Ferro-self-assembly: Magnetic and electrochemical adaptation of a multiresponsive zwitterionic metalloamphiphile showing a shape-hysteresis effect

Article abstract of DOI:10.1039/d0sc05249c

Metallosurfactants are molecular compounds which combine the unique features of amphiphiles, like their capability of self-organization, with the peculiar properties of metal complexes like magnetism and a rich redox chemistry. Considering the high relevance of surfactants in industry and science, amphiphiles that change their properties on applying an external trigger are highly desirable. A special feature of the surfactant reported here, 1-(Z)-heptenyl-1′-dimethylammonium-methyl-(3-sulfopropyl)ferrocene (6), is that the redox-active ferrocene constituent is in a gemini-position. Oxidation to 6+ induces a drastic change of the surfactant's properties accompanied by the emergence of paramagnetism. The effects of an external magnetic field on vesicles formed by 6+ and the associated dynamics were monitored in situ using a custom-made optical birefringence and dual dynamic light scattering setup. This allowed us to observe the optical anisotropy as well as the anisotropy of the diffusion coefficient and revealed the field-induced formation of oriented string-of-pearls-like aggregates and their delayed disappearance after the field is switched off.

Full text of DOI:10.1039/d0sc05249c

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Ferro-self-assembly: magnetic and electrochemical
adaptation of a multiresponsive zwitterionic
metalloamphiphile showing a shape-hysteresis
effect†
Cite this: Chem. Sci., 2021, 12, 270
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Stefan Bitter,a Moritz Schlotter,a Markus Schilling,a Marina Krumova,a
¨
ab
a
*
Sebastian Polarz
and Rainer F. Winter
Metallosurfactants are molecular compounds which combine the unique features of amphiphiles, like their
capability of self-organization, with the peculiar properties of metal complexes like magnetism and a rich
redox chemistry. Considering the high relevance of surfactants in industry and science, amphiphiles that
change their properties on applying an external trigger are highly desirable. A special feature of the
surfactant reported here, 1-(Z)-heptenyl-10-dimethylammonium-methyl-(3-sulfopropyl)ferrocene (6), is
that the redox-active ferrocene constituent is in a gemini-position. Oxidation to 6+ induces a drastic
change of the surfactant's properties accompanied by the emergence of paramagnetism. The effects of
an external magnetic field on vesicles formed by 6+ and the associated dynamics were monitored in situ
using a custom-made optical birefringence and dual dynamic light scattering setup. This allowed us to
observe the optical anisotropy as well as the anisotropy of the diffusion coefficient and revealed the
field-induced formation of oriented string-of-pearls-like aggregates and their delayed disappearance
after the field is switched off.
Received 22nd September 2020
Accepted 21st October 2020
DOI: 10.1039/d0sc05249c
rsc.li/chemical-science
Amphiphiles (or surfactants) combine hydrophilic (the so- using an external input such as the pH, temperature, ionic
called headgroups) and lipophilic entities (the so-called tails) as strength, or redox state.19–26
integral parts of their molecular structures. This particular
Paramagnetic amphiphiles, recently reviewed by Eastoe and
construction principle provides them with the ability to display coworkers, constitute a fascinating family of stimuli-responsive
concentration-dependent self-organization in nonpolar and surfactants.27 Particular attention has been paid to magnetic
polar solvents.1 Amphiphiles with advanced functions that go ionic liquids based on amphiphilic transition metal complexes,
far beyond the traditional ones as emulsiers, stabilizing agents as their properties are oen superior to those of conventional
for interfaces, or detergents were meanwhile realized by skillful magnetic uids (ferrouids).28–31 Self-assembly results in high
manipulation of any of its constituents.2–4 Recent examples are effective concentrations of the paramagnetic metal centers, and
micellar LEDs,5,6 catalysts,7–9 or batteries.10 Such applications this in turn allows us to control their physico-chemical prop-
are important hallmarks on the way to even more sophisticated erties and the morphologies of their superstructures through an
amphiphiles such as the ones found in nature, e.g. in the external magnetic eld. Such a scheme has the added advantage
pockets of enzymes.11–18 An important milestone is the advent of that the external stimulus is non-invasive. In many current
(multi-) stimuli-responsive amphiphiles, whose encoded func- realizations of such systems, however, the magneto-active
tionalities respond to (different) external triggers. Such systems (transition) metal ion is only present as a constituent of the
are capable of adaptive self-assembly, which can be controlled counterion of a cationic surfactant, but is not an integral
constituent of the surfactant itself.21,30,31
Some of us have previously reported redox-switchable as well
as paramagnetic stimuli-responsive amphiphiles of relevance to
a
¨
Department of Chemistry, University of Konstanz, Universitatsstrasse 10, 78457
the current work.32,33 We thought that ferrocene would be an
ideal building block in order to combine both these kinds of
stimuli within one single amphiphile.34–37 On oxidation, the
diamagnetic, hydrophobic ferrocene nucleus is transformed
into a paramagnetic S ¼ 1/2 ferrocenium ion with a distinct
hydrophilic character.38–41 Oxidation does hence not only
generate a magnetic moment, but also transfers the ferrocene
Konstanz, Germany. E-mail: rainer.winter@uni-konstanz.de
bInstitute of Inorganic Chemistry, Leibniz-University Hannover, Callinstrasse 9, 30167
Hannover, Germany. E-mail: sebastian.polarz@aca.uni-hannover.de
† Electronic supplementary information (ESI) available: Experimental procedures
and methods are included in the supporting information. The cif-le with the
structural details of
6 was deposited as CCDC 2003544. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc05249c
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nucleus from the lipo- to the hydrophilic part of the eld is switched off. Thus, the anisotropy of larger aggregates
amphiphile, thereby changing its entire structure. persists for more than 5 min, while the structural alignment of
A
1,10-disubstitution pattern of the ferrocene scaffold, which is smaller ones vanishes at a signicantly faster rate.
synthetically well accessible,34,42–44 seemed particularly suited
for such an endeavor.
Results
Studies on paramagnetic amphiphiles are oen thwarted by
Synthesis and characterization of FcNMe2SO3Heptene (6)
the non-trivial analytics involved in their characterization.
Detailed investigations oen rely on small-angle neutron scat-
tering (SANS), which is time-consuming and costly and suffers
from poor availability.27,30,31,45–47 Moreover, SANS is only of
limited value for following kinetically fast processes which
would be desirable for the live monitoring of structural changes
occurring in solution. Optical birefringence is a well-established
method to monitor the dynamic response of materials to
external elds.48–50 Although of high intrinsic value, optical
birefringence measurements in magnetic elds were only rarely
applied for the study of paramagnetic amphiphiles.29
FcNMe2SO3Heptene was obtained in four consecutive synthetic
steps from literature-known 1,10-dibromoferrocene (2) (Fig. 1).51
Formylation of 2 yields asymmetrically functionalized 1-formyl-
10-bromoferrocene (3). 3 was then subjected to a Wittig reaction
with hexyltriphenylphosphonium bromide to provide 1-(Z)-
heptenyl-10-bromoferrocene (4).52–54 Subsequent conversion
with Eschenmoser's salt under Mannich-like conditions55 fol-
lowed by quaternization with 1,3-propane sultone ultimately
yielded the target compound 6.56
1
6 was fully characterized by H and 13C{1H} NMR spectros-
We here report the zwitterionic, ferrocene-based amphiphile
FcNMe2SO3Heptene 6 (see Fig. 1, Fc ¼ ferrocenyl) with a sultone
headgroup. Compound 6 is unique in that its self-assembly
properties can be controlled by three different external stimuli,
namely the (i) addition of an electrolyte, (ii) addition of an
oxidant/reductant, and (iii) exposure to an external magnetic
eld. We also demonstrate that optical birefringence in
combination with dynamic light scattering (DLS) measure-
ments in two orthogonal directions provides detailed insights
into the functional response of aggregated magnetic nano-
particles formed by 6+ to an external magnetic eld in real time.
Specically, we have observed the formation of string-of-pearls-
like aggregates of 6+ in a magnetic eld (0.8 T), the eld-induced
anisotropy of the diffusion of aggregated nanoparticles, and
a hysteresis effect for their disappearance aer the magnetic
copy, electrospray ionization mass spectrometry (ESI MS), IR
spectroscopy, UV-Vis spectroscopy and X-ray crystallography.
Fig. S1(a–g) of the ESI† provide graphical representations of the
corresponding spectra and the assignments of IR peaks. The ESI
MS signals and their isotope patterns are in perfect agreement
with the simulated ones for the M+ and (M + H)+ peaks of 6. Of
particular note is the observation of intense mass peaks corre-
sponding to a dimer of 6 and to higher oligomers up to the
tetramer as well as their Na+ adducts. This is a token of the high
propensity of 6 to aggregate. UV-Vis absorption spectra recorded
in acetonitrile show an intense p / p* absorption band at lmax
¼ 284 nm and a weaker, characteristic band of the dd / dp/Cpp
excitation of a ferrocene species (the HOMO / LUMO transi-
tion) at lmax ¼ 444 nm (3 ¼ 225 Mꢀ1 cmꢀ1), which accounts for
the pale orange color of 6.57–59 The respective DFT calculated
frontier MOs of 6 are shown in Fig. S2 of the ESI.†
Surfactant properties of 6 in the presence and absence of
added salts
As is known for other sulfobetaine-containing polymers,60 the
solubility and the preferred conformation of 6 are highly
sensitive to the solvent polarity and the kind and concentration
of an added electrolyte. In nonpolar solvents, 6 undergoeꢀs
intramolecular ion pairing by backfolding of the C3H6SO3
moiety onto the ammonium part of the headgroup as was
veried by NOESY experiments in CDCl3 (Fig. S3a of the ESI†).
In spite of its zwitterionic sulfobetaine headgroup, 6 is only
sparingly soluble in pure water, already saturating at a concen-
tration of 0.87 mM. Under these conditions, 6 adopts an open
conguration as indicated by the absence of NOESY cross peaks
between the methylene protons of the propylene connector and
the methyl and methylene protons of the Fc-CH2NMe2 segment
(see Fig. 2a). In this open conguration, individual molecules of
6 associate with neighboring molecules by intermolecular ion
pairing between the oppositely charged ammonium and sulfo-
nate groups, thereby annihilating the individual charges. This
accounts for the low water solubility of 6.
Fig. 1 Synthesis of FcNMe2SO3Heptene (6). (a) Synthesis of 6; (b)
molecular structure of 6 crystallized from acetonitrile. C; dark grey, N;
This conformation and intermolecular ion pairing are
turquoise, Fe; orange, S; yellow, O; red, H atoms are omitted for clarity. preserved in the solid state. Packing diagrams in Fig. S4 of the
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Fig. 2 Triggered aggregation of compound 6 by the addition of an external electrolyte. (a) NOESY spectrum of 6 (22.98 mM) in pure D2O; (b)
NOESY spectrum of 6 (22.98 mM) in 11.5 mM KPF6 (0.5 eq.) in D2O; (c) graphical illustrations of the closed (left) and open (right) headgroup
conformations of 6; (d) graphical illustrations of aggregates (micelle and vesicle).
ESI† show that, in the crystal, molecules of 6 associate with four from 0.87 mM to 23 mM, with a concomitant change of the zeta
neighboring molecules via close interionic contact O/N of potential from +50 mV to ꢀ49 mV.
3.764 A to 4.098 A between the NR4+ and the RSO3ꢀ parts of the
headgroups.
In aqueous 0.1 M KNO3 or 0.01 M KPF6 solutions compound
6 shows remarkable surfactant properties and exhibits a surface
˚
˚
These interionic interactions are supported by a total of 10 activity g of 36 mN mꢀ1. The surface tension curve can be
strong, pairwise hydrogen bonds C–H/O between hydrogen treated as a Gibbs isotherm, and a surface excess area G of 6.93
2
atoms at the NMe groups or at a cyclopentadienide ligand and mmol mꢀ2 and an average area Am of 24 A per molecule at the
˚
˚
the sulfonate oxygen atoms. The latter range from 2.187 A, water–air interface are obtained. Dynamic light scattering (DLS)
˚
which is as much as 0.533 A shorter than the sum of their van and surface tension measurements (Fig. 3a) provide a critical
˚
der Waals radii, to 2.527 A. Additional hydrophobic interactions aggregation concentration cac of ca. 1.0 mM. At concentrations
between the oleophilic Z-heptenyl chains organize crystalline 6 > cac, and immediately aer preparation, aggregates with a size
into a lamellar structure resulting from head-to-head/tail-to-tail of DH z 4 nm form in both 0.1 M KNO3 and 0.01 M KPF6
arrangements of individual molecules. SAXS and PXRD (Fig. S8a and b†). Considering that 6 has a diameter of ca. 2.1
measurements of solid samples of 6 obtained from aqueous nm, one can infer that under these conditions spherical
solutions show a microcrystalline pattern with d100 ¼ 2.68 nm micelles are formed. Aer 5 days in solution, the size of the
and the corresponding higher-order reexes (see Fig. S5 of the aggregates has increased to 40–120 nm in aqueous KNO3 and to
ESI†). The observed PXRD pattern matches with that calculated 120–180 nm in aqueous KPF6. The dhkl value of 3.26 nm ob-
from the experimental X-ray structure, which proves that tained from SAXS/PXRD measurements corresponds to roughly
intermolecular ion pairing with an alignment of the oppositely twice the molecular length of 6. This is a typical feature of
charged headgroups prevails under these conditions.
vesicles. Cryo-transmission electron microscopy (cryo-TEM)
Disruption of this packing motif and the typical foam images consequently show large spherical aggregates, whose
formation of an amphiphile are, however, observed aer the diameters correspond well with the DLS data, besides smaller
addition of KPF6 or KNO3 to aqueous solutions of 6. According micelles. Area-selected energy-dispersive X-ray spectroscopy
to the Hofmeister salt series, large and polarizable ions, in (EDX), electron spectroscopic imaging (ESI), bright eld (BF)/
particular anions, have the largest effect (Fig. S7†).60,61 Aggre- dark eld (UDF) measurements (Fig. 3c–e), and electron energy
gation upon electrolyte addition can be monitored via NOESY loss spectroscopy (EELS) measurements in the scanning trans-
(Fig. 2b) and DOSY (Fig. S6 of the ESI†) measurements and is mission electron microscopy (STEM) mode (Fig. S9 of the ESI†)
further supported by the SAXS and PXRD data (see Fig. S5 of the conrm the presence of the respective elements (Fe, C, N, O, S
ESI†). Ion-pairing of the ammonium and sulfonate entities of and K) in the vesicle walls. At concentrations c z 4.34 mM the
the sultone headgroup with the appropriate ions of an external further growth of the primary aggregates into vesicles is effec-
electrolyte breaks the intermolecular ionic interactions. As tively suppressed as shown by the respective number distribu-
a consequence, the solubility of 6 in water increases 26 fold, tions (Fig. S8c and d of the ESI†).
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Fig. 3 Surfactant properties of 6. (a) Concentration-dependent surface tension and DLS measurements of 6 at t ¼ 0 d and after 5 d; (b) cryo-TEM
image of 6 (micelles and vesicles are denoted by black arrows); (c) area-selected EDX results of 6; (d) area-selected ESI images (TEM) of a dried
sample of 6 for Fe, O and C; (e) BF and UDF measurements in the STEM mode proving the presence of heavy atoms in the double layer
membranes of the vesicles. Scale bar: 25 nm.
A temperature increase from 20 ꢁC to 50 ꢁC has no detectable is sharper and more intense than the cathodic return wave. This
effect on aggregate formation (Fig. S8e of the ESI†). At even matches with earlier observations on ammonium-functional-
higher concentrations, typical amphiphilic properties are ized amphiphilic ferrocenes with a dodecyl chain at the
preserved and lyotropic liquid crystals are formed (Fig. S8f of ammonium nitrogen atom.62,63
the ESI†). Hence, addition of an electrolyte converts intermo-
Preparative oxidation of 6 was performed either chemically
lecular ion pairs or lamellar aggregates into micelles and vesi- with cerium ammonium nitrate ((NH4)2[Ce(NO3)6], CAN) as the
cles and simultaneously increases the solubility of 6 26 fold.
oxidant or by electrochemical means and is accompanied by
a color change from yellow to green (Fig S10a of the ESI†). The
UV-Vis absorption spectrum of 6+ in water features the typical
ferrocenium absorptions at lmax ¼ 585 nm (3 ¼ 220 Mꢀ1 cmꢀ1
)
Redox-induced changes of the surfactant properties of 6
and at lmax ¼ 820 nm (3 ¼ 382 Mꢀ1 cmꢀ1) (Fig. S10b of the
ESI†).64,65 Chemical (Na2S2O3) or electrochemical reduction
converts 6+ back to the neutral state. The NMR spectra of
a sample recovered aer a full oxidation/reduction cycle indi-
cate isomerization of the heptenyl tail from the Z to the E
conguration (see Fig. S10c of the ESI†). This is, however, not
expected to affect the amphiphilic properties.
An important feature of ferrocenes is their propensity to
undergo reversible one-electron oxidation. In the case of ferro-
cene-derived amphiphiles, the formerly hydrophobic ferrocene
nucleus is transformed into a hydrophilic ferrocenium species
and thus becomes part of the headgroup. The calculated elec-
trostatic potential surfaces shown in Fig. 4a clearly demonstrate
this.
Cyclic voltammograms (CVs) of 6 (le middle panel of
Fig. 4b) and 6+ (right panel) indicate that the 6/6+ wave consti-
tutes a chemically reversible redox couple with a half-wave
potential E1/2 of 438 mV versus the Ag/AgCl reference. While the
reduction peak of 6+ has the typical shape of a diffusion-
controlled process, the sharp, symmetric anodic counterpeak
indicates that electrogenerated 6 is adsorbed on the electrode
surface. Concomitantly, the anodic forward peak in the CV of 6
In a 0.1 M aqueous KNO3 solution, 6+ still forms a foam.
Concentration-dependent surface-tension measurements show
a surface activity g of 34 mN mꢀ1, which is virtually identical to
that of neutral 6 (Fig. S11a of the ESI†). The surface excess area
G of 4.84 mmol mꢀ2 is, however, ca. 30% smaller than that of 6
2
˚
and corresponds to an average area Am of 34 A per molecule of
6+ at the water–air interface. DLS measurements as depicted in
Fig. S11b of the ESI† indicate that aggregates of 6+ exhibit very
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Fig. 5 Schematic drawing of the setup to detect magnetic field-
induced changes in DLS and birefringence.
is an integral part of the amphiphilic structure of 6+, an external
magnetic eld might exert a large effect on the morphologies of
its aggregates. Seeking a method to monitor any such changes,
we combined two DLS setups with optical birefringence detec-
tion inside a split-coil electromagnet. The additional dual DLS
system allows for an independent determination of the diffu-
sion coefficients D and Dt, parallel and perpendicular to the
k
external eld. The ratio D /Dt represents the effect of the
k
structural alignment upon Brownian motion and strongly
supports our interpretation of how the aggregates respond to an
external magnetic eld. The experimental setup is schematically
shown in Fig. 5, and further details are provided in the Methods
section.48
Fig. 4 Redox-switchability of compound 6. (a) Electrostatic potential
surfaces of 6 (left) and 6+ (right); (b) CVs of 6 (left) and 6+ (right) ob-
tained in 0.1 M aqueous KNO3 with a sweep rate of 100 mV sꢀ1; (c)
cryo-TEM image of 6+ (left) and area-selected EDX spectrum (right);
The following results will show that our approach is indeed
capable of monitoring eld-induced morphological changes in
(d) graphical illustrations of the monomer and vesicles of 6+. Scale bar: real time. Moreover, we were also able to show that large
superstructures of aggregates of 6+ form and align parallel to
50 nm.
the magnetic eld when the eld is switched on and reorient
and/or disassemble with a sizable delay aer the eld is
switched off.
broad size distributions while their average size gradually
increases over time (see Fig. S11c of the ESI†). DLS yielded a cac
Optical birefringence Dn relies on the anisotropy of
of ca. 1.4 mM (Fig. S11d of the ESI†). The rather broad size
a molecular orientation distribution, according to eqn (1).
distribution of the dispersed aggregates matches with the
results of cryo-TEM measurements on a freshly prepared
Dn ¼ n ꢀ nt
(1)
k
sample of 6+ (Fig. 4c). Individual spherical objects (vesicles)
with double layer membranes of vastly different sizes and
curvatures are observed besides more irregularly shaped and
larger, fused vesicles as well as large quantities of smaller
micelles. No thermodynamically favored morphology seems to
prevail under these conditions. As the sulfonate moiety of 6+ can
interact not only with the ammonium but also with the cationic
ferrocenium moiety, 6+ may exist in several conformations
which differ with respect to their effective headgroup sizes. This
may lead to large variations in effective headgroup charges and
areas and, hence, different packing parameters. Area-selected
EDX measurements on chemically oxidized samples (Fig. 4c)
prove the presence of the elements Fe, N, S, O, C, and Ce in
these aggregates. EELS measurements (Fig. S12 of the ESI†) in
the STEM mode agree with the EDX data. Hence, while oxida-
tion does not have much of an inuence on the typical surfac-
tant properties of 6/6+, it inicts drastic changes on their self-
assembly properties and aggregation behavior in solution.
n is the refractive index for light that is polarized parallel to
k
the external magnetic eld Bext and nt corresponds to a polar-
ization perpendicular to Bext. The external magnetic eld
imposes the energy DcBext2, which in turn will act to deform any
magnetic aggregates. The magnetic energy is counteracted by
the thermal energy kT and the mechanical properties of the
particles' membranes.
Negligible birefringence features (Dn < 10ꢀ7) were obtained
for neutral 6 and the oxidant CAN, indicating that the observed
response is indeed due to 6+. The onset of a detectable response
to the external magnetic eld matched with the cac of ꢂ1.4 mM,
thus showing that it is tied to the presence of aggregates of 6+.
Under these conditions, an external magnetic eld of 0.8 T was
switched on and off for 10 min, respectively. Fig. 6 shows the
data collected for three freshly prepared, differently concen-
trated samples of 6+ (c ¼ 2.2, 3.3, and 4.3 mM). Magnetically
induced optical birefringence can be observed for all three
investigated samples, but with strongly differing intensities.
The rst indications of optical birefringence as a response to
the applied external magnetic eld appear ca. 2 h aer oxidation
of 6 to 6+. More substantial intensity increases are noted aer
ca. 3.5 h for the 2.2 mM sample, aer ca. 3 h for an amphiphile
The triggering of the surfactant properties of 6+ by magnetic
elds
Ferrocenium ions are associated with an effective magnetic
moment meff of 2.3–2.6 mB.38,39,65 As the ferrocenium constituent
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The magnetically induced birefringence was measured with
modulated laser light under simultaneous DLS monitoring of
the diffusion coefficients D and Dt, i.e. parallel and orthogonal
k
to the applied magnetic eld. The effective hydrodynamic
radius as determined from the Stokes–Einstein relation as
shown in Fig. 6b and the direction-dependent analysis of the
diffusion anisotropy as shown in Fig. S11 of the ESI† agree with
the observed changes of the optical birefringence. Since the
initial aggregates present
a broad size distribution, the
appearance and amount of higher aggregates that result from
exposing them to the magnetic eld do not scale linearly with
amphiphile concentration c (Dn ¼ 1$10ꢀ7 at c ¼ 2.2 mM, Dn ¼
3.0$10ꢀ6 for c ¼ 3.3 mM, Dn ¼ 3.8$10ꢀ6 for c ¼ 4.3 mM). The
birefringence, which represents the degree of orientation,
increases signicantly as the amphiphile concentration is
changed from 2.2 mM to 3.3 mM, but only moderately so on
increasing c further to 4.3 mM. Even larger differences are
encountered for the diffusion anisotropy, especially for the
most concentrated sample (4.3 mM, see Fig. S11 of the ESI†).
This indicates that higher aggregates are already present in the
3.3 mM solution. While being sufficiently long to be well-
aligned, they still exert only a modest inuence on the diffu-
sional behavior. At the even higher concentration of 4.3 mM,
the aggregates can grow into very long chains, whose diffusion
perpendicular to the axis of orientation is consequently
hindered. The latter are detected by their diffusion anisotropy
and their persistent orientational order (Fig. 8). These indirectly
observed, collective phenomena are in line with TEM and cryo-
Fig. 6 Magnetic field-induced (0.8 T) response of birefringence (top
panel (a)) and of the hydrodynamic radius (bottom panel (b)). The x-
axis indicates the time passed after oxidation. The field was switched
on and off for 10 minutes, respectively. The shaded, grey areas denote
the time during which the field was switched on (B0 ¼ 0.8 T). (a) The
magnetically induced optical birefringence; (b) effective hydrody-
namic radius obtained by averaging the two DLS measurements. The
dots represent the mean hydrodynamic radius Rh. The width of the
semi-transparent area at Rh is a measure of the polydispersity PDI,
scaled by a factor of 0.2.
concentration of 3.3 mM, and even aer ca. 2.5 h for the 4.3 mM
sample of 6+. Every sample reached a plateau value within ca. 4–
5 h aer oxidation.
Fig. 7 (a) TEM (2.2 mM) and (b)–(d) cryo-TEM images of 3.3 and 4.3 mM samples of 6+ collected from our setup after exposing them to the
magnetic field (0.8 T) for 5 h. (e) Schematic drawing of the reversible magnetic field-induced deformation of vesicles of paramagnetic amphiphile
6+ in a magnetic field and the fusion of individual vesicles into string-of-pearls-like assemblies, and their stepwise disassembly or loss of
orientational order after the field is switched off (openings are drawn for better visualization of their inner structures).
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A second, considerably slower component appears only for the
more concentrated samples with c ¼ 3.3 mM (s2 ¼ 30 s) or 4.3
mM (s2 ¼ 60 s). Free rotational diffusion on the timescale of s2
corresponds to a structure length of 6–8 mm. The contribution
of s2 can thus be attributed to the population of mid-sized
chain-like arrays of primary aggregates as sketched in the
middle right panel of Fig. 7e. The birefringence of the sample at
the highest investigated concentration of 4.3 mM displays an
additional, even slower decay with a time constant s3 of ca. 10
min. Such a long time constant corresponds to even more
extended structures with lengths of >15 mm (note that, due to
their kinked structures, the actual chainlike aggregates are
probably even longer, see Fig. 7b–d). Dual DLS measurements of
the higher concentrated sample (4.3 mM) reveal that the
diffusion coefficient parallel to the external eld is ca. 1.5 times
larger than that measured in a perpendicular direction. This
indicates a preferential alignment of the chain-like super-
structures parallel to the eld (Fig. S14 of the ESI†). The derived
diffusion anisotropy ratio of 1.5 is quite large and is fully
consistent with the kinked structures of these chains (note that
for an ideal, perfectly aligned rod an isotropy factor of 2 would
be expected).68
Fig. 8 Response and relaxation of magnetically induced birefrin-
gence. The field was switched on/off for 10 minutes each. Solid lines:
samples aged for 5:10 h; dashed lines: samples aged for 4:30 h; dotted
lines: samples aged for 3:10 h after oxidation; the blue, orange or
green colors represent the three different concentrations employed in
this study. Panel (a) shows the full cycle from switching the field on
until full relaxation, and panel (b) shows the relaxation that is observed
upon switching the field off. Three distinct timescales can be identified.
A fast relaxation occurring within a few seconds is present in all
samples. A second relaxation s2 of ca. 30–60 s is indicated by the
shaded gray area with white circles. The birefringence of the sample
with a concentration of 4.3 mM does fully not vanish with s2 but then
decays with a much slower relaxation time s3 of 10 min (indicated by
the shaded gray area with white star symbols) until ca. 5 min (two
exponential functions with s2 and s3 are provided as a guide to the eye).
This is followed by a final, faster decay with a relaxation time s02 of 60 s.
5 min aer switching the magnetic eld off, these extended
structures lose their orientational order and probably collapse
with a concomitant decay of the birefringence and an associated
time constant s02 of 60 s. This nal, faster relaxation thus
indicates that the larger chains collapse into smaller fragments
with a maximum length of ca. 8 mm, similar to the length limit
obtained at the intermediate concentration. The bottom right
image of Fig. 7e provides a sketch of these transformations.69,70
The overall behavior of concentrated solutions of 6+ is therefore
reminiscent of shape-hysteresis materials, as the morphology
changes only occur with a delay aer an outer perturbation is
applied.
TEM measurements on samples retrieved aer exposing 2.2, 3.3
and 4.3 mM solutions of 6+ to the 0.8 T eld in our birefringence
setup. Representative images as shown in Fig. 7a–d and S13 of
the ESI† clearly reveal the formation of deformed aggregates at
the lower c and of string-of-pearls-like aggregates of slightly
deformed vesicles with lengths of up to 30 mm, particularly for
4.3 mM solutions.
Most remarkably, for every pulse of the magnetic eld, i.e.
aer switching the magnet on or off, a fast response of the
aggregates in solution can be observed (see Fig. 8). Within ca.
30–60 s a plateau is reached and only slight changes of the
optical birefringence can be observed aerwards. The plateau
value of Dn for each pulse scales with the concentrations and
average sizes of the spherical aggregates present in solution.
The time-dependent changes of the optical birefringence
aer the magnetic eld is switched off are particularly revealing.
The time constant for relaxation is proportional to r3, where r is
half the length of the long axis of the particle for both response
modes, deformation and orientation. The rotational relaxation
time srot for a diffusing rigid particle is given by
Conclusions and outlook
The novel surfactant FcNMe2SO3Heptene (6) presented herein
offers a unique combination of features that are rooted in
utilizing ferrocene as the central building block in conjunction
with a zwitterionic sultone headgroup. This renders 6 a multi
stimuli-responsive surfactant, whose aggregation behavior can
be altered by three different inputs, namely (i) a change of the
ionic strength, which increases the solubility of 6 26 fold with
a concomitant structure change from intermolecular ion pairs
or lamellar structures to rst micelles and then vesicles; (ii)
reversible oxidation of the ferrocene constituent to para-
magnetic 6+, which leads to a distinct broadening of the size
distribution of these aggregates; and (iii) application of an
external magnetic eld (0.8 T), which induces a transformation
from individual aggregates to chain-like superstructures. Most
importantly, we have employed magnetic optical birefringence
as a powerful means to monitor the assembly and disassembly
of larger, chain-like superstructures in situ and in real time. This
allowed us to detect a shape-memory-like hysteresis of the
structural anisotropy, as orientationally ordered, larger chains
persist for ca. 5 min aer the external magnetic eld is switched
s
rot z 6phr3/kT,
(2)
where h denotes the viscosity of the medium. By analyzing the
relaxation behavior aer the eld is switched off one can esti-
mate the sizes of the largest aggregates that were formed while
the eld was switched on.66,67
The relaxation behavior of the birefringence Dn is presented
in Fig. 8. A fast component with a time constant s1 of ca. 1 s is
present for all samples at every concentration. This component
is attributed to the fast relaxation of the population of deformed
small aggregates such as the ones shown in Fig. 7a with a size of
roughly 1 mm, most probably along the magnetic eld axis (Bext).
276 | Chem. Sci., 2021, 12, 270–281
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off. This may open new horizons in dynamic, non-equilibrium representations of molecular orbitals were generated with the
self-assembly. At the same time we have demonstrated that help of GNU parallel and plotted using the vmd program
magnetic optical birefringence with simultaneous double DLS package in combination with POV-Ray.78–80 Electron microscopy
detection provides a powerful means for the live monitoring of was performed on a Zeiss Libra 120 TEM instrument and a JEOL
the formation of oriented aggregates of paramagnetic nano- JEM 2200FS with accelerating voltages of 120 and 200 kV,
particles and their subsequent disassembly and the loss of respectively. Both microscopes were equipped with in column
orientational order aer the eld is switched off. Given our energy lters. For cryo-TEM measurements the samples were
success, we expect that this technique will be of high utility for prepared using a Grid-Plunger Leica EM GP. 3 mL of the
the future investigation of such systems.
dispersion were dropped onto both sides of a Quantifoil holey
Because this effect and the associated hysteresis are the carbon lm grid. The excess liquid was removed with a lter
consequences of a eld-induced polarization and a molecular paper and the resulting thin water lms were vitried by rapidly
reorientation process, we see similarities to certain aspects immersing (plunging) into liquid ethane. The specimens
from a totally different area, namely the transition from para- to were then inserted into a cryo-transfer sample holder Gatan 914
ferromagnetic or from piezo- to ferroelectric materials. As the and transferred to the TEM instrument. X-Ray diffraction
advent of typical “ferro”-characteristics, such as remanescent analysis on single crystals of 6 grown from acetonitrile was
behavior, hysteresis patterns, etc., have provided a boost in performed on
a STOE IPDS-II diffractometer equipped
ensuing properties and applications, we are condent this can with a graphite-monochromated Mo Ka radiation source
˚
also be the case for surfactants with ferro-self-assembly (l ¼ 0.71073 A) and an image plate detection system at 100.15 K.
features.
Using Olex2, the structures were solved with the SIR2004
structure solution program using direct methods or the ShelXT
structure solution program using Intrinsic Phasing and rened
with the ShelXL renement package using Least Squares
minimization.81–84 Hydrogen atoms were introduced at their
Methods
General
NMR experiments were carried out on a Varian Unity Inova 400, calculated positions. Optical in situ spectroscopy was performed
a Bruker Avance III DRX 400, or a Bruker Avance Neo 800 MHz as follows: the sample is placed in an electromagnet and illu-
1
spectrometer. H and 13C NMR spectra were referenced to the minated with a laser (HeNe 632 nm, 10 mW). The birefringence
respective solvent signal. 2D-NMR experiments were used to is detected from the transmitted beam. Enhanced detection
unequivocally assign the NMR resonances. The numbering of sensitivity is achieved with the use of a photoelastic modulator
the nuclei is provided in Fig. S1 of the ESI.† ESIMS data were (PEM II/FS84 from Hinds Instruments), two Glan–Thompson
acquired on a Bruker microTOF focus II system. Cyclic vol- polarizers (aligned at ꢃ45ꢁ with respect to the magnetic eld)
tammetry was performed in a one-compartment cell with 5–7 and a lock-in amplier (Stanford ResearchSR830-DSP). The
mL of deionized water as the solvent and KNO3 (0.1 M) as the value of birefringence is measured by compensation of the
supporting electrolyte. A gold electrode (B ¼ 1.1 mm, BASI) was signal with a pockels cell.85,86 The two DLS detection optics are
used as the working electrode. A computer controlled BASi aligned at a scattering angle of 9ꢁ, one with a horizontal and the
EPSILON potentiostat was used for recording the voltammo- other with a vertical scattering plane. The scattered light is
grams. An Ag/AgCl (3 M KCl) reference electrode in water was collected using mono-mode bers and detected with single
used in combination with a platinum wire auxiliary electrode. photon detectors (ALV SO-SIPD) that are connected to a hard-
IR/NIR spectra were recorded on a FT-IR Bruker Tensor II ware correlator (Flex02-12D/C from http://correlator.com). The
instrument. UV/Vis spectroscopy was performed on a diode- intensity autocorrelation functions are analyzed separately with
array unit TIDAS manufactured by J&M ANALYTIK. ATR-IR a cumulant method. The diffusion anisotropy is calculated
spectra were measured on a PerkinElmer 100 spectrometer directly from the ratio of the rst cumulants. The effective
equipped with an ATR unit. DLS measurements were performed hydrodynamic radius is calculated from the average diffusion
on a Malvern Zen5600 instrument. The liquid crystal picture coefficient, using the Stokes–Einstein relation. The poly-
was obtiained with an Olympus CX41 light microscope. For dispersity is estimated from the second cumulant.87
¨
surface tension measurements, a Kruss K100 instrument was
used. Additional surface tension measurements were per-
formed by the capillary method. SAXS measurements were
Synthesis and characterization
acquired on a Bruker Nanostar system equipped with a pinhole The reactions were performed using standard Schlenk tech-
collimator and a Cu Ka radiation source. Powder X-ray diffrac- niques under a N2 atmosphere. Solvents were dried according to
tion measurements (PXRD) were performed with a Bruker AXS standard procedures and stored under an argon atmosphere.
D8 Avance diffractometer using Cu-Ka radiation and a lynxeye Water was deionized using a Millipore Milli-Q. C6D6, CD2Cl2,
detector. Geometry optimization and orbital calculations were CDCl3, D2O and MeOH-d4 were supplied by Eurisotop. Starting
performed using DFT with the Gaussian16 package for ab initio materials for syntheses were purchased from commercial
electronic structure calculation using the pbe1pbe/def2-TZVP sources unless stated otherwise. Hexyltriphenylphosphonium
level of theory.71–75 The GaussSum program package was used to bromide (Ph3PHexyl+Brꢀ),88 1,10-dibromoferrocene (FcBr2, (2)),51
analyze the results, while the visualization of the results was and 1-formyl-10-bromoferrocene (FcBrCHO, (3))54 were prepared
performed with the Avogadro program package.76,77 Graphical according to the literature. Detailed procedures are provided in
© 2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci., 2021, 12, 270–281 | 277

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the ESI.† The atom numbering pertinent to the NMR discussion (vt, 3JHH ¼ 1.9 Hz, 2H, H-8), 4.37 (s, 2H, H-5), 4.34 (vt, 3JHH ¼ 1.9
is provided in the ESI† together with the corresponding NMR Hz, 2H, H-9), 3.45–3.39 (m, 2H, H-3), 2.96 (s, 6H, H-4), 2.88 (t,
spectra.
3JHH ¼ 6.9 Hz, 2H, H-1), 2.31–2.15 (m, 4H, H-2, H-14), 1.54–1.45
(m, 2H, H-15), 1.43–1.35 (m, 4H, H-16, H-17), 0.95 (t, 3JHH ¼ 7.1
Hz, 3H, H-18). 13C NMR (151 MHz, CDCl3) d 131.67 (s, C-13),
124.53 (s, C-12), 84.07 (s, C-11), 72.86 (s, C-7), 72.35 (s, C-9),
71.97 (s, C-6), 70.36 (s, C-10), 70.15 (s, C-8), 65.68 (s, C-5), 63.21
(s, C-3), 49.55 (s, C-4), 48.07 (s, C-1), 31.80 (s, C-16), 29.53 (s, C-
15), 29.19 (s, C-14), 22.74 (s, C-17), 19.73 (s, C-2), 14.25 (s, C-18).
ESIMS: [g molꢀ1]: (4M + Na + H)+ ¼ (C92H141Fe4N4O12S4Na)+
calc.: 1868.67, found: 1868.67; (4M + Na)+ ¼ (C92H140Fe4N4-
O12S4Na)+ calc.: 1867.66, found: 1867.66; (3M + Na)+ ¼ (C69-
1-(Z)-Heptenyl-10-bromoferrocene (FcBrHeptene, 4)
BrPPh3Hex (3.25 g, 7.61 mmol, 1 equiv.) was dissolved in 60 mL
of THF. Aer the addition of KOtBu (0.85 g, 7.61 mmol, 1 equiv.)
to the turbid solution, a colour change to red was observed. The
solution was stirred for 1 h. A solution of FcBrCHO (2.23 g, 7.61
mmol, 1 equiv.) in 20 mL of THF was added dropwise over
a period of 20 min. The solution was stirred overnight. 40 mL of
n-pentane were added, and the precipitate was ltered off. The
solvent was removed in vacuo and the crude product was puri-
ed by column chromatography (50% EE/PE) yielding
FcBrHeptene (2.52 g, 7.00 mmol, 92%) as a brown oil.
H
105Fe3N3O9S3Na)+ calc.: 1406.50, found: 1406.50; (2M + Na)+ ¼
(C46H70Fe2N2O6S2Na)+ calc.: 945.33, found: 945.33; (2M + H)+ ¼
(C46H71Fe2N2O6S2)+ calc.: 923.34, found: 923.34; (M + K)+
(C23H35FeNO3SK)+ calc.: 500.13, found: 500.13; (M + Na)+
(C23H35FeNO3SNa)+ calc.: 484.16, found: 484.15; (M + H)+
¼
¼
¼
1
FcBrHeptene was obtained selectively in (Z)-conguration. H
NMR (400 MHz, CDCl3) d 6.06 (dt, 3JHH,cis ¼ 11.3 Hz, 4JHH ¼ 1.8
Hz, 1H, CpC-H), 5.55 (dt, 3JHH,cis ¼ 11.3 Hz, 3JHH ¼ 7.2 Hz, 1H,
(C23H36FeNO3S)+ calc.: 462.18, found: 462.17; (M)+ ¼ (C23H35
-
3
3
FeNO3S)+ calc.: 461.17, found: 461.17; (M ꢀ NMe2SO3)+
¼
C]CH), 4.32 (vt, JHH ¼ 1.9 Hz, 2H, Cp-H), 4.30 (vt, JHH ¼ 1.9
(C18H23Fe)+ calc.: 295.11, found: 295.11. IR (powder): 3067,
Hz, 2H, Cp-H), 4.26 (vt, 3JHH ¼ 1.9 Hz, 2H, Cp-H), 4.05 (vt, 3JHH
¼
3037, 2955, 2918, 2853, 1638. UV-Vis (MeCN): ¼ 225 Mꢀ1 cmꢀ1
.
1.9 Hz, 2H, Cp-H), 2.25–2.22 (m, 2H, CH2), 1.51–1.42 (m, 2H,
3
CH2), 1.39–1.32 (m, 4H, CH2), 0.92 (t, JHH ¼ 7.2 Hz, CH3).
Conflicts of interest
1-(Z)-Heptenyl-10-dimethylaminomethylferrocene
(FcNMe2Heptene, 5)
The authors declare no competing nancial interests.
FcBrHeptene (1.14 g, 3.16 mmol, 1 equiv.) was dissolved in 50
mL of THF and the solution was cooled to ꢀ78 ꢁC. Then, 2.0 mL
of a 1.6 M solution of n-BuLi in hexane (3.16 mmol, 1 equiv.)
were added dropwise over a period of 5 min. The solution was
Acknowledgements
The current research was funded by an ERC consolidator grant
(I-SURF; project 614606). We gratefully acknowledge the Euro-
pean Research Council for nancial support. The authors also
ꢁ
stirred at ꢀ78 C for another 10 min. Eschenmoser's salt (585
mg, 3.16 mmol, 1 equiv.) was added and the temperature was
kept at ꢀ78 ꢁC for one hour. Then, 20 mL of distilled water and
40 mL of ethyl acetate were added. The phases were separated,
and the aqueous phase was extracted twice with ethyl acetate.
The combined organic layers were washed with brine and dried
over MgSO4. The solvent was removed in vacuo and the crude
product was puried by column chromatography (5% NEt3/PE)
yielding 400 mg of FcNMe2Heptene (1.18 mmol, 37%) as
a brown oil. 1H NMR (400 MHz, CDCl3) d 6.02 (d, 3JHH,cis ¼ 11.5
Hz, 1H, CpC-H), 5.51 (dt, 3JHH,cis ¼ 11.5 Hz, 3JHH ¼ 7.2 Hz, 1H,
C]CH), 4.28–4.24 (m, 2H, Cp-H), 4.18–4.15 (m, 2H, Cp-H),
4.11–4.06 (m, 4H, Cp-H), 3.25 (s, 2H, NCH2), 2.26–2.24 (m, 2H,
CH2), 2.18 (s, 6H, NCH3), 1.52–1.42 (m, 2H, CH2), 1.41–1.34 (m,
¨
acknowledge the state of Baden-Wurttemberg and the Deutsche
Forschungsgemeinscha (DFG) for providing us with access to
the supercomputing facilities of the bwHPC program (Grant No.
INST 40/467-1 FUGG). We are indebted to Georg Maret for
helpful discussions and thank Stephan Siroky for his help with
the 3D graphics and Michael Linseis for DFT calculations and
XRD measurements.
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