Aggregation emission properties of oligomers based on tetraphenylethylene

Article abstract of DOI:10.1021/jp911311j

A series of eight derivatives based on tetraphenylethylene were prepared, and two of these, i.e., 1,1-bis(4-phenylcarbonyl)-2,2-diphenylethylene (2), 1,1,2,2-tetrakis(4-phenylcarbonyl)phenylethylene (4), were characterized crystallographically. Because the rigidity and steric hindrance in the molecular structure enhanced regularly from sample 5 to 8, UV-visible absorption and PL spectra of 5-8 show the transition from aggregation-induced emission (AIE) to aggregation-induced emission enhancement (AIEE) behavior. Solid fluorescence lifetime characterization shows that samples with less steric hindrance and more interaction in or between molecules will result in a short fluorescence lifetime. All samples 5-8 become more emissive when their chains are induced to aggregate by adding water into their acetonitrile solutions. Cyclic voltammetry measurements taken give the band gap of sample 5-8 as 2.88, 2.70, 2.56, and 2.43 eV, and theoretical calculations also support these bad gap results. Conformational simulations also suggest that the origin of transition from AIE to AIEE behavior is due to the restricted intramolecular rotations of the aromatic rings in samples.

Full text of DOI:10.1021/jp911311j

J. Phys. Chem. B 2010, 114, 5983–5988
5983
Aggregation Emission Properties of Oligomers Based on Tetraphenylethylene
Weizhi Wang,*,† Tingting Lin,‡ Min Wang,§ Tian-Xi Liu,† Lulu Ren,† Dan Chen,† and
Shu Huang†
Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of
Macromolecular Science, Fudan UniVersity, Shanghai 200433, Institute of Materials Research and
Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602,
and Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai200438
ReceiVed: NoVember 29, 2009; ReVised Manuscript ReceiVed: March 30, 2010
A series of eight derivatives based on tetraphenylethylene were prepared, and two of these, i.e., 1,1-bis(4-
phenylcarbonyl)-2,2-diphenylethylene (2), 1,1,2,2-tetrakis(4-phenylcarbonyl)phenylethylene (4), were character-
ized crystallographically. Because the rigidity and steric hindrance in the molecular structure enhanced regularly
from sample 5 to 8, UV-visible absorption and PL spectra of 5-8 show the transition from aggregation-
induced emission (AIE) to aggregation-induced emission enhancement (AIEE) behavior. Solid fluorescence
lifetime characterization shows that samples with less steric hindrance and more interaction in or between
molecules will result in a short fluorescence lifetime. All samples 5-8 become more emissive when their
chains are induced to aggregate by adding water into their acetonitrile solutions. Cyclic voltammetry
measurements taken give the band gap of sample 5-8 as 2.88, 2.70, 2.56, and 2.43 eV, and theoretical
calculations also support these bad gap results. Conformational simulations also suggest that the origin of
transition from AIE to AIEE behavior is due to the restricted intramolecular rotations of the aromatic rings
in samples.
Introduction
used without further purification. All of the solvents were used
after purification according to conventional methods when
Tetraphenylethenes (TPE) display interesting chemical and
physical properties1 and can be used as electron transfer
catalysts.2 Furthermore, its rich electrochemical and excited state
properties provide excellent research modes for energy transfer.3
On the other hand, TPE’s extended π-systems are also important
candidates for incorporation into various organic optoelectronic
and optomechanical switching and storage devices,4 as well as
for fluorescent chemosensors.5
As novel and unusual phenomena, aggregation-induced
emission (AIE),6,7 and aggregation-induced emission enhance-
ment (AIEE)8 have recently been given much attention. These
findings challenge our current understanding of photolumines-
cence (PL) processes and aid in deciphering the causes and
mechanisms of PL, which may help spawn new photophysical
theories and technological innovations. AIE is usually observed
in small molecules, such as benzoxazole-based dye by Lee9 and
in organoiridium complexes by Huang.10 On the other hand,
AIEE is only found in polymers and large structure
nanoparticles.8b Both phenomena are caused by steric interac-
tions in the materials.
required.
1
Measurements and Characterization. H and 13C NMR
spectra were recorded with Varian Mercury Plus 400 spectrom-
eters, and chemical shifts were reported in ppm units with
tetramethylsilane as an internal standard. The UV-visible
absorption spectra were obtained in chloroform on a Shimadzu
UV-3150 spectrophotometer. The photoluminescence spectra
were recorded on a Shimadzu RF-5301 PC fluorometer at room
temperature. Lifetime studies were performed with an Edinburgh
FL-920 photocounting system with a hydrogen-filled lamp as
the excitation source. The data were analyzed by iterative
convolution of the luminescence decay profile with the instru-
ment response function using the software package provided
by Edinburgh Instruments. The PL quantum yields (QF) of the
oligomer solutions were measured with 9,10-diphenylanthracene
(DPA) in cyclohexane (QF ) 90%) as the standard. The samples
were analyzed by Voyager DE-STR matrix assisted laser
desorption-time-of-flight mass spectrometry (MALDI-TOF).
Cyclic voltammetry (CV) measurements of the oligomer films
coated on a glassy carbon electrode (0.08 cm2) were performed
in an electrolyte of 0.1 M tetrabutylammonium hexafluorophos-
phate (TBAPF6) in acetonitrile using ferrocene (4.8 eV under
vacuum) as the internal standard at a scan rate of 100 mV s-1
at room temperature under the protection of argon. A Pt wire
was used as the counter electrode and an Ag/AgNO3 electrode
was used as the reference electrode. X-ray crystallographic data
were collected on a P4 Bruker diffractometer equipped with a
Bruker SMART 1K CCD area detector (employing the program
SMART) and a rotating anode utilizing graphite-monochromated
Mo KR radiation (λ ) 0.710 73 Å). Data processing was carried
out by use of the program SAINT, while the program SADABS
was utilized for the scaling of diffraction data, the application
One question naturally arises: Can we build steric oligomers
to observe the critical change from AIE to AIEE? In this paper,
we successfully designed and synthesized tetraphenylethylene
derivatives that display this change.
Experimental Section
Materials. Chemicals and reagents were purchased from
Aldrich and Acros Chemical Co. unless otherwise stated and
* To whom correspondence should be addressed. Fax: +86 21-65640293.
Tel: +86 21-55664180. E-mail: wzwang@ntu.edu.sg.
† Department of Macromolecular Science, Fudan University.
‡ A*STAR.
§ Laboratory of Advanced Materials, Fudan University.
10.1021/jp911311j  2010 American Chemical Society
Published on Web 04/21/2010

5984 J. Phys. Chem. B, Vol. 114, No. 18, 2010
Wang et al.
of a decay correction and an empirical absorption correction
based on redundant reflections. The structures were solved by
using the direct-methods procedure in the Bruker SHELXL
program library and refined by full-matrix least-squares methods
on F2. All non-hydrogen atoms were refined using anisotropic
thermal parameters, and hydrogen atoms were added as fixed
contributors at calculated positions, with isotropic thermal
parameters based on the carbon atom to which they are bonded.
Theoretical Calculations. All periodic DFT calculations were
performed using SIESTA code with numerical-orbital basis sets.
The exchange-correlation functional used is the generalized-
gradient-approximation method, known as GGA-PBE. A dou-
ble-ꢀ plus polarization basis set was employed. The orbital-
confining cutoff radii were determined from an energy shift of
0.01 eV. The energy cutoff for the real space grid used to
represent the density was set as 150 Ry. To further speed up
calculations, Kohn-Sham equation was solved by an iterative
parallel diagonalization method that utilizes the ScaLAPACK
subroutine pdsygvx with two-dimensional block cyclicly dis-
tributed matrix. The accuracy of the SIESTA method was
carefully benchmarked with the plane-wave code used previ-
ously. The geometry of single molecule (5-8) was optimized
by employing DFT methods at the level of GGA-PW91/DNP
(cutoff 3.7 Å) and pseudopotentials available in the DMol3
module in the Materials Studio Modeling package (Accelrys).
The optimization convergences for the energy, force, displace-
added, dropwise, a solution of 1.41 g (0.01 mol) of benzoyl
chloride in 5 mL of CS2. When addition was complete, the flask
content was reacted for 5 h under room temperature. After that,
the reaction mixture was stirred in an ice bath while water was
cautiously added. The reaction mixture was extracted with
methylene chloride, and the extract was washed with dilute,
aqueous sodium hydroxide, dried over magnesium sulfate, and
evaporated to give a white solid (3.0 g, 70%). This material
was purified by a silica column (petroleum ether:ethyl acetate
) 10:1, Rf ) 0.76), mp 57-58 °C. 1H NMR (CDCl3, 400 MHz):
δ 7.20-7.65 (m, 2H), 7.51-7.60 (m, 3H), 7.41-7.47 (m, 2H),
7.08-7.18 (m, 11H), 7.01-7.08 (m, 6H). 13C NMR (CDCl3,
100 MHz): δ 126.9, 127.1, 127.9, 128.1, 128.4, 129.8, 130.1,
131.5, 132.4, 135.4, 138.0, 140.1, 142.8, 143.4, 148.6, 196.5.
Anal. Calcd for C33H24O (436.54): C, 90.79; H, 5.54. Found:
C, 90.66; H, 5.61. HRMS (MALDI-TOF): m/z 436.2 (M+, 30%),
459.3 ([M + Na]+, 100%).
1,1-Bis(4-phenylcarbonyl)-2,2-diphenylethylene (2). Syn-
thesis steps are the same as for compound 1: aluminum chloride
(2.66 g, 0.02 mol), TPE (3.32 g, 0.01 mol) in 20 mL of CS2,
2.82 g (0.02 mol) of benzoyl chloride in 5 mL of CS2, 7 h under
room temperature. A green yellow solid (3.1 g, 57%) was
purified by silica column (petroleum ether:ethyl acetate ) 10:
1
2, Rf ) 0.35), mp 209-210 °C. H NMR (CDCl3, 400 MHz):
δ 7.72-7.77 (m, 4H), 7.52-7.62 (m, 6H), 7.42-7.48 (m, 4H),
7.08-7.18 (m, 10H), 7.02-7.08 (m, 4H). 13C NMR (CDCl3,
100 MHz): δ 127.5, 128.1, 128.4, 130.1, 131.5, 132.5, 135.7,
137.8, 139.1, 142.9, 144.4, 147.9, 196.5. Anal. Calcd for
C40H28O2 (540.65): C, 88.86; H, 5.22. Found: C, 88.80; H, 5.41.
HRMS (MALDI-TOF): m/z 539.8 (M+, 100%), 562.8 ([M +
Na]+, 55%).
ment, and SCF density are 1.0 × 10-5, 2.0 × 10-3, 5.0 × 10-3
,
and 1.0 × 10-6, respectively. The HOMO and LUMO were
obtained at the optimized structures. Calculations were carried
out at T ) 0 K (DFT), and the gas phase (single molecule) and
experimental measurements were done at room temperature and
in dilute solutions.
1,1,2-Bis(4-phenylcarbonyl)-2-phenylethylene (3). Synthesis
steps are the same as for compound 1: aluminum chloride (7.98
g, 0.06 mol), TPE (3.32 g, 0.01 mol) in 50 mL of nitrobenzene,
benzoyl chloride (8.46 g, 0.06 mol) of in 10 mL of nitrobenzene,
12 h under 80 °C. A yellow solid (2.97 g, 46%) was purified
by silica column (petroleum ether:ethyl acetate ) 10:4, Rf )
0.62), mp 140-141 °C. 1H NMR (CDCl3, 400 MHz): δ
7.72-7.78 (m, 6H), 7.52-7.64 (m, 9H), 7.42-7.49 (m, 6H),
7.12-7.21 (m, 9H), 7.05-7.10 (m, 2H). 13C NMR (CDCl3, 100
MHz): δ 127.8, 128.5, 130.1, 131.4, 132.6, 136.0, 136.3, 137.7,
140.6, 142.3, 143.2, 147.2, 147.3, 196.3. Anal. Calcd for
C47H32O3 (644.76): C, 87.55; H, 5.00. Found: C, 87.51; H, 5.11.
HRMS (MALDI-TOF): m/z 643.3 (M+, 100%), 666.3 ([M +
Na]+, 90%).
1,1,2,2-Tetrakis(4-phenylcarbonyl)phenylethylene (4). Syn-
thesis steps are the same as for compound 1: aluminum chloride
(13.3 g, 0.1 mol), TPE (3.32 g, 0.01 mol) in 50 mL of
nitrobenzene, benzoyl chloride (14.1 g, 0.1 mol) in 10 mL of
nitrobenzene, 15 h under 100 °C. A light yellow solid (4.1 g,
55%) was purified by silica column (petroleum ether:ethyl
acetate ) 10:4, Rf ) 0.30), mp 233-234 °C. 1H NMR (CDCl3,
400 MHz): δ 7.71-7.77 (m, 8H), 7.60-7.64 (m, 8H), 7.54-7.60
(m, 4H), 7.43-7.49 (m, 8H), 7.17-7.22 (m, 8H). 13C NMR
(CDCl3, 100 MHz): δ 128.5, 130.1, 131.4, 132.7, 136.5, 137.6,
142.1, 146.7, 196.2. Anal. Calcd for C54H36O4 (748.86): C,
86.61; H, 4.85. Found: C, 86.49; H, 4.92. HRMS (MALDI-
TOF): m/z 748.3 (M+, 40%), 771.3 ([M + Na]+, 100%).
1,4-Bis(1,2,2-triphenylWinyl)benzene (5). To a solution of
diphenylmethane (2.02 g, 12 mmol) in dry tetrahydrofuran (20
mL) was added 4 mL of a 2.5 M solution of n-butyllithium in
hexane (10 mmol) at 0 °C under an argon atmosphere. The
resulting orange-red solution was stirred for 30 min at that
temperature. To this solution was added an appropriate amount
Synthetic Procedures. 1,1,2,2-Tetraphenylethylene (TPE).
A solution of diphenylmethane (2.02 g, 12 mmol) in dry
tetrahydrofuran (20 mL) was added with 4 mL of a 2.5 M
solution of n-butyllithium in hexane (10 mmol) at 0 °C under
an argon atmosphere. The resulting orange-red solution was
stirred for 30 min at that temperature. An appropriate amount
of benzophenone (9 mmol) was added to this solution and the
reaction mixture was allowed to warm to room temperature with
stirring during a 6 h period. The reaction was quenched with
the addition of an aqueous solution of ammonium chloride, the
organic layer was extracted with dichloromethane (30-50 mL),
and the combined organic layers were washed with a saturated
brine solution and dried over anhydrous MgSO4. The solvent
was evaporated, and the resulting crude alcohol (containing
excess diphenylmethane) was subjected to acid-catalyzed de-
hydration as follows.
The crude alcohol was dissolved in about 80 mL of toluene
in a 100 mL Schlenk flask fitted with a Dean-Stark trap. A
catalytic amount of p-toluenesulfonic acid (342 mg, 1.8 mmol)
was added, and the mixture was refluxed for 3-4 h and cooled
to room temperature. The toluene layer was washed with 10%
aqueous NaHCO3 solution (20-25 mL) and dried over anhy-
drous magnesium sulfate and evaporated to afford the crude
tetraphenylethylene. The crude product was purified by a simple
recrystallization from a mixture of dichloromethane and metha-
nol to give a white solid, yield 84%, mp 222-224 °C. 1H NMR
(CDCl3, 400 MHz): δ 7.05-7.08 (m, 8H), 7.10-7.13 (m, 12H).
13C NMR (CDCl3, 100 MHz): δ 126.6, 127.8, 131.5, 141.1,
143.9.
1-(4-Phenylcarbonyl)-1,2,2-triphenylethylene (1). To a stirred
slurry of 1.33 g (0.01 mol) of aluminum chloride and 3.32 g of
TPE (0.01 mol) in 15 mL of CS2 in room temperature was

Oligomers Based on Tetraphenylethylene
J. Phys. Chem. B, Vol. 114, No. 18, 2010 5985
SCHEME 1: Synthesis of TPE Derivatives
of compound 1 (9 mmol), and the reaction mixture was allowed
to warm to room temperature with stirring over a 6 h period.
The reaction was quenched with the addition of an aqueous
solution of ammonium chloride, the organic layer was extracted
with dichloromethane (30-50 mL), and the combined organic
layers were washed with a saturated brine solution and dried
over anhydrous MgSO4. The solvent was evaporated, and the
resulting crude alcohol (containing excess diphenylmethane) was
subjected to acid-catalyzed dehydration as follows.
The crude alcohol was dissolved in about 80 mL of toluene
in a 100 mL Schlenk flask fitted with a Dean-Stark trap. A
catalytic amount of p-toluenesulfonic acid (342 mg, 1.8 mmol)
was added, and the mixture was refluxed for 3-4 h and cooled
to room temperature. The toluene layer was washed with 10%
aqueous NaHCO3 solution (20-25 mL), dried over anhydrous
magnesium sulfate, and evaporated to afford the crude 5 (5.2
g, 99%). The crude product was purified by a simple recrys-
tallization from a mixture of dichloromethane and methanol,
1
mp 250-251 °C. H NMR (CDCl3, 400 MHz): δ 7.08-7.14
(m, 4H), 7.02-7.08 (m, 12H), 6.95-7.02 (m, 12H), 6.73-6.77
(m, 6H). 13C NMR (CDCl3, 100 MHz): δ 126.6, 127.8, 130.9,
131.6, 141.0, 142.0, 143.7, 144.0. Anal. Calcd for C46H34
(586.76): C, 94.16; H, 5.84. Found: C, 93.89; H, 6.11. HRMS
(MALDI-TOF): m/z 586.5 (M+, 40%), 693.4 ([M + Ag]+, 68%).
1-(2,2-Diphenyl-1-(4-(1,2,2-triphenylWinyl)phenyl)Winyl)-4-
(1,2,2-triphenylWinyl)benzene (6). Steps are the same as for
compound 5: diphenylmethane (2.02 g, 12 mmol) in 20 mL of
dry tetrahydrofuran, 4 mL of n-butyllithium in hexane (2.5 M,
10 mmol), 0 °C, 30 min. After that, compound 2 (4.5 mmol)
was added and reacted for 6 h to get crude 6 (3.7 g, 100%).
The crude product was purified by a simple recrystallization
Anal. Calcd for C106H76 (1349.74): C, 94.32; H, 5.68. Found:
C, 94.31; H, 5.69. HRMS (MALDI-TOF): m/z 1350.0 (M+,
100%), 1458.0 ([M + Ag]+, 95%).
1
from a mixture of dichloromethane and methanol. H NMR
Results and Discussion
(CDCl3, 400 MHz): δ 7.08-7.11 (m, 8H), 7.06-7.08 (m, 16H),
6.93-7.00 (m, 16H), 6.69-6.74 (m, 8H). 13C NMR (CDCl3,
100 MHz): δ 126.6, 127.7, 128.3, 128.8, 130.7, 130.9, 131.6,
133.1 140.9, 140.1, 141.9, 142.1, 143.8, 144.0. Anal. Calcd for
C66H48 (841.09): calcd. C 94.25, H 5.75, found C 94.20, H 5.80.
HRMS (MALDI-TOF): m/z 840.4 (M+, 30%), 948.3 ([M+Ag]+,
100%).
Synthesis. Symmetrical TPE are easily prepared via McMurry
coupling11 of the corresponding benzophenones, catalyzed by
low-valent titanium. However, it is difficult to prepare unsym-
metrical tetraarylethylenes in high yield, even when one of the
components is used in sacrificial excess. Using two different
benzophenones, the McMurry reaction will produce a mixture
of symmetrical and unsymmetrical products. Rathore et al. have
recently reported12 a new method involving dehydration of the
corresponding alcohols to afford the unsymmetrical products
in nearly quantitative yield. This method is used here to
synthesize the tetraphenylethene derivatives 5-8 listed in
Scheme 1.
The details of reaction of TPE with benzoyl chloride are listed
in Table S1 (Supporting Information). It was noticed that when
the Lewis acid was added to the reaction mixture containing
TPE and the acylating agent in nitrobenzene with the mole ratio
1:4:4 (TPE/benzoyl chloride/AlCl3), a strong dark violet color
change occurred for the aluminum chloride and the product
forms complexes. It is noteworthy that the desired products 1,
2, 3, and 4 can be synthesized in one step. However, the low
yields and difficult separation make it unacceptable. As shown
for entries 2, 3, 4, and 5 in Table S1 (Supporting Information),
the separated four steps with different TPE/benzoyl chloride/
AlCl3 mole ratios can obtain good yield. Most importantly, the
separation process can be easily realized. If the solvent was
changed to CS2, the results of products 1 and 2 are better because
the solvent can be easily discarded by vacuum.
1-(2-Phenyl-1,2-bis(4-(1,2,2-triphenylWinyl)phenyl)Winyl)-4-
(1,2,2-triphenylWinyl)benzene. (7). Steps are the same as for
compound 5: diphenylmethane (2.02 g, 12 mmol) in 20 mL of
dry tetrahydrofuran, 4 mL of n-butyllithium in hexane (2.5 M,
10 mmol), 0 °C, 30 min. The following steps are same as above
to get crude 7 (3.2 g, 100%). The crude product was purified
by a simple recrystallization from a mixture of dichloromethane
1
and methanol. H NMR (CDCl3, 400 MHz): δ 7.09-7.18 (m,
12H), 7.01-7.09 (m, 20H), 6.91-7.01 (m, 20H), 6.64-6.86
(m, 10H). 13C NMR (CDCl3, 100 MHz): δ 126.7, 127.7, 130.6,
131.0, 131.6, 140.7, 141.7, 142.1, 142.4, 143.9, 144.1. Anal.
Calcd for C86H62 (1095.41): C, 94.30; H, 5.70. Found: C, 94.32;
H, 5.68. HRMS (MALDI-TOF): m/z 1094.6 (M+, 32%), 1203.5
([M + Ag]+, 100%).
1,1,2,2-Tetrakis(4-(1,2,2-triphenylWinyl)phenyl)ethene (8).
Steps are the same as for compound 5: diphenylmethane (2.02
g, 12 mmol) in 20 mL of dry tetrahydrofuran, 4 mL of
n-butyllithium in hexane (2.5 M, 10 mmol), 0 °C, 30 min. After
that, compound 4 (2.3 mmol) was added and reacted for 6 h to
get crude 8 (3.1 g, 100%). The crude product was purified by
a simple recrystallization from a mixture of dichloromethane
1
and methanol. H NMR (CDCl3, 400 MHz): δ 7.07-7.15 (m,
Friedel-Crafts-type acylation of TPE in the presence of a
Lewis acid (AlCl3) gave four differently substituted aromatic
products (1, 2, 3, and 4) readily in high yield. This electrophilic
aromatic substitution allows the synthesis of monoacylated
28H), 6.98-7.07 (m, 24H), 6.90-6.94 (m, 8H), 6.71-6.76 (m,
8H), 6.61-6.66 (m, 8H). 13C NMR (CDCl3, 100 MHz): δ 126.7,
127.8, 130.7, 131.0, 131.7, 140.6, 141.1, 142.0, 142.2, 143.8.

5986 J. Phys. Chem. B, Vol. 114, No. 18, 2010
Wang et al.
Figure 2. (a) UV-visible absorption and photoluminescence spectra
of samples 5-8 in THF at room temperature with a concentration of
10 µM. (b) Photoluminescence spectra of solid films of 5-8 (obtained
by spin-coating a solution with a concentration of 100 µM).
shifts typically indicate conformational reorganization in ge-
ometry from the pyramidal ground state to a more planar excited
state.14 The PL intensity and fluorescent quantum yield (ΦF) of
7 and 8 were detected in THF solution (ΦF for 7 is 0.15%, and
ΦF for 8 is 0.21%).
Unlike the solution PL spectra, the solid films of all the four
samples emitted light when they were exposed to UV irradiation.
The PL spectra of the solid films of 5-8 are shown in Figure
2b, and the corresponding emission peaks and ΦF are 469, 480,
486, and 490 nm, and 25.8, 30.2, 44.0, and 47.8%, respectively.
Comparing the samples in solution and in solid films, we observe
two obvious differences. One difference is in the fluorescence
intensity. Samples 5 and 6 emitted light in the solid state but
were almost nonemissive in solution. However, the change in
fluorescence intensity for samples 7 and 8 with more steric
hindrance was not so obvious. This phenomenon can probably
be explained by the existence of a nonradiative decay pathway
of the singlet excited state. It is known that rotational energy
relaxation can nonradiatively deactivate excited species.15 In
solutions at room temperature, phenyl rings against the ethylene
core of samples 5 and 6 have enough space for intramolecular
motions [motions include rotation (e.g., CPhsCd torsion) and
vibration (e.g., CPhsCd stretching and bending); the space
structures of samples 5 and 6 can be found in the Supporting
Information]. This type of steric hindrance will effectively
deactivated the excited states by the rotation, thus making the
molecules nonemissive. However, for samples 7 and 8, which
have a sterically crowded structure (see Supporting Information
Figure 1. ORTEP drawing of 2 and 4. The ellipsoid probability is
30%.13
products between a benzene ring of TPE and different acyl
chlorides. The acylated benzene is deactivated and does not
undergo a second substitution. Further acylation only occurs
on other benzene rings in TPE. The products are controlled by
changing the TPE/benzoyl chloride/AlCl3 molar ratios and
reaction conditions. As an example, the structures of 2 and 4
were characterized by X-ray crystallographic analysis in Figure
1 and ref 13.
Optical Properties. To investigate the transition between AIE
and AIEE, UV-visible absorption and PL spectra of solutions
of 5-8 were obtained (Figure 2a). Absorption peaks of 5-8
were observed at 313, 320, 328, and 331 nm, respectively.
However, the corresponding PL spectra of the solutions were
different. 5 and 6 are practically nonluminescent in dilute THF
solution. The PL curves of 7 and 8 had maxima at 406 and 462
nm, respectively. The Stokes shifts for compounds 7 and 8 were
78 and 131 nm, respectively, suggesting that there is a large
and fast geometrical conversion between the TPE units as well
as between the neighboring aromatic units. These large Stokes

Oligomers Based on Tetraphenylethylene
J. Phys. Chem. B, Vol. 114, No. 18, 2010 5987
Figure 3. Fluorescence decay curves of solid film of samples 5-8.
Figure 4. Dependence of quantum yield on the water content in
acetonitrile/water mixtures of samples 5-8. Sample concentration: 5
and 6, 10 µM; 7 and 8, 1 µM. Excitation wavelength: 350 nm.
Figures S30 and S31), this nonradiative decay pathway of its
singlet excited state is blocked. Therefore, 7 and 8 can emit
light in solution with fluorescence intensities depending on the
extent of steric hindrance in the molecule. In the solid aggregate
state, interactions involved in the crystal packing may restrict
the intramolecular rotations, which may block the nonradiative
channel and stop the radiative decay, thus making the film
sample luminescent. This is why samples 5 and 6, as derivatives
of TPE, show AIE. It is also possible that samples 7 and 8 show
reduced AIE (in other words, samples 7 and 8 exhibit AIEE)
by increasing the steric hindrance of the structure.
Another obvious difference in solid state PL of the com-
pounds is the shift of emission wavelength. Extending the length
of conjugated chains results in a red shift of fluorescence spectra.
However, the difference in the emission wavelength of the solid
films of 5-8 decreases steadily (for 5 to 6, 11 nm; for 6 to 7,
6 nm; for 7 to 8, 4 nm). If we take the changes in steric
hindrance into account, a possible explanation is that the red
shift usually observed by extending the conjugated chain is
counteracted by the blue shift caused by the increase in steric
hindrance. The effect of steric hindrance on the emission
wavelength has also been proven by the characterization of the
fluorescence lifetimes (τ) of the solid films, as shown in Figure
3. Sample 5, which is inclined to crystallize with less steric
hindrance, has more intramolecular or intermolecular interac-
tions, thus resulting in a short fluorescence lifetime of 2.10 ns.
Following the above concept, it is clear that the τ increases from
2.28 to 2.64 to 2.83 ns for 6, 7, and 8, respectively, due to the
increased steric hindrance within the sample.
molecules begin to aggregate in the solvent mixture with this
composition. The ΦF values of both 5 and 6 continuously
increase with an increase in the water content, reaching a
maximum of 6.7% and 8.8%, respectively, at water content of
90%. This is an increase in ΦF of more than 250-fold for both
5 and 6 over their respective dilute acetonitrile solutions under
the same measurement conditions. Further increasing the water
content to 95% results in a decrease in ΦF due to a morphologi-
cal change, as suggested by Tang.6d The emission properties of
samples 5 and 6 clearly proves that they exhibit AIE. Compared
to 5 and 6, derivatives 7 and 8 show little AIE, instead exhibiting
AIEE. The acetonitrile solutions of 7 and 8 are luminescent
(ΦF for 7 is 5.8%, ΦF for 8 is 9.5%), because the intramolecular
rotations are more restricted for more sterically hindered groups.
The ΦF of 7 and 8 display different trends to those of 5 and 6.
At a water content of 0-70%, the ΦF values of 7 and 8
gradually increase. When the water content is increased to 80%,
the ΦF values for 7 and 8 reach a maximum of 16.1% and
20.4%, respectively, which are 2.8 and 2.1 times higher than
the ΦF vlues of their respective acetonitrile solutions. At very
high water contents, ΦF decreases a little.
Electrochemical Properties. Cyclic voltammetry (CV) mea-
surements of 5-8 films coated on a glassy carbon electrode
(∼0.08 cm2) were performed in an electrolyte of 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) in aceto-
nitrile using ferrocene (4.8 eV below vacuum) as the internal
standard at a scan rate of 100 mV/s at room temperature under
the protection of argon. A Pt wire was used as the counter
electrode, and an Ag/AgNO3 electrode was used as the reference
electrode. The corresponding data are summarized in Table 1.
On the basis of the onset potentials of the oxidation (0.85 to
0.70 V) and reduction (-2.03 to -1.73 V), we estimated the
HOMO and LUMO energy levels of 5-8 to be 5.60, 5.53, 5.47,
5.45 eV and 2.72, 2.65, 2.57, 2.46 eV, respectively, with regard
To provide further evidence for the above hypothesis, the
dependence of the ΦF of 5-8 on the solvent composition of
the acetonitrile/water mixture is shown in Figure 4. In the
mixtures with a water content of less than 50%, the ΦF for 5
and 6 are very small (0.026% for 5 and 0.032% for 6) because
their molecules are dissolved in the solvent mixtures. ΦF starts
to increase at a water content of >50%, suggesting that the
TABLE 1: Electrochemical Properties of Samples 5-8a,b
HOMO (eV)
LUMO (eV)
Eg (eV)
ox
red
compound
Eonset (V)
Eonset (V)
exptl
calc
exptl
calc
exptl
calc
5
6
7
8
0.85
0.78
0.72
0.70
-2.03
-1.92.
-1.84
-1.73
-5.60
-5.53
-5.47
-5.45
-4.731
-4.625
-4.591
-4.564
-2.72
-2.65
-2.57
-2.46
-2.454
-2.589
-2.556
-2.562
2.88
2.70
2.56
2.43
2.277
2.036
2.035
2.002
a Determined from cyclic voltammetry in acetonitrile for oxidation potentials and in THF for reduction potentials (0.1 M n-Bu4N+PF6 as a
supporting electrolyte) using Ag/Ag+ (0.01 M) as a reference electrode at a scan rate of 100 mV/s. b HOMO and LUMO levels were
determined using the following equations: HOMO/LUMO ) -e(Eonset -0.0468 V) - 4.8 eV, where the value 0.0468 V is for FOC vs Ag/Ag+.
-

5988 J. Phys. Chem. B, Vol. 114, No. 18, 2010
Wang et al.
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to the energy level of ferrocene. The band gaps of samples 5-8
are 2.88, 2.70, 2.56, and 2.43 eV, which have the same trends
with the calculated data in Table S2 (Supporting Information).
DFT calculations show that, for the TPEs, the lowest unoccupied
molecular orbital (LUMO) is stabilized by a bonding interaction
between the π-system and the double bond (Figure S27-S31
in Supporting Information). The DFT calculations also reveal
that the HOMO diminishes significantly as the oligomer length
increases (from 5 to 8).
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In summary, we have studied aggregation emission properties
of oligomers based on tetraphenylethylene in solution and
aggregate states. Due to the rigidity and steric hindrance in
molecular structure enhanced regularly, UV-visible absorption
and PL spectra of 5-8 reveal the transition from AIE to AIEE
behavior. Lifetime measurements reveal that the excited species
of samples become longer-lived in the aggregates. All samples
5-8 become more emissive when their chains are induced to
aggregate by adding water into their acetonitrile solutions. Cyclic
voltammetry measurements and theoretical calculations also
support the bad gap results. The conformational simulations also
suggest that the origin of transition from AIE to AIEE behavior
is the restricted intramolecular rotations of the aromatic rings
in samples.
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Commun. 2008, 685–687.
Acknowledgment. This paper was inspired by Prof. Ben
Zhong Tang’ s lecture in Fudan 2008. This work was financially
supported by the National Natural Science Foundation of China
(20974022, 20704009, 20774019, 50873027), the Opening
Project of Key Lab for Special Functional Materials, Henan
University (0701).
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(13) Crystal data for 2: Crystals were grown from CH2Cl2. The structure
was solved on a Bruker SMART CCD diffractometer using Mo KR
radiation. C40H28O2 (Mr ) 540.62); monoclinic, space group C2/c, Dc )
1.233 g cm-3, a ) 16.698(3) Å, b ) 10.5516(19) Å, c ) 16.706 (3) Å, R
) 90°, ꢁ ) 98.437(2)°, γ ) 90°, V ) 2911.6(9) Å3, Z ) 4, λ ) 0.710 73
Å, µ ) 0.075 mm-1, T ) 296(2) K, R ) 0.0264 for 11 361 observed
reflections [I > 2σI] and Rw ) 0.0939 for all 3138 unique reflections. Crystal
data for 4: Crystals were grown from CH2Cl2. The structure was solved on
a Bruker SMART CCD diffractometer using Mo KR radiation. C54H36O4
Supporting Information Available: Crystal date (cif file).
Figures showing the 1H NMR, 13C NMR, and MALDI-TOF of
compounds 1-8, 1H-1H COSY of compound 2, byproducts in
the synthesis of TPE derivatives, and frontier orbitals. Tables
of reaction data and theoretical calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
(Mr ) 748.83); triclinic, space group P1, Dc ) 1.240 g cm-3, a ) 10.9819(4)
j
References and Notes
Å, b ) 12.2293(4) Å, c ) 15.9624 (7) Å, R ) 78.216(2)°, ꢁ ) 85.799(2)°,
γ ) 72.961(2)°, V ) 2006.26(13) Å3, Z ) 2, λ ) 0.710 73Å, µ ) 0.077
mm-1, T ) 296(2) K, R ) 0.0259 for 18 656 observed reflections [I > 2σI]
and Rw ) 0.1139 for all 8657 unique reflections.
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