Redox-Driven Folding, Unfolding, and Refolding of Bis(tetrathiafulvalene) Molecular Switch

Article abstract of DOI:10.1021/acs.joc.9b00541

We report redox-driven folding, unfolding, and refolding motions of a synthetic molecular system, in which two tetrathiafulvalene (TTF) units are tethered onto a conformationally rigid yet torsionally flexible π-conjugated backbone. Upon one-electron oxid

Full text of DOI:10.1021/acs.joc.9b00541

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Article
Redox-Driven Folding, Unfolding, and Refolding
of Bis(Tetrathiafulvalene) Molecular Switch
Jiyoung Jung, Wenjun Liu, Seyong Kim, and Dongwhan Lee
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00541 • Publication Date (Web): 17 Apr 2019
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Redox-Driven Folding, Unfolding, and Refolding of
Bis(Tetrathiafulvalene) Molecular Switch
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Jiyoung Jung,†,* Wenjun Liu,‡ Seyong Kim,§ and Dongwhan Lee§,*
† Penn State Scranton, 120 Ridge View Drive, Dunmore, PA 18512, United States
‡ Analytical Research & Development, Merck Research Laboratories, Merck & Co., Inc.,
Rahway, NJ 07065, United States
§ Department of Chemistry, Seoul National University, 1 Gwanak–ro, Gwanak–gu, Seoul 08826,
Korea
ABSTRACT
We report redox-driven folding, unfolding, and refolding motions of a synthetic molecular system,
in which two tetrathiafulvalene (TTF) units are tethered onto a conformationally rigid yet
torsionally flexible π-conjugated backbone. Upon one-electron oxidation, this molecular switch
undergoes swiveling motions from a fully relaxed and freely-rotating Z-shaped conformation to a
compact folded conformation stabilizing π-stacked radical species. Subsequent one-electron
oxidation produces dicationic intermediate, which either engages in intimate π–π interaction, or
transitions to an open structure. A further oxidation, however, brings the molecule back to the
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initial conformation to minimize the repulsion between doubly-charged TTF units. Intriguingly,
the reaction coordinates of this redox-driven structural change have strong dependence on the
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–
–
environment, such as solvent (THF vs CH2Cl2) and supporting electrolyte (PF6 vs B(C6F5)4 ).
With proper design, factors that are typically considered as “secondary effects” could dictate the
solution dynamics and reaction pathways of structural folding and unfolding, all driven by
controlled delivery of electrons.
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INTRODUCTION
Conformationally dynamic molecules and assemblies can be made to sample multiple stable states.
With appropriate signal transduction mechanisms, they serve as key functional components in
chemical sensing, switching, and actuation.1–6 For implementations, the switching can be
controlled by (i) direct interaction with chemical species,7–20 (ii) surrounding environments such
as solvent and temperature,21–24 or (iii) other types of external stimuli including light, force, and
electrons.25–39 Among such stimuli-responsive molecules, redox-active switches have received
much attention as artificial molecular machine.40–43
As a redox-active structural motif, tetrathiafulvalene (TTF) has been used extensively in
molecular devices and conductive materials.44–48 As shown in Scheme 1, TTF can undergo
stepwise one-electron redox processes to switch among charge-neutral TTF, cationic radical
TTF•+, and dicationic TTF2+. The reversibility of this process, in conjunction with the characteristic
electronic transitions in the UV–vis–NIR region as spectroscopic handles, further enhances the
utility of TTF in redox-driven molecular devices.49–57
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Scheme 1. Sequential One-Electron Oxidations of TTF to TTF•+ and TTF2+ (top), and Face-
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•+
to-Face Stacking of two TTF Units in the Mixed-Valent Complex [TTF]2 and π-Dimer
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[TTF•+]2 (bottom).
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As shown in Scheme 1, the TTF•+ cation radical can associate with either the charge-neutral
•+
TTF to produce a mixed-valent dimer, [TTF]2 (= TTF•+–TTF), or self-associate to form a
dicationic π-dimer, [TTF•+]2 (= TTF•+–TTF•+), all readily distinguished by straightforward
spectroelectrochemical methods. To provide structural stability to these otherwise labile adducts
at room temperature, [TTF]2 and [TTF•+]2 have previously been encapsulated within molecular
•+
or supramolecular hosts. Prominent examples include cucurbit[8]uril,58 coordination cages,59
rotaxanes,60 and catenanes.61–64 Alternatively, multiple TTF units can be covalently linked by
bridging units.65–73
In this paper, we report a redox-driven molecular switch,74–86 which undergoes sequential
folding, unfolding, and refolding motions through intramolecular self-association of tethered TTFs
(Scheme 2). This mechanical process is reminiscent of structural folding and unfolding of
biological molecules through redox reactions, although intimate π–π contacts, rather than covalent
bond formation, such as disulfide bridge, is the main driving force here.
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Scheme 2. (a) Metal Binding-Induced vs (b) Redox-Driven Folding (F) and Unfolding (U) of
Z-Shaped Molecular Switches.
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This chemistry is built upon our previous work on C2-symmetric π-scaffolds that restrict
torsional motions of the “half-ligand” units to facilitate metal binding (Scheme 2a).87,88 By
installing two TTF redoxphores at each end of the Z-shaped phenyleneethynylene-derived
skeleton, the reaction coordinates of structural folding–unfolding is made simple, while the
efficiency of intramolecular [TTF–TTF]n+ (n = 1 or 2) association enhanced. These key structural
features contribute synergistically to the switching behavior that we observe for 1 (= 1U; Scheme
2b), which is dictated by the choice of solvent and electrolytes. By changing the combination of
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solvents and anionic electrolytes, the oxidized species [1]+ and [1]2+ can be made to either fold (for
[1F]+ and [1F]2+), unfold (for [1U]+ and [1U]2+), or exist as mixed conformations (Scheme 2b).
With solvent–electrolyte combination as key parameters to define and tweak the reaction
coordinates, a fine control is thus gained over the electrochemically-driven structural folding and
unfolding. Described in the following sections are our efforts to understand this phenomenon by a
combination of synthetic, electrochemical, and spectroscopic studies.
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RESULTS AND DISCUSSION
Modular Synthesis to Build Mono- and Bis-TTF Molecules
Our synthesis, as summarized in Scheme 3, commenced with the known compound 8. A
Sonogashira-Hagihara coupling reaction between 8 and acetone-protected acetylene proceeded
exclusively on the iodo-substituted position at r.t. to produce the mono-coupling product 7. This
reaction intermediate was subjected to a second round of cross-coupling reaction with
triisopropylsilyl-protected acetylene to furnish 6. By taking advantage of the orthogonal reactivity
of the two protecting groups, i.e. propargylic vs silyl, compound 6 was treated with either nBu4NF
or KOH to afford 5 or 9, respectively.
An oxidative homocoupling of 5 completed the construction of the C2-symmetric
phenyleneethynylene backbone of 4, which was converted to 3. A final Sonogashira-Hagihara
coupling between 3 and iodo-TTF 2 furnished the target compound 1.89 By cross-coupling with 2,
compound 9 was converted to the mono-TTF molecule 10, which serves as a half-site model of
the bis-TTF molecule 1 in comparative studies (vide infra).
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Scheme 3. Synthetic Rutes to Bis-TTF (1), and Mono-TTF (10) as a Half-Unit Model.
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The chemical structures of 1 and 10 were confirmed by 1H and 13C NMR spectroscopy, and
high-resolution mass spectrometry. The similar chemical shifts of the proton resonances of the
TTF units in 1 and 10 support the notion that the charge-neutral 1 adopts an extended Z-shaped
planar conformation (denoted as 1U in Scheme 2 to emphasize its “unfolded” open
conformation),87 with no intimate π–π contact between the two TTF faces. Charge-neutral and
electron-rich TTF does not self-associate in solution, either.
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Electrochemical Studies: Molecular Structure, Solvent, and Anion-Dependent Redox
Behavior
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The electrochemical properties of 1 and 10 were studied by cyclic voltammetry on solution
samples (Figure 1, Table 1), prepared with various combinations of solvents (THF or CH2Cl2) and
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n
electrolytes (nBu4NPF6 or Bu4N[B(C6F5)4]). Under all conditions, the mono-TTF compound 10
displays two well separated and reversible oxidation waves (Figure 1a). These stepwise one-
electron oxidation processes, however, have a strong dependence of the E1/2ox values (in particular,
the E1/2ox(2) value) on the solvent/anion used (Table 1).
Figure 1. Cyclic voltammograms of (a) 10 (1.2 mM) and (b) 1 (0.95 mM) in (i) CH2Cl2 with
nBu4NPF6, (ii) CH2Cl2 with Bu4N[B(C6F5)4], (iii) THF with Bu4NPF6, and (iv) THF with
nBu4N[B(C6F5)4] as supporting electrolytes (0.1 M). Scan rate = 100 mV/s. The red vertical lines
denote the positions of E1/2ox (see Table 1 for a summary of numerical values). All potentials are
externally referenced to the (C5H5)2Fe/(C5H5)2Fe+ redox-couple.
n
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Table 1. Half-wave potentials E1/2 (mV) and potential differences ΔE1/2 (mV) of the first two
oxidation waves of 1 measured under different conditions.a
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ox(1)
ox(2)
ox(3)
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CV
Solvent Electrolyte
CH2Cl2 nBu4NPF6
E1/2
E1/2
E1/2
E1/2
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189
–
Kcomp
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d
Figure 1b(i)
Figure 1b(ii)
Figure 1b(iii)
–82 (1 e)
+38 (1 e)
+86 (1 e)
–
1.0 × 102
1.7 × 103
–
CH2Cl2 nBu4N[B(C6F5)4] –103 (1 e)
+835 (2 e)
+344 (2 e)
+504 f
e
THF
THF
nBu4NPF6
nBu4N[B(C6F5)4] –87 (1 e)
–22 (2 e)
–
Figure 1b(iv)
+37 (1 e)
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1.2 × 102
Externally referenced to the (C5H5)2Fe/(C5H5)2Fe+ redox couple; shown in parentheses is the
a
b
ox(2)
number of electrons involved in each electrochemical oxidation process. ΔE1/2 = E1/2
–
E1/2ox(1). log Kcomp = 16.9 × ΔE1/2. Appearing as stripping peak from species adsorbed on the
electrode surface. e A seemingly concerted two-electron wave with E1/2 = –22 mV, rather than two
consecutive one-electron waves. f Quasi-reversible wave.
c
d
For the corresponding bis-TTF molecule 1, a total of four electrons, associated with the two
TTF units, can be removed. This oxidation reaction could proceed as either sequential one-
electron, or concerted two-electron processes. In THF (solvent donor number = 20.6),90 1 shows
two reversible two-electron oxidation waves at E1/2 = –22 and +344 mV when nBu4NPF6 is used
as supporting electrolyte (Figure 1b(iii) and Table 1). In contrast, two consecutive one-electron
oxidation waves were observed at E1/2 = –87 and +37 mV, along with a third quasi-reversible wave
n
at E1/2 = +504 mV, when Bu4N[B(C6F5)4] was used instead as supporting electrolyte in THF
(Figure 1b(iv) and Table 1). This behavior implicates the involvement of ion-pairing effects91–94
in the stepwise oxidation of 1, which was not observed for the mono-TTF model 10 (Figure 1a(iv)).
Solvents of low donor number90 solvate ions less effectively, so that the ion-pair interaction
is further promoted. Indeed, when CH2Cl2 (solvent donor number = 1.2)90 was used in conjunction
with nBu4NPF6 as the supporting electrolyte, 1 displays two reversible oxidation waves at E1/2 = –
82 and +38 mV, followed by a third oxidation wave appearing as a stripping peak (Figure 1b(i)
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and Table 1). A direct side-by-side comparison with the half-site model 10 (Figure 1a(i)) indicates
that the first two oxidation waves of 1 here arise from one-electron oxidation of each of the two
TTF units, whereas the sharp feature at Ep,a = +490 mV is from the putative three-electron oxidized
[1]3+[PF6]3 and/or four-electron oxidized [1]4+[PF6]4 species that apparently adsorb on the
electrode surface to give rise to narrow spiky waves. Apparently, the ion-pair interactions of highly
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–
charged [1]3+ or [1]4+ cations and PF6 anion are strong in the low-polarity solvent CH2Cl2 (ε =
8.9), which also has weak donor ability.91–94
The electrochemical behavior of 1, as compared and contrasted in Figure 1, convincingly
demonstrates that the oxidation of tethered-TTFs is heavily dependent on the choice of solvents
and anionic electrolytes. In addition to the intrinsic thermodynamics of electron shuttling, what are
usually considered as “secondary effects”, such as solvation and ion-pair interactions, provide
additional driving forces to shift the redox equilibrium. This point is made further evident by the
last oxidation wave of 1, which is shifted cathodically by as much as ~490 mV depending on the
choice of solvent and electrolyte (Figure 1b(ii) vs Figure 1b(iii); Table 1). For multi-electron redox
processes, molecule–solvent and molecule–ion interactions should have increasingly larger
impacts as additional positive charges build up with successive oxidation.93
In addition to the large shift in the third oxidation wave described above, solvent and
ox(2)
electrolyte also impact the separation between the first two oxidation waves: ΔE1/2 = E1/2
–
E1/2ox(1) (Table 1 and Figure 2). Specifically, a single two-electron oxidation wave is observed for
n
1 in the donor solvent THF when Bu4NPF6 is used as supporting electrolyte (Figure 2(c)).
Presumably, this behavior is due to the nearly identical oxidation potentials associated with the
first (E1/2ox(1)) and second (E1/2ox(2)) one-electron process.
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n
Figure 2. Differential pulse voltammograms of 1 (0.95 mM) in (a) CH2Cl2 with Bu4NPF6, (b)
n
n
n
CH2Cl2 with Bu4N[B(C6F5)4], (c) THF with Bu4NPF6, and (d) THF with Bu4N[B(C6F5)4] as
supporting electrolytes (0.1 M).
Under all other conditions (Figure 2(a), 2(b), and 2(d)), however, two sequential and well-
resolved one-electron oxidation waves were observed. The largest separation of ΔE1/2 = 189 mV,
corresponding to the comproportionation constant of Kcomp = 1.7 × 103, was observed in the non-
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donor solvent CH2Cl2 with the weakly coordinating Bu4N[B(C6F5)4] electrolyte (Figure 2b). A
large ΔE1/2 value here reflects the increased stability of the mixed-valent species 1+ (= TTF•+–TTF),
in which only one of the two TTF units in 1 is oxidized while the other still remain charge-neutral.
To gain experimental evidence for this putative 1+ cation and its subsequent oxidation to dicationic
12+, we proceeded to carry out spectroscopic studies under electrochemical conditions.
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UV–vis–NIR Spectroelectrochemical Studies to Probe Redox-Driven Conformational
Changes
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A face-to-face interaction between two TTF units (Scheme 1), either as one-electron oxidized
mixed–valent dimer, [TTF]2 (= TTF•+–TTF), or two-electron oxidized dicationic π–dimer
•+
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[TTF•+]2 (= TTF•+–TTF•+) was investigated by spectroelectrochemical analysis. To probe the
molecular conformations adopted by [1]+ and [1]2+, we first carried out UV–vis–NIR spectroscopic
studies on 1 (Figure 3) under the four types of electrochemical reaction conditions that we tested
in Figure 1b.
For the solvent–electrolyte combination of THF–nBu4NPF6, the first two one-electron
oxidation processes, corresponding to [1] → [1]+, and then [1]+ → [1]2+, merge to appear as a
concerted two-electron oxidation wave in the cyclic voltammogram (Figure 1b(iii) and Figure 2c).
Consistent with this behavior, electrochemical oxidation of 1 under this condition resulted in the
immediate appearance of two absorptions at around 550 and 800 nm (Figure 3a), which fully
develop into two sharp bands at λmax = 570 and 809 nm (Figure 3b). Such optical transitions are
characteristic of the π-dimeric [TTF•+]2.49,52,58,61,62,66,67
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Figure 3. UV–vis–NIR spectral change of 1 (1.0 mM) recorded upon electrochemical oxidation
in a 1-mm pathlength cell using either THF–nBu4NPF6 (for (a) and (b)) or THF–nBu4N[B(C6F5)4]
(for (c) and (d)) solvent–electrolyte (= 0.1 M) combination. Spectral traces are color-coded as [1]
(blue), [1]1+ (red), and [1]2+ (green). In THF with nBu4NPF6, the applied potential is increased from
225 mV (vs Ag/AgCl; corresponding to the blue line in (a)) to 575 mV (corresponding to the red
line in (a) and (b)), and finally to 625 mV (corresponding to the green line in (b)). In THF with
nBu4N[B(C6F5)4], the applied potential is increased from 200 mV (vs Ag/AgCl; corresponding to
the blue line in (c)) to 500 mV (corresponding to the red lines in (c) and (d)), and finally to 750
mV (corresponding to the green line in (d)). The broad absorption at λmax ~ 2800 nm (*) is from
solvent background. Colored vertical bars in each spectrum indicate signature wavelengths of each
chromophore discussed in the main text.
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Unlike the situation with Bu4NPF6 electrolyte (Figure 3a and 3b), the use of weakly
coordinating electrolyte nBu4N[B(C6F5)4] in THF resulted in the initial build-up of a single sharp
transition at λmax = 618 nm, as well as a broad NIR intervalence charge-transfer (IVCT) band
centered at ~1900 nm (Figure 3c). These spectroscopic features point toward the formation of
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•+
dimeric [TTF]2 , which are well known in the TTF literature.49,52,59–64 In contrast, the mono-TTF
model 10 at similar concentration undergoes stepwise conversion of TTF to TTF•+, and then to
TTF2+ (Figure 1a), without the formation of the mixed-valent dimer [TTF]2 in between. This
•+
structure-dependent behavior highlights the importance of tethering two TTF units to achieve the
intramolecularly π–π stacked [1F]+ upon oxidation, as shown in Scheme 2b. Here, the cationic
TTF•+ unit at one end of the Z-shaped molecular skeleton is brought in close proximity to the
charge-neutral TTF unit at the other end to establish an intimate π–π contact, thereby stabilizing
the mixed-valent form.
Due to the rapid formation of such mixed-valent species, subsequent oxidation of the second
TTF unit occurs at a more anodically shifted potential than the oxidation of the first TTF unit
(Figure 1). On the other hand, the lack of a significant optical feature at ~1900 nm obtained in
n
THF with Bu4NPF6 electrolyte (Figure 3(a) and (b)), characteristic of the mix-valent complex,
indicates that the primary oxidative pathway under this particular set of conditions does not involve
[TTF]2•+ as main intermediate. Consequently, the oxidation of the second TTF unit here occurs at
a similar potential to the oxidation of the first TTF unit (Figure 1).
Upon further oxidation of [1F]+ in THF with nBu4N[B(C6F5)4] electrolyte, the intensity of the
NIR IVCT transition diminished, with concomitant build-up of two electronic transitions at λmax
=
585 and 811 nm (Figure 3d). The absorption at λmax = 811 nm arises from the two-electron oxidized
π-dimer [1F]2+,49,52,58,61,62,66,67 but its intensity (Amax = 0.29) was much smaller than that observed
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in THF–nBu4NPF6 combinations (Amax = 1.00, Figure 3b). One interpretation is the involvement
of folding–unfolding equilibrium of [1F]2+-to-[1U]2+ (Scheme 2b), which would change the relative
concentration of the π-dimer chromophore comprised of two closely stacked TTF•+ units (vide
infra).
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To further probe the effect of solvents on the redox-induced spectral and structural changes,
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spectroelectrochemical studies were also carried out in CH2Cl2 (Figure 4). With PF6 as anionic
electrolyte in CH2Cl2, one-electron oxidation of 1 gave rise to an intense electronic transition at
λ
max = 618 nm, as well as a broad NIR IVCT band centered at ~1900 nm (Figure 4a). Removal of
the second equivalent of electron led to a decrease in the intensity of this IVCT band, with
concomitant development of two new sharp peaks at λmax = 580 and 817 nm (Figure 4b), in a
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manner similar to the behavior of π-dimer generated in THF with PF6 electrolyte (Figure 3b).
These diagnostic features suggest the dominant formation of folded [1F]2+ (Scheme 2) with two
electrochemically generated TTF•+ units stacking on top of over each other to form a stable π-
dimer.
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Figure 4. UV–vis–NIR spectral change of 1 (= 1.0 mM) recorded upon electrochemical oxidation
in a 1-mm pathlength cell using either CH2Cl2–nBu4NPF6 (for (a) and (b)) or CH2Cl2–
nBu4N[B(C6F5)4] (for (c) and (d)) solvent–electrolyte (= 0.1 M) combination. Spectral traces are
n
color-coded as [1] (blue), [1]1+ (red), and [1]2+ (green). In CH2Cl2 with Bu4NPF6 , the applied
potential is increased from 50 mV (vs Ag/AgCl; corresponding to the blue line in (a)) to 450 mV
(corresponding to the red line in (a) and (b)), and finally to 700 mV (corresponding to the green
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line in (b)). In CH2Cl2 with Bu4N[B(C6F5)4], the applied potential is increased from 50 mV (vs
Ag/AgCl; corresponding to the blue line in (c)) to 425 mV (corresponding to the red lines in (c)
and (d)), and finally to 700 mV (corresponding to the green line in (d)). Colored vertical bars in
each spectrum indicate signature wavelengths of each chromophore discussed in the main text.
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–
When B(C6F5)4 was used instead (Figure 4c and 4d), a similar build-up and decay of the
IVCT band (λmax ~ 2000 nm; ε = 2.0 × 102 M–1cm–1) was observed upon sequential one-electron
oxidations. This spectral change reflects the formation and decay of the mixed-valent [1F]+ species,
which was consistently observed for all solvent–electrolyte combinations (Figure 3 and 4). Unlike
any other conditions, however, the absorption at λmax = 629 nm continued to grow in its intensity
upon further oxidation, and became a slightly red-shifted single peak at λmax = 635 nm with two
shoulders (Figure 4d). Such spectral feature is distinctively different from that of the π-dimer
[TTF•+]2 (Figure 3b, 3d, and 4b),49,52,58,61,62,66,67 but is similar to that of unstacked (and thus
spectroscopically independent) TTF•+, as anticipated from unfolded [1U]2+ (Scheme 2).61
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EPR Spectroscopic Studies to Probe the TTF Redox Chemistry
As shown in Figure 4, redox-induced UV–vis–NIR spectral changes are more pronounced and
better resolved in CH2Cl2. We thus decided to carry out chemical oxidation of 1 in CH2Cl2, and
track the generation and fate of TTF•+ radicals. Due to the co-existence and equilibrium of
1
diamagnetic and paramagnetic species in solution, however, H NMR spectroscopy is less ideal
for capturing the details of conformational changes of 1 upon oxidation in solution. An EPR
spectroscopic analysis thus became a more attractive choice to pinpoint changes in the spin–spin
interactions upon structural folding and unfolding.
The oxidation potential window of the Ag/Ag+ redox-couple indicates that it would remove
only one electron from TTF to generate TTF•+.95 Therefore, reaction of 1 with silver(I)-based
oxidant, even in excess amount (> 2 equiv), would result in the formation of [1]2+ with no further
removal of electrons. To probe the effect of anions in this redox chemistry, AgPF6 and AgB(C6F5)4
were employed as one-electron oxidants. The choice of these particular silver(I) reagents takes
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into account the anions used in comparative electrochemical (Figure 1 and 2) and
spectroelectrochemical (Figure 3 and 4) studies.
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As shown in Figure 5a, a redox titration of 1 with AgPF6 in CH2Cl2 produced a sharp EPR
signal at g = 2.01, which reached its maximum intensity when 1 equiv of oxidant was delivered.
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Figure 5. EPR spectroscopic titration of 1 (= 0.095 mM) in CH2Cl2 at T = 298 K with addition of
(a) 0–1, (b) 1–2.5, and (c) 2.5–4.0 equiv of AgPF6, and (d) 0–1, (e) 1–2.5, and (f) 2.5–4.0 equiv of
AgB(C6F5) relative to 1.
A further addition of the oxidant resulted in the diminution of the signal intensity (Figure
5b), despite that conversion of the remaining TTF groups (NOTE: each molecule of 1 has two TTF
units) to TTF•+ should still continue under this condition. As such, the attenuation of the
paramagnetic signal here could best be explained by invoking antiferromagnetic (AF) coupling
between two TTF•+ radicals that are brought in close proximity through the formation of the π-
dimer in the folded conformer [1F]2+ (Scheme 2). The residual signal intensity even after the
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addition of an excess amount (up to 4 equiv) of AgPF6 (Figure 5c) might reflect the presence of a
small amount of unfolded conformer [1U]2+, which is in equilibrium with folded [1F]2+ (Scheme
2). For [1U]2+, no strong magnetic coupling is anticipated between the two unstacked TTF•+ groups.
When 1 was treated with the oxidant AgB(C6F5)4, a similar EPR signal developed until
addition of up to 1 equiv of AgB(C6F5)4 (Figure 5d). In stark contrast to the spectral change
observed with AgPF6 (Figure 5b), however, this paramagnetic signal continued to grow with
increasing amount of oxidant (Figure 5e), reaching its maximum intensity with 2.5 equiv of
AgB(C6F5)4. No further change was observed with additional amount of oxidant (Figure 5f). The
continuous increase in the intensity of the EPR signal after addition of 1 equiv of AgB(C6F5)4 is
consistent with the dominance of the unfolded [1U]2+, in which two TTF•+ units are magnetically
decoupled, and behave as two independent paramagnetic centers (Scheme 2).
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Solvent- and Anion-Dependent Structural Folding and Unfolding
As discussed in the previous sections, both the thermodynamic stability and conformational
preference of the [1]+ and [1]2+ species are modulated by an interplay between molecule–solvent
and molecule–ion interactions. It should be noted that local environmental changes, such as solvent,
temperature, and counter-ion, often induce conformational isomerism.96–99 In most cases, the
relative population of such species is followed primarily by NMR spectroscopy, the interpretation
of which becoming increasingly complicated when multiple paramagnetic and diamagnetic species
co-exist and equilibrate at different exchange rates. In our system, a spatial confinement of two
TTF units inside a rigid π-scaffold greatly simplifies the reaction coordinates, the details of which
were interrogated by a combination of electrochemical, UV–vis–NIR spectroelectrochemical, and
EPR spectroscopic techniques. Our empirical observations from these studies are summarized in
Scheme 4.
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Scheme 4. Solvent and Anion-dependent Conformational Switching between the Folded
[1F]n+ and Unfolded [1U]n+ (n = 1 or 2).a
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a Shown in the boxes are conditions under which the corresponding conformation is favored. In
THF with B(C6F5)4 electrolyte, the two confomers, i.e. folded (F) and unfolded (U), coexist.
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First, the combination of non-donor solvent CH2Cl2 and weakly coordinating anion
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B(C6F5)4 favors the folded conformer [1F]+ over the unfolded [1U]+. On the other hand, the
interaction of positively-charged TTF•+ with either donor solvent (such as THF) or coordinating
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anion (such as PF6 ) weakens the charge transfer (CT)-type interaction between TTF and TTF•+ in
[1F]+, thus shifting the equilibrium toward [1U]+. Such non-covalent interactions, however,
compensate for the Coulombic repulsion between two positively-charged TTF•+ groups in the π-
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dimer [TTF•+]2, so that folded [1F]2+ is favored over [1U]2+ in THF with PF6 as the electrolyte.
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Second, the effect of one factor, either solvent or anion, sometimes outcompetes the other.
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B(C6F5)4 (Figure 4; Scheme 4).
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Third, regardless the choice of solvent, PF6 electrolyte always stabilizes the folded π-dimer
[1F]2+ as the two-electron oxidation product of 1 (Figure 3, 4 and 5; Scheme 4).
Fourth, for the oxidized species with no stabilizing π–π contacts, both solvent and anion need
to work cooperatively to provide additional stability. For example, the unfolded [1U]2+ is observed
as the dominant species only when both the non-donor solvent CH2Cl2 and weakly coordinating
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B(C6F5)4 anion are employed (Figure 4 and 5; Scheme 4). When the effects of electrolyte and
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solvent work in opposite directions, such as weakly coordinating B(C6F5)4 in the donor solvent
THF, both folded and unfolded conformations co-exist (Figure 3; Scheme 4).
Undoubtedly, such medium-dependent shifts in the relative stabilities and conformational
preferences, as summarized in Scheme 4, add extra complexity to the system. With a judicious
choice of the solvent–electrolyte combination, however, this complexity can be exploited to
control redox-driven molecular motions, as summarized in Scheme 5.
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Scheme 5. Reaction Pathways of Redox-Driven Structural Folding and Unfolding of 1.
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For example, 1 in CH2Cl2 undergoes folding and unfolding motions upon sequential removal
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of two electrons. As shown in Pathway A (Scheme 5), when PF6 is used as the electrolyte, one-
electron oxidation converts unfolded [1U] to folded [1F]+. With positively charged [TTF]•+ unit
stacked on top of charge-neutral [TTF], it becomes more difficult to remove the second electron
from [1F]+, and hence a large ∆E value and increased stability of the mixed-valent [TTF2]•+
intermediate (Figure 1 and 2; Table 1). Removal of the second electron at a higher potential
produces [1F]2+ having the π•+···π•+ stacked conformation still intact, and hence AF-coupling
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between two [TTF]•+ radicals and net cancelling of the spins (Figure 5). The final two-electron
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oxidation (Figure 1) converts [1F]2+ to [1U]4+, which would unfold to minimize the repulsion
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electrolyte, however, [1F]+ unfolds to [1U]2+ upon loss of an electron (Pathway B, Scheme 5; see
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also Scheme 4 for a summary of conformational preferences under given condition).
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The situation becomes quite different with the coordinating PF6 anion in the donor solvent
THF (Pathway C, Scheme 5). The two TTF units in 1 are oxidized at nearly identical potentials
since the mixed-valent [1F]+ cannot be stabilized by π···π•+ stacking under this condition.
Consequently, the oxidation proceeds directly to [1F]2+ (Figure 1b(iii) and Figure 2(c)). A
subsequent two-electron oxidation of [1F]2+ would result in the unfolded [1U]4+, as in all other cases.
A rather complicated reaction coordinate emerges for the combination of weakly
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coordinating anion B(C6F5)4 in the donor solvent THF (Pathway D, Scheme 5). As shown in
Figure 3, our UV–vis–NIR spectroelectrochemistry results implicate the co-existence of both the
folded and unfolded forms for [1]1+ and [1]2+. The presence of two parallel and competing electron
transfer pathways, involving the folded and unfolded conformers in equilibrium, might also
explain the quasi-reversible nature of the last oxidation wave observed in the CV obtained under
this particular condition (Figure 1b(iv)). From the intensities of the IVCT band assigned to [1F]+,
and the absorption at λmax = 811 nm assigned to [1F]2+ (Figure 3), we estimate equilibrium
constants K of ~1 for the [1U]+-to-[1F]+, and ~0.4 for the [1U]2+-to-[1F]2+ conversion (Pathway D,
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Scheme 5). This calculation is based on the assumption that (i) in CH2Cl2 with B(C6F5)4 , and in
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THF with PF6 , [1F]+ and [1F]2+, respectively, are the only species existing in solution, and (ii) the
intensity of the IVCT transitions are directly proportional to the amount of [1F]+ regardless of the
solvent system used.
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CONCLUSION
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With proper structure design and judicious control, factors that are typically considered as
“secondary effects” could redefine and redirect the solution dynamics and pathways. By installing
two TTF redoxphores at each end of a Z-shaped π-skeleton, we have achieved a redox-driven
molecular switch that undergoes externally-triggered structural folding and unfolding. In our
system, restricted swiveling motions around the rigid phenyleneethynylene-derived scaffold assist
intramolecular π-π stacking of two TTF units, which functions as a key driving force for structural
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folding upon stepwise removal of electrons.
A
combination of UV–vis–NIR
spectroelectrochemical and EPR spectroscopic studies helped map out the reaction coordinates of
conformational switching, which respond sensitively to the solvent and electrolyte environment.
Important lessons that we have learned from this exploratory study should help future design of
stimuli-responsive molecules and materials.
EXPERIMENTAL SECTION
General Considerations. All reagents were obtained from commercial suppliers and used
as received unless otherwise noted. The solvents THF and CH2Cl2 were saturated with nitrogen
and passed through an activate Al2O3 column under nitrogen atmosphere (Innovative Technology
SPS 400) prior to use. The solvents toluene and anhydrous DMF purchased from Aldrich were
sparged with N2 prior to use. The compounds iodo-TTF (2),100 4-(tert-butyl)-2-iodophenyl
trifluoromethanesulfonate
(8),88
tetrabutylammonium
tetrakis(pentafluorophenyl)borate
(nBu4N[B(C6F5)4]),101 and silver(I) tetrakis(pentafluorophenyl)borate etherate complex
AgB(C6F5)4·Et2O102 were prepared according to literature procedures. All air-sensitive
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manipulations were carried out under nitrogen atmosphere in a M.Braun drybox or by standard
Schlenk line techniques.
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Physical Measurements. 1H NMR, 13C{1H} NMR, and 19F NMR spectra were recorded on
300 MHz Varian Gemini 2000, 300 MHz Bruker Advance DPX-300, 400 MHz Agilent 400-MR
DD2 Magnetic Resonance System, and 400 MHz Varian Inova NMR Spectrometers. Chemical
shifts were reported vs tetramethylsilane, and referenced to the residual solvent peaks. 19F NMR
spectrum was referenced to CFCl3 using an internal standard of fluorobenzene (δ = –112.96 ppm)
in CDCl3.103 High-resolution electrospray ionization (ESI) mass spectra were obtained on a
Thermo Electron Corporation MAT 95XP-Trap. FT-IR spectra were recorded on a Nicolet Avatar
360 FT-IR Spectrometer with EZ OMNIC ESP software. X-band EPR spectra were recorded on a
Bruker EMX-A spectrometer with an ER041X microwave bridge.
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4-tert-Butyl-2-(3-hydroxy-3-methylbut-1-ynyl)phenyl trifluoromethanesulfonate (7). A
flask loaded with 8 (6.00 g, 14.7 mmol), PdCl2(PPh3)2 (206 mg, 0.29 mmol), and CuI (84 mg, 0.44
mmol) was evacuated and back-filled with N2 three times. A portion of 2-methyl-3-butyn-2-ol (1.7
i
mL, 0.29 mmol) was delivered under N2 atmosphere, followed by a mixture of Pr2NH/THF (90
mL, 1:2, v/v). The reaction mixture was stirred under N2 at r.t. for 12 h. After removal of volatile
fractions under reduced pressure, the residual material was purified by flash column
chromatography on SiO2 (hexanes: EtOAc = 1:3, v/v) to afford 7 as a yellow oil (4.87 g, yield =
1
91%). H NMR (300 MHz, CDCl3, 298 K): δ 7.54–7.55 (d, J = 2.1 Hz, 1H), 7.41–7.37 (dd, J =
2.1, 8.8 Hz, 1H), 7.18–7.15 (d, J = 8.7 Hz, 1H), 2.57 (br, 1H), 1.65 (s, 6H), 1.32 (s, 9H); 13C{1H}
NMR (75 MHz, acetone-d6, 298 K): δ 152.7, 148.4, 131.6, 128.3, 122.1, 125.9–113.2 (q, J = 319.6
Hz), 118.2, 102.6, 75.5, 65.3, 35.3, 31.6, 31.3; 19F NMR (376 MHz, CDCl3, 298 K): δ –73.7. FT-
IR (thin film on KBr, cm–1): 3355, 2969, 2872, 1492, 1425, 1365, 1249, 1212, 1174, 1142, 1118,
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1085, 973, 939, 887, 864, 829, 811, 754, 638, 614. HRMS (ESI) m/z: [M + Na]+ Calcd for
C16H19O4F3SNa 387.0854; Found 387.0846.
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4-(5-(tert-Butyl)-2-((triisopropylsilyl)ethynyl)phenyl)-2-methylbut-3-yn-2-ol (6).
A
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flask loaded with 7 (1.47 g, 4.06 mmol), PdCl2(dppf)·CH2Cl2 (66 mg, 0.081 mmol), and CuI (23.2
mg, 0.122 mmol) was evacuated and back-filled with N2 three times. A portion of triisopropylsilyl
acetylene (1.10 mL, 4.87 mmol) was delivered under N2 atmosphere, followed by a mixture of
iPr2NH/DMF (20 mL, 1:1, v/v). The reaction mixture was stirred at 80 °C under N2 for 12 h. After
removal of volatile fractions under reduced pressure, the residual material was purified by flash
column chromatography on SiO2 (hexanes: EtOAc = 5:1, v/v) to afford 6 as a yellow oil (1.5 g,
yield = 94%). 1H NMR (400 MHz, CDCl3, 298 K): δ 7.43–7.42 (d, J = 1.6 Hz, 1H), 7.42–7.39 (d,
J = 8.4 Hz, 1H) 7.26–7.24 (dd, J = 1.6, 8.4 Hz, 1H), 2.05 (s, 1H), 1.62 (s, 6H), 1.28 (s, 9H), 1.14
13
(s, 21H); C{1H} NMR (100 MHz, CDCl3, 298 K): δ 151.4, 132.8, 129.3, 125.3, 124.6, 122.8,
105.6, 96.8, 93.7, 81.6, 65.8, 34.7, 31.5, 31.1, 18.8, 11.3. FT-IR (thin film on KBr, cm–1): 3332,
2960, 2865, 1491, 1463, 1396, 1363, 1165, 996, 972, 937, 883, 827, 799, 676. HRMS (ESI) m/z:
[M + Na]+ Calcd for C26H40OSiNa 419.2746; Found 419.2745.
4-(5-(tert-Butyl)-2-ethynylphenyl)-2-methylbut-3-yn-2-ol (5). A THF (70 mL) solution of
6 (6.78 g, 17.1 mmol) was cooled to 0 °C in an ice bath. A THF solution of TBAF (20.5 mL, 20.5
mmol) was added dropwise. The reaction mixture was stirred at r.t. for 1 h. After removal of
volatile fractions under reduced pressure, the residual material was purified by flash column
chromatography on SiO2 (hexanes: EtOAc = 4:1, v/v) to afford 5 as a pale yellow oil (4.0 g, yield
= 98%). 1H NMR (400 MHz, CDCl3, 298 K): δ 7.42–7.41 (d, J = 2.0 Hz, 1H), 7.40–7.38 (d, J =
8.0 Hz, 1H), 7.26–7.24 (dd, J = 2.6 , 8.0 Hz, 1H), 3.24 (s, 1H), 2.48 (br, 1H), 1.63 (s, 6H), 1.27 (s,
9H); 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ 151.9, 132.2, 128.8, 125.4, 125.3, 121.8, 97.4,
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82.3, 81.1, 80.2, 65.7, 34.7, 31.4, 31.0. FT-IR (thin film on KBr, cm–1): 3387, 3289, 2965, 2869,
1716, 1598, 1488, 1396, 1363, 1165, 936, 832. HRMS (ESI) m/z: [M + Na]+ Calcd for C17H20ONa
263.1412; Found 263.1401.
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4,4'-(Buta-1,3-diyne-1,4-diylbis(3-(tert-butyl)-6,1-phenylene))bis(2-methylbut-3-yn-2-
ol) (4). To a suspension of 5 (2.00 g, 8.32 mmol), PdCl2(PPh3)2 (117 mg, 0.166 mmol), and
Cu(OAc)2 (302 mg, 1.66 mmol) in a mixture of iPr2NH/pyridine/THF (36 mL, 5:3:10, v/v/v) was
added I2 (1.06 g, 4.16 mmol). The reaction was stirred for 3 h in air. After removal of volatile
fractions under reduced pressure, the residual material was purified by flash column
chromatography on SiO2 (hexanes: EtOAc = 3:1, v/v) to afford 4 as a yellow solid (1.88 g, yield
= 95%). 1H NMR (400 MHz, CDCl3, 298 K): δ 7.41–7.40 (d, J = 1.6 Hz, 2H), 7.40–7.38 (d, J =
8.0 Hz, 2H), 7.26–7.24 (dd, J = 1.6, 8.0 Hz, 2H), 3.28 (br, 2H), 1.65 (s, 12H), 1.26 (s, 18H);
13C{1H} NMR (100 MHz, CDCl3, 298 K): δ 152.4, 132.1, 128.6, 126.4, 125.3, 121.7, 98.3, 81.2,
80.9, 77.0, 65.7, 34.8, 31.4, 30.9. FT-IR (thin film on KBr, cm–1): 3354, 2965, 2868, 1595, 1483,
1400, 1363, 1262, 1166, 971, 935, 830. . HRMS (ESI) m/z: [M + Na]+ Calcd for C34H38O2Na
501.2769; Found 501.2791.
1,4-Bis(4-(tert-butyl)-2-ethynylphenyl)buta-1,3-diyne (3). A flask loaded with 4 (1.85 g,
3.86 mmol) and pulverized KOH (542 mg, 9.66 mmol) was evacuated and back-filled with N2
three times. A portion of toluene (30 mL) and five drops of distilled water were added to the
reaction mixture, which was heated at reflux for 12 h and cooled to r.t. The reaction mixture was
washed with water, and extracted with Et2O. The organic layer was dried over anhyd MgSO4, and
filtered. After removal of volatile fractions under reduced pressure, the residual material was
purified by flash column chromatography on SiO2 (hexanes: EtOAc = 4:1, v/v) to afford 3 as a
yellow solid (1.03 g, yield = 73%). 1H NMR (400 MHz, CDCl3, 298 K): δ 7.52–7.51 (d, J = 2.0
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Hz, 2H), 7.47–7.45 (d, J = 8.4 Hz, 2H), 7.34–7.31 (dd, J = 2.0, 8.0 Hz, 2H), 3.34 (s, 2H), 1.29 (s,
18H); 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ 152.4, 132.9, 129.8, 126.0, 125.2, 122.0, 82.2,
81.0, 80.7, 77.2, 34.9, 31.0. FT-IR (thin film on KBr, cm–1): 3291, 2964, 2904, 2868, 1594, 1485,
1394, 1363, 1261, 1190, 1127, 895, 833, 739, 643. HRMS (ESI) m/z: [M + H + CH3OH]+ Calcd
for C29H31O 395.2375; Found 395.2372.
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1,4-Bis(2-([2,2'-bi(1,3-dithiolylidene)]-4-ylethynyl)-4-(tert-butyl)phenyl)buta-1,3-diyne
(1). A flask loaded with 3 (400 mg, 1.10 mmol), 2 (1.09 g, 3.31 mmol), PdCl2(PPh3)2 (31 mg,
0.044 mmol), and CuI (17 mg, 0.088 mmol) was evacuated and back-filled with N2 three times. A
mixture of Et3N/THF (30 mL, 1:1, v/v) was delivered under N2 atmosphere. The reaction mixture
was stirred at 60 °C under N2 for 12 h, and cooled to r.t. After removal of volatile fractions under
reduced pressure, the residual material was purified by flash column chromatography on SiO2
(hexanes: EtOAc = 4:1, v/v) to afford 1 as a red solid (219 mg, yield = 26%). 1H NMR (400 MHz,
CDCl3, 298 K): δ 7.48–7.46 (d, J = 8.4 Hz, 2H), 7.43–7.45 (d, J = 2.0 Hz, 2H), 7.33–7.28 (dd, J =
2.0, 8.4 Hz, 2H), 6.63 (s, 2H), 6.26 (br, 4H), 1.29 (s, 18H); 13C{1H} NMR (100 MHz, CDCl3, 298
K): δ 152.5, 132.6, 128.6, 126.0, 125.7, 125.6, 121.6, 119.1, 118.8, 115.9, 92.4, 81.1, 77.6, 77.2,
35.0, 31.0. FT-IR (thin film on KBr, cm–1): 3423, 2962, 2872, 1643, 1481, 1395, 1363, 1262, 831,
737, 638. HRMS (ESI) m/z: [M]+ Calcd for C40H30S8 766.0114; Found 766.0114.
((4-(tert-Butyl)-2-ethynylphenyl)ethynyl) triisopropylsilane (9). A flask loaded with 6
(1.00 g, 2.52 mmol) and pulverized KOH (0.212 g, 3.78 mmol) was evacuated and back-filled with
N2 three times. A portion of toluene (10 mL) was delivered, and the reaction mixture was heated
at reflux for 12 h and cooled to r.t. The reaction mixture was washed with water, and extracted
with Et2O. The combined extracts were dried over anhyd MgSO4 and filtered. After removal of
volatile fractions under reduced pressure, the residual material was purified by flash column
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chromatography on SiO2 (hexanes) to afford 9 as a yellow oil (819 mg, yield = 96%). H NMR
(400 MHz, CDCl3, 298 K): δ 7.51–7.50 (d, J = 2.0 Hz, 1H), 7.42–7.40 (d, J = 8.4 Hz, 1H), 7.31–
7.28 (dd, J = 2.0, 8.4 Hz, 1H), 3.22 (s, 1H), 1.29 (s, 9H), 1.19–1.09 (m, 21H); 13C{1H} NMR (100
MHz, CDCl3, 298 K): δ 151.4, 132.2, 129.5, 125.8, 124.6, 123.9, 105.1, 94.3, 82.9, 80.4, 34.7,
31.1, 18.7, 11.4. FT-IR (thin film on KBr, cm–1): 3427, 3307, 2960, 2865, 2156, 2062, 1488, 1463,
1394, 1364, 1262, 1127, 1072, 996, 883, 827, 781, 677, 643. HRMS (ESI) m/z: [M + H + CH3OH]+
Calcd for C24H39OSi 371.2770; Found 371.2771.
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((2-([2,2'-Bi(1,3-dithiolylidene)]-4-ylethynyl)-4-(tert-butyl)phenyl)ethynyl)tri-
isopropylsilane (10). A flask loaded with 9 (207 mg, 0.610 mmol), 2 (300 mg, 0.915 mmol),
PdCl2(PPh3)2 (17 mg, 0.024 mmol), and CuI (7.0 mg, 0.037 mmol) was evacuated and back-filled
with N2 three times. A mixture of Et3N/THF (20 mL, 1:1, v/v) was delivered under N2 atmosphere.
The reaction mixture was stirred at 60 °C under N2 for 12 h, and then cooled to r.t. After removal
of volatile fractions under reduced pressure, the residual material was purified by flash column
1
chromatography on SiO2 (hexanes) to afford 10 as a red solid (191 mg, yield = 58%). H NMR
(400 MHz, CDCl3, 298 K): δ 7.43–7.42 (d, J = 1.6 Hz, 1H), 7.42–7.39 (d, J = 8.4 Hz, 1H), 7.30–
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7.27 (dd, J = 2.0, 8.4 Hz, 1H), 6.50 (s, 1H), 6.31 (s, 2H), 1.28 (m, 9H), 1.13 (s, 21H); C{1H}
NMR (100 MHz, CDCl3, 298 K): δ 151.5, 132.6, 129.1, 128.4, 128.3, 126.0, 124.3, 124.1, 123.1,
119.2, 118.8, 116.3, 104.9, 94.8, 92.8, 60.4, 34.8, 31.0, 18.8, 11.3. FT-IR (thin film on KBr, cm–
1): 3432, 2959, 2864, 2154, 1706, 1462, 1364, 1263, 1016, 882, 825, 739. HRMS (ESI) m/z: [M]+
Calcd for C29H36SiS4 540.1469; Found 540.1469.
Electrochemistry. Electrochemical studies were carried out under ambient conditions using
an Autolab model PGSTA30 potentiostat (Eco Chemie). A three-electrode configuration
consisting of a platinum button working electrode, a Ag/AgNO3 (0.01 M in MeCN with 0.1 M
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nBu4NPF6) reference electrode, and a platinum coil counter electrode was used. All
electrochemical potentials were reported to the (C5H5)2Fe/(C5H5)2Fe+ redox couple unless
otherwise noted.
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UV–Vis–NIR Spectroelectrochemistry. UV–vis–NIR spectroelectrochemistry studies
were conducted in a custom-built spectroelectrochemical cell with 1-mm pathlength, equipped
with a platinum mesh working electrode, a Ag/AgCl reference electrode, and a platinum coil
counter electrode. Solution samples 1 (1.0 mM) with electrolyte (nBu4NPF6 or nBu4N[B(C6F5)4];
0.1 M) were delivered into the cell. The potential was applied through a Princeton Applied
Research Potentiostat/Galvanostat model 263A as a step function with 10 min interval for every
step and with stepwise voltage increase of 25 mV. UV–vis–NIR spectra as a function of applied
potential were recorded on a Varian Cary 5000 UV–Vis–NIR Spectrophotometer until no further
change could observed at each potential applied.
ASSOCIATED CONTENTS
Supporting Information. The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.joc.xxxxxxx.
Characterization data (1H NMR, 13C{1H} NMR, and 19F NMR spectra)
AUTHOR INFORMATION
Corresponding Author
* E-mail: dongwhan@snu.ac.kr ; juj23@psu.edu
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