Aggregation induced ratiometric fluorescence change for a novel boron-based carbazole derivative

Article abstract of DOI:10.1039/c2cc34345b

Nanoparticles of a novel boron-based carbazole derivative have been reported. They exhibit efficient green fluorescence, aggregation induced ratiometric fluorescence change and green/blue fluorescent switching to sense VOCs.

Full text of DOI:10.1039/c2cc34345b

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COMMUNICATION
Aggregation induced ratiometric fluorescence change for a
novel boron-based carbazole derivativew
He-ping Shi,* Jian-xin Dai, Li-wen Shi, Mei-hua Wang, Li Fang,* Shao-min Shuang and
Chuan Dong*
Received 18th June 2012, Accepted 6th July 2012
DOI: 10.1039/c2cc34345b
Nanoparticles of a novel boron-based carbazole derivative have been
reported. They exhibit efficient green fluorescence, aggregation
induced ratiometric fluorescence change and green/blue fluores-
cent switching to sense VOCs.
biomolecule assays.6 A fluorescent probe which is dependent
on the changes of the fluorescence intensity can be easily
influenced by the excitation energy and the detection sensitivity.
Furthermore, fluorescence intensity changes are not suitable for
direct observation with eye.16 Therefore, the fluorescent probe
with fluorescence color change is highly preferred because it can
be measured directly with a colorimeter or even distinguished
easily by eye.
The design and investigation of organic chromophores possessing
excellent photophysical properties in the solid or nanoparticles
state have been an imperative research topic1 for their
potential applications in non-doped organic light emitting
diodes (OLEDs),2 liquid crystals3 and fluorescence sensors.4–6
It is well known that organic compounds are usually weakly
emissive with an increase in their solution concentrations
or become non-emissive when they are aggregated due to
aggregation-caused quenching (ACQ).7 Since the chromo-
phores are commonly used as thin films, nanoparticles or
sensory materials in the aqueous media, this phenomenon
has greatly limited the scope of their applications. Fortunately,
an intriguing phenomenon that some silole derivatives are weakly
or non-emissive in solutions but become highly fluorescent when
they are aggregated, which is referred to as aggregation induced
emission enhancement (AIEE), was successfully developed by Tang
et al.8 Various kinds of organic molecules with the AIEE pheno-
menon have been developed by many research groups since then,
such as hetero-oligophenylene,5 poly(N-alkyl-2,7-carbazole-
diyl) derivatives,9 poly(phenylenevinylene) derivatives,10 and
9,10-distyrylanthracene derivatives.11
In this communication, we report a novel ICT compound,
named 1,4-bis(3,6-bis(dimesitylboryl)carbazol-9-yl)benzene (1)
(Scheme 1). This compound belongs to the D–p-A system
containing peripheral dimesitylboron groups (electron-acceptors)
and 1,4-bis(carbazol-9-yl)benzene as the core (electron-donor).
Photophysical properties and AIEE behavior of 1 are investigated
in this work. The unique AIEE phenomenon with ratiometric
fluorescence changes and remarkable color change can provide
a new way to design useful ratiometric biomolecular sensors.
Moreover, the response to volatile organic compounds (VOCs)
is discussed to demonstrate their potential applications as
sensory materials.
The AIEE phenomenon is possibly attributed to the intra-
molecular rotation, intramolecular planarization, restricted excimer
formation or a specific aggregation (H- or J-aggregation) in the
aggregated state,17 but the causes of this phenomenon in ICT
compounds are still unclear. Therefore, a derivative of 1 has
also been synthesized by the introduction of hexyloxy groups
at the 2,5-positions of the benzene ring, in order to understand
the mechanism of the special fluorescence changes.
Until now examples of the AIEE systems are still quite
limited especially for the intramolecular charge transfer (ICT)
compounds.12 D–p-A structures of the ICT compounds
generally cause strong intermolecular H-aggregate p–p inter-
actions in the aggregated state and consequently exhibit weak
or no fluorescence in most cases.13 For this reason, only a small
amount of the ICT compounds has been reported. However,
emission wavelengths of these compounds in the aggregated
state are similar to those in solution14 and many of the ICT
compounds possess low fluorescence efficiency,15 which may
restrict their applications as sensing materials in the area of
Compounds 1 and 2 were readily prepared by Ullmann reaction,
subsequent bromination with NBS and lithiation with t-BuLi
School of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan, 030006, P. R. China. E-mail: hepingshi@sxu.edu.cn;
Tel: +86 351 7018094
w Electronic supplementary information (ESI) available: Synthesis,
spectra and theoretical calculations. See DOI: 10.1039/c2cc34345b
Scheme 1 Chemical structures of 1 and 2.
c
8586 Chem. Commun., 2012, 48, 8586–8588
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and dimesitylboron fluoride. We initially investigated the
absorption and fluorescence spectra of 1 in various solvents
(5 mM) of different polarity (Fig. S1, ESIw). The absorption
spectra of 1 demonstrate quite similar maxima in the different
solvents meaning the slight solvent-dependence. Compound 1
exhibits a strong absorption band at 350–360 nm. In contrast
to the slight solvatochromism in absorption spectra, fluores-
cence spectra are dependent on the solvent system polarity,
compound 1 displays remarkable solvatochromism. With the
increasing polarity of the solvents, the fluorescence intensity
decreased and a bathochromic shift of 45 nm ranging from
371 nm (in hexane) to 416 nm (in acetone) was observed. It
indicated that ICT took place from the carbazole core to the
dimesitylboron groups during the excitation process because
polar solvents are propitious to the ICT process.18 Theoretical
calculations have proven to be a reliable tool for qualitative
indication of the excitation and emission properties. There-
fore, we calculated frontier molecular orbitals, absorption and
fluorescence spectra in THF solvent. Further details are given
in the ESIw and the results are in good agreement with the
experimental absorption and fluorescence spectra.
Fig. 2 Absorption spectra (a) and fluorescence spectra (b) of 1
(5 mM) in the THF–water mixture with different fractions of water
(fw). Excited at 350 nm.
the system. Upon further addition of water, when the water
fraction is higher than 40% (Fig. 1b), the nanoparticles start to
form, the blue emission band centered at 395 nm gradually
‘‘turns-off’’ along with the green emission band at 485 nm rapidly
‘‘turning-on’’, that is, the color of the emission changes vividly from
blue to green after addition of water (Fig. 2b). In addition, the
fluorescence quantum yield (F) increases from 0.23 in THF solution
to 0.36 in THF–water mixture due to the formation of nano-
particles. Notably, 1 exhibits high fluorescence quantum yields in
the nanoparticles state, indicating its potential applications as an
excellent non-doped material in OLEDs. Moreover, such a ratio-
metric fluorescence change with a large bathochromic shift of
90 nm in an ICT compound has so far never been reported, not
to mention the colour changes. This unique AIEE phenomenon
is very significant in the field of biomolecular sensors.
Then, the nanoparticles of 1 were prepared by using a
simple precipitation method in the absence of surfactants. 1
must have aggregated into nanoparticles in the THF–water
mixture because water is a non-solvent for 1 in THF. The
mixture was stable even after two months with no precipitates.
Such long-term stability has already been reported for several
surfactant-free organic nanoparticles in the THF–water
mixture.19,20 The shape of nanoparticles was investigated by
FE-SEM and TEM. Fig. 1a shows FE-SEM images of nano-
particles from THF–water (1 : 9) mixture solution onto a
conductive tape substrate, which displays a large number
of discrete nanoparticles. In addition, nanoparticles with a
diameter of about 40–60 nm are clearly observed by TEM
images (the inset of Fig. 1a), which are in good agreement with
those shown in the FE-SEM images.
In view of the different fluorescence spectra in solution and
nanoparticles, the response of compound 1 to volatile organic
compounds (VOCs) was investigated through thin-layer chroma-
tography (TLC). Under illumination of a UV light (365 nm) at
room temperature, the nanoparticles on a TLC plate emit green
fluorescence which switches to sapphire blue in the atmosphere of
chloroform vapor. The color can be recovered after the vapor is
removed. Because of the liquid formation of the solvent on the
TLC plate, compound 1 on the TLC plate is dissolved, leading
to fluorescence colour change. Similar remarkable fluores-
cence variation for 1 can be also observed for other VOCs
such as acetone, dichloromethane and THF.
The absorption spectra of 1 in THF solution and THF–water
mixture are quite similar (Fig. 2a). Only a slight bathochromic
shift and a level-off tail can be observed. The tail is attributed to
the Mie scattering effect suggesting the formation of nanoparticles.
In contrast to the absorption behavior, a dramatic change of
the fluorescence spectra of 1 from the THF solution to the
THF–water mixture can be observed. Upon addition of water,
the emission intensity and wavelength show a slight decrease
and red shift, respectively, owing to the increasing polarity of
Encouraged by the significant results, we designed and
synthesized compound 2 to investigate the structural effect
on the AIEE phenomenon. As shown in Fig. 3a, 1 and 2 have
similar maximum absorption and fluorescence wavelengths in
pure THF solution. Theoretical calculations of 2 were also
carried out. The calculated MOs, absorption and fluorescence
spectra of 2 are quite similar to those of 1. The calculated data
are in good agreement with experimental results, indicating
Fig. 1 (a) FE-SEM and TEM (the inset) images of the nanoparticles
prepared in the THF–water (1 : 9) mixture. (b) Fluorescence intensity
ratio changes (I485/I395) of 1 (5 mM) in the THF–water mixture with
different fractions of water (fw). Excited at 350 nm.
Fig. 3 (a) Absorption and fluorescence spectra of 1 and 2 (5 mM) in
THF. (b) Fluorescence spectra of 2 (5 mM) in the THF–water mixture
with different fractions of water (fw). Excited at 350 nm.
c
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that hexyloxy groups do not have much effect on the optical
properties of the isolated molecular state.
Notes and references
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In conclusion, we have reported the nanoparticles of a novel
boron-based carbazole derivative with a diameter of about
40–60 nm. They exhibit not only efficient green fluorescence
which is suitable for non-doped OLED materials but also
unique aggregation induced emission enhancement with ratio-
metric fluorescence change which is very useful in the field of
biomolecular sensors. Theoretical calculations were carried
out and the results were in good agreement with the experi-
mental spectra. Moreover, the response to VOCs with green/
blue fluorescent switching was also demonstrated in this
communication. This ratiometric fluorescence change is
probably attributed to the intermolecular donor–acceptor
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This work was supported by National Science Foundation
of China (21175086 and 21175087).
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8588 Chem. Commun., 2012, 48, 8586–8588
This journal is The Royal Society of Chemistry 2012