Circularly polarized luminescence of AIE-active chiral O-BODIPYs induced via intramolecular energy transfer

Article abstract of DOI:10.1039/c5cc01994j

Two AIE-active chiral BINOL-based O-BODIPY enantiomers (R/S-5) were synthesized and showed mirror-image red-color CPL induced via intramolecular energy transfer. The chiroptical properties of the molecules indicate that the chirality of electronic ground

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DOI: 10.1039/C5CC01994J
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COMMUNICATION
Circularly Polarized Luminescence of AIE-active
Chiral O-BODIPYs Induced from Intramolecular
Energy Transfer
Cite this: DOI: 10.1039/x0xx00000x
Shuwei Zhanga, Yuxiang Wanga, Fandian Menga, Chunhui Daia, Yixiang
Cheng*a and Chengjian Zhu*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two
AIE-active
chiral
BINOL-based
O-BODIPY CPL signals both in molecular dispersal and aggregated states.
enantiomers (R/S
-5) were synthesized and showed mirror- In the molecules, three moieties have different effects: i) BODIPY
image red-color CPL induced from intramolecular energy as the acting chromophore; ii) BINOL as the chiral perturbing
transfer. The chiroptical properties of the molecules indicate moiety; iii) TPE as AIE and UVꢀabsorbing moiety probably due to
that the chirality of electronic ground and excited states is TPE moieties having emission overlapping with the absorption of
stable and independent of aggregation.
the BODIPY moiety.9, 10
The synthesized procedures of R/Sꢀ5 are outlined in Scheme 1.
Circularly polarized luminescence (CPL) is defined as the
differential emission of rightꢀ versus leftꢀ circularly polarized light.
In the past few decades, CPL has attracted great interest not
only due to the valuable information on the chirality of the
electronic excited states, but also its potential applications in
future optical technologies.1 To date, various chiral systems have
been reported with CPL observations, such as chiral lanthanide
complexes, organic πꢀconjugated molecules, polymers, and so
on.2 However, only a few organic compounds show CPL in redꢀ
color region except chiral europium complexes.3 On the other
hand, most reported CPL materials show CPL signals either in
molecular dispersal or aggregated state. Therefore, CPL
materials with red emission in both states are required eagerly.
Aggregationꢀinduced emission (AIE) originally proposed by
Tang shows strong fluorescence in aggregation or solid states,4
which could overcome the problem of aggregationꢀcaused
quenching (ACQ).5 Recently, Tang's group reported several AIEꢀ
active compounds with CPL properties.6 In the past few years,
optically active 1,1'ꢀbinaphthol (BINOL), which is one of the most
important C2 symmetric compounds, has been studied for CPL.7
Santiago de la Moya and coꢀworkers designed a novel way to
prepare CPL organic molecules from an achiral chromophore,
boron dipyrromethene (BODIPY), by perturbing with chiral
BINOL.8 As it is wellꢀknown, BODIPYs always have small Stokes’
shifts, which could cause reabsorption affecting the optical
properties.9 Herein, we synthesized two enantiomers based on
BODIPY chromophore and chiral BINOL, as well as on an AIE
chomophore, tetraphenylethene (TPE), which exhibit redꢀcolor
Compounds
R/Sꢀ
were prepared according to the reported methods.8, 12 R/Sꢀ
were synthesized from R/Sꢀ and by palladiumꢀcatalyzed
1, 2 and 4
were synthesized from previous works.11
3
5
3
4
Sonogashira coupling reaction. The detailed procedures and
characterizations are described in ESI.
Scheme 1 Synthesis of R/S-5.
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shown in Fig. 1(a), the UVꢀvis spectra of Rꢀ
5
in
dichloromethane/hexane mixed solvents of different ratios did not
show obvious changes. The absorption spectra exhibit two broad
bands centered at 580 and 340 nm, which are ascribed to πꢀ
π*
transitions of the extended conjugated BODIPY and TPE
moieties, respectively. In Fig. 1(b), the fluorescence spectra
display emission band at 618 nm excited by 580 nm. With the
hexane addition, the emissive intensity of Rꢀ
5 increases
dramatically. And the relative emission intensity can reach to 44ꢀ
folds compared the hexane fraction at 90% with that of 0%. The
fluorescence quantum yields (ΦF) of Rꢀ5 were measured by
using cresyl violet as a reference in the molecular dispersal and
aggregated states.13 Its ΦF is 1.4% in dichloromethane solution,
and the ΦF value can reach to 58.2% with hexane fraction at 90%
in the mixed solvents. These results demonstrate the molecule
having typical AIE feature.
The fluorescence spectra of Rꢀ5 display broad emission band
in the mixed solvents. Compared the absorption with emission
spectra, there is an overlapped region from 580 to 620 nm, which
could cause reabsorption influencing on optical properties. As
shown in Figure S1 and S2, the emission of isolated
4 takes
place in the BODIPY absorptionꢀregion of Rꢀ , and the excitation
6
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 exciting at 340 nm, and displayed a little smaller
fluorescence intensity compared to those excited by 580 nm. The
molecule also showed AIE feature in this case, and the relative
intensities can reach to 42ꢀfolds with the hexane fraction at 90%.
The results indicate that intramolecular energy transfer could
take place from TPE to BODIPY moieties effectively. The AIE
feature of Rꢀ5 with redꢀcolor emission can be observed directly
by 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
fraction at 30, 60 and 90%, respectively, and the sizes of them
increased with hexane fractions increasing (Figure S3).
The CD spectra are shown in Fig. 2. R/Sꢀ
image CD bands in the solvent mixtures. The bands at 580 nm of
R/Sꢀ with negative/positive signals indicate that the chirality of
5 exhibited mirrorꢀ
5
BINOL successfully transfers to the BODIPY chromophore.
Similar to the UVꢀvis spectra, there are no obvious changes in
the CD spectra with hexane fraction increased. To illustrate this
phenomenon, we performed the CD spectra of Rꢀ5 by changing
the temperature and concentration (Figure S4 to S7). The CD
spectra in different solvents showed no obvious changes. Herein
we used chloroform instead of dichloromethane for temperatureꢀ
dependent CD spectra due to its higher boiling point. The results
showed that the concentrationꢀdependent CD spectra increased
obviously, but no apparent changes for those of temperature,
which indicates that the molecules are molecular dispersal in
good solvents. Meanwhile, we measured the temperatureꢀ
dependent CD spectra in different ratios of the mixed solvents,
which also showed no obvious changes. These results indicate
that the CD spectra are independent of aggregation. The
Fig. 1 (a) UV-vis spectra; (b) Fluorescence spectra excited by 580 nm; (c)
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:
Photographs taken under UV illumination (365 nm); of R-
5
in
dichloromethane/hexane mixtures. Solution concentration: 1.0×10-5 mol L-1.
In this work, we used dichloromethane/hexane mixed solvents
for optical measurements of the enantiomers due to their good
and bad solubility in the two solvents, respectively. We
investigated their absorption and emission behaviors in the
mixed solvents at a fixed concentration (1.0×10ꢀ5 mol Lꢀ1). As
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absorption dissymmetry factors (gabs) of R/Sꢀ
CD spectra were about 0.001 at 580 nm.
The AIEꢀactive molecules exhibited efficient fluorescence and
CD responses in both states, which inspired us to investigate R/Sꢀ
5
obtained from the at 630 nm were about
±
0.002, which are in the range of most
±
CPL materials from 10ꢀ5 to 10ꢀ2.
In summary, we designed and synthesized the enantiomers
5
with obvious AIE feature. The intramolecular energy
their CPL behaviors. As shown in the CD spectra, there is a transfer from the TPE to BODIY moieties in the molecules could
broad CD band in the region from 500 to 610 nm, which could induce the two enantiomers with redꢀcolor mirrorꢀimage CPL
affect their CPL signals (Figure S8). To reduce the influence on signals. Interestingly, the chiroptical properties on CD and CPL
CPL signals, we used intramolecular energy transfer by exciting do not change both in molecular dispersal and aggregated states,
at 340 nm to enlarge the Stokes’ shift. Meanwhile, to better which indicates that the chirality of electronic ground and excited
compare the CPL spectra, we collected the CPL data by states is stable and independent of aggregation. Based on AIE
adjusting fluorescence with the same intensity in the mixed feature, the intensity of CPL (
solvents. In Fig. 3, R/Sꢀ showed mirrorꢀimage CPL signals adjusted via changing the ratios of the mixed solvents.
centered at 630 nm with negative/positive signals. Interestingly, We would like to thank Prof. Zhiyong Tang and Dr. Lin Shi at
ΔI) for the molecules can be
5
the CPL spectra did not have apparent changes by changing the NCNST for their kind help on CPL measurement. This work was
ratios of the mixed solvents, which are coincident with those of supported by the National Natural Science Foundation of China
CD. These results indicate that the chirality of electronic ground (21174061, 51173078, 21172106 and 21474048) and open
and excited states of the molecules is stable and independent of project of Beijing National Laboratory for Molecular Sciences.
aggregation.
Notes and references
a 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/c000000x/
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