Synthesis, Two-photon absorption and AIE properties of multibranched thiophene-based triphenylamine derivatives with triazine core

Article abstract of DOI:10.1007/s11426-013-4924-7

Two new multibranched thiophene-based triarylamine derivatives with 1,3,5-triazine core are synthesized and characterized. Their one- and two-photon absorption properties and aggregation-induced emission effect have been investigated. Both the STAPA-based compounds are AIE active. The two-photon absorption (2PA) cross sections measured by the open aperture Z-scan technique are determined to be 620 and 1610 GM for STAPA-a and STAPA-b in chloroform, respectively, which dramatically increase with the introduction of alkyl chains. The relationship between their structures and properties on one- and two-photon absorption and aggregation-induced emission is discussed, which allows us to examine the effect of introducing alkyl chains. In addition, solvent effects also show a significant influence on the 2PA cross section. The two compounds with excellent AIE and 2PA properties provide attractive alternatives for the biophotonic materials.

Full text of DOI:10.1007/s11426-013-4924-7

SCIENCE CHINA
Chemistry
September 2013 Vol.56 No.9: 1204–1212
doi: 10.1007/s11426-013-4924-7
• ARTICLES •
· SPECIAL TOPIC ·Aggregated-Induced Emission
Synthesis, two-photon absorption and AIE properties of
multibranched thiophene-based triphenylamine derivatives
with triazine core
GAO YuTing1, ZHANG Hao1, JIANG Tao1, YANG Ji1, LI Bo2*, LI Zhen3 & HUA JianLi1*
1Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology,
Shanghai 200237, China
2Key Laboratory of Polar Materials and Devices, Ministry of Education; East China Normal University, Shanghai 200241, China
3Department of Chemistry, Wuhan University, Wuhan 430072, China
Received May 27, 2013; accepted June 17, 2013; published online July 19, 2013
Two new multibranched thiophene-based triarylamine derivatives with 1,3,5-triazine core are synthesized and characterized.
Their one- and two-photon absorption properties and aggregation-induced emission effect have been investigated. Both the
STAPA-based compounds are AIE active. The two-photon absorption (2PA) cross sections measured by the open aperture
Z-scan technique are determined to be 620 and 1610 GM for STAPA-a and STAPA-b in chloroform，respectively, which
dramatically increase with the introduction of alkyl chains. The relationship between their structures and properties on one-
and two-photon absorption and aggregation-induced emission is discussed, which allows us to examine the effect of introduc-
ing alkyl chains. In addition, solvent effects also show a significant influence on the 2PA cross section. The two compounds
with excellent AIE and 2PA properties provide attractive alternatives for the biophotonic materials.
two-photon absorption, aggregation-induced emission, thiophene-based triphenylamine, 1,3,5-triazine
photon microscopes are commercially available and become
1 Introduction
common tools for biologists, which contribute to more fo-
cus being put on biological fluorescent probes [4]. Although
two-photon excitation has applications in many areas, in
most cases, these applications have been demonstrated by
using conventional dyes with modest 2PA cross-sections (σ
< 50 GM) [3]. The significant issue is how to synthesize
large 2PA cross section materials in order to realize full
potential applications, which stimulated substantial research
on structure-property relationships. Some efficient molecu-
lar design strategies were proposed for the development of
materials with large two-photon absorption cross sections,
such as molecular coplanarity, extension of -conjugated
bridge, strong donor and acceptor groups [5].
The concept of the two-photon absorption (2PA) process
was first proposed by Göppert-Mayer in 1931 [1] and
demonstrated experimentally by Kaiser and Garrett in 1961
[2]. The rate of absorption takes on a quadratic dependency
on the incident intensity in the 2PA process, resulting in
deep penetration and high three-dimensional spatial selec-
tivity. Owing to these superiorities, 2PA materials have re-
cently aroused considerable interest as promising materials
for applications in various areas, such as two-photon dy-
namic therapy, three-dimensional optical data storage,
up-converted lasing, optical power limiting materials, two
photon microscopy and bioimaging [3]. Nowadays, two
Even some 2PA chromophores have been synthesized
with large σ values, in aqueous solution, aggregation-caused
quenching (ACQ) effect and intramolecular charge transfer
*Corresponding authors (email: jlhua@ecust.edu.cn; bli@ee.ecnu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com www.springerlink.com

Gao YT, et al. Sci China Chem September (2013) Vol.56 No.9
1205
(ICT) often lead to low fluorescence quantum yield (φ) and
small σ data which limit their application especially in fluo-
rescent chemosensors and bioprobes. The annoying problem
brought by ACQ effect was solved by Tang and coworkers
with the discovery of aggregation-induced emission (AIE)
phenomenon [6]. The AIE molecules which behave exactly
opposite to the conventional ACQ luminophores are none-
missive when dissolved in good solvents but become highly
luminescent when aggregated in the solid state. Research
indicated that the AIE effect may be caused by the re-
strictions of intramolecular rotations of the chromophoric
molecules in the aggregate state which associated with their
molecular shapes, conformational flexibility, intramolecular
movements, packing structure, and organizational mor-
phology [7]. Very recently, Tang’s group has investigated
the structure–property relationship on AIE behavior. Twist-
ing amplitude, steric effects and adding bulky aromatic rings
were found to play a crucial role in their fluorescence behav-
iors in the aggregated state [8]. Now AIE luminogens have
showed significant academic value and diverse promising
applications in cell imaging [9], fluorescent sensors and bi-
oprobes [10], mechanafluorochromic materials [11], etc.
2PA materials with triarylamine as electron donor have
aroused great interest and become the focus of intensive
research in the field of 2PA materials owing to their good
electron donating and transporting capability [3, 12]. The
introduction of a thiophene moiety with richer -electron
density was expected to enrich the -electron density of the
system and improve the optoelectronic properties. In addi-
tion, the alkyl groups can enhance the materials elec-
tron-donating ability and solubility. In particular, octupolar
molecules consisting of a strong triazine electron- accepting
center and an electron-donating end group linked through a
-conjugated bridge have been shown to be excellent 2PA
materials [13]. In our previous work, we studied multi-
branched triarylamine end-capped triazines with large
two-photon absorption cross-sections and AIE properties
[14]. One representative example of these fluorophores was
TAPA, which was functionalized by folic acid for targeted
MCF-7 cancer cell imaging [15]. To enrich the research
field of triarylamine derivatives with triazine acceptor and
to gain more information about their structure–property
relationships, in this work, we have synthesized two octu-
polar triazine molecules (STAPA-a and STAPA-b, Scheme
1) containing multibranched thiophene-based triphenyla-
mine derivatives with and without alkyl groups and investi-
gated their properties on one- and two-photon absorption
and aggregation-induced emission.
benzophenone ketyl immediately prior to use. Triethyla-
mine was distilled under normal pressure and dried over
potassium hydroxide. N,N-dimethyl formamide (DMF) and
dichloromethane (DCM) were refluxed with calciumhydride
and distilled before use. Starting materials 2,4,6-tri(p-tolyl)
-1,3,5-triazine, 4-(diphenylamine)benzaldehyde were pre-
pared according to published procedures [16]. All other
chemicals were purchased from Aldrich and used as re-
ceived without further purification.
2.2 Characterization
1H and 13C NMR spectra were recorded on a Bruker
AM-400 spectrometer using chloroform-d as solvent and
tetramethylsilane (δ = 0) as internal references. The UV-Vis
spectra were recorded on a Varian-Cary 500 spectropho-
tometer with 2 nm resolution at room temperature. The flu-
orescence spectra were taken on a Varian-Cary fluorescence
spectrophotometer. SEM micrographs were obtained on a
JEOL JSM-6360 scanning electron microscope. 2PA cross
section of STAPA-a and STAPA-b was measured by
femtosecond open-aperture Z-scan technique according to
previously described method [17]. Two-photon excited flu-
orescence (TPF) was excited by the fs pulses with different
intensities at wavelength of 800 nm. The repetition rate of
the laser pulses is 250 kHz and the pulse duration is 80 fs.
2.3 Synthesis
4-[N,N-Bis(4-iodophenyl)amino]benzaldehyde (1)
A modified version of a previously reported method [19]
was used. In a 500 mL three-necked round-bottom flask,
4-(N,N-diphenylamino)benzaldehyde (14.00 g, 51.28 mmol),
potassium iodide (11.43 g, 68.85 mmol), acetic acid (210
mL) and water (20 mL) were heated to 80 °C. After stirring
for 1 h, potassium iodate (10.97 g, 51.26 mmol) was added
and the reaction was stirred at 80 °C for 4 h. The solution
was allowed to cool and the solid was collected, washed
with water and recrystallised from dichloromethane/ethanol
1
(v/v = 20/1). H NMR (CDCl3, 400 MHz), δ (TMS, ppm):
9.89 (s, 1H), 7.75 (d, J = 9.0 Hz, 2H), 7.67 (d, J = 9.0 Hz,
4H), 7.09 (d, J = 9.0 Hz, 2H), 6.93 (d, J = 9.0 Hz, 4H).
2-(N,N-Diphenylamino)thiophene (2a)
To a 250 mL round-bottom flask containing a mixture of
(C6H5)2NH (8.45 g, 50.0 mmol), sodium tert-butoxide (5.75
g, 6.0 mmol) and Pd(OAc)2 (225 mg, 1.0 mmol) were added
dry toluene (100 mL), 2-bromothiophene (4.82 mL, 50.0
mmol), and P(t-Bu)3 (1.25 mL, 1.25 mmol, 1 M in toluene)
sequentially. The solution mixture was slowly heated to
reflux, stirred for 16 h, and then cooled, and 2 mL of water
was added. The solution was pumped dry, and the residue
was extracted with dichloromethane/water. The organic
layer was dried over magnesium sulfate, filtered, and dried.
2 Experimental
2.1 Materials
Tetrahydrofuran (THF) was pre-dried over 4 Å molecular
sieves and distilled under argon atmosphere from sodium

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Gao YT, et al. Sci China Chem September (2013) Vol.56 No.9
Scheme 1 Synthetic routes to the STAPA-based compounds.
The residue was chromatographed through silica gel using
petroleum ether/dichloromethane (v/v = 6/1) as the eluent to
give the pure 2a as a pale yellow solid (6.66 g, yield: 53%).
1H NMR (CDCl3, 400 MHz), δ (TMS, ppm): 7.25 (dd, J =
8.1, 7.0 Hz, 4H), 7.12 (d, J = 8.4 Hz, 4H), 7.05– 6.95 (m,
3H), 6.92–6.83 (m, 1H), 6.76–6.66 (m, 1H).
pure 2b as a pale yellow thick liquid (7.4 g, yield: 62%). 1H
NMR (400 MHz, CDCl3) δ (TMS, ppm): 7.23 (d, J = 8.7
Hz, 4H), 7.01 (d, J = 8.7 Hz, 4H), 6.92 (dd, J = 5.6, 1.1 Hz,
1H), 6.85 (dd, J = 5.6, 3.7 Hz, 1H), 6.65 (dd, J = 3.6, 1.1
Hz, 1H), 1.69 (s, 4H), 1.35 (s, 12H), 0.74 (s, 18H). 13C
NMR (400 MHz, CDCl3) δ (TMS, ppm): 146.86, 139.93,
138.90, 121.27, 120.24, 116.16, 114.63, 114.13, 51.66,
32.67, 26.93, 26.30, 26.01.
2-(N,N-Bis(4-tert-octylphenyl)amino)thiophene (2b)
To a 250 mL round-bottom flask containing a mixture of
Bis(4-tert-octylphenyl)amine (9.82 g, 25.0 mmol), sodium
tert-butoxide (2.86 g, 3.0 mmol), and Pd(OAc)2 (113 mg,
0.5 mmol) were added dry toluene (50 mL), 2-bromothio-
phene (2.41 mL, 25.0 mmol), and tri-tert-butylphosphine
solution (0.6 mL, 0.6 mmol, 1 M in toluene) sequentially.
The solution mixture was slowly heated to reflux, stirred for
16 h, and then cooled, and 2 mL of water was added. The
solution was pumped dry, and the residue was extracted
with dichloromethane/water. The organic layer was dried
over magnesium sulfate, filtered, and dried. The residue was
chromatographed through silica gel using petroleum
ether/dichloromethane (v/v = 6/1) as the eluent to give the
5-(N,N-Diphenylamino)-2-(tri-n-butylstannyl)thiophene (3a)
A modified version of a previously reported method [18]
was used. A solution of n-BuLi (7.5 mL, 12 mmol, 1.6 M in
hexane) was added to a solution of 2a (2.51 g, 10 mmol) in
40 mL of THF at 78 °C. The solution was stirred for 30
min, then slowly warmed to room temperature over 30 min,
and stirred for another 30 min. The mixture was cooled to 
78 °C again, and 5.42 mL (20 mmol) of tri-n-butylchloros-
tannane was added. After that the mixture was stirred for 30
min at the same condition. The mixture was then warmed to
room temperature and stirred for 16 h. The mixture was
quenched with water (2 mL) and extracted with dichloro-

Gao YT, et al. Sci China Chem September (2013) Vol.56 No.9
1207
methane and water. The organic layer was dried over mag-
nesium sulfate and filtered. The solvent was removed to
give a yellow oily 3a in quantitative yield. The compound
was used without further purification. Dissolving 3a to 20
mL DMF to make a 0.5 mmol/L solution.
and extracted with dichloromethane/brine. The dichloro-
methane was removed by rotary evaporator. The residue
was purified by chromatography on silica (petroleum
ether/dichloromethane, v/v = 4/1) to afford the product as a
yellow powder (300 mg, yield: 19%). 1H NMR (400 MHz,
CDCl3) δ (TMS, ppm): 9.82 (s, 1H), 7.70 (d, J = 8.7 Hz,
2H), 7.47 (d, J = 8.5 Hz, 4H), 7.29 (s, 2H), 7.25 (s, 4H),
7.18 (d, J = 7.7 Hz, 9H), 7.13–7.02 (m,13H), 6.66 (d, J =
3.9 Hz, 2H). 13C NMR (CDCl3, 100 MHz) δ (TMS, ppm):
190.39, 152.72, 151.01, 147.66, 144.75, 137.14, 131.34,
129.62, 128.21, 126.31, 126.19, 123.16, 122.66, 121.79,
121.62, 120.14.
5-(N,N-Bis(4-tert-Octylphenyl)amino)-2-(tri-n-butylstannyl)-
thiophene (3b)
A modified version of a previously reported method was
used [18]. A solution of n-BuLi (5.6 mL, 9.0 mmol, 1.6 M
in hexane) was added to a solution of 2b (3.3 g, 6.9 mmol)
in 40 mL of THF at 78 °C. The solution was stirred for 30
min, then slowly warmed to room temperature over 30 min,
and stirred for another 30 min. The mixture was cooled to
78 °C again, and 2.42 mL (9.0 mmol) of tri-n-butyl-
chlorostannane was added. After that the mixture was
stirred for 30 min at the same condition. The mixture was
then warmed to room temperature and stirred for 16 h. The
mixture was quenched with water (2 mL) and extracted with
dichloromethane/water. The organic layer was dried over
magnesium sulfate and filtered. The solvent was removed to
give a yellow oily 3b in quantitative yield. The compound
was used without further purification. Dissolving 3b to 13.8
ml DMF to make it 0.3 mmol/L in DMF.
4-[N,N-Bis(5-(N,N-bis(4-tert-octylphenyl)amino)-2-thienyl)-
amino]benzaldehyde (5b)
In a 100 mL two-neck round-bottom flask, 1 (0.79 g , 1.5
mmol), PdCl2(PPh3)2 (24 mg, 0.034 mmol) were dissolved
in 34 mL dry DMF. The mixture was heated to 50 °C. 3b
(6.0 mL, 3.0 mmol) diluted by DMF was added. The solu-
tion was slowly warmed to 80 °C and stirred for 1 h. The
mixture was stirred for an additional 16 h at 60 °C. After
cooling, the mixture was quenched with water (2 mL) and
extracted with dichloromethane/brine. The dichloromethane
was removed by rotary evaporator. The residue was purified
by column chromatography on silica (petroleum ether:
dichloromethane, v/v = 4:1) to afford the product as a yel-
(4,4',4''-(1,3,5-Triazine-2,4,6-triyl)tris(benzene-4,1-diyl))-
1
low powder (350 mg, yield: 19%). H NMR (400 MHz,
tris(methylene)triphosphonate (4)
THF) δ (TMS, ppm): 9.69 (s, 1H), 7.59 (d, J = 8.7 Hz, 2H),
7.40 (d, J = 8.6 Hz, 4H), 7.19 (d, J = 8.7 Hz, 8H), 7.09–6.87
(m, 16H), 6.46 (d, J = 3.9 Hz, 2H), 1.65 (s, 8H), 1.26 (s,
24H), 0.66 (s, 36H). 13C NMR (100 MHz, THF) δ (TMS,
ppm): 192.53, 156.11, 155.31, 148.89, 148.55, 148.24,
139.81, 135.12, 134.33, 134.03, 130.43, 129.67, 129.61,
125.60, 125.18, 124.06, 123.83, 60.47, 41.59, 35.73, 34.90,
34.66.
In a 100 mL round- bottom flask, 2,4,6-tri (p-tolyl)-1,3,5-
triazine (3.51 g, 0.01 mol), NBS (5.34 g 0.03 mol) and BPO
(0.3 g, 1.2 mmol) were dissolved into 50 mL chlorobenzene
and heated at 110 °C for 7 h. The mixture was filtered and
the solvent was removed under vacuum. The residue was
dissolved into trimethyl phosphite (10 mL) and refluxed for
9 h. The excessive trimethyl phosphite was removed under
vacuum. The residue was purified by column chromatog-
raphy on silica (ethanol/dichloromethane, v/v = 1/10) to
afford the product as a white powder (5.3 g, yield: 78 %).
1H NMR (CDCl3, 400 MHz), δ (TMS, ppm): 8.71 (d, 6H, J
= 8.0 Hz), 7.51 (m, 6H), 3.61 (d, 18H, J = 10.8 Hz), 3.30 (d,
6H, J = 22.0 Hz). 13C NMR (CDCl3, 100 MHz), δ (TMS,
ppm): 171.3, 136.3, 136.2, 135.0, 130.1, 129.3, 129.2, 53.1,
53.0, 33.9, 32.5. HRMS (EI) m/z: Calcd. for C30H36N3O9P3:
675.1664, Found: 675.1663.
2,4,6-Tris[(4-(N,N-Bis(5-(N,N-diphenylamino)-2-thienyl)
amino))(benzene-4,1-diyl)(ethene-2,1-diyl)(benzene-4,1-diyl))]-
1,3,5-triazine (STAPA-a)
In a 100 mL two-neck round-bottom flask were added 4
(33.78 mg, 0.05 mmol), 5a (154.4 mg, 0.20 mmol),
potassium tert-butoxide (112.0 mg, 1.0 mmol), 18-crown-6
(10 mg, 0.04 mmol), and 40 mL DCM under argon
atmosphere. After stirring at 45 °C for 6 h, the mixture was
poured into distilled water and extracted with DCM and
water. The combined organic phases were dried over
4-[N,N-Bis(5-(N,N-diphenylamino)-2-thienyl)amino]benzal-
dehyde (5a)
anhydrous MgSO4 and concentrated using
a rotary
In a 100 mL two-neck round-bottom flask, 4-[N,N-bis(4-
iodophenyl)amino]benzaldehyde (4) (1.05 g , 2.0 mmol),
PdCl2(PPh3)2 (24 mg, 0.034 mmol) were dissolved in 34 mL
dry dimethylformamide (DMF). The mixture was heated to
50 °C. 3a (12.0 mL, 6.0 mmol) diluted by DMF was added.
The solution was slowly warmed to 80 °C and stirred for 1 h.
The mixture was stirred for an additional 16 h at 60 °C.
After cooling, the mixture was quenched with water (2 mL)
evaporator. The residue was purified by column
chromatography on silica (petroleum ether/dichloromethane,
v/v = 2:1) to afford 40 mg of product as a yellow powder
1
(yield: 31 %). H NMR (400 MHz, CDCl3) δ (TMS, ppm):
8.74 (d, J = 8.3 Hz, 6H), 7.68 (d, J = 8.4 Hz, 6H), 7.44 (dd,
J = 14.0, 8.6 Hz, 18H), 7.29 (s, 6H), 7.26 (s, 18H), 7.18 (d,
J = 7.7 Hz, 24H), 7.14–7.01 (m, 42H), 6.66 (d, J = 3.8 Hz,

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Gao YT, et al. Sci China Chem September (2013) Vol.56 No.9
6H). 13C NMR (CDCl3, 100 MHz) δ (TMS, ppm): 171.63,
151.97, 148.40, 146.71, 138.67, 130.24, 130.00, 129.85,
128.46, 127.12, 126.76, 125.29, 124.34, 123.68, 123.33,
123.22, 122.79, 121.67. MALDI-TOF: Calcd. for
C177H126N12S6: 2613.4, Found: 2612.4.
2,4,6-Tris[(4-(N,N-bis(5-(N,N-bis(4-tert-octylphenyl)amino)-
2-thienyl)amino))(benzene-4,1-diyl)(ethene-2,1-diyl)(benzene-
4,1-diyl))]-1,3,5-triazine (STAPA-b)
In a 100 mL two-neck round-bottom flask were added into 1
(27.02 mg, 0.04 mmol), 5b (195.33 mg, 0.16 mmol),
potassium tert-butoxide (56 mg, 0.5 mmol), and 18-crown-6
(10 mg, 0.04 mmol), and 40 mL DCM under argon
atmosphere. After stirring at 45 °C for 6 h, the mixture was
poured into distilled water and extracted with
dichloromethane and water. The combined organic phases
were dried over anhydrous MgSO4 and concentrated using a
rotary evaporator. The residue was purified by column
chromatography on silica (petroleum ether/dichloromethane,
v/v = 2/1) to afford 100 mg of product as a yellow powder
Figure 1 Normalized one-photon absorption of STAPA-a and STAPA-b
in THF and in dispersion of the nanoaggregate form (90% water) at 1 ×
105 M.
the absorption of STAPA-b was obviously broadened and
its λmax (λmax = 426 nm) was bathochromically shifted by 17
nm in comparison with that of STAPA-a (λmax = 409 nm) in
dispersion of the nanoaggregate form (90% water). The
phenomena implied that the planarization of the distorted
conjugation is induced due to the intermolecular steric
effect by aggregation and the alkyl chain has affected the
distortion degree of the conjugated backbone and its
packing modes (J-aggregation) [20]. The absorption tails
extending well into the long wavelength region further
indicated the luminogens aggregated into nanoparticles in
the presence of water, as it is well known that the Mie effect
of nanoparticles causes such level-off tails in the absorption
spectra [21].
1
(yield: 51 %). H NMR (400 MHz, THF) δ (TMS, ppm):
8.68 (d, J = 8.1 Hz, 2H), 7.66 (d, J = 8.2 Hz, 2H), 7.44 (d, J
= 8.3 Hz, 3H), 7.35 (d, J = 8.5 Hz, 4H), 7.27–7.09 (m, 11H),
7.07–6.84 (m, 18H), 6.47 (d, J = 3.8 Hz, 2H), 1.65 (s, 8H),
1.26 (s, 24H), 0.66 (s, 36H). 13C NMR (CDCl3, 100 MHz) δ
(TMS, ppm): 170.72, 150.74, 145.67, 144.95, 144.10,
137.86, 136.56, 129.60, 127.35, 126.44, 126.36, 125.40,
124.15, 121.47, 120.56, 120.35, 56.47, 37.56, 31.86, 30.84,
30.60. MALDI-TOF: Calcd for C153H102N12S6: 3959.9,
Found: 3962.3.
3 Results and discussion
All the STAPA-based derivatives are soluble in common
organic solvents such as tetrahydrofuran (THF), toluene and
dichloromethane but are insoluble in water. Stable water
dispersions of STAPA-a/b nano-aggregates were prepared
by the precipitation method using THF as a water-miscible
solvent for dyes. Figure 2(a) shows the corresponding emis-
sion spectra of STAPA-a in aqueous THF with different
water/THF ratios at 1 × 105 M. The emission from the THF
solution of STAPA-a was so weak that almost no photolu-
minescence (PL) signal was recorded. However, an obvious
enhancement of luminescence was observed at the 30% (v/v)
water–THF mixture and the PL signal was gradually en-
hanced with the increasing water/THF ratios. While the
water/THF ratio reached to 90 %, the PL intensity boosted
to the maximum. The similar behavior was observed for
STAPA-b in Figure 2(b), verifying that these two lumino-
gens were AIE-active. The fluorescene behavior of STAPA-
a and STAPA-b is well visualized through fluorescence
images of solution and nanoaggregate dispersions in Figure
2(c) and Figure 2(d), respectively. To make the comparison,
we estimated the integrals of the PL spectra shown in Figure
2(c) and 2(d). The PL intensity of STAPA-a and STAPA-b
in THF solution was very low and almost unchanged when
3.1 Synthesis
Scheme 1 illustrates synthetic procedures of the two chro-
mophores. Yamamoto coupling [18] of bromothiophene
with diarylamine afforded diarylthienylamine (2), which
was then converted to thienylstannane (3). Double thienyla-
tion of triphenylamine aldehyde iodide (1), using Stille’s
cross-coupling reaction, yielded the air-stable starburst
compounds (5a and 5b). Finally, condensation of the alde-
hyde (5a and 5b) with triazine moiety (4) by the Horner-
Wadsworth-Emmons reaction gave the target compounds
STAPA-a and STAPA-b. Their structures were character-
ized by spectroscopy methods.
3.2 AIE properties
Figure 1 shows the normalized one-photon absorption of
STAPA-a and STAPA-b in THF and in dispersion of the
nanoaggregate form (90% water) at 1 × 105 M. The
absorption maxima (λmax) of them were appearing at 400 nm
and 402 nm，respectively. The λmax of STAPA-b is only
red-shifted by 2 nm compared with STAPA-a. However,

Gao YT, et al. Sci China Chem September (2013) Vol.56 No.9
1209
Figure 2 (a, b) Corresponding emission spectra of compound STAPA-a and STAPA-b in THF-water mixtures with different water fraction at 1×105 M;
(c, d) plot of I/I0 vs water content of the solvent mixture, where I0 is the PL intensity in pure THF solution. Inset: photos of STAPA-a and STAPA-b in the
THF–water mixtures with 0% and 90% water contents taken under illumination of a UV light. Dye concentration: 105 M; Excitation wavelength: 405 nm. (e)
emission spectra of compound STAPA-a and STAPA-b in solid; Excitation wavelength: 405 nm. Inset: photos of STAPA-a and STAPA-b in solid taken
under illumination of a UV light (Excitation wavelength: 365 nm).
water ratio was added up to 20% and 10% (v/v), respective-
ly, but started to increase swiftly upon addition of water to
30% and 20% (v/v), respectively. While the water/THF ratio
reached to 90%, the PL intensity of them emitted strongly
orange-red luminescence with 23-fold and 37-fold increase
in the I/I0 ratio, respectively. The AIE effect is enhanced by
adding long octyl chains.
The formation of nanoparticles in the mixtures with high
water contents can be further certificated by the scanning
electron microscopy (SEM) image of the two dyes (Figure
3). We can see the different appearance between them. A
nanoscopic aggregate of STAPA-b gives more homogene-
ous and regular size particles. We presume that the branch-
ing alkyl chains make the stucture a more twisted skeleton,
which superiorly restrict intramolecular rotation and hinder
intermolecular close stacking, such as π-π interaction, in
dispersion of the nanoaggregate form. This illustrates why
they have the similar absorbance (∆λmax = 2 nm ) in THF,
but an obvious bathochromic shift of 25 nm in dispersion of
the nanoaggregate form (90% water). The similar phenom-
ena can be observed in emission spectra of solid state (Fig-
ure 2(e)). The emission maxima of the STAPA-a and
STAPA-b in solid state are appearing at 587 nm and 607
nm, respectively. At the same time, the PL quantum yields
(Φf) of STAPA-b (Φf = 7.4%) is about 3 times higher than
that of STAPA-a (Φf = 2.0 %). Introduction of alkyl chains
results in a redshift of 20 nm in the emission spectra of the
two luminogens in solid state. The alkyl chains are sup-
posed to affect the distortion degree of the conjugated
backbone. Presumably as a result of the more twisted con-
formation, the moleculars arrange in a mode of head-to-tail
instead of in parallel. The mode of head-to-tail which is
called J-aggregation gives rise to an increasing quantum
yield and a bathochromic shift in the emission spectra [22].
3.3 Two-photon absorption properties
2PA cross sections of two compounds were determined by
femtosecond open aperture Z-scan technique according to
previously described method [15]. Figure 4 shows the
Figure 3 SEM image of STAPA-a (a) and STAPA-b (b) nanoaggegates
prepared in THF/H2O (v/v = 1:9) at 1×105 M.

1210
Gao YT, et al. Sci China Chem September (2013) Vol.56 No.9
Figure 4 Open-Aperture Z-scan trace of STAPA-a (a, c) and STAPA-b (b, d) (scattered circle experimental data, straight line theoretic fitted data) in
CHCl3 and CH2Cl2; Two-photon fluorescence emissions spectra and TPA emissions images for STAPA-a (e) and STAPA-b (f) in solution (THF) and in
dispersion of the nanoaggregate form (90% water) at 1 × 105 M, excited at 800 nm.
open-aperture Z-scan data and 2PA coefficient got by data
fitting. The 2PA cross section σ can be calculated by using
the equation of σ = hνβN0, where N0 = NAC is the number
density of the absorption centers, NA is the Avogadro
constant and C represents the solute molar concentration.
The values of 2PA cross section (σ) for STAPA-a in chlo-
roform and dichloromethane are 620 and 405 GM at
wavelength of 800 nm, respectively. Interestingly,
STAPA-b gives much larger σ value of 1610 and 909 GM
in chloroform and dichloromethane, respectively. We sus-
pect the electron-donating ability of triarylamine is en-
hanced with the introduction of alkyl chains, which increase
the extent of intramolecular charge transfer from the ends to
the core, resulting in a large increase of 2PA cross section.
In addition, the solubility of STAPA-b improved by alkyl
chains also plays a role in increasing σ value. It is notewor-
thy that solvent effect has a significant influence on σ and
the 2PA cross section, in which they show better σ data in
chloroform. As the solvent polarity increases, consistent
with more pronounced ICT in the excited state, larger
changes are produced by solvent stabilization. The correla-
tion between the solvent effect and 2PA cross section value
is complicated and has not made very clear by far, which
relates to solute-solvent interactions and solubility EHOMO
difference besides solvent polarity [23].
(Figure 4), respectively. Comparing to the one photon
fluorescence of STAPA-a and STAPA-b with the peaks
located at 575 nm and 580 nm (Figure 2 (a, b)), the good
overlap between one- and two-photon excitation fluores-
cence indicates that the emissions result from the same
excited state, regardless of the different mode of excitation.
It shall be noted that the two compounds have performed
two-photon excitation studies at one wavelength (800 nm).
As shown in Figure 4(e, f), two-photon fluorescence (TPF)
was remarkably intensified by nanoaggregation.
3.4 Electrochemical properites
It is known that the stronger electron-donating ability of the
donor can result in a higher HOMO energy level. To further
demonstrate that the introduction of alkyl chains does en-
hance the electron-donating ability of triarylamine. Figure 5
shows the cyclic voltammetry (CV) diagrams of the com-
pounds using 0.1 M tetrabutylammonium hexafluorophos-
phate as supporting electrolyte in dichloromethane solution
with platinum button working electrodes, a platinum wire
counter electrode, and an SCE reference electrode. The SCE
reference electrode was calibrated using a ferrocene/
ferrocenium (Fc/Fc+) redox couple as an external standard.
The data are summarized in Table 1. It can be speculated
from Figure 5 that the first half-wave potentials (Eox) of
STAPA-a and STAPA-b were 0.53 and 0.46 V, respec-
tively. Therefore, the ground state oxidation potential cor-
Under the excitation of 80 fs, 800 nm pulse, STAPA-a
and STAPA-b in a mixture of THF and water emit intense
fluorescence with the peaks located at 580 nm and 590 nm

Gao YT, et al. Sci China Chem September (2013) Vol.56 No.9
1211
Figure 5 Cyclic voltammograms of STAPA-a (a) and STAPA-b (b) in DCM with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electro-
lyte.
Table 1 Electrochemical properties of STAPA-a and STAPA-b
λcut offd) (nm)
559
Compound
STAPA-a
STAPA-b
EHOMOa) (eV)
5.32
ELUMOb) (eV)
3.10
E0-0c) (eV)
2.22
2.20
564
5.16
2.96
a) EHOMO was measured in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode: Pt; reference
electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an external reference. Counter electrode: Pt wire); b) ELUMO was estimated by subtracting
E0-0 to HOMO; c) E0-0 was estimated from the intercept of the normalized absorption and emission spectra; d) estimated from the absorption thresholds.
1
2
3
Goppert-Mayer M. Elementary processes with two quantum jumps.
Ann Phys, 1931, 9: 273–294
responding to the HOMO energy levels were 5.32
and5.16 eV (vs. vacuum) according to the equations
EHOMO = e(Eox + 4.7) (eV), respectively. It is clear that the
HOMO level of STAPA-b is higher than that of STAPA-a,
which is caused by the stronger electron-donating ability of
N,N-dioctylaniline. The above results further verify that the
electron-donating ability of triarylamine is indeed increased
with the introduction of alkyl chains.
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4 Conclusions
In summary, we have synthesized and characterized two
new octupolar triazine-based chromophores with large two-
photon absorption cross sections and excellent aggrega-
tion-induced emission properties. Their one- and two-
photon absorption properties, aggregation-induced emission
effect and electrochemical properties have been investigated.
The two-photon absorption cross sections of STAPA-a
and STAPA-b can reach as large as 620 and 1610 GM in
chloroform, respectively. Introduction of alkyl chains which
has improved the electron-donating ability and solubility of
STAPA-b results in a considerable increase in the value of
2PA cross section. STAPA-b with alkyl chains exhibits a
more excellent AIE effect. The study on structure-property
relationships will be useful in engineering novel chromo-
phores with great AIE and 2PA properties.
playing high two-photon absorption.
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This work was supported by the National Natural Foundation of China
(2116110444, 21172073), National Basic Research Program of China (973
program, 2013CB733700, 2013CB834701), and the Fundamental Re-
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