Monodisperse Six-Armed Starbursts based on Truxene-Cored Multibranched Oligofluorenes: Design, Synthesis, and Stabilized Lasing Characteristics

Article abstract of DOI:10.1002/chem.201805813

A series of monodisperse six-armed conjugated starbursts (Tr1F, Tr2F, and Tr3F) containing a truxene core and multibranched oligofluorene bridges capped with diphenylamine (DPA) units has been designed, synthesized, and investigated as robust gain media for organic semiconductor lasers (OSLs). The influence of electron-rich DPA end groups on their optoelectronic characteristics has been discussed at length. DPA cappers effectively raise HOMO levels of the starbursts, thus enhancing the hole injection and transport ability. Solution-processed electroluminescence devices based on the resulting six-armed starbursts exhibited efficient deep-blue electroluminescence with clear reduced turn-on voltages (3.2–3.5 V). Moreover, the resulting six-armed molecules showed stabilized electroluminescence and amplified spontaneous emission with low thresholds (27.4–63.9 nJ pulse^?1), high net gain coefficients (80.1–101.3 cm^?1), and small optical loss (2.6–4.4 cm^?1). Distributed feedback OSLs made from Tr3F exhibited a low lasing threshold of 0.31 kW cm^?2 (at 465 nm). The results suggest that the construction of truxene-centered six-armed conjugated starbursts with the incorporation of DPA units can effectively enhance EL properties by precisely regulating the HOMO energy levels, and further optimizing their optical gain properties.

Full text of DOI:10.1002/chem.201805813

A Journal of
Accepted Article
Title: Monodisperse Six-Armed Starbursts based on Truxene-Cored
Multibranched Oligofluorenes: Design, Synthesis, and Stabilized
Lasing Characteristics
Authors: Hao Zhang, Xu Liu, Ting-Ting Lu, Peng Lv, and Wen-Yong
Lai
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DOI: 10.1002/ ((please add manuscript number))
Article type: (Full Paper)
Monodisperse Six-Armed Starbursts based on Truxene-Cored Multibranched
Oligofluorenes: Design, Synthesis, and Stabilized Lasing Characteristics
Hao Zhang,† Xu Liu,† Ting-Ting Lu, Peng Lv, and Wen-Yong Lai*
–––––––––
H. Zhang, Dr. X.Liu, T.-T. Lu, P. Lv, and Prof. W.-Y. Lai
Key Laboratory for Organic Electronics and Information Displays (KLOEID) & Institute of
Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials
(SICAM), Nanjing University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing
210023, China
E-mail: iamwylai@njupt.edu.cn
†These authors contributed equally to this work.
–––––––––
1
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ABSTRACT: A series of monodisperse six-armed conjugated starbursts (Tr1F, Tr2F and Tr3F)
containing a truxene core and multibranched oligofluorene bridges capped with diphenylamine
(DPA) units has been designed, synthesized, and investigated as robust gain media for organic
semiconductor lasers (OSLs). The influence of electron-rich DPA end-groups on their
optoelectronic characteristics has been discussed at length. DPA cappers effectively raise HOMO
levels of the starbursts, thus enhancing the hole injection and transport ability. Solution-processed
electroluminescence devices based on the resulting six-armed starbursts exhibited efficient deep-
blue electroluminescence with clear reduced turn-on voltages (3.2~3.5 V). Moreover, the resulting
six-armed molecules showed stabilized electroluminescence and amplified spontaneous emission
with low thresholds (27.4~63.9 nJ pulse-1), high net gain coefficients (80.1~101.3 cm-1), and small
optical loss (2.6~4.4 cm-1). Distributed feedback OSLs made from Tr3F exhibited low lasing
threshold of 0.31 kW/cm2 (at 465 nm). The results suggest that the construction of truxene-centered
six-armed conjugated starbursts with the incorporation of DPA units can effectively enhance EL
properties by precisely regulating the HOMO energy levels, and further optimizing their optical
gain properties.
Keywords: Organic semiconductor lasers (OSLs); Organic gain media; Conjugated starburst
molecules; HOMO levels; Organic light-emitting diodes (OLEDs)
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1. Introduction
Organic light-emitting devices based on organic -conjugated semiconductors are receiving
increasing attention due to their great promising application in the fundamental field of biological
probes, chemical sensors and especially organic light-emitting diodes (OLED)[1-4] and organic
semiconductor lasers (OSLs).[5-8] Over the past decade, great progress made in novel molecular
designs of -conjugated semiconductors, such as thermally activated delayed fluorescence (TADF)
materials,[2] aggregation-induced emission (AIE) materials,[3] and room-temperature
phosphorescence (RTP) materials,[4] have made an important contribution to the advancement of
organic electronics. Among them, OSLs are particularly challenging.[9] Especially, electrically
pumped OSLs, which ask organic gain materials with good emission efficiency, strong optical gain,
low lasing thresholds, and high mobilities, have not been realized yet.[10-12] Although many organic
emitters show great promise in OLEDs, only a few organic materials can afford low thresholds and
high optical gain, which are essential for OSLs. Consequently, it is highly required to develop
prominent gain media with superior optical gain and lasing characteristics for OSLs.
As far as we are concerned, the most widely employed materials for OSLs mainly include π-
conjugated materials.[13-15] Organic materials containing fluorene units have served as attractive
candidates as organic optical gain materials due to the high photoluminescence quantum yields
(PLQYs), good film-forming abilities, and facile tunability of optical and electrical characteristics
by substitution or copolymerization.[8] However, these materials tend to form aggregates/excimers,
which are detrimental to luminescence efficiency, spectral stability and corresponding electrical
characteristics.[16-18] To tackle the problems mentioned above, various strategies on incorporating
bulky moieties with steric hindrance, constructing starburst architectures, or carefully adjusting the
polymer side chain structures have been proposed.[19-24] Despite great development in this aspect, it
still has a long way to go for achieving electrically pumped OSLs. The design and synthesis of
prominent molecules possessing superior thermal stability and optoelectronic characteristics for
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OSLs is worthy of attention and effort. Investigation on the relationships between the structures and
optoelectronic characteristics is of great importance to provide a deep insight on developing
reasonable design strategies for organic gain media with high performance.
According to our previous studies, a variety of monodisperse π-conjugated starbursts with
multifarious cores such as triazatruxene,[25-29] truxene,[30-32] and pyrene,[33-37] have been explored.
Such starburst compounds, which combine both the merits of small molecules and polymers, appear
rather promising as active materials for OLEDs and OSLs owing to their high chemical purity,
excellent stability, facile processibility via solution-based methods, good film-forming ability,
promising lasing properties. Among them, truxene-centered multiarmed starburst derivatives are
rather attractive as robust organic emitters.[38,39] A set of monodisperse six-armed starburst
molecules composed of truxene core and oligofluorene arms (Tr1–Tr4)[38] exhibited not only stable
pure-deep-blue emission, but also excellent lasing characteristics with ultralow lasing threshold
laser
(Eth
≈ 0.4 nJ/pulse, 1.3 J cm-2) due to depressed aggregation tendency. Nevertheless, their
electrical behaviors remain to be further improved to meet the challenges for electrically pumped
OSLs, which require high charge carrier mobility for high current injection and transport.
Especially, the energy level mismatch between HOMO levels of Tr1-Tr4 (-5.66 ~ -5.74 eV) and
the work function of the common anodes (i.e. ITO: 4.5~4.8 eV) generally causes weak carrier
injection and transport, which restricts their practical applications in OLEDs and electrically
pumped OSLs.
In this contribution, on continuing our further efforts, a novel series of monodisperse six-armed
starburst conjugated molecules (Tr1F, Tr2F and Tr3F) based on truxene core with six
oligofluorenes as bridge arms end-capped with diphenylamine (DPA) units have been designed,
synthesized, and investigated, aiming at developing optimal gain materials possessing promising
electrical characteristics to overcome obstacles of OSLs. DPA units as the electron-donating
arylamine moieties were incorporated into the six-armed starburst molecular structures because of
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their strong electron-donating characteristics, which are beneficial for raising the HOMO levels and
precisely modulating main electrical behaviors of the resulting molecules.[40-46] For instance, Neher
et al. achieved blue OLEDs with high efficiency and excellent color stability by end-capping of
polyfluorene homopolymer with DPA.[47] Oligofluorenes were incorporated with the aim to increase
PLQYs and film-forming characteristics.[48,49] The effect of the structural modification and the
incorporation of DPA moieties on their physical and optoelectronic characteristics, especially lasing
characteristics have been systematically explored. Solution-processed OLEDs based on these
starbursts showed relatively low turn-on voltages (3.2~3.5 V) and bright emission (1974~3478 cd
m-2). Ultra-low amplified spontaneous emission (ASE) thresholds (27.4~63.9 nJ pulse-1), high net
gain coefficients (80.1~101.3 cm-1), outstanding ASE stability upon temperature, and low loss
properties (2.6~4.4 cm-1) were achieved. It is indicated that the design strategy with incorporating
DPA end-groups is advantageous for enhancing the efficiency by precisely adjusting the HOMO
levels as well as optimizing the gain characteristics of the resulting molecules. Combining the large
gain and small waveguide loss with outstanding stability, the resulting six-armed starburst
conjugated molecules manifest great potentials as robust gain media for OSLs.
2. Experimental
The synthetic routes of DPA-capped six-armed truxene derivatives, Tr1F, Tr2F, and Tr3F, are
depicted in Scheme 1. The synthesis of the target starbursts adopted a stepwise convergent synthetic
strategy, including microwave-assisted Pd-catalytic multiple Suzuki coupling reaction of DPA-
functionalized oligofluorene arms with the 2,3,7,8,12,13-hexaaryltruxene core structure. The Suzuki
cross-coupling reaction assisted by microwave irradiation ((30 min at 150 °C) gave the resulting
materials in 40%–55% yields. The chemical purity and structures of Tr1F, Tr2F, and Tr3F were
confirmed by MALDI-TOF mass spectrometry, 1H NMR and 13C NMR data (see Figure S19−S27 in
the Supporting Information). Details on synthetic processes and measurements can be found in
Supporting Information.
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Scheme 1. Synthetic routes of Tr1F, Tr2F, and Tr3F.
3. Results and discussion
3.1 Thermal and morphological properties
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) experiments
were performed to investigate the thermal stabilities of Tr1F, Tr2F, and Tr3F. High Td (5% weight
loss temperatures) over 410oC for the resulting starbursts (Figure 1) indicated excellent thermal
stability, which guaranteed their good thermal stabilities for practical applications. Furthermore,
DSC curves suggested glassy morphologies of Tr1F, Tr2F, and Tr3F. With an increment of the
oligofluorene length, a slight increase in glass-transition temperature (Tg) (85°C for Tr1F, 90°C for
Tr2F, 93°C for Tr3F) (inset of Figure 1) was observed. The Tg results are higher in comparison to
those of their corresponding analogues Tr1–Tr3 with no DPA end-groups (Tg = 49-85°C).[38,39]
According to wide-angle X-ray diffraction (WAXD) curves (Figure S28), broad amorphous halos
with low intensity at around 2 = 22o further confirmed the amorphous glassy morphology of the
resulting starbursts. According to these results, DPA moieties are advantageous in limiting the
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crystallization process and producing amorphous films, thus increasing the thermal stability and
morphological stability of the films.
Tr1F-TGA
Tr2F-TGA
Tr3F-TGA
100
Tg=93 o
C
80
60
40
-1
-2
-3
Tg=90 o
C
Tg=85 o
C
50
100
150
Temperature
(
oC
)
0
100 200 300 400 500 600 700
Temperature
oC
(
)
Figure 1. TGA thermograms of Tr1F, Tr2F, to Tr3F. Inset: DSC traces.
Atomic force microscopy (AFM) of Tr1F, Tr2F and Tr3F was conducted to investigate the
film-forming ability (Figure 2a-c) on quartz substrates. With the increase of the oligofluorene arm
length, the film-forming ability was effectively improved, which was confirmed by the surface
morphology of pristine Tr1F, Tr2F and Tr3F films. A bit rough film was observed for Tr1F film
with a root mean-square (RMS) roughness of 1.281 nm, while Tr2F and Tr3F gave smooth films
with RMS roughness of 0.251 and 0.228 nm, respectively.
Figure 2. AFM images of a) Tr1F, b) Tr2F and c) Tr3F. Scan size is 2 μm×2 μm and scan rate is 1
Hz at room temperature in N2 atmosphere.
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Tr1F-UV
Tr2F-UV
Tr3F-UV
Tr1F-PL
Tr2F-PL
Tr3F-PL
(b)
(a)
Tr1F-UV
Tr2F-UV
Tr3F-UV
Tr1F-PL
Tr2F-PL
Tr3F-PL
1.0
0.8
0.6
0.4
0.2
0.0
1.0
1.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.8
0.6
0.4
0.2
0.6
0.4
0.2
0.0
0.0
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Figure 3. Absorption and PL spectra of Tr1F, Tr2F, Tr3F in (a) THF and (b) film.
Table 1. Photophysical and electrochemical data of Tr1F–Tr3F.
d)
λabs (nm)
THFa)
λem (nm)
THF
τ (ns)c)
ФPL
HOMOe)/LUMOe)(eV)
Filmb)
Film
438
THF
Film
THF Film
Film
Compd
Tr1F
300,362
303,373
304,377
304,367
435
1.50 0.58(90%) 0.63
3.23(10%)
0.56
-5.17/-2.03
-5.20/-2.08
-5.22/-2.14
309,378
311,381
438
442
427,445
430,450
1.13 0.38(85%) 0.85
2.76(15%)
0.82
0.89
Tr2F
Tr3F
1.03 0.45(88%) 0.93
2.63(12%)
a) Measured in THF with a concentration of 10-6 M; b) Measured in solid states; c) PL maxima as the
d)
excitation wavelength; Fluorescence quantum yields in films. e) Based on the onset of oxidation
and reduction potentials.
3.2 Photophysical characteristics
The absorption and photoluminescence (PL) spectra of Tr1F, Tr2F, and Tr3F in both
tetrahydrofuran (THF) solution (10-6 M) and solid films were recorded (Figure 3, Table 1). These
starbursts exhibited bathochromic absorption and PL spectra relative to their analogues Tr1–Tr3
without DPA end-groups.[38] In films, the absorption spectra of Tr1F–Tr3F (Figure 3b) remain
similar to those in dilute solutions (Figure 3a). The normalized UV–Vis absorption showed two
distinctive peaks at 300 and 362 nm for Tr1F, 303 and 373 nm for Tr2F, and 304 and 377 nm for
Tr3F, respectively. The peaks of absorption spectra located in the range of 362~377 nm mostly
result from π-π* transitions of DPA end-capped arms. Under this circumstance, the absorption peaks
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are gradually red-shifted by extending the length of oligofluorene arm. Additionally, the maxima
absorption spectra of each molecule in films exhibit a slight red-shift (4~5 nm) as compared with
those in dilute solutions. According to the similar absorption behaviors, the starbursts demonstrate
similar ground-state electronic structures, with the absence of large aggregates or conformational
changes in the solid-state films. The bulky six-armed starburst architectures with strong steric
hindrance are believed to play important roles on effectively inhibiting the intermolecular close
packing. The results follow similar trends to the previous studies on truxene derivatives[38,39] and
DPA dendrimers.[47]
PL spectral shapes of Tr1F–Tr3F in thin films (Figure. 3b) are different from those in dilute
solutions (Figure. 3a). The solution PL emission bands of these starburst molecules show major
peaks in the range of 435~442 nm. Tr1F showed one main emission peak with a very slightly red-
shifted PL band peaked at 438 nm in films, which suggested no obvious intermolecular interactions
and intramolecular ordering in films in this system. For Tr2F and Tr3F in solid films, the major
emission peaks located at λem = 445 and 450 nm, respectively, with a shoulder emission at 427 nm.
The six-armed bulky architectures likely aid to maintain an isolated-molecule emission induced by
increasing the oligofluorene arm length.[50,51] This is significative for high luminescence efficiency
because intermolecular interactions induced by π-π stacking in most conjugated molecules generally
depress the luminescence in condensed states. On the basis of the results, the luminescence
properties of the resulting starbursts just slightly rely on the arm length. Nevertheless, it appears as
a perfect fluorene-based platform for further exploring the influence of structures on the film
morphology and resulting optoelectronic characteristics.
Upon excitation at their absorption maxima, Tr1F, Tr2F, and Tr3F manifested high PLQYs of
56%, 82%, and 89% even in films, quite close to those in solutions of 63%, 85%, and 93%,
respectively. The results indicate that the bulky six-armed starburst architecture effectively
suppresses the intermolecular interactions in solid states. PLQYs for Tr2F and Tr3F became quite
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close, indicating that the effect of the oligofluorene chain on depressing the intramolecular
interactions will be passivated with increasing the arm length. This tendency was further confirmed
by transient PL characterization in films. According to the PL decay transients of dilute solutions
and solid states (Figure S29), the PL decay transients in dilute solutions showed single exponential
decays (estimated fluorescence lifetimes (τ) of 1.50 ns, 1.13 ns and 1.03 ns for Tr1F, Tr2F, and
Tr3F, respectively), while the PL decay transients in thin films manifested bi-exponential decay
profiles, demonstrating τ1 = 0.58 ns (90.15%), τ2 = 3.23 ns (9.85%) for Tr1F, τ1 = 0.38 ns (85.15%),
τ2 = 2.76 ns (14.85%) for Tr2F, and τ1 = 0.45 ns (88.03%), τ2 = 2.63 ns (11.97%) for Tr3F,
respectively (Table 1).
3.3 Electrochemical characteristics
Cyclic voltammetry (CV) was conducted with the aim to study the electrochemical behaviors
of Tr1F, Tr2F, and Tr3F. The results are depicted in Figure 4 and listed in Table 1 and Table S1.
The onset oxidation potentials (Eox) of Tr1F, Tr2F, Tr3F are 0.45, 0.48, and 0.50 eV, respectively.
HOMO energy levels of Tr1F, Tr2F, and Tr3F are thus determined as -5.17, -5.20, and -5.22 eV,
respectively, which are 0.49~0.52 eV higher than those of their analogues without DPA end-cappers
(Tr1: -5.66 eV, Tr2: -5.70 eV, Tr3: -5.74 eV),[38] respectively. At the same time, the onset reduction
pontentials of Tr1F, Tr2F, and Tr3F were measured as -2.69, -2.64, and -2.58 eV, respectively. The
corresponding LUMO levels of Tr1F, Tr2F, and Tr3F are calculated to be -2.05, -2.07, and -2.14
eV based on ELUMO = - [Ered + 4.72] eV. The band gaps (Eg) of Tr1F, Tr2F, and Tr3F were
estimated to be 3.14, 3.12, and 3.08 eV (Table S1), according to Eg = Eox - Ered,. The tendency
matches well with their optical band-gaps (Egopt: 2.97, 2.86, and 2.85 eV for Tr1F, Tr2F, and Tr3F,
respectively), which were obtained on the basis of the edges of the longest absorption wavelength of
films. The results indicate that incorporating DPA end-groups can greatly raise HOMO energy
levels, further adjusting the electrical characteristics to enhance the charge injection and transport
properties.
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0
200
0
Tr1F-OX
-200
-400
0
Tr1F-RED
Tr2F-RED
-200
400
200
0
Tr2F-OX
Tr3F-OX
-200
-400
-200
100
50
0
0
-200
-400
Tr3F-RED
-3
-2
-1
0
1
Potential V vs. Ag/Ag+
Figure 4. CV curves of Tr1F, Tr2F, and Tr3F in acetonitrile for oxidation and reduction.
3.4 Electroluminescence (EL) properties
With the aim to further study the EL characteristics of the six-armed conjugated starbursts, EL
devices were manufactured with the construction of indium tin oxide (ITO)/PEDOT:PSS/Emissive
layer (EML)/TPBI/LiF/Al. EML was spin-coated from chlorobenzene solutions (A: Tr1F; B: Tr2F;
C: Tr3F). PEDOT:PSS and LiF was used as the hole-injection and electron-injection layer,
respectively. As shown in Figure S30a-c, deep-blue emission was observed for Tr1F, Tr2F, and
Tr3F at 437, 450, and 453 nm, respectively. As presented, the EL spectra maintained almost the
same with their PL spectra in both solutions and solid states, implying that exciplex or excimer was
greatly suppressed. Remarkably, the EL spectra maintained almost the same and the CIE
coordinates were almost not varied with the increment of the driving voltages from 5 to 9 V.
According to the results, EL emission is independent on the voltages, which probably results from
the steric effect of the starbursts effectively depressing the molecular packing in films.
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The current density−voltage−brightness (J−V−L) characteristics, luminance efficiency-voltage
(LE−V), and external quantum efficiency (EQE) vs. current density curves of the devices are
depicted in Figure S30d-f. OLEDs based on Tr1F, Tr2F, and Tr3F exhibited reduced turn-on
voltages (3.2~3.5 V), which were much lower than those of their analogues without DPA end-
groups (4.1~4.3 V for Tr1-Tr3).[38] This is mainly attributed to the incorporation of DPA end-groups
that aid to raise the HOMO levels (from -5.66 eV ~ -5.74 eV for Tr1-Tr3 to -5.17 ~ -5.22 eV for
Tr1F-Tr3F), which are consistent with the anode work function (PEDOT:PSS/ITO, ~-5.2 eV) and
thus enhance the hole-injection ability and charge balance. Superior device performance were
obtained. Device A demonstrated good performance with a maximum luminance (Lmax) of 1974 cd
m-2 (9.0 V), a maximum luminance efficiency (LEmax) of 2.28 cd A−1, and a maximum external
quantum efficiency (EQEmax) of 2.00% (9.9 V, 393.4 mA cm-2, 1926.1 cd m-2). Improved
performance was obtained from device B [Lmax: 2675 cd m-2 (11.5 V), LEmax: 2.61 cd A-1, and
EQEmax: 2.24% (11.3 V, 350.1 mA cm-2, 2665.2 cd m-2)] and device C [Lmax: 3478 cd m-2 (9.4 V),
LEmax: 2.12 cd A-1, and EQEmax: 1.85% (12.5 V, 760.1 mA cm-2, 2534.1 cd m-2)]. Apparently, the
devices based on Tr1F-Tr3F achieved much better EL performance than that of Tr1-Tr3 without
DPA end-groups.[38] Besides, the devices based on Tr1F-Tr3F gave better performance than that of
polyfluorene homopolymers end-capped with DPA units (ca. 1.1 cd/A at 1600 cd/m2).[47] Our results
indicated that construction of DPA end-capped starbursts is beneficial to enhancing the electrical
and luminous characteristics.
Table 2. EL characteristics of OLEDs from Tr1F-Tr3F.
Device
Compd
Von
(V)
3.4
3.2
3.5
Luminance a)
(cd m-2)
1974
LE a)
(cd A-1)
2.28
EQE a)
(%)
CIE (x, y)
at 6 V
2.00
(0.16, 0.08)
(0.16, 0.11)
(0.17, 0.15)
A
B
C
Tr1F
Tr2F
Tr3F
2675
2.61
2.24
3478
2.12
1.85
a) Maximum values recorded from the devices.
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The superior EL properties of these devices could result from reasons as follows: (1) the
introduction of DPA are beneficial to raise HOMO energy levels; (2) the raised HOMO levels are in
good agreement with the anode work function and thus promote the charge injection and transport;
(3) the steric effect of starbursts can restrain the intermolecular interactions that aid to improve PL
and EL ability; (4) the superior EL performance for device B based on Tr2F may originate from its
proper molecular sizes and suitable HOMO and LUMO levels.
3.5 ASE and lasing characteristics
ASE spectra for Tr1F-Tr3F in films are depicted in Figure S31 in comparison with their
absorption and PL spectra. With high excitation, the ASE peaks from Tr1F, Tr2F, and Tr3F are
located at around 434, 443, and 450 nm, respectively. As far as we are concerned, the peaks
correspond to the 0–1 vibrations, manifasting a quasi-four-level system for the film samples of
Tr2F and Tr3F, which is beneficial to decreasing the ASE thresholds. The output intensity of edge
ASE
emission is plotted as a function of the pump energy density (Figure 5a). The lowest Eth
were
recorded accordingly as 63.9 nJ pulse-1 (3.5 µJ cm-2 @ 498 nm) for Tr1F, 45.7 nJ pulse-1 (2.5 µJ
cm-2 @ 489 nm) for Tr2F, and 27.4 nJ pulse-1 (1.5 µJ cm-2 @ 479 nm) for Tr3F, respectively. The
results indicate that increasing the arm length aids to decrease the ASE thresholds.[37] Remarkably,
Tr3F shows high fluorescence quantum yields, excellent morphology stability, and low ASE
thresholds, which are comparable to the state-of-art blue organic lasers reported so far.[51, 52] Based
on the results, we speculate that the construction of starburst structures can effectively decrease the
energy loss caused by molecular vibration and promote higher radiation transition efficiency, thus
give superior ASE performance.
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4
3
2
1
0
(a)
(b)
Tr1F
Tr2F
Tr3F
Tr1F
Tr2F
Tr3F
Tr1F 25 0C
Tr1F 200 0C
Tr2F 25 0C
Tr2F 220 0C
Tr3F 25 0C
Tr3F 260 0C
1.0
0.8
0.6
0.4
0.2
0.0
250
200
150
100
50
360 380 400 420 440 460 480
Wavelength (nm)
0
2.8 5.6 8.3 11.1 13.9 16.7
Pump Energy Density (J/cm2
0
50
100
150
200
oC
250
)
Annealing Temperature
(
)
Figure 5. (a) The curves of ASE thresholds (r. t.) vs. pump energy density. (b) The curves of ASE
thresholds vs. annealing temperature. The inset shows normalized ASE spectra at r. t. (solid line)
and various annealing temperature (dashed line) under open ambient conditions.
The roadmap towards electrically pumped OSLs requires good thermal and optical stability of
gain media. TGA and DSC measuremrnts confirmed good thermal stability of these starbursts. For
the purpose of further evaluating the photostability, the impact of annealing temperature on the ASE
thresholds were studied (Figure 5b). During the test, the films were heated at different temperatures
for 10 min under open ambient conditions. For each measurement, the excitation stripe was put to a
different place on the specific sample for the purpose of eliminating the degradation of
ASE
photodegradable molecules during the test. Most noticeably of all, Eth
of Tr1F was enhanced
twice compared with original samples by increasing room temperature to 120°C, while the
annealing temperatures for Tr2F and Tr3F is up to 160°C. At the annealing temperature of 200°C,
ASE
50.3-fold and 10.1-fold of Eth
were recorded for Tr1F and Tr2F, respectively, whereas only 5-
fold was observed for Tr3F. It is worthwhile to mention that ASE of Tr3F can be detected even by
heating at 260°C. The ASE spectra showed quite small shifts (around 4 nm) with the annealing
temperature of 220°C and 260°C for Tr2F and Tr3F. Meanwhile, a small shift was also obversed
for Tr1F below 200°C, as depicted in the inset of Figure 6b. It is expected that the outstanding ASE
thermal stability is beneficial for enhancing the optical gain stability to realize electrically pumped
lasing.
14
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10.1002/chem.201805813
Chemistry - A European Journal
100
80
60
Tr1F
Tr2F
Tr3F
40
0
20
40
60
80
100 120
J/cm2
Pump Energy Density
(

)
Figure 6. The plot of net gain coefficients (g) as a function of different pumping intensity for
Tr1F–Tr3F.
The variable stripe length (VSL) method is applied to study the optical gain properties. ASE
emission intensity as a function of the stripe length with various pump intensities is plotted as
shown in Figure S32. At the stripe lengths <2 mm, by increasing the pump energy intensity, the
output intensity exponentially increased, but then approached gain saturation at longer stripe lengths.
To verify if the saturation resulted from the degradation, the characterization were carried out at the
sidentical location repeatedly, and the results still remained. Before the gain grows saturated, by
fitting the yielded results can obtain solid lines (Figure 6). The net gain was progressively enhanced
by increasing the pump energy intensity; the light travelling in devices became magnified. With the
emergence of gain saturation, no additional magnified light was obtained with the increase of the
stripe length while the gain coefficient decreased. According to the data, the maxima net gain (g)
coefficients are calculated as 80.1±2, 94.1±2, and 101.3±2 cm-1, with pump energy of 49.5, 72.5,
and 52.5 µJ cm-2, for Tr1F, Tr2F, and Tr3F, respectively.
15
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10.1002/chem.201805813
Chemistry - A European Journal
104
103
102
105
104
103
Tr1F
Epump= 16.5

J/cm2

=4.4
0.1 cm-1
±
Tr2F
Tr3F
Epump= 14.5

J/cm2

=3.5
0.1 cm-1
±
105
104
103
Epump= 10.5

J/cm2

=2.6
±
0.1 cm-1
0.20
0.00 0.05
0.10
0.15
0.25
Position (cm)
Figure 7. ASE loss coefficients of the waveguides for Tr1F, Tr2F, and Tr3F, respectively.
To gain deeper understanding about the optical gain characteristics of Tr1F, Tr2F, and Tr3F,
the loss coefficients were calculated. Figure 7 shows the plot of loss coefficient versus stripe
position. Low loss coefficients as 4.4±0.1, 3.5±0.1, and 2.6±0.1 cm-1 were recorded for Tr1F, Tr2F,
and Tr3F, respectively. Waveguide losses are attributed to absorption behaviors and scattering
processes at the interfaces induced by surface roughness and large aggreagates in the samples.[50]
On the basis of the small loss coefficients, it is inferred that the resulting six-armed starbursts
possess good waveguide properties and film-forming abilities. The low loss values and high gain
render these materials rather promising as optical gain media for OSLs.
(b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
5
4
3
2
1
0
2 3
4
1
6
5
7
448 nm
450 nm
454 nm
460 nm
465 nm
474 nm
480 nm
465 nm
0.31 kW/cm2
0.2
0.4
0.8
1.6
440
450
460
470
480
490
KW/cm2
)
Wavelength (nm)
Pump Intensity
(
16
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10.1002/chem.201805813
Chemistry - A European Journal
Figure 8. (a) Output intensity of Tr3F laser vs. pump energy. The detailed structure of the
corresponding device is presented in the inset. (b) DFB lasing spectra of Tr3F possessing various
film thickness and grating periods: (1) 160 nm with  = 280 nm for λ = 448 nm. (2) 180 nm with 
= 280 nm for λ = 450 nm. (3) 200 nm with  = 280 nm for λ = 454 nm. (4) 160 nm with  = 290
nm for λ = 460 nm. (5) 180 nm with  = 290 nm for λ = 465 nm. (6) 160 nm with  = 300 nm for λ
= 474 nm. (7) 180 nm with = 300 nm for λ = 480 nm.
Tr3F was chosen as a typical material to study the lasing behaviors because of its smooth film
morphologies and excellent ASE properties. One-dimensional distributed-feedback (DFB) gratings
were prepared via nanoimprint lithography (NIL) methods. Second order gratings with 50% fill
factor and 50 nm depth consisted of lines with periods of 280, 290, and 300 nm, respectively. As
shown in Figure 8a, there emerged a sudden variation in the slope of the output intensity for Tr3F
laser with the increment of excitation, which was recorded as the threshold. The minimum lasing
threshold for Tr3F was 0.31 kW cm-2 (3.7 nJ pulse-1) at 465 nm with a grating period of 290 nm.
Moreover, FWHM of narrow lasing peaks were about 0.2 nm in DFB resonators. Generally, the
lasing peaks strongly depend on the gratings and thickness of films. As depicted in Figure 8b, by
simply modulating various gain layer thickness (160~200 nm) and grating periods (280, 290, 300
nm), the lasing spectra of Tr3F were adjustable ranging from 448 nm to 480 nm across a spectral
window of 32 nm.
4. Conclusions
To summarize, a novel set of monodisperse six-armed starbursts based on multibranched
oligofluorenes with a truxene core and six DPA end-groups (Tr1F, Tr2F, and Tr3F) were designed
and synthesized. As revealed by TGA, DSC, WAXD, optoelectronic, ASE and lasing
characterizations, detailed studies were performed to provide a deep insight into the relationships
between the electron-donating DPA end-groups and their corresponding optoelectronic properties.
The resulting monodisperse six-armed starbursts manifested excellent thermal stabilities, facile
17
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10.1002/chem.201805813
Chemistry - A European Journal
solution processability and pronounced PL efficiencies. The incorporation of peripheral DPA
groups helps to increase the HOMO levels (0.49~0.52 eV higher than those of their analogues
without DPA end-groups) and thereby enhance their hole-injection and transport. Noticeably, their
raised HOMO energy levels consist with the anode work function, which thus facilitates the
subsequent charge injection and transport. Consequently, relatively low turn-on voltages (3.2~3.5
V) and high luminescence (1974~3478 cd m-2) were achieved for the devices. By means of
increasing the conjugation length of the six-armed starbursts, the ASE threshold decreases
accordingly. Ultra-low ASE threshold about 27.4~63.9 nJ pulse-1, high net gain coefficients around
80.1~101.3 cm-1, outstanding ASE stability upon temperature, and low loss coefficients
approximately 2.6~4.4 cm-1 were recorded. Especially for Tr3F with the longest conjugation length,
the films exhibited low EthASE of 27.4 nJ pulse-1 (2.1 µJ cm-2 @ 479 nm), high net gain (101.3 cm-1),
low loss (2.6 ± 0.1) cm-1, and excellent optical stability. Moreover, DFB lasers based on Tr3F
exhibited lasing threshold of 0.31 kW cm-2 (3.7 nJ pulse-1) at 465 nm in the case of a grating period
of 290 nm. The results indicated that construction of DPA end-capped starburst oligofluorenes can
effectively boost the electrical behaviors by raising the HOMO levels, and meanwhile, obviously
improve their optical gain characteristics, endowing this kind of materials rather promising for
attempting electrically pumped OSLs.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
We thank the National Natural Science Foundation of China (21835003, 21674050, 91833304,
21422402, 20904024), the National Key Basic Research Program of China (973 Program,
2014CB648300, 2017YFB0404501), the Natural Science Foundation of Jiangsu Province
(BK20140060,
BM2012010), Program for
Jiangsu
Specially-Appointed
Professor
(RK030STP15001), the Six Talent Peaks Project of Jiangsu Province (TD-XCL-009), the 333
18
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10.1002/chem.201805813
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Project of Jiangsu Province (BRA2017402), the NUPT "1311 Project" and Scientific Foundation
(NY218164, NY217169, NY215062, NY217087), the Leading Talent of Technological Innovation
of National Ten-Thousands Talents Program of China, the Excellent Scientific and Technological
Innovative Teams of Jiangsu Higher Education Institutions (TJ217038), the Synergetic Innovation
Center for Organic Electronics and Information Displays, and the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD) for financial support.
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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Chemistry - A European Journal
Monodisperse six-armed starburst conjugated molecules with diphenylamine (DPA) end-
cappers are synthesized, and their optical gain and lasing characteristics have been
investigated. The incorporation of DPA end-groups effectively raises the HOMO levels thus
effectively enhancing the hole injection and transport ability. The resulting starbursts manifested
stabilized optical gain and lasing performance with low thresholds (27.4~63.9 nJ pulse-1), high net
gain coefficients (80.1~101.3 cm-1) and small waveguide loss (2.6~4.4 cm-1).
Organic Semicondutor lasers
H. Zhang, X. Liu, T.-T. Lu, P. Lv, and W.-Y. Lai*
Monodisperse Six-Armed Starbursts based on Truxene-Cored Multibranched Oligofluorenes:
Design, Synthesis, and Stabilized Lasing Characteristics
1.0
UV
PL
ASE
0.8
Lasing
0.6
0.4
0.2
0.0
300
400
500
600
Wavelength (nm)
23
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