Blue-light-emitting multifunctional triphenylamine-centered starburst quinolines: Synthesis, electrochemical and photophysical properties

Article abstract of DOI:10.1039/c2ob25120e

A series of triphenylamine-centered starburst quinolines (1a-1g) have been synthesized by Friedlaender condensation of the 4,4′,4′′- triacetyltriphenylamine (2) and 2-aminophenyl ketones (3a-3g) in the presence of catalytic sulfuric acid and characterized well. They are thermally robust with high glass transition temperatures (above 176.4 °C) and decomposition temperatures (above 406 °C). These compounds emit blue fluorescence with λEmmax ranging from 433 to 446 nm in dilute toluene solution and 461 to 502 nm in the solid-state and have a relatively high efficiency (Φ_u = 0.98-0.57). 1a-1g have estimated ionization potentials (IP) of 4.54 to 6.45 eV which are significantly near or higher than those of well-known electron transport materials (ETMs), including tris(8- hydroxyquinoline)aluminium (Alq₃) (IP = 5.7-5.9 eV), and previously reported oligoquinolines (IP = 5.53-5.81 eV). Quantum chemical calculations using DFT B3LYP/6-31G* showed the highest occupied molecular orbital (HOMO) of -5.05 to -4.81 eV, which is close to the work function of indium tin oxide (ITO). These results demonstrate the potential of 1a-1g as hole-transporting/light-emitting/electron-transport materials and the host-materials of a dopant for hole-injecting for applications in organic light-emitting devices.

Full text of DOI:10.1039/c2ob25120e

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PAPER
Blue-light-emitting multifunctional triphenylamine-centered starburst
quinolines: synthesis, electrochemical and photophysical properties†
Peng Jiang,a Dong-Dong Zhao,a Xiao-Li Yang,a Xiao-Lin Zhu,a Jin Changb and Hong-Jun Zhu*a
Received 16th January 2012, Accepted 12th April 2012
DOI: 10.1039/c2ob25120e
A series of triphenylamine-centered starburst quinolines (1a–1g) have been synthesized by Friedländer
condensation of the 4,4′,4′′-triacetyltriphenylamine (2) and 2-aminophenyl ketones (3a–3g) in the
presence of catalytic sulfuric acid and characterized well. They are thermally robust with high glass
transition temperatures (above 176.4 °C) and decomposition temperatures (above 406 °C). These
Em
max
compounds emit blue fluorescence with λ
ranging from 433 to 446 nm in dilute toluene solution and
461 to 502 nm in the solid-state and have a relatively high efficiency (Φu = 0.98–0.57). 1a–1g have
estimated ionization potentials (IP) of 4.54 to 6.45 eV which are significantly near or higher than those of
well-known electron transport materials (ETMs), including tris(8-hydroxyquinoline)aluminium (Alq3)
(IP = 5.7–5.9 eV), and previously reported oligoquinolines (IP = 5.53–5.81 eV). Quantum chemical
calculations using DFT B3LYP/6-31G* showed the highest occupied molecular orbital (HOMO) of −5.05
to −4.81 eV, which is close to the work function of indium tin oxide (ITO). These results demonstrate the
potential of 1a–1g as hole-transporting/light-emitting/electron-transport materials and the host-materials
of a dopant for hole-injecting for applications in organic light-emitting devices.
commonly used as building blocks for increasing electron
Introduction
affinity of current electron transport materials for OLEDs.25
Thus the combination of the hole-transporting triphenylamine
moiety and the electron-transporting quinoline moiety in tris(8-
quinoline)-triphenylamine (TQTPA) gives it bipolar charge
transport property. Above studies and reports on TQTPA showed
that the bipolar starburst molecule, TQTPA exhibited desirable
thermal stability, luminescence property and excellent hole-trans-
porting ability.26
Organic light-emitting devices (OLEDs) have attracted a great
deal of attention because of their wide applications in high-resol-
ution, full-color, flat-panel displays and solid-state illumination
sources, since the first application of a multilayer thin-film struc-
ture in OLEDs by Tang et al.1–8 In the past two decades, small
molecules and polymers have received the most attention in the
field of OLEDs.9 An important factor for achieving high device
efficiency in the field is maintaining a balance between electron
and hole currents.10 Different strategies have been developed to
meet the requirement of charge balance, which includes the
employment of multilayered structures and the introduction of
bipolar or multifunctional molecules like dendrimers or
starbursts.11–21 Meanwhile, triarylamine (TPA) and its deriva-
tives have long been exploited as hole-transport layers (HTLs)
for OLEDs, and their range of application is continuously
widening.22–24 Quinoline and its derivatives are also most
Moreover, owing to its excellent thermal stabilities from its
three-dimensional propeller structure, TPA is a good choice as
the central core for starburst molecules. The starburst molecules,
tris(4-(quinolin-5-yl)phenyl)amine (TPAQ-1)27 and 3-(naphtha-
len-1-yl)-N-(3-(naphthalen-1-yl)phenyl)-N-(3-(quinolin-8-yl)-
phenyl)aniline (TPAQ-2)28 had been synthesized and used for
light-emitting materials in OLEDS, with excellent thermal stab-
ility and luminescence properties. However, the reports concern-
ing the electrochemical and photophysical properties of the
starbursts based on triphenylamine-centered starburst 4-substi-
tuted quinolines are limited. In this context, we aim to develop
new triphenylamine-centered starburst 4-substituted quinolines
molecules and further study the underlying structure–property
relationships.
aDepartment of Applied Chemistry, College of Science, Nanjing
University of Technology, Jiangsu Key Laboratory of Industrial Water-
Conservation & Emission Reduction, Nanjing 210009, China. E-mail:
zhuhjnjut@hotmail.com; Fax: +86 25 83587443; Tel: +86 25 83172358
bDiscipline of Chemistry, Queensland University of Technology, 2
George St, Brisbane (QLD), 4000, Australia
†Electronic supplementary information (ESI) available: Experimental
details and characterization data of new compounds 3d, 3f–3g, 1a–1g;
mass spectra of 3d, 3f–3g and NMR spectra of 3d, 3f–3g, 1a–1g; calcu-
lation results of 1a–1g. See DOI: 10.1039/c2ob25120e
In this paper, we report the synthesis and investigation of the
thermal stabilities, electrochemical and photophysical properties
of a series of novel triphenylamine-centered starburst quinolines:
tris(4-(4-phenylquinolin-2-yl)phenyl)amine (1a), tris(4-(4-(4-
bromophenyl)quinolin-2-yl)phenyl)amine
(1b),
tris(4-(4-
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(pyridin-4-yl)quinolin-2-yl)phenyl)amine (1c), tris(4-(4-(9H-
fluoren-2-yl)quinolin-2-yl)phenyl)amine (1d), tris(4-(4-(9,9-
dibutyl-9H-fluoren-2-yl)quinolin-2-yl)phenyl)amine (1e), tris(4-
(4-(4′-methoxybiphenyl-4-yl)quinolin-2-yl)phenyl)amine (1f)
and tris(4-(4-(4′-tert-butylbiphenyl-4-yl)quinolin-2-yl)phenyl)
amine (1g) (shown in Chart 1). These starbursts show superior
properties including relatively high fluorescence quantum
efficiency, excellent thermal stabilities, high ionization potential
and highest occupied molecular orbitals (HOMO) proximate to
the work function of indium tin oxide (ITO), which make them
possible candidates for a class of hole-injecting/hole-transport-
ing/light-emitting/electron-transport material in OLEDs.
Results and discussion
Synthesis
As shown in Scheme 1, all seven new target compounds 1a–1g
were synthesized by a sulfuric acid-catalyzed Friedländer reac-
tion with 4,4′,4′′-triacetyltriphenylamine (2) and 2-aminophenyl
ketones (3a–3g) in 41–33% yield, respectively (Scheme 1).29
The sulfuric acid catalyst and acetic acid as a solvent were
readily removed by precipitation into a 25% ammonia solution
in water. The materials were passed through silica-gel columns
via flash chromatography to remove polar byproducts. They were
subsequently recrystallized twice in tetrahydrofuran–methanol
mixtures ranging from 30 to 50% methanol. The compound 2
was synthesized by acetylation of triphenylamine with acetyl
chloride and aluminium trichloride in dichloromethane at reflux
temperature (Scheme 2) in high yields 92%.30 The intermediates
3a–3e were obtained via the Sugasawa reaction, which starts
from an aniline and a series of different nitriles 4a–4e in the
Chart 1 Structures of seven triphenylamine-centered starburst quinolines.
Scheme 1 Synthesis of starburst multifunctional molecules 1a–1g by a Friedländer reaction.
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Scheme 2 Reagents and conditions: (a) acetyl chloride, AlCl3, DCM, reflux; (b) NBS, propylene carbonate, 60 °C;47 (c) n-butyl bromide, KOH, KI,
DMSO, 60 °C;48 (d) CuCN, NMP, reflux;49 (e) (i) BCl3, 1,1,2,2-tetrachloroethane, reflux; (ii) HCl–H2O, 80 °C; (f) (i) Mg, THF, reflux; (ii)
B(OCH3)3, −78 °C;50,51 (iii) HCl–H2O; (g) Pd(AcO)2, K2CO3, DMF + H2O, 80 °C.
presence of stoichiometric amounts of BCl3 in 54.6%–35.3%
yield, respectively.31 The decrease in the yield might be due to
the increase in the conjugate structure. The other intermediates 3f
and 3g were obtained by the palladium-catalyzed Suzuki cross-
coupling with compound 3b and compound 7f or 7g, respectively
(Scheme 2) in the desired yield of 80.5% and 79.3%.32 The syn-
NMR spectra, 13C NMR spectra, elemental analysis and FTIR
spectra on 1a–1g confirmed the proposed structures.
Thermal properties
The thermal properties, including glass transition and decompo-
sition temperatures, of these molecules (1a–1g) are shown in
1
thetic routes to other intermediates are outlined in Scheme 2. H
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Table 1 Photophysical properties of starbursts 1a–1g
Abs
max
Em
max
λ
(nm)
λ
Toluene
433
(nm)
Solid-state
482
Eogpt (eV)
Toluene Solid-state
Stokes shift (nm)
Toluene Solid-state
Compound
Toluene
Solid-state
Φu
Tg/Td
1a
1b
1c
1d
1e
1f
295(3.46a)
400(6.232)
297(9.11)
401(15.97)
299(3.82)
403(7.34)
309(8.52)
400(9.04)
313(7.18)
401(7.61)
296(9.62)
400(10.45)
294(7.21)
401(8.01)
405
388
407
397
425
410
413
0.71
0.98
0.61
0.58
0.61
0.57
0.58
2.35
2.33
2.31
2.34
2.33
2.34
2.32
1.67
1.65
1.64
1.65
1.65
1.66
1.65
33
38
43
38
36
38
36
77
95
79
86
36
92
73
229.8/>500
205.3/>500
301.9/>500
366.7/>500
176/405.9
298.7/>500
248.8/>500
439
483
446
486
437
483
437
461
438
502
1g
437
486
a The values in brackets are molar extinction coefficient, 104 M−1 cm−1
.
Table 1. Differential scanning calorimetry (DSC) was used to
investigate phase transitions (1a and 1b are shown in Fig. 1, the
others are shown in Table S1 in ESI†). All the seven compounds
(1a–1g) exhibit very high glass transition temperatures (Tg)
ranging from 176.4 °C for 9,9-n-dibutyl-fluorenyl substituted 1e
to 366.7 °C for fluorenyl substituted 1d. 1a–1g are thermally
stable up to 400 °C as shown by their high decomposition temp-
eratures (Td) in the thermogravimetric analysis (TGA) measure-
ments. The Tg of 1a–1g is much higher than that of the
commonly used hole-transporting material (e.g., 98 °C for NPB)
and previously reported TQTPA (Tg = 130 °C).26,33 Therefore,
all of these results demonstrate that the series of triphenylamine-
centered starburst quinolines 1a–1g are very robust molecules.
Photophysical properties
Comparisons of 1a–1g in visible light and UV light in toluene
solution are shown in Fig. 2. The solution of 1a–1g appears as
light blue fluorescence in visible light and emits blue fluor-
escence in UV light (band 254 nm).
The absorption and photoluminescence spectra of 1a–1g were
investigated both in solutions and in solid-states. Fig. 3 shows
the absorption spectra of 1a–1g in toluene solution. The solution
absorption spectra of 1a–1g show two bands, a lower intensity
band in the 260 to 320 nm range and a higher intensity band in
the 390 to 430 nm range. With the exception of the starbursts
with fluorenyl and 9,9-dibutyl-fluorenyl substituents, 1d and 1e,
all the others have very similar absorption spectra with two
peaks in the range of 294 to 299 nm and 400 to 403 nm. Both
peaks have large molar extinction coefficients (ε), shown in
Table 1. These absorption bands in the spectra of 1a–1g are
associated with π–π* transitions. As expected, the absorption
peaks are red-shifted to 309 nm, 400 nm in 1d and 313 nm,
401 nm in 1e owing to the increase in conjugation length with
fluorenyl and 9,9-dibutyl-fluorenyl substitution.
Fig. 1 DSC thermograms of 1a and 1b under a nitrogen atmosphere at
a heating rate of 10 °C min−1
.
from the dilute solution spectra for 1a, 1c, 1e, 1f, 1g. The
4–24 nm red shifts in the absorption maximum for 1a, 1c, 1e, 1f
and 1g suggest there is a slight increase in conjugation length in
The absorption spectra of 1a–1g in the solid-state are shown
in Fig. 3. In addition to their broad featureless characteristics,
these solid-state absorption bands are significantly red-shifted
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Fig. 2 (a) Color comparison of 1a–1g in visible light in toluene
(10−5 M); (b) color comparison of 1a–1g in UV in toluene (10−5 M).
Fig. 4 (a) Photoluminescence spectra of 1a–1g in toluene solution
(10−8 M); (b) photoluminescence spectra of 1a–1g in solid-state.
All seven molecules 1a–1g emit blue light with the emission
maximum ranging from 433 nm for 1a to 446 nm for 1c with a
narrow full-width-at-half-maximum (FWHM) of 40–45 nm
Em
max
(Fig. 4). All the λ
of 1b–1g are slightly red shifted compared
to 1a. Similarly pyridyl substituted 1c has the maximum red
shift (13 nm). The Stokes shift is small for all the compounds,
ranging from 33 to 43 nm. Using quinine sulfate (Φs = 0.546 in
0.1 N H2SO4)34–36 as a reference, the fluorescence quantum
yields (Φu) of 1a–1g in toluene have been shown in Table 1. The
Φu of compounds 1a–1g in dilute solution in toluene was rela-
tively high, ranging from 0.57 for 1f to 0.98 for 1b (Table 1).
1c–1g have almost identical values of 0.57–0.61, whereas 1a
and 1b have the highest PL quantum yields at 0.71 and 0.98.
The decrease in the quantum yield may be due to the increase in
the conjugate structure.
The blue emission observed in solution is retained in the
solid-state with PL emission maxima in the range of 461 nm for
1e to 502 nm for 1f (Fig. 4). Compared to the solutions, the
maximum emission peaks of the solid state have a red-shift of
23 nm to 64 nm. The similarities of the emission spectral fea-
tures in conjunction with the π-stacking of the molecules in the
solid-state suggest that excimer emission37,38 best describes the
solid-sate luminescence of 1a–1g. The solid-state PL emission
spectra are also much broader compared to the solution spectra,
with a FWHM ranging from 44 nm to 80 nm. In contrast, 1c has
a broad PL spectrum with a shoulder peak at 502 nm and a
FWHM of 80 nm. The broad PL emission spectrum of 1c
Fig. 3 (a) UV–vis absorption spectra of 1a–1g in toluene solution
(10−5 M); (b) UV–vis absorption spectra of 1a–1g in solid-state.
the solid-state. Such an increase in conjugation length in the
solid-state is expected from the more planar conformations due
to the π-stacking in the solid-state. Optical band gaps (Egopt
)
determined from the absorption edge of the solution and solid-
state spectra are given in Table 1. The optical band gap varies
from 2.31 to 2.35 eV for the solution and from 1.64 to 1.67 eV
for the solid-state.
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Fig. 5 Fluorescence spectra of 1a and 1b in toluene solutions at differ-
ent excitation wavelengths.
Fig. 6 The reduction cyclic voltammograms of 1b and 1e in solution.
suggests that it favors strong aggregation compared to the other
compounds.
level of 4.4 eV versus vacuum.41–43 These compounds had EA
values of 2.16–2.35 eV (below vacuum). The reduction poten-
tials of all the molecules are relatively unchanged by the substi-
tuent (R groups), indicating that the effect of the substituent is
weak.
Molecules 1a, 1b, 1d, 1e, 1g had one irreversible oxidation
peak. In the case of 1c and 1f, the irreversible oxidation peak
was not seen (1b and 1e in solution are shown in Fig. 7, while
the oxidation CVs of 1a, 1d, 1g are shown in Table S3 in the
ESI†). The five molecules 1a, 1b, 1d, 1e, 1g display very similar
When the starbursts are excited in the absorption range of
290–320 nm or 390–410 nm, the emission spectra for all seven
molecules 1a–1g only show one emission peak from the core.
The single emission upon excitation of quinoline arms suggests
the occurrence of efficient intramolecular energy transfer39,40 (1a
and 1b are shown in Fig. 5, the others are shown in Table S2 in
the ESI†).
Electrochemical properties
CVs with a formal potential of 1.52–2.18 V (vs. SCE) and
onset
ox
The reduction cyclic voltammograms (CVs) of 1b and 1e in sol-
ution are shown in Fig. 6 while the reduction CVs of 1a, 1c, 1d,
1f, 1g are shown in the ESI (Table S3†). Irreversible reduction
peaks were seen in the CVs of 1a–1b and 1d–1g, whereas
quasireversible reduction peaks were seen in the CVs of 1c. A
formal reduction potential varies from −2.18 V for 1c to −2.44
V for 1e (vs. SCE) and the onset reduction potentials of 1a–1g
are in the range of −2.05 to −2.24 V (vs. SCE). Table 2 shows
the solution electrochemical data for 1a–1g. Electron affinities
E
in the range of 1.46–2.05 V (vs. SCE). Ionization poten-
tials (IP) of 1c and 1f were estimated using the optical bandgap
determined from optical absorption edge (Table 1): IP = EA +
Eogpt 44 and 1a, 1b, 1d, 1e, 1g were estimated using the
onset
ox
onset
ox
41–43
E
(Table 2): IP = E
+ 4.4 eV.
As a result the IP is
reduced from 4.54 eV in 1f to 6.45 eV in 1a. The estimated IP
show rather deep HOMO energy levels for this class of electron
transport materials. These IP values are significantly near or
higher than those of well-known electron transport materials
(ETMs), including tris(8-hydroxyquinoline)aluminium (Alq3)
(IP = 5.7–5.9 eV),45 and previously reported oligoquinolines
(EA) were estimated from the onset of the reduction waves in the
onset
red
solution CVs (EA = E
+ 4.4 eV) by using an SCE energy
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Table 2 Electrochemical properties of solution and energy values theoretically calculated for molecules of 1a–1g in the gas phase
onset
red
peak
ox
onset
Compound
Epreedak (V)
E
(V)
EA (eV)
E
(V)
E
ox
(V)
IP (eV)
HOMO (eV)
LUMO (eV)
Ecgal (eV)
1a
1b
1c
1d
1e
1f
−2.29
−2.29
−2.18
−2.43
−2.44
−2.30
−2.30
−2.14
−2.16
−2.05
−2.20
−2.24
−2.20
−2.18
2.26
2.24
2.35
2.20
2.16
2.20
2.22
2.18
2.12
—
1.52
1.57
—
2.05
2.02
—
1.46
1.50
—
6.45
6.42
4.66
5.86
5.90
4.54
5.98
−4.86
−4.99
−5.05
−4.84
−4.84
−4.81
−4.84
−1.29
−1.47
−1.57
−1.32
−1.33
−1.27
−1.31
3.57
3.52
3.48
3.52
3.51
3.54
3.53
1g
1.67
1.58
molecules shows the three-dimensional propeller structure, qui-
noline groups are nearly coplanar with triphenylamine group and
the substituents on the quinoline unit (R group) are twisted from
the plane of the quinoline unit. This geometry could result in
good solution processability which was confirmed with exper-
imental results that show 1a–1g were soluble in most solvents
like chloroform, tetrahydrofuran, toluene and so on. The HOMO
of 1a–1g is located on the TPA unit, whereas the LUMO is
located predominantly on the two arms of the molecule (see
Table S4 in the ESI†). The broad distribution of the HOMO
should be good for the hole transport through the molecules
when 1a–1g are used as hole-injecting/hole-transporting
materials in OLEDs. 1a–1g have the HOMO (−5.05 to −4.81
eV, shown in Table 2) close to the work function of ITO and
thus allow efficient hole injection from the ITO. Therefore 1a–
1g were expected to have hole-injecting/hole-transport capacity.
The LUMO is shown in Table 2. The LUMO energy values of
1a–1g were from −1.27 to −1.57 eV, respectively. The HOMO–
LUMO energy differences (energy band gaps, calculated Ecgal) at
B3LYP/DFT level of theory are presented in Table 2. The result
indicates that Ecgal values decrease as the length of π-conjugated
system increases. These predicted Ecgal values are much higher
than those estimated from the onset of UV–vis absorption (Eogpt).
There are factors which may be responsible for the errors
because the orbital energy difference between HOMO and
LUMO is still an approximate estimation to the transition energy
since the transition energy also contains significant contributions
from some two-electron integrals. The real situation is that an
accurate description of the lowest singlet excited state requires a
linear combination of a number of excited configurations.
Conclusions
Fig. 7 The oxidation cyclic voltammograms of 1b and 1e in solution.
We have synthesized seven triphenylamine-centered starburst
quinolines 1a–1g, and characterized them with 1H and 13C NMR
spectroscopy, elemental analysis and IR. The new triphenyl-
amine-centered starburst quinolines combined relatively high flu-
orescence quantum yields (0.98 to 0.57) with high ionization
potentials (4.54 to 6.45 eV), high glass transition temperatures
(above 176.4 °C) and high decomposition temperatures (above
406 °C). These compounds also have the HOMO (−5.05 to
−4.81 eV, respectively) close to the work function of ITO. These
results demonstrate that the triphenylamine-centered starburst
quinolines 1a–1g are very promising hole-transporting, blue
emitting, electron-transport materials and the host-materials of a
dopant for hole-injecting for applications for developing high-
efficiency OLEDs with a simple architecture. Furthermore, we
(IP = 5.53–5.81 eV).43 This suggests that the seven triphenyla-
mine-centered starburst quinolines (1a–1g) can be used as wide-
energy gap ETMs and they could also be used for an excellent
hole-blocking layer in OLEDs.
Theoretical calculations
The geometry and the HOMO and LUMO energy levels of 1a–
1g were optimized using density functional theory (DFT)
method at the B3LYP/6-31G* level, as implemented in Gaussian
09 program.46 The core triphenylamine (TPA) group in these
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26 S. Tao, L. Li, J. Yu, Y. Jiang, Y. Zhou, C.-S. Lee, S.-T. Lee, X. Zhang and
O. Kwon, Chem. Mater., 2009, 21, 1284–1287.
27 Jilin University, , CN Pat., 101423757A, 2009.
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30 Y.-Q. Xu, J.-M. Lu, N.-J. Li, F. Yan, X.-W. Xia and Q.-F. Xu, Eur. Polym.
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