Star-shaped donor-π-acceptor conjugated oligomers with 1,3,5-triazine cores: Convergent synthesis and multifunctional properties

Article abstract of DOI:10.1021/jp104710y

Monodispersed oligomers have been widely developed and used in different optoelectronic areas due to their well-defined molecular structures, high purity, and solution processability. Star-shaped oligomers are especially interesting for OLED application because of their antiaggregation abilities and stable electroluminescence. In addition, star-shaped donor-π-acceptor conjugated molecules are known to afford good nonlinear optical and two-photon absorption properties due to the intramolecular charge transfer and cooperative enhancement effects. In this context, three generations of highly soluble 1,3,5-triazine based donor-π-acceptor compounds, TFT1, TFT2, and TFT3, were prepared through a convergent synthetic strategy and their optoelectronic properties were fully studied, which showed distinct correlations with the structures. Closed-aperture and open-aperture Z-scan methods were employed to measure the nonlinear refractive index and two-photon absorption properties of the oligomers, respectively. TFT1 showed a high nonlinear refractive index of 4.14 × 10^-12 esu in THF solution with an excitation wavelength of 800 nm. Also, TFT1 exhibited a large two-photon absorption cross section of 234 GM and a frequency up-converted two-photon excited fluorescence with a λ_TPEF^max value of 527 nm under 800 nm laser irradiation with a pulse duration of 140 fs. OLED devices using the spin-coated films of these oligomers as an active layer showed intensive blue electroluminescence with a maximum luminance of 3093 cd/m² at a current efficiency of 1.47 cd/A from TFT1.

Full text of DOI:10.1021/jp104710y

10374
J. Phys. Chem. B 2010, 114, 10374–10383
Star-Shaped Donor-π-Acceptor Conjugated Oligomers with 1,3,5-Triazine Cores:
Convergent Synthesis and Multifunctional Properties
Shijie Ren,† Danli Zeng,† Hongliang Zhong,† Yaochuan Wang,‡ Shixiong Qian,*,‡ and
Qiang Fang*,†
Key Laboratory of Organofluorine Chemistry and Laboratory for Polymer Materials, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. China, and
Surface Physics Lab (National Key Lab) and Physics Department, Fudan UniVersity, 220 Handan Road,
Shanghai 200433, P. R. China
ReceiVed: May 22, 2010; ReVised Manuscript ReceiVed: July 15, 2010
Monodispersed oligomers have been widely developed and used in different optoelectronic areas due to their
well-defined molecular structures, high purity, and solution processability. Star-shaped oligomers are especially
interesting for OLED application because of their antiaggregation abilities and stable electroluminescence. In
addition, star-shaped donor-π-acceptor conjugated molecules are known to afford good nonlinear optical and
two-photon absorption properties due to the intramolecular charge transfer and cooperative enhancement effects.
In this context, three generations of highly soluble 1,3,5-triazine based donor-π-acceptor compounds, TFT1,
TFT2, and TFT3, were prepared through a convergent synthetic strategy and their optoelectronic properties
were fully studied, which showed distinct correlations with the structures. Closed-aperture and open-aperture
Z-scan methods were employed to measure the nonlinear refractive index and two-photon absorption properties
of the oligomers, respectively. TFT1 showed a high nonlinear refractive index of 4.14 × 10-12 esu in THF
solution with an excitation wavelength of 800 nm. Also, TFT1 exhibited a large two-photon absorption cross
max
section of 234 GM and a frequency up-converted two-photon excited fluorescence with a λTPEF value of
527 nm under 800 nm laser irradiation with a pulse duration of 140 fs. OLED devices using the spin-coated
films of these oligomers as an active layer showed intensive blue electroluminescence with a maximum
luminance of 3093 cd/m2 at a current efficiency of 1.47 cd/A from TFT1.
Introduction
in the solid state, and the fact that more functional groups can
be merged into the branched structure means there are more
opportunities to fine-tune the optoelectronic performances and
to study the relationship between structures and properties.
Putting an electron donor and acceptor into one molecule can
normally lead to broad absorption, lower band gap, easily tuned
fluorescence, and good nonlinear optical properties.8 Also,
multibranched structures are known to bring cooperative
enhancement in terms of conjugation expansion and two-photon
absorption (TPA) properties.9
In the past two decades, extensive research attention has been
paid to high-performance organic materials used in optoelec-
tronic fields, such as organic light-emitting diodes (OLEDs),1
organic solar cells,2 and organic thin-film transistors.3 At present,
there are manly two kinds of materials being developed for
organic electronics: small molecules1a,4 and conjugated poly-
mers.5 However, both groups of material have certain draw-
backs. Organic small molecules are prone to crystallization, and
typically high temperature vacuum deposition has to be used
to fabricate the devices. On the other hand, although they can
be processed simply with methods such as spin-coating and
inkjet printing, polymer materials are more difficult to purify
and thus stability and reproducibility remain big problems. In
recent years, monodispersed oligomers or dendrimers which sit
between small molecules and polymers have gradually attracted
more and more attention and become a third choice of the
organic optoelectronic materials.6 Ideally, they can combine the
advantages of both conjugated polymers and small molecules
together, such as good solution processability and well-defined
molecular structure.
1,3,5-Triazine containing materials have been widely used
in industry owing to their high thermal stability derived from
the structural symmetry of 1,3,5-triazine units,10 and recently
quite a bit of attention has been paid to 1,3,5-triazine containing
π-conjugated systems because of the unique properties such as
high electron deficiency and spatial coplanarity.11 1,3,5-Triazine
based small molecules and conjugated polymers have been
employedaselectrontransportingandholeblockingmaterials,11b,c,12
emissive layers,13 or host materials for phosphorescent dyes14
in OLED devices. For TPA application, a series of star-shaped
1,3,5-triazine based chromophores were reported in 2004,15
which showed promising TPA properties because of strong
intramolecular charge transfer (ICT). In that paper, the authors
discussed the effects of different side chains on the properties.
Recently, star-shaped novel donor-π-acceptor (D-π-A) mol-
ecules containing 1,3,5-triazine-2,4,6-trithiophene units were
published and therein different donor groups and π-conjugated
bridges were studied according to their influences on the TPA
performance.16
Compared with their linear counterparts, conjugated star-
shaped molecules have exhibited some special merits guaranteed
by the unique structures.7 Branched molecular configuration can
afford better electroluminescent stability due to less aggregation
* To whom correspondence should be addressed. E-mail:
qiangfang@mail.sioc.ac.cn.
† Chinese Academy of Sciences.
‡ Fudan University.
10.1021/jp104710y  2010 American Chemical Society
Published on Web 07/22/2010

Star-Shaped Donor-π-Acceptor Conjugated Oligomers
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10375
However, there have only been limited reports on the use
of 1,3,5-triazine based oligomers or dendrimers as organic
electronic materials,17 which are supposed to be advantageous
in terms of film-forming and solution processability. Here,
we report the synthesis and characterization of a series of
1,3,5-triazine cored star-shaped oligomers. The performance
of the novel D-π-A dendrimers in OLEDs and their nonlinear
optical properties were investigated. The results show good
structure-property correlation and promising OLED device
behavior and decent TPA cross sections were achieved.
was separated, washed twice with water, and then dried over
Na2SO4. The solvent was removed under reduced pressure to
give a crude product, which was further purified by column
chromatography using CH2Cl2/petroleum ether (10/80, v/v) as
eluent to offer FT1 as a transparent viscous liquid in a yield of
1
91%. H NMR (300 MHz, CDCl3, δ): 8.85 (3H, d), 8.76 (3H,
s), 7.94 (3H, d), 7.84 (3H, m), 7.38-7.43 (9H, m), 2.12 (12H,
m), 1.02-1.20 (60H, m), 0.65-0.82 (30H, m). 13C NMR (100
MHz, CDCl3, δ): 171.83, 151.97, 151.13, 145.55, 140.38,
135.34, 128.28, 128.04, 126.95, 123.31, 123.10, 120.52, 119.77,
55.28, 40.37, 31.80, 30.08, 29.27, 29.24, 23.86, 22.59, 14.04,
13.85. FT-IR (NaCl pellets, cm-1): 2928, 2854, 1609, 1579,
1513, 1466, 1370, 823, 740. MS (MALDI-TOF): m/z 1247.1
(M+). Anal. Calcd for C90H123N3: C, 86.69; H, 9.94; N, 3.37.
Found: C, 87.00; H, 10.14; N, 3.36.
Experimental Section
Reagents and Instruments. Chemical reagents and solvents
were purchased from Acros or Aldrich and used without further
purification unless otherwise noted. Toluene and THF were
distilled from sodium, and dichloromethane and chloroform were
distilled over KOH. The starting compounds 2-bromo-9,9-
dioctyl-fluorene (1),18 4-bromo-N,N-diphenylaniline (3),19 and
2,7-dibromo-9,9-dioctyl-fluorene (5)18 were prepared according
to published procedures. All air and water sensitive reactions
were performed under a dry argon atmosphere.
N,N-Diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)aniline (4). 13.4 mL (21.5 mmol, 1.25 equiv) of n-BuLi (1.6
M in hexane) was added through a syringe over about 30 min
to a solution of 3 (5.575 g, 17.2 mmol) in dry THF (150 mL)
at -78 °C. The mixture was stirred for 1 h at the temperature,
and then, 9.7 g (51.6 mmol, 3 equiv) of boronic acid triisopro-
pylester was added. After being stirred overnight at room
temperature, the reaction mixture was poured into 200 mL of
water and extracted with ethyl acetate. The organic layer was
washed with water three times and dried over Na2SO4. The
solvent was removed under reduced pressure to give a crude
product. The crude product was purified by column chroma-
tography using ethyl acetate/petroleum ether (20/80, v/v) as
eluent to give 4-(diphenylamino)phenylboronic acid, and the
product was used in the next step directly.
1H NMR and 13C NMR spectra were recorded on a Bruker
DPX 400 or Varian mercury 300 spectrometer using CDCl3 as
solvent and TMS as internal standard, respectively. Mass spectra
were taken with a HP 5989A or Finnigan 4021 mass spec-
trograph. Elemental analysis was performed with an Elementer
Vario EL elemental analyzer. UV-vis and PL spectra were
obtained with a Hitachi UV-2910 and a Hitachi F-4500
spectrophotometer, respectively. Thermal stability was deter-
mined with a TGA Q500 thermogravimetric analyzer at a
heating rate of 10 °C min-1 in nitrogen. Cyclic voltammetry of
the cast films of the compounds on ITO glass (working
electrode) was measured in an acetonitrile solution of Bu4NBF4
(0.10 M) under argon using (0.10 M AgNO3)/Ag and platinum
wire as reference and counter electrodes, respectively. The
ferrocenium/ferrocene (Fc+/Fc) couple was used as the internal
standard. A CHI 600B analyzer was used for the cyclic
voltammetry.
Thus, the boronic acid obtained was dissolved in 300 mL of
toluene and 4.059 g (34.4 mmol) of pinacol was added into the
mixture. The reaction solution was refluxed with a Dean-Stark
apparatus for 10 h. Then, the organic layer was separated and
washed twice with water. After being dried over Na2SO4, the
solvent was removed under reduced pressure. The crude product
was purified by column chromatography using ethyl acetate/
petroleum ether (10/80, v/v) as eluent to give 4 as white solid
in a yield of 64% totally. 1H NMR (300 MHz, CDCl3, δ): 7.66
(2H, d), 7.26 (4H, m), 7.01-7.12 (8H, m), 1.33 (12H, s).
7-Bromo-9,9-dioctyl-9H-fluorene-2-carbonitrile (6). A mix-
ture of 5 (24.08 g, 43.94 mmol) and CuCN (3.54 g, 39.55 mmol,
0.9 equiv) in NMP (100 mL) was purged with argon and
refluxed overnight. Then, the reaction temperature was reduced
to 120 °C and 20 g of FeCl3 in 10 mL of concentrated HCl and
40 mL of H2O were added into the dark solution. The mixture
was stirred for a further 30 min. After being cooled at room
temperature, the mixture was extracted with toluene (150 mL)
twice and the organic layer was washed with 100 mL of 6 N
HCl, water, 10% NaOH solution, and water again prior to being
dried over Na2SO4. The solvent was then removed under reduced
pressure to give a crude product. The crude product was purified
by column chromatography using CH2Cl2/petroleum ether (20/
80, v/v) as eluent, followed by recrystallization from ethanol
Synthesis. 9,9-Dioctyl-9H-fluorene-2-carbonitrile (2). A
mixture of 1 (2.192 g, 4.674 mmol) and CuCN (0.627 g, 7
mmol, 1.5 equiv) in NMP (40 mL) was purged with argon and
refluxed overnight. Then, the reaction temperature was reduced
to about 120 °C and 4 g of FeCl3 in 2 mL of concentrated HCl
and 10 mL of distilled H2O were added into the dark solution.
The mixture was stirred for a further 30 min. After being cooled
at room temperature, the mixture was extracted with toluene
(150 mL) twice and the organic layer was washed with 100
mL of 6 N HCl, water, 10% NaOH solution, and water again
prior to being dried over Na2SO4. The solvent was then removed
under reduced pressure to give a crude product. The crude
product was purified by column chromatography using CH2Cl2/
petroleum ether (20/80, v/v) as eluent to give 2 as a colorless
1
viscous liquid in a yield of 68%. H NMR (300 MHz, CDCl3,
δ): δ 7.73-7.78 (2H, m), 7.60-7.65 (2H, m), 7.36-7.39 (3H,
m), 1.97 (4H, m), 1.04-1.22 (20H, m), 0.82 (6H, t), 0.54 (4H,
m). FT-IR (NaCl pellets, cm-1): 2927, 2855, 2224, 1610, 1458,
1380, 740. MS (EI), m/z 416 (M+).
2,4,6-Tris(9,9-dioctyl-9H-fluoren-2-yl)-1,3,5-triazine (FT1).
To a vigorously stirring solution of trifluoromethanesulfonic acid
(2 mL) was added dropwise 2 (1.313 g, 3.164 mmol) in dry
CHCl3 (3 mL) over about 10 min at 0 °C under an Ar
atmosphere. After being stirred at room temperature for 2 days,
the mixture was diluted with 100 mL of CHCl3 and poured into
100 mL of 10% NH4OH aqueous solution. The organic layer
1
to offer 6 as white crystals in a yield of 53%. H NMR (300
MHz, CDCl3, δ): 7.74 (1H, dd), 7.63 (3H, m), 7.52 (2H, m),
1.95 (4H, t), 1.03-1.25 (20H, m), 0.83 (6H, t), 0.55 (4H, m).
FT-IR (NaCl pellets, cm-1): 2928, 2854, 2223, 1611, 1458,
1376, 821. MS (EI): m/e 496, 494 (M+).
7-(4-(Diphenylamino)phenyl)-9,9-dioctyl-9H-fluorene-2-car-
bonitrile (7). To a 100 mL Schlenk tube charged with 6 (1.381
g, 2.8 mmol), 4 (1.296 g, 3.5 mmol), Pd(PPh3)4 (0.161 g, 0.14
mmol), and Na2CO3 (2.12 g, 20 mmol) were added 50 mL of
THF and 10 mL of H2O. The mixture was frozen with liquid

10376 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Ren et al.
N2 and pumped to a vacuum for 20 min and then thawed with
ethanol in an Ar atmosphere. The procedure above was repeated
three times. The mixture was heated to 80 °C and stirred for
72 h at this temperature. After being cooled to room temperature,
the mixture was diluted with 100 mL of toluene. The organic
layer was separated, washed with water, and dried over
anhydrous Na2SO4. The solvent was then removed under
reduced pressure to give a crude product. The crude product
was purified by column chromatography using CH2Cl2/
petroleum ether (10/80, v/v) as eluent, followed by recrystal-
lization from ethanol to offer 7 as yellowish green crystals in a
yield of 87%. 1H NMR (300 MHz, CDCl3, δ): 7.74-7.79 (2H,
m), 7.53-7.65 (6H, m), 7.25-7.31 (4H, m), 7.14-7.18 (6H,
m), 7.05 (2H, m), 2.09 (4H, m), 1.05-1.20 (20H, m), 0.87 (6H,
t), 0.62 (4H, m). 13C NMR (100 MHz, CDCl3, δ): 152.17,
151.64, 147.60, 145.55, 141.52, 137.96, 134.77, 131.41, 129.37,
127.92, 126.46, 125.97, 124.57, 123.78, 123.18, 121.18, 121.01,
120.19, 120.01, 109.68, 55.62, 40.21, 31.80, 29.92, 29.21, 23.80,
22.63, 14.11. FT-IR (NaCl pellets, cm-1): 2926, 2854, 2223,
1593, 1515, 1494, 1465, 1375, 1281, 817, 753, 696. MS
(MALDI-TOF): m/z 658.5 (M+). Anal. Calcd for C48H54N2: C,
87.49; H, 8.26; N, 4.25. Found: C, 87.18; H, 8.30; N, 4.07.
1H NMR (300 MHz, CDCl3, δ): 7.78-7.83 (2H, m), 7.60-7.74
(8H, m), 7.49-7.53 (2H, m), 2.02 (8H, m), 1.07-1.25 (40H,
m), 0.63-0.87 (20H, m), 0.31 (9H, s).
7′-Iodo-9,9,9′,9′-tetraoctyl-9H,9′H-2,2′-bifluorene-7-carbo-
nitrile (11). 1.89 g (2.16 mmol) of 10 was dissolved in 25 mL
of dry dichloromethane (DCM), and the solution was cooled
down to 0 °C. Then, 2.59 mL (2.59 mmol, 1.2 equiv) of ICl (1
M in DCM) was added through a syringe over about 20 min.
After being stirred for 2 h at room temperature, the reaction
mixture was poured into 200 mL of 15% aqueous NaHSO3
solution and extracted with DCM. The organic layer was washed
with water three times and dried over Na2SO4. The solvent was
removed under reduced pressure to give a crude product. The
crude product was purified by column chromatography using
CH2Cl2/petroleum ether (10/80, v/v) as eluent, followed by
recrystallization from chloroform/ethanol to offer 11 as a white
1
solid in a yield of 100%. H NMR (300 MHz, CDCl3, δ):
7.78-7.83 (2H, m), 7.60-7.74 (8H, m), 7.49-7.53 (2H, m),
2.02 (8H, m), 1.07-1.25 (40H, m), 0.63-0.87 (20H, m), 0.31
(9H, s). 13C NMR (100 MHz, CDCl3, δ): 153.40, 152.22, 151.66,
151.01, 145.35, 142.27, 140.51, 140.25, 139.76, 138.38, 135.94,
132.14, 131.38, 126.61, 126.49, 126.36, 121.51, 121.38, 121.08,
120.25, 120.12, 119.88, 109.82, 92.68, 55.63, 55.48, 40.18,
40.07, 31.75, 31.72, 29.89, 29.85, 29.14, 23.77, 23.73, 22.59,
22.56, 14.03. FT-IR (NaCl pellets, cm-1): 2926, 2854, 2222,
1609, 1455, 1402, 1377, 1261, 1002, 892, 811, 755, 722. MS
(MALDI-TOF): m/z 803.5 (M-I)+. Anal. Calcd for C59H80IN:
C, 76.18; H, 8.67; N, 1.51. Found: C, 76.63; H, 8.68; N, 1.49.
4,4′,4′′-(7,7′,7′′-(1,3,5-Triazine-2,4,6-triyl)tris(9,9-dioctyl-9H-
fluorene-7,2-diyl))tris(N,N-diphenylani-line) (TFT1). This com-
pound was prepared using a similar procedure to that used for
the synthesis of FT1. Yellow solid. Yield of 78%. M.p.: 123
°C. 1H NMR (300 MHz, CDCl3, δ): 8.87 (3H, d), 8.78 (3H, s),
7.95 (3H, d), 7.88 (3H, d), 7.59-7.65 (12H, m), 7.25-7.32
(12H, m), 7.16-7.22 (18H, m), 7.06 (6H, t), 2.11-2.22 (12H,
m), 1.09-1.26 (60H, m), 0.73-0.88 (30H, m). 13C NMR (100
MHz, CDCl3, δ): 171.90, 152.74, 151.40, 147.72, 147.32,
145.37, 140.58, 139.28, 135.41, 135.26, 129.38, 128.43, 127.94,
125.77, 124.49, 124.00, 123.33, 123.07, 121.13, 120.87, 119.81,
55.44, 40.51, 31.87, 30.15, 29.31, 23.96, 22.67, 14.12. FT-IR
(NaCl pellets, cm-1): 2926, 2853, 1593, 1511, 1492, 1467, 1369,
1280, 814, 752, 695. MS (MALDI-TOF): mz 1977.6 (M+).
Anal. Calcd for C144H162N6: C, 87.49; H, 8.26; N, 4.25. Found:
C, 87.28; H, 8.14; N, 4.06.
7′-(4-(Diphenylamino)phenyl)-9,9,9′,9′-tetraoctyl-9H,9′H-
2,2′-bifluorene-7-carbonitrile (12). This compound was pre-
pared using a similar procedure to that used for the synthesis
of 7 in a yield of 69%. 1H NMR (300 MHz, CDCl3, δ):
7.77-7.84 (4H, m), 7.56-7.71 (10H, m), 7.25-7.31 (4H, m),
7.15-7.19 (6H, m), 7.06 (2H, m), 2.05 (8H, m), 1.01-1.25
(40H, m), 0.58-0.83 (20H, m). 13C NMR (100 MHz, CDCl3,
δ): 152.19, 151.82, 151.67, 147.68, 147.13, 145.44, 142.52,
140.57, 139.67, 139.46, 138.21, 135.51, 131.37, 129.28, 127.79,
126.59, 126.48, 126.23, 125.60, 124.38, 123.98, 122.94, 121.49,
121.42, 121.06, 121.00, 120.21, 120.07, 119.94, 109.72, 55.62,
55.30, 40.39, 40.10, 31.76, 31.73, 29.99, 29.87, 29.70, 29.18,
29.15, 23.83, 23.79, 22.57, 14.04. FT-IR (NaCl pellets, cm-1):
3035, 2926, 2854, 2224, 1593, 1515, 1493, 1461, 1377, 1278,
815, 753, 695. MS (MALDI-TOF): m/z 1047.2 (M+). Anal.
Calcd for C77H94N2: C, 88.28; H, 9.04; N, 2.67. Found: C, 88.02;
H, 9.34; N, 2.25.
(7-Bromo-9,9-dioctyl-9H-fluoren-2-yl)trimethylsilane (8).
5.48 g (10 mmol) of 5 was dissolved in 100 mL of dry
tetrahydrofuran (THF), and the solution was cooled down to
-78 °C. Then, 6.9 mL (11 mmol, 1.1 equiv) of n-BuLi (1.6 M
in hexane) was added through a syringe over about 30 min.
After stirring for 1 h at -78 °C, 20 g (20 mmol, 2 equiv) of
chlorotrimethylsilane (1 M in THF) was added. After being
stirred overnight at room temperature, the reaction mixture was
poured into 200 mL of water and extracted with petroleum ether.
The organic layer was washed with water three times and dried
over Na2SO4. The solvent was removed under reduced pressure
to give a crude product. The crude product was purified by
column chromatography using petroleum ether as eluent to give
TFT2. This compound was prepared using a similar proce-
dure to that used for the synthesis of FT1 in a yield of 40% as
1
a yellow solid. M.p.: 135 °C. H NMR (300 MHz, CDCl3, δ):
8.90 (3H, d), 8.82 (3H, s), 8.01 (3H, d), 7.93 (3H, d), 7.58-7.84
(30H, m), 7.25-7.32 (12H, m), 7.16-7.21 (18H, m), 7.05 (6H,
t), 2.08-2.24 (24H, m), 1.03-1.27 (120H, m), 0.71-0.89 (60H,
m). 13C NMR (100 MHz, CDCl3, δ): 171.90, 152.76, 151.78,
151.73, 151.43, 147.70, 147.08, 145.25, 141.59, 140.29, 140.19,
139.63, 139.54, 135.62, 135.31, 129.28, 128.40, 127.79, 126.34,
126.21, 125.57, 124.37, 124.02, 123.38, 122.92, 121.58, 121.49,
121.00, 120.77, 120.02, 119.92, 55.43, 55.30, 40.43, 31.80,
30.08, 30.04, 29.25, 29.22, 23.94, 23.87, 22.59, 14.06, 14.03.
FT-IR (NaCl pellets, cm-1): 2926, 2854, 1592, 1515, 1494,
1460, 1370, 1280, 814, 752, 696. MS (MALDI-TOF): m/z
3142.7 (M+). Anal. Calcd for C231H282N6: C, 88.28; H, 9.04;
N, 2.67. Found: C, 88.46; H, 9.24; N, 2.31.
1
8 as a colorless oil liquid in a yield of 94%. H NMR (300
MHz, CDCl3, δ): 7.64 (1H, d), 7.56 (1H, d), 7.42-7.50 (4H,
m), 1.92 (4H, m), 1.05-1.22 (20H, m), 0.82 (6H, t), 0.61 (4H,
m), 0.30 (9H, s).
(9,9-Dioctyl-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9H-fluoren-2-yl)trimethylsilane (9). This compound was pre-
pared using a similar procedure to that used for the synthesis
1
of 4 in a yield of 74%. H NMR (300 MHz, CDCl3, δ): 7.80
(1H, d), 7.69-7.74 (3H, m), 7.46-7.50 (2H, m), 1.97 (4H, m),
1.03-1.21 (20H, m), 0.81 (6H, t), 0.60 (4H, m), 0.31 (9H, s).
9,9,9′,9′-Tetraoctyl-7′-(trimethylsilyl)-9H,9′H-2,2′-bifluorene-
7-carbonitrile (10). This compound was prepared using a similar
procedure to that used for the synthesis of 7 in a yield of 87%.
13. This compound was prepared using a similar procedure
to that used for the synthesis of 7 in a yield of 78%. H NMR
(300 MHz, CDCl3, δ): 7.78-7.84 (5H, m), 7.63-7.74 (11H,
1

Star-Shaped Donor-π-Acceptor Conjugated Oligomers
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10377
m), 7.50-7.53 (2H, m), 2.00-2.13 (12H, m), 1.07-1.25 (60H,
m), 0.59-0.83 (30H, m), 0.33 (9H, s).
SCHEME 1: Molecular Structures of the Star-Shaped
Oligomers
14. This compound was prepared using a similar procedure
to that used for the synthesis of 11 in a yield of 99%. 1H NMR
(300 MHz, CDCl3, δ): 7.80-7.84 (4H, m), 7.72-7.77 (2H, m),
7.60-7.69 (11H, m), 7.49 (1H, d), 1.99-2.12 (12H, m),
1.01-1.24 (60H, m), 0.59-0.84 (20H, m), 0.31 (9H, s). FT-IR
(NaCl pellets, cm-1): 2927, 2854, 2224, 1608, 1459, 1402, 1377,
1259, 1001, 887, 817, 756, 722. MS (MALDI-TOF): m/z 1318.1
(M+), 1192.2 (M-I)+. Anal. Calcd for C88H120IN: C, 80.14; H,
9.17; N, 1.06. Found: C, 80.36; H, 9.12; N, 0.87.
15. This compound was prepared using a similar procedure
1
to that used for the synthesis of 7 in a yield of 63%. H NMR
(300 MHz, CDCl3, δ): 7.77-7.85 (6H, m), 7.64-7.73 (10H,
m), 7.57-7.60 (4H, m), 7.25-7.31 (4H, m), 7.15-7.20 (6H,
m), 7.05 (2H, m), 2.08 (12H, m), 1.02-1.26 (60H, m),
0.60-0.88 (30H, m). 13C NMR (100 MHz, CDCl3, δ): 152.21,
151.90, 151.79, 151.73, 151.68, 147.69, 147.07, 145.44, 142.52,
140.72, 140.53, 140.28, 140.14, 139.79, 139.64, 139.49, 138.24,
135.62, 131.38, 129.27, 127.78, 126.60, 126.49, 126.27, 126.19,
126.11, 124.36, 124.02, 122.91, 121.51, 121.07, 120.99, 120.22,
120.03, 119.88, 109.73, 55.64, 55.36, 55. 27, 40.42, 40.33,
40.10, 31.77, 31.74, 30.02, 29.99, 29.87, 29.19, 23.85, 23.79,
22.57, 14.04. FT-IR (NaCl pellets, cm-1): 2926, 2854, 2224,
1593, 1515, 1494, 1460, 1377, 1280, 814, 753, 696. MS
(MALDI-TOF): m/z 1436.0 (M+). Anal. Calcd for C106H134N2:
C, 88.65; H, 9.40; N, 1.95. Found: C, 88.02; H, 9.32; N, 1.72.
TFT3. This compound was prepared using a similar proce-
dure to that used for the synthesis of FT1 in a yield of 38% as
then spin-coated on the top of the PEDOT:PSS layer from a
toluene solution of TFT1 (or TFT2 and TFT3) (15 mg/mL),
followed by drying at 60 °C for 2 h in a glovebox. The electron
transport layer, electron injection layer (LiF), and the cathode
Al were sequentially evaporated onto the emitting layer. During
the deposition, the thickness of the layers was monitored by an
oscillating quartz crystal and optimized device to device. The
properties of the devices were measured at room temperature
in air. The EL spectra were recorded on a photometer PR 705.
1
a yellow solid. M.p.: 168 °C. H NMR (300 MHz, CDCl3, δ):
Results and Discussion
8.91 (3H, d), 8.83 (3H, s), 8.03 (3H, m), 7.95 (3H, m),
7.44-7.87 (48H, m), 7.25-7.32 (12H, m), 7.16-7.23 (18H,
m), 7.05 (6H, m), 2.08-2.26 (36H, m), 1.05-1.28 (180H, m),
0.69-0.89 (90H, m). 13C NMR (100 MHz, CDCl3, δ): 171.92,
152.79, 152.02, 151.85, 151.75, 151.46, 150.10, 147.72, 147.08,
145.27, 141.60, 141.51, 141.38, 140.61, 140.54, 140.44, 140.29,
140.06, 139.86, 139.64, 139.57, 139.43, 135.69, 135.34, 129.29,
128.43, 127.80, 126.37, 126.20, 125.56, 124.37, 124.07, 123.41,
123.25, 123.15, 122.92, 121.55, 120.96, 120.80, 120.01, 119.91,
118.41, 118.30, 55.46, 55.31, 55.09, 55.02, 40.42, 31.82, 30.11,
29.27, 23.98, 22.61, 14.08. FT-IR (NaCl pellets, cm-1): 2926,
2852, 1591, 1515, 1493, 1461, 1371, 1277, 814, 752, 695. Anal.
Calcd for C318H402N6: C, 88.65; H, 9.40; N, 1.95. Found: C,
88.62; H, 9.27; N, 2.34.
Synthesis and Characterization. Molecular structures of the
star-shaped molecules are shown in Scheme 1. FT1 is a
molecule centered with the electron-withdrawing 1,3,5-triazine
group and armed with three fluorene units, whereas TFT1,
TFT2, and TFT3 have bridges made of different numbers of
fluorene units between the 1,3,5-triazine core and electron-
donating triphenylamine peripherals. Synthesis of 1,3,5-triazine
derivatives is most commonly started with nucleophilic aromatic
substitutionofcyanurichalides,followedbyfurthermodifications.14a,d
The problem with this method is that in every step there are
three reacting sites, which makes the purification procedure very
tedious. In this contribution, we adopted a convergent synthetic
strategy and left 1,3,5-triazine formation as the last step, making
the whole synthesis more efficient.
Measurement of Nonlinear Optical Properties. For the
study on the nonlinear optical properties of these new materials,
we employed a femtosecond laser system consisting of a mode
locked Ti:sapphire oscillator (Tsunami, Spectra Physics) and a
regenerative amplifier (spitfire). The average output power was
about 90 mW with a repetition rate of 1 kHz, a pulse duration
of 140 fs, and the wavelength at 800 nm. We used the open-
aperture Z-scan technique to measure the TPA cross section
and closed aperture for the third-order nonlinear refractivity.
The laser beam with 1.3 mJ pulse energy was focused on the
solution in a 1 mm cell by a lens of 10 cm focal length and the
transmitted light, after the sample was collected by a photodiode
detector connected with a lock-in amplifier.
OLED Device Fabrication and Measurement. Indium tin
oxide (ITO)-coated glass substrates were cleaned by ultrasoni-
cally washing in isopropyl alcohol, detergent solution, deionized
water, and isopropyl alcohol successively before being dried
under N2 flushing and UV-ozone treatment. Then, PEDOT:PSS
was spin-coated onto the ITO glass from its aqueous solution
and dried at 150 °C for 20 min in air. The emitting layer was
Synthetic routes of these molecules are shown in Schemes 2
and 3. The synthesis started from cyanization of 2-bromo-9,9-
dioctylfluorene (1) to give the cyano compound 2 in a yield of
68%. Then, compound 2 underwent a trimerization reaction
under strong acid conditions with trifluoromethanesulfonic acid
in chloroform to afford the star-shaped compound FT1 in a high
yield of 91%.
For the first generation D-π-A type compound TFT1, 2,7-
dibromo-9,9-dioctylfluorene 5 was used as the starting material
to undergo a monocyanization reaction using 0.9 equivalent
CuCN to give compound 6. Meanwhile, borylation of 4-bromo-
N,N-diphenylbenzenamine 3 gave rise to the boronic easter
compound 4. Suzuki coupling between 4 and6 gave the triazine
precursor 7 in a yield of 86%. With the same conditions as the
synthesis of FT1, TFT1 was obtained as yellowish solid in a
reasonable yield (78%). The key point of the trimerization
reaction was the feeding sequence, with adding the reactant
solution to the acid necessary for better yield.13a
Borolane compound 9 was synthesized through two steps
from compound 5, monoprotection by TMS and borylation.

10378 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Ren et al.
SCHEME 2: Synthesis of Star-Shaped Oligomers FT1 and TFT1
SCHEME 3: Synthesis of Star-Shaped Oligomers TFT2 and TFT3
Then, compound 9 underwent a Suzuki coupling reaction with
compound 6 to afford the compound 10, which was subsequently
converted to compound 11 through an iodination reaction in a
quantative yield. Another Suzuki reaction was employed to
synthesize the second generation triazine precursor 12, followed
by the cyclization to make TFT2.
Suzuki coupling reaction between the iodo compound 11 and
the boronic easter compound 9 gave rise to the trisfluorene
compound 13, followed by an iodination reaction to produce
compound 14. Suzuki coupling between 14 and 4 gave the third
generation triazine precursor 15, which then underwent a
trimerization reaction to give TFT3.
molecules, as shown in Figure 1 and Table 1. The GPC results
showed that the molecular weight of these star-shaped
molecules was monodispersed and correlated well with their
calculated molecular weight.
Thermogravimetric analysis (TGA) performed in a nitrogen
atmosphere showed good thermal stability of the star-shaped
compounds (Figure 2 and Table 1). The 5% weight loss
temperatures of the star-shaped compounds were all more than
425 °C with the best 439 °C of TFT1.
All four final compounds dissolved readily in common
organic solvents such as dichloromethane, chloroform,
toluene, and tetrahydrofuran, which allowed us to do
conventional characterization by normal spectroscopic tech-
niques. The identity and purity of all new compounds were
1
confirmed by various methods, such as H NMR, 13C NMR
spectra, mass spectrometry, and elemental analysis, as
detailed in the experimental part. The 13C NMR of the star-
shaped molecules showed characteristic signals of the 1,3,5-
triazine unit around 172 ppm. Mass spectra of TFT3 were
not able to be obtained after several tries, so gel permeation
chromatography (GPC) was used to further characterize these
Figure 1. GPC traces of the star-shaped oligomers.

Star-Shaped Donor-π-Acceptor Conjugated Oligomers
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10379
TABLE 1: Summary of the Experimental Data of the Star-Shaped Oligomers
max a
max b
λSPA
(nm)
λSPEF
(nm)
d
f
compound Mn (PDI)c Td (°C)
CYC
TOL
THF (lg εe) DCM DMF film
CYC
TOL (ΦPL
)
THF DCM DMF film
FT1
1500 (1.02)
2600 (1.01)
4200 (1.01)
5600 (1.01)
425
439
436
433
333/351 336/352 351 (4.96)
351
391
389
379
352 366 360/378 385 (0.08) 393
395 399 435/456 465 (0.83) 522
401
544
544
473
413 394
574 479
453 471
452 464
TFT1
TFT2
TFT3
387
385
373
392
388
375
391 (5.18)
388 (5.50)
378 (5.51)
393 394 429
381 378 423
445 (0.93) 508
431 (0.99) 461
a λSPAmax, maximum single-photon absorption wavelength in different solutions (cyclohaxane, CYC; toluene, TOL; tetrahydrofuran, THF;
b
dichloromethane, DCM; N,N-dimethylformamide, DMF) and solid state (film). λSPEFmax, maximum single-photon excited fluorescence
wavelength in different solutions and solid state. c Determined by GPC using polystyrene as reference and THF as elute. d 5% weight loss
temperature determined by TGA. e Molar extinction coefficient. f Photoluminescence quantum yield determined in toluene solutions with a 0.5
M H2SO4 solution of quinine (10-5 M, ΦPL ) 0.55) as reference.
Single-Photon Photophysical and Electrochemical Proper-
ties. The single-photon excited absorption and fluorescence
properties of the star-shaped compounds are summarized in
Table 1. The single-photon absorption (SPA) and single-photon
excited fluorescence (SPEF) spectra were measured in various
solvents with different polarities, which ranged from nonpolar
cyclohexane (dielectric constant Dk ) 2.02), medium polar
toluene (Dk ) 2.38), THF (Dk ) 7.5), and DCM (Dk ) 9.1) to
highly polar DMF (Dk ) 38), to investigate the influence of
solvent polarity on the optical properties of these triazine based
compounds.
meanwhile, the Stokes shifts between the absorption and
emission of FT1 in the solvents cyclohexane, toluene, THF,
DCM, and DMF were 9, 34, 42, 50, and 61 nm, respectively.
The solvatochromism is caused by photoinduced intramolecular
charge transfer (ICT) in the excited state. Such chromic behavior
is associated with the stabilization of the polar emissive excited
states by the polar solvents.9a Similar results have been observed
in linear donor-acceptor oligomers.20
Compared with FT1, the SPFE spectra of the D-π-A molecule
TFT1 showed a much stronger solvatochromism phenomenon.
The SPEF maximum of TFT1 shifted from 435 nm in
cyclohaxane to 574 nm in DMF, which was a 139 nm change,
while, under the same circumstances, there was only a 53 nm
red-shift for FT1. The enhancement of solvatochromism
indicated that the attachment of the strong electron-donating
triphenylamine group significantly increased the ICT effects in
the D-π-A molecule. The Stokes shifts between the absorption
and emission of TFT1 in the solvents cyclohexane, toluene,
THF, DCM, and DMF were 48, 73, 131, 153, and 179 nm,
respectively. The SPEF spectra of TFT2 and TFT3 were also
measured in different solvents, and they basically showed the
same solvatochromism behaviors, except for the abnormality
in DMF due to poor solubility.
The SPA spectra of all of the star-shaped oligomers were
slightly changed in solvents of different polarities, with the SPA
maxima (λSPAmax) located at around 351 nm for FT1, 393 nm
for TFT1, 390 nm for TFT2, and 378 nm for TFT3 (Table 1
and Figure 3). In THF solutions, molar absorption coefficients
(ε) of the star-shaped compounds were measured. The ε value
was increased from 9.12 × 104 for FT1 to 1.51 × 105 for TFT1
with the addition of the electron donating group of tripheny-
lamine, and it was further enhanced by the expansion of the
π-spacer, with TFT2 having a ε value of 3.16 × 105 and TFT3
of 3.24 × 105.
In contrast to the absorption process, SPEF of all the star-
shaped compounds exhibited solvatochromism in different
solvents. With increasing solvent polarity, the SPEF maxima
(λSPEFmax) showed bathochromic shifts of different extent ac-
cording to each specific compound. FT1 showed two emission
peaks in cyclohexane, located at 360 and 378 nm; however, in
the other four more polar solvents, there was only one emission
max
As shown in Table 1, in the same solvent, both λSPA and
max
λSPEF
values of the star-shaped compounds could be se-
quenced as TFT1 > TFT2 > TFT3 > FT1. For example, in
toluene solutions, the λSPAmax values of FT1, TFT1, TFT2, and
TFT3 were 352, 392, 388, and 375 nm and the λSPEFmax values
were 385, 465, 445, and 431 nm, respectively, suggesting that
the addition of electron donating groups could effectively lower
the band gap of the molecules and the expansion of the fluorenyl
bridge could contrarily increase the band gap, which was not
surprising because of the high band gap properties of fluorene
based materials. Photoluminescence quantum yields (ΦPL) of
the star-shaped oligomers in toluene solutions were measured
with a 0.5 M H2SO4 solution of quinine (10-5 M, ΦPL ) 0.55)
as reference. The ΦPL of TFT1 was more than 10 times of that
of FT1 (the emission of FT1 fell in the ultraviolet region, so
the quinine solution was not a very good reference), and when
the number of the bridging fluorene units was increased, the
ΦPL value was further enhanced, up to 99% for TFT3.
Normally, the ICT process in a molecule leads to the decrease
of ΦPL,8g so the results here also evidenced the weakening of
the ICT effects as the length of the oligomeric fluorene arms
increased.
max
peak left and λSPEF red-shifted regularly with the increasing
solvent polarity. In detail, the maximum emission wavelength
moved from 360 nm in cyclohexane to 413 nm in DMF. In the
In order to investigate the condensed optical properties of
the star-shaped oligomers, we studied their SPA and SPEF
behaviors in the solid states, as shown in Table 1 and Figure 4.
Uniformly smooth thin films were prepared on glass substrates
by spin-coating from toluene solutions. The SPA maximum
Figure 2. TGA curves of the star-shaped oligomers.

10380 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Ren et al.
Figure 3. SPA (solid lines) and SPEF (dashed lines) spectra of the star-shaped oligomers in different solvents (CYC, black; TOL, blue; THF,
green; DCM, red; DMF, cyan).
The optical band gaps of these oligomers were estimated
from the onset wavelength of their solid state optical
absorption, as summarized in Table 2. The band gap of FT1
was 3.16 eV, and that of TFT1 was effectively lowered to
2.75 eV due to the strong ICT effect. The band gaps of TFT2
(2.83 eV) and TFT3 (2.90 eV) were increased gradually as
the π-spacer became longer. To further study the effect of
arm length on the electrochemical properties of the D-π-A
star-shaped oligomers, cyclic voltammetry (CV) was per-
formed with the cast films on ITO glass, as shown in the
Figure 4. SPA (solid lines) and SPEF (dashed lines) spectra of the
star-shaped oligomers in the solid states (FT1, black; TFT1, red; TFT2,
green; TFT3, blue).
Supporting Information (Figure S15). The potentials were
reported relative to an internal Fc+/Fc standard with a scan
rate of 0.1 V/s. All oligomers exhibited reversible electro-
chemical oxidation behavior with the p-doping onset located
at 0.49, 0.53, and 0.54 eV versus Ag+/Ag, respectively. The
wavelength of FT1 film was 366 nm, about 15 nm red-shifted
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of these
from the λSPAmax value in solution, while the D-π-A compounds
max
TFT1-3 showed almost the same λSPA
values in both
oligomers were then calculated and shown in Table 2. In
OLED applications, the work function of commonly used
anode material ITO is 4.8 eV and the work function of widely
used cathode Al is 3.7 eV; thus, compared to TFT2 and
TFT3, TFT1 has better energy level matching with the
electrodes, which would be favorable for hole and electron
transporting to afford better electroluminescence properties.
solutions and solid states. These results indicated that the
attachment of the nonplanar triphenylamine group could ef-
fectively suppress the solid state stacking. All of the films of
FT1 and TFT1-3 were intensively fluorescent under UV-vis
max
excitation, with λSPEF
at 394, 479, 471, and 464 nm,
respectively. Annealing experiments were employed to test the
fluorescence stability of the oligomers in films, and it was found
that all of the D-π-A compounds showed almost unchanged
fluorescent properties after thermal annealing, which was
favorable for the overall stability of the electroluminescent
devices.
Nonlinear Optical and Two-Photon Absorption and Emis-
sion Properties. The nonlinear optical properties of the D-π-A
oligomers were measured by closed-aperture and open-aperture
Z-scan techniques. As shown in Figure 5, both TFT1 and TFT2
TABLE 2: Summary of Redox and Nonlinear Optical Data of the Star-Shaped Oligomers
σg (GM)
opt a
onset b
c
d
e
max f
compound
Eg
(eV)
Eox
(V)
EHOMO (eV)
ELUMO (eV)
n2 (esu)
λTPEF
(nm)
FT1
3.16
TFT1
TFT2
TFT3
2.75
2.83
2.90
0.49
0.53
0.54
-5.29
-5.33
-5.34
-2.54
-2.50
-2.44
4.14 × 10-12
2.48 × 10-12
527
514
470
234
196
189
a Estimated from the absorption onset of the oligomer thin films. b Determined by CV with reference to (0.10 M AgNO3)/Ag. c Determined
+ 4.8) (eV). d Calculated from ELUMO ) EHOMO + Eg e Nonlinear refractive index measured by the closed-aperture
.
onset
opt
from EHOMO ) -(Eox
Z-scan technique. f Two-photon excited fluorescence maximum wavelength excited by 800 nm lasers in THF solutions. g Two-photon absorption
cross sections measured by the open-aperture Z-scan technique upon excitation of 800 nm laser pulses with a pulse width of 140 fs in 5 × 10-3
M THF solutions. 1 GM ) 1 × 10-50 cm4 s per photon per molecule.

Star-Shaped Donor-π-Acceptor Conjugated Oligomers
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10381
Figure 5. Normalized closed- and open-aperture Z-scan traces of TFT1 and TFT2 in THF solutions.
Figure 6. TPEF spectra of TFT1 and TFT2 in THF solutions at different average laser powers.
exhibited active nonlinear absorption and refraction behaviors.
Peak-valley characteristics were shown in closed-aperture
Z-scan measurement, indicating negative nonlinear refraction
or self-defocusing effect. Using carbon disulfide (CS2) as a
reference nonlinear material (n2[CS2] ) 1.3 × 10-11 esu),21 the
third-order nonlinear refractive indices (n2) of TFT1 and TFT2
in THF solutions were calculated to be 4.14 × 10-12 and 2.48
× 10-12 esu, respectively. It was reported22 that the n2 value of
poly(9,9-dioctylfluorene) (PFO) was 2.04 × 10-12 esu. Com-
pared with TFT2 and PFO, TFT1 showed higher nonlinear
refractivity, which could be attributed to the stronger ICT effect
in the molecule.
Two-photon excited fluorescence (TPEF) spectra of the
oligomers TFT1-3 were recorded in THF solution at room
temperature (Figure 6). Upon excitation of 800 nm laser pulses
Figure 7. Linear dependence of peak fluorescence intensity on the
square of the excitation powers.
with a pulse width of 140 fs, all of the oligomers emitted
extensive frequency upconverted fluorescence with the maxi-
mum TPEF wavelength (λTPEFmax) at 527 nm for TFT1, 514
nm for TFT2, and 470 nm for TFT3. As shown in Figure 7,
the output intensity of two-photon excited fluorescence is
linearly dependent on the square of the input laser intensity,
indicating the occurrence of two-photon absorption (TPA).
TPA cross sections of the D-π-A oligomers TFT1-3 were
measured by open-aperture Z-scan experiments performed with
a femtosecond 800 nm laser source. As summarized in Table
2, the TPA cross sections of TFT1-3 were 234, 196, and 189
GM, respectively, which were higher than the reported cross
section value of triazine derivative AF-450 (137 GM) measured
under the excitation of the 790 nm laser.15 These results
indicated that the triphenylamine donor part in TFT1-3 could
max
λTPEF of TFT1-3 shifted to longer wavelength by 5-9 nm
compared with their maximum SPEF wavelength (Tables 1 and
2). The red-shift could be attributed to the reabsorption of the
fluorescence under more concentrated solution conditions (5 ×
10-3 to 1 × 10-5 M).

10382 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Ren et al.
Figure 8. Schematic energy level diagram of the OLED devices
and molecular structure of the ETL material. The energy level data
were derived from CV and UV-vis absorption data.
Figure 10. L-V and J-V curves of the OLED devices based on star-
shaped D-π-A oligomers.
voltages compared with OLEDs fabricated by vacuum evapora-
tion. The luminance versus operation voltage (L-V) and current
density versus operation voltage (J-V) curves of the devices
were shown in Figure 10. The device using TFT1 as an emitting
material showed a maximum luminance of 3093 cd/m2 and a
current efficiency of 1.47 cd/A at a relatively low driving voltage
of 7.5 V, while the devices based on TFT2 and TFT3 exhibited
less efficient electroluminescence. The reason why TFT1
showed the best EL properties could be ascribed to easier
electron and hole transportation and more balanced charge
recombination. The device performances obtained here will
likely be improved by optimizing the device conditions, such
as varying the thickness of the active layers, the electron
transport materials or the cathodes having been used.
Figure 9. EL spectra of the devices with the star-shaped D-π-A
oligomers as active layers (TFT1, solid; TFT2, dash; TFT3, dash dot).
supply stronger ICT effects than the diphenylamine counterpart
in AF-450 and, within the TFT series, TFT1 had the strongest
nonlinear TPA ability.
Conclusions
1,3,5-Triazine based star-shaped oligomer FT1 and three
generations of highly soluble donor-π-acceptor compounds,
TFT1, TFT2, and TFT3, were prepared through an efficient
convergent synthetic strategy, and their photophysical and
electrochemical properties were characterized, which showed
distinct correlations with their specific structures. Closed-
aperture and open-aperture Z-scan measurements showed that
the D-π-A oligomers were both nonlinear refraction and
nonlinear absorption active. TFT1 showed a high nonlinear
refractive index of 4.14 × 10-12 esu in THF solution with
an excitation wavelength of 800 nm. Among the oligomers,
TFT1 exhibited the largest two-photon absorption cross
section of 234 GM under an 800 nm laser irradiation with a
pulse duration of 140 fs due to the strongest ICT effect. All
of the D-π-A oligomers TFT1-3 showed frequency up-
Electroluminescent Properties. The star-shaped D-π-A
oligomers can be easily deposited as thin films by spin-coating,
and OLED devices with TFT1-3 as active layers were
fabricated with a configuration of ITO/PEDOT:PSS/oligomer/
ETL/LiF/Al. In these devices, PEDOT:PSS was employed as
the hole injection material and a bitriazine compound developed
by our group12c was used as the electron-transporting layer (ETL)
to improve electron-transporting ability and avoid using active
metals such as calcium. The energy diagram of these devices
was shown in Figure 8. Incorporation of an ETL could lower
the electron transporting barrier between the emitting layer and
the cathode. All of the electroluminescent (EL) spectra of the
devices based on TFT1-3, shown in Figure 9, were similar to
the corresponding solid state photoluminescent spectra of
TFT1-3, indicating that holes from the anode and electrons
from the cathode were combined in the active layer and the
light was indeed emitted from the oligomers as expected.
As shown in Table 3, all of the devices had very low turn-on
max
converted two-photon excited fluorescence with λTPEF
located at 527, 514, and 470 nm, respectively. OLED devices
using the spin-coated films of these oligomers as an active
layer showed intensive blue electroluminescence and a
TABLE 3: Summary of the OLED Device Data of the Star-Shaped Oligomers
turn-on voltageb
(V)
drive voltagec
(V)
current densityc
luminancec
current efficiencyc
(cd/A)
power efficiencyc
(lm/w)
max a
λEL
compound
(nm)
(mA/cm2)
(cd/m2)
TFT1
TFT2
TFT3
466
450/466
436/460
3.0
3.5
3.5
7.5
10.5
10.5
211
600
411
3093
1647
1610
1.47
0.27
0.39
0.61
0.08
0.12
a Maximum electroluminescent wavelength of the OLED devices. b Voltage when the luminance is visible. c Measurement conditions and
device performances at the maximum luminance.

Star-Shaped Donor-π-Acceptor Conjugated Oligomers
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10383
maximum luminance of 3093 cd/m2 at a current efficiency
of 1.47 cd/A was achieved by using TFT1 as the emitting
material. The convenient synthesis and multifunctional ap-
plications of the star-shaped oligomers claimed a novel series
of useful optoelectronic materials.
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Acknowledgment. Financial support from the Natural Sci-
ence Foundation of China (NSFC, Nos. 20872171 and 60978055)
is gratefully acknowledged.
Supporting Information Available: Spectroscopic analyti-
cal data for all new compounds and extra CV and TPEF spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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