Synthesis and characterization of highly stable and efficient star-molecules

Article abstract of DOI:10.1016/j.dyepig.2012.11.014

A series of well-defined star-shaped molecules have been synthesized by Pd-catalyzed Suzuki cross-coupling starting from very simple reactants with 1,3,5-trisubstituted benzene, 2,4,6-trisubstituted pyridine and trisubstituted phenylcarbazole as the backbones. These star-molecules are all soluble in common organic solvents and electrochemically stable with reversible cyclic voltammographs and high lying HOMOs. They exhibit excellent blue-fluorescence with quantum yield up to 0.87 and high glass transition temperatures. These molecules offer potential as pure blue-light emitting, hole-transport or host materials for optoelectronic applications.

Full text of DOI:10.1016/j.dyepig.2012.11.014

Dyes and Pigments 96 (2013) 705e713
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Dyes and Pigments
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Synthesis and characterization of highly stable and efficient star-molecules
*
Hai-Fang Huang, Shi-Hua Xu, Yan-Bo He, Cai-Cai Zhu, He-Liang Fan, Xue-Hua Zhou, Xi-Cun Gao ,
*
Yan-Feng Dai
Department of Chemistry, School of Science, Nanchang University, Xue-Fu Road 999, Hong-Gu-Tan New District, Nanchang 330031, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2012
Received in revised form
10 November 2012
Accepted 12 November 2012
Available online 23 November 2012
A series of well-defined star-shaped molecules have been synthesized by Pd-catalyzed Suzuki cross-
coupling starting from very simple reactants with 1,3,5-trisubstituted benzene, 2,4,6-trisubstituted
pyridine and trisubstituted phenylcarbazole as the backbones. These star-molecules are all soluble in
common organic solvents and electrochemically stable with reversible cyclic voltammographs and high
lying HOMOs. They exhibit excellent blue-fluorescence with quantum yield up to 0.87 and high glass
transition temperatures. These molecules offer potential as pure blue-light emitting, hole-transport or
host materials for optoelectronic applications.
Keywords:
Star-molecule
Suzuki coupling
Stable
Ó 2012 Elsevier Ltd. All rights reserved.
Efficient
Synthesis
High Tg
1. Introduction
electron-donating property, long conjugation chain, high thermal
stability, and high photoluminescence efficiency. These physical
In the past two decades, organic light-emitting diodes (OLEDs)
based on small molecules have received considerable attention
primarily due to the ease of fabrication into large-area flat-panel
displays and solid-state lighting sources [1e4]. However, as one of
the most important components for display and white lighting, the
ideal blue light-emitting pixel still remains a great challenge [5e7].
Low efficiency, low thermal stability, low chemical purity, easy
crystallization and poor structural uniformity are some of the
detrimental factors restricting performance [8]. To improve the
performance from a chemical approach, molecules with a rigid
structure either star-shaped or dendrite-shaped structure have been
designed and effectively used [9e12]. These high-molecular-weight
star-molecules or dendrite molecules often include an arylamine or
carbazole backbone [13e15].
Compared with linear molecules, star-shaped molecules with
well-defined three-dimensional spherical architectures have a site
isolation effect and possess the following advantages: (i) high
degree of purity, (ii) good solution processability, (iii) good thin film
formation, (iv) good thermal stability [16e18]. Molecules with
carbazole and arylamine backbones have been extensively studied
as blue light-emitting as well as hole and host materials due to their
properties can be realized through vigorous chemical designs and
structure modification [19e22]. Many reports have revealed that
the morphological instability of hole-transporting and host mate-
rials are responsible for the device lifetime, especially at higher
temperature [23e28]. From the synthetic organic chemistry point
of view, many hole-transporting and host materials with high
glass-transition temperature (Tg) have been synthesized [29e32].
This is very important because such devices not only occasionally
need to be operated at high temperature but also need to be
operated at high current densities with high luminance in lighting
sources. In these circumstances, materials with a high Tg are better,
to be able to resist heat. Therefore, materials with a high glass-
transition temperature are highly desirable.
In this paper, we have designed a new series of star-shaped
molecules containing carbazole/arylamine backbone: S1L7
(Scheme 1). Their synthesis, thermal stability, UV absorption,
electrochemical behavior, and the photoluminescent properties
were discussed.
2. Experimental
2.1. Chemicals and instruments
All reactants and solvents, unless otherwise stated, were
purchased from commercial sources such as Alfa Aesar and Acros,
* Corresponding authors. Tel./fax: þ86 0791 83969386.
E-mail addresses: xcgao@ncu.edu.cn (X.-C. Gao), yfdai@ncu.edu.cn (Y.-F. Dai).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.11.014

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H.-F. Huang et al. / Dyes and Pigments 96 (2013) 705e713
Scheme 1. Chemical structures of the star-shaped molecules S1e7
.
and used as received. Melting points were measured on an X-4
microscope electrothermal apparatus (Shanghai Jingmi Instru-
ments). FT-IR data were obtained from a Nicolet 5700. 1H NMR
spectra were performed on a Bruker 400 MHz spectrometer.
Elemental analysis was carried out on a Vario III elemental
analyzer. UVevis absorption spectra were recorded on a Perkine
Elmer Lambda-35 spectrometer. Photoluminescence (PL) spectra
were obtained using a Hitachi F-4500 florescence spectropho-
tometer. Differential scanning calorimetery (DSC) and thermogra-
vimetric analysis (TGA) were performed on a PerkineElmer MAS-
5800 instrument with a heating/cooling rate of 10 ꢀC minꢁ1 and
a nitrogen flow rate of 90 mL minꢁ1. Cyclic voltammetry (CV) was
performed on an Autolab Potentiostat 30 with a glassy-carbon
electrode in CH2Cl2 solutions containing 0.0005 M of compound
and 0.10 M (n-Bu)4NBF4 against Ag/AgCl, with ferrocene (Fc) as the
internal standard. The HOMO energy level can be estimated from
reference (positive), Eox is the oxidation potentials of the sample
(positive).
2.2. Synthesis
The chemical structure of the star-shaped molecules S1e7 was
illustrated in Scheme1. Suzukicouplingwas applied as a keyreaction
technique to construct the intermediates and the target compounds.
Schemes 2e5 outlines the synthetic routes. Our synthetic strategy
started from commercially available reagents and intermediates 1, 2
and 10 were prepared by Ullmann coupling reaction [33e38].
The treatment of 2 with three equiv. of N-bromosuccinimide
(NBS) in dichloromethane afforded 16 [39]. Intermediates 3, 4, 11,
12 and 17 were obtained from lithiated 1, 2, 8, 9, 16 with trime-
thylborate, respectively [36e41]. Compounds 5, 8, 9 were synthe-
sized according to the literature methods [38,42e44].
Intermediates 6, 7, 13e15, 18e20 were synthesized according to
reported procedures [38,45,46]. S1 was obtained by Suzuki cross-
the onset potentials through the following equation: EHOMO
¼
(ꢁ4.72 þ Eref e Eox) eV, where Eref is the potential of the Fc

H.-F. Huang et al. / Dyes and Pigments 96 (2013) 705e713
707
Scheme 4. Synthetic routes of intermediates 13e15.
Scheme 2. Synthetic routes of intermediates 1e5, (a) CuI, 1,10-phenanthroline, K2CO3,
o-xylene, reflux; (b) n-BuLi, trimethylborate, THF, ꢁ78 ꢀC; (c) n-BuLi, THF, ꢁ78 ꢀC.
11: Compound 8: (13.66 g, 0.05 mol) was dissolved in anhydrous
THF (60 mL) in a three-necked round-bottom flask fitted with
a magnetic stirrer, a condenser, a N2 purge and a ꢁ78 ꢀC dry ice-
ethanol bath. n-BuLi (2.5 M in hexane, 40.50 mL, 0.065 mol) was
added dropwise at ꢁ78 ꢀC and under N2 gas. After stirring for 1 h at
this temperature, B(OCH3)3 (16.7 mL, 0.15 mol) was added quckly
and the mixture was stirred at ꢁ78 ꢀC for 4 h. As the temperature
was raised to 0 ꢀC, HCl (2.0 M, 300 mL) was added into the flask and
the reaction continued for another 6 h. The reaction mixture was
extracted 3 times with Et2O and the organic layer was evaporated
under reduced pressure. The residue was purified by silica gel
chromatograph with petroleum ether/dichloromethane (4:1) as the
eluent to afford an orange-yellow liquid (11).
coupling reaction between 13 and 4. Similar procedures were
performed to obtain S2 - S7.
2.2.1. 9,9-Dimethyl-7-(9H-9- carbazolyl)-2-bromofluorene (10)
Carbazole (5.01 g, 0.03 mol), compound 9 (31.68 g, 0.09 mol),
1,10-phenanthroline (1.98 g, 0.01 mol), K2CO3 (37.26 g, 0.09 mol) was
dissolved in anhydrous 1,2-dimethylbenzene (100 mL) in a three-
necked round-bottom flask fitted with a magnetic stirrer and
a condenser. The mixture was stirred at room temperature for 0.5 h
under N2 followed by adding CuI (1.91 g, 0.01 mol) and then by
heating slowly to 150 ꢀC for 48 h. After cooling to room temperature,
the reaction mixture was filtered and the filtrate was evaporated
under reduced pressure. The residue was purified by silica gel
chromatograph with petroleum ether/dichloromethane (10:1) as
the eluent. Recrystallization from ethanol yielded 7.33 g (55.8%) of
(10) as a white crystalline power. Mp: 191e193 ꢀC; 1H NMR
2.2.2.1. 9,9-Dimethyl-2-fluorene boronic acid (11). Orange-yellow
liquid, yield: 77.0%, 1H NMR (400 MHz, CDCl3, TMS)
d: 8.34e8.32 (t,
J ¼ 3.43 Hz, 2H); 7.93e7.91 (d, J ¼ 7.74 Hz, 1H); 7.86e7.84 (m, 1H);
7.78e7.76 (d, J ¼ 7.2 Hz, 1H); 7.56e7.55 (m, 1H); 7.45e7.38 (m, 1H);
1.63 (s, 6H).
(400 MHz, CDCl3, TMS)
J ¼ 8.00 Hz,1H); 7.66e7.64 (d, J ¼ 8.03 Hz,1H); 7.61 (s, 2H); 7.56e7.51
(m, 2H); 7.46e7.40 (m, 4H); 7.32e7.28 (m, 2H); 1.54 (s, 6H).
d
: 8.18e8.16 (d, J ¼ 7.76 Hz, 2H); 7.90e7.88 (d,
2.2.2.2. 9,9-Dimethyl-7-(9H-9-carbazolyl)-2-fluorene boronic acid
(12). Orange-yellow power, yield: 70.2%, Mp: 102e115 ꢀC; 1H NMR
(400 MHz, CDCl3, TMS)
d: 8.25e8.16 (m, 3H); 7.82e7.80
2.2.2. Generalprocedureforthesynthesisof thecompounds(11,12,17)
These compounds were obtained following an essentially
similar procedure. An illustrative example is provided for
(d, J ¼ 7.72 Hz, 1H); 7.73 (s, 1H); 7.66e7.64 (d, J ¼ 8.10 Hz, 1H);
7.57e7.55 (d, J ¼ 5.33 Hz, 2H); 7.51e7.47 (m, 2H); 7.35e7.28 (m, 4H);
1.66 (s, 6H).
Scheme 3. Synthetic routes of intermediates 6e12, (d) Pd(PPh3)4, 2 M Cs2CO3, toluene:ethanol ¼ 3:1, reflux; (e) t-BuONa, CH3I, THF, 0 ꢀC.

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H.-F. Huang et al. / Dyes and Pigments 96 (2013) 705e713
Scheme 5. Synthetic routes of the star-shaped molecules S1e7, (f) NBS, CH2Cl2,
CH3COOH, 80 ꢀC; (j) PBr3, CHCl3, reflux.
0
ꢀC; (g) H2O2, F3CCOOH, 80 ꢀC; (h) The mixture of H2SO4 and HNO3, 90 ꢀC; (i) Acetyl bromide,
2.2.2.3. 9-(Phenylboronic-4-yl)-9H-carbazole-3,6-diboronic
acid
Anal. Calcd. For C29H19BrN2 (%): C, 73.27; H, 4.03; N, 5.89. Found:
C, 73.69; H, 4.71; N, 5.97.
(17). Gray power, yield: 59.1%, Mp: > 350 ꢀC. 1H NMR (400 MHz,
DMSO) d: 8.69.
(s, 2H), 8.27 (s, 2H), 8.09 (d, J ¼ 7.8 Hz, 2H), 8.04 (s, 4H), 7.89 (d,
2.2.4.3. 1-(30,50-Dibromophenyl)-4-diphenylaminobenzene
White power, yield: 85.0%, Mp: 181e184 ꢀC; 1H NMR (400 MHz,
CDCl3, TMS)
2H); 7.30e7.28 (t, J ¼ 5.12 Hz, 4H); 7.13e7.06 (m, 8H).
(13).
J ¼ 8.3 Hz, 2H), 7.61 (d, J ¼ 7.8 Hz, 2H), 7.38 (d, J ¼ 8.3 Hz, 2H).
d
: 7.62 (s, 2H); 7.57 (s, 1H); 7.39e7.37 (d, J ¼ 7.84 Hz,
2.2.3. 4-Bromophenyltriphenylsilane (5)
1,4-Dibromophenyl (7.08 g, 0.03 mol) was dissolved in anhy-
drous THF (120 mL) in a three-necked round-bottom flask fitted
with a magnetic stirrer, a condenser, a N2 purge and a ꢁ78 ꢀC dry
ice-ethanol bath. n-BuLi (2.5 M in hexane, 18.7 mL, 0.03 mol) was
added dropwise at ꢁ78 ꢀC and under N2 gas. After stirring for 1 h
at this temperature, triphenylchlorosilane (9.74 g, 0.03 mol) was
added quickly and the mixture was stirred at ꢁ78 ꢀC for 2 h. The
reaction mixture was stirred overnight allowing the temperature
to rise gradually to room temperature and evaporated under
reduced pressure. The residue was purified by silica gel chroma-
tography with petroleum ether as the eluent to afford a white
power (5) (10.58 g, 85.0%). Mp: 161e163 ꢀC; 1H NMR (400 MHz,
2.2.4.4. 2-(30,50-Dibromophenyl)-9,9-dimethylfluorene
(14). White
power, yield: 64.0%, Mp: 143e145 ꢀC; 1H NMR (400 MHz, CDCl3, TMS)
d
: 7.77 (s, 1H); 7.75e7.73 (d, J ¼ 9.2 Hz, 1H); 7.71 (s, 2H); 7.63 (s, 1H);
7.55 (s, 1H); 7.49e7.44 (m, 2H); 7.35e7.34 (t, J ¼ 3.4 Hz, 2H); 1.53
(s, 6H).
2.2.4.5. 2-(30,50-Dibromophenyl)-7-(9H-9-carbazolyl)-9,9-dimethyl-
fluorene (15). White crystalline power, yield: 81.3%, Mp: 253e
255 ꢀC; 1H NMR (400 MHz, CDCl3, TMS)
d: 8.21e8.20 (d,
J ¼ 7.74 Hz, 2H); 7.99e7.97 (d, J ¼ 7.94 Hz, 1H); 7.89e7.87 (d,
J ¼ 7.84 Hz, 1H); 7.78 (s, 2H), 7.70e7.69 (d, J ¼ 3.24 Hz, 2H); 7.64 (s,
1H); 7.61e7.59 (d, J ¼ 7.90 Hz, 2H); 7.53e7.47 (m, 4H); 7.36e7.32 (t,
J ¼ 7.36 Hz, 2H); 1.63 (s, 6H).
CDCl3, TMS)
d
: 7.56e7.53 (t, J ¼ 5.60 Hz, 6H); 7.53e7.7.51 (d,
J ¼ 8.00 Hz, 2H); 7.49e7.46 (t, J ¼ 5.40 Hz, 2H); 7.45e7.37 (m, 9H).
Anal. calcd. for C24H19BrSi (%): C, 69.39; H, 4.61. Found: C, 69.73;
H, 3.57.
2.2.5. General procedure for the synthesis of the compounds (8,9)
These compounds were obtained following an essentially
similar procedure. An illustrative example is provided for 8: 2-
bromofluorene (4.90 g, 0.02 mol) was dissolved in anhydrous THF
2.2.4. General procedure for the synthesis of the compounds (6,7,13,
14, 15)
These compounds were obtained following an essentially
similar procedure. An illustrative example is provided for 6: 4-
bromophenyl boronic acid (5.00 g, 0.025 mol), 2,5-
dibromopyridine (7.10 g, 0.03 mol) was dissolved in the mixture
of toluene (60 mL), ethanol (20 mL) and cesium carbonate aqueous
solution (2.0 M, 40 mL) in a three-necked round-bottom flask fitted
with a magnetic stirrer, a condenser and a N2 purge. The mixture
was stirred at room temperature for 0.5 h under N2 gas followed by
adding Pd(PPh3)4 (0.0003 mol, 0.35 g) and then heated slowly to
95 ꢀC for 24 h. After cooling to room temperature, the reaction
mixture was filtered and the filtrate was evaporated under reduced
pressure. The residue was purified by silica gel chromatograph with
petroleum ether/dichloromethane (8:1) as the eluent. Recrystalli-
zation from ethanol yielded 5.84 g (74.7%) of (6) as a white crys-
talline power.
(100 mL) in a three-necked round-bottom flask fitted with
a magnetic stirrer, a condenser and a N2 purge. t-BuONa (in THF,
8.64 g, 0.09 mol) was added dropwise at 0 ꢀC and under N2 gas. The
reaction mixture was stirred for 1.5 h at room temperature. Iodo-
methane (3.12 mL, 0.05 mol) was added dropwise and then the
reaction was continued for another 2 h the reaction mixture was
filtered and the filtrate was evaporated under reduced pressure. The
residue was purified by silica gel chromatography with petroleum
ether as the eluent to afford a white liquid (8) (5.21 g, 95.1%).
2.2.5.1. 2-Bromo-9,9-dimethylfluorene (8). Yield: 95.1%, 1H NMR
(400 MHz, CDCl3, TMS) d: 7.69e7.68 (m, 1H); 7.58 (s, 1H); 7.56e7.55
(t, J ¼ 2.80 Hz, 1H); 7.46e7.44 (m, 1H); 7.43e7.40 (m, 1H); 7.35e7.32
(m, 2H); 1.47 (s, 6H).
2.2.5.2. 2,7-Dibromo-9,9-dimethylfluorene (9). White liquid, yield:
2.2.4.1. 5-Bromo-2-(4-bromophenyl)pyridine (6). Yield: 74.7%, Mp:
93.5%, 1H NMR (400 MHz, CDCl3, TMS)
7.44 (m, 2H); 1.46 (s, 6H).
d: 7.55e7.53 (m, 4H); 7.47e
127e128 ꢀC; 1H NMR (400 MHz, CDCl3, TMS)
7.89e7.83 (m, 3H); 7.61e7.59 (d, 3H).
d: 8.73e8.72 (s, H);
2.2.6. 3,6-Dibromo-9-(40-bromophenyl)-9H-carbazole (16)
2.2.4.2. 2-(4-Bromophenyl)-5-[9-(benzyl-4-yl)-9H-carbazolyl] pyri-
dine (7). Yellow crystalline power, yield: 60.7%, Mp: 250e252 ꢀC;
Compound 2 (3.0 g, 0.09 mol) was dissolved in dichloromethane
(50 mL) in a three-necked round-bottom flask fitted with

H.-F. Huang et al. / Dyes and Pigments 96 (2013) 705e713
709
a magnetic stirrer. NBS (in dichloromethane, 4.10 g, 0.023 mol) was
added dropwise from a dropping funnel at 0 ꢀC and in the darkness.
After the addition was completed, the reaction mixture was stirred
for 6 h. It was quenched with water and extracted 3 times with
dichloromethane. The organic layer was evaporated under reduced
pressure. The residue was purified by silica gel chromatography
with petroleum ether/dichloromethane (10:1) as the eluent to
afford a gossypine solid (16) (3.37 g, 78.0%). Mp: 195e199 ꢀC; Anal.
calcd. for C18H10Br3N (%): C, 45.05; H, 2.10; N, 2.92. Found: C, 45.24;
H, 2.25; N, 2.67.
compound 13 (2.40 g, 0.005 mol), compound 4 (2.87 g, 0.01 mol)
was dissolved in the mixture of toluene (75 mL), ethanol (25 mL)
and cesium carbonate aqueous solution (2.0 M, 15 mL) in a three-
necked round-bottom flask fitted with
a magnetic stirrer,
a condenser and a N2 purge. The mixture was stirred at room
temperature for 0.5 h under N2 gas followed by adding Pd(PPh3)4
(0.0003 mol, 0.35 g) and then heated slowly to 95 ꢀC for 48 h. After
cooling to room temperature, the reaction mixture was filtered
and the filtrate was evaporated under reduced pressure. The
residue was purified by silica gel chromatography with petroleum
ether/dichloromethane (2:1) as the eluent to afford a white power.
The white power was sublimated by train sublimation (Model D-8-
25-880, Nanchang Choice Laboratory Instruments Ltd.) [47] to
afford a white crystalline power (S1). 2.2.10.1. (S1) White power,
2.2.7. 2,6-Dibromopyridine-N-oxide (18)
2,6-Dibromopyridine (6.0 g, 0.025 mol) was dissolved in tri-
fluoroacetic acid (20 ml) in a three-necked round-bottom flask
fitted with a magnetic stirrer and a condenser. H2O2 (30%, 60 ml)
was added dropwise from a dropping funnel at 80 ꢀC and then the
reaction continued for another 4 h; The reaction mixture was
poured into water and then cooled to ꢁ2 ꢀC, The cooling liquid was
filtered and the filtrate was extracted 3 times with dichloro-
methane and the organic layer was evaporated under reduced
pressure to afford a white power. Recrystallization from ethanol
yielded 3.70 g (58.5%) of (18) as a white crystalline power. Mp: 173e
175 ꢀC; Anal. calcd. for C5H3Br2NO (%): C, 23.75; H, 1.20; N, 5.54.
Found: C, 23.95; H, 1.30; N, 5.86.
yield: 78.6%, Mp: 271e273 ꢀC; FT-IR (KBr)
n
¼ 3035 (AreH), 1334
(AreN), 1H NMR (400 MHz, CDCl3, TMS)
d
: 8.19e8.17 (d, J ¼ 7.74 Hz,
4H); 7.99e7.97 (d, J ¼ 8.30 Hz, 4H); 7.94e7.92 (d, J ¼ 8.62 Hz, 3H);
7.73e7.71 (d, J ¼ 8.31 Hz, 4H); 7.67e7.65 (d, J ¼ 8.58 Hz, 2H); 7.53e
7.51 (d, J ¼ 8.10 Hz, 4H); 7.47e7.43 (t, J ¼ 7.62 Hz, 4H); 7.34e7.28
(m, 10H); 7.19e7.17 (d, J ¼ 7.73 Hz, 4H); 7.09e7.05 (t, J ¼ 7.31 Hz,
2H). 13C NMR (101 MHz, CDCl3þDMSO)
d: 147.25, 147.13, 141.77,
141.10, 140.32, 139.66, 136.67, 134.07, 129.04, 128.48, 127.70, 126.94,
125.80, 124.46, 124.10, 123.43, 122.94, 122.84, 119.99, 119.82,
109.52; Anal. calcd. for C60H41N3 (%): C, 89.63; H, 5.14; N, 5.23.
Found: C, 89.60; H, 5.16; N, 5.24. ESI - MS (m/z): calcd. for C60H41N3
803.33; Found 803.38 ([M]þ). 2.2.10.2. (S2) White crystalline
2.2.8. 2,6-Dibromo-4-nitropyridine-N-oxide (19)
Compound 18 (2.0 g, 0.008 mol) was dissolved in concentrated
sulfuric acid (20 ml) in a three-necked round-bottom flask fitted
with a magnetic stirrer and a condenser. Mixed acid (concen-
trated sulfuric acid (20 mL) and fuming nitric acid (10 mL)) was
added dropwise from a dropping funnel at 90 ꢀC and then the
reaction continued for another 2 h; The reaction mixture was
poured into crushed ice and was filtered to afford a yellow power.
Recrystallization from ethanol yielded 1.85 g (77.7%) of (19) as
a yellow crystalline power. Mp: 212e215 ꢀC; Anal. calcd. for
C5H2Br2N2O3 (%): C, 20.16; H, 0.68; N, 9.40. Found: C, 20.82; H,
0.69; N, 9.53.
power, yield: 75.4%, Mp: 182e183 ꢀC; FT-IR (KBr)
n
¼ 3050 (AreH),
2957 (CeH), 1337 (AreN), 1H NMR (400 MHz, DMSO, TMS)
d
: 8.30e
8.27 (t, J ¼ 6.66 Hz, 8H); 8.21 (s, 1H); 8.19(s, 3H); 8.01e7.96 (m, 2H);
7.93e7.91 (d, J ¼ 6.32 Hz, 1H); 7.83e7.81 (d, J ¼ 8.29 Hz, 4H); 7.61e
7.60 (d, J ¼ 6.81 Hz, 1H); 7.54e7.47 (m, 8H); 7.41e7.38 (m, 2H);
7.35e7.31 (m, 4H); 1.58(s, 6H). 13C NMR (101 MHz, CDCl3þDMSO)
d
: 159.19, 158.63, 147.69, 146.28, 145.40, 144.74, 144.58, 143.57,
143.29, 141.78, 133.66, 132.30, 132.05, 131.91, 131.22, 130.91, 130.04,
129.71, 128.02, 127.48, 126.32, 125.24, 125.09, 124.93, 114.62, 51.75,
32.07; Anal. calcd. for C57H40N2 (%): C, 90.92; H, 5.35; N, 3.72.
Found: C, 90.90; H, 5.39; N, 3.71. ESI - MS (m/z): calcd. for C57H40N2
752.32; Found 752.32 ([M]þ). 2.2.10.3. (S3) White crystalline
2.2.9. 2,4,6-Tribromopyridine (20)
power, yield: 72.8%, Mp: 244e245 ꢀC; FT-IR (KBr)
n
¼ 3040 (AreH),
Compound 19 (2.0 g, 0.007 mol) was dissolved in glacial
acetic acid (35 mL) in a three-necked round-bottom flask fitted
with a magnetic stirrer, a condenser and a N2 purge. Acetyl
bromide (1.0 mL, 0.013 mol) was added slowly at 60 ꢀC under N2
gas. The mixture was stirred at room temperature for 0.5 h
under N2 gas and then heated to 80 ꢀC for 2 h. After cooling to
room temperature, the reaction mixture was filtered and the
residue was washed with a little glacial acetic acid and was dried
with KOH under vacuum. The dry residue was dissolved in
chloroform (25 mL) in a three-necked round-bottom flask fitted
with a magnetic stirrer, a condenser and a N2 purge. PBr3
(7.9 mL, 0.084 mol) was added dropwise at room temperature
under N2 gas and continued stirring for 1 h and then heated to
65 ꢀC for 24 h. After cooling to room temperature, the reaction
mixture was neutralized with NaHCO3 (aq) and then was
extracted 3 times with dichloromethane and the organic layer
was evaporated under reduced pressure to afford a white power.
Recrystallization from ethanol yielded 1.33 g (62.7%) of (20) as
a white crystalline power. Mp: 100e102 ꢀC; Anal. calcd. for
C5H2Br3N(%): C, 19.02; H, 0.64; N, 4.44. Found: C, 18.88; H, 0.59;
N, 4.84.
2956 (CeH), 1334 (AreN), 1H NMR (400 MHz, DMSO, TMS)
d
: 8.31e
8.29 (d, J ¼ 7.78 Hz,11H); 8.23e8.20 (d, J ¼ 10.80 Hz, 4H); 8.14e8.12
(d, J ¼ 7.91 Hz, 1H); 8.06e8.04 (d, J ¼ 7.82 Hz, 1H); 7.91 (s, 1H);
7.84e7.82 (d, J ¼ 8.31 Hz, 4H); 7.65e7.63 (d, J ¼ 7.90 Hz, 1H); 7.54e
7.48 (m, 12H); 7.35e7.30 (m, 6H); 1.68 (s, 6H). 13C NMR (101 MHz,
CDCl3þDMSO)
d: 160.54, 159.45, 147.60, 146.35, 145.56, 145.41,
144.95, 144.72, 142.82, 142.68, 141.83, 141.38, 133.63, 132.05,
131.48, 130.87, 130.57, 130.11, 129.83, 128.06, 127.98, 126.41, 126.18,
126.00, 125.52, 125.07, 124.91, 124.79, 114.61, 52.10, 32.02; Anal.
calcd. for C66H47N3 (%): C, 89.87; H, 5.37; N, 4.76. Found: C, 90.21;
H, 5.19; N, 4.60. ESI - MS (m/z): calcd. for C69H47N3 917.38; Found
917.3 ([M]þ). 2.2.10.4. (S4) White crystalline power, yield: 70.9%,
Mp: 202e210 ꢀC; FT-IR (KBr)
n
¼ 3064 (AreH), 1367 (AreN), 702
(SieC), 1H NMR (400 MHz, CDCl3, TMS)
d
: 7.88 (d, J ¼ 8.2 Hz, 2H);
7.82e7.68 (m, 17H), 7.65 (d, J ¼ 7.1 Hz, 18H); 7.57 (d, J ¼ 8.5 Hz, 2H);
7.44 (m, J ¼ 6.4 Hz, 28H). 13C NMR (101 MHz, DMSO)
d: 141.70,
139.93, 136.65, 136.03, 133.85, 133.70, 132.63, 130.10, 129.19,
128.73, 128.40, 128.01, 126.99, 126.64; Anal. calcd. for C90H67NSi3
(%): C, 86.70; H, 5.42; N, 1.12. Found: C, 86.75; H, 5.30; N, 1.20. ESI -
MS (m/z): calcd. for C90H67NSi3 1245.46; Found 1245.1 ([M]þ).
2.2.10.5. (S5) White crystalline power, yield: 62.3%, Mp: 202e
204 ꢀC; FT-IR (KBr)
n
¼ 3051 (AreH), 1597 (C]N, pyridine), 1336
2.2.10. General procedure for the synthesis of the compounds (S1, S2,
S3, S4, S5, S6, S7)
These compounds were obtained following an essentially
similar procedure. An illustrative example is provided for S1:
(AreN), 1H NMR (400 MHz, CDCl3, TMS)
d
: 8.18 (d, J ¼ 7.7 Hz, 7H);
8.13e8.02 (m, 11H); 7.88 (m, J ¼ 8.2 Hz, 10H); 7.72 (d, J ¼ 8.2 Hz,
7H); 7.50 (m, J ¼ 7.5 Hz, 25H); 7.33 (t, J ¼ 7.4 Hz, 7H). 13C NMR
(101 MHz, CDCl3þDMSO)
d: 155.72, 147.47, 140.23, 138.31, 137.12,

710
H.-F. Huang et al. / Dyes and Pigments 96 (2013) 705e713
136.13, 134.92, 133.62, 128.88, 128.53, 128.12, 127.13, 126.48,
125.84, 123.00, 120.13, 120.01, 119.92, 109.51; Anal. calcd. for
C, 87.59; H, 5.48; N, 6.93. Found: C, 87.11; H, 5.42; N, 6.97. ESI - MS
(m/z): calcd. for C59H44N4 808.36; Found 808.1 ([M]þ).
C
105H67N7 (%): C, 88.39; H, 4.73; N, 6.87. Found: C, 87.71; H, 4.60; N,
7.10. ESI - MS (m/z): calcd. for C105H67N7 1425.55; Found 1425.4
([M]þ). 2.2.10.6. (S6) White power, yield: 60.6%, Mp: 331e338 ꢀC;
3. Results and discussion
FT-IR (KBr)
n
¼ 3052 (AreH), 1602 (C]N, pyridine), 1338 (AreN),
1H NMR (400 MHz, CDCl3, TMS)
d
8.54 (d, J ¼ 8.5 Hz, 4H), 8.18
3.1. Thermal analysis
(dd, J ¼ 9.7, 6.2 Hz, 8H), 8.10 (d, J ¼ 8.4 Hz, 2H), 7.86e7.75 (m, 6H),
7.56 (d, J ¼ 8.2 Hz, 6H), 7.48 (dd, J ¼ 13.0, 5.9 Hz, 6H), 7.34 (dd,
For optoelectronic applications, thermal stability of organic
materials is crucial for device stability and lifetime, because during
the electroluminescent process heat is generated which affects the
material morphology and the device performance leading to the
degradation of the OLED [48]. The thermal properties of S1 e S7
were investigated by TGA and DSC. Results are shown (Fig. 1) and
summarized in Table 1. The TGA measurement reveals that S1e7 all
are thermally stable materials, with the onset of decomposition
temperature Td (5% weight loss) as high as 595 ꢀC for S4. In the DSC
measurement, very high Tg values were observed. The Tg values of
all compounds are above 162.0 ꢀC, while that of compound S6 is the
highest (336.0 ꢀC); to our knowledge, this value is the highest Tg
ever reported for electroluminescent materials.
J ¼ 13.9, 6.8 Hz, 6H). 13C NMR (101 MHz, CDCl3þDMSO)
d: 158.34,
154.21, 146.82, 145.56, 141.22, 139.82, 135.44, 131.22, 130.9, 127.66,
127.2, 122.11, 121.73, 119.64, 114.71, 108.22; Anal. calcd. for
C
59H44N4 (%): C, 88.25; H, 4.77; N, 6.98. Found: C, 87.75; H, 5.13; N,
6.95. ESI - MS (m/z): calcd. for C59H38N4 802.31; Found 803.2
([M]þ). 2.2.10.7. (S7). Yellow power, yield: 68.2%, Mp: 255e257 ꢀC;
FT-IR (KBr)
n
¼ 3032 (AreH), 1590 (C]N, pyridine), 1375 (AreN),
1H NMR (400 MHz, CDCl3, TMS)
d
: 8.05 (d, J ¼ 8.6 Hz, 4H): 7.75
(s, 2H); 7.61 (d, J ¼ 8.6 Hz, 2H); 7.36e7.27 (m, 12H); 7.16 (t,
J ¼ 8.6 Hz, 18H); 7.06 (m, J ¼ 7.3 Hz, 6H). 13C NMR (101 MHz, CDCl3)
d
: 156.91, 148.72, 147.56, 147.38, 129.45, 129.33, 128.03, 127.86,
124.93, 124.72, 123.53, 123.20, 115.35; Anal. calcd. for C59H38N4 (%):
Fig. 1. a) TGA curves of S1e7; b) DSC thermograms (second heating) of S1e7 at a heating rate of 10 ꢀC/min; c) DSC curve of S1,5 at a heating rate of 10 ꢀC/min, a cooling rate of 20 ꢀC/
min; d) TGA curve of S6 at a heating rate of 20 ꢀC/min.

H.-F. Huang et al. / Dyes and Pigments 96 (2013) 705e713
711
Table 1
Physical properties of star-shaped molecules S1eS7.
S1
S2
S3
S4
S5
S6
S7
lAbs/nm
lex/nm
V/%
222
424
54.2
3.76
221
387
55.7
3.63
223
400
72.9
3.46
219
396
35.0
3.64
221
419
87.2
3.58
342
425
38.8
3.52
362
456
46.2
3.34
Eg/eV
Eox/eV
HOMO/eV
LOMO/eV
Tm/ꢀC
1.19
e5.36
ꢁ2.15
287.7
162.0
527
1.22
e5.39
ꢁ2.31
168.4
164.6
499
1.36
e5.53
ꢁ2.62
204.7
201.2
558
1.50
e5.67
ꢁ2.58
208.2
203.7
595
1.65
e5.82
ꢁ2.79
214.6
196.8
360
1.20
e5.37
ꢁ2.40
342.0
336.0
544
1.01
e5.18
ꢁ2.39
267.4
256.3
473
Tg/ꢀC
Td/ꢀC
Fig. 3. Absorption of S1L7 in dilute CH2Cl2 solution.
3.2. Electrochemical properties
In order to investigate the electrochemical behavior of the star-
shaped molecules, CV measurements were performed at 25 ꢀC in
CH2Cl2 solution containing 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as supporting electrolyte with a glassy
carbon working electrode and Ag/AgCl as the reference electrode
[49,50]. The results are shown in Fig. 2 and the electrochemical
potentials are summarized in Table 1. Compounds S2e6 demon-
strate two quasi-reversible oxidation processes, while compound
S1 and S7 exhibit one quasi-reversible oxidation process and three
reversible oxidation processes, respectively.
Based on the onset oxidation potentials of the samples and Fc
(0.55 V), the HOMO energy levels were estimated by assuming the
energy level of ferrocene/ferrocenium (Fc/Fc) to be ꢁ4.72 eV below
the vacuum level [51e53]. The LOMO energy levels and the energy
gaps were calculated from the spectroscopic and electrochemical
data (Table 1). The compounds with high electron density moieties
such as S5 have relatively high HOMO levels.
results are shown in Fig. 3 and Fig. 4 and are listed in Table 1 [54].
Compounds S1e7 exhibited three major absorption bands. One is in
the high energy 220e250 nm region, corresponding to the spin-
allowed,
one covers the lower energy 270e310 nm region, corresponding to
the * absorption induced by the large conjugative phenyl
chain. Finally, the weakest one at the lowest energy 310e400 nm
pep* adsorption of peripheral phenyl groups. Another
pep
region were attributed to the forbidden n-p* absorption localized
at the N atoms.
For S7, with pyridine as the central group and triphenylamine as
the substituent groups, the emission moves toward the lower
energy region with lmax lying at 455 nm which constitutes a pure
blue emission. For S2,4, with the fewer N atoms compared with
other molecules (therefore, fewer n-p* transition), lmax lies at
about 385e395 nm which is a violet-blue. By comparing the
chemical structure of S2 with that of S3, the fewer n-p* transitions
result in a hypsochromicly shift [52e54].
S
1L7 in this study all are highly fluorescent with emission color
ranging from near-violet to pure-blue. The quantum yields (V) of
1e7 were determined in dilute CH2Cl2 solution by comparison with
3.3. Photophysical properties
S
the emission of quinine sulfate dihydrate (V ¼ 0.48) as the standard
The optical properties of the S1e7 were investigated by UV
absorption and PL spectroscopy in dilute CH2Cl2 solution, the
[55]. The fluorescence quantum yield of those compounds ranges
Fig. 2. CV curves of S1L7 in CH2Cl2 containing 0.1 M TBAPF6 at a scan rate of 100 mV minꢁ1
.

712
H.-F. Huang et al. / Dyes and Pigments 96 (2013) 705e713
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Fig. 4. Emission spectra of S1L7 in dilute CH2Cl2 solution excited at 313 nm.
from 0.350 to 0.872 (Table 1) with S5 as the highest. The high
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-conjugation between the three long arms of star-shaped
p
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4. Conclusions
In summary, we have successfully synthesized and fully charac-
terized a series of well-defined, thermally and electrochemically
highly stable star-shaped molecules via Pd-catalyzed Suzuki cross-
coupling reactions in good yields. The materials exhibited high
fluorescence quantum yields in solution and they should be prom-
ising pure-blue light-emitting, hole-transport and host materials for
OLED devices. The OLED performance based on these materials is
presently under investigation.
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Acknowledgment
This work was financially supported by the National Natural
Science Foundation of China (60868001, 21162017).
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