Two classes of alongside charge-transfer interactions defined in one [2]catenane

Article abstract of DOI:10.1021/ja069047w

A [2]catenane, which incorporates hydroquinone (HQ) and a sterically bulky tetrathiafulvalene (TTF) into a bismacrocycle, has been designed to probe the alongside charge-transfer (CT) interactions taking place between a TTF unit and one of the bipyridiniu

Full text of DOI:10.1021/ja069047w

Published on Web 05/18/2007
Two Classes of Alongside Charge-Transfer Interactions
Defined in One [2]Catenane
Sune Nygaard,†,‡ Stinne W. Hansen,† John C. Huffman,‡ Frank Jensen,†
Amar H. Flood,*,‡ and Jan O. Jeppesen*,†
Contribution from the Department of Physics and Chemistry, UniVersity of Southern Denmark,
Odense UniVersity, CampusVej 55, 5230, Odense M, Denmark and Department of Chemistry,
UniVersity of IndianasBloomington, 800 East Kirkwood AVenue, Bloomington, Indiana 47405
Received December 27, 2006; E-mail: aflood@indiana.edu; joj@ifk.sdu.dk
Abstract: A [2]catenane, which incorporates hydroquinone (HQ) and a sterically bulky tetrathiafulvalene
(TTF) into a bismacrocycle, has been designed to probe the alongside charge-transfer (CT) interactions
taking place between a TTF unit and one of the bipyridinium moieties in the tetracationic cyclophane cyclobis-
(paraquat-p-phenylene) (CBPQT4+). A template-directed strategy employs the HQ unit as the primary
template for formation of the tetracationic cyclophane CBPQT4+, affording the desired [2]catenane structure
but as an uncharacteristic green solid. The X-ray crystal structure and detailed 13C NMR assignments
have identified a stereoselective preference for catenation about the cis isomer. The 1H NMR spectroscopy,
electrochemistry, and X-ray crystallography all confirm that the CBPQT4+ cyclophane encircles the HQ
unit, thereby defining a structure which would normally determine a red color. The visible-NIR region of
the absorption spectrum displays a band at ∼740 nm that is unambiguously assigned to a TTF f CBPQT4+
CT transition on the basis of resonance Raman spectroscopy using 785 nm excitation. The profile of the
CT band changes depending on the ratio of the cis- to trans-TTF isomers in the [2]catenane for which the
molar absorptivity of each isomer is estimated to be significantly different at ꢀmax ) 380 and 3690 M-1
cm-1, respectively. Molecular modeling studies confirmed that the observed difference in the absorp-
tion spectroscopic profile can be accounted for by both a better overlap of the HOMO(TTF) and
LUMO+1(CBPQT4+) as well as a more stable face-to-face (π‚‚‚π) conformation in the trans isomer compared
to the edge-to-face cis isomer of the [2]catenane. The latter is arranged for π-orbital overlap through the
sulfur atoms of the TTF unit, thereby defining an [Sπ‚‚‚π] interaction.
employing the concepts of self-assembly4 and self-organiza-
tion.4,5 Of particular note are the fabrication of structures with
increasing complexity such as catenanes6 and rotaxanes,6a,b,7 as
well as molecular lifts,8 unidirectional motion systems,9 and
light-harvesting entities.10 These structures emerge from the
control over a range of noncovalent interactions including edge-
Introduction
Through the last couple of decades a wide range of different
interlocked structures,1 macromolecular assemblies,2 and charge-
transfer (CT) conducting salts3 have been constructed by
† University of Southern Denmark.
‡ University of IndianasBloomington.
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J. AM. CHEM. SOC. 2007, 129, 7354-7363
10.1021/ja069047w CCC: $37.00 © 2007 American Chemical Society

Alongside Charge-Transfer Interactions
A R T I C L E S
to-face [CH‚‚‚π] interactions,11 hydrogen bonding, and CT
interactions.12 One class of CT interactions occurs when the
central cavity of a cyclophane is blocked, thereby forcing the
π-donor and -acceptor pair to interact in an alongside13 manner.
These alongside interactions can be further subdivided based
upon the relative orientation of the donor/acceptor pair into face-
to-face11c,13 and edge-to-face of which the latter arise in specific
situations.14 For both synthetic purposes and machine-like
functionality, a significant number of the more complex
interlocked structures have been relying on the sum of the
noncovalent interactions that occur between the tetracationic
π-electron-deficient cyclophane15 cyclobis(paraquat-p-phen-
ylene) (CBPQT4+) serving as a host and the efficient π-elec-
tron-rich guests, such as dioxynaphthalene (DNP),6c,d,e,16
tetrathiafulvalene (TTF),16b,d,17 monopyrrolotetrathiafulvalene
(MPTTF),18 and 1,4-dihydroquinone (HQ) units.6c,f,g,16a Once
the guests are accommodated inside the CBPQT4+ cyclophane’s
cavity, a characteristic color is observed that originates from
the CT interaction between the guest and the host. It is well
known that aromatic entities such as DNP and HQ give rise to
a distinct red or purple color, respectively,8a,18b-d,19 originating
from a weak CT electronic absorption band at ∼500 nm (ꢀ ≈
100’s M-1 cm-1), whereas inclusion of various TTF units
invariably results in formation of a more intense characteristic
CT band in the 750-850 nm (ꢀ ≈ 1000’s M-1 cm-1) region.18,20
The fact that the CT interaction between the CBPQT4+
cyclophane and the π-electron-rich guests gives rise to such
well-characterized and well-assigned CT bands has determined
that the analysis of CT bands has been both ubiquitous and of
prime importance particularly in cases when the structures and
dynamics of more complex interlocked molecules, such as
bistable [2]rotaxanes, have been examined.18
Most [2]catenanes and [2]rotaxanes prepared so far have been
characterized16-19,21 by a high degree of flexibility on account
of the fact that the various π-electron-rich units in the interlocked
systems have historically been linked by flexible poly(ethylene
glycol) (PEG) chains, a feature which has conferred a high
degree of folding and thereby allowed for various conformations
to exist in solution as well as in more condensed phases.10c,22
Building on the inherent flexibility of the interlocked com-
pounds, it has long been known that the dynamic nature of
nondegenerate two-station [2]catenanes and [2]rotaxanes can
lead to competing co-conformations in which some of them can
be surprisingly stable.21 In particular, [2]rotaxanes18b-d,21,23 have
been found to exist in co-conformations in which the tetracat-
ionic cyclophane is positioned on the weakest π-electron donor.
In such examples it was observed that the CBPQT4+ cyclophane
interacts in an alongside manner with the stronger π-electron
donor, confirmed by analysis18c,24 of the UV-vis-NIR absorp-
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J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007 7355

A R T I C L E S
Nygaard et al.
In this paper we have chosen to employ a bulky macrocyclic
TTF derivative25 as a stepping stone to the construction of a
bismacrocyclic compound containing a weak π-electron-donor
HQ in the second expanded macrocycle. Subsequently, forma-
tion of a [2]catenane by a template-directed synthesis creates a
situation where the CBPQT4+ cyclophane is left no other choice
than to encircle the weaker but readily accessible π-electron
donor, i.e., the HQ unit. The linkers between the TTF unit and
the HQ moiety were chosen so that they will position the
CBPQT4+ cyclophane at an ideal distance to be engaged in
alongside π-π interactions with the bulky TTF unit.6c This
design also allows us to test and eventually refine the hypothesis
put forward in the past, namely, that distortion of the TTF unit
should actually prevent formation of the tetracationic cyclophane
around another π-electron-rich station in the macrocycle.
Construction of the [2]catenane such that the CBPQT4+ cyclo-
phane can only encircle one of the recognition units also helps
Figure 1. Molecular structure of the [2]catenane 1‚4PF6.
tion spectroscopy, whereas it is usually assumed that the
CBPQT4+ cyclophane will exclusively encircle the most π-elec-
tron-rich unit. These findings can be rationalized by taking
into account that the favored co-conformation is determined by
host-guest π-π stacking as well as by a wide array of weaker
[CH‚‚‚π], [CH‚‚‚O], and alongside CT interactions. These
noncovalent bonding interactions can all be accommodated when
there is sufficient flexibility in the interlocked molecule. In the
drive toward exercising greater control and providing a deeper
understanding of these weak noncovalent interactions,12 all
insights that can ultimately lead to the rational utilization of
the least studied alongside interaction are considered to be of
paramount importance for the development of increasingly
complex interlocked systems. In order to investigate and
understand the alongside interaction involving the CPBQT4+
cyclophane and a strong donor we constructed a [2]catenane
1‚4PF6 (Figure 1) composed by a TTF unit which, for steric
reasons, will not be encircled by the tetracationic cyclophane.
1
to simplify the assignment of both the H NMR spectroscopic
data and the UV-vis-NIR photophysical data. These assign-
ments serve as signatures that can be utilized in the analysis of
more complex interlocked molecules. Through the course of
this study the serendipitous crystallization25 of the cis and the
trans isomers of a protected macrocyclic TTF precursor 5
(Scheme 2) has also allowed for an unambiguous determination
1
of H and 13C NMR spectroscopic assignments of the cis and
trans isomers of the TTF-containing bismacrocyclic compound
6 to be carried out. This assignment, in combination with the
solid-state structure of the [2]catenane 1‚4PF6, subsequently
allowed us to determine the predominately cis-isomer nature
of the isolated target [2]catenane. Identification of a single co-
conformation of the catenated structure allows us to clearly
assign the solution-phase spectroscopic features in the UV-
vis-NIR spectra wherein a face-to-face alongside CT interaction
leads to strong band intensity while an edge-to-face alongside
[Sπ‚‚‚π] interaction26 leads to weak band intensity. Resonance
Raman spectroscopy has been used to provide an unambiguous
assignment of the related absorption band at ∼740 nm to the
TTF f CBPQT4+ CT transition arising from either of the two
alongside CT interactions. Taken as a whole, this study provides
access to a wide array of heretofore unobtainable catenated
structures and detailed spectroscopic assignments.
The design of viable synthetic strategies to prepare the desired
[2]catenane 1‚4PF6 demanded a reevaluation of previous studies
(Scheme 1) that focused on the methodology of how to construct
mono- and bismacrocycles from TTF units incorporating one
or two polyether crown-like moieties. It was believed6c (Scheme
1) that distortion of the TTF unit in the bismacrocycle A was
the reason why catenation of the tetracationic cyclophane
CBPQT4+ around the HQ unit at room temperature and ambient
pressure was prevented. However, in light of the success by
which ultrahigh pressure (10 kbar) has been used to facilitate
the self-assembly of interlocked structures containing the
tetracationic cyclophane CBPQT4+ we were encouraged to
reexamine if the desired [2]catenane 1‚4PF6 could in fact be
formed under these conditions despite the inherent distortion
of the TTF unit.
Results and Discussion
Synthesis. The bismacrocycle 6 was prepared (Scheme 2)
by following a convergent synthetic strategy in which the two
components, containing either the bulky TTF unit or HQ moiety,
were prepared in two separate pathways and then combined in
a final step using the pseudo-high-dilution approach. Synthesis
Scheme 1. Previously Attempted Catenation of a Bismacrocycle A Containing a Distorted TTF Unit
9
7356 J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007

Alongside Charge-Transfer Interactions
A R T I C L E S
Scheme 2. Synthesis of the HQ-Containing Thread 4 and the Bismacrocyclic Compound 6
Scheme 3. Synthesis of the [2]Catenane 1‚4PF6
(Scheme 2) of the HQ moiety 4, which turns out to be a very
useful building block for the construction of various interlocked
molecules, is not well described in the literature, and the
synthetic route was therefore optimized in this study. The
dialcohol27 2 was mesylated in quantitative yield (MsCl/Et3N/
CH2Cl2), and a subsequent Finkelstein reaction (NaI/Me2CO)
gave the desired 1,4-bis-{2-[2-(2-iodoethoxy)ethoxy]ethoxy}-
benzene (4) in good yield (85%).
We previously described25 the synthesis and full characteriza-
tion of the bulky TTF macrocycle 5, and this was used to form
(Scheme 2) the bismacrocycle 6 by deprotecting the cis/trans
mixture of the bulky TTF unit 5 in situ using 2.05 equiv of
CsOH, whereafter the resulting dithiolate and the HQ-containing
diiodo compound 4 were reacted under pseudo-high-dilution
conditions in DMF employing a perfusor pump to yield the
bismacrocycle 6 in an overall yield of 82%. The initially isolated
1
bismacrocycle 6 was found by H and 13C NMR spectroscopy
to exist as a 1:1 mixture of the cis and trans isomer that could
be converted into the predominant cis isomer (9:1) by treat-
ment28 with p-toluenesulfonic acid (PTSA) in Me2CO (see
Supporting Information).
The desired [2]catenane 1‚4PF6 was obtained (Scheme 3)
from the template-directed high-pressure29 reaction in DMF
where the bismacrocycle 6, as either the 1:1 or the 9:1 mixture
of the cis/trans isomers, provides the HQ unit as a template
around which the CBPQT4+ cyclophane forms from its dicat-
ionic precursor15a 7‚2PF6 and 1,4-bis(bromomethyl)benzene (8).
In both cases, the [2]catenane 1‚4PF6 was isolated as an
analytically pure dark green solid after column chromatography.
Starting from the 1:1 cis/trans-TTF isomeric mixture of 6 a 9%
yield of 1‚4PF6 was obtained, whereas 1‚4PF6 was isolated in
a 16% yield starting from the 9:1 cis/trans-TTF isomeric mixture
(25) Nygaard, S.; Flood, A. H.; Jeppesen, J. O.; Bond, A. D. Acta Crystallogr.,
Sect. C 2006, 62, 677-680.
of the bismacrocycle 6. The observation that the [2]catenane
1‚4PF6 can be isolated in acceptable yields, despite the distortion
of the TTF backbone, is in contrast to the previously published
assumption6c that twisting the TTF backbone totally prevents
formation of catenanes. It was initially quite surprising that in
both instances the [2]catenane was isolated as a green solid and
also produced a green solution upon dissolution. Given the fact
(26) For an edge-to-face TTF-to-xylene [S‚‚‚π] interaction of a model host-
guest complex, see: Ercolani, G.; Mencarelli, P. J. Org. Chem. 2003, 68,
6470-6473.
(27) Heilmayer, W.; Wallfisch, B.; Kappe, C. O.; Wentrup, C.; Gloe, K.; Kollenz,
G. Supramol. Chem. 2003, 15, 375-383.
(28) Souizi, A.; Robert, A. J. Org. Chem. 1987, 52, 1610-1611. (b) Liu, Y.;
Flood, A. H.; Moskowitz, R. M.; Stoddart, J. F. Chem. Eur. J. 2005, 11,
369-385.
(29) Kla¨rner, F.-G.; Wurche, F. J. Prakt. Chem. 2000, 7, 609-636.
9
J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007 7357

A R T I C L E S
Nygaard et al.
Figure 2. Full 1H NMR (200 MHz) spectrum of the initially isolated cis-
and trans-isomeric mixture of the bismacrocycle 6 recorded in CD3CN at
298 K. The bismacrocycle is contaminated with the starting diiodo
compound 4 as most clearly observed by the signals marked by an asterisk
(*).
that the tetracationic cyclophane is forced by the steric bulk of
the TTF unit to reside on the HQ unit, a characteristic red color
in both the solid state and solution is expected. The green color,
therefore, seems to indicate the presence of a moderately strong
alongside CT interaction between the TTF unit and one of the
bipyridinium moieties of the CBPQT4+ cyclophane.
Structural Characterization of the Catenane 1‚4PF6 by
Mass Spectrometry. The electrospray ionization mass spectrum
(ESI-MS) of the [2]catenane 1‚4PF6 shows a diagnostic peak
at m/z 809 corresponding to the doubly charged [M - 2PF6]2+
ion, which unambiguously confirms the presence of the
[2]catenane structure.
Figure 3. (a) Full 1H NMR spectrum of the [2]catenane 1‚4PF6 (67:33
cis/trans) obtained from the non-PTSA-treated 1:1 isomeric mixture of the
bismacrocycle 6 recorded in CD3CN (500 MHz) at 298 K. (b) Comparison
of the low-field region (9.50-5.00 ppm) of the 1H NMR spectra of the
[2]catenane 1‚4PF6 (67:33 cis/trans) (black solid trace) or the [2]catenane
1‚4PF6 (95:5 cis/trans) (blue solid trace).
Structural Characterization of the Bismacrocycle 6 and
the [2]Catenane 1‚4PF6 by 1H NMR Spectroscopy and X-ray
Crystallography. The 1H NMR spectroscopic investigation
(Figure 2) of the bismacrocycle 6 carried out in CD3CN (200
MHz, 298 K) revealed that the initially isolated compound
consisted of an approximately 1:1 mixture of the cis and trans
isomers. The most diagnostic evidence supporting this conclu-
sion is the presence of two singlets at δ ) 6.81 and 6.87 ppm
which originate from the resonances associated with the Ar-H
protons in the HQ moiety in the cis and trans isomers of the
bismacrocycle 6, respectively.
1‚4PF6 (Figure 3a), synthesized from the 1:1 cis/trans-isomeric
mixture of the bismacrocycle 6, unambiguously confirms the
presence of the CBPQT4+ cyclophane as indicated by two sets
of indicative resonances (Figure 3b). These signals correspond
to the bipyridinium HR (δ ) 8.96 ppm) and Hâ (δ ) 7.87 ppm)
protons and to the N+CH2 protons (δ ) 5.71 ppm, singlet) of
the cyclophane in the aromatic region of the spectrum.
Noteworthy is the appearance of two singlets in varying
intensities originating from the resonances associated with the
protons in the p-phenylene moieties (Hxy) (δ ) 7.79 and 7.77
ppm) of the tetracationic cyclophane CBPQT4+. The fact that
two singlets are observed for the p-phenylene moieties indi-
cates30 that the catenane exists as both the cis and the trans
isomer when synthesized from the 1:1 isomeric mixture of the
bismacrocycle 6. By integration of the area under the two
observed Hxy signals, the major isomer constitutes 67% of the
total population.
1
The aliphatic region of the H NMR spectrum consists of
series of multiplets arising from the resonances of the remaining
protons in the cis/trans-isomeric mixture. The multiplets can
be categorized into three classes, namely, resonances associated
with (i) the CH2 protons in the saturated decamethylenedithio
linker, (ii) the CH2 protons next to an O atom, and (iii) the
CH2 protons next to an S atom. Previous28 investigations have
shown that it is possible to alter the cis/trans-isomeric ratio in
TTF derivatives by treatment with PTSA in Me2CO. A similar
isomerization was found to take place in the bismacrocyclic
compound 6, and addition of small amounts of PTSA to a
solution (Me2CO) of the initially 1:1 cis/trans-isomeric mixture
of 6 changed the ratio to approximately 9:1 in favor of the cis
isomer (for a detailed NMR spectroscopic analysis and assign-
ment of the two TTF isomers, see Supporting Information,
Figures S2-S6).
On the other hand, when the [2]catenane 1‚4PF6 was
synthesized from the PTSA-treated and predominately (9:1) cis-
(30) The Hxy signal is normally observed as one singlet in the 1H NMR spectrum
recorded at room temperature on account of the rotational freedom of the
CBPQT4+ cyclophane which renders all the Hxy protons undistinguishable
by 1H NMR spectroscopy. The existence of two Hxy signals in 1‚4PF6 with
different relative intensities at almost identical positions, therefore, indicates
that the Hxy experiences two slightly different chemical environments which
only can be accounted for by the presence of both a cis- and a trans-TTF
isomer in the isolated [2]catenane 1‚4PF6.
Analysis of the 1H NMR spectrum (CD3CN, 500 MHz)
recorded at 298 K of the green solution of the catenated structure
9
7358 J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007

Alongside Charge-Transfer Interactions
A R T I C L E S
isomeric bismacrocycle 6, only one singlet (δ ) 7.79 ppm) was
observed for the protons of the p-phenylene moieties of the
CBPQT4+ cyclophane (Figure 3b). This observation and the
overall simplification of the entire spectrum (see Supporting
Information, Figure S7) indicate that the isolated [2]catenane
1‚4PF6, when synthesized from a predominately cis-isomeric
(9:1) macrocycle 6, is found mainly as a single cis isomer (95%)
retaining its original stereochemistry. The cis isomer was
confirmed from an X-ray crystal structure determination (vide
infra). This assignment is consistent with the following experi-
mental observations. First, the 1H NMR spectrum of 1‚4PF6 is
simpler, displaying fewer resonances, when the [2]catenane is
synthesized from the predominately cis-isomeric (9:1) bismac-
rocycle 6 as compared to starting from the 1:1 isomeric mixture
of the bismacrocycle 6. Second, the yield of the high-pressure
clipping reaction to form the [2]catenane 1‚4PF6 is observed to
almost double from 9% to 16% when the [2]catenane is
synthesized from the predominately cis-isomeric (9:1) macro-
cycle 6 as compared to the 1:1 cis/trans-isomeric mixture of
the bismacrocycle 6. These observations indicate that the
template-directed reaction shows isomeric selectivity31 and
predominately takes place around the HQ moiety in the cis
isomer of the bismacrocycle 6.32 Finally, this is one of a few
examples33 in the literature of a TTF-containing [2]catenane
system that is both translational isomer free and where the exact
stereochemistry of the isolated compound can be determined
unambiguously.
The encirclement of the HQ moiety by the CBPQT4+
cyclophane significantly affects the resonance of the aromatic
protons on the HQ moiety, and it is evident that these protons
have experienced a significant upfield shift as they are now
found (Figure 3a) to resonate as a singlet at δ ) 3.57 ppm in
both the cis and trans isomers of 1‚4PF6. This new position
corresponds to a 3.2 ppm upfield shift of the localization of the
HQ protons when compared to the bismacrocycle 6. No major
shifts are observed for the resonances of the SCH2 protons, but
these features are not clearly evident given the multiplet nature
and extensive overlap of signals in this region. Nevertheless,
these spectroscopic data are entirely consistent with the cyclo-
phane CBPQT4+, as expected, exclusively encircling the HQ
moiety in the [2]catenane 1‚4PF6, irrespective of its isomeric
setup.
Figure 4. X-ray crystal structure of the cis isomer of the [2]catenane
1‚4PF6 depicted as a capped stick representation. Hydrogens, solvent
molecules, and counterions are omitted for clarity.
cis configuration. It is noteworthy that the [2]catenane units are
arranged in such a way that the TTF unit from one [2]catenane
participates in a face-to-face interaction with a TTF unit in a
neighboring [2]catenane. By adopting such a face-to-face
conformation in the solid state, the TTF units from two different
cis-[2]catenanes can form favorable ring-over-bond overlap,
which is a common feature observed in many crystal structures
of TTF derivatives.35
Electrochemical Investigations. Electrochemical investiga-
tions were carried out in argon-purged MeCN solutions at room
temperature using cyclic voltammetry (CV) and differential
pulse voltammetry (DPV). It is well known36 that TTF can be
oxidized reversibly in two discrete steps to the radical cation
and the dication. For the bismacrocycle37 6 two reversible,
monoelectronic oxidation processes associated with the TTF unit
are observed in the CV at potentials less than +1.0 V, whereas
one reversible two-electron process associated with the oxidation
of the HQ moiety is seen (Table 1) at positive potentials greater
than +1.0 V. Comparing the electrochemical data of the
bismacrocycle 6 to those of the bulky macrocyclic TTF building
block38 5 reveals that that the presence of cyanoethylthio groups
on the TTF unit greatly affects the donor properties of the
molecule. As expected, one observes that the TTF unit 5 again
is being oxidized in two reversible discrete steps but at
significantly higher potential than in the bismacrocycle 6. This
difference is thought to be a result of the presence of two
cyanoethylthio groups in 5 which by an inductive process as
well as by an internal CT interaction in which the -CN groups
receive electrons from the TTF core withdrawing electrons from
the TTF unit, thereby decreasing the TTF core’s ability to
stabilize a positive charge. A better model of the TTF subunit
within the bismacrocycle is tetrakis(thiomethyl)TTF 9 (see
Slow diffusion of iPrOH into a CD3CN solution of the
[2]catenane 1‚4PF6, obtained from the clipping reaction of the
9:1 cis-isomeric bismacrocycle 6 gave orange needle-like
crystals suitable for X-ray diffraction analysis.34 The crystal
structure shown in Figure 4 reveals that (i) in the solid state of
1‚4PF6 the tetracationic cyclophane CBPQT4+ is positioned
around the HQ moiety in accordance with the 1H NMR
spectroscopic studies and (ii) the [2]catenane 1‚4PF6 adopts a
(35) (a) Moore, A. J.; Bryce, M. R.; Cooke, G.; Marshallsay, G. J.; Skabara, P.
J.; Batsanov, A. S.; Howard, J. A. K.; Daley, S. T. A. K. J. Chem. Soc.,
Perkins Trans. 1993, 1, 1403-1410. (b) Fujiwara, H.; Arai, E.; Kobayashi,
H. J. Mater. Chem. 1998, 8, 829-831. (c) Kimura, S.; Nii, H.; Kurai, H.;
Takeuchi, H.; Katsuhara, M.; Mori, T. Bull. Chem. Soc. Jpn. 2003, 76,
89-96.
(36) (a) Bryce, M. R. J. Mater. Chem. 2000, 10, 589-598. (b) Segura, J. L.;
Mart´ın, N. Angew. Chem., Int. Ed. 2001, 40, 1372-1409. (c) Schukat, G.;
Fangha¨nel, E. Sulfur Rep. 2003, 24, 1-190. (d) Becher, J.; Jeppesen, J.
O.; Nielsen, K. A. Synth. Met. 2003, 133-134, 309-315. (e) Otsubo, T.;
Takimiya, K. Bull. Chem. Soc. Jpn. 2004, 77, 43-58. (f) Jeppesen, J. O.;
Nielsen, M. B.; Becher, J. Chem. ReV. 2004, 104, 5115-5132. (g) Gorgues,
A.; Hudhomme, P.; Salle´, M. Chem. ReV. 2004, 104, 5151-5184.
(37) No difference in the electrochemical data collected was observed when
comparing the electrochemical data of the predominately cis isomer
(9:1) bismacrocycle 6 to the 1:1 isomeric cis/trans mixture of the bis-
macrocycle 6.
(31) For another example of isomeric selectivity in the formation of [2]catenanes,
refer to ref 6d.
(32) This assignment is further substantiated and confirmed by the relationship
between the observed UV-vis-NIR absorption spectroscopic data and the
cis/trans isomer ratio of the [2]catenane as confirmed by molecular
modeling.
(33) The exact stereochemistry of a TTF-containing “[2]pseudocatenane” has
been reported in refs 6c,d in which the TTF unit, and not the HQ unit, is
encircled by the CBPQT4+ cyclophane.
(34) Crystallographic data for 1‚4PF6‚(CD3CN)7 at 133(2) K with Mo KR
radiation (λ ) 0.71069 Å). 1‚4PF6: triclinic, space group P-1, a ) 13.571-
(8) Å, b ) 14.252(8) Å, c ) 27.108(15) Å, R ) 97.841(12)°, â ) 91.323-
(12)°, ø ) 106.338(12)°, Z ) 2, R1 ) 0.0887, wR2 (all data) ) 0.2497,
GOF ) 0.810.
(38) No observable difference in the electrochemical data was observed when
the experiments were carried out on either a 1:1 mixture of the cis and the
trans isomer or the purified TTF isomers.
9
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A R T I C L E S
Nygaard et al.
Table 1. Electrochemical Dataa for the TTF Macrocyclic
Compound 5, Bismacrocycle 6, Model Compound 9,b and
[2]Catenane 1‚4PF6
compound
bipyridiniumc Ered (V)
TTF Eox (V)
HQ Eox (V)
5
6
9
+0.65
+0.90
+0.81
d
+0.57
+0.55
+0.65
+1.35
+1.70
1‚4PF6
-0.28
+0.88
a Argon-purged MeCN, room temperature, and tetrabutylammmonium
hexafluorophosphate (TBAPF6) as supporting electrolyte. Glassy-carbon
working electrode. Potential values are stated in V vs SCE. b Compound 9
is a tetrakis(thiomethyl)-substituted TTF derivative (see Supporting Informa-
tion); data obtained from ref 20 (MeCN, adjusted for SCE from Ag/AgCl).
c The second reduction process corresponding to the complete reduction of
both bipyridinium moieties in the cyclophane, to yield the neutral compound,
was observed to be irreversible. d Not reported.
Supporting Information),20 which displays its first oxidation at
a position, +0.55 V, that is very similar to 6, consistent with
the equivalent substitution patterns.
Figure 5. UV-vis-NIR absorption spectrum of the bismacrocycle 6
recorded at room temperature in Me2CO (solid line), the [2]catenane
1‚4PF6 (67:33 cis/trans) in MeCN (dotted line), and the [2]catenane
1‚4PF6 (95:5 cis/trans) in MeCN at room temperature (dashed line).
It is clearly evident from the electrochemical data of the
[2]catenane39 1‚4PF6 that the tetracationic cyclophane does not
encircle the TTF unit, but as expected, it encircles the HQ
moiety. This gives rise to the very dominant (+350 mV) positive
shift of the two-electron process corresponding to oxidation of
the HQ moiety inside the cavity of CBPQT4+. At a lower
positive potential the two processes corresponding to the
oxidations of the TTF unit are observed. On account of the close
proximity of the tetracationic cyclophane, it is observed that
the two TTF-based oxidation processes in 1‚4PF6 are shifted to
higher potential by approximately +80 mV when compared to
the same processes in the bismacrocycle 6. The observed
positive shift of the TTF oxidations can be accounted for by
the repulsive and through-space electrostatic interactions when
the TTF2+/+ unit and the CBPQT4+ cyclophane are alongside40
each other. This interpretation is in good agreement with the
observed40 positive shifts previously assigned to similar along-
side interactions taking place in bistable [2]pseudorotaxanes.
At negative potentials the CV of 1‚4PF6 exhibits a fully
reversible reduction peak, corresponding to the first reduction
of both bipyridinium moieties of the cyclophane from the 2+
state to the 1+ state.
Photophysical Investigations and Molecular Modeling of
the Alongside Charge-Transfer Interaction. It is well
documented8a,18b-d,20,41 that inclusion of π-electron-rich guests
inside the cavity of the tetracationic cyclophane CBPQT4+ gives
rise to a wealth of [2]pseudorotaxanes in solution as evidenced
by the formation of colored solutions and the appearance of
characteristic CT bands in the absorption spectra. The location
of these CT bands is highly dependent on which guest is being
included in the cyclophane. The interaction between TTF
derivatives and CBPQT4+ is particularly strong, and a large
number of [2]pseudorotaxanes have been reported in the
literature20,41 for which it is possible to obtain X-ray crystal
structures of some of these superstructures.42 Notwithstanding
all of these instances of CT interactions, there have been few if
any UV-vis-NIR absorption studies that specifically43 probe
the alongside interaction between a TTF unit and the tetracat-
ionic cyclophane CBPQT4+ in either a [2]pseudorotaxane or a
[2]catenane. The chosen molecular design of the [2]catenane
1‚4PF6 is ideally suited for this purpose. In particular, spectro-
scopic markers for the situation where the maximum extent of
an alongside CT interaction between the TTF unit and the
CBPQT4+ cyclophane can be realized. Structurally, the flexible
PEG chains can accommodate a positioning of the TTF unit
approximately 3.5 Å from the cyclophane in order to maximize6c
the π-π overlap of the CT interaction.
The absorption spectrum recorded in Me2CO of the bismac-
rocyclic compound 6 reveals (Figure 5) an absorption band at
380 nm indicating the presence of a TTF unit. Formation of
the [2]catenane 1‚4PF6 leads to a green-colored solution
regardless of the isomeric distribution in the isolated [2]catenane.
It is evident from the solution-phase 1H NMR spectroscopy and
CV studies and from the solid-state structure that the tetracat-
ionic cyclophane CBPQT4+ encircles the HQ unit. Conse-
quently, the green color observed in solution must originate from
alongside interactions occurring between the TTF unit and one
of the bipyridinium units of the CBPQT4+ cyclophane. Inspec-
tion of the UV-vis-NIR absorption spectra recorded in MeCN
of the two different isomeric mixtures of 1‚4PF6 reveals the
presence of a CT band in the 740 nm region giving rise to the
green color. Surprisingly, it can be observed that there is a
marked difference in both the location and the intensity of the
CT band depending on the cis/trans-isomeric mixtures. The
spectrum recorded of the 67:33 cis/trans [2]catenane 1‚4PF6
shows a moderate intensity CT band (ꢀ ) 1475 M-1 s-1) at
(41) (a) Castro, R.; Nixon, K. R.; Evanseck, J. D.; Kaifer, A. E. J. Org. Chem.
1996, 61, 9591-9595. (b) Anelli, P.-L. et al. Chem. Eur. J. 1997, 3, 1113-
1135. (c) Ashton, P. R.; Balzani, V.; Becher, J.; Credi, A.; Fyfe, M. C. T.;
Mattersteig, G.; Menzer, S.; Nielsen, M. B.; Raymo, F. M.; Stoddart, J. F.;
Venturi, M.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 3951-3957.
(d) Bryce, M. R.; Cooke, G.; Duclairoir, F. M. A.; Rotello, V. M.
Tetrahedron Lett. 2001, 42, 1143-1145.
(42) (a) Philp, D.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D.
J. J. Chem. Soc., Chem. Commun. 1991, 1584-1586. (b) Asakawa, M.;
Ashton, P. R.; Balzani, V.; Credi, A.; Hamers, C.; Mattersteig, G.; Montalti,
M.; Shipway, A. N.; Spencer, N.; Stoddart, J. F.; Tolley, M. S.; Venturi,
M.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 1998, 37, 333-
337.
(39) No difference in the electrochemical data collected was observed when
the [2]catenane (95:5 cis/trans) prepared from the predominately cis isomer
(9:1) bismacrocycle 6 or the [2]catenane (67:33 cis/trans) prepared from a
1:1 isomeric mixture of the bismacrocycle 6 was used in individual
experiments.
(40) For a detailed analysis of the electrochemical profile of the alongside
interaction in a two-station [2]pseudorotaxane, please refer to refs 18c
and 24.
(43) The proposed existence of an alongside CT interaction between TTF
derivatives and CBPQT4+ has been reported in the literature. Please refer
to refs 18c and 24. In both cases, the alongside interaction was observed
as a weak shoulder in the UV-vis-NIR absorption spectra and arose
surreptitously.
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7360 J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007

Alongside Charge-Transfer Interactions
A R T I C L E S
740 nm, whereas the spectrum recorded of the 95:5 cis/trans
isomer shows a reduced intensity CT band (ꢀ ) 550 M-1 s-1
)
at 745 nm. In both cases, a less defined HQ f CBPQT4+ CT
band at around 475 nm is observed as a shoulder which is
consistent6c,f,g,16a with the HQ moiety being encircled by the
CBPQT4+ cyclophane.
It is possible to estimate the intensity of the vis-NIR CT band
associated with the wholly cis or the trans isomer based on the
known isomeric distribution in the [2]catenane in the two cases
(see Supporting Information). The all-cis isomer of 1‚4PF6
would show a weak CT band with an approximate intensity of
380 M-1 s-1, whereas the all-trans isomer of 1‚4PF6 would show
a strong intensity CT band (ꢀ ) 3690 M-1 s-1). From these
experiments it is evident that the configuration of the TTF unit
is extremely important for maximizing the alongside CT
interaction on account of the 10-fold increase in intensity of
the CT band going from the all-cis to the all-trans TTF isomer
of 1‚4PF6. However, based on the crystal structure, wherein the
face-to-face TTF-to-CBPQT4+ arrangement (i.e., with the TTF
oriented face-on to one bipyridinium unit of the CBPQT4+
cyclophane) appears optimally suited for a favorable overlap
between the TTF-based highest occupied molecular orbital
(HOMO) and one of the lowest occupied molecular orbitals
(LUMOs) on the CBPQT4+ cyclophane, this result is unex-
pected. Consequently, the difference in overlap between the
HOMO on the TTF and one of the LUMOs on the CBPQT4+
cyclophane is believed to arise out of the detailed orientation
of the two isomers that occur in solution. In support of this
interpretation, the crystal structure25 of the cis and trans isomers
of the macrocycle 5 display very different structures.
To get a better understanding of these observations, we
employed molecular modeling to investigate the orientational
effects. The conformational space of the trans- and cis-1‚4PF6
was investigated by running a 10 ns molecular dynamics
simulation using the MMFF force field.44 Apart from the
conformational flexibility of the decamethylenedithio linker,
both isomers displayed only one orientation of the TTF and
CBPQT4+ units during the whole simulation. The trans isomer
adopts a face-to-face arrangement between the TTF and the
CBPQT4+ cyclophane, while the cis isomer settles into an edge-
to-face alignment (see Supporting Information, Figure S9). For
each isomer we optimized 1000 structures selected from the 10
ns simulation and reoptimized the lowest energy structure at
the B3LYP/6-31G(d) level. These results confirmed that each
of the 1000 trans isomers is locked in a face-to-face geometry
by the constraints of the PEG linker, while the cis isomer has
substantially more conformational flexibility and prefers to adopt
an edge-to-face alignment in all 1000 cases. The latter is in
disagreement with the crystal structure, and in order to
investigate this discrepancy, we generated two additional starting
conformations for the cis isomer. One was prepared from the
face-to-face trans isomer but with the decamethylenedithio
linker reconnected into a cis-isomeric structure, and a second
started from the face-to-face crystal structure. Optimization of
both starting geometries at the B3LYP/6-31G(d) level relaxed
into the edge-to-face arrangement. We attribute this discrepancy
between the observed solid-state conformation and calculated
conformations of the cis isomer of the [2]catenane 14+ to the
Figure 6. TDHF/6-31G*-generated HOMO (a) and LUMO+1 (b) orbitals
for the trans isomer of the [2]catenane 1‚4PF6 which shows significant
mixing of the orbitals for the trans system, whereas the HOMO (c) and
LUMO+1 (d) orbitals for the cis isomer of the [2]catenane 1‚4PF6 shows
that the molecular orbitals are localized in this case.
presence of crystal-packing forces. In particular, the sum of the
favorable ring-over-bond overlap35 and the face-to-face along-
side CT interactions dominate over the edge-to-face CT inter-
action.
With the calculated cis- and trans-isomeric structures as a
starting point, we investigated the electronic features of the
alongside CT interaction. From TDHF/6-31G(d) calculations
performed on both [2]catenanes, the LUMO is actually located
on the bipyridinium furthest away, leaving the LUMO+1
available for the CT interaction For the cis isomer, a small
degree (16%) of mixing (defined as the overlap of the orbital
charge densities) is present45 between the HOMO localized on
the TTF unit and the LUMO+1 localized on the CBPQT4+
cyclophane when it adopts the edge-to-face conformation,
whereas significant mixing (34%) of the HOMO and LUMO+1
is observed for the trans isomer (Figure 6) in the face-to-face
conformation. From the excitation energy calculations the
oscillator strength of the predominately HOMO f LUMO+1
transition is roughly five times larger for the trans system than
the cis system, which is in agreement with the experimentally
observed UV-vis-NIR absorption spectroscopic behavior of
the trans isomer compared to the cis isomer of the [2]rotaxane
1‚4PF6. The calculated CT band position is predicted to be at
somewhat shorter wavelength for the trans isomer (707 nm)
compared to the cis isomer (774 nm), which again matches the
trend for the observed difference in location of the CT bands
found in solution at 740 and 745 nm for the (67:23 cis/trans)
1‚4PF6 and the (95:5 cis/trans) 1‚4PF6, respectively. From these
calculations, we infer that the trans-1‚4PF6 catenane has a stable
face-to-face conformation that produces the intense alongside
(44) Jensen, F. Introduction to Computational Chemistry, 2nd edition; VCH-
(45) Lee, E. C.; Hong, H.; Lee, J. Y.; Kim, J. C.; Kim, D.; Kim, Y.; Tarakeshwar,
Wiley: Chichester, 2006.
P.; Kim, K. S. J. Am. Chem. Soc. 2005, 127, 4530-4537.
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A R T I C L E S
Nygaard et al.
the bismacrocycle 6 and the tetracationic cyclophane CBPQT4+
,
the spectra of both isomeric ratios of the [2]catenane 1‚4PF6
display strong resonantly enhanced spectra. For example, the
CBPQT4+-based marker band at ∼1640 cm-1 is enhanced 6-fold
for the 95:5 cis/trans [2]catenane 1‚4PF6, whereas a remarkable
30-fold enhancement is seen for the 67:33 cis/trans [2]catenane
1‚4PF6 when utilizing the MeCN 919 cm-1 band as an internal
standard (see Supporting Information, Table S2). All of the
remaining bands are resonantly enhanced to a similar degree.
One band that can definitively be assigned to the TTF unit in
the 95:5 cis/trans sample of the [2]catenane 1‚4PF6 is the
CdC interring stretch at 486 cm-1, displaying a significantly
large enhancement compared to the spectrum of the bismacro-
cycle 6. This band is also seen to be enhanced strongly in the
67:33 cis/trans [2]catenane 1‚4PF6. The greater degree of inten-
sity enhancement of the entire spectrum in the latter isomeric
ratio corresponds to the larger molecular absorptivity ob-
served in the UV-vis-NIR absorption spectrum for the 67:33
cis/trans sample of the [2]catenane 1‚4PF6. In order to verify
the chromophore-specific spectrum for the TTF f CBPQT4+
CT transition, the same spectra for a model TTF unit 9 and the
associated [2]pseudorotaxane 9⊂CBPQT4+ have been recorded
(see Supporting Information, Scheme S1 and Figure S10). All
of the important features are retained, and a ∼5-fold enhance-
ment of the CBPQT4+ marker band and the νCdC TTF interring
band are observed for the [2]pseudorotaxane. While the overall
degree of enhancement appears to follow the difference in
extinction coefficient of each sample, one new band is observed
at 1585 cm-1 (arrow in Figure 7a) in the 67:33 cis/trans isomer.
This band is not seen in the model TTF system 9, which does
not have any isomers, therefore implying that the C2h mi-
crosymmetry of the trans isomer allows for one additional
Raman-active vibrational mode to gain intensity compared with
the C2V cis isomer. Consistent with the CT interaction and the
mixing of the CBPQT4+ LUMO+1 into the occupied HOMO,
the ∼1647 cm-1 marker band of CBPQT4+ has shifted down
in energy by 7 cm-1. This shift is attributed to the CT interaction
with the TTF unit and not with the HQ moiety on account of
the fact that the CT band intensity of the former far exceeds
the latter and that the marker band does not shift position when
the CBPQT4+ cyclophane is threaded with the HQ moiety 2
(see Supporting Information, Figure S11). Surprisingly, the TTF-
based band at 486 cm-1 does not shift due to the CT interaction.
It is important to note that the impact of the alongside CT
interaction is observed most acutely in the photophysical
properties (UV-vis-NIR and Raman spectroscopy) compared
Figure 7. Raman spectroscopic data of a MeCN solution (4.5 mM) of the
(a) 67:33 cis/trans [2]catenane 1‚4PF6, (b) 95:5 cis/trans [2]catenane
1‚4PF6, (c) CBPQT4+, and (d) 1:1 cis/trans isomer mixture of the
bismacrocycle 6. MeCN features as marked by an asterisk (*).
CT absorption band. By contrast, the cis form of the catenane
derives its weakened CT intensity from poorer HOMO f
LUMO+1 overlap of the edge-to-face conformation. This
arrangement determines that one of the sulfur atoms in the TTF
unit lies above the center of a pyridinium ring at a distance of
3.67 Å and the other sulfur is situated above the interring carbon
of the adjacent pyridinium ring at a distance of 3.72 Å. This
relative orientation in the cis isomer corresponds to a [Sπ‚‚‚π]
CT interaction arising from the small degree of π-orbital overlap.
The contribution of the [Sπ‚‚‚π] CT interaction to host-guest
complexation has been described computationally26 for the TTF-
to-xylene orientation in a model complex with CBPQT4+
.
Finally, we believe that this [Sπ‚‚‚π] CT interaction does not
contribute significantly to the total sum of the noncovalent
bonding interactionssit has weak CT band intensity and its
influence on structure can be overturned in the solid state.
Structural Characterization of the Model Compounds and
the Catenane 1‚4PF6 by Resonance Raman Spectroscopy.
In order to verify that the band at ∼740 nm is a TTF f
CBPQT4+ transition arising from an alongside interaction, the
resonance Raman spectra of the two isomer distributions of the
[2]catenane 1‚4PF6 were recorded and compared to the Raman
spectra of model compounds 6 and CBPQT4+. When the laser
excitation wavelength (λexc) is selected such that it comes into
resonance with a molecular chromophore two features of the
resulting resonance Raman spectra allow for the nature of the
electronic transition to be assigned.46 First, the spectrum’s signal
intensity becomes resonantly enhanced. Second, the specific
vibrational bands that gain intensity by the resonance effect are
chromophore specific and therefore, directly correspond to the
donor and acceptor molecular orbitals. The 740 nm band is
assigned to a HOMO(TTF) f LUMO+1(CBPQT4+) electronic
transition. Laser excitation within the absorption profile (λexc
) 785 nm) is therefore expected to selectively enhance47 (Figure
7) diagnostic bands attributed to the TTF unit48 and the
CBPQT4+ cyclophane,49 as observed.
1
with either H NMR spectroscopy or electrochemistry. This
brings out an important comment that when alongside TTF f
CBPQT4+ CT interactions are playing a role in influencing
distributions of co-conformations, one would expect to detect
this band by the presence of the CT chromophore in the UV-
vis-NIR spectrum and/or through the chromophore-specific
resonance Raman spectrum.
Conclusion
Compared to the solution-phase Raman spectra (MeCN,
Figure 7) of the individual components of the [2]catenane, i.e.,
We have demonstrated that it is possible to synthesize a
[2]catenane 1‚4PF6 wherein the cyclophane component
(46) Hirakawa, A. Y.; Tsuboi, M. Science 1975, 188, 359-361.
(47) Flood, A. H.; Wong, E. W.; Stoddart, J. F. Chem. Phys. 2006, 324, 280-
290.
(48) Joy, V. T.; Srinivasan, T. K. K. Chem. Phys. Lett. 2000, 328, 221-226.
(49) (a) Lee, P. C.; Schmidt, K.; Gordon, S.; Meisel, D. Chem. Phys. Lett. 1981,
80, 242-249. (b) Hennessy, B.; Megelski, S.; Marcolli, C.; Shklover, V.;
Ba¨rlocher, C.; Calzaferri, G. J. Phys. Chem. B. 1999, 103, 3340-3351.
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Alongside Charge-Transfer Interactions
A R T I C L E S
CBPQT4+ is forced to reside on the weakest π-electron donor
by employing a bulky and distorted TTF unit that does not fit
inside the cavity of the CBPQT4+ cyclophane. Furthermore, we
demonstrated that it is possible to optimize the yield by utilizing
the cis/trans stereoselectivity of the high-pressure clipping
reaction leading to the [2]catenane. Bearing in mind that the
tetracationic cyclophane is found exclusively to encircle the HQ
moiety, it was quite surprising to find for both samples of the
[2]catenane 1‚4PF6 that the alongside CT band at ∼740 nm
dominates over the CT band originating from the HQ f
CBPQT4+ transition at 475 nm to such an extent that a green
color is observed in solution instead of the expected red solution.
Furthermore, the position and intensity of the alongside CT band
assigned to the TTF f CBPQT4+ transition is strongly
dependent on the cis/trans configuration of the TTF unit.
Molecular dynamics and electronic structure calculations al-
lowed us to distinguish between a single low-energy face-to-
face conformation of the trans isomer and an edge-to-face
arrangement for the cis isomer, which defines a weak [Sπ‚‚‚π]
CT interaction. The trans isomer, with a high degree of HOMO-
(TTF)-to-LUMO+1(CBPQT4+) overlap, contrasts well with the
smaller overlap for the cis isomer. Employing the incisive
concept of resonance Raman spectroscopy allowed us to
unambiguously assign the 740 nm band to the TTF f CBPQT4+
transition of the alongside CT interaction on account of the
selective enhancement of diagnostic bands of the TTF unit and
the CBPQT4+ cyclophane. In conclusion, our findings indicate
that a weaker CT interaction based on a [Sπ‚‚‚π] edge-to-face
arrangement can be identified in instances when the more
common face-to-face orientation is structurally eliminated.
Acknowledgment. This work was funded in part by a Ph.D.
Scholarship from the University of Southern Denmark to S.N.,
the Danish Natural Science Research Council (SNF, project #21-
03-0317), the Danish Strategic Research Council in Denmark
through the Young Researchers Programme (#2117-05-0115),
and the Danish Center for Scientific Computing.
Supporting Information Available: Synthetic procedure of
all compounds as well as relevant NMR spectroscopic data,
X-ray crystal data details, UV-vis-NIR absorption spectra,
modeling details, and Raman spectroscopic data. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA069047W
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