Butterfly-Shaped Thiophene-Pyridine Hybrids: Green Electroluminescence and Large Third-Order Optical Nonlinearities

Article abstract of DOI:10.1002/cplu.202000435

A set of four symmetric, butterfly-shaped 4-(4-(decyloxy)phenyl)-2,6-di(thiophen-2-yl)pyridine (TPY) derivatives 2TPA-TPY (TPY center and triphenylamine end groups), 2CBZ-TPY (TPY center and N-ethyl carbazole end groups), 2TPY-TPA (triphenylamine center and TPY at the periphery) and 2TPY-CBZ (N-ethyl carbazole center and TPY at the periphery) was synthesized. The molecules show reverse saturable absorption (RSA) which is consistent with two-photon absorption (2PA) associated with excited-state absorption (ESA) when excited using a 532 nm laser beam. The molecules 2TPA-TPY and 2TPY-TPA possess extremely low limiting thresholds of 1.73 and 2.68 J cm^?2, respectively. An organic light-emitting diode (OLED) fabricated from 2TPA-TPY exhibits green emission with a maximum luminance of 207 cd m^?2, a current efficiency (η_CE) of 1.51cd A^?1, a maximum power efficiency (η_Pmax) of 0.46 lm W^?1 and an external quantum efficiency (η_EQE) of 0.48 % at 100 cd m^?2.

Full text of DOI:10.1002/cplu.202000435

A Multidisciplinary Journal Centering on Chemistry
Accepted Article
Title: Butterfly-Shaped Thiophene-Pyridine Hybrids: Green
Electroluminescence and Large Third-Order Optical
Nonlinearities
Authors: Udaya Kumar Dalimba, Viprabha Kakekochi, Sathish
Chatnahalli Gangadharappa, Nikhil P.P, Chandrasekharan
Keloth, Vibhu Darshan, and Narayanan Unni K. N.
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0
1/2020

ChemPlusChem
10.1002/cplu.202000435
Butterfly-Shaped Thiophene-Pyridine Hybrids: Green Electroluminescence and Large
Third-Order Optical Nonlinearities
[a]
[a]
[b]
Viprabha Kakekochi , Sathish Chatnahalli Gangadharappa , Nikhil P. P. , Dr. Keloth
[b]
[c]
[c]
Chandrasekharan , Vibhu Darshan , Dr. Narayanan Unni K. N. and Dr. Udaya Kumar
*
[a]
Dalimba .
[a]Organic and Materials Chemistry Laboratory, Department of Chemistry, National Institute
of Technology Karnataka, Srinivasnagar 575025, Mangaluru, India.
[b]Laser and Nonlinear Optics Laboratory, Department of Physics, National Institute of
Technology Calicut, Kozhikode 673601, Kerala, India.
[c]Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIR-
National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, Kerala
6
95019, India.
Corresponding Author
Contact email: udayaravi80@gmail.com, udayakumar@nitk.ac.in
*
Abstract
A set of four symmetric, butterfly-shaped 4-(4-(decyloxy)phenyl)-2,6-di(thiophen-2-
yl)pyridine (TPY) derivatives viz., 2TPA-TPY containing TPY moiety at the center and
triphenylamine units as end groups, 2CBZ-TPY possessing TPY moiety at the center and N-
ethyl carbazole units as end groups, 2TPy-TPA comprising triphenylamine unit at the center
with TPY moiety at the periphery and 2TPY-CBZ consisting of N-ethyl carbazole at the center
with TPY moiety at the periphery was synthesized. The synthesized molecules show reverse
saturable absorption (RSA) which is consistent with two-photon absorption (2PA) associated
with excited state absorption (ESA) when excited using a 532 nm laser beam. The molecules
2
TPA-TPY and 2TPY-TPA possess extremely low limiting thresholds of 1.73 and 2.68 J
−
2
cm , respectively. The organic light emitting diode (OLED) fabricated as a proof of concept
using selected molecule 2TPA-TPY as emitter exhibits green emission with a maximum
−
2
−1
luminance of 207 cd m , a current efficiency (ηCE) of 1.51cd A , a maximum power efficiency
η
Pmax) of 0.46 lm W and an external quantum efficiency (ηEQE) of 0.48 % at 100 cd m−2.
−1
(
1
. Introduction
Designing and developing new families of organic molecules with the specific features is at
the heart of optoelectronics. Indeed, a number of organic molecules have drawn immense
attention due to their prime applications in the fields like OLEDs, nonlinear optics (NLO),
organic solar cells (OSCs) and field effect transistors (FETs). In particular, ample interest has
1
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10.1002/cplu.202000435
been developed towards OLEDs owing to their advantages such as light weight, low cost,
thinness, flexibility, high brightness and wide viewing angles. In addition, they are also
considered as one of the most anticipated device architectures for solid state lighting sources
as well as a key technology for flat-panel displays[1–4]. The prerequisite of monochromatic red-
green-blue (RGB) primaries with narrow bandwidth to the BT.2020 standard demands great
[5]
challenge for the development of RGB luminescent materials . However, the ultrapure green
emission strictly needs narrower half width spectrum and defined emission peak wavelength
as compared to the other colors[6,7]. Although it is difficult to develop high color purity emitters
and to achieve excellent devices, from the point of chemical approach one way of improving
the major factors such as low thermal stability, chemical purity and poor efficiency that restrict
the device performance is designing of rigid either star/butterfly-shaped or dendrite-shaped
molecules[8–10]. Moreover, due to the advantages like excellent thermal stability, high purity,
good thin film formation and solution processability of star/butterfly-shaped molecules over
the linear molecules, they have gained enormous attention in the recent past[11–14]
To date, there has been a rapid technological development in NLO which demands the rational
design and synthesis of organic materials to achieve enhanced nonlinear optical properties[15]
.
.
Among the organic materials studied, the high molecular weight π–conjugated molecules with
donor–acceptor–donor (D–A–D) arrangement have been the attracting candidates due to their
extensive π–delocalization which enables intramolecular charge transfer (ICT) between D and
A units, providing high polarizability to the system, which results in superior nonlinear optical
properties in addition to their fast-optical responses, ease of fabrication, light weight and
flexibility[16,17]. In addition, materials with 2PA or RSA property are remarkably used in many
photonic industry such as optical limiters for protecting human eye and sensitive devices from
high energy lasers[18], 3D optical data storage[19], microfabrication[20], energy up-conversion[21]
and two-photon photodynamic therapy[22]. However, there is still room for developing novel
molecules which can exhibit TPA/RSA.
Among a wide varieties of electron donors exploited in the optoelectronic field, thiophene is
found to be one of the most explored heterocyclic compounds due to its unique optical and
electronic properties[23]. The degree of π–conjugation of thiophene-based materials affects the
FMO energies, which in turn affect the energy gap, photophysical and electrochemical
properties. Moreover, the extent of π–conjugation also governs the highest occupied molecular
orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels and electrical
conductivity, making the thiophene based materials as an inimitable part in synthetic chemistry
and optoelectronics[24–26]. Another excellent electron donor is, propeller shaped triphenylamine
2
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10.1002/cplu.202000435
moiety possessing a continuous conjugation between the central nitrogen atom and peripheral
phenyl groups[27]. In addition, it has a high triplet energy of 3.04 eV and possesses good hole
transporting property[28–30]. Similarly, carbazole, another electron donating group, which when
incorporated into a conjugated backbone, provides good charge-transport function, high
thermal stability, solubility, moderately high oxidation potential and extended glassy state[31,32]
and hence, carbazole containing molecules also have attracted much attention in the
optoelectronic industry. On the other hand, the simplest electron accepting unit in the category
of small band gap material containing D–A units is pyridine ring, possessing an electron
deficient imine nitrogen[33]. The high thermal stability, chemical stability and effective
electron-transporting abilities of pyridine make pyridine to have significant role in
optoelectronic industry[31]
.
On the basis of foregoing points, herein, two sets of four symmetric, novel butterfly-shaped
molecules (2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ) were designed and
synthesized. In one set (2TPA-TPY and 2CBZ-TPY) the central pyridine core (served as
electron acceptor) was connected to terminal triphenylamine (2TPA-TPY)/carbazole (2CBZ-
TPY) moiety (served as electron donor) through thiophene (served as π–linker), which
provided a D–π–A–π–D structural configuration to the system, whereas, in another set (2TPY-
TPA and 2TPY-CBZ), the central triphenylamine (2TPy-TPA)/carbazole (2TPY-CBZ)
moiety was connected to the peripheral thiophene–pyridine–thiophene unit (D–A–π) to provide
D–A–π–D–π–A–D configuration. Their thermal, photophysical and electrochemical properties
were discussed in detail along with the theoretical evidence. In addition, the third-order NLO
properties of all the four molecules were discussed and the electroluminescent property of the
selected molecule 2TPA-TPY was also investigated by fabricating OLED.
3
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Figure 1. Molecular structures of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ.
4
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5
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Scheme 1. Synthetic pathways of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ.
Reagents and reaction conditions: (a) ammonium acetate, microwave, 400 W, 120–180°C, 0.5
h, yield: 68%; (b)1-bromodecane, K
0–95°C, 48 h, yield: 26–49%; (d) sodium borohydride, CH
PPh HBr, CHCl , 60°C, 3 h, yield: 93–94%; (f) NaH, C Br, DMF, RT, 12 h, yield: 82%; (g)
POCl , DMF, 60–65°C, 24 h, yield: 38–49%; (h) C ONa, CHCl , C OH, RT, 12 h, yield:
6–63%.
2
CO
3
, DMF, 85–90°C, 8 h, yield: 90%; (c) POCl
3
, DMF,
9
3
OH, RT, 12 h, yield: 90–92%; (e)
3
3
2
H
5
3
2
H
5
3
2
H
5
5
2
2
. Results and discussion
.1 Synthesis
Scheme 1 depicts the synthesis of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ, and
their chemical structures are shown in Figure 1. The molecules were synthesized using
multistep synthetic protocol. The green synthesis of core moiety 3 involves one-pot
multicomponent reaction under solvent-free (neat) condition via microwave irradiation. The
intermediate 3 was alkylated to improve the solubility to achieve intermediate 4 which was
formylated to obtain intermediates 5 and 6. The reduction of intermediates 5 and 6 yielded the
intermediates 7 and 8, respectively, which were later converted into their corresponding Wittig
salts 9 and 10. Finally, Wittig reaction of intermediate 9 with bis(4-formylphenyl)phenylamine
and intermediate 14 produced 2TPY-TPA and 2TPY-CBZ, respectively, while, Wittig
reaction of intermediate 10 with 4-formyltriphenylamine and intermediate 13 produced 2TPA-
6
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TPY and 2CBZ-TPY, respectively. The characterization details of all the intermediates and
final molecules are given in the ESI.
2
.2 Photophysical studies
The UV–vis absorption spectra of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ
exhibit two absorption bands (Figure 2a–d). One absorption band is located at higher energy
UV region (280–330 nm) and the other is located at lower energy visible region (350–460 nm).
The band at higher energy region is attributed to π-π* transition and that at lower energy region
ascribes to ICT transition from the electron donating triphenylamine/carbazole moiety to the
electron accepting pyridine group. The absorption maxima (λmax) of 2TPA-TPY, 2CBZ-TPY,
2
TPY-TPA and 2TPY-CBZ are observed at 409, 394, 419 and 404 nm, respectively. The
absorption spectra clearly display the effect of variation of peripheral groups on the conjugation
and hence, the absorption of the molecules. As it is seen in Figure 2a–d, there is a considerable
red shift in the absorption of 2TPA-TPY and 2TPY-TPA compared to that of 2CBZ-TPY and
2
TPY-CBZ. This bathochromic shift is attributed to the higher electron donating ability of
triphenylamine group in 2TPA-TPY and 2TPY-TPA compared to that of carbazole in 2CBZ-
TPY and 2TPY-CBZ. Moreover, the higher λmax of 2TPY-TPA is resulted from the higher
degree of conjugation in 2TPY-TPA compared to that of 2TPA-TPY, 2CBZ-TPY and 2TPY-
CBZ. The optical band gap (E
emission spectra is 2.64, 2.71, 2.62 and 2.76 eV for 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and
TPY-CBZ, respectively.
g
) determined from the point of intersection of absorption and
2
The fluorescence spectra at RT show an emission peak at 503, 450, 488 and 461 nm for 2TPA-
TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ, respectively (Figure 2a–d), by exciting at
their corresponding λmax. In addition, there is a considerable red shift in the absorption (Figure
3
a) and emission spectra (Figure 3b) of all the four molecules in the solid state compared to
that of solution state. This is due to the increased intermolecular interactions in the solid state.
For an ideal fluorophore, fluorescence quantum yield (Φ ) is one of the deciding factors. In view of
this, their absolute solid-state fluorescence quantum yields were obtained, and they are 0.153, 0.021,
.041 and 0.038 for 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ, respectively. The
F
0
emission profiles of the four molecules were also measured at 77 K in degassed toluene to
ensure the emission from the triplet state. It is observed that the low temperature emission
profiles are overlapping with the RT emission profiles (Figure 2a–d), which suggests the
nonexistence of phosphorescence and validates the pure fluorescence emission (with the
enhanced vibronic bands). Further, the time-resolved photoluminescence decay dynamics were
studied at RT to measure the lifetime of the molecules. The time-resolved photoluminescence
7
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decay profiles for the molecules in the degassed toluene are given in Figure S31. The
molecules 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ possess a short lifetime (
of 2.93, 2.42, 1.17 and 1.18 ns, respectively, which is attributing to the relaxation from S to
. And there is no any delayed component observed, indicating the pure fluorescence
emission. The data of absorption and emission studies are summarized in Table 1.
F
)
1
S
0
2
TPA-TPY
Abs
FL at 77K
FL at RT
2CBZ-TPY
Abs
.0 (a)
1.0
0.8
0.6
0.4
0.2
0.0
1
.0 (b)
1.0
1
0
0
0
0
0
FL at 77K
FL at RT 0.8
.8
.6
.4
.2
.0
0.8
0.6
0.4
0.6
0.4
0.2
0
.2
.0
0
0.0
700
3
00
400
500
600
700
300
(d)
400
500
600
Wavelength (nm)
Wavelength (nm)
(c)
2TPY-TPA
2TPY-CBZ
1
0
0
0
0
0
.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Abs
FL at 77K
FL at RT
Abs
FL at 77K
FL at RT
.8
.6
.4
.2
.0
3
00
400
500
600
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
−
5
Figure 2. UV-Vis absorption spectra at RT (measured in CHCl , 10 M) and fluorescence
3
−
5
emission spectra at RT and 77K (measured in toluene, 10 M) of a) 2TPA-TPY, b) 2CBZ-
TPY, c) 2TPY-TPA and d) 2TPY-CBZ.
(a)
2TPA-TPY
1.0 (b)
2TPA-TPY
1.0
0.8
0.6
0.4
0.2
0.0
2
2
CBZ-TPY
TPY-TPA
2CBZ-TPY
2TPY-TPA
2TPY-CBZ
2TPY-CBZ
0.8
0.6
0.4
0.2
0.0
3
50
400
450
500
550
600
650
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Figure 3. a) Absorption in thin film and b) emission profiles in the solid state of 2TPA-TPY,
CBZ-TPY, 2TPY-TPA and 2TPY-CBZ.
2
8
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Table 1. Optical absorption and emission data of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and
2
TPY-CBZ.
abs
max
pl
max
[b]
F[c]
(ns)
Molecules
λ
λ
E
g
Φ
F
[a]
[
solution/
thin film]
nm)
[solution/
solid state]
(nm)
[optical /
calculated]
(eV)
(
±0.02)
(
2
TPA-TPY
CBZ-TPY
TPY-TPA
TPY-CBZ
409/420
394/408
419/430
404/417
503/543
450/569
488/538
461/542
2.64/3.05
2.71/3.26
2.62/2.98
2.76/3.19
0.153
0.021
0.041
0.038
2.93 ± 0.03
2.42 ± 0.01
1.17 ± 0.05[*]
1.19 ± 0.005
2
2
2
[
a] Optical band gap calculated from the intersection of normalized absorption and emission
spectra.
b] Absolute fluorescence quantum yield determined using an integrated sphere in solid state at RT.
c] Amplitude weighted average lifetime biexponential, [*] triexponential.
.3 Electrochemical properties
Figure 4a–d depict the cyclic voltammograms of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and
TPY-CBZ, respectively, that were obtained by cyclic voltammetry (CV) measurements. The
[
[
,
2
2
observed onset oxidation peaks of first oxidation peak (Eoxonset) are 0.77, 0.80, 0.71 and 0.82 V
vs SCE for 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ, respectively. As observed,
the molecules undergo multistep oxidation process. The oxidation peaks other than the first
oxidation peak could be due to the oxidation of different units present in the molecule. The
small oxidation peaks observed at ~1.67–1.7 V in all the four molecules could be associated
with vinylene bond oxidation[34]. The oxidation peak observed at ~1.2 V may be due to the
oxidation of N-substituted carbazole in 2CBA-TPY and 2TPY-CBZ[34] whereas, the potential
range between 0.8–1.5 V in 2TPA-TPY and 2TPY-TPA could be related to the oxidation of
triphenylamine unit which depends on the substituents attached to the phenyl ring of
[35]
triphenylamine unit . The energy of the HOMO (EHOMO) calculated from the equation (1) is
5.31, −5.34, −5.25 and −5.36 eV for 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ,
respectively.
−
ox
EHOMO = −E + 4.8eV − EFOC ........................................................(1)
onset


where, Eoxonset and EFOC are the onset oxidation potentials of the molecules and ferrocene,
respectively, and −4.8 eV is the HOMO energy level of ferrocene against vacuum.
The energy of the LUMO (ELUMO) determined from the equation (2) is −2.67, −2.63, −2.63 and
−2.60 eV for 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ, respectively.
9
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ELUMO = EHOMO + E .............................................................................(2)
g
The presence of stronger electron donating triphenylamine group contributes to a lower
oxidation potential and higher HOMO energy level in 2TPA-TPY and 2TPY-TPA compared
to carbazole containing 2CBZ-TPY and 2TPY-CBZ. The electrochemical data are listed in
Table 2.
2TPA-TPY
2TPA-CBZ
(a)
(b)
0.08
0.04
0.00
0
.04
.00
0
-
0.04
0.08
-
0.04
0.08
0.3
-
-
-
2
-1
0
1
2
-2
-1
0
1
2
Potential (V vs SCE)
Potential (V vs SCE)
2
TPY-TPA
2TPY-CBZ
0
0
0
.08 (c)
(d)
0
.2
.1
.04
0
.00
0.0
0.1
-
-0.04
-
2
-1
0
1
2
-2
-1
0
1
2
Potential (V vs SCE)
Potential (V vs SCE)
Figure 4. Cyclic voltammograms of a) 2TPA-TPY, b) 2CBZ-TPY, c) 2TPY-TPA and d)
TPY-CBZ.
2
Table 2. Electrochemical, theoretical and thermal results of 2TPA-TPY, 2CBZ-TPY, 2TPY-
TPA and 2TPY-CBZ.
Molecules Eoxonset
HOMO
LUMO
T
/T
g
d
(V)
[optical/calculated] [optical/calculated]
(°C)
(eV)
(eV)
2
TPA-TPY 0.77
CBZ-TPY 0.80
TPY-TPA 0.71
TPY-CBZ 0.82
−5.31/−4.73
−5.34/−4.76
−5.25/−4.70
−5.36/−4.77
−2.67/−1.68
−2.63/−1.50
−2.63/−1.72
−2.60/−1.58
72.32/426
66.12/432
27.63/417
36.33/428
2
2
2
2
.4 Theoretical calculations
Figure 5 represents the ground state optimized geometry and the HOMO/LUMO distribution
of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ. The long decyloxy chain is
replaced by methoxy group to reduce the calculation time. As seen in Figure 5, in the case of
1
0
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2
TPA-TPY and 2CBZ-TPY, except the decyloxy substituted phenyl group which is twisted
by an angle of ~41º from the thiophene-pyridine plane, the HOMO is distributed majorly on
spacer thiophene and vinylic linkage. The HOMO distribution is also extended on nitrogen
atom and on one of the phenyl group (which is attached to the vinylic bond) of peripheral
triphenylamine in 2TPA-TPY and of ethyl carbazole in 2CBZ-TPY. Further, a partial
distribution of HOMO over central pyridine ring is also seen. Since the free phenyl groups in
terminal triphenylamine are twisted by an angle of ~42º from the thiophene-pyridine plane,
HOMO is distributed to a lesser extent on triphenylamine group in 2TPA-TPY. Similarly, due
to the small twisting from its co-planarity of unsubstituted phenyl group of terminal ethyl
carbazole, HOMO is spread to a lesser extent on ethyl carbazole in 2CBZ-TPY. On the other
hand, the LUMO is localized mainly on central pyridine and spacer thiophene units; and
partially distributed over vinylic linkage both in 2TPA-TPY and 2CBZ-TPY. In addition,
there is a minor distribution of LUMO on substituted phenyl group of triphenylamine and ethyl
carbazole in 2TPA-TPY and 2CBZ-TPY, respectively. Correspondingly, in the case of 2TPY-
TPA and 2TPY-CBZ, the HOMO is majorly localized on spacer thiophene, vinylic linkage,
central triphenylamine in 2TPY-TPA and on central carbazole in 2TPY-CBZ, while the
LUMO is localized primarily on electron acceptor pyridine ring, spacer thiophene and vinylic
linkage. Also, there is a partial distribution of LUMO over substituted phenyl rings of donor
triphenylamine and carbazole unit in 2TPY-TPA and 2TPY-CBZ, respectively. It is observed
that, the HOMO of all the four molecules is mainly situated on electron donors, while LUMO
is mainly localized on electron withdrawing pyridine unit, which indicates the D–A type charge
transfer character of the molecules. And, a significant overlap of orbitals is observed from the
electron density distribution of HOMO and LUMO levels, suggesting the ambipolar property,
which may support for the proper transport of holes/electrons and to maintain the active
electronic communication between the acceptor and the donor[36,37]. The calculated HOMO and
LUMO values are given in Table 2.
1
1
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Figure 5. The optimized molecular structures and HOMO/LUMO distributions of 2TPA-TPY,
2
2
CBZ-TPY, 2TPY-TPA and 2TPY-CBZ.
.5 Thermal properties
Thermal properties of the molecules were analyzed by differential scanning colorimetry (DSC)
and thermogravimetric analysis (TGA). The DSC and the TGA plots of 2TPA-TPY, 2CBZ-
TPY, 2TPY-TPA and 2TPY-CBZ are depicted in Figure 6 and Figure 7, respectively and
the results are summarized in Table 2. The DSC analysis shows that the molecules are
amorphous in nature, as there are no exothermic melting peaks observed during the two heating
cycles (Figure 6). And, all the four molecules exhibit a clear glass transition temperature (T
during the second heating cycle. In fact, molecules 2TPY-TPA and 2TPY-CBZ show T of
7.63 and 36.33°C, respectively, whereas, molecules 2TPA-TPY and 2CBZ-TPY exhibit
comparatively better T of 72.32 and 66.12°C, respectively. The higher T of 2TPA-TPY and
CBZ-TPY compared to that of 2TPY-TPA and 2TPY-CBZ is attributed to the presence of
bulky side groups in 2TPA-TPY and 2CBZ-TPY, which hinders the rotational flexibility,
thereby increases the T
[38].
The TGA results reveal that the molecules possess extremely good thermal stabilities with the
high degradation temperatures. The decomposition temperature (T )of 2TPA-TPY and 2CBZ-
TPY (corresponding to 5% weight loss) is observed at 426 and 432°C, respectively, whereas,
the T of 2TPY-TPA and 2TPY-CBZ is observed at 417 (corresponding to 20% weight loss)
and 428°C (corresponding to 10% weight loss), respectively. In addition, a slight degradation
g
)
g
2
g
g
2
g
d
d
1
2
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10.1002/cplu.202000435
is observed at 218 and 240°C (corresponding to 2% weight loss), in 2TPY-TPA and 2TPY-
CBZ, respectively, which is due to the decomposition of peripheral, low molecular weight
component (4-(4-(decyloxy)phenyl)-2,6-di(thiophen-2-yl)pyridine), present in them. The
relatively higher T of 2CBZ-TPY and 2TPY-CBZ compared to that of triphenylamine
d
containing 2TPA-TPY and 2TPY-TPA is due to the presence of carbazole (structurally more
rigid compared to triphenylamine) unit in them, which improves the structural rigidity of
2
CBZ-TPY and 2TPY-CBZ. Apparently, the excellent thermal stability with high T of all
d
the four molecules is attributed to the bulky D–π–A–π–D/ D–A–π–D–π–A–D structural
arrangement[39] with relatively large molecular weights, which leads to a homogeneous and
amorphous film and thus, improves the quality of the film at high temperatures, which is a
criterion for their practical use in OLEDs.
2
TPA-TPY
2TPA-CBZ
First heating
3
2
2
2
2
2
1
0 (a)
40 (b)
First heating
Cooling
Second heating
Cooling
Second heating
8
6
4
2
0
8
38
36
34
32
o
Tg=66.12 C
o
T =72.32 C
g
30
28
0
100
200
300
400
0
50
100
150
200
250
o
o
Temperature ( C)
Temperature ( C)
24
23
22
21
20
19
(c)
(d)
26
24
22
20
o
T = 36.33 C
g
o
Tg= 27.63 C
2
TPY-TPA
2TPY-CBZ
First heating
Cooling
First heating
Cooling
Second heating
Second heating
18
1
6
4
1
0
50
100
150
o
200
0
50
100
150
200
o
Temperature ( C)
Temperature ( C)
Figure 6. DSC plots of a) 2TPA-TPY, b) 2CBZ-TPY, c) 2TPY-TPA and d) 2TPY-CBZ.
2TPA-TPY
2CBZ-TPY
2TPY-TPA
2TPY-CBZ
100
80
60
40
1
00 200 300 400 500
o
Temperature ( C)
Figure 7. TGA plots of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ.
1
3
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2
2
.6 Third–order NLO studies
.6.1 Nonlinear absorption (NLA) studies
Under ultra-short 50 µJ laser excitation (532 nm, 7 ns pulse width), corresponding to on–axis
−
2
intensity of 1.386 GW cm , the open aperture (OA) Z–scan analysis was carried out, wherein
the transmitted beam was measured without the aperture in front of the detector, to determine
the NLA of the molecules[40]. The linear transmittance was maintained at 70–72% (the details
of the experiment are given in ESI). Figure 8a–d show the OA Z–scan signatures of 2TPA-
TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ in CHCl solution, respectively. The
3
interaction of laser beam with the molecules produces a valley pattern i.e., the transmittance of
the molecules decreases gradually towards the focal point and reaches a minimum with the
deep transmittance trough at the focus where the curves are symmetric at z=0, which signifies
the RSA behavior of the molecules with the positive NLA of the incident light. On the
nanosecond time scale the RSA is combined with TPA and ESA, which is collectively called
as “the effective TPA” process[41,42]. The experimental data are matching well with the
theoretical model for ESA assisted TPA process in all the four molecules.
Following the intensity dependent absorption coefficient (α(I)) for ESA assisted TPA
(associated with saturable absorption (SA)), OA Z–scan recordings were theoretically fitted
using equation (3) to determine the nonlinear absorption coefficient (βeff) and is given by,
o
+ I Is

(I) =
+  I.........................................................................(3)
eff
1
where, α
0
is the linear absorption coefficient, I is the incident laser intensity, I is the saturation
s
intensity, βeff is the nonlinear absorption coefficient associated to the RSA response[43]
Pulse propagation equation to calculate βeff is given by,
.




dI
dz
o
2
=
−
 I −  I ....................................................................(4)
eff
I






1
+
Is
where, z is the propagation distance within the sample. The first term in equation (4) expresses
the SA and the next term indicates effective TPA part.
The normalized transmittance is given by,

1
2
−


d...............................................(5)
T
(
z
)
=
ln 1+ q
(
z
)
e

0



q0
z
( )
−
1
4
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−
 L
o
2
1
(
−e



z 
)
where, q z =  I L
1+
and L =

(
)
eff
0
eff
0
eff
2
z0
0

L
eff is the effective sample length
,
L is the sample length, α
0
is the unsaturated linear absorption
2
o
coefficient, z is the position of the sample, z =  is the Raleigh range, ω
0
is the beam
o
waist radius at the focal point and λ is the wavelength of laser beam.
(3)
The imaginary part of the third-order nonlinear susceptibility (χ ) is given by,
2
o
n  c
(
3
)
o
eff
Im  =
........................................................................(6)
2
where, n
0
is the linear refractive index, c is the speed of light and ε is the permittivity of free
0
space.
Using the equations (3) and (5), the experimental data were fitted to the theoretical model to
obtain NLA parameters (Table 3).
The numerically fitted OA Z–scan results disclose enhanced NLO responses of the synthesized
molecules. The obtained βeff values for 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ
are 4.45, 3.65, 3.91 and 2.58×10−10 m W , respectively, which are very much closer and
predominantly better than those reported in the literature (Table 3).
−1
Figure 8. Z–scan curves of a) 2TPA-TPY, b) 2CBZ-TPY, c) 2TPY-TPAand d) 2TPY-CBZ
under OA configuration.
1
5
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2
.6.2 Optical limiting (OL) studies
The OL data of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ were extracted from
the graphs of normalized transmittance obtained from OA Z–scan analysis against input
fluence (Figure 9a–d). From the graph, the onsets of limiting action (the value of input fluence
at which the intensity of output transmittance starts decreasing) are observed at 0.18, 0.49, 0.38
−
2
and 0.65 J cm and the limiting threshold (LT) values (the value of input fluence at which the
intensity of output pulse becomes 50% of the initial value) are 1.73, 2.72, 2.68 and 4.68 J cm−2
for 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ, respectively. From the results it is
evidenced that the molecules in the present study exhibit exceptional OL action with very low
onset and LT values. These significantly reduced values are analogous and better than those of
many reported values (Table 3). For an instance, under similar conditions, in our previous
communication, one of the thiophene-pyridine based small molecule exhibited excellent NLO
properties with βeff and LT of 2.08×10−10 m W and 4.16 J cm , respectively[44]. Shiju et al.[45]
−1
−2
investigated the NLO response of phenothiazine (PTZ)–silver (Ag) organometallic hybrid
system, which showed βeff of 3.60×10−10 m W and LT of 3.43 J cm , which is two orders of
−1
−2
magnitude higher than that of pristine PTZ. Zhang et al.[46] studied the NLO and optical limiting
properties of graphene oxide–Fe
3
O
4
hybrid material, which showed an enhanced βeff of
.6×10−10 m W−1 with the LT of 2.82 J cm , whereas new quinoxaline based push–pull
−2
2
molecule with 1,3-indandione as acceptor was synthesized and its third-order NLO properties
were studied by Gopi et al.[47], which showed an effective TPA with very high βeff of 2.0×10−10
−
1
−2
[48]
m W and low LT of 1.15 J cm . Further, Mi et al. synthesized Zn-complexed D–π–A type
porphyrin derivative, Por−Zn−N, through [2+2] click reactions and studied the NLO response
of the material, which showed RSA with βeff of 4.8×10−10 m W . Specifically, 2TPA-TPY and
−1
2
CBZ-TPY of the present study having similar structural arrangement as that of our previously
reported molecules wherein the same core structure (i.e., TPY) was connected to small end
groups such as thiophene/phenyl acrylonitriles[44], show enhanced NLO properties compared
to the previous molecules. This is mainly attributed to the improved ICT, increased polarity
and extended conjugation existed due to the presence of bulkier end groups
(triphenylamine/carbazole) in the present molecules compared to thiophene/phenyl
−
2
acrylonitriles. Moreover, the LT value of 2TPA-TPY (1.73 J cm ) is similar to that of our
−2 [44]
previously reported polymer, P2TPy (1.42 J cm ) which suggests that the proper selection
of bulkier end group and structural modifications in small molecules could lead to comparable
NLO properties as that of polymers and could replace the polymeric materials in optical
limiting devices.
1
6
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From the literature reports it is noticed that the enhancement in the nonlinearity is mainly due
to the doping of organic compounds with some inorganic materials with the exception of some
of the pure organic materials, which showed improved nonlinearity without any doping.
However, the materials in the present study have shown remarkable nonlinear response with
substantial increment in the βeff of the order 10−10 m W and exceptionally well optical limiting
behavior with very low LT in their pristine form, which is comparable and better than some of
the well-known reported materials, making them capable materials for optical power limiting
devices in photonics.
−1
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(
a)
Experimental value of 2TPA-TPY
Theoretical fit
(b)
Experimental value of 2CBZ-TPY
Theoretical fit
2.72 Jcm-2
1
.73 Jcm-2
0
.07
0.7
7
21
0.07
0.7
7
21
Input intensity (J/cm2)
Input intensity (J/cm2)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(
c)
Experimental value of 2TPY-TPA
Theoretical fit
Experimental value of 2TPY-CBZ
Theoretical fit
1
1
0
0
0
.2 (d)
.0
.8
.6
.4
2
.68 Jcm-2
.68 Jcm-2
4
0.2
0.07
0
.07
0.7
7
21
0.7
7
14
Input intensity (J/cm2)
Input intensity (J/cm2)
Figure 9. Optical limiting curves of a) 2TPA-TPY, b) 2CBZ-TPY, c) 2TPY-TPA and d)
−
2
2
2
TPY-CBZ at an input intensity of 1.386 GW cm .
.6.3 Nonlinear refraction (NLR) studies
The pure NLR components were evaluated by dividing closed aperture (CA) data by the
corresponding normalized OA Z–scan data to eliminate the NLA part present in CA Z–scan
data. The closed by open signatures of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ
are depicted in Figure 10a–d, respectively. As it is seen from the figure, there is a prefocal
peak followed by null and postfocal valley, which indicates the negative sign of refractive
index, representing a self-defocusing nature and thus, negative nonlinearity of 2TPA-TPY,
2
CBZ-TPY, 2TPY-TPA and 2TPY-CBZ. As a result, molecules act like diverging lens. By
fitting experimental data to the equation (7), the normalized transmittance (T) at CA condition
was determined, and is given by,
1
7
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


z 
4

zo

T
(
z,o
)
=1+
 .....................................(7)
o
2
2






z 
 z 



+1
+ 9


 z
o


zo





where, T is the normalized transmittance, ∆Φ
o
is the on–axis nonlinear phase shift at the focus.
Therefore, the nonlinear refractive index (ɳ
cn 
2
) (the NLR coefficient) is calculated by,
0
o
n2
(
esu
)
=
............................................................................(8)
2
8
0 I L
o
eff
(3)
The real χ is related to ɳ
2
by the relation (in electrostatic unit),
(
3
)
2
o
Real = 2n  cn ..............................................................................(9)
o
2
The NLR parameters of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA and 2TPY-CBZ are listed in
Table 3.
Figure 10. Z–scan curves of a) 2TPA-TPY, b) 2CBZ-TPY, c) 2TPY-TPA and d) 2TPY-CBZ
under CA configuration.
1
8
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Table 3. Comparison of NLA and NLR parameters of 2TPA-TPY, 2CBZ-TPY, 2TPY-TPA
and 2TPY-CBZ with similar results reported.
Sample
β
eff
Limiting
ɳ2
χ3
References
(
×10−10 mW-1) threshold
(esu)
(esu)
−
2
−12
(
Jcm )
1.73
2.72
2.68
4.68
4.16
3.43
(10 )
2
TPA-TPY
4.45
3.65
3.91
2.58
2.08
3.60
7.06357×10−11 14.0157 This work
6.21902×10−11 11.5239 This work
7.63576×10−11 12.3421 This work
4.78997×10−11 8.1398 This work
2
CBZ-TPY
TPY-TPA
2
2
TPY-CBZ
[
44]
TPyP
-
6.56524
[45]
Phenothiazine
PTZ)–silver (Ag)
organometallic
hybrid system
8.0
-
(
[
46]
47]
Graphene oxide–
Fe
2.6
2.82
1.15
-
-
-
-
3
O
4
[
2
.0
[48]
Zn-complexed D–
π–A type porphyrin
derivative,
4.8
-
-
-
(Por−Zn−N)
The butterfly-shaped molecules in the present study possess highly delocalized π–electronic
cloud, resulting from the structural planarity, which provides a prolonged π–conjugation
between the electron donor and acceptor groups via excellent ICT interactions, inducing the
1
9
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polarization in the molecules, which leads to a remarkably high βeff value and extremely low
LT values of the pristine molecules.
Further, the higher extent of electron donating ability of triphenylamine moiety than the
carbazole unit brings about a notable increment in the βeff value and exceptional reduction in
the LT value in the case of 2TPA-TPY and 2TPY-TPA compared to that of 2CBZ-TPY and
2
TPY-CBZ. Thus, it is worth to note that these materials would be efficient limiters which
would provide safety for damages caused by high intensity sources.
.7 Electroluminescence properties
2
Out of four molecules synthesized, 2TPA-TPY exhibits comparatively good film formation
and higher solid-state fluorescence quantum yield. As a result, a multilayer non-doped OLED
was fabricated employing 2TPA-TPY as the emissive material and the electroluminescence
properties were investigated. The device configuration is as follows: ITO (150
nm)/PEDOT:PSS (30 nm)/NPB (20 nm)/2TPA-TPY (40 nm)/TPBi (35 nm)/LiF (1 nm)/Al
(100 nm). In the device, patterned ITO (indium tin oxide)-coated glass substrate was used as a
transparent anode, PEDOT:PSS (Poly(3,4-ethylene dioxythiophene)-poly(styrenesulfonate))
as hole injecting material, NPB (N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′biphenyl)-4,4′-
diamine) as hole transporting layer (HTL), 2TPA-TPY as emissive material, TPBi (2,2′,2"-
(
1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) as electron transporting layer (ETL)
and hole blocking layer, LiF (lithium fluoride) served as electron injecting layer (EIL) and Al
aluminium) as cathode (Figure 11a), and the energy level diagram is shown in Figure 11b.
(
The current density-voltage-luminance (J-V-L) characteristic, current density-luminance,
current density-current efficiency and electroluminescence (EL) spectra of the fabricated
device are presented in Figure 12a–d, respectively. As sown in Figure 12a, the device exhibits
−
2
good diode characteristic with a turn-on voltage of 7.74 V (at 1 cd m ), a maximum luminance
−
2
−1
−1
−2
of 207 cd m , ηCE of 1.51cd A , ηPmax of 0.46 lm W and ηEQE of 0.48 % at 100 cd m (Table
). The device emits bright green light having maximum emission (λmaxem) at 512 nm with a
4
narrow full width half maximum (FWHM) of 68 nm and commission internationale de
leclairage (CIE) coordinates of (0.290, 0.606) at 12 V (Figure 12e). It is seen that the presence
of triphenylamine unit as end groups with D–π–A–π–D configuration effectively reduced the
aggregation and crystallinity, and thus, facilitated the formation of homogeneous film by
solution processing. Also, there is no any change in the EL peak shape, that reveals the
excellent spectral stability of the device. The fabricated device is unoptimized and there is a
large scope for the optimization of various parameters which can further improve the device
performance.
2
0
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Figure 11. a) Device configuration and b) the energy level diagram showing the HOMO and
LUMO energies of different component materials.
Figure 12. Device characteristics (a) J–V–L characteristic, (b) J–L curve, (b) J– ηCE curve, (d)
EL spectrum of solution processed un-doped OLED fabricated using 2TPA-TPY as emissive
material, (e) CIE 1931 chromaticity coordinates (x,y) of OLED based on 2TPA-TPY, (f)
photograph of the device in dark and (g) photograph of the device in light .
2
1
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Table 4. Electroluminescence data of 2TPA-TPY in OLED device.
Molecule
V
onset
λemmax
(nm)
512
L
max
η
c
η
P
EQE
(%)
−
2
(cd A−1)
(lm W−1)
(
V)
(cdm )
2
TPA-TPY
7.74
207
1.51
0.46
0.48
3
. Conclusion
In conclusion, four butterfly-shaped molecules possessing D–π–A–π–D/ D–A–π–D–π–A–D
configuration, comprising of 2,4,6-trisubstituted pyridine, thiophene, triphenylamine and
carbazole moieties were synthesized, characterized and their optoelectronic properties were
investigated. The molecules 2TPA-TPY and 2TPY-TPA possess red shifted absorption and
emission maxima, and also possess higher HOMO energy level compared to 2CBZ-TPY and
2
TPY-CBZ. Localization of HOMO on electron donor and LUMO on acceptor group renders
D–A type charge transfer and the overlapping of orbitals provides ambipolar property for all
the four molecules. All the four molecules exhibit excited state assisted TPA with the βeff as
high as 10−10 m W . The molecules 2TPA-TPY and 2TPY-TPA show remarkably high βeff
−1
of 4.45 and 3.91×10−10 m W , and extremely low limiting threshold of 1.73 and 2.68 J cm ,
respectively. The un-doped solution processed OLED using 2TPA-TPY as emitter exhibits
bright green electroluminescence at 512 nm with narrow FWHM of 68 nm, turn-on voltage of
−1
−2
−
2
−2
−1
7
.74 V (at 1 cd m ), a maximum luminance of 207 cd m , ηCE of 1.51cd A , ηPmax of 0.46 lm
−
1
−2
W and ηEQE of 0.48 % at 100 cd m . The striking results of 2TPA-TPY demonstrate its
potential use as optical limiter in photonics and emissive material in OLEDs.
4
4
. Experimental Section
.1 Materials and methods
The chemicals were purchased from commercial suppliers and used as received without further
purification unless specially stated. The chemicals used were, 2-acetylthiophene (Avra
Synthesis Pvt. Ltd, 95%), 4-hydroxybenzaldehyde (Loba Chemie, 98%), ammonium acetate
(
Merck, 97%), 1-bromodecane (Sigma Aldrich, 98%), K
borohydride (Spectrochem Pvt. Ltd., 96%), triphenylphosphine hydrobromide (Sigma Aldrich,
7%), 9H-carbazole (Loba Chemie, 95%), 1-bromoethane (Sigma Aldrich, 98%), sodium
2
CO (Loba Chemie, 99%), sodium
3
9
hydride (NaH) (Sigma Aldrich, 60% dispersion in mineral oil), 4-formyltriphenylamine (Sigma
Aldrich, 97%) and bis(4-formylphenyl)phenylamine (Sigma Aldrich, 95%). Organic solvents
were purified according to the standard procedures prior to the usage. All the reactions were
carried out under argon atmosphere in round bottomed (RB) flask. The progress of the reaction
2
2
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was monitored by thin layer chromatography (TLC) performed on aluminum plates pre-coated
with 60 F254 silica gel (Merck KGaA). The developed TLCs were observed under a short/long
wavelength ultra-violet-visible (UV-Vis) lamp. The intermediates and final compounds were
purified by silica gel column chromatography (60–120 mesh size) using suitable solvent
mixtures.
4
.2 Synthesis
Synthesis of 4-(4-(hydroxy)phenyl)-2,6-di(thiophen-2-yl)pyridine (3)
The intermediate 3 was synthesized following the procedure mentioned in the literature[44]
Synthesis of 4-(4-(decyloxy)phenyl)-2,6-di(thiophen-2-yl)pyridine (4)
.
The intermediate 4 was synthesized according to the procedure mentioned in the literature[44]
.
Synthesis of 5-(4-(4-(decyloxy)phenyl)-6-(thiophen-2-yl)pyridin-2-yl)thiophene-2-
carbaldehyde (5) and 5,5'-(4-(4-(decyloxy)phenyl)pyridine-2,6-diyl)bis(thiophene-2-
carbaldehyde) (6)
The intermediate 6 was synthesized following the procedure mentioned in the literature[44]
.
The intermediate 5 was separated from the crude product of intermediate 6 by column
chromatography using a mixture of pet ether/ethyl acetate (9:1, 8:2, v/v) (1.1 g, yield: 26%) as
+
1
yellow solid. ESI–MS (m/z) calcd. for C30
400 MHz, CDCl ) δ (ppm): 9.96 (s, 1H, –CHO), 7.79–7.71 (m, 5H, Ar–H), 7.66 (d, J = 8.0
Hz, 2H, Ar–H), 7.46 (d, J = 4.0 Hz, 1H, Ar–H), 7.17–7.15 (m, 1H, Ar–H), 7.05 (d, J = 8.0 Hz,
H, Ar–H), 4.04 (t, J = 6.8 Hz, 2H, –OCH ), 1.87–1.83 (m, 2H, –CH –), 1.53–1.49 (m, 2H, –
CH –), 1.31–1.28 (m, 12H, –CH –), 0.91 (t, J = 6.4 Hz, 3H, –CH ); C NMR (100 MHz,
CDCl ) δ (ppm): 183.2, 160.4, 154.2, 152.9, 151.0, 149.9, 144.3, 144.1, 136.7, 129.8, 128.1,
28.0, 125.1, 126.0, 115.9, 115.2, 115.1, 68.2, 31.8, 29.5, 29.3, 30.0, 29.2, 26.0, 22.6, 14.0;
2
H33NO S
2
: 503.20 found: 504.37 [M+H] ; H NMR
(
3
2
2
2
1
3
2
2
3
3
1
Anal. calcd. for C30
N, 2.03; S, 12.27.
H33NO
2
2
S : C, 71.53; H, 6.60; N, 2.78; S, 12.73; found: C, 70.99; H, 6.87;
Synthesis
of
(5-(4-(4-(decyloxy)phenyl)-6-(thiophen-2-yl)pyridin-2-yl)thiophen-2-
yl)methanol (7) and ((4-(4-(decyloxy)phenyl)pyridine-2,6-diyl)bis(thiophene-5,2-
diyl))dimethanol (8)
In a clean and dry RB flask, a solution of intermediate 5 (1.00 g, 1.98 mmol) in methanol (10
mL) was prepared. To this solution, sodium borohydride (0.15 g, 3.97 mmol) was added and
the mixture was stirred at room temperature (RT) for 4 h. After the completion of the reaction,
the solution was quenched using ice-cold water, the obtained solid was filtered, washed with
excess of distilled water, dried and purified by column chromatography using a mixture of pet
ether/ethyl acetate (8:2, v/v) as eluent to obtain intermediate 7 as white solid (0.92 g, yield:
2
3
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1
9
5
2%). H NMR (400 MHz, CDCl
3
) δ (ppm): 7.75 (d, J = 2.8 Hz, 1H, Ar–H), 7.67–7.58 (m,
H, Ar–H), 7.52 (d, J = 4.0 Hz, 1H, Ar–H), 7.39 (d, J = 4.0 Hz, 1H, Ar–H), 7.12–7.10 (m, 1H,
OH), 3.99 (t, J = 6.8 Hz, 2H, –OCH ), 1.99
–), 1.47–1.45 (m, 2H, –CH –), 1.28 (m, 12H, –CH
); C NMR (100 MHz, CDCl ) δ (ppm): 160.3, 152.5, 152.2,
49.4, 144.8, 144.5, 130.3, 127.7, 127.5, 126.2, 124.7, 124.4, 115.1, 114.3, 114.2, 68.2, 60.3,
Ar–H), 7.01–6.99 (m, 2H, Ar–H), 4.84 (s, 2H, –CH
2
2
(
s, 1H, –CH
2
OH), 1.82–1.79 (m, 2H, –CH
2
2
2
–
1
3
)
, 0.87 (t, J = 6.4 Hz, 3H, –CH
3
3
1
3
1
1.8, 29.5, 29.3, 26.0, 22.5, 14.0; Anal. calcd. for C30
2.68; found: C, 71.98; H, 6.32; N, 2.89; S, 12.41.
H35NO
2
2
S : C, 71.25; H, 6.98; N, 2.77; S,
The above-mentioned procedure was followed to get intermediate 8 wherein, intermediate 6
2.00 g, 3.76 mmol) was made to react with NaBH (0.56 g, 15.04 mmol) (1.82 g, yield: 90%).
ESI–MS (m/z) calcd. for C31 : 535.22 found: 536.44 [M+H] ; H NMR (400 MHz,
CDCl ) δ (ppm): 7.58 (d, J = 8.8 Hz, 2H, Ar–H), 7.54 (s, 2H, Ar–H), 7.50 (d, J = 4.0 Hz, 2H,
Ar–H), 7.00–6.97 (m, 4H, Ar–H), 4.83 (s, 4H, –CH OH), 4.01 (t, J = 6.8 Hz, 2H, –OCH ), 2.50
s, 2H, –CH OH), 1.86–1.79 (m, 2H, –CH –), 1.51–1.46 (m, 2H, –CH –), 1.36–1.31 (m, 12H,
) δ (ppm): 160.1, 152.2,
49.3, 146.3, 144.7, 130.2, 128.0, 126.0, 124.4, 115.0, 114.1, 68.2, 60.3, 31.8, 29.5, 29.4, 29.3,
(
4
+
1
3 2
H37NO S
3
2
2
(
2
2
2
1
3
–
1
2
CH
2
–), 0.91 (t, J = 7.2 Hz, 3H, –CH
3
); C NMR (100 MHz, CDCl
3
9.2, 26.0, 22.6, 14.1; Anal. calcd. for C31
H37NO
3
2
S : C, 69.50; H, 6.96; N, 2.61; S, 11.97;
found: C, 69.08; H, 6.50; N, 2.93; S, 12.08.
Synthesis of
5
2-(5-((bromotriphenyl-λ -phosphanyl)methyl)thiophen-2-yl)-4-(4-
(
decyloxy)phenyl)-6-(thiophen-2-yl)pyridine (9) and 2,6-bis(5-((bromotriphenyl-λ5-
phosphanyl)methyl)thiophen-2-yl)-4-(4-(decyloxy)phenyl)pyridine (10)
A mixture of intermediate 7 (0.7 g, 1.38 mmol) and triphenylphosphine hydrobromide (0.57 g,
o
1
.66 mmol) was taken in chloroform (CHCl
3
) (10 mL) and refluxed at 60 C for 3 h. Then, the
solvent was removed under reduced pressure and diethyl ether (5 mL) was added to solidify
the product. The intermediate 9 was obtained as yellow solid and used as such for the next step
1
(
1.08 g, yield: 94%). H NMR (400 MHz, CDCl3) δ (ppm): 7.86–7.77 (m, 10H, Ar–H), 7.69
m, 6H, Ar–H), 7.61–7.56 (m, 3H, Ar–H), 7.50 (s, 2H, Ar–H), 7.43 (d, J = 4.8 Hz, 1H, Ar–H),
(
7
1
.22 (s, 1H, Ar–H), 7.14–7.12 (m, 1H, Ar–H), 7.00 (d, J = 8.4 Hz, 2H, Ar–H), 5.83 (d, J =
3.6 Hz, 2H, –CH
2
–P), 4.02 (t, J = 6.8 Hz, 2H, –OCH
2
), 1.84–1.81 (m, 2H, –CH
2
–), 1.49–1.47
1
3
(
m, 2H, –CH
2
–), 1.29 (m, 12H, –CH
2
–), 0.89 (t, J = 6.8 Hz, 3H, –CH
3
); C NMR (100 MHz,
CDCl ) δ (ppm): 160.6, 151.5, 151.3, 150.8, 136.6, 135.2, 134.5, 134.4, 133.5, 130.5, 130.4,
3
1
2
1
30.2, 129.1, 128.4, 128.3, 126.4, 125.6, 118.1, 117.1, 116.8, 115.1, 68.3, 59.4, 31.8, 29.6,
9.3, 29.2, 28.3, 27.5, 25.9, 22.7, 14.0; Anal. calcd. for C48
.69; S, 7.72; found: C, 69.14; H, 5.56; N, 1.91; S, 7.85.
H
49BrNOPS : C, 69.38; H, 5.94; N,
2
2
4
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Intermediate 10 was synthesized according to the above-mentioned procedure wherein,
intermediate 8 (1.0 g, 1.86 mmol) was made to react with triphenylphosphine hydrobromide
1
(
0.76 g, 2.23 mmol) (2.05 g, yield: 93%). H NMR (400 MHz, CDCl
3
) δ (ppm): 7.87–7.77 (m,
1
7
7H, Ar–H), 7.72–7.64 (m, 12H, Ar–H), 7.45–7.40 (m, 4H, Ar–H), 7.20–7.18 (m, 2H, Ar–H),
.07 (s, 2H, Ar–H), 6.88 (d, J = 8.0 Hz, 2H, Ar–H), 5.94 (d, J = 14.0 Hz, 4H, –CH –P), 3.96
), 1.84–1.79 (m, 2H, –CH –), 1.47–1.29 (m, 14H, –CH ), 0.88 (t, J
); C NMR (100 MHz, CDCl ) δ (ppm): 160.6, 150.2, 149.9, 143.1, 135.0,
2
(t, J = 6.8 Hz, 2H, –OCH
2
2
2
13
=
6.4 Hz, 3H, –CH
3
3
1
2
34.6, 134.5, 133.0, 131.0, 130.2, 130.1, 128.3, 126.9, 118.0, 114.9, 68.2, 59.3, 31.8, 29.5,
9.4, 29.2, 27.1, 26.7, 26.0, 22.6, 14.0; Anal. calcd. for C67 NOP : C, 67.85; H, 5.52;
H65Br
2
S
2 2
N, 1.18; S, 5.41; found: C, 67.67; H, 5.32; N, 1.58; S, 5.97.
Synthesis of 9-ethyl-9H-carbazole (12)
To a solution of 9H-carbazole (2.0 g, 11.96 mmol) in DMF (15 mL), sodium hydride (0.86 g,
o
3
5.88 mmol) was added portion wise under argon at 0 C and the reaction mixture was stirred
at RT for 30 min. Later, 1-bromoethane (1.07 mL, 14.35 mmol) was added drop wise and the
reaction mixture was stirred at RT for 6ꢀh. After the completion of the reaction, the resulted
solution was quenched using ice cold water, then the solid product obtained was filtered and
the crude product was purified using column chromatography with pet ether as eluent to get
1
intermediate 12 as white solid (2.93 g, yield: 82%). H NMR (400 MHz, CDCl
3
) δ (ppm): 8.17
(d, J = 6.8 Hz, 2H, Ar–H), 7.52 (t, J = 6.8 Hz, 2H, Ar–H), 7.45 (d, J = 8.0 Hz, 2H, Ar–H), 7.30
1
3
(
t, J = 7.2 Hz, 2H, Ar–H), 4.38 (q, J = 7.2 Hz, 2H, –NCH
) δ (ppm): 139.9, 125.6, 122.9, 120.4, 118.7, 108.4, 37.4, 13.7; Anal.
13N: C, 86.12; H, 6.71; N, 7.17; found: C, 86.28; H, 6.36; N, 7.27.
2
), 1.46 (t, J = 7.2 Hz, 3H, –CH ); C
3
NMR (100 MHz, CDCl
calcd. for C14
3
H
Synthesis of 9-ethyl-9H-carbazole-3-carbaldehyde (13) and 9-ethyl-9H-carbazole-3,6-
dicarbaldehyde (14)
Intermediates 13 (1.12 g, yield: 49%) and 14 (0.98 g, yield: 38%) were synthesized according
to the procedure mentioned for intermediate 5 in which precursors used were POCl
3
(3.81 mL,
4
0.96 mmol), DMF (3.15 mL, 40.96 mmol) and intermediate 12 (2.0 g, 10.24 mmol) and the
1
reaction was carried out at 65°C for 24 h. 13: H NMR (400 MHz, CDCl
H, –CHO), 8.55 (s, 1H, Ar–H), 8.11 (d, J = 8.0 Hz, 1H, Ar–H), 7.97 (d, J = 8.4 Hz, 1H, Ar–
H), 7.51 (t, J = 7.2 Hz, 1H, Ar–H), 7.42–7.39 (m, 2H, Ar–H), 7.30 (t, J = 7.2 Hz, 1H, Ar–H),
3
) δ (ppm): 10.05 (s,
1
13
4
.32 (q, J = 7.2 Hz, 2H, –NCH
2
), 1.42 (t, J = 7.2 Hz, 3H, –CH
3
); C NMR (100 MHz, CDCl )
3
δ (ppm): 191.7, 143.5, 140.6, 128.5, 127.1, 126.7, 123.9, 123.1, 123.0, 120.7, 120.2, 109.1,
1
5
08.6, 37.8, 13.7; Anal. calcd. for C15
.76; N, 6.38.
H13NO: C, 80.69; H, 5.87; N, 6.27; found: C, 80.31; H,
2
5
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1
1
4: H NMR (400 MHz, CDCl
3
) δ (ppm): 10.16 (s, 2H, –CHO), 8.69 (s, 2H, Ar–H), 8.11 (d, J
=
8.0 Hz, 2H, Ar–H), 7.58 (d, J = 8.0 Hz, 2H, Ar–H), 4.48 (q, J = 7.2 Hz, 2H, –NCH
2
), 1.53
1
3
(
t, J = 7.2 Hz, 3H, –CH
3
); C NMR (100 MHz, CDCl
3
) δ (ppm): 191.6, 144.2, 129.5, 127.8,
1
7
27.0, 124.0, 123.1, 120.7, 109.4, 109.1, 108.6, 37.9, 13.8; Anal. calcd. for C16
6.48; H, 5.21; N, 5.57; found: C, 76.62; H, 5.33; N, 5.21.
H
13NO : C,
2
Synthesis of 4,4'-((1E,1'E)-((4-(4-(decyloxy)phenyl)pyridine-2,6-diyl)bis(thiophene-5,2-
diyl))bis(ethene-2,1-diyl))bis(N,N-diphenylaniline) (2TPA-TPY)
In a two necked RB flask, 4-formyltriphenylamine (0.14 g, 0.50 mmol) and intermediate 10
(0.30ꢀg, 0.25ꢀmmol) were dissolved in a solvent mixture of ethanol and chloroform (1:1) (6ꢀmL)
under argon atmosphere. Then, sodium ethoxide (0.023ꢀg, 1.01ꢀmmol, 1 mL) was added in
portion wise and the reaction mixture was stirred at RT for 12 h. The excess of solvent was
removed under reduced pressure and the crude product was purified by column
chromatography using pet ether/dichloromethane (DCM) (8:2, v/v) as mobile solvent to get
2
TPA-TPY as a yellow solid (0.163 g, yield: 63%). MALDI–TOF–MS (m/z) calcd. for
+
1
C
69
H
63
N
3
OS
2
: 1013.44 found: 1013.849 [M+H] ; H NMR (400 MHz, CDCl ) δ (ppm): 7.65
3
(d, J = 8.0 Hz, 2H, Ar–H), 7.61 (s, 2H, Ar–H), 7.58 (d, J = 4.0 Hz, 2H, Ar–H), 7.38 (d, J = 8.0
Hz, 4H, Ar–H), 7.29–7.25 (m, 10H, Ar–H), 7.15–7.11 (m, 9H, Ar–H), 7.06–7.02 (m, 13H, Ar–
H), 4.02 (t, J = 6.8 Hz, 2H, –OCH
2
), 1.86–1.81 (m, 2H, –CH
2
–), 1.51–1.47 (m, 2H, –CH
2
–),
) δ
1
3
1
.28–1.25 (m, 12H, –CH –), 0.88 (t, J = 6.4 Hz, 3H, –CH ); C NMR (100 MHz, CDCl
2
3
3
(ppm): 160.2, 152.4, 149.3, 147.5, 145.4, 143.0, 131.0, 130.5, 129.3, 128.7, 128.1, 127.3,
1
2
26.6, 125.3, 124.6, 123.4, 123.1, 120.3, 115.1, 114.2, 68.2, 31.9, 29.7, 29.6, 29.6, 29.4, 29.3,
9.2, 26.0, 22.7, 14.1; Anal. calcd. for C69 OS : C, 81.70; H, 6.26; N, 4.14; S, 6.32; found:
H
63
N
3
2
C, 81.65; H, 6.17; N, 4.23; S, 6.44.
Synthesis of 3,3'-((1E,1'E)-((4-(4-(decyloxy)phenyl)pyridine-2,6-diyl)bis(thiophene-5,2-
diyl))bis(ethene-2,1-diyl))bis(9-ethyl-9H-carbazole) (2CBZ-TPY)
The molecule 2CBZ-TPY was synthesized following the procedure for 2TPA-TPY, using the
precursors intermediate 13 (0.113 g, 0.50 mmol) and intermediate 10 (0.30ꢀg, 0.25ꢀmmol)
(0.137 g, yield: 59%). MALDI–TOF–MS (m/z) calcd. for C61
H
59
N
3
OS
2
: 913.41 found: 913.652
+
1
[
M+H] ; H NMR (400 MHz, CDCl
3
) δ (ppm): 8.26 (s, 2H, Ar–H), 8.15 (d, J = 8.0 Hz, 2H,
Ar–H), 7.70–7.66 (m, 4H, Ar–H), 7.63–7.62 (m, 4H, Ar–H), 7.51–7.47 (m, 2H, Ar–H), 7.43–
7
2
.39 (m, 4H, Ar–H), 7.37–7.31 (m, 4H, Ar–H), 7.28–7.25 (m, 2H, Ar–H), 7.10 (d, J = 4.0 Hz,
H, Ar–H), 7.04 (d, J = 8.0 Hz, 2H, Ar–H), 4.38 (q, J = 7.2 Hz, 4H, –NCH ), 4.03 (t, J = 6.4
), 1.85–1.81 (m, 2H, –CH –), 1.50–1.48 (m, 2H, –CH –), 1.46 (t, J = 7.0 Hz,
), 1.29–1.25 (m, 12H, –CH –), 0.89 (t, J = 6.8 Hz, 3H, –CH
2
Hz, 2H, –OCH
2
2
2
13
3
H, –CH
3
2
3
); C NMR (100 MHz,
2
6
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CDCl
3
) δ (ppm): 160.1, 152.5, 149.2, 145.9, 142.7, 140.4, 139.8, 130.6, 130.3, 128.2, 126.2,
1
3
8
25.8, 125.3, 124.4, 123.4, 123.0, 120.5, 119.4, 119.1, 118.8, 115.0, 114.1, 108.7, 108.6, 68.2,
7.7, 31.9, 29.7, 29.6, 29.3, 29.2, 26.0, 22.7, 14.1, 13.9; Anal. calcd. for C61
0.14; H, 6.50; N, 4.60; S, 7.01; found: C, 80.31; H, 6.37; N, 4.77; S, 7.22.
H
59
N
3
OS : C,
2
Synthesis of 4-((E)-2-(5-(4-(4-(decyloxy)phenyl)-6-(thiophen-2-yl)pyridin-2-yl)thiophen-
-yl)vinyl)-N-(4-((E)-2-(5-(4-(4-(decyloxy)phenyl)-6-(thiophen-2-yl)pyridin-2-
2
yl)thiophen-2-yl)vinyl)phenyl)-N-phenylaniline (2TPY-TPA)
The molecule 2TPY-TPA was synthesized following the procedure for 2TPA-TPY, using the
precursors bis(4-formylphenyl)phenylamine (0.054 g, 0.18 mmol) and intermediate 9 (0.30ꢀg,
0
.36ꢀmmol) (0.136 g, yield: 61%). MALDI–TOF–MS (m/z) calcd. for C80
H
81
N
3
O
2
S
4
: 1243.52
) δ (ppm): 7.71–7.69 (m, 3H, Ar–H),
.67–7.64 (m, 7H, Ar–H), 7.60–7.59 (m, 2H, Ar–H), 7.44–7.30 (m, 8H, Ar–H), 7.21–7.13 (m,
H, Ar–H), 7.10–7.00 (m, 11H, Ar–H), 4.03 (t, J = 6.8 Hz, 4H, –OCH ), 1.86–1.89 (m, 4H, –
–), 1.52–1.45 (m, 4H, –CH –), 1.29–1.25 (m, 24H, –CH –), 0.88 (t, J = 6.4 Hz, 6H, –CH );
13C NMR (100 MHz, CDCl
) δ (ppm): 160.2, 153.0, 152.5, 149.5, 147.0, 145.4, 145.0, 143.1,
31.5, 131.3, 130.5, 129.8, 128.2, 127.9, 127.6, 127.3, 125.3, 124.7, 123.9, 115.1, 114.4, 68.2,
1.9, 29.7, 29.6, 29.4, 29.3, 29.2, 26.0, 22.7, 14.1; Anal. calcd. for C80 : C, 77.19;
+
1
found: 1243.866 [M+H] ; H NMR (400 MHz, CDCl
3
7
8
2
CH
2
2
2
3
3
1
3
H
81
N
3
O
S
2 4
H, 6.56; N, 3.38; S, 10.30; found: C, 77.38; H, 6.67; N, 3.44; S, 10.09.
Synthesis of 3,6-bis((E)-2-(5-(4-(4-(decyloxy)phenyl)-6-(thiophen-2-yl)pyridin-2-
yl)thiophen-2-yl)vinyl)-9-ethyl-9H-carbazole (2TPY-CBZ )
The molecule 2TPY-CBZ was synthesized following the procedure for 2TPA-TPY, using the
precursors intermediate 14 (0.046 g, 0.18 mmol) and intermediate 9 (0.30ꢀg, 0.36ꢀmmol) (0.121
g, yield: 56%). MALDI–TOF–MS (m/z) calcd. for C76
H
79
N
3
O
2
S
4
: 1193.51 found: 1193.815
) δ (ppm): 7.73–7.62 (m, 12H, Ar–H), 7.57–7.41 (m, 8H,
Ar–H), 7.30–7.29 (m, 3H, Ar–H), 7.17–7.15 (m, 2H, Ar–H), 7.12–7.10 (m, 2H, Ar–H), 7.05–
.03 (m, 4H, Ar–H), 6.98–6.96 (m, 1H, Ar–H), 4.39 (q, J = 7.2 Hz, –NCH ), 4.03 (t, J = 6.8
Hz, –OCH ), 1.85–1.80 (m, 4H, –CH –), 1.50–1.48 (m, 4H, –CH –), 1.45 ( t, J = 7.0 Hz, 3H,
CH ), 1.28–1.25 (m, 24H, –CH –), 0.88 (t, J = 6.4 Hz, 6H, –CH
CDCl ) δ (ppm): 160.2, 152.4, 149.4, 145.8, 145.1, 142.6, 140.2, 130.5, 128.2, 128.1, 127.9,
27.6, 125.4, 124.7, 123.4, 115.0, 114.3, 108.9, 68.2, 31.9, 29.6, 29.4, 29.3, 29.2, 26.0, 22.7,
+
1
[
M+H] ; H NMR (400 MHz, CDCl
3
7
2
2
2
2
1
3
–
3
2
3
); C NMR (100 MHz,
3
1
1
6
4.1; Anal. calcd. for C76
.51; N, 3.32; S, 10.90.
H
79
N
3
O
2
4
S : C, 76.41; H, 6.67; N, 3.52; S, 10.73; found: C, 76.62; H,
2
7
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Associated content
Supplementary data
Characterization details, spectral characterizations of the intermediates and final compounds,
time-resolved photoluminescence decay curves, experimental set up of Z–scan measurements
and OLED device fabrication procedure.
Conflicts of interest
None.
Acknowledgment
Viprabha Kakekochi, Sathish Chatnahalli Gangadharappa and Udaya Kumar Dalimba
acknowledge Prof. Cristian A. Strassert for providing access to spectroscopic instrumentation
at the CeNTech, WWU Münster. And they are thankful to the Department of Chemical
Engineering, NITK, Surathkal for the TGA facility and are also thankful to Deziy Melita
DSouza and Meghana K J, M. Sc. Students, Department of Chemistry NITK, Surathkal for
helping in the synthesis of molecules.
Keywords: electroluminescence • light-emitting diodes • nonlinear optics • photo physics •
thiophenes.
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