Cation exchange reversibly switches rotor speed and is monitored by a networked fluorescent reporter

Article abstract of DOI:10.1039/c9dt01633c

Framework 1, a freely rotating turnstile, is transformed by sequential metal ion addition into the coordination-based double-minimum rotors [M₂(1)]²ⁿ⁺ that operate at 8 kHz (M = Zn²⁺; n = 2) and 30 kHz (M = Cu⁺; n = 1). In a network with the fluorescent receptor 2, the metal ion exchange at [M₂(1)]²ⁿ⁺ and thus indirectly the rotor speed is reported by distinct fluorescence changes at 2.

Full text of DOI:10.1039/c9dt01633c

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Cation exchange reversibly switches rotor speed
and is monitored by a networked fluorescent
reporter†
Cite this: DOI: 10.1039/c9dt01633c
Received 17th April 2019,
Accepted 29th May 2019
Merve S. Özer, Indrajit Paul, Abir Goswami and Michael Schmittel
*
DOI: 10.1039/c9dt01633c
rsc.li/dalton
Framework 1, a freely rotating turnstile, is transformed by sequen- protocol constitutes an adaptable toolbox that could be
tial metal ion addition into the coordination-based double- applied to a variety of rotor and optical reporter systems.
minimum rotors [M2(1)]2n+ that operate at 8 kHz (M = Zn2+; n = 2)
and 30 kHz (M = Cu+; n = 1). In a network with the fluorescent state behaviour of rotor 1 that is reversibly regulated from slow
receptor 2, the metal ion exchange at [M2(1)]2n+ and thus indirectly to fast motion by addition/exchange of different metal ions is
Here, we report on a networked system in which the two-
the rotor speed is reported by distinct fluorescence changes at 2.
monitored by luminescence changes at a remote receptor 2
(Scheme 1). Our design was guided by the following consider-
ations: intrinsic rotation within the framework 1 occurs about
a tolane unit9 and thus should proceed without a sizable
barrier as in a typical turnstile. The situation ought to change
once metal ions (Zn2+ or Cu+) ions are added, since the pyr-
idine terminals are expected to bind to the metal-loaded
phenanthroline stations via HETPYP-I (HETeroleptic PYridine
and Phenanthroline Metal)10 complexation. Now the arm is
expected to exchange at defined speed between the two degen-
erate metal-loaded stations. The luminescent aza-crown ether
2 was designed for communication with 1 as we expected
different emission for the free and cation-loaded receptor.
Among the many types of molecular devices,1 thermally acti-
vated turnstiles2 and rotors3 allow us to study and comprehend
the relationship between motion and smart molecular pro-
perties,4 as lately demonstrated in catalytic rotary machinery.5
The finding that product inhibition and catalytic activity
directly depend on the machine speed5 suggests that one does
not need a unidirectional motor for sophisticated effects in
catalysis. Such results ask for a more systematic exploration of
rotor systems with speed regulation.6
Despite recent advances,6 reversible toggling of rotational
speed in rotors needs further exploration, in particular when
the course of switching is to be reported by an optical signal.
As shown by Hosseini, it is possible to control the motion of
turnstiles by addition of metal ions (stop-go) and monitor the
process by luminescence changes.2a,7 For instance, in a turn-
stile containing a bis-pyridyl terminated rotator and a triden-
tate-appended stator, drastic emission changes in the open vs.
closed state (with Ag+ and Pd2+) allowed monitoring of the
reversible ON/OFF switching of motion.2b
Implementation of speed regulation in rotors linked with
optical monitoring requires matching of two functions in one
device at the same time. Due to our expertise in networking
devices8 we considered another approach, i.e. to link an exter-
nal optical reporter to the rotor by chemical signalling. Such
Center of Micro and Nanochemistry and Engineering, Organische Chemie I,
Universität Siegen, Adolf-Reichwein-Str. 2, D-57068 Siegen, Germany.
E-mail: schmittel@chemie.uni-siegen.de; Tel: +49(0) 2717404356
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, compound characterizations, spectral data, UV-vis titrations data. See
DOI: 10.1039/c9dt01633c
Scheme 1 Communication between nanorotor 1 and reporter 2 upon
sequential addition of Zn2+ and Cu+ ions.
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Scheme 3 Brief retrosynthetic analysis of ligand 1.
CD2Cl2 : CD3CN (98 : 2). In the 1H NMR all proton signals of
phenanthroline 3 were shifted upfield while both aromatic
and aliphatic protons of aza-crown ether 4 showed downfield
shifts as a result of zinc(II) binding (Fig. 1c). Finally, two equiv.
of cyclam were added for removal of Cu+ and Zn2+ in order to
regenerate the initial state represented by the free ligands 3, 4
and 5 (see NMR, Fig. 1d).
Guided by this model study, the design of the rotor and
reporter was finalized (Scheme 1). In scaffold 1 the pyridine
arm and shielded phenanthroline stations are covalently con-
nected preventing the possibility of pyridine translocation in
presence of Zn2+ and Cu+ as in the model self-sorting.
Scheme 2 Self-sorting of ligands 3, 4 and 5 in the presence of Zn2+
and Cu+ ions.
Before addressing the precise design (Scheme 1) of the
rotor and luminescence reporter, we optimized the involved
binding sites by studying various two-step self-sorting events.
We observed that the 1 : 1 : 1 blending of the shielded phenan-
throline 3, aza-crown ether 4 and 4-iodopyridine (5) in pres-
ence of one equivalent of Zn2+ (Scheme 2) cleanly furnished
the HETPYP-I10 complex [Zn(3)(5)]2+. In the 1H NMR, all
phenanthroline protons of 3 were downfield shifted while
those of pyridine, e.g. α-protons, were moved upfield from 8.25
to 7.78 ppm (Fig. 1a and b). The aza-crown ether 4 remained
unaltered attesting a two-fold incomplete11 self-sorting. In the
second self-sorting, addition of Cu+ triggered a translocation
of Zn2+ from [Zn(3)(5)]2+ to aza-crown ether 4 to furnish
[Zn(4)(5)]2+ while copper(I) coordinated to phenanthroline 3
(85%). Importantly, as the binding of Cu+ to phenanthroline 3
in CH2Cl2 (log K = 5.42 0.10, see Fig. S63†) is stronger than
Scaffold 1 was synthesized through a series of Sonogashira
couplings in 15 steps and characterized by NMR, ESI-MS and
elemental analysis (Scheme 3, ESI†). Characteristic signals in
1
the H NMR proved the formation of 1, e.g. peaks of the pyri-
dine protons α-H and β-H appeared at 8.57 and 7.44 ppm in
addition to distinctive phenanthroline proton signals. The MS
peak at m/z = 1270.8 showed the expected mass for [1 + H]+.
For fluorescence monitoring, the aza-crown receptor in 2
was decorated with an anthracene unit. It was synthesized as
described in the ESI† and characterized by spectroscopic
methods and ESI-MS. After excitation at λ = 410 nm, the
system 2 showed a bright yellow emission at λ = 552 nm as a
result of intramolecular charge transfer (ICT). When the aza-
crown ether part was coordinated to zinc(II), the fluorescence
wavelength shifted to λ = 448 and 475 nm (vide infra).
that of Zn2+ (log K = 4.58
0.58, see Fig. S64†), copper(I)
replaced zinc(II) at the phenanthroline binding site of 3 in
Firstly, [Zn2(1)]4+ was prepared by mixing 1 and Zn(OTf)2
in a 1 : 2 ratio. Phenanthroline protons 4-H to 8-H shifted
downfield owing to zinc(II) ion coordination, while signals
of pyridine protons α-H broadened and moved upfield, indi-
cating that the zinc(II)-bound pyridine terminal was located in
the shielding region of the mesityl groups of the zinc(II)
phenanthroline stations (Fig. 2a and b). ESI-MS confirmed
the formation by
a peak at m/z = 858.0 representing
[Zn2(1)(H2O)](OTf)22+. Its hydrodynamic radius was calculated
to 11 Å by using D = 4.7 × 10−10 m2 s−1 from DOSY measure-
ments (the DFT-computed theoretical radius is 9.6 Å). The
1
Fig. 1 Partial 1H NMR (500 MHz, 25 °C) of (a) 3, 4 and 5 (1 : 1 : 1 ratio)
single set of signals in the H NMR suggested rapid exchange
in CD2Cl2, (b) [Zn(3)(5)]2+ and
4 in CD2Cl2 : CD3CN (98 : 2) after of the pyridine arm between both zinc(II)-loaded phenanthro-
Zn(OTf)2 addition, (c) after [Cu(CH3CN)4]PF6 addition to b: [Zn(4)(5)]2+
and [Cu(3)]+, (d) after cyclam addition to c: 3, 4 and 5 in CD2Cl2 : CD3CN
(98 : 2). (‡: The corresponding signal belongs to 2’-H caused by
line stations. The kinetics of motion in [Zn2(1)]4+ was deter-
mined by a VT 1H NMR study from 25 °C to −75 °C in CD2Cl2/
CD3CN (80 : 20) (Fig. 3a); the solvent mixture12 was required to
keep the complex [Zn2(1)]4+ soluble. At 25 °C the single set of
signals for the phenanthroline protons attested fast exchange,
interference of
3 with 4; self-sorting is >98%; *: Corresponding
proton signals of [Zn(3)]2+ from which 15% of Zn2+ was not
translocated).
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Table 2 1H NMR shift comparison of some characteristic protons of
ligand 1, [Zn2(1)]4+ and [Cu2(1)]2+ (ligand 1 and [Cu2(1)]2+ were measured
in CD2Cl2 and [Zn2(1)]4+ was measured in CD2Cl2 : CD3CN (96 : 4))
Protons
δ/ppm ligand 1
δ/ppm [Zn2(1)]4+
δ/ppm [Cu2(1)]2+
a-H
b-H
8.57
7.44
6.56
7.01
6.67
7.10
4-H
5 and 6-H
7-H
8.49
7.91, 7.80
8.32
8.94
8.27, 8.17
8.87
8.77
8.13, 8.06
8.66
8-H
9 and 9′-H
7.57
6.97, 6.96
7.97
6.94
7.90
6.96
mation of [Cu2(1)(CH3CN)]2+. The diffusion coefficient D = 4.4
× 10−10 m2 s−1 (DOSY) suggested a hydrodynamic radius of
12 Å (compare to 9.6 Å from the DFT-computed structure). As
Fig. 2 Partial 1H NMR (500 MHz, 25 °C) of (a) 1 and 2 (1 : 2 ratio) in
CD2Cl2, (b) after Zn(OTf)2 addition to a: [Zn2(1)]4+ and
2
1
before, the H NMR showed fast exchange of the pyridine arm
in CD2Cl2 : CD3CN (98 : 2), (c) after [Cu(CH3CN)4]PF6 addition to b:
[Cu2(1)]2+ and [Zn(2)]2+ in CD2Cl2 : CD3CN (98 : 2), (d) after cyclam between both copper(I)-loaded phenanthroline stations. In the
VT 1H NMR performed in CD2Cl2 from +25 to −75 °C (Fig. 3b),
addition to c: 1 and 2 in CD2Cl2 : CD3CN (98 : 2) (*: Corresponding
proton signals of [Zn2(1)]4+ from which 9% of Zn2+ was not
translocated).
a splitting of protons 4-, 5-, 6-, 7-, 8-, 9- and 9′-H was observed.
Kinetic parameters were calculated from the changes of proton
4-H in 1, which split into copper(I)-loaded (8.74 ppm) and pyri-
dine → Cu+ complexed (8.85 ppm) phenanthroline stations at
−50 °C. The exchange rate at room temperature was deter-
mined to k25 = 30 000 s−1 (Table 1).
The stochastic rotational exchange in nanorotors [Zn2(1)]4+
‡
and [Cu2(1)]2+ occurs at activation barriers ΔG25 of 50.5 and
47.4 kJ mol−1, respectively. Since, departure of the rotator arm
is endergonic and rate determining3a it was expected that rotor
[Cu2(1)]2+ has a slightly higher rotational rate than [Zn2(1)]4+
because the pyridine → copper(I) (18.3 kJ mol
gonic than the pyridine → zinc(II) dissociation (22.2 kJ mol−1
Fig. S65†). Moreover, dissociation of pyridine from the metal
center may be accelerated via an SN2 displacement by nucleo-
philic components in the solvent, e.g. acetonitrile.12
)
−1 3e is less ender-
;
Fig. 3 Partial VT 1H NMR (600 MHz) at various temperatures of
(a) [Zn2(1)]4+ in CD2Cl2 : CD3CN (80 : 20), (b) [Cu2(1)]2+ in CD2Cl2.
To probe the interdependent system of rotor and fluo-
rescence reporter, ligands 1 and 2 were mixed (1 : 2). The UV-
vis analysis in 0.2% CH3CN in CH2Cl2 displayed absorptions at
λ = 255, 438 and 458 nm, characteristic of receptor 2 (Fig. 4a).
After addition of two equiv. of Zn(OTf)2, nanorotor [Zn2(1)]4+
while rotational exchange slowed down visibly at −25 °C. Now
two sets of signals emerged for phenanthroline proton 4-H,
one resonating at 8.92 ppm for the pyridine → Zn2+ complexed
site and the other at 8.81 ppm for the exclusively zinc(II)-
loaded phenanthroline. The exchange frequency was deter-
mined as k25 = 8000 s−1 using WinD-NMR;13 the full set of acti-
vation data is depicted in Table 1.
Similarly, the reaction of two equiv. of [Cu(CH3CN)4]PF6
with 1 furnished [Cu2(1)]2+ as proven by 1H NMR (Fig. 2a
and c). Compared to nanorotor [Zn2(1)]4+, the phenanthroline
protons were shifted upfield upon Cu+ coordination while the
pyridine protons were slightly shifted downfield (Table 2). In
the ESI-MS a signal at m/z = 719.5 (Fig. S58†) showed the for-
was afforded while receptor
2 remained unloaded, as
suggested in NMR (Fig. 2b) and also by UV-vis and emission
spectra. The UV-vis spectrum still displayed the absorption
bands of the free receptor 2 at λ = 255, 438 and 458 nm (Fig. 4a
and b). Equally, the emission profile did not change after
addition of zinc(II); the maximum remained at λ = 552 nm as
in the case of receptor 2 only (Fig. 4c and d). It is important to
note that the zinc(II) rotor does not exhibit any emission when
excited at λ = 410 nm.
To test the communication via metal translocation, two equiv.
of [Cu(CH3CN)]PF6 were added. 1H NMR results suggested that
91% of the copper(I) rotor [Cu2(1)]2+ formed while parallel Zn2+
was translocated to receptor 2 (Fig. 2c). The NMR signals of 2
changed on the way to [Zn(2)]2+, e.g. proton k-H shifted down-
field from 6.71 to 6.87 ppm. At the same time, the UV-vis ana-
lysis equally validated that Zn2+ had translocated to receptor 2
to afford [Zn(2)]2+ (Fig. 4b), because absorption bands
Table 1 Experimental activation parameters of nanorotors and
rotational frequency at 25 °C
ΔH‡
ΔS‡
ΔG‡
k25
Nano-rotors
(kJ mol−1
)
(J mol−1 K−1
)
(kJ mol−1
)
(s−1
)
[Zn2(1)]4+
[Cu2(1)]2+
53.4 0.4
50.4 0.3
9.7 2.0
9.9 1.2
50.5
47.4
8000
30 000
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as rotator, whose rotational barrier at room temperature is esti-
mated as 2.4–2.7 kJ mol−1.9 Nanorotor [Zn2(1)]4+, formed upon
addition of Zn2+, operates at k25 = 8000 Hz and the Cu+-loaded
system [Cu2(1)]2+ rotates at k25 = 30 000 Hz. The three-step
transformation of 1 (>109 kHz) → [Zn2(1)]4+ (8 kHz) →
[Cu2(1)]2+ (30 kHz) was achieved by the sequential addition of
zinc(II) and copper(I) ions with the last transformation being
monitored by 2 as fluorescence reporter. While the ensemble
of rotor [Zn2(1)]4+ and free receptor 2 exhibited an emission at
552 nm, the addition of copper(I) triggering zinc(II) transloca-
tion (t1/2 = 762 s) to afford receptor [Zn(2)]2+ shifted the emis-
sion to λ = 448 and 475 nm. Fully reversible ion exchange
between 1 and receptor 2 was demonstrated over two cycles,
indicating that multifunctional (i.e. rotation & emission) and mul-
ticomponent systems work in a reliable and coherent manner.
Fig. 4 Comparison of UV-vis spectra (25 °C, 0.2% CH3CN in CH2Cl2) of
(a) 2 and [Zn(2)]2+ at 5.00 μM, (b) 1 (2.50 μM) + 2 (5.00 μM), [Zn2(1)]4+ + 2,
Conflicts of interest
[Cu2(1)]2+ + [Zn(2)]2+ and 1 + 2 + [Cu(cyclam)]+ + [Zn(cyclam)]2+
;
There are no conflicts to declare.
comparison of emission spectra (25 °C, λexc = 410 nm, 0.2% CH3CN in
CH2Cl2) of (c) 2 and [Zn(2)]2+ at 1.25 μM, (d) 1 (0.625 μM) + 2 (1.25 μM),
[Zn2(1)]4+ + 2, [Cu2(1)]2+ + [Zn(2)]2+ and 1 + 2 + [Cu(cyclam)]+
+
[Zn(cyclam)]2+
.
Acknowledgements
We acknowledge generous financial support from the
Deutsche Forschungsgemeinschaft (DFG Schm 647/20-2).
appeared at λ = 268, 412 and 438 nm, identical to the absorp-
tions of separately prepared [Zn(2)]2+. Parallel, emission spec-
troscopy exhibited a blue-shifted fluorescence at λ = 448 and
475 nm (Fig. 4d). Finally, four equivalents of cyclam were
added to capture all copper(I) and zinc(II) ions and to resume
the initial state (Fig. 2d). The process of sequential addition of
metal ions (Zn2+ and Cu+) and of cyclam was repeated up to
two cycles (ESI) proving full reversibility of the networking by
1H NMR and emission data.
Upon addition of two equiv. of [Cu(CH3CN)4]PF6 with
respect to rotor [Zn2(1)]4+ (3.60 μM) in the presence of receptor
2 (7.20 μM), translocation of Zn2+ from ligand 1 to receptor 2
took approximately 60 min in 0.2% CH3CN in CH2Cl2 by
UV-Vis. Due to an intermittent absorbance at 268 nm, attributed
to [Cu(2)]+, the mechanism is suggested as follows: first, the
copper(I) ion coordinates to aza-crown 2, then the copper(I) and
zinc(II) ions slowly exchange their binding sites due to global
thermodynamics since the coordination of Cu+ to the phenan-
throline ligand (log K = 5.42) is stronger than that of Zn2+ (log K
= 4.58) while both ions have almost identical binding data to 2
(log K = 4.08 vs. 3.91). Kinetic analysis suggests a first-order
kinetics (t1/2 = 762 s at 25 °C) for translocation.
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