New tetraphenylpyridinium-based luminogens with aggregation-induced emission characteristics

Article abstract of DOI:10.1007/s11426-013-4933-6

The research on aggregation-induced emission (AIE) has drawn increasing interests in the past decade. With the efforts scientists paid, a variety of AIE systems have been developed, among which the tetraphenylethelene and silole derivatives are the most s

Full text of DOI:10.1007/s11426-013-4933-6

SCIENCE CHINA
Chemistry
• COMMUNICATIONS •
· SPECIAL TOPIC · Aggregated-Induced Emission
doi: 10.1007/s11426-013-4933-6
doi: 10.1007/s11426-013-4933-6
New tetraphenylpyridinium-based luminogens with
aggregation-induced emission characteristics
WANG ZongTan1, FANG Yuan1, SUN JingZhi1, QIN AnJun1* & TANG Ben Zhong1, 2, 3*
1MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang
University, Hangzhou 310027, China
2Department of Chemistry, Institute for Advanced Study, Institute of Molecular Functional Materials, The Hong Kong University of Science &
Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China
3Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and
Devices, South China University of Technology (SCUT), Guangzhou 510640, China
Received June 26, 2013; accepted July 7, 2013
The research on aggregation-induced emission (AIE) has drawn increasing interests in the past decade. With the efforts scien-
tists paid, a variety of AIE systems have been developed, among which the tetraphenylethelene and silole derivatives are the
most studied. Development of new AIE systems could further enrich the AIE molecules and promote the development of AIE
area. In this communication, we prepared a new AIE system based on 1,2,4,6-tetraphenylpyridinium ions according to the re-
striction of intramolecular rotation mechanism. These molecules could be facilely synthesized via one-step and one-pot reac-
tion. The ionic AIE-active molecules could find wide application in sensing and optoelectronic areas.
aggregation-induced emission, cation, restriction of intramolecular rotation, tetraphenylpyridinium
used as solid films in their real-world applications [7, 8].
Various chemical, physical, and engineering approaches
1 Introduction
(e.g. attachment of alicyclic pendants, encapsulation by
amphiphilic surfactants, and blending with transparent
polymers) have been taken to alleviate the ACQ effect.
These attempts, however, have met with limited success
[9–11]. Dye aggregation is a spontaneously occurred physi-
cal phenomenon in nature. It will be better if the dye aggre-
gation plays a positive instead of negative role for its emis-
sion in condensed phase. Thus, researchers never need to
painfully fight against this process but amusedly utilize it to
generate efficient emitters in the solid state [12–14].
We have observed an unusual phenomenon of “aggrega-
tion-induced emission” (AIE) in 2001 [15], which is exactly
opposite to the aforementioned ACQ effect. A series of
propeller-shaped molecules are nonluminescent when mo-
lecularly dissolved but become highly emissive in the ag-
gregate state. Restriction of intramolecular rotation (RIR) of
their multiple phenyl rings in the aggregate or solid states
Development of efficient luminescent materials in the solid-
state is a hot research topic because of their promising prac-
tical applications in organic light-emitting diodes (OLEDs),
organic solid-state lasers, chemical sensors, biological
probes, logic circuitry, etc [1–6]. With the enthusiastic ef-
forts scientists paid, tremendous fluorogenic dyes have been
generated. However, many of them are highly emissive
when dissolved in good solvents but faintly luminescent or
totally nonemissive when increased the concentration or
aggregated in poor solvents. This is the thorny problem of
aggregation-caused quenching (ACQ) effect, which should
be properly tackled because these materials are commonly
*Corresponding authors (email: qinaj@zju.edu.cn; tangbenz@ust.hk)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com www.springerlink.com

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Wang ZT, et al. Sci China Chem January (2013) Vol.56 No.1
has been proven experimentally and theoretically as the
main cause for the AIE effect [16–25].
would consume the energy of excited state and make the
molecule nonemissive or weakly luminescent. While, in the
aggregate or solid states, the rotations are greatly restricted
physically, and the emission will turn on.
Attracted by the intriguing photophysical process of AIE
effect, the researchers throughout the world have been ac-
tively involved in this area. As a result, a variety of AIE
systems have been developed in the past decade, among
which the tetraphenylethelene (TPE) and silole derivatives
have been widely investigated and identified as the arche-
type AIE systems [12, 13, 26]. They have been widely ap-
plied in fluorescent chemosensors and bioprobes, stimuli-
responsive nanomaterials, and as active layers in efficient
OLEDs [27]. However, the emission efficiency of TPEs and
the tedious synthesis and purification procedures of siloles
have greatly limited the expansion of the researches on AIE.
To further promote the development of the AIE area, new
AIE system is highly demanded. Furthermore, for biological
applications, the AIE molecules have always been decorated
with hydrophilic groups on their periphery to make them
water soluble, which requires additional functionalization
procedures. [28, 29].
We are interesting in exploring new AIE systems, espe-
cially the ionic AIE-active molecules because of their
promising application in biological field. According to the
RIR mechanism of AIE, a propeller-shaped molecule with
multiple phenyl rotors could feature such characteristics,
and pyridine derivatives could generate ionic species by
substitution reaction with ease. Following these rules, a type
of cationic molecules named 1,2,4,6-tetraphenylpyridinium
drew our attention [3032]. In such molecule, four phenyl
rings are integrated on a pyridinium cation. We thus antici-
pated that the rotation of these phenyl rings in solution
2 Results and discussion
The 1,2,4,6-tetraphenylpyridinium-based ionic molecules
could be facilely synthesized. As shown in Scheme 1, the
reactions between 2,4,6-triphenylpyrylium tetrafluoroborate
and mono- or di-amines in the refluxed ethanol solutions
readily furnished corresponding compounds 1, 2 and 3 after
simply workup procedures [33]. Unlike other ionic
AIE-active luminogens, which were prepared via post-
functionalization reactions, the cationic pyridiniums are
formed in situ with the substitution reactions of oxygen by
nitrogen. The molecules are soluble in dichloromethane and
other polar solvents like DMSO and DMF, but partially
soluble in water. The molecules were fully characterized by
1H (Figure S1) and 13C (Figure S2) NMR and mass spectra
(Figure S3), and satisfactory data corresponding their struc-
tures were obtained [see Supporting Information for de-
tails].
Then, we studied their photophysical properties. As can
be seen from Figure S4, the dilute DMSO solutions of 1, 2
and 3 show absorption maximum (max) at 309, 313, and
317 nm, respectively. The red-shift of the max is probably
due to the conjugation extension from 1 to 3.
According to the RIR mechanism of AIE, the 1,2,4,6-
tetraphenylpyridinium derivatives should possess such
Scheme 1 Synthetic routes to 1,2,4,6-tetraphenylpyridiniums 1, 2 and 3.

Wang ZT, et al. Sci China Chem January (2013) Vol.56 No.1
3
was recorded. Furthermore, the emission peaks were gradu-
ally red shifted from 422 to 446 nm under the same meas-
urement conditions, which is probably due to the solvent
polarity effect with more water added. Since 2 is partially
soluble in water, addition of large amount water into its
DMSO solution will make the molecules aggregated. 2 was
thus induced to emit by aggregate formation, i.e. 2 is
AIE-active. While, for 1 and 3, their DMSO solutions could
emit weak light but the intensities were strengthened upon
addition of water (Figures S5 and S6). In other words, they
feature the characteristics of aggregate-enhanced emission
(AEE).
The plots of relative emission intensity of the molecules
in DMSO/water mixtures vs. fw further confirm their
AIE/AEE features. As shown in Figure 1(b), the emission
intensity of 1, 2 and 3 was gradually increased with addition
of water, and the highest values were all recorded in
DMSO/water mixtures with fw of 90%, which are 12.6, 85.0,
and 6.1-fold higher than that those in their DMSO solutions,
respectively. The relatively low intensity enhancement of 1
and 3 is because their solutions are weakly emissive.
These results indicate that the emission behaviors of
1,2,4,6-tetraphenylpyridinium-based molecules could be
easily fine-tuned through molecular engineering.
3 Conclusions
In summary, according to the RIR mechanism, we have
developed a new AIE system based on 1,2,4,6-tetraphen-
ylpyridinium ion. Compounds 1, 2 and 3 could be facilely
synthesized from commercially available starting materials
of 2,4,6-triphenylpyrylium tetrafluoroborate and mono- or
di-amines in the refluxed ethanol solutions. The photophys-
ical measurements showed that these molecules possess
AIE/AEE characteristics, which in turn proves the reasona-
bility of the RIR mechanism. Unlike the reported ionic
AIE-active molecules, in which the charges were on their
periphery, these molecules have charges on their cores,
which are promising to be applied as chemosensors and
bioprobes with high sensitivity. The generation of more
AIE-active 1,2,4,6-tetraphenylpyridinium ions and explora-
tion of their applications are on-going in our research
groups.
Figure 1 (a) PL spectra of 1,2,4,6-tetraphenylpyridinium derivative 2 in
the DMSO/H2O mixtures with different fractions of water (fw). Concentra-
tion: 10 µM; excitation wavelength: 313 nm. (b) Plot of the change in PL
intensity with fw of 1,2,4,6-tetraphenylpyridinium derivative 2, where I0
and I are the maxima PL intensity. Inset: Plots of the changes in PL inten-
sity with fw of 1,2,4,6-tetraphenylpyridinium derivatives 1 and 3.
characteristics. We thus investigated the photoluminescence
(PL) behaviors of 1, 2 and 3 in the solution and aggregate
states. The aggregates were prepared by adding water into
their DMSO solutions under vigorous stirring. The re-
sultant mixtures are visually transparent and macroscopi-
cally homogenous, suggesting that the molecular aggregates
are nanometer-sized [34].
Variations of the PL spectra of 2 with fw in the
DMSO/water mixtures are shown in panel (a) of Figure 1 as
an example. When excited at 313 nm, the PL spectrum of
the diluted DMSO solution of 2 gives almost a flat line par-
allel to the abscissa, manifesting that 2 is a weak emitter
when molecularly dissolved. In contrast, when a large
amount of water is added into the solution, intense emission
This work was partially supported by the National Science Foundation of
China (21222402, 21174120, 20974098), the National Basic Research
Program of China (2013CB834702, 2009CB623605), and the Research
Grants Council of Hong Kong (603509, HKUST2/CRF/10, 604711,
NHKUST620/11).
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