Aggregation-induced emission in tetraphenylthiophene-derived organic molecules and vinyl polymer

Article abstract of DOI:10.1021/jp1012297

In this study, organic molecules of tetraphenylthiophene (TP) and the derived model compound of TP-Qu and vinyl polymer of PS-Qu with the pendant group of TP-Qu were prepared and characterized to identify their photoluminescent (PL) responses toward the effect of aggregation-induced emission (AIE). During aggregate formation by adding the nonsolvent water to the THF solvent, the corresponding TP solutions greatly gained the emission intensity. In contrast, TP-Qu and PS-Qu in THF/water solution mixtures emitted strongly with nearly the same emission intensity despite the composition differences on the applied THF/water mixtures. Restricted intramolecular rotation (RIR) is the key factor deciding the AIE effect in different media. With four small phenyl rotors around the central thiophene stator, the RIR of the TP molecules in dilute solution is low but increases upon aggregate formations. In contrast, the bulky C-2 quinoline rotor of the TP-Qu molecule enhances the RIR effect and its PL emissions in the aggregate and in the solution states are essentially the same (irrespective of the aggregate formation by adding nonsolvent water). With the inherent TP-Qu pendant groups, the vinyl polymer PS-Qu behaves similarly to the TP-Qu molecule; that is, solutions of PS-Qu have similar emission spectra in different solution mixtures. Several methods including PL excitation (PLE), low-temperature PL emission study, and computer simulation were used in this study to estimate the relative extents of RIR and therefore, the resulting AIE effect in all the TP-derived materials.

Full text of DOI:10.1021/jp1012297

10302
J. Phys. Chem. B 2010, 114, 10302–10310
Aggregation-Induced Emission in Tetraphenylthiophene-Derived Organic Molecules and
Vinyl Polymer
Chung-Tin Lai and Jin-Long Hong*
Department of Materials and Optoelectronic Science, National Sun Yat-Sen UniVersity,
Kaohsiung 80424, Taiwan, ROC
ReceiVed: February 8, 2010; ReVised Manuscript ReceiVed: July 8, 2010
In this study, organic molecules of tetraphenylthiophene (TP) and the derived model compound of TP-Qu
and vinyl polymer of PS-Qu with the pendant group of TP-Qu were prepared and characterized to identify
their photoluminescent (PL) responses toward the effect of aggregation-induced emission (AIE). During
aggregate formation by adding the nonsolvent water to the THF solvent, the corresponding TP solutions
greatly gained the emission intensity. In contrast, TP-Qu and PS-Qu in THF/water solution mixtures emitted
strongly with nearly the same emission intensity despite the composition differences on the applied THF/
water mixtures. Restricted intramolecular rotation (RIR) is the key factor deciding the AIE effect in different
media. With four small phenyl rotors around the central thiophene stator, the RIR of the TP molecules in
dilute solution is low but increases upon aggregate formations. In contrast, the bulky C-2 quinoline rotor of
the TP-Qu molecule enhances the RIR effect and its PL emissions in the aggregate and in the solution states
are essentially the same (irrespective of the aggregate formation by adding nonsolvent water). With the inherent
TP-Qu pendant groups, the vinyl polymer PS-Qu behaves similarly to the TP-Qu molecule; that is, solutions
of PS-Qu have similar emission spectra in different solution mixtures. Several methods including PL excitation
(PLE), low-temperature PL emission study, and computer simulation were used in this study to estimate the
relative extents of RIR and therefore, the resulting AIE effect in all the TP-derived materials.
Introduction
rotors against the central silole stator, which reduces the
nonradiative pathways and leads to the enhanced emission in
the aggregated states. By internally hindering the intramolecular
rotation, one particular regioisomer of diisopropyl-attached
hexaphenylsilole compound was reported to emit strongly even
in the pure solution state.30 It suggested that with hindered
intramolecular rotation, organic fluorophores have the possibility
to emit strongly in both the aggregated and the solution states.
In addition to small-molecule fluorophores, several high-
molecular-weight polymers were also studied to exhibit AIE
effect.12,13,16,31-36 Among them, poly(diphenylacetylene)s (PD-
PAs) had been thoroughly studied and the computer simulation
results suggested that the two bulky phenyl rings in every
monomer repeat unit hinder the conformational transformation
process, which enhances the RIR process and leads to the
observed AIE phenomenon.32
By the common chemical framework with multiple peripheral
phenyl rings connected to one central stator, silole compounds
have the required RIRs responsible for the observed AIE
property. Previously, we had prepared several fluorescent main-
chain polymers with the fundamental 2,3,4,5-tetraphenylth-
iophene (TP, Scheme 1) moiety.37,38 With chemical structure
similar to silole compounds, TP molecule was expected to
behave like silole compounds and to have the prospect to exhibit
the AIE property. The small-molecule TP compound can be
easily prepared by one-step reaction from the inexpensive
starting materials of benzyl chloride and sulfur.39,40 Furthermore,
chemical formulations such as bromination, acetylation, or
nitration can be readily achieved on TP molecule by simple,
classical electrophilic substitution reactions.37-40
With the planar, disklike aromatic structures, many “tradi-
tional” organic fluorophores experience strong π-π stacking
interactions and the accompanying aggregations. Upon light
irradiation, the excited states of the aggregations often decay
nonradiatively and cause reduction in the emission intensity.
Therefore, traditional organic fluorophores are commonly emis-
sive in the dilute solution but become weak or nonemissive when
aggregated in the concentrated solution or in the solid state due
to this aggregation-caused quenching (ACQ).1-8 In view that
fluorescent materials are normally used in the solid film state,
development of efficient fluorophores with enhanced film
emission is of particular interest.
In 2001, Tang’s groups discovered that one particular silole
molecule (1-methyl-1,2,3,4,5-pentaphenylsilole) emits strongly
in the aggregated or solid state even though it is nonemissive
in the dilute solution.9,10 This interesting phenomenon, in contrast
to the normal ACQ behavior, was designated as “aggregation-
induced emission” (AIE) to emphasize the phenomenon that
the originally nonluminescent solution of silole can be induced
to emit strongly when the corresponding nanoaggregates formed
after introduction of the nonsolvent water.9-15 AIE-active
fluorophores rendered hope to the fabrications of organic light-
emitting diodes (OLEDs) with high performance since it directly
confronts the ACQ behavior encountered in the condensed
states. With this aspect, lots of organic fluorophores with the
silole framework or with other chemical functions were prepared
and found to exhibit the AIE effect.16-29 A plausible cause for
the novel AIE property of the silole compounds was considered
to be the restricted intramolecular rotation (RIR) of the phenyl
To the best of our knowledge, no report has been devoted to
the AIE effect of TP or TP-derived compounds. With the ease
of chemical formulations, TP and the TP-derived model
* To whom correspondence should be addressed. E-mail: jlhong@
mail.nsysu.edu.tw. Tel: +886-7-5252000 ext 4065.
10.1021/jp1012297  2010 American Chemical Society
Published on Web 07/27/2010

Tetraphenylthiophene-Derived Organic Molecules
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10303
SCHEME 1: Syntheses of Organic Compounds of TP, TP-NO2, TP-Bz, TP-Pm, and TP-Qu and Vinyl Polymer of PS-Qu
compound of TP-Qu (Scheme 1) and the vinyl polymer of PS-
Qu (Scheme 1) with the pendant TP-Qu group were prepared
and their photoluminescence (PL) responses toward AIE effect
were evaluated. Structural difference between the TP and the
model TP-Qu was primarily established to account for their
varied responses toward AIE effect. The PL emission of the
model TP-Qu was then used to probe the emission behavior of
the vinyl polymer of PS-Qu. Relationship between the chemical
structures and the extent of RIR leading to the observed
equivalent aggregated and solution emissions are the focuses
of this study.
to obtain the crude product. Continuous washing of the crude
product with water was performed until pH ) 7 was achieved.
Column chromatography with hexane eluent yielded the final
product (2.4 g; 24%). Mp: 144 °C; IR (KBr pellet, cm-1) 3061,
1591, 1515, 1346, 1115, 853, 700; 1H NMR (500 MHz, CD2Cl2)
δ 6.9-7.5 (m, 15H, aromatic Hs), 8.03 (d, 2H), 8.08 (d, 2H)
(Figure S-1, Supporting Information); MS m/e: calcd for
C28H19NO2S, 433.11; found, 433.2 (M+); Anal. Calcd for
C28H19NO2S: C, 77.57; H, 4.42; N, 3.23; O, 7.38; S, 7.40. Found:
C, 77.40; H, 4.02; N, 3.12; O, 7.31; S, 7.38.
Preparation of 3-Phenyl-5-(3,4,5-triphenylthiophen-2-yl)-
benzo[c]isoxazole (TP-Bz). To an ice-cooled mixture of sodium
hydroxide (3 g, 75 mmol), methanol (15 mL), and THF (6 mL),
benzyl nitrile (2 mL, 17 mmol) was added slowly. After stirring
for 5 min, TP-NO2 (2.5 g, 5.77 mmol) was added and the
mixture was then heated at 80 °C for 20 h. The reaction mixture
was then cooled in an ice bath, and the resulting precipitate
was isolated by filtration and washed with cold methanol
exhaustively. The crude product was purified by column
chromatography (CH2Cl2) to give TP-Bz (2.15 g; 73.8%). Mp:
247 °C; IR (KBr pellet, cm-1) 3059, 1634, 1597, 1558, 1490,
Experimental Section
Materials. Reagent-grade benzyl chloride, sulfur powder,
methanol, ethyl acetate, acetic acid, sodium hydroxide, 2-phe-
nylacetonitrile, iron powder, acetophenone, aluminum chloride,
acetyl chloride, m-cresol, dichloromethane, n-butyllithium, and
diphenyl phosphate were purchased from Aldrich Chemical Co.
and used directly without further purification. Styrene monomer
(Aldrich) was distilled from calcium hydride and stored under
nitrogen before use. Tetrahydrofuran (THF) and dichlo-
romethane were refluxed over sodium/benzophenone under
nitrogen for more than 2-3 days before distillation. TP
compound was prepared according to the reported procedures.39,40
Syntheses of Small Molecules and Polymer. Small mol-
ecules and polymers were prepared according to Scheme 1 and
the detailed procedures are given below.
1
1435, 1343, 1206, 1069, 1030, 796, 766, 727, 688, 600; H
NMR (500 MHz, CD2Cl2) δ 6.9-7.7 (m, 23H, aromatic Hs)
(Figure S-2, Supporting Information); MS m/e: calcd for
C35H23NOS, 505.15; found, 505.2 (M+); Anal. Calcd for
C35H23NOS: C, 83.14; H, 4.58; N, 2.77; O, 3.16; S, 6.34. Found:
C, 82.83; H, 4.12; N, 2.59; O, 3.23; S, 6.30.
Preparation of 2-(4-Nitrophenyl)-3,4,5-triphenylthiophene
(TP-NO2). A mixture of TP (10 g, 25.8 mmol) and glacial acetic
acid (75 mL) was vigorously stirred at 60 °C to result in a
homogeneous suspension, to which mixed liquids of glacial
acetic acid (35 mL) and concentrated nitric acid (0.83 mL) were
added quickly. After stirring at 70 °C for another 1 h, the yellow
suspension was filtered. The filtrate was precipitated from water
Preparation of (2-Amino-5-(3,4,5-triphenylthiophen-2-
yl)phenyl)(phenyl) methanone (TP-Pm). Iron powder (2 g, 36.3
mmol) and water (2.5 mL) were added to a stirred suspension
of TP-Bz (1.5 g, 2.96 mmol) in acetic acid (45 mL) at 95 °C.
The reaction mixture was heated for 1 h before being cooled to
room temperature to remove the iron powder by filtration. The
filtrate was poured into water (250 mL) and the resulting

10304 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Lai and Hong
precipitate was collected and purified by column chromatog-
raphy (EtOAc/hexane/CHCl3 1:5:45) to give TP-Pm (1.43 g,
95.7%). Mp: 207 °C; IR (KBr) 3463, 3350, 3047, 1613, 1579,
was then dried at 40 °C under vacuum to give PS-Qu (1.40 g,
83%). GPC (eluent: THF): Mn ) 153 800, Mw ) 168 000 and
PDI ) 1.09; Tg ) 160 °C; IR (KBr pellet, cm-1) 3056, 3023,
1587, 1543, 1492, 1480, 1442, 1357, 1073, 1028, 836, 754, 697;
1H NMR (500 MHz, CD2Cl2) δ 6.54-7.79 (m, 28H, aromatic
Hs), 1.56 (broad, 3H, backbone Hs′) (Figure S-5, Supporting
Information).
1
1490, 1423, 1299, 1255, 1164, 758, 694, 659 cm-1; H NMR
(500 MHz, CD2Cl2) δ 6.6-7.4 (m, 23H, aromatic Hs), 6.2 (s,
2H, Ph-NH2) (Figure S-3, Supporting Information); MS m/e:
calcd for C35H25NOS, 507.17; found, 507.2 (M+); Anal. Calcd
for C35H25NOS: C, 82.81; H, 4.96; N, 2.76; O, 3.15; S, 6.32.
Found: C, 82.47; H, 4.53; N, 2.58; O, 3.21; S, 6.29.
Instrumentation and Sample Preparation. 1H NMR spectra
were recorded with a Varian VXR-500 MHz FT-NMR instru-
ment. Tetramethylsilane was used as internal standard. A VG
Quattro GC/MS/MS/DS instrument was used to determine the
molar mass of the organic molecules. The sample was charged
into the rapidly moving gas and converted into the corresponding
ions, which were further separated based on their mass-to-charge
ratios (m/z). Molecular weight and molecular weight distribution
were determined at 40 °C from GPC, using a Waters 510 HPLC
model equipped with three Ultrastyragel columns (100, 500,
and 1000 Å) connected in series and a 410 differential
refractometer and an UV detector. Polymer solution was eluted
by THF solvent with a flow rate of 0.6 mL/min. The melting
point (mp) of the organic molecules and the glass transition
temperatures (Tg) of polymers were obtained from a TA Q-20
DSC calorimeter with a scan rate of 20 °C/min.
PL emission spectra were obtained from a LabGuide X350
fluorescence spectrophotometer using a 450 W Xe lamp as the
continuous light source. PL excitation (PLE) spectra were
recorded from a Hitachi F-4500 fluorescence spectrometer with
collection at an angle of 90° and with a resolution of 2.5 nm
(light source 150 W Xe lamp). UV-vis absorption spectra were
recorded with an Ocean Optics DT 1000 CE 376 spectropho-
tometer. Quartz cell with dimensions of 0.2 × 1.0 × 4.5 cm3
was used for the UV-vis absorption, PL excitation, and PL
emission spectra measurements. Stock solutions of the organic
and polymeric fluorophores with a concentration of 10-4 M were
primarily prepared. Aliquots of these stock solutions were
transferred to 10 mL volumetric flasks, into which appropriate
volumes of THF and water were added dropwise under vigorous
stirring to furnish 10-5 M solutions with different water contents
(0-90 vol %). UV-vis, PL emission, and PL excitation (PLE)
spectroscopy were immediately performed once the solutions
were prepared. FT-IR spectra were obtained on a Nicolet IR-
200 spectrometer. Sample solution in methanol was dropped
on a KBr pellet and dried at 100 °C under vacuum to prepare
the solid film for FT-IR analysis. The rotation barrier (ER) of
small fluorophores was formulated from the Materials Studio
(MS) commercial software of Accelrys Inc. X-ray diffraction
intensity data were collected at room temperature on a Nonius
Kappa CCD, Bruker Kappa APEX II single crystal XRD. Single
crystal of TP-Qu was grown from a dilute DMF solution in a
closed chamber saturated with hexane vapor. Fluorescence
quantum yields (Φf) of the TP and TP-Qu in the solution
mixtures (10-5 M) with varied compositions were determined
by comparison with a quinine sulfate standard (10-5 M in 0.1
N H2SO4, Φf ) 0.55).
Preparation of 2,4-Diphenyl-6-(3,4,5-triphenylthiophen-2-
yl)quinoline (TP-Qu). A solution of diphenyl phosphate (10 g,
39.97 mmol) in m-cresol (6.5 g) was added to a mixture of TP-
Pm (0.2 g, 0.39 mmol) and acetophenone (0.084 g, 0.7 mmol).
The reaction mixture was purged with argon for 20 min before
the temperature was gradually raised to 90 °C. Reaction was
conducted at 90 °C for 1 h and 140 °C for 4 h. After cooling,
methylene chloride (100 mL) and 10% NaOH (100 mL) were
added to the reaction mixture. The organic layer was separated
and washed with water (250 mL) and then dried over MgSO4
and evaporated under reduced pressure to yield an off-white
solid with some reddish liquid on top. The crude product was
filtered and washed with pentane (15 mL) to afford crystalline
solid (0.16 g, 70%). Mp: 189 °C; IR (KBr pellet, cm-1) 3055,
3025, 1588, 1548, 1480, 1446, 1358, 1031, 838, 794, 774, 750,
696; 1H NMR (500 MHz, CD2Cl2) δ 6.9-8.2 (m, 29H, aromatic
Hs) (Figure S-4, Supporting Information); MS m/e: calcd for
C43H29NS, 591.20; found, 591 (M+); Anal. Calcd for C43H29NS:
C, 87.27; H, 4.94; N, 2.37; S, 5.42. Found: C, 87.18; H, 4.80;
N, 2.34; S, 5.40.
Preparation of PS-Qu. PS-Qu was prepared according to the
procedures described in Scheme 1. The starting polystyrene used
in this study was prepared from anionic polymerization of
styrene at 50 °C with sec-BuLi as initiator and cyclohexane as
solvent and has a narrow molecular weight distribution of 1.06
(Mn ) 45 600 and Mw ) 48 200). Tg ) 102 °C.
Poly(4-acetylstyrene) (PacS). PAcS was prepared according
to the reported procedures41 and the details are as follows:
polystyrene (3.5 g, 33.7 mmol repeat unit), anhydrous chloro-
form (100 mL), and AlCl3 (10 g, 0.13 mol) were added to a
three-neck flask under nitrogen atmosphere. Acetyl chloride (6
mL, 85 mmol) was added dropwise to the stirred reaction
mixture at 45 °C, and the reaction was allowed to proceed
overnight. The resulting mixture was poured into ice water (300
mL) and the organic layer was collected, concentrated, and
poured into a large amount of ethanol. A white powder product
(4.6 g, 94%) was collected after standing for 1 day. GPC (eluent:
THF): Mn ) 50 500, Mw ) 54 300 and PDI ) 1.08; Tg ) 119
°C; IR (KBr) 2925, 1687, 1609, 1365, 1273, 842 cm-1; 1H NMR
(500 MHz, CD2Cl2) δ 7.38 (broad, 2H, aromatic Hs), 6.50
(broad, 2H, aromatic Hs), 2.52 (s, 3H, Ph-COCH3), 1.43-1.68
(m, broad, 3H, backbone sCH2-CH-) (Figure S-5, Supporting
Information).
PS-Qu. A solution of diphenyl phosphate (10 g, 39.97 mmol)
in m-cresol (6.5 g) was dropped into mixtures of TP-Pm (1.3
g, 6.59 mmol) and poly(4-acetylstyrene) (0.80 g, 5.48 mmol
repeat unit). The reaction mixture was purged with argon for
20 min before being gradually heated to 115 °C. Reaction was
conducted at 115 °C for 1 h and 140 °C for 3 h. The product
after cooling to room temperature was precipitated from a 10%
triethylamine/ethanol (150 mL) solution. The collected polymer
was dissolved in chloroform and precipitated from a 10%
triethylamine/methanol (330 mL) solution. This procedure was
repeated for three times before Soxhlet extraction by 10%
triethylamine/methanol solution for 24 h. The resulting product
Results and Discussion
TP and model compound of TP-Qu and vinyl polymer of
PS-Qu with the pendant TP-Qu group were synthesized ac-
cording to the preparative steps illustrated in Scheme 1. In order
to prepare the key intermediate of TP-Pm, the starting TP
molecule had to be converted to the mononitrated TP-NO2 and
to the heterocyclic TP-Bz compounds. The key intermediate of
TP-Pm can react with 2-aminobenzophenone and poly(4-
acetylstyrene) (PAcS) by Friedla¨nder condensation41 to yield

Tetraphenylthiophene-Derived Organic Molecules
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10305
Figure 1. (A) Solution (10 µM) PL emission spectra (excitation wavelength: 320 nm) and (B) the relative quantum yields of TP in THF/water
mixtures with varied compositions.
the model compound of TP-Qu and the vinyl polymer PS-Qu,
respectively. Substitution level of PS-Qu depends on the first-
step acetylation41 of the starting polystyrene and the following
Friedla¨nder condensation between PAcS and TP-Pm. The extent
of acetylation in PAcS is 95% as calculated from the corre-
sponding 1H NMR spectrum (Figure S-5, Supporting Informa-
tion). Conversion of the subsequent Friedla¨nder condensation
can be calculated from the proton ratio between the main-chain
the aggregate formations, or in other words, TP compound is
AIE-active. Further support for the AIE effect is from the
estimated fluorescence quantum yield (Φf) of TP in THF/H2O
solvent mixtures (Figure 1B). The resulting Φf value of TP in
the pure THF solvent is as low as 0.0023. Values of Φf remain
unchanged for solvent mixtures with H2O < 50 vol %, but with
further increase of H2O content in the solvent mixtures, the
corresponding Φf increases rapidly. Solution of TP in 10:90
THF:H2O has the highest Φf value of 0.41, which is about 178
times higher than that of TP in pure THF.
1
aliphatic (-CH2-CH-) and the aromatic protons in the H
NMR spectrum of PS-Qu (Figure S-5, Supporting Information).
The result indicates that a high value of 91.7% of the side chain
in the modified PS-Qu has been substituted by the TP-Qu
fluorophores. Calculation also suggests that there are 3.3% of
acetylated styrene pendant groups remaining in the modified
In contrast to the weak emission of TP in pure THF, the TP-
Qu compound emits strongly even in the dilute solution state
(Figure 2A). All spectra in Figure 2A possess a near emission
intensity disregarding the difference in the applied composition
of the water/THF mixtures. Dilute solution (10 µM) of TP-Qu
compound in THF emitted intensely and, with the addition of
water, the resulting TP-Qu aggregates emitted with approxi-
mately the same emission intensity. The summarized Φf values
from different solvent mixtures were summarized in Figure 2B
and are relatively constant (in the ranges between 0.34 and 0.36),
which is consistent with the constant emission intensity observed
in Figure 2A. The observed equivalent aggregate and solution
emissions (EASEs) are unusual and may be attributed to the
operative RIR mechanism originating from the inherent chemical
structure of the TP-Qu compound. We will discuss this later.
To gain insight into the cause of the unique PL properties,
we checked the absorption spectra of TP and TP-Qu in the THF/
water mixtures. The solution (10-5 M) of TP in pure THF
(Figure S-8, Supporting Information) contains one minor
absorption band at 267 nm besides the major absorption band
at 313 nm. With increasing water content in the solution mixture,
the minor absorption band became ambiguous in shape due to
the overlap with the major absorption band. Nevertheless, we
still can recognize the trend that both absorption bands showed
a bathchromic shift with increasing water content in the solution
mixtures. Similarly, the two absorption bands observed for
solutions of TP-Qu (Figure S-9, Supporting Information) also
red-shifted with increasing water content in the solutions. For
both samples solutions, the absorptions at the short- and the
long-wavelength regions are supposed to originate from the
isolated (monomeric) and the aggregated molecules, respec-
tively. The red shift of the short-wavelength absorptions
indicates that effective conjugation length of the corresponding
fluorophore is extended from the isolated free-rotated molecule
PS-Qu, which resulted in the tiny carbonyl (CdO; 1688 cm-1
)
(Figure S-6, Supporting Information) stretching in PS-Qu. Other
resolved resonances such as the new aromatic proton resonances
at 7-8 ppm in the spectrum of PS-Qu also correlate with the
structure assignment for the heterocyclic quinoline ring.41-44
With the chemical structure similar to the silole compounds,
we anticipated the same AIE properties of TP and the derived
TP-Qu compounds. The same method by use of the solvent
mixtures (good solvent and nonsolvent pair) was then applied
to examine the AIE effect. For TP and TP-Qu compounds, THF
is the good solvent and water is used as nonsolvent. Solution
emissions of TP compound in water/THF solvent mixtures of
different compositions were then evaluated as in Figure 1A.
Upon excitation at 320 nm, dilute solution (10 µM) of TP in
pure THF is very weak in emission. In contrast, PL spectra of
TP solutions (10 µM) in THF/water mixtures exhibited enhanced
emissions with the corresponding intensity progressively in-
creased by the increasing content of water in the solvent
mixtures. All solutions emit a bright blue light with nearly a
constant peak maximum (λem) of 404 nm. Solution mixtures
with high water content (g60%) became opaque, which is
attributed to the aggregate formation for the hydrophobic TP
molecules in the water/THF mixtures. The opaque suspension
solutions are macroscopically homogeneous with no visible
precipitates even after prolonged standing. The solution of TP
in 10:90 THF/H2O was analyzed by dynamic light scattering
(DLS) and the result (Figure S-7, Supporting Information)
revealed an average hydrodynamic diameter of 147.7 ( 2.4 nm.
Suggestively, the TP aggregates formed in the solvent mixtures
are nanosize in scale. The emission of TP is thus enhanced by

10306 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Lai and Hong
Figure 2. (A) Solution (10 µM) PL emission spectra (excitation wavelength: 350 nm) and (B) the relative quantum yields of TP-Qu in THF/water
mixtures with varied compositions.
Figure 3. Molecular packing arrangement of TP-Qu obtained from
single-crystal X-ray diffraction.
Figure 4. Solution PL excitation (PLE) spectra of TP (10 µM) in THF/
water mixtures with varied compositions. The PLE spectra were
monitored at 404 nm.
to more planar one in nanoparticles, the same result also reported
for the cyano-functionalized organic nanoparticle.21 Red shift
of the aggregation band with the addition of nonsolvent had
been also found for other AIE-active materials such as silole,
butadiene, and dibenzofulvene series.9,10,19,26,27 For the 80 and
90% water solutions, the observed long-wavelength (>400 nm)
tails can be attributed to the Mie scattering from the corre-
sponding nanoaggregates, a common phenomenon reported in
other AIE-active materials.11,15,18,19,21,26,45-50
A single crystal of TP-Qu molecule (the growth of TP single
crystal was unsuccessful) was then prepared and its X-ray
diffraction was studied. The molecular arrangement of the unit
cell deduced from the corresponding X-ray diffraction pattern
is shown in Figure 3. As the pair molecules in the central line
was focused, we noticed that the nitrogen atoms of the quinoline
ring interact with the hydrogen atoms of the phenyl ring in the
other molecule and form two C-H· · ·N hydrogen bonds with
a bonding distance of 2.984 Å. The hydrogen bond interlocks
the free rotation of the large C-2 quinoline substituents of the
TP-Qu molecules and should reduce the nonradiative decay
pathways through interreaction among the neighboring mol-
ecules when in close contacts.29,51 The three phenyl (C-3, C-4,
and C-5) substituents other than the C-2 quinoline ring adopt
noncoplanar geometry relative to the central thiophene ring and
with this arrangement, intermolecular face-to-face molecular
stacking will be largely inhibited. In general, the resolved edge-
to-edge arrangement of the central paired molecules may refer
to the formation of J-aggregate, which was suggested to have
enhanced emission compared to the face-to-face (or sandwich-
like) H-aggregate.52-55 In general, the J-aggregation absorption
is distinctly red-shifted and appears as an intense narrow
absorption band due to the motional narrowing, but that of TP-
Qu nanoparticles (Figure S-9, Supporting Information) is
relatively broad. This result suggests that the molecules in
nanoparticles may be oriented in a less optimal way of
J-aggregate due to the possible lattice disorder.21
PL excitation (PLE) spectra of TP and TP-Qu were further
conducted in order to locate the possible energy resources
responsible for the PL emission. The solution PLE spectra were
conducted by monitoring the variations of the respective PL
emission maxima (404 nm for TP and 464 nm for TP-Qu). For
dilute solution (10-5 M) of TP (Figure 4) in pure THF, the feeble
PLE signal is correlated with the corresponding weak PL
emission observed in Figure 1A. With increasing water content
in the solution mixtures, two bands in the short- and long-
wavelength regions started to emerge and increased the intensi-
ties, which correlates with the increasing PL emissions (excited
at 320 nm) in Figure 1A. In contrast to the TP case, two PLE
bands were clearly viewed in solution of TP-Qu in pure THF
(Figure 5). Here, the long-wavelength absorption is the main
energy source responsible for the strong PL emission (excited

Tetraphenylthiophene-Derived Organic Molecules
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10307
rings and the central thiophene ring, indicated in the lower panels
of Figure 6) in relation to the neighboring aryl substituents to
avoid parallel arrangements among them. For TP-Qu, the results
from chemical computation are quite close to those obtained from
the single-crystal analysis (the torsion angles in Figure 3 are θ2 )
4.9°, θ3 ) 62.2°, θ4 ) 64.1°, and θ5 ) 3.7°, respectively). Here,
the 4-phenyl ring of TP-Qu (marked by the red arrow in the lower-
right panel of Figure 6) is perpendicular to the attached C-2
quinoline ring and points on the opposite side of the C-3 phenyl
ring in order to minimize the mutual steric repulsions between them.
Similar to the results from the single-crystal X-ray diffraction, all
these noncoplanar conformations of TP and TP-Qu compounds
will suppress the face-to-face molecular packing fashion when in
the aggregate forms, an important prerequisite for the generation
of AIE effect.9-16,18-25,28,29,31,35
By rotating the C-2 aryl substituents from the minimum
energy positions (from a θ2 of 1.5° for TP and 3.5° for TP-Qu)
to any other angle, the corresponding energy difference (as ER)
as a function of the rotation angle can be formulated and the
results are summarized in Figure 7. For TP with small phenyl
ring on C-2, we observed little energy variations among all
conformers and the maximum energy differences resolved here
are in the ranges of 16 and 19 kcal/mol. On the contrary, energy
difference abruptly arose once the rotation angle exceeded
∼150° and the maximum energy barrier located at a rotation
angle of 170° is ∼160 kcal/mol for TP-Qu. Supposedly, this
result is directly related to the large steric repulsion forces
between the bulky heterocyclic quinoline ring on C-2 and the
phenyl ring on C-3. The concept can be best illustrated by the
diagrams shown in the upper panels of Figure 6. For TP
molecule (upper left), the four phenyl rotors connected by the
central thiophene stator are small and equal in size while for
the TP-Qu molecule the large C-2 quinoline rotor affects the
free rotation of the phenyl ring on C-3, which inevitably imposes
steric restriction on the C-4 and so the C-5 phenyl rings.
Rotations of the small phenyl rotors of TP molecule are
considered to be easier than the rotors of TP-Qu, in which the
large, immobile C-2 quinoline rotor imposes steric restrictions
on other phenyl rotors. The built-in large C-2 rotor in TP-Qu
makes its RIR an inherent property, which is operative and
effective in either the dilute solution or the aggregated states.
The operative RIR of TP-Qu leads to the pronounced blue color
of the dilute TP-Qu solution when irradiated by the 365 nm
UV light (upper right in Figure 6). On the contrary, no
luminescence was observed for the dilute TP solution (upper
left in Figure 6) under identical irradiation condition. The
inherent RIR process of TP-Qu compound is considered to be
so effective that the nonradiative decay pathways through
molecular collisions with the environmental molecules became
less probable, which contributes to the strong emission of TP-
Qu compound in dilute solution and aggregate states (Figure
2A).
Figure 5. Solution PL excitation (PLE) spectra of TP-Qu (10 µM) in
THF/water mixtures with varied compositions. The PLE spectra were
monitored at 464 nm.
at 350 nm) shown in Figure 2A. With increasing water content
in the solution mixtures, the long-wavelength absorptions nearly
maintain constant strength, which corresponds to the constant
PL emission observed in Figure 2A. The two PLE bands from
both the TP and TP-Qu solutions locate at the wavelength
positions correlated with those observed in the corresponding
UV-vis absorptions spectra (Figures S-8 and S-9, Supporting
Information). In addition, the short- and long-wavelength bands
red-shifted with increasing water content in the solution
mixtures, which is also consistent with observed red shift of
the absorption maxima in the UV-vis spectra. Therefore, we
suggested that the short- and the long-wavelength PLE bands
are due to the respective energy sources from the monomeric
and the aggregated absorptions.
It is unusual that the PLE monomer absorption band (Figure
5) increases with the increasing water content in the TP-Qu
solutions, which is distinctly different from the coherent
increases of the two PLE bands of the TP solutions (Figure 4).
The distinct responses between two systems may be due to the
structural dissimilarity. J-aggregate of TP-Qu (Figure 3) is a
bulky ensemble surrounded by eight nonparallel phenyl rings,
which may retard its development in the small entity of
aggregated nanoparticles. Through the favorable hydrogen-bond
interactions, J-aggregates are supposed to be prevalent species
in dilute THF solution of TP-Qu (judged by the corresponding
PLE spectrum in Figure 5). With the bulky ensemble and the
unparallel phenyl rings, J-aggregates initially formed will impose
geometrical constraint for the rest of TP-Qu molecule to pack
in a regularly paired manners during the aggregation process.
The situation is supposed to be different for TP without any
polar quinoline ring as TP-Qu does.
The difference between TP and TP-Qu compounds was also
evaluated from the rotation barrier (ER) by the formula ER ) E
- E0 from the Molecular Studio (MS) software. Accordingly,
rotation barrier is considered as the average energy required
for the C2-ring substituent to rotate from the equilibrium position
of the minimum-energy conformer to other conformers with
specific rotation angles deviated from the starting equilibrium
ones. The simulated conformations of TP and TP-Qu with
minimum energy are depicted in Figure 6, in which the four
aryl substituents connected to the central thiophene core are
twisted with certain angles (TP θ2 ) 1.5°, θ3 ) 54.3°, θ4 )
60.3°, and θ5 ) 1.7°; TP-Qu θ2 ) 3.5°, θ3 ) 65.9°, θ4 ) 51.4°,
and θ5 ) 1.1°; θ is the torsion angle between the individual
With the pendant TP-Qu group, the vinyl polymer PS-Qu
was supposed to have similar RIR effect observed in the model
compound of TP-Qu. As expected, the PL emissions (Figure
8) of the PS-Qu polymer in THF/water mixtures resemble those
of the model TP-Qu (Figure 2A) in regard to emission
wavelength and intensity. Irrespective of the composition
difference in the applied solvent mixtures, the resolved PL
emission intensities remain approximately the same for all
emission spectra shown in Figure 8. In either pure THF or in
mixed water/THF, the vinyl polymer of PS-Qu all exhibited
intense blue emissions (as B and D insets in Figure 8). Similar
to the model TP-Qu, the operative RIR of PS-Qu polymer is

10308 J. Phys. Chem. B, Vol. 114, No. 32, 2010
Lai and Hong
Figure 6. TP (upper left) and TP-Qu (upper right) solutions (10 µm in THF) under illumination with a 365 nm UV lamp, and the simulated
conformations of TP (lower left) and TP-Qu ((lower right) molecules with minimum energy.
Figure 8. Solution PL emission spectra of the PS-Qu (10 µM) in THF/
Figure 7. Rotational energy barrier as the function of the rotational
water mixtures with varied compositions. Excitation wavelength ) 320
angle of the C-2 rotors in the TP and TP-Qu molecules.
nm. Insets: (A) PS-Qu in THF/H2O ) 100/0; (B) solution A under
irradiation by a 365 nm UV lamp; (C) PS-Qu in THF/H2O ) 10/90;
considered to be pronounced and effective. Relative effective-
ness of the operative RIRs in TP, TP-Qu, and PS-Qu may be
compared if feasibility of rotation in each sample can be
evaluated by their PL emission responses toward cooling.
The emission intensities of TP, TP-Qu, and polymer PS-Qu
at low temperatures were then measured to qualitatively evaluate
the relative extent of RIR process in the three systems. In
general, the emission intensity becomes stronger when the
sample temperature is decreased because of the reductions in
the molecular collisions and in the nonradiative pathways. For
the reported cases of silole compounds,11,13,15,18,22,24,29,32 tem-
perature lowering resulted in the reductions in the revolving
motions of the peripheral phenyl rings and led to the high
fluorescence intensity. At temperatures below the melting point
of the media solvent applied, the revolving motions are
suggested to be greatly reduced. Herein, we chose DMF as
solvent to study the effect of temperature and the RIR effect
on the fluorescence intensity. The results summarized in Figure
9 clearly show that all materials have enhanced emissions with
decreasing temperature until the point (∼160 K; however, the
and (D) solution C under irradiation by a 365 nm UV lamp).
experimental freezing point of the mixture solution is supposed
to be lower than mp ()212 K) of the pure DMF) that serious
scattering originating from the frozen DMF media plays the
decisive role.56 For TP molecule with the small phenyl rings as
peripheral rotors, we observed the drastic enhancements (over
8 × 104 times) in the integrated emission intensity when cooled
from room temperature to 160 K. In contrast, much smaller
changes (1.39 times) were resolved for TP-Qu and PS-Qu
systems when the same experimental condition was applied.
The size of the C-2 rotor connected to the central thiophene
stator leads to the behavior difference between TP-Qu (or PS-
Qu) and TP. Rotation of the large C-2 quninoline rotors on the
model TP-Qu (or on the pendant TP-Qu of the vinyl polymer
PS-Qu) are restricted to a great extent due to the large rotation
barriers suggested above. In contrast, free rotation of the small
C-2 phenyl rotor in dilute solution can be greatly reduced by
lowering the temperature, which resulted in the promoted RIR
in the frozen DMF media and led to the enhanced emissions at

Tetraphenylthiophene-Derived Organic Molecules
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10309
rings are twisted at certain angles with respect to the neighboring
quinoline rings due to the large steric bulkiness among them.
The bulky quinoline rings occupy most of the space in between
the 1,3-pendant groups, which make the rotations of the pendant
TP-Qu rings a rather difficult task, and therefore, the RIR of
the pendant groups in PS-Qu will be just like the small-molecule
model of TP-Qu and functions effectively in the dilute and
aggregate states. Suggestively, the hindered rotations among the
involved 1,3-pendant pairs are so efficient that the nonradiative
decay pathways due to collisions with the surrounding molecules
can be largely prohibited and this leads to the observed
equivalent aggregated and solution emissions for PS-Qu. The
PL emission spectra of the model TP-Qu and the PS-Qu polymer
resemble each other, which suggests inherently effective RIRs
in both systems.
Conclusions
Figure 9. Summary of relative fluorescence intensities from the dilute
solutions (10 µM) of TP, TP-Qu, and PS-Qu in DMF at different
temperatures.
Small molecules of TP and model compound of TP-Qu and
vinyl polymer of PS-Qu were successfully prepared, and their
PL emission responses toward AIE phenomena were character-
ized. TP compound showed the typical AIE effect that its
aggregate emission is stronger than the emission in the dilute
solution state. In contrast, TP-Qu and PS-Qu have nearly
equivalent emissions in the aggregate and in the solution states.
Irrespective of the composition difference in the applied solvent
mixtures, the PL emission spectra of the model TP-Qu and the
PS-Qu polymer resemble each other, which suggests the
operative RIRs in both systems are so effective that the resolved
PL emissions are little affected by the surrounding media and
the nonradiative decays involved can be largely reduced due to
the limited interactions among the fluorophores and the media
molecules. The computer simulation suggests a high rotation
barrier (ER) for the bulky C-2 quinoline rotor in TP-Qu
molecule, which contributes to the hindered rotations and the
pronounced RIR effect. The pendant TP-Qu groups in the vinyl
polymer of PS-Qu also exert similar RIR effect and lead to the
strong emissions in the dilute and the aggregated solution states.
The PL emissions at reduced temperatures suggest that there
are little PL emission gains (about 1.39 times) for TP-Qu and
PS-Qu but, for TP molecule, large emission enhancement (about
8 × 104 times) resulted when cooled from room temperature to
-190 °C. The inherent restrictions on the molecular rotations
of TP-Qu and PS-Qu are considered to be much larger than for
the TP molecule.
Figure 10. Simulated conformation of chain segments of PS-Qu with
10 monomer units (upper A: magnified portion of the selected green
area in the lower panel B).
low temperature. The effective RIR process inherent in TP-Qu
and PS-Qu results in less temperature-enhanced emission when
cooled from room temperature to low temperatures before
substantial scatterings occurred.
Acknowledgment. We appreciate the financial support from
the National Science Council, Taiwan, Republic of China, under
Contract No. NSC 98-2221-E-110-005-MY2.
As suggested above, emissions of the vinyl polymer PS-Qu
were affected little by the applied solution media. To gain insight
into this effective AIE phenomenon observed for the vinyl
polymer, chain conformation of PS-Qu was simulated. The
Smart Minimizer incorporated in the Discover MS Program32
was used to broadcast the minimum-energy conformer of PS-
Qu. The description of the chain structures underwent confor-
mation optimization, and energy minimization of a 10-monomer
unit can be viewed from the resulting chain arrangement shown
in Figure 10. The enlarged diagram in the upper panel of Figure
10 suggests that all the phenylene rings in the adjacent monomer
units along the main chain are facing each other in a parallel
fashion. The parallel arrangement among the 1,3-phenylene rings
is correlated with the previous study for the amorphous
polystyrene.57,58 Despite the parallel arrangement for the poly-
styrene framework, the resulting conformation of the PS-Qu
main chain is determined by the connecting bulky quinoline
ring in the rest of the pendant group. With the parallel stacking
between the 1,3-phenylene linkages, the connecting quinoline
Supporting Information Available: All supporting data
including the sample characterizations (1H NMR, FTIR spectra),
the results from the dynamic light scattering, and the UV-vis
absorption spectra from the solution mixtures. This material is
available free of charge via the Internet at http://pubs.acs.org.
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