Modulation of aggregation-induced emission and electroluminescence of silole derivatives by a covalent bonding pattern

Article abstract of DOI:10.1002/chem.201500002

The deciphering of structure-property relationships is of high importance to rational design of functional molecules and to explore their potential applications. In this work, a series of silole derivatives substituted with benzo[b]thiophene (BT) at the 2,5-positions of the silole ring are synthesized and characterized. The experimental investigation reveals that the covalent bonding through the 2-position of BT (2-BT) with silole ring allows a better conjugation of the backbone than that achieved though the 5-position of BT (5-BT), and results in totally different emission behaviors. The silole derivatives with 5-BT groups are weakly fluorescent in solutions, but are induced to emit intensely in aggregates, presenting excellent aggregation-induced emission (AIE) characteristics. Those with 2-BT groups can fluoresce more strongly in solutions, but no obvious emission enhancements are found in aggregates, suggesting they are not AIE-active. Theoretical calculations disclose that the good conjugation lowers the rotational motions of BT groups, which enables the molecules to emit more efficiently in solutions. But the well-conjugated planar backbone is prone to form strong intermoelcular interactions in aggregates, which decreases the emission efficiency. Non-doped organic light-emitting diodes (OLEDs) are fabricated by using these siloles as emitters. AIE-active silole derivatives show much better elecroluminescence properties than those without the AIE characterisic, demonstrating the advantage of AIE-active emitters in OLED applications. Stay connected! Connecting benzo[b]thiophene with a silole ring through its 2- or 5-position furnishes silole derivatives with totally different photoluminescence and electroluminescence properties (see figure).

Full text of DOI:10.1002/chem.201500002

DOI: 10.1002/chem.201500002
Full Paper
&
Luminescence
Modulation of Aggregation-Induced Emission and
Electroluminescence of Silole Derivatives by a Covalent Bonding
Pattern
[
a]
[b]
[a]
[b]
[b]
[a]
Han Nie, Bin Chen, Changyun Quan, Jian Zhou, Huayu Qiu, Rongrong Hu, Shi-
[
a]
[a]
[a, b]
[a, c]
Jian Su, Anjun Qin, Zujin Zhao,*
and Ben Zhong Tang*
Abstract: The deciphering of structure–property relation-
ships is of high importance to rational design of functional
molecules and to explore their potential applications. In this
work, a series of silole derivatives substituted with benzo[b]-
thiophene (BT) at the 2,5-positions of the silole ring are syn-
thesized and characterized. The experimental investigation
reveals that the covalent bonding through the 2-position of
BT (2-BT) with silole ring allows a better conjugation of the
backbone than that achieved though the 5-position of BT
strongly in solutions, but no obvious emission enhance-
ments are found in aggregates, suggesting they are not AIE-
active. Theoretical calculations disclose that the good conju-
gation lowers the rotational motions of BT groups, which en-
ables the molecules to emit more efficiently in solutions. But
the well-conjugated planar backbone is prone to form
strong intermoelcular interactions in aggregates, which de-
creases the emission efficiency. Non-doped organic light-
emitting diodes (OLEDs) are fabricated by using these siloles
as emitters. AIE-active silole derivatives show much better
elecroluminescence properties than those without the AIE
characterisic, demonstrating the advantage of AIE-active
emitters in OLED applications.
(5-BT), and results in totally different emission behaviors. The
silole derivatives with 5-BT groups are weakly fluorescent in
solutions, but are induced to emit intensely in aggregates,
presenting excellent aggregation-induced emission (AIE)
characteristics. Those with 2-BT groups can fluoresce more
Introduction
solid state are of high academic and practical value. Conven-
tional chromophores that show good emissions in solutions
often suffer from emission quenching effect when their mole-
cules aggregate because of the formation of detrimental spe-
[1]
Since the pioneering work by Tang and co-workers in 1987,
organic light-emitting diodes (OLEDs) have attracted consider-
able scientific and industrial interests because of their great
potential in full-color flat panel displays and mercury-free
[4]
cies such as excimers in the condensed phase, which has ob-
structed the development of high-performance OLEDs to some
extent. To solve this notorious aggregation-caused quenching
(ACQ) problem, various molecular engineering approaches and
[
2]
solid-state lighting sources. Organic light emitters usually
have to be fabricated into solid thin films in a real-world appli-
[
3]
[5]
cation, such as in OLEDs. Hence, the design and synthesis of
robust organic materials that can fluoresce efficiently in the
device fabrication techniques had been proposed. But these
attempts often ended with only limited success and even led
[6]
to some side effects in many cases.
To address this issue, an abnormal emission phenomenon of
aggregation-induced emission (AIE), firstly reported on propel-
ler-like silole derivatives, such as 1-methyl-1,2,3,4,5-pentaphe-
nylsilole (MPPS) and 1,1-dimethyl-2,3,4,5-tetraphenylsilole
[
a] H. Nie, C. Quan, Dr. R. Hu, Prof. S.-J. Su, Prof. A. Qin, Prof. Z. Zhao,
Prof. B. Z. Tang
Guangdong Innovative Research Team
State Key Laboratory of Luminescent Materials and Devices
South China University of Technology
Guangzhou 510640 (P.R. China)
[7]
(
DMTPS), has caught the attention of researchers. These silole
derivatives are almost non-fluorescent in solutions but show
strong fluorescence when assembled as nanoparticles or fabri-
cated into solid films. The fact that light emission is enhanced
rather than quenched by aggregate formation challenges the
common knowledge of the ACQ effect. A series of designed
experiments and theoretical calculations were performed and
rationalized that restriction of intramolecular rotations (RIR) is
the main working mechanism of this new photophysical phe-
E-mail: mszjzhao@scut.edu.cn
tangbenz@ust.hk
[
b] B. Chen, Dr. J. Zhou, Prof. H. Qiu, Prof. Z. Zhao
College of Material, Chemistry and Chemical Engineering
Hangzhou Normal University, Hangzhou 310036 (P.R. China)
[
c] Prof. B. Z. Tang
Department of Chemistry, Division of Biomedical Engineering
Division of Life Science, The Hong Kong University of Science
&
Technology, Kowloon, Hong Kong (P.R. China)
[8]
nomenon. This important finding paves a new avenue to
create efficient solid-state emitters. Many luminescent materi-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500002.
Chem. Eur. J. 2015, 21, 1 – 12
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als bearing AIE-active units dis-
play high fluorescence efficien-
[
9]
cies in solid films and exhibit
promising application in
[
10]
OLEDs.
So far, a great many new AIE
luminogens have been devel-
oped and studied worldwide,
[
11]
such as cyanostilbenes, tetra-
[
12]
phenylethenes, diphenyldiben-
[
13]
zofulvenes,
phosphole and
[
14]
phosphindole oxides, 9,10-dis-
[
15]
tyrylanthracenes,
and so on.
As the archetypal AIE lumino-
gens, siloles are still drawing
considerable attention and have
been applied to diverse research
frontiers, such as chemosen-
[
16]
[17]
sors, bioprobes, and so on.
Their highly efficient emissions
in the solid state allow them to Scheme 1. Molecular structures and synthetic routes of the new silole derivatives. LiNaph=lithium naphthalenide,
TMEDA=N,N,N,N-tetramethylethylenediamine.
serve as light-emitting layers to
construct
high-performance
OLEDs. In addition, siloles pos-
sess unique low-lying LUMO levels owing to the s*–p* conju-
gation arising from the effective interaction between the s* or-
bital of two exocyclic single CꢀSi bonds and the p* orbital of
tovoltaic cells, field-effect transistors, nonlinear optics, and
[26]
OLEDs, is attached onto the silole ring through its different
positions (-5 and -2). The 2,5-BT-substituted siloles (Scheme 1)
were synthesized and fully characterized. The correlation be-
tween molecular structures and photoluminescence (PL) and
electroluminescence (EL) properties of these new siloles is dis-
cussed based on the results from experimental measurements
and theoretical calculations. It is demonstrated that different
connection patterns between BT and silole ring impacts conju-
gation degree of the backbone, and alters the AIE characteris-
tics of the silole derivatives. The AIE-active silole derivatives
show superior EL performance compared with those without
AIE characteristics, directly highlighting the advantage of AIE
effect in OLED applications.
[
18]
the butadiene moiety. Siloles hence show good electron af-
[
19]
finity and fast electron mobility, and have been utilized as
[20]
electron-transporting layers in the construction of OLEDs.
The AIE characteristic and high electron mobility enables si-
loles to function as bifunctional materials for the fabrication of
configuration-simplified OLEDs without sacrificing device per-
[
21]
formances.
The study of the structure–property relationship is funda-
mentally important as it helps us to understand the structure
impacts on the properties, and thus further wisely design mol-
ecules for desired functionalities and applications. The emis-
sion wavelengths as well as the AIE characteristics of silole de-
[22]
rivatives are also reported to be sensitive to the substituents.
Some papers concluded that the substituents at the 1,1-posi-
tions of silole ring exert inductive effect primarily upon the ab- Results and Discussion
sorption properties of the silole ring but have only a minor
Synthesis
[
18,23,8a]
impact on fluorescence spectra.
,4-positions can affect the emission behavior of siloles mainly
The substituents at the
3
Scheme 1 illustrates the synthetic routes to 2,5-BT-substituted
[
24]
by the steric effect, and are considered to be crucial compo-
nents to keep the AIE attribute of the silole derivatives. On the
other hand, the substituents at the 2,5-positions can tune the
electronic structures and photophysical properties of silole de-
silole derivatives. The starting materials 1a and 1b, which
were facilely prepared according to the reported methods,
[27]
underwent endo–endo intramolecular reductive cyclizations
with treatment of lithium naphthalenide (LiNaph) and subse-
[
25]
rivatives through p-conjugation with the silole ring. The p-
extended planar chromophores at the 2,5-positions of silole
ring can increase the emission efficiency in solutions but de-
crease that in the solid state. To further depict how the cova-
lent bonding pattern between substituents and silole ring
impact the optical properties of siloles, a typical planar group,
benzo[b]thiophene (BT), which is widely utilized to construct
functional materials that are applied as active materials in pho-
quent ZnCl -N,N,N’,N’-tetramethylethylenediamine (TMEDA) to
2
[28]
yield 2,5-metalated silole intermediates (2). The palladium-
catalyzed cross-couplings of 2 with 2-bromobenzo[b]thiophene
or 5-bromobenzo[b]thiophene provided target silole deriva-
tives. All of these final products are characterized by standard
spectroscopic methods with satisfactory results. The detailed
procedures and characterization data are given in Experimental
section. They show good solubility in common organic sol-
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vents including THF, dichloromethane, chloroform, toluene,
and so on, but are insoluble in water and ethanol.
formed between the phenyl hydrogen atom in one molecule
and the p-electron cloud of the phenyl ring of BT group in the
adjacent molecule. The collective effect of these weak intermo-
lecular interactions can help to rigidify the molecular confor-
mation, lock the intramolecular motions, and thus, enhance
the emission efficiency in the condensed phase.
Crystal structure
Single crystals of 5-BTDMS were grown from THF/ethanol mix-
tures and analyzed by using X-ray diffraction crystallography.
The ORTEP drawing of the crystal structure of 5-BTDMS is dis-
played in Figure 1. The torsion angles between silole and 5-BT
groups are 29.818, which are smaller than those between silole
and phenyl rings at the 2,5-positions in DMTPS (34.82–
Optical property
Figure 3A shows the absorption spectra of the new silole de-
rivatives in dilute THF solutions. The spectral profiles of 5-
BTDMS and 5-BTMPS are similar, and so are those of 2-BTDMS
and 2-BTMPS. A close survey of the absorption spectra disclo-
ses that the absorption maximum of 5-BTMPS is located at
375 nm, associated with the p–p* transition, which is slightly
redshifted by 5 nm relative to that of 5-BTDMS (370 nm). A
similar small redshift is also observed by comparing the ab-
sorption spectra of 2-BTMPS (437 nm) and 2-BTDMS
[
19d]
48.798).
Figure 2 illustrates the molecular packing manner
of 5-BTDMS in the crystalline state. It can be seen that no close
p–p stacking is found, owing to the twisted molecular confor-
mation of 5-BTDMS, which prevents the strong redshift in the
PL spectrum and emission quenching in the solid state. Multi-
ple CꢀH···p hydrogen bonds with a distance of 2.681 ꢁ are
(
432 nm). This phenomenon is attributed to the in-
ductive effect of the phenyl ring at the 1-position of
[23,8a]
silole ring.
On the other side, the absorption
maxima of 2-BTDMS and 2-BTMPS are redshifted ap-
parently with much larger molar absorptivities than
those of 5-BTDMS and 5-BTMPS, suggesting that the
former two silole derivatives own a better conjuga-
tion backbone.
Figure 3B and C present the PL spectra of these
silole derivatives in dilute THF solutions and in solid
films, respectively. The emission maximum of 5-
BTDMS in solid film is 494 nm, being slightly redshift-
ed by 6 nm compared with that in THF solution
(
488 nm) on account of its propeller-like molecular
conformation that can impede close p-stacking inter-
actions. However, a large redshift of 19 nm is ob-
served in the PL spectrum of 2-BTDMS in solid film
[
34]
Figure 1. ORTEP drawing of the crystal structure of 5-BTDMS.
(
541 nm) relative to that in THF solution (522 nm),
which suggests that strong p-stacking interactions
occur in the solid film of 2-BTDMS. The PL spectra of
2-BTDMS in both solution and solid film are also red-
shifted relative to those of 5-BTDMS, owing to the
better conjugation of 2-BTDMS. Similar emission
trends are also observed for 2-BTMPS and 5-BTMPS.
Like most silole derivatives in the literature, 5-BTDMS
and 5-BTMPS emit weakly in THF solutions, with low
fluorescence quantum yields (F ) of 0.8 and 1.2%, re-
F
spectively (Table 1). The F values of 5-BTDMS and 5-
F
BTMPS are increased to 60.4 and 53.0%, respectively,
revealing that they are AIE-active, and are excellent
solid-state emitters. What surprises us is that 2-
BTDMS and 2-BTMPS exhibit quite different emission
behaviors under the same conditions. They can fluo-
resce more strongly than 5-BTDMS and 5-BTMPS,
with much higher F values of 4.5 and 6.5% in THF
F
solutions. However, their emission efficiencies in solid
films are only 5.5 and 8.5%, being barely increased
with regards to those in solutions. This should also
be ascribed to the p-stacking interactions in solid
Figure 2. Molecular packing of 5-BTDMS in the crystalline state. Intermolecular CꢀH···p
hydrogen bonds are indicated (d=2.681 ꢁ).
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used as a good solvent but water as a poor solvent for these
silole derivatives. Figure 4A and B exemplify the PL spectra of
5-BTDMS and 2-BTDMS in THF/water mixtures, respectively. It
can be seen that the PL emission of 5-BTDMS is very weak
when the water fraction (f ) is low. But the emission efficiency
w
increases swiftly (Figure 4C) when the f becomes high. Since
w
5-BTDMS is insoluble in water, its molecules must have aggre-
gated in the mixtures with a high f . The intramolecular rota-
w
tion is active when 5-BTDMS molecularly dissolved in THF/
water mixtures with a low f , which efficiently deactivates the
w
excited state through the nonradiative relaxation channel. But
in the aggregated state, the intramolecular rotations are re-
stricted by steric constraint, and thus the nonradiative energy
decay is blocked, rendering the molecules highly emissive.
Thus, the result confirms that 5-BTDMS is AIE-active indeed.
For 2-BTDMS, addition of a large amount of water (f ꢁ70%)
w
into its THF solution causes obvious redshifts in the PL spectra
(
Figure 4B), which is consistent with those observed in solid
films. As shown in Figure 4C, 2-BTDMS exhibits similar FF
values in both the isolated molecule and the aggregated state.
Namely, 2-BTDMS does not possess a typical AIE characteristic.
Clearly, although 5-BTDMS and 2-BTDMS are comprised of the
same building blocks, the different connection manner has re-
sulted in vastly varied optical properties.
Theoretical calculations
To gain a deeper insight into the significant distinctions in
photophysical properties of these silole derivatives, theoretical
calculations were performed using the Gaussian 09 program.
Time-dependent density functional theory (TD-DFT) calcula-
tions (the Supporting Information, Table S1) show that the first
singlet excited state (S ) is dominated by the transition from
1
the highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO) for both isolated 5-
BTDMS and 2-BTDMS molecules; that is, their photophysical
properties are mainly determined by their HOMO and LUMO
energy levels. As illustrated in Figure 5, the HOMOs and
LUMOs are mainly located on the central silole ring and the BT
substituents at the 2,5-positions. In addition, 2-BTDMS has
a higher HOMO energy level but a lower LUMO one than
those of 5-BTDMS, which is equal to a narrower energy band
gap of 2.91 eV than that of 5-BTDMS (3.58 eV). This also reflects
that 2-BTDMS has a better conjugation between silole and BT
rings than 5-BTDMS.
As the saying goes, structure determines property. We thus
investigated the optimized molecular structures of 5-BTDMS
and 2-BTDMS in the isolated single-molecule state. The impor-
tant geometrical parameters of the ground state (S ) and S for
0
1
both isolated molecules are listed in Table 2. Two important
structural diversities should be noted: 1) The lengths of single
bonds C2ꢀC2’ and C5ꢀC2“ in 2-BTDMS are obviously shorter
than the corresponding bond lengths (C2ꢀC5’ and C5ꢀC5”) of
5-BTDMS in both S and S ; 2) In S , the dihedral angles Si-C2-
Figure 3. A) Absorption spectra in THF solutions and PL spectra, B) in THF
solution (10 m), and C) in solid films of the silole derivatives.
ꢀ
5
film due to the planar conjugated backbones of 2-BTDMS and
0
1
0
2
-BTMPS.
C2’-C3’ and Si-C5-C2“-C3” of 2-BTDMS (both ꢀ14.68) are dra-
matically smaller than those of Si-C2-C5’-C6’ and Si-C5-C5’’-C6’’
in 5-BTDMS (both 49.98). An identical trend is also found in S1
The PL properties of these new silole derivatives were fur-
ther investigated in THF/water mixtures, in which THF was
&
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Figure 4. PL spectra of A) 5-BTDMS and B) 2-BTDMS in THF/water mixtures with different water fractions (f
w
). Concentration: 10 mm; excitation wavelengths:
370 nm for 5-BTDMS and 420 nm for 2-BTDMS. C) Plots of fluorescence quantum yields versus water fractions in THF/water mixtures. Fluorescence quantum
w
yields were determined by a calibrated integrating sphere. Inset: photos of A) 5-BTDMS and B) 2-BTDMS in THF/water mixtures (f =0 and 95%), taken under
the illumination of a UV lamp (365 nm).
Table 1. Optical properties, thermal stabilities, and energy levels of the silole deriva-
tives.
ring in BT is smaller than phenyl ring in size, and thus
it can connect with silole ring in a more coplanar pat-
tern with less steric congestion, which facilitates the
p-electron delocalization. From S to S , the changes
[
c]
[d]
[e]
l
abs
l
em
F
F
T
[ C]
g
o
/T
m
/T
d
HOMO/LUMO
[eV]
E
g
[eV]
[
a]
[
nm]
[nm]
[%]
0
1
[
a]
[b]
of dihedral angles (Si-C2-C5’-C6’ and Si-C5-C5’’-C6’’)
are 19.08 for 5-BTDMS, whereas those of 2-BTDMS
are as low as 1.08, revealing that the BT groups in 2-
BTDMS experience only slight structural alteration
upon photoexcitation. In other words, the rotational
motions of the BT rings are lowered in isolated 2-
BTDMS. However, the corresponding changes of C3-
C4-C6-C7 and C4-C3-C8-C9 present a contrary ten-
dency: 2.48 for 5-BTDMS and 12.38 for 2-BTDMS.
This indicates that the rotations of the phenyl rings
Soln
488
499
522
522
Film
494
502
541
552
Soln Film
5
5
2
2
-BTDMS 370
-BTMPS 375
-BTDMS 432
-BTMPS 437
0.8
1.2
4.5
6.5
60.4 n.d/198/305 ꢀ5.37/ꢀ2.58
53.0 84/263/322
5.5
8.5
2.79
2.76
2.48
2.44
ꢀ5.55/ꢀ2.79
n.d/262/315 ꢀ5.33/ꢀ2.85
107/239/351 ꢀ5.35/ꢀ2.91
ꢀ
5
[
a] In THF solution (10 m). [b] Vacuum-deposited film. [c] Fluorescence quantum yield
determined by a calibrated intergrating sphere. [d] n.d=not detectable. [e] Deter-
mined by cyclic voltammetry.
at the 3,4-positions of silole ring are easier in isolated 2-BTDMS
than in isolated 5-BTDMS.
It is generally known that the fluorescence quantum yield
can be expressed as: F =k /(k +k )=k /(k +k +k ), in
F
r
r
nr
r
r
ic
isc
which k is the radiative decay rate from S to S , k is the inter-
r
1
0
ic
nal conversion rate from S to S , and k is the intersystem
1
0
isc
[29]
crossing rate from S to the first triplet excited state (T ).
1
1
Since the spin-orbital coupling between S and T1 in these
1
[19d]
silole derivatives is very small,
the intersystem crossing pro-
cess from S to T is reasonably negligible. Hence, F is deter-
1
1
F
mined by the competition between k and k . The radiative
r
ic
decay rate can be evaluated through the Einstein spontaneous
emission relationship, which can be cast into a simple working
2
formula k =fE /1.499, in which f is the oscillator strength of
r
ꢀ
1
emission, E is the transition energy of emission (in cm ), and
ꢀ
1
then k is in unit of s . The calculated data of f and E are
r
shown in Table S1 (the Supporting information). Consequently,
Figure 5. Calculated molecular orbital amplitude plots and energy levels of
HOMOs and LUMOs of 5-BTDMS and 2-BTDMS.
8
ꢀ1
the k value of isolated 2-BTDMS (1.33ꢂ10 s ) is slightly
r
8
ꢀ1
higher than that of isolated 5-BTDMS (0.87ꢂ10 s ) because
of the better conjugation of 2-BTDMS, which could only mar-
of both molecules. These results directly indicate that the BT
group conjugates better with the silole ring through its 2-posi-
tion in 2-BTDMS. This is understandable because the thiophene
ginally increase the F of 2-BTDMS compared to 5-BTDMS. So
F
the difference between 5-BTDMS and 2-BTDMS in F values in
F
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total reorganization energy of S is from the rotation-
1
Table 2. Selected bond lengths [ꢁ], dihedral angles [8] of the S
BTDMS and 2-BTDMS molecules.
0
and S
1
for isolated 5-
Crystal data
al motions of BT rings at the 2,5-positions
ꢀ
1
(
1953 cm ), accounting for 42.1% of the total reor-
Molecule
Structural parameter
S
0
S
1
jD(S
1
ꢀS )j
0
ganization energy, which is much higher than the
contributions from phenyl rings at the 3,4-positions
C2ꢀC5’
1.479
1.479
49.9
49.9
58.0
58.0
1.448
1.448
ꢀ14.6
ꢀ14.6
ꢀ73.6
ꢀ73.6
1.448
1.448
30.9
30.9
55.6
55.6
1.419
1.419
ꢀ15.6
ꢀ15.6
ꢀ61.3
ꢀ61.3
0.031
0.031
19.0
19.0
2.4
2.4
0.029
0.029
1.0
1.0
12.3
12.3
1.475
1.475
33.7
33.7
61.9
C5ꢀC5“
(
5.4%). On the contrary, in isolated 2-BTDMS, the con-
Si-C2-C5’-C6’
Si-C5-C5“-C6”
C3-C4-C6-C7
C4-C3-C8-C9
C2ꢀC2’
5
-BTDMS
-BTDMS
tribution from the rotational motions of phenyl rings
at the 3,4-positions of silole ring takes a larger por-
ꢀ
1
61.9
tion (22.4%, 572 cm ), whereas that from the BT
ꢀ
1
groups is only 0.3% (8 cm ). It is also noteworthy
that the reorganization energy from the rotational
motions of BT rings for isolated 5-BTDMS is dramati-
C5ꢀC2“
Si-C2-C2’-C3’
Si-C5-C2“-C3”
C3-C4-C6-C7
C4-C3-C8-C9
2
ꢀ
1
cally larger than that of 2-BTDMS (8 cm , 0.3%), but
the contributions of other motion modes for 5-
ꢀ
1
BTDMS and 2-BTDMS (2657 vs. 2541 cm ) are com-
parable. These results are fully consistent with the
above structural modification from S to S .
the solution state is largely determined by the diversity of the
0
1
internal conversion rate k in the single-molecule state.
Therefore, it is reasonable that the rotational motions are
easier for the BT groups than for the phenyl rings at the 3,4-
positions, and are mainly responsible for the weak emission of
5-BTDMS in solution. In 2-BTDMS, the rotational motions of BT
ic
[
30,8c]
According to the previous researches,
the total reorgani-
zation energy of S is a crucial factor in determining the k , in
1
ic
other words, a larger total reorganization energy predicatively
leads to a higher k , which results in a smaller F value. The
ic
F
calculated total reorganization energies of isolated 5-BTDMS
and 2-BTDMS are given in Table 3. Evidently, 5-BTDMS has
Table 3. Selected calculated results for isolated 5-BTDMS and 2-BTDMS
molecules.
[
a]
[b]
[c]
[d]
k
[
r
l
es
l
2,5
l
3,4
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
s
]
[cm
]
[cm
]
[cm ]
8
8
5
2
-BTDMS
-BTDMS
0.87ꢂ10
1.33ꢂ10
4640
2549
1953 (42.1%)
8 (0.3%)
251 (5.4%)
572 (22.4%)
[a] Calculated radiative decay rate from S
1 0
to S . [b] The total reorganiza-
tion energy of S . [c] Contribution to the total reorganization energy from
1
the BT ring rotations at 2,5-positions. [d] Contribution to the total reor-
ganization energy from the phenyl ring rotations at 3,4-positions.
ꢀ
1
a much larger total reorganization energy of S (4640 cm )
1
ꢀ
1
than that of 2-BTDMS (2549 cm ), revealing that the isolated
-BTDMS possesses a higher k , and hence a smaller F value
5
ic
F
in the solution state. In a word, it is the block of nonradiative
channel, internal conversion process from S to S , that enhan-
1
0
ces the emission of 2-BTDMS in the solution state. In addition,
the reorganization energies versus normal modes are plotted
in Figure 6. It can be clearly seen that the contribution of the
low frequency modes that represent the ring rotational mo-
tions to the total reorganization is suppressed in 2-BTDMS in
comparison with that of 5-BTDMS. This suggests that the re-
striction of rotational motions is the main cause for blocking
the nonradiative channel and enhancing the emission efficien-
cy of 2-BTDMS in the solution state, which again offers a direct
evidence to support the RIR mechanism of AIE phenomenon.
To further study the relationship between their emission be-
haviors and the molecular structures, we project their total re-
organization energies into the structural parameter relaxations
Figure 6. The calculated reorganization energy versus the normal mode
(Table 3). For isolated 5-BTDMS, the main contribution to the
wavenumber for isolated molecules of A) 5-BTDMS and B) 2-BTDMS
&
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Full Paper
rings are lowered greatly by a good conjugation, and the rota-
tional motions of the phenyl rings at the 3,4-positions are pro-
moted to some extent. However, the former effect is domina-
tive still, which allows the 2-BTDMS to emit more efficiently
than 5-BTDMS in solutions.
It is also noteworthy that the connection between silole ring
and thiophene ring in BT (2-position of BT) endows the mole-
cules with higher T and T than the connection between the
d
g
silole ring with phenyl ring in BT (5-position) (Table 1). The
good thermal and morphological stabilities enable these silole
derivatives to be utilized as light-emitting materials for OLEDs.
Thermal stability
Electrochemical property
The thermal properties of these silole derivatives were evaluat-
ed by thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC). As shown in Figure 7, these com-
The electrochemical properties of these silole derivatives were
estimated by using cyclic voltammetry (CV) in acetonitrile solu-
tion with 0.1m tetrabutylammonium hexafluorophosphate as
the supporting electrolyte at a scan rate of 50 mVs and plati-
num as the working electrode and saturated calomel electrode
ꢀ
1
(
SCE) as the reference electrode. The voltammograms are pre-
sented in Figure S1 (the Supporting Information) and the ob-
tained energy levels are summarized in Table 1. 5-BTDMS and
ox
5
1
-BTMPS show oxidation onset potentials (E onset) at 0.97 and
.15 V, respectively, which are slightly higher than those of 2-
BTDMS and 2-BTMPS (0.93 and 0.95 V, respectively). According
ox
to the following Equation: HOMO=ꢀ(4.4+E onset) eV, the
HOMO energy levels are calculated to be ꢀ5.37, ꢀ5.55, ꢀ5.33,
and ꢀ5.35 eV for 5-BTDMS, 5-BTMPS, 2-BTDMS, and 2-BTMPS,
respectively. The LUMO energy levels [LUMO=ꢀ(HOMO+
E ) eV] are determined by HOMO values and optical band gaps
g
(
estimated from the onset wavelength of their UV absorptions).
The LUMO energy levels are ꢀ2.58, ꢀ2.79, ꢀ2.85, and
2.91 eV for 5-BTDMS, 5-BTMPS, 2-BTDMS, and 2-BTMPS, re-
ꢀ
spectively.
Electroluminescence
As discussed above, the connection pattern between BT group
and silole ring has impacted the PL property of the silole deriv-
atives greatly, and an influence on the EL property of the mole-
cules is also expected. Therefore, multilayer OLEDs with a con-
figuration of ITO/NPB (60 nm)/EML (20 nm)/TPBi (40 nm)/LiF
(
1 nm)/Al (100 nm) were fabricated, in which the silole deriva-
tives function as light-emitting layers (EML), N,N’-di(1-naph-
thyl)-N,N’-diphenyl-benzidine (NPB) acts as a hole-transporting
layer (HTL) and 2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-ben-
zimidazole) (TPBi) serves as an electron-transporting layer (ETL).
As illustrated in Figure 8A, 5-BTDMS and 5-BTMPS exhibit
green EL with peaks located at 512 nm. 2-BTDMS and 2-
BTMPS, however, emit yellow EL peaks at 548 and 560 nm, re-
spectively. These EL peak positions are only slightly redshifted
compared with those of the PL peak positions in solid films,
confirming that the EL emissions are indeed from the emitting
layers of the silole derivatives. The EL performance data of the
devices are summarized in Table 4. The device of 5-BTDMS
Figure 7. A) TGA and B) DSC thermograms of the silole derivatives recorded
ꢀ
1
under nitrogen at a heating rate of A) 20 and B) 108Cmin
.
pounds have good thermal stability with high decomposition
temperatures (T ) of 305–3518C, according to 5% loss of initial
weight. DSC analysis shows that all the compounds exhibit ob-
shows good performance with a turn-on voltage (V ) of 4.4 V,
d
on
ꢀ2
a maximum luminance (Lmax) of 20107 cdm , a maximum cur-
ꢀ
1
vious melting temperatures (T ) in the range of 198–2638C.
rent efficiency (hC,max) of 10.29 cdA , a maximum power effi-
m
ꢀ
1
Glass-transition temperatures (T ) of 5-BTMPS and 2-BTMPS are
ciency (hP,max) of 4.63 lmW , and a maximum external quan-
tum efficiency (hext,max) of 3.63%. The device of 5-BTMPS pro-
g
detected at 84 and 1078C, respectively. One thing that should
be pointed out is that the T and T become higher as the re-
vides comparable EL performances, with an even lower V of
d
g
on
ꢀ
1
placement of methyl group by phenyl ring at the 1,1-positions.
3.7 V, and a higher hP,max of 6.08 lmW . However, the devices
Chem. Eur. J. 2015, 21, 1 – 12
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Figure 8. A) EL spectra, B) current density–voltage–luminance characteristics, and (C) changes in current efficiency with the current density in multilayer EL de-
vices of these silole derivatives.
aggregated state. However, the competition between
[
a]
Table 4. EL performance of the new silole derivatives.
EML
the restriction of the rotational motions of phenyl
rings at 3,4-positions of silole ring and intermolecular
interactions between well-conjugated backbones
makes 2-BTDMS present only a slight increase in
emission efficiency in the aggregated state relative to
that in solution. Multilayer OLEDs were fabricated by
using these new siloles as emitters. The AIE-active 5-
BTDMS and 5-BTMPS exhibit superior EL properties
than 2-BTDMS and 2-BTMPS. The highest device per-
formance was recorded with 5-BTMPS as the emitter,
l
EL
V
on
L
max
h
C
h
P
EQE
[%]
CIE
[x, y]
ꢀ2
ꢀ1
ꢀ1
[
nm]
[V]
[cdm
]
[cdA
]
[lmW
]
5
5
2
2
-BTDMS
512
512
548
560
4.4
3.7
4.4
5.2
20107
27070
5010
10.29
10.23
3.66
4.63
6.08
1.64
0.68
3.63
3.58
1.22
0.56
[0.25, 0.49]
[0.25, 0.50]
[0.41, 0.56]
[0.43, 0.54]
-BTMPS
-BTDMS
-BTMPS
3950
1.61
[a] With a device configuration of ITO/NPB/EML/TPBi/LiF/Al. Abbreviations: EML=
ꢀ2
light-emitting layer; lEL =EL maximum; Von =turn-on voltage at 1 cdm ; Lmax =maxi-
mum.
with V , L
hC,max, hP,max, and h
of 3.7 V,
on
max
ext,max
ꢀ1
ꢀ2
ꢀ1
27070 cdm , 10.23 cdA , 6.08 lmW , and 3.58%,
of 2-BTDMS and 2-BTMPS exhibit much inferior EL data
respectively. The EL properties of these siloles once again dem-
onstrate that AIE emitters have promising applications in
OLEDs. The results presented here not only provide more evi-
dences to support the RIR mechanism of the AIE phenomenon,
but also offer a deeper insight into the structure–property cor-
relation of silole derivatives, which is conducive for the further
design of silole-based functional materials.
ꢀ
2
ꢀ1
ꢀ1
(
5010 cdm , 3.66 cdA , 1.64 lmW and 1.22% for 2-BTDMS;
ꢀ
2
ꢀ1
ꢀ1
3
950 cdm , 1.61 cdA , 0.68 lmW and 0.56% for 2-BTMPS),
which is rationally attributed to their poor solid-state PL emis-
sions. Such a huge difference is virtually demonstrating the
positive effect of AIE characteristic on the OLED device per-
formance.
Conclusion
Experimental Section
A series of silole derivatives substituted with BT groups
through different covalent bonding patterns have been suc-
cessfully synthesized and fully characterized. These new silole
derivatives possess a similar molecular structure but a different
conjugation degree, and thus, quite different absorption and
emission behaviors. For instance, 5-BTDMS with a poorly conju-
gated backbone is weakly fluorescent in solution but becomes
highly emissive in the aggregated state, presenting a typical
AIE characteristic. 2-BTDMS that is of a high conjugation
degree, however, does not exhibit the classical AIE effect: It
Materials and instruments
THF was distilled from sodium benzophenone ketyl under dry ni-
trogen immediately prior to use. All other chemicals and reagents
were purchased from J&K Scientific Ltd., and used as received
1
13
without further purification. H and C NMR spectra were mea-
sured on a Bruker AV 500 spectrometer in deuterated chloroform
using tetramethylsilane (TMS; d=0 ppm) as internal reference. UV/
Vis absorption spectra were measured on a Shimadzu UV-2600
spectrophotometer. PL spectra were recorded on a Horiba Fluoro-
max-4 spectrofluorometer. High-resolution mass spectra (HRMS)
were recorded on a GCT premier CAB048 mass spectrometer oper-
ating in MALDI-TOF mode. Single-crystal X-ray diffraction intensity
data were collected at 293 K on a Bruker-Nonices Smart Apex CCD
diffractometer with graphite monochromated MoKa radiation. Proc-
essing of the intensity data was carried out using the SAINT and
SADABS routines, and the structure and refinement were conduct-
ed using the SHELTL suite of X-ray programs (version 6.10). TGA
analysis was carried on a SHIMADZU TGA-50 under dry nitrogen at
has a much higher F value than 5-BTDMS in THF solution but
F
a much lower F value in solid film. Combining the experimen-
F
tal results and theoretical calculations, it is concluded that the
good conjugation between BT groups and silole ring lowers
rotational motions of BT groups, and makes 2-BTDMS more flu-
orescent in the solution state. It is the restriction of rotational
motions of BT rings that makes 5-BTDMS emit strongly in the
&
&
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ꢀ
1
a heating rate of 208C min . Thermal transitions were investigated
reflux for 12 h, the reaction mixture was cooled to room tempera-
ture and terminated by addition of 1m hydrochloric acid. The mix-
ture was poured into water and extracted with dichloromethane.
The organic layer was washed successively with aqueous sodium
chloride solution and water, and dried over anhydrous magnesium
sulfate. After filtration, the solvent was evaporated under reduced
pressure and the residue was purified by silica-gel column chroma-
by DSC using NETZSCH DSC-204(F1) instrument under dry nitrogen
at a heating rate of 108Cmin .
ꢀ1
Computational methods
The geometry optimization and harmonic vibrational frequency
calculations of ground state (S ) were carried out at the level of
density functional theory (DFT). Then, for the first single excited
tography using n-hexane/dichloromethane as eluent. 5-BTDMS was
0
1
obtained as green solid in 54% yield. H NMR (500 MHz, CDCl ): d
3
state (S ), the time-dependent density functional theory (TD-DFT)
(TMS)=7.59 (d, 2H, J=8.0 Hz), 7.45 (s, 2H), 7.35 (d, 2H, J=5.0 Hz),
7.16 (d, 2H, J=5.5 Hz), 7.03–6.98 (m, 6H), 6.89 (d, 2H, J=8.5 Hz),
1
[31]
was applied. The B3LYP functional was applied with 6–31G(d,p)
basis set. We have confirmed that all the frequencies obtained at
the minima of S and S states are positive, namely, the optimized
13
6.86–6.84 (m, 4H), 0.53 ppm (s, 6H); C NMR (125 MHz, CDCl ): d
3
(TMS)=155.0, 142.9, 140.4, 139.1, 138.3, 133.4, 129.9, 128.3, 127.5,
124.3, 124.1, 123.9, 123.0, 121.9, ꢀ1.8 ppm; HRMS: m/z calcd for
0
1
structures are stable. All these electronic structure calculations
[32]
+
were performed with Gaussian 09 program package. The reor-
ganization energy was obtained through the DUSHIN program,
with the total reorganization energy projected to the molecular
C H S Si: 526.1245 [M ]; found: 526.1224.
34
26 2
2
,5-Bis(benzo[b]thiophen-2-yl)-1,1-dimethyl-3,4-diphenylsilole
(
2-BTDMS): The procedure was analogous to that described for 5-
[33]
1
structure parameters relaxation.
BTDMS. Yellow solid, yield 36%. H NMR (500 MHz, CDCl ): d
3
(
(
TMS)=7.65 (d, 2H, J=8.0 Hz), 7.55 (d, 2H, J=8.0 Hz), 7.25–7.16
m, 12H), 7.05–7.03 (m, 4H), 0.81 ppm (s, 6H); C NMR (125 MHz,
13
Device fabrication
CDCl ): d (TMS)=155.0, 142.9, 140.4, 139.1, 138.3, 133.4, 129.9,
3
Glass substrates pre-coated with a 170 nm thin layer of indium tin
oxide (ITO) with a sheet resistance of 10 W per square were thor-
oughly cleaned in ultrasonic bath of acetone, isopropyl alcohol, de-
tergent, deionized water, and isopropyl alcohol and treated with
1
28.3, 127.5, 124.3, 124.1, 124.0, 123.0, 121.9, ꢀ1.8 ppm; HRMS: m/
+
z calcd for C H S Si: 526.1245 [M ]; found: 526.1221.
34
26 2
2
,5-Bis(benzo[b]thiophen-5-yl)-1-methyl-1,3,4-triphenylsilole (5-
BTMPS): The procedure was analogous to that described for 5-
BTDMS. Green solid, yield 28%. H NMR (500 MHz, CDCl ): d
O plasma for 20 min in sequence. Organic layers were deposited
1
2
ꢀ
4
3
onto the ITO-coated substrates by high-vacuum (<5ꢂ10 Pa)
thermal evaporation. A 60 nm thin hole-transporting layer NPB was
deposited. Next, a 20 nm thin of 5-BTDMS, 5-BTMPS, 2-BTDMS, or
(TMS)=7.69 (d, 2H, J=8.0 Hz), 7.49 (d, 2H, J=8.5 Hz), 7.40–7.35
(m, 5H), 7.29 (d, 2H, J=5.0 Hz), 7.06–7.01 (m, 8H), 6.91–6.90 (m,
13
4
H), 6.83 ppm (dd, 2H, J =8.5 Hz, J =1.5 Hz); C NMR (125 MHz,
1 2
2
-BTMPS was deposited to form EML. Finally, a 40 nm thin ETL of
CDCl ): d (TMS)=155.6, 140.4, 139.6, 138.9, 137.2, 135.7, 134.7,
1
1
3
TPBi was deposited to transport electrons, and to confine excitons
in the emission zone. Cathodes, consisting of a 1 nm thin layer of
LiF followed by a 100 nm thin layer of Al, were patterned using
a shadow mask with an array of 3 mmꢂ3 mm openings. Deposi-
33.5, 130.1, 129.9, 128.3, 127.6, 126.4, 126.0, 125.9, 124.0, 123.8,
+
21.7, ꢀ6.3 ppm; HRMS:): m/z calcd for C H S Si: 588.1402 [M ];
39
28 2
found: 588.1522.
ꢀ
1
ꢀ1
2,5-Bis(benzo[b]thiophen-2-yl)-1-methyl-1,3,4-triphenylsilole (2-
BTMPS): The procedure was analogous to that described for 5-
BTDMS. Yellow solid, yield 19%. H NMR (500 MHz, CDCl ): d
tion rates are 1~2 As for organic materials, 0.1 As for LiF, and
6
ꢀ
1
As for Al, respectively. EL spectra were taken by an optical ana-
1
3
lyzer, Photo Research PR705. The current density and luminance
versus driving voltage characteristics were measured by Keithley
(TMS)=7.90–7.89 (m, 2H), 7.53–7.46 (m, 7H), 7.25–7.23 (m, 6H),
1
3
7
.19–7.10 (m, 8H), 7.02 (s, 2H), 1.10 ppm (s, 3H); C NMR
2
420 and Konica Minolta chromameter CS-200, respectively.
(125 MHz, CDCl ): d (TMS)=156.2, 142.4, 140.4, 139.0, 138.3, 134.7,
3
1
1
33.4, 132.5, 130.5, 129.9, 128.6, 128.3, 127.6, 124.6, 124.2, 123.9,
Preparation of nanoaggregates
23.0, 121.8, ꢀ4.6 ppm; HRMS m/z calcd for C H S Si: 588.1402
39
28 2
+
[M ]; found: 588.1425.
Stock THF solutions of the silole derivatives with a concentration
ꢀ4
of 10 m were prepared. Aliquots of the stock solution were trans-
ferred to 10 mL volumetric flasks. After appropriate amounts of
THF were added, water was added dropwise under vigorous stir-
X-ray crystallography
Crystal data for 5-BTDMS : C34
P2/n, a=7.7430(15), b=10.568(2), c=17.025(3) ꢁ, b=77.727(2)8,
V=1388.9(5) ꢁ , Z=2, 1cald =1.260 gcm , m=0.256 mm (MoKa,
ꢀ
5
[34]
ring to furnish 10 m solutions with different water contents (0–
5vol%). The PL measurements of the resultant solutions were
H
26
S
2
Si, M =526.76, monoclinic,
w
9
3
ꢀ3
ꢀ1
then performed immediately.
l=0.71073), F(000)=552, T=293(2) K, 2qmax =52.008, 11590 mea-
sured reflections, 2728 independent reflections (R =0.0142), GOF
int
Synthesis
2
on F =0.994, R =0.0373, wR =0.1300 (all data), De 0.256 and
1
2
ꢀ
3
2
,5-Di(benzo[b]thiophen-5-yl)-1,1-dimethyl-3,4-diphenylsilole (5-
ꢀ0.348 eꢁ .
BTDMS): A solution of lithium naphthalenide (LiNaph) was pre-
pared by stirring a mixture of naphthalene (1.28 g, 10 mmol) and
lithium granular (0.07 g, 10 mmol) in dry THF (30 mL) for 4 h at Acknowledgement
room temperature under nitrogen. A solution of bis(phenylethy-
nyl)dimethylsilane (0.65 g, 2.5 mmol) in THF (20 mL) was then
added into the solution of LiNaph, and the resultant mixture was
stirred for 1 h at room temperature. After the solution was cooled
We acknowledge the financial support from the National Natu-
ral Science Foundation of China (51273053, 21404029 and
21274034). B.Z.T. thanks the support of the Guangdong Inno-
to ꢀ108C, ZnCl -TMEDA (3.2 g, 12.5 mmol) and THF (20 mL) were
2
vative Research Team Program of China (20110C0105067115),
the Research Grants Council of Hong Kong (16301614, N_
HKUST604/14 and N_HKUST620/11), the Innovation and Tech-
added. The fine suspension was stirred for 1 h at room tempera-
ture, and [PdCl (PPh ) ] (105 mg, 0.15 mmol) and 5-bromobenzo[b]-
2
3 2
thiophene (1.28 g, 6 mmol) were then added. After heating at
Chem. Eur. J. 2015, 21, 1 – 12
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nology Commission (ITCPD/17-9), and the University Grants
Committee of Hong Kong (AoE/P-03/08 and T23-713/11-1). We
thank Tian Zhang, Prof. Qian Peng, and Prof. Zhigang Shuai for
valuable discussions on theoretical calculations.
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FULL PAPER
Stay connected! Connecting benzo[b]-
thiophene with a silole ring through its
&
Luminescence
H. Nie, B. Chen, C. Quan, J. Zhou, H. Qiu,
R. Hu, S.-J. Su, A. Qin, Z. Zhao,*
B. Z. Tang*
2
- or 5-position furnishes silole deriva-
tives with totally different photolumi-
nescence and electroluminescence
properties (see figure).
&& – &&
Modulation of Aggregation-Induced
Emission and Electroluminescence of
Silole Derivatives by a Covalent
Bonding Pattern
&
&
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www.chemeurj.org
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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