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Circularly polarized luminescence of AIE-active

chiral O-BODIPYs induced via intramolecular
Cite this: Chem. Commun., 2015,
51, 9014 energy transfer†
Received 9th March 2015,
Accepted 16th April 2015 Shuwei Zhang, Yuxiang Wang, Fandian Meng, Chunhui Dai, Yixiang Cheng* and
Chengjian Zhu*
DOI: 10.1039/c5cc01994j

www.rsc.org/chemcomm

Two AIE-active chiral BINOL-based O-BODIPY enantiomers (R/S-5) Herein, we synthesized two enantiomers based on a BODIPY
were synthesized and showed mirror-image red-color CPL induced chromophore and chiral BINOL, as well as an AIE chromo-
via intramolecular energy transfer. The chiroptical properties of the phore, tetraphenylethene (TPE), which exhibit red-color CPL
molecules indicate that the chirality of electronic ground and signals both in the molecular dispersion and aggregated states.
excited states is stable and independent of aggregation. In the molecules, three moieties have different effects: (i) BODIPY
acting as the chromophore; (ii) BINOL as the chiral perturbing
Circularly polarized luminescence (CPL) is defined as the moiety; and (iii) TPE as the AIE and UV-absorbing moiety probably
differential emission of right- versus left-circularly polarized due to TPE moieties having emissions overlapping with the
light. In the past few decades, CPL has attracted great interest absorption of the BODIPY moiety.9,10
not only due to the valuable information it provides on the The synthesized procedures of R/S-5 are outlined in Scheme 1.
chirality of the electronic excited states, but also due to its Compounds 1, 2 and 4 were synthesized according to previous
potential applications in future optical technologies.1 To date, studies.11 R/S-3 were prepared according to the reported methods.8,12
various chiral systems have been reported with CPL observa- R/S-5 were synthesized from R/S-3 and 4 via a palladium-catalyzed
tions, such as chiral lanthanide complexes, organic p-conjugated Sonogashira coupling reaction. The detailed procedures and char-
molecules, polymers, and so on.2 However, only a few organic acterizations are described in the ESI.†
compounds show CPL in the red-color region except chiral
europium complexes.3 On the other hand, most reported CPL
materials show CPL signals either in the molecular dispersion or
in the aggregated state. Therefore, CPL materials with red
emission in both states are required eagerly.
Aggregation-induced emission (AIE), originally proposed by
Tang, shows strong fluorescence in aggregation or solid states,4
which could overcome the problem of aggregation-caused quench-
ing (ACQ).5 Recently, Tang’s group reported several AIE-active
compounds with CPL properties.6 In the past few years, optically
active 1,10 -binaphthol (BINOL), which is one of the most important
C2 symmetric compounds, has been studied for CPL.7 Santiago
de la Moya and co-workers designed a novel way to prepare
CPL organic molecules from an achiral chromophore, boron
dipyrromethene (BODIPY), by perturbation with chiral BINOL.8
As is well-known, BODIPYs always have small Stokes shifts,
which could cause reabsorption affecting the optical properties.9

Key Lab of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing 210093, China.
E-mail: yxcheng@nju.edu.cn, cjzhu@nju.edu.cn; Fax: +86-25-88317761;
Tel: +86-25-83686508
† Electronic supplementary information (ESI) available: Detailed experimental
methods and additional data. See DOI: 10.1039/c5cc01994j Scheme 1 Synthesis of R/S-5.

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In this work, we used dichloromethane–hexane mixed As shown in Fig. 1(a), the UV-vis spectra of R-5 in dichloro-
solvents for optical measurements of the enantiomers due to methane–hexane mixed solvents of different ratios did not
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their good and bad solubility in the two solvents, respectively. show obvious changes. The absorption spectra exhibit two broad
We investigated their absorption and emission behaviors in the bands centered at 580 and 340 nm, which are ascribed to p–p*
mixed solvents at a fixed concentration (1.0  105 mol L1). transitions of the extended conjugated BODIPY and TPE
moieties, respectively. In Fig. 1(b), the fluorescence spectra dis-
play emission bands at 618 nm excited by 580 nm. With the
addition of hexane, the emissive intensity of R-5 increases
dramatically and the relative emission intensity can reach up
to 44-fold by comparing the hexane fraction at 90% with that at
0%. The fluorescence quantum yields (FF) of R-5 were measured
by using cresyl violet as a reference in the molecular dispersion
and aggregated states.13 Its FF is 1.4% in dichloromethane
solution, and the FF value can reach up to 58.2% with hexane
fraction at 90% in the mixed solvents. These results demonstrate
the molecule as having typical AIE features.
The fluorescence spectra of R-5 display broad emission
bands in the mixed solvents. Comparing the absorption with
emission spectra, there is an overlapped region from 580 to
620 nm, which could cause reabsorption, influencing the optical
properties. As shown in Fig. S1 and S2 (ESI†), the emission of
isolated 4 takes place in the BODIPY absorption-region of R-6,
and the excitation spectrum was found to match the absorption
spectrum recorded over the entire spectral range. Therefore,
intramolecular energy transfer could take place in the molecule
if 340 nm was used as the excited wavelength. The fluorescence
spectra in the Fig. 1(c) are recorded by excitation at 340 nm, and
displayed a slightly smaller fluorescence intensity compared to
those excited by 580 nm. The molecule also showed AIE features
in this case, and the relative intensities can reach up to 42-fold
with the hexane fraction at 90%. The results indicate that intra-
molecular energy transfer could take place from TPE to BODIPY
moieties effectively. The AIE features of R-5 with red-color
emission can be observed directly via the fluorescence images
of their solutions taken under a commercially available UV lamp
(Fig. 1(d)). DLS data showed that the molecule aggregated
to nanoparticles with hexane fractions at 30, 60 and 90%,
respectively, and their sizes increased with increasing hexane
fractions (Fig. S3, ESI†).
The CD spectra are shown in Fig. 2. R/S-5 exhibited mirror-
image CD bands in the solvent mixtures. The bands at 580 nm
of R/S-5 with negative/positive signals indicate that the chirality
of BINOL successfully transfers to the BODIPY chromophore.
Similar to the UV-vis spectra, there are no obvious changes in
the CD spectra with the increasing hexane fraction. To illustrate
this phenomenon, we performed the CD spectra of R-5 by
changing the temperature and concentration (Fig. S4 to S7,
ESI†). The CD spectra in different solvents showed no obvious
changes. Herein we used chloroform instead of dichloromethane
for temperature-dependent CD spectra due to its higher boiling
point. The results showed that the concentration-dependent
CD spectra increased obviously, but no apparent changes were
Fig. 1 (a) UV-vis spectra; (b) fluorescence spectra excited by 580 nm; (c)
observed for those of temperature, which indicates that the mole-
fluorescence spectra excited by 340 nm; (d) plot of (I/I0) values versus the
compositions of the hexane fractions excited by 580 and 340 nm, inset:
cules are molecular dispersed in good solvents. Meanwhile, we
photographs taken under UV illumination (365 nm); of R-5 in dichloro- measured the temperature-dependent CD spectra in different ratios
methane–hexane mixtures. Solution concentration: 1.0  105 mol L1. of the mixed solvents, which also showed no obvious changes.

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circularly polarized luminescence, respectively.14 The glum values

of R/S-5 at 630 nm were about 0.002, which are in the range of
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most CPL materials from 105 to 102.

In summary, we have designed and synthesized the enantio-
mers R/S-5 possessing obvious AIE features. The intramolecular
energy transfer from the TPE to BODIY moieties in the molecules
could induce the two enantiomers with red-color mirror-image
CPL signals. Interestingly, the chiroptical properties on CD and
CPL do not change in both molecular dispersion and aggregated
states, which indicates that the chirality of electronic ground and
excited states is stable and independent of aggregation. Based on
the AIE features, the intensity of CPL (DI) for the molecules can
be adjusted via changing the ratios of the mixed solvents.
Fig. 2 CD spectra of R/S-5 in dichloromethane–hexane mixtures. We would like to thank Prof. Zhiyong Tang and Dr Lin Shi at
Solution concentration: 1.0  105 mol L1. NCNST for their kind help with CPL measurements. This work
was supported by the National Natural Science Foundation of China
(21174061, 51173078, 21172106 and 21474048) and the open
These results indicate that the CD spectra are independent of project of Beijing National Laboratory for Molecular Sciences.
aggregation. The absorption dissymmetry factors (gabs) of R/S-5
obtained from the CD spectra were about 0.001 at 580 nm.
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