Monomer emission and aggregate emission of TPE derivatives in the presence of γ-cyclodextrin

Article abstract of DOI:10.1021/ol500638c

It was found for the first time that neutral amphiphilc tetraphenylethylene (TPE) derivatives showed an enhanced monomer emission and a decreased aggregate emission when they were included in the cavity of γ-cyclodextrin. This result provided a new insight into the aggregation-induced emission (AIE) effect.

Full text of DOI:10.1021/ol500638c

Letter
pubs.acs.org/OrgLett
Monomer Emission and Aggregate Emission of TPE Derivatives in
the Presence of γ‑Cyclodextrin
Song Song,† Hua-Fei Zheng,‡ Dong-Mi Li,† Jin-Hua Wang,† Hai-Tao Feng,† Zhi-Hua Zhu,†
Yi-Chang Chen,† and Yan-Song Zheng*,†
†Key Laboratory for Large-Format Battery materials and System, Ministry of Education, School of Chemistry and Chemical
Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
‡School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China
S
* Supporting Information
ABSTRACT: It was found for the first time that neutral
amphiphilc tetraphenylethylene (TPE) derivatives showed an
enhanced monomer emission and a decreased aggregate
emission when they were included in the cavity of γ-
cyclodextrin. This result provided a new insight into the
aggregation-induced emission (AIE) effect.
ecently, a new class of organic compounds with an
Ramazing property of aggregation-induced emission (AIE)
effect is attracting increasing interest because of their
tremendous potential in chemosensors, bioprobes, and solid-
state emitters.1,2 Since the AIE effect was discovered in 2001,2 a
huge number of AIE compounds have been garnered, and
successful studies regarding application have been conducted.1
However, the mechanism underlying this novel AIE phenom-
enon is still bewildering and in dispute1,3,4 due to limited direct
Figure 1. Synthesis of the AIE compounds 2a−2c.
evidence.5 The AIE compounds, especially the most studied
typical AIE property of the known 1,11 compounds 2a−2c, as
tetraphenylethylene (TPE) derivatives, do not emit light but
display a strong fluorescence in the aggregation state. At low
temperature, in viscous solvent, and even under high pressure,
colorless sticky solids, emitted a brilliant green light under a
365 nm lamp. A solution of 2a, 2b, or 2c in 1,2-dichloroethane
had no fluorescence but started to emit fluorescence at 485 nm
when turbidity appeared upon addition of nonsolvent hexane
up to 70% volume fraction. Afterward, the fluorescence
the AIE compounds also have an AIE enhancement because of
slowed motions.1b After absorption onto biopolymers such as
DNA,6 protein,7 and peptides,8 these compounds also exhibit
strong aggregation fluorescence resulting from the restriction of
their motions by the biopolymer backbone or groove.
Moreover, upon loading into mesopores of silica nanoparticles,
the AIE molecules have an increased AIE effect due to narrow
space that limits their motions.9 If an AIE molecule is put into a
cavity of a host compound such as crown ethers, cyclodextrins
(CDs),10 calixarenes, curcubiturils, and so on, the cavity will
confine the motions of the AIE molecules and result in an
enhanced AIE effect. Here we report for the first time that a
stable 1:1 inclusion complex of neutral amphiphilic tetraphenyl-
ethylene derivatives with γ-CD arouses a significantly
attenuated aggregate emission and an increased monomer
emission, which provided a new insight into the fluorescence of
AIE compounds.
intensity increased rapidly with hexane (Supplementary Figure
S13). Therefore, 2a−2c were AIE compounds.
Compounds 2a−2c were soluble and light-emitting in water.
The fluorescence intensity became larger with increasing
concentration. Moreover, there appeared a minor emission
with two fine vibration bands at 405 and 388 nm in addition to
the main emission at 485 nm (Supplementary Figure S14). It is
well-known that the neutral amphiphilic compounds bearing
hydrophilic oligo(ethylene glycol) chains at both ends of one
hydrophobic core easily self-assembly into one-dimensional
aggregates.12 For compounds 2a−2c, they were prone to
aggregate into a linear micelle by overlapping of molecules in
order to avoid the hydrophobicity of the TPE core
(Supplementary Figure S15A). The formation of the micelles
As shown in Figure 1, compounds 2a, 2b, and 2c were
synthesized by the reaction of tetra(p-hydroxyphenyl)ethylene
111a with oligo(ethylene glycol) monotosylate. Because of a
Received: February 28, 2014
Published: April 4, 2014
© 2014 American Chemical Society
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Letter
will lead to the main fluorescence emission at 485 nm, and the
emission maximum wavelength was the same as that of the
aggregates in the mixed solvent of 1,2-dichloroethane and
hexane. Although the aqueous solutions of 2a−2c were very
clear and transparent, dynamic light scattering (DLS) disclosed
that there were nanoparticles with a diameter of 5−12 nm
(Supplementary Figure S16), indicating the formation of
micelles or nanoaggregates.
Because of the two fine vibration bands at 405 and 388 nm,
the weak emission at this area could probably be ascribed to the
monomers of 2a−2c. However, this weak emission was not
observed in organic solvent, and therefore 2a−2c in water
should exist in a special state when they dismantle as
monomers from a micelle. Due to the hydrophobicity of the
TPE unit, the four hydrophilic and flexible chains will have a
fold around the TPE core instead of pointing away from it in
order to reduce its hydrophobic force (Supplementary Figure
S15B). The four hydrophilic chains that wound around the
TPE unit not only made the compound more compatible with
water but also restricted the rotation of the phenyl rings, which
would result in a monomer fluorescence of 2a−2c.13 The 2D
NMR NOESY spectrum of 2c in D2O confirmed that the
hydrophibic chains were truly bent toward the TPE unit instead
of pointing off it (Supplementary Figure S15).
Figure 3. Change of fluorescence spectra of 2c with γ-CD in water. λex
= 330 nm, ex/em slits = 5/5 nm. [2c] = 5.0 × 10−5 M. Inset, curves of
fluorescence intensity vs concentration of γ-CD measured at 485 nm
▲
(
) and 405 nm (Δ).
complex of 2c and γ-CD with 1:1 molar ratio. The HRMS
spectrum of the mixture of 2c and γ-CD in water showed an
obvious molecular ion peak (2597.0797) of 2c-γ-CD
(calculated 2597.0762 (M + Na)), demonstrating the
possibility of a 1:1 complex (Supplementary Figure S18).
The 2c-γ-CD complex was further corroborated by 1H NMR
1
spectrum (Figure 4). The H NMR titration of 2c by γ-CD
Interestingly, 2a−2c exhibited a selective interaction with
CD compounds. As shown in Figure 2 and Supplementary
Figure 2. Fluorescence spectra of 2c without and with α-CD, β-CD,
and γ-CD in water. (Right) Photos of a solution of 2c, 2c-γ-CD, and
2c-γ-CD-potassium ursodeoxycholate in water under a 365 nm lamp.
[2c] = [α-CD] = [β-CD] = [γ-CD] = 1/3[cholate] = 5.0 × 10−5 M,
λex = 330 nm, ex/em slits = 5/5 nm.
Figure 4. Change of 1H NMR spectra of 2c with γ-CD in D2O. [2c] =
2.0 mM. The number over the spectrum is the molar ratio of γ-CD vs
2c.
Figure S17, α-CD or β-CD had almost no effect on the
fluorescence of compounds 2a−2c, but γ-CD significantly
weakened the emission at 485 nm. Meanwhile, two weak
emission peaks at 405 and 388 nm became strong and even
much larger than that at 485 nm, and therefore a color change
from green to blue could be observed under a 365 lamp (Figure
2). It is known that the inside diameter of the cavity of α-CD,
β-CD, and γ-CD is 0.47−0.53, 0.60−0.65, and 0.75−0.85 nm,
respectively.14 The distance between two phenyl rings at
different carbons and at the same carbon of the TPE double
bond is about 0.74 and 0.90 nm, respectively;15 therefore, only
γ-CD could encapsulate just one molecule of 2a−2c and affect
their fluorescence, displaying a high selectivity for the size of
CD compounds
The fluorescence titration of 2c with γ-CD showed that the
fluorescence intensity rapidly decreased at 485 nm while the
emission significantly increased at 405 and 385 nm with
addition of γ-CD. After the 1:1 molar ratio of γ-CD vs 2c, all of
the emissions displayed a slowed change (Figure 3). At 437 nm,
a clear isoemissive point was observed, indicating formation of a
disclosed that two pairs of new doublets (6.84, 6.79 and 6.66,
6.64 ppm) that were ascribed to aromatic protons appeared and
became stronger, while one pair of doublets (6.73 ppm, 6.45
ppm) from aromatic protons of free 2c decreased gradually
with addition of γ-CD. After a 1:1 molar ratio of γ-CD vs 2c,
this pair of doublets from free 2c completely disappeared and
the two pairs of new doublets did not grow further. This
unambiguously demonstrated that the 2c-γ-CD complex with a
1:1 binding ratio was produced. During the titration, all signals
of both free 2c and the complex had no shift, indicating no
distinct interaction between free 2c and the complex. The pair
of doublets at 6.66 and 6.64 ppm, which seemed to be a singlet
snuggled by one tiny peak at each side, should be ascribed to
the protons of one phenyl ring, and the pair of doublets at 6.84
and 6.79 ppm should result from another phenyl ring, just like
the pair of doublets for free 2c. Therefore, the 2c-γ-CD
complex had two sets of phenyl signals with a big downfield
shift compared to free 2c which had only one set of phenyl
signals, hinting that all four phenyl rings of the TPE unit were
no longer the same but divided into two groups. Probably, two
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Letter
phenyl rings completely inserted into the cavity of γ-CD, while
the other two phenyl rings were partially included by γ-CD.
The 1H NMR titration of γ-CD with 2c (Supplementary
Figures S19 and S20) also confirmed the formation of a 2c-γ-
CD complex. Since the concentration of the complex, free 2c,
and γ-CD at equilibrium could be directly measured from the
1H NMR spectra, the associated constant of the complex could
be calculated by using the thermodynamic equilibrium equation
to be 1.22 ( 0.21) × 104 M−1, which is a large value.
In the 2D NOESY spectrum of the 2c and γ-CD mixture, the
triplet peak of the 2c-γ-CD complex at 3.45 ppm should be
ascribed to the proton H2 of γ-CD since it had a strong cross-
peak with H1 of γ-CD. The new proton signals as a doublet at
3.26 ppm, which was at the upmost field of all the signals, was
correlated with the proton H2 of γ-CD. Therefore, this signal
should belong to proton H3 of γ-CD since H3 and H2 were in a
neighboring position. These two protons have been reported to
have a strong NOE, although they are in a trans position.16
Now the proton H3 had a strong cross-peak with the aromatic
protons of 2c, indicating that 2c was encapsulated into the
cavity of γ-CD by its two phenyl rings from the wide rim of γ-
CD because the proton H3 is near to the wide rim and points to
the inside of the γ-CD cavity. In addition, the NOE of H3 with
the aromatic proton Ha was stronger than that with the
aromatic proton Hb, further demonstrating that the phenyl ring
deeply inserted into the cavity until Ha was nearer to it than Hb.
Due to the strong shielding role from the encapsulated phenyl
ring, the proton H3 of the complex displayed the most obvious
upfield shift compared with that of the free γ-CD.
Figure 5. Schematic structure of 2c-γ-CD inclusion complex and its
partial 2D NOESY spectrum in D2O. [2c] = [γ-CD] = 2 mM.
The 13C NMR spectrum of the 2c-γ-CD complex disclosed
that four carbon peaks of the four phenyl rings in free 2c
changed into eight carbon peaks in the complex (each carbon
peak was divided into two peaks) (Supplementary Figure S22),
indicating that one set of phenyl rings in free 2c became into
1
two sets of ones in the complex, which was in line with H
NMR test. However, the carbon signal from the double bond
was still one peak rather than two peaks, demonstrating that the
two carbons of the double bond were still the same in the
complex. Therefore, 2c was included into the cavity of γ-CD
along a direction of two phenyl rings at different carbons of the
double bond rather than at the same carbon (Figure 5, 2c-γ-
CD). In this including way, all four phenyl rings could go into
the cavity of γ-CD due to a 0.79 nm depth of the cavity,14 so
that they all had cross-peaks with H3 of γ-CD in 2D NOESY
spectrum. Due to the restriction of the phenyl ring rotations,
the monomer emission of 2c significantly increased. In
addition, the absorbance maximum wavelength (312 nm) of
the 2c-γ-CD complex was less by 5 nm than that (317 nm) of
2c in water (Supplementary Figure S23), also demonstrating
that the phenyl rings were confined in the cavity of γ-CD and a
more twisted propeller conformation of the TPE unit was
produced.
As expected, the aggregate emission increased while the
monomer emission decreased with the potassium salt of
ursodeoxycholic acid (here abbreviated as cholate), a kind of
clinical drug for dissolving gallstones that can be easily included
by γ-CD (Figure 6).17 A distinct color change from blue to
green, which was inverse to the color change when 2c and γ-
CD were mixed, could be seen under a 365 nm lamp after 2−3
equiv of the cholate was added (Figure 2). At 437 nm, a clear
isoemissive point was observed, indicating the dissociation of
the 2c-γ-CD complex and the formation of a new complex. The
2a-γ-CD and 2b-γ-CD complexes also had a similar change
Figure 6. Change of fluorescence spectra of 2c-γ-CD complex with the
cholate in water. λex = 330 nm, ex/em slits = 5/5 nm. [2c] = [γ-CD] =
5.0 × 10−5 M. Inset, curves of intensity vs the cholate concentration
▲
measured at 485 nm ( ) and 405 nm (Δ).
with the cholate (Supplementary Figure S24). This result
further confirmed the monomer emission and the aggregate
emission of 2.
In conclusion, it was found for the first time that neutral
amphiphilic TPE derivatives with AIE effect could selectively
insert into the cavity of γ-CD by its TPE unit and form a stable
inclusion complex. After the TPE fluorogen was confined in the
cavity, the monomer emission at the short wavelength
increased while the AIE effect at the long wavelength had a
significant attenuation due to deaggregation by the host
compound. This finding provided a new insight into the
fluorescence of AIE compounds.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic and experimental details, DLS diagrams, and
additional spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zyansong@hotmail.com.
Notes
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The authors declare no competing financial interest.
̈
ACKNOWLEDGMENTS
(c) Gorl, D.; Zhang, X.; Wurthner, F. Angew. Chem., Int. Ed. 2012, 51,
2−23.
̈
̈
■
The authors thank National Natural Science Foundation of
China (21072067) for financial support and thank the
Analytical and Testing Centre at Huazhong University of
Science and Technology for measurement.
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