A pyridine based: Meta -linking deep-blue emitter with high conjugation extent and electroluminescence efficiencies

Article abstract of DOI:10.1039/c6tc01689h

We designed and synthesized a bipolar deep-blue emitter 2,6-bis(4-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)pyridine (26BTPIPy) based on a meta-linking D-π-A-π-D structure. Compared to its para-linking analogue (25BTPIPy), the me

Full text of DOI:10.1039/c6tc01689h

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DOI: 10.1039/C6TC01689H
Pyridine based meta-linking deep-blue emitter with high conjugation extent
and electroluminescent efficiencies
ZeꢀLin Zhu, ab WenꢀCheng Chen,b LiangꢀDong Zhang,a XiaoꢀLe Liu,a QingꢀXiao Tong,a* Fuꢀ
Lung Wong,b Feng Lua and ChunꢀSing Leeb*
a. Department of Chemistry, Shantou University, Guangdong 515063, China
*Eꢀmail: qxtong@stu.edu.cn
b. Center of SuperꢀDiamond and Advanced Films (COSDAF) & Department of Physics and
Materials Science, City University of Hong Kong, Hong Kong SAR, China
*Eꢀmail: apcslee@cityu.edu.hk
We designed and synthesized a bipolar deepꢀblue emitter 2,6ꢀbis(4ꢀ(1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ
1Hꢀphenanthro[9,10ꢀd]imidazolꢀ2ꢀyl)phenyl)pyridine (26BTPIPy) based on a metaꢀlinking Dꢀπꢀ
AꢀπꢀD stucture. Comparing to its paraꢀlinking analogue (25BTPIPy), the metaꢀlinking in
26BTPIPy effectively shortens molecular conjugated length and restricts intramolecular charge
transfer. Interestingly, unlike most other metaꢀlinking emitters, a high fluorescent yield can be
maintained in 26BTPIPy. This may be attributed to a relatively planar structure at the benzeneꢀ
pyridineꢀbenzene joint in 26BTPIPy leading to considerable overlapping of its frontier
molecular orbitals. These suitable combination in properties endow 26BTPIPy with efficient
deepꢀblue emission and good bipolar carrier tranporting characteristics. An organic lightꢀ
emitting device using 26BTPIPy as emitter shows a low turnꢀon voltage (2.8 V), deepꢀblue
emission with color index of (0.15, 0.09) and high current and external quantum efficiencies
(4.16 cd Aꢀ1 and 5.15%). Besides, a bilayer device using 26BTPIPy as both emitting and
electronꢀtransporting material also gives high performance with current efficiency of 4.22 cd Aꢀ1
and color purity of (0.15, 0.11).
Keyword: metaꢀlinkage, conjugation, pyridine, deepꢀblue electroluminescence
1. Introduction
Blue emitter in organic lightꢀemitting diodes (OLEDs) is of vital importance for fullꢀcolor
display and lighting applications.1 Among emitters of all colors, blue emitter has the widest
energy gap,2 especially deepꢀblue emitters often possess energy gaps > 3 eV. This often leads to
poor carrier injection and transportation. An effect stragegy for remediating the electrical
properties of deepꢀblue emitter is to combine electronꢀdonating (D) and electronꢀaccepting (A)
moieties to form a DꢀA or DꢀπꢀA molecule. Because donor group often possesses hole

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transporting ability and its shallow energy level of the highest occupied molecular orbital
(HOMO) is beneficial to hole injection, while acceptor group will help in electron transporting
and its deep energy level of the lowest unoccupied molecular orbital (LUMO) facilitates electron
injection.3ꢀ5 Concurrently, there is often considerable intramolecular charge transfer (ICT) in
such DꢀA and DꢀπꢀA molecules. This would deteriorate color purity of the deepꢀblue materials
with spectral redꢀshift and broadening.6ꢀ9 Besides, to function as an efficient emitter, high
photoluminescent quantum yeild (ϕPL) is necessary. To achieve this, many emitters adopt a paraꢀ
linking mode to guarantee effective conjugation and thus larger overlapping of molecular frontier
orbitals for high ϕPL. However, linear extension of π system often increases emission wavelength
and results in higher probability of πꢀπ stacking intermolecularly. These will undermine the
deepꢀblue color purity and quench emission.10, 11 These contradictory effects among electrical
properties and fluorescent efficiency against color purity explain why the development of deepꢀ
blue emitters is still considerably lagging behind.
To restrict conjugation length and suppress ICT effect of molecules for deepꢀblue emissions,
metaꢀlinkage mode of benzene ring is a commonly used strategy.12ꢀ17 For instance, skyꢀblue
emitter BTPE with an electroluminescent (EL) peak at 488 nm shows 36 nm hypochromatic shift
when adopting metaꢀlinking to be mTPEꢀmTPE (λmax, EL = 452 nm).15 Blue emitter TPAꢀPPI with
a Commission Internationale de L'Eclairage (CIE) coordinates (0.15, 0.11) fails to match the
National Television System Committee (NTSC) blue standard (0.14, 0.08), but its meta analogue
mTPAꢀPPI shows good CIE coordinates of (0.161, 0.049).18 Unfortunately, accompanying with
the shortening of emission wavelength, the conjugated nodes between metaꢀpositions of benzene
ring also hinder delocalization of molecular frontier orbitals and most of the metaꢀderivatives
have small overlaps between their HOMOs and LUMOs, leading to low ϕPL.15, 18ꢀ21
Herein, we apply both the DꢀπꢀAꢀπꢀD structure and the metaꢀlinking strategy to design a new
emitter 2,6ꢀbis(4ꢀ(1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazolꢀ2ꢀyl)phenyl)pyridine
(26BTPIPy) (Fig. 1a), to endow this molecule with bipolar feature and deepꢀblue emission. For
comparison, its para analogue 2,2'ꢀ(pyridineꢀ2,5ꢀdiylbis(4,1ꢀphenylene))bis(1ꢀ(4ꢀ(tertꢀ
butyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazole) (25BTPIPy) is also synthesized. 26BTPIPy
shows shortened conjugation length and restricted ICT as expected from its meta architecture. To
our surprise, 26BTPIPy still maintains a high fluorescence quantum yield of 79%, which is
considerably higher than those observed in most other emitters with meta architecture.

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Theoretical study showed that metaꢀlinkage at 2ꢀ and 6ꢀposition of pyridine ring has less steric
hindrance because the missing hydrogen atom comparing to a benzene ring (Fig. 1b). This leads
to a more planar benzeneꢀpyridineꢀbenzene structure and more conjugation. Both HOMO and
LUMO of 26BTPIPy are well delocalized, which is conducive to achieving larger ϕPL. A nonꢀ
doped OLED using this material as emitter achieved deepꢀblue emission with CIE coordinates of
(0.15, 0.09) and high efficiencies: current efficiency (CE) of 4.16 cd Aꢀ1 and external quantum
efficiency (EQE) of 5.15% are obtained, which are comparable to some highꢀefficiency paraꢀ
linking emitters. Moreover, 26BTPIPy was found to be a good bifunctional material. A bilayer
device with 26BTPIPy as emitter and electron transporting material still presented high
efficiency (CE: 4.22 cd Aꢀ1 and EQE: 4.26%).
2. Experiment
2.1. Synthetic procedures
All reagents and solvents were purchased from commercial sources and used as received without
further purification unless otherwise stated. Specific details of the new compounds’ synthetic
routes are summarized in ESI.
2.2. General procedures
1H NMR was recorded with a Varian Geminꢀ400 or ꢀ300 spectrometer. Mass spectra were
recorded on a PE SCIEX APIꢀMS spectrometer. Elemental analysis (C, H, N) was performed
using a Vario EL III CHNS elemental analyzer. UVꢀvis absorption and photoluminescence (PL)
spectra were measured on a PerkinꢀElmer Lambda 950 UV/vis Spectrometer and a PerkinElmer
LS50 fluorescence spectrometer, respectively. Absolute ϕPL was measured with a LabsphereTM
integrating sphere using a monochromatized Xe lamp (NewportTM) as the exciting source (at 365
nm). Thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) were
performed on a TA Instrument TGAQ50 and a TA Instrument DSC2910, respectively. Cyclic
voltammetry (CV) was carried out in nitrogenꢀpurged CH2Cl2 (positive scan) at room
temperature using a CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate
(TBAPF6) (0.1 M) was used as the supporting electrolyte. The measurements were carried out at
a scan rate of 100 mV sꢀ1 with a conventional threeꢀelectrode configuration consisting of a glassy
carbon working electrode, a platinum wire auxiliary electrode, and a saturated calomel reference
electrode calibrated by ferrocene/ferrocenium (Fc/Fc+). For the theoretical calculation,
geometrical properties was optimized at B3LYP/6ꢀ31g(d) level using the Gaussian 09 program.

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Molecular orbitals were obtained at the same level of theory. Devices were fabricated on preꢀ
cleaned ITOꢀcoated glass substrates with a sheet resistance of 15 ꢁ square−1. After a 15 min UVꢀ
ozone treatment, the ITO substrates were immediately transferred into a deposition chamber with
a base pressure of 5 × 10−6 torr for organic and metal deposition. Current densityꢀVoltage (J–V)
characteristics and EL luminescence properties of the devices were recorded with a Keithley
2400 SourceMeter and a Spectrascan PR650 photometer respectively.
3. Results and discussion
3.1. Synthesis and characterization
Scheme S1 in ESI summarizes the synthetic routes of 26BTPIPy and 25BTPIPy. Two steps of
reactions are involved in the synthesis process, including a Suzukiꢀcoupling22, 23 reaction to link
the intermediates 4,4'ꢀ(pyridineꢀ2,6ꢀdiyl)dibenzaldehyde (26BCHOPy) and 4,4'ꢀ(pyridineꢀ2,5ꢀ
diyl)dibenzaldehyde (25BCHOPy), and a “oneꢀpot”24 reaction to add phenanthroimidazole (PI)
endcaps to 26BCHOPy and 25BCHOPy. The products were isolated with moderate yields and
1
their molecular structures were confirmed with H NMR, mass spectrometry and elemental
analysis (see ESI).
3.2. Thermal property
Thermal properties of 26BTPIPy and 25BTPIPy were evaluated with TGA and DSC under N2
atmosphere. Fig. 2 gives the corresponding results and the key data are summarized in Table 1.
Both these materials possess high decomposition temperature (Td, 5% weight loss) of 519 and
499 °C for 26BTPIPy and 25BTPIPy respectively. The metaꢀlinking compound is slightly better
than the paraꢀlinking one in term of Td. This maybe because the metaꢀlinkage mode reduces the
free rotation possibility and stretching vibration length compared to its paraꢀcounterpart. Thus a
more rigid structure endows the metaꢀlinking material with a higher Td. Besides, DSC shows that
26BTPIPy has a glassꢀtransition temperature (Tg) as high as 204 °C, which is high enough to
guarantee its morphological stability in thinꢀfilm device application. We did not observe obvious
endothermic peak through the whole measuring process (twoꢀcycle scanning between 50 and 420
°C) of 25BTPIPy, indicating no obvious phase change occurred. Overall, both materials show
good thermal stabilities for OLED applications.
3.3. Photophysical property
Fig. 3a shows absorption spectra of 26BTPIPy and 25BTPIPy in dichloromethane (DCM) (10ꢀ5
mol Lꢀ1) as well as their PL spectra in DCM and as thin films on quartz substrates. Key

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photophysical parameters of the two emitters are also listed in Table 1. A similar sharp
absorption peak at around 260 nm were observed in both compounds, which is attributed to the
πꢀπ* transition of benzene ring.22 However, their broad absorptions ranging from 330 ~370 nm
are quite different. The 347 nm absorption peak of 26BTPIPy should be ascribed to πꢀπ*
transition of the PI units19, 24 and the 367 nm absorption peak is originated from the absorption of
the whole molecule skeleton.25 Generally, the use of metaꢀlinking mode in molecular
modification often results in hypochromic shift in emission spectrum due to the suppression of
conjugated effect. In our molecules, this conjugation suppression effect also works. Comparing
to the spectrum of 25BTPIPy, there are obvious blue shifts in absorption peak (367 nm vs 372
nm) and onset (397 nm vs 413 nm) positions, evidencing a shortened conjugated length. PL
peaks of 26BTPIPy (427 nm) and 25BTPIPy (452 nm) in solution (Fig. 3a) show the same trend
as their absorptions. PL spectra of the emitters in neat film are peaked at 438 nm and 457 nm
respectively, which are redshifted by ~5ꢀ11 nm from their solutions. Besides, the solvatochromic
PL spectra (Fig. 3b) shows that 25BTPIPy has stronger ICT character (peaked at 416 nm with
full width at half maximum (FWHM) of 47 nm in hexane; 474 nm with 78 nmꢀFWHM in
acetonitrile) than 26BTPIPy (401 nm with 52ꢀnm FWHM in hexane; 434 nm with 66ꢀnm
FWHM in acetonitrile), indicating the suppressed ICT nature. Therefore, metaꢀlinking mode is
effective in achieving bluer emission and suppressing ICT redshifts in our pyridineꢀbased
system. Interestingly, ϕPL of 26BTPIPy in solution and solid state are reasonably high (0.90 in
solution and 0.79 in neat film), which is comparable to its paraꢀcounterpart (0.96/0.83) and
surpass those of most other metaꢀlinking molecules (e.g. 0.70/0.59 and 0.55/0.35 for mꢀBBTPI19
and ZꢀBBPI20 respectively). Comparing absorption spectra of mꢀBBTPI (an analogue molecule
of 26BTPIPy with its pyridine spacer replaced with a benzene ring) and 26BTPIPy (see Fig. S2
in ESI), an obvious redshift from mꢀBBTPI (peaks: 334 nm; onset: 386 nm) to 26BTPIPy can be
observed, demonstrating a higher conjugation extent in 26BTPIPy. It is reported that ϕPL will
increase with conjugation extent in similar molecular system.26, 27 Considering the very similar
structure of mꢀBBTPI and 26BTPIPy (Fig. 1a), we speculated that the better conjugation may be
responsible for the higher ϕPL in the present metaꢀarchitecture molecule. From their absorption
onset, we estimated the optical energy gaps of 26BTPIPy and 25BTPIPy to be 3.12 eV and 3.00
eV. Combined with 26BTPIPy and 25BTPIPy’s HOMO energy levels of ꢀ5.52 eV and ꢀ5.50 eV

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obtained from CV measurements, LUMO energy levels are projected to be ꢀ2.40 eV and ꢀ2.50
eV respectively.
3.4. Theoretical calculation
For a better understanding of the relationship between the molecular structures and properties of
26BTPIPy and 25BTPIPy, quantum chemistry calculations were performed using B3LYP
method with a basic set of 6ꢀ31g(d). As shown in Fig. 4, the dihedral angle between pyridine ring
and the phenyl ring connected to its 5ꢀposition is 35.8°, which is similar to that of two benzene
rings (e.g. 36.7° in mꢀBBTPI). But the twist angle decreased to 19.3° between pyridine and its
adjacent benzene rings at 2ꢀ and 6ꢀposition in 26BTPIPy. This will enhance conjugation as
suggested by previous report.28 Distributions of molecular orbital electron cloud further support
this point. Both the HOMO and the LUMO of 26BTPIPy are delocalized over the whole
molecular skeleton with considerable overlap comparable to that of its para analogue,
25BTPIPy. In fact, in most metaꢀlinking molecules, the HOMO is often confined by the metaꢀ
linking node14, 29, 30 and mostly localizes on only one side of the node. The unusual delocalization
in 26BTPIPy is in accordance with the above discussed interesting photophysical properties and
can explain its high ϕPL.31, 32 More carefully, we manipulated the twisting angle at 2ꢀ and 6ꢀ
position in simulation and observed the change in orbital distribution of 26BTPIPy. As shown in
Fig.5, increase of twisting does not significantly change the HOMO distribution of 26BTPIPy
until 60°. Similarly, in the case of mꢀBBTPI, twist angle change also induce little variation of
HOMO distribution (Fig. S4). Therefore, we proposed the electronic resonance33 between
position 2 and 6 of pyridine ring may be another important factor to enhance the conjugation
extent of meta molecule (Scheme 1). Because of the larger electronegativity of N atom in the
pyridine ring, its adjacent C atoms (position 2 and 6) show some extent of positive charge.
Assuming single electron transfer of CT transition from donor to pyridine acceptor, electronic
resonance effect could move the electron from one side to the other, which thus helps the
electronic communication of the two donor parts and benefits conjugative effect. Therefore,
electronic resonance effect in the metaꢀlinking 26BTPIPy helps to maintain high conjugation
extent and emission efficiency.
3.5. Electrical properties
To investigate the electrical properties of the two DꢀπꢀAꢀπꢀD structure molecules, we fabricated
holeꢀ and electronꢀonly devices with configurations of indium tin oxide (ITO)/4,4’ꢀbis[Nꢀ(1ꢀ

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naphthyl)ꢀNꢀphenylamino]ꢀbiphenyl (NPB) (20 nm)/26BTPIPy or 25BTPIPy (50 nm)/NPB (20
nm)/Al (100 nm) and ITO/1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀbenzimidazolꢀ2ꢀyl)ꢀbenzene (TPBI) (20
nm)/26BTPIPy or 25BTPIPy (50 nm)/TPBI(20 nm)/LiF (1 nm)/Al (100 nm), respectively. Fig.
6 are JꢀV plots of these devices. Both molecules show obvious bipolar transporting capacities.34,
35
Devices with 26BTPIPy show higher currents for both hole and electron, especially the
electron current, indicating that 26BTPIPy has better charge transporting abilities.
3.6 Electroluminescent properties
Motivated by their good optical and electrical properties, we used these materials as emitters to
fabricate nonꢀdoped OLEDs with configurations of ITO/hexaazatriphenylene hexacarbonitrile
(HATCN) (5 nm)/NPB (60 nm)/4,4’,4”ꢀtris(carbazolꢀ9ꢀyl)ꢀtriphenylamine (TCTA) (5
nm)/26BTPIPy or 25BTPIPy (20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (120 nm). HATCN is used
as hole injection layer and TCTA is a buffer layer to prevent leakage of exciton into the hole
transporting layer (HTL, NPB),19 while TPBI is used as electron transporting material. Fig. 7a is
the current densityꢀvoltageꢀluminescence (JꢀVꢀL) characteristics of the two devices, from which
turnꢀon voltages (voltage at 1 cd mꢀ2) of 2.8 and 3.0 V for 26BTPIPyꢀ and 25BTPIPyꢀbased
devices were obtained respectively. As discussed above, 26BTPIPy has better carrier
transporting abilities, leading to higher current density in the 26BTPIPyꢀbased device comparing
to the 25BTPIPy device. 26BTPIPyꢀbased device achieved maximum CE and EQE (Fig. 7b) of
4.16 cd Aꢀ1 and 5.15%, while the corresponding efficiencies for the 25BTPIPyꢀbased device
were 6.22 cd Aꢀ1 and 4.72% respectively. Deepꢀblue emission with CIE coordinates of (0.15,
0.09) was obtained from the 26BTPIPyꢀbased device. This EL emission with peak at 440 nm
showed no observable change from 3 to 8 V (Fig 7c). Linear increase of EL luminescence with
current density (inset of Fig. 7c) suggests minimal involvement of tripletꢀtriplet annihilation and
current induced fluorescence quenching.25, 36 For the 25BTPIPyꢀbased device, EL emission
peaked at 460 nm displays color coordinates of (0.15, 0.16) which was in accord with PL
emission of 25BTPIPy. In these multiꢀlayer devices, we found the EQE of the 26BTPIPyꢀbased
OLED is higher than that of the 25BTPIPyꢀbased device, although the 26BTPIPy displays
relatively lower ϕPL in solid film. Theoretically, the EQE of a device can describe as,
EQE = γ ηout ϕPL ηr
(1)
where γ is recombination efficiency of hole and electron carriers, ηout is the light outꢀcoupling
efficiency and ηr is the ratio of radiative exciton. In fact, EQE depends on four factors. In our

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case, the difference of ϕPL of the two molecules is not large (83 vs 79%), so the EQEs are mainly
influenced by other three factors. For example, due to the hole transport is slightly better than
electron transport in our molecules (Fig. 6), a thicker HTL layer was used in the device structure
to balance charge transport. However, the electron transport of 26BTPIPy is better than that of
25BTPIPy, which may lead to higher γ in 26BTPIPyꢀbased OLED with the same device
structure especially in the low bias region (where highest efficiency achieved). Besides, the
efficiency rollꢀoff of the 26BTPIPyꢀbased device is slightly larger than that of 25BTPIPyꢀbased
device (10.3% vs 7.2% at 1000 cd mꢀ2), implying that different γ can affect EQE performance.
Key device performances data are summarized in Table 2. Noticeably, performance of
26BTPIPy is among the best of metaꢀderivatives and even comparable to some high efficiency
nonꢀdoped deepꢀblue materials with paraꢀlinkage.6, 15, 19, 20, 37ꢀ42
Inspired by its excellent electron injection and transporting abilities, 26BTPIPy were further
used in both emitting and electron transporting layer to fabricate a simpler structured OLED:
ITO/HATCN (5 nm)/NPB (60 nm)/TCTA (5 nm)/26BTPIPy (50 nm)/LiF (1 nm)/Al (120 nm).
As shown in Fig. S5a, slight redshifts of EL emission (448 nm) was observed from the device.
High device performance was still maintained in this simple device configuration, as shown in
Fig. 8 and Table 2, with CEmax of 4.22 cd Aꢀ1, EQEmax of 4.26% and CIE coordinates of (0.15,
0.11). These show that 26BTPIPy can be used as a high performance bifunctional material. With
the same structure, bilayer device using 25BTPIPy in emitting and electronꢀtransporting layer
was also fabricated. However, the device efficiencies are far inferior to its corresponding
multilayer device with a TPBI electronꢀtransporting layer (Table 2).
4. Conclusion
In conclusion, we designed and synthesized a meta DꢀπꢀAꢀπꢀD molecule 26BTPIPy as deepꢀ
blue emitter for OLED applications. Comparing to its paraꢀcounterpart 25BTPIPy, the material
not only effectively suppresses ICTꢀinduced redshifts and shortens conjugated length to give
deep blue emission (CIE coordinates: (0.15, 0.09) vs (0.15,0.16)), but also keep high conjugation
extent and high ϕPL. Superior electrical properties endows 26BTPIPy with a better device
performance than 25BTPIPy (EQE: 5.15% vs 4.72%). Besides, 26BTPIPy can be used as a
bifunctional material and achieve high performance (CE: 4.22 cd Aꢀ1, EQE: 4.26%) in doubleꢀ
layer device. The outstanding character presented by this meta molecule can provide us with a
good reference for future deepꢀblue emitter design for OLED.

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Acknowledgements
We would like to appreciate Dr. Ji Zheng at Shantou University for the fruitful discussions.
This work was supported by the National Natural Science Foundation of China (Project No.
51473138 and 51273108) and the National Basic Research Program of China (973 Program No.
2013CB834803).
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Page 11 of 16
Journal of Materials Chemistry C
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DOI: 10.1039/C6TC01689H
Fig. 1 a) Molecular structures of 26BTPIPy and 25BTPIPy (structures of mꢀBBTPI and BBTPI
are also presented for comparison); b) Schematic of reduced hindrance in metaꢀlinkage of
pyridine ring
Fig. 2 TGA and DSC (inset) measurement of 26BTPIPy and 25BTPIPy.
Table 1 Summary of the physical data of 26BTPIPy and 25BTPIPy
a)
b)
e)
e)/f)
Td
Tg
HOMOc)
[eV]
LUMOd)
[eV]
λabs
λem
Φf (soln/film)g)
compound
[°C]
[°C]
[nm]
[nm]
%

Journal of Materials Chemistry C
Page 12 of 16
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DOI: 10.1039/C6TC01689H
519
499
512
481
204
n.o.
190
192
ꢀ5.52
ꢀ5.50
ꢀ5.34
ꢀ5.56
ꢀ2.40
ꢀ2.50
ꢀ2.13
347, 365
372
427/438
452/457
406/432
427/448
90/79
96/83
70/59
93/82
26BTPIPy
25BTPIPy
m-BBTPIh)
BBTPIh)
334
ꢀ2.48
367
a)
c)
d)
5% weight loss temperature; b) n.o. = not observed; Detected via CV; Estimated by Eg from HOMO
e)
f)
g)
levels; Measured in dilute CH2Cl2 solution (10ꢀ5 mol Lꢀ1); Measured in film; The fluorescence quantum
yield in solution using a 10ꢀ6 mol Lꢀ1 cyclohexane solution of 9,10ꢀdiphenylanthracene (0.90) as standard, and
the solid state quantum yield on the quartz plate using an integrating sphere apparatus; h) data of these two
compounds are taken from ref. 19.
Fig. 3 a) Absorption and PL spectra and b) comparison of solvatochromic effect of 26BTPIPy
and 25BTPIPy.

Page 13 of 16
Journal of Materials Chemistry C
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DOI: 10.1039/C6TC01689H
Fig. 4 Optimized geometry of and calculated HOMO and LUMO distributions of 26BTPIPy and
25BTPIPy.
Fig. 5 Calculated HOMO distributions of 26BTPIPy with arbitrary fixed twisting angle between
the pyridine ring and two connected benzene rings.

Journal of Materials Chemistry C
Page 14 of 16
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DOI: 10.1039/C6TC01689H
Scheme 1 Electronic resonance between electronꢀdeficient positions of pyridine ring.
Fig. 6 Current density versus voltage of the single carrier devices based on 26BTPIPy and
25BTPIPy.

Page 15 of 16
Journal of Materials Chemistry C
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DOI: 10.1039/C6TC01689H
Fig. 7 a) Current densityꢀvoltageꢀluminescence characteristic curves, b) Plots of current
efficiencyꢀluminanceꢀEQE, c) and d) EL spectra at different voltages (inset is luminescenceꢀ
current density plot) of 26BTPIPy and 25BTPIPyꢀbased OLEDs.
Table 2. Key performance of new compoundsꢀbased devices
Von λEL
Luminescencemax CE *
EQE *
[%]
PE*
[lm Wꢀ1]
CIE
Device
[V] [nm]
[cd mꢀ2]
[cd Aꢀ1]
[x, y]
A
B
C
D
2.8 440
4.16, 4.12, 3.61 5.15, 5.15, 4.62 4.35, 3.70, 1.95 (0.15, 0.09)
6.22, 6.20, 5.70 4.72, 4.70, 4.38 5.89, 4.62, 2.89 (0.15, 0.16)
4.22, 4.20, 3.78 4.26, 4.25, 3.96 4.34, 3.63, 1.83 (0.15, 0.11)
3.42, 3.23, 2.89 2.47 ,2.38, 2.14 3.33, 2.44, 1.33 (0.16, 0.17)
4416
6762
3019
3150
3
3
3
460
448
460
Device structure: A and B: ITO/HATCN (5 nm)/NPB (60 nm)/TCTA (5 nm)/26BTPIPy or 25BTPIPy
(20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (120 nm); C and D: ITO/HATCN (5 nm)/NPB (60 nm)/TCTA (5
nm)/26BTPIPy or 25BTPIPy (50 nm)/LiF (1 nm)/Al (120 nm).
*The three values for each device are the efficiency at maximum, 100 and 1000 cd mꢀ2 respectively.
Fig. 8 a) Current densityꢀvoltageꢀluminescence characteristic curves, b) plots of current
efficiencyꢀluminanceꢀEQE of the 26BTPIPy and the 25BTPIPyꢀbased bilayer devices.

Journal of Materials Chemistry C
Page 16 of 16
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DOI: 10.1039/C6TC01689H
Pyridine based meta-linking deep-blue emitter with high conjugation
extent and electroluminescent efficiencies
Ze-Lin Zhu, Wen-Cheng Chen, Liang-Dong Zhang, Xiao-Le Liu, Qing-Xiao Tong, Fu-Lung
Wong, Feng Lu and Chun-Sing Lee
A
meta-linking pyridine based compound with high conjugation extent was reported as high
efficiency deep-blue OLED emitter.