Influence of the number and substitution position of phenyl groups on the aggregation-enhanced emission of benzene-cored luminogens

Article abstract of DOI:10.1039/c5cc00241a

The influence of the number and substitution position of phenyl groups on the aggregation-enhanced emission of benzene-cored luminogens is unambiguously revealed.

Full text of DOI:10.1039/c5cc00241a

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DOI: 10.1039/C5CC00241A
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Influence of number and substitution position of
phenyl groups on the aggregation-enhanced emission of
benzene-cored luminogens
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
b
a
ab
Received 00th January 2012,
Accepted 00th January 2012
Lingzhi Li, Ming Chen, Haoke Zhang, Han Nie, Jing Zhi Sun, Anjun Qin * and
abc
Ben Zhong Tang *
DOI: 10.1039/x0xx00000x
www.rsc.org/
The influence of the number and substitution position of polyphenyls via varying the number and position of substituted
phenyl groups on the aggregation-enhanced emission of phenyl rings on the benzene cores.
benzene-cored luminogens was unambiguous unveiled
.
Aggregation induced emission (AIE) conceptually developed in
001 is drawing increasing attention for it could solve the
2
notorious aggregation-caused quenching (ACQ) problem
1
encountered in most traditional fluorephores. Objectively, AIE is
TPE-An
TPE-Py
both of great academic and practical implies. It can help to enrich
the basic photo-physical knowledge by rationalizing the working
2
mechanism of restriction of intramolecular rotation (RIR).
Furthermore, it satisfies the application requirements of
luminogens that are always used in their aggregate or solid states,
such as solid films in organic light emitting diode (OLED) and
nanoparticles with high contrast and photo-bleaching stability in
N
N
3
,4
TPE-TPA
TPE-Cz
chemo- and biosensor.
Chart 1. Molecular structures of AIEgens with TPE as rotors (red) and
Since AIE luminogens (AIEgens) exhibit remarkable
advantages over typical ACQ molecules, especially in their
aggregate and solid states, the exploration of new AIEgens is of
great importance. One direct and effective strategy is to convert
an ACQ fluorophore to an AIE one by decorating an AIEgen on
ACQ moieties (blue) as stators.
We thus embarked a research on this aspect in order to develop
new AIEgens and to further deciphering the AIE mechanism.
Delightfully, an unambiguous structure-property relationship for
such system was established. The AIE-inactive polyphenyls
could be successfully converted to AIE-active ones by attached
certain amount of phenyl rings on the benzene cores with subtle
change of the substitution positions.
5
it. For example, anthracene (An), pyrene (Py), carbazole (Cz)
and triphenylamine (TPA) show typical ACQ effects due to their
inevitable π-π stacking in the aggregate states. The attachment of
the AIE-active tetraphenylethene (TPE) readily generates new
AIEgens of TPE-An, TPE-Py, TPE-Cz and TPE-TPA,
6
The molecular structures of polyphenyls studied in this work are
shown in Chart 2. Among which, 1,3,5-triphenylbenzene (1,3,5-
respectively (Chart 1). Inspired by such a powerful strategy, we
thus made our efforts on investigating whether simply adding
rotatable phenyl rings to an ACQ fluorophore behaves similarly.
In the family of organic conjugated emitters, polyphenyl is one
of the most popularly used species, because phenyl group is an
TPB),
1,2,4-triphenylbenzene
(1,2,4-TPB),
1,2,4,5-
tetraphenylbenzene (1,2,4,5-TPB) and pentaphenylbenzene (PPB)
9
were synthesized according to the reported methods. Their
7
structures were fully characterized by spectroscopic methods and
satisfactory analysis data were obtained (Fig. S1-S8, electronic
supplementary information, ESI†). The relationship of the
structure-photophysical property of these multiple phenyl
elementary unit that can extend conjugation effectively.
Nevertheless, the ACQ effect in such system has hampered its
8
further application.
While, rare efforts have been paid on
analysing and understanding the photo-physical properties of
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substituted benzenes was studied using fluorescence spectroscopy photo-physical property of this molecule is similar to that of
m-
technique and X-ray diffraction analysis.
terphenyl. It also displays the atypical AEE effectVbieuwtAertmicleitOanltinae
p-terphenyl, longer wavelength of 354 nm due to the lengthening of the
We first investigated the emission behaviour of
DOI: 10.1039/C5CC00241A
in which the central benzene was substituted by two phenyl rings conjugation (Fig. 1B and S10, ESI†). It is worth noting that the
on its para-positions (Chart 2). The photoluminescence (PL) maximum emission peaks remain almost unchanged, suggesting
measurements showed that its THF solution emits intensely at that no strong
- interaction existed due to the non-planar
3
50 nm with absolute quantum yield (F) of 64.5% and fine structure conformation. Fortunately, single crystal of 1,3,5-TPB
emission structure could be observed in its PL spectrum. suitable for X-ray diffraction analysis was obtained (CCDC
However, upon addition of water, the non-solvent of it, into the 1032746). The measurement showed that 1,3,5-TPB adopts a
THF solution, distinctly decrease of the absolute F was recorded non-planar conformation with twisted angles in the range of
(
Fig. S9; ESI†), indicative of a typical ACQ effect.
34.58-38.60° (Fig. S11, ESI†), further confirming its propeller-
shaped conformation. The linear increase of the emission
intensity upon addition of water is probably ascribed to the poor
solubility of 1,3,5-TPB in THF.
p-terphenyl
AIE-inactive
Water fraction
A
(vol%)
1,2,4-TPB
1,2,4,5-TPB
90
80
Typical AEE-active
70
m-terphenyl
60
Atypical AEE-active
50
40
30
20
10
0
1,3,5-TPB
PPB
HPB
310
340
370
400
Chart 2. Molecular structures of polyphenyls.
Wavelength (nm)
4
B
The linear structure of
p-terphenyl is apt to adopt a stable
planar conformation to maintain the minimal potential energy for
10
intramolecular rotation with a maximal electronic conjugation.
3
2
1
Such conformation of the molecule tends to form quenching
species like excimers via π-π stacking interaction in the aggregate
3i
state, making
p-terphenyl AIE-inactive.
Interestingly, one of the isomers of
p-terphenyl, i.e. m-
terphenyl, in which the central benzene was substituted by two
phenyl rings on its metal-positions, behaves differently. Its THF
solution is weakly emissive at 341 nm. Since the absolute F
could not be measured due to the equipment test limit, we used
the PL intensity to characterize the photo-physical property. The
PL intensity linearly increased upon addition of non-solvent of
water into its THF solution, exhibiting the atypical aggregation-
enhanced emission (AEE) characteristics (Fig. 1A). The slight
1,3,5-TPB
m-terphenyl
0
30
60
90
Water fraction (vol%)
Figure 1. (A) PL spectra of
different water fractions; ex: 248 nm, concentration: 10
-terphenyl and 1,3,5-TPB in THF/water mixtures
m
-terphenyl in THF/water mixtures with


M. (B) Changes
drop of PL intensity in the THF/water mixture with 90% water in the PL intensity of
m
with different water fractions.
fraction (fw) is probably due to the formation of less emissive
amorphous nanoaggregates via random packing of the
11
Surprisingly, slight changing the structure of 1,3,5-TPB to its
isomer of 1,2,4-TPB, in which only one substitution position of
phenyl group was different, remarkably differentiates their photo-
physical properties. As aforementioned, 1,3,5-TPB is atypically
AEE-active, whereas, 1,2,4-TPB is an typical AEE-active
molecules. The AEE feature of
m-terphenyl could be ascribed
to its V-shaped structure, which makes two substituted phenyl
rings twisted out the central plane of benzene core and prohibits
the π-π stacking interaction between molecules in the aggregate
state. Meanwhile, the RIR enables this AIEgen to emit intensely
in its aggregate state.
molecule. This AIEgen emits weakly with maximum peak at ca
75 nm in THF solution and THF/water mixtures with fw lower
than 80%, but efficiently in THF/water mixtures with w of 90%
.
3
Addition of another phenyl ring on the meta position of
m-
f
terphenyl will generate another AIEgen of 1,3,5-TPB. The
due to the RIR (Fig. 2A and Fig. S12A; ESI†). Moreover, the
2
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(1.5%) in THF and THF/water mixtures with
absolute

F
measurements show similar behaviours (Fig. S12B; absolute

F
f
w
ESI†). In comparison with its congener of 1,3,5-TPB, the strong lower than 60%. Afterwards, the emission intensViietyw AirnticclereOanslienes
steric hindrance leads to high twisting angles between the gradually but without shifting its maximDuOmI: 1e0m.10is3s9i/oCn5CpCe0a0k2s41aAt
surrounding phenyl rings and the central benzene. As can be seen 380 nm. The highest intensity and absolute F (6.4%) were
from the single crystal structure (CCDC 736467) of 1,2,4-TPB, recorded in THF/water mixtures with fw of 90%. It is worth
o
the dihedral angles are in the range of 42.03-50.06 (Fig. S13, noting that the absolute F (20.9%, Table S1, ESI†) is much
1
2
ESI†), which are much larger than that of 1,3,5-TPB. This higher than the aggregates formed in THF/water mixture with w
f
structural conformation enables 1,2,4-TPB to dissolve in THF of 90% probably due to more tight molecular packing, which
well and emit weakly in its THF solution/water mixtures with fw further restrict the intramolecular rotations.
less than 80%.
Similarly, thanks to the intramolecular rotation in certain
degree of substituted phenyl rings on the benzene cores, PPB and
HPB are also AEE-active (Fig. 2B, S16 and S17, ESI†).
Moreover, due to the more crowded substituted phenyl rings on
the benzene cores as indicated by their single crystals, PPB
A
Water fraction
vol%
90
80
70
60
50
40
30
20
10
0
13
14
(CCDC 769270) and HPB (CCDC 609800) possess the
o
o
dihedral angles of 56.09-72.92 and 65.14-88.59 , respectively
Fig. S18 and S19, ESI†), which are more twisted than that of
(
1,2,4,5-TPB. Such large dihedral angles between substituted
phenyl rings and central benzene cores inevitably shorten the
effective conjugation length and blue-shifted their emission 30
nm shorter than that of 1,2,4,5-TPB (Fig. S16 and S17, ESI†).
Interestingly, 37 nm of red-shift of p-terphenyl was observed
from its solution to film states, further suggesting the π-π
stacking interaction occurred in this molecule (Fig. S20 and
Table S2, ESI†). The AEE-active luminogens, however, show no
shift or blue-shift of their maximum emission from their solution
to film states, which are general photophysical properties for
3
30
370
410
450
490
Wavelength (nm)
1
1
9
7
5
3
1
B
3h
AIEgens.
In summary, we successfully establish an unambiguous
structure-property relationship of polyphenyls via tuning the
number and position of substituted phenyl rings on the benzene
core. The general rule concluded from this study is that more
phenyl rings attached on the central benzenes will induce a
pronounced AEE effect due to its much more twisted
conformation influenced by steric effect. Unlike TPE, the
HPB
PPB
1,2,4-TPB
1,2,4,5-TPB
polyphenyls contain no double-bonds, which exclude the
E-Z
isomerization, simplifies the mechanism understanding for the
AIE/AEE phenomenon, and enhance their thermal stabilities (Fig.
S21, ESI†), making them a new kind of AIEgens.
0
30
60
90
Water fraction (vol%)
This work was partially supported by the key project of the
Ministry of Science and Technology of China (2013CB834702);
Figure 2. (A) PL spectra of 1,2,4,5-TPB in THF/water mixtures with
different water fractions; ex: 275 nm, concentration: 10 M. (B) Changes the National Science Foundation of China (21490571, 21222402
in the fluorescence intensity of 1,2,4-TPB, 1,2,4,5-TPB, PPB and HPB in
THF/water mixtures with different water fractions.

and 21174120); the Research Grants Council of Hong Kong
604711, 602212 and HKUST2/CRF/10). A.J.Q. and B.Z.T.
(
thank the support from Guangdong Innovative Research Team
Program (201101C0105067115).
These results indicate that more twisted the structural
conformation and more phenyl rings attached on the central
benzene, more pronounced AEE features will be achieved. Is this
law applicable for much more complex system of polyphenyls?
To answer this question, we investigated the molecular structure
and photo-physical property of 1,2,4,5-TPB, PPB and HPB. As
could be found from their single crystal structures (CCDC
Notes and references
a
MOE Key Laboratory of Macromolecular Synthesis and
Functionalization, Department of Polymer Science and Engineering,
Zhejiang University, Hangzhou 310027, China.
b
Guangdong Innovative Research Team, State Key Laboratory of
7
03163), the 1,2,4,5-TPB possesses the moderate dihedral angles
o
Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510640, China.
of 40.50-65.98 (Fig. S14, ESI†), which are larger than 1,2,4-
TPB. It is worthy to note that 1,2,4,5-TPB also features the AEE
characteristics (Fig. 2B and S15). It remains weak emissive with
c
Department of Chemistry, Institute for Advanced Study, Institute of
Molecular Functional Materials, and State Key Laboratory of Molecular
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Neuroscience, The Hong Kong University of Science and Technology,
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†
Electronic Supplementary Information (ESI) available: Synthesis and
Zhao, S. Chen, J. W. Y. Lam, P. Lu, Y. Zhong, K. S. Wong, H. S.
DOI: 10.1039/C5CC00241A
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The influence of the number and substitution position
of phenyl groups on the emission of benzene-cored
AIEgens was unambiguous unveiled.
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1‐3 | 5