Combined experimental and density functional theory studies on novel 9-(4/3/2-cyanophenyl)-9H-carbazole-3-carbonitrile compounds for organic electronics

Article abstract of DOI:10.1002/poc.4207

We have synthesized a series of novel hybrid molecules 9-(2-cyanophenyl)-9H-carbazole-3-carbonitrile (o-CNCbzCN), 9-(3-cyanophenyl)-9H-carbazole-3-carbonitrile (m-CNCbzCN) and 9-(4-cyanophenyl)-9H-carbazole-3-carbonitrile (p-CNCbzCN), comprising electron-donating carbazole and electron-accepting nitrile groups. Three positional isomers were synthesized with a view to tune photophysical and electrochemical properties of the hybrids. The photophysical study displayed absorption maxima in the range of 281–340 nm and 277–298 nm whereas emission maxima in the range of 349–366 nm and 366–369 nm in toluene and dimethylformamide (DMF), respectively. These molecules demonstrated suitable frontier molecular orbital (FMO) energy levels and ensure good thermal and morphological stability. Among these synthesized molecules, m-CNCbzCN showed very high decomposition temperature (T_d = 341°C) whereas p-CNCbzCN exhibited good glass transition (T_g = 182°C) as well as melting temperature (T_m = 236°C), indicating its significant stability and potential utility as a bipolar host material for efficient phosphorescent organic light-emitting diodes (PhOLEDs).

Full text of DOI:10.1002/poc.4207

Received: 1 January 2021
DOI: 10.1002/poc.4207
Revised: 2 February 2021
Accepted: 11 March 2021
R E S E A R C H A R T I C L E
Combined experimental and density functional theory
studies on novel 9-(4/3/2-cyanophenyl)-9H-carbazole-
3-carbonitrile compounds for organic electronics
Bhausaheb Patil1 | Jatin Lade1 | Pratima Sathe1 | Anuj Tripathi2
|
B. Pownthurai1 | Prabhakar Chetti2
| Yogesh Jadhav1 | Atul Chaskar1
1National Centre for Nanosciences and
Abstract
Nanotechnology, University of Mumbai,
Mumbai, India
We have synthesized a series of novel hybrid molecules 9-(2-cyanophenyl)-
9H-carbazole-3-carbonitrile (o-CNCbzCN), 9-(3-cyanophenyl)-9H-carbazole-
3-carbonitrile (m-CNCbzCN) and 9-(4-cyanophenyl)-9H-carbazole-3-carbonitrile
(p-CNCbzCN), comprising electron-donating carbazole and electron-accepting
nitrile groups. Three positional isomers were synthesized with a view to tune
photophysical and electrochemical properties of the hybrids. The photophysical
study displayed absorption maxima in the range of 281–340 nm and 277–298 nm
whereas emission maxima in the range of 349–366 nm and 366–369 nm in tolu-
ene and dimethylformamide (DMF), respectively. These molecules demonstrated
suitable frontier molecular orbital (FMO) energy levels and ensure good
thermal and morphological stability. Among these synthesized molecules,
m-CNCbzCN showed very high decomposition temperature (Td = 341ꢀC)
whereas p-CNCbzCN exhibited good glass transition (Tg = 182ꢀC) as well as
melting temperature (Tm = 236ꢀC), indicating its significant stability and
potential utility as a bipolar host material for efficient phosphorescent organic
light-emitting diodes (PhOLEDs).
2Department of Chemistry, National
Institute of Technology, Kurukshetra,
Haryana, India
Correspondence
Prabhakar Chetti, Department of
Chemistry, National Institute of
Technology, Kurukshetra 136119,
Haryana, India.
Email: chetty_prabhakar@yahoo.com
Yogesh Jadhav and Atul Chaskar,
National Centre for Nanosciences and
Nanotechnology, University of Mumbai,
Mumbai 400098, India.
Email: yajadhav@nano.mu.ac.in;
achaskar25@gmail.com;
achaskar@nano.mu.ac.in
Funding information
Council of Scientific and Industrial
Research, India (CSIR), Grant/Award
Number: 02(0339)/18/EMR-II; Defense
Research and Development Organization
(DRDO), Grant/Award Number: ERIP/
ER/1503212/M/01/1666; University Grant
Commission
K E Y W O R D S
carbazole, carbonitrile, density functional theory, donor–acceptor system, photophysical study
1 | INTRODUCTION
advantage of these organic semiconductor materials is
low cost, lightweight, and possibility of producing flexible
Over the past few decades, an intense research effort has
been made for the design, synthesis, and application of
organic materials. At the present time, these organic
materials are emerging as the best alternative for inor-
ganic semiconductor materials and found applications in
organic light-emitting diodes (OLEDs),[1,2] organic photo-
voltaics (OPVs),[3,4] dye-sensitized solar cells (DSSCs),[5]
and organic thin-film transistors (OTFTs).[6] A further
and great area devices which make them the best substit-
uents for inorganic semiconductor materials.[7–10] Among
above stated optoelectronic devices, an OLED-based
flexible flat panel display has already conquered the
markets in the form of digital cameras, television, and
cell phones.[11–15] Nevertheless, phosphorescent OLEDs
(PhOLED) are promising owing to their ability to achieve
100% internal quantum efficiency. In these devices
J Phys Org Chem. 2021;e4207.
wileyonlinelibrary.com/journal/poc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.4207

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PATIL ET AL.
phosphorescent metal-complex emitters are commonly
doped into a suitable bipolar host material in order to
reduce concentration quenching and triplet–triplet anni-
hilation. The bipolar materials with donor–acceptor
(D-A) system comprising π or σ bridge have engaged with
great significance because of their distinctive properties
like well-defined structural system, balance hole and
electron transport, ease of purification along with
authentic reproducibility, high luminescence quantum
yield,[16–18] and easy fabrication of large area
devices.[19–27] Over a wide range of tailored D-A moieties
along with unique architecture system with wise selec-
tion and proper linking of donor and acceptor motifs
allows to tune the photophysical, thermal, and electro-
chemical properties.[28] Indeed, a host material for PhO-
LEDs device should possess some inherent features:
(I) higher triplet energy (ET) than the phosphorescent
guest, which prevent reverse energy transfer, (II) suitable
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) for effective
charge injections into the emitting layer from the
adjacent charge transporting layers, (III) large HOMO–
LUMO energy gap (Eg) than the phosphorescent guest,
and (IV) high glass transition temperature (Tg) to provide
good film morphology and avoid morphological changes
at device operating voltage and so on.
exhibited maximum external quantum efficiencies of
17.1 % and 24.4 %, respectively with significantly small
efficiency roll-off.[50,51] Li and coworkers reported three
positional isomers o-CzCN, m-CzCN, and p-CzCN with
donor:acceptor ratio of 2:1. m-CzCN-hosted blue phos-
phorescent OLED exhibited an external quantum effi-
ciency of 23.14% with a maximum current efficiency of
46.81 cd A−1.[52]
Herein, we report the synthesis of three novel D-A
materials 9-(2-cyanophenyl)-9H-carbazole-3-carbonitrile
(o-CNCbzCN),
9-(3-cyanophenyl)-9H-carbazole-3-
carbonitrile (m-CNCbzCN), and 9-(4-cyanophenyl)-
9H-carbazole-3-carbonitrile (p-CNCbzCN) by using a
combination of carbazole core as donor unit with periph-
eral cyano ( CN) as an acceptor unit encompassing ratio
of 1:2 (shown in Scheme 1). This investigation shed a
light on effect of position of CN group on properties of
molecules such as photophysical, electrochemical, and
intramolecular charge transfer (ICT), HOMO–LUMO
energy levels, and energy band gap. Density functional
theory (DFT) and time-dependent density functional
theory (TDDFT) studies are also performed to support
experimental findings. Additionally, calculated charge
transport parameters such as ionization potential, elec-
tron affinity, and reorganization energies of synthesized
molecules are also reported.
Unlike unipolar host, bipolar hosts promote balance
holes and electrons mobility which leads to generation of
broad charge recombination zone and eventually
alters the device structure; this results into augmentation
of the device performance.[29–34] In this context,
diverse D-A molecules consist of high triplet energy (ET)
donors such as triphenylamine,[35–37] carbazole,[38–40]
dibenzothiophene,[41] and dibenzofuran[42,43] and accep-
tors such as oxadiazole, triazole, azine, phenanthroline,
and benzimidazole have been synthesized and success-
fully used in the device fabrication. Indeed, carbazole
exhibit several meritorious properties like high triplet
energy level, small conjugated system, and chemical
stability, which made it as favorable donor moiety in
synthesis of bipolar materials.[44–46] Hitherto, cyano
( CN)-containing compounds are not so extensively
studied as bipolar host materials for PhOLEDs. It was
noted that the strong electron-withdrawing cyano group
( CN) should be a good candidate as n-type unit to form
D-A host molecules.[47–49] Lin and coworkers used a typi-
cal CN group to decorate the well-known benchmark
blue host material mCP (N,N-dicarbazolyl-3,5-benzene)
and realized the excellent device results.[47] Recently,
two novel host materials CNPhCz and DCNPhCz com-
prehending carbazole and cyanophenyl units were
designed, synthesized, and fully characterized by Liu
et al. and Li et al.. CNPhCz-hosted blue and green devices
2 | EXPERIMENTAL SECTION
2.1 | Materials and methods
Chemical reagents were purchased from commercial
suppliers. All reactions were done in a round bottom
flask and monitored by TLC performed on aluminum
plates (0.25 mm, E. Merck) precoated with silica gel
(Merck 60 F-254). Developed TLC plates were visualized
under a short-wavelength UV lamp. Reactions were
conducted an open-air atmosphere in solvent such as
dimethylformamide (DMF). Yields refer to spectroscopi-
cally (1H and 13C NMR) homogeneous material
obtained after column chromatography performed on
silica gel (100–200 mesh size) supplied by S.D. Fine
Chemicals Limited, India. 1H and 13C NMR were
recorded in DMSO solution with a Bruker 300-MHz
spectrometer. Chemical shifts (δ) are quoted in ppm,
relative to SiMe4 (δ = 0.0) as an internal standard. The
number of protons (n) for a given resonance is indicated
by nH. Peak multiplicities are designated by the
following abbreviations: s, singlet; d, doublet; t, triplet;
q, quartet; m, multiplet; dd, doublet of doublet; and
J, coupling constant in Hz. High-resolution mass spectra
(HRMS) were obtained by using positive electrospray

PATIL ET AL.
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SCHEME 1 Synthetic routes for bipolar host materials p-CNCbzCN, m-CNCbzCN, and o-CNCbzCN
ionization (ESI) by time-of-flight (TOF) method. Melting
points were recorded on a standard melting point
apparatus from Gautam Lab Equipment, Mumbai, and
are uncorrected. Electronic absorption spectra were
recorded on UV-Vis-NIR spectrophotometer. Emission
spectra were recorded on Varian Cary-Eclipse fluores-
cence spectrophotometer. The thermal properties were
measured on Mettler Toledo system with a heating rate
of 20ꢀC/min and N2 flow of 20 ml/min. Electrochemical
studies were performed using Metrohm potentiostat/
galvanostat Autolab PGSTAT 100 with a scan rate of
100 mV/s. A standard three-electrode cell comprising
Ag/AgCl, a platinum wire, and a glassy carbon electrode
as the reference, counter, and working electrodes,
respectively, was used. HOMO is calculated from the
oxidation peak potentials.
(0.5 g, 1.44 mmol) and CuCN (0.19 g, 2.12 mmol) were
dissolved in N,N0-DMF (5 ml). After stirring for 24 h,
reaction mixture was allowed to cool at room tempera-
ture. After completion of reaction (as indicated by TLC
Rf: 0.7), 60 ml of aqueous ammonia and 60 ml of water
were added, and the reaction mixture was extracted with
dichloromethane (2 × 50 ml). The combined organic
layer was dried over anhydrous Na2SO4 and evaporated
under vacuum. Crude solid obtained was then purified
by silica gel column chromatography with hexane:ethyl
acetate (80:20).
2.2.1 | 9-(4-Cyanophenyl)-9H-carbazole-
3-carbonitrile (6)
Pale yellow solid. Yield. 80% (0.33 g). 1H NMR
(300 MHz, DMSO, ppm): δ 8.96–8.87 (m, 1H), 8.38
(d, J = 7.8 Hz, 1H), 8.19 (d, J = 7.8 Hz, 2H), 7.93–7.74
(m, 3H), 7.57–7.38 (m, 4H); 13C NMR (75 MHz, DMSO,
ppm): 166.9, 141.6, 140.6, 138.2, 133.8, 129.5, 127.7,
126.5, 125.8, 123.2, 121.9, 121.5, 120.0, 110.8, 110.2,
102.1. HRMS-ESI (m/z): calcd for C20H11N3, [M + H]+:
293.0953; found, 293.0955.
2.2 | General procedure for the synthesis
of 9-(4/3/2-cyanophenyl)-9H-carbazole-
3-carbonitrile (6/11/16)
A
round bottom flask containing mixture of
4/3/2-(3-bromo-9H-carbazol-9-yl) benzonitrile (5/10/15)

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PATIL ET AL.
2.2.2 | 9-(3-Cyanophenyl)-9H-carbazole-
3-carbonitrile (11)
intermediates were then converted to 4-(9H-carbazol-
9-yl)benzonitrile (4), 3-(9H-carbazol-9-yl)benzonitrile (9),
and 2-(9H-carbazol-9-yl)benzonitrile(14) by replacing
bromo group by cyano group using copper cyanide.
Subsequently, intermediate 4, 9, and 14 were brominated
at C3 position of carbazole which eventually converted
to o-CNCbzCN, m-CNCbzCN, and p-CNCbzCN using
copper cyanide. These compounds were confirmed by
1H and 13C NMR spectroscopy and high-resolution mass
analysis.
Pale yellow solid. Yield. 76% (0.32 g). 1H NMR (300 MHz,
DMSO, ppm):δ 8.88 (s, 1H), 8.53 (s, 1H), 8.40–8.31
(m, 1H), 8.15–8.04 (m, 2H), 7.81–7.79 (m, 2H), 7.58–7.33
(m, 4H); 13C NMR (75 MHz, DMSO, ppm): 166.7, 141.9,
140.8, 136.4, 135.8, 130.4, 129.5, 127.7, 127.1, 125.7, 123.0,
121.7, 121.3, 120.6, 120.0, 112.2, 111.6, 110.8, 110.2, 101.9;
HRMS-ESI (m/z): calcd for C20H11N3, [M + H]+:
293.0953; found, 293.0952.
3.2 | Thermal properties
2.2.3 | 9-(2-Cyanophenyl)-9H-carbazole-
3-carbonitrile (16)
Differential scanning calorimetry (DSC) and ther-
mogravimetric analysis (TGA) were used to probe the
thermal properties of o-CNCbzCN (16), m-CNCbzCN
(11), and p-CNCbzCN (6) at a heating rate of 20ꢀC/min
under a nitrogen flow. Earlier reported cyano derivatives
by Lin et al. (mCPN and mCP),[47] Zhang et al. (CBP-CN
and CBP),[48] and Liu et al. and Li et al. (m-PyCNmCP,
3-PyCNmCP, CNPhCz, and DCNPhCz)[50,51] exhibited
glass transition temperatures in the range of 49ꢀC to
162ꢀC and decomposition temperature (Td) in the range
of 280ꢀC to 440ꢀC. However, our molecules o-CNCbzCN,
m-CNCbzCN, and p-CNCbzCN showed distinctly
higher glass transition temperatures (131ꢀC to 182ꢀC)
and decomposition temperatures (Td) in the range of
338ꢀC to 341ꢀC. The high Tg values are observed due to
encapsulation of two CN groups on N-phenyl carbazole
molecule. In addition, melting temperature of
m-CNCbzCN and p-CNCbzCN is found at 212ꢀC and
Pale yellow solid. Yield. 72% (0.30 g). 1H NMR (300 MHz,
DMSO, ppm): δ 8.95–8.81 (m, 1H), 8.65–8.52 (m, 1H),
8.40–8.30 (m, 1H), 8.16–8.05 (m, 2H), 7.88–7.79 (m, 2H),
7.57–7.32 (m, 4H); 13C NMR (75 MHz, DMSO, ppm):
166.8, 140.4, 138.8, 136.4, 130.3, 129.5, 128.7, 127.5, 127.1,
125.5, 124.7, 123.7, 123.1, 121.3, 120.6, 112.2, 111.6, 110.8,
109.8, 101.9. HRMS-ESI (m/z): calcd for C20H11N3,
[M + H]+: 293.0953; found, 293.0956.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis
The intermediates 9-(4-bromophenyl)-9H-carbazole (3),
9-(3-bromophenyl)-9H-carbazole (8), and 9-(2-bromophenyl)-
9H-carbazole (13) were prepared in good yields from
carbazole using Ullmann coupling reaction.[53–56] These
236ꢀC, respectively (Figure
significantly high glass transition temperature (Tg) and
1 and Table 1). The
FIGURE 1 Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) thermograms of o-CNCbzCN,
m-CNCbzCN, and p-CNCbzCN recorded at a heating rate of 20ꢀC/min

PATIL ET AL.
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TABLE 1 Photophysical, thermal, and electrochemical data of the synthesized molecules
λabs.[a] (nm) λfluor.[b] (nm)
[h]
Molecule
DCM
DCM
HOMO (eV) LUMO[c] (eV) Eoxi[d] (V) Ered[e] (V) Td[f]/Tm[g]/Tg (ꢀC)
o-CNCbzCN
274
353, 363
350, 366
351, 367
−6.19
−6.29
−6.32
−2.21
−2.29
−2.76
1.84
1.94
1.97
−0.43
−0.40
−0.49
337.7/–/167.7
m-CNCbzCN 275
p-CNCbzCN 274
341.5/212/131.3
338.7/236/181.6
Abbreviations: LUMO, lowest unoccupied molecular orbital; HOMO, highest occupied molecular orbital.
aAbsorption maximum.
bFluorescence maximum.
cLUMO levels were derived via eq. LUMO = HOMO + Eg.
dEoxi, oxidation peak potential (V).
eEred, reduction peak potential (V).
fTd, thermal decomposition temperature.
gTm, melting temperature.
hTg, glass transition temperature.
decomposition temperature (Td) values of our host mate-
rials imply its capability for enduring stable amorphous
thin films through vacuum thermal sublimation and
suppress aggregation formation.
polarity increases, bathochromic shifts were observed in
their PL spectra of all the three compounds, which could
be due to the strong ICT character at the excited state.
On comparing with the UV-Vis absorption spectra, all
the three compounds showed larger Stokes shifts in dif-
ferent solutions, which also means that all of them have
ICT character. The gradual red shifted emission suggests
rapid photoinduced electron transfer (PET) within the
D-A moiety. Due to PET, dipole moment of excited state
changes significantly which could be stabilized by
dielectric environment. The HOMO–LUMO gap, that is,
band gap is estimated from the Tauc plot (refer to the
Supporting Information) (Table 2).
3.3 | Photophysical properties
Electronic absorption and photoluminescence (PL)
spectra (recorded at room temperature) of p-CNCbzCN,
m-CNCbzCN, and o-CNCbzCN in various solvents are
depicted in Figure 2, and respective data are summarized
in Table 1.
Figure
2 displays UV-Vis and PL spectra of
o-CNCbzCN, m-CNCbzCN, and p-CNCbzCN in various
solvents. Previous reports on cyano derivatives by Lin
et al. (mCPN and mCP),[47] Zhang et al. (CBP-CN and
CBP),[48] and Liu et al. and Li et al. (m-PyCNmCP,
3-PyCNmCP, CNPhCz, and DCNPhCz)[50,51] exhibited
UV absorbance maxima in the range of 284–339 nm and
emission maxima in the range of 346–434 nm. However,
our molecules o-CNCbzCN, m-CNCbzCN, and
p-CNCbzCN showed absorption maxima in the range
of 274–275 nm and emission maxima in the range of
350–367 nm (Table 1). In the UV-Vis spectra of
o-CNCbzCN, m-CNCbzCN, and p-CNCbzCN, the
solvent polarity-independent absorption peaks around
257 nm are ascribed to the π–π* transition.[57–59] The
absorption spectra of these three compounds measured
in cyclohexane, toluene, CH2Cl2, MeCN, and DMF
showed nearly similar absorption properties. Such
solvent-independent absorption spectra suggest that the
Franck–Condon excited states are subjected to rather
small changes in dipole moment with respect to the
ground states. The PL spectra of o-CNCbzCN,
m-CNCbzCN, and p-CNCbzCN illustrated prominent
solvent-dependent emission properties. As the solvent
3.4 | Electrochemical properties
The electrochemical behavior particularly band
structure parameters of o-CNCbzCN, m-CNCbzCN, and
p-CNCbzCN were investigated by using cyclic
voltammetry (CV) method. Three-electrode cell system
encompassing a glassy carbon as working electrode,
silver/silver chloride (Ag/AgCl) as reference electrode,
and a platinum wire as counter electrode was used.
A ferrocene (Fc) was used as an internal standard for
CV. The CV experimental measurements are included in
Section 2, and the result is summarized in Figure 3
and Table 1, where tetra(n-butyl)ammonium hexa-
fluorophosphate (TBAPF6) in CH2Cl2 as a supporting
electrolyte (0.1 M) was used for the oxidation scans.[60]
All
the
three
compounds
o-CNCbzCN,
m-CNCbzCN, and p-CNCbzCN exhibited an irreversible
oxidation peak potential. The oxidation onsets of
o-CNCbzCN, m-CNCbzCN, and p-CNCbzCN observed
at 1.84, 1.94, and 1.97 V (vs. Ag/AgCl), respectively. It
was worthy to note that o-CNCbzCN and m-CNCbzCN
exhibited nearly the same oxidation peak potential value

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PATIL ET AL.
FIG URE 2 The (A) absorption and
(B) emission spectra o-CNCbzCN,
m-CNCbzCN, and p-CNCbzCN in
various solvents
TABLE 2 Calculated band gap (in
eV) from Tauc plot, cyclic voltammetry,
and DFT study
Band gap (Eg)
Tauc plot
Band gap (Eg)
CV
Band gap (Eg)
DFT
Molecule
o-CNCbzCN
m-CNCbzCN
p-CNCbzCN
3.98
4.00
3.56
2.33
2.34
2.46
4.07
4.16
4.16
Abbreviations: CV, cyclic voltammetry; DFT, density functional theory.
which correlates to the degree of π-conjugation due to
9H-phenyl carbazole moiety. HOMO energy level
was calculated using formula HOMO (eV) =
−(Eoxonset − EFc/Fc+onset) − 4.80 eV and those appear at
−6.19, −6.29, and −6.32 for o-CNCbzCN, m-CNCbzCN,
and p-CNCbzCN, respectively (Table 2). LUMO energy
levels were calculated from the HOMO energy levels
using the equation LUMO = HOMO + Eg, where Eg is
absorption wavelength band gap, that is, −2.21, −2.29,
and −2.76 for o-CNCbzCN, m-CNCbzCN, and
p-CNCbzCN, respectively.
To get more insight into absorption properties, quan-
tum mechanical calculations are performed for these
reported molecules. The electronic ground-state geome-
tries of all the three molecules are fully optimized in gas
phase without any symmetric considerations by using
Becke's three-parameter hybrid exchange with the
Lee–Yang–Parr correlation functional (B3LYP) with a

PATIL ET AL.
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6-31G(d,p) basis set. Vibrational frequencies were
evaluated for the optimized geometries at the same level
of calculations to ensure that the structure obtained was
a minimum on the potential energy surface. To predict
the absorption maxima, vertical excitation energies
were calculated with TDDFT method at the CAM-
B3LYP/6-31G(d,p) level. All the calculations were carried
out using ab initio software Gaussian 09W program.[61]
To get absorption energies of the molecules, TDDFT
calculations have been carried out for the molecules
under study. The calculated absorption energies are in
good agreement with the experiment and are having
absorption maxima in the range of 281 to 291 nm. Calcu-
lated lowest three states absorption energies (S1, S2, and
S3), oscillator strength, major transitions along with per-
centage weight are shown in Table 3a. The first excitation
(S1) in o-CNCbzCN and p-CNCbzCN molecules is due
to the major transition from HOMO to LUMO, whereas
m-CNCbzCN is due the transition between HOMO to
LUMO + 1, and corresponding molecular orbitals are
shown in Figure 4.
It is evident from Figure 4. That HOMO is localized
on backbone of the molecule whereas its LUMO is dis-
persed on the benzene ring with CN group as substitution
at ortho, meta, and para positions. This indicates that
these molecules are exhibiting D-A-type charge transfer
character.
The calculated HOMO and LUMO energies are fol-
lowing the experimental trend and the HOMO–LUMO
gap calculated from DFT are in excellent agreement
with HOMO–LUMO gap estimated from the CV and
FIGURE 3 Cyclic voltammograms of o-CNCbzCN,
m-CNCbzCN, and p-CNCbzCN
TABLE 3a Absorption energies (λmax in nm), oscillator strength ( f ), major transitions (MT), % weight (% Ci) obtained at TD-CAM-
B3LYP method
Molecule
State
S1
λmax
291
f
MT
% Ci
89
14
71
50
20
16
76
11
86
32
50
77
11
15
67
58
23
o-CNCbzCN
0.075
0.018
H ! L
S2
278
H − 1 ! L + 3
H ! L + 1
H − 1 ! L + 1
H ! L + 2
H ! L + 3
H ! L + 1
H − 1 ! L + 3
H ! L
H − 1 ! L + 1
H ! L + 2
H ! L
H ! L + 1
H ! L
S3
S1
260
281
0.024
0.035
m-CNCbzCN
p-CNCbzCN
S2
S3
277
262
0.053
0.062
S1
S2
S3
284
281
259
0.361
0.023
H ! L + 1
H − 1 ! L + 1
H ! L + 3
Note: % weight (% Ci), weight more than 10% indicated in the table.

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PATIL ET AL.
FIGURE 4 Molecular orbital pictures of
synthesized molecules
TABLE 3b Calculated HOMO energy, LUMO energy,
HOMO–LUMO gap (HLG in eV), and ground-state (μg) and
excited-state (μe) dipole moments (in Debye) obtained at B3LYP
method
The dihedral angles obtained from B3LYP/6-31G(d,p)
S0 and TD-B3LYP/6-31G(d,p) S1 geometries of
o-CNCbzCN, m-CNCbzCN, and p-CNCbzCN are
shown in Table 4.
Molecule
HOMO
−6.03
−6.12
−6.15
LUMO
−1.96
−1.96
−1.99
HLG
4.07
4.16
4.16
μg
μe
o-CNCbzCN
m-CNCbzCN
p-CNCbzCN
8.13
4.62
4.18
9.35
6.78
4.39
3.5 | Charge transport and charge
injection parameter of the synthesized
molecules
UV-Vis spectroscopy experimentally. The calculated
dipole moment of excited molecule changes signifi-
cantly which could be stabilized by dielectric environ-
ment (Table 3b).
According to hopping model, charge transfer reaction is
self-exchange electron transfer reaction between neutral
and radical cation (hole transfer) shown in Equation 1:
The geometrical inequality between ground (S0) and
excited (S1) states in the dihedral angles of o-CNCbzCN,
m-CNCbzCN, and p-CNCbzCN are apparent from
Table 4. In o-CNCbzCN, dihedral angle increased from
−68.35ꢀ to −90.91ꢀ, whereas in m-CNCbzCN, dihedral
angle increased from −56.42ꢀ to −87.94ꢀ. On the other
hand, in p-CNCbzCN, the same was increased from
−53.59ꢀ to 88.89ꢀ. The above outcomes specify that the
molecular configuration of carbazole and cyanophenyl in
all the three molecules is twisted in S1 state and the angle
of twisting in between S1 and S0 varies significantly,
which concur with the donor to acceptor charge transfer
behavior in these molecules.
M + M + ! M + + M:
ð1Þ
Marcus expresses the rate constant (K) for charge
transfer reaction between two molecules shown in
Equation 2 as[62,63]
ꢀ
ꢁ
4π2
h
−1
=
2
−λ
K =
H2ð4πλkTÞ expð
=
4kTÞ:
ð2Þ
Here, H is charge transfer integral matrix between
neighboring molecules,[64,65] λ is reorganization energy,
k is Boltzmann constant, and T is the temperature. Total
reorganization energy is the sum of internal and external

PATIL ET AL.
9 of 12
TABLE 4 Ground- and excited-state optimized geometries
Molecule
Ground-state geometry
Excited-state geometry
o-CNCbzCN
m-CNCbzCN
p-CNCbzCN
reorganization energies; however, external λ is negligible
as compared with internal λ.[66] The calculation of inter-
nal λ is done by using the following equations.
Calculation of ionization potentials (adiabatic [IPa]
and vertical [IPv]) and electron affinities (adiabatic [EAa]
and vertical [EAv]) are done by Equations (5) and (6),
respectively, as they are important parameter regarding
charge injection phenomenon.
Â
Ã
λh = λ1 + λ2 = ½E + ðM0Þ−E+ ðM + Þꢁ + E0ðM + Þ−E0ðM0Þ ,
ð3Þ
IPa = E+ ðM + Þ−E0ðM0Þ,
IPv = E+ ðM0Þ−E0ðM0Þ,
ð5aÞ
ð5bÞ
Â
Ã
λe = λ3 + λ4 = ½E− ðM0Þ−E− ðM− Þꢁ + E0ðM− Þ−E0ðM0Þ :
ð4Þ

10 of 12
PATIL ET AL.
TABLE 5 Calculated ionization potential (IP, in eV), electron affinity (EA, in eV), hole and electron (λh and λe) reorganization energy
(in meV), and singlet–triplet gap (ES-T in eV)
Name
IPa
IPv
EAa
0.49
0.49
0.58
EAv
0.35
0.37
0.46
λh
λe
ES-T
3.19
3.19
3.16
o-CNCbzCN
m-CNCbzCN
p-CNCbzCN
7.43
7.50
7.52
7.49
7.55
7.56
119
90
290
252
241
81
EAa = E0ðM0Þ−E− ðM− Þ,
EAv = E0ðM0Þ−E− ðM0Þ:
ð6aÞ
ð6bÞ
the molecules, λh < λe, suggesting that the studied mole-
cules are better candidates as hole-transporting materials.
Also, the λh calculated for the isomers are smaller
than that of N,N0-diphenyl-N,N0-bis(3-methylphenyl)-
(1,10-biphenyl)-4,40-diamine (TPD) which is typical
hole-transport material (λh = 290 meV).[69] Furthermore,
the λe calculated for the isomers are smaller than tris
(8-hydroxyquinolinato)aluminum(III) (Alq3) which is
considered as typical electron-transport material
(λe = 276 meV).[70] In conclusion, the theoretical shows
that the studied isomers are showing smaller reorganiza-
tion energy than standard molecule reported in literature.
Also, the synthesized isomers are showing very small
hole reorganization energy suggesting these isomers are
better candidate for hole-transport materials.
To know singlet–triplet gap (ES-T in eV), the mole-
cules are also optimized in their triplet state, calculated
(ES-T) are tabulated in Table 5 using Equation 7. From
Table 5, it is clear that the singlet–triplet gap energies are
in the range of 3.16–3.19 eV. This showed that these
could be potential bipolar host materials for realization of
phosphorescent LEDs.
Here, in the above equations, E+(M0)/E−(M0) is the
energy of cation/anion obtained at ground-state neutral
molecule, and E0(M+)/E0(M−) is the energy of neutral
molecule obtained at optimized cationic/anionic state.
The lowest energies of cation, anion, and neutral state
are represented by E+(M+), E−(M−), and E0(M0),
respectively.
The calculation of IP and EA is carried out for the
reported molecules, as they are responsible for energy
barrier regarding injection of the holes and electrons,
respectively. IP is defined as the energy required to
remove an electron, and it must be sufficiently low for
efficient hole injection into the HOMO of the molecule.
The smaller IP of the hole-transport layer (HTL), the eas-
ier will be the injection of holes from indium tin oxide
(ITO) to HTL. EA is defined as the energy released when
an electron is added to the system, and it must be high
enough to allow an efficient electron injection into
LUMO of the molecule. The EAa (EAv) values are higher
for the molecule p-CNCbzCN among all the molecules,
ES−T = Etriplet −Esinglet
:
ð7Þ
suggesting there is efficient electron injection in LUMO
[67,68]
level of this molecule.
In Table 5, calculated values
of IP and EA are tabulated. From Table 5, it is observed
that the calculated IPa (IPv) values for ortho-substituted
molecule is 7.43 eV (7.49 eV) and is smallest among all
the isomers. Hence, it is easy to create a hole in ortho-
substituted molecule compared to para- and meta-
substituted isomers.
The calculated hole (λh) and electron (λe) reorganiza-
tion energy for ortho-, meta-, and para-substituted mole-
cule are shown in Table 5. As evident from Equation 2,
smaller reorganization energy leads to the higher rate
constant value which in turn increases the charge
transport properties in the molecule. From Table 5,
para-substituted isomer is showing smallest λh value of
81 meV, whereas meta- and ortho-substituted isomers are
showing 90 and 119 meV, respectively. Similar is the case
for λe values, para isomers is showing smallest (241 meV)
and ortho being the highest (290 meV). Moreover in all
4 | CONCLUSIONS
A series of dicyano derivatives of N-phenyl carbazole was
synthesized via Ullmann coupling reaction and other
synthetic methods. The absorption, emission, thermal,
and electrochemical properties as well as DFT studies of
the synthesized materials were reported. The properties
of the molecules are significantly influenced by the posi-
tion of the cyano segment attached to the phenyl ring of
N-phenyl carbazole. The HOMO energy levels of these
three compounds were found in the range of −6.19 to
−6.32 eV and were comparable with those of the com-
monly used hole-transporting materials. Experimental
data are in correlation with the theoretical data calcu-
lated by DFT and TDDFT methods. The thermal study
showed that these derivatives have high glass transition
temperature and good thermal stability. The results

PATIL ET AL.
11 of 12
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obtained for the HOMO–LUMO positions and the energy
gap from DFT calculation show very close agreement
with the experimentally estimated values using spectros-
copy and CV. Lastly, on the basis of the theoretical and
experimental study, we believe that these compounds
have the potential to be used as bipolar host materials in
optoelectronic device fabrication.
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B.N.P. thanks University Grant Commission for financial
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greatly appreciates the generous financial support from
the Defense Research and Development Organization
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The authors declare no competing financial interest.
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
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How to cite this article: Patil B, Lade J, Sathe P,
et al. Combined experimental and density
functional theory studies on novel 9-(4/3/2-
cyanophenyl)-9H-carbazole-3-carbonitrile
compounds for organic electronics. J Phys Org
Chem. 2021;e4207. https://doi.org/10.1002/poc.
4207