Donor-acceptor type silole compounds with aggregation-induced deep-red emission enhancement: Synthesis and application for significant intensification of near-infrared photoluminescence

Article abstract of DOI:10.1039/c1cc00066g

Through efficient Foerster resonance energy transfer, the near-infrared emission (850 nm) of a guest (weak near-infrared emitter)-host (strong deep-red emitter) system is greatly enhanced by excitation of the host molecule at 508 nm, in comparison with ex

Full text of DOI:10.1039/c1cc00066g

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COMMUNICATION
Donor–acceptor type silole compounds with aggregation-induced deep-red
emission enhancement: synthesis and application for significant
intensification of near-infrared photoluminescencew
Xiaobo Du and Zhi Yuan Wang*
Received 5th January 2011, Accepted 15th February 2011
DOI: 10.1039/c1cc00066g
¨
Through efficient Forster resonance energy transfer, the near-
Nearly all the known AIEE compounds emit the light in the
visible region. To realize a red shift of emission into the NIR
spectral region, one could introduce an extended p-conjugation
or a D–A moiety in a chromophore.
infrared emission (850 nm) of a guest (weak near-infrared
emitter)–host (strong deep-red emitter) system is greatly
enhanced by excitation of the host molecule at 508 nm, in
comparison with excitation of the guest molecule at 660 nm.
Herein, we report a new series of D–A type silole compounds
with aggregation-induced enhancement of deep-red photo-
luminescence and describe a method for significant enhancement
of NIR photoluminescence through an efficient energy transfer
from an AIEE host to a weak NIR emitter.
Luminescent organic materials are fundamentally interesting
and practically useful in commercial applications such as
organic light-emitting diodes (OLEDs), chemosensors, and
bioimaging.1–3 Although many chromophores are highly
luminescent in dilute solutions, the photoluminescence is
often quenched in the solid state due to aggregation of
their chromophoric units. The aggregation-caused emission
quenching (ACEQ) is generally attributed to a nonradiative
deactivation process, such as excitonic coupling, excimer
formation, and excitation energy migration to the impurity
traps.4 Low bandgap compounds or the donor–acceptor
(D–A) type of near-infrared (NIR) chromophores are
relatively more prone to ACEQ and usually have low emission
efficiency despite their many potential applications.5 Examples
of organic solids that exhibit intense deep-red or NIR emission
with high absolute quantum yield are quite limited.6 Diluting
or doping fluorescent compounds in a host material is a
common approach to emission enhancement.7 Another unique
approach is based on the discovery made by Tang in 2001
that the luminescence of silole molecules is stronger in the
aggregate state than that in solution.8 A variety of other
luminogens, such as distyrylbenzene, fluorene, pentacene,
and pyrene derivatives, have also shown the property
of aggregation-induced emission enhancement (AIEE).9
Conceptually, high NIR photoluminescence could be realized
by using either the AIEE NIR chromophores if available or
using an AIEE chromophore as a host for the ACEQ-type
A new series of siloles 2a–e (Fig. 1) are designed to have a
D–A–p–A–D structure, in which the central silole unit acts a
p-spacer that links the two D–A units and also ensures the
AIEE property. The widely used benzothiadiazole group is
chosen as an acceptor and attached to several common
donors, in order to probe the structure–property relationship.
The synthesis of these new siloles requires the use of an
intermediate 1, which was prepared from bis(phenylethynyl)-
dimethylsilane and 4,7-dibromo-2,1,3-benzothiadiazole. The
silole core (1) was then coupled with different donor molecules
by the Stille cross-coupling reaction to afford the D–A type
silole compounds 2a–e (Fig. 1). These siloles are all soluble in
common organic solvents such as THF, dichloromethane and
toluene and fully characterized by spectroscopic methods and
elemental analysis (ESI).
The absorption and photoluminescence spectra of 2a–e were
recorded in THF and as solid films (Fig. S7, ESIw). The
absorption bands from 300 to 400 nm are attributed to p–p*
and n–p* transitions of the conjugated aromatic segments and
those at longer wavelengths are due to the intramolecular
charge transfer between the donors and acceptors. An
anticipated bathochromic shift was observed with an increase
of the electron-donating strength of donors and extension of
NIR emitters through effective Forster energy transfer.10
¨
State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences and Graduate School of Chinese Academy of Sciences,
Changchun, 130022, China. E-mail: wwjoy@ciac.jl.cn
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for 2a–2e, PL spectra of 2b in the
THF–hexane mixtures, and excitation wavelength-dependence study.
See DOI: 10.1039/c1cc00066g
Fig. 1 D–A type silole-based AIEE compounds 2a–e derived from 1.
c
4276 Chem. Commun., 2011, 47, 4276–4278
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dropped with the water fractions of 30% or less (Fig. 2).
At 40% of water fraction or higher, the molecules of 2c began
to aggregate, which results in restriction of intramolecular
rotation and the emission enhancement. At 99% of water
fraction, the PL intensity was 29 times higher than that at 30%
of water fraction. Accordingly, the fluorescence quantum yield
of 2c in the solid film is 14 times higher than that in solution.
All these D–A type silole derivatives have the AIEE
property. Although the emission enhancement of 2b and 2d
is relatively more significant than the others, their fluorescence
quantum yields are lower than 2a and 2c in the solid state. The
low quantum yield is mainly due to a heavy atom effect
brought by the thiophene group in 2b, 2d and 2e. The
fluorescence quantum yield of 2e in solution and as a film
was quite similar, presumably due to the presence of planar
fluorene units for a better p–p stacking in film or relatively
more PL quenching against the AIEE.
Fig. 2 PL spectra of 2c in the THF–water mixtures with different
water fraction (fw). Inset: correlation between the maximum PL
intensity of 2c and water fraction (fw). Excitation wavelength: 493 nm.
The efficient long-wavelength (635 nm) emission of 2c in the
solid state implies its potential use as a donor chromophore in
a guest–host system that may emit the light above 750 nm
p-conjugation length. For example, going from benzene to
thiophene donor (2a to 2b) and from thiophene to diphenyl-
aminothiophene donor (2b to 2d), a red-shift of 28 nm
(ca. 1384 cmÀ1) and 56 nm (ca. 2321 cmÀ1) in absorption
was observed respectively. In the photoluminescence spectra,
compounds without smaller thiophene groups did not have a
bathochromic shift in films than in solution, due to the
molecules not being coplanar in aggregation to increase the
conjugation degree. With the strongest donor among all these
siloles, 2d emits deep red light at 643 nm (in solution) and
666 nm (as film). A further red shift or NIR emission above
700 nm can be expected by introduction of stronger donors
and acceptors in this type of silole compounds.
from an acceptor chromophore through Forster resonance
¨
energy transfer.7b Accordingly, compound 3 was selected
(ESI), with reference to the previously known NIR emitters,12
because its absorption spectrum overlaps well with the
PL spectrum of 2c in the region of 550–750 nm (Fig. 3).
Compound 3 emits weakly above 800 nm in dilute non-polar
solvents (FF = 3.7% in toluene) and is totally non-luminous
in polar solvents and as a film (Fig. S2, ESI).
Thus, the films on quartz plate were prepared by casting a
chloroform solution of a mixture of 2c containing 3 in an
amount of 0.5%, 1%, 2%, 5% and 10% by weight. As shown
in Fig. 4, the peak in the visible region is attributed to the
emission of the host (2c), while the NIR emission comes from
the guest (3). It should be noted that the emission of 2c
decreased rapidly with increase of the doping concentration
of 3. With 5% of 3 in 2c, the visible fluorescence was almost
completely quenched, indicating an efficient energy transfer
from 2c to 3. Meanwhile, the NIR emission reached a
maximum when the doping concentration increased to 1%.
The intensity of the NIR emission gradually decreased
with further increase of doping concentration due to the
aggregation-caused quenching. The observed red shift in the
NIR emission band is also attributed to aggregation of 3
at a higher concentration which leads to enhanced radiative
transitions from lower vibronic energy levels.13
The AIEE property of the siloles 2a–e is clearly evident by
comparison of the photoluminescence in solution and in the
solid state (Table 1 and Fig. S6 in ESI). Fig. 2 clearly shows
the photoluminescence enhancement of 2c in THF–water
mixture upon addition of a nonsolvent (water) in THF. In
THF solution, 2c emits the red light of 644 nm with a rather
low quantum yield of 0.64%. By gradually adding water to its
THF solution, 2c undergo a transition from a solute to
aggregate or precipitate. Because the D–A type molecules tend
to adopt a more twisted conformation in a more polar solvent,
the excited luminogens are in the twisted intramolecular
charge transfer state that is susceptible to various nonradiative
quenching processes.4c,11 Therefore, by diluting with water
before the formation of aggregates, the PL intensity of 2c
The excitation-dependent PL of 3 in the film of 2c was then
investigated in order to study and compare the effect of the
energy transfer on the NIR emission from 3 (Fig. S3, ESI).
First of all, the films of a blend of 2c and 3 in any ratio only
emit weakly at 850 nm when being excited at 660 nm.
Secondly, the film of 3 in PMMA also emits weakly when
being excited at 660 nm and is not fluorescent at all when being
excited at 508 nm where 2c absorbs intensely (Fig. S5, ESI).
Finally, when being excited at 508 nm, the NIR emission of 3
at 850 nm is several times more intense in comparison with
excitation at 660 nm for various doping concentrations
(Fig. 5). For example, at 1% doping concentration, the NIR
PL intensity is enhanced by four times by exciting the host 2c
(508 nm) rather than the guest 3 (660 nm). The results clearly
Table 1 Absorption and emission properties of 2a–ea
Abs
PL
l
[nm]
l
[nm]
FF [%]b
max
max
THF
Film
THF
Film
THF
Film
F
F,f/FF,s
2a
2b
2c
2d
2e
436
464
493
520
501
451
482
511
544
518
575
573
644
643
588
575
624
635
666
635
2.17
0.17
0.64
n.d.
1.01
10.3
3.70
9.10
1.70
2.10
4.8
21
14
n.d.
2.1
a
In THF (10 mM) and as the film spin-coated on quartz plate.
b
Fluorescence quantum yield in THF relative to Nile Red (FF = 0.78,
= 615 in acetonitrile) and as film determined using an integrating
max
PL
l
sphere; n.d. = not detectable (signal too weak to be accurately
determined).
c
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Chem. Commun., 2011, 47, 4276–4278 4277

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Notes and references
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Fig. 4 PL spectra in the visible (left) and NIR (right) regions of
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¨
is believed to be dominant in the 2c/3 doped system.
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(Fig. S4, ESI). Similarly, the PL intensity is enhanced by
several times by exciting the host 2b (476 nm) rather than
the guest 3 (660 nm). However, the PL of 2b/3 film is weaker
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In conclusion, significant enhancement of the NIR emission
from a weak NIR emitter 3 doped in the film of 2c
demonstrates an efficient energy transfer in the guest–host
system containing the D–A type AIEE donor chromophore
and a low bandgap acceptor chromophore and implies a
potential application in NIR light-emitting diodes.
This work was supported by the National Natural Science
Foundation of China (21074132, 20920102032 and 20834001).
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Fig. 5 PL enhancement at 850 nm of the films of 2c doped with
different amounts of 3 with excitation at 508 nm vs. 660 nm.
c
4278 Chem. Commun., 2011, 47, 4276–4278
This journal is The Royal Society of Chemistry 2011