
HOMO
HOMO
Fig. 6. HOMO and LUMO energy levels of chromophores 5f—i.
4-Methylcoumarin (2).13 A free-neck flask was charged with
phenol (20 g, 0.21 mmol) and ethyl acetoacetate (27.6 g,
0.21 mmol), and anhydrous nitrobenzene (30 mL). Aluminum
chloride (56.7 g, 0.42 mol) was dissolved in nitrobenzene
(200 mL) and the obtained solution was poured into the flask
maintaining the temperature at 100—130 C. Then the mixture
was heated at 130 C for 3 h, cooled to ~20 C, and treated with
a solution of HCl (50 mL) in the equimolar ratio (Caution! The
reaction mixture warms up). The mixture was stirred and shacked,
the organic layer was separated, extracted with either ethyl
acetate or acetone (to remove resinous products) for 3—4 times.
The volatiles were removed in vacuo, the residue was vacuum
distilled to obtain the product 2 with b.p. 170—180 C (15 Torr),
m.p. ~80 C. Yield 29%. Spectral properties of compound 2 are
in agreement with those published earlier.13
spectral properties of the synthesized chromophores were
studied. For the most promising compounds, solvatochro-
mic effects were studied and the non-linear optical prop-
erties were computed at M06-2x/6-31+G* level of theory.
The synthesized compounds absorb in a wide range of
wavelengths, from 350 to 770 nm. It was revealed that
compound 5i bearing the divinylthiophene bridge and
characterizing by the lowest band gap value, pronounced
solvatochromic effect, and the highest first polarizability
value is the most promising of the synthesized derivatives.
In general, the analysis of the obtained results indicatives
that the synthesized compounds are promising for further
development of electro-optical modulators.
4-Methyl-2-thiocoumarin (3).14 A mixture of 4-methylcou-
marin (16 mmol) and phosphorus pentasulfide (20 mmol) in
benzene (50 mL) was refluxed for 5 h. After cooling down, the
solution was decanted. The residue was diluted with benzene,
refluxed for 3—5 min and the solution was decanted. This extrac-
tion was repeated twice. The combined organic extracts were
filtered and concentrated in vacuo. The residue was crystallized
from ethanol to give compound 3 in 67% yield. 1H NMR
(300 MHz, CDCl3), : 7.68—7.56 (m, 2 H); 7.48 (d, 1 H,
J = 8.1 Hz); 7.37 (t, 1 H, J = 7.6 Hz); 7.18 (s, 1 H); 2.39 (s, 3 H).
13C NMR (75 MHz, CDCl3), : 197.42, 156.12, 144.06, 132.04,
129.04, 125.31, 124.48, 121.45, 116.97, 17.96.
Experimental
1H and 13C NMR spectra were run on a Bruker AM-300
spectrometer (working frequencies of 300 (1H) and 75 MHz
(
13C)). The chemical shifts are given in the  scale relative to the
residual proton signal and the signal of the carbon atom of the
solvent. 1H and 13C NMR spectra and 2D correlation NMR
spectra of compound 5a were recorded with a BrukerAvance 600
instrument (working frequencies of 600 (1H) and 150 MHz (13C))
in CDCl3 at 293 K. High resolution electrospray ionization (ESI)
mass spectrometry was performed with a Bruker micrOTOF II
instrument. The reaction course and the purify of the isolated
compounds were monitored by TLC using Merck Silica gel 60
F254 UV254 plates. Spectral studies of the solutions of the syn-
2-(4-Methyl-2H-chromen-2-ylidene)malononitrile (4) was
synthesized following the procedure described earlier for
2-(2H-chromen-2-ylidene)malononitrile.9 To a solution of
malononitrile (1.27 g, 19 mmol) in ethanol (20 mL), 4-methyl-

1700
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 9, September, 2019
Levchenko et al.
2-thiocoumarin (3) (3.4 g, 19 mmol) and several drops of triethyl-
amine were added. The reaction mixture was refluxed for 5 h.
The precipitate formed was crystallized from ethanol to give
product 4 in the yield of 30—40%. 1H NMR (300 MHz, CDCl3),
: 7.71–7.60 (m, 2 H); 7.52—7.40 (m, 2 H); 6.96 (s, 1 H);
2.54 (s, 3 H). 13C NMR (75 MHz, CDCl3), : 171.15, 152.04,
148.89, 133.23, 126.19, 124.88, 120.70, 117.15, 114.68, 113.61,
112.48, 18.92.
The Knoevenagel reaction (general procedure). A flask was
charged with 2-(4-methyl-2H-chromen-2-ylidene)malononitrile
(4) (1 mmol) and the corresponding aldehyde (1 mmol). Then
ethanol (5—7 mL) and few drops of piperidine was added. The
resulting mixture was refluxed for the time indicated in Table 2.
The reaction course was monitored by TLC (development with
EtOAc—hexane, 1 : 3). After the reaction completion, the reac-
tion mixture was cooled down, the precipitate formed was col-
lected by filtration and washed with ethanol. The yields of the
products 5a—i are given in Table 2.
(E)-2-{4-[2-(Furan-2-yl)vinyl]-2H-chromen-2-ylidene}mal-
ononitrile (5a). 1H NMR (600 MHz, CDCl3), : 7.90 (d, 1 H,
J = 9.0 Hz); 7.64 (t, 1 H, J = 7.8 Hz); 7.57 (s, 1 H); 7.51 (d, 1 H,
J = 7.9 Hz); 7.44 (t, 1 H, J = 8.0 Hz); 7.31 (dd, 2 H, J = 16.0 Hz,
J = 2.1 Hz); 7.19 (s, 1 H); 6.71 (d, 1 H, J = 3.4 Hz); 6.56
(dd, 1 H, J = 3.4 Hz, J = 1.8 Hz). 13C NMR (151 MHz, CDCl3),
: 171.20, 152.66, 151.76, 145.74, 145.41, 133.43, 126.27, 124.69,
119.40, 117.79, 116.39, 115.68, 114.49, 113.18, 113.12, 108.74,
57.64. HRMS, found: m/z 287.0813. Calculated: 287.0815.
(E)-2-{4-[2-(Thiophen-2-yl)vinyl]-2H-chromen-2-ylidene}-
malononitrile (5b). 1H NMR (300 MHz, CDCl3), : 7.87 (d, 1 H,
J = 7.8 Hz); 7.73—7.62 (m, 2 H); 7.56—7.43 (m, 3 H); 7.37
(d, 1 H, J = 3.5 Hz); 7.24—7.09 (m, 3 H). 13C NMR (75 MHz,
CDCl3), : 171.05, 152.58, 145.53, 140.79, 133.36, 132.70,
131.28, 128.99, 128.67, 126.18, 124.54, 119.22, 117.73, 117.59,
114.27, 112.95, 109.05, 57.83. HRMS, found: m/z 303.0578.
Calculated: 303.0587.
(E)-2-{4-[2-(5-Bromothiophen-2-yl)vinyl]-2H-chromen-2-
ylidene}malononitrile (5c). 1H NMR (300 MHz, DMSO-d6),
: 8.19 (d, 1 H, J = 8.1 Hz); 7.93 (d, 1 H, J = 16.0 Hz); 7.75
(t, 2 H, J = 7.0 Hz); 7.62—7.21 (m, 6 H). 13C NMR (75 MHz,
DMSO-d6), : 170.82, 151.81, 145.71, 142.62, 133.66, 132.40,
131.89, 126.23, 125.55, 118.69, 118.27, 116.98, 114.17, 113.61,
108.36 (the signals are overlapped). HRMS, found: m/z 382.9682.
Calculated: 382.9672.
J = 4.4 Hz); 7.35 (t, 4 H, J = 7.8 Hz); 7.29 (s, 1 H); 7.24—7.05
(m, 9 H). 13C NMR (75 MHz, CDCl3), : 171.08, 152.59, 150.35,
146.76, 146.19, 139.86, 133.12, 129.66, 129.35, 128.06, 126.01,
125.73, 124.48, 121.54, 119.55, 117.69, 115.56, 114.54, 113.25,
108.28, 56.92. HRMS, found: m/z 464.1757. Calculated:
464.1757.
(E)-2-{4-[4-(Diethylamino)styryl]-2H-chromen-2-ylidene}-
malononitrile (5g). 1H NMR (300 MHz, CDCl3), : 7.93 (d, 1 H,
J = 8.0 Hz); 7.62 (t, 1 H, J = 7.7 Hz); 7.55—7.38 (m, 5 H);
7.24—7.07 (m, 2 H); 6.73 (s, 2 H); 3.47 (q, 4 H, J = 7.0 Hz);
1.25 (t, 6 H, J = 7.0 Hz). 13C NMR (75 MHz, CDCl3), : 170.98,
152.61, 146.51, 141.07, 133.95, 132.88, 130.54, 125.86, 124.36,
119.70, 117.66, 116.24, 115.16, 113.87, 111.65, 106.28, 44.78,
12.68. HRMS, found: m/z 368.1748. Calculated: 368.1757.
(E)-2-{4-[2-(1,2,3,5,6,7-Hexahydropyrido[3,2,1-ij]quinolin-
9-yl)vinyl]-2H-chromen-2-ylidene}malononitrile (5h). 1H NMR
(300 MHz, CDCl3), : 7.95 (d, 1 H, J = 7.6 Hz); 7.62 (t, 1 H,
J = 7.8 Hz); 7.50—7.38 (m, 3 H); 7.18—7.06 (m, 4 H); 3.37—3.28
(m, 4 H); 2.80 (t, 4 H, J = 6.3 Hz); 2.07—1.97 (m, 4 H). 13C NMR
(75 MHz, CDCl3), : 170.79, 152.54, 146.34, 141.48, 132.67,
127.98, 125.67, 124.26, 122.29, 121.53, 119.71, 117.56, 115.27,
114.04, 113.98, 110.78, 105.60, 50.11, 27.67, 21.41. HRMS,
found: m/z 392.1743. Calculated: 392.1757.
2-[4-((E)-2-{5-[(E)-4-(Diethylamino)styryl]thiophen-2-yl}-
vinyl)-2H-chromen-2-ylidene]malononitrile (5i). 1H NMR
(300 MHz, CDCl3), : 7.86 (d, 1 H, J = 7.8 Hz); 7.69—7.56
(m, 2 H); 7.51—7.41 (m, 2 H); 7.33 (d, 2 H, J = 8.2 Hz); 7.20
(d, 1 H, J = 3.6 Hz); 7.14 (s, 1 H); 7.09—6.90 (m, 4 H); 6.66
(d, 2 H, J = 8.1 Hz); 3.42 (q, 4 H, J = 6.6 Hz); 1.22 (t, 6 H,
J = 6.8 Hz). 13C NMR (75 MHz, CDCl3), : 170.76, 152.45,
149.02, 148.11, 145.92, 145.18, 137.97, 133.42, 133.04, 132.83,
132.13, 128.34, 125.92, 124.23, 123.31, 119.19, 117.62, 116.01,
115.54, 113.33, 111.58, 107.48, 103.44, 44.45, 12.67. HRMS,
found: m/z 476.1788. Calculated: 476.1791.
X-ray diffraction study of compound 5g. Crystals of compound
5g was grown from ethanol. Crystals of 5g (C24H21N3O,
M = 367.44,  = 0.78 cm–1, dcalc = 1.246 g cm–3) at 120 K are
monoclinic, space group P21/c, a = 15.657(3), b = 7.7650(17),
c = 17.018(4) Å,  = 108.768(4), V = 1959.0(7) Å3. Intensities
of 10792 reflections were collected on a Bruker APEX II CCD
diffractometer ((Mo-K) = 0.71072 Å,  scan mode, 2 < 58).
For the refinement, 5186 independent reflections (Rint = 0.0323)
were used. He structure was solved by the direct method and
refined anisotropically by full-matrix least-squares method against
F2hkl. Hydrogen atoms were positioned geometrically and refined
isotropically using a riding model. The final divergence para-
meters are as follows: wR2 = 0.15071 and GOOF = 1.024 for all
independent reflections (R1 = 0.03794 was calculated on F for
4957 all observed reflections with I > 2(I)). All calculations
were performed with SHELXTL software.20 Atomic coordinates
and full tables of structural parameters were deposited with the
Cambridge Crystallographic Data Center (CCDC 1909963) and
are available free of charge via www.ccdc.ac.uk.
(E)-2-{4-[2-(Benzofuran-2-yl)vinyl]-2H-chromen-2-ylid-
ene}malononitrile (5d). 1H NMR (300 MHz, DMSO-d6), : 8.15
(d, 1 H, J = 6.4 Hz); 7.85 (d, 1 H, J = 15.4 Hz); 7.78—7.66
(m, 2 H); 7.65—7.50 (m, 4 H); 7.44—7.25 (m, 4 H). 13C NMR
(75 MHz, DMSO-d6), : 153.37, 151.85, 133.65, 127.25, 126.73,
126.37, 125.26, 123.57, 122.10, 119.29, 117.02, 114.09, 113.45,
111.58, 111.16, 109.08 (the signals are overlapped). HRMS,
found: m/z 337.0962. Calculated: 337.0972.
(E)-2-[4-(3,4,5-Trimethoxystyryl)-2H-chromen-2-ylidene]-
malononitrile (5e). 1H NMR (300 MHz, CDCl3), : 7.91 (d, 1 H,
J = 8.0 Hz); 7.68 (t, 1 H, J = 7.7 Hz); 7.55—7.42 (m, 4 H); 7.25
(s, 1 H); 6.88 (s, 2 H); 3.98 (s, 6 H); 3.94 (s, 3 H). 13C NMR (75
MHz, CDCl3), : 171.14, 153.81, 152.57, 145.97, 140.78, 140.15,
133.36, 130.62, 126.15, 124.63, 119.38, 117.92, 117.74, 115.42,
109.47, 105.53, 61.15, 57.86, 56.47. HRMS, found: m/z 387.1327.
Calculated: 387.1339.
This work was financially supported by the Russian
Science Foundation (Project No. 17-73-10433).
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Received April 16, 2019;
in revised form June 19, 2019;
accepted July 1, 2019