Enhanced emission of a pyridine-based luminogen by hydrogen-bonding to organic and polymeric phenols

Article abstract of DOI:10.1039/c3ra22217a

Fluorescent bis(pyridinylvinyl)anthracene (An2Py) with two pyridine terminals was synthesized and used to prepare miscible blends with hydroxyl-containing components (organic bisphenol A (BPA) and polymeric poly(vinyl phenol) (PVPh)) through the facile intermolecular hydrogen-bond (H-bond) interactions between the pyridine and the hydroxyl functions. Before blending, the solution of An2Py already emits appreciably due to its aggregation-induced emission enhancement (AIEE) behavior; after blending with the hydroxyl components, the fluorescence can be further intensified due to the restricted molecular rotation, which leads to the blockage of non-radiative decay channels, imposed by the H-bond interactions. The role of the H-bonding on the restricted molecular rotation of the An2Py/BPA (and the An2Py/PVPh) blends was characterized by solution ¹H NMR and solid infrared spectroscopy. The effectiveness of the organic BPA and the polymeric PVP as molecular anchors to lock the free rotation of the An2Py luminogen were compared and discussed in this study.

Full text of DOI:10.1039/c3ra22217a

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Enhanced emission of a pyridine-based luminogen by
hydrogen-bonding to organic and polymeric phenols3
Cite this: RSC Advances, 2013, 3,
6
930
Wei-Lun Chien, Chih-Min Yang, Tai-Lin Chen, Shu-Ting Li and Jin-Long Hong*
Fluorescent bis(pyridinylvinyl)anthracene (An2Py) with two pyridine terminals was synthesized and used to
prepare miscible blends with hydroxyl-containing components (organic bisphenol A (BPA) and polymeric
poly(vinyl phenol) (PVPh)) through the facile intermolecular hydrogen-bond (H-bond) interactions
between the pyridine and the hydroxyl functions. Before blending, the solution of An2Py already emits
appreciably due to its aggregation-induced emission enhancement (AIEE) behavior; after blending with
the hydroxyl components, the fluorescence can be further intensified due to the restricted molecular
rotation, which leads to the blockage of non-radiative decay channels, imposed by the H-bond
interactions. The role of the H-bonding on the restricted molecular rotation of the An2Py/BPA (and the
Received 19th September 2012,
Accepted 25th February 2013
1
An2Py/PVPh) blends was characterized by solution H NMR and solid infrared spectroscopy. The
DOI: 10.1039/c3ra22217a
effectiveness of the organic BPA and the polymeric PVP as molecular anchors to lock the free rotation of
the An2Py luminogen were compared and discussed in this study.
www.rsc.org/advances
3
4
butylthio)phenyl)-fluorenone (DSFO) was reported to have
enhanced excimer emissions due to intermolecular H-bonds.
The dimer structure of DSFO was locked by intermolecular
H-bonds and the corresponding excimer emission therefore
underwent fewer non-radiative decay pathways. The salicyli-
Introduction
1
,2
In 2001, Tang’s group found that the non-coplanar 1-methyl-
,2,3,4,5-pentaphenylsilole (MPS) luminogen has interesting
1
fluorescence behavior where it emits strongly when aggregated
in solution and in the solid film state, despite its non-emissive
character in the dilute solution state. In contrast to the
aggregation-caused quenching (ACQ) observed in planar
organic luminogens, this enhanced emission in the aggre-
gated state had been designated as aggregation-induced
emission (AIE). With beneficial emission efficiency in the film
state, lots of organic and polymeric luminogens with AIE or
35
deneaniline (SA) compound forms a gel in certain organic
solvents. The gel solution emits strongly with a fluorescent
quantum yield (W ) 600 times higher than that of the
homogeneous solution. The AIEE property is ascribed to the
formation of J-aggregates and the inhibition of molecular
rotation by H-bond interactions. Other compounds containing
hydrazine , benzoxazole , acrylamido and naphthalide
functions were also found to exhibit enhanced emission in the
gels, promoted by H-bond interactions. A polymeric system of
F
36
37
38
39
3
–31
AIE enhancement (AIEE) properties
have been prepared
and studied in order to improve fluorescent emission
efficiency and to understand the operative mechanism leading
to the observed enhanced emissions. Among several possibi-
40
poly(fluorene-alt-naphthol) (PFN, Scheme 1A) prepared in
our laboratory was also found to exhibit AIEE properties due to
the restricted molecular rotation promoted by inter and
intrachain H-bonds among the inherent hydroxyl groups.
The restricted molecular rotation of PFN can be further
enhanced by blending it with different amounts of poly(N-vinyl
3
2,33
lities, restricted intramolecular rotation
had been pre-
viously identified to be the main mechanism responsible for
the observed AIE or AIEE behavior of silole compounds. In the
aggregated states of silole compounds, the restricted mole-
cular rotation of the phenyl groups reduces the possibilities of
non-radiative decay pathways and results in the enhanced
emission.
4
1
pyrrolidone)
H-bond interactions between PFN and PVR, a PFN/PVR blend
with small amounts (2.3 wt%) of PFN can emit with a high W
(PVR). Through the facile intermolecular
F
Physical forces, such as hydrogen-bond (H-bond) interac-
tions, can be used to restrict the molecular rotation of organic
value of 93%. Use of H-bond interactions to lock the molecular
rotations of organic and polymeric luminogens was therefore
illustrated by the above examples. With effective restriction on
molecular rotation, AIEE-active luminogens possess a bene-
ficial strong emission in aggregated solution and in the solid
state, which facilitates their solid-state applications (e.g. as a
light-emitting layer in an organic light-emitting diode (OLED)).
Under the premise that the emission performance can be
luminogens.
A
fluorenone-derivative of 2,7-bis(4-(tert-
Department of Materials and Optoelectrical Science, National Sun Yat-Sen
University, Kaohsiung, 80424, Taiwan. E-mail: jlhong@mail.nsysu.edu.tw;
Tel: +886 07 5252000 ext. 4065
3
1
Electronic supplementary information (ESI) available: Fig. S1–S5. See DOI:
0.1039/c3ra22217a
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Experimental section
Materials
Reagent grade anthracene, chloroform (CHCl
o-tolylphosphine, 4-vinylpyridine, poly(vinyl phenol) (M
2 000) and dichloromethane (DCM) were purchased from
Aldrich Chemical Co. and used directly without purification.
N,N-Dimethylformamide(DMF) and Pd(OAc) was purchased
3 2
), Br , tri-
W
y
2
2
from Echo chemical Co. and DMF was refluxed over CaH2
under nitrogen for 5 h before distillation for use.
Synthesis of 9,10-dibromoanthracene (An2Br)
Synthesis of An2Br was performed according to the reported
4
2
procedures but with minor modification on purification
step: To a solution of 5.0 g (28.05 mmol) anthracene in CHCl3
(
100 mL), two eq. Br
was dropped slowly in the dark. After stirring for an additional
h the whole reaction mixture was poured into methanol (500
mL) to removed excess Br and the crude reaction product was
recrystallized from CH Cl to yield the product as yellow
2 3
(2.9 mL, 56.1 mmol) in 50 mL of CHCl
4
2
2
2
1
needles; Yield: 9.05 g (96%); H-NMR (500 MHz, chloroform-
Scheme 1 (A) Structure of PFN and (B) synthesis of An2Py and its mixing with
BPA and PVPh to form An2Py/BPA(x/y) and An2Py/PVPh(x/y) blends, respec-
tively.
d): d 8.62–8.56 (m, 4H, Ar–H), 7.66–7.61 (m, 4H, Ar–H) (Fig. S3,
ESI
3
14 8 2
); MS m/z: found, 336.2. Anal. calcd for C H Br : C, 49.69;
H, 2.26. Found: C, 49.75; H, 2.20.
Synthesis of bis(pyridinylvinyl)anthracene (An2Py)
A reaction mixture of 4-vinylpyridine (3.3 ml, 29.77 mmol),
An2Br (5 g, 14.89 mmol), triethylamine (4 mL), Pd(OAc) (0.33
g, 1.47 mmol) and tri-o-tolylphosphine (0.22 g, 0.72 mmol) in
dry DMF (100 mL) was placed in a predried round-bottomed
flask (250 mL). The reaction mixture was then degassed by
freeze–pump–thaw 3 times before being heated at 110 uC for
maintained or enhanced, blending the valuable AIEE-active
luminogens with other inexpensive, commercialized materials
provides alternative routes to cut down the costs in practical
applications. Because the H-bond interaction can effectively
hinder the molecular rotation of non-coplanar AIEE-active
fluorophores, blending AIEE-active luminogens with other
H-bond interaction pairs was therefore attempted in this
study. Here, the AIEE-active luminogen of bis(pyridinylvinyl)
anthracene (An2Py in Scheme 1B) was used as a H-bond
acceptor to react with H-bond donors of small-mass bisphenol
A (BPA) and polymeric poly(vinyl phenol) (PVPh). Distinct from
2
2
4 h. After cooling to room temperature, the reaction mixtures
were poured into water and the resultant suspensions were
extracted with CH Cl (3 6 40 mL). The combined organic
extracts were washed with brine, dried over MgSO and
2
2
4
concentrated by rotary evaporator. The crude product was
then purified by flash column chromatography (v/v hexane/
ethyl acetate = 4/1) to give 1.51 g yellow An2Py as the final
4
1
the above-mentioned PFN/PVR system, the use of small-mass
An2Py here was based on its higher molecular mobility to
produce more H-bond pairs compared to the polymeric PFN.
The preferential H-bond interaction of the pyridine ring with
the hydroxyl group of BPA (and PVPh) should operate to inhibit
molecular rotation and further enhance fluorescence of the
AIEE-active An2Py luminogen. With respect to molecular
mobility and chain viscosity, the small-mass BPA and the
polymeric PVPh are supposed to have different extents of
restriction forces on the molecular rotation of the An2Py and
therefore were used to form two blend systems for comparison.
The correlations of H-bond interaction to restricted molecular
rotation and to the emission efficiency are characterized
1
product. Yield: 1.51 g (26%); H NMR (500 MHz, THF-d ): d
8
6.57–6.66 (d, 2H, –CLCH, Jtrans = 16.5Hz), 7.12–7.49 (m, 8H, Ar–
H), 7.92–8.02 (d, 2H, –CLCH, Jtrans = 16.5Hz), 8.05–8.11 (m, 4H,
Ar–H), 8.31–8.45 (d, 4H, Ar–H) (Fig. S4, ESI ); MS m/z: found,
3
384.3; Anal. calcd for C H N : C, 87.47; H, 5.24; N, 7.29.
2
8
20 2
Found: C, 87.57; H, 5.19; N, 7.24.
Instrumentation
1
H NMR spectra were recorded with a Varian Unity Inova-500
MHz FT-NMR instrument. Tetramethylsilane (TMS) was used
as the internal standard. The molar masses of the organic
molecules were determined by a Bruker Autoflex III MALDI/
TOF mass spectrometer. PL emission spectra were obtained
from a LabGuide X350 fluorescence spectrophotometer using
a 450 W Xe lamp as the continuous light source. UV-vis
absorption spectra were recorded with an Ocean Optics
DT1000 CE 376 spectrophotometer. A quartz cell with
1
through the application of H NMR, infrared and emission
spectroscopies and the results are summarized in this study.
3
dimensions of 0.2 6 1.0 6 4.5 cm was used for the UV-Vis
absorption and PL emission spectra measurements. Stock
solutions of the organic and polymeric fluorophores with a
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2
4
concentration of 10 M in THF were first prepared. Aliquots
of these stock solutions were transferred to 10 mL volumetric
flasks, into which appropriate volumes of THF and hexane
2
5
were added dropwise under vigorous stirring to furnish 10
M solutions with different hexane contents (0–90 vol%). UV-
Vis and PL emission spectroscopy were immediately per-
formed once the solutions were prepared. Solid samples were
prepared by drop-casting sample solutions over quartz plates.
Fluorescence quantum yields (W ) of the solution mixtures
F
with varied compositions were determined by comparison
with a quinine sulfate standard (W = 54% in 0.05 M H SO ).
F 2 4
An integrating sphere was used for the film sample. FT-IR
spectra were obtained on a Nicolet IR-200 spectrometer.
Sample solution was dropped on a KBr pellet and dried under
vacuum to prepare the solid film for FT-IR analysis. The wide-
angle X-ray (WAXR) diffraction pattern of the solid blend was
obtained from a Siemens D5000 diffractometer. Particle sizes
of the aggregates in solution were measured by dynamic light
scattering (DLS) using a Brookhaven 90 plus spectrometer
equipped with a temperature controller. An argon ion laser
operating at 658 nm was used as the light source.
Results and discussion
As illustrated in Scheme 1B, the An2Py luminogen can be
readily prepared from the corresponding Heck-coupling
reaction between 9,10-dibromoanthracene and 4-vinylpyri-
dine. The two pyridine terminal rings of An2Py can be
H-bonded to the hydroxyl groups of BPA and PVPh to form
An2Py/BPA(x/y) and An2Py/PVPh(x/y) (x/y: refer to the molar
ratio between the pyridine and the hydroxyl functions) blends,
respectively. Except for the terminal rings (pyridine vs.
benzene), the chemical structure of An2Py is similar to the
reported AIEE-active 9,10-bis((E)-2-phenyl)anthracene lumi-
nogen; therefore, An2Py is expected to have enhanced
emission in the aggregated state and is characterized before-
hand.
Fig. 1 (A) The PL emission spectra (excited at 400 nm) and (B) the hydrodynamic
26
diameters of An2Py (10 M) in THF/hexane solvent mixtures of different
volumetric (v/v) ratios.
aggregated particles due to the steric constraints imposed by
the congested environment; thereby, the non-radiative decay
channels can be more effectively blocked to result in intense
fluorescence in the mixture solutions. Molecular rotations are
more effectively hampered when the An2Py molecules are
located in a more crowded environment, such as in a smaller
particle formed in solutions containing more hexane.
Therefore, the An2Py luminogens in THF/hexane mixtures
emit with higher intensity than An2Py in pure THF solvent.
4
3
AIEE properties of An2Py
The AIEE property of An2Py was primarily evaluated by its PL
emission spectra in THF/hexane solvent/poor solvent mixtures
2
6
Blending of An2Py with small-mass bisphenol-A (BPA)
(Fig. 1A). Despite the high dilution (10
M) in pure THF
solvent, the An2Py luminogen already emits with appreciable
intensity and the solution fluorescence can be further
intensified with increasing hexane content (while keeping
the concentration of An2Py at 10 M) to generate aggregates
in the solution mixtures. Formation of aggregated particles in
the solution mixtures can be confirmed from the DLS analysis.
The results summarized in Fig. 1B suggest that aggregated
To emphasize the role of H-bond interaction on the molecular
rotation and on the emission efficiency of An2Py, different
amounts of BPA were used to mix with An2Py in the solution
state. Dilute solutions of An2Py/BPA(x/y) mixtures in THF were
prepared under the condition that the concentration of An2Py
2
6
2
5
was kept at a constant value of 10
M. As illustrated in
Fig. 2A, addition of non-emissive BPA to pure An2Py causes
conceivable emission intensification. The emission intensity
of An2Py/BPA(1/1) solution is more than 4-fold that of the pure
An2Py solution. The An2Py luminogens in the dilute solution
mixtures are supposed to be locked by the BPA molecules in
order to exhibit the emission enhancements. Most of the
An2Py luminogens in An2Py/BPA(1/1) solution are assumed to
be molecularly anchored by BPA since further increase of BPA
content to An2Py/BPA(3/7) results in little emission variation,
particles with an average hydrodynamic diameter (D
h
) between
185 to 125 nm are resolved in the solution mixtures. With
increasing hexane content in the solution mixtures, the
resultant aggregated particles reduced their sizes and An2Py
molecules in THF/hexane (v/v) = 1/9 solution form nanopar-
ticles with the smallest size (with Dh y 125 nm) among all
three solutions. Suggestively, free rotation of An2Py luminogen
is supposed to be restricted within the confined spaces of the
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1
25
25
Fig. 3 Solution H NMR spectra of (A) An2Py (=10 M), (B) BPA (=10 M) and
(
8
C) An2Py/BPA(1/1) mixture (THF-d ).
experiments on solutions of An2Py, BPA and the An2Py/BPA(1/
1
1) mixture in THF-d
8
. The H NMR band shape analysis had
been previously utilized to study the rotation-induced con-
4
4
45
formational changes on AIE-active organic and polymeric
systems. In general, the fast conformational exchanges caused
by the fast molecular rotations of the axes of single bonds
linked to the central planar ring give sharp resonance peaks,
whereas the slower exchanges due to restricted molecular
1
rotation broaden the resonance peaks. The H NMR spectra of
the An2Py/BPA(1/1) solution (Fig. 3C) were performed under
the condition that the concentrations of An2Py and BPA were
fixed at 10 M; therefore, if there are no mutual associations
2
Fig. 2 The emission spectra of (A) solution (with [An2Py] = 10 M) and (B) solid
An2Py/BPA(x/y) of different compositions (excited at 400 nm).
5
25
between An2Py and BPA, the resonance peaks of the An2Py/
BPA(1/1) solution should be the same as those in the pure
2
5
25
An2Py (=10 M) (Fig. 3A) and the pure BPA (=10 M) (Fig. 3B)
which indicates that H-bond interactions between An2Py and
BPA proceed efficiently even in dilute solution. To verify the
effectiveness of the H-bond interactions, a large excess of BPA
molecules were intentionally added in separate experiments
and the results suggest that overdose of BPA to a high value of
solutions. The solution of pure An2Py in THF-d exhibits the
8
sharp, multiple resonance peaks of the aromatic (H
a b e
, H , H
and H ) and the vinylenic (H and H ) protons in the range of d
f
c
d
6.0 to 8.7. The sharp multiplets in the pure An2Py solution,
however, became broader and indistinguishable in the
spectrum of An2Py/BPA(1/1) solution. The H-bond interaction
forces should give the same constraint on the BPA component;
therefore, the same peak broadening was observed on the
100-fold (i.e. An2Py/BPA = 1/100) gives no intensity variations
compared to the An2Py/BPA(3/7) solution. Under the premise
that restricted molecular rotation is the key leading to
enhanced emission, the amounts of BPA applied in the
An2Py/BPA(3/7) solution are already sufficient to impose
effective restriction to result in an emission behavior similar
i j
aromatic (H ) and isopropylic (H ) proton peaks of the BPA
molecules in the spectrum of An2Py/BPA(1/1) solution.
Suggestively, intermolecular H-bond interactions between
An2Py and BPA will exert the same locking mechanism on
both components and the molecular anchoring on the An2Py
luminogen caused the observed enhanced emission in the
3
to the An2Py/BPA(1/100) solution. DLS analysis (Fig. S1A, ESI )
indicates that aggregated particles also formed in the
corresponding An2Py/BPA(x/y) solution mixtures and the
aggregated particles progressively shrunk in size with increas-
ing BPA content in the solution. The congestion in the shrunk
nanoparticles imposes steric constraints on the molecular
rotations of the An2Py lumnogens; thus, An2Py molecules in
the smaller particles will have stronger emission than An2Py in
the larger particles due to the reduced non-radiative decay
pathways
1
mixture solutions. The H NMR spectra therefore identify the
existence of H-bond interactions between An2Py and BPA in
the solution mixtures.
Because the molecular rotation of the AIEE-active lumino-
gens should be more effectively restricted in the solid when
compared to their solution states, we therefore expected strong
fluorescence in the pure An2Py solid. Furthermore, the
H-bond interactions in the solid state should be more
pronounced than in the solution state due to more H-bond
To experimentally verify the restricted molecular rotation
1
promoted by H-bond interactions, we carried out H NMR
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Fig. 4 Wide-angle X-ray diffraction spectra of BPA, An2Py and An2Py/BPA(x/y)
solids of different compositions.
Fig. 5 Infrared spectra of solid An2Py/BPA(x/y) blends on (A) the hydroxyl OH
and (B) the pyridine absorptions.
interaction opportunities in the closely-packed condensed
solid state. To understand the solid-state fluorescence proper-
ties, preliminary characterization on the pure An2Py, BPA and
the blended An2Py/BPA(x/y) solids was therefore conducted.
Pure An2Py and BPA solids are both crystalline materials, as
verified from the X-ray diffraction patterns in Fig. 4.
Interestingly, all the An2Py/BPA solid blends are still crystal-
line materials with the resolved crystalline diffraction peaks
located at positions essentially different from those in the pure
An2Py and BPA components, which indicates that a new
crystalline structure different from the starting An2Py and BPA
forms after blending. Supposedly, this new crystalline struc-
ture should be correlated with the An2Py/BPA H-bonded
species. Identical to compounds with crystalline-induced
An2Py/BPA(9/1) in Fig. 5A). With increasing An2Py content
from An2Py/BPA(3/7) to (9/1) blends, we observed the
transformation of the O–H stretching bands from the
2
1
predominantly self-associated (3350 cm ) to the intermole-
2
1
cular (3150 cm ) H-bonded OH absorptions. With excess
An2Py component, An2Py/BPA(9/1) and (6/4) blends exhibit
basically the intermolecular H-bonded OH absorption at 3150
2
1
cm . The characteristic absorption band of the free pyridine
2
1
48
ring in pure An2Py appears at 993 cm (Fig. 5B). With the
presence of BPA component, a new band due to the
intermolecular H-bonded pyridine absorption emerged at
2
1
around 997 cm . It is clear that the free pyridine absorption
for pure An2Py will gradually reduce in intensity. In An2Py/
BPA(6/4) blend, certain pyridine rings of An2Py still remain
non-bonded but in An2Py/BPA(1/1) and (3/7), most of the
pyridine rings are already H-bonded to hydroxyl groups. The
high fractions of H-bonded pyridine rings should contribute to
the high fluorescence in An2Py/BPA(1/1) and (3/7) samples. As
suggested above, the intermolecular H-bond interactions
impose restriction on the An2Py luminogen and lead to
enhanced emission of the blending mixtures. The influence of
intermolecular H-bonds on the solid blends can be clearly
demonstrated in the solid emission spectra (Fig. 2B) that the
emission intensity increases with the increasing BPA content
from the pure solid An2Py to the An2Py/BPA(3/7) samples.
Emissions of solid samples are all higher in intensity than
their respective mother solutions (cf. Fig. 2A), a result
indicative of the effective restriction on the molecular
rotations of the crystalline solid samples. The emission
4
emission (CIE) behavior, the regularly-packed crystalline
arrays should be the additive factor contributing to effective
restriction on the molecular rotation of the luminescent An2Py
in the An2Py/BPA blends. Molecular rotations of the fluor-
escent An2Py are considered to be largely restricted in the
crystalline arrays and emission intensity of the solid blends is
therefore comparatively larger than their preparative solution
mixtures (compare Fig. 2A and 2B). The H-bond interactions in
the solid blends can be conveniently evaluated from the FTIR
2
1
analysis on the hydroxyl (3500–3100 cm ) and the pyridine
2
1
(
900–1000 cm ) absorption regions. The hydroxyl stretching
band of the starting BPA (Fig. 5A) is discussed first: the self-
associated H-bonded O–H groups contribute to the broad
2
1
asymmetric absorption at 3350 cm
and the free O–H
2
1 46,47
stretching absorption should locate at 3525 cm
.
In
general, a dynamic equilibrium between free O–H and self-
associated H-bonded O–H should exist in the pure BPA solid;
however, the scarcity of the free O–H stretching absorption at
efficiency can be also followed by the quantum yields (W
measured from an integrating sphere; as listed in Table 1, the
solid An2Py possesses a W value of 48% and with increasing
BPA content in the solid, we observe the increase of W until
F
)
2
1
3
525 cm (Fig. 5A) indicates that most O–H groups form self-
F
associated H-bonds in the crystalline BPA solid. Upon
blending with An2Py, fractions of self-associated H-bonds in
BPA are broken and replaced by the intermolecular H-bonds
between BPA and An2Py, which should result in the broad
F
the ultimate value of 69% was attained for An2Py/BPA(3/7)
blend. Similar to the solution samples, further increase of BPA
content gains no difference to the emission efficiency (e.g. the
2
1
F
An2Py/BPA(1/100) and (3/7) blends have the same W value).
stretching absorption at around 3150 cm (cf. the spectra of
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a
F
Table 1 Quantum yields (W ) of the solid An2Py/BPA and An2Py/PVh blends
b
Solid An2Py/BPA(x/y) blends
x/y
W (%)
F
1/0
48
9/1
50
6/4
57
1/1
66
3/7
69
b
Solid An2Py/PVPh(x/y) blends
x/y
2/0
48
20/1
51
4/1
54
2/1
57
1/1
66
2/3
71
W (%)
F
a
b
Quantum yield, obtained from integrating sphere. x/y refers to
the applied molar ratio between the pyridinyl ring and the hydroxyl
groups.
When the BPA content is more than 70 mol% (as in An2Py/
BPA(3/7)), most of the An2Py luminogens should already be
surrounded and locked by the BPA molecules through the
mutual intermolecular H-bonds, further increase of the BPA
(as in An2Py/BPA(1/100)) component gains no more increase of
the H-bonded An2Py pairs. For a blend of An2Py/BPA(1/100),
all the An2Py luminogens should be well isolated by and
bonded by large amounts of BPA molecules. With less
luminescent component, the solid An2Py/BPA(1/100) blend
has the same emission efficiency as An2Py/BPA(3/7); therefore,
effective molecular anchoring, not the content of the lumino-
gens, is the key leading to enhanced emission in the solid
blends.
An2Py to poly(vinyl phenol) (PVPh)
As similar with small-mass BPA, polymeric PVPh has the same
hydroxyl functions capable of binding to An2Py with preferable
H-bond interactions. Primarily, the viscous polymeric chain of
the PVPh component should have more effective restriction on
molecular mobility compared to small-mass BPA; however, the
multiple hydroxyl pendant groups along the polymer chain of
PVPh will generate lots of intermolecular physical junction
points when H-bonded to BPA molecules in the solution state
and the resultant crosslinked networks in the solution should
prevent the hydroxyl functions from complete H-bonding. To
avoid the premature precipitation of the crosslinked gels, all
solution mixtures of An2Py/PVPh were agitated during the
solution mixing step and the resultant solutions were sent for
spectral measurement immediately after preparation (gelled
masses were visible after two hours). Despite the incomplete
H-bond interactions involve in the preparation step, the
solution emission spectra in Fig. 6A still show the same trend
that increasing PVPh content results in the increase of the
emission intensity. Here, solution of An2Py/PVPh(2/3) has the
highest emission intensity among all solutions and, similar to
the An2Py/BPA case, further increase of PVPh content resulted
in little variation of the emission intensity. It is then envisaged
that H-bond interactions cannot be driven to completion in
the viscous polymeric system with multiple H-bond reaction
sites along the long-chain segments. The DLS analysis (Fig.
25
Fig. 6 The emission spectra of (A) solution An2Py/PVPh(x/y) ([An2Py] = 10 M)
and (b) solid An2Py/PVPh(x/y) of different compositions (excited at 400 nm).
the enhanced molecular restriction on shrunk particles also
contributes to the emission intensification. The restricted
molecular rotation imposed by the polymeric PVPh was also
1
verified from the solution H NMR spectra (Fig. S2, ESI
3
) of
An2Py, PVPh and An2Py/PVPh(1/1) samples. Under the
condition that the concentration of An2Py was kept at 10
25
M, the aromatic resonance peaks of the An2Py/PVPh(1/1)
solution is comparatively broader than those in pure An2Py
solution. The solid An2Py/PVPh blends from the mother
solution mixtures were also inspected by WAXS. Despite the
amorphous PVPh being applied in the blend, the resultant
An2Py/PVPh(20/1), (4/1) and (2/1) blends have a new crystalline
structure because the sharp crystalline peaks (Fig. 7) formed
are at positions different from those in An2Py. With the lowest
PVPh content, the An2Py/PVPh(20/1) blend has the highest
crystallinity among all samples. With increasing PVPh content
from An2Py/PVPh(20/1) to (2/1) blends, the crystallinity
obviously decreases and no crystalline diffraction peaks are
resolved in An2Py/PVPh(1/1) and (2/3) blends. The new crystal
formed in An2Py/PVPh(20/1), (4/1) and (2/1) should be due to
the H-bonded An2Py. Theoretically, in the An2Py/PVPh(20/1)
blend, the maximum number of hydroxyl groups loaded by
PVPh is only 1/20 of the pyridine rings in An2Py; therefore,
S1B, ESI
contain nano-sized (D
3
) indicates that the An2Py/PVPh(x/y) solutions also
= 220–140 nm) particles and the
h
corresponding particles shrunk in size with increasing PVPh
content in the solutions. Consistent with the small-mass case,
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Fig. 7 Wide-angle X-ray diffraction spectra of An2Py, PVPh and An2Py/PVPh(x/
y) blends of different compositions.
Fig. 8 Solid infrared spectra of An2Py/PVPh(x/y) blends on (A) the hydroxyl OH
and (B) the pyridine absorptions.
only a low fraction of An2Py molecules can be H-bonded to
PVPh and cannot account for the high crystallinity resolved for
An2Py/PVPh(20/1). It is possible that the small amounts of
H-bonded An2Py serve as the starting crystallization loci for
further deposition of non-H-bonded An2Py molecules to result
in new crystals with high crystallinity. By increasing the
amorphous PVPh in the blend, the regular packing of the
amounts of H-bonded An2Py is reduced due to the imposed
geometrical restraints from the random hydroxyl bonding sites
of the amorphous crosslinked networks. In An2Py/PVPh(1/1)
and (2/3) blends, the random hydroxyl bonding sites in the
crosslinked networks are supposed to bind to An2Py irregu-
larly to result in the incomplete H-bonding sites along the
crosslinked networks. With increasing randomness intro-
duced by the crosslinking network, the An2Py/PVPh(1/1) and
2
bonded pyridine absorption at 1005 cm . As indicated by the
deconvoluted results (dashed curves) shown in Fig. 8B, the free
1
2
1
pyridine absorptions at 993 cm decrease their contribution
with increasing PVPh content in the blends. Nevertheless, even
with the large amounts of PVPh, the An2Py/PVPh(2/3) blend
still contains a certain amount of residual pyridine rings
remaining non-bonded, which is consistent with the observed
2
1
free OH absorption at 3525 cm
.
Qualitative comparisons of the An2Py/BPA, An2Py/PVPh and
the previous PFN/PVR systems
The qualitative comparison on the small-mass BPA and the
polymeric PVPh as molecular anchors to restrict rotation of the
An2Py are illustrated here. For An2Py/PVPh system, the viscous
chain segments in the physically crosslinked networks are
primarily considered to have efficient restriction forces on the
molecular rotation. Nevertheless, the geometrical constraints
on the highly-crosslinked samples also prohibit complete
H-bond interactions and result in less H-boned pairs to
provide fluorescence with enhanced efficiency. On the other
hand, the mobile small-mass BPA generates no viscous,
physical-crosslinked product as PVPh does but the thorough
H-bond interactions in the highly-crystalline blended products
furnish effective restriction on molecular rotation and result in
fluorescence with intensity comparable to the polymeric
An2Py/PVPh system. The ultimate emission efficiencies (69
vs. 71%) attained for both An2Py/BPA and An2Py/PVPh are
therefore comparable. Our initiative for the present study was
aimed at the high molecular mobility of the small-mass An2Py,
which should furnish high contact possibility with other
H-bond donators to result in a high number of H-bonded pairs
in the preparative solution and therefore the solid blend
states. However, the ultimate emission yields (69 and 71%)
obtained in this study are comparatively lower than those
(2/3) blends are identified to be amorphous. The resultant
solid emission spectra (Fig. 6B) also exhibit the similar trend
of increasing emission intensity with increasing PVPh content
in the blends, which can be also confirmed by the resultant W
F
values summarized in Table 1. A highest W value of 71% was
F
identified for the solid An2Py/PVPh(2/3) blend and no more
emission enhancement can be attained with more PVPh
component loaded in the blend. The H-bond interactions are
also confirmed from the corresponding infrared spectra on the
hydroxyl (Fig. 8A) and the pyridine (Fig. 8B) absorption
regions. Similar to An2Py/BPA system, the same transforma-
tion from the intra to the intermolecular H-bonded OH groups
is resolved when the PVPh content in the blends is increased.
However, in contrast to the small-mass system, certain
2
1
fractions of free hydroxyl absorptions at 3525 cm are clearly
viewed as shoulder-like peaks in the spectra of An2Py/PVPh(2/
1), (1/1) and (2/3) blends. The presence of free hydroxyl groups
in blends of high PVPh content is consistent with the above-
mentioned concept that complete intermolecular H-bond
interactions are difficult to attain in the crosslinking system.
2
1
41
Pyridine absorptions (Fig. 8B) in the range of 985 to 1020 cm
(93%) from the PFN/PVR blends. The pure rigid-chain PFN
were subjected to curve-fitting to have a clear view of the
bonding situation on the pyridine ring. Here, the phenol
solid already has a high quantum yield of 61% and an increase
of PVR content in the PFN/PVR blends raised the quantum
efficiency to an ultimate value of 93% for the PFN/PVR(1/200)
4
9
21
absorption at 1012 cm
should be separated from the
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936 | RSC Adv., 2013, 3, 6930–6938
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blend. Instead of molecular mobility to create a high number
of H-bond pairs, the preliminary structural factor of chain
rigidity seems to be more crucial for deciding the final
emission performances of the blend systems. The molecular
rotations of the luminogenic units in PFN polymer were
supposed to be effectively hampered by the rigid main-chain
framework and addition of PVR continuously introduced
rotational constraints by H-bond interactions and resulted in
the ultimate quantum efficiency of 93% for PFN/PVE(1/200)
with small amounts (y2.3 wt%) of fluorescent PFN compo-
nent. For the present system, the molecular mobility of the
mobile An2Py can be frozen by H-bond interaction, too;
however, to a lesser extent compared to the rigid PFN system
due to the mobile nature of the small-mass An2Py. Therefore,
molecular rotation of the rigid main-chain polymeric lumino-
gens are said to be more efficiently restricted by H-bond
interactions compared to the small-mass organic luminogens.
2 B. Z. Tang, X. Zhan, G. Yu, P. P. S. Lee, Y. Liu and D. Zhu, J.
Mater. Chem., 2001, 11, 2974.
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5
6
7
J. Liu, J. W. Y. Lam and B. Z. Tang, J. Inorg. Organomet.
Polym. Mater., 2009, 19, 249.
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3
7, 182.
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4
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S. Dong, Z. Li and J. Qin, J. Phys. Chem. B, 2009, 113, 434.
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1
1
1
1
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0 Z. Wang, H. Shao, J. Ye, L. Tang and P. Lu, J. Phys. Chem. B,
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3 W. Z. Yuan, P. Lu, S. Chen, J. W. Y. Lam, Z. Wang, Y. Liu, H.
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4 B. Xu, Z. Chi, Z. Yang, J. Chen, S. Deng, H. Li, X. Li,
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2
Conclusion
An2Py with two pyridine terminal rings was successfully prepared
by a Heck coupling reaction and was used to H-bond with small-
mass BPA and the polymeric PVPh, respectively, to construct
fluorescence systems with enhanced emissions. Primarily, An2Py
was proven to be AIEE-active according to the emission behavior
in the THF/hexane solvent/poor solvent mixtures. With increas-
ing content of H-bond donors (i.e. BPA and PVPh), both An2Py/
BPA and An2Py/PVPh systems show enhanced emissions in the
16 W. Wang, T. Lin, M. Wang, T. X. Liu, L. Ren, D. Chen and
S. Huang, J. Phys. Chem. B, 2010, 114, 5983.
17 K. Kokado and Y. Chujo, Macromolecules, 2009, 42, 1418.
18 A. Pucci, R. Rausa and F. Ciardelli, Macromol. Chem. Phys.,
1
solution and in the solid states. Results from the solution H
NMR spectra suggest that molecular rotations of the An2Py
luminogens are hampered by H-bond interactions. Solution and
solid blends of An2Py/BPA(3/7) and An2Py/PVPh(2/3) possess the
highest emission efficiencies and no more intensity gain is
observed when more H-bond donors were loaded. Polymer PVPh
provides the beneficial viscous chain to impose effective
molecular restriction on the An2Py luminogens; however, the
incomplete H-bond interactions in the highly-crosslinked pro-
ducts give less H-bond pairs and less emission enhancements.
On the other hand, the thorough H-bond interactions and the
highly-crystalline structures in the An2Py/BPA blends furnish
effective restriction on molecular rotation and result in
fluorescence with intensity comparable to the polymeric An2Py/
PVPh system. The ultimate emission efficiencies (69 vs. 71%) are
therefore quite close for both systems.
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9 J. Liu, Y. Zhong, J. W. Y. Lam, P. Lu, Y. Hong, Y. Yu, Y. Yue,
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Acknowledgements
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We appreciate the financial support from National Science
Council, Taiwan under the contract no. 101-2221-E-110-022.
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