Pyrene-Oxadiazoles for Organic Light-Emitting Diodes: Triplet to Singlet Energy Transfer and Role of Hole-Injection/Hole-Blocking Materials

Article abstract of DOI:10.1021/acs.joc.5b02423

Three pyrene-oxadiazole derivatives were synthesized and characterized by optical, electrochemical, thermal, and theoretical investigations to obtain efficient multifunctional organic light emitting diode (OLED) materials. Synthesized molecules were used as emitters and electron transporters in three different device configurations, involving hole-injection/hole-blocking materials that showed good current and power efficiencies. To understand the underlying mechanisms involved in the application of these molecules as emitters and transporters, a detailed photophysical characterization of molecules 4-6 was carried out. The absorption, steady-state fluorescence, phosphorescence, fluorescence lifetime, and phosphorescence lifetime measurements were carried out. The high quantum yield and efficient reverse intersystem crossing leading to delayed fluorescence emission makes the molecule a good emitter, and the charge delocalization properties leading to excimer formation make them efficient electron transporters. Isoenergetic singlet and triplet states of the molecules make the reverse intersystem crossing feasible at room temperature even in the absence of thermal activation.

Full text of DOI:10.1021/acs.joc.5b02423

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Article
Pyrene-Oxadiazoles for Organic Light Emitting Diodes: Triplet to Singlet
Energy Transfer and Role of Hole-injection/Hole-blocking Materials
Swetha Chidirala, Hidayath Ulla, Anusha Valaboju, M. Raveendra Kiran, Maneesha Esther Mohanty, M.
Nagarajan Satyanarayan, Govindarao Umesh, Kotamarthi Bhanuprakash, and Vaidya Jayathirtha Rao
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b02423 • Publication Date (Web): 10 Dec 2015
Downloaded from http://pubs.acs.org on December 10, 2015
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PyreneꢀOxadiazoles for Organic Light Emitting Diodes: Triplet to Singlet Energy Transfer
and Role of Holeꢀinjection/Holeꢀblocking Materials
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Swetha Chidiralaa, Hidayath Ullac, Anusha Valabojub, M. Raveendra Kiranc, Maneesha Esther
Mohantya, M. N. Satyanarayan*c, G. Umeshc, Kotamarthi Bhanuprakash*b, Vaidya Jayathirtha
Rao*a,d
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aCrop Protection Chemicals Division and AcSIR, CSIRꢀIndian Institute of Chemical
Technology, Uppal Road Tarnaka, Hyderabad 500007, India, Eꢀmail: jrao@iict.res.in
bInorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal
Road Tarnaka, Hyderabad 500007, India, Eꢀmail: bhanu2505@yahoo.co.in
cOptoelectronics Laboratory, Department of Physics, National Institute of Technology
Karnataka, Surathkal, Mangalore, 575025, India, Eꢀmail: mn.satyanarayan@gmail.com
dNetwork Institute for Solar Energy, New Delhi, India
Corresponding author: jrao@iict.res.in
Keywords: PyreneꢀOxadiazoles, Electron Transport Materials, Green Emitters, DFT/ TDꢀDFT,
Energy Transfer, Delayed Fluorescence, Organic Light Emitting Devices.
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Graphical Abstract
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Abstract
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Three pyreneꢀoxadiazole derivatives were synthesized and characterized by optical,
electrochemical, thermal, and theoretical investigations to obtain efficient multifunctional
organic light emitting diode (OLED) materials. Synthesized molecules were used as emitters and
electron transporters in three different device configurations, involving holeꢀinjection/holeꢀ
blocking materials that showed good current and power efficiencies. To understand the
underlying mechanisms involved in the application of these molecules as emitters and
transporters a detailed photophysical characterization of molecules 4ꢀ6 was carried out. The
absorption, steady state fluorescence, phosphorescence, fluorescence lifetime and
phosphorescence lifetime measurements were carried out. The high quantum yield and efficient
reverse intersystem crossing leading to delayed fluorescence emission makes the molecule a
good emitter and the charge delocalization properties leading to excimer formation makes them
efficient electron transporters. Isoꢀenergetic singlet and triplet states of the molecules make the
reverse intersystem crossing feasible at room temperature even in absence of thermal activation.
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Introduction
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The work by Tang and VanSlyke in 19871 introduced organic light emitting diodes (OLEDs)
having applications in flat panel displays and solid state lighting resources. Subsequently, a huge
academic interest has developed in the field of OLEDs.2ꢀ3 OLEDs have several advantages like
low driving voltage, high brightness, fullꢀcolor emission, fast response time, wide viewing angle
and self emitting properties with low cost and improved performances.4 The search for stable and
highly efficient primary colour (red, green, and blue) emitters is extremely important for the
commercial application of OLEDs.5ꢀ7 Developing new emitting materials and optimizing the
device architectures can lead to improved efficiency and lifetime of OLEDs.8 Spin statistics
indicate that the fluorescent OLEDs can have a maximum of 25% quantum efficiency (ηint),
whereas phosphorescent OLEDs (PhOLEDs) can achieve a ηint value of 100%.9 Although
phosphorescent OLEDs have reached their ηint limit,10 they have some disadvantages, including
lower electroluminescence (ECL) efficiency under high current densities11 and rather low
reliability in the blue region of the visible spectrum for practical applications. The thermally
activated delayed fluorescence (TADF) phenomenon facilitates the harvesting of triplets in to
singlets resulting in enhanced fluorescence leading to higher efficiencies.12ꢀ14 The TADF
phenomenon attracted researchers to design new molecules with improved properties and higher
efficiencies.15ꢀ16
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The electron deficient moieties like pyridine,17 oxadiazole,18ꢀ20 triazine,21 phenathroline,22
benzimidazole,23 quinolines,24 phosphine oxide25 and quinoxalines26 are known to be good
electron conductors and with various core structures have been used as electron transport
materials in OLEDs. Oxadiazoles are excellent electron transporting/hole blocking materials
used in OLEDs due to their electron deficient nature.18ꢀ20 Their derivatives have also been
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reported to have over 20% external quantum efficiency in red, green and blue PhOLEDs.
Incorporation of oxadiazole group enhances thermal stability which is an essential property of
OLED material.27
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Pyrene is employed as a building block for organic optoelectronic materials, particularly in
OLEDs due to its strong fluorescence emission and charge transport ability.28ꢀ30 However, the
high tendency towards πꢀπ stacking of the pyrene moieties generally lends the pyrene containing
emitters strong intermolecular interactions in the solid state, which leads to a substantial red shift
of their fluorescence emission and a decrease of the fluorescence quantum yields (Фf).31 Various
pyreneꢀbased lightꢀemitting materials have been reported in recent literature including
functionalized pyrene based light emitting molecules,32ꢀ33 functionalized pyrene based lightꢀ
emitting dendrimers,34ꢀ35 oligomers and polymers.36 To date, many kinds of pyrene based
materials have been synthesized and considered for several applications, and some of them
proved to be promising blue emitters for OLEDs. Pyrene coupled with carbazole dendrimers
were synthesized by Keawin et al. to obtain multifunctional materials that behave as blue
emitters and hole transporting materials.37 Another pyrene based multifunctional molecule was
synthesized by Mallesham et al. which could be used as a blue emitter and electron transporting
material.25 Pyrene appended dimethyl phenyl derivative was used (Chou et al.) to harvest the
triplet states through reverse intersystem crossing.38 Previously, there have been reports on
pyrene compounds, but most of them are blue emitting materials.39ꢀ40 Pyrene functionalized
triphenylamine based dyes as greenꢀemitting materials in OLEDs have been reported by Lu et
al.41 However, pyrene functionalized green emitters for OLEDS were less explored.
With an aim to develop efficient electron transporting green emitters, we investigated three new
oxadiazole appended pyrene derivatives wherein pyrene acts as an emitter and oxadiazole as an
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electron transporter. These compounds exhibited excellent thermal stability, high fluorescence
quantum yields and appropriate HOMO/LUMO energy levels making them promising
multifunctional green emitters. Device data indicated that these compounds showed excellent
device characteristics which can be attributed to reverse intersystem crossing involving
oxadiazole and pyrene units of the molecule and further excimer formation leading to
luminescence. Use of suitable hole injection agent or holeꢀblocking material played an important
role in obtaining higher efficiencies. The observed delayed fluorescence emission along with
complete overlap of oxadiazole phosphorescence with pyrene fluorescence is cited as supporting
evidence for the proposed mechanism.
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Results and Discussion
Synthesis and characterization
Scheme 1 illustrates the design and synthesis of three pyreneꢀoxadiazole derivatives (Fig. 1)
namely, 4 (2ꢀ(pyrenꢀ1ꢀyl)ꢀ5ꢀpꢀtolylꢀ1,3,4ꢀoxadiazole), 5 (2ꢀ(4ꢀtertꢀbutylphenyl)ꢀ5ꢀ(pyrenꢀ1ꢀyl)ꢀ
1,3,4ꢀoxadiazole, 6 (2ꢀ(pyrenꢀ1ꢀyl)ꢀ5ꢀ(3,4,5ꢀtrimethoxyphenyl)ꢀ1,3,4ꢀoxadiazole and 7 2ꢀpꢀtolylꢀ
1,3,4ꢀoxadiazole. The synthetic procedure adopted to obtain the target materials 4ꢀ7 (Fig. 1) is
given in Scheme 1. Pyrene was brominated42 to get bromopyrene 1. Conversion of the bromo
derivative to nitrile 2 was achieved by Rosenmundꢀvon Braun synthesis43 using cuprous cyanide.
Freshly prepared Bu3SnN3 was used to obtain tetrazole intermediate 3.44 In the final step
compound 3 was treated with the different acid chlorides to get 4, 5 and 6 target compounds. 4ꢀ
Methylbenzoic acid was esterified, treated with hydrazine hydrate and then reacted with
trimethylorthoformate to get the targeted 2ꢀpꢀtolylꢀ1,3,4ꢀoxadiazole 7. All the compounds were
fully characterized by spectroscopic methods. The details of the experimental characterization
data are available in the supporting information(SI).
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N
N
O
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9
(4)
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N
N
O
N
O
N
(5)
(7)
O
N
N
O
O
O
(6)
Figure 1: Chemical structures of the target compounds 4ꢀ7
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N
N
N
Br
CN
NaN3, Bu3SnCl
N
HBr, H2O2
CH3OH+Et2O (1:1)
H
CuCN, DMF
reflux
0ꢀ5 oC ꢀ rt
m-xylene,
reflux
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R1
R1
N
N
N
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Cl
N
R
R
N
H
O
O
R1
R1
pyridine, reflux
3
4 R = CH3, R1 = H
5 R = tꢀBut, R1 = H
6 R = R1 = OCH3
O
O
O
NHNH2
O
OH
O
N
CH3OH
H2SO4
(C2H5O)3CH
Heat
N2H4
.
H2O
N
reflux 12ꢀ16h
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Scheme 1: Synthetic route of compounds 4ꢀ7
Photophysical properties
UV–Vis absorption spectra and photoluminescence of the compounds 4, 5 and 6 were recorded
in a series of solvents of different polarities viz hexane, toluene, THF, chloroform, acetonitrile
and methanol. The absorption and photoluminescence spectral data of the compounds 4, 5 and 6
are presented in Table 1 and 2. Representative absorption and emission spectra in solution and
thin film are shown in Fig. 2 and 3. A typical UVꢀVisible spectrum of pyrene has a structured
absorption between 250ꢀ350 nm. In comparison with pyrene the absorption spectra of
compounds 4, 5 and 6 are red shifted due to attachment of oxadiazole moiety to pyrene and
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slightly broadened due to the introduction of charge delocalisation in the system. The bands
between 360ꢀ394 nm are ascribed to the πꢀπ* transition of the pyrene moiety (Fig. 2). The
absorption spectra in thin films are redshifted by ~10 nm relative to those in solution (Fig 2b).
Fluorescence emission maxima observed for these three compounds 4, 5 and 6 are in the range of
420ꢀ460 nm (Table 2) in solution whereas in thin film it is red shifted to 510ꢀ530 nm. On
changing the solvent polarity from hexane to acetonitrile a small 5ꢀ10 nm shift was observed in
the samples (4ꢀ6). The fluorescence emission spectra of the compounds in chloroform is
structured and on casting films the spectra becomes broadened (Fig 3). Thin film fluorescence
studies indicate that there are intermolecular interactions in the excited state, leading to over 60
nm red shift. The structured shape of fluorescence in solution (Fig 3a) and the broad structure
less fluorescence in thin film (Fig 3b) also indicates intermolecular interactions in the excited
state. The quantum yield of fluorescence measured for these compounds are over 85% (Table 2).
Further fluorescence studies were carried out by varying the concentration in order to monitor
any possible excimer formation in the molecule and it was found that there is no significant
difference (SI). Compound 7 2ꢀpꢀtolylꢀ1,3,4ꢀoxadiazole was synthesized and its photophysical
properties were evaluated for comparison purpose. UVꢀVisible absorption (Fig 4a), fluorescence
(Fig 4b) and phosphorescence (Fig 5) measurements were carried out. Phosphorescence lifetime
for 7 was measured, data was fitted monoꢀexponentially and the lifetime was 464 µs.
Phosphorescence of the oxadiazole derivative (7) has an excellent overlap with fluorescence of
all the compounds 4, 5 and 6 (Fig 5). The spectral overlap established (Fig 5) indicates that the
triplet to singlet intraꢀmolecular energy transfer is an energetically feasible process. The
fluorescence lifetime of compounds 4ꢀ6 at 10ꢀ5 and 10ꢀ2 M concentrations were measured using
time correlated single photon counting technique and the decays are shown in Figure 6. The
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decays were fitted monoꢀexponentially and the fluorescence lifetime of molecules 4ꢀ6 were
found to be between 1ꢀ3 ns (Table 3). The excited state lifetime of pyrene monomer is known to
be between 100ꢀ450 ns in different solvents45ꢀ46. Tremendous decrease in the fluorescence
lifetime of the synthesized molecules in comparison to pyrene monomer was observed probably
due to addition of phenyl oxadiazole group. This is in accordance with the earlier reports where
pyrene lifetime reduced on attaching single47 or multiple phenyl rings.48 Phosphorescence
measurements were carried out to identify the triplet states. Interestingly delayed fluorescence
emission band was observed using 0.06 ms (60 µs) delay after flashing the lamp (SI). Time
resolved delayed fluorescence studies were carried out at emission maxima and the decays fitted
monoꢀexponentially (Table 4). Delayed fluorescence (DF) lifetimes are in microsecond time
scales which are important for display applications. It is well established that a long lived triplet
species may reduce the performance of OLED devices.49 Thermally activated delayed
fluorescence (TADF) is recently studied extensively for applications in OLED and molecules
having TADF show increase in device efficiency. Close lying triplet and singlet states are a preꢀ
requisite for TADF. The intensity of TADF emission increases with temperature. However for
samples 4ꢀ6 there was no increase in fluorescence intensity on increasing the temperature from 0
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o
to 35 C implying that the intraꢀmolecular triplet to singlet energy transfer/conversion does not
require thermal activation. The other known form of delayed fluorescence is via tripletꢀtriplet
annihilation. No emission was observed from the triplet state hence it is impossible to estimate
the role of tripletꢀtriplet annihilation as it is characterized by comparing the triplet and singlet
lifetimes.50 Similar observation was made by Chou. et. al. 2014 in pyrene based molecules which
showed delayed flourescence and it could not be assigned as TADF. Due to high fluorescence
quantum yield the phosphorescence peak could not be experimentally observed.38 However due
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to delocalization of charges within the molecule as observed by the structure less fluorescence
emission and our earlier report on pyrene based blue emitters, a CT state might improve the
possibility of reverse intersystem crossing.25 Another possibility was explained by measuring the
triplet emission of pyrene and oxadiazole separately. The pyrene phosphorescence emission
maxima was observed at 590 nm as reported earlier51 and the oxadiazole triplet peak is observed
at 440 nm (Fig. 5) with no significant spectral overlap. However the spectral overlap between the
oxadiazole triplet state and the singlet states of pyrene derivatives (4ꢀ6) may support the triplet to
singlet energy transfer (Fig. 5).
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Figure 2.UVꢀVis absorption spectra of compounds 4ꢀ6 in (a) CHCl3 at 10ꢀ5 M concentration (b)
thin film state.
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Table 1.UVꢀVis absorption data of compounds 4ꢀ6 in different solvents (10ꢀ5 M) and in thin film
3
4
5
state.
6
7
8
9
Thin
film
(nm)
292,
Comp. Hexane Toluene THF CHCl3 CH3CN CH3OH
λabs(calculated)ꢀ
(f)a
(nm)
(nm)
(nm) (nm)
(nm)
(nm)
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286,
364,
391
289,
370,
394
287, 268,
367, 369,
284,
364,
390
285,
364,
391
4
5
6
390
381,
403
(0.8545)
392
394
287,
365,
391
289,
370,
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279, 288,
367, 364,
285,
364,
390
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391
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390,
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388
(0.8613)
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396
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371, 372,
281,
367,
391
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368,
392
291,
394,
407
390(0.8523)
393
396
acalculated at PBE0/6ꢀ311g(d,p) level of theory, in chloroform solvent, f=Oscillator Strength
Figure 3. Photoluminescence spectra of compounds 4ꢀ6 in (a) CHCl3 at 10ꢀ5 M concentration (b)
thin film state. λex = 370 nm.
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Table 2. Fluorescence data of compounds 4ꢀ6 in different solvents and in thin film state.
c
Compound Hexane Toluene THF CHCl3 CH3CN CH3OH
Thin
Φfb
λem
6
7
(nm)
(nm)
(nm) (nm)
(nm)
(nm)
filma(nm)
8
9
418,
425,
425,
426,
423,
426,
511
0.85
0.85
444
445
4
5
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447
418,
451
426,
449
423,
450
426,
449
423,
452
425,
517
443
423,
450
453
428,
459
450
426,
458
456
428,
457
452
427,
456
458
430,
457
531
0.90
478
6
All the compounds were measured with 10−5 M concentration at RT; excitation at 390 nm. Measured in the solid state
at RT. bFluorescence yields relative to 9,10ꢀdiphenyl anthracene (Φ = 0.95 in cyclohexane).ccalculated at SSꢀ
PCM//PBE0/6ꢀ31+g(d,p) level of theory, in chloroform solvent, Emissions at LRꢀPCM//PBE0/6ꢀ31+g(d,p) level of
theory, in chloroform solvent are 480 nm, 480 nm, and 486 nm for 4, 5 and 6 respectively.
a
Figure 4. (a) UVꢀVis absorption (b) fluorescence of 2(pꢀtolyl) 1,3,4ꢀoxadiazole (7) at 10ꢀ5M
concentration. λex = 260 nm.
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Figure 5. Flourescence spectra of compounds 4ꢀ6 (λex = 370 nm) and phosphorescence spectra
of 2(pꢀtolyl) 1,3,4ꢀoxadiazole (7) in green(λex = 260 nm)
Table 3: Fluorescence lifetimes of compounds 4ꢀ6 in nanoseconds
Compound
10ꢀ5 M
1.87
10ꢀ2 M
3.01
4
5
6
1.88
2.77
1.73
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Figure6. Flourescence lifetime decay curves of compounds (a) 4 (b) 5 (c) 6
Table 4. Delayed Fluorescence lifetimes of compounds 4ꢀ6 at RT in microseconds
Compound
10ꢀ5 M
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420 nm
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Theoretical calculations
5
6
7
8
To gain further insight into the electronic structures, and their excited states, DFT and TDDFT
calculations were performed. The DFT calculated optimized geometries of molecules 4, 5 and 6
(Fig. 7) in gas phase show slightly twisted conformations. Oxadiazole is having ~25o tilt with
respect to the pyrene plane in all the molecules. Whereas the excited state optimized geometries,
at both S1 and T1, are very planar and rigid (Fig. 7). The HOMO and LUMO of the molecules is
majorly on pyrene part and HOMOꢀ1 and LUMO+1 is mostly upon the nonꢀpyrene part.
The singlet excitation energies were obtained from TDDFT calculations and are included
in Table 1 for comparison with experimental data. The most intense peak in 4 is at 390 nm with
oscillator strength f = 0.8545. In molecule 5, it is at 388 nm with f = 0.8613. In molecule 6, it is
at 390 nm, with f = 0.8523. It is clear that a small charge transfer occurs from pyrene part to nonꢀ
pyrene part. The substituent showed less impact on the absorption phenomenon in these
molecules. The calculated emission energies using TDDFT are included in Table 2. The
emission energies are 444 nm for 4, 445 nm for 5, and 478 nm in 6, and are in very good
agreement with the experimental emissions.
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To assess the possibility of triplet to singlet intersystem crossing, the energy gap between
lowest excited singlet state and lowest triplet state (ꢁEST) is calculated. ꢁEST is 165 meV, 165
meV and 153 meV for the molecules 4, 5 and 6 respectively. And it is 173 meV for 7 and 259
meV for unsubstituted pyrene. There is an obvious decrease in the ꢁEST upon introduction of
substituents on basic pyrene and further decrease upon addition of methoxy substituents on the
molecules. In general, the molecules 4, 5 and 6 show proficiently smaller ꢁEST, facilitating T1ꢀ
S1 reverse intersystem crossing.
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4
5
6
C12ꢀC11ꢀC26ꢀN43 = ꢀ23.918°
(ꢀ0.002°, 0.001°)
C12ꢀC11ꢀC26ꢀN40 = ꢀ24.794°
(ꢀ0.002°, ꢀ0.003°)
C12ꢀC11ꢀC26ꢀN37 = ꢀ25.612°
(0.428°, 0.464°)
C12ꢀC11ꢀC26ꢀO27 = 153.089°
(179.998°, ꢀ179.996°)
N44ꢀC28ꢀC29ꢀC31 = ꢀ2.349°
(ꢀ0.003°, 0.002°)
C12ꢀC11ꢀC26ꢀO27 = ꢀ152.123°
(179.999°, 179.995°)
N41ꢀC28ꢀC29ꢀC31 = ꢀ1.972°
(ꢀ0.007°, 0.006°)
C12ꢀC11ꢀC26ꢀO27 = 151.101°
(ꢀ179.521°, ꢀ179.465°)
N38ꢀC28ꢀC29ꢀC31 = ꢀ2.618°
(0.700°, 0.467°)
O27ꢀC28ꢀC29ꢀC30 = ꢀ2.729°
(ꢀ0.003°, ꢀ0.002°)
O27ꢀC28ꢀC29ꢀC30 = ꢀ2.201°
(ꢀ0.006°, 0.005°)
O27ꢀC28ꢀC29ꢀC30 = ꢀ3.058°
(0.0290°, 0.180°)
Figure7. Optimized geometries of molecules 4, 5 and 6 at B3LYP/6ꢀ311g(d,p) level in gas
phase. Hydrogens are omitted for clarity. To compare S0, S1, T1states, all
geometries at PBE0/6ꢀ31+g(d,p) level of theory in chloroform solvent are
considered. (Torsion angles in singlet and triplet excited state optimized geometries
are given in parentheses (S1, T1)).
Electrochemical properties
Electrochemical properties of the molecules were studied (SI) to estimate the ionization potential
(IP) and the electron affinity for compounds 4, 5 and 6 using cyclic voltammetry (CV) in a
standard three electrode electrochemical cell and the results are summarized in Table 5. The
onset oxidation potentials of 4ꢀ6 were estimated to be ~1.3 V, corresponding to the IP ~5.7 eV
(Table 5). The very small variation in IP value indicates that the oxidation occurs from the same
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moiety in all the molecules, namely pyrene. The onset reduction potentials were estimated to be
~1.5 V, corresponding to the EA of ~2.9 eV. IP and EA estimated by CV can be correlated to the
HOMO and LUMO energy values obtained from DFT calculations. Hence, for comparison of the
experimental values the frontier orbital data obtained from DFT, is given in the same Table.
While the HOMO values obtained from DFT are in good agreement with the IP obtained from
CV measurements, the LUMO values are slightly underestimated by the DFT method. EA using
DFT methods are underestimated and here this could also be due to the different conformations
adopted by the molecule in the solvent (the variation of LUMO with change in geometry is larger
in these molecules).52 Scheme 2 depicts the typical energy level diagram for a bilayer device with
4,4’ꢀbis[Nꢀ(1ꢀnaphthyl)ꢀNꢀphenylꢀlꢀamino]ꢀbiphenyl (αꢀNPD) as hole transport layer (HTL) and
synthesized materials as nꢀtype emitters. The IP of compounds offer a minimal energy barrier for
holes to be injected from αꢀNPD, while EA is relatively closer to the work function of cathode
LiF/Al. The HOMO and LUMO energy levels of the host materials were estimated from their
oxidation onset and first reduction onset potentials.
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Table 5. Electrochemical data of compounds 4ꢀ6
f
IPc
HOMOf LUMOf µground
ꢁꢃꢄꢅꢆ
ꢀ
ꢁꢂ
Compa
EAe (eV)
2.88
d
ꢁꢃꢄꢅꢆ
ꢀ
(V)
(V)b (eV)
1.32 5.72
(eV)
(eV)
(Debye)
ꢇꢅꢈ
ꢀ1.52
ꢀ5.67
2.30
3.96
4
5
1.3
1.4
5.7
5.8
ꢀ1.5
2.9
ꢀ5.67
ꢀ5.69
2.29
2.31
3.99
5.48
ꢀ1.48
2.92
6
a
Cyclic voltammetry was performed to calculate IP and EA of 4ꢀ6 in 0.1 M NBu4ClO4
b
c
d
ꢁꢃꢄꢅꢆ
+ 4.4) eV; Onset
ꢁꢂ
anhydrous DCM solvent;
potential of reduction; EA = ꢀ(ꢀ
Onset potential of oxidation; IP = ꢀ(ꢀ
e
+4.4) eV; f,gcalculated at B3LYPꢀ6ꢀ311G(d,p) level of
ꢁꢃꢄꢅꢆ
ꢇꢅꢈ
theory in gas phase.
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Thermal properties
5
6
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The thermal properties like decomposition temperature (Td), melting temperature (Tm) and glass
transition temperatures (Tg) of the compounds (4, 5 and 6) were evaluated using thermo
gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements (SI). The
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o
data generated informed that the Td values are in the range of 314‒363 C (Table 6) for these
materials indicating their excellent thermal stability over a wide range of temperatures. No glass
transition was noticed for 4 and 6, while the compound 5 had a low Tg value.
Table 6. Thermal properties of compounds 4ꢀ6
a
b
c
Compound
Tg
Tm
Td
182
178
194
314
354
363
ꢀ
4
5
6
52
ꢀ
aTg: glass transition temperature(not observed);bTm:
melting point; cTd: decomposition temperature
(corresponding to 5% weight loss);
Electrochemiluminescence
To study the utility of the synthesized pyrene derivatives in device applications, undoped OLEDs
were fabricated and characterized. The related HOMO/LUMO energy levels of the materials are
illustrated in Scheme 2.The electrochemiluminescent properties of pyrene derivatives 4, 5 and 6
as green emitters were evaluated by fabricating OLEDs with the device configuration I: Indium
tinoxide (ITO) (120 nm)/ 4,4’ꢀbis[Nꢀ(1ꢀnaphthyl)ꢀNꢀphenylꢀlꢀamino]ꢀbiphenyl (αꢀNPD) (30
nm)/Pyreneꢀoxadiazole(35 nm)/Bathocuproine (BCP)(6 nm)/ trisꢀ(8ꢀhydroxyquinoline)
aluminum (Alq3) (35 nm)/LiF(1 nm)/Al(150 nm). Here ITO (indiumꢀtin oxide) was used as a
transparent anode, αꢀNPD was used as the holeꢀtransporting layer (HTL), BCP(bathocuproine)
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as holeꢀblocking layer (HBL) and trisꢀ(8ꢀhydroxyquinoline) aluminum (Alq3) as electronꢀ
transporting layer (ETL); LiF(lithium fluoride) and Al(aluminum) were used as the electronꢀ
injecting layer (EIL) and cathode, respectively. The current densityꢀvoltageꢀluminance (JꢀVꢀL)
characteristics of the fabricated OLEDs are shown in Figure 8a. The characteristics of the current
density as a function of applied voltage reveal good diode behaviour. The devices utilizing 4, 5
and 6 as emitter materials exhibit drive voltage (corresponding to 1 cd/m2) of 3.28, 3.17 and 2.98
V, respectively. The current densityꢀluminance (JꢀL) characteristics (Figure 8b) show a linear
relationship. The current, power and external quantum efficiencies for all the devices along with
other parameters of the devices are summarized in Table 7. Out of the three derivatives
investigated, device fabricated with 5 as emissive material showed highest luminance(Lmax) of
6627 cd/m2, current efficiency(ηc) of 13.27 cd/A, power efficiency(ηp) of 9.73 lm/W and external
quantum efficiency (ηext) of 4.99%. The ECL spectra of the devices with 4, 5 and 6 as emitter
materials with green light emission with peaks centered at 522, 518 and 527 nm, respectively
with fullꢀwidth at half maxima (FWHM) of ~ 60 nm. The narrow FWHM in ECL spectra suggest
that the derivatives are capable of forming good chromaticity devices. The CIE chromaticity
coordinates of the derivatives as ECL materials are determined using the ECL spectra of the
devices at 10 V. The chromaticity diagram is shown in Figure 9 and the CIE coordinates are
tabulated in Table 7. It can also be observed from Figure 7 that the variation of applied voltage
has no effect on the shape and the peak of the ECL spectra. Compared to the thin film
fluorescence spectra with the ECL emission peaks for all the derivatives may be ascribed to the
intermolecular interactions in the excited states.53
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Scheme 2. Energyꢀlevel diagram of the materials used in the devices (a) pyrene as emitter
material (b) Compound 5 as emitter and ETL material (c) Compound 5 and Alq3 as
ETL
Figure 8.(a) Current densityꢀvoltageꢀluminance (JꢀVꢀL) (b) Current densityꢀluminance (JꢀL)
characteristics for OLEDs with pyrene as EL material
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Table 7. Electroluminescent performance data of pyrene derivatives as emissive materials
5
6
7
8
h
Device Vonset
Lmax
(cd mꢀ2)c
ηc
ηp
ηext
(%)f
λem(FWHM)
(nm)g
CIE (x,y)
a
(V)b
(cd Aꢀ1) d
(lm Wꢀ1)e
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3.28
3.17
2.98
4390
6627
5034
10.86 (8V)
6.93 (6V)
4.25 (8V)
4.99 (8V)
3.46 (7V)
522 (58)
518 (56)
527 (62)
(0.311, 0.588)
(0.271, 0.547)
(0.323, 0.572)
4
5
6
13.27 (8V) 9.73 (5.5V)
11.24 (7V) 7.42 (4.5V)
a Device Configuration I: ITO (120 nm)/ αꢀNPD (30 nm)/ 5 (35 nm)/BCP (6 nm)/ Alq3 (35 nm)/ LiF (1 nm)/ Al
(150 nm). bVonset: turnꢀon voltage at luminance of 1 cd mꢀ2. cLmax: Maximum luminance at 10 V. dηc: Maximum
current efficiency measured at applied voltage (in parentheses). eηp: Maximum power efficiency measured at
applied voltage (in parentheses). fηext: Maximum external quantum efficiency measured at applied voltage (in
parentheses). gλem: Emission wavelength maximum. FWHM: full width half maximum at 10 V. hCIEcolor
coordinate
Figure 9. EL spectra of the OLEDs with pyrene derivatives (a) 4 (b) 5 and (c) 6 as EL materials
at different applied voltages and (d) chromaticity diagram
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The role of pyrene derivatives as electron transporting and emitting materials were
studied by fabricating OLEDs with the device configuration IIꢀIV: ITO (120 nm)/ 2,3,5,6ꢀ
Tetrafluoroꢀ7,7,8,8ꢀtetracyanoquinodimethane (F4TCNQ) (4 nm)/ αꢀNPD (40 nm)/ Pyrene (50
nm)/ BCP (6 nm)/ LiF (1 nm)/ Al (150 nm). Here F4TCNQ (2,3,5,6ꢀTetrafluoroꢀ7,7,8,8ꢀ
tetracyanoquinodimethane) acts as holeꢀinjecting layer (HIL). An optimized thickness of
F4TCNQ was used for better hole injection as reported earlier.54 The device performance data is
presented in Table 8. It is evident from Table 8 that the device II without F4TCNQ and BCP has
low efficiencies of 2.67 cd Aꢀ1 (ηc), 1.36 lm Wꢀ1 (ηp) and 0.77 % (ηext). With the inclusion of 4
nm of F4TCNQ layer (device III there is an increase of efficiencies; 13.56 cd Aꢀ1 (ηc), 9.56 lm
Wꢀ1 (ηp) and 5.34 % (ηext). This can the ascribed to the efficient injection of holes from the ITO
anode to the hole transport layer αꢀNPD.54ꢀ56 Furthermore, in device IV, with the use of BCP as a
HBL, there is further increase in the efficiencies. This increase in efficiencies may be due to the
confining of redundant holes in the emitting layer which didn’t recombine with the electrons in
the emitting zone.57 The device structure IV was used for the other two derivatives (4 and 6). It is
evident from Table 8 that the devices show high efficiencies close to 15 cd Aꢀ1 (ηc), 11 lm Wꢀ1
(ηp) and 6 % (ηext).
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Table 8. Electroluminescent performance data of OLEDs using pyrene as both EL and ETL
material
6
7
Device
Vonset
Lmax
ηc
ηp
ηext
(V)d
(cd mꢀ2)e
(cd Aꢀ1) f
(lm Wꢀ1)g
(%)h
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5a
6.78
3.23
2.76
2.93
2.61
809
2.67 (9.0)
13.56 (7.0)
15.13 (7.5)
13.06 (8.5)
14.30 (7.5)
1.36 (8.5)
9.56 (6.5)
10.95 (6.5)
8.34 (5.5)
9.46 (5.0)
0.77 (9.5)
5.34 (7.0)
5.96 (7.5)
5.14 (8.5)
5b
5c
4c
6 c
7605
8524
6387
7050
5.49 (7.5)
aDevice Configuration II: ITO (120 nm)/ αꢀNPD (40 nm)/ 5(50 nm)/ LiF (1 nm)/ Al (150 nm). Device
Configuration III: ITO (120 nm)/ F4TCNQ (4 nm)/ αꢀNPD (40 nm)/ 5(50 nm)/ LiF (1 nm)/ Al (150 nm).
cDevice Configuration IV: ITO (120 nm)/ F4TCNQ (4 nm)/ αꢀNPD (40 nm)/ Pyrene(50 nm)/ BCP (6
b
nm)/ LiF (1 nm)/ Al (150 nm) Vonset: turnꢀon voltage at luminance of 1 cd mꢀ2. Lmax: Maximum
d
e
f
g
luminance at 10 V. ηc: Maximum current efficiency measured at applied voltage (in parentheses). ηp:
h
Maximum power efficiency measured at applied voltage (in parentheses). ηext: Maximum external
quantum efficiency measured at applied voltage (in parentheses)
The electron transporting properties of pyrene derivatives were investigated by
fabricating three different devices with tris[2ꢀphenylpyridinatoꢀC2,N]iridium(III) (Ir(ppy)3)
doped 4,4’ꢀbis(9ꢀcarbazolyl)ꢀbiphenyl (CBP) as phosphorescent emitter with the device
configuration V: ITO (120 nm)/αꢀNPD (40 nm)/Ir(ppy)3 doped CBP (35 nm)/BCP (6 nm)/
without ETL or 5 or Alq3 (30 nm)/LiF (1 nm)/Al (150 nm). Here only ETL was changed while
the rest of the device configurations were kept same. The first device was fabricated without any
ETL material, while the second with Alq3 as ETL and third with 5 as ETL. The data from each of
these devices are depicted in Table 9. The JꢀVꢀL characteristics and EL spectra of the devices are
shown in Figure 10a and Figure 10b, respectively. A similar EL spectrum for Ir(ppy)3with λmax at
516 nm was observed for all the three devices prepared without any ETL, with 5 and Alq3 as
electron transporters. It can be seen from Figure 8a and Table 9 that a drastic increase in the
luminance and efficiencies can be achieved by the inclusion of ETL in the device configuration.
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Also, the device with 5 as ETL shows an increase in luminescence at all voltages and better
efficiencies in comparison to Alq3 as ETL. Hence we can infer that the performance of the
device with 5 as ETL is superior compared to commercially available Alq3.
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Table 9. Electroluminescent performance data of OLEDs using triplet emitter Ir(ppy)3 doped
with CBP (with changing ETL)
Device a
Vonset
Lmax
ηc
ηp
ηext
λem
(V)b
(cd mꢀ2)c
(cd Aꢀ1) d
(lm Wꢀ1)e
(%)f
(nm)g
12.76
9.09
8.24
1714
9650
5.39 (19 V)
9.79 (20 V)
1.23 (18 V)
2.71 (19 V)
0.94 (18 V)
1.70 (18 V)
1.73 (19 V)
516
516
516
Without ETL
Alq3
12769
10.10 (19 V) 3.04 (18 V)
5
a Device Configuration II: ITO (120 nm)/ αꢀNPD (40 nm)/ Ir(ppy)3 doped CBP (35 nm)/ BCP (6 nm)/ Without
b
ETL or Alq3 or 5 (30 nm)/ LiF (1 nm)/ Al (150 nm). Vonset: turnꢀon voltage at luminance of 1 cd mꢀ2. cLmax:
Maximum luminance at 21 V. dηc: Maximum current efficiency measured at applied voltage (in parentheses)
eηp: Maximum power efficiency measured at applied voltage (in parentheses). fηext: Maximum external quantum
efficiency measured at applied voltage (in parentheses). gλem: Emission wavelength maximum at 21 V
Figure 10. (a) Current densityꢀvoltageꢀluminance (JꢀVꢀL) characteristics of triplet emitter
Ir(ppy)3 doped with CBP without ETL/with Alq3/TBUT as ETL (b) EL spectra of
triplet emitter Ir(ppy)3 doped with CBP without ETL/with Alq3/5 as ETL.
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Conclusion
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Three new pyreneꢀoxadiazole compounds 4, 5 and 6 were synthesized, characterized and their
optical, thermal and electrochemical properties were evaluated. Excellent emissive nature of the
compounds studied coupled with their good thermal and electrochemical properties make them
suitable candidates for OLEDs as electron transporting and green emitting materials. The device
performance is improved in terms of current efficiency 13.56 cd Aꢀ1 (ηc), power efficiency 9.56
lm Wꢀ1 (ηp) and quantum yield 5.34 % (ηext) by using F4TCNQ as holeꢀinjecting and BCP as
holeꢀblocking agents. The triplet states could be harnessed which was evident by the delayed
fluorescence emission at room temperature. The excellent overlap between the phosphorescence
of 2ꢀpꢀtolylꢀ1,3,4ꢀoxadiazole 7 and flourescence of pyrene leads to reverse intersystem crossing.
The excited state charge transfer nature of the compound can facilitate the triplet to singlet
energy transfer and result in red shifting the emission to green region. The green emission as
observed in the film state can also arise due to formation of pyrene excimers.
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Experimental section:
Synthesis and characterization:
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All chemicals are reagent/analytical grade and used without further purification. H NMR and
13C NMR were recorded on a 75 MHz spectrometer in CDCl3 with TMS as the internal standard.
Mass spectra was obtained using electron ionization (EI) ion trap mass spectrometry .
Synthesis of 1ꢀbromopyrene(1)42
To a stirred solution of pyrene (10.0 g, 49.5 mmol) in methanol and diethyl ether mixture (60
mL, 1:1), hydrobromic acid (9.2 mL of 48% aqueous solution, 54.4 mmol) and hydrogen
peroxide (5.6 mL of 30% aqueous solution, 49.5 mmol) mixture was added over a period of 15
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minutes at 5ꢀ10oC temperature. After complete addition, the reaction mixture was allowed to
reach room temperature and stirred at room temperature for 12 h. The reaction progress was
monitored by TLC. After the maximum conversion to mono bromopyrene, water (50 mL) was
added to the reaction mixture and compound was extracted with DCM (3 × 100 mL). Combined
organic layers were dried over anhydrous Na2SO4, filtered and the solvent was removed under
reduced pressure. Thus obtained residue was subjected to column chromatography (60ꢀ120 mesh
size silica) using hexane as eluent to get pure compound. Yield: 86% (11.9 g, colorless solid),
m.p: 81ꢀ83 oC. 1H NMR (300 MHz, CDCl3): δ 8.43 (d, J = 9.8 Hz, 1H), 8.25ꢀ8.13 (m, 4H), 8.11ꢀ
7.98 (m, 4H).
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Synthesis of Pyreneꢀ1ꢀcarbonitrile(2)43
A mixture of CuCN (3.16 g, 35.37 mmol), 1ꢀbromopyrene (5g, 17.685mmol) in 30 mL dry DMF
was charged into a single neck RB flask. The reaction mixture was refluxed for 24 h. The
reaction mass was then cooled to room temperature and 30% aq. NH3 solution (60 mL) was
added. The precipitate obtained was filtered and washed with dilute aq. ammonia solution (10
mL) and then with water. This precipitate was then washed with dichloromethane thoroughly.
The dichloromethane filtrate was dried over sodium sulfate and the solvent was removed by
rotary evaporator. The residue obtained was purified by column chromatography on silica gel
60–120 mesh, initially using 2% CHCl3/Hexane and cautiously increasing the polarity to 10%
CHCl3/hexane. The compound obtained had a dark brown tinge which was removed by washing
the solid with hexane. The compound was recrystallized twice from toluene to give fluorescent
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green crystals. Yield: 1.93 g, 48%. H NMR (300 MHz, CDCl3, ppm): δ8.48(d, 1H), 8.37ꢀ8.23
(m, 5H), 8.21ꢀ8.10(m, 3H). MS (ES+): m/z 226 (M+).
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Synthesis of 5ꢀ(pyrenꢀ1ꢀyl)ꢀ1Hꢀtetrazole(3)44
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A mixture of pyreneꢀ1ꢀcarbonitrile (1g, 4.42 mmol), NaN3, (0.43g, 6.63 mmol), Bu3SnCl (1.79
mL, 6.63 mmol) in mꢀxylene (30 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 colourless solid. Yield 0.82 g, 96%, m.p. 162oC. 1H NMR
(300 MHz, CDCl3, ppm): δ 8.43 (d, J = 9.8 Hz, 1H), 8.25ꢀ8.13 (m, 4H), 8.11ꢀ7.98 (m, 4H), 7.71.
13C NMR (75 MHz, CDCl3, ppm): δ163.5, 134.32, 133.3, 128.8, 126.6, 126.3, 125.6, 124.8,
123.3, 121, 120.4. MS (ES+): m/z 271 (M + H)+.
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General Procedure(A): Compounds obtained from tetrazole derivatives (4ꢀ6)
To a stirred solution of pyrenylꢀ1Hꢀtetrazole derivative 3 (1 equiv) in dry pyridine, benzoyl
chloride (1.2 equiv) derivatives were added slowly via syringe under nitrogen atmosphere. The
reaction temperature was raised to reflux and continued for 6 h. Later slowly it was brought to
ambient temperature, and then 20 mL of water was added to the reaction mixture to precipitate
out the white solid compound. The reaction mass 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 titled
compounds as colorless solids.
Synthesis of 2ꢀ(pyrenꢀ1ꢀyl)ꢀ5ꢀpꢀtolylꢀ1,3,4ꢀoxadiazole (4): Compound 4 was synthesized
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according to procedure A. Yield: 0.97g, 73%, m.p 180ꢀ182oC. H NMR (300 MHz, CDCl3,
ppm): δ 9.6 (d, J = 9.3 Hz ,1H), 8.70(d, J = 8.0 Hz, 1H), 8.33ꢀ8.26(m, 4H), 8.20(d, J = 9.0 Hz,
1H), 8.15ꢀ8.07(m, 4H), 7.39(d, J = 7.7 Hz, 2H), 2.48(s, 3H). 13C NMR (75 MHz, CDCl3, ppm): δ
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165.0, 164.4, 142.3, 129.8, 129.5, 129.4, 127.1, 127.0, 126.5, 126.3, 125.2,124.6, 116.9, 21.7;
MS (ESI) calcd for m/z 360[M+]; EIꢀHRMS: m/z Calcd for C25H16N2O, 360.12626 [M]+; found:
360.12615;(SI)
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Synthesis of 2ꢀ(4ꢀtertꢀbutyl)phenyl)ꢀ5ꢀ(pyrenꢀ1ꢀyl)ꢀ1,3,4ꢀoxadiazole (5): Compound 5 was
synthesized according to procedure A. Yield: 1.13 g, 76%, m.p. 178ꢀ181oC. 1H NMR (300 MHz,
CDCl3, ppm): δ 9.58(d, J = 9.3 Hz, 1H), 8.69(d, J = 8.1 Hz, 1H), 8.30(t, J = 7.7 Hz, 2H), 8.27(d,
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J = 8.2 Hz, 2H), 8.20ꢀ8.17(m, 3H), 8.12ꢀ8.06(m, 2H), 7.61(d, J = 8.5 Hz, 2H), 1.41(s, 9H); C
NMR (75 MHz, CDCl3, ppm): δ165.0, 164.3, 155.3, 133.5, 131.1, 129.7, 129.4, 126.3, 126.2,
126.0, 125.1, 124.6, 124.1, 121.1, 116.9, 35.1, 31.1; MS (ESI) calcd for m/z 402[M+]; EIꢀ
HRMS: m/z Calcd for C28H22N2O, 402.17321 [M]+; found: 402.17315;(SI)
Synthesis of 2ꢀ(pyrenꢀ1ꢀyl)ꢀ5ꢀ(3,4,5ꢀtrimethoxyphenyl)ꢀ1,3,4ꢀoxadiazole (6): Compound 6
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was synthesized according to procedure A. Yield: 1.1g, 69%, m.p. 203ꢀ205oC. H NMR (300
MHz, CDCl3, ppm): δ 9.57(d, J = 9.8 Hz, 1H), 8.69(d, J = 8.3 Hz, 1H). 8.36ꢀ8.28(m, 4H),
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MHz, CDCl3, ppm): δ 165.0, 164.3, 142.3, 131.1, 127.1, 127.0, 126.4, 126.3, 126.2, 125.1,
124.9, 124.6, 130.5, 129.8, 129.5, 129.39, 124.1, 116.9, 21.7 MS (ESI) calcd for m/z 436[M+];
EIꢀHRMS: m/z Calcd for C27H20N2O4, 436.14231[M]+; found: 436.14220;(SI)
Synthesis of 2ꢀpꢀtolylꢀ1,3,4ꢀoxadiazole (7) 58ꢀ60
To a solution of the pꢀtoluene benzoic acid (0.01 mol) in anhydrous methanol (25 mL),
sulphuric acid (0.015 mol) was added and refluxed for 6h. The solvent was evaporated and the
ester formed was used in the next step without purification. To the crude ester, hydrazine hydrate
(0.025 mol) was added and this mixture was kept at room temperature until a precipitate of acyl
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