Heteroleptic Zn(II) Complexes: Synthesis, Characterization and Photoluminescence Properties

Article abstract of DOI:10.1007/s10895-019-02397-x

Heteroleptic Zn (II) complexes containing 8-hydroxy quinoline as preliminary ligand and pyrazolone based derivatives as secondary ligand were synthesized and their structures confirmed by NMR, Mass, FT-IR, UV-vis and Elemental analysis. Theses complexes s

Full text of DOI:10.1007/s10895-019-02397-x

Journal of Fluorescence
https://doi.org/10.1007/s10895-019-02397-x
ORIGINAL ARTICLE
Heteroleptic Zn(II) Complexes: Synthesis, Characterization
and Photoluminescence Properties
1
1
Jaydip Solanki & Kiran Surati
Received: 23 March 2019 /Accepted: 28 May 2019
Springer Science+Business Media, LLC, part of Springer Nature 2019
#
Abstract
Heteroleptic Zn (II) complexes containing 8-hydroxy quinoline as preliminary ligand and pyrazolone based derivatives as
secondary ligand were synthesized and their structures confirmed by NMR, Mass, FT-IR, UV-vis and Elemental analysis.
Theses complexes show good photoluminescence properties in solid and solution state in the range of 505–544 nm with quantum
yield 0.38 to 0.50. Whereas these complexes also show good life time in the range of 0.037 to 0.043 ms. These complexes show
shift in the range of 25–30 nm. in different polar and nonpolar solvents due to intramolecular charge transfer (ICT). The bandgap
of these complexes is around ~2.60 eV. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(
LUMO) of all complexes are determine by cyclic voltammetry it obtained in the range of and ~ ( 5.29 eV) and ~( 2.69 eV). The
energy band gap, frontier molecular orbitals (FMO) energy levels and geometrical structures were optimized using density
functional theory (DFT) with B3LYP/6-31G* basic set on Spartan’18 software. All complexes displayed high thermal stability.
.
.
Keywords Luminescence Heteroleptic Zn(II) complexes Solvent effect
Introduction
hydroxyquinoline) aluminum (Alq3) and bis(8-
hydroxyquinoline) zinc (Znq2) broadly considered to be
one of the most credible electron transporting and emitting
materials, but Znq2 exhibit higher quantum yield and im-
prove injection efficiency than Alq3 [18–22]. The maximum
OLED light output obtained when the excitons generate due
to the conjunction of an electron-hole at the interface of emit-
ting layer [23]. This circumstance is accomplished by the
distribution of the electron donor (D) and electron acceptor
(A) environment or both are connected with a suitable spacer
[24–27]. Generally, the donor-acceptor (D-A) and donor-
bridge-acceptor (D-B-A) systems of organic and inorganic
molecules are used for controllable tuning theemissioncolor
and bandgap, the formation of π-conjugation charge and
expeditious charge transfer from donor to acceptor [28,
29]. In view of these photophysical and electrochemical
properties are essential for developing a highly competent
donor-acceptor category for OLED application. The emis-
sion properties of the metal complex will be regulated by
variation the ligands or by substitution the electron with-
drawing and donating group in the ligands. Some research
articles are available in the literature, where the replacement
of electron withdrawing atoms at 2 or 5 or 5,7 positions ex-
hibited a hypsochromic shift in the emission spectra as relat-
ed to unsubstituted zincquinolate complex [30–35].
Since the past two-three decades, the field of Organic Light
Emitting Diodes (OLEDs) has shown tremendous growth
due to their potential applications in low cost, more efficient
flat-panel displays and solid-state lighting [1]. Generally,
these devices are fabricated by using organic, inorganic mol-
ecules or organic polymers as hole transport layers, electron
transport layers, dopants and hosts [2–8]. Particularly,
Iridium(III), Platinum(II), Aluminum(III), Zinc(II) com-
plexes are seen their attractive electroluminescence proper-
tiesonthebasisof anintroductorystudybyC. W. TangandS.
A. Van Slyke [9–13]. Among these, zinc complexes are
promising candidates in comparison to other metal because
of zinc complex having some superior features like good
emission, less toxicity, low molecular weight, high thermal
stability, easy color tunability with simple synthesis proce-
dure and higher reaction yields [14–17]. Metal complexes of
8
-hydroxyquinoline (Mqn) and its derivatives like tris(8-
*
Kiran Surati
kiransurati@yahoo.co.in
1
Department of Chemistry, Sardar Patel University, Vallabh
Vidyanagar, Anand, Gujarat 388120, India

J Fluoresc
In the present work mainly focus on photophysical and
electrochemical properties of the synthesized heteroleptic
or ternary zinc complexes by using 5,7 dihalogen (Br and
Cl) substituted 8-hydroxyquinoline as primary ligand and
β-diketonate ligand (5-hydroxy-3-methyl-1-phenyl-1H-
pyrazole-4-carbaldehyde) as a secondary ligand. In this
complexes, disubstituted 8-hydroxyquinoline behave as
an electron donor (D) ligand, β-diketonate ligand behave
as an electron acceptor (A) ligand and both ligands
linking with bridge of zinc(II) metal ion [36, 37]. The
complexes [(4-formyl-3-methyl-1-phenyl-1H-pyrazol-5-
yl) oxy) (5,7-dibromo-quinolin-8-yl) oxy) zinc(II)] and
General Procedure for the Synthesis of Zinc Complex
The complexes were synthesized by DBHQ (0.303 g, 1 mmol)
or DCHQ (0.214 g, 1 mmol) and secondary ligand (L)
(0.202 g, 1 mmol) in absolute ethanol at 70 °C followed by
dropwise addition of zinc acetate dihydrate (0.219 g, 1 mmol)
in 10 ml of same solvent at 70 °C with continuous stirring and
pale yellow colored precipitates were obtained. Wash with
deionized water to remove excess metal ions and then wash
with absolute ethanol followed by diethyl ether and dried it.
Complex 1; [Zn(DBHQ)(L)];
[
(4-formyl-3-methyl-1-phenyl-1H-pyrazol-5-yl)oxy)(5,7-
[(4-Formyl-3-Methyl-1-Phenyl-1H-Pyrazol-5-Yl)Oxy)
(5,7-Dibromo-Quinolin-8-Yloxy)Zinc(II)]
dichloro-quinolin-8-yloxy)zinc(II)] are exhibited high
thermal stability, effective photo-luminescence perfor-
mance in solid as well as different solvents with a higher
quantum yield.
(Yield: 61%) Anal. calcd for C H Br N O Zn; C 50.09, H
2
0
13
2
3 3
1
2.73, N 8.76; Found: C 50.22, H 2.87, N 8.89; H NMR:
δ(ppm) 9.04(1H, s), 8.65(1H, d),8.49–8.41(1H, m), 7.98(2H,
d), 7.77–7.69(2H, m), 7.41(2H, t), 7.18(1H, t), 2.24(3H, s);
1
3
C NMR (APT): δ(ppm) 165.12, 160.26, 146.31, 140.47,
Experimental Section
1
1
1
39.49, 137.83, 135.16, 129.14, 127.81, 123.62, 119.93,
05.80, 98.10, 12.78; FT-IR (cm ): 2923, 1643, 1535,
488, 1330, 1234, 1110, 1083, 817, 748, 686,586.
−
1
Materials and Measurement
1
-Phenyl-3-methyl-2-pyrazoline-5-one purchase from Sigma-
Complex 2; [Zn(DCHQ)(L)];
Aldrich. 5,7-dibromo-8-hydroxyquinoline (DBHQ) and 5,7-
dichloro-8-hydroxyquinoline (DCHQ) were purchase from
TCI. Zinc acetate dihydrate used was of analytical grade.
The H NMR and C NMR spectra were recorded
u s i n g B r u k e r 4 0 0 M H z s p e c t r o m e t e r w i t h
tetramethylsilane as internal standard and deuterated di-
methyl sulfoxide (DMSO-d ) used as a solvent. The
Fourier transforms infrared spectroscopy (FT-IR) spectra
were recorded on MB-3000 infra-red spectrophotometer
in the range of 4000–400 cm . The UV-Visible absorp-
tion spectra of complexes was performed using a
Shimadzu 160A spectrophotometer. Mass spectra were
obtained by Shimadzu LCMS-2010 spectrometer and cy-
clic voltammetry was recorded on CHI660E electrochem-
ical workstation. Both differential scanning calorimetry
[
(
(4-Formyl-3-Methyl-1-Phenyl-1H-Pyrazol-5-Yl)Oxy)
5,7-Dichloro-Quinolin-8-Yloxy)Zinc(II)]
1
13
(Yield: 68%) Anal. calcd for C H Cl N O Zn: C 42.25, H
20 13 2 3 3
1
2.30, N 7.39; Found: C 42.32, H 2.39, N 7.51; H NMR:
δ(ppm) 9.04(1H, s), 8.67(1H, d), 8.56–8.50(1H, m),
.97(2H, d), 7.76–7.67(2H, m),7.40(2H, t), 7.16(1H,
6
7
1
3
t),2.24(3H, s); C NMR (APT): δ(ppm) 165.33, 158.69,
1
1
1
1
50.15, 146.37, 140.47, 139.48, 135.46, 129.90, 129.13,
26.18, 124.63, 123.18, 119.36, 115.21, 108.85, 106.07,
2.78; FT-IR(cm ) 2931, 1650, 1542,1458,1450, 1373,
218, 1118, 1095, 810,748, 671, 578.
−
1
−
1
Theoretical Calculation
(
DSC) and thermogravimetry analysis (TGA) were done
by using a Perkin Elmer Pyris 1 instrument at a heating
rate of 10 °C/min under a nitrogen atmosphere.
Photoluminescence(PL) spectra, quantum yield and life
time study were measured by a Perkin Elmer LS-55 fluo-
rescence spectrometer.
In additionally to understand the different photophysical prop-
erties of zinc(II) complexes 1 and 2 like the HOMO, LUMO
and energy band gap, frontier molecular orbitals (FMO) ener-
gy levels and geometrical structures were optimized using
density functional theory (DFT) with B3LYP/6-31G* basic
set on Spartan’18 software. The optimized 3D structure with
minimal energy are shown in Fig. 1 and both complexes show
tetrahedral geometry.
Synthesis
Synthesis of Ligand (L)
The FMOs of both complexes are shown in Fig. 2. In the
complexes, the FMOs are dominated by the molecular orbitals
arising from the ligands and the Zn2+ ions are appeared to be
small contribution in FMO. The HOMO and LUMO of
The ligand 5-hydroxy-3-methyl-1-phenyl-1H-pyrazole-4-
carbaldehyde (L) was prepared by our previously reports [38].

J Fluoresc
Fig. 1 3D Optimized Structure
with minimal energy of a
complex-1 and b complex-2
complexes are particularly localized on phenoxide and pyridyl
ring of primary ligand as DBHQ and DCHQ and HOMO-1
and LUMO+1 are on secondary ligand (L). The calculated
HOMO energy levels of complex 1 and 2 are −5.58 and −
Results and Discussion
All ligands and complexes were stable at atmospheric tem-
peratureand pressure. They weresoluble in common organic
solvents and characterized by spectroscopic methods, the
results were good agreement with proposed structure as
shown in scheme 1. The photophysical and electrochemical
characteristic measurement by luminescence spectra, UV-
visible spectra and cyclic voltammetry respectively.
Thermal stability was measured by TGA and DSC.
Theoretical calculations performed to estimate FMOs and
optimized structure.
5
.55 eVand the LUMO energy levels are −2.25 and − 2.23 eV,
respectively. So, the band gap of complex 1 and 2 are 3.33 and
.32 eV, as per theoretical study of complexes we can be
3
concluded that substitution of halogen on primary ligand
was negligible effect and further preliminary ligand (DBHQ
and DCHQ) stabilized the HOMO and LUMO and β-
diketone as secondary ligand taking part to stabilized the
HOMO (−1) and LUMO (+1) of complexes [34, 35].
Fig. 2 Frontier molecular orbitals (FMOs) of the complexes calculated using B3LYP/6-31G*

J Fluoresc
Where,
Complex 1: R =R = Br; [Zn(DBHQ)(L)] and
1
2
1
2
Complex 2: R =R = Cl; [Zn(DCHQ)(L)]
1
2
1
2
Scheme 1 Synthesis scheme of ligand and complexes. Where, Complex 1: R = R = Br; [Zn(DBHQ)(L)] and Complex 2: R = R = Cl;
[Zn(DCHQ)(L)]
1
13
Complex Characterization
magnetic resonance spectroscopy (like H NMR and
C
1
NMR), Elemental analysis and FTIR. In H NMR, li-
gands are shown the peak of enolic proton at δ 11.561
in case of secondary ligand and the singlet peak due to
the hydroxyl proton at δ 9.26 in case of DBHQ and
The synthesis scheme of Zn(II) complex was shown in
scheme 1. The complexes were characterized by nuclear
[Zn(DBHQ)(L)]
[Zn(DCHQ)(L)]
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1.0
0.8
0.6
0.4
0.2
0.0
[
Zn(DBHQ)(L)]
Zn(DCHQ)(L)]
1.0
[
0
0
0
0
0
0
.12
.10
.08
.06
.04
.02
0.8
0.6
0.4
0.2
0.0
2
80
300
320
340
360
380
400
420
440
300
400
500
600
700
Wavelength (nm)
450
500
550
600
650
Wavelength (nm)
Fig. 3 Absorption and Photoluminescence Spectra of [Zn(DBHQ)(L)]
and [Zn(DCHQ)(L)] measured in acetonitrile solvent
Fig. 4 Photoluminescence spectra of both complex in solid state

J Fluoresc
Table 1 UV-vis absorption, photoluminescence (PL), relative PL quantum yield and lifetime studies data of zinc complex
Complex
UV-vis absorbance (nm)
Photoluminescence (PL)
DMF Solution
Solid state
Lifetime (ms)
PL λmax (nm)
QY
CIE (x, y)
PL λmax (nm)
QY
CIE (x, y)
[
Zn(DBHQ)(L)]
Zn(DCHQ)(L)]
268,342,405
266,344,405
539
544
0.46
0.50
(0.36,0.58)
(0.38,0.57)
505
520
0.38
0.41
(0.19,0.48)
(0.25,0.61)
0.037
0.043
[
Absorption spectra measured in acetonitrile (1.0 × 10–4 M)
DCHQ, both signal disappear after complexation with
Zn(II) ion, it indicates that both the enolic and hydroxyl
proton takes part in complex formation. From the FT-IR
Photophysical Properties
The Photophysical properties of complex 1 and 2 in
dimethylformamide (DMF) (1.0 × 10 M) solution were
−
1
−4
spectra, the peak between 2920 and 2930 cm indicate
the stretching vibration of C-H bond in aromatic system.
measured by UV-vis absorption and photoluminescence spec-
troscopy (in Fig. 3). The acquired data are summarized in
Table 1. Both complex display a multiple UV-absorption peak
in the range of 230–350 nm, which are emanate from ligands
centered π-π* and n-π* electronic transition and one distinct
weaker band at 400–410 nm assigned as intramolecular inter-
action between metal-ligand charge transfer within the whole
complex [40]. The energy band gap of both complex was
−
1
Other peak at 1542 and 1373 cm which were related
to the υ(C = N) bond of the pyrazolone ring and the υ(C–
bond, respectively. The observed peaks in the region
O)
−
1
from 800 to 600 cm have been shown the presence of
quinoline ring. The metal sensitive stretching vibrational
band at 578 cm− for both complexes can be related to
the υ(Zn-O) vibration [39].
1
Fig. 5 CIE coordinate of both
complexes in solution and solid
state

J Fluoresc
estimated by Eq. 1 and calculated value are shown in Table 3.
using the reference standard in the same solvent [42]. The
quantum yields were calculated by using Eq. 2 [43],
Band gap; Eg ¼ hc=λonsetðUV−visÞ
ð1Þ
AbsðrÞ * AreaðxÞ * η2ðxÞ
Quantum Yield ΦðxÞ ¼ ΦðrÞ*
ð2Þ
AbsðxÞ AreaðrÞ η2ðrÞ
The both complex showed good luminescence proper-
ties in solution and solid states. The emission spectra of
synthesized complexes were measured in DMF at room
temperature, the values are 539 nm with CIE coordinate
Where, Φ(r) and Φ(x) are the quantum yields of refer-
ence standard and unknown sample, Abs(r) and Abs(x) are
the solution absorbance, Area(r) and Area(x) are area of
the entire emission, η(r) and η(x) are the refractive index
of the solvent.
(
0.36,0.58) and 544 nm with CIE coordinate (0.38,0.57)
for 1 and 2 respectively (see Fig. 5). In solid state, com-
plex 1 and 2 are exhibited strong luminescence broad
band at 505 nm and 520 nm, respectively. (see Fig. 4),
Complex 1 and 2 are shown luminescence quantum yield
where Znq showed emission at 560 and 496 nm in solu-
2
(Φ) 0.46 and 0.50 which is greater than of Znq (0.30) in
2
tion and solid state respectively [41].
The photoluminescence quantum yield of both complexes
were measured in solution of complexes by a relative method
acetonitrile, while 0.38 and 0.41 are Φ of both complexes in
the solid state and it was also close to literature value of Znq2
(0.50) [41].
Fig. 6 Absorption spectra of a
Zn(DBHQ)(L)
MeOH
Acetonitrile
[Zn(DBHQ)(L)] and b
1.2
[Zn(DCHQ)(L)] in different
DMSO
DMF
solvents
1.0
0.8
0.6
0.4
0.2
0.0
Acetone
DCM
CHCl3
THF
Toluene
300 320 340 360 380 400 420 440 460 480
Wavelength (nm)
(a)
Zn(DCHQ)(L)
MeOH
Acetonitrile
1
1
0
0
0
0
0
.2
DMSO
DMF
.0
.8
.6
.4
.2
.0
DMSO
DCM
CHCl3
THF
Toluene
300 320 340 360 380 400 420 440 460 480
Wavelength (nm)
(b)

J Fluoresc
Solvent Effect on Absorption and Photoluminescence
Spectra and its Correlation of Solvatochromic Shift
with Solvent Polarity
of Stoke shift (Δυ) and orientation polarizability (Δf) is ob-
tained Lippert-Mataga plot [55–57]. The value of stoke shift
and Δf of both complexes are summarized in Table 3 and
Lippert-Mataga plot of both complexes are shown in Fig. 8.
In π-conjugated molecules with donor-acceptor (D-A) or
donor-bridge-acceptor (D-B-A) type systems have been intro-
duced intramolecular charge-transfer (ICT) character [44–46].
Such ICT character of complexes were studied by measuring
the absorption and photoluminescence spectra with different
polarity of solvents [47–51]. So, solvent effect of complexes
studied by the absorption and photoluminescence (PL) spectra
of both complexes in different solvents (Figs. 6 and 7). The
absorption and luminescence spectra of both complexes re-
corded in different solvents with λabs and λPL obtained in
range of 384–420 nm and 522–550 nm. respectively. The
absorption and PL peaks values of both complexes in different
solvents are summarized in Table 2. In absorption and PL
spectra of both complexes blue shift on increasing polarity
of solvents but in the case of acetone or DMF give red shift
compered to Methanol [52–54]. Further, the liner correlation
Electrochemical Analysis
The electrochemical properties of synthesized zinc com plexes
containing disubstituted 8-hydroxyquinoline and β-
diketonate ligand were studied by cyclic voltammetry (CV).
The CV experiment was performed on a CHI660E electro-
chemical workstation using a three assembly consist of a
Platinum (Pt) disk as working electrode, a non-aqueous Ag/
Ag + as a reference electrode and a subsidiary Pt electrode.
Figure 9 shows the cyclic voltammograms of zinc complex
1 and 2, It was obtained by using DMF containing 0.1 M
tetrabutylammonium perchlorate (TBAP) as a supporting
electrolyte as well as ferrocene (Fc) as an internal standard
in the potential range of −1.2 to +0.68 V for complex 1 and
−1
− 0.8 to +0.8 V for complex 2 at scan rate of 50 mV s . The
Fig. 7 PL emission spectra of a
Zn(DBHQ)(L)
MeOH
ACN
1.0
[Zn(DBHQ)(L)] and b
DMSO
DMF
[Zn(DCHQ)(L)] in different
solvents at same excitation
400 nm)
0.8
Acetone
DCM
(
CHCl3
THF
0.6
0.4
0.2
0.0
Toluene
460 480 500 520 540 560 580 600 620 640 660 680 700
Wavelength (nm)
(
a)
[
Zn(DCHQ)(L)]
MeOH
ACN
1.0
0.8
0.6
0.4
0.2
0.0
DMSO
DMF
Acetone
DCM
CHCl3
THF
Toluene
460 480 500 520 540 560 580 600 620 640 660 680 700
Wavelength (nm)
(
b)

J Fluoresc
Table 2 Spectroscopic characteristics of investigated complexes in
selected solvents
1
1
1
1
1
1
50
40
30
20
10
00
[Zn(DBHQ)(L)]
[Zn(DCHQ)(L)]
Solvents
[Zn(DBHQ)(L)]
abs (nm)
[Zn(DCHQ)(L)]
abs (nm) λPL (nm)
λ
λPL (nm)
λ
MeOH
394
404
407
410
408
389
405
414
418
535
540
539
539
539
525
527
533
519
398
402
407
413
409
384
406
415
420
537
550
544
544
541
522
533
537
522
3
CH CN
DMSO
DMF
Equation
Plot
y = a + b*x
[Zn(DBHQ)(L)]
01.28015 ± 4.71657
16.34323 ± 19.48786
[Zn(DCHQ)(L)]
-
-
--
1
102.96213 ± 5.73531
123.19212 ± 23.69708
Acetone
DCM
Intercept
Slope
1
9
0
CHCl
3
0
.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
THF
orientation polarizability Δf
Fig. 8 Lippert-Mataga Plot of both complexes
Toluene
3
Where, MeOH methanol, CH CN acetonitrile, DMSO dimethyl sulfox-
specified electrochemical properties of both zinc complexes
are summarized in Table 4.
ide, DMF dimethylformamide, DCM dichloromethane, CHCl chloro-
3
form, THF tetrahydrofuran
The highest occupied molecular orbital (HOMO) and low-
est unoccupied molecular orbital (LUMO) energy levels of
both complexes were evaluated using Eqs. 3 and 4 [58],
of halogen substitution on 8-hydroxyquinoline ligand. The
bromine atoms have higher effect on the half-point potential
compare to chlorine atom in complex [60]. Therefore,
dihalogen substituted 8-hydroxy quinoline and pyrazolone li-
gands are more favorable for tuning the bandgap for HOMO
and LUMO levels of zinc complexes for OLEDs application.
À
Á
EHOMO ¼ − E
ELUMO ¼ EHOMO þ Eg eV
þ 5:1 eV
ð3Þ
ð4Þ
onsetðFcþ=FcÞ
À
Á
The HOMO energy levels of complexes 1 and 2 were cal-
culated to be −5.213 and − 5.299 eV from the onset oxidation
potential, assuming the absolute energy level of ferrocenium/
ferrocene (Fc+/Fc) redox couple to be 5.10 eV below the
vacuum energy level [59]. The LUMO energy levels of com-
plexes 1 and 2 were estimated to be −2.523 and − 2.699 eV,
achieved from its HOMO energy level and optical band gap
Thermal Properties
A necessary property for construction of OLEDs device is the
formation of morphologically stable and uniform amorphous
thin films by chemical vapor deposition (CVD) method.
Therefore, thermal properties and the amorphous behavior of
the synthesized zinc complexes were measured by
thermogravimetry (TGA) and differential scanning
(
calculated from its absorption λonset using Eq. 1). Therefore,
the complexes are shown the difference in HOMO level of
.086 eVand LUMO level of 0.176 eV may due to the effect
0
Table 3 The value of orientation polarizability(Δf) of selected solvent
1
2
9
6
3
0
3
for Lippert-Mataga Plot
[
Zn(DBHQ)(L)]
Solvents Polarity
(H O 100)
Δf
Stoke Shift Δυ (nm)
Zn(DBHQ)(L)] [Zn(DCHQ)(L)]
[Zn(DCHQ)(L)]
2
[
MeOH
CH CN
76.2
46.0
44.4
40.4
35.5
30.9
25.9
21.0
9.9
0.309 141
0.306 136
0.262 132
0.274 129
0.284 131
0.218 136
0.148 122
0.210 119
0.013 101
139
148
137
131
132
138
127
122
102
3
DMSO
DMF
Acetone
DCM
CHCl
3
-
THF
-1.2
-0.9
-0.6
-0.3
0.0
0.3
0.6
0.9
Toluene
Potential (V) vs. non aqueous Ag/Ag+
2
ɛ−1
η −1
2
Where, Δf ¼
−
; ε = dielectric constant of solvents and η =
Fig. 9 Cyclic Voltammograms of [Zn(DBHQ)(L)] and [Zn(DCHQ)(L)]
2ɛþ1 2η þ1
refractive index of solvents
in DMF solvent

J Fluoresc
Table 4 Electrochemical behavior of zinc complexes 1 and 2
infrastructural and instrumental facility, SICART, Vallabha Vidyanagar
for instrumental facility. We also thankful to Prof. S. S. Soni for their
support and cyclic voltammetry measurements
Complex
E
g
(eV)
Eonset (V)
HOMO (eV)
LUMO (eV)
1
2
2.69
2.60
0.113
0.199
−5.213
−5.299
−2.523
−2.699
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melting transition temperature (T ) occur at 320 and 375 °C
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for complexes 1 and 2 respectively. In additional significance
from DSC, the endothermic peak of both complexes are ob-
served at the higher temperature, that means both complexes
existence in monomeric structure and they are not form any
type of dimer or oligomer [62]. Such higher value of T , T
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and monomeric structure at high temperature indicate that
both complexes are stable, they also offer a great potential to
be fabricated device by chemical vapor deposition (CVD) and
vacuum thermal evaporation technology [19].
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Conclusion
In summary, two heteroleptic complexes were design based
on D-B-A system. These complexes structure were character-
ized by spectroscopy. The photophysical and luminescence
properties were revels that these complexes 1 and 2 were
suitable emitters with luminescence quantum yield 0.31 and
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.41 respectively as compared with Znq (0.30), whereas lu-
2
7
992–8003
minescence lifetime of complex 2 obtained value 0.041 ms
which is higher as compared with complex 1 value 0.037 ms.
Here, ligand DCHQ in complex 2 shows higher lifetime and
luminescence quantum yield due to their less non-radiative
decay as compared with complex 1. Moreover, the band gap
of complex 2 were also favorable 2.60 eV with other host
materials. These complexes were thermally stable up to
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