Electrochemical Switching of a Fluorescent Molecular Rotor Embedded within a Bistable Rotaxane

Article abstract of DOI:10.1021/jacs.0c03701

We report how the nanoconfined environment, introduced by the mechanical bonds within an electrochemically switchable bistable [2]rotaxane, controls the rotation of a fluorescent molecular rotor, namely, an 8-phenyl-substituted boron dipyrromethene (BODIP

Full text of DOI:10.1021/jacs.0c03701

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Article
Electrochemical Switching of a Fluorescent Molecular
Rotor Embedded within a Bistable Rotaxane
Yilei Wu, Marco Frasconi, Wei-Guang Liu, Ryan M. Young, William
A. Goddard, Michael R. Wasielewski, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 May 2020
Downloaded from pubs.acs.org on May 29, 2020
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Electrochemical Switching of a Fluorescent
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Molecular Rotor Embedded within a Bistable Rotaxane
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Yilei Wu,† Marco Frasconi,*,§ Wei-Guang Liu,‡ Ryan M. Young,† William A. Goddard III,‡
Michael R. Wasielewski*,†,‖ and J. Fraser Stoddart*,†, ∆,┴
‖
†Department of Chemistry and Institute for Sustainability and Energy at Northwestern (ISEN),
Northwestern University, 2145, Sheridan Road, Evanston, Illinois 60208, United States
§Department of Chemical Sciences, University of Padova, Via Marzolo 1, Padova 35131, Italy
‡Materials and Process Simulation Center, California Institute of Technology, Pasadena,
California 91125, United States
∆ Institute for Molecular Design and Synthesis, Tianjin University, 92 Weijin Road, Nankai District,
Tianjin 300072, China
┴School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
*Correspondence Addresses
Professor Marco Frasconi
Department of Chemical Sciences
University of Padova
Via Marzolo 1, Padova 35131, Italy
Email: marco.frasconi@unipd.it
Professor Michael R. Wasielewski
Department of Chemistry and Institute for Sustainability and Energy at
Northwestern (ISEN), Northwestern University
2145 Sheridan Road, Evanston, IL 60208-3113 (USA)
Fax: (+1)-847-467-1425
Email: m-wasielewski@northwestern.edu
Professor J. Fraser Stoddart
Department of Chemistry
Northwestern University
2145 Sheridan Road, Evanston, IL 60802-3113 (USA)
Fax: (+1)-847-491-1009
Email: stoddart@northwestern.edu
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ABSTRACT: We report how the nanoconfined environment, introduced by the mechanical bonds
within an electrochemically switchable bistable [2]rotaxane, controls the rotation of a fluorescent
molecular rotor, namely an 8-phenyl-substituted boron dipyrromethene (BODIPY). The
electrochemical switching of the bistable [2]rotaxane induces changes in the ground-state co-
conformation and in the corresponding excited-state properties of the BODIPY rotor. In the
starting redox state, when no external potential is applied, the cyclobis(paraquat-p-phenylene)
(CBPQT4+) ring component encircles the tetrathiafulvalene (TTF) unit on the dumbbell component,
leaving the BODIPY rotor unhindered and exhibiting low fluorescence. Upon oxidation of the
TTF unit to a TTF2+ dication the CBPQT4+ ring is forced toward the molecular rotor leading to an
increased energy barrier for the excited-state to rotate the rotor into the state with the high non-
radiative rate constant, resulting in an overall 3.4-fold fluorescent enhancement. On the other hand,
when the solvent polarity is high enough to stabilize the excited charge transfer state between the
BODIPY rotor and the CBPQT4+ ring, the movement of the ring towards the BODIPY rotor
produces an unexpectedly strong fluorescent signal decrease as the result of the photoinduced
electron transfer from the BODIPY rotor to the CBPQT4+ ring. The nanoconfinement effect
introduced by mechanical bonding can effectively lead to the modulation of the physicochemical
properties as observed in this bistable [2]rotaxane. On account of the straightforward synthetic
strategy and the facile modulation of switchable electrochromic behavior, our approach could pave
the way for the development of new stimuli-responsive materials based on mechanically
interlocked molecules for future electro-optical applications, such as sensors, molecular memories
and molecular logic gates.
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INTRODUCTION
Molecular devices and machines are of considerable interest on account of their potential use
in sensing, catalytic, electronics, and nanotechnological applications.1 Mechanically interlocked
molecules (MIMs) constitute promising nanoscale molecular assemblies for the development of
switches,2 actuators,3 ratchets4 and motors.5 Well-known examples are bistable [2]rotaxanes,2a,6
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which are MIMs comprising a ring component mechanically bonded onto a linear dumbbell
component with two (or more) recognition sites for occupation by the ring component. The ability
to manipulate reversibly the relative positioning of the ring component with respect to the
dumbbell component is crucial to exploring their use in operating molecular devices. The
controlled rotary movement of the ring component around the dumbbell has been achieved7 by
introducing suitable steric hindrance between the two components of the rotaxane. On the other
hand, the net linear translation of the ring with respect to a constitutionally asymmetric axle has
been demonstrated in molecular shuttles4b,c and pseudorotaxanes8 by exploiting ratchet
mechanisms.9 Recently, a rotaxane-based molecular shuttle, combined with an overcrowded
alkene rotary motor, has led to the transformation of the unidirectional rotation of the molecular
motor into a reciprocating shuttling motion,10 thus coupling rotary and translational movements in
a MIM. Indeed, rotary motors are a class of molecular machines that display controlled rotation of
one component with respect to the other around single11 or double12 bonds driven, for the most
part, by either light or heat.
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Fluorescent molecular rotors are compounds whose photoluminescence is modulated by
segmental mobility (twisting)—that is, the locally excited (LE) electronic state can relax either by
(i) the radiative emission of a photon or by (ii) the formation of a “dark state” that relaxes non-
radiatively to the ground state (GS) on account of internal rotation.13 If the local environment
around the fluorophore permits rapid internal rotation in the excited state, fast non-radiative decay
processes can effectively quench its fluorescence. On the other hand, any environmental restriction
to twisting in the excited state because of free volume, molecular crowding or solvent viscosity,
slows down rotational relaxation, enhancing fluorescence efficiency from the LE state. This
environmental sensitivity of fluorescent molecular rotors has been exploited13 extensively in
biological applications to probe, in real time, local microviscosity in biofluids and biomembranes.
One of the most widely used14 fluorescent molecular rotors is (Scheme 1a) the BODIPY-rotor.
The introduction of steric constrains in the form of substituents onto the phenyl ring or the dipyrrin
units of the BODIPY is an effective approach to increasing the quantum yield of these rotors by
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preventing free rotation of the phenyl group, thus reducing loss of energy from the excited states
via non-radiative molecular motions.15 Indeed, a theoretical study shows15a that the phenyl ring
rotation and accompanying boron-dippyrin distortions allow access to an excited-state
conformation with low radiative probability and facile nonradiative deactivation to the ground
state, thereby limiting the fluorescence yields of the dyes. Such a distorted conformation is
energetically inaccessible in a system bearing the sterically hindered o-tolyl or mesityl group,
leading to a high radiative probability—and high fluorescence quantum yield—involving
conformations at or near the initial Franck-Condon form of the excited state.15b An increase in the
fluorescence quantum yield of BODIPY-rotor is also observed14 by increasing the solvent viscosity,
that results in the restricted rotation of the phenyl group, thus preventing non-radiative relaxation.
This property of the BODIPY-rotor and its derivatives has been employed successfully to measure
viscosity in model lipid membranes,16 in protocells,17 and in the inner membranes of living cells.18
One of the main advantages of BODIPY-rotor over other reported molecular rotors is the wide
dynamic range of its fluorescence response,14 corresponding to a broad range of viscosities, and a
very weak sensitivity to solvent polarity19 and temperature.16 The control of the rotation of a
fluorescent molecular rotor, imposed by mechanical bonds within the nanoconfinement20 provided
by a MIM, however, has not been explored so far to the best of our knowledge.
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Herein, we have designed (Scheme 1b) and synthesized a novel electrochemically switchable
bistable [2]rotaxane (64+) with an embedded fluorescent rotor where the fluorescence output is
actuated by an electrochemical stimulus. The bistable [2]rotaxane (64+) is formed between the π-
electron-poor cyclobis(paraquat-p-phenylene) CBPQT4+ ring, and a dumbbell containing a π-
electron-rich tetrathiafulvalene (TTF) unit with a fluorescent BODIPY rotor, acting as a stopper.
This particular bistable [2]rotaxane has only one switchable recognition unit and therefore exists
as one translational co-conformation in its ground state. Upon complete oxidation of the TTF unit
to the stable, doubly charged TTF2+ dication, the CBPQT4+ ring is forced into juxtaposition with
the BODIPY rotor, as a result of Coulombic repulsions. The nature of the switching process has
been investigated using (i) 1-D and 2-D NMR spectroscopy, (ii) steady-state and ultrafast time-
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resolved spectroscopy, (iii) electrochemical experiments and (iv) quantum mechanical calculations.
In low polarity solvents—e.g. PhMe—the oxidation of the TTF unit to a TTF2+ dication drives the
CBPQT4+ ring towards the BODIPY rotor, increasing the fluorescence quantum yield of the latter.
This remarkable fluorescent enhancement is the consequence of the increased energy barrier
associated with its excited-state rotation which is responsible for its high non-radiative rate
constant. The novelty of our approach lies in the ability to control the rotation of a molecular rotor
by the constrained nanoconfined20 environment introduced by the mechanical bonding associated
with this electrochemically switchable bistable [2]rotaxane (Scheme 1b). The bistable [2]rotaxane
can therefore be considered as a model electromechanical molecular brake based on Coulombic
repulsion and actuated by redox inputs where the co-conformational change is readily monitored
through the fluorescence output. On the other hand, in high polarity solvents—e.g. MeCN—an
excited charge-shifted (CS) state involving electron transfer from the BODIPY rotor to the
CBPQT4+ ring become energetically accessible, enforcing association between the two
components and, hence, decreasing the fluorescence output of the BODIPY rotor. The ultrafast,
photoinduced electron transfer from the BODIPY rotor to the CBPQT4+ ring in MeCN is
corroborated by femtosecond transient absorption (fsTA) spectroscopy, which reveals the
characteristic absorption features of the reduced CBPQT4+ ring. The unconventional
electrochromic behavior of the fluorescent molecular rotor embedded within this bistable
[2]rotaxane, generated by the mechanical motion of the redox-actuated switching process,
demonstrates that mechanical bonding remains a powerful strategy to create unpredictable
emergent properties in MIMs through the induced nanoconfinement effect. This electrochemically
addressable fluorescent-bistable rotaxane is of particular interest for the development of electro-
optical applications because, in contrast with conventional electro-optical molecule switches, the
change in the fluorescence output in the rotaxane-based system is modulated by the mechanical
movement within the MIM which can lead to more sophisticated outcomes.
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RESULTS AND DISCUSSION
Synthesis and NMR Spectroscopy. The 1,3-dipolar cycloadditions21 of azides with
alkynes, otherwise known as the copper-catalyzed azide-alkyne cycloaddition (CuAAC) approach
to synthesis, has been demonstrated22 to be highly effective when employed in the final step,
leading to the formation of mechanical bonds in the syntheses of MIMs, such as rotaxanes. The
CuAAC reaction occurs readily under very mild conditions and at room temperature, a quality that
is desirable for the template-directed synthesis of MIMs on account of the optimal stabilization of
the supramolecular intermediates.
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TTF and its derivatives have been used widely in the design and synthesis of reversible,
electrochemically switchable, supramolecular systems23 and MIMs.24,25 The π-electron-rich
neutral state has the ability to be oxidized reversibly, not just once but twice at mild potentials to
give a stable radical cation and dication, respectively. In its neutral state, the TTF unit is bound
strongly23a inside the π-electron-poor CBPQT4+. Oxidation, however, leads to the ejection of the
oxidized TTF2+ dication from inside the tetracationic cyclophane as a consequence of Coulombic
repulsions.
The BODIPY-alkyne 4, which was synthesized following a method reported in the literature
by Wagner and Lindsey26, was obtained (Scheme 2) in four steps. As for the synthesis of the meso-
substituted dipyrromethanes 2, we adopted a one-flask synthesis, which was developed previously
for the preparation of trans-substituted porphyrins.27 Pyrroles readily undergo acid-catalyzed
condensation at room temperature in presence of highly electrophilic carbonyl compounds, such
as the aldehyde 1, which is used to form the meso-bridge.28 In order to avoid oligomerization, a
large excess of pyrrole is employed. Moreover, pyrrole serves as the solvent for the reaction,
leading to the direct formation of the dipyrromethane 2.
The π-electron-rich TTF derivative 5 was prepared using a previously reported procedure.29
Upon mixing of 5 with 1 molar equiv of CBPQT•4PF6 in Me2CO, the solution immediately turns
in to an intense emerald-green color, indicating the formation of a donor-acceptor pseudorotaxane
complex—namely, [5  CBPQT]•4PF6. Reaction with tetrakis(acetonitrile) copper(I)
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hexafluorophosphate in presence of the tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA)
at 20 °C for 5 days promoted the desired click reaction, affording the bistable [2]rotaxane 6•4PF6
as a dark green solid in 20% yield. Similarly, the reference dumbbell compound DB was obtained
(Scheme 2) under similar reaction condition in the absence of CBPQT•4PF6. The molecular
structures and compound purities were ascertained by mass spectrometry and HPLC, as well as by
1-D and 2-D NMR spectroscopy. See the Supporting Information (SI) for detailed synthetic
procedures and characterization.
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The 1H NMR spectrum (Figure 1) of 6•4PF6 confirms its mechanically interlocked nature and
the fact that the CBPQT4+ ring encircles the TTF unit in the ground state. Notably, the separation
of the signals for both the α- and β-bipyridinium protons on the CBPQT4+ ring is observed at room
temperature. This observation is to be expected, as the constitutional asymmetry of the dumbbell
component imposes its lack of symmetry on the CBPQT4+ ring, rendering the α- and β-
bipyridinium protons heterotopic and thus, in each case, giving rise to separate signals. We were
not able to observe (Figure S8, Supporting Information) any ground-state shuttling by dynamic
1H NMR spectroscopy, within the temperature range from 238 to 308 K, indicating that the
CBPQT4+ ring resides, to all intents and purposes, solely on the TTF unit, at least within the limits
1
of detection provided by variable-temperature H NMR spectroscopy. This observation is
consistent with the one-station nature of the bistable [2]rotaxane when the TTF unit is neutral.
The bistable [2]rotaxane 6•4PF6 can be actuated (Scheme 1) by chemical oxidation of 64+ to
66+ in CD3CN using 2 equiv of tris(4-bromophenyl)ammoniumyl hexachloroantimonate as an
oxidizing agent.30 The effects of chemical switching can be monitored by 1H NMR spectroscopy,
steady-state UV-Vis absorption and emission spectroscopy. The 1H NMR spectrum (Figure 2) is
characterized by a substantial change, as a consequence of large downfield shifts of the TTF2+
proton resonances, as well as those associated with the neighboring methylene groups. These
changes are in agreement with previously reported31 spectroscopic data. Before the oxidation, the
spectrum of the bistable rotaxane 64+ reveals two pairs of peaks of almost equal integration (56:44)
at δ = 6.19 and 6.14 ppm, and δ = 6.02 and 5.98 ppm, which can be assigned to the constitutionally
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heterotopic TTF methine protons in the cis and trans isomers, respectively. These isomers are in
dynamic equilibrium on the laboratory timescale. Following oxidation, the signals for the
constitutionally heterotopic methine protons on the TTF2+ dication resonate32 at δ = 9.30 and 9.17
ppm, while those associated with the methylene protons on the adjacent CH2 groups are shifted
downfield to 5.21 and 4.96 ppm. In this oxidized state, cis-trans isomerism is removed since the
TTF2+ dication is no longer planar.30a The movement of the CBPQT4+ ring from the TTF unit to
the BODIPY rotor was probed (Figures S9 and S10) by two-dimensional (2D) Nuclear Overhauser
Effect (NOE) measurements, where the presence of through-space correlations between the
resonances for the BODIPY protons and those on the CBPQT4+ ring in 66+ were observed. It should
be noted that the interaction between the CBPQT4+ ring and the BODIPY rotor was absent in the
reduced 64+ state. The disappearance of NOE correlations between the methylene protons next to
the TTF unit and those of the CBPQT4+ ring corroborates the switching to 66+. Upon treatment
with Zn dust, the TTF2+ was reduced (Figure 2) to its neutral form and the CBPQT4+ ring moved
back to reside on the π-electron-rich TTF unit, as indicated by the 1H NMR spectrum of the product,
in order to re-establish the CT interactions between the TTF and the CBPQT4+ ring.
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Steady-State UV/Vis Spectroscopy. The UV/Vis absorption spectrum (Figure 3a,
green trace) of the bistable [2]rotaxane 6•4PF6 shows the characteristic charge transfer (CT)
absorption band33 centered on 843 nm (ε = 3500 M-1cm-1) for the TTF unit residing inside the
CBPQT4+ ring. Furthermore, in the visible region, the strong absorption band at 499 nm (ε = 48000
M-1cm-1), typical of an S1←S0 electronic transition of BODIPY chromophore unit, is observed.
The UV region is characterized by the S2←S0 transition of BODIPY (376 nm, ε = 16000 M-1cm-1)
and the CBPQT4+ absorption at 260 nm (ε = 40000 M-1cm-1). Switching of 64+ was investigated in
–
a 6.25 μM solution of the 4PF6 salt in MeCN when Fe(ClO4)3 was added.34 Addition of 1 mol
equiv of this chemical oxidant led (Figure 3a, orange trace) to disappearance of the CT absorption
band at 843 nm and the rise of absorption bands centered on 450 and 600 nm characteristic35 of
the mono-oxidized form of the TTF. Further addition of the oxidant (up to 2 mol equiv) led the
disappearance of the absorption bands for the mono-oxidized TTF unit and the enhancement of
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the band at 375 nm (Figure 3b, red trace), indicative35 of TTF2+ dication formation. Notably, the
movement of the CBPQT4+ ring away from the TTF unit is also accompanied by a small red-shift
of the absorption maxima of BODIPY from 499 to 505 nm. This observed ground-state electronic
perturbation is most likely caused by the enforced encirclement of the BODIPY rotor by the
tetracationic cyclophane as the reference DB does not show (Figure 3c,d) any red-shift upon
oxidation of the TTF unit. After the reduction with Zn powder, the original spectrum is
quantitatively restored. In summary, both the 1H NMR spectroscopic and UV/Vis
spectrophotometric experiments showed clearly that the redox switching of the TTF unit forces
the CBPQT4+ ring toward and away from the BODIPY rotor.
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Electrochemistry. The switching of the bistable [2]rotaxane can also be enacted
electrochemically. The CBPQT4+ ring shows (Figure 4a) two characteristic reversible two-
electron reductions with E1/2 at –270 and –715 mV vs Ag/AgCl while the BODIPY rotor reveals
(Figure 4b) a reversible one-electron reduction at E1/2 = –652 mV and an irreversible oxidation
peak at +1630 mV. The TTF oxidation processes, that are TTF → TTF•+ followed by TTF•+ →
TTF2+ one-electron events, are well-resolved in the CV of DB (Figure 4c) with E1/2 at +392 and
+755 mV, respectively. On the other hand, the oxidative scan of the bistable [2]rotaxane 6•4PF6
(Figure 4d) shows only one peak centered at +772 mV encompassing both of the one-electron
processes which together generate the TTF2+ dicationic state from its neutral form to the radical
TTF•+ intermediate. The complete disappearance of the first oxidation peak matches the fact that
the TTF unit within the bistable [2]rotaxane 6•4PF6 is encircled completely by the electron poor
CBPQT4+ ring, such that the first TTF oxidation is shifted substantially to more positive potentials,
close to the potential for the second oxidation.13 We also performed variable scan-rate CV
measurements in order to elucidate the kinetics associated with the return of the CBPQT4+ ring
from the oxidation-induced co-conformation to the ground-state co-conformation (GSCC). As
observed in other bistable MIMs,24e the initial oxidation of the TTF unit forces the CBPQT4+ ring
onto the alternative recognition site and the subsequent re-reduction of the TTF unit back to its
neutral form does not result immediately in the regeneration of the GSCC. The transient co-
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conformation, where the CBPQT4+ ring still encircles the weaker binding site, while the TTF unit
is back in the neutral state, is referred to as the metastable-state co-conformation (MSCC). As the
conversion of the MSCC to the GSCC is usually an activated process, its kinetics have a
measurable rate, from seconds to hours, which depends on the nature of the linker, the temperature
and the environment.31 In case of the bistable [2]rotaxane 64+, however, even at high scan rate
(2000 mV/s, Figure S11), we could not detect any presence of the MSCC, which would appear as
emergence of the first oxidation of free TTF in the second scan. The absence of any detectable
MSCC suggests that there is not much, if any, affinity between CBPQT4+ ring and BODIPY rotor
and their dissociation after the oxidation step is driven largely by the Coulombic repulsion between
the tetracationic cyclophane and the TTF2+ dication.
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Quantum Mechanical Calculations. In order to examine the energetics that govern the
intramolecular noncovalent bonding interactions in the bistable [2]rotaxane 6•4PF6 in its different
redox states, we performed density functional theory (DFT) calculations at the M06-2X/6-
311++G**//M06-2X/6-31G* level that includes corrections for van der Waals attraction (normally
not included in DFT).36 Although 64+ has multiple rotatable C‒O and C‒C single bonds, we started
with the linear conformation that has the longest possible distance between the TTF at the center
and two ends of 64+. This minimizes the electrostatic repulsion between TTF2+ and CBPQT4+ once
TTF is oxidized and CBPQT4+ is driven to the two ends. We assume that the entropic contribution
to the free energy from the multiple rotatable single bonds will cancel out as CBPQT4+ relocates.
The ground-state geometries and energy landscapes predicted from the DFT methods (Figure 5),
carried out on the 64+ and 66+ states, are in agreement with the experimental observations. In 64+
the CBPQT4+ ring prefers to reside on the TTF unit rather than on the BODIPY rotor by 19.8
kcal/mol due to stronger donor-acceptor interactions between CBPQT4+ and TTF. Once two
electrons are removed from the TTF unit, the strong electrostatic repulsion between TTF2+ and
CBPQT4+ drives the CBPQT4+ ring to relocate preferentially beside the BODIPY rotor leading to
47.9 kcal/mol energy stabilization. The other co-conformation of 66+ has the CBPQT4+ ring move
to the di-t-butyl benzene unit, which is 10.0 kcal/mol higher in energy than when the CBPQT4+
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ring resides beside the BODIPY rotor.
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Redox Modulation of Excited-State Properties. In order to demonstrate the ability of
the bistable [2]rotaxane to modulate fluorescence outputs and excited-state dynamics on the
BODIPY rotor, we performed steady-state fluorescence titration and fsTA measurements before
and after chemical oxidation. A dilute solution of 6•4PF6 in PhMe excited at 467 nm shows (Figure
6a, green trace) the characteristic emission spectra of BODIPY around 515 nm. The measured
fluorescence quantum yield is low (Φem = 0.19 %) and insensitive to the solvent polarity, as
observed (Figure 6b and 6c, green traces) in both MeCN and THF. These results are consistent37
with those reported for sterically unhindered BODIPY derivatives. Upon addition of 2 mol equiv
of Fe(ClO4)3, a 3.4-fold increase in fluorescence intensity was observed for 66+ in PhMe. For
comparison, the fluorescence signal of the BODIPY unit in the reference dumbbell DB, measured
(Figure S12) before and after oxidation of the TTF unit, does not show appreciable fluorescence
changes in the tested solvents. We attribute the fluorescence signal enhancement present in the
oxidized bistable [2]rotaxane to the expected increase in energy barrier for the intramolecular
rotation of BODIPY rotor induced by the enforced positioning of the CBPQT4+ ring close to the
rotor in the 66+ state. Unexpectedly, when the same fluorescence titration experiment of the bistable
[2]rotaxane is performed in the MeCN (Figure 6c), a nearly 3-fold decrease in the fluorescence
quantum yield is observed. In THF (Figure 6b), which has an intermediate polarity compared to
PhMe and MeCN, neither an increase nor a decrease in the fluorescence signal is observed upon
addition of Fe(ClO4)3. The different fluorescence response of [2]rotaxane 6•4PF6, upon chemical
switching in the three solvents, suggests that an additional excited state reaction, which is highly
sensitive to the solvent polarity, must be involved. We hypothesize that the quenching of
fluorescence in MeCN could be indicative of a photoinduced electron transfer reaction from the
BODIPY singlet excited state to the electron accepting CBPQT4+ ring. In order to obtain support
for this hypothesis, we performed fsTA spectroscopy investigations.
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Photoexcitation of the bistable [2]rotaxane 6•4PF6 in MeCN at 497 nm with a 150 fs laser pulse
populated the lowest excited singlet state of the BODIPY rotor, which displays a prominent
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ground-state bleach (GSB) at 500 nm, and which overlaps partially with stimulated emission (SE)
signals in the 500–600 region (Figure 7a). These transient features decay bi-exponentially with
time constants of  S1*S1 = 4.1  0.3 ps and S1S0 = 31.7  0.6 ps. The first decay component can
be assigned to the solvation evolution or vibrational/conformational relaxation associated with
BODIPY-core in the excited state, while the relaxed excited singlet state decays with the slower
time constant to the ground state. On the other hand, photoexcitation of the oxidized [2]rotaxane
66+ under the same conditions generates 1*BODIPY, which decays rapidly (CS = 1.4  0.3 ps) to
produce (Figure 7c) a new species with an absorption maximum at 610 nm that is indicative38 of
the presence of the CBPQT3+• (Figure S13). The free energy change for such process in polar
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MeCN can be estimated as
, where
is the oxidation potential
∆퐺퐶푆 = 푒(퐸푂푋 ― 퐸푅퐸퐷) ― 퐸푆
퐸푂푋
(+1.69 V, Figure 4) of the BODIPY rotor and
is the reduction potential (–0.27 V, Figure
퐸푅퐸퐷
1
4) of the CBPQT4+, while
is the energy of *BODIPY (+2.44 eV, from the average of
퐸푆
absorption and fluorescence maxima), so that
observed fluorescence quenching results. The
–0.51 eV, and is thus consistent with the
∆퐺퐶푆 =
for the formation of an ion pair in a solvent
∆퐺퐼푃
of arbitrary polarity can be determined by using an expression developed by Weller:39
3푒2 푒2
1
1
1
1
∆퐺퐼푃 = 푒(퐸푂푋 ― 퐸푅퐸퐷) +
+
+
―
(1)
(
)(
)
4휋휀0휀푠푟12 4휋휀0 2푟퐷 2푟퐴 휖푠 휖푠푝
where the redox potentials are measured in a high polarity solvent with a static dielectric constant
,
is the charge of the electron,
is the ion pair distance,
and
are the ionic radii,
푟퐴
휀푠푝
푒
푟12
푟퐷
and
is the static dielectric constant of a solvent of arbitrary polarity. While the term involving
휀푠
the ion pair distance is the Coulombic interaction of the ions, the last term accounts for the lesser
ability of the lower polarity solvents to stabilize charges as compared to the higher polarity used
in the electrochemical measurements. However, there are several limitations with this treatment
for estimating
for large π-stacked donor−acceptor systems. First, the ionic radii of the donor
∆퐺퐼푃
and acceptor are larger than the distance between them, which violates the assumption of a hard-
sphere model surrounded by a continuous dielectric that is intrinsic to eq 1. Second, the BODIPY
unit is highly shielded from the surrounding medium by the presence of the CBPQT4+ ring; thus,
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BODIPY experiences a significant electrostatic influence from the CBPQT4+ ring, which will
deviate from that of the bulk solvent. Nevertheless, one can estimate that the energy of CS state is
destabilized by at least 0.6 eV in PhMe relative to its energy in MeCN, making the photoinduced
electron transfer reaction endoenergetic by ≥ 0.1 eV.40 Notably, the charge recombination proceeds
biexponentially. The shorter lifetime (8.0  0.3 ps) can be safely assigned to the geminate
recombination from TTF- CBPQT3+•-BODIPY+• to the ground state. A smaller fraction of the
population, however, can undergo to a charge-shift reaction to generate TTF+•- CBPQT4+-
BODIPY+•, which ultimately decays in 1644  87 ps. The energy levels and excited state decay
pathways under various conditions are summarized in Figure 8.
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CONCLUSIONS
We have reported the synthesis of an electrochemically switchable bistable [2]rotaxane with
an embedded fluorescent molecular rotor—the BODIPY rotor—and demonstrated that the redox
actuation of this mechanically interlocked molecule can impose a nanoconfined constrained
environment that controls the rotation of the fluorescent rotor, resulting in a unique electrochromic
effect. The electrochemically switchable [2]rotaxane, 6•4PF6, composed of a CBPQT4+ ring
mechanically interlocked with a dumbbell component containing a TTF recognition site and a
functional fluorescent molecular rotor in the form of a BODIPY stopper, was synthetized in good
yield by a template-directed protocol utilizing a “threading-followed-by-stoppering” approach, in
combination with the CuAAC reaction. The bistable [2]rotaxane can be switched reversibly so that
the CBPQT4+ ring is positioned next to the BODIPY rotor upon oxidation of TTF unit to its TTF2+
dication. We have investigated the switching of the ground state co-conformation by means of 1-
D and 2-D NMR spectroscopies and investigated the corresponding electronic excited state
dynamics changes using a variety of steady-state and time-resolved optical spectroscopies.
Remarkably, two completely different mechanisms of fluorescence co-exist within this
fluorescent-bistable rotaxane upon the switching of its ground state co-conformation; (i)
fluorescence enhancement by reducing the loss of energy from the excited states to the non-
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radiative molecular motions of the BODIPY-rotor in low polarity solvents and (ii) fluorescence
quenching on account of the photoinduced electron transfer reaction from the BODIPY singlet
excited state to the electron accepting CBPQT4+ ring in polar solvents. This unconventional
electrochromic effect is generated by the constrained nanoconfined environment introduced by the
mechanical bond in this electrochemically switchable bistable [2]rotaxane which imparts the
forced association between the CBPQT4+ ring and the BODIPY rotor driven purely by the
Coulombic repulsion between the tetracationic ring and the TTF2+ dication on the dumbbell. We
believe that the extension of the concept of the nanoconfinement effect,20 introduced by
mechanical binding in the context of stimuli-responsive materials based on MIMs, could lead to
novel electro-optical switchable materials that can perform complex operations, holding the
promise for future applications as sensors, molecular memories and molecular logic gates.
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■ ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
Experimental details, including synthesis, NMR, and supportive figures; Additional
electrochemical studies and fluorescence spectra; quantum mechanical calculations.
■ AUTHOR INFORMATION
Corresponding Authors
marco.frasconi@unipd.it; m-wasielewski@northwestern.edu; stoddart@northwestern.edu
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
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The authors would like to thank NU for their continued support of this research. The synthesis was
supported by the National Science Foundation under CHE-1308107 (J.F.S.). This project was also
supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences
under Award DE-FG02-99ER14999 (M.R.W.). W.-G.L. and W.A.G. were supported by NSF
(CBET). Y.W. thanks the Fulbright Scholar Program for a Fellowship and the NU International
Institute of Nanotechnology for a Ryan Fellowship. We thank the personnel in the Integrated
Molecular Structure Education and Research Center (IMSERC) at Northwestern University (NU)
for their assistance in the collection of the analytical data.
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Dimerization in a Bistable Tripodal [4]Rotaxane. Chem. Eur. J. 2008, 14, 3889–3895. (d)
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(30) For a description of the oxidizing agent, tris(4-bromophenyl)ammoniumyl hexachloroantimonate,
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2003, 42, 1491–1495. For a review on this and other chemical oxidants commonly use in
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NMR spectroscopic investigations, see: (b) Connelly, N. G.; Geiger, W. E. Chemical Redox
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Agents for Organometallic Chemistry. Chem. Rev. 1996, 96, 877–910.
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Motions in Pseudorotaxanes: a Case of Dual Mode (Oxidative and Reductive) Dethreading.
Chem. - Eur. J. 1997, 3, 1992–1996.
(34) The chemical oxidant Fe(ClO4)3 was chosen for the UV/Vis spectroscopic studies because it
does not show any significant optical absorption band in the visible region, making changes
in UV/Vis absorption spectra easier to interpret.
(35) Hünig, S.; Kiesslich, G.; Quast, H.; Scheutzow, D. Über Zweistufige Redoxsysteme, X1)
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of Light-Harvesting Arrays Comprised of a Porphyrin and One, Two, or Eight Boron-Dipyrrin
Accessory Pigments. J. Am. Chem. Soc. 1998, 120, 10001–10017. (b) Alamiry, M. A. H.;
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McGonigal, P. R.; Gibbs-Hall, I. C.; Diercks, C. S.; Sarjeant, A. A.; Stern, C. L.; Goddard, III
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Soc. 2016, 138, 10214–10225.
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(39) (a) Weller, A. Z. Photoinduced Electron Transfer in Solution: Exciplex and Radical Ion Pair
Formation Free Enthalpies and their Solvent Dependence. Phys. Chem. 1982, 133, 93–98. (b)
Young, R. M.; Dyar, S. M.; Barnes, J. C.; Juricek, M.; Stoddart, J. F.; Co, D. T.; Wasielewski,
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(40) Complementary fsTA measurements in PhMe, in an attempt to corroborate the excited state
dynamics, were unsuccessful because of the low solubility of both 6•4PF6 and 6•6PF6 in low
polarity solvents. Additional efforts to tune the polarity have been made using a binary solvent
mixture of PhMe and MeCN (90:10 vol/vol). The fluorescence spectra, resulting from
carrying out a titration experiment on the bistable [2]rotaxane in this solvent mixture, show
(Figure S13) a decrease in the fluorescence upon addition of the oxidizing agent. This result
might be, in part, related to a selective solvation effect of the bistable [2]rotaxane in the binary
solvent system.
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Scheme 1. (a) Covalent-bond approach to enhancing the fluorescence of the BODIPY by
introducing steric constraints on the aryl ring and dipyrrin core. Box: Structure of the BODIPY-
rotor employed as a viscosity sensitive probe. (b) Mechanical-bond approach to controlling the
rotation of the BODIPY rotor by redox-actuation of a bistable [2]rotaxane 64+. Structural formulas
and graphical representation of 64+ and its redox switching from 64+ to 66+.
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Scheme 2. Synthesis of the BODIPY-alkyne 4, the bistable [2]rotaxane 6•4PF6 and the dumbbell DB.
Figure 1. 1H NMR spectra (600 MHz, CD3CN, 298 K) of bistable [2]rotaxane 6•4PF6.
Figure 2. 1H NMR spectra (600 MHz, CD3CN, 298 K) of the bistable [2]rotaxane 6•4PF6, fully
oxidized 66+ after addition of 2 equiv of the chemical oxidant tris(4-bromophenyl)ammoniumyl
hexachloroantimonate, and re-reduced 64+ after the addition of Zn dust. Resonances due to the
chemical oxidant and residual solvent are marked with red (X) symbols.
Figure 3. UV/Vis absorption spectra of a-b) the bistable [2]rotaxane 6•4PF6 and c-d) the reference
dumbbell DB in MeCN (6.25 μM, 10 mm optical pathway) following addition of Fe(ClO4)3 as the
chemical oxidant.
Figure 4. Cyclic voltammograms of (a) the ring component CBPQT•4PF6, (b) the BODIPY-
alkyne 4, (c) the dumbbell DB and (d) the bistable [2]rotaxane 6•4PF6.
Figure 5. DFT Calculated relative energy surface diagram of a) 64+ and b) 66+. Compound 64+
exists as a single ground state co-conformation that may be switched wholly to its inverse state as
a result of a translation of the CBPQT4+ ring on the TTF-containing dumbbell unit to afford 66+,
where the CBPQT4+ ring is located on BODIPY unit as a consequence of Coulombic repulsions
originating from the TTF2+ unit.
Figure 6. Fluorescence spectral changes of the bistable [2]rotaxane 6•4PF6 (green traces) in a)
PhMe, b) THF, and c) MeCN following addition of 1 equiv (orange traces) and 2 equiv (red traces)
of Fe(ClO4)3 as the chemical oxidant.
Figure 7. fsTA spectroscopy of the bistable [2]rotaxane 6•4PF6 in MeCN a-b) before and c-d)
after addition of 2 equiv of Fe(ClO4)3 as the chemical oxidant. Samples were excited at 497 nm
with a ~150 fs laser pulse. a,c) fsTA spectra at selected delay times; b,d) kinetic fits at selected
wavelengths.
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Figure 8. Energy diagram showing excited state decay pathways of the bistable [2]rotaxane 6•4PF6
in MeCN a) before and b) after the addition of 2 equiv of Fe(ClO4)3 as the chemical oxidant. In
the low polarity solvent such as PhMe (c), the photoinduced electron-transfer reaction is
energetically unfavorable and the fluorescence decay enhances on account of hindered
intramolecular rotation responsible of nonradiative decay process.
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Scheme 1
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Scheme 2
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Figure 1
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Figure 3
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