Thermal, spectroscopic, electrochemical, and electroluminescent characterization of malononitrile derivatives with triphenylamine structure

Article abstract of DOI:10.1016/j.saa.2018.11.025

Three push-pull molecules with linear, quadrupolar and tripodal arrangements, consisting of triphenylamine (electro-donor) substituted with malononitrile groups (electro-acceptor), were synthesized with high yield by a simple procedure. Impact of the numb

Full text of DOI:10.1016/j.saa.2018.11.025

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Thermal, spectroscopic, electrochemical, and electroluminescent
characterization of malononitrile derivatives with
triphenylamine structure
Danuta Sęk a, Sonia Kotowicz b, Sławomir Kula b, Mariola Siwy a, Agata Szłapa-Kula b, Jan Grzegorz Małecki b,
Sebastian Maćkowski c, Ewa Schab-Balcerzak a,b,
⁎
a
Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Str., 41-819 Zabrze, Poland
Institute of Chemistry, University of Silesia, 9 Szkolna Str., 40-006 Katowice, Poland
Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, 5 Grudziadzka Str., 87-100 Torun, Poland
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 August 2018
Received in revised form 2 October 2018
Accepted 12 November 2018
Available online 13 November 2018
Three push-pull molecules with linear, quadrupolar and tripodal arrangements, consisting of triphenylamine
(electro-donor) substituted with malononitrile groups (electro-acceptor), were synthesized with high yield by a
simple procedure. Impact of the number of malononitrile substituents on optoelectronic properties was investigated
with cyclic voltammetry, absorption and emission spectroscopy, as well as density functional theory calculation. The
derivatives formed amorphous materials and exhibited low energy band gaps ranging from 2.06 to 2.49 eV. UV–Vis
absorption and photoluminescence emission spectra were investigated in solutions (CHCl3, NMP) and in solid-state
as thin films and two kinds of blends (with PMMA and PVK:PBD). Quantum yield of photoluminescence was depen-
dent on the molecule structure, solvent, and solid-state layer formulation. The compounds exhibited high
photoluminescence quantum yield in the range of 15–42% and 12–59% in solid-state as film and blend with
PMMA (1 wt%), respectively, being promising for applications in light emitting diodes. The diodes with active
layer consisting of neat derivatives and compounds molecularly dispersed in PVK:PBD (50:50 wt%) matrix showed
orange and green electroluminescence.
Keywords:
Malononitrile
Triphenylamine
Photoluminescence
Electroluminescence
Electrochemistry
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
groups were presented. However, here we focus only on the results
which concern triphenylamine substituted with malononitrile groups.
Organic push-pull chromophores have become very popular being
widely investigated and used as materials for optical devices as organic
light-emitting diodes, organic photovoltaic cells or field effect transis-
tors. Such a push-pull molecule exhibits D-π-A structure, i.e. donor
unit, (D)-π-conjugated unit, and acceptor unit (A), where the intramo-
lecular charge transfer can occur. Among many chemical structures
having electron donor properties, triphenylamine derivatives are com-
monly used, while malononitrile unit is very often used as electron ac-
ceptor moieties in creation of push-pull molecules [1,2]. In the case of
triphenylamine (TPA) mono-, di- and tri- substituted derivatives with
malononitrile group can be synthesized having linear, quadrupolar
and tripodal structures. Synthesis and some properties of substituted
TPA with malononitrile groups were described in few publications. It
is necessary to stress that in many papers, investigations of compounds,
in which triphenylamine rings were also substituted with another
Y. Cao et al. [3] described the dependence of fluorescence emission on
various external stimuli, such as grinding, annealing and solvent fuming
for triphenylamine substituted with one malononitrile group. They
found that photoluminescence of the compound after grinding changed
from 581 nm to 602 nm, which is a typical mechanochromic phenome-
non. Interactions of triphenylaminomonomalonitrile molecules and
crystal growth process were investigated by L. Kong et al. [4]. L. Kong
et al. [5]. investigated dependence of photoluminescence emission
wavelength and quantum yield (using fluorescein as standard) and
also fluorescence decay lifetime of triphenylamine-mononitrile and
other compounds on various solvents (benzene, CH2Cl2, THF, DMF).
They found that increasing solvent polarity caused red-shift of the emis-
sion wavelength. Z. Fuke et al. [6] described the results of study
concerning the influence of solvent nature and its viscosity on fluores-
cence wavelength and quantum yield of series of compounds. They
found that emission of triphenylamine-monomalonitrile was sensitive
to the polarity of solvents and also that its emission is enhanced in sol-
vents with high viscosity. The effect of solvent polarity on absorption
and photoluminescence spectra of triphenylamine with mono-, di-
⁎
Corresponding author at: Centre of Polymer and Carbon Materials, Polish Academy of
Sciences, 34 M. Curie-Sklodowska Str., 41-819 Zabrze, Poland.
E-mail address: ewa.schab-balcerzak@us.edu.pl (E. Schab-Balcerzak).
https://doi.org/10.1016/j.saa.2018.11.025
1386-1425/© 2018 Elsevier B.V. All rights reserved.

D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
137
and trimalonitrile units was reported by B. Li et al. [7] and S.-H. Kim et al.
[8], because the solute-solvent interactions are crucial to understand
molecular behavior as solvent effect may lead significant changes
[9–11]. Moreover, the S.-H. Kim investigated electrochemical properties
of prepared triphenylamine-mono- and di-malonitrile [8].
Triphenylamine being substituted with one malononitrile group was
found to be a highly selective and sensitive fluorescent chemosensor for
detection of CN−, SO32−and Fe3+ ions [12]. After addition of Fe3+
solution in water into the compound solution (in DMSO:H2O) emission
decreased gradually along with Fe3+ concentration, being absolutely
quenched after addition of Fe3+ due to the consumption of the total
amount of malononitrile compound in the complexation process. It is
interesting that presence of other metal ions did not disturb this
process. Decrease of emission band and quenching at proper amount
of anions CN− and SO32− was also observed and the presence of other
anions did not influence the process.
M. Klikar et al. [13]. synthesized and investigated the properties of a
series of push-pull compounds including mono, di and three substituted
triphenylamine with malononitrile group. They studied their thermal
and electrochemical properties, absorption in CH2Cl2 solution and
also photoinduced piezooptical coefficients at a He-Ne laser probing
wavelength at 1150 nm. However, photoluminescence of the com-
pounds was not investigated. A series of compounds consisting of
triphenylamine as donor and different substituents as electron
accepting units were synthesized by R. Lartia et al. [14]. Investigations
were carried out from the point of view for those materials being
suitable for coupling to biomolecules. The main emphasis was put on
their two-photon absorption properties. Among the other compounds,
triphenylamine substituted with one, two or three malononitrile groups
were investigated. It was found that three substituted triphenylamine
belongs to the most efficient two photon absorption compound de-
scribed in [14]. Additionally triphenylamine with two malononitrile
group was found to be promising compound for labeling of biomole-
cules from the point of view of its two-photon absorption efficiency/
molecular weight ratio. D. Cvejn et al. [15]. synthesized and compared
the properties of a series of triphenylamine being tri-substituted with
various electron accepting units, among them also with malononitrile
one. The authors detected their thermal properties, electrochemistry
and absorption and photoluminescence in THF solution.
Triphenylamine with one malononitrile group dispersed in poly
(9-vinylcarbazole) (PVK) was tested in the light emitting diode with
configuration ITO/PVK + dye/Al and emission of orange light was ob-
served [16,17].
In none of these works there is any investigation concerning
photophysical properties of triphenylamines substituted with
malononitrile groups measured in solid-state as blend and testing
their electroluminescence ability in diodes, in which a neat compound
create of active layer (ITO/PEDOT:PSS/compound/Al) and is dispersed
in a binary matrix consists of PVK: 2-(4-biphenylyl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole (PBD), (ITO/PEDOT:PSS/
PVK:PBD:compound (1, 2 and 15 wt%)/Al). Thus, the aim of the
presented study was the synthesis, using a simple procedure, without
complicated catalytic systems and with high yields, small molecules as
prospective materials for light emitting applications. The malononitrile
derivatives were selected because they are solid-state emissive organic
materials due to aggregation-induced emission (AIE), in contrast to
most of other compounds, whose emission is usually quenched in
solid-state [2,18].
chromatography was done on Merck silica gel. Thin layer chromatogra-
phy (TLC) was carreid out on silica gel (Merck TLC Silica Gel 60). Poly
(3,4 (ethylenedioxy)thiophene): poly-(styrenesulfonate) (PEDOT:PSS)
(0.1–1.0 S/cm) and substrates with pixilated ITO anodes were supplied
by Ossila. Poly(9-vinylcarbazole) PVK (Mn = 25,000–50,000), 2-(4-
biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), poly
(methyl methacrylate) (PMMA), Bu4NPF6, NaOH, 4-(diphenylamino)
benzaldehyde, 4,4′-diformyltriphenylamine, tris(4-formylphenyl)
amine were purchased from Sigma Aldrich.
2.2. Typical Procedure for the Synthesis of Malononitrile Derivatives
All compounds were prepared according to the method described in
our previously published paper [19].
Aluminum trioxide (0.8 g, 1.6 g and 2.4 g) and appropriate aldehyde
(4 mmol) were placed in a glass-stoppered conical flask. Then solution
of malononitrile (8 mmol, 16 mmol and 24 mmol) in methylene chlo-
ride (20 cm3) was added. The reaction mixture was stirred until the
complete disappearance of the starting material was determined by
thin layer chromatography. Crude products were purified using column
chromatography (SiO2, CH2Cl2).
[4-(Diphenylamino)benzylidene]propanedinitrile (MN-M)
Orange Solid. Yield: 84%. 1H NMR (400 MHz, CDCl3) δ 7.74
(d, J = 9.0 Hz, 2H), 7.52 (s, 1H), 7.43–7.35 (m, 4H), 7.27–7.18
(m, 6H), 6.95 (d, J = 9.0 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ
157.9, 153.5, 145.1, 133.0, 130.0, 126.7, 126.2, 122.7, 118.4, 115.2,
114.1, 75.3. FTIR (KBr, cm−1): 3062 (C\\H stretching aromatic),
2220 (-C ≡ N), 1591 (C\\C stretching in the aromatic ring), 1190
(C\\N stretching), 706 (C\\N deformation). Anal. Calcd for
(C22H15N3) (321.37 g/mol): C, 82.22; H, 4.70; N, 13.08; Found:
C,82.42; H, 4.85; N, 12.77. DSC: I run: Tm = 137 °C, Lit. Tm
=
139 °C [6]; II run: Tg = 27 °C. TGA: T5% = 247 °C, Tmax = 337 °C.
Bis[4-(2,2-dicyanovinyl)phenyl]phenylamine (MN-D)
Red Solid. Yield: 72%. 1H NMR (400 MHz, CDCl3) δ 7.84 (d, J =
8.8 Hz, 4H), 7.63 (s, 2H), 7.48–7.41 (m, 2H), 7.38–7.32 (m, 1H),
7.18 (d, J = 8.9 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 157.8, 151.5,
144.6, 132.8, 130.7, 127.6, 126.2, 123.0, 114.4, 113.3, 79.9. FTIR
(KBr, cm−1): 3027 (C\\H stretching aromatic), 2220 (-C ≡ N), 1575
(C\\C stretching in the aromatic ring), 1183 (C\\N stretching), 696
(C\\N deformation). Anal. Calcd for (C26H15N5) (397.43 g/mol): C,
78.57; H, 3.80; N, 17.62; Found: C, 78.27; H, 3.64; N, 17.48. DSC: I
run: Tm = 230 °C Lit. Tm = 226 °C [6]; II run: Tg = 75 °C, Tc =
154 °C, Tm = 230 °C. TGA: T5% = 297 °C, Tmax = 298,405 °C.
Tris[4-(2,2-dicyanovinyl)phenyl]amine (MN-T)
Orange Solid. Yield: 67%. 1H NMR (400 MHz, CDCl3) δ 7.92 (d, J =
8.8 Hz, 6H), 7.69 (s, 3H), 7.25 (d, J = 8.7 Hz, 6H). 13C NMR
(100 MHz, CDCl3) δ 157.5, 150.0, 132.8, 127.7, 124.9, 113.8,
112.8, 81.7. FTIR (KBr, cm−1): 3069 (C\\H stretching aromatic),
2217 (-C ≡ N), 1571 (C\\C stretching in the aromatic ring), 1193
(C\\N stretching), 705 (C\\N deformation). Anal. Calcd for
(C30H15N7) (473.48 g/mol): C, 76.10; H, 3.19; N, 20.71; Found:
C,75.74; H, 3.47; N, 20.60. DSC: I run: Tm = 155 °C; II run: Tg
99 °C. TGA: T5% = 271 °C, Tmax = 337 °C.
=
2. Experimental Section
2.3. Blends and Films Preparation
2.1. Materials
Films and blends on glass substrates were prepared from a homoge-
neous chloroform solution (10 mg/ml) of malononitrile derivatives
with PVK:PBD (50:50 in weight %) (1, 2 or 15 wt% content) and
PMMA (1 wt% compound content). Films and blends with PVK:PBD
were performed by spin-coating method (1000 rpm, 60 s) and blends
Some of the commercially available chemicals were used without
further purification. Solvents were purified by distillation using the
standard methods and were purged with nitrogen before use. Column

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D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
with PMMA by casting technique. Then films and blends were dried for
24 h in a vacuum oven at 50 °C.
stable temperature (t = 20 °C). The measurements were recorded with
moderate scan rate equal to 0.100 V/s. The concentration of compounds
was equal 1.0·10−6 mol/dm3. Deaeration of the solution was assured by
argon bubbling through the solution for about 10 min before every
measurement.
2.4. OLED Preparation and Electroluminescence Measurements
Devices with sandwich configuration ITO/PEDOT:PSS/compound/Al
and ITO/PEDOT:PSS/compound:PVK:PBD/Al with 1, 2 and 15 wt% of
malononitrile derivatives content in blend were prepared. Devices
were prepared on OSSILA substrates with pixilated ITO anodes, cleaned
with detergent, deionized water, 10% NaOH solution, water and isopro-
pyl alcohol in an ultrasonic bath. Substrates were covered with PEDOT:
PSS film by spin coating at 5000 rpm for 60s and annealed for 5 min at
120 °C. Active layer was spin-coated on top of the PEDOT:PSS layer
from chloroform solution (10 mg/cm3) at 1000 rpm for 60 s and
annealed for 5 min at 100 °C. Finally Al was vacuum-deposited at a pres-
sure of 5·10−5 Torr. Electroluminescence (EL) spectra were measured
with the voltage applied using a precise voltage supply (GwInstek
PSP-405) and the sample was fixed to an XYZ stage. Light from the
OLED device was collected through a 30 mm lens, focused on the en-
trance slit (50 μm) of a monochromator (Shamrock SR-303i) and de-
tected using a CCD detector (AndoriDus 12,305). Typical acquisition
times were equal to 10 s. The pre-alignment of the setup was done
using a 405 nm laser. All spectra were corrected with respect to the
spectral dependence of quantum efficiency of the CCD detector.
2.6. Crystallography
The crystals of MN-M and MN-D were mounted in turn on a Gemini
Ultra Oxford Diffraction automatic diffractometer equipped with a CCD
detector. X-ray intensity data were collected with graphite
monochromated MoKα radiation (λ = 0.71073 Å) at 295(2) K, in the
ω scan mode. Details concerning crystal data and refinement are gath-
ered in Table 1. Lorentz, polarization and empirical absorption correc-
tion using spherical harmonics implemented in SCALE3 ABSPACK
scaling algorithm [20] were applied. The structure was solved by the di-
rect method and subsequently completed by the difference Fourier
recycling. All the non hydrogen atoms were refined anisotropically
using the full-matrix, least-squares technique. The Olex2 [21] and
SHELXS, SHELXL [22] programs were used for all the calculations.
Atomic scattering factors were incorporated in the computer programs.
3. Result and Discussion
3.1. Synthesis and Characterization
2.5. Characterization Methods
The designed malononitrile derivatives with triphenylamine struc-
ture (MNs) were synthesized with high yield by a simple procedure
using one step Knoevenagel reaction of 4-(diphenylamino)benzalde-
hyde, 4,4′-diformyltriphenylamine, tris(4-formylphenyl)amine with
dicyanomethane. In this condensation reaction, a dehydrating agent
(aluminum oxide) was used. They form push-pull small molecules
with the donor (D)-π-acceptor (A) structure (cf. Fig. 1).
The chemical structure of the synthesized compounds was con-
firmed by typical techniques including 1H NMR, 13C NMR, FTIR and ele-
mental analysis. The signals of protons in dicyanovinyl units in the
molecules were observed as singlets in the range of 7.69–7.52 ppm
(cf. Supplementary Material, Fig. S1, S3 and S4). The signals in the
range of 157.89–157.47 ppm in 13C NMR spectra confirmed the
presence of carbon atoms bonded to hydrogen in vinyl units, while
the signals in the range of 113.78–115.22 ppm were due to the carbon
atom in –C ≡ N groups. Signals in the range of 75.29–81.70 ppm
confirmed the presence of carbon atom bounded to cyano group in
the vinyl unit (cf. Supplementary Material, Fig. S2, S4 and S6).
There no signals from the aldehyde group seen in target compounds.
The absorption of –C ≡ N groups in the FTIR spectra was detected
at 2220 cm−1 for MN-M and MN-D, while the absorption band at
2217 cm−1 was found for MN-T. The elemental analysis results
confirmed the chemical structures and purity of the synthesized
compounds.
Nuclear magnetic resonance (1H and 13C NMR) spectra were re-
corded on a Bruker AC400 spectrometer in DMSO d6 as solvent and
TMS as the internal standard. Infrared spectra (FTIR) were recorded
on a Thermo Scientific Nicolet iS5 FT-IR Spectrometer in the range of
4000–400 cm−1 as KBr pressed pellets (KBr before use was dried).
Elementary analysis was performed using Vario EL III apparatus
(Elementar, Germany). Differential Scanning Calorimetry (DSC) was
performed using a Du Pont 1090B apparatus with a heating/cooling
rate of 20 °C·min−1 under nitrogen and using aluminum sample pans
in the range of 0–250 °C. The glass transition temperature was recorded
in the second scan after cooling. Thermogravimetric analysis (TGA)
was done with a Mettler Toledo TGA STARe system with a heating
rate of 10 °C·min−1 in a constant stream of nitrogen (20 ml·min−1
)
and temperature range from 50 °C to 800 °C. UV–Vis absorption spectra
were performed using a Evolution 220 UV–Visible Spectrophotometer
at the concentration of 10−5 mol/dm3 and 1 cm quartz cell and Jasco
V-550 Spectrophotometer for films and blends. Photoluminescence
spectra (PL) in solutions were performed by using Varian Carry Eclipse
Spectrometer and using Hitachi F-2500 Spectrometer for blends and
films PL measurements. Quantum yields (Φf) measurements were
performed by using the integrating sphere Avantes AvaSphere-80
(Edinburgh Instruments) and absolute method. The lifetime (τ) of
photoluminescence was measured with a time correlated single photon
counting (TCSPC). The time-resolve measurements were performed
using the picosecond pulsed diode laser, EPL-405 nm and using a
60 W microsecond Xe flash lamp. Pulse period for all measurements
was equal 50 ns and PMT (Hamamatsu, R928P) as a detector. The fluo-
rescence decay analysis was receive an instrument response function
(IRF) using ludox solution and results were presented as average values
of decay after exponential fitting. Electrochemical measurements (CV)
were performed with Eco Chemie Autolab PGSTAT128n potentiostat.
Glassy carbon (diam. 2.0 mm) served as working electrode, while plat-
inum coil and silver wire were used as auxiliary and reference electrode
respectively. All potentials were referenced with respect to the stable
internal standard, which was ferrocene couple (Fc/Fc+). Electrochemi-
cal measurements were performed in a one-compartment cell, in
CH2Cl2 (ACROSS Organics, 99.9% for biochemistry grade), under argon
purging, with 0.2 M Bu4NPF6 (Aldrich, 99%) used as the supporting elec-
trolyte salt. Each experiment was performed in air-conditioned room in
Compounds were received as an orange (MN-1 and MN-3) and a
red (MN-2) solids easily soluble in polar and non-polar solvents
(e.g. dimethylsulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP),
methylene chloride (CH2Cl2), chloroform (CHCl3)).
3.2. Crystal and Molecular Structures
To gain insight into the molecular organization of the
triphenylamine malononitrile derivatives architecture, single crystals
of MN-M and MN-D were analyzed at room temperature. The crystal
were obtained by slow evaporation of the acetone:methanol (1:2)
solutions of the compounds. Unfortunately it has failed to obtain well-
formed MN-T crystals. X-ray crystallography analysis indicated that
MN-M and MN-D molecules existed in the monoclinic P21/c and ortho-
rhombic Pbcn space groups respectively. Crystal data and processing
parameters for the compounds are given in Table 1 while ORTEP views

D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
139
Table 1
cyano nitrogen and phenyl hydrogen in the crystal lattice. In the MN-M
crystal the molecules are packed in antiparallel patterns and in MN-D
the molecules are twisted by 90° (cf. Supplementary Material, Fig. S9).
The parameters of the hydrogen bonds and stacking interactions were
collected in Tables S1 and S2 in Supplementary Material. The intermo-
lecular interactions are analyzed by means of the Hirshfeld surfaces
and of the corresponding 2D fingerprint plots [24–26].
Crystal data and structure refinement details of MN-M and MN-D compounds.
MN-M
22H15N3
MN-D
Empirical formula
Formula weight
Temperature [K]
Crystal system
C
C26H15N5
397.43
295(2)
orthorhombic
Pbcn
321.37
295(2)
monoclinic
P21/c
Space group
The fingerprint plots and the histogram of the relative contributions
to the Hirshfeld surface for the major intermolecular contacts are pre-
sented on the Fig. 2. From the histogram immediately can be seen a
larger share of the C\\H…N contacts (hydrogen bonds) in dicyano
MN-D derivative than in MN-M. The C…C and C…H close contacts are
attributed to π…π and C\\H…π stacking interactions, respectively and
in both cases the share of the interactions in total Hirshfeld surface
area is similar (32.2% in MN-M and 30% in MN-D). Although in the
case of MN-D the stacking interactions are stronger as indicated by the
geometric parameters collected in Supplementary Material Table S2.
The N..C interaction corresponds with C`N…π contact and their partici-
pation in Hirshfeld surface area in the case of MN-D is much higher than
in monosubstituted MN-M. The stronger intermolecular interactions in
dicyano derivative can help to rigidify the conformations of the molecules,
thereby yielding enhanced emissions. On the other hand the twisted
stacking architecture of MN-D could result in shorter effective conjuga-
tion lengths and blue shift emission, while the planar conformation
(MN-M) could result in increased effective lengths and red shift emission
(vide infra, cf. Table 4 emission maxima in film).
Unit cell dimensions
a [Å]
b [Å]
c [Å]
7.0132(3)
15.9088(6)
16.1060(6)
90
95.218(4)
90
1789.53(12)
4
1.193
15.8829(7)
7.4025(6)
36.853(3)
90
90
90
4333.0(5)
8
1.218
0.075
1648
0.25 × 0.15 × 0.04
3.317 to 29.358
−21 ≤h ≤16
−10 ≤k ≤7
−50 ≤l ≤36
16,066
α
β
γ
Volume [Å3]
Z
Calculated density [Mg/m3]
Absorption coefficient [mm−1
F(000)
]
0.072
672
Crystal dimensions [mm]
θ range for data collection [°]
0.24 × 0.17 × 0.11
3.330 to 29.431
−9 ≤h ≤9
−16 ≤k ≤21
−22 ≤l ≤16
8957
4217 [R(int) = 0.0217] 5262 [R(int) = 0.0356]
4217/0/226
1.022
Index ranges
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
5262/0/280
1.026
R1 = 0.0470
R1 = 0.0660
wR2 = 0.1542
R1 = 0.1187
wR2 = 0.1819
0.408 and − 0.304
1,822,695
Final R indices [I N 2σ(I)]
wR2 = 0.1019
R1 = 0.0803
wR2 = 0.1206
0.128 and − 0.174
1,821,956
R indices (all data)a
3.3. Thermal Properties
Largest diff. peak and hole
CCDC number
Thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) were applied to estimate the thermal behavior of the pre-
pared compounds. Thermogravimetric analysis carried out in nitrogen
atmosphere showed that the obtained compounds in dependence on
their structures, started decomposition at 247 (MN-M), 297 (MN-D)
and 271 °C (MN-T), detected as temperature of 5% weight loss (T5).
All the compounds gave similar DSC thermograms and during first
heating run (20 °C/min) endothermic peak confirming their melting
point (Tm) was found at 137 °C, 230 °C and 155 °C for MN-M, MN-D
and MN-T, respectively. Thus, MNs were obtained as crystalline
compounds after synthesis. However, they could form also amorphous
materials. Under the second heating scan after cooling (15 °C/min)
only glass transition temperature (Tg) was seen for MN-M and MN-T
at 27 and 99 °C, respectively. DSC thermogram of the disubstituted
triphenylamine (MN-D) during the second heating run showed glass
transition temperature at 75 °C, then exotherme at 154 °C due to cold
crystallization (Tc) and melting endotherm at 230 °C. Thus the investi-
gated malononitrile derivatives exhibited typical behavior for molecular
1
=
2
P
P
a
R1
¼
jjF0j−jFcjj= jFcj; wR2 ¼ ð∑jwðF20−Fc2Þ2j=∑ðwF02Þ2Þ
.
and packing diagrams are presented on the Figs. S7 and S8.The structure
of MN-M compound is known and determined in this work is similar to
that reported in Ref. [22]. The main difference was in torsion of the
phenyl ring. The dihedral angles between the three phenyl rings in Li
et all work [23] were 70.05(1)°, 72.667(3)° and 74.16(3)°, respectively
while in this work they were 68.28(18)°, 70.05(19)°, 70.89(18)°. The
difference in the morphology of a crystal was dependent on the
crystallization solvent. In the cited work crystals were obtained from
hexane while there was used acetone/methanol mixture. The difference
in polarity of the used to crystallization solutions are responsible for
changes in the molecules geometry in crystals.
In both cases the π⋯π and CN…π-ring stacking interactions formed
dimers that was further linked to a 1D structure by H-bonding between
CN
CN
CN
CN
CN
CN
N
N
N
CN
NC
NC
CN
NC
CN
MN-T
MN-M
MN-D
Fig. 1. Chemical structures of the investigated malononitrile derivatives (MNs).

140
D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
MN-M
MN-D
Fig. 2. Fingerprint plots and percentage contributions to the Hirshfeld surface area for the various close intermolecular contacts for MN-M and MN-D molecules.
glasses. It is interesting that MN-D melted at higher temperature and
also exhibited higher T5 that MN-M and MN-T (cf. Supplementary
Material, Fig. S10).
The reported molecular orbital calculations on DFT differ from pre-
sented herein HOMO, LUMO and consequently Eg value [8]. However,
the same trend in HOMO's and LUMO's values was described in men-
tioned work
(−4.86 and − 2.92 eV for MN-M, −5.37 and − 3.57 eV for MN-D
and − 5.72 and − 3.32 eV for
3.4. DFT Calculations
MN-T).
The quantum theoretical calculations was used by density functional
theory (DFT), with an exchange correlation hybrid functional B3LYP and
the basis 6-311+G** for all atoms. The calculations were carried out
with the use of Gaussian 09 program [27]. Simulated geometries of ob-
tained compounds were optimized in vacuum. Based on the optimized
geometries molecular frontier orbitals were calculated in dichlorometh-
ane. The contours of HOMOs and LUMOs are presented in Fig. 3.
The HOMO orbitals were situated on the whole molecules in the case
of all compounds. As we can see, the number of malononitrile groups do
not affect the location of these orbitals. On the other hand, if we take ac-
count of HOMO's values we see that amount of malononitrile groups
lowers values of these orbitals (−5.79 eV for MN-M, −6.01 eV for
MN-D and − 6.16 eV for MN-T). LUMO orbitals for MN-M are placed
only on one phenyl ring with malononitrile group. In the case of MN-
D and MN-T are situated on two phenyl rings and two malononitrile
groups. Interestingly, for the compound MN-T the LUMO orbital doesn't
occur on all three phenyl ring with malononitrile groups, what could be
expected. Increasing the number of -CN groups from two to three does
not change the location of this orbital. Moreover, similar LUMO's values
for MN-D and MN-T confirm it (−3.23 eV and − 3.32 eV, cf. Table 2).
3.5. Electrochemical Investigations
The electrochemical behavior of the triphenylamino-malonitriles
was investigated using cyclic voltammetry (CV) in CH2Cl2 solution
with glassy carbon electrode as the working electrode. The cyclic volt-
ammograms for MNs are presented in Fig. 4. The electrochemical oxida-
tion and reduction onset potentials were applied for estimation of the
ionization potentials (IP) and electron affinities (EA) of the materials
(assuming that IP of ferrocene equals −5.1 eV) [28]. The electrochemi-
cal data together with DFT calculated HOMO, LUMO and band gap (Eg)
are presented in Table 2.
The first reduction process (Ered) (between −1.42 and −1.34 V) of
malononitrile derivatives was irreversible (ΔE ≥ 100 mV). The reduction
process is situated on strongly electron deficient 2,2-dicyanovinyl
(\\CH_C(CN)2) moieties [13,29]. The number of cyano groups weakly
affects the first reduction onset potentials (Ered (onset)), however a signif-
icant differences were seen in the case of oxidation process of MN-M
and MN-D (cf. Table 2). Similar values of Ered for mono (MN-M) and di

D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
141
Fig. 3. HOMO, LUMO contours of the studied MNs.
(MN-D) substituted triphenylamine were seen. The electron affinities
3.6. Photophysical Properties
difference between attached one (MN-M), two (MN-D) and three
(MN-T) malononitrile groups suggest similar acceptor strength for all
malononitrile derivatives. The anodic shift of Ered with the increasing
of acceptors was more expected [13,30].
The optical properties of malononitrile derivatives were studied by ab-
sorption and photoluminescence (PL) spectroscopy. UV–Vis absorption
and emission properties were investigated both in solutions (CHCl3
and NMP) and in solid-state as thin film and two kinds of blend, that
is, one with poly(methylmethacrylate) (PMMA) and the other with a
mixture of poly(9-vinylcarbazole) (PVK) (50 wt%) and (2-(4-tert-
butylphenyl)-5-(4-biphenylyl)-1,3,4-oxdiazole) (PBD) (50 wt%). The
binary blends contained 1, 2 and 15 wt% of the compounds, while
PMMA contained 1 wt% of compounds. The electronic absorption data
are collected in Table 3, while UV–Vis spectra in solutions are presented
in Fig. 5.
UV–Vis absorption spectra of the compounds in CHCl3 (cf. Fig. 5a)
and NMP consist of two bands, one below 305 nm and the other at
longer wavelengths that depended on the compounds structure and
solvent polarity. Maximum absorption bands (λmax) at lower wave-
lengths can be attributed to π → π* transition in triphenylamine struc-
ture and λmax above 350 nm to intermolecular charge transfer (ICT)
[31]. Taking into consideration the solvent polarity it can be seen that
maxima band absorption at longer wavelengths in less polar chloroform
solution were bathochromically shifted in comparison with the ones in
NMP, and the shift ranged from 65 to 123 nm depending on the com-
pound structure. The largest shift was observed for the triphenylamine
substituted with two malononitrile groups (MN-D) (cf. Table 3, Fig. 5a).
It is interesting that in chloroform solution absorption bands of
mono (MN-M) and tri (MN-T) substituted triphenylamine were
found at the same wavelength λmax = 444 nm, while for the disub-
The oxidation process is predominantly on the triphenylamine
donor, nevertheless the 2,2-dicyanovinyl moieties influence was seen.
The first oxidation potential for MN-M (Eox = 0.94 V) is quite similar
to oxidized unsubstituted triphenylamine (Eox = 0.98 V) [31]. The Eox
of MN-D and MN-T in cyclic voltammetry (CV) showed the same
potential peak position shifted to higher potential compare to MN-M
(cf. Table 2). The stronger electron-donating ability of the triphenylamine
donor was seen in the case of MN-M, as expected. S.H. Kim et al. also car-
ried out electrochemical measurements of malononitryle substituted
triphenylamines [8]. Despite the fact that, they performed the CV investi-
gations in different experimental conditions and used a value of −4.8 as
attributed to the Fc/Fc+ couple for calculation of EA and IP, the differences
compare to ours results are significant. Markedly, lower Eg value of
malononitrile derivatives (1.49 for MN-M and 1.67 eV for MN-D) was re-
ported [8].
Table 2
Redox potentials (vs Fc/Fc+), IP and EA together with HOMO and LUMO and the energy
band gaps of MNs.
Code
Eox onset Ered onset IPa
EAb
HOMOc LUMOc Eg
Eg
(DFT) (CV)
d
e
(CV)
[V]
(CV)
[V]
(CV)
[eV]
(CV)
[eV]
(DFT)
[eV]
(DFT)
[eV]
[eV]
[eV]
stituted compound (MN-D) absorption was red shifted to λmax
=
MN-M 0.85
MN-D 1.22
−1.21
−1.23
−1.28
−5.95 −3.89 −5.79
−6.32 −3.87 −6.01
−6.31 −3.82 −6.16
−2.8
−3.23
−3.32
2.99
2.78
2.84
2.06
2.45
2.49
477 nm. The red shift of disubstituted triphenylamine compare to
mono and tri MN was also reported [13]. It is worth pointing out
that the molecular absorption coefficient (ε) for the maximum
bands is much lower (except for the second band of MN-T) for the
compounds in NMP solution in comparison with the values in
CHCl3 solution. The effect of solvent polarity on UV–vis spectra of
investigated compounds was reported by B. Li et al. [7], but not in
MN-T
1.21
a
IP = −5.1 − Eox onset
EA = −5.1 − Ered onset
B3LYP/6–31++G**(d,p).
Eg = LUMO − HOMO.
Eg = Eox onset − Ered onset
.
b
c
.
d
e
.

142
D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
Fig. 4. Cyclic voltammogramms (CV) during (a) reduction and (b) oxidation process (GC as working electrode; scan rate 0.100 V·s−1; 0.2 M Bu4NPF6 in CH2Cl2).
CHCl3 and NMP and the opposite tendency, that is, positive
solvatochromism was observed. S.H. Kim et al. [8] presented UV–vis
spectra of MNs also in chloroform solution. However, considering the
detected in CHCl3 solution (cf. Table 3, Fig. 5). However, it is necessary
to stress that the spectrum of the disubstituted triphenylamine (MN-D)
film is broader and unsymmetrical in comparison with the spectra of
MN-M and MN-T films. Absorption maxima of the MNs molecularly
dispersed in PMMA were hypsochromically shifted in comparison with
the λmax detected for their films and chloroform solution. UV–Vis spectra
of the disubstituted triphenylamine (MN-D) in PMMA exhibited two
separated absorption bands similarly to the spectrum of its film, however
in the last case they were overlapped. In UV–vis spectra of compounds in
a binary matrix absorption originating from PVK:PBD was also seen at
300 nm.
λmax position of MN-M seen in its absorption spectrum at about 570 nm
is significant different compare to our result [8].
The observed spectral shifts in solvents with different polarities
mainly depends on the strength of the intermolecular hydrogen bonds
between the substituent groups of the spectrally active molecule and
the solvent molecules and whether the intramolecular hydrogen
bonds are present or not. In the case of the MNs compounds no intramo-
lecular hydrogen bonds occur and therefore the spectral shifts are sen-
sitive to the solvent polarity. In many molecules the π-π* bands are
shifted bathochromically when the solvent polarity increases. In the
case of the substituted triphenylamine compounds the hypsochromic
shift in solutions is observed which suggest that the formation of H-
aggregates plays an important role. The presence of aggregates is ex-
pected to diminish as the concentration of the solution is decreased
and the spectra recorded in films are usually used to obtain electronic
spectra from the isolated molecules. Thus the comparison of the absorp-
tion maxima of solutions and film also indicates the role of aggregates.
The stronger shift in NMP suggests increase in the order within H-
aggregate in more polar solvent. This conjecture is confirmed by the
analysis of packing and molecular interactions in the solid-state accord-
ing to which in the case of MN-D there are stronger stacking and H-
bond interactions than in MN-M.
The studied derivatives exhibited significantly higher
photoluminescence in CHCl3 than in more polar solvent, that is, NMP. In
Fig. 6 emission spectra and pictures taken under UV irradiation (λex
=
366 nm) of MNs are presented, whereas the PL spectroscopic data are
collected in Table 3.
In all the cases only one emission band was detected and the PL
intensity was dependent on the excitation wavelengths (λex) in the
solutions (cf. Table 4). Generally the investigated compounds in CHCl3
solution emitted light at longer wavelengths than in NMP. MN-D and
MN-T in chloroform emitted green and orange light, respectively
(cf. Fig.6a), whereas in NMP they emit blue light. It was reported that
the emission spectra of MNs may showed both positive and negative
solvatochromism with increasing solvent polarity [8]. It was found
that the emission wavelengths of MN-M and MN-T differ very little in
CHCl3, while the emission band of MN-D is blue shifted in comparison
with the MN-M and MN-T. The PL quantum yield (ΦPL) in chloroform
was found to be in the range of 11.9–32.4%, with the highest result for
Electronic spectra were also detected for films of the compounds and
their blends with PMMA and PVK:PBD. Maxima absorption bands in the
films were slightly bathochromic shifted in comparison with the ones
Table 3
The electronic absorption data of malononitrile derivatives.
Code
UV–Vis λmax [nm] (ε·104)a
b
CHCl3
NMPb
Film
445
Blend PMMA
1 wt%
Blend PVK:PBD
1 wt%
Blend PVK:PBD
2 wt%
Blend PVK:PBD
15 wt%
ε = 4.81c
ε = 33.00c
290 (3.1)
444 (9.3)
281 (2.0)
395 (2.2)
477 (6.3)
250 (9.9)
444 (5.6)
298 (1.8)
379 (2.3)
354 (1.6)
438
460
485
450
485
450
MN-M
MN-D
452
393
470
402
485
502sh
305 (3.0)
340 (2.6)
468
431
450
475
473
MN-T
a
ε - Absorption coefficient, [dm3·mol−1·cm−1].
b
c = 10−5 mol/dm3, solvents: chloroform, N-methyl-2-pyrrolidone.
Dielectric constant. PVK:PBD – 50:50 wt%.
Shoulder.
c
sh

D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
143
Fig. 5. UV–vis of the compounds in (a) chloroform solution and (b) – (d) in film, blend PMMA and PVK:PBD together with PL spectrum of PVK:PBD matrix (λex = 340 nm).
trisubstituted triphenylamine (MN-T). This confirms that in CHCl3 solu-
tion quantum yield increased with the number of malononitrile groups
in the compound. The reported relative photoluminescence quantum
yield determined using standards (quinine sulfate and/or fluorescein)
in various solvents was in the range of 0.01–0.192 [7,8]. The life-time
(τPL) measurements were carried out and the values of τPL were calcu-
lated, the longest decay was found for monosubstituted compound
(MN-M) (cf. Table 4). Maxima emission bands in the films were red
shifted in comparison with the ones in CHCl3 solution for MN-D and
MN-T. The longest wavelength of emitted light in film was recorded
for MN-T (cf. Table 4).
In order to clarity the observed blue shift in the PL maxima in a polar
NMP solvent, the geometries of the first singlet and triplet excited states
of the compounds were optimized at the TD-B3LYP/6-311+G** level in
both solvents. The calculated dipole moments in the first excited states
are lower than in ground state (cf. Supplementary Material, Table S3)
therefore in less polar solvent (CHCl3 vs NMP) the stabilization of the
excited state are expected. Moreover the emission originates from
LUMO→HOMO transition and the energy difference between these
levels is lower in chloroform than in NMP (cf. Supplementary Material,
Fig. S11). The maximum difference between S1 energy gaps in chloro-
form and NMP was calculated for MN-D compound, that is for the one
for which the greatest solvent effect is observed. Therefore, it can be
concluded that the blue shift of absorption and emission bands in chlo-
roform compared with NMP is caused, on the one hand, by formation of
H-aggregates in solutions, and on the other by a lower dipole moment of
the excited state compared to the ground one, which affects its stabiliza-
tion in less polar solvents.
Maxima emission bands (λem) of the compounds in blends with
PMMA were hypsochromic shifted in comparison with the emission of
the films and the differences were observed in the range of 63 nm
(MN-D) to 83 nm (MN-T) (cf. Supplementary Material, Fig. S12, S13).
In the case of films the ϕPL were higher than in solution for MN-M
and MN-T and reached the values of 26.5% and 42.5%, respectively.
Triphenylamine with two malononitrile units (MN-D) exhibited slightly
lower ΦPL in film (17.7%) than in solution. The PL quantum yield of the
compounds in PMMA blends increased with the increasing of the
nitrile groups and was found as 12.4, 55.0 and 58.7% for MN-M, MN-D
and MN-T, respectively. In the case of a binary matrix (PVK:PBD) the
hypsochromic shift of the λem was also observed in comparison with
the one detected in a neat film (cf. Fig. 6, Table 4). Increasing the com-
pounds content in a binary blends caused the red shift of the maximum
emission band originating from MNs (cf. Table 4, cf. Supplementary
Material, Fig. S14). The opposite behavior was registered in the case of
PVK:PBD with 1 and 2 wt% for MN-T. In the PL spectra of the most of
the blends with PVK:PBD, the emission of matrix in the range of
369–413 nm was seen and it suggests not complete energy transfer
from the matrix to the luminophore (cf. Fig. 6b,c,d). In the case of
PVK:PBD blend with the lowest MNs content the emission of matrix is
dominating, expect for MN-T. Energy transfer from host (binary matrix)

144
D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
(a)
MN-1 MN-2 MN-3
MN-1 MN-2 MN-3
MN-1 MN-2 MN-3
in CHCl3 solution
in NMP solution
in blend with PMMA
(c)
(b)
(d)
(e)
Fig. 6. (a) Photographs of investigated malononitrile derivatives under UV irradiation λex = 366 nm, (b) - (e) PL spectra of MNs in chloroform solution and in solid-state.
to guest (chromophore) via Förster energy transfer (FRET) can
takes place when the PL spectrum of the host matrix overlaps with
the UV–vis spectrum of the guest luminophore [32]. Energy transfer
may occur as well by Dexter energy transfer, however, Dexter energy
transfer requires matching the energy levels of the single and triplet ex-
citons in the host with the energy levels of the excitons in the guest [33].
The absorption of the investigated MNs overlapped with PL of a matrix
(cf. Fig. 5). The negligible PL of matrix for blend with 15 wt% of MN-D
was found, which let us to conclude that the energy transfer is almost
complete. The quantum yield of binary blends containing 1 wt% of
the compounds increased with the malononitrile groups number and
maxima of ΦPL ranged from 28.5% to 51.9%. Increase of compound con-
tent in blend affected in a different way PL quantum yield in depen-
dence on the number of malononitrile units. It was observed that the
highest ΦPL was estimated for 15, 2 and 1 wt% of compound MN-M,
MN-D and MN-T dispersed in a binary matrix, respectively. One can no-
tice that the photoluminescence quantum yield of MNs in the PVK:PBD
matrix for the lowest luminophore content (1 wt%) is higher than the
quantum yield measured for the photoluminescence in solution
(cf. Table 4). The decrease of ΦPL of PVK:PBD blends together with
increase of MN-D and MN-T content may indicate that in these layers
there exist intermolecular interactions which quench the emission,

D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
145
Table 4
Photoluminescence spectroscopic data of the malononitrile derivatives.
3.7. Electroluminescence Study
Electroluminescence (EL) ability of the MNs was tested in diodes
with two structure types. Diodes with configuration of ITO/PEDOT:
PSS/compound/Al and ITO/PEDOT:PSS/PVK:PBD:compound (1, 2
and 15 wt%)/Al were fabricated. The EL spectra of selected diodes
and HOMO (IP) and LUMO (EA) energy levels of device compo-
nents are presented in Fig. 7 and in Supplementary Material
(cf. Fig. S15). All prepared diodes under external voltage emitted
light (cf. Table 5). We are aware, that for detailed discussion of
OLED performance the luminance or power intensities of EL should be
given.
However, we present in this work preliminary investigations aimed
at testing only the ability of MNs for EL and at this stage we have exper-
imental capability to register electroluminescence spectra only. Thus,
the position of λEL and intensity of the EL emission band can
be disused. The maximum emission band (λEL) in the range of
532–653 nm was observed. Diodes with configuration ITO/PEDOT:
PSS/compound/Al emitted red (MN-M, MN-T) and orange (MN-D)
light under external voltage in the range of 6–10 V (cf. Table 5, Fig. 7),
whereas the diodes with compounds dispersed in PVK:PBD matrix
emitted green light. The red shift of EL with increasing of the MNs
content in matrix was seen except for MN-T with 2 wt%. It should
be stressed that the highest EL intensity was found for diodes
based on neat compounds. Considering the devices with such active
layers the EL intensity increased in the following order MN-T b MN-D b
MN-M. This rule also concerns the diodes with 1 wt% of MNs in PVK:
PBD matrix. Generally, the increase of MNs content in active layer re-
sulted in lowering the light intensity. Together with increase of external
voltage the increase of EL intensity without changes of λEL position
was seen.
Code
Medium
PL
λex
[nm]
λem
[nm]
Stokes shift
Φ [%]
(τ [ns])
a
[cm−1
]
310
340
450
310
340
310
340
450
440
310
340
450
310
340
450
310
340
450
310
340
400
470
340
310
340
400
480
400
470
310
340
490
310
340
490
310
340
400
490
310
340
450
470
310
340
310
340
410
470
430
450
310
340
470
310
340
470
310
340
470
596
596
596
443
443
591
591
591
516
385,523
385,523
523
374,550
374,550
550
369,564
385,564
564
526
526
526
526
453
574
574
574
574
511
511
385,532
385,532
532
383,536
383,536
536
371,557
557
557
557
593
593
593
5744
5744
5744
3812
3812
5551
5551
5551
3451
–
–
2619
–
–
4040
–
–
–
b
CHCl3
11.9 (2.0)
–
NMPb
–
15.4
20.1
26.5
12.4
28.6
2.7;28.4
8.8
31.4
22.9
33.7
30.1
30.8
38.7
–
Film
Blend PMMA 1 wt%c
MN-M
Blend PVK:PBD 1 wt%c
Blend PVK:PBD 2 wt%c
Blend PVK:PBD 15 wt%c
–
1049
1953
1953
1953
1953
6174
4702
4702
4702
4702
1707
1707
–
–
1962
–
–
1962
–
2665
2665
2665
5659
5659
5659
5659
7434
7434
4811
4811
4811
4811
4008
4008
–
–
b
CHCl3
15.4
18.7(1.1)
–
NMPb
Film
17.7
17.1
13.2
15.4
50.3
54.9
32.8
3.4;30.6
22.7
35.8
2.4;34.5
32.0
23.9
23.7
18.9
25.2
–
Blend PMMA 1 wt%c
MN-D
Blend PVK:PBD 1 wt%c
4. Conclusions
Blend PVK:PBD 2 wt%c
Blend PVK:PBD 15 wt%c
Three molecules consisting of triphenylamine being functionalized
with one, two and three malononitrile groups were prepared and
investigated. The impact of the number of malononitrile substituents
in the molecules on their selected properties was presented. These com-
pounds can formed glassy state and together with increase of number of
malononitrile units the increased of Tg from 27 °C to 99 °C was detected.
They showed the similar low Eg value, slightly lower about 0.1 eV for
molecule with two malononitrile units (MN-D). The results of CV mea-
surements show that the ionization potential decrease with increase of
number malononitrile groups from one to two in molecule. The studied
MNs were emissive in solution and in solid-state. The polarity of the sol-
vent significantly influenced on the absorption and photoluminescence
range and negative solvatochromic effect was observed. It can be
noticed that compounds with one and three malononitrile groups
(MN-M and MN-T) exhibited the similar UV–vis absorption and
photoluminescence range. Whereas, the absorption and emission of
disubstituted triphenylamine (MN-D) was bathochromically and
hyspsochromically shifted, respectively. Together with increase on
malononitrile units number more efficient PL with ΦPL up to 58.7% in so-
lution and in solid state solid-state as film and blend with PMMA was
seen (except for MN-D in film). Considering the PL in a binary matrix
only in the case of compound with one malononitrile unit (MN-M)
the highest its content resulted in raising of ΦPL. Optimal content of
MN-D and MN-T in PVK:PBD which gives the highest PL was 2 and
1 wt%. It seems that an AIE phenomenon is weaker in di- and trisubsti-
tuted triphenylamine. It was demonstrated that the compounds are
electroluminescent and diode with a neat malononitrile derivatives as
active layer emitted light with λEL the located at 603–653 nm. Whereas,
utilization of them as component of active layer gives emission of
green light by devices. To the best of our knowledge, the first time,
ability for electroluminescence of triphenylamines functionalized with
malononitrile groups is presented.
–
b
CHCl3
21.1
32.4(1.9)
–
593
455
455
604
604
604
604
521
NMPb
Film
–
19.6
16.8
42.5
15.0
42.7
58.7
51.9
51.3
39.0
14.1
13.3
24.0
8.0
MN-T
Blend PMMA 1 wt%
521
377,542
377,542
542
381,526
381,526
526
375,553
413,553
553
Blend PVK:PBD 1 wt%c
–
3772
–
–
2041
–
Blend PVK:PBD 2 wt%c
Blend PVK:PBD 15 wt%c
–
9.1
11.5
3058
Bold data indicates the most intense PL. Underlined data indicates the dominant band.
a
Stokes shifts calculated according to the equation Δν = (1/λabs-1/λem)·107 [cm−1].
b
c = 10−5 mol/dm3.
c
wt% - concentration of compound in blend PMMA or PVK:PBD (50 wt%:50 wt%).
contrary to the behavior seen for MN-M. It can be concluded that the
number of malononitrile units linked with triphenylamine structure
significantly affects the luminescence ability of the studied compounds.
The AIE phenomena seems to be weaker together with the increase of
number of malononitrile groups.

146
D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
Fig. 7. (a) HOMO and LUMO energy levels of diode components and (b) - (g) electroluminescence spectra with photographs of working devices of investigated malononitrile derivatives.

D. Sęk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 210 (2019) 136–147
147
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Table 5
Maximum of EL band and its intensity.
Code
ELa
intensity
λEL
ELb
intensity
λEL
ELc
intensity
λEL
ELd
intensity
λEL
[counts] [nm] [counts] [nm] [counts] [nm] [counts] [nm]
(2016) 383–390.
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MN-M
MN-D
MN-T
61,290
55,820
41,700
632
603
653
58,580
55,270
1112
532
537
559
52,660
1856
25,520
551
545
535
53,064
25,700
56,450
584
553
573
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a
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ITO/PEDOT:PSS/MNs/Al.
b
c
ITO/PEDOT:PSS/PVK:PBD:MNs 1 wt%/Al.
ITO/PEDOT:PSS/PVK:PBD:MNs 2 wt%/Al.
ITO/PEDOT:PSS/PVK:PBD:MNs 15 wt%/Al.
d
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Acknowledgments
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This work was supported by National Science Centre of Poland
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using resources provided by Wroclaw Centre for Networking and
Supercomputing (http://wcss.pl), grant No. 18. We thank dr H. Janeczek
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Appendix A. Supplementary Data
CIF files giving crystallographic data for MN-M and MN-D
(CCDC 1821956, 1822695) can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223-336-033. Supplementary data to this article can be
found online at https://doi.org/10.1016/j.saa.2018.11.025.
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