Quantification of the π-π Interactions that govern tertiary structure in donor-acceptor [2]pseudorotaxanes

Article abstract of DOI:10.1021/ja210861v

Flexibility in pseudorotaxanes and interlocked molecules that rely on interactions between π-donor-acceptor subunits provides access to folded structures reminiscent of the tertiary structure of proteins. While they have been described before, only now have we been able to quantify one such tertiary structure by making use of pseudorotaxanes designed for the purpose. Here, the enhanced stability of a pseudorotaxane inside a folded structure is measured to be ΔG = ca. 0.5 kcal mol^-1. The tertiary structure is stabilized by a charge-transfer interaction between a tetrathiafulvalene-based π-donor that can situate alongside a π-accepting paraquat-based macrocycle by folding of a flexible linker. At room temperature, it was estimated that 70% of the pseudorotaxanes examined here exist in their folded state. This quantitative information is critical for the creation of interlocked molecular machines that have predictable energetics and structures and for revealing a complexity approaching biological molecules.

Full text of DOI:10.1021/ja210861v

Article
pubs.acs.org/JACS
Quantification of the π−π Interactions that Govern Tertiary Structure
in Donor−Acceptor [2]Pseudorotaxanes
Stinne W. Hansen,†,‡ Paul C. Stein,† Anne Sørensen,† Andrew I. Share,‡ Edward H. Witlicki,‡
Jacob Kongsted,† Amar H. Flood,‡ and Jan O. Jeppesen*,†
†Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark
‡Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States
S
* Supporting Information
ABSTRACT: Flexibility in pseudorotaxanes and interlocked
molecules that rely on interactions between π−donor−
acceptor subunits provides access to folded structures
reminiscent of the tertiary structure of proteins. While they
have been described before, only now have we been able to
quantify one such tertiary structure by making use of
pseudorotaxanes designed for the purpose. Here, the enhanced
stability of a pseudorotaxane inside a folded structure is
measured to be ΔG = ca. 0.5 kcal mol−1. The tertiary structure is stabilized by a charge-transfer interaction between a
tetrathiafulvalene-based π-donor that can situate alongside a π-accepting paraquat-based macrocycle by folding of a flexible linker.
At room temperature, it was estimated that 70% of the pseudorotaxanes examined here exist in their folded state. This
quantitative information is critical for the creation of interlocked molecular machines that have predictable energetics and
structures and for revealing a complexity approaching biological molecules.
INTRODUCTION
■
The field of supramolecular chemistry was initiated approx-
imately 40 years ago and is today a vibrant area of
contemporary research1 that has recently branched out into
nanoscience.2−4 Supramolecular systems rely on weak non-
covalent interactions such as hydrogen bonding, π−π
interactions, and donor−acceptor interactions and exploit
naturally occurring phenomena like self-organization, self-
assembly, self-replication, and molecular recognition. The
remarkable properties of proteins and DNA are only possible
because the same noncovalent interactions that control their
folding and assembly are sufficient to overcome conformational
chaos. In addition to sharing these phenomena and
interactions, nature also serves as an inexhaustible source of
inspiration and analogy in supramolecular science. Here, we
apply the concepts of a protein’s primary (1°), secondary (2°),
and tertiary (3°) structure (Figure 1) to the emergence of
ordered structures in pseudorotaxanes as a means to help
understand the energetics of folded states that have hitherto
been inaccessible despite being spotted in a few instances.5
The focus on pseudorotaxanes is based on their ability to
forecast rotaxanes, which are interlocked molecules with “rod-
in-ring” rotaxanes, capable of performing molecular motions6
and acting as nanoscale machinery. Success with this functional
outcome has almost always relied upon the selection and
sequence of stations along the rod (1° structure, Figure 1) to
generate a predictable 2° structure where the localization of the
ring at one station over another is determined by well-known
noncovalent bonds. The 3° structures, however, have usually
been overlooked presumably because they are difficult to
Figure 1. Hierarchical organization of protein structure serves as an
analogy for threaded complexes and interlocked molecules. The 3°
protein structure shown is the crystal structure of human recombinant
MTCP-1.8
observe. Yet, the folded states can cause the properties of
molecular machines to diverge from predictions when they are
based solely on 1° and 2° design considerations. For example, a
bistable rotaxane unexpectedly lost ca. 50% of its switchable
character.5a This outcome was believed to arise from a folded
state; a 3° structure that was later supported qualitatively.5b
Thus, when the same depth of understanding that is enjoyed in
the creation of 2° structures is extended to the 3° level,7 both
Received: November 18, 2011
Published: January 17, 2012
© 2012 American Chemical Society
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Figure 2. Molecular structures of the semidumbbells 1−3 and CBPQT4+ and their corresponding cartoon representations.
the accuracy of our designs and opportunities for new ones can
grow.
In the 2° structure of the complex, the CBPQT4+ ring is
restricted to encircle the TTF unit (Figure 3, [1⊂CBPQT4+]2°)
since the combination of the SEt group (purple) and the
central thioglycol linker (purple) provide a kinetic barrier, i.e.,
they act together as a stopper. The thioglycol linker was
introduced to provide the flexibility necessary to allow the
monopyrroloTTF12 (MPTTF) unit within the stopper on the
west side to fold back onto the CBPQT4+ ring. In this way, it
can interact in an alongside manner generating a 3° structure
(Figure 3, [1⊂CBPQT4+]3°). Semidumbbell 3 has been
designed to consist solely of a TTF unit similar to the east
side of semidumbbells 1 and 2. It is assumed that the TTF unit
of semidumbbell 3 will have the same binding energy with the
CBPQT4+ ring as the east sides of semidumbbells 1 and 2. This
assumption makes it possible to quantify the additional binding
energy provided by the folding of the supermolecule into a 3°
structure stabilized by CT interactions between the west side
MPTTF unit and the exterior of the CBPQT4+ ring. The
semidumbbells 1−3 all exist as two different isomers on
account of the inherent E/Z isomerism of the TTF unit. Since
the ratios of the isomers are all the same, this is thought to be of
minor importance.
Studies identifying 3° structures in interlocked molecules are
small in number. It is the flexible poly(ethyleneglycol) (PEG)
linkers present in the majority of rotaxanes containing π-
electron rich stations that allow the folded 3° structure to exist.
There has been early focus on the C−H···O hydrogen bonding
between the cyclobis(paraquat-p-phenylene)9 (CBPQT4+) α-
protons and the PEG linker oxygens,2d,10 taking place on the
exterior of the complex. However, what has emerged in recent
works5a,b is the need to deliver a quantitative account of the
charge−transfer (CT) interactions that help determine the
folded 3° structurein addition to the C−H···O hydrogen
bonds. These CT interactions are prevalent, if not universal,
when π-derived donors and acceptors are used in redox-active
molecular machines for demonstrations of molecular elec-
tronics,2 molecular muscles,3 and RGB electrochromic
materials4 that highlight the potential of interlocked molecules
in nanotechnology.
In this work, we present the synthesis and investigation of
foldable [2]pseudorotaxanes based on CBPQT4+ and semi-
dumbbells (1 or 2, Figure 2) incorporating two different
tetrathiafulvalene11 (TTF) units. These designs render it
possible to investigate and quantify the CT interactions that
govern the 3° alongside structure in π-donor−acceptor
[2]pseudorotaxanes.
RESULTS AND DISCUSSION
■
Synthesis. The semidumbbells 1−3 were synthesized
(Scheme 1) using a convergent synthetic strategy. The
MPTTF building block2d 4 was alkylated with 1-iodo-2-[2-(2-
methoxyethoxy)ethoxy]ethane13 following its in situ depro-
tection with CsOH·H2O to give MPTTF 5 in 88% yield. The
tosyl protecting group was removed by NaOMe in a THF/
MeOH mixture and the resulting free pyrrole nitrogen in
compound 6 was N-alkylated (NaH/DMF) with 1,2-bis(2-
iodoethoxy)ethane,14 affording the MPTTF building block 7 in
85% yield over two steps. The thione15 8 was S-alkylated with
1-iodo-2-[2-(2-methoxyethoxy)ethoxy]ethane after in situ
deprotection with CsOH·H2O in 87% yield to afford the
thione 9 which was transchalcogenated using Hg(OAc)2 in
CH2Cl2 to the corresponding ketone 10 in 96% yield. The
ketone 10 was cross coupled with the thione16 11 yielding the
TTF building block 12 in 52% yield. This building block was in
situ deprotected with CsOH·H2O and afterward S-alkylated in
THF/MeOH with either 1-iodo-2-[2-(2-methoxyethoxy)-
ethoxy]ethane to give semidumbbell 3 in 85% yield or with
the MPTTF building block 7 giving semidumbbell 1 in 80%
yield. Semidumbbell 2 was also prepared from the MPTTF
building block 4 and 3,5-di-tert-butylbenzylbromide17 13 in a
series of reactions identical to those used for the synthesis of 2
Figure 3. Representations of the complexation and folding equilibria
between the semidumbbell 1 and CBPQT4+ (blue) show different
levels of structural organization. After complexation of CBPQT4+ with
TTF (light green), the MPTTF unit (dark green) can fold back and
interact with CBPQT4+ in an alongside manner stabilized by CT
interactions.
The 1° structures of all the semidumbbells (Figures 2 and 3)
are designed to be nonsymmetric along the east−west axis. The
entire west side is a stopper with internal structure, whereas the
east side is the binding site. Consequently, the CBPQT4+ ring
can only access the TTF unit (light green) by threading over
the triethylene glycol (TEG) linker on the east side of 1 and 2.
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Scheme 1. Synthesis of the Semi-Dumbbells 1−3
and in comparable yields (Scheme 1). All of the compounds
immediate color change of the solution from yellow to green
and the concomitant appearance of CT bands in the 790−813
nm region of the absorption spectra. The Ka values were
determined from the linear plots of c/A against 1/A1/2 (Figure
4a) for 1:1 mixtures of CBPQT4+ and the semidumbbells 1−3
(Supporting Information) at 298 K in MeCN and the data
obtained are summarized in Table 1. The binding constants
containing the TTF unit were isolated as mixtures of E/Z
1
isomers and H NMR spectroscopy confirmed (Supporting
Information) that the ratio between the isomers was 1:1 in all
cases.
Binding Constant Determinations. Comparisons of the
binding between the CBPQT4+ ring and semidumbbells 1−3
indicate that the presence of the alongside interaction affords a
0.5 kcal mol−1 boost to the free energy of binding. The
individual binding constants were characterized (Figure 4) by
Table 1. Comparison of Binding Constants, Extinction
Coefficients, and Derived Free Energies of Complexation
a
between CBPQT·4PF6 and the Semi-Dumbbells 1−3
complex
Ka [M−1
]
ε [M−1 cm−1
]
ΔG° [kcal mol−1
]
1 ⊂ CBPQT·4PF6
2 ⊂ CBPQT·4PF6
3 ⊂ CBPQT·4PF6
10300 1400
9000 1500
4200 600
2500 600
2200 500
1700 500
−5.5 0.1
−5.4 0.1
−4.9 0.1
a
Determined by absorption spectroscopy at 298 K in MeCN using the
NIR CT band as probe. The errors were obtained as described by
Nygaard et al.
Figure 4. (a) Plots of c/A against 1/A1/2 for 1:1 mixtures of CBPQT4+
and the semidumbbells 1−3. The absorbance (A) was measured at
several different absolute concentrations (c) obtained from dilution of
at least two independent stock solutions in MeCN at 298 K. (b) A
comparison of the CT bands for 1:1 mixtures of CBPQT4+ and the
semidumbbells 1−3 at the conditions 0.5 mM + 1 equiv CBPQT4+.
(Ka) for the semidumbbells 1 and 2 are more than twice as
large as 3. This finding indicates that the presence of the
MPTTF unit results in a stabilizing effect of approximately 0.5
kcal mol−1 on the complex formed between CBPQT4+ and the
TTF unit. We ascribe this to a folded 3° structure (Figure 3)
stabilized by CT interactions taking place between the MPTTF
unit and one of the bipyridinium moieties of CBPQT4+.
Consistently, the extinction coefficients (Table 1) of the CT
electronic transitions generated (Supporting Information) in
the fitting analysis are greater for both 1 and 2 when compared
to 3, and the bandwidths are larger (Figure 4b). The additional
absorption spectroscopy using the dilution method.10c,18
Mixing equimolar amounts of the CBPQT4+ ring and each of
the semidumbbells 1−3 in MeCN results in the formation of
the corresponding [2]pseudorotaxanes confirmed by the
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CT interaction is expected to be accompanied by another
electronic transition that, based on prior work,19 is likely to be
situated at another location. Therefore, both factors would lead
to the enhanced and broadened band in the NIR region.
The Ka values for pseudorotaxanes 1⊂CBPQT4+ and
2⊂CBPQT4+ are the same to within error. This outcome was
not predictable on the basis of the structures (Figure 2). The
TEG substituent on the far west side of MPTTF unit in
semidumbbell 1 has the possibility of forming hydrogen
bonding interactions between the acidic α-CH protons in the
bipyridinium moieties of CBPQT4+ and some of the oxygen
atoms in the TEG chain.10c These interactions would be absent
in semidumbbell 2. Thus, the equivalence of the binding
affinities indicate that neither the potentially “sticky” TEG nor
the “bulky” stopper groups, which are located beyond the
MPTTF unit folded onto the side of the CBPQT4+ ring do not
have a major impact on the stability of the 3° structure.
Molecular Modeling. In order to get a better under-
standing of the nature of the binding interactions present in the
two [2]pseudorotaxanes 1⊂CBPQT4+ and 2⊂CBPQT4+,
density functional theory has been employed with the M06-L
exchange-correlation functional,20 implemented in Jaguar 7.6,21
to provide a quantum mechanically based description of the
geometry of the 3° superstructures. This exchange-correlation
functional has previously been used to provide accurate
structural and energetic descriptions of superstructures and
mechanically interlocked molecules.22 We have performed
geometry optimizations for the pseudorotaxanes with
CBPQT4+ encircling the TTF unit of semidumbbells 1 and 2
at the M06-L/6-31G** level of theory including solvent
corrections in MeCN (ε = 37.5, R0 = 2.18 Å) based on the
Poisson−Boltzmann dielectric continuum model implemented
in Jaguar.
MPTTF unit of either 1 or 2 indicating that an alongside CT
interaction can exist. In addition, the optimized structures show
geometries consistent with hydrogen bonding interactions
taking place between one of the CBPQT4+pyridinium α-
protons and TEG oxygens (ca. 2.5 Å) with the glycol linkers on
either side of the TTF unit. However, the glycol chain on
MPTTF in the semidumbbell 1 folds back on itself neither
favoring nor disfavoring the folded 3° structure. The bulky di-
tert-butylbenzylic stopper from semidumbbell 2 also folds away
from the CBPQT4+ ring, and therefore it will not sterically
disfavor the folded 3° structure.
These findings are in complete agreement with the results
obtained from the solution-phase binding studies showing a
negligible difference in the binding constants (Table 1)
between the CBPQT4+ and the semidumbbells 1 and 2,
respectively. That is, the MPTTF unit seems to contribute the
same degree of extra stabilization to the [2]pseudorotaxanes
1⊂CBPQT4+ and 2⊂CBPQT4+, regardless of substitution on
the MPTTF unit with either the TEG or bulky di-tert-
butylbenzylic stopper.
1H NMR Spectroscopy. The complexation between the
CBPQT4+ ring and semidumbbells 1 and 3 and the subsequent
folding in 1⊂CBPQT4+ have been studied by variable
temperature (VT) and two-dimensional (2D) 1H NMR
1
spectroscopy. Complete assignments of the H NMR spectra
are complicated by the need to count for 87 and 68 protons
present in 1⊂CBPQT4+ and 3⊂CBPQT4+, respectively,
together with the fact that both complexes are in equilibrium
with their free constituents and the semidumbbells exist as two
E/Z isomers.
In the optimized structures (Figure 5), CBPQT4+ encircles
the TTF unit with TTF···bipyridinium distances of 3.5 Å in
Figure 6. Partial 1H NMR spectra (500 MHz, 233 K, CD3CN) of: top,
green: semidumbbell 1 + 0.75 equiv. CBPQT·4PF6, middle, blue:
semidumbbell 3 + 0.75 equiv. CBPQT·4PF6, bottom, gray:
CBPQT·4PF6.
Figure 5. M06-L/6-31G** geometry optimized superstructures
(solvent corrected, MeCN) of the two [2]pseudorotaxanes
[1⊂CBPQT4+]3° (left) and [2⊂CBPQT4+]3° (right). For clarity no
hydrogens are shown and the TTF unit is colored light green, MPTTF
dark green, CBPQT4+ blue, other atoms: carbon = dark gray, sulfur =
light gray and oxygen = red.
In the case of the complex formed between the semidumb-
bell 3 and CBPQT4+, exchange between the complexed and
uncomplexed species occurs rapidly (Supporting Information,
Figure S5) on the 1H NMR time scale (CD3CN, 500 MHz) at
293 K. Decreasing the temperature of the CD3CN sample
favors complexation, and at 233 K, the kinetics is in the regime
of slow exchange, and both complexed and uncomplexed
agreement23 with the optimal distance for donor−acceptor
interactions. The MPTTF unit is engaged in an alongside CT
interaction with one of the bipyridinium moieties in CBPQT4+
with a MPTTF···bipyridinium distance of 3.5 Å. Consistent
with the enhanced stability and UV−vis-NIR absorption
properties, these ground state CT interactions are furthermore
theoretically supported by natural population analysis. When
conducted at the same level as described above they reveal that
the charge of CBPQT4+ is reduced by 0.85 and 0.92 atomic
units (au) in [1⊂CBPQT4+]3° and [2⊂CBPQT4+]3°, respec-
tively, upon complexation. Furthermore, for both complexes
0.43 au of the transferred charge is found to originate from the
1
semidumbbell can be observed (Figure S4 of the SI) in the H
NMR spectrum recorded of 3 and 0.75 equivalents of
CBPQT4+. However, no signals associated with uncomplexed
1
CBPQT4+ are present (Figure 6) in the H NMR spectrum
indicating that the complexation is very strong at 233 K. A close
examination of the 1H COSY (correlation spectroscopy)
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spectrum (Figure S7 of the SI) reveals that eight doublets (J =
6.5 Hz) are present between 8.93 and 9.15 ppm and eight
doublets (J = 6.5 Hz) are present between 7.96 and 8.17 ppm.
It is well-known4b,5c,18,24 that signals in these regions for these
particular types of complexes can be associated with the
resonances for the bipyridinium Hα and Hβ protons (Figure 2),
respectively. The presence of eight Hα and Hβ resonances
confirms that 3⊂CBPQT4+ exists as a mixture of E/Z isomers.
On account of the low degree of symmetry of the semidumb-
bell 3 and the lack of free rotation in the CBPQT4+ ring at 233
K, desymmetrization of the complexed CBPQT4+ ring will take
place and four signals5b,25 of Hα and Hβ will appear for both the
E and the Z isomer. These effects were also seen when studying
the complexation between 1 and CBPQT4+ but with one
noteworthy difference. Even though the cyclophane in
1⊂CBPQT4+ resides on a TTF unit that is identical to the
one in 3, the resonances of the cyclophane Hα and Hβ protons
are shifted upfield in comparison to 3⊂CBPQT4+. This upfield
shift is indicative5e of π−π interactions and is assigned to
stacking between one of bipyridinium moieties of CBPQT4+
and the MPTTF unit residing alongside it in the folded 3°
structure (Figure 3). The upfield shift is consistent with
increased shielding effect of the protons in the CBPQT4+ ring.
The effect is also evident in some of the +NCH2 protons of the
CBPQT4+ ring (Figure 6), but less so in the positions of the p-
xylyl protons as they are not directly engaged in the structure-
determining π−π interactions.
NOESY experiments were also carried out in CD3CN at 233
K and in CD3COCD3 at 203 K. However, it was not possible to
observe any NOE effect between the protons in MPTTF unit
and the protons on the CBPQT4+ ring, which most likely can
be accounted for the by dynamic nature of the complex even at
low temperatures.26
Electrochemical Investigations. With strong evidence for
the folded 3° structures available, cyclic voltammetry (CV) was
used to probe the changes in the redox potentials of the
semidumbbell 1 when the alongside CT interaction is in place.
The first oxidation process of the free semidumbbell 1 (Figure
7, black trace) appears at E1/21 = +0.39 V and is associated with
solution, the first oxidation process associated with the MPTTF
unit shifts anodically by +75 mV to +0.475 V. Therefore, the
peak observed at +0.475 V can be assigned to two different
overlapping oxidation processes. The oxidation of the MPTTF
unit shifts in a manner consistent with the alongside interaction
with CBPQT4+; MPTTF units become more difficult to oxidize
when engaged in a CT interaction.23b The first oxidation of the
TTF unit, now encircled by CBPQT4+, also occurs at +0.475 V.
Such a change was also seen (Supporting Information) when
CBPQT4+ was titrated into the semidumbbell 3. These changes
are subtle, highlighting how easy they are to overlook in two-
station pseudototaxanes5b and rotaxanes5a based on TTF units.
Resonance Raman Spectroscopy. The 1⊂CBPQT4+
pseudorotaxane was studied by resonance Raman spectroscopy
to inspect for signatures of the alongside interaction; as has
been previously observed.19 On the basis of the broadened CT
bands (Figure 4b), it was hoped that MPTTF vibrations would
be enhanced at different locations to TTF. Unfortunately, it
was not possible to detect the alongside interaction by that
technique presumably because the MPTTF → CBPQT4+
transition was too weak and/or its Raman bands coincided
with the TTF → CBPQT4+ transition.
CONCLUSIONS
■
The alongside π−π interactions observed in rotaxanes with
more than one π-electron donor were seen here in numerous
forms: chemical shift changes, altered redox properties,
enhanced binding affinities, modified absorption band
intensities, and band widths, as well as supporting computer-
generated structures. By absorption spectroscopic investiga-
tions, it has been possible, for the first time, to quantify the π−π
interactions. The binding studies carried out on 1−
3⊂CBPQT4+ revealed that the complexation ability of
CBPQT4+ with semidumbbells 1 and 2 is 0.5
0.2 kcal
mol−1 higher as compared to the model compound 3.
Assuming that the relative distribution between the 2° and 3°
structures is related to this free energy difference, it can be
calculated27 that the equilibrium constant (Keq) between the
unfolded 2° structure and the folded 3° structure is 2.3 0.8.
This Keq value implies that approximately 70% of 1⊂CBPQT4+
and 2⊂CBPQT4+ exists (Figure 8) in their folded 3° structures
in MeCN at 298 K.
Figure 8. The equilibrium between the 2° and 3° structures is shifted
toward the folded 3° structure on account of the stabilizing effect of
0.5 kcal mol−1 from the π−π interactions between the bipyridinium
moieties of CBPQT4+ (blue) and the MPTTF unit (dark green).
Figure 7. Partial cyclic voltammograms of a titration of the
semidumbbell 1 with 0−2.50 equiv. of CBPQT·4PF6. CV measure-
ments were carried out at 265 K in argon-purged MeCN solutions (1.0
mM) with tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) as
supporting electrolyte (0.1 M) and glassy carbon as working electrode,
platinum wire as auxiliary electrode at a scan rate of 200 mV s−1.
Potential values in V vs Ag/AgCl.
An extra stabilization of the pseudorotaxanes by 0.5 kcal
mol−1 is a reasonable adjustment to include in designs of
bistable [2]rotaxanes. While small, the 2-fold enhancement in
the binding affinity can become significant, particularly when it
is structure-determining; taking a rotaxane and folding it back
upon itself. The impact of this interaction on the tertiary
structure must also be accounted for in the designed
functionality of molecular machines just as nature accom-
modates and uses them in the performance of biomachines.
oxidation of the MPTTF unit (Supporting Information),
2
whereas the second oxidation process appearing at E1/2
=
+0.46 V is assigned to the oxidation of the TTF unit
(Supporting Information) and both generate monocations.
Upon addition of increasing amounts of CBPQT4+ to the
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures for synthesis of the semidumbbells 1−
3, description of binding studies, XYZ coordinates for
calculated super structures, variable temperature 1H NMR
studies, and description of electrochemical measurements. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
joj@ifk.sdu.dk.
■
(7) Meijer, E. W. Nature 2011, 469, 23−25.
Notes
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Natl. Acad. Sci. U.S.A. 1998, 95, 3413−3418.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Center for Fundamental
Living Technology (FLinT), University of Southern Denmark
(SDU), the Villum Foundation, and The Danish Natural
Science Research Council (FNU, projects no. 272-08-0578).
A.H.F. thanks the Dreyfus Foundation for support.
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applicable to TTF compounds with and without thio substituents on
one of the dithiole positions.
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describing NOE effects for complexes based on TTF derivatives and
CBPQT4+ have been reported. However, NOE effects have been
reported for complexes based on CBPQT4+ and other electron donors,
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Chem.Eur. J. 2005, 11, 369−385. (b) Caldwell, S. T.; Cooke, G.;
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(27) The equilibrium constant Keq for the equilibrium between the
unfolded 2° structure and the folded 3° structure was calculated using
the relationship Keq = exp(ΔG/RT), where R is the gas constant and T
is the absolute temperature.
3863
dx.doi.org/10.1021/ja210861v | J. Am. Chem. Soc. 2012, 134, 3857−3863