Design and synthesis of novel anthracene derivatives as n-type emitters for electroluminescent devices: A combined experimental and DFT study

Article abstract of DOI:10.1039/c3pp50284h

Six novel anthracene-oxadiazole derivatives, 4a (2-(4-(anthracen-9-yl) phenyl)-5-p-tolyl-1,3,4-oxadiazole), 4b (2-(4-(anthracen-9-yl)phenyl)-5-(4-tert- butylphenyl)-1,3,4-oxadiazole), 4c (2-(4-(anthracen-9-yl)phenyl)-5-(4- methoxyphenyl)-1,3,4-oxadiazole), 8a (2-(4-(anthracen-9-yl)phenyl)-5-m-tolyl-1, 3,4-oxadiazole), 8b (2-(3-(anthracen-9-yl)phenyl)-5-(4-tert-butylphenyl)-1,3,4- oxadiazole) and 8c (2-(3-(anthracen-9-yl)phenyl)-5-(3,4,5-trimethoxyphenyl)-1,3, 4-oxadiazole) have been synthesized and characterized for use as emitters in organic light emitting devices (OLEDs). They show good thermal stability (T _d, 297-364 °C) and glass transition temperatures (T_g) in the range of 82-98 °C, as seen from the thermo gravimetric analysis and differential scanning calorimetric studies. The solvatochromism phenomenon and electrochemical properties have been studied in detail using UV-Vis absorption, fluorescence spectroscopy and cyclic voltammetry. TD-DFT calculations have been carried out to understand the electrochemical and photophysical properties. The spatial structures of 4b and 8c are further confirmed by X-ray diffraction analysis. Un-optimized non-doped electroluminescent devices were fabricated using these anthracene derivatives as emitters with the following device configuration: ITO (120 nm)/α-NPD (30 nm)/4a-4c or 8a-8c (35 nm)/BCP (6 nm)/Alq₃ (28 nm)/LiF (1 nm)/Al (150 nm). Among all the six compounds, 8a displays the maximum brightness of 1728 cd m^-2 and current efficiency 0.89 cd A^-1. Furthermore, as an electron transporter, 8a exhibited superior performance (current efficiency is 11.7 cd A^-1) than the device using standard Alq₃ (current efficiency is 8.69 cd A^-1), demonstrating its high potential for employment in OLEDs. These results indicate that the new anthracene-oxadiazole derivatives could play an important role in the development of OLEDs. The Royal Society of Chemistry and Owner Societies.

Full text of DOI:10.1039/c3pp50284h

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DOI: 10.1039/C3PP50284H.
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Page 1 of 46
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DOI: 10.1039/C3PP50284H
Design and Synthesis of Novel Anthracene Derivatives as
n-type Emitters for Electroluminescent Devices: A
Combined Experimental and DFT Study
G. Mallesham,a,b S. Balaiah,b M. Ananth Reddy,a,b B. Sridhar,*c Punita Singh,d Ritu
Srivastava,*d,e K. Bhanuprakash*b,e and V. Jayathirtha Rao*a,e
a Crop Protection Chemicals Division, b Inorganic and Physical Chemistry Division,
c
Laboratory of XꢀRay Crystallography, Indian Institute of Chemical Technology, Hyderabad
500007, India
d Organic Electronics, National Physical Laboratory, New Delhi, India
e National Institute for Solar Energy, New Delhi, India
Keywords: Organic light emitting devices, Anthraceneꢀoxadiazole derivatives, Emitting and
Electronꢀtransport materials, DFT calculations, Electroluminescence.
Corresponding author: jrao@iict.res.in
1

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DOI: 10.1039/C3PP50284H
Abstract
Six novel anthraceneꢀoxadiazole derivatives, 4a (2ꢀ(4ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀpꢀtolylꢀ1,3,4ꢀ
oxadiazole), 4b (2ꢀ(4ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀ(4ꢀtertꢀbutylphenyl)ꢀ1,3,4ꢀoxadiazole), 4c (2ꢀ
(4ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀ(4ꢀmethoxyphenyl)ꢀ1,3,4ꢀoxadiazole), 8a (2ꢀ(4ꢀ(anthracenꢀ9ꢀ
yl)phenyl)ꢀ5ꢀmꢀtolylꢀ1,3,4ꢀoxadiazole),
butylphenyl)ꢀ1,3,4ꢀoxadiazole) and
8b
8c
(2ꢀ(3ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀ(4ꢀtertꢀ
(2ꢀ(3ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀ(3,4,5ꢀ
trimethoxyphenyl)ꢀ1,3,4ꢀoxadiazole) have been synthesized, characterized for usage as
emitters in organic light emitting devices (OLEDs). They show good thermal stability (Td,
o
o
297–364 C) and glass transition temperatures (Tg) in the range of 82–98 C as seen from
the thermo gravimetric analysis and differential scanning calorimetric studies.
Solvatochromism phenomenon using UVꢀVis absorption and fluorescence spectroscopy and
electrochemical properties have been studied in detail. TDꢀDFT calculations have been
carried out to understand the electrochemical and photo physical properties. The spatial
structures of 4b and 8c are further confirmed by Xꢀray diffraction analysis. Unꢀoptimized
nonꢀdoped electroluminescent devices were fabricated using these anthracene derivatives
as emitters with following the device configuration: ITO (120 nm)/αꢀNPD (30 nm)/4a-4c or
8a-8c (35 nm)/BCP (6 nm)/Alq3 (28 nm)/LiF (1 nm)/Al (150 nm). Among all the six
compounds, 8a displays maximum brightness of 1728 cd mꢀ2 and current efficiency 0.89 cd
Aꢀ1. Furthermore, 8a as an electron transporter exhibited superior performance (current
efficiency is 11.7 cd Aꢀ1) than the device using standard Alq3 (current efficiency is 8.69 cd Aꢀ1)
demonstrating their high potential for employment in OLEDs. These results indicate that the
new anthraceneꢀoxadiazole derivatives could play an important role in the development of
OLEDs.
1. Introduction
Organic lightꢀemitting diodes (OLEDs) have received considerable attention in both scientific
and commercial applications since the pioneering work by Tang and VanSlyke in 1987
2

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because of their characteristic low driving voltage, high brightness, fullꢀcolor emission, fast
response time, wide viewing angle and selfꢀemitting properties.1,2 Extensive research has
been carried out to promote OLEDs into commercial applications as flatꢀpanel displays and
solidꢀstate lighting resources due to their low cost.3,4 OLEDs have been successfully applied
in mobile phones, computers, car stereos, digital cameras,5 wrist watches and white solidꢀ
state lighting etc.6,7 The search for new and efficient emitting materials and charge transport
layers remains as one of the most active areas in this field.8,9 While bringing out efficient
devices is one challenge, understanding the interactions, properties due to substitution,
geometry and packing of the molecules in the solid state is another. The latter would help us
not only to improve the device efficiency but also give us a more detailed insight into the
mechanism of OLED.
The basic structures of the OLED, which have been widely reported in the literature
are the multi layer fluorescent emitters and phosphorescent organic light emitting devices
(PHOLEDs).10,11 Generally in an OLED, three layers such as hole transport layer (HTL), an
emitting layer (EML) and an electron transport layers (ETL) are deposited in between anode
and cathode with each layer having their own function.12 In the multi layer fluorescent
emitters and PHOLEDs deposition of different layers provides complexity and also increases
the cost of the mass production of OLEDs.9 To reduce the complexity, overall cost and to
increase the extent of OLED commercialization, several organic lightꢀemitting materials
possessing electron or hole transporting characteristics or even having both properties in a
single molecule are being developed, such that their devices can be simplified to a double or
even singleꢀlayer device architecture.13 The major drawbacks in the doped devices
(PHOLEDs) are that the electrochemiluminescence (ECL) properties including color purity
are extremely sensitive to the dopant concentration (usually <5%)14,15 and tripletꢀtriplet
annihilations.16,17 Precise control of the dopant concentration using vacuum evaporation or
spin coating methods is not an easy task and to prevent exothermic reverse energy transfer
in PHOLEDs materials with high triplet energy than the dopant are required.18,19 In this
regard, it is a good strategy to develop non doped OLEDs based on small molecules with
3

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charge transporting abilities to decrease complexity and cost and to increase existing
possibilities for commercialization of OLEDs. In addition, small organic molecules are also
having other advantages like easy synthesis, purification and analysis over polymers and
inorganic compounds.
Fullꢀcolor displays require three primary colors (red, green and blue) of relatively
equal stability, efficiency and color purity. Among these, a wide range of efficient green and
redꢀemitting devices have been reported.20 However, blueꢀemitting devices remain relatively
less explored due to their intrinsic characteristics such as wide bandꢀgap and low highest
occupied molecular orbital (HOMO) energy level, which make it hard to inject charges into
the emitting layer.21,22 Therefore the development of blueꢀemitting diodes having more
stability and good color purity with longer life times and high efficiency especially with non
doped device architecture has become a test bench in the recent years. Many types of blue
light emitting materials have been investigated, such as derivatives of anthracene,23,24
pyrene,25,26
fluorene,27,28
spirofluorene,29
triarylamine,30,31
carbazole,32,33,34
and
phenanthrene35 etc. Among these, anthracene has been widely used as a building block to
produce blue emitters due to its high photoluminescence efficiency in blue region, high
carrier mobility and better holeꢀinjection ability, wide band gap (~3.90 eV),36 structural
rigidity, excellent thermal stability for better morphology and easy to modify at 9 and 10
positions of the anthracene moiety. In continuation of our research in the development of
OLED materials here we present new series of anthracene derivatives which show
promising properties.37,38,39
In this contribution, we have designed and synthesized a series of six small organic
molecules, 4a-4c and 8a-8c (Scheme 1 and 2) with anthracene core and oxadiazole moiety
as electron transporter, which are connected through phenyl spacer with para and meta
linkage. The introduction of oxadiazole moiety enhances electron transporting capability
because of two withdrawing C=N groups and also facilitates in improving thermal stability for
better morphology. Thermal, electrochemical and photo physical properties for all six
compounds are studied in detail. Solid state structure for compounds 4b and 8c are
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determined by single crystal Xꢀray diffraction studies. TDꢀDFT calculations are carried out for
all the molecules to understand photo physical and electrochemical properties. Unꢀoptimized
nonꢀdoped electroluminescent devices were fabricated from these anthracene derivatives
with the following configuration: ITO (120 nm)/αꢀNPD (30 nm)/4a-4c or 8a-8c (35 nm)/BCP
(6 nm)/Alq3 (28 nm)/LiF (1 nm)/Al (150 nm). In addition, a three layer device using 8a as an
electron transport material and iridium complex as emitter was also fabricated.
2. Experimental details
2.1. Measurements and Instruments
All chemicals are reagent/analytical grade and used without further purification. 1H NMR and
13C NMR spectra were recorded on a Bruker Avance (75 MHz) spectrometer in CDCl3 with
TMS as internal standard. Mass spectra were obtained by using electro spray ionization
(ESI) ion trap mass spectrometry (Thermofinnigan, Sanzox, CA, USA) were recorded on
VGꢀAUTOSPEC spectrometer. Elemental analyses were performed using a VarioꢀEL
elemental analyzer. UV–Vis absorption spectra were measured using Jasco Vꢀ550
spectrophotometer. Steady state fluorescence spectra were recorded using a Spex
Fluorologꢀ3 spectrofluorometer. The fluorescence quantum yields (Φ) were estimated by
integrating the fluorescence bands and by using 9,10ꢀdiphenylanthracene (DPA) as
standard.40 The glassꢀtransition temperatures (Tg) of the compounds were measured using
differential scanning calorimetry (DSC) under nitrogen atmosphere at a heating rate of 10 oC
minꢀ1 using DSC Q200 (TA instruments). The Tg was determined from the second heating
scan. Thermo gravimetric analysis (TGA) was performed using a TGA/SDTA 851e (Mettler
Toledo) in the temperature range of 33–550 oC under nitrogen atmosphere at a heating rate
o
of 10 C minꢀ1. Cyclic voltammetric measurements were performed on a PCꢀcontrolled CH
instruments model CHI 620C electrochemical analyzer. Cyclic voltammetric experiments
were performed in 1 mM solution of degassed dry dichloromethane at a scan rate of 100
mV/s using 0.1M tetrabutylammoniumperchlorate (TBAP) as supporting electrolyte. The
5

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glassy carbon was used as working electrode, Ag/AgCl as the reference electrode and
platinum wire as the counter electrode. The working electrode surface was first polished with
1 mm alumina slurry, followed by 0.3 mm alumina slurry on a micro cloth. It was then rinsed
with mille cure water and also sonicated in water for 5 min. The polishing and sonication
steps were repeated twice.
For the ECL studies, Indiumꢀtin oxide (ITO) coated glass substrates with sheet
resistance of 20 ꢀ were photo lithographically patterned and cleaned using trichloroethylene,
acetone, isopropyl alcohol and de ionized water sequentially for 20 minutes using an
ultrasonic bath and dried in flowing nitrogen. Prior to film deposition, the ITO substrates were
treated with UVꢀozone for 5 minutes. On the substrate, the hole transport layer, emitting
layer, hole blocking layer, electron transport layer, electron injecting layer and cathode were
deposited sequentially under a high vacuum (1 x 10ꢀ5 torr). The deposition rate of organic
materials were kept at 1 Å /sec whereas that of LiF was 0.5 Å /sec and Al at 5 Å /sec.
Thickness of the deposited layers were controlled by a quartz crystal monitor. The cathode
was deposited on the top of the structure through a shadow mask. N,NꢀdiphenylꢀN’N’ꢀbis(1ꢀ
naphthyl)ꢀ1,1’ꢀbiphenylꢀ4,4’ꢀdiamine, (αꢀNPD) (Sigma Aldrich) was used as a hole transport
layer, BCP was used as hole blocking layer, Tris(8ꢀhydroxyquinoline)aluminium (Alq3)
(Sigma Aldrich) was used as electron transport layer, LiF (Merck, Germany) was an electron
injection layer and the Al as cathode. The size of each pixel was 3ꢀ4 mm. The ECL spectra
were measured using an Ocean Optics high resolution Spectrometer (HRꢀ2000CG UVꢀNIR).
The current densityꢀvoltageꢀluminescence (J–V–L) characteristics were measured with a
luminance meter (LMT lꢀ1009) and a Keithley 2400 programmable voltageꢀcurrent digital
source meter. All the measurements were carried out at room temperature under ambient
conditions.
2.2. Synthesis and characterization (Scheme 1 and 2)
6

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2.2.1. Synthesis of compounds 2 and 6.38,41,42 The compounds 5ꢀ(4ꢀbromophenyl)ꢀ1Hꢀ
tetrazole (2) and 5ꢀ(3ꢀbromophenyl)ꢀ1Hꢀtetrazole (6) were synthesized according to the
following procedure. A mixture of 4ꢀbromobenzonitrile (1) / 3ꢀbromobenzonitrile (5) (10 g,
55.2 mmol), NaN3, (5.4 g, 82.9 mmol), Bu3SnCl (26.9 g, 82.9 mmol) in mꢀxylene (150 mL)
was refluxed for 24 h under nitrogen atmosphere. The reaction mixture was cooled to room
temperature and acidified with 40 mL of 2N HCl. A white solid suspension formed, filtered on
Buchner funnel and washed thoroughly with water followed by nꢀhexane to get titled
compound as colorless solid.
2.2.2. 5-(4-bromophenyl)-1H-tetrazole (2).38,41 Isolated yield: 96 % (12.0 g). mp: 256 oC. 1H
NMR (300 MHz, CDCl3): δ 8.00 (d, 2H, J = 8.3 Hz), 7.69 (d, 2H, J = 8.3 Hz). MS (ES+): m/z
225 (M+H)+.
2.2.3. 5-(3-bromophenyl)-1H-tetrazole (6).38,42 Isolated yield: 95 % (11.9 g). mp: 151 oC. 1H
NMR (300 MHz, CDCl3): δ 8.25 (s, 1H), 8.05 (d, 1H, J = 7.2 Hz), 7.63 (d, 1H, J = 7.2 Hz),
7.44 (t, 1H, J = 7.2 Hz). MS (ES+): m/z 225 (M+H)+.
2.2.4. 2-(4-bromophenyl)-5-p-tolyl-1,3,4-oxadiazole (3a).43 To a stirred solution of 2 (8.0 g,
35.5 mmol) in dry pyridine (15 mL), 4ꢀmethylbenzoyl chloride (5.1 mL, 39.1 mmol) was
added slowly via syringe under nitrogen atmosphere. The reaction temperature was raised
to reflux and continued for 6 h, later slowly brought to ambient temperature, and then 20 mL
of water was added to precipitate out colorless solid compound. Thus obtained solid was
filtered, washed thoroughly with water, dried and purified by silica gel column
chromatography using ethyl acetate/ hexane (1:10) as eluent. The obtained solid was further
recrystallized from toluene/ nꢀhexane to get 3a as colorless solid. Isolated yield: 85% (9.5 g).
mp: 205 oC. 1H NMR (300 MHz, CDCl3): δ 7.98 (d, 4H, J = 8.3 Hz), 7.65 (d, 2H, J = 8.3 Hz),
7.30 (d, 2H, J = 8.3 Hz), 2.44 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 164.80, 163.50, 142.40,
132.31, 129.72, 128.18, 126.81, 126.20, 122.83, 120.82, 21.61. MS (ES+): m/z 316 (M+H)+.
2.2.5. 2-(4-bromophenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (3b).18,38 This compound
was synthesized according to the procedure similar to that of 3a, using 2 (8.0 g, 35.5 mmol),
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4ꢀtertꢀbutylbenzoyl chloride (8.1 mL, 39.1 mmol) and dry pyridine (15 mL) to get colorless
o
1
solid. Isolated yield: 79% (10.1 g). mp: 142 C. H NMR (300 MHz, CDCl3): δ 7.99ꢀ8.01 (m,
4H), 7.65 (d, 2H, J = 8.8 Hz), 7.51 (d, 2H, J = 7.8 Hz), 1.38 (s, 9H). 13C NMR (75 MHz,
CDCl3): δ 164.52, 163.31, 155.14, 132.45, 128.36, 126.93, 126.25, 125.91, 123.27, 121.25,
35.24, 31.38. MS (ES+): m/z 358 (M+H)+.
2.2.6. 2-(4-bromophenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole (3c).44 This compound
was synthesized according to the procedure similar to that of 3a, using 2 (8.0 g, 35.5 mmol),
4ꢀmethoxybenzoyl chloride (5.8 mL, 39.1 mmol) and dry pyridine (15 mL) to get colorless
solid. Isolated yield: 83% (9.8 g). mp: 159 oC. 1H NMR (300 MHz, CDCl3): δ 8.10 (d, 2H, J =
9.0 Hz), 7.96 (d, 2H, J = 8.0 Hz), 7.64 (d, 2H, J = 8.0 Hz), 6.98 (d, 2H, J = 9.0 Hz), 3.87 (s,
3H). 13C NMR (75 MHz, CDCl3): δ 163.13, 162.36, 132.32, 128.66, 128.14, 126.09, 123.06,
116.26, 114.47, 55.33. MS (ES+): m/z 331 (M+H)+.
2.2.7. 2-(3-bromophenyl)-5-p-tolyl-1,3,4-oxadiazole (7a). This compound was synthesized
according to the procedure similar to that of 3a, using 6 (8.0 g, 35.5 mmol), 3ꢀmethylbenzoyl
chloride (5.1 mL, 39.1 mmol) and dry pyridine (15 mL) to get colorless solid. Isolated yield:
o
1
72 % (8.1 g). mp: 140 C. H NMR (300 MHz, CDCl3): δ 8.22 (s, 1H), 8.07 (d, 1H, J = 7.7
Hz), 8.01 (d, 2H, J = 8.1 Hz), 7.64 (d, 1H, J = 8.1 Hz), 7.42ꢀ7.36 (t, 1H, J = 7.7 Hz), 7.30 (d,
2H, J = 8.1 Hz), 2.45 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 164.72, 162.69, 142.32, 134.29,
130.41, 129.60, 129.38, 126.69, 125.58, 125.14, 122.86, 120.56, 21.48. MS (ES+): m/z 316
(M+H)+.
2.2.8. 2-(3-bromophenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (7b).38 This compound
was synthesized according to the procedure similar to that of 3a, using 6 (8.0 g, 35.5 mmol),
3ꢀtertꢀbutylbenzoyl chloride (8.1 mL, 39.1 mmol) and dry pyridine (15 mL) to get colorless
solid. Isolated yield: 72% (9.2 g). mp: 115 oC. 1H NMR (300 MHz, CDCl3): δ 8.25ꢀ8.24 (t, 1H),
8.09ꢀ8.02 (m, 3H), 7.67ꢀ7.63 (dd, 1H, J = 8.3 Hz), 7.52 (d, 2H, J = 8.3 Hz), 7.42ꢀ7.37 (t, 1H,
J = 8.3 Hz), 1.38 (s, 9H). 13C NMR (75 MHz, CDCl3): δ 164.77, 162.84, 155.38, 134.40,
8

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130.53, 129.63, 126.84, 125.99, 125.93, 125.33, 123.09, 120.84, 35.08, 31.15. MS (ES+):
m/z 358 (M+H)+.
2.2.9.
2-(3-bromophenyl)-5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazole
(7c).
This
compound was synthesized according to the procedure similar to that of 3a, using 6 (8.0 g,
35.5 mmol), 3,4,5ꢀtrimethoxybenzoyl chloride (9.0 g, 39.1 mmol) and dry pyridine (15 mL) to
o
1
get colorless solid. Isolated yield: 73 % (10.2 g). mp: 152 C. H NMR (300 MHz, CDCl3): δ
8.23ꢀ8.22 (t, 1H), 8.08ꢀ8.05 (m, 1H), 7.68ꢀ7.64 (dd, 1H, J = 8.3 Hz), 7.42ꢀ7.37 (t, 1H, J = 8.3
Hz), 7.29 (s, 2H), 3.97 (s, 6H), 3.90 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 164.63, 163.00,
153.55, 141.21, 134.46, 130.48, 129.44, 125.54, 125.29, 122.91, 118.47, 104.11, 60.87,
56.30. MS (ES+): m/z 391 (M+H)+.
2.2.10. 2-(4-(anthracen-9-yl)phenyl)-5-p-tolyl-1,3,4-oxadiazole (4a). To a suspension of
3a (1.5 g, 4.8 mmol) in dry THF (30 mL), nꢀBuLi (2.6 mL, 5.2 mmol, 2.0M in cyclohexane)
was added drop wise at ꢀ78 oC (using dry iceꢀacetone) for 10 minutes. The reaction mixture
was stirred for 1 h and then anthrone (0.83 g, 4.3 mmol) which was dissolved in 30 mL of dry
THF added to the reaction mixture via syringe at same temperature for 15 minutes. It was
stirred at same temperature for 1 h, brought to ambient temperature and continued for 3 h.
Excess nꢀBuLi was destroyed using saturated NH4Cl solution (5 mL). The organic compound
was extracted with ethyl acetate (3 x 60 mL), combined organic layers were dried over
sodium sulphate and solvent was removed under reduced pressure to get foamy residue.
Thus obtained compound was dissolved in acetone (50 mL), conc. HCl (1.0 mL) was added
and stirred for 3 h. Then water (20 mL) was added to reaction mixture and extracted the
compound with ethyl acetate, washed with brine solution followed by water. Combined
organic layers were dried over sodium sulphate and the solvent was removed under reduced
pressure. Thus obtained crude compound was purified by silica gel column chromatography
using ethyl acetate/ hexane (1:10) as eluent to get product. Thus product was further
precipitated using CHCl3/MeOH (1:4) to get 4a as colorless solid. Isolated yield: 43% (0.77
g). mp: 236 oC. 1H NMR (300 MHz, CDCl3): δ 8.47 (s, 1H), 8.34 (d, 2H, J = 7.8 Hz), 8.05 (d,
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2H, J = 7.8 Hz), 8.01 (d, 2H, J = 7.8 Hz), 7.61 (d, 4H, J = 8.8 Hz), 7.44ꢀ7.41 (t, 2H, J = 7.8
Hz), 7.35ꢀ7.32 (t, 4H, J = 7.8 Hz), 2.46 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 142.61, 141.94,
132.05, 131.38, 129.92, 129.75, 129.66, 128.93, 128.44, 127.16, 126.93, 126.87, 126.26,
125.71, 125.12, 123.35, 121.34, 21.70. FTIR (KBr, cmꢀ1): 3043, 2960, 1611, 1491, 1066,
731. MS (ES+): m/z 413 (M+H)+. Anal. Calcd (%) for C21H20N2O: C 84.44, H 4.89, N 6.79.
Found: C 84.42, H 4.90, N 6.78.
2.2.11. 2-(4-(anthracen-9-yl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (4b). This
compound was synthesized according to the procedure similar to that of 4a, using 3b (1.5 g,
4.2 mmol), nꢀBuLi (2.3 mL, 4.6 mmol, 2.0M in cyclohexane), anthrone (0.73 g, 3.8 mmol),
dry THF 60 (30+30) mL, acetone (50 mL) and conc. HCl (1.0 mL) to get light green solid.
Isolated yield: 49% (0.85 g). mp: 210 oC. 1H NMR (300 MHz, CDCl3): δ 8.47 (s, 1H), 8.35 (d,
2H, J = 8.1 Hz), 8.08 (d, 2H, J = 9.0 Hz), 8.01 (d, 2H, J = 8.1 Hz), 7.61 (d, 4H, J = 8.1 Hz),
7.54 (d, 2H, J = 9.0 Hz ), 7.44ꢀ7.41 (t, 2H, J = 7.2 Hz), 7.35ꢀ7.32 (t, 2H, J = 8.1, 7.2 Hz), 1.39
(s, 9H). 13C NMR (75 MHz, CDCl3): δ 164.74, 164.31, 155.36, 142.58, 135.38, 132.04,
131.23, 129.86, 128.43, 127.16, 126.93, 126.77, 126.22, 126.06, 125.73, 125.18, 123.23,
121.07, 35.07, 31.10. FTIR (KBr, cmꢀ1): 3050, 2961, 2905, 2866, 1611, 1492, 1063, 843,
729. MS (ES+): m/z 455 (M+H)+. Anal. Calcd (%) for C32H26N2O: C 84.55, H 5.77, N 6.16.
Found: C 84.53, H 5.78, N 6.15.
2.2.12. 2-(4-(anthracen-9-yl)phenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole (4c). This
compound was synthesized according to the procedure similar to that of 4a, using 3c (1.5 g,
4.5 mmol), nꢀBuLi (2.5 mL, 5.0 mmol, 2.0M in cyclohexane), anthrone (0.79 g, 4.1 mmol),
dry THF 60 (30+30) mL, acetone (50 mL) and conc. HCl (1.0 mL) to get light green solid.
Isolated yield: 40% (0.70 g). mp: 220 oC. 1H NMR (300 MHz, CDCl3): δ 8.50 (s, 1H), 8.35 (d,
2H, J = 7.9 Hz), 8.12 (d, 2H, J = 8.9 Hz), 8.04 (d, 2H, J = 7.9 Hz), 7.62 (d, 4H, J = 7.9 Hz),
7.45 (t, 2H, J = 6.9, 7.9 Hz ), 7.37ꢀ7.35 (t, 2H, J = 6.9, 7.9 Hz), 7.04 ( d, 2H, J = 8.9 Hz), 3.91
(s, 3H). 13C NMR (75 MHz, CDCl3): δ 169.54, 164.02, 142.55, 135.42, 132.08, 131.32,
129.95, 128.73, 128.49, 127.21, 126.89, 126.32, 125.76, 125.21, 123.40, 116.54, 114.56,
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55.39. FTIR (KBr, cmꢀ1): 3042, 2957, 2832, 1609, 1493, 1253, 1170, 823, 729. MS (ES+):
m/z 429 (M+H)+. Anal. Calcd (%) for C29H20N2O2: C 81.29, H 4.70, N 6.54. Found: C 81.27,
H 4.73, N 6.55.
2.2.13. 2-(4-(anthracen-9-yl)phenyl)-5-m-tolyl-1,3,4-oxadiazole (8a). This compound was
synthesized according to the procedure similar to that of 4a, using 7a (1.5 g, 4.8 mmol), nꢀ
BuLi (2.6 mL, 5.2 mmol, 2.0M in cyclohexane), anthrone (0.83 g, 4.3 mmol), dry THF 60
(30+30) mL, acetone (50 mL) and conc. HCl (1.0 mL) to get colorless solid. Isolated yield:
o
1
36% (0.61 g). mp: 207 C. H NMR (300 MHz, CDCl3): δ 8.50 (s, 1H), 8.37 (d, 1H, J = 7.9
Hz), 8.15 (s, 1H), 8.03 (d, 2H, J = 8.3 Hz), 7.96 (d, 2H, J = 8.1 Hz), 7.78ꢀ7.73 (t, 1H, J = 7.7
Hz), 7.60 (d, 3H, J = 9.1 Hz), 7.46ꢀ7.41 (t, 2H, J = 6.8, 7.5 Hz ), 7.36ꢀ7.32 (t, 2H, J = 6.8, 7.5
Hz), 7.24 (d, 2H, J = 7.5 Hz), 2.40 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 164.69, 164.11,
142.07, 140.04, 135.22, 134.42, 131.33, 130.17, 129.69, 129.40, 129.28, 128.48, 127.20,
126.93, 126.38, 126.18, 125.79, 125.20, 124.52, 121.14, 21.68. FTIR (KBr, cmꢀ1): 3045,
2922, 1615, 1494, 1261, 1066, 735. MS (ES+): m/z 413 (M+H)+. Anal. Calcd (%) for
C21H20N2O: C 84.44, H 4.89, N 6.79. Found: C 84.43, H 4.90, N 6.78.
2.2.14. 2-(3-(anthracen-9-yl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (8b). This
compound was synthesized according to the procedure similar to that of 4a, using 7b (1.5 g,
4.2 mmol), nꢀBuLi (2.3 mL, 4.6 mmol, 2.0M in cyclohexane), anthrone (0.73 g, 3.8 mmol),
dry THF 60 (30+30) mL, acetone (50 mL) and conc. HCl (1.0 mL) to get colorless solid.
Isolated yield: 42% (0.72 g). mp: 204 oC. 1H NMR (300 MHz, CDCl3): δ 8.50 (s, 1H), 8.35 (d,
1H, J = 7.7 Hz), 8.18 (s, 1H), 8.04ꢀ7.97 (m, 4H), 7.77ꢀ7.72 (t, 1H, J = 7.5, 7.7 Hz), 7.61 (d,
3H, J = 8.7 Hz ), 7.46ꢀ7.41 (t, 4H, J = 5.7, 8.3 Hz), 7.37ꢀ7.31 (t, 2H, J = 7.0, 8.3 Hz), 1.34 (s,
9H). 13C NMR (75 MHz, CDCl3): δ 164.54, 164.05, 155.03, 140.07, 135.23, 134.39, 131.36,
130.20, 129.44, 129.23, 128.49, 127.20, 126.83, 126.40, 126.17, 125.90, 125.79, 125.19,
124.65, 121.21, 35.09, 31.23. FTIR (KBr, cmꢀ1): 3048, 2956, 2864, 1611, 1491, 1265, 842,
730. MS (ES+): m/z 455 (M+H)+. Anal. Calcd (%) for C32H26N2O: C 84.55, H 5.77, N 6.16.
Found: C 84.55, H 5.78, N 6.17.
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2.2.15. 2-(3-(anthracen-9-yl)phenyl)-5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazole (8c).
This compound was synthesized according to the procedure similar to that of 4a, using 7c
(1.5 g, 3.8 mmol), nꢀBuLi (2.1 mL, 4.2 mmol), anthrone (0.67 g, 3.5 mmol), dry THF 60
(30+30) mL, acetone (50 mL) and conc. HCl (1.0 mL) to get colorless solid. Isolated yield:
o
1
40% (0.68 g). mp: 232 C. H NMR (300 MHz, CDCl3): δ 8.51 (s, 1H), 8.38 (d, 1H, J = 7.7
Hz), 8.15 (s, 1H), 8.04 (d, 2H, J = 8.5 Hz), 7.79ꢀ7.73 (t, 1H, J = 7.7, 7.5 Hz ), 7.62ꢀ7.59 (m,
3H), 7.47ꢀ7.42 (t, 2H, J = 7.2, 7.5 Hz), 7.37ꢀ7.32 (t, 2H, J = 6.8, 8.3 Hz), 7.26 (d, 2H), 3.90
(s, 6H), 3.86 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 164.58, 164.38, 153.58, 141.01, 139.89,
135.19, 134.48, 131.19, 130.06, 129.32, 129.19, 128.43, 127.15, 126.29, 126.25, 125.76,
125.23, 124.21, 118.82, 104.02, 60.95, 56.35. FTIR (KBr, cmꢀ1): 3048, 2966, 2939, 2833,
1592, 1496, 1236, 1126, 845, 740. MS (ES+): m/z 489 (M+H)+. Anal. Calcd (%) for
C31H24N2O4: C 76.21, H 4.95, N 5.73. Found: C 76.22, H 4.96, N 5.71.
2.3 Computational methods
Density functional theory calculations were carried out for the anthracene derivatives to
know absorption, HOMO and LUMO values. Ground state geometries of the molecules were
obtained at B3LYP/6ꢀ311G(d,p) level of theory. Frequency calculations were performed at
the same level of theory to ensure the optimized geometries are local minima on the
potential energy surface. Time dependant density functional theory was employed for the
simulation of absorption spectra using B3LYP/6ꢀ311G(d,p) in different solvents (Hexane,
CHCl3 and CH3OH) using Polarizable Continuum Model (PCM)45 as implemented in
Gaussian09 software46 for better understanding of the excitation. The absorption maxima
reported here was the excitation energy taken from vertical S0ꢀS1 transition.
3. Results and discussion
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3.1 Synthesis
In this contribution, we present the synthesis of six new compounds, namely 2ꢀ(4ꢀ
(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀpꢀtolylꢀ1,3,4ꢀoxadiazole (4a), 2ꢀ(4ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀ(4ꢀ
tertꢀbutylphenyl)ꢀ1,3,4ꢀoxadiazole (4b), 2ꢀ(4ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀ(4ꢀmethoxyphenyl)ꢀ
1,3,4ꢀoxadiazole (4c), 2ꢀ(4ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀmꢀtolylꢀ1,3,4ꢀoxadiazole (8a), 2ꢀ(3ꢀ
(anthracenꢀ9ꢀyl)phenyl)ꢀ5ꢀ(4ꢀtertꢀbutylphenyl)ꢀ1,3,4ꢀoxadiazole
(8b),
2ꢀ(3ꢀ(anthracenꢀ9ꢀ
yl)phenyl)ꢀ5ꢀ(3,4,5ꢀtrimethoxyphenyl)ꢀ1,3,4ꢀoxadiazole (8c), which features anthracene as
an emitter, oxadiazole as an electron transporter and phenyl group as spacer and end group
to influence thermal, optical and electrochemical properties designed for OLED applications.
The synthetic details are illustrated in Scheme 1 and 2. The intermediates 2 and 6 were
obtained38,41,42 from 4ꢀbromobenzonitrile (1), 3ꢀbromobenzonitrile (5) upon treating with NaN3
and Bu3SnCl in mꢀxylene. The key intermediates 3a-3c and 7a-7c were synthesized18,38,43,44
upon treatment of 2 and 6 with the corresponding acid chlorides refluxed in dry pyridine.
Further, upon metal halogen exchange of 3a-3c and 7a-7c by treating with nꢀBuLi followed
by addition of anthrone produces corresponding alcohols. Thus obtained alcohols, without
further purification were treated with catalytic amount of conc. HCl in acetone to obtain final
compounds (4a-4c, 8a-8c). Products were purified by column chromatography followed by
recrystallization before spectral characterization. All these compounds are readily soluble in
common organic solvents like chloroform, DCM and THF etc. The molecular structures of 4b
and 8c were further confirmed by singleꢀcrystal Xꢀray diffraction analysis.
3.2. Molecular geometry and crystal structure
The anthracene derivatives 4b and 8c were crystallized from CH3OH/CHCl3 (1:4) and the
single crystal structures determined. Both crystallize into the monoclinic P21/c space group.
The summary of crystal data and structure refinement details are given in supporting
information. The Xꢀray molecular structures are shown in Fig. 1. In both crystals there are
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four independent molecules of anthracene derivatives in the unit cell. The phenyl ring forms
a twist angle of 98.9o with the anthracene and oxadiazole is almost in the same plane as the
phenyl ring (a twist angle of only ꢀ7.0o). Similarly the corresponding twist angles in 8c are
observed to be 70.6o, and 25.8o respectively which is shown in the same figure. The
geometrical parameters from B3LYP method are in line with the crystal data. But the
dihedral angles from theory are a little deviated from Xꢀray values which could be attributed
to the intermolecular forces in the solid state environment. The anthracene–phenyl
connecting C–C bond length C(12)ꢀC(15) is 1.484(3) Å and C(13)ꢀC(15) (see Fig. 2 for
carbon atom numbering in SI) is 1.497(3) Å for 4b and 8c respectively which indicates more
single bond character giving chance for free rotation.
Crystal structure of 4b is mainly stabilized by CH π and π π interactions.
Intermolecular stacking interactions (Fig. 2A) are observed between the inversionꢀrelated
rings of the anthracene moiety (centroid–centroid separation between the inversionꢀrelated
C15 C28 rings = 3.950(1) Å, centroid–centroid separation between C15 C28 and C23 28
rings = 3.860(1) Å and centroid–centroid separation between C16 C21 and C23 28 rings =
3.884(2) Å (see Fig. 2A for carbon numbering in SI) symmetry code (1ꢀx, 2ꢀy, 2ꢀz) and forms
a dimmer in the crystal packing. The anthracene rings are packed inversely parallel whereas
oxadiazole rings are in slip parallel stacking. The crystal structure of 8c is stabilized by CH
O, CH π and π π interactions (Fig. 2B). The intermolecular CH O interaction (C O =
3.472(3) Å, x, 1/2+y, 1/2ꢀz) involving atoms C25 of the anthracene and atom O1 of the
oxadiazole forms an infinite helical chain along the bꢀaxis. Intermolecular stacking via π π
interactions are also observed between the symmetry related oxadiazole ring (centroid–
centroid separation = 3.838(1) Å, symmetry code x, 1/2ꢀy,ꢀ1/2+z) and between the
oxadiazole and the C1 C6 phenyl ring (centroid–centroid separation = 3.751(1) Å,
symmetry code = x, 1/2ꢀy,ꢀ1/2+z).
3.3. Thermal properties
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Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC)
measurements were performed for all the six new compounds (4a-4c and 8a-8c) to evaluate
thermal properties and the results are summarized in Table 1. Representative DSC scans of
4a and 8a are shown in Fig. 3 while the further DSC and TGA plots are given in SI. Thermal
decomposition temperatures (Td) corresponding to 5% weight loss of these compounds are
in the range of 297 364 oC informing good thermal stability. A key physical parameter which
indicates the stability of the amorphous nature of a given material is the glass transition
temperature (Tg). In a repeated differential scanning calorimetric (DSC) scan, all the thermo
grams displayed endothermic baseline shifts owing to glass transitions (Tg) in the range of
o
o
82 98 C without crystallization except for 4c (142 C) indicating stable amorphous nature
o
of materials. The melting points of these six compounds are in the range of 204 236 C.
These results reveal that the synthesized compounds are thermally stable with good Tg
values to form films with better morphology.
3.4. Electrochemical properties
Cyclic voltammetric measurements were performed to investigate the redox properties of the
six (4a-4c and 8a-8c) compounds using three electrode systems. The representative cyclic
voltammograms are shown in Fig. 4 and the relevant data is summarized in Table 2. During
the anodic voltage sweep all compounds exhibit reversible oxidation and upon cathodic
voltage sweep exhibit quasi reversible reduction. The oxidation and reduction onset
potentials47 are used to calculate the HOMO, LUMO energy values relative to ferrocene as
48
49,50
ꢁꢃꢄꢅꢆ
) eV, respectively.
ꢇꢅꢈ
ꢁꢃꢄꢅꢆ
reference using equations – (4.40 + ꢀ
)
eV and – (4.40 + ꢀ
ꢁꢂ
The HOMO and LUMO energy values are in the range of ꢀ5.72 to ꢀ5.77 eV and 2.59 2.66
eV, respectively. Small variations in HOMO, LUMO energy values for all compounds indicate
similar electronic structure. The computationally calculated HOMO/LUMO energy values are
in consistent with experimental values (Table 2, Fig. 5 for 4a and 8a and see SI for other
data). The electrochemical band gaps and optical band gaps of the synthesized compounds
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are in between 3.08–3.18 eV and 3.11 3.17 eV, respectively. The electrochemical band
gaps are in good agreement with optical band gaps.
3.5. Photophysical properties
UVꢀVis absorption and photoluminescence (PL) properties of 4a-4c and 8a-8c in different
solvents investigated are shown in Fig. 6, 7 (in CHCl3) and 8 (for 4a and 8a in different
solvents and other data are shown in SI) and the pertinent data are listed in Table 3a and 4.
All these compounds have similar structured absorption spectra (Fig. 6a and 6b) with three
bands located ranging between 340–400 nm, which were assigned to π π* transition
arising from the characteristic vibrational structures of isolated anthracene moiety.51,52,53
Absorption band in the range of 280 300 nm for all compounds could be assigned to the
group at the 9ꢀposition of the central anthracene core.54 Compounds 4a, 4b, 8a and 8b have
similar absorption peak at 290 nm, where as for 4c and 8c are ~10 nm bathochromically
shifted probably due to strong donating methoxy and trimethoxy substituents, respectively.
Similar pattern is observed for all compounds in seven different solvents (Table 3a) which
suggests that small dipole moment is associated with these molecules in the ground
state.55,56
To have a deeper understanding of the absorption spectra, we have carried out
TDDFT calculations, including the solvent effect, in various solvents (Hexane, CHCl3 and
CH3OH) using PCM model. The results obtained are shown in Table 3b. From these results
we observe that there is no noticeable solvatochromic effect for these molecules which is in
line with the experimental results (Table 3a). Calculated oscillator strengths (f) and
corresponding CI of wave function values are also given in the same Table. The major
transitions at the orbital level in all the molecules are observed to be HOMOꢀLUMO, which is
localized on the anthracene moiety. The MO picture for 4a and 8a of the molecules is shown
in Figure 5. The simulated spectra of 4a and 8a are presented in SI (Fig.10).
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In comparison with absorption spectra, the emission spectra (PL) of certain
compounds are sensitive to the polarity of solvents and the fluorescence data generated is
arranged in Table 4. PL spectra of compounds 4a-4c (Fig. 7a in chloroform and for other
data see SI) show a broad structure less emission, where as compounds 8a-8c (Fig. 7b in
chloroform and for other data see SI) display structured emission indicating the emission
behavior linked with para and meta substitution. Broad structure less emission is an
indication of charge transfer character of the excited state.55,57 Photoluminescence recorded
for 4a-4c (Fig. 8a for 4a and for other compounds see SI) in various solvents exhibited
fluorescence solvatochromism, where as compounds 8a-8c (Fig. 8b for compound 8a and
for other compounds see SI) did not respond to solvent polarity. The fluorescence
solvatochromism phenomenon (407 453 nm; Table 4) can be explained by solvent
interaction with the excited state and also an indication of polarized nature of the excited
state.55 Compounds 8a-8c do not show any solvatochromic effects indicating less dipole
interactions with different polar solvents may be because of twisted geometry and may not
possess excited state polar character. PL spectra are also recorded in solid state for this
series of compounds. Compounds in the solid state exhibited more red shifted (444 513
nm) emission spectra than in dilute solution indicating stronger intermolecular interactions.
Fluorescence quantum yields (Φ) were measured in dichloromethane relative to 9,10ꢀ
diphenylanthracene (Φ = 0.9).40 The quantum yields are very good for all the compounds
(Table 4). The higher emission yields for 8a-8c (0.92–0.95), may be due to anthracene and
oxadiazole moieties connected through meta linkage. There is a decrease of quantum yields
for 4a-4c (0.61–0.68) compared to 8a-8c because of para linkage of anthracene and
oxadiazole moieties, which increases the conjugation and planarity.38,58 The high quantum
yields of these anthracene derivatives make them excellent candidates for use as an
efficient lightꢀemitting materials in OLEDs. Optical band gaps obtained from absorption
threshold are in the range of 3.08 3.19 eV.
3.6. Electroluminescent properties
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Unꢀoptimized OLEDs were fabricated using anthracene derivatives 4a-4c and 8a-8c as an
emitters with the device configuration: ITO (120 nm)/αꢀNPD (30 nm)/4a-4c or 8a-8c (35
nm)/BCP (6 nm)/Alq3 (28 nm)/LiF (1 nm)/Al (150 nm), where 4,4’ꢀbis[Nꢀ(1ꢀnaphthyl)ꢀNꢀ
phenylꢀlꢀamino]ꢀbiphenyl (αꢀNPD) was used as the holeꢀtransporting layer, BCP was used
as hole blocking layer, Alq3 was used as the electronꢀtransporting layer, LiF and Al were
used as the electron injecting layer and cathode, respectively. The corresponding current
densities vs. voltage curves were obtained for all fabricated devices (Fig. 9 for 4a and 8a
and for other compounds see SI). The characteristics of the current density against the
applied voltage reveal good diode behavior. The driving voltage for device 4a-4c is high
around 9–12 V, whereas for devices with 8a-8c show decrease in turn on voltage to about 7
V (Table 5). The ECL emission in 8aꢀ8c series is slightly blue shifted in comparison to 4a-4c
series due to meta linkage (Fig. 10 for 4a and 8a and for other compounds see SI), which
decreases conjugation. The color of the light achieved is bluishꢀgreen in all the cases. The
CIE coordinates calculated for the compounds are shown in Table 5. The device
characteristic for 8b is slightly different from its series, ECL emission peaking at 515 and 553
nm. Moreover, its emission is entering into the yellowishꢀwhite region of the CIE color space
(0.32, 0.42). The devices with each emitter show good stability up to high voltage range.
Compared to the fluorescence spectra (Table 4) the ECL spectra are red shifted (Table 5).
Among the six anthracene derivatives (4a-4c and 8a-8c) investigated, 8a displayed highest
luminescence of 1728 cd/m2 at a current density of 326.7 mA cmꢀ2 as an emitter. The
maximum current efficiency and power efficiency for all devices are shown in Table 5 (Fig.
11 for 4a and 8a and for other compounds see SI).
In addition, 8a was investigated as electron transport layer (ETL) by fabricating two
different devices with phosphorescent emitter Ir(ppy)3 doped 4,4’ꢀbis(9ꢀcarbazolyl)ꢀbiphenyl
(CBP) with the device configuration: ITO (120 nm)/αꢀNPD (30 nm)/Ir(ppy)3 doped CBP (35
nm)/BCP (6 nm)/8a or Alq3 (28 nm)/LiF (1 nm)/Al (150 nm). Only the ETL was changed in
the device while the rest of the device was kept same. The first device was with 8a as an
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ETL, while in the second case Alq3 was used as ETL and the pertinent data is given in Table
6. These results reveal that by using 8a as an electron transport layer a significant increase
in current efficiency at all voltages and almost equivalent luminescence is achieved to the
standard Alq3. Thus it can be summarized that the anthracene derivative 8a shows superior
electron transport property to that of the commercially reported Alq3.
Further, compound 8a was tested in bi layer device architecture to know the electron
transport emitting character (Table 7). From the performance data generated it is evident
that 8a can act as electron transporting emitter with good current and power efficiency.
4.0 Conclusion
In summary, a series of novel anthraceneꢀoxadiazole based compounds 4a-4c and 8a-8c
have been designed and synthesized in a facile and easy synthetic strategy, as emitters for
nonꢀdoped organic light emitting devices. Thermal properties like Td (297 364 oC), Tm (204
236 oC) and Tg (82 98 oC) were evaluated, which indicate good stability of the compounds.
Electrochemical properties like HOMO, LUMO energy values determined using cyclic
voltammetry (CV), are in correlation with the data generated by computational analysis (TDꢀ
DFT). The compounds studied have reversible oxidation and quasi reversible reductions and
those values indicate that the compounds could be suitable for electron transporting as well
as hole blocking material for electroluminescent devices. The study of photo physical
properties in different solvents as well as in solid state reveal blue emission originating from
electronic π π* excited states. The fluorescence solvatochromism observed in 4a-4c
indicate the polar nature in excited state. The absorption maxima determined experimentally
are in concurrence with the values determined by TDꢀDFT calculations. The fluorescence
quantum yields measured are found to be excellent for 8a-8c and good for 4a-4c. In an unꢀ
optimized, nonꢀdoped OLED device configuration, 8a displayed high brightness 1728 cd mꢀ2
and a current efficiency of 0.89 cd Aꢀ1 compared to other compounds. Electron transport
property evaluated for 8a shows current efficiency of 11.7 cd Aꢀ1 (compared to the standard
electron transport material Alq3, 8.69 cd Aꢀ1) demonstrating its potential use in OLED.
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Current results demonstrate that the anthraceneꢀoxadiazole based compounds are
promising electronꢀtransport material for the fabrication of OLEDs. Further, these molecules
can serve as basis in designing better performing small molecules for nonꢀdoped OLEDs.
Acknowledgements
The authors GM, MAR thank UGC and SB thank CSIR for fellowships. We thank the
Director, CSIRꢀIICT for encouragement. We acknowledge NWPꢀ0055 project for funding.
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Figures and Tables:
R
Cl
O
N N
NH
N
N
N
O
CN
NaN3, Bu3SnCl
R
pyridine,
reflux
m-xylene,
reflux
Br
Br
Br
1
2
3a R = CH3,
3b R = t-Bu,
3c R = OCH3
R2
R1
R2
Cl
O
N
N
N N
O
N
NH
CN
R2
R2
NaN3, Bu3SnCl
R1
m-xylene,
Br
Br
pyridine,
reflux
Br
reflux
5
7a R1 = CH3, R2 = H
7b R1 = t-Bu, R2 = H
7c R1 = R2 = OCH3
6
Scheme 1: Reaction scheme for the synthesis of the key intermediates.

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Scheme 2. Reaction scheme for the synthesis of the target compounds 4a-4c and 8a-8c.