Acid/Base-Controllable FRET and Self-Assembling Systems Fabricated by Rhodamine B Functionalized Pillar[5]arene-Based Host-Guest Recognition Motifs

Article abstract of DOI:10.1021/acs.orglett.7b03612

A novel supramolecular F?ster resonance energy transfer (FRET) system was fabricated by utilizing rhodamine B (RB) functionalized pillar[5]arene (EtP5-RB) and cyano-modified boron dipyrromethene (BDP-CN) based on their host-guest recognition at 5.0 × 10

Full text of DOI:10.1021/acs.orglett.7b03612

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Acid/Base-Controllable FRET and Self-Assembling Systems
Fabricated by Rhodamine B Functionalized Pillar[5]arene-Based
Host−Guest Recognition Motifs
Jifu Sun,* Bin Hua, Qing Li, Jiong Zhou,* and Jie Yang*
State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of
Chemistry, Zhejiang University, Hangzhou 310027, P. R. China
*
S Supporting Information
ABSTRACT: A novel supramolecular Fo
̈
ster resonance energy
transfer (FRET) system was fabricated by utilizing rhodamine
B (RB) functionalized pillar[5]arene (EtP5-RB) and cyano-
modified boron dipyrromethene (BDP-CN) based on their
−5
host−guest recognition at 5.0 × 10 M, which could be turned
on” and “off” by adding trifluoroacetic acid (TFA) and
triethylamine (TEA), respectively. At a higher concentration
“
4
(
1.0 × 10− M) in acetone, EtP5-RB self-assembled into
vesicles while EtP5-RBH self-assembled into nanoribbons.
After the addition of BDP-CN, both EtP5-RB⊃BDP-CN and
EtP5-RBH⊃BDP-CN self-assembled into nanoparticles, which
caused the fluorescence of the host−guest complexes to be
quenched.
upramolecular fluorescent systems based on both of the
tron or energy transfer is still a great challenge, and only few
examples without stimuli-responsiveness have been reported to
S
host−guest recognition and various electron- or energy-
12
transfer mechanisms, such as photoinduced electron transfer
date.
1
2
(
PET), Fo
̈
ster resonance energy transfer (FRET), and triplet−
On the other hand, there are many stimuli-responsive
compounds, such as dithienylethene (DTE) derivatives,
azobenzene (AB) compounds,
compounds,
3
1d,2a,d,13
triplet energy transfer (TTET), have stimulated tremendous
interest in recent years due to their promising applications in
1
d,2a,5a
spiropyrane (SP)
and rhodamine B (RB), that have been
1b,c,e
1d,2a
14
numerous fields, including fluorescent probes,
chemo-
1
h
1d,g,2c,d,3b
sensors, molecular devices,
and biological applica-
etc. The host−guest recognition plays a significant role
widely utilized in fabricating functional molecular systems.
Among them, RB, which is well-known for the acid/base or metal
ion-controllable spirolactam ↔ opened amide reversible trans-
formation, was widely utilized in ratiometric fluorescent
1
a,2a
tions,
in the construction of advanced supramolecular fluorescent
systems as a result of their intriguing binding abilities, facile
1
−3
14a,b,d
14c
tunable functionality, and abundant stimuli responsiveness.
probes
and molecular logic gates; however, it was rarely
investigated in supramolecular fluorescent systems based on the
For instance, the supramolecular fluorescent systems based on
the host−guest recognition can be controlled via a variety of
1c,e
15
host−guest recognition. Furthermore, to the best of our
knowledge, the research of fabricating FRET-based supra-
molecular fluorescent systems upon the modification of RB
moiety onto the pillararenes has not been reported so far.
Herein, we fabricated an acid/base-controllable FRET system
via RB functionalized pillar[5]arene-based host−guest recog-
nition. A novel functionalized pillar[5]arene which was modified
with RB was synthesized by adopting click reaction to conjugate
the RB moiety to the pillar[5]arene. Based on the host−guest
interactions between the RB functionalized pillar[5]arene
1
b,g,3b
external stimulations, such as acid/base,
redox,
1
d,2b,d
1a,e,f,h,2c
light,
and metal ion
by tuning the formation/
dissociation of the host−guest complexes.
As a new generation of macrocyclic hosts, pillar[n]arenes were
4
synthesized by Ogoshi et al. in 2008 for the first time. Since then,
due to the accessible one-step synthesis, convenient tunable
functionalization, unique symmetric pillar-shaped structures, and
intriguing host−guest binding properties, pillararenes have been
widely explored in the construction of numerous supramolecular
5
(
(
EtP5-RB) and the cyano-modified boron dipyrromethene
BDP-CN), the FRET process between EtP5-RB and BDP-
systems, such as supramolecular polymers, controlled release
6
7
8
systems, transmembrane channels, and porous materials. In
addition, pillararene-based fluorescent systems have also
attracted considerable attention owing to their promising
CN could be turned “on” and “off” by adding trifluoroacetic acid
TFA) and triethylamine (TEA), respectively (Scheme 1).
(
9
applications in cell diagnostic imaging, controlled release
1
0
11
systems, and chemosensors. However, the investigation of
Received: November 21, 2017
pillararene-based supramolecular fluorescent systems via elec-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03612
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Top: Structures and Cartoon Representations of
EtP5-RB, BDP-CN, the Protonated EtP5-RB (EtP5-RBH),
and EtP5. Bottom: Cartoon Representations of Acid/Base
Responsive FRET and the Self-Assemblies of EtP5-RB, EtP5-
RBH, EtP5-RB⊃BDP-CN, and EtP5-RBH⊃BDP-CN
NMR time scale between EtP5 and BDP-CN (Figures S8 and
S9). The association constant (K ) of EtP5⊃BDP-CN was
a
2
−1
1
calculated to be 3.56 × 10 M by using the H NMR single
16
point method. In addition, from the UV−vis absorption
titration experiments in acetone, Job plots of both complexes
EtP5-RB⊃BDP-CN and EtP5-RBH⊃BDP-CN showed a 1:1
12b
stoichiometry (Figures S15 and S16). These results confirmed
that the cyano moiety of BDP-CN could be threaded into the
electron-rich cavities of EtP5, EtP5-RB, and EtP5-RBH in
5b
acetone. However, there was no complexation for EtP5 and
BDP-CN in DMSO-d , the polarity of which was very high so
6
that the electron-rich cavity of EtP5 could not complex with
5b
BDP-CN but DMSO-d instead (Figures S12 and S13).
6
In order to investigate the acid/base responsiveness, the UV−
vis absorptions of EtP5-RB, BDP-CN, and EtP5-RB⊃BDP-CN
under different conditions were studied in acetone (Figure 1).
Figure 1. UV−vis absorption spectra changes of (a) EtP5-RB and (b)
−
5
BDP-CN and EtP5-RB⊃BDP-CN, by adding TFA/TEA (1.0 × 10
M
in acetone, 298 K). Photographs: Etp5-RB (No. 1), EtP5-RB + TFA
No. 2), EtP5-RB + TFA + TEA (No. 3), BDP-CN (No. 4), EtP5-
(
RB⊃BDP-CN (No. 5), EtP5-RB⊃BDP-CN + TFA (No. 6) and EtP5-
RB⊃BDP-CN + TFA + TEA (No. 7).
No absorption of EtP5-RB in the visible spectral region was
observed (Figure 1a); in contrast, upon addition of TFA, a new
absorption band at 554 nm was observed and the color of EtP5-
RB changed from colorless to red (the inset photographs in
Figure 1a), which was attributed to the absorption of protonated
RB moiety due to forming into a π electron conjugated system.
Furthermore, the absorption band at 554 nm disappeared upon
the addition of TEA and the color changed from red to colorless
(the inset photographs in Figure 1a) due to forming into a
spirolactam of the RB moiety whose π electron conjugated
It is well-known that pillar[5]arene could associate with nitrile
guests in CDCl ; in contrast, the investigation of the host−guest
3
chemistry between pillar[5]arenes and nitrile guests in other
5b
14
solvents such as acetone-d has been rarely reported. To
system was destroyed. Similarly, the maximum absorption
6
confirm the formation of the host−guest complexes between
wavelength of BDP-CN and EtP5-RB⊃BDP-CN in the visible
spectral region was 497 and 498 nm, respectively. Upon the
addition of TFA into EtP5-RB⊃BDP-CN, a new absorption
band appeared at 554 nm, which was the absorption of the
opened amide form of RB (Figure 1b). However, the absorption
band disappeared again when excess TEA was added (Figure 1b).
Moreover, the changes of colors of EtP5-RB⊃BDP-CN in acid
or base conditions could be clearly visualized by the naked eye
(the inset photographs in Figure 1b). Therefore, the absorption
of EtP5-RB and EtP5-RB⊃BDP-CN at 554 nm could be turned
“on” and “off” by the spirolactam ↔ opened amide trans-
formation of the RB moiety.
EtP5-RB and BDP-CN as well as EtP5-RBH and BDP-CN in
1
acetone-d , partial H NMR spectra were first investigated by
6
using BDP-CN as a guest compound and EtP5 as a model host
compound. As shown in Figure S10, compared with the free
BDP-CN (Figure S10c), the peaks of protons H1−4 on BDP-CN
shifted upfield significantly (Δδ = −2.99, −3.24, −2.88, and
−
2.16 ppm for H , H , H , and H , respectively) and became
1 2 3 4
broad as a consequence of complexation dynamics after the
addition of EtP5 (Figure S10b). Meanwhile, the peaks of protons
Ha−d on EtP5 shifted downfield slightly (Δδ = 0.07, 0.09, 0.11,
and 0.03 ppm for H , H , H , and H , respectively). Second, 2D
a
b
c
d
NOESY NMR spectrum of EtP5 and BDP-CN in acetone-d was
Then, the concentration-dependent fluorescent emission
spectra of EtP5-RBH⊃BDP-CN (molar ratio was 1:1) were
conducted to investigate the FRET between EtP5-RBH and
BDP-CN upon excitation at 460 nm in acetone (Figures S20 and
S21). With increasing concentration, the luminescence intensity
of BDP-CN increased first and then was intensely quenched
while the luminescence intensity of EtP5-RBH gradually
increased. When the concentration of both EtP5-RBH and
6
also carried out to prove the formation of a host−guest complex
(Figure S14). Nuclear overhauser effect (NOE) correlations
were observed between protons H and H , H , H (B, F, and D
a
1
2
3
points, Figure S14), H , H and H , H , H (A, C, and E points,
b
d
1
2
3
1
Figure S14). Third, in the H NMR titration experiments in
acetone-d , two sets of peaks were clearly observed for H on
6
1−3
BDP-CN, indicating the slow-exchange complexation on the
B
DOI: 10.1021/acs.orglett.7b03612
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 2. (a) Fluorescence changes of BDP-CN and EtP5-RBH by adding EtP5-RBH to the acetone solution of BDP-CN (5.0 × 10−5 M, 298 K) from
.0 to 1.5 equiv upon excitation at 460 nm. The inset picture was the amplified fluorescence changes of EtP5-RBH under the same conditions. (b)
0
−
5
Maximum emission intensity changes of EtP5-RBH under the same condition as (a). (c) Fluorescence changes of EtP5-RB⊃BDP-CN (5.0 × 10 M,
molar ratio was 1:1, 298 K) at 591 nm upon addition of TFA/TEA, respectively. The inset photographs show the corresponding fluorescence changes of
EtP5-RB⊃BDP-CN under the same conditions. (d) Corresponding color changes of EtP5-RBH⊃BDP-CN under the same conditions as (a).
BDP-CN was 5.0 × 10−5 M, the emission intensity of EtP5-RBH
reached the highest value. Herein, according to the integral
intensities of uncomplexed and complexed BDP-CN, respec-
tively (Figure S9), the concentration of the host−guest complex
ing fluorescent color of EtP5-RBH⊃BDP-CN changed from
green to yellow (Figure 2c). Moreover, upon addition of TEA to
the system the emission band at 591 nm disappeared because the
π electron conjugated system of RB moiety was destroyed by
TEA and the corresponding fluorescent color of EtP5-
−5
was calculated to be approximately 1.6 × 10 M at which there
was FRET occurring, and the emission intensity of EtP5-RBH
was highest. However, when the concentration of EtP5-RBH
14
RBH⊃BDP-CN returned from yellow to green (Figure 2c).
Therefore, the FRET process could be turned “on” and “off” over
several cycles upon the addition of TFA/TEA, respectively.
Finally, in order to verify the self-assembling behaviors of
EtP5-RB, EtP5-RBH, EtP5-RB⊃BDP-CN, and EtP5-
−5
and BDP-CN was higher than 5.0 × 10 M, the luminescence
intensity of EtP5-RBH decreased intensely (Figure S21). It was
speculated that EtP5-RBH⊃BDP-CN aggregated and self-
assembled at a higher concentration, which caused its
−4
RBH⊃BDP-CN at a higher concentration (1.0 × 10 M) in
acetone, scanning electron microscope (SEM), transmission
electron microscope (TEM), and dynamic laser scattering (DLS)
were investigated (Figure 3 and Figures S33 and S34). The TEM
17
fluorescence to be quenched. In addition, although there was
no complexation between EtP5-RBH and BDP without CN
moiety, the FRET still occurred between them (Figure S31),
which was in accordance with the experiment results in DMSO
(Figures S26, S27, and S30 and Table S3). It is noteworthy that
the FRET efficiency (Φ ) was enhanced by formatting of the
ET
host−guest complex between EtP5-RBH and BDP-CN, which
could ensure the efficient distance between the donor (BDP-
CN) and acceptor (EtP5-RBH) for FRET (Tables S3 and
2
,14b
S4).
To further confirm the FRET process between BDP-CN and
EtP5-RBH, fluorescence titration experiments were conducted
in acetone (Figure 2). As shown in Figure 2a,b, with the addition
of EtP5-RBH from 0.0 to 1.5 equiv, the emission intensity of
BDP-CN at 521 nm decreased significantly while the emission
intensity of EtP5-RBH at 591 nm increased slightly upon
excitation at 460 nm, which proved that the FRET process
occurred between EtP5-RBH and BDP-CN. When the molar
ratio of EtP5-RBH was 1:1, the fluorescence emission intensity
−4
Figure 3. (a, b): TEM images of EtP5-RB (1.0 × 10 M). (d, e): TEM
of EtP5-RBH reached the highest value (Φ = 71.1%).
−4
ET
images of EtP5-RBH (1.0 × 10 M) and EtP5-RBH⊃BDP-CN (1.0 ×
−
4
−4
Moreover, the corresponding fluorescence changes of EtP5-
RBH⊃BDP-CN could be clearly visualized by the naked eye
10 M), respectively. (c, f): DLS study of EtP5-RB (1.0 × 10 M) and
4
−
EtP5-RBH⊃BDP-CN (1.0 × 10 M), respectively.
(Figure 2d). In addition, the acid/base responsiveness of the
FRET in EtP5-RB⊃BDP-CN was also investigated (Figure 2c).
Upon excitation at 460 nm, the maximum fluorescence emission
wavelength of EtP5-RB⊃BDP-CN was at 521 nm, which was the
fluorescence emission of BDP-CN, while upon addition of TFA a
new emission band appeared at 591 nm, which was the
fluorescence emission of EtP5-RBH as a result of the FRET
between EtP5-RBH and BDP-CN. Meanwhile, the correspond-
images revealed that the aggregates of EtP5-RB were vesicles
(Figure 3a,b), which was further supported by SEM image
(Figure S33). The average diameter of the vesicles was
approximately 150 nm supported by TEM and DLS results
(Figure 3a−c). However, by addition of TFA, the aggregates of
EtP5-RBH were nanosheet assemblies (Figure 3d). Moreover,
after the addition of BDP-CN, both of the aggregates of EtP5-
17
C
DOI: 10.1021/acs.orglett.7b03612
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
RB⊃BDP-CN and EtP5-RBH⊃BDP-CN were spherical
assemblies approximately 200 nm in diameter (Figure 3e and
Figure S34), which were consistent with the DLS results (Figure
Q.-Z. Chem. Soc. Rev. 2015, 44, 6143. (d) Qu, D.-H.; Wang, Q.-C.;
Zhang, Q.-W.; Ma, X.; Tian, H. Chem. Rev. 2015, 115, 7543.
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3f). As we know, in acetone, the pillar[5]arene had a good
solubility but the solubility of RB and RBH was poor. It was
speculated that EtP5-RB, EtP5-RBH, EtP5-RB⊃BDP-CN, and
EtP5-RBH⊃BDP-CN self-assembled in acetone at a higher
concentration as a result of the difference in solubility and the
π−π interactions of the moieties in the hosts or host−guest
(2) (a) Peng, H.-Q.; Niu, L.-Y.; Chen, Y.-Z.; Wu, L.-Z.; Tung, C.-H.;
Yang, Q.-Z. Chem. Rev. 2015, 115, 7502. (b) Liu, G.; Xu, X.; Chen, Y.;
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1
7
complexes. Similarly, EtP5-RBH and EtP5-RBH⊃BDP-CN
self-assembled into nanosheets and nanoparticles in CHCl ,
3
(3) (a) Ye, C.; Wang, J.; Wang, X.; Ding, P.; Liang, Z.; Tao, X. Phys.
respectively (Figure S35). In contrast, EtP5-RB and EtP5-
RB⊃BDP-CN could aggregate but not self-assemble in CHCl3
because each moiety of the host and guest (EtP5, RB and BDP)
Chem. Chem. Phys. 2016, 18, 3430. (b) Ma, X.; Zhang, J.; Cao, J.; Yao, X.;
Cao, T.; Gong, Y.; Zhao, C.; Tian, H. Chem. Sci. 2016, 7, 4582.
(
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had good solubility in CHCl (Figure S35).
3
In conclusion, a novel supramolecular FRET system was
constructed via a RB functionalized pillar[5]arene-based host−
guest recognition motif. An efficient FRET process ocurred in
(
Y.; Li, X.; Yang, H. J. Am. Chem. Soc. 2014, 136, 8577. (b) Wang, Y.; Ping,
G.; Li, C. Chem. Commun. 2016, 52, 9858.
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Huang, F. Chem. Soc. Rev. 2017, 46, 7021.
−5
EtP5-RBH⊃BDP-CN at 5.0 × 10 M, which could be turned
on” and “off” over several cycles upon the addition of TFA/
TEA, respectively. At a higher concentration in acetone (1.0 ×
“
−4
(7) (a) Si, W.; Chen, L.; Hu, X.; Tang, G.; Chen, Z.; Hou, J.; Li, Z.
10
M), EtP5-RB self-assembled into vesicles with the average
Angew. Chem. 2011, 123, 12772. (b) Wang, R.; Sun, Y.; Zhang, F.; Song,
M.; Tian, D.; Li, H. Angew. Chem., Int. Ed. 2017, 56, 5294.
diameter of 150 nm, while EtP5-RBH self-assembled into
nanosheets. Moreover, after the addition of BDP-CN, both of
EtP5-RB⊃BDP-CN and EtP5-RBH⊃BDP-CN self-assembled
into nanoparticles with an average diameter of 200 nm, which
caused the fluorescence emission of the host−guest complexes to
be quenched. Therefore, we developed a new pathway for
fabricating acid/base-controllable FRET and self-assembling
systems based on the host−guest recognition of EtP5-RB/EtP5-
RBH with BDP-CN. These research results will be significant for
designing novel molecular sensors and devices.
(
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ASSOCIATED CONTENT
Supporting Information
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*
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03612.
1
35, 10190. (b) Cui, X.; Zhao, J.; Zhou, Y.; Ma, J.; Zhao, Y. J. Am. Chem.
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Syntheses, NMR data, MALDI-TOF MS spectra, 2D
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AUTHOR INFORMATION
Corresponding Authors
80, 568. (d) Chen, H.; Dong, B.; Tang, Y.; Lin, W. Acc. Chem. Res. 2017,
0, 1410.
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■
*
*
*
5
(
E-mail: sunjifu@zju.edu.cn.
E-mail: jiongzhou@zju.edu.cn.
E-mail: jieyang@zju.edu.cn.
ORCID
Jifu Sun: 0000-0001-7536-9759
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Fundamental Research Funds
for the Central Universities and China Postdoctoral Science
Foundation (2016M601928).
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DOI: 10.1021/acs.orglett.7b03612
Org. Lett. XXXX, XXX, XXX−XXX