Emission enhancement and color adjustment of silicon-cored structure in tetraphenyl benzene with aggregation-enhanced emission

Article abstract of DOI:10.1039/c4tc00570h

In this paper, we report the synthesis and optical behavior of tetraphenyl benzene and its two derivatives. All the three compounds exhibited aggregation-enhanced emission (AEE) properties at a low concentration. The AEE mechanism was investigated and is due to restricted intramolecular rotation and unique packing structure. Furthermore, this kind of dendritic benzene derivatives exhibited interesting optical properties with increased concentration. Furthermore, we found a new phenomenon that the silicon-cored structure could efficiently enhance emission intensity and adjust the emission colors of dendritic benzene. The phenomenon was called "silicon-cored effect." This effect may give some guidance to the design of new luminescent materials with AIE and AEE properties. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00570h

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Materials Chemistry C
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Emission enhancement and color adjustment of
silicon-cored structure in tetraphenyl benzene with
aggregation-enhanced emission†
Cite this: J. Mater. Chem. C, 2014, 2,
5601
a
b
a
a
a
*
Hua Wang, Yan Liang, Huanling Xie, Linglong Feng, Haifeng Lu
a
*
and Shengyu Feng
In this paper, we report the synthesis and optical behavior of tetraphenyl benzene and its two derivatives. All
the three compounds exhibited aggregation-enhanced emission (AEE) properties at a low concentration.
The AEE mechanism was investigated and is due to restricted intramolecular rotation and unique packing
structure. Furthermore, this kind of dendritic benzene derivatives exhibited interesting optical properties
with increased concentration. Furthermore, we found a new phenomenon that the silicon-cored
structure could efficiently enhance emission intensity and adjust the emission colors of dendritic
benzene. The phenomenon was called “silicon-cored effect.” This effect may give some guidance to the
design of new luminescent materials with AIE and AEE properties.
Received 21st March 2014
Accepted 9th May 2014
DOI: 10.1039/c4tc00570h
www.rsc.org/MaterialsC
Currently, these dendritic benzene derivatives are mostly
used to modify the plane conjugate chromophore in optical
applications.6–10 The introduction of the dendritic benzene
Introduction
The derivatives of benzene with multiple contiguous phenyl
substituents have many potential applications in many areas of
science and technology, such as modication of luminescent
materials for optoelectric devices, template materials for
supramolecular assembly, and the preparation of analogues of
graphene.1–22 Aryl substituents are not constrained by addi-
tional bonds in the plane of the aromatic core, and thus, the aryl
substituents exhibit large, twisted angles with the central
benzene.6–9 Therefore, the derivatives of benzene with multiple
contiguous phenyl substituents have much more complex
nonplanar topologies than planar molecules.15 These
nonplanar topologies could efficiently limit conjugation and
inhibit extensive aromatic p–p and C–H/p interactions.6–9,15
Therefore, these compounds typically show greater highest-
occupied molecular orbital lowest-unoccupied molecular
orbital gaps, lower degrees of self-association, less efficient
packing, and higher solubility than planar analogues with
similarly high thermal stability.15,23
could efficiently restrain the aggregation of luminescent
compounds, which can broaden their emission bands, shi
the frequencies emitted, and cause quenching. These poten-
tial effects allow thin lms to be processed from solution as
long-lived amorphous phases with high values of Tg, excellent
thermal stability, efficient luminescence, and other attractive
properties. Most studies focused on the assistant function of
dendritic benzenes in optical application, but information on
their self-optical properties is lacking. Tang's group found that
benzene-cored luminophors exhibited aggregation-induced
emission properties and the mechanism could be attributed to
the restriction of intramolecular rotations.24–26 Based on this
result, we designed and synthesized two derivatives of benzene
with multiple contiguous phenyl substituents, 1,2,3,4-tetra-
phenyl benzene (TPB) and 1,2,3,4,5-pentaphenyl benzene
(PPB) (see Scheme 1). The synthesis process is provided in
ESI.† We found that the two compounds exhibited common
aggregation-enhanced emission (AEE) at low concentration.
However, these compounds exhibited distinctive optical
aKey Laboratory of Special Functional Aggregated Materials& Key Laboratory of
Colloid and Interface Chemistry (Shandong University), Ministry of Education,
School of Chemistry and Chemical Engineering, Shandong University, Jinan,
250100, People's Republic of China. E-mail: fsy@sdu.edu.cn; Fax: +86-531-
88564464; Tel: +86-531-88364866
bCollege of Food and Bioengineering, Qilu University of Technology, Jinan, Shandong
250353, People's Republic of China
† Electronic supplementary information (ESI) available: The synthesis,
characterization, absorption and TGA spectra of TPB, PPB and BTPB-DPS.
CCDC 991012, 991014 and 991015. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4tc00570h
Scheme 1 The molecular structures of TPB, PPB and BTPB-DPS.
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properties, which were caused by aggregation at high
concentration.
The silicon-cored aromatic molecules are appropriate lumi-
nescent materials because of their high values of Tg, excellent
thermal stability, efficient luminescence, and other attractive
properties that can be attributed to the non-planar topologies of
the silicon-cored tetrahedron structure.27–31 When the silicon-
cored structure is introduced to non-planar phenyl-substituted
benzenes, the product would have much more complicated
topologies. These unique topologies may generate some new
signicant properties in many areas of science and technology.
In this paper, we also report the synthesis of bis(1,2,3,4-tetra-
phenyl benzene-yl) diphenylsilane (BTPB-DPS) (see Scheme 1).
The synthesis process is provided in ESI.† BTPB-DPS exhibits an
interesting AEE phenomenon. Furthermore, this silicon-cored
derivative shows higher uorescence emission and larger
emission wavelength than the pure dendritic benzene
derivatives.
Fig. 2 The absorption spectra of TPB (a), PPB (b) and BTPB-DPS (c)
with water/THF content at 0% and 99%.
70% water was added to the THF solutions, but intensity rapidly
increased thereaer because the solvation power of the mixture
had lowered to such an extent that the dyes had begun to
aggregate. When the water fraction was increased to 99%, the
PL peak intensities increased by up to elevenfold. The uores-
cent quantums in solution and aggregate were also measured.
The uorescence quantum was just 4.3% in THF solution.
When the water fraction was increased to 99%, the uorescence
quantum increased to 10.3%.
PPB and BTPB-DPS were also tested to further investigate the
optical behavior. The two compounds exhibited AEE properties
at low concentration. When the water fraction was increased to
99%, the PL peak intensities were increased by up to eleven and
twelvefold, and the uorescent quantums increased from 3.9%
to 7.6% for PPB and 4.5% to 12.7% for BTPB-DPS. According to
Tang's work on the AIE mechanism of benzene-cored lumino-
phors, the AEE of the three compounds was due to the restricted
intramolecular rotation.24,25
As shown in Fig. 2, the experimental UV data of TPB in THF
and water were tested to further investigate the aggregation-
enhanced emission properties. In solution, TPB exhibited the
maximum peak absorption spectra at 244 nm. The 244 nm band
is associated with the p–p transition of the phenyl group of TPB
molecule in dilute THF solution. When the volume of water
fractions increased to 99%, the maximum peaks in the
absorption spectra of TPB are red shied and a new shoulder
band appeared at around 293 nm. The absorption spectra of
PPB and BTPB-DPS also exhibited a bathochromic shi of
maximum UV absorption when the water content was 99% in
contrast to those in pure THF solution (from 245 nm to 284 nm
and 246 nm to 280 nm, respectively).
Results and discussion
Optical properties of tetraphenyl benzene derivatives at low
concentration
The PL behavior of 1,2,3,4-tetraphenyl benzene (TPB) as the
simplest model in solution and aggregation states was initially
examined. Fig. 1 shows that its maximum emission wavelength
was 343 nm in tetrahydrofuran (THF) solution and the PL
intensity was very weak. When large amounts of water were
added to their THF solutions (with nal concentrations
constant at 2 Â 10À5 mol LÀ1), PL intensity gradually increased
with increased water content in THF. Therefore, this simple
molecule exhibited aggregation-enhanced emission (AEE). The
PL peak intensity remained almost unchanged when less than
Optical properties of tetraphenyl benzene derivatives in
different concentrations
Aer discussing the AEE properties of the three compounds in
water/THF systems, we further investigated the optical proper-
ties in pure THF solutions at different concentrations. The
results were interesting.
TPB was dissolved in THF at concentration of 2 Â 10À5 mol
LÀ1, 2 Â 10À4 mol LÀ1, 2 Â 10À3 mol LÀ1 and 2 Â 10À2 mol LÀ1
.
Fig. 1 (a) PL spectra of the dilute solutions of TPB (2 Â 10À5 mol LÀ1) in
As the concentration increased in THF, the emission bands of
water/THF mixtures with different water contents. (b) PL spectra of the TPB exhibited a gradual red-shi tendency from 341 nm to
dilute solutions of PPB (2 Â 10À5 mol LÀ1) in water/THF mixtures with
377 nm when the emission intensity increased (see Fig. 3
and Table 1). Similarly, the excitation bands also exhibited a
bathochromic shi from 276 nm to 323 nm (Fig. 4 and
different water contents. (c) PL spectra of the dilute solutions of BTPB-
DPS (10À5 mol LÀ1) in water/THF mixtures with different water
contents. The excitation wavelength was 280 nm. (d) Changes in the
Table 1). Meanwhile, the uorescence emission intensity
nally increased by ninefold. At low concentration, the
PL peak intensities of TPB, PPB and BTPB-DPS with different water
fractions in the water/THF mixtures.
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not only uorescence emission intensity enhancement, but
also a bathochromic shi.
Encouraged by this result, we also tested PPB and BTPB-DPS
to further investigate the optical properties of the compounds
in this series. The concentrations in THF were set at 2 Â 10À5
mol LÀ1, 2 Â 10À4 mol LÀ1, 2 Â 10À3 mol LÀ1 and 2 Â 10À2 mol
LÀ1 for PPB, and 10À5 mol LÀ1, 10À4 mol LÀ1, 10À3 mol LÀ1 and
10À2 mol LÀ1 for BTPB-DPS. The two derivatives also exhibited
the uorescence enhancement and wavelength red-shi prop-
erties in the THF solution (see Fig. 3 and Table 1).
As the concentration increased in THF, the emission bands
of PPB exhibited a gradual red-shi tendency from 343 nm to
378 nm, and the excitation bands exhibited a bathochromic
shi from 273 nm to 324 nm. Meanwhile, the uorescence
emission intensity also increased by sixfold. The optical prop-
erty of PPB was similar to the TPB molecules. It indicated that
the presence of an additional phenyl group did not have a
signicant inuence on the optical properties of TPB.
Fig. 3 (a–c) PL spectra of the THF solutions of TPB, PPB and BTPB-
DPS with different concentrations; (d) PL spectra of the THF solutions
of TPB, PPB and BTPB-DPS, 2 Â 10À2 mol LÀ1 for TPB and PPB, 10À2
mol LÀ1 for BTPB-DPS.
For the silicon-cored derivative BTPB-DPS, the results were
distinctive and exciting. BTPB-DPS exhibited a gradual red-shi
tendency from 346 nm to 408 nm, and the excitation bands
exhibited the bathochromic shi from 272 nm to 336 nm (Fig. 4
and Table 1). Meanwhile, the uorescence emission intensity
also increased by ninefold.
We found an interesting phenomenon in the optical prop-
erties of tetraphenyl benzene and silicon-cored derivative. As
shown in Fig. 5, emissions were all colorless for TPB at different
concentrations, and the same phenomenon was observed for
PPB under a 365 nm UV lamp. However, BTPB-DPS exhibited
obvious blue emissions in 10À3 and 10À2 mol LÀ1 THF solution.
We dropped the THF solution of TPB, PPB and BTPB-DPS on
thin-layer chromatography (TLC) plate to further investigate the
solid state uorescent properties. Aer solvent evaporation,
uorescence was observed under UV lamp. As shown in Fig. 3,
Table 1 The fluorescence emission and excitation data of three
compounds in different concentrations
Concentration/
mol LÀ1
2 Â 10À5 2 Â 10À4 2 Â 10À3 2 Â 10À2
TPB
lem/nm 341
lex/nm 276
lem/nm 343
lex/nm 273
347
290
347
290
357
309
359
310
377
323
378
324
PPB
Concentration/
mol LÀ1
10À5
10À4
10À3
10À2
BTPB-DPS
lem/nm
lex/nm
346
272
349
281
378
323
408
336
aggregation of TPB molecules only enhanced the intensity
of uorescence emission with no red-shi (Fig. 1). At higher
concentration, the aggregation of TPB molecules exhibited
Fig. 5 Images of TPB and BTPB-DPS in THF solutions with different
concentrations under 365 nm UV lamp. For TPB: (a) 2 Â 10À5 mol LÀ1
,
(b) 2 Â 10À4 mol LÀ1, (c) 2 Â 10À3 mol LÀ1, (d) 2 Â 10À2 mol LÀ1; for
Fig. 4 The excitation spectra of (a) TPB, (b) PPB and (c) BTPB-DPS BTPB-DPS: (A) 10À5 mol LÀ1, (B) 10À4 mol LÀ1, (C) 10À3 mol LÀ1, (D)
with different concentrations.
10À2 mol LÀ1
.
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under the UV lamp (365 nm), TPB on TLC plate showed no derivative exhibits higher uorescence and larger emission
luminescence and PPB showed the same phenomenon, whereas wavelength than the dendritic benzene. It means that the
BTPB-DPS exhibited obvious blue light emission.
silicon-cored structure could efficiently enhance the uores-
According to the above-mentioned phenomenon, BTPB-DPS cence and adjust the emission wavelength of dendritic benzene.
is an interesting kind of AIE system. The uorescence variation This phenomenon can be called the “silicon-cored effect” in this
from colorless to blue emission of BTPB-DPS is due to the series of dendritic benzene derivatives.
emission bathochromic shi and intensity enhancement. It is
different from other molecules, which are only based on the
variation of uorescent intensity.25,32–36
The crystal structures of tetraphenyl benzene derivatives
Furthermore, when the concentration of tetraphenyl
The crystal structures of the three compounds were also further
investigated to explain the above-mentioned unique optical
phenomenon.
benzene groups were the same at 2 Â 10À2 mol LÀ1, BTPB-DPS
exhibited higher uorescence emission and blue emission in
contrast to the colorless emission of TPB and PPB (see Fig. 2d).
The molecules of TPB exhibited a symmetrical structure.
Therefore, the silicon-cored derivative BTPB-DPS exhibited the
With four contiguous phenyl substituents, TPB is forced to
emission enhancement and color adjustment effect of tetra-
adopt a conformation with large torsional angles (50.3ꢀ and
phenyl benzene. Because the silicon-cored structure did not
68.7ꢀ) between the central and peripheral aromatic rings. In one
enlarge the conjugation of tetraphenyl benzene, the reason may
direction, each molecule engages in two C–H/p interactions
be due to the molecular packing ways in aggregate states. To our
knowledge, this enhancement and adjustment effect of silicon-
cored structure was the rst report in the investigation of AIE
systems. Particularly, when the silicon-cored structure was
introduced to the dendritic benzene, the silicon-cored
˚
with two neighbors (C–H/p centroid distances of 3.5 A) to
dene chains that pack to form corrugated sheets (Fig. 6a). The
molecules were packed side by side. In the other direction,
adjacent sheets are joined by four additional C–H/p interac-
˚
tions per molecule (C–H/p centroid distances of 3.2 and 3.7 A),
Fig. 6 The packing modes of TPB (a and b), PPB (c and d), BTPB-DPS (e and f); (hydrogen atoms are omitted for clarity in e and f).
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with each central aromatic TPB ring serving as a double exhibited the gradual bathochromic shi and emission
acceptor (Fig. 6b). The molecules were packed head to tail. enhancement in THF solution with increased concentration.
As expected, the average torsional angles between the central The silicon-cored derivative BTPB-DPS showed a unique AIE
aromatic ring and the ve phenyl groups of PPB are slightly property and exhibited better optical properties than the
larger than those of TPB. PPB formed sheets with prominent hydrocarbon compounds (TPB and PPB). This phenomenon of
embraces, which dene dimers joined by two C–H/p interac- emission enhancement and color adjustment of the silicon-
˚
tions (C–H/p centroid distances of 3.3 A). Adjacent sheets are cored structure is called the “silicon-cored effect”, which may be
joined by four additional C–H/p interactions per molecule (C– due to the unique silicon-cored structure that results in a highly
˚
H/p centroid distances of 3.6 and 4.0 A), with each central complicated topological structure in aggregated states. This
aromatic ring of PPB serving as a double acceptor (Fig. 6c and class of highly uorescent dendritic benzene compounds can
d). In one direction, the molecules were packed side by side. In lead to new applications, such as optoelectronics, sensors, and
the other direction, the molecules were packed head to tail. biological devices.
According to the above mentioned discussion, PPB has similar
molecular packing ways to TPB. These molecular packing
modes made molecules of TPB and PPB have efficient uores-
cence without p/p quenching in their aggregate states.37–44
Acknowledgements
This work was nancially supported by the National Natural
BTPB-DPS exhibited a different crystal structure. The whole Science Foundation of China (no. 21274080 and 21204043), the
molecule shows a tetrahedral structure. The two tetraphenyl Key Natural Science Foundation of Shandong Province of China
benzene groups attached on the Si atom exhibited different (no. ZR2011BZ001 and ZR2009BZ006) and the Doctor Station
topological structures. The torsional angles between the central Foundation of Chinese Education Department (no. 200603086).
and peripheral aromatic rings are 43.8ꢀ, 69.3ꢀ, 75.4ꢀ, and 76.3ꢀ
for one and 66.1ꢀ, 68.0ꢀ, 68.8ꢀ, and 83.5ꢀ for the other. The
torsional angles in silicon-cored derivatives of BTPB-DPS are
Notes and references
¨
¨
larger than those of TPB and PPB. Larger torsional angles
resulted in weaker intermolecular interactions. However, it was
interesting that the intramolecular C–H/p interaction was
found in BTPB-DPS molecules. For the intermolecular interac-
tions, in one direction, only the C–H/p interaction (centroid
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