Towards Monodisperse Star-Shaped Ladder-Type Conjugated Systems: Design, Synthesis, Stabilized Blue Electroluminescence, and Amplified Spontaneous Emission

Article abstract of DOI:10.1002/chem.201605885

A novel series of monodisperse star-shaped ladder-type oligo(p-phenylene)s, named as TrL-n (n=1–3), have been explored. Their thermal and electrochemical properties, fluorescence transients, photoluminescence quantum yields, density functional theory calc

Full text of DOI:10.1002/chem.201605885

A Journal of
Accepted Article
Title: Towards Monodisperse Star-Shaped Ladder-Type Conjugated
Systems: Design, Synthesis, Stabilized Blue Electroluminescence
and Amplified Spontaneous Emission
Authors: Wen-Yong Lai, Yi Jiang, Mei Fang, Si-Ju Chang, Jin-Jin
Huang, Shuang-Quan Chu, Shan-Ming Hu, Cheng-Fang Liu,
and Wei Huang
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.201605885
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Towards Monodisperse Star-Shaped Ladder-Type Conjugated
Systems: Design, Synthesis, Stabilized Blue Electroluminescence
and Amplified Spontaneous Emission
[
a]
[a]
[a]
[a]
[a]
[a]
Yi Jiang, Mei Fang, Si-Ju Chang, Jin-Jin Huang, Shuang-Quan Chu, Shan-Ming Hu, Cheng-
[a]
[a,b]
[a,b]
Fang Liu, Wen-Yong Lai,* Wei Huang
Abstract: A novel series of monodisperse star-shaped ladder-type
oligo(p-phenylene)s, named as TrL-n (n = 1-3), have been explored.
Their thermal and electrochemical properties, fluorescence
transients, photoluminescence quantum yields, density functional
theory calculations, electroluminescence (EL) and amplified
spontaneous emission (ASE) properties have been systematically
investigated to unravel the molecular design on optoelectronic
properties. The resulting materials showed excellent structural
perfection free of chemical defects, exhibiting great thermal stability
considerable interest in the field of organic optoelectronics,
which possess huge advantages of well-defined structures,
excellent flexibility, great thermal stability, good film-forming
[3]
characteristics, and superior optoelectronic properties.
Previous studies have shown that various cores such as
[
4]
[5]
[6]
[3a,7]
[8]
truxene, isotruxene, benzene, pyrene,
triazatruxene,
[9]
[10]
[11]
triphenylamine, triazine,
and spirofluorene
have been
employed to construct various star-shaped molecules.
Consequently, it is particularly gratifying that recent progress in
organic synthesis provides the possibility of designing and
synthesizing novel materials with complex branches and bulky
structures to achieve promising functional characteristics like
polymers with high molecular weights.
Phenylene-based conjugated oligomers and polymers are
promising candidates for organic electronics due to their high
photoluminescence quantum efficiency, prominent charge-
carrier mobility, and great electrochemical and thermal stability.
(T
d
:
404-418°C and
g
T :147-184°C) and amorphous glassy
morphologies. Compared with their corresponding linear
counterparts FL-m (m = 1-3), TrL-n showed only little bathochromic
shifts (5-12 nm) for the absorption maxima λmax in both solution and
films. The star-shaped ladder-type compounds exhibited enhanced
optical stability and suppressed low-energy emission. Their EL
spectra exhibited excellent stability with increasing the driving
voltage from 6 to 12 V. Moreover, superior low ASE thresholds were
recorded for TrL-n compared with FL-m. Rather low ASE threshold
Since the first report on ladder-type poly(p-phenylene)s (LPPP)
2
[12]
(29 nJ/pulse or 1.60 μJ/cm ) was recorded for TrL-3, demonstrating
by Scherf and Müllen in 1991,
various ladder-type materials
their promising potential as excellent gain media. This study
provides a novel design concept to develop monodisperse star-
shaped ladder-type materials with excellent structural perfection,
which are vital for shedding light on exploring robust organic emitters
for optoelectronic applications.
based on phenylene building blocks have been synthesized and
[13]
investigated in the field of organic optoelectronics. Due to their
efficient blue emission, ladder-typed π-conjugated polymers are
promising conjugated materials for optoelectronic applications,
such as organic light-emitting diodes (OLEDs), and organic
[14]
lasers. However, in the solid state, the emission of ladder-type
polymers is unstable due to the appearance of long-wavelength
emission bands, which has been attributed to physical or
Introduction
[13b, 15]
chemical degradation processes.
Apart from these, in the
process of polymer analogous reactions, fully bridged polymers
are subjected to incomplete formation of bridges. A tiny quantity
of ketonic defects in the polymers, which even can’t be
detectable by means of UV-visible and infrared (IR)
spectroscopy measurements, can lead to a drastic change of the
emission spectra in the solid state especially under driving
voltages. This is mainly because the ketonic defects in films are
low-energy trapping sites, which are generally accompanied by
efficient energy transfer. Therefore, it is highly desirable to
develop ladder-type conjugated materials with excellent
structural perfection free of ketonic defects to achieve superior
spectral stability. Intense efforts have been attempted to address
this challenge. For instances, by modifying the synthesis of
Nowadays, π-conjugated organic semiconductors have been
widely explored with the rapid advancement of organic
[1]
electronics. Among them, star-shaped molecules constitute an
[2]
important class of organic electronic materials, which usually
comprise a central core and multiple conjugated arms as the
functionalized units. By means of combining the merits of small
molecules and polymers, star-shaped molecules have received
[
a] Y. Jiang, M. Fang, S.-J. Chang, J.-J. Huang, S.-Q. Chu, S.-M. Hu, Dr. C.-F.
Liu, Prof. Dr. W.-Y. Lai, Prof. Dr. W. Huang
Key Laboratory for Organic Electronics and Information Displays, Institute
of Advanced Materials (IAM), Jiangsu National Synergetic Innovation
Center for Advanced Materials (SICAM)
[12]
LPPP previously reported by Scherf and Müllen,
Ma's group
Nanjing University of Posts & Telecommunications
replaced benzene-diboronic acid with 9,9-dihexylfluorene-2,7-
bis(boronic acid pinacol ester) to accomplish a novel series of
ladder-type conjugated polymers with improved structural
9
Wenyuan Road, Nanjing 210023, China
E-mail: iamwylai@njupt.edu.cn
[
b] Prof. Dr. W.-Y. Lai, Prof. Dr. W. Huang
[13a]
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced
Materials (IAM), Jiangsu National Synergetic Innovation Center for
Advanced Materials (SICAM)
perfection.
Bo et al. developed a new synthetic approach to
develop spiro-bridged ladder-type polymers and oligomers with
[13c]
excellent thermal and color stability.
Nevertheless, the
Nanjing Tech University (NanjingTech)
attempt on constructing star-shaped ladder-type conjugated
30 South Puzhu Road, Nanjing 211816, China
Supporting information for this article is given via a link at the end of the
document.
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Chemistry - A European Journal
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systems with excellent structural perfection free of chemical
defects is rare.
obtained by palladium-catalyzed Suzuki cross coupling of
excess monomer 4 with N,N-diphenyl-4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)aniline in a yield of 65%. The stepwise
preparation of the key intermediate 2 commenced with the
synthesis of 5, which was subjected to the amination of the aryl
bromide by a modified, mild Buchwald cross-coupling reaction.
Subsequent borylation using bis(pinacolato)diboron afforded the
desired boronic acid pinacol ester 6. By using Suzuki cross-
coupling reactions of building block 4 with 6, the intermediate 2
was obtained. Reaction of excess 4 with 1,4-benzenedi-boronic
acid pinacol ester afforded 7 in a higher yield of 63% via
[13k, 16]
In this contribution, we present a design strategy to explore a
novel series of monodisperse star-shaped ladder-type oligo(p-
phenylene)s, named as TrL-n (n = 1-3), with the aim to achieve
excellent structural perfection free of chemical defects. The
novel star-shaped systems are composed of truxene as the
central core, three fused ladder-type oligo(p-phenylene) arms
with varying π-conjugated length and diphenylamine units as
functional end-cappers. Ladder-type oligo(p-phenylene) arms
were fused into the star-shaped conjugated system in order to
take the significant advantages of rigid conjugated ladder-type
architectures to improve the carrier mobility and thermal and
electrical stability of monodisperse star-shaped systems.
Truxene comprising three overlapping fluorene units was
selected as the central core to construct fully-fused conjugated
star-shaped ladder-type system with phenylene building blocks.
Diphenylamine units were used as the end-cappers, which are
expected to modulate the electronic energy levels and thus the
[4b]
microwave-accelerated Suzuki reactions. Palladium catalyzed
Suzuki coupling of N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)aniline and monomer 7 afforded ladder-type
arm 3 in 70% yield.
[6, 13e, 17]
charge injection/transport properties.
In the interest of
unraveling how the molecular design affects the optoelectronic
properties, corresponding linear counterparts named as FL-m (m
=
1-3), which contained the same π-conjugated system in every
arm of TrL-n, were synthesized. The thermal, photophysical,
electrochemical, electroluminescence (EL) and amplified
spontaneous emission (ASE) properties of the resulting star-
shaped ladder-type macromolecules were systematically
investigated in comparison with those of their linear counterparts.
Significantly, no obvious low energy green emission was
observed from the resulting large star-shaped ladder-type
conjugated systems for the TrL-n films annealed even at 200 °C
for 30 min. Their EL spectra exhibited excellent stability with
increasing the driving voltage from 6 to 12 V. Moreover, superior
low ASE thresholds were recorded for TrL-n compared with their
corresponding linear ladder-type counterparts FL-m.
Results and Discussion
Synthesis and Structure Characterization
Scheme 1. Molecular structures of TrL-n and FL-m.
The chemical structures of TrL-n (n = 1-3) and FL-m (m = 1-3)
are depicted in Scheme 1. Although truxene derivatives mainly
based on three-substituted core scaffold have been widely
The synthesis of TrL-n adopted a convergent synthetic
strategy by linking a truxene core and three corresponding arms.
The synthetic strategy and detailed procedures for TrL-n are
illustrated in Scheme 3. The monomer 8 was prepared via the
route published by Pei et al. Suzuki-Miyaura cross coupling of
the boronic acid functionalized truxene core 8 and the bromo
functionalized ladder-type arms formed the phenyl ketone–
containing phenylene precursor Tr-1, Tr-2, Tr-3, respectively.
[4,17b,18]
studied,
the star-shaped ladder-type systems based on
[13k,16]
truxene core remain largely unexplored.
Due to the facile
functionalization at C-2, C-7, and C-12 positions and C-5, C-10,
C-15 positions of truxene units, star-shaped ladder-type oligo(p-
phenylene)s were constructed successfully by stepwise
synthetic strategy.
The synthetic route to the ladder-type conjugated arms is
outlined in Scheme 2. To prepare the diphenylamine end-
capped ladder-type arms, it is necessary to synthesize the key
monomer 4. Starting from the known reagent 1, 4-dibromo-2,5-
benzenedicarboxylic acid, monomer 4 was obtained via a two-
step synthesis in a total yield of 72%. The intermediate 1 was
[19]
The
subsequent
nucleophilic
carbonyl
addition
and
intramolecular Friedel-Crafts alkylation reactions afforded the
fully ring-bridged ladder scaffolds of TrL-n.
The synthesis of FL-m was similar to the preparation of TrL-n,
as shown in scheme S1. The well-defined structures and
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Chemistry - A European Journal
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FULL PAPER
[
4a,20]
[4b,6,21]
chemical purities of the intermediates and the final ladder-type
macromolecular stars and linear counterparts have been
armed
and even six-armed
oligofluorene counterparts.
The results suggested the high rigidity of the fused star-shaped
ladder-type conjugated systems. As revealed by DSC curves,
both TrL-n and FL-m exhibited glassy morphologies with no
obvious melting processes during the repeated scan cycles.
According to wide-angle X-ray diffraction (WAXD) patterns
(Figure S25), no obvious diffraction peak was observed, further
confirming their amorphous glassy morphologies.
1
13
adequately verified by H NMR, C NMR, MALDI-TOF mass
1
13
spectrometry. The H and C NMR spectra of the phenyl
ketone-containing phenylene precursors (Tr-1, Tr-2, Tr-3, F-1, F-
2, F-3) and final targets (TrL-n, FL-m) were given in Figure S1-
S24. The eluting curves from the gel permeation
chromatography (GPC) (Figure 1 and Table S1) exhibited
symmetrical narrow peaks with polydispersities of ≤1.01,
suggesting a very high monodispersity and purity. No distinct
precursors or partially substituted byproducts were observed.
OBO
Br
O
O
1
, 2, 3
R
1
B B
R
1
O
O
R1
Br
R
1
1
R
1
R
1
R
1
C
4
H
9
C
4
H
9
R
Pd(pph
3
4
) /NaHCO
3
2 2
KOAc/Pd(dppf )Cl
R
1
2
THF, H O
O
O
O
Pd(pph
Toluene, H
3
)
4
/K
2
CO
3
R
1
B
R
1
R1
B
O
O
O
HOOC
Br
SOCl2
AlCl , CH
2
O
Br
O
Br
O
Br
N
B
N
Br
Br
O
8
2
Cl
2
O
3
COOH
N
O
4 9
C H
4 9
C H
4
1
R
2
1) BuLi, 1-bromo-4-butylbenzene
) BF Et O, MeOH
O
O
O
B B
TrL-1
Diphenylamine
Dioxane, CuI,
O
O
R
2
O
N
B
O
2
3
2
Br
Br
N
Br
R
R
1
1
KOAc/Pd(dppf
2
)Cl
2
R
1
1
R
t-BuONa, L-proline
R
2
O
5
6
R
2
O
R
1
R
1
R
2
4 9
C H
N
O
Pd(pph
3
)
4
/K
2
CO3,
R
2
O
N
Toluene, H
2
O
O
Br
N
Tr-1
N
4
O
2
4 9
C H
4 9
C H
4
C H
9
4 9
C H
C
4
H
9
O
O
O
O
O
O
O
O
B
B
Br
Br
N
Br
O
R
2
O
O
1
) BuLi, 1-bromo-4-butylbenzene
Pd(pph
3
)
4
/K
2
CO3,
O
O
Pd(pph
3
)
4
/K
2
CO3,
R2
Toluene, H
2
O
Toluene, H
2
O
TrL-2
O
C
4
H
9
C
4
H
9
C
4
H
9
C
4
H
9
R1
R
3 2
2) BF Et O, MeOH
1R
1
R
1
7
3
R2
O
O
R
2
R
1
R
1
O
O
Scheme 2. Synthetic routes towards the ladder-type arms (Compounds 1-3).
R
2
R2
N
N
Tr-2
Thermal stability
N
The thermal stabilities of the resulting ladder-type star-shaped
macromolecules TrL-n and linear counterparts FL-m were
investigated by thermogravimetric analysis (TGA) and
O
R
2
R2
O
O
differential scanning calorimetry (DSC) in an inert
N
2
R
2
1) BuLi, 1-bromo-4-butylbenzene
) BF Et O, MeOH
O
atmosphere, as shown in Figure 2 and Table 1. For the star-
shaped ladder-type macromolecules TrL-n, good thermal
stability of the compounds was confirmed by TGA with high
TrL-3
R
1
2
3
2
R
R1
1
R
1
R
1
R
2
O
R
2
decomposition temperatures (T
d
, corresponding to 5% weight
O
R
1
R1
O
R
2
R
2
R
2
loss) in the range of 404-418 °C. The reason for the weight lost
to around 50% of the original weight might be the cleavage of
alkyl or aryl substitution at the bridgeheads in ladder-type
oligophenylenes.
O
O
R2
O
R
1
6 13
= n-C H
O
N
R
2
R
2
O
N
R
2
= *
4 9
C H
Tr-3
According to DSC curves, TrL-n exhibited their glass transition
temperatures (T
increasing the π-conjugated phenylene rings. Moreover,
compared with the linear counterparts FL-m, T of the ladder-
type macromolecules rised obviously by ca. 70 °C. The T
values of TrL-n are also generally higher than those of the three-
g
) between 147-184 °C, which increased with
Scheme 3. The synthesis towards the ladder-type macromolecular stars TrL-n.
g
g
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FULL PAPER
-6
dilute THF (1×10 M) and neat films are shown in Figure 3 and
Figure 4, respectively. The results are summarized in Table 1.
According to our previous report, different flexible side chain
plays an important role on modulating the functional properties
TrL-1
TrL-2
TrL-3
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
[22]
of organic semiconductors for optoelectronics.
We here
mainly focus on the effect of the phenylene rings number on the
photochemical and electrochemical properties. The impact of
molecular design will be elucidated by comparing TrL-n with the
linear ladder-type counterparts FL-m.
In dilute solution, the UV/Vis absorption spectra of all
compounds are mainly composed of two bands: the n→π
transition of triarylamine moieties and the π→π transition of the
ladder-type skeleton. For TrL-n, the UV/Vis absorption spectra
have presented a well-defined vibronic structure with two main
absorption peaks at 388 nm and 410 nm for TrL-1, 395 nm and
20
22
24
26
Elution time (min)
4
19 nm for TrL-2, 405 nm and 430 nm for TrL-3, respectively.
Obviously, the absorption spectra undergo bathochromic shifts
9-11 nm) with increasing the number of phenylene rings. In
Figure 1. The GPC elution curves of TrL-n.
(
comparison with the corresponding linear ladder counterparts
FL-m, the absorption spectra of TrL-n show obvious
bathochromic shifts (7-10 nm), indicating the existence of
(
a) 100
TrL-1
TrL-2
TrL-3
additional
π delocalization between the rigid ladder-type
90
80
70
60
50
40
skeleton arms through the truxene core.
o
1
84
C
TrL-1
FL-1
o
1
65
C
o
1
47 C
1
00
120
140
160
180
200
TrL-2
FL-2
Temperature (℃)
1
00
200
300
400
500
600
700
Temperature (℃)
TrL-3
FL-3
(
b)
FL-1
FL-2
FL-3
1
00
90
80
70
60
50
40
3
00
350
400
450 400
450
500
550
600
Wavelength (nm)
o
1
28
C
o
9
7 C
o
7
1
C
Figure 3. Normalized absorption and fluorescence spectra for TrL-n (n = 1-3)
-6
and FL-m (m = 1-3) in 10 M THF at room temperature. The four vertical lines
denote the positions of 395, 425, 435 and 465 nm for the view of the results of
absorption and fluorescence spectra.
7
5
100
125
150
Temperature (℃)
100
200
300
400
500
600
700
Temperature (℃)
Unlike most linear rigid π-system such as LPPP and
[13a,13c]
MeLPPP,
the absorption and fluorescence spectra of TrL-
Figure 2. TGA and DSC curves of the materials recorded at a heating rate of
n (n = 1-3) and FL-m (m = 1-3) show poor mirror symmetry,
which is also obviously different from other linear and star-
shaped π-conjugated systems based on oligofluorenes.
-
1
10 °C min .
[4b]
Furthermore, TrL-n show large Stokes shifts (12-18 nm), quite
different from those rigid ladder-type counterparts without
Optical properties
diphenylamine end-cappers that exhibit only very small Stokes
Optical properties of TrL-n were investigated in comparison with
those of their corresponding linear counterparts (FL-m). The
absorption and emission spectra of the samples measured in
[13a-c, 13g, 23]
shifts.
But the fluorescence spectra of TrL-n are quite
similar to those of the corresponding linear FL-m, suggesting
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Table 1. Photophysical Properties of the TrL-n and FL-m.
λ
max,abs
λ
max, PL
[
e]
[
a]
[b]
Estimated fluorescence lifetimes
HOMO
LUMO
[eV]
/
T
d
T
g
[nm]
[nm]
[f]
[g]
[g]
Compd
η
sol.
η
film
[
°C]
[°C]
[
c]
[d]
[c]
[d]
THF
Film
THF
Film
τ
THF [ns]
1.29
1.11
0.93
1.50
1.42
1.14
τfilm[ns]
TrL-1
TrL-2
TrL-3
FL-1
FL-2
FL-3
405
418
404
396
420
392
147
167
184
71
388, 410
395, 419
405, 430
381, 400
391, 411
392, 416
401, 425
409, 435
387, 408
399, 420
405, 423
428
437
427, 451
436, 463
451, 473
427, 446
434, 457
448, 468
-5.39/-2.55
-5.37/-2.57
-5.34/-2.61
-5.37/-2.47
-5.37/-2.55
-5.33/-2.63
0.81
0.80
0.89
0.89
0.78
0.30
0.27
0.32
0.22
0.26
0.24
0.28(68%), 2.98(32%)
0.26(69%), 2.73(31%)
0.26(63%), 2.96(37%)
0.28(72%), 2.93(28%)
0.24(61%), 3.18(39%)
0.25(70%), 3.26(30%)
442, 467
432
97
440
128
400, 423
445
0.82
[
a]
[b]
[c]
-6
[d]
[e]
T
d
: decomposition temperature.
g
T : glass transition temperature. Measured in 10 M THF solution. Film samples. Estimated from the onset oxidation and
[f]
reduction potential by using EHOMO g=] -[Eox+4.77] eV, E(Fe/Fe+) = 0.03 eV. Deduced from HOMO and E
wavelength for the solid-state films. Fluorescence quantum yields in solution and solid-state films, measured on the quartz plate using an integrating sphere.
g
estimated from the red edge of the longest absorption
[
similar emissive decays for both the star-shaped and linear
ladder-type conjugated systems.
ladder-type macromolecules showed relatively shorter lifetimes
compared with the linear counterparts, suggesting more efficient
intramolecular energy transfer and faster decays for the star-
shaped ladder-type conjugated systems.
The UV/Vis spectra of TrL-n and FL-m in thin films are almost
identical to their corresponding solution spectra, except for 3-7
nm bathochromic shifts. The fluorescence spectra of TrL-n in
neat films show two well resolved emission bands, which can be
assigned to the 0-0 and 0-1 singlet transitions. In addition, the
emission spectra of TrL-n in neat films are practically identical to
those obtained from corresponding FL-m, except for little slight
red shifts of 2-6 nm. The results suggest that the bulky star-
shaped architectures can effectively maintain the photophysical
properties of the linear ladder-type skeletons even in the
aggregation states.
TrL-1
FL-1
TrL-2
FL-2
The photoluminescence quantum yields (PLQYs) for TrL-n
and FL-m in 10 M THF and neat films are summarized in Table
-5
TrL-3
FL-3
1. All of the materials exhibited similar PLQYs in solution (η =
0.78-0.89), which are larger than the solid-state PLQYs, which
are in the range from 0.22 to 0.32. The rapid decrease (over 2.5-
fold) of η in the neat film suggested strong concentration
quenching due to the large coplanar π-conjugated system.
Among these materials, TrL-n show relatively higher PLQYs in
the solid states, mainly due to the bulky star-shaped
architectures that hinder the intermolecular interactions.
300
350
400
450 400
450
500
550
600
Wavelength (nm)
Figure 4. Normalized absorption and fluorescence spectra for TrL-n (n = 1-3)
and FL-m (m = 1-3) in neat films at room temperature. The four vertical blue
lines denote the positions of 395, 425, 435 and 465 nm for the discussion of
the results of time-resolved fluorescence anisotropy.
To study the excited state relaxation dynamics in the star-
shaped ladder-type macromolecules, fluorescence transients of
-5
TrL-n and FL-m were investigated in 10 M THF solution and in
neat films (Figure 5). The corresponding data of fluorescence
lifetime (τ) for TrL-n and FL-m are summarized in Table 1. The
transients at the spectral peak of 0-1 band in neat films of all
samples are shown in Figure S26.
Preliminary studies showed that fluorescence transients of
thin films manifested bi-exponential behaviors, which included
comparatively faster excited relaxation during the first 2 ns upon
excitation and a slower relaxation state at a later stage. The bi-
exponential transient together with the redshifted fluorescence
emission in thin films suggested that energy transfer occurred.
The fluorescence decay profiles were well described by two-
component exponential decay models to clarify different
components and their corresponding intensities, as summarized
Excited state relaxation of the samples in dilute solution was
found to follow a quite similar single exponential decay profile
with estimated fluorescence lifetimes (τ) of 1.14-1.50 ns for FL-m
and a little shorter ones (0.93-1.29 ns) for TrL-n. The values of τ
showed a progressive decrease with the increase of the π-
conjugated phenylene rings in the star-shaped system, which
was similar to the trend of the linear counterparts. Star-shaped
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in Table 1. The fractional intensity of different components is
listed in the brackets adjacent to the τ value, denoting various
contributions to the whole excited-state decay. Obviously, the
major contribution provided by the short component with τ < 1.0
ns, which was about one third of that recorded from the dilute
solution, was probably ascribed to the fast radiative relaxation
arising from the efficient exciton decays in thin films at the initial
stage. The longer decay component mainly resulted from the
stronger exciton delocalization or exciton migration, which was
able to evade the fast radiative decays. According to the results,
no significant differences in transient decays were noticed
between the star-shaped ladder-type macromolecules and the
corresponding linear counterparts, indicating that the molecular
architecture has little impact on the fluorescence transients in
film states.
which are estimated from the onset reduction potentials
assuming the absolute energy level of Fc/Fc+ to be 4.8 eV
below vacuum. The band gaps (E ) of TrL-n, on the basis of
g
the absorption onset of the solid-film samples, are estimated to
be 2.84 eV, 2.80 eV, 2.73 eV for TrL-1, TrL-2 and TrL-3,
respectively. By increasing the linear length of the ladder
systems, the band gap becomes narrower (Table S2). The
lowest unoccupied molecular orbital (LUMO) levels are thus
[24]
calculated according to the following equation: ELUMO = EHOMO
+
Egap. The results are summarized in Table 1 and Table S2.
TrL-1
1
TrL-1
TrL-2
TrL-3
FL-1
0
.1
0
.01
1
E-3
1
TrL-2
FL-2
0
.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
+
0.1
Potential vs. Fc/Fc (V)
0
.01
1
E-3
1
TrL-3
FL-3
2 2
Figure 6. Cyclic voltammograms of TrL-n in CH Cl for oxidation.
0
.1
0.01
Theoretical Calculations
1E-3
0
3
6
9
12
15
Time (ns)
To gain insight into the electronic structure of TrL-n, density
functional theory (DFT) calculations were carried out for the
0
gound-state (S ) geometry optimization. To expedite the
-
5
calculations, all the solubilizers on the saturated carbons are
replaced with methyl groups. Singlet transition energies and
spatial distributions of electron density for HOMOs (0, -1, -2) and
LUMOs (0, +1, +2) are obtained by means of the semi-empirical
ZINDO calculations. The energy diagram of the HOMO, LUMO
and the nearby orbitals (HOMO-2, HOMO-1, LUMO+1,
LUMO+2) for TrL-n and FL-m are shown in Figure 7. The frontier
molecular orbitals (MOs) for TrL-1 are shown in Figure 8, and
the corresponding frontier MOs for other molecules are shown in
Figure S28-S32.
Figure 5. Fluorescence transients of 10 THF solutions (open points) and
neat films (solid points) of (a) TrL-1 and FL-1, (b) TrL-2 and FL-2, (c) TrL-3
and FL-3 excited at the first peak of fluorescence spectra. All of the lines
indicate single or double exponential fits to the experimental data.
Fluorescence lifetimes (τ) are indicated.
Electrochemical Properties
The electrochemical behaviors of the compounds were
investigated by cyclic voltammetry (CV) with a standard three-
electrode electrochemical cell in acetonitrile solution containing
.1 M tetrabutylammonium hexafluorophosphate (TBAPF
room temperature (Figure 6 and Figure S27). The oxidation
potentials were measured versus Ag/AgNO as the reference
electrode and a standard ferrocene/ferrocenium redox system
as the internal standard for estimating the highest occupied
molecular orbital (HOMO) energy levels of the star-shaped
ladder-type macromolecules and the linear counterparts. Figure
As shown in Figure 7, phenylene rings and molecular
dimensions affect the orbital energy levels. It should be noted
that the S ← S (n = 1, 2, 3) excitation is primarily described by
n 0
0
6
) at
the energy transition from HOMOs (HOMO-2, HOMO-1, HOMO)
to LUMOs (LUMO, LUMO+1, LUMO+2), although increasing the
3
[1d,
conjugated π-system can facilitate configuration interactions.
13k, 25]
What’s more, the little energy difference between HOMO-2
and HOMO-1 orbitals and LUMO+1 and LUMO+2 orbitals for
TrL-n (Figure 7) can significantly increase the participation of the
configurations associated with the HOMO-2 and LUMO+2
orbitals in the allowed transitions. The close HOMOs as well as
LUMOs in the configuration of TrL-n is beneficial for the
intramolecular charge transfer.
6
shows the CV curves of TrL-n in films at a scan rate of 20
mV/s in the range of 0-1.4 V. All the films exhibit irreversible
oxidation process. The oxidation onset potentials are around 0.6
V, which can be attributed to the oxidation of the terminal
diphenylamine groups. The corresponding HOMO energy levels
are thus estimated to be around -5.37 (± 0.03) eV for TrL-n,
Although TrL-n have different phenylene rings, great similarity
is observed in the electron distribution (Figure 8 and Figure S28-
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2
9). In the case of TrL-1 (Figure 8), all the HOMOs are mainly
much better spectral stability without obvious energy increase in
0-2 emission band. But the energy increase is still recorded in
the low energy band. In addition, TrL-3 shows no sign of an
localized on the selected end-capped diphenylamine moieties
and adjacent ladder-type skeletons, which matches well with
diphenylamine moieties as the strong electron donor. In contrast, ongoing formation of low-energy emission at 200 °C. According
all the LUMOs have electron density localized on the partially
phenylene backbone including the central truxene core.
Evidently, electron density distribution of every arm is not the
same in all frontier MOs, which indicate that the π delocalization
and excition migration between the rigid ladder- type skeleton
arms are suppressed. It is consistent with the similar absorption
and fluorescence spectra for TrL-1 and FL-1. In this context, the
same phenomena are also observed in the frontier MOs of TrL-2
and TrL-3 (Figure S28-29) and the electron distribution of FL-n
to the analysis, the star-shaped ladder-type configuration can
result in not only increasing spectral stability but also decreasing
the low-energy emission, which holds great promise as blue
emitters. The intensity change of 0-0, 0-1 and 0-2 emission
peaks upon heating is mainly related to the structural relaxation
[26]
energy and the associated vibrational mode. It is thus inferred
that the alkyl substitution at the truxene core and the
bridgeheads in ladder-type oligophenylenes could reduce the
[13b]
stability towards oxidative degradation.
(
Figure S30-32).
0
1
2
3
4
LUMO+2
LUMO+1
-
0.32
-0.32
-0.32
-0.85
-
0.34
-
0.64
1.35
-
1.13
-
-
-
-
-
-
-
1.28
-1.37
LUMO
-1.25
-
-1.3
-
-
1.42
-1.39
-1.4
-1.45
-1.46
1.31
FL-1
FL-2
FL-3
TrL-1
TrL-2
TrL-3
-
-
4.7
4.71
-4.68
-4.69
-
4.72
HOMO
-4.73
5.38
-4.71
-5.21
-4.7
5.09
-4.73
-
-4.76
-4.73
-4.71
5 HOMO-1
-
-5.7
-5.94
6
HOMO-2
-
6.27
Figure 7. B3LYP/6-31G** orbital energy level diagrams for TrL-n and FL-m.
Optical Stability and Blue Electroluminescence
To determine their overall stability against the unwanted low-
energy emission band at 2.2-2.4 eV, spin-coated films of all
samples on quartz substrates were exposed to oxygen under
thermal stress conditions in an accelerated lifetime test. The film
samples were annealed at various temperatures (100 °C, 120 °C,
1
50 °C, 180 °C and 200 °C) for 30 min in open ambient
atmosphere with room temperature of 25 °C and humidity of
0%. Figure S33-36 show UV/Vis spectra and fluorescence
3
Figure 8. B3LYP/6-31G** orbital energy level diagrams for TrL-n and FL-m.
spectra of as-prepared films and the subsequently annealed
ones in air at different temperatures. For the linear counterparts,
upon thermal annealing at 180 °C or 200 °C (180 °C for FL-1,
To further investigate the luminescence properties of these
star-shaped ladder-type macromolecules and the corresponding
linear counterparts, especially their color stability under electrical
bias, solution-processed OLEDs with the configuration of indium
tin oxide (ITO)/ PEDOT:PSS (40 nm)/emissive layer (EML, 50
nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene (TPBI, 30
nm)/lithium fluoride (LiF, 1 nm)/aluminum (Al, 100 nm) (emitter:
TrL-n, (n = 1-3) Device A-C, respectively; FL-n, (n = 1-3) Device
D-F, respectively) were fabricated, where TPBI was used as a
buffer electron-transporting and hole-blocking layer.
200 °C for FL-2 and FL-3), the fluorescence spectra show broad
bands between 500 and 600 nm. By comparison, Figure 9
shows the emission spectra of as-prepared films and the
subsequently annealed ones at 150 °C and 200 °C in air for the
star-shaped
ladder-type
macromolecules.
From
room
temperature to 150 °C, all of the films exhibit no notable spectral
shifts. Further increasing the temperature up to 200 °C, for TrL-1,
the 0-2 emission band (2.4-2.7 eV) exhibits an obvious increase,
and the low energy band at 2.2-2.4 eV increases with ongoing
degradation. Upon increasing the temperature, TrL-2 exhibits
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The recorded EL spectra of TrL-1 (Device A, Figure 10a) are
appreciably broader than the corresponding PL spectra (Figure
process, and eventually grow to dominate the spectra, namely at
457 nm for TrL-1, 465 nm for TrL-2, 477 nm for TrL-3, 450 nm
4
), with a central peak at 452 nm, shoulder peaks at 432 m and
for FL-1, 460 nm for FL-2, 473 nm for FL-3. ASE threshold (Eth)
2
480 nm. With increasing the π-system, the EL spectra (Device B
values are 76 nJ/pulse (4.22 μJ/cm ) for TrL-1, 74 nJ/pulse (4.12
2
2
and C, Figure 10b and c) show narrower emission and relatively
more stable spectra across the range of driving voltages from 6
to 12 V. Compared with their PL spectra in the thin film states,
the 0-0 emission bands are suppressed in the EL spectra,
indicating more efficient energy transfer during the EL process.
Notably, with increasing the driving voltage from 6 to 12 V, the
EL spectra of the resulting devices based on TrL-n remain
nearly unchanged, and the CIE coordinate values show
negligible variation (Table S3), suggesting a remarkable voltage-
independent EL emission. While the low-energy emission
becomes more pronounced for the devices based on the linear
counterparts (Device D-F) with an obvious change of the EL
spectra observed as the devices were driven to higher
luminance (Figure S38). The EL results presented here
suggests that the bulky star-shaped ladder-type architectures
with suppressed molecular alignment in the solid state are less
prone to low-energy emission during the device operation.
Additional EL characteristics of the devices are also supplied as
summarized in Table S3.
μJ/cm ) for TrL-2, 29 nJ/pulse (1.60 μJ/cm ) for TrL-3, 132
2
2
nJ/pulse (7.36 μJ/cm ) for FL-1, 161 nJ/pulse (8.40 μJ/cm ) for
FL-2, 80 nJ/pulse (4.43 μJ/cm ) for FL-3, respectively (Figure
2
S40 and S41, and Table 2). All the ASE results of TrL-n and FL-
m are summarized in Table 2. Compared with the linear
counterparts, the star-shaped ladder-type macromolecules
exhibit much lower ASE thresholds, suggesting that the star-
shaped ladder-type macromolecules are potential excellent gain
media. Emphatically, the largest star-shaped ladder-type
conjugated system TrL-3 achieves the lowest ASE threshold of
2
29 nJ/pulse (1.60 μJ/cm ), which is also among the best results
[
3b, 22, 28]
reported for organic gain media.
12V
(a)
(b)
(c)
11V
10V
9
8
7
V
V
V
Wavelength (nm)
6
20
564
517
477
443
413
6V
(a)
(b)
400 480 560 640 400 480 560 640 400 480 560 640
Wavelength (nm)
(c)
Figure 10. Normalized EL spectra of Devices (a) A (b) B (c) C at various
driving voltages.
2
.0
2.2
2.4
2.6
2.8
3.0
Energy (eV)
TrL-1
4
57 nm
Figure 9. The normalized PL spectra of (a) TrL-1, (b) TrL-2, (c) TrL-3 before
and after annealing in air. The solid line: room temperature; dash line: 30 min
at 150 °C; dot line: 30 min at 200 °C. Room temperature is 25 °C and the
humidity is 30%.
TrL-2
TrL-3
465 nm
Amplified Spontaneous Emission (ASE) Properties
477 nm
Organic emitters with good thermal and spectral stabilities are
promising gain media for organic solid-state lasers. To explore
this potential, ASE characteristics of TrL-n and FL-m have been
investigated. In order to quantify our results, the pulse energy of
exciting light at which the full width at half-maximum (FWHM)
intensity of the emission spectra drops to half of its PL value was
defined as the ASE threshold (Eth).
As shown in Figure 11 and Figure S39, the ASE peaks occur
at 0-1 vibronic, corresponding to a favorable four-level gain
3
50
400
450
500
550
600
Wavelength (nm)
[27]
Figure 11. Normalized absorption, emission and ASE spectra (excited at 390
nm for TrL-1 and TrL-2 and excited at 405 nm for TrL-3) of the films of star-
shaped ladder-type macromolecules.
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Chemistry - A European Journal
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China (21422402, 21674050, 20904024, 61136003), the Natural
Science Foundation of Jiangsu Province (BK20140060,
BK20130037, BM2012010), Program for Jiangsu Specially-
Appointed Professors (RK030STP15001), Program for New
Century Excellent Talents in University (NCET-13-0872),
Specialized Research Fund for the Doctoral Program of Higher
Education (20133223110008), the Ministry of Education of
China (IRT1148), the NUPT "1311 Project" and Scientific
Foundation (NY213119, NY213169), the Synergetic Innovation
Center for Organic Electronics and Information Displays, the
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD), the 333 Project (BRA2015374)
and the Qing Lan Project of Jiangsu Province.
Table 1. ASE properties of TrL-n and FL-m.
ASE threshold
λ
max,
λ
ASE
FWHM
(nm)
areal
energy
Compd
PL
energy/
pulse
areal power
density
(KW/cm )
(nm)
(nm)
density
2
(nJ/pulse)
2
(
μJ/cm )
4
4
27,
51
TrL-1
TrL-2
TrL-3
FL-1
FL-2
FL-3
457
465
477
450
460
473
4.0
4.2
4.1
4.3
4.1
4.4
76
74
4.22
0.84
0.82
0.32
1.47
1.68
0.89
436,
4.12
1.60
7.36
8.40
4.43
4
63
451,
29
4
73
427,
132
151
80
Keywords: conjugated systems • star-shaped ladder-type
molecules • oligo(p-phenylene)s • electroluminescence •
amplified spontaneous emission
4
46
434,
4
57
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Chemistry - A European Journal
10.1002/chem.201605885
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Entry for the Table of Contents
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[a]
[a]
[a]
Yi Jiang, Mei Fang, Si-Ju Chang,
[a]
[a]
Jin-Jin Huang, Shuang-Quan Chu,
[a]
[a]
Shan-Ming Hu, Cheng-Fang Liu,
[a,b]
[a,b]
Wen-Yong Lai,*
Wei Huang
Page No. – Page No.
Towards Monodisperse Star-Shaped
Ladder-Type Conjugated Systems:
Design, Synthesis, Stabilized Blue
Electroluminescence and Amplified
Spontaneous Emission
A novel series of monodisperse star-shaped ladder-type oligo(p-phenylene)s, named as TrL-n (n = 1-3), have been explored with
excellent structural perfection free of chemical defects. The resulting materials TrL-n exhibited enhanced spectral stability,
suppressed low-energy emission, and superior low ASE thresholds, compared with their corresponding linear ladder-type
counterparts FL-m.
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