Strong circularly polarized luminescence induced from chiral supramolecular assembly of helical nanorods

Article abstract of DOI:10.1039/c7cc04363e

Chiral supramolecular assemblies (BNS-BPP) can be self-assembled by electrostatic and π-π stacking interactions between the chiral binaphthyl sulphonate (BNS) anion and the achiral bipyrene-based pyridinium (BPP) cation chromophore. BNS-BPP can exhibit st

Full text of DOI:10.1039/c7cc04363e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Strong Circularly Polarized Luminescence Induced from Chiral
Supramolecular Assembly Helical Nanorods
Yuxiang Wang,a Xiaojing Li,a Fei Li,a Weiying Sun,b Chengjian Zhu*b and Yixiang Cheng*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The chiral supramolecular assemblies (BNS-BPP) can be self- supramolecular assembly system8b,8e Therefore, it is of great
assembled by electrostatic and π-π stacking interactions between significance on developing novel CPL emission materials via
chiral binaphthyl sulphonates (BNS) anion and the achiral chiral supramolecular assembly process.
bipyrene-based pyridinium (BPP) cation chromophore. BNS-BPP
Chiral supramolecular assemblies, which are the
can exhibit strong CD and circularly polarized luminescence (CPL) combinations of both nanoscale and chirality, have now played
signals, and the dissymmetry factor glum values can be up to 0.079.
a vital role in the development of new materials for
enantioselective sensing and separation,10 chiral catalysis11 and
CPL emission12. Kawai’s group reported that the chiral
bichromophoric perylene bisimides (PBI) possessing
cyclohexane as the chiral central core unit could exhibit CPL
signals at both monomeric and assembled states, while the
self-assembled structures showed more than an order of
magnitude higher CPL dissymmetry factors compared with
monomeric state of the molecule.9a Recently, our group also
found that no CD or CPL signals of two perylene diimide (PDI)
enantiomers incorporating a D/L-alanine moiety could be
observed in CHCl3 solution, but the D/L-PDI enantiomers could
exhibit strong Cotton effects and CPL signals in the
Circularly polarized luminescence (CPL) is a unique chiroptical
property of chiral luminescent systems (such as metal
complexes,1
small
organic
molecules,2
polymers,3
supramolecular assemblies4). As the emission analog of
circular dichroism (CD) absorption spectrum, CPL emission has
been regarded as one of the most important tools to
investigate the excited-state features of chiral dyes.5 Recently,
there have been many reports on various CPL-emission
materials for promising applications in information storage,
photoelectric devices and chiroptical sensors.6 Normally, the
strategy of designing novel CPL materials is to introduce the
chosen chiral moieties to the chromophores through covalent
bonds or coordinate covalent bonds.7 So far there have been
few reports on CPL materials based on noncovalent
interactions (such as electrostatic, hydrogen bonding and van
der Waals interactions) via supramolecular self-assembly
between the chiral sources and achiral chromophores.8
Compared with common small organic molecules, chiral
supramolecular assemblies can provide the enhanced CPL
response signals as the molecular chirality can be effectively
amplified through the assembly system.9 Moreover, for
multicomponent supramolecular assembly system, a series of
CPL materials can be easily obtained by introducing different
chiral sources and achiral chromophores into the
supramolecular
self-assembly
aggregations
via
the
intermolecular π-π interaction upon the addition of the poor
solvent MeOH to 99% fraction.9b However, chromophores like
perylene bisimides suffered from aggregation-caused
quenching (ACQ) phenomenon in aggregation state, which
leads to the low fluorescence quantum yield of the
supramolecular self-assemblies. In this paper, the chiral
supramolecular assemblies can exhibit red fluorescent
emission with good fluorescence quantum yield.
Optically active 1,1’-binaphthol (BINOL) and its derivatives
have been widely studied as one of the most important chiral
sources for designing CPL materials.13 Akagi’s group first
reported blue CPL material of an interchain helically assembly
between cationic π-conjugated polymers and anionic chiral
binaphthyl derivatives through electrostatic and π-π
interactions.8d Recently, Lu’s group also discovered that the
helical fiber assemblies between a TPE cationic and an anionic
binaphthyl derivative could produce strong CPL response
signals in cast films, and the glum values can reach as high as
+3.8 × 10-2 and -4.2 × 10-2 for S and R configuration of
binaphthyl derivatives in the assemblies, respectively.14
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Herein, we designed the chiral supramolecular assemblies fraction of MeOH-H2O solution system. As can be seen in Fig.
(BNS-BPP) between chiral binaphthyl sulphonates (BNS) and a 2, the absorption peaks at 475 nm slightly decreased as the
bipyrene-based pyridinium (BPP) through electrostatic and π-π water fraction (fw) increased from 0 to 40%. Strangely, the
interactions. The aim of our design is to investigate the chiral absorption peak of BNS-BPP at 576 nm can be observed only
transfer mechanism of BNS to chromophore BPP through when fw is exactly 50%. When fw is above 50%, this absorption
electrostatic interaction. Then BPP can self-assemble via π-π peak disappears, but there appears a broad absorption band
stacking interactions under the chiral restriction of BNS. As a around 500 nm. We can draw a conclusion that 50% water
result, R/S-BNS-BPP can form M/P-helical nanorods and fraction is the most suitable ratio of MeOH-H2O mixed solvents
produce strong red-color CPL emission signals with +0.079/- for the formation of the chiral supramolecular assemblies BNS-
0.070 of glum values.
BPP. As is evident from the Job’s plot at the 576 nm absorption
The synthesis procedures of BPP and R/S-BNS are outlined in peak of BNS-BPP in MeOH-H2O (50:50 v/v) (Fig. S2), the
Scheme 1. Compound 1 and 2 were synthesized according to optimum mixing ratio of BNS: BPP is found to be 1.0: 2.0,
reported literatures.15 Compound 3 was synthesized by Pd- which consists with the charge numbers of the two monomers.
catalyzed Sonogashira coupling reaction of 1-ethynylpyrene
The CD spectra of BNS-BPP were also measured in MeOH-
with 2, 6-dibromopyridine in 71% yield. BPP could be obtained H2O solution at 50% water fraction. R/S-BNS-BPP can show
from the quaternization of compound 3 with iodomethane in obvious mirror-image CD signals (Fig. 1). The weaker positive-
86% yield. The detailed procedures and characterizations are negative Cotton effects at 225 and 240 nm can be regarded as
described in the ESI. We have measured the FTIR spectra of the feature absorption signals of chiral binaphthyl moiety in
BPP, S-BNS and S-BNS-BPP assemblies. As can be seen in Fig. the chiral supramolecular assemblies BNS-BPP(Fig. S4). Strong
S1, the peak at 2193 cm-1 can be assigned to the stretching Cotton effect of CD signals at long wavelength region can be
vibration of triple bond in BPP. The two peaks at 1204 cm-1 and observed, and the absorption dissymmetry factors (gabs) can
1055 cm-1 can be ascribed to the asymmetric vibration and the reach as high as +4.2 × 10-2 for R-BNS-BPP at 577 nm and -4.1 ×
symmetric vibration of the sulfonic group in BNS.14 However, 10-2 for S-BNS-BPP at 576 nm, respectively, which can be
the characteristic peaks of sulfonic group shift to 1173 cm-1 regarded as the absorption bands of the chiral supramolecular
and 1049 cm-1 in the chiral supramolecular assemblies S-BNS- assemblies and demonstrate that the chirality has been
BPP, which indicates the electrostatic interaction between BPP successfully transferred from the chiral BNS anionic moiety to
and BNS.
the achiral BPP cationic chromophore by electrostatic
interaction. We observed that the further addition of BNS can
form more BNS-BPP assemblies and lead to the enhanced CD
absorption signals until the molar ratio of BNS/BPP reaches 1.5
(Fig. S3). We also investigate the CD behaviors in MeOH-H2O
solution by changing different water fractions. As can be seen
in Fig. S4-S6, the CD spectra of BNS-BPP can only exhibit the
CD signals of BNS when water fraction is below 40%.
Compared to the CD signals at 576 nm at fw =50%, only weaker
positive-negative bisignals at 480 nm and 530 nm can be
observed at fw =80%, which indicates that the chirality can be
greatly amplified at the most suitable ratio of MeOH-H2O
mixed solvents through the orderly and regular arrangement
of the chiral supramolecular assemblies BNS-BPP.
R-BNS
S-BNS
-
BPP
150
100
50
-BPP
Scheme 1 The synthesis procedures of BPP and R/S-BNS.
0
The UV-vis absorption spectra of BNS, BPP and the chiral
supramolecular assemblies (BNS-BPP) (molar ratio 1: 2, 1.0 ×
10-5 mol/L for BPP) in MeOH-H2O solution (50:50 v/v) are
presented in Fig. 1. As shown in Fig. 1, BNS exhibits absorption
bands at 228 nm and 275 nm, which can be assigned to the π-
π* transition of the binaphthyl. The absorption band of BPP at
475 nm can be ascribed to the conjugated structure between
pyrene and pyridine. But the assemblies BNS-BPP appears two
additional absorption bands situated at 530 nm and 576 nm,
which can be attributed to the formation of the chiral
supramolecular assemblies BNS-BPP between BPP and BNS. In
this paper the solvent effect experiments for UV-vis absorption
spectra of BNS-BPP were carried out by changing water
-50
-100
-150
0.5
0.4
0.3
0.2
0.1
0.0
BNS-BPP
R-BNS
BPP
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 1 UV-vis absorption spectra of BNS-BPP, R-BNS and BPP, CD spectra of BNS-BPP in
MeOH-H2O (50:50 v/v) solution (0.5 × 10-5 mol/L for BNS and 1.0 × 10-5 mol/L for BPP).
2 | J. Name., 2012, 00, 1-3
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f
/ %
w
0.5
0.4
0.3
0.2
0.1
0.0
0
300
R-BNS-BPP
S-BNS-BPP
10
200
20
∆
∆
∆
∆
100
30
40
50
60
70
80
90
95
0
-100
-200
-300
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
BNS-BPP
300
400
500
600
700
500
550
600
650
700
750
800
Wavelength(nm)
Wavelength (nm)
Fig. 2 UV-vis absorption spectra of BNS-BPP in MeOH-H2O solution (0.5 × 10-5
mol/L for BNS and 1.0 × 10-5 mol/L for BPP).
Fig. 3 Fluorescent and CPL spectra of BNS-BPP in MeOH-H2O (50:50 v/v) solution
(excitation: 467 nm, 0.5 × 10-5 mol/L for BNS and 1.0 × 10-5 mol/L for BPP).
The fluorescence and CPL spectra of BNS-BPP are presented
in Fig. S7 and 3. BNS-BPP can exhibit red fluorescent emission
at 605 nm in MeOH-H2O (50:50 v/v) solution. There is almost
no fluorescent emission at 605 nm for BPP in the absence of
BNS (Fig. S7). Interestingly, the emission intensity of BPP at
605 nm gradually increases as the molar ratio increase of BNS
until BNS: BPP is 2.0: 1.0, indicating the emission peak at 605
nm belongs to the chiral supramolecular assemblies BNS-BPP.
The fluorescece quantum yield of BNS-BPP in MeOH-H2O
(molar ratio=1:2, fw=50%) was measured to be 11.3%. Herein,
CPL signals were performed by using JASCO CPL-300. As is
evident from Fig. 3, the CPL response signals of BNS-BPP at 605
nm can exhibit obvious mirror-image positive (R-BNS-BPP) and
negative (S-BNS-BPP) monosignals at fw=50% of MeOH-H2O
solution. Meanwhile, we also found that almost no CPL signals
can be detected at fw=0%, 40%, 60% and 80% (Fig. S9 and S10),
which further demonstrates that 50% water fraction is the
most suitable ratio of MeOH-H2O mixed solvents for the
formation of the stable chiral supramolecular assemblies.
According to the experimental determination of the CPL of R-
BNS-BPP assembly with different molar ratio of BNS/BPP as
shown in Fig. S8, the CPL signals can reach the maximum value
at the 1:2 molar ratio of BNS/BPP although the molar ratio of
BNS further increases, which also demonstrates that the chiral
supramolecular assemblies can well consist with the charge
numbers of BNS/BPP at the 1:2 molar ratio. Normally, the
magnitude of circular polarization in the ground and excited
The morphology study of the BNS-BPP assemblies was
performed by scanning electron microscope (SEM). As is
evident from Fig. 4, the morphologies of the chiral
supramolecular assemblies are helical nanorods when the
water fractions are 50%. The R and S-BNS in the assemblies
can result in the left-handed (M-helicity) and right-handed (P-
helicity) helical nanorods, which can well correspond to the
positive and negative CPL signals of R-BNS-BPP and S-BNS-BPP.
Meanwhile, we can discover that there are no helical nanorods
when the water fractions are 0, 40%, 60% and 80% (Fig. S11),
which is also coincident with the CD and CPL responses in
different methanol-water mixtures. The self-assembly
behaviors can be greatly influenced by the water fraction,
which indicates that the chiral supramolecular assemblies can
be formed by the co-effect of electrostatic interaction and π-π
stack of pyrene.
Fig. 4 SEM images of the chiral supramolecular assemblies of (a) R-BNS-BPP and (b) S-
BNS-BPP obtained from drying the solutions of BNS-BPP in MeOH-H2O (50:50 v/v). (0.5
× 10-5 mol/L for BNS and 1.0 × 10-5 mol/L for BPP)
states can be evaluated by dissymmetry factors gabs and glum
,
respectively. The gabs values of BNS-BPP are ±4.2 × 10-2 at
about 576 nm, while the glum values of BNS-BPP are +7.9 × 10-2
(at 605 nm for R configuration) and -7.0 × 10-2 (at 605 nm for S
configuration). The gabs and glum values are on the order of 10-2
at the maximum absorption or emission wavelength, which
can be attributed to the helical structures in the chiral
supramolecular assemblies.16
Fig. 5 Schematic illustration of hypothetical mechanism of the formation of R-BNS-BPP
helical nanorods
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Herein we proposed the formation mechanism of the chiral
supramolecular assemblies. As can be seen in Fig. 5, R-BNS first
combines with BPP through electrostatic interaction and forms
the left-handed helical nanowires via π-π stacking interaction.
And then the left-handed helical nanowires can further bundle
and form left-handed helical nanorods as shown in the SEM
images (Fig. 4a).
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In summary, the chiral binaphthyl sulphonates (BNS) and
bipyrene-based pyridinium (BPP) were synthesized and then
mixed together to generate the chiral supramolecular
assemblies BNS-BPP. The chiral supramolecular assemblies can
exhibit strong CD and CPL signals because of the chirality
transfer from the chiral resources to achiral luminophores
through the both effects of electrostatic and π-π interactions.
The SEM images further support the optical properties of the
chiral supramolecular assemblies. The dissymmetry factors
glum can reach as high as 0.079, which is the most excellent CPL
materials of the reported chiral organic molecules.
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We would like to thank Prof. Lianhui Wang and Prof. Biqing
Bao at IAM for their kind help with CPL measurements.This
work was supported by the National Natural Science
Foundation of China (21674046, 21474048 and 51673093)
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