Synthesis and characterization of a green-light-emitting (pbi-Br)2Ir (acac) metal complex for OLEDs

Article abstract of DOI:10.1002/bio.3565

We designed and synthesized a 2-(4-bromophenyl)-1-phenyl-1H-benzimidazole (pbi-Br) ligand, which was then employed to create an innovative phosphorescent cyclometallated iridium(III) (pbi-Br)₂Ir(acac) metal complex with acetyl acetone as an ancillary ligand using the Suzuki coupling reaction. The complex was then characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra and thermogravimetric analysis (TGA)/differential thermal analysis (DTA) for structural and thermal analysis, respectively. XRD confirmed its amorphous nature and the FTIR spectrum revealed the molecular structure confirmation of the metal complex. The TGA/DTA curve disclosed its thermal stability up to 310°C. Ultraviolet (UV)–vis absorption and photoluminescence (PL) spectra were measured to explore the photo-physical properties of the (pbi-Br)₂Ir(acac) complex in basic and acidic media respectively. With the variation in solvent from acidic to basic media, optical absorption peaks blue shifted with variation in optical densities. These results facilitated the calculation of various photo-physical parameters. When excited at 379?nm in the solid state, the synthesized complex gave out a green light emission, peaking at λ_emi?=?552?nm. Staggering differences in optical density were observed in the PL spectra of the solvated complex. A Stokes’ shift of 7140.45?cm^?1 and 7364.94?cm^?1 was observed when the complex was solvated in acetic acid and chloroform, respectively. Hence the synthesized iridium metal complex can be considered as promising green emissive material for optoelectronic applications.

Full text of DOI:10.1002/bio.3565

Received: 30 June 2018
DOI: 10.1002/bio.3565
Revised: 17 September 2018
Accepted: 23 September 2018
R E S E A R C H A R T I C L E
Synthesis and characterization of a green‐light‐emitting
(pbi‐Br)2Ir (acac) metal complex for OLEDs
Neha Khotele1 N. Thejo Kalyani2
S.J. Dhoble3
|
|
1 Department of Physics, Shivaji Science
Abstract
College, Congress Nagar, Nagpur 440012,
India
We designed and synthesized a 2‐(4‐bromophenyl)‐1‐phenyl‐1H‐benzimidazole (pbi‐
Br) ligand, which was then employed to create an innovative phosphorescent
cyclometallated iridium(III) (pbi‐Br)2Ir(acac) metal complex with acetyl acetone as an
ancillary ligand using the Suzuki coupling reaction. The complex was then character-
ized by X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectra and thermo-
gravimetric analysis (TGA)/differential thermal analysis (DTA) for structural and
thermal analysis, respectively. XRD confirmed its amorphous nature and the FTIR
spectrum revealed the molecular structure confirmation of the metal complex. The
TGA/DTA curve disclosed its thermal stability up to 310°C. Ultraviolet (UV)–vis
absorption and photoluminescence (PL) spectra were measured to explore the
photo‐physical properties of the (pbi‐Br)2Ir(acac) complex in basic and acidic media
respectively. With the variation in solvent from acidic to basic media, optical absorp-
tion peaks blue shifted with variation in optical densities. These results facilitated the
calculation of various photo‐physical parameters. When excited at 379 nm in the solid
state, the synthesized complex gave out a green light emission, peaking at
2 Department of Applied Physics,
Laxminarayan Institute of Technology, Nagpur
440033, India
3 Department of Physics, Rashtrasant Tukadoji
Maharaj Nagpur University, Nagpur 440033,
India
Correspondence
N. ThejoKalyani, Department of Applied
Physics, Laxminarayan Institute of Technology,
Nagpur 440033, India.
Email: thejokalyani@rediffmail.com
λemi = 552 nm. Staggering differences in optical density were observed in the PL spec-
tra of the solvated complex. A Stokes’ shift of 7140.45 cm−1 and 7364.94 cm−1 was
observed when the complex was solvated in acetic acid and chloroform, respectively.
Hence the synthesized iridium metal complex can be considered as promising green
emissive material for optoelectronic applications.
KEYWORDS
iridium complex, Lippert–Mataga plot, molecular absorption cross‐section, OLEDs, solid‐state
lighting, Suzuki coupling reaction
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1
INTRODUCTION
complexes, have attracted much more attention due to the theoretical
utilization of both singlet and triplet excitons for light emission to real-
ize an internal quantum efficiency of 100%.[4,5] Prior state of art illus-
trates phosphorescent heavy metal complexes such as Ru(II), Os(II),
Re(III) and Ir(III) as green light‐emitting photo‐luminophores.[6] Out of
these, Ir(III) complexes have received more attention due to their
broad color tunability, efficient emission and high thermal stability.[7,8]
Photo‐physical properties of Ir(III) complexes are governed by the
type of cyclometallated ligand.[9] Among many, the benzimidazole
ligand has been reported as an important component for the
In recent years, phosphorescent iridium complexes with benzimidazole
ligands have gained momentum in various optoelectronic applications
such as organic light‐emitting diodes (OLEDs), flat panel displays and
solid‐state lighting as they exhibit a broad range of emission colors
in the green region of the visible spectrum. In particular, iridium‐based
complexes with benzimidazole ligand are envisioned to be superior
green light emitters for the next generation of display and lighting
technology.[1–3] OLED technology, based on higher transition metal
Luminescence. 2018;1–8.
wileyonlinelibrary.com/journal/bio
© 2018 John Wiley & Sons, Ltd.
1

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KHOTELE ET AL.
preparation of Ir(III) complexes in OLEDs due to its excellent electron
mobility and flexible modification ability.[10] It is also expected that irid-
ium complexed with benzimidazole ligand can be employed in fabricat-
ing OLED devices and displays by either a vacuum deposition method
or solution techniques due to good volatility and solubility. Keeping this
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2.1
Synthesis of the [(pbi‐Br)2Ir(acac)] complex
The (pbi‐Br)2Ir(acac) complex was synthesized according to Scheme 2
as shown in Figure 2. A mixture of 2‐methoxyethanol (S) and water
(3:1 v/v, 25 ml) was added to a flask containing IrCl3.nH2O (176 mg,
0.5 mmol) and the ligand 2‐(4‐bromophenyl)‐1‐phenyl‐1H‐benzimid-
azole (pbi‐Br) (700 mg, 2.0 equiv.). The mixture was refluxed to react
for 48 h and then cooled to room temperature. Later, the reaction
was quenched with water, extracted with dichloromethane, and
dried under vacuum. The solid yield was collected by filtration and
evaporation to give the crude product. The crude product, i.e.,
m‐chloro‐bridged Ir(III) dimmer, was mixed with Na2CO3 (0.30 g,
3.0 mmol), 2,4‐pentanedione (0.30 g, 3.0 mmol), and 2‐
methoxyethanol (20 ml) in a flask. The mixture was heated to react
for 24 h. After cooling, the reaction was quenched with water and
the mixture was extracted with dichloromethane. The combined
extracts were then washed with brine, dried over MgSO4, and evap-
orated to dehydrate it to produce a yellow solid compound with
65% yield.
in mind, we synthesized
a (pbi‐Br)2Ir(acac) complex with 2‐(4‐
bromophenyl)‐1‐phenyl‐1H‐benzimidazole (pbi‐Br) as the main ligand
and acetyl acetone as an ancillary ligand to study the structural, thermal
and photo‐physical properties of this complex.
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2
EXPERIMENTAL
All reagents and solvents used were of analytical reagent grade. All
reactions were performed under an argon atmosphere. A 2‐(4‐
bromophenyl)‐1‐phenyl‐1H‐benzimidazole (pbi‐Br) ligand was synthe-
sized conveniently, according to Scheme 1, from the condensation of
N‐phenyl‐O‐phenylenediamine and 4‐bromobenzaldehyde by the
Suzuki coupling reaction[11–13] as shown in Figure 1. N‐Phenyl‐O‐
phenylenediamine (1 equiv.) and 4‐bromobenzaldehyde (1 equiv.) were
dissolved in 50 ml of 2‐methoxyethanol. The mixture was refluxed for
48 h and volatiles were removed under vacuum. The resulting organic
solid was extracted by dichloromethane and washed with brine solution
two or three times. It was then dried over MgSO4 and evaporated
under a natural environment to yield a brownish solid [C19H13BrN2]
with a 60% yield.
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3
RESULTS AND DISCUSSION
X‐ray diffraction was carried out on an X‐ray Diffractometer System
XPERT‐PRO (PAN analytical), while Fourier transform infrared (FTIR)
spectra were measured a on Bruker‐Alpha instrument at room temper-
ature over the range 4000–600 cm−1 by averaging 64 scans at a
maximum resolution of 4 cm−1. Thermogravimetric analysis (TGA)
was carried out on Perkin Elmer Diamond instrument. The optical
absorption spectrum of the (pbi‐Br)2Ir(acac) complex in acetic acid
was obtained on Perkin Elmer Lamda35 spectrophotometer and the
photoluminescence (PL) spectrum was obtained using a Shimadzu RF
5301 spectrofluorometer. The Commission International d'Eclairage
(CIE) coordinates of the synthesized complex were probed using a
CIE1931 system.
FIGURE
1
Synthesis route of 2‐(4‐bromophenyl)‐1‐phenyl‐1H‐
benzimidazole (pbi‐Br) ligand
FIGURE 2 Synthesis route of the (pbi‐Br)2Ir(acac) complex (Scheme 2)

KHOTELE ET AL.
3
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3.1
Fourier transform infrared spectroscopy
FTIR spectroscopy is an analytical technique employed to categorize
molecular structures and validate the packing arrangements and chain
conformational properties of polymeric compounds. This technique
measures the absorption of a range of infrared light wavelengths of
the material of interest. FTIR spectra of the (pbi‐Br)2Ir (acac) complex
were measured over the range 4000–600 cm−1 by averaging 64 scans
at a maximum resolution of 4 cm−1 as shown in Figure 3.
Numerous absorbance peaks were observed in the fingerprint
region of the FTIR spectra, which may be attributed to C–H bending
(690–900 cm−1) and C–O stretching (970–1250 cm−1). A peak at
1243.26 cm−1 may be assigned to C–H bonding.
A peak at
1547.18 cm−1 may be attributed to C=C stretching and the peaks in
the range 1500–1450 cm−1 can be assigned to the aromatic ring
stretch C–C. Variations in the range of2850–3000 cm−1 may be due
to double bonded =CH2 stretching, peaking at 2843.71 and
2951.24 cm−1 for –CH3 stretching, while 3294–3950 cm−1 can be
assigned to single bond –N–H stretching vibrations showing peaks at
3631.37, 3555.21 and 3440.89 cm−1.This result confirmed the
formation of the cyclometallated iridium(III) (pbi‐Br)2Ir(acac) metal
complex with acetyl acetone as the ancillary ligand.
FIGURE 4 XRD pattern of the (pbi‐Br)2Ir(acac) complex
A thermogram of the (pbi‐Br)2Ir(acac) complex exhibited no weight
loss in the temperature range below 310°C, indicating its thermal sta-
bility, as shown in Figure 5. With further increase in temperature, a
weight loss of about 6.20% was observed up to 376°C. The DTA curve
of the (pbi‐Br)2Ir(acac) complex shows a combination of endothermic
and exothermic peaks.
The first endothermic peak was centered at 155.71°C, corre-
sponds to the distortion of water from the synthesized complex, while
the second peak at 381.94°C corresponded to the evaporation of
residual moisture. However, no exothermic peaks were observed for
the DTA curve. This result indicated that the synthesized Ir(III)
complex is highly stable and cannot be distorted easily, even under
high temperatures associated with heat generation during the
operation of OLEDs, and leading to long device lifetimes.
|
3.2
X‐ray diffraction
X‐ray diffraction (XRD) is one of the most promising techniques for
structural analysis. The XRD pattern of the (pbi‐Br)2Ir(acac) complex
in the solid state showed diffused diffraction peaks, revealing its amor-
phous nature. The peak with maximum relative intensity was observed
at 2θ = 40.593° having interplanar distance 4.3048 Å as shown in
Figure 4. Another three peaks with relative intensity of 44%, 23%
and 12% were observed at 2θ = 74.547°, 30.581° and 17.547° having
interplaner distances as 1.26924 Å , 2.9209 Å and 5.05018 Å
respectively.
|
3.4
Photo‐physical properties
Photo‐physical properties such as absorbance, absorption maxima
(λabs), molecular absorption cross‐section, molar absorption coeffi-
cient, radiative life time, optical band gap (Eg), excitation maxima
(λexct), emission maxima (λemi) and Stokes’ shift (υabs – υem) of the
molecules of the solvated (pbi‐Br)2Ir(acac) complex were established
by UV–visible absorption and photoluminescence spectroscopy.
|
3.3
Thermogravimetric analysis and differential
thermal analysis
TGA is used for precise measurement of specimen weight changes as
a function of time and temperature in various gaseous environments.
This analysis is very useful for investigation of reactivity and stability.
FIGURE 3 FTIR spectra of (pbi‐Br)2Ir(acac) complex
FIGURE 5 TGA and DTA curve of the (pbi‐Br)2Ir(acac) complex

4
KHOTELE ET AL.
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3.4.1
UV–visible light absorption spectra
The UV–vis light absorption spectra of the (pbi‐Br)2Ir(acac) metal
complex in acetic acid and chloroform at 10−3 M concentration, and
recorded at room temperature is depicted in Figure 6 and Figure 7,
respectively. The absorption spectra of the (pbi‐Br)2Ir(acac) complex
displayed the characteristic vibration pattern of an iridium complex.
The absorption peaks at 258, 272 and 308 nm in acetic acid and
265, 294 and 316 nm in chloroform could be assigned to the spin‐
allowed π–π* transition of the cyclometallated ligand.
The broad absorption bands in the range of 360–400 nm in acetic
acid and 350–400 nm in chloroform can be assigned to spin‐allowed
metal‐to‐ligand charge‐transfer (MLCT) transition and spin‐forbidden
MLCT transition, respectively, having peaks at 361, 376, nm in acetic
acid and 364 and 376 nm in chloroform, respectively. Optical
absorption parameters and the probability transition parameters of
the (pbi‐Br)2Ir (acac) complex at 10−3 in acetic acid and chloroform
are listed in Tables 1 and 2, respectively.
FIGURE 6 UV–vis spectra of (pbi‐Br)2Ir(acac) complex in acetic acid
|
3.4.2
Determination of energy band gap
By using the procedure described by Morita et al.,[14] the optical band
gap (Eg) of the solvated (pbi‐Br)2Ir(acac) metal complex was deter-
mined. The same measurement was found to be 2.35 eV and
2.42 eV in acetic acid and chloroform, as shown in Figure 8.
Theoretically, the energy is calculated using Einstein's equation
given by.
hc 6:626 × 10−34 × 3 × 108 1239:83
E ¼ hv ¼
¼
¼
λ
λ
λ
Where h = Planck's constant, c = velocity of light and λ is the
absorption wavelength. The same analogy can be applied to calculate
the amount of energy absorbed by phosphor when exposed to UV
light. When the absorption wavelength is employed to the above
equation, the amount of energy that is required by the phosphor to
make transition from the highest occupied molecular orbital (HOMO)
FIGURE 7 UV–vis spectra of (pbi‐Br)2Ir(acac) complex in chloroform
TABLE 1 Optical absorption parameters of the (pbi‐Br)2Ir (acac) complex at 10−3 in acetic acid and chloroform
Complex
Solvent
Molar conc. mol/L
Band
λmax (nm)
Transition
Optical density (a.u.)
(pbi‐Br)2Ir(acac)
Acetic acid
10−3
I
II
I
308
376
316
376
π–π*
n–π*
π–π*
n–π*
2.926
2.777
2.703
2.536
Chloroform
10−3
II
TABLE 2 Transition probability parameters of (pbi‐Br)2Ir (acac) complex at 10−3 in acetic acid and chloroform
Molecular
absorption
cross‐section
σ(λ) = 0.382
× 10−23. εmax
(cm)
Molar
Decadic
absorption
coefficient
Aλ
Calculated
radiative
lifetime
Naperian
absorption
coefficient
Amax
absorption
coefficient
α(λ) = a(λ)
ln 10 (cm−1
Number
density (N)
Absorbance
Absorption
wavelength
λmax (nm)
0
Iλ
aðλÞ
10−4
εmax
¼
CL
ΑðλÞ ¼ log
aðλÞ ¼
¼ AðλÞ
Iλ
l
N ¼
ðλÞ τ0
¼
(μs)
Solvent
¼ ελLC (a.u.)
(a.u.)
)
(mol. L−1. cm−1
)
σ
εmax
Acetic acid 272
2.47
2.92
2.77
2.47
2.92
2.77
5.6933
6.7233
6.3779
2474.6
2926.23
2777.21
9.45 × 10−21
0.117 × 10−21
0.106 × 10−21
8.44 × 1020 404
308
376
341
360
Chloroform 294
2.61
2.70
2.53
2.61
2.70
2.53
6.0095
6.2167
6.3779
2612.9
2703.6
2536.9
9.98 × 10−21
0.103 × 10−21
9.69 × 10−21
3.78 × 1020 382
316
376
369
394

KHOTELE ET AL.
5
FIGURE 8 Determination of band gap: (pbi‐Br)2Ir(acac) in (a) acetic
acid and (b) chloroform
FIGURE 9 Photoluminescence (a) excitation and (b) emission spectra
of the (pbi‐Br)2Ir(acac) complex
to the lowest unoccupied molecular orbital (LUMO), i.e. energy band
gap, can be calculated.
The calculated values of the optical band gap were found to
match with the theoretical values, as listed in Table 3.
|
3.4.3
Photoluminescence spectra
The PL spectra of the (pbi‐Br)2Ir(acac) complex in the solid state exhib-
ited intense green phosphorescence, peaking at 552 nm under an
excitation wavelength of 379 nm, while in acetic acid and chloroform
it peaked at λemi = 514 nm and 520 nm, respectively, as shown in
Figures 9 and 10, respectively. This result reflected a noticeable
bathochromic shift with a hyperchromic shift (increase in emission
intensity) and a hypochromic shift (decrease in emission intensity)
when (pbi‐Br)2Ir(acac) was solvated in chloroform and acetic acid
respectively. These results can be attributed to the fact that chloro-
form is non‐polar in nature, while acetic acid is polar protic solvent
due in which there could be poor intermolecular interaction and
dissociation of the complex.
FIGURE 10 Photoluminescence (a) excitation and (b) emission
spectra of the (pbi‐Br)2Ir (acac) complex in various organic solvents
|
3.4.4
Stokes’ shift
Stokes’ shift (υabs– υem) indicates the difference in absorption and
emission wavelengths. The Stokes’ shift (cm−1) can be determined by
the formula:
7364.94 cm−1 in acetic acid and chloroform as shown in Figure 11 and
Figure 12, respectively.
|
3.4.5
Lippert–Mataga plot
Stoke0 s shift ¼ υabs − υemi
¼
−
× 107cm−1
1
1
This plot is between Stokes’ shift (Δυ) and solvent polarity function
(Δ f ), which estimates the solvent sensitivity to chromophore.[15] It is
a known fact that the energy difference (ΔE) between the ground
state and excited state is affected by the interaction between the
λabs λemi
Substituting the values of absorption and emission wavelength,
the calculated values for Stokes’ shift were found to be 7140.45 and
TABLE 3 Photo‐physical properties of (pbi‐Br)2Ir(acac) complex in solvated state
Photo‐physical properties of complex
Nature of
solvent
Polarity
index
Solvent polarity
function (Δf)
Dielectric
constant (є)
Refractive
index (n)
λexe
(nm)
λemi
(nm)
λab
(nm)
Eg (eV)
(plot)
Eg (eV)
(the.)
Stokes’ shift
Solvent
(cm−1
)
Acetic acid
Chloroform
Polar protic
6.2
4.1
0.2028
0.1483
6.15
4.81
1.3716
1.4458
455
456
514
520
376
376
2.35
2.42
2.724
2.718
7140.45
7364.94
Non‐polar

6
KHOTELE ET AL.
FIGURE 13 Plot of Stokes’ shift versus solvent polarity function
FIGURE 11 Stokes’ shift of the (pbi‐Br)2Ir (acac) complex in acetic
acid
FIGURE 12 Stokes’ shift of the (pbi‐Br)2Ir (acac) complex in
chloroform
FIGURE 14 CIE 1931 (x, y) diagram of the (pbi‐Br)2Ir(acac) complex
considered solvent and chromophore. Stokes’ shift of the complex
versus solvent polarity function is shown in Figure 13. Photo‐physical
properties of the (pbi‐Br)2Ir(acac) complex in the solvated state are
listed in Table 3.
respectively. Correlated color temperature,[17] a graphical method for
correlating CIE coordinates using an RGB diagram was obtained based
on the CIE 1931 system.
Correlated color temperature is the temperature of a black body
whose chromaticity closely resembles that of a light source. For high
quality white light (RGB) illumination the CCT should lie between
2500 K to 6500 K. Hence the same parameter for our complex fell
inbetween these values. CIE coordinates and CCT values for the
(pbi‐Br)2Ir (acac) complex in various environments are given in
Table 4.
|
3.4.6
CIE coordinates
The color of the phosphorescence is represented by color coordinates,
which could be calculated from the emission spectrum using the
chromaticity coordinate calculation method based on the CIE 1931
system.[16] CIE coordinates of the (pbi‐Br)2 Ir (acac) complex in the
solid state were found to be (0.313, 0.680) with color correlating tem-
perature (CCT) at 5778 K, indicating that the synthesized complex
emitted deep green light, as shown in Figure 14. Whereas in the
solvated complex, the same was found to be (0.035, 0.800), (0.073,
0.831) with 9607 and 8801 as CCT in acetic acid and chloroform,
Figure 15 shows the plot of the solvent polarity function (Δf) ver-
sus CIE coordinates. The nature of the graph between polarity
function (Δf) and CIE coordinates demonstrates its linear nature.
The absorption and emission spectra of phosphorescent metal
complexes are in general influenced by various parameters such as

KHOTELE ET AL.
7
physical behavior of transition metal complexes can be properly
described with molecular orbitals, however the performance of accu-
rate quantum mechanical calculations is far more complicated. It has
been shown that frontier molecular orbitals are not equally delocalized
in heavy metal complexes, resulting in different electronically excited
states. Among these, metal‐centered (MC) excited states are typically
present in metal complexes with partially filled d shells at the metal
center. The corresponding d–d transitions are Laporte forbidden and,
consequently, exhibit very low transition probabilities. MLCT states
involve electronic transitions from metal‐based d orbitals to a ligand‐
centered π* anti‐bonding orbitals. Emissive MLCT states are particu-
larly observed in d6 and d8 transition metal complexes and therefore
play a major role in the photo‐physics of iridium(III) compounds.
Intraligand (IL) π–π* excited states originate from the electronic
transitions of the ligand.[21–24] If the metal perturbation upon coordi-
nation is minimized, their spectral properties often closely resemble
the free ligand states. Finally, ligand‐to‐metal charge‐transfer (LMCT)
excited states are occasionally observed in complexes with metal
atoms in high oxidation states. All these transitions determine the
photo‐physical properties of transition metal complexes and can be
used for the interpretation of experimentally observed spectra or the
prediction of absorption and emission characteristics of novel com-
pounds. Comparative analysis of our results with the results of W‐S
Huang et al.[13] is shown in Table 5. This comparison revealed that
our ligand and complex were synthesized at well defined temperature
of about 130°C and 105°C respectively. In view of characterization,
noticeable differences were observed in the absorption peak, energy
gap and emission wavelength. Other important supporting character-
izations such as FTIR spectra, which confirmed the structure forma-
tion, Stokes’ shift, which reflected the difference between positions
of the band maxima of the absorption and emission spectra and CIE
coordinates, which helped to evaluate the quality of a color regardless
TABLE 4 CIE coordinates and CCT values for the (pbi‐Br)2Ir (acac)
complex in various environments
CIE coordinates
Co‐related
color
Emission
wavelength
(nm)
temperature
(K)
Complex in
Cx
Cy
Solid state
Acetic acid
Chloroform
552 nm
514 nm
520 nm
0.313
0.035
0.073
0.680
0.800
0.831
5778
9607
8801
FIGURE 15 Plot of solvent polarity function (Δf) versus CIE
coordinates
the valence electron configuration at the metal, the type of the elec-
tronic transitions or the correlation among lower lying electronic
excited states.[18–20] As in fluorescent small molecules, the photo‐
TABLE 5 Comparative analysis of our results with the results of W‐S Huang et al.[13]
S.
no.
Characterization
techniques
Parameters
IR peaks
Referred results
Obtained result
1.
FTIR
XRD
–
–
1243.26, 1547.18, 1500–1450, 2843.71,
2951.24,3631.37, 3555.21, 3440.89 cm−1
2.
3.
Nature
Amorphous
TGA‐DTA
Thermal stability
–
310°C
Tg
Td
–
115.71 °C
381.94°C
363°C
4.
5.
6.
UV–vis absorption spectra
Energy band gap
Abs. Peak in chloroform
Abs. Peak in DCM
Abs. Peak in acetic acid
–
265, 294, 316, 364, 376
–
258, 272, 308, 361,376 nm
303, 316, 388, 416, 450 a.u.
–
By cyclic voltammetry
By Morita et al. chloroform
Acetic acid
3.02 eV
–
–
–
2.42 eV
2.32 ev
Photoluminescence spectra
Solid state
Chloroform
DCM
–
–
λexc = 379 nm and λems = 552 nm
λexc = 456 nm and λems = 520 nm
–
λemi = 518 nm
Acetic acid
–
λexc = 454 nm and λems = 514 nm
7.
8.
CIE coordinates (x,y)
Solid state
Chloroform
DCM
–
–
–
–
(0.313, 0.680)
(0.073, 0.831)
‐
Acetic acid
(0.035, 0.800)
Stokes’ shift
Chloroform
Acetic acid
–
–
7364.94
7140.45 cm−1

8
KHOTELE ET AL.
of its luminance have also been measured to check the suitability of
the complex for OLED applications.
REFERENCES
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B. H. Tong, M. K. Fung, L. Fu, Dyes Pigm, 2018, 150, 284.
[2] D. Chitnis, N. Thejokalyani, H. C. Swart, S. J. Dhoble, Renew. Sustain.
Energy Rev. 2016, 64, 727.
|
4
CONCLUSIONS
[3] C. Xiang, X. Fu, W. Wei, R. Liu, Y. Zhang, V. Balema, Adv. Funct. Mater.
2016, 26, 1463.
The synthesized green light‐emitting Ir(III) phosphorescent complex,
(pbi‐Br)2Ir(acac), bearing the 2‐(4‐bromophenyl)‐1‐phenyl‐1H‐benz-
imidazole (pbi‐Br) ligand was proven to be amorphous in nature. FTIR
spectrum results confirmed the formation of the (pbi‐Br)2Ir(acac)
complex by indicating peaks at 1243.26 cm−1, 1547.18 cm−1,1500–
1450 cm−1, 2843.71 cm−1, 2951.24 cm−1, 3631.37 cm−1, 3555.21
cm−1 and 3440.89 cm−1 attributed to C–H bonding, C=C stretching,
aromatic C–C stretching, –CH2 stretching, –CH3 stretching and N–H
stretching respectively. DTA/TGA curves inferred that the complex
had the ability to maintain its properties unchanged upon heating up
to 310°C. UV–vis spectra showed two prominent absorption bands
in acetic acid (258 nm, 272 nm and 308 nm), (361 nm and 376 nm)
and in chloroform (265 nm, 294 nm and 316 nm), (364 nm and
376 nm) attributed to π→π* and spin‐allowed 1MLCT transition. The
energy gap of the solvated (pbi‐Br)2Ir(acac) complex was found to be
2.35 eV and 2.42 eV in acetic acid and chloroform, respectively. When
the metal complex was excited at 379 nm in the solid state, it
displayed green emission, peaking at λmax = 552 nm. However, the
PL spectra of the solvated complex reflected noticeable bathochromic
shift with hyperchromic shift (increase in emission intensity) and
hypochromic shift (decrease in emission intensity) when (pbi‐
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Br)2Ir(acac) was in chloroform and acetic acid respectively.
A
Lippert–Mataga plot, established the relationship between Stokes’
shift and solvent polarity function. Stokes’ shifts of 7140.45 cm−1
and 7364.94 cm−1 were observed when the complex was solvated in
acetic acid and chloroform. CIE coordinates of the (pbi‐Br)2Ir(acac)
complex in the solid state were found to be (0.313, 0.680) with CCT
at 5778 K, indicating that the synthesized complex emitted deep
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0.831) with CCT at 9607 K and 8801 K in acetic acid and chloroform,
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How to cite this article: Khotele N, Kalyani NT, Dhoble SJ.
Synthesis and characterization of
a green‐light‐emitting
(pbi‐Br)2Ir (acac) metal complex for OLEDs. Luminescence.
2018;1–8. https://doi.org/10.1002/bio.3565
ORCID
N. Thejo Kalyani http://orcid.org/0000-0001-8115-9010