Fluorescent molecular spring that visualizes the extension and contraction motions of a double-stranded helicate bearing terminal pyrene units triggered by release and binding of alkali metal ions

Article abstract of DOI:10.1039/c9cc06126f

A novel double-stranded spiroborate helicate bearing terminal pyrene residues exhibited reversible fluorescence color changes (green and blue colors) due to the unique unidirectional springlike motions of the helicate triggered by catch and release of alk

Full text of DOI:10.1039/c9cc06126f

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Fluorescent molecular spring that visualizes the extension and
contraction motions of a double-stranded helicate bearing terminal
pyrene units triggered by release and binding of alkali metal ions†
Received 00th January 20xx,
Accepted 00th January 20xx
Daisuke Taura,ab Kaori Shimizu,b Chiaki Yokota,b Riho Ikeda,a Yoshimasa Suzuki,b Hiroki Iida,bc
DOI: 10.1039/x0xx00000x
Naoki Ousakaab and Eiji Yashima*ab
A novel double-stranded spiroborate helicate bearing terminal introduction of substituents at the meta-position of the terminal
pyrene residues exhibited reversible fluorescence color changes phenyl residues (rac-DH2aBNaB–·Na+ and rac-DH2bBNaB–·Na+)7
(green and blue colors) due to the unique unidirectional springlike (Fig. 1a) maintained their double-stranded helicate structures,
motions of the helicate triggered by catch and release of alkali thus showing reversible ion-triggered springlike motions.8
metal ions.
On the other hand, pyrene and its derivatives have been
extensively used as a powerful fluorescent probe for the sensing
and detection of specific molecules, ions and biomolecules9 as
well as for studying the structures of synthetic and biological
macromolecules10 because of their characteristic excimer
formations, thus displaying a green fluorescence, being
different from the corresponding monomer’s blue
fluorescence.9,10 However, to the best of our knowledge,
fluorescent color changes have never been utilized to visualize
unidirectional springlike molecular motions of artificial helical
systems. To this end, we have synthesized a novel spiroborate-
based double-stranded racemo-helicate (rac-DH3BNaB–·Na+)
composed of ortho-biphenylene-linked tetraphenol strands (L3)
bearing pyrene units at both terminals as a fluorophore and
investigated its reversible fluorescent color change during the
ion-triggered extension and contraction motions (Fig. 2).
The development of artificial helical molecules and polymers
which undergo reversible musclelike extension and contraction
(elastic) motions by responding to external stimuli has attracted
continuous interest because of their possible application to
molecular
actuators,
nanodevices
and
molecular
nanomachines.1 In contrast to biological helical systems,
however, most of synthetic helical systems rarely exhibit a
unidirectional twisting motion.2,3
We previously reported that an ortho-linked tetraphenol
strand with a biphenylene unit in the middle formed a unique
double-stranded helicate bridged by two spiroborates in which a
Na+ ion is encapsulated in the center of the helicate (rac-
DH1BNaB–·Na+)4 (Fig. 1a). This helicate displayed an intriguing
Na+ ion-triggered, reversible extension and contraction motion
coupled with a unidirectional twisting motion (Fig. 1b) upon
the release and binding of the Na+ ion.4 Detailed
thermodynamic studies using its extended tetrabutylammonium
(TBA) salts (rac-DH1BB2–·(TBA+)2) revealed that the extended
helicate also formed similar inclusion complexes with a series
of monovalent cations and their binding constants (Ka)
decreased with the increasing size of the cations (Fig. 1b).5 The
replacement of the central biphenylene units of the rac-
DH1BNaB–·Na+ with other functional units, such as the 4,4′-
linked 2,2′-bipyridine and its N,N’-dioxide residues6 and the
The double-stranded spiroborate helicate bearing the pyrene
units at both ends (rac-DH3BNaB–·Na+) was first attempted to
prepare in a similar manner for the synthesis of the analogous
–
spiroborate helicates (rac-DH1BNaB–·Na+ and rac-DH2BNaB
·Na+) (Fig. 1a)4,6-8 by the reaction of the pyrene-bound
tetraphenol L3 with NaBH4 (1 equiv) in 1,2-
dichloroethane/EtOH (6/1, v/v) at 80 °C for 18 h (Fig. 1c),
resulting in the precipitate as the major product in 55% yield,
which was composed of the spiroborate helicate as confirmed
by the electron-spray ionization (ESI) mass spectrometry (Fig.
S14a, ESI†). However, the X-ray crystallographic analysis of
the single crystals of the helicate grown from its CH3CN
solution in the presence of n-tetrabutylammonium bromide
(TBA+·Br−) (10 equiv) revealed that the resulting double-
stranded spiroborate helicate is not a racemo-helicate, but an
extended meso-helicate (meso-DH3BB2–·(TBA+)2) with a plane
of symmetry (Fig. 1c).11 The two tetraphenol strands are
bridged by two spiroborates in a parallel way with each other
with a B-B distance (dB-B) of 12.8 Å, which is similar to those
of the extended meso- (11.8 Å) and racemo- (13.0 Å) helicates
a. Department of Molecular and Macromolecular Chemistry, Graduate School of
Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan. E-mail:
yashima@chembio.nagoya-u.ac.jp
b. Department of Molecular Design and Engineering, Graduate School of
Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
c. Present Address: Department of Chemistry, Graduate School of Natural Science
and Technology, Shimane University, 1060 Nishikawatsu, Matsue 690-8504,
Japan
† Electronic supplementary information (ESI) available: CCDC 1914679 (meso-
DH3BB2–·(TBA+)2). Experimental procedures, characterizations of ligand (L3) and
helicates (meso-DH3BB2–·(Na+)2 and rac-DH3BNaB–·Na+) and additional
spectroscopic data. See DOI: 10.1039/XXXX
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DOI: 10.1039/C9CC06126F
Fig. 2 Schematic representation of a reversible switching of the fluorescence emission
induced by the extension and contraction motions of rac-DH3BNaB–·Na+ upon the release
and catch (or binding) of a Na+ ion, respectively. The structures were obtained by DFT
calculations (Fig. S3, ESI†).
rings significantly shifted upfield due to the ring current effects
of the benzene rings of the other strand and those of the pyrene
units, respectively. The calculated contracted structure of rac-
DH3BNaB–·Na+ by the density functional theory (DFT) supports
this speculation (Fig. S3, ESI†). Further concrete evidence was
obtained by a chiral HPLC analysis of rac-DH3BNaB–·Na+
(CHIRALPAK IB, DAICEL), demonstrating complete
separation into the enantiomers (Fig. S10, ESI†).
The ion-triggered extension and contraction motions of rac-
1
DH3BNaB–·Na+ were then investigated by H NMR, absorption
and fluorescent spectroscopy. Upon the addition of 2.5 equiv of
–
cryptand[2.2.1] to a solution of the contracted rac-DH3BNaB
·Na+ in CH2Cl2/CH3CN (6/1, v/v),12,13 which completely
removed the weakly bound Na+ ion within the contracted
helicate at the middle, the 1H NMR signals of the aromatic and
tBu protons remarkably shifted downfield (Fig. 3a(iv)),
resulting from the formation of the extended helicate rac-
DH3BB2–·(Na+[2.2.1])2. Further addition of 2.5 equiv of
NaN(SO2CF3)2 to the extended helicate quantitatively
–
Fig. 1 (a) Chemical structures of double-stranded spiroborate helicates rac-DH1BNaB
·Na+, rac-DH2aBNaB–·Na+ and rac-DH2bBNaB–·Na+. (b) Schematic illustration of the
unidirectional springlike motion of the double-stranded spiroborate helicate rac-
DH1BB2–·(TBA+)2 upon binding (or catch) and release of alkali metal (Li+, Na+, K+, Rb+
–
regenerated the original contracted helicate (rac-DH3BNaB
and Cs+) ions. TBA: tetrabutylammonium. (c) Synthesis of
a double-stranded
1
·Na+) as shown in the H NMR spectral changes (Fig. 3a(v)).
spiroborate meso-helicate meso-DH3BB2–·(Na+)2. Capped-stick representation of the X-
ray crystal structure of meso-DH3BB2–·(TBA+)2 (side (left) and top (right) views). All the
hydrogen atoms, solvent molecules and countercations are omitted for clarity. A
photograph of meso-DH3BB2–·(Na+)2 in CH3CN under irradiation at 365 nm is also
Since the extension is coupled with the twisting motion of the
helicate in one direction, this (unidirectional) springlike motion
could be reversibly repeated by the sequential addition of
cryptand[2.2.1] and Na+ ions as previously demonstrated for
rac-DH1BNaB–·Na+ and rac-DH2BNaB–·Na+.4,5,7
shown in (c). (d) Isomerization of meso-DH3BB2–·(Na+)2 to
spiroborate racemo-helicate rac-DH3BNaB–·Na+.
a double-stranded
2– 4,5
Of particular interest is that this ion-triggered springlike
extension and contraction motions of rac-DH3BNaB–·Na+ are
accompanied by a fluorescence color change due to a structural
change in the terminal pyrene units between a free monomer
and a stacked dimer, which are readily visible with the naked
eye and can be quantified by fluorescence spectroscopy. The
pyrene units of the contracted rac-DH3BNaB–·Na+ tilt away from
each other (Fig. 2), thereby exhibiting the characteristic blue
emission of the pyrene monomer at 440 nm in CH2Cl2/CH3CN
(6/1, v/v) under irradiation at 365 nm (Fig. 3c(i)). The addition
of 2.5 equiv of [2.2.1]cryptand resulted in the formation of the
extended rac-DH3BNaB–·Na+, in which the terminal pyrene
residues in each strand are positioned in close spatial proximity
(Fig. 2), thus showing the green excimer emission around 500
nm (Fig. 3c(ii)). Further addition of 2.6 equiv of NaN(SO2CF3)2
completely restored the fluorescence spectrum accompanied by
of DH1BB
.
The resulting meso-DH3BB2–·(Na+)2 was then quantitatively
converted to the contracted racemo-helicate (rac-DH3BNaB
·Na+) in 1,2-dichloroethane/CH3CN (6/1, v/v) at 60 °C for 48 h
(Fig. 1d and Fig. S1, ESI†) based on a method for the meso-to-
racemo isomerization of the meso-DH1BB helicate through
water-mediated dynamic B–O bond cleavage/reformation of the
spiroborate groups.5 The formation of the intertwined double-
stranded helical structure of rac-DH3BNaB–·Na+, in which a Na+
ion was embraced within the helicate at the middle, was
confirmed by ESI mass spectrometry (Fig. S14b, ESI†) and
further supported by the interstrand ROE cross-peak (a··f) (Fig.
–
2–
1
S7, ESI†) and characteristic proton resonances of its H NMR
spectrum due to the contracted spiroborate-based helicate like
the contracted rac-DH1BNaB–·Na+ (Fig. 3a(iii)); the aromatic
proton signals of the terminal (a–c) and central (f–h) benzene
2 | J. Name., 2012, 00, 1-3
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tBu
tBu
tBu
g
tBu
h
of [2.2.1]cryptand in CD3CN12 (Fig. S8, ESI†). The pyrene
View Article Online
Py
Py
Na+
Py
Py
Py
f
Py
Py
cryptand
[2.2.1]
units of meso-DH3BB ·(Na+)2 are in closDeOpIr:o1x0.i1m03i9ty/C, 9thCeCr0e6f1o2r6eF,
2–
i
O
O
O
O
O
O
O
O
O
O
O
O
B
O
Na+
B
O
B
O
B
O
i
NaN(SO2CF3)2
CH2Cl2/MeCN
= 6/1
the helicate exhibited an intense interstrand excimer emission
(green color) (Fig. 1c) around 500 nm in CD3CN, which also
remained unchanged upon the addition of 3 equiv of
[2.2.1]cryptand (Fig. S9, ESI†), indicating its structural feature
which prevents the ion-triggered extension and contraction
motions.
a
b
a
b
h
e
e
Py
g
f
tBu
j
tBu
tBu
c d
tBu
c
d
25 °C
j
contracted rac-DH3BNaB– ·Na+
extended rac-DH3BB2– ·(Na+[2.2.1])2
Na+
OH
OH
i
a
(a)
Py
j
(i) L3
c d e f
h
g
tBu
b
2
ed
a
L3
c
g
i
b
OH
OH
h
f
Recently, we have determined the Ka values of the extended
rac-DH1BB–·(TBA+)2 for a series of alkali metal ions in
CH3CN, which decreased in the following order:5 Li+ ≥ Na+ >
K+ > Cs+ > Rb+. The extended rac-DH3BB2–·(Na+[2.2.1])2 was
anticipated to have a similar trend in CH3CN, but in
CH2Cl2/CH3CN (6/1, v/v), the binding affinities toward alkali
metals were significantly enhanced to form the contracted rac-
DH3BMB–·M+ (M+ = Li+, Na+, K+ or Cs+).13,14 If this is the case,
the extended pyrene-bound helicate could be applied to the
visual fluorogenic sensing of alkali metals,9a,16 since the
contracted rac-DH3BMB–·M+ embracing the larger cations
tended to adopt more elongated structures by an induced-fit
mechanism.5 The addition of 2.0 equiv of Li+, Na+ or K+ ions to
a solution of the extended rac-DH3BB2–·(Na+[2.2.1])2 induced
a drastic change in its fluorescence color from the green
excimer emission to the blue monomer emission due to the
contracted rac-DH3BMB–·M+ (M+ = Li+, Na+ or K+) formation,
whereas the excimer emission was predominant in the presence
of 2.0 equiv of Cs+ ions (Fig. 4). This is because the contracted
helicate embracing the larger Cs+ cation resulted in a more
elongated structure being suitable for the excimer emission in
part, indicating that the extended pyrene-bound helicate can be
used as a fluorescent chemosensor for alkali metals based on
the unique ion-triggered springlike motion.
(ⅱ ) meso-DH3BB2– ·(Na+)2
h,a,e
j
d
c
g
i
b
f
(ⅱ ) rac-DH3BNaB– ·Na+
j
e
Na+
d
c
f
a
g
i
b
h
*
*
a
*
e
(ⅱ ) (ⅱ ) + [2.2.1] (2.5 equiv)
j
d
b
c
g
i
h
f
(ⅱ ) (ⅱ ) + NaN(SO2CF3)2 (2.5 equiv)
Na+
8.4 8.2 8.0 7.8 7.6 7.4 7.2 7.0 6.8 6.6 6.4 6.2 6.0
/ ppm
1.6 1.5 1.4 1.3

(b)
(c)
3
2
1
0
(i) rac-DH3BNaB– ·Na+
(i) rac-DH3BNaB– ·Na+
(ii) (i) + [2.2.1]
Na+
(ii) (i) + [2.2.1]
(iii) (ii) + NaN(SO2CF3)2
(iii) (ii) + NaN(SO2CF3)2
(i), (iii)
(i) (ii) (iii)
(i), (iii)
(ii)
(ii)
225
300
400
500
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 3 (a) Partial 1H NMR spectra (500 MHz, CD2Cl2/CD3CN = 6/1 (v/v), 0.5 mM, 25
°C) of (i) L3, (ii) meso-DH3BB2–·(Na+)2, (iii) rac-DH3BNaB–·Na+, (iv) (iii) + [2.2.1] (2.5
equiv) and (v) (iv) + NaN(SO2CF3)2 (2.5 equiv). CD3CN was used as the solvent for
meso-DH3BB2–·(Na+)2 (ii).12 * in (iii) denotes the Bu and aromatic protons from meso-
t
DH3BB2–·(Na+)2 (ca. 1 mol%). The peak assignments were done on the basis of gCOSY
and ROESY spectra (Fig. S4 and S5, ESI†). Other sharp and broad peaks in (ii–iv) were
assigned to the terminal pyrene protons. (b) Absorption and (c) fluorescence spectra
(CH2Cl2/CH3CN = 6/1 (v/v), 10 M, ca. 25 °C) of (i) rac-DH3BNaB–·Na+, (ii) (i) +
[2.2.1] (2.5 equiv) and (iii) (ii) + NaN(SO2CF3)2 (2.6 equiv). Excited wavelength: 365
nm. Photographs of (i) rac-DH3BNaB–·Na+ (10 M), (ii) (i) + [2.2.1] (2.5 equiv) and (iii)
(ii) + NaN(SO2CF3)2 (2.6 equiv) in CH2Cl2/CH3CN under irradiation at 365 nm are also
shown in (c).
In summary, we have successfully visualized a reversible
springlike motion of a double-stranded helicate by introducing
pyrene units at the terminal phenyl residues. The extension and
contraction motions of the helicate are triggered by the release
and binding of alkali metal ions, which eventually displays
fluorescence color changes from green to blue emissions due to
a pyrene excimer formation and its monomer, respectively. The
fluorescent helicate can be resolved into optically-pure one-
handed helicates (Fig. S10, ESI†), which will be further applied
the fluorescence color change from green to blue (Fig. 3c(iii)),
therefore, these changes could be reversibly repeated by the
alternative addition of cryptand[2.2.1] and Na+ ions. As a
consequence, for the first time, we have achieved the
fluorescent visualization of the molecular springlike motion
triggered by the binding and release of Na+ ions.
2–
As expected from the crystal structure of meso-DH3BB
·(TBA+)2 (Fig. 1c), its H NMR spectral pattern was similar to
Li+
K+
1
Ionic radius
2–
Na+
that of the extended racemo-helicate rac-DH3BB
Small
Large
Cs+
Na+ Li+ K+ Cs+
·(Na+[2.2.1])2 (Fig. 3a(ii,iv)) and cross-peaks between the
[2.2.1]
strands (j··g and j··h)5 were observed in the 2D ROESY
2–
spectrum (Fig. S5, ESI†), indicating that the meso-DH3BB
·(Na+)2 maintained its meso-structure in solution as well as in
the solid state. Because of the extended rigid meso-structure,
meso-DH3BB2–·(Na+)2 cannot accommodate a Na+ ion in the
center. As a result, the springlike extension and contraction
motions observed in rac-DH1BNaB–·Na+ did not proceed upon
the Na+ ion release by [2.2.1]cryptand and further binding of a
Monomer
Excimer
Emission
300
400
500
600
700
Wavelength (nm)
Fig. 4 Fluorescence spectra (CH2Cl2/CH3CN = 6/1 (v/v), 10 M, ca. 25 °C) of rac-
DH3BB2–·(Na+[2.2.1])2 (10 M) in the presence of 2.0 equiv of Li+, Na+, K+ and Cs+
ions, respectively. Excited wavelength: 365 nm. Photographs of rac-DH3BB
·(Na+[2.2.1])2 (10 M) in the presence of 2.0 equiv of Li+, Na+, K+ and Cs+ ions,
2–
1
Na+ ion as supported by negligible changes in the H NMR
spectrum of meso-DH3BB2–·(Na+)2 after the addition of 3 equiv
respectively, in CH2Cl2/CH3CN under irradiation at 365 nm are also shown.
This journal is © The Royal Society of Chemistry 20xx
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N. Ousaka, K. Shimizu, Y. Suzuki, T. Iwata, M. Itakura, D.
Taura, H. Iida, Y. Furusho, T. Mori aDndOIE: 1.0Y.10a3s9h/iCm9aC,CJ0.61A2m6F.
Chem. Soc., 2018, 140, 17027–17039.
Y. Suzuki, T. Nakamura, H. Iida, N. Ousaka and E. Yashima,
J. Am. Chem. Soc., 2016, 138, 4852–4859.
K. Miwa, K. Shimizu, H. Min, Y. Furusho and E. Yashima,
Tetrahedron, 2012, 68, 4470–4478.
For other functional spiroborate-based helicates containing
porphyrin and photoresponsive stilbene units in the middle,
see: (a) S. Yamamoto, H. Iida and E. Yashima, Angew.
Chem., Int. Ed., 2013, 52, 6849–6853; (b) N. Ousaka, S.
Yamamoto, H. Iida, T. Iwata, S. Ito, Y. Hijikata, S. Irle and
E. Yashima, Nat. Commun., 2019, 10, 1457; (c) D. Taura, H.
Min, C. Katan and E. Yashima, New J. Chem., 2015, 39,
3259–3269.
to the development of unique chirality-responsive molecular
springs toward chiral guests, such as chiral organic ammoniums
and π-electron-deficient aromatic molecules; the latter may be
sandwiched between the pyrene units in a diastereoselective
fashion, resulting in fluorescent color changes.17 Research
along this line is now in progress in our laboratory.
This work was supported in part by JSPS KAKENHI
(Grant-in-Aid for Specially Promoted Research, no. 18H05209
(E.Y.) and Grant-in-Aid for Scientific Research (C), no.
18K05059 (D.T.)).
View Article Online
6
7
8
Conflicts of interest
There are no conflicts to declare.
9
For reviews of pyrene-based supramolecular chemical
sensors, see: (a) S. Karuppannan and J.-C. Chambron,
Chem.–Asian J., 2011, 6, 964–984; (b) E. Manandhar and K.
J. Wallace, Inorg. Chim. Acta, 2012, 381, 15–43.
Notes and references
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11 A similar meso-helicate (meso-DH1BB2–·(Na+)2) has recently
been found to form as a precipitate during the reaction of the
corresponding tetraphenol strand with NaBH4.5
12 Acetonitrile was used as the solvent for the 1H NMR,
2
For examples of stimuli-responsive springlike motions with
artificial helical systems, see: (a) O.-S. Jung, Y. J. Kim, Y.-A.
Lee, J. K. Park and H. K. Chae, J. Am. Chem. Soc., 2000,
122, 9921–9925; (b) M. Barboiu, G. Vaughan, N. Kyritsakas
and J. M. Lehn, Chem.–Eur. J., 2003, 9, 763–769; (c) K.
2–
absorption and fluorescent measurements of meso-DH1BB
·(Na+)2 because the meso-to-racemo isomerization readily
took place with the increasing amount of CH2Cl2 in
acetonitrile (Fig. S1a, ESI†).
2–
Maeda, H. Mochizuki, M. Watanabe and E. Yashima, J. Am. 13 The binding constant (Ka) of the extended rac-DH3BB
Chem. Soc., 2006, 128, 7639–7650; (d) E. Berni, B.
Kauffmann, C. Y. Bao, J. Lefeuvre, D. M. Bassani and I.
Huc, Chem.–Eur. J., 2007, 13, 8463–8469; (e) H.-J. Kim, E.
Lee, H.-s. Park and M. Lee, J. Am. Chem. Soc., 2007, 129,
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·(Na+[2.2.1])2 to a Na+ ion estimated in CD3CN by 1H NMR
titration (3.4 × 103 M-1) (Fig. S11, ESI†) was three orders of
magnitude lower than that of the extended rac-DH1BB2– (2.68
× 106 M-1),4 probably because of the bulky pyrene units at
both ends, while the Ka value increased to 3.3 × 106 M-1 in
CH2Cl2/CH3CN (6/1, v/v) (Fig. S12, ESI†) which was
estimated by fluorescence titrations. In addition, rac-
DH1BNaB–·Na+ almost retained its contracted form in
CH2Cl2/CH3CN (6/1, v/v) at the concentration range between
10-3 and 10-6 M during dilution absorption and fluorescence
experiments (Fig. S13a,b, ESI†), indicating the Ka value was
as high as ca. 106 M-1 in CH2Cl2/CH3CN (6/1, v/v).
Therefore, a CH2Cl2/CH3CN (6/1, v/v) mixture was employed
as the solvent throughout the course of the ion-triggered
extension and contraction experiments.
14 The Ka value of the extended rac-DH3BB2–·(Na+[2.2.1])2 to
a Cs+ ion in CH2Cl2/CH3CN (6/1, v/v) roughly estimated by
dilution experiments was ca. 106 M-1 (Fig. S13c,d, ESI†),
which is of the same order of magnitude as that to a Na+ ion,
probably due to the solvent effect of nonpolar CH2Cl2.15
15 R. M. Izatt, K. Pawlak, J. S. Bradshaw and R. L. Bruening,
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Rev., 2015, 44, 4619–4644.
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Chem. Rev., 2004, 104, 1687–1716; (b) G. A. Hembury, V.
V. Borovkov and Y. Inoue, Chem. Rev., 2008, 108, 1–73; (c)
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4 | J. Name., 2012, 00, 1-3
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Journal Name
COMMUNICATION
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ChemComm
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Graphical abstract
View Article Online
DOI: 10.1039/C9CC06126F
Fluorescent molecular spring
Na+
Cs+
Contracted helicate
Extended helicate
Excimer
emission
Monomer
emission
A unique springlike motion of a fluorescent pyrene-terminated double-stranded helicate is visualized by the
catch and release of alkali metal ions.
5
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