Effective blocking of the molecular aggregation of novel truxene-based emitters with spirobifluorene and electron-donating moieties for furnishing highly efficient non-doped blue-emitting OLEDs

Article abstract of DOI:10.1039/c5tc01018g

Several truxene-based blue fluorescent emitters bearing different functional moieties and peripheral spirobifluorene groups have been successfully designed and synthesized. Through tuning the chemical structures and electronic characters of the functional moieties, both optimized molecular configuration and elevated highest occupied molecular orbital (HOMO) energy levels have been afforded to the truxene-based emitters, representing a new effective strategy for solving the problems of molecular aggregation and poor charge carrier injection/transport associated with the truxene-based blue emitters. Owing to the sophisticated strategy, the concerned emitters can furnish highly efficient non-doped blue-emitting OLEDs with a maximum current efficiency (η_L) of 7.41 cd A^-1, an external quantum efficiency (η_ext) of 4.33%, and a power efficiency (η_P) of 6.79 lm W^-1, representing the state-of-the-art EL efficiencies ever achieved by the truxene-based blue emitters. All promising results will not only show the great potential of the concerned truxene-based blue fluorescent emitters in the field of OLEDs, but also furnish valuable clues for developing high-performance truxene-based emitters.

Full text of DOI:10.1039/c5tc01018g

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DOI: 10.1039/C5TC01018G
Effective Blocking the Molecular Aggregation of Novel TruxeneꢀBased
Emitters with spiroBifluorene and ElectronꢀDonating Moieties for
Furnishing Highly Efficient NonꢀDoped BlueꢀEmitting OLEDs
Chunliang Yao,† Yue Yu,‡ Xiaolong Yang,† Huiming Zhang,† Zuan Huang,† Xianbin Xu,†
Guijiang Zhou,*,† Ling Yue*,† and Zhaoxin Wu*,‡
†MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key
Laboratory for Mechanical Behavior of Materials, and Institute of Chemistry for New Energy Materials,
Department of Chemistry, Faculty of Science, Xi’an Jiaotong University, Xi’an 710049 (P.R. China)
‡Key Laboratory of Photonics Technology for Information, Faculty of Electronic and Information
Engineering, Xi’an Jiaotong University, Xi’an, 710049 (P.R. China)

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Several truxeneꢀbased blue fluorescent emitters bearing different functional moieties and
peripheral spirobifluorene groups have been successfully designed and synthesized. Through
tuning the chemical structures and electronic characters of the functional moieties, both
optimized molecular configuration and elevated highest occupied molecular orbital (HOMO)
energy levels have been afforded to the truxeneꢀbased emitters, representing new effective
strategy for solving the problems of molecular aggregation and poor charge carrier
injection/transporting associated with the truxeneꢀbased blue emitters. Owing to the
sophisticated strategy, the concerned emitters can furnish highly efficient nonꢀdoped
blueꢀemitting OLEDs with the maximum current efficiency (ŋL) of 7.41 cd A–1, external
quantum efficiency (ŋext) of 4.33%, and power efficiency (ŋP) of 6.79 lm W–1, representing the
stateꢀofꢀtheꢀart EL efficiencies every achieved by the truxeneꢀbased blue emitters. All promising
results will not only show the great potential of the concerned truxeneꢀbased blue fluorescent
emitters in the field of OLEDs, but also furnish valuable clues for developing highꢀperformance
truxeneꢀbased emitters.

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Introduction
Blue fluorescent (singlet) emitters occupy critical position in the field of organic lightꢀemitting
diodes (OLEDs) which have shown great potential in developing next generation flatꢀpanel
displays and energyꢀsaving solidꢀstate lighting sources, since both fullꢀcolor display and
whiteꢀlight lighting will require blue emitters to fulfill three primary colors, i.e. red (R), green
(G), and blue (B).1ꢀ4 For the lighting purpose, the blue fluorescent emitters can also be employed
to combine with orange (O) emitters to produce white light through OꢀB complementary colors
in simplified device structures.5ꢀ7 Through replacing the unstable phosphorescent (triplet)
counterparts, blue fluorescent emitters can be used to construct white OLEDs (WOLEDs) to
both guarantee high electroluminescent efficiencies via harnessing their unemissive triplet
excitons by the longꢀwavelength triplet emitters and furnish longer device life expectancy.8, 9
Besides these critical functions, a blue fluorescent emitter can also be utilized to generate light
of long wavelength by an energy cascade to a suitably emissive dopant. However, the progress
in blue OLEDs is lagged far behind the analogs of green and red due to the difficulty in
developing blueꢀemitting materials with a high efficiency, color purity, and long lifetime.9ꢀ11
Hence, searching high performance blue emitters definitely represent very crucial research lines
in the field of OLEDs.
With the continuous efforts from the concerned researchers, various kinds of blue fluorescent
emitters based on fluorene,12ꢀ14 carbazole,15, 16 anthracene,17, 18 styrylarylene,19 pyrene,20, 21
triphenylene,22, 23 biphenyl24 and quinoline25 derivatives have been developed to fabricate
efficient blue OLEDs. From the structural diversity of the blue fluorescent emitters and their
encouraging EL performance, it can be clearly seen that developing new chemical structures
should be the most feasible way to cope with the problems in searching blueꢀemitting materials

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with high EL performance.
Truxene possesses large πꢀconjugation system and multi reactive sites to afford diverse
derivatives which can find applications in varies fields.26 Hence, besides the buildingꢀblocks
aforementioned, truxene should also show great potential for developing novel blue fluorescent
emitters due to the following facts: 1) Truxene possesses rigid planar skeleton with large
πꢀconjugating structures, which can afford the indispensable characters to lead to high
fluorescent quantum yield (ΦF), which represent the most crucial parameter to furnish high EL
performance. 2) The energyꢀlevel for the S1 states associated with the truxene πꢀconjugating
system is high enough to produce fluorescence in blue region. Unfortunately, the poor solubility
and large steric hindrance make truxene difficult to modify and the large planar πꢀconjugating
structures and structure of truxene will make it prone to aggregate to induce undesired
quenching effect.27 Obviously, these characters will disfavor the truxene application in OLEDs.
In order to cope with these problems, many truxene derivatives have been developed. The
luminescent starꢀshaped conjugates with truxene as either the core28ꢀ30 or the arms31 have been
prepared. The truxeneꢀbased oligomers and polymers have also been developed as lightꢀemitting
materials.32, 33 With truxene as the core or/and dendrons, novel luminescent dendrimers have
been obtained.34ꢀ36 In all of these truxeneꢀbased luminescent materials, long alkyl groups are
typically introduced to the three internal bridging sp3 hybrid carbon atoms of the truxene units
with the aim to both enhance good solubility and block effectively aggregation of these emissive
molecules.34, 37, 38 With all these improving strategies, the truxeneꢀbased luminescent molecules
can exhibit high fluorescent quantum yield (ΦP).29, 39 Restraining the aggregation of the
lightꢀemitting truxene derivatives should be crucial for their application in blueꢀemitting OLEDs
with high color quality. Despite that doping can solve the aggregation problem of the blue

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emitters, doped emitting layer (EML) in OLEDs will both show the susceptibility of undesired
phase separation which will decrease the lifetime of the concerned devices and complicate
device fabrication.
Despite many lightꢀemitting truxeneꢀbased materials have been developed for various
applications,40ꢀ44 the ones applied in OLEDs as emitters are in limited number.30, 35, 45, 46 The first
starꢀshape truxeneꢀbased blue emitting can show the current efficiency (ηL) of 0.60 cd Aꢀ1.45 The
truxeneꢀbased blue emitters with anthraceneꢀbased arms can show maximum ηL of 2.98 cd Aꢀ1.30
Some novel truxene derivatives with sophisticated 3D configuration can bring narrow blue EL
spectra with maximum external quantum efficiency (ηext) of 2.9%.46 Recently, the starꢀshaped
truxeneꢀbased blue emitter can bring maximum ηL of 5.3 cd Aꢀ1.35 Despite of these encouraging
results, the concerned devices still show inferior EL performance with respect to the
blueꢀemitting OLEDs prepared by vacuumꢀdeposition with other blue emitters.47 However,
solution strategy has afforded a convenient and lowꢀcost method to construct OLEDs. Hence, it
is really necessary to develop novel truxeneꢀbased blue emitters. So, along this respect, several
truxene derivatives bearing different functional moieties and peripheral spirobifluorene groups
have been successfully designed and prepared to furnish highly efficient solutionꢀprocessed blue
OLEDs. The concerned results should providing valuable information for design highly efficient
blue fluorescent emitters.
Experimental section
General information. All commercially available starting materials were used directly with no
further purification. The solvents were carefully dried prior to use. All reactions were monitored
using thinꢀlayer chromatography (TLC) purchased from Merck & Co., Inc. Flash column

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chromatography and preparative TLC were made from silica gel bought from Shenghai Qingdao
(200ꢀ300 mesh). 1H NMR, 13C NMR and 31P NMR spectra were recorded in CDCl3 on a Bruker
Avance 400 MHz spectrometer and chemical shifts were referenced to the solvent residual peak
1
at δ 7.26 for H and 77.0 for 13C, respectively. Elemental analyses were performed on a Flash
EA 1112 elemental analyzer. The thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) data were collected on a NETZSCH STA 409C instrument and a NETZSCH
DSC 200 PC unit, respectively. UVꢀvis absorption spectra were measured at room temperature
on a Shimadzu UVꢀ2250 spectrophotometer. Emission spectra and lifetimes of these compounds
were recorded on an Edinburgh Instruments Ltd (FLSP920) fluorescence spectrophotometer
using the software package provided by Edinburgh Instruments. The fluorescent quantum yields
(ΦF) were determined in CH2Cl2 solutions at 298 K against quinine sulphate standard (ΦP =
0.56).48 Cyclic voltammetry was performed using a Princeton Applied Research model 273A
potentiostat at a scan rate of 100 mV sꢀ1. All experiments were carried out in a threeꢀelectrode
compartment cell with a Ptꢀsheet counter electrode, a glassy carbon working electrode, and a
Ptꢀwire reference electrode. The supporting electrolyte used was 0.1 M [nBu4N]BF4 solution in
acetonitrile. The concentration for all the samples was set at ca. 1.0 mM in acetonitrile. The
onset oxidation (Epo) potentials were used to determine the HOMO energy levels using the
equation EHOMO = – (Epo + 4.8) eV, which was calculated using the internal standard ferrocene
value of – 4.8 eV with respect to the vacuum.49 The LUMO energy level was determined by the
HOMOꢀLUMO gap (Eg) with the equation ELUMO = EHOMO + Eg. Fast atom bombardment (FAB)
mass spectra were recorded on a Finnigan MAT SSQ710 system.
OLED fabrication and measurements. The preꢀcleaned ITO glass substrates were treated with
ozone for 20 min. Then, the PEDOT: PSS was deposited on the surface of ITO glass by

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spinꢀcoating method to form a 40 nmꢀthick holeꢀinjection layer which was treated at 120 °C for
30 min in the air. The emitting layer (40 nm) was obtained by spinꢀcoating a chlorobenzene
solution of each corresponding emitters. The ITO glass was dried in a vacuum oven at 50 °C for
15 min and transferred to the deposition system for organic and metal deposition. TPBi (40 nm),
LiF (1 nm) and Al cathode (100 nm) were successively evaporated at a base pressure less than
10–6 Torr. The EL spectra and CIE coordinates were measured with a PR650 spectra colorimeter.
The JꢀVꢀL curves of the devices were recorded by a Keithley 2400/2000 source meter and the
luminance was measured using a PR650 Spectra Scan spectrometer. All the experiments and
measurements were carried out under ambient conditions.
Computational details. Geometrical optimizations were conducted using the popular B3LYP
functional theory. The basis set used for C, H, N, and S atoms was 6ꢀ31G(d, p).50, 51 All
calculations were carried out by using the Gaussian 09 program. In the vertical excitation energy
calculation, all excited states were obtained using TDꢀDFT in the adiabatic approximation, using
the same functional and basis set. A projection of the deformation in the relaxed excitedꢀstate
onto the vibrational eigenvectors defined the HuangꢀRhys factor Sj for each vibrational mode
ћωj. As well, the vibrational frequency (ћωj) and the normal mode displacements (∆Qj) for each
normal mode are calculated using the DUSHIN program.52
Results and discussion
Synthesis. The chemical structures of the designed truxeneꢀbased molecules are shown in
Figure 1. Their synthetic details have been presented in Scheme 1 and Electronic Supplementary
Information (ESI). All the target molecules are developed from truxene prepared from
3ꢀphenylpropionic acid. In order to afford the processibility to the blue emitters, the nꢀhexyl

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groups have been attached to the sp3 hybrid carbon atoms of the truxene core TrꢀC, which was
converted into TrꢀCBr with high yield (ca. 93%) through the reaction with bromine in CH2Cl2
(Scheme 1). The blue emitter Tr1 was obtained through the Suzuki crossꢀcoupling reaction
between TrꢀCBr and 9,9'ꢀspirobifluorenꢀ7ꢀylboronic acid. The other blue emitters Tr2 − Tr4
have been prepared in three steps of reaction. Firstly, through Suzuki crossꢀcoupling reaction
with corresponding arylboronic acid, TrꢀCBr can be transferred into intermediate compounds
TrꢀAr1 − TrꢀAr3. Secondly, the compounds TrꢀAr1 − TrꢀAr3 can react with NBS to form
TrꢀArBr1 − TrꢀArBr3. Finally, the target molecules Tr2 − Tr4 had been successfully prepared
by Suzuki crossꢀcoupling reaction in good yield (Scheme 1). Owing to their good solubility, all
the truxeneꢀbased compounds can be purified by silica chromatography to be obtained as white
or light yellow powder with high purity. Their wellꢀdefined chemical structures have been
confirmed by nuclear magnetic resonance (NMR) (Figure S1), mass spectrometry (MS) (Figure
S2), and elemental analysis.

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S
S
S
Tr1
Tr2
N
N
N
N
N
N
N
N
N
N
N
N
Tr4
Tr3
Figure. 1 The chemical structures of the truxeneꢀbased compounds.

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Br
1) BuLi/THF
2) nꢀC6H13Br
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
COOH
C6H13
Br2
PPA
C6H13
Br
C6H13
C6H13
C6H13
C6H13
Br
1
TrꢀC
TrꢀCBr
Br
S
S
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
S
Br
B(OH)2
S
C6H13
C6H13
S
C6H13
C6H13
C6H13
C6H13
S
S
Br
TrꢀAr1
TrꢀArBr1
N
N
Br
Br
N
C6H13
C6H13
C6H13
C6H13
C6H13
2M Na2CO3
Pd(PPh3)4
C6H13
B(OH)2
C6H13
C6H13
N
NBS
N
THF
C6H13
C6H13
C6H13
C6H13
N
TrꢀAr2
TrꢀArBr2
N
Br
Br
Br
N
N
Br
N
B(OH)2
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
N
N
C6H13
C6H13
C6H13
C6H13
Br
Br
N
N
TrꢀArBr3
TrꢀAr3
Br
2M Na2CO3
Pd(PPh3)4
2M Na2CO3
Pd(PPh3)4
B(OH)2
N
B(OH)2 TrꢀArBr3
Tr4
+
Tr1/Tr2/Tr3
+
TrꢀBr/TrꢀArBr1/TrꢀArBr2
THF
THF
Scheme 1. Synthetic procedures for the truxeneꢀbased compounds.
Thermal and photophysical properties. The thermal characters of the truxeneꢀbased
compounds have been characterized by thermal gravimetric analysis (TGA) and differential
o
scanning calorimetry (DSC) with a scanning rate of 10 C /min under a nitrogen atmosphere
(Table 1 and Figure S3). The TGA results indicate that these compounds show excellent

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thermodynamic stability, with the 5% weightꢀreduction temperature (ꢀT5%) ranging from 396 to
o
421 C. In addition, the DSC traces of the compounds revealed their high glassꢀtransition
temperatures (Tg) in the range from 151 to 178 °C and no crystallizing processes had been
observed, suggesting the good morphological stability of the blue emitters. The excellent
thermal properties associated with the truxeneꢀbased molecules will guarantee the stabilities of
the OLEDs based on them.
The UVꢀvis absorption spectra of the truxeneꢀbased molecules in solution are shown in Figure
2a and the corresponding data are listed in Table 1. The highꢀenergy absorption bands around
230 nm should be induced by the absorption of the discrete aromatic units constructing the
molecular skeletons. The sharp absorption band at ca. 308 nm can be ascribed to the absorption
of the truxene core.32 The strong absorption bands at longer wavelength region from ca. 330 nm
to 390 nm are assigned to the absorption transitions from the whole πꢀconjugation systems of
the molecular skeletons of Tr1 − Tr4. The UVꢀvis spectra of the truxeneꢀbased molecules in
neat film have also been measured (Figure 2b). Compared with those in solution (Figure 2a), the
emission bands of these truxeneꢀbased molecules have been broadened, which have been
observed in other truxeneꢀbased emitters.38 However, the absorption maxima of Tr1 − Tr4 in
solid film just exhibit slightly redꢀshift (Figure 2). The molecules Tr2 and Tr4 exhibit obvious
bathochromic shift in their predominant absorption band with respect to that of Tr1 (Figure 2).
Notably, despite of bearing thiophene unit with small conjugation, Tr2 can even possess lower
absorption energy compared with Tr4. However, Tr1 and Tr3 show very similar absorption
onset (Figure 2).
As aforementioned, the predominant absorption bands of these truxeneꢀbased molecules come
from the transition of their expanded πꢀconjugation systems, which can be greatly affected by

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the molecular geometries. In order to interpret their absorption behaviors, the optimized
molecular geometries together with the patterns of the frontier molecular orbitals (MO) have
been obtained by theoretical calculations using DFT//B3LYP/6ꢀ31G(d, p) level with the
Gaussian 09 program (Figure 3). From the molecular geometry of Tr2, it can be seen clearly
that the twisted angle of the thiophene unit between the truxeneꢀcore and peripheral
spirobifluorene blocks is ca. 28ꢀ29º, which is noticeably smaller than that between the
truxeneꢀcore and peripheral spirobifluorene blocks in Tr1 (ca. 38º) (Figure 3). This result
indicates that the thiophene unit can effectively extend πꢀconjugation across the truxeneꢀcore
and the peripheral groups to form large conjugation in Tr2, which will facilitate the absorption
transition. Hence, the dominate absorption band assigned to the absorption transition in Tr2
exhibits bathochromic shift in regard of that of Tr1 (Figure 2). In Tr4, the inner triphenylamine
(TPA) unit show twisted angle ca. 36ꢀ38º between the truxeneꢀcore and peripheral TPA groups
(Figure 3), indicating the poor πꢀconjugation extending ability of the inner TPA units in Tr4.
Why does Tr4 still show bathochromic shift in its UVꢀvis absorption with respect to Tr1 as
aforementioned (Figure 2)? From the MO patterns of Tr4, it can be concluded that the
absorption transition of Tr4 exhibit obvious intraꢀmolecular charge transfer characters from the
electronꢀdonating TPA groups to the truxeneꢀcore (Figure 3). Clearly, the molecular geometry of
the Tr4 will show trivial influence on its transition process. On the contrary, the strong
electronꢀdonating ability associated with the TPA groups will promote the transition process in
Tr4. Thus, Tr4 can show bathochromic shift in its UVꢀvis absorption in regard of Tr1. Due to
the effective conjugation interruption from 3,6ꢀcarbazole substitution mode,53ꢀ55 the carbazole
unit in Tr3 cannot extend πꢀconjugation from the truxeneꢀcore to the peripheral spirobifluorene
blocks. As a result, Tr1 and Tr3 show very similar lowꢀenergy absorption bands (Figure 2).

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(b)
1.0
(a)
1.0
0.8
0.6
0.4
0.2
0.0
Tr1
Tr2
Tr3
Tr4
Tr1
0.8
Tr2
Tr3
Tr4
0.6
0.4
0.2
0.0
300
350
400
450
500
550
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure. 2 The UVꢀvis absorption spectra of truxeneꢀbased compounds. (a) CH2Cl2 solution; (b)
Thin neat film.

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Figure. 3 The HOMO (left) and LUMO (right) patterns of the truxeneꢀbased compounds.
Figure 4 illustrates the PL spectra of the truxeneꢀbased compounds in both CH2Cl2 solution
and neat films. Except for Tr4, all compounds show two major emission peaks in CH2Cl2
solution, which can be assigned to the 0–0 and 0–1 singlet state transitions.56 The shoulder

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emission peaks ca. 420 nm for Tr1 and 489 nm for Tr2 should come from the 0–2 transition. All
the compounds exhibit major emission bands before 470 nm. Thus, they can be regarded as blue
fluorescent emitters. The lifetimes associated with the blueꢀemitting bands are in the order of
nanosecond (Table 1). It is well accepted that the LUMO←HOMO transition typically
correspond with the lowest energy transition to form the first excited states, i.e. S1 states for
fluorescent emitters, leading to the emission of the luminescent materials by their radiative
decay. Hence, the LUMO and HOMO patterns for Tr1, Tr2, Tr3 and Tr4 have been obtained by
the DFT calculation (Figure 3). The S1 states should response the fluorescence of these blue
emitters. Thus, the LUMO←HOMO transitions for Tr1, Tr2, Tr3 and Tr4 have be employed to
interpret their emission behaviors. From the MO patterns for Tr1, Tr2 and Tr3, it can be seen
clearly that their LUMO←HOMO transitions show obvious π–π* characters (Figure 3). Thus,
the PL spectra for Tr1, Tr2 and Tr3 all exhibit obvious structured lineꢀshape (Figure 4a).
Differently, LUMO←HOMO transition for Tr4 shows intraꢀmolecular charge transfer
characters (i.e. from triphenylamine moiety to truxene core) features (Figure 3). As a result, the
PL spectrum for Tr4 displays structureless features (Figure 4a). Owing to the fact that the
lowꢀenergy absorption bands for Tr2 and Tr4 locate in longer wavelength region than those of
Tr1 and Tr3 (Figure 2), Tr2 and Tr4 possess longer emission wavelength than that of Tr1 and
Tr3 (Figure 4a and Table 1). The PL spectra in solid film display different pattern compared
with that in solution, especially for Tr1 – Tr3 (Figure 4). This result can be ascribed the
selfꢀabsorption due to the broadening of the absorption band which will enhance the overlapping
between absorption and emission bands in solid film.

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(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
Tr1
Tr2
Tr3
Tr4
Tr1
0.8
Tr2
Tr3
Tr4
0.6
0.4
0.2
0.0
350
400
450
500
550
600
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Figure. 4 The photoluminescence (PL) spectra for the truxeneꢀbased blue emitters at 298 K. (a)
CH2Cl2 solution; (b) Thin neat film.
Table 1. Photophysical and thermostability data for the truxeneꢀbased blue emitters.
d
e
Absorption λabs (nm) a
Emission
λem(nm) b
Solution
Emission
λem (nm) c
Film
ΦF
τF
ꢁT5%/Tg
(oC)
Compound
Solution/Film
(ns)
Tr1
Tr2
Tr3
228 (5.14), 264 (4.74), 296 (4.72), 380, 400, 420sh
308 (4.84), 342 (5.17)
406, 426, 466
442, 466
397, 411
432
0.76/0.39
2.11
410/169
396/178
421/163
420/151
230 (5.26), 296 (4.61), 308 (4.74), 432, 460, 489sh
392 (5.38)
0.59/0.45
0.47/0.38
0.58/0.49
0.70
3.91
1.66
228 (5.29), 262 (5.09), 308 (5.24), 388, 406
326 (5.29), 338 (5.28)
Tr4
a
232 (5.18), 304 (5.21) , 370 (5.49)
447
Measured in CH2Cl2 at a concentration of 10–6 M at 298K, and log ε values are shown in parentheses. Measured in CH2Cl2 at a
b
concentration of 10–6 M, λex = 334 nm. c Maximum emission wavelength measured in a film state, λex = 334 nm. Fluorescence quantum
d
yields (ΦF) were measured in CH2Cl2 using quinine sulfate in 1.0 M H2SO4 as the standard (ΦF = 0.56), λex = 334 nm. The ΦF for solid
film is obtained by an integrating sphere with λex = 334 nm. e Measured in degassed CH2Cl2 solutions at a concentration of ca. 10–6 M, and
the excitation wavelength was set at 334 nm for all the samples at 298 K.
The PL spectra of the neat films with thickness of ca. 100 nm for these truxeneꢀbased
compounds have also been obtained (Figure 4b). The dominate emission peak of Tr1 in neat
film shows an obvious redꢀshift ca. 26 nm as compared with that in solution (Figure 4 and Table
1). Furthermore, a new emission band at ca. 466 nm appears (Figure 4 and Table 1). Comparing
with its PL spectrum in solution (Figure 4a), the emission bands at 406 and 426 nm in the film
of Tr1 can be easily assigned to the 0–0 and 0–1 vibronicallyꢀresolved transitions (Figure 4b).

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In order to find out the origin of the new emission band at ca. 466 nm in Tr1 film, the PL
spectrum of the Tr1 5ꢀwt% doped PMMA thin film with thickness of ca. 100 nm has also been
obtained to see whether the new emission band is induced by individual molecule of Tr1 or the
interaction among Tr1 molecules (Figure 5a). The PL maxima of the Tr1 doped PMMA film are
almost identical to those in the PL spectrum of the Tr1 solution (Figure 5a). However, there is
no emission band at ca. 466 nm have been detected for the Tr1 doped PMMA film (Figure 5a).
Clearly, the new emission band at ca. 466 nm should be induced by the multiꢀmolecular
behavior of Tr1. It should come from the aggregation of the Tr1 molecules. In addition, the
substantial redꢀshift effect in solid film for Tr1 also indicated the molecular aggregation
according to the reported data.32 In order to support this conclusion, various discrete vibronic
transitions contributing to the PL spectra have been calculated using vibrational progressions
(Figure 5a).57 The theoretical calculations can show excellent agreement with the experimental
results from the PL spectra of both solution and doped PMMA film of Tr1, indicating the
validity of the calculation method. From solution to neat film, both the 0–0 and 0–1 emission
peaks redꢀshift ca. 26 nm. However, wavelength margin between the new emission band (ca.
466 nm) in neat film of Tr1 and the 0–2 emission peak ca. 420 nm for Tr1 in both solution and
doped PMMA film is ca. 46 nm. Furthermore, the theoretical results indicate that the 0–2
emission peak for Tr1 should be much weaker than the 0–0 and 0–1 emission peaks. However,
the new emission band (ca. 466 nm) in neat film of Tr1 exhibits nearly the same intensity of the
0–0 and 0–1 emission peaks. All these results indicate that the new emission band at ca. 466 nm
in neat film of Tr1 might not come from the highꢀfrequency vibrational modes. Even there is
contribution from the 0–2 emission to the PL band at ca. 466 nm in the neat film of Tr1, the
contribution should also in subordinate place due the low transition possibility (Figure 5a). The

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excitation spectra for different emission bands of Tr1 solid film have been measured (Figure 5b).
In Figure 5b, the new emission band at ca. 466 nm shows very similar excitation spectrum to the
other two emission bands, which also exhibit the inherent character of the eximer emission due
to the molecular aggregation.58 The ΦFs in both solution and solid film of these truxeneꢀbased
compounds should also indicate the molecular aggregation in Tr1 (Table 1). From solution to
solid film, Tr1 exhibits the highest reduction of ΦF (ca. 49%) compared with other analogs (ca.
24% for Tr2, 19% for Tr3 and 16% for Tr4), which can be ascribed to the quenching effect
induced by molecular aggregation in Tr1. Hence, the nꢀhexyl groups and spirobifluorene blocks
might not effectively block the molecular aggregation among the Tr1 molecules.
It has been accepted that introducing long alkyl chains and effectively block the aggregation
the concerned truxene derivatives.37, 38 In some compounds, there is only very slight redꢀshift
effect in PL maxima can be observed in going from solution to solid film. However, from
solution to neat film, the pronounced redꢀshift of ca. 23 nm in some truxene derivatives with
long alkyl chains have been ascribed to the molecular aggregation effect.26, 37 The truxeneꢀbased
compound with both nꢀhexyl groups and six fluorene sustitutents still shows inefficient ability in
blocking the molecular aggregation. The truxene derivatives with six nꢀhexyl groups and three
oligo(pꢀphenylene) arms still show molecular aggregation indicated by the large redꢀshift effect
of PL maxima in solid film.32 Thus, it seems that ability of the long alkyl chains in blocking the
aggregation of the truxene derivatives can be influenced by the molecular structures. So, the
molecular structure of Tr1 might disfavor the blocking molecular aggregation of the hexyl
groups.

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2500
(b)
(a)
Solution
5ꢀwt% doped PMMA
Neat Film
1.0
0.8
0.6
0.4
0.2
0.0
2000
1500
1000
500
0
0-0
0-1
406 nm
426 nm
466 nm
0-2
0-3
350
400
450
500
550
600
250
300
350
400
Wavelength (nm)
Wavelength (nm)
Figure 5. (a) The PL spectra of Tr1 in different states together with the calculated discrete
vertical vibronic transitions (vertical columns); (b) Excitation spectra for different emission
bands in the solid film of Tr1.
For Tr2 and Tr3, the redꢀshift effect is ca. 10 and 5 nm for the major emission band on going
from solution to neat film, respectively. However, the emission band at ca. 447 nm for Tr4 in
solution even show redꢀshift ca. 15 nm compared with that from its neat film. This result can be
explained as follows: As aforementioned, the emissive excited states S1 of Tr4 possess obvious
intraꢀmolecular charge transfer characters, which will make them possess obvious polar
characters. Accordingly, the polar excited states S1 of Tr4 will be stabilized in the polar solvent
(such as CH2Cl2) to show positive solvatochromism effect which can lead to obvious redꢀshift of
the emission band of the concerned molecules in polar solvent (Figure S4). In order to eliminate
the influence of the solvatochromism effect, the PL spectra of Tr4 in lowꢀpolar toluene has been
recorded. The PL maximum of the neat film of Tr4 shows very slight redꢀshift effect ca. 2 nm
with respect to that in toluene (Figure S4). This result indicates that there is nearly no
aggregation among the molecules of Tr4.
As aforementioned, the large planar skeleton of truxene typically induces serious molecular
aggregation, which will impede the EL efficiency and color purity. Hence, the concerned

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researchers have done their best to block this undesired aggregation process associated with
truxene. In this study, the spirobifluorene and triphenylamine blocks have been introduced to
fulfill the purpose, since spirobifluorene possesses two fluorene units adopting perpendicular
configuration and triphenylamine shows twisted propeller geometry. However, the molecules of
Tr1 still show aggregation in the neat film (Figure 4b), indicating that the nꢀhexyl groups
attached to the truxene core might not show enough ability in effective coping with the
molecular aggregation problem in this case. Clearly, the molecular aggregation should have tight
relationship with the configuration of the concerned molecules. Hence, the molecular
configurations have been optimized (Figure 6). Despite of the deviation from theoretical
configuration, the actual molecules tends to adopt the optimized configurations since they
represent the lowꢀenergy and stable states. From the molecular configuration of Tr1, it can be
seen clearly that the spirobifluorene moieties are located at one side of the truxene core (Figure
6 and Figure S5), which will leave another side of the truxene core to pack with the other
truxene core. Hence, the molecules of Tr1 exhibit aggregation in the neat film (Figure S5).
Through inserting thiophene unit in between spirobifluorene and truxene core, Tr2 should
adopting more favorable configuration in blocking molecular aggregation due to the fact that
one fluorene unit in spirobifluorene moieties points to both sides of the truxene core. Thus, only
10 nm redꢀshift has been observed in the neat film of Tr2 with respect to its solution despite of
its similar molecular “thickness” to that of Tr1. Owing to the large twist angle ca. 40º between
the peripheral groups and the truxene core (Figure 3), the molecules of Tr3 and Tr4 possess
much larger “thickness” (ca. 20.64 Å for Tr3 and 25.96 Å for Tr4) compared with Tr1 and Tr2
(Figure 6), which can effectively block the aggregation of the truxene core in Tr3 and Tr4.
Hence, very slight redꢀshift effect has been detected for the emission maxima of the neat films

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for Tr3 and Tr4 as aforementioned (Table 1 and Figure 4). The slight redꢀshift effect in PL
maxima of their neat film indicates that the strategy employed in this study should represent
very effective strategy to overcome the problem of molecular aggregation associated with
truxene core. Importantly, all these results have clearly indicated that inserting different
aromatic unit in the arms of the branched truxeneꢀbased emitters can effectively alter the
molecular configuration to block the undesired molecular aggregation effect. It can provide
important information for the design and synthesis of truxeneꢀbased emitters free of molecular
aggregation.
Figure. 6 The sideꢀview of the optimized molecular configuration of the truxeneꢀbased
compound. The nꢀhexyl groups have been replaced by methyl group for both the calculation
convenience and their little effect in blocking the molecular aggregation.
Electrochemical properties. The electrochemical properties of the truxeneꢀbased emitters have
been characterized by cyclic voltammetry (CV) measurement calibrated with ferrocene as
standard under a nitrogen atmosphere (Table 2 and Figure S6). Except for Tr4, all the
compounds show a quasiꢀreversible oxidation wave with potential (Epa) at 0.93 V for Tr1, 0.79

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V for Tr2 and 0.69 V for Tr3, respectively. However, the compound Tr4 exhibits three
quasiꢀreversible oxidation waves with Epa of 0.40, 0.59, and 0.79 V. Compared with Tr1, the
compounds Tr2, Tr3 and Tr4 display noticeably lower first oxidation potentials (Table 2),
indicating their better hole injection ability. The lower Epa for Tr2 should benefit from the
extended conjugation across the truxene core and the arms due to the small twisted angel
between the thiophene units and both the truxene core and the spirobifluorene moieties (Figure
3), which will destabilize the HOMO level of Tr2. Differently, the reduced Epa for Tr3 and Tr4
should be induced by the electronꢀdonating property of the carbazole and triphenylamine
moieties, since the conjugation in Tr3 and Tr4 cannot be effectively extended due to their
twisted molecular configuration (Figure 6). Owing to the stronger electronꢀdonating ability of
triphenylamine unit than that of the carbazole group, Tr4 can show the first Epa as low as 0.23 V.
The oneset oxidation potentials (Epo) of these molecules have also been obtained to calculate the
ox
HOMO levels (EHOMO): EHOMO = – (Epo + 4.8) eV.38 Based on the optical gap (Egopt) of these
blue emitters, their LUMO levels (ELUMO) can be obtained (Table 2). Clearly, Tr2 possesses the
lowest ELUMO, which can be assigned to the fact that both the HOMO and LUMO locate on
nearly the same area of Tr2 (Figure 3). Thus, it will facilitate LUMO←HOMO transition and
make the ELUMO of Tr2 closer to its EHOMO
.
Table 2. Redox properties of the truxeneꢀbased compounds.
Compound
Tr1
Tr2
Epa (V)a
0.93
0.79
0.69
Epo (V) b
0.77
0.56
0.47
Egopt (eV)c
3.28
2.88
3.27
EHOMO (eV) ELUMO (eV)
ꢀ5.57
ꢀ5.36
ꢀ5.27
ꢀ5.03
ꢀ2.29
ꢀ2.48
ꢀ2.00
ꢀ2.01
Tr3
0.40, 0.59, 0.79 0.23
3.01
Tr4
a
Oxidation potential. b Onset oxidation potential. c Calculated from the absorption onset of the UVꢀvis
absorption spectra.
Electroluminescent characterization. In order to investigate their electroluminescent

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characteristics, nonꢀdoped blueꢀemitting OLEDs have been constructed with these
truxeneꢀbased emitters as emission layer. Due to their good solubility in common solvents such
as CH2Cl2, toluene and chlorobenzene etc, the concerned devices were easily fabricated with
solutionꢀprocessing approach with the configuration of ITO/PEDOT: PSS (40 nm)/emitters
(EML) (40 nm)/TPBi (40 nm) /LiF (1 nm)/Al (100 nm). Figure 7 depicts the structure of the
multiꢀlayer OLEDs and the functional materials involved in the fabrication of the devices. The
PEDOT: PSS layer was firstly deposited by spinꢀcoating on the preꢀcleaned ITO surface to form
holeꢀinjection layer (HIL) for its matching HOMO level with that of ITO. The emission layer
(EML) is deposited on the PEDOT: PSS layer by spinꢀcoating the chlorobenzene solution of the
concerned emitters. The 1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)benzene (TPBi) layer
plays the role of both holeꢀblocking and electronꢀtransporting, while LiF acts as an
electronꢀinjection layer.

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Figure. 7 The general configuration for OLEDs made from the truxeneꢀbased emitters and the
chemical structures of the relevant compounds used in these devices.
1.0
Device A
Device B
Device C
Device D
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
Figure. 8 The EL spectra for the OLEDs made from the truxeneꢀbased emitters at 9 V.

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When proper voltage applied, except device A, all devices emit blue electroluminescence
(Figure 8). Compared with the PL spectrum of Tr1 in both solution and neat film, the
longꢀwavelength band at ca. 500 nm in the EL spectrum of device A should come from the
molecular aggregation of Tr1 as aforementioned. This has seriously affected the EL blue color
quality by shifting the CIE coordinates towards green region in the chromaticity diagram (Table
3). Hence, the EL emission from device A can be regarded as bluishꢀgreen color. Owing the
favorable configuration of Tr2, Tr3, and Tr4 in blocking molecular aggregation, the concerned
device can show EL spectra without longꢀwavelength EL bands induced by molecular
aggregation (Figure 8), maintaining the blue EL color. According to the CIE coordinates (Table
3), device B can show greenishꢀblue EL emission, while both device C and D emit blue EL
color. These results have clearly indicated the crucial role played by our molecular design
strategy in relieving the molecular aggregation problem associated with the truxeneꢀbased
emitters.
The current density (J)–voltage (V)–luminance (L) curves for the concerned devices are
shown in Figure 9, while the relationship between EL efficiencies and current density for the
devices are presented in Figure 10 and Figure S7. Owing to the serious molecular aggregation of
Tr1 in the EML, device A shows very poor EL efficiencies (Table 3). On the contrary, the other
devices can exhibit much improved EL performances. The device B can furnish maximum EL
ability with a maximum brightness (Lmax) of 11629 cd m–2 at 11.3 V, a peak external quantum
efficiency (ηext) of 1.68%, a current efficiency (ηL) of 3.88 cd A–1, and a power efficiency (ηp) of
1.33 lm W–1. Notably, device C and D can even exhibit better EL performances compared with
device B (Table 3). Device C can show a low turnꢀon voltage (Vturnꢀon) of 4.0 V, Lmax of 5306 cd
m–2 at 9.3 V, ηext of 4.61%, ηL of 4.85 cd A–1, and ηp of 2.71 lm W–1. Encouragingly, device D

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can furnish even better EL performances with Vturnꢀon of 2.8 V, Lmax of 9461 cd mꢀ2, ηext of 4.33%,
ηL of 7.41 cd A–1, and ηp of 6.79 lm W–1. Even at high luminance of 500 cd mꢀ2, device C/D can
still show high EL efficiencies with ηext of 4.40/3.96%, ηL of 4.62/6.77 cd A–1, and ηp of
2.41/3.79 lm W–1 (Table 3 and Figure 10). The good hole injection/transporting ability
associated with the carbazole and triphenylamine units should benefit the improved EL
performances of device C and D. Besides the contribution from its good hole
injection/transporting ability, the admirable EL performance of Tr4 can also be attributed to its
proper HOMO level of ca. 5.08 eV, which is almost the same to that of PEDOT: PSS (ca. 5.1
eV). Thus, the hole injection from PEDOT: PSS layer to EML of device D should be nearly
barrierꢀfree (Figure 7). In addition, the energy barrier between the EML of device D and the
holeꢀblocking layer (TPBi) is the largest (ca. 1.22 eV) (Figure 7), which can effectively restrain
the injected holes in the EML to combine with the injected electrons rather than drift to the
cathode and form current without combination. Hence, the charge recombination efficiency
should be greatly improved to bring high EL efficiency for device D based on Tr4. From device
A to device D, the Vturnꢀon decreases gradually, which can be ascribed to the continuously
elevating of the HOMO levels from Tr1 to Tr4 to facilitate the hole injection to the EMLs in the
concerned devices, indicating the advantage of employing functional moieties in developing
truxeneꢀbased emitters. There are also some highꢀperformance truxeneꢀbased emitters employed
in OLEDs. Yang and coworkers have reported some truxeneꢀbased blue emitters that can
achieve peak ηL of 4.01 cd A–1 and ηP of 3.60 lm W–1.38 Some truxeneꢀbased blue emitters with
anthracene substituents can furnish ηL of 2.98 cd A–1 and ηP of 1.52 lm W–1.30 The deepꢀblue
emitting OLEDs made from truxeneꢀbased dendrimer can exhibit high EL efficiencies with ηL of
5.3 cd A–1 and ηext of 6.6%.39 Compared all these good documented results, the EL performances

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in this study have not only indicated the great potential of these novel truxeneꢀbased emitters in
furnishing highly efficient blueꢀemitting OLEDs, but also shown the important value of the
conception for molecular design in coping with the problems associated with truxeneꢀbased
emitters.
Table 3. The EL performance for the OLEDs made from the truxeneꢀbased fluorescent emitters.
Device Vturnꢀon(V)
Luminance
(cd mꢀ2)a
ηext
ηL
(cd Aꢀ1)
ηp
λmax
(nm)c
CIE (x,y) c
(%)
(lm Wꢀ1)
0.52 (8.1) a
0.51 b
1.13 (8.1)a 0.44 (8.1)a
A
B
C
D
6.5
4.3
4.0
2.8
821 (10.1)
11629 (11.3)
5306 (9.3)
9461 (9.3)
388, 400, 500 (0.21, 0.30)
436, 464, 492 (0.18, 0.22)
1.11 b
0.42 b
1.68 (9.1)
0.38
3.88 (9.1)
0.88
1.33 (9.1)
0.41
4.61 (5.7)
4.40
4.85 (5.7)
4.62
2.71 (5.5)
2.41
392, 404
436
(0.18, 0.15)
(0.17, 0.15)
4.33 (4.0)
7.41 (4.0)
6.79 (2.6)
3.96
6.77
3.79
a Maximum values of the devices. Values in parentheses are the voltages at which data were obtained. b Values
at 500 cd mꢀ2. c Values collected at 9V.
10000
400
300
200
100
0
Device A
Device B
Device C
Device D
1000
100
10
1
0.1
4
6
8
10
Voltage (V)
Figure. 9 The current density (J)–voltage (V)–luminance (L) curves for all the devices.

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10
Device C
1
ηL
ηP
η
η
η
η
ηext
η
η
η
η
η
η
η
η
0.1
10
0
100
200
mA cmꢀ1
)
300
Current density
(
Device D
1
ηL
ηP
η
η
η
η
ηext
η
η
η
η
η
η
η
η
0.1
0
50
100
150
Current density ( )
mA cmꢀ1
Figure. 10 The relationship between EL efficiencies and current density for the device C and D.
Conclusion
Through introducing spirobifluorene unit and electronꢀdonating moieties, such as thiophene,
carbazole, and triphenylamine groups, four truxeneꢀbased blue emitters have been successfully
developed with the optimized molecular configuration which can effectively block the undesired
aggregation of the truxene core. Furthermore, good electron injection/transporting ability has
been afforded to the blue fluorescent emitters by the electronꢀdonating blocks. Benefiting from
all these advantages of these truxeneꢀbased novel blue emitters, the concerned nonꢀdoped

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solutionꢀprocessed blueꢀemitting OLEDs can furnish very high EL performances with Vturnꢀon of
2.8 V, Lmax of 9461 cd mꢀ2, ηext of 4.33%, ηL of 7.41 cd A–1, and ηp of 6.79 lm W–1, which might
represent the stateꢀofꢀtheꢀart performance for truxeneꢀbased blueꢀemitting OLEDs. All these
encouraging data will not only show the great potential of these blue emitters, but also provide
important information for design and synthesis of highꢀperformance truxeneꢀbased fluorescent
emitters.
Associated content
Electronic supplementary information
Synthetic scheme for the truxeneꢀbased emitters, synthetic details for the key intermediate
compounds, and some EL data. This information is available free of charge via internet.
Author information
Corresponding author
*Fax:
+86ꢀ029ꢀ82663914
(G.Z.).
Eꢀmail:
zhougj@mail.xjtu.edu.cn
(G.Z.);
zhaoxinwu@mail.xjtu.edu.cn (Z.W.); heartstar@mail.xjtu.edu.cn (Y.L.).
Acknowledgements
This work was financially supported by Tengfei Project from Xi'an Jiaotong University, the
Fundamental Research Funds for the Central Universities, The Program for New Century
Excellent Talents in University, the Ministry of Education of China (NECTꢀ09ꢀ0651), the Key
Creative Scientific Research Team in Shaanxi Province (2013KCTꢀ05), the China Postdoctoral
Science Foundation (Grant no. 20130201110034), and the National Natural Science Foundation