Structural modulation of solid-state emission of 2,5- bis(trialkylsilylethynyl)-3,4-diphenylsiloles

Full text of DOI:10.1002/anie.200903698

Communications
DOI: 10.1002/anie.200903698
Emissive Siloles
Structural Modulation of Solid-State Emission of
2,5-Bis(trialkylsilylethynyl)-3,4-diphenylsiloles**
Zujin Zhao, Zhiming Wang, Ping Lu, Carrie Y. K. Chan, Dandan Liu, Jacky W. Y. Lam,
Herman H. Y. Sung, Ian D. Williams, Yuguang Ma, and Ben Zhong Tang*
Development of efficient luminescent materials is a topic of
current interest. Understanding and manipulation of their
light-emitting behaviors and performances in the solid state is
of great importance because the materials are commonly used
as thin films in their real-world applications.[1] In the solid
state, the luminogenic molecules are located in close vicinity
and are inclined to form aggregates. Chromophore aggrega-
tion can quench light emission, as short-range intermolecular
interactions favor the formation of such detrimental species
as excimers.[2] Besides this negative effect, aggregation can
positively restrict intramolecular rotation (IMR), which
rigidifies molecular conformations and blocks nonradiative
decay channels.[3] Whether the aggregation weakens or
strengthens light emission is decided by the competition
between these two antagonistic physical effects, which is
internally determined by the chemical structures of the
luminogenic molecules.[4]
the substituents affect the packing arrangements and emission
behaviors of the luminogens in the solid state.
The synthetic routes to the silole derivatives are presented
in Scheme 1.[5,6] The key intermediate 2,5-dibromosiloles (3)
were prepared by bromination of 2 with NBS. Cross-coupling
In our previous studies, we found that the positive effect
prevails in propeller-shaped molecules, such as hexaphenylsi-
lole (HPS) and tetraphenylethylene.[3] These molecules are
non-emissive in solutions because of the active IMR processes
of their multiple rotors but are induced to luminesce by
aggregate formation owing to the restriction of their IMR
processes in the condensed phase. These molecules have been
referred to as “aggregation-induced emission” (AIE) lumi-
nogens. To enlarge the family of AIE luminogens and to gain
insight into their structure–property relationships, herein we
synthesized a group of new silole derivatives with systemati-
cally varied substituents at the 1,1- and 2,5-positions and
studied how molecular structure, especially steric effects, of
Scheme 1. Synthesis of silole derivatives. Np=naphthyl, tme-
da=N,N,N’,N’-tetramethylethylenediamine, NBS=N-bromosuccini-
mide.
[*] Dr. Z. Zhao, Dr. P. Lu, C. Y. K. Chan, Dr. J. W. Y. Lam,
Dr. H. H. Y. Sung, Prof. Dr. I. D. Williams, Prof. Dr. B. Z. Tang
Department of Chemistry, The Hong Kong University of Science &
Technology
of 3 with alkynes gave the target products (4–6) in high yields
(83–95%). The detailed synthetic procedures and character-
ization data are given in the Supporting Information. Single
crystals of 4a, 5a, 5c, and 6a–c were grown from their THF/
methanol solutions and analyzed by X-ray diffraction crys-
tallography. The crystal structures of the silole derivatives are
shown in Figure 1, and their detailed crystal analysis data are
given in Tables S1 and S2 in the Supporting Information.
The energy levels of the highest occupied and lowest
unoccupied molecular orbitals (HOMO and LUMO) of 4–6
were calculated using the B3LYP/6-31G* basis set. The data
are summarized in Tables S3–S5 and Figure S1 in the
Supporting Information, and those for 6a are plotted in
Figure 1 as an example. The orbitals are dominated by the
contributions from the silole rings and the 2,5-ethynyl triple
bonds, thanks to their orbital overlap and electronic commu-
Clear Water Bay, Kowloon, Hong Kong (China)
Fax: (+852)2358-1594
E-mail: tangbenz@ust.hk
Homepage: http://home.ust.hk/~tangbenz/
Z. Wang, D. Liu, Prof. Dr. Y. Ma
State Key Laboratory of Supramolecular Structure and Materials
Jilin University, Changchun 130012 (China)
[**] We thank the Research Grants Council of Hong Kong (603008 and
601608), the Ministry of Science & Technology of China
(2009CB623605) and the National Science Foundation of China
(20634020). B.Z.T. is grateful to Cao Gaungbiao Foundation of
Zhejiang University.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903698.
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Similar to HPS, the silole derivatives become stronger
emitters when they are aggregated. As can be seen from the
example shown in Figure 2a, the emission of 4b is intensified
when a large amount of water (fw > 70%) is added into THF.
Figure 1. Molecular structures of 4a, 5a, 6a, 6b, 5c, and 6c (see the
Supporting Information for detailed crystal analysis data) and molec-
ular orbital amplitude plots of HOMO and LUMO energy levels of 6a
calculated using the B3LYP/6-31G* basis set (see the Supporting
Information for the plots for other silole derivatives).
nication. The HOMO energy levels of the silole derivatives
are located in the range of ꢀ5.28 to ꢀ5.31 eV, comparable to
that of dimethyltetraphenylsilole (DMTPS; ꢀ5.29 eV).[7]
Their LUMO energy levels, however, are from ꢀ2.10 to
ꢀ2.18 eV, lower than that of DMTPS (ꢀ1.59 eV), thus
indicating that these new silole derivatives have higher
electron affinities.
Dilute THF solutions of 4–6 show absorption peaks (lab)
at approximately 400 nm, which arise from the p–p* tran-
sition of the 2,5-diethynylsilole moiety (Table 1). Changing
the 1,1-substituents from two methyl groups to one methyl
and one phenyl to two phenyl groups progressively red-shifts
the lab of the silole derivatives, owing to the gradually
strengthened inductive effect.[5] The silole solutions emit
weakly, because their active IMR processes annihilate the
excitons. Their fluorescence quantum yields (FF) are low
(0.32–1.67%), but are somewhat higher than that of HPS
(0.1%),[6] owing to their more conjugated electronic struc-
tures.
Figure 2. a) Emission spectra of 4b in THF/water mixtures with differ-
ent fractions of water (fw). b) Emission spectra of amorphous powders
of 4b, 5b, and 6b. c) Polarized emission spectra of crystals of 6a;
excitation wavelength: 370 nm; Iv =intensity in vertical direction
(c), Ih =intensity in horizontal direction (a), P=degree of polar-
ization [P=(IvꢀIh)/(Iv + Ih)]. d) Photographs of the powders of 4b, 5b,
and 6b and the crystals of 6a taken under UV illumination.
The higher the water content, the stronger the light emission.
Since water is a nonsolvent of 4b, the silole molecules must
have aggregated in the aqueous mixtures with high water
contents. This result verifies that the silole emission is
enhanced by aggregate formation. High water content
populates the silole aggregates, thereby boosting the light
emission. A similar emission-enhancement effect is observed
in other silole systems, thus revealing that the silole deriva-
tives are all AIE luminogens.
Table 1: Absorption and emission characteristics of silole derivatives 4–6
in solution, amorphous, and crystalline states[a]
The nice results obtained from the aggregates suspended
in aqueous media prompted us to further study the silole
emissions in the solid state. The amorphous powders of 4b,
5b, and 6b fluoresce at 542, 524, and 489 nm (Figure 2b) in
efficiencies of 33.3, 76.4, and 99.9% (Table 1), respectively. A
similar trend is observed in the series of 4a!5a!6a and
4c!5c!6c (Table 1). Evidently, when 2,5-substituents in a
silole molecule become bulkier (methyl!ethyl!isopropyl),
its emission color (lem) is hypsochromically shifted (Fig-
ure 2d) and its emission efficiency (FF) is generally enhanced.
It is worth noting that the silole molecules with dissimilar
1,1-substituents always stand out and show the highest FF
values among their congeners: for example, in the family of
6a–c, 6b exhibits the highest emission efficiency (99.9%).
This finding is similar to what we have observed in other AIE
Silole
lab [nm]
Soln
lem [nm]
Amor
FF [%]
Amor[c]
Soln
Cryst
520
Soln[b]
4a
4b
4c
5a
5b
5c
6a
6b
6c
400
397
393
403
398
394
403
399
396
485
486
486
488
486
484
491
488
489
518
542
544
486
524
535
484
489
481
0.75
0.41
0.32
1.03
0.46
0.24
1.67
0.74
0.39
26.8
33.3
26.1
39.9
76.4
62.6
76.6
99.9
57.1
496
536
495
501
498
[a] Abbreviations: Soln=solution (10 mm in THF), Amor=amorphous,
Cryst=crystal, lab =absorption maximum, lem =emission maximum,
FF =fluorescence quantum yield. [b] Estimated using 9,10-diphenyl-
anthracene (FF =90% in cyclohexane) as the standard. [c] Measured
using an integrating sphere.
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systems: the silole molecules with different 1,1-substitutions
formed between two phenyl rings in neighboring molecules.
ꢀ
display higher FF values, because they are difficult to pack
compactly in the solid state, and their excitons are less likely
to be quenched by intermolecular interactions.[3]
These multiple C H···p hydrogen bonds help rigidify the
molecular conformation and restrict IMR processes, thus
making the crystals of 4a highly emissive. During crystalliza-
tion, the silole molecules may adjust their conformations to fit
into the crystalline lattices, and the silole core and 2,5-ethynyl
rods may become more coplanar. The planarized silole
molecules possess better electronic conjugation and experi-
ence stronger intermolecular interactions in the crystal cells,
thereby causing the silole emission to red shift.
In our previously studied silole systems, aggregate for-
mation dramatically boosted the emission efficiency but
caused little shift in the emission color in comparison to the
data for dilute solution.[3] In contrast, the emission colors of 4–
6 vary with aggregate formation (Table 1). The emission
maximums (lem) for amorphous powders of 4 and 5 are
generally red-shifted from those of their solutions (e.g.,
Dlem = + 58 nm for 4c), while the opposite effect is observed
in the system of 6 (e.g., Dlem = ꢀ8 nm for 6c). Another
difference is that the lem values for the amorphous powders of
On the other hand, the steric effects of the bulky
triisopropylsilyl substituents give rise to wide separation
between the molecules in the crystal structure, as evidenced
by the large interplane distance (d3 = 6.629 ꢀ) between the
ethynyl rods in two neighboring molecules of 6a (Figure 3b).
This situation leads to weakened intermolecular interactions
and accounts for the smaller red shift of the crystal emission
from the solution emission (Dlem = 4 nm). The molecules of
6a are stuck in an antiparallel fashion in the crystal lattice
(Figure 3c), which effectively restricts IMR processes and
dramatically boosts the emission efficiency.
Orderly packed molecules in the solid state can emit
polarized light. Polarized emission, however, is often
quenched in “conventional” luminophoric materials owing
to strong p stacking interactions in their solids. Thanks to the
unique AIE nature, 4–6 emit intensely in the condensed
phase. Remarkably, their crystals display emission anisotropy,
with stronger emission in the vertical direction. The polarized
emission spectra for the crystals of 6a are shown in Figure 2c.
The ratio of emission intensities (Iv/Ih) and the degree of
polarization (P) are 5.3 and 0.68, respectively. These values
are rather high, close to those for single-crystal nanorods of
CdSe, a highly emissive inorganic semiconductor.[8] While
crystals cannot be grown from the solution of 4c, it readily
forms microfibers on a quartz plate. The microfibers emit
green, yellow, and red light when excited at different wave-
lengths (Supporting Information, Figure S2), thus realizing
multiple color emissions from one luminogenic molecule by
simply changing the excitation wavelength.
4 and 5 are red-shifted in the order of a!b!c (cf. lem
=
518 nm for 4a, lem = 544 nm for 4c), but those in the system of
6 are generally blue-shifted (cf. lem = 484 nm for 6a, lem
481 nm for 6c).
=
We further investigated the emission behaviors of the
aggregates of 4–6 in the crystalline state. Compared to their
isolated species in the dilute solutions, the crystals of the silole
derivatives all emit further into the red spectral region. The
extent of the spectral shift varies with the molecular structure.
For example, the crystals of the silole dye carrying smaller
trimethylsilyl substituents (4a) furnish a lem of 520 nm, which
is 35 nm red-shifted from the corresponding value in solution.
The silole derivatives bearing larger triisopropylsilyl groups
(6) display much smaller red shifts when they are crystallized.
This finding suggests that the bulky 2,5-substituents help
reduce intermolecular interactions. The light emissions of the
crystals of 4–6 are also influenced by the 1,1-substitutions. The
phenyl group works better in terms of suppressing intermo-
lecular interactions than the methyl group because of the
larger size of the aromatic ring. This property is manifested by
the crystal emissions of 5a and 5c: the former emits at
496 nm, while the latter emits at 536 nm.
In an effort to understand the mechanism operating in the
AIE system, we checked geometric structures and packing
arrangements of the silole molecules in the crystalline state.
ꢀ
As can be seen from the example shown in Figure 3a, a C
In summary, we have synthesized a group of silole
derivatives with systematically varying 1,1- and 2,5-substitu-
ents. The solid-state emissions of the luminogens are found to
be tunable by changing the molecular structure, especially the
steric effects, of the substituents. Bulkier substituents help
weaken the intermolecular interactions and restrict the IMR
process, thereby blue-shifting the emission color and dramat-
ically boosting the emission efficiency (FF up to 99.9%).
Polarized and multicolor emissions are realized in silole
crystals and fibers, respectively. The information gained in
this work on the structure–property relationships in the AIE
system is of great value in terms of guiding our future
molecular engineering endeavors in designing molecular
structures for new luminogenic materials with desired light-
emitting properties in the solid state.
H···p hydrogen bond (d1 = 3.077 ꢀ) is formed between a
hydrogen atom of the trimethylsilyl group in one molecule of
4a and the p electron cloud of a phenyl ring in another
ꢀ
molecule. A C H···p hydrogen bond (d2 = 2.749 ꢀ) is also
Figure 3. a) Packing arrangement in the crystal structure of 4a, with
ꢀ
C H···p hydrogen bonds marked by dashed lines. b) Top and c) side
views of adjacent molecules of 6a in its crystal structure.
Received: July 7, 2009
Published online: September 4, 2009
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Keywords: aggregation-induced emission · fluorescence ·
luminescence · siloles · structure–activity relationships
.
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