Pyrene-Capped Conjugated Amorphous Starbursts: Synthesis, Characterization, and Stable Lasing Properties in Ambient Atmosphere

Article abstract of DOI:10.1002/adfm.201501337

A family of trigonal starburst conjugated molecules (TrFPy, TrFPy, and TrF2Py) composed of a truxene core and pyrene cappers with various bridge lengths is synthesized and characterized. The incorporation of pyrene cappers successfully depress the crystallization tendency, resulting in enhanced glassy temperature and improved morphological stability of the thin films. The high photoluminescence yield in neat films and excellent thermal stability render these pyrene-capped starbursts promising lasing optical gain media. Low amplified spontaneous emission (ASE) thresholds (E_th^ASE) of 180 nJ pulse^-1 and 101 nJ pulse^-1 were recorded for TrFPy and TrF2Py, respectively. One dimensional distributed feedback (1D DFB) lasers demonstrated lasing threshold of 9.3 kW/cm² and 7.3 kW/cm² for TrFPy (at 457 nm) and TrF2Py lasers (at 451 nm), respectively. The ASE performance of TrFPy and TrF2Py in an ambient condition was recorded with various annealing temperature (from 80 to 250 °C, 10 min). Surprisingly, TrFPy exhibited excellent ASE stability in an ambient condition, which is still detectable even after annealing at 250 °C for 10 min. The results suggest the pyrene-capped molecular design strategy is positive on improving the optical gain stability and meanwhile maintaining excellent lasing properties. Well-defined starburst conjugated macromolecules with pyrene moieties as end-capped groups are explored as the optical gain media for organic semiconductor lasers. The length of oligofluorene bridges between the truxene core and pyrene cappers is carefully tuned. The relationships between their morphology, thermal stability, and amplified spontaneous emission stability in an ambient condition are systematically investigated, suggesting that the pyrene-capped molecular design improves the optical gain stability.

Full text of DOI:10.1002/adfm.201501337

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Pyrene-Capped Conjugated Amorphous Starbursts:
Synthesis, Characterization, and Stable Lasing Properties in
Ambient Atmosphere
Weidong Xu, Jianpeng Yi, Wen-Yong Lai,* Li Zhao, Qi Zhang, Wenbo Hu,
Xin-Wen Zhang, Yi Jiang, Ling Liu, and Wei Huang*
of being pumped directly by electrical
[
1–4]
A family of trigonal starburst conjugated molecules (TrFPy, TrFPy, and TrF2Py)
composed of a truxene core and pyrene cappers with various bridge lengths
is synthesized and characterized. The incorporation of pyrene cappers suc-
cessfully depress the crystallization tendency, resulting in enhanced glassy
temperature and improved morphological stability of the thin films. The
high photoluminescence yield in neat films and excellent thermal stability
render these pyrene-capped starbursts promising lasing optical gain media.
injection.
Although numerous solu-
tion-processed emitters have been dem-
onstrated in organic light emitting diodes
(
OLEDs), it should be noted that not all
materials that show good performance
in OLEDs exhibit efficient optical gain at
low pump thresholds. Unrevealing the
structure-function relationships is a vital
research task with the aim to shed light
on rational design and development of
high-performance optical gain materials.
Particularly, to address the challenge of
electrically pumped organic laser diodes,
researchers have devoted considerable
efforts to realize large stimulated emission
Low amplified spontaneous emission (ASE) thresholds (EthASE) of 180 nJ
−
pulse 1 and 101 nJ pulse–1 were recorded for TrFPy and TrF2Py, respectively.
One dimensional distributed feedback (1D DFB) lasers demonstrated lasing
2
2
threshold of 9.3 kW/cm and 7.3 kW/cm for TrFPy (at 457 nm) and TrF2Py
lasers (at 451 nm), respectively. The ASE performance of TrFPy and TrF2Py in
an ambient condition was recorded with various annealing temperature (from
cross-sections (σ ), low lasing thresholds,
8
0 to 250 °C, 10 min). Surprisingly, TrFPy exhibited excellent ASE stability in
e
high carrier mobility, and excellent power-
an ambient condition, which is still detectable even after annealing at 250 °C
for 10 min. The results suggest the pyrene-capped molecular design strategy
is positive on improving the optical gain stability and meanwhile maintaining
excellent lasing properties.
[
5–9]
conversion efficiencies.
Nevertheless,
the development of efficient optical gain
materials is still highly demanded.
From a fundamental viewpoint, mole-
cules with well-defined structures are crit-
ical for understanding the structural prop-
1
. Introduction
erties and functions. This rule also applies to the study of the
lasing and/or spontaneous emission property of organic semi-
conductors. Good solution processability is one of the attrac-
tive prerequisites, which make organic semiconductor lasers
suffer little from the concentration quenching that plagues
typical dye-doped organic lasers. In our previous contribution,
we have explored a novel series of monodisperse starburst
conjugated macromolecules based on truxene or triazatruxene
Solution-processed organic semiconductor emitters are prom-
ising broadly tunable coherent sources which have the benefits
of low-cost processing, light weight physical characteristics,
tunable optoelectronic properties, and the attractive possibility
of direct integration into flexible substrates. In contrast to the
widely utilized lasing dyes, one of the most attractive proper-
ties is that organic semiconductors possess the possibilities
[
10–14]
cores with three or six oligofluorene arms.
Promising
W. Xu, J. Yi, Prof. W.-Y. Lai, L. Zhao, Q. Zhang, W. Hu, X.-W. Zhang,
Y. Jiang, L. Liu, Prof. W. Huang
Key Laboratory for Organic Electronics and Information
Displays (KLOEID)
Prof. W.-Y. Lai, Prof. W. Huang
Key Laboratory of Flexible Electronics (KLOFE)
and Institute of Advanced Materials (IAM)
Jiangsu National Synergetic Innovation Center
for Advanced Materials (SICAM)
Institute of Advanced Materials (IAM)
Jiangsu National Synergetic Innovation Center
for Advanced Materials (SICAM)
Nanjing Tech University (NanjingTech)
30 South Puzhu Road, Nanjing 211816, China
Nanjing University of Posts and Telecommunications
9
Wenyuan Road, Nanjing 210023, China
E-mail: iamwylai@njupt.edu.cn; wei-huang@njtech.edu.cn
DOI: 10.1002/adfm.201501337
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electroluminescence, optical gain, and lasing performance have
been achieved. Well-defined chemical structures, high purity,
excellent reproducibility, good solution processability, and gen-
eral superior optoelectronic properties are the main advantages
charge transfer between the core and the end cappers, and to
understand the impact of the photoelectric property variation
on the lasing properties. The relationships between their mor-
phology, thermal stability, and corresponding ASE stability in
an ambient condition were systematically investigated, which
suggests the pyrene-capped molecular design strategy is posi-
tive on improving the optical gain stability. Consequently,
promising low-threshold and stable lasing properties have been
achieved with 1D DFB structures.
[
10–14]
of these monodiperse star-shaped multiarmed materials.
These studies have enabled us to systematically investigate the
relationships between the arm length, arm number, core type
and the resulting lasing and electroluminence properties. Fur-
thermore, an extremely low-threshold lasing has been dem-
onstrated by a 1D distributed feedback (DFB) laser with a six-
–2 [12]
armed starburst material as gain media (T3, 1.3 µJ cm ).
From a practical standpoint, high environmental stability
is strongly demanded in organic lasers since the gain media
have to be exposed under high-fluence pumped sources. For
achieving electrically pumped organic laser diodes, a much
higher current density is necessary in comparison to those
typical OLEDs. This requires that the laser gain media should
possess not only high carrier mobility but also good thermal
and optical stability. The widely investigated polyfluorene,
2
. Results and Discussion
2.1. Synthesis and Characterization
The synthetic routes toward the pyrene-capped starbursts are
shown in Scheme 1. The key step for the synthesis involves
microwave-assisted Pd-mediated Suzuki cross-coupling
reaction of 1-bromopyrene or pyrene-capped oligofluorene
e.g., poly(9,9-dioctylfluorene) (PFO), shows
increase in amplified spontaneous emission (ASE) threshold
a
continuous
with
2,7,12-tri-(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolane)-
hexaethyltruxene. The Suzuki cross-coupling reaction with
Pd(PPh ) , K CO , and tetrabutylammonium bromide (TBAB)
as catalyst under microwave irradiation (CEM system) for
30 min at 150 °C afforded target compounds in 70%–90%
yields. The well-defined structures and chemical purities of the
intermediates and the final starbursts have been adequately ver-
by annealing from room temperature up to 130 °C (0.32 µJ
3
4
2
3
−
1
to 2.0 µJ pulse ), and the ASE tends to be undetectable with
the annealing temperature above 150 °C, as suggested by
Sim et al. [ Pyrene derivatives have attracted much recent
interest due to their high photoluminescence quantum yields
15]
1
13
(
PLQYs), high charge carrier mobility, and excellent thermal
ified by H and C NMR spectroscopy and MALDI-TOF mass
spectrometry.
[16]
stability. Incorporating with other highly emissive building
blocks, such as fluorene and anthracene units, pyrene deriva-
tives have been demonstrated as one of the most promising
candidates for organic lasing applications.[ In contrast to the
fluorene-based materials that commonly show a poor spectral
stability, the outstanding thermal stability for pyrene derivatives
is rather attractive.[ However, planar pyrene molecules tend
to form π aggregates/excimers in the solid state, resulting in
11]
2.2. Photophysical Properties
Figure 1 shows the normalized UV–vis absorption and
photoluminescence (PL) spectra for dilute THF solutions
(Figure 1a) and spin-coated thin films (Figure 1b) of the star-
bursts. The normalized UV–vis absorption exhibited a peak
at λabs = 359 nm for TrPy, 362 nm for TrFPy, and 372 nm for
TrF2Py, respectively. Slight bathochromic shifts to 368 nm for
TrPy, 369 nm for TrFPy, and 375 nm for TrF2Py were observed
for the thin film absorption spectra, consistent with denser
packing and extended conjugation in the solid states. The
corresponding maximum absorption coefficient (αmax) were
8.1 × 10 , 10.4 × 10 , and 10.8 × 10 cm for TrPy, TrFPy, and
TrF2Py, respectively. The solution PL emission bands of all
the starburst compounds showed a 0–0 emission band peak
at around 417–418 nm. For TrFPy and TrF2Py in film states,
the 0–1 emission peaked at λem = 451 and 449 nm, respectively,
with a 0–0 shoulder at around 427 nm. For TrPy, a broadened
and redshifted PL band was observed with a peak at 475 nm in
film states, which indicated that the planar pyrene unit directly
attached to the truxene core was not effective to circumvent the
aggregation tendency, resulting in an obvious emission band
from aggregates.
17]
redshifted aggregates/excimers emission with reduced fluores-
cence efficiency.[
17–23]
In order to address these issues, many
new materials with hindered structures have been explored and
[24]
investigated, such as polypyrene molecules, tetraarylpyrene
[
17]
[25–27]
molecules, pyrene–fluorene
and pyrene–carbazole sys-
tems.[
28,29]
Generally, it is essential to integrate either large
bulky groups or multisubstituted rigid moieties to suppress
aggregates and excimers. We have recently developed a series
of monodisperse multiarmed starbursts with pyrene as the core
and oligofluorenes as the arms.[11,17] Such a novel molecular
design has been proven to be effective to restrain the aggrega-
tion tendency of pyrene units, resulting in not only excellent
electroluminescence performance and extremely low-threshold
organic lasing, but also improved ASE stability in comparison
with those of linear polyfluorenes. Moreover, a modest increase
in ASE thresholds with increasing the annealing temperature
4
4
4
−1
[11]
has been found in N -filled atmosphere.
2
In this contribution, we report the design, synthesis, and
optical gain properties of a series of new trigonal starburst
conjugated molecules with a truxene core end-capped with
pyrene moieties (TrPy, TrFPy, and TrF2Py). Pyrene units were
incorporated as the end functional groups and the length of
oligofluoene bridges was varied with the aim to provide a set
of model samples to reveal the influence of intramolecular
The PLQYs of the compounds in film states were measured
by an integrating sphere with λabs in film states as the excitation
wavelength. The PLQY increased with the increment of oligoflu-
orene bridge lengths from 35.7% for TrPy, 56.2% for TrFPy to
60.1% for TrF2Py. The PLQY for the starbursts TrFPy and TrF2Py
became close, suggesting that the incorporation of one fluorene
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Scheme 1. Synthetic routes toward pyrene-capped starbursts.
unit as the bridge was effective to depress the intramolecular
interactions. This was also supported by the observation from the
solid-state transient PL measurements. The PL decay transients
were plotted as shown in Figure 2, which was measured from
a second fast component with a lifetime of τ = 4.84 ns (43.2%),
2
and a third longer component with a lifetime of τ = 15.47 ns
3
(41.2%). The longest lifetime (τ ) was probably attributed to
3
the excited-state excimers of the rigid pyrene substituents. In
contrast, TrFPy and TrF2Py films showed only two exponential
60 nm thick films of each compound with 1 MHz, 85 ps pulse
excitation. The film of TrPy showed a tri-exponential decay with
component decays with a fast component at τ (TrFPy) = 0.68 ns
1
an initial fast component with a lifetime of τ = 0.84 ns (15.6%),
(67.8%), τ (TrF2Py) = 0.66 ns (71.9%), and a longer component
1
1
Figure 1. a) UV–vis absorption and PL spectra of TrPy, TrFPy, and TrF2Py in dilute THF solutions and b) in the films spin-cast on quartz from a toluene
solution.
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starburst molecules (T1–T4) with only oligofluorene arms, the
T values have been enhanced significantly (from T = 63 °C
g
g
[
13,32]
for T1 to 116 °C for T4).
It is implied that the presence of
pyrene end-cappers can effectively improve the rigidity of the
molecular backbone and limit the crystallization and induce
the formation of an amorphous morphology, which in turn
improves the thermal and morphological stability of the films.
Atomic force microscopy (AFM) measurements were con-
ducted to understand the film-forming ability. The results were
recorded from the pristine films of TrPy, TrFPy, and TrF2Py
on quartz, as shown in Figure 4a–c, respectively. The pristine
TrPy film showed an extremely rough surface with a root-
mean-square (rms) roughness of 6.31 nm. Generally, such poor
film-forming ability is not desirable for organic semiconductor
devices. In contrast, the films of TrFPy and TrF2Py exhibited
a smooth and uniform surface with the rms roughness of
Figure 2. Normalized PL decay transients for TrPy (squares), TrFPy (cir-
cles), and TrF2Py (triangles) in the film states (the excitation wavelength
is 375 nm).
0
.285 and 0.268 nm, respectively. These results suggested that
the increase of arm length of the starbursts could effectively
improve the solubility, and therefore enhance their film-forming
ability. Figure 4d–f presents a set of fluorescence optical micro-
scope (FOM) images of the same films after thermal annealing
at 150 °C for 10 min in ambient condition. Different from the
uniform surface of TrF2Py, the annealed TrPy and TrFPy films
exhibited micrometer-sized features caused by their relatively
at τ (TrFPy) = 5.05 ns (32.2%), τ (TrF2Py) = 3.05 ns (28.1%),
2
2
respectively. The prolonged relaxation (τ ) was ascribed to exciton
2
migration from the truxene core or oligofluorene units to pyrene
cappers, which was typically observed in solid states.[ The radi-
30]
ative lifetime (τ ) was determined by decay time and PLQY. For
low T . This result is consistent with their DSC measurements.
R
g
TrPy, a long radiative lifetime of 24.08 ns was recorded. The radi-
ative lifetimes of these starbursts decreased with the increment
of the arm length. For TrFPy and TrF2Py, the radiative lifetimes
were determined as 3.72 and 2.50 ns, respectively. All the optical
properties are summarized in Table 1.
The good film-forming ability and thermal stability for TrF2Py
is highly attractive for lasing.
2.4. Electronic and Electroluminescent Properties
Density functional theory calculations of the starburst com-
pounds were carried out using a suite of Gaussian 03 pro-
gram in order to understand their electronic properties at the
molecular level. The nonlocal density functional of B3LYP
with 6-31G(d) basis set was used for the calculation. The opti-
mized structures and the orbital distributions of HOMO and
LUMO energy levels of the starbursts are shown in Figure
S2 and Table S1 (Supporting Information), respectively.
The excited electrons are polarized from the truxene core to
pyrene-capped units through the oligofluorene bridge, which
indicates that the pyrene units are more likely to accept elec-
trons. It is consistent with the transient PL results that these
2
.3. Thermal Properties and Morphologies
The thermal properties of trigonal materials were investigated
by thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) in N atmosphere. The results are shown in
2
Figure 3. All these compounds were thermally stable with 5%
weight loss temperatures (T ) of 403 °C for TrPy, 402 °C for
d
TrFPy, and 385 °C for TrF2Py, respectively. Distinct glass transi-
tion temperatures (T ) were observed at 131 °C for TrPy and 168
g
°
C for TrFPy. For TrF2Py, no obvious T was found within the
heating scans from room temperature to 280 °C (Figure 3, inset).
g
The T value of trigonal materials increased obviously with the
materials show a long lifetime components (τ ) around 3–5 ns
g
2
increment of fluorene units at each bridge from TrPy, TrFPy to
TrF2Py, resulting from the increased molecular weight or the
enhanced steric hindrance by increasing the arm length.
In contrast to previously reported trigonal truxene-cored
in neat films. Slight variation of calculated energy levels
is found, implying a weak influence for the pyrene cappers
on the electronic properties of the molecules. The electro-
chemical properties of the starburst materials were further
[
31]
Table 1. Photophysical and electrochemical properties of TrPy, TrFPy, and TrF2Py.
Compound
λabs
λPL
λabs
[nm] (film)
λPL
[nm] (film)
αmax
[10 cm
PLQY
[%]
Decay time
[ns]
Radiative lifetime
a)
a)
b)
b)
4
−1 b)
b,c)
b)
d)
[
nm] (solution) [nm] (solution)
]
[ns]
TrPy
359
362
372
418
417
417
368
369
375
475
451
449
8.1
35.7
56.2
60.1
0.84 (15.6%), 4.84 (43.2%), 15.47 (41.2%)
0.68 (67.8%), 5.05 (32.2%)
24.08
3.72
2.50
TrFPy
TrF2Py
10.4
10.8
0.66 (71.9%), 3.65 (28.1%)
a)
5 × 10−7 mol L−1 in THF; b)PLQY of spin-coated films measured in an integrating sphere; c)Prepared by spin-coating at 1200 rpm in toluene (15 mg mL−1). The excitation
≈
wavelength is 370 nm. d)Calculated by the equation of τR = τ/PLQY.
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with an increased EthASE of 356 nJ pulse−1 (22.3 µJ cm ). The
−2
1D DFB lasers based on TrFPy and TrF2Py were fabricated by
spin-coated thin films on top of the DFB gratings which were
obtained by the method of nanoimprint lithography (NIL). The
fill factor of the gratings was 50%, and the periods we chose
were 280, 290, and 300 nm. The pulse beam of the frequency-
tripled Nd:YAG pulse laser (with the pump wavelength at 355
nm, a repetition rate of 10 Hz, a pulse duration of 5 ns) was
used as the pump source, which was focused into a circle-
shaped spot with a diameter of 681 µm. As shown in Figure 6a ,
two transitions of the slopes were observed by increasing
the pump energy density, indicating lasing threshold of
−
2
9
.3 kW cm for TrFPy (120 nm) with the output wavelength
−2
peaked at 457 nm and 7.3 kW cm for TrF2Py (108 nm) with
the output wavelength peaked at 451 nm, respectively. By regu-
lating the periods of the gratings, the lasing wavelength can be
tuned across a spectral window of 17 nm from 442 to 451 nm
with the FWHM of 0.2 nm for TrFPy, as depicted in Figure 6b.
The ASE characteristics of TrFPy and TrF2Py were moni-
tored as a function of annealing temperatures (from 80 to
250 °C) to assess the corresponding thermal stability of the
Figure 3. TGA thermograms and DSC traces (inset) of the materials
−
1
recorded at a heating rate of 10 °C min .
investigated by cyclic voltammetry. As shown in Figure S3 and
Table S1 (Supporting Information), all the molecules showed
similar HOMO levels at around −5.79 to −5.80 eV, indicating
p-type characteristics of these materials. The LUMO levels
of the starbursts raised slightly from −2.51 eV for TrPy to
[
33]
gain properties.
The measurements were conducted in an
ambient condition with room temperature of 20 °C and the
humidity of 30%. After cooling back to room temperature
fast, standard stripe-pumped ASE measurements were per-
formed. It is worthwhile to mention that the excitation stripe
is moved to a fresh location on the films for each successive
measurement to prevent annealing effects from photodegrada-
tion. The increased folds of the ASE threshold as a function of
−
2.45 eV for TrFPy and −2.28 eV for TrF2Py, demonstrating
the influence of increasing the arm length. To further inves-
tigate their electroluminescence properties of these novel
starburst materials, and especially their color stability under
electrical bias, solution-processed OLEDs with the configura-
tion of ITO/PEDOT:PSS/Emissive Layer/LiF/Al were fabri-
cated. The EL spectra of TrFPy and TrF2Py based devices are
shown in Figure S1 (Supporting Information). Because of the
poor film-forming ability of TrPy, no good EL performance
was recorded. In contrast, both TrFPy and TrF2Py-based
OLEDs showed good EL spectral stability with the operating
voltage up to 8 V.
the annealing temperature are demonstrated in Figure 7. The
ASE
Eth
of TrF2Py did not show significant changes from room
temperature to 160 °C (1.2-fold to pristine films), while slightly
ASE
increased Eth
of TrFPy to 1.97-fold was noticed by annealing
the pristine film at 160 °C. These results agreed well with the
thermal and morphology stability results. The slightly higher
loss of ASE for TrFPy before 160 °C may originate from crys-
tallization that induces scattering, which would frustrate the
2
.5. Amplified Spontaneous Emission (ASE)
Properties
The ASE properties of TrFPy and TrF2Py in
neat films were measured by an Nd:YAG laser
(a repetition rate of 10 Hz, a pulse duration of
12 ns) with the pumped source at their max-
imum absorption wavelength, and the surface
emission was collected and analyzed with a
fiber-coupled spectrograph and charge-cou-
pled device (CCD) detector. The lowest ASE
thresholds (EthASE) for the TrFPy and TrF2Py
films with optimized thickness are 180 nJ
−1
−2
pulse
and 105 nJ pulse (6.3 µJ cm , peaked at
45 nm), respectively. As shown in Figure 5b ,
(11.2 µJ cm , peaked at 449 nm)
−1
−2
4
by controlling the TrF2Py film thickness,
we can access lasing wavelengths across
a 13 nm spectral window (433–445 nm).
It is noted that the ASE phenomenon is even
Figure 4. AFM images (top, as spun) and FOM images (annealed at 150 °C for 10 min in
ambient condition) of a,d) TrPy, b,f) TrFPy, and c,g) TrF2Py on quartz. Scan size is 2 µm × 2 µm
and scan rate is 1 Hz for AFM. The size of each FOM image corresponds to 88.8 µm × 67 µm,
detectable in an ultrathin film of 25 nm, and all the FOM images were recorded at 550 nm.
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Figure 5. a) Normalized output intensity versus pump intensity for the TrFPy and TrF2Py films (90 nm); b) Normalized ASE spectra for TrF2Py films
with various thicknesses: the emission peak at 433, 440, 443, 444, and 445 nm for TrF2Py films, corresponding to the thickness of 25, 45, 75, 90, and
1
50 nm, respectively.
required in-plane waveguide propagation. However, the EthASE
of TrF2Py films increased severely up to 17.6-fold relative
to that of the pristine films when the annealing temperature
approached 200 °C. In contrast, TrFPy showed relatively stable
ASE performance for subsequent annealing temperature from
degradation caused by the existence of oxygen and moisture in
a rather high temperature (annealing at 250 °C) should have
a significant impact on the optical properties. It is also sug-
gested that long oligofluorene arms may favor intra-/inter-chain
[
12]
interactions between the arms. Therefore, the only slightly
enhanced green emission band for TrF2Py films after thermal
annealing at high temperature (250 °C) may probably origi-
nate from the excimer formation of the stacked fluorene units,
1
60 °C (1.97-fold relative to pristine films) to 200 °C (3.11-fold
relative to pristine films). The ASE of TrFPy maintained good
quality upon annealing even up to 250 °C with only 4.5-fold
increment compared with that of the pristine films; while it was
completely not detectable for TrF2Py. The results manifest that
high thermal stability is helpful for improving the optical gain
stability.
[
34]
causing optical gain loss.
3
. Conclusion
To further understand the origin for the different ASE behav-
iors of TrFPy and TrF2Py at high temperatures, the PL spectra
of the same films after annealing are investigated and the
results are shown in Figure S4 (Supporting Information). All
the PL characteristics of both materials showed little variation
by increasing the annealing temperature from 80 to 200 °C.
However, for the TrF2Py film annealed at 250 °C, a weak green
band (from 500 to 600 nm) appeared. In contrast, for TrFPy
with a single fluorene bridge, the PL spectra remained almost
the same with no obvious green emission band emerging even
after annealing at 250 °C. The formation of the green band may
result from both chemical and physical defects.[ Since the
films were annealed in an ambient atmosphere, the chemical
In summary, we have presented the synthesis and characteriza-
tion of a series of new trigonal starburst molecules comprising
a truxene core and pyrene end-cappers with (TrFPy and TrF2Py)
and without (TrPy) oligofluorene bridges. The π–π stacking of
planar pyrene cappers were prevented by the introduction of
fluorene bridges, resulting in the blueshifted PL spectra and
enhanced PLQYs. In comparison to TrPy (35.7%), enhanced
PLQYs of 56.2% for TrFPy and 60.1% for TrF2Py were achieved.
The PL decay transients were reduced with the increase of flu-
orene bridge units. The incorporation of rigid pyrene cappers
led to an enhancement of glass transition temperatures, which
was beneficial for the morphology stability while pumped source
31]
Figure 6. a) The output intensity as a function of the pump intensity for the TrFPy (top) and TrF2Py (bottom) lasers; b) The lasing spectra obtained
by tuning the grating periods: 1, 3, and 5 for TrFPy films (thickness of 120 nm) with grating periods of 280, 290, and 300 nm respectively; 2 and 4 for
TrF2Py films (thickness of 108 nm) with grating periods of 280 and 300 nm, respectively.
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2:1) as eluent to afford 4 (516 mg, 55.2%) as a white powder. 1
www.MaterialsViews.com
(
H
NMR (400 MHz, CDCl , δ): 8.29–8.16 (m, 4 H), 8.11 (t, J = 5.0 Hz,
3
2
7
H), 8.07–7.99 (m, 3 H), 7.86 (d, J = 7.7 Hz, 1 H), 7.68–7.58 (m, 3 H),
.54–7.50 (m, 2 H), 2.10–1.93 (m, 4 H), 1.21–1.03 (m, 12 H), 0.84–0.75
1
3
(m, 10 H). C NMR (100 MHz, CDCl , δ): 153.35, 150.53, 140.42,
3
139.93, 139.24, 138.10, 131.56, 131.01, 130.64, 130.11, 129.53, 128.63,
127.64, 127.49, 127.48, 126.29, 126.09, 125.36, 125.27, 125.21, 125.11,
124.98, 124.86, 124.69, 121.21, 121.18, 119.78, 55.60, 40.30, 31.56,
29.70, 23.91, 22.64, 14.08.
Synthesis
tetramethyl-1,3,2-dioxaborolane (5): A flask charged with 4 (400 mg,
.33 mmol), bis(pinacolato)diborane (100 mg, 0.39 mmol), potassium
acetate (121 mg, 1.24 mmol), Pd(dppf) Cl2 (14 mg, 0.01 mmol), and
of
2-(9,9-Dihexyl-7-(pyren-1-yl)-9H-fluoren-2-yl)-4,4,5,5-
0
2
1
5 mL of anhydrous dioxane was degassed for 15 min. After the mixture
was stirred at 95 °C for 12 h, it was cooled to room temperature and
then poured into ice water (100 mL). The mixture was then extracted
with CHCl , and the combined organic layers were dried over anhydrous
3
MgSO . After the solvent was evaporated, the residue was purified by
4
chromatography using silica gel (hexane: CHCl = 2:1) to afford 5 (353
3
1
mg, 81.8%) as a white solid. H NMR (400 MHz, CDCl , δ) : 8.29–8.16
m, 4H), 8.15–8.08 (m, 3H), 8.07–8.00 (m, 2 H), 7.96–7.82 (m, 2 H),
3
Figure 7. The increase of the ASE threshold as a function of the annealing
(
temperature for 90 nm thick films (TrFPy and TrF2Py).
7
1
.84 (d, J = 2.8 Hz, 2 H), 7.66–7.61 (m, 2 H), 2.07 (d, J = 7.3 Hz, 4 H),
.44 (s, 12 H), 1.21–1.07 (m, 12 H), 0.81 (t, J = 6.9 Hz, 10 H). 13C NMR
(
100 MHz, CDCl , δ): 151.49, 150.29, 143.90, 140.40, 140.15, 138.33,
performing on them. The lowest ASE threshold for neat films
3
was found to be 180 nJ pulse − for TrFPy and 101 nJ pulse for
1
−1
133.92, 131.55, 131.02, 130.57, 129.31, 128.96, 128.64, 127.69, 127.49,
127.42, 127.41, 126.06, 125.44, 125.40, 125.14, 125.10, 124.98, 124.82,
124.69, 120.13, 119.20, 83.79, 55.32, 40.26, 31.57, 29.74, 25.01, 23.88,
22.65, 14.11.
TrF2Py. A tunable optical window of 13 nm for TrF2Py (from
4
33to 445 nm) was achieved by controlling the thickness, and
the ASE was even detectable for a thin film of 25 nm. 1D DFB
lasing (50% fill factor, pumped source at 355 nm) demonstrated
Synthesis of 1-(7′-Bromo-9,9,9′,9′-tetrahexyl-9H,9′H-[2,2′-bifluoren]-7-yl)
pyrene (6): A flask charged with 5 (400 mg, 0.60 mmol), 2,7-dibromo-
lasing threshold of 9.3 kW cm− and 7.3 kW cm for TrFPy (at
2
−2
9,9-dihexyl-9H-fluorene (3) (0.88 g, 1.80 mmol), Pd(PPh3)4 (23 mg,
0
.02 mmol ), and K CO3 (20 mg) in toluene/water (2:1, 60 mL) was
2
4
57 nm) and TrF2Py lasers (at 451 nm), respectively. The lasing
degassed for 15 min. After being refluxed for 24 h, it was cooled to
room temperature and then extracted with CHCl3 (25 mL × 3). The
organic layer was washed with water, brine and dried with anhydrous
output wavelength could be fine-tuned by changing the periods
of the gratings. In comparison to a slightly enhanced ASE
threshold for TrFPy, the ASE for TrF2Py was found to be rela-
tively robust against annealing in an ambient condition, which
was less affected by heating up to 160 °C. The result was con-
sistent with an absence of scattering-induced losses. However,
the easier tendency of oxidation for TrF2Py in an ambient con-
dition generated a significantly raised ASE threshold (17.6-fold
relative to the as-spun films) when the annealing temperature
was increased to 200 °C. In contrast, the TrFPy films showed
excellent optical gain stability and the ASE can even be detect-
able after annealing at a very high temperature of 250 °C. These
results suggest the utilization of pyrene as building blocks for
exploring novel organic semiconductor gain media can effec-
tively improve the thermal stability while maintain excellent
lasing properties.
MgSO . The solvent was removed and the residue was purified by silica
4
gel column using hexane/CH Cl (2:1) as eluent to afford 6 (146 mg,
2
2
1
47.3%) as a white powder. H NMR (400 MHz, CDCl3, δ): 8.29–8.17 (m,
4
7
7
0
1
H), 8.14–8.08 (m, 3 H), 8.07–8.00 (m, 2 H), 7.93 (d, J = 8.1 Hz, 1 H),
.89 (d, J = 7.8 Hz, 1 H), 7.77 (d, J = 7.9 Hz, 1 H), 7.72–7.60 (m, 7 H),
.49 (d, J = 8.7 Hz, 2 H), 2.15–1.95 (m, 8 H), 1.12 (d, J = 16.1 Hz, 24 H),
13
.94–0.75 (m, 20 H). C NMR (100 MHz, CDCl , δ): 153.31, 151.93,
3
51.20, 151.18, 141.06, 140.49, 140.25, 139.99, 139.97, 139.90, 139.29,
138.36, 131.59, 131.06, 130.60, 130.05, 129.46, 128.68, 127.72, 127.50,
127.44, 127.42, 126.36, 126.27, 126.07, 125.46, 125.43, 125.16, 125.03,
24.83, 124.71, 121.55, 121.49, 121.12, 121.04, 120.14, 120.07, 119.77,
5.59, 55.42, 40.39, 40.33, 31.56, 31.50, 29.74, 29.67, 24.01, 23.79, 22.62,
2.61, 14.08, 14.05.
General Procedure for Synthesis of TrPy: The monomer 1 (229 mg,
.815 mmol), 8 (200 mg, 0.163 mmol), TBAB (50 mg, 0.155 mmol)
and Pd(PPh3)4 (62 mg) were placed in a 30 mL vessel. A mixture
1
5
2
0
of 2 M K CO3 aqueous solution (5 mL) and THF (15 mL) was added
2
to the vessel. The mixture was then irradiated at 250 W for 30 min in
microwave cavity. After the mixture was cooled to room temperature, it
was extracted with CHCl3 (25 mL × 3). The organic layer was washed
with water, brine, and dried with anhydrous MgSO4. The solvent was
removed and the residue was purified by silica gel column using hexane/
4
. Experimental Section
Materials: All reagents and solvents, unless otherwise specified,
were obtained from Aldrich, Acros, and TCI Chemical Co., and used
as received. All manipulations involving air-sensitive reagents were
performed under an atmosphere of dry nitrogen.
1
CH2Cl2 (2:1) as eluent to afford TrPy (213 mg, yield: 89.6%): H NMR
(400 MHz, CDCl3, δ): 8.60 (d, J = 8.2 Hz, 3 H), 8.40–8.30 (m, 6 H),
8.25–8.00 (m, 21H), 7.80 (s, 3 H), 7.72 (d, J = 8.0 Hz, 3 H), 3.17 (d, J
= 6.8 Hz, 6 H), 2.27 (d, J = 7.2 Hz, 6 H), 1.14–1.00 (m, 36 H), 0.82 (s,
Synthesis of 1-(7-Bromo-9,9-dihexyl-9H-fluoren-2-yl)pyrene (4): A flask
charged
with
4,4,5,5-tetramethyl-2-(pyren-1-yl)-1,3,2-dioxaborolane
1
3
(
(
(
2) (500 mg, 1.52 mmol), 2,7-dibromo-9,9-dihexyl-9H-fluorene (3)
2.25 g, 4.56 mmol), Pd(PPh3)4 (53 mg, 0.04 mmol), and K CO3
20 mg) in toluene/water (2:1, 60 mL) was degassed for 15 min. After
12 H), 0.73 (t, J = 6.5 Hz, 18 H). C NMR (100 MHz, CDCl3, δ): 153.86,
145.50, 139.56, 139.23, 138.41, 138.24, 131.62, 131.11, 130.62, 128.72,
128.61, 127.77, 127.54, 127.50, 127.45, 126.08, 125.53, 125.22, 125.16,
125.09, 124.84, 124.77, 124.74, 124.71, 55.98, 37.06, 31.97, 31.65, 29.74,
29.70, 29.62, 29.40, 24.17, 22.73, 22.44, 14.16, 14.09. MALDI-TOF-MS
2
being refluxed for 24 h, it was cooled to room temperature and then
extracted with CHCl (25 mL × 3). The organic layer was washed with
water, brine and dried with anhydrous MgSO . The solvent was removed
3
+
+
(m/z): calcd for C111H114, exact mass: M 1446.89; Found: 1446.5 (M ),
1361.2 ([M-C6H13] ).
4
+
and the residue was purified by silica gel column using hexane/CH Cl2
2
Adv. Funct. Mater. 2015, 25, 4617–4625
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www.MaterialsViews.com
TrFPy: The monomer
.163 mmol), TBAB (50 mg, 0.155 mmol), and Pd(PPh ) (62 mg) were
placed in a 30 mL vessel. A mixture of 2 M K CO aqueous solution
5 mL) and THF (15 mL) was added to the vessel. The mixture was then
irradiated at 250 W for 30 min in microwave cavity. After the mixture was
cooled to room temperature, it was extracted with CHCl (25 mL × 3).
The organic layer was washed with water, brine, and dried with anhydrous
4
(501 mg, 0.815 mmol),
8
(200 mg,
Fabrication of OLEDs: OLEDs were prepared on commercial glass
slides coated with patterned indium tin oxide (ITO), which were cleaned
in sequential ultrasonic baths using detergent solution, deionized
water, acetone, and isopropanol. A thin layer of poly(styrene sulfonic
acid)-doped-poly(3,4-ethylenedioxy thiophene) (PEDOT:PSS) (40 nm)
was spin-coated onto ITO substrate, which was then treated with O3
for 0.5 h. After baking at 120 °C for 0.5 h in an oven installed inside a
glovebox with argon, a thin layer (80 nm) of TrFPy or TrF2Py were spin-
coated on top of the treated ITO substrate from its toluene solution
0
3 4
2
3
(
3
MgSO . The solvent was removed and the residue was purified by silica
4
gel column using hexane/CH Cl (2:1) as eluent to afford TrFPy (313 mg,
2
2
1
−1
yield: 78.3%). H NMR (400 MHz, CDCl , δ): 8.58 (d, J = 8.4 Hz, 3 H),
(18 mg mL ) at 1000 rpm. At last, a thin layer of LiF (1.2 nm) and Al
3
8
3
(
.35–7.95 (m, 33 H), 7.92–7.84 (m, 12 H), 7.69 (d, J = 6.8 Hz, 6 H),
(80 nm) was deposited by thermal evaporation through a shadow mask
−
6
.21–3.06 (m, 6 H), 2.40–2.07 (m, 18 H), 1.26–1.16 (m, 36 H), 1.07–0.91
in a high vacuum chamber under a pressure of 10 Torr.
1
3
m, 48 H), 0.86 (t, J = 6.7 Hz, 18 H), 0.69 (t, J = 6.9 Hz, 30 H). C NMR
(
100 MHz, CDCl , δ): 173.39, 154.52, 151.95, 151.21, 145.26, 140.30,
3
1
1
1
1
2
40.20, 140.10, 139.93, 139.72, 139.51, 138.40, 138.21, 131.59, 131.06,
30.59, 129.47, 128.68, 127.75, 127.53, 127.46, 126.21, 126.09, 125.57, Supporting Information
25.46, 125.33, 125.15, 125.03, 124.84, 124.73, 121.35, 120.56, 120.21,
Supporting Information is available from the Wiley Online Library or
from the author.
19.79, 55.93, 55.47, 40.54, 37.23, 31.65, 31.62, 30.36, 29.82, 29.75,
9.68, 24.15, 24.05, 22.69, 22.42, 14.13, 14.02. MALDI-TOF-MS (m/z):
+
+
calcd for C186H210, exact mass: M 2443.6; Found: 2443.7 (M ), 2357.9
+
(
M-C H ] ).
6
13
Acknowledgements
TrF2Py: The monomer 6 (772 mg, 0.815 mmol), 8 (200 mg,
.163 mmol), TBAB (50 mg, 0.155 mmol), and Pd(PPh ) (62 mg) were
placed in a 30 mL vessel. A mixture of 2 M K CO aqueous solution
5 mL), and THF (15 mL) was added to the vessel. The mixture was
then irradiated at 250 W for 30 min in microwave cavity. After the
mixture was cooled to room temperature, it was extracted with CHCl3
0
3
4
This study was supported by the National Key Basic Research Program
of China (973 Program, Grant No. 2014CB648300), the National
Natural Science Foundation of China (Grant Nos. 21422402, 20904024,
2
3
(
5
1173081, and 61136003), the Natural Science Foundation of Jiangsu
Province (Grant Nos. BM2012010, BK20140060, and BK20130037),
Program for New Century Excellent Talents in University (NCET-13-
(
25 mL × 3). The organic layer was washed with water, brine, and dried
with anhydrous MgSO . The solvent was removed and the residue was
4
0
872), Specialized Research Fund for the Doctoral Program of Higher
purified by silica gel column using hexane/CH Cl (1:1) as eluent to
2
2
Education (Grant No. 20133223110008), the Ministry of Education
of China (IRT1148), the Synergetic Innovation Center for Organic
Electronics and Information Displays, the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD), the Six
Talent Plan (Grant No. 2012XCL035) and the Qing Lan Project of Jiangsu
Province.
afford TrF2Py (401 mg, yield: 73.1%) (400 MHz, CDCl , δ): 8.57 (d, J
3
=
6
6
7.8 Hz, 3 H), 8.33–8.04 (m, 27 H), 8.01–7.73 (m, 36 H), 7.69 (d, J =
.9 Hz, 6 H), 3.22–3.04 (m, 6 H), 2.33-2.12 (m, 30 H), 1.24–1.18 (m,
1
3
0 H), 1.09–0.81 (m, 108 H), 0.69 (t, J = 6.9 Hz, 30 H). C NMR
(
100 MHz, CDCl , δ): 154.50, 151.92, 151.87, 151.22, 145.25, 140.73,
3
1
1
1
1
5
2
40.57, 140.20, 140.17, 140.12, 140.05, 139.92, 139.68, 139.57, 138.40,
38.21, 131.59, 131.07, 130.59, 129.46, 128.68, 127.74, 127.52, 127.45,
26.28, 126.16, 126.08, 125.46, 125.32, 125.16, 125.03, 124.84, 124.72,
21.58, 121.38, 120.59, 120.15, 120.11, 120.08, 119.76. 55.92, 55.45,
5.43, 40.54, 40.44, 37.22, 31.63, 31.59, 31.55, 29.78, 29.75, 29.66,
4.13, 24.04, 23.94, 22.65, 22.41, 14.12, 14.10, 14.00. MALDI-TOF-MS
Received: April 3, 2015
Revised: May 19, 2015
Published online: June 18, 2015
+
+
(
3
m/z): calcd for C261H306, exact mass: M 3440.4; Found: 3440.8 (M ),
+
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6
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[
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1
13
Shield Plus 400 MHz NMR ( H: 400 MHz, C: 100 MHz). The
molecular weight of intermediates was measured by a Bruker matrix-
assisted laser desorption/ionization time of flight mass spectrometry
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Adv. Funct. Mater. 2015, 25, 4617–4625
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