Synthesis, solution and solid-state fluorescence of 2-diethylaminocinchomeronic dinitrile derivatives

Article abstract of DOI:10.1039/c7ra06217f

A facile approach to the synthesis of novel 2-diethylaminocinchomeronic dinitriles, which are found to be fluorescent both in the solution and in a solid states, was developed. Absorption, fluorescence and solvatochromic properties as well as a crystal st

Full text of DOI:10.1039/c7ra06217f

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Synthesis, solution and solid-state fluorescence of
2-diethylaminocinchomeronic dinitrile derivatives†
Cite this: RSC Adv., 2017, 7, 34886
a
a
a
A. I. Naidenova,a
*
O. V. Ershov,
M. Yu. Ievlev,
M. Yu. Belikov,
V. N. Maksimovaa and V. A. Tafeenkob
A facile approach to the synthesis of novel 2-diethylaminocinchomeronic dinitriles, which are found to be
fluorescent both in the solution and in a solid states, was developed. Absorption, fluorescence and
solvatochromic properties as well as a crystal structure of the synthesized compounds were investigated.
It was found that the 2-diethylaminocinchomeronic dinitrile derivatives with methoxy groups in the
aryl moiety possess the most intensive emission in nonpolar solvents with fluorescence quantum yields
up to 0.59.
Received 3rd June 2017
Accepted 5th July 2017
DOI: 10.1039/c7ra06217f
rsc.li/rsc-advances
moiety. It was noted that amino substituted pyridines possess
a higher uorescence quantum yield in compare to unsub-
stituted pyridines, and 2-aminopyridines has the most one.12
Therefore, the combination of a cyano group and an amino
group in one molecule is a promising approach to structures
with promising optical properties.
The known compounds with such a donor–acceptor moiety
are 2-aminopyridine-3-carbonitriles possessing uorescent and
other practically useful properties.7,8,13 It was also mentioned
that diethylamino group leads to the highest uorescence
quantum yield.14
We have reported previously, that pyridines containing
two cyano groups as acceptors – 2-oxo-1,2-dihydropyridine-3,4-
dicarbonitriles and 2-chlorocinchomeronic dinitriles possess
uorescence properties in solutions (an intensive blue emis-
sion) and in solid state (an emission in the blue, green and
yellow regions of the spectrum).15 Moreover, the other authors
reported that various porphyrazines could be synthesized from
these cinchomeronic dinitriles.16 Taking into account the
importance of diethylamino group as well as the described
prospects of using the cinchomeronic dinitrile moiety we
decided to construct structures with improved uorescent
properties. Therefore, we had synthesized a series of 2-dieth-
ylaminocinchomeronic dinitriles combining key fragments in
the structure, and studied uorescence properties of these
compounds both in solutions and in a solid state.
Introduction
Luminescent dyes are widely used in various elds of industry
and science. There are many examples of optical devices,
chemo- and biosensors, and nanomaterials which are based on
luminescent substances.1 Because of the theoretical and prac-
tical interest, compounds of the cyano pyridine series should be
mentioned separately among the diverse uorescent organic
molecules.2–8 They are commonly used in light-emitting diodes
(OLED),2 photovoltaics,3 nonlinear optical (NLO) materials,4
liquid crystalline,5 dyes,6 sensors for the detection of metal ions7
and uorescent security markers.8 Such a diverse practical use
of uorophores causes an interest for the targeted synthesis of
new uorescent structures with desired properties.
It was reported earlier, that the cyano group is able to
signicantly increase uorescence intensity.9 Because of its
structural
simplicity,
compactness,
strong
electron-
withdrawing properties and high polarizability, the cyano
group has been frequently utilized as a functional unit in the
design of advanced optical materials.10 Various «push–pull»
chromophores wherein cyano group or a polynitrile moiety is an
acceptor are widely presented in the modern scientic litera-
ture. A number of reports describes that including of cyano
group in the molecule could increase such characteristics of the
substances as solubility, thermal stability, sublimability, optical
and electrochemical properties, and so on, making them
interesting as advanced functional materials for novel opto-
electronic devices.11 Amino (or alkylamino) group, in such
a kind donor–acceptor structures, is oen used as a donor
Results and discussions
2-Diethylaminocinchomeronic dinitrile derivatives
3 were
aUlyanov Chuvash State University, Moskovsky pr., 15, Cheboksary, Russia. E-mail:
oleg.ershov@mail.ru
bLomonosov Moscow State University, Leninskie Gory 1, Moscow, Russia
synthesized by the method shown in Table 1. Starting 4-
oxoalkane-1,1,2,2-tetracarbonitriles 1a–c were prepared by
interaction of TCNE and carbonyl compounds in 1,4-dioxane in
the presence of hydrochloric acid17 as well as compounds 1d–h
were obtained according to the «solvent-free» protocol.18
† Electronic supplementary information (ESI) available. CCDC 1545662–1545665.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7ra06217f
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Table 1 Synthesis of compounds 3
of compound 3f in various polar and nonpolar solvents are
shown in Fig. 1, the obtained spectral characteristics are
summarized in Table 2. A position of absorption maxima
(342–346 nm and 409–415 nm) does not change signicantly in
various solvents.
The compound 3f exhibits uorescence in various solvents,
the most intensive emission was noted in benzene, an emission
band undergoes a slight red-shi and signicantly loses its
intensity with an increase of the polarity of the solvent. The
relative uorescence quantum yield (FF) was also calculated
(see Table 2). It decreased when the solvent polarity increased,
probably, because of the charge transfer phenomena. The
intermolecular charge transfer (ICT)4c phenomena occurs, when
an electron is transferred from the amino group to the acceptor
moiety upon light absorption, producing an (ICT) excited state,
which is more sensitive to the solvent polarity. A change in the
excited state leads to the change in the lifetime and hence the
results are a decrease of radiative emission.
2
R1
R2
aYield%
3a
3b
3c
3d
3e
3f
Me
(CH2)5
(CH2)6
Ph
4-MeO-C6H4
3,4-Di-MeO-C6H3
Ph
Me
82
71
78
88
94
92
91
96
H
H
H
Me
Me
3g
3h
4-MeO-C6H4
a
Yield has been reported for isolated crude product.
The other compounds 3 possess similar spectral character-
istics which are presented in Table 3. Fig. 2 shows absorption
and emission spectra of 2-diethylaminocinchomeronic dini-
triles 3 in benzene. Solutions of all the studied compounds 3 are
characterized with an absorption maximum in the violet region
of the spectrum (399–416 nm), corresponding, probably, to the
intramolecular charge transfer (ICT) according to TD-DFT
calculations. The intensity of this band increases with the
growth of the number of donor fragments in the molecule.
Moreover, in the spectra of the compounds 3e,f,h containing
MeO– groups a new shortwave absorption band is appeared
which is corresponds to n–p* electronic transition in the aryl
substituent.
It was observed that the structure of aminopyridines 3 has
a little inuence on the positions of the absorption and emis-
sion maxima. Compounds 3d–h with aryl substituent are
characterized with a slight (up to 15 nm) bathochromic shi of
the absorption band and a more intensive uorescence.
A relative uorescence quantum yield of the dimethoxy
derivative 3f is the highest (FF 0.59) and comparable to the used
Compounds 1 were further transformed to corresponding 2-
chlorcinchomeronic dinitriles 2 by an interaction with hydro-
chloric acid for the synthesis of compounds 2a–d,g or with
generated in situ dry hydrogen chloride to obtain compounds
2e,f,h.15b
Targeted 2-diethylaminocinchomeronic dinitrile derivatives
3 were synthesized as a result of an interaction between
2-chlorocinchomeronic dinitriles 2 and diethylamine. The
reactions were carried out in an excess of diethylamine. It was
observed, that the interaction carried out in the organic solvents
(alcohols, dioxane, THF were tested) took much more time. For
example, the reaction of 2-chloro-5,6-dimethylpyridine-3,4-
dicarbonitrile 3a with diethylamine in THF takes 30 h.16a
Moreover, we have found that nal products are not required
a further purication aer the reaction carried out in the excess
of diethylamine without heating. An addition of metal carbon-
ates as a catalyst did not increase a yield of compounds 3 but
led to the formation of byproducts obtaining as a result of
the addition of diethylamine to cyano group.19 According to
the developed facile protocol
a
series of 2-dieth-
ylaminocinchomeronic dinitrile derivatives 3 were synthesized
containing alkyl, cycloalkyl and aryl substituents, including the
moieties with electron-donating methoxy groups (Table 1).
Compounds 3b–h are new and were not described in the
literature.
2-Diethylaminocinchomeronic dinitrile derivatives 3 are
crystalline powders from green to yellow-green color, they were
found to be uorescent both in the solution and in the solid
state. There are very few dyes that exhibit intensive uorescence
both in the solid state and in solutions because generally the
molecular aggregation in the solid state causes uorescence
quenching (ACQ). It causes an increased interest to study the
compounds 3.
Spectroscopic properties in solution
The structure of the synthesized pyridines 3 containing conju-
gated donor and acceptor moieties prompted us to explore their
Fig. 1 Absorption (solid) and emission (doted) spectra of 3f in various
photophysical properties. The absorption and emission spectra solvents, excitation wavelength is 375 nm.
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Table 2 Spectral characteristics of the compounds 3 in various
solvents
substituent. The full characteristics of the luminescence are
given in the Table 3.
labs;max
,
lu;max
,
Stokes shi,
cmꢀ1 (nm)
FF
Solid-state uorescence of synthesized compounds
Solvent
Benzene
DCM
nm
log 3max
nm
Microcrystalline compounds 3a–f are bright green or yellow-
green. Upon illumination by an UV lamp, compounds
3b–e,g,h are brightly emissive in the blue-green region of the
spectrum while compounds 3a,f are less emissive. The solid-
state uorescence spectra were recorded at room temperature
and showed a main band around 480 nm and a shoulder
around 510 nm. The emission bands appeared in a narrow
region (478–491 nm) (see Fig. 3, Table 4). Nevertheless,
a substituent in the aryl moiety possesses a signicant inuence
on the emission intensity. Methoxy group insertion led to the
decreasing of solid-state uorescence intensity for compound
3h by about 20% in compare to unsubstituted compound 3g. In
compare to unsubstituted compound 3d methoxy derivative 3e
has a 55% less intensive emission, and dimethoxy derivative 3f
has an 85% less one. It should be noted that an observed
regularity is opposite to the uorescence in solution.
343
412
346
412
343
409
349
417
343
413
342
411
343
415
346
415
4.04
4.10
3.92
3.96
4.01
4.06
3.97
4.04
4.08
4.09
4.02
4.11
4.05
4.12
4.09
4.18
449
456
456
465
468
459
468
466
2000 (37)
2342 (44)
2520 (47)
2475 (48)
2845 (55)
2544 (48)
2729 (53)
2637 (51)
0.59
0.33
0.29
0.06
0.11
0.14
0.04
0.05
Dioxane
Pyridine
MeCN
AcOEt
AcOH
EtOH
Also, an interesting phenomenon is the wide region of the
excitation of solid-state uorescence which is located in area of
320–470 nm. The highest emission intensity is observed for
compounds 3 upon excitation by irradiation of the blue region
Table 3 Spectral properties of the compounds 3 in benzene
labs;max
,
lu;max
,
Stokes shi,
Compound
nm
log 3max
nm
cmꢀ1 (nm)
FF
3a
3b
3c
3d
3e
3f
399
399
403
410
339, 410
344, 412
416
3.44
3.63
3.52
3.77
3.73, 3.89
4.04, 4.10
3.93
453
450
453
457
449
449
460
457
2988 (54)
2840 (51)
2739 (50)
2508 (47)
2119 (39)
2000 (37)
2299 (44)
2157 (41)
0.46
0.45
0.53
0.20
0.41
0.59
0.11
0.35
3g
3h
319, 416
4.12, 4.07
Fig. 3 Solid-state emission spectra of compounds 3, excited by
351 nm.
Table 4 Photophysical parameters of the crystalline compounds 3
lex; max
,
lem; max
,
HOMOb,
eV
LUMOb,
eV
GAPb,
eV
3
nm
nm
R.I.a
3a
3b
3c
3d
3e
3f
444
446
451
455
450
460
453
459
489
478
481
486
486
491
487
488
0.18
1.16
0.71
1.21
0.54
0.18
1.23
1.00
ꢀ6.2
ꢀ2.29
ꢀ2.24
ꢀ2.29
ꢀ2.57
ꢀ2.42
ꢀ2.41
ꢀ2.44
ꢀ2.36
3.91
3.91
3.85
3.79
3.76
3.62
3.76
3.73
ꢀ6.15
ꢀ6.14
ꢀ6.36
ꢀ6.18
ꢀ6.03
ꢀ6.20
ꢀ6.09
Fig. 2 Absorption (solid) and emission (doted) spectra of compounds
3 in benzene, excitation wavelength is 375 nm.
standard (7-hydroxy-4-methylcoumarine in 0.1 M phosphate
buffer with pH ¼ 10, FF 0.63, excitation wavelength is 375 nm).
Compounds 3d and 3g possess the lowest emission intensity
probably due to the absence of the donating moieties in the aryl
3g
3h
a
Excited by 351 nm, R.I. – relative intensity of emission is given in
compare to 3h. b Calculated in the basis TDDFT(B3LYP)/6-31+G(d,p).
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of the spectrum with wavelengths of 450–460 nm, which is
clearly shown in Fig. 4.
The quantum-chemical calculations in the basis
TDDFT(B3LYP)/6-31+G(d,p) (see Fig. 5) showed that electron
density of the highest occupied molecular orbital (HOMO) is
localized on the diethylamino moiety and the lowest unoccu-
pied molecular orbital (LUMO) involved cyano group. It indi-
rectly conrms that absorption of light could lead to the
intramolecular charge transfer from the donor moiety to
acceptor. The energy levels of Frontier orbitals and gap values
are also given in Table 4.
Continuing our earlier studies,15b it is important to note that
the uorescence intensity increases signicantly, and its band
shis to the long-wavelength region upon transformation of 2-
chloropyridines
2 to 2-diethylaminopyridine derivatives 3
(Fig. 6). It demonstrates the promise of this work, as well as the
subsequent studies of spectral properties of other 3,4-dicyano-
substituted aminopyridines.
Fig. 6 Solid-state emission spectra of compounds 2d, 3d and photos
under UV (365 nm) irradiation.
Crystal structures characterization
The pyridine ring of the 3g (Fig. 7d) is overloaded with
substituents, which leads to a substantial deformation of the
heterocycle. The deviation of the atoms C3, C2, N1 from the
The obtained single crystal X-ray crystallographic data of the
synthesized compounds can be used to gain a better under-
standing of the relationship between the optical properties and
the molecular conformation and packing mode. Single crystals
of 3c, 3d, 3f and 3g suitable for X-ray structural analysis were
prepared by slow evaporation of acetonitrile solution, at room
temperature (Fig. 7).
˚
plane through C4/C5/C6 – are 0.05, ꢀ0.20, ꢀ0.12 A, respec-
tively. The angle along the C6/C3/N3 line of pyridine cycle
and cyano group at the third position is 171.1, while for another
cyano group at the forth position, the angle along the N1/C4/
N4 line is 177.8. The heterocycle forms a dihedral angle ꢀ130.5
with a phenyl ring. Despite the signicant steric interactions,
the amino group (nitrogen atom N2) actively interacts with the
˚
p-heterocycle system. The bond length C2–N2 (1.351 A) coin-
˚
cides with the value of the delocalized bond N1 C2 (1.354 A).
˚
The nitrogen atom N2 is located outside (0.1 A) of plane running
through atoms C2, C17, C18. This means, that the nitrogen
atom, despite its conjugation with the p-system, retains, to
some extent, the tetrahedral (sp3) conguration.
Interestingly, there are two independent molecules in the
structure of 3f (Fig. 7c). The two molecules possess the different
conformations with the different torsion angles. The greatest
difference is observed in twisted conformation in which the
torsion angles between pyridine ring and ethyl moieties.
Torsion angles C3/C2/N2/C17 and C3/C2/N2/C18 are
ꢀ34.7 and 167.5 respectively, while angles C3A/C2A/N2A/
C17A and C3A/C2A/N2A/C17A are – 25.9 and 171.6.
Pyridine ring in the molecule 3f as well as in the compound
3g is signicantly deformed. The deviations from the plane
running through the atoms C4/C5/C6 (C4A/C5A/C6A)
Fig. 4 Solid-state excitation spectrum of compound 3h and photos
under daylight (a), UV (b: 365 nm) and visible (c: 400 nm, d: 455 nm)
irradiation.
˚
have C2 ꢀ0.12 and C2A ꢀ0.07 A. The angle C6/C3/N3 (C6A/
C3A/N3) of the line along the pyridine cycle and the cyano
group at the third position is 171.7 (173.7), while for the cyano
group at the forth position the angle N1/C4/N4 (N1A/C4A/
N4A) is 178.9 (177.6).
Amino groups of N2, N2A are conjugated to pyridine rings
˚
with parameters close to 3g N2/C2 is 1.356 A and N2A/C2A is
˚
1.354 A. In the crystal, molecules form dimers, as a result of the
Fig. 5 (a) HOMO and (b) LUMO for compound 3a, computed with
TDDFT(B3LYP)/6-31+G(d,p). The pink (purple) lobes indicate a positive
(negative) isocontour value.
˚
p–p interaction. The shortest distance C12/C6A is 3.364 A.
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Fig. 7 Single-crystal structures of 3c (a), 3d (b), 3f (c) and 3g (d).
Phenyl ring in structure of 3f lie in the same plane with the
Conformational features of the 8-membered cycle of the
pyridine rings in contrast to the molecule 3g. This difference molecule 3c can be seen in Fig. 7a. As in the structures pre-
can be explained by the inuence of the methyl group at the sented earlier, the amino group (ethyl moieties) prevents from
h position of the pyridine ring, which creates steric the p–p interacting adjacent molecules in the stack. The angle
hindrances for the planar arrangement of the rings in the case C12/C3/N3 along the pyridine ring and the cyano group at
of 3g. In the crystal of compound 3f, wherein the hydrogen atom the third position is 173.0, while with the cyano group at the
in the h position, the repulsion between the atoms H5/H8 fourth position the angle N1/C4/N4 is to 179.4.
(H5/H8a) is compensated by the interaction of the lone elec-
To summarize the obtained data for the compounds studied,
tron pair of the nitrogen atom, N1 (N1A), and the hydrogen the deviation of angles for cyano groups in the third position of
atoms, H12 (H12A), respectively, for each of the independent pyridine cycle for all the crystals obtained is observed in crys-
molecules.
talline form and is in the range of 171–173ꢁ. It is probably the
The structure of the molecule 3d (Fig. 7b) in the crystal is result of the inuence of the adjacent diethylamine fragment,
similar to the structure of the 3f molecule. Small differences are but on the other hand it also can indicate a signicant contri-
observed in the packing of molecules in a crystal. Molecules bution of the intermolecular charge-transfer (ICT) structure.
form stacks in which the values of the distances between The latter is supported by the fact that exactly the acceptor
neighboring molecules are substantially similar, but greater cyano group in the third position is in effective conjugation with
˚
than 3.5 A, that is, more than the sum of van der Waals radii of the donor diethylamine fragment.
carbon–carbon.
Moreover, the obtained data for crystal 3f shows a difference
Pyridine cycles in molecule 3d are planar, in contrast to in the torsion angles for the diethylamino group in two inde-
molecules 3f and 3g. Nevertheless, the cyano group in the third pendent molecules of the dimer (Fig. 7c). A different conjuga-
position deviates from the plane of the pyridine ring, and the tion degree of the amino group with the p-heterocycle system is
angle along the line C6/C3/N19 is 173.1, while the cyano the consequence of the above. This is probably the reason for
group in the fourth position does not deviate, and the angle the appearance of a “shoulder” in the emission spectra in the
N1/C4/N12 is 178.3.
solid state and in solutions for all compounds. It is especially
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noticeable for the solid state emission of compounds 3a and 3f
(Fig. 3).
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Conclusions
In conclusion, we have developed the facile method for the
synthesis of 2-diethylaminocinchomeronic dinitrile derivatives
and have studied uorescence properties thereof in the solu-
tions and in the solid state. High uorescence quantum yield
was observed for the solutions of pyridine derivatives with
electron donating groups in nonpolar solvents. Moreover, an
intensive solid-state emission in the blue-green region of the
spectrum was observed for alkyl substituted 2-dieth-
ylaminocinchomeronic dinitriles, as well as compounds with
unsubstituted aryl moieties. This work is a continuation of the
series of papers aimed to the study of the correlation of the
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Acknowledgements
The study was performed within the framework of the baseline
of the State Assignment for scientic activity of the Ministry of
Education and Science of Russia No. 4.6283.2017/8.9. The XRD
study was carried out using the equipment purchased from the
funds of the Program of development of Moscow University and
within the framework of the Agreement on collaboration
between the Chemical Department of the Lomonsov Moscow
State University and the Chemical-Pharmaceutical Department
of the I. N. Ulyanov Chuvash State University
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RSC Adv., 2017, 7, 34886–34891 | 34891