Photoswitchable rotaxanes using the photolysis of alkoxyacridanes

Article abstract of DOI:10.1039/b815848g

9-Aryl-9-alkoxy-acridanes and their counterparts, 9-aryl-acridinium ions, have been incorporated into the axles both of one- and two-station [2]rotaxanes. The ring component of the rotaxanes consists of the tetracationic ring cyclobis(paraquat-4,4′-bisphe

Full text of DOI:10.1039/b815848g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Photoswitchable rotaxanes using the photolysis of alkoxyacridanes†
Werner Abraham,* Andre Wlosnewski, Karin Buck and Sabine Jacob
Received 10th September 2008, Accepted 2nd October 2008
First published as an Advance Article on the web 10th November 2008
DOI: 10.1039/b815848g
9-Aryl-9-alkoxy-acridanes and their counterparts, 9-aryl-acridinium ions, have been incorporated into
the axles both of one- and two-station [2]rotaxanes. The ring component of the rotaxanes consists of
the tetracationic ring cyclobis(paraquat-4,4¢-bisphenylene). The electron-rich acridanes represent
suitable recognition sites for the electron-poor ring because charge transfer interaction plays an
important role. The 9-aryl groups at the acridane unit bearing substituents such as the alkoxy and the
amino groups influence the strength of the recognition site. Photoexcited acridanes bearing a suitable
leaving group such as the methoxy substituent in the 9-position undergo heterolysis, resulting in the
formation of the acridinium methoxides. The acridanes are regenerated by the nucleophilic attack of the
methoxide ion at the acridinium ion formed. The lifetime of the ionic state is strongly dependent on the
solvent composition. Because the positively charged acridinium ions repel the positively charged ring
component of the interlocked molecules, a movement of the ring is initiated provided the molecular
axle contains an evasive recognition site. Two-station rotaxanes presented here possess as the second
station an anisol unit. Both the photoreaction and the thermal back-reaction render the
thermodynamic driving force of the interaction of the ring with one of the two recognition stations.
Accordingly, movement of the ring forward and back, driven by Brownian motion, occurs. The
switching cycle can also be triggered by acid–base titration. The photoexcitation of the acridane unit
present in one-station rotaxanes leads to a very unfavourable acridinium recognition station. However,
because of the absence of a second station, the ring remains at the unfavourable acridinium station
having interaction with 9-aryl group.
tion is related to the type of the noncovalent interaction. Pho-
Introduction
=
=
toinduced E/Z-isomerisation (N N or C C double bonds) and
photoinduced electron transfer are often used.6 The hydrophobic
interactions in rotaxanes based on cyclodextrins (CDs) can be
disturbed by simple isomerisation of stilbene- or azobenzene-units.
Due to steric interference of the Z-isomers with CDs, an alternative
recognition station can interact with the CD.7 In comparison with
the steric interference, the isomerisation of fumaramide plays a
more active role. The hydrogen bonding between a tetramide-
based macrocycle and the formed maleamide station is drastically
reduced because the intramolecular hydrogen bonds within the
maleamide predominate. The reset can be accomplished either
photochemically or thermally.8 Recently, the photoisomerisation
of a spiropyrane was shown to switch the movement of a
macrocycle due to the strong hydrogen-bond acceptor property
of the opened merocyanine isomer.9
Rotaxanes are mechanically interlocked molecules composed of
a threadlike molecule (axle) incorporating recognition stations
and a ring. Bistable rotaxanes, having at least two recognition
stations, are molecular shuttles in which the ring position may be
altered. Energy for the shuttling process stems from the Brownian
motion.1 Provided that the two stations have sufficiently different
interaction strength with the ring, the ring will reside preferentially
on one of the two recognition stations. If an outer stimulus
weakens the strong interaction of the station on which the ring
resides, the ring will move to the station with the originally weaker
interaction. Thus, a complete rearrangement of the mechanically
bonded components will occur, e.g. the co-conformation,2 and the
shuttle is referred to as a switch.1
Solvents,3 chemicals,4 electrons5 or light can all serve as stimuli
for the switching process, the advantage of the light-driven systems
being that no chemical fuels are necessary and no waste products
are formed.
The way in which a photoreaction may control the interaction
between the ring and the photoactive recognition station is an
important consideration. The usefulness of a type of photoreac-
Photoinduced electron transfer is most suitable for controlling
the strength of charge-transfer interaction. The bipyridinium unit
either in the threadlike component or in the ring of rotaxanes
introduced by the Stoddart group10 is the most often-used strong
electron acceptor. However, direct excitation at the charge-transfer
absorption band results in formation of an intimate radical ion pair
which undergoes rapid charge recombination that is faster than
the ring movement.11 A detailed study of time resolved absorption
spectra of the rotaxane in the presence of an external electron
donor as a dual-action redox reagent has shown the function of an
autonomous mechanism which allows about 10% ring movement
in competition with back electron transfer. Even in the absence of
the external donor, about 4% ring movement could be estimated.12
Institute of Chemistry, Humboldt-Universitaet zu Berlin, Brook-Taylor-Str.
2, D-12489, Berlin, Germany. E-mail: abraham@chemie.hu-berlin.de
† Electronic supplementary information (ESI) available: Additional exper-
imental information, UV-vis and NMR spectra of selected compounds and
transients, absorption maxima of rotaxanes and molecular axles observed
in different solvents, assignment of protons of rotaxanes, fluorescence
spectrum of rotaxane 29. See DOI: 10.1039/b815848g
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It is also worth noting that a hydrogen-bonded rotaxane can be
switched by photoinduced electron transfer.13 In each case, the
lifetime of the charge separated state of the rotaxanes is very short.
We have introduced an alternative switching principle for rotax-
anes based on charge-transfer interaction of the tetracationic ring
cyclobis(paraquat-4,4¢-bisphenylene) with a photoactive electron
donor. The switch is characterized by the generation of cations by
photoheterolysis of alkoxyarylcycloheptatrienes which transform
an electron donor (arylcycloheptatriene) into an electron acceptor
(aryltropylium ion).14 The lifetime of the rotaxanes in their tropy-
lium state is in the range of seconds. However, side reactions such
as the hydrogen shift disturbed the photoheterolysis.15 In order to
improve the functionality of the photoswitchable rotaxanes, we
have recently presented a preliminary communication which de-
scribed the introduction of another photoswitch based upon pho-
toheterolysis of 9-alkoxy-9-aryl-N-methylacridanes which form
acridinium ions with methoxide counterions. The photoactive
alkoxyphenyl acridanes bearing the methoxide leaving group in the
9-position are used as recognition station in rotaxanes such as 1.16
The [2]rotaxane 1 (Scheme 1) based on the acridane photoswitch
has two different recognition sites, namely the 9-alkoxy-9-phenyl-
acridane (A), which is also a stopper unit, and the photoinactive
alkoxyphenyl station (B).
and 9-aminophenyl-9-methoxy-9,10-dihydroacridines (acridanes)
and their acridinium counterparts. By using the aminophenyl
group as station A, a further improvement of the selectivity may
result due to the increased donor capability of the aryl group at
9-position of the acridane moiety. We compare the efficiency of the
photoreaction of the acridane rotaxanes and their free molecular
axles, and we discuss the influence of the tetracationic cyclophane
present in rotaxanes on the reactivity and photophysical properties
of the acridinium and acridane stations.
Results and discussion
The photoswitches 9-aryl-9-methoxy-N-methyl-9,10-
dihydroacridines (9-aryl-9-methoxyacridanes)
Until now the photochemistry of compounds 2 and 4 (Scheme 2)
has not been studied.
Scheme 1 Rotaxane 1.
Surprisingly, the interaction of the tetracationic ring with the
alkoxyphenyl unit A within the acridane moiety is favored over
the alkoxyphenyl station B. This finding was attributed to the
additional edge-to-face interaction between the ring and the
acridane unit.16 The only observed photoreaction of the acridane is
photoheterolysis. Consequently, these photoswitchable rotaxanes
are an alternative for the direct electron transfer used by the groups
of Balzani and Stoddart.12 9-Arylacridinium are Janus molecules
because the acridinium part is an electron acceptor while the aryl
group in the 9-position is an electron donor. Due to the torsional
angle between both subunits they are decoupled from each other.
Therefore the question arises as to whether the 9-aryl acridinium
station is able to act as recognition station for the tetracationic
cyclophane competing with the photoinactive station present in
the molecular axle of bistable rotaxanes. In particular, this problem
may become relevant if the electron donor strength of the 9-aryl
group is increased, as in the 9-aminophenyl group. One-station
rotaxanes containing only the 9-arylacridinium station should
be suitable for studying the interaction between the subunits of
the acridinium station and the tetracationic cyclophane with the
help of NMR-spectroscopy. In this paper we report on newly
designed one- and two-station rotaxanes incorporating 9-alkoxy-
Scheme 2 Two types of photoswitches used in rotaxanes.
The UV-vis spectra of 2 and 4 recorded after consecutive
irradiations in acetonitrile solution (Fig. 1 of the ESI†) exhibit
three isosbestic points indicating a single step photoreaction
resulting in the formation of the corresponding acridinium salts
3 and 5, respectively. The complete recovery of the acridanes by
nucleophilic attack of methanol is only observed in acetonitrile
solution containing at least 5% methanol. The formed methoxide
supports the reaction by abstracting the proton (see Scheme 3).
Therefore, the thermal reaction is pseudo first order.
The analysis of the exponential decay of the absorption of
the intermediate acridinium ions at 360 nm yields the lifetime
(1/k) of the ionic state. It depends strongly on the composition
of the solvent. For example, the lifetime in acetonitrile solution
containing 20% methanol is 990 s for 3 and 900 s for 5. The
lifetime of 5 is shortened to 25 s in methanol solution (see ESI).
Compounds 2 and 4 represent photochromic systems with large
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Fig. 1 UV-Vis spectra of the molecular thread 15 (—) and the rotaxane 29 (– · –) in acetonitrile solution.
Scheme 3 Thermal back-reaction of acridinium methoxides.
differences between the UV-vis absorption maxima of the involved
compounds (see Scheme 2).
Syntheses of molecular axles
There are two possibilities for connecting the acridine unit
with the various threadlike ethylene glycol chains. (i) In the
case of the 9-aryl-N-methyl-acridinium compounds 7 and 9,
N-methylacridinone was reacted with lithium-organic com-
pounds. The primarily formed acridanes are converted to the
acridinium compounds during the purification procedure with
solvents containing NH4PF6 (Scheme 4). (ii) The aniline com-
pounds are able to react directly with N-methylacridinium iodide17
to yield the acridinium salts 15–18 (Scheme 5). The corresponding
acridane compounds are easily available by reacting the acridinium
salts with methanol in the presence of a base (Scheme 6).
Scheme 4 Synthesis of molecular axles 7 and 9.
Rotaxanes
clearly indicate the formation of pseudorotaxanes. Threading of
the molecular axles through the ring is accompanied by the typical
down-field shift of the proton signal of the benzyl spacer and
the upfield shift of the b-protons (relative to nitrogen) of the
bipyridinium moiety. In contrast, molecular axles incorporating
the acridinium unit such as 9 and 16 do not form pseudorotaxanes
with 26.
Pseudorotaxanes. In order to prepare rotaxanes, the acridane
axles has to thread through the cyclophane 26. The acridane
moiety cannot pass through the cyclophane opening, and therefore
only the alcohol site is able to thread. The formation of pseudoro-
taxanes has only exemplarily been proven with compounds 19, 22
and 25 to explore the influence of the 9-aryl substituent of the
acridane moiety and the presence of the second station within the
axle. The association constants as determined by means of NMR-
titration in CD3CN solution are independent of these parameters
(K19/26 = 550, K22/26 = 570, K25/26 = 580 l mol-1). 1H NMR spectra
Rotaxane synthesis. Rotaxanes were prepared by the acylative
capping method. The alcohol site of the acridane axles threading
through the cyclophane was reacted with adamantoyl chloride
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to give the acridane rotaxanes which were converted into
the corresponding acridinium rotaxanes by the purification
procedure (Schemes 7–9).
The compound 20 differs from the previously reported molecu-
lar axle of rotaxane 1 by flipping the photoinactive station within
the two-station molecular thread. Therefore, in the course of the
rotaxane preparation, the electron donor strength is diminished
by esterification of the phenolic OH-group (see Scheme 8). We
questioned whether the weaker electron donor capacity of the
photoinactive station B has any influence on the distribution
between the two stations.
Despite the comparable association constants obtained for
one- and two-station pseudorotaxanes the yield of the one-
station rotaxanes 27, 29 and 31 (10, 5 and 12%, respectively) is
considerably lower than that of the two-station rotaxanes 33 (26%),
1 (28%) and 37 (22%). Obviously the second recognition station
supports the rotaxane formation. In most cases the acylated free
axles (28, 30, 32, 34 and 38) were formed as the main products.
The acylation of 21 takes place only at the more nucleophilic
amino group yielding compound 30. In contrast, only the rotaxane
29 with the free amino group was formed, indicating that the
ring resides on this group during the synthesis, thus protecting
the amino group. In contrast to the synthesis of the one-station
rotaxane 29, the amino group of the axle 23 was acylated during
the synthesis of the two station rotaxane 36. Obviously both the
station A and the station B are populated by the cyclophane 26
in pseudorotaxanes formed as precursors during the rotaxane
synthesis. The rotaxanes 35 and 36 were formed in a 1:1 ratio,
but due to their very similar properties, the separation of both
compounds was not successful. In order to avoid the acylation
of the amino group the axle 24 containing the methylated amino
group has been used to prepare the rotaxane 37 (Scheme 9).
In order to prepare the photoactive acridane rotaxanes, the
corresponding acridinium rotaxanes were treated with a small
volume of methanol in acetonitrile solution in the presence of weak
bases such as NaHCO3 or di-i-propylethylamine (Scheme 10).
During the reaction, the solution turns colourless because both
the acridane component and the ring absorb only UV-light.
It is worthy to note that the acridane rotaxanes are also
formed without a base simply upon dissolution of the acridinium
rotaxanes in methanol (2–5 ¥ 10-5 M).
Scheme 5 Synthesis of molecular axles incorporating 4-aminophenyl
acridinium moieties.
Whereas 29 and 31 are quantitatively transformed into the
acridanes 40 and 41, 18% of 27 remains in equilibrium with
39. In the case of the two-station rotaxane 37, the interaction
energy between the cyclophane and the acridane station causes
the formation of the acridane upon dissolution in methanol
containing 20% acetonitrile (eqn 1).
37+MeOH ꢀ 43+HPF
(1)
6
52%
The pseudo acid – pseudo base equilibrium is ~1:1 in diluted
solution (2–5 ¥ 10-5 M). The presence of the second station gives
rise to only 50% turnover while the one-station rotaxane 31 reacts
quantitatively to produce the acridane rotaxane (see above). In
contrast, the molecular axle 24 reacts as a pseudo base to give
60% acridinium methoxide in acetonitrile/methanol 1:1 solution
(eqn 2).
Scheme 6 Axle molecules incorporating the acridane moiety and cyclo-
phane 26.
24+MeOH ꢀ 18 MeO-
(2)
60%
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Scheme 7 Synthesis of one-station rotaxanes.
Scheme 8 Synthesis of the two-station rotaxane 33.
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Scheme 9 Synthesis of two-station rotaxanes.
Scheme 10 Synthesis of acridane rotaxanes.
The free molecular axle 21 incorporating the acridane sta-
tion is completely transformed to the acridinium compound in
dilute solution (methanol/acetonitrile 1:1). This emphasises the
higher stability of the acridinium compound 15 with respect
to 21. The acridane rotaxanes do not react with methanol.
The transformation of the acridinium rotaxanes to the acridane
rotaxanes is obviously driven by the energy won by the stronger
interaction of the acridane moiety with 26 compared with the
interaction with the acridinium unit. Accordingly, acridinium
compounds 9, 15 and 16 do not react with methanol to give
the acridane. Thus, the acridane unit within the rotaxane and
the acridane unit present in the corresponding molecular axle
exhibit opposite behaviour in their reactivity with the nucleophile
methanol.
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Rotaxane properties
taining the aminophenyl substituent at the 9-position (rotaxanes
35–37).
Co-conformation of rotaxanes.
Rotaxanes 35 and 36 could not be separated. Nevertheless, the
complete assignment of proton resonances to the two rotaxanes
was possible with the help of two dimensional NMR spectra such
as COSY and ROESY. Because only the proton resonances of
station B are exclusively upfield shifted by the same amount (see
ESI), this station is populated by the ring. Due to the voluminous
adamantoyl group at the acridane moiety the population of the
aniline subunit is not possible.
Also the proton resonances of station B within the rotaxane
37 appear at the same values as seen for 35 and 36. NOEs taken
from ROESY spectra prove the population of station B by the
cyclophane within the rotaxane 37. In conclusion, the aryl groups
of the acridinium moieties are not capable of competing with the
rather weak electron-donating second station even in the case of
the ester group at this station in the rotaxane 33.
Acridinium rotaxanes. Rotaxanes with only one recognition
station should exist in only a single co-conformation with the
ring residing on this station. Therefore, the strongest possible
interaction should exist between the cyclophane and the station
since there would be no disturbances due to shuttling between two
or more stations. The one-station rotaxanes 27, 29 and 31 consist
of two parts: the acridinium station A and the flexible polyether
chain. We expected that this chain would be a relatively high
barrier against the ring location. The question arises as to whether
the strongly twisted 9-aryl substituent18 of the acridinium unit
aryl group is able to accommodate the tetracationic ring despite
the presence of the positively charged acridinium subunit. The
chemical induced shift (CIS) of specific proton signals both of
the cyclophane and of the axle manifested in the 1H-NMR spectra
indicated the strength of the interaction between specific regions of
the axle with the ring compound. The resonances of both the ortho-
protons (with respect to the substituent in the 4-position) of the
aryl group attached to the acridinium moiety and of the adjacent
methylene group of the chain are up-field shifted by more than
3 ppm with respect to those arising from the free axle compound
(see ESI). Other resonances of the chain protons are not shifted.
In contrast, the aryl protons adjacent to the acridinium subunit
are upfield-shifted by less than 1 ppm. These spectral shifts clearly
demonstrate that the positively charged ring 26 resides on the aryl
group at the 9-position of the acridinium moiety. Further evidence
comes from correlation-peaks arising from protons in the ring
component and from both the ortho-protons of the aryl group and
the adjacent protons of the chain observed in ROESY-spectra. The
proton resonance of the N-methyl group present in rotaxane 31 is
also upfield shifted by 2 ppm. Taken together, the ring resides on
the aryl group of the acridinium station and the adjacent chain as
far away as possible from the positively charged acridinium moiety.
The N-methyl-group does not disturb the interaction of the aryl
group with the ring. In conclusion, while pseudorotaxanes of the
acridinium axles with 26 dissociate, the stopper units in rotaxanes
formed from acridane rotaxanes prevent dissociation into the two
components. Therefore the cyclophane has to arrange with in the
unfavorable situation.
The proton resonances of the ring 26 are well resolved in the 1H
NMR spectra both of the one- and two-station rotaxanes. While
the b-protons (with respect to N+) of the ring are upfield shifted,
the proton resonances of the aromatic groups of the xylylene
spacers are low-field shifted. Due to the asymmetry of the axle,
the proton resonances of the methylene groups of 26 are split into
two doublets of an AB-system.
Acridane rotaxanes. Because the acridane station is more
suitable to accommodate the tetracationic ring than the acridinium
station,16 the ring does not change its position by transforming the
acridinium rotaxanes to the one-station acridane rotaxanes 39–41.
For the acridane rotaxanes, the upfield shifts of the aryl protons
are comparable with those of the same group of the acridinium
rotaxanes. Because there is no competition between the different
stations, the observed upfield shift should approach the maximum
value that can ever be achieved due to the interaction between the
acridane units and the ring component. Therefore, the observed
proton resonances of the protons of the aryl group at the 9-
position of the acridane unit can be used as a probe to monitor the
localisation of the ring in two-station rotaxanes. The population
of the aryl group of station A by 26 is also indicated by NOEs
(according to correlation peaks between the aromatic protons of
the benzylic spacer of 26 and the aromatic protons of the aryl
group of station A in the ROESY spectra).
The situation is completely changed for the two-station ro-
taxanes. The distribution between the acridinium station and
the second station having weak electron donor capacity depends
on the interaction of the tetracationic cyclophane 26 with the
two stations. According to the CIS-values observed for the two
stations in all two-station acridinium rotaxanes the cyclophane
resides exclusively on the station B. The proton resonances of the
acridinium unit are not shifted with respect to those of the free
axles. In contrast, the interactions between the ring and the station
B are accompanied by significant upfield shifts of the aromatic
proton resonances and the adjacent protons of the polyether chain
caused by strong diamagnetic shielding due to the p-stacking
of the bipyridinium ring and station B aromatic systems. The
assignments of proton signals were made by using standard
COSY and NOE (ROESY) data (see ESI). Cross-peaks of the
ROESY spectrum reveal the interaction between the cyclophane
and the station B (see ESI). The exclusive occupation of the
station B holds true also for those acridinium rotaxanes con-
While the cyclophane 26 of acridinium rotaxanes 27, 29 and
31 does not interact with the acridinium part (see above), all
proton resonances of the acridane core of the rotaxanes 39–41 are
low-field shifted (see ESI), indicating an edge-to-face interaction
between the aromatic rings of 26 and the plane of the acridane
subunit enabled by the shape of this part of the molecular axle (see
Scheme 11). Obviously, with respect to the acridinium counterpart,
this additional interaction is the driving force of the occupation
of the acridane station as an entity while also accounting for
the formation of the acridane rotaxanes from the corresponding
acridinium rotaxanes in methanol solution (see above). The proton
resonance of the N-methyl group present in rotaxane 41 is upfield
shifted by 1.1 ppm. This finding indicates that the N-methyl
group does not disturb the interaction of the aryl group with
the cyclophane.
In two-station acridane rotaxanes, the acridane station A
competes with the station B. Any distribution between the two
stations by shuttling of the cyclophane 26 should result in a
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ROESY spectrum of the rotaxane 43 recorded at 233 K reveals
the occupation of station A because correlation peaks between
the cyclophane proton signals (b-protons) and the protons of the
acridane ring (protons in 1- and 8-positions) are present.
Dynamic processes in acridane rotaxanes. Apart from the
aromatic protons of the benzylic groups, cyclophane signals
are very broad for all acridane rotaxanes in both CD3CN and
CD3OD/CD3CN (5:1) solutions at room temperature. The b-
protons (with respect to N+) of the bipyridinium moiety are even
merged into the base line.
Broadening of the ring signals indicates dynamic processes. By
lowering the temperature to 233 K and 213 K, respectively, the
broad signals of the a- and b-protons of the ring (with respect to
N+) observed for the rotaxanes 40 and 43 are split into eight signals
of two protons each. The signals of the benzylic protons exhibit the
opposite behavior. At room temperature, a AB system is observed,
indicating that the two sites of the ring are non-equivalent due to
the asymmetry of the molecular axle. At 213 K, this signal is
broadened (see ESI), indicating that the dynamic process is slow
on the NMR time scale. Two different rotational processes may be
responsible for these observed phenomena: 1) the rotation of the
ring around the axle, which is frozen at low temperature, making
the ring protons above and below the axle non-equivalent; and
2) the internal rotation of the bipyridinium units is rapid at room
temperature and slow at 213 K with respect to the NMR time scale
(see Scheme 12).
Scheme 11 Calculated structures (MM2-level) of one-station rotaxanes
represent the changes in the shape of the molecular axle.
lower CIS-value of the acridane station compared with the value
1
observed in one-station rotaxanes. The H NMR spectra clearly
show that the acridane station A is exclusively populated by the
cyclophane 26. The upfield shifts of the aromatic protons of the
aryl group at the 9-position of the acridane moiety of rotaxane 42
are exactly as that observed with the one-station rotaxane 39 (see
above and the ESI). The aromatic proton resonances of the second
station B are not shielded. Surprisingly, the proton resonances of
the adjacent propyl chain are moderately upfield shifted (see ESI).
Possibly a back-folding of the whole chain occurs, bringing the
CH2 groups in contact with the ring from outside.
The acridane rotaxane 43 exhibits very broad signals corre-
sponding to the proton resonances both of the ring and of the
molecular axle at room temperature. Only the proton signals of
the chain adjacent to station B are sharp and assignable with
the help of two dimensional methods such as HH- and CH-
COSY. The population of station A can be concluded from the
proton resonances of station B. The signals of the propoxy chain
appear at the same values as observed for compound 24, and this
finding is an appropriate probe for the failed threading of this part
of the axle.16 At moderately increased temperature (343 K), the
proton resonances of the phenyl group of station A appear at d
= 4.4 and 2.5 ppm indicating the occupation of station A by the
ring. The assignment of these signals was done with the help of
two-dimensional C-H-COSY spectra at 343 K. Again the observed
CIS-values match those observed with the one-station rotaxane 41.
The proton resonances of the phenyl group of station B are shifted
slightly up-field by 0.5 and 0.3 ppm. This shift might be caused by
an interaction of station B with the outside ring by back-folding
of the molecular axle. CIS-values of other proton resonances are
given in the ESI. The proton resonances of the aryl group attached
to the acridane moiety that accommodates the ring also appear at
low temperature as two broad sets of signals. This indicates that
the asymmetry of the two sites of the 9-aryl group due to both
the slow rotary motion of the ring and the 9-aryl substituent. The
Scheme 12 Depiction of two rotational phenomena in 40 that influence
spectral data.
With respect to rotaxane 42 upon lowering the temperature, the
a- and b-proton signals (with respect to N+) of the ring appear as
four broad singlets, indicating that a thermally activated motion
of the bipyridinium units is frozen (see ESI). The same behaviour
was observed with the rotaxane 1. The a-proton resonances are
split by 0.6 ppm and the b-proton resonances by 1 ppm. If the
ring resides on the 9-aryl-9-methoxy-acridane station, one side
of the bipyridinium units is involved in edge-to-face interaction
with the dihydroacridine plane. The other side of the bipyridinium
plane is directed toward the part of the axle with the second
station B. The protons of the methylene groups of the ring are
also topologically non-equivalent and appear as two doublets of
an AB-system in rotaxanes 1 and 42 even at room temperature.
Because only the a- and b-protons act as a probe for the dynamic
process, we may exclude the possibility that the entire ring rotates
around station A. Therefore it seems likely that the rotation of
the bipyridinium unit around itself exchanges the protons of the
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two sides, thereby enabling the alternating interaction with the
acridane plane. Accordingly, ROESY spectra at 233 K exhibit
exchange cross-peaks between the a- and b-proton signals in
addition to the NOE cross peaks (see ESI).
Provided that the acridinium ions carry electron-donating
substituents at the 9-position, the locally excited state occupied
by light absorption is accompanied by a very fast electron transfer
to a charge shift state which quenches the fluorescence.19 Due to
the interaction of 26 with the electron-rich phenyl group of the
acridinium unit, the electron-donating strength of this subunit is
reduced. Accordingly, in contrast with the molecular axles 9, the
rotaxane 27 exhibits fluorescence characteristic of the acridinium
subunit (see ESI).
Photochemistry
UV-Vis-spectroscopy. Acridinium salts of the types 9 and 15 or
16 exhibit different features. Both 9 and 15,16 show two absorption
bands at ~360 and ~420 nm typical for the acridinium chro-
mophore and localised in this part of the molecule. Compounds
15 and 16 have an additional long-wavelength transition >500 nm
that is assigned as an internal charge-transfer transition from the
aminophenyl substituent to the acridinium moiety (see Fig. 1).18
This charge-transfer transition exhibits negative solvatochromism.
The encapsulation of the aryl group of the acridinium unit by the
ring gives rise to a number of interesting spectroscopic properties.
While the longest wavelength band of the rotaxane 27 is not
different from that of the molecular axle 9, the longest wavelength
absorption bands of rotaxanes 29 and 31 are blue shifted with
respect to the corresponding compounds 15 and 16 (Fig. 1). The
different characters of the electronic transitions of 27 on one side
and 29 and 31 on the other side give rise to this different behavior.
The longest wavelength absorption bands of rotaxanes 29 and 31
monitored in acetonitrile solution are blue-shifted by 1.2 ¥ 103 and
1.7 ¥ 103 cm-1, respectively, with respect to the absorption bands of
the corresponding molecular axles. The most likely explanation of
these findings is the reduced electron-donating strength of the aryl
groups due to the interaction with the electron acceptor 26 and the
polar microenvironment provided by the tetracationic cyclophane.
The internal charge-transfer transition is more sensitive to the
solvent polarity than the localised transition of the rotaxane 27.
In summary, the longest wavelength absorption band of the
rotaxanes 29 and 31 may serve as a probe for the localisation
of the ring. Accordingly, the absorption spectrum of the two-
station rotaxane 37 corresponds to that of axle 18 and reveals the
population of station B.
In 9-arylacridanes, the phenyl group is electronically decoupled
from the acridane core. For this reason, the absorption spectra of
all acridane derivatives are nearly identical, exhibiting absorption
bands at around 320 nm (as shoulder) and 280 nm (Fig. 2).
Photoreaction and thermal back-reaction. Compounds 19–24
in acetonitrile solution undergo bond fission upon excitation with
313 nm light to form the corresponding acridinium methoxides.
The recovery of the acridanes is very slow (lifetimes >1 week) and
incomplete in acetonitrile solution so that UV-Vis spectra can be
recorded after consecutive irradiations (see Fig. 2 for an example).
The quantum yield of the photoreaction of compound 21 in
acetonitrile solution is 0.4. Using a mixture of acetonitrile and
methanol, the nucleophilic attack of the methoxide ion on the
formed acridinium ion is relatively fast. The lifetime of the ionic
state is 50 s observed in acetonitrile/methanol solution (5:1).
We monitored the transient absorption spectrum by subtracting
the absorptions spectrum recorded before irradiation from that
recorded immediately after irradiation with light of a wavelength
>300 nm. The spectrum consists of two parts: The negative
absorption corresponds to the decomposition of the acridane
while the positive absorption clearly matches that of the acridinium
ion 15 (compare Figs. 2 and 3).
The one-station rotaxanes 39–41 also undergo photoheterolysis
upon excitation with light between 254 and 330 nm in acetoni-
trile solution containing methanol. Due to the charge-transfer
interaction16 between the acridane station and the ring component,
the efficiency of the photoreaction is decreased. Furthermore, the
Fig. 2 UV-vis spectra recorded after consecutive irradiation periods (0.5, 2, 4, 8, 16 min) at 313 nm of compound 21 (8.4 ¥ 10-5 M) in acetonitrile
solution (the acridinium ion 15 is formed).
150 | Org. Biomol. Chem., 2009, 7, 142–154
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rotaxanes 29 and 31, which have hexafluorophospate anions. After
the photochemical conversion of the acridane moiety into the
acridinium methoxide, the ring is in a unfavorable high-energy
position. According to the co-conformation found for one-station
acridinium rotaxanes, the cyclophane has only to move from a
position close to the acridinium core to a position at the end
of the 9-aryl substituent. Since this is also an energy-rich co-
conformation the driving force for changing the co-conformation
should be small. If we assume a slow movement of the ring,
the absorption spectrum of the transient acridinium methoxide
rotaxane would be recorded with a non-relaxed intermediate state.
Here, the electronic transition may be of lower energy compared
with the same transition within the relaxed acridinium rotaxane
with hexafluorophosphate counter ions. Acidifying the solution of
the rotaxanes 29 and 31 with trifluoroacetic acid (TFA) restored
the blue-shifted acridinium absorption band.
The lifetime of the intermediate acridinium methoxides ob-
tained from the rotaxanes 39–41 depends on the content of
methanol in the solvent mixture MeCN/MeOH. Some values are
given in Table 2 of the ESI.
Fig. 3 Transient absorption spectrum recorded immediately after the
irradiation of compound 21 (1 ¥ 10-4 M) for 5 s in acetonitrile/methanol
(5:1) solution.
The two-station rotaxanes have low-energy co-conformation
both in the acridinium and in the acridane state. Accordingly, the
transient absorption spectra recorded after the photoexcitation
both of the acridane rotaxanes 42 and 43 match that of the axle
24 (see Fig. 5 for 43).
absorption spectrum of the rotaxanes is superimposable upon the
absorbance spectrum of the cyclophane and the acridane station.
Because both subunits absorb light in the same wavelength
range, the ring acting as an inner light filter. From the comparison
of the absorbance of the transient absorption at 360 nm after
photoexcitation of the molecular axle 21 and the rotaxane 40,
respectively, a quantum yield of about 0.03 can be estimated for
40.
Transient absorption spectra observed upon photoexcitation of
rotaxanes 40 and 41 are similar and exhibit two characteristic
features (see Fig. 4 for an example).
Fig. 5 Transient absorption spectra recorded upon photolysis of molecu-
lar axle 24 and two-station acridane rotaxane 43 (in each case 3 ¥ 10-5 M)
in methanol/acetonitrile solution (1:1).
The lifetime of the acridinium rotaxane 33 (as methoxide)
is 110 s in methanol solution; it is increased to 1000 s in
MeCN/MeOH 5:1. The efficiency of the photoreaction is only
10% as good as that of axle 34.
The quantum yield of the photoheterolysis of the rotaxane 43 is
reduced to about 0.02. The lifetimes of the intermediate acridinium
rotaxanes formed from 43 are 60 s in methanol solution and 220 s
in a mixture of methanol and acetonitrile (1:1). The same lifetime
of the transient acridinium ion was observed for compound 24.
Because the NMR spectra of both acridinium rotaxanes with
hexafluorophosphate anions have shown that the ring resides on
station B, it is beyond doubt that the ring will move from station
Fig. 4 Transient absorption spectra observed upon photoexcitation of
the rotaxane 40 (3 ¥ 10-5 M) in acetonitrile/methanol (5:1) solution.
The typical spectrum of an acridinium salt with the two
characteristic transitions around 260, 360, 410 and 550 nm besides
the depletion in the region of the acridane unit absorption (270 nm)
was observed. The long wavelength absorption bands are red-
shifted by 30 nm with respect to the absorption band of the stable
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A to B upon photolysis of the acridane rotaxanes. In the case of
the rotaxane 43, support for this idea comes from the transient
UV-vis spectra. The longest wavelength absorption band matches
that of the molecular axle 24 (see Fig. 5). A blue-shift of the
longest-wavelength absorption band would be expected if the ring
resided on station A (in the acridinium state) as it was found for
the one-station rotaxane 31 (see above).
The preparation and isolation of the acridane rotaxanes was
deemed unnecessary. They can be prepared in situ by the reaction
of the corresponding acridinium rotaxanes in diluted solution
with a base such as N-ethyldiisopropylamine or NaHCO3. By
acidifying with trifluoroacetic acid the acridinium rotaxanes are
regenerated. This procedure can be repeated at least ten times. The
transformation can be easily followed by UV-vis spectroscopy. This
means that the switch cycle can also be carried out by acid–base
titration (see Scheme 13).
at both the ring and the acridinium moiety, the ring resides on
the 9-aryl-acridinium station of the one-station rotaxanes. The
occupation of the 9-aryl group by the ring results in changed
chemical and photophysical properties of the acridinium subunit
with respect to the reference compounds without the ring 26.
Shuttling processes between a 9-aryl-9-methoxy acridane recog-
nition station and an alternate station can be initiated both
photochemically by photoheterolysis followed by the thermal
back-reaction and by acid/base titration.
Experimental
General methods
Commercially available chemicals and solvents (UVASOL,
Merck) were used as received unless otherwise noted; solvents were
dried according to standard procedures. Column chromatography
(CC) was carried out on 200 mesh silica gel (Merck). Melting
points (m.p.) were determined with a Boetius heating microscope.
ESI mass spectroscopy was carried out on LTQ FT, Finnigan
MAT, Bremen, Germany) equipped with an electrospray ion
source (Thermo Electron).
NMR spectra were recorded on a Bruker DPX 300 (300 MHz)
and a Bruker Advance 400 (400 MHz). The proton signals
were attributed to the different subunits with the aid of two-
dimensional NMR spectroscopy, such as C-H-COSY, H-H-COSY,
and ROESY.
UV/Vis measurements were performed with a Shimadzu UV
2101 PC spectrometer.
Absorption spectra of the solutions were recorded before and
after repeated irradiations in order to detect irreversible degrada-
tion. Transient absorption spectra between 220 and 650 nm were
recorded with the UV 2101 PC spectrometer using the fastest scan
mode.
Steady state photolysis experiments were carried out with
acetonitrile solutions using a 500 W or 200 W high pressure
mercury lamp operated in conjunction with a glass filter (cut off
300 nm) or a metal interference filter (Carl Zeiss Jena) (313 nm).
The solutions were irradiated and measured in 1 cm quartz
cuvettes. The rate constant of the formation of acridinium ion
15 from 21 was calculated by the tangent method from plots of
conversion vs. the time of irradiation. The concentration of 15 was
obtained by measuring the optical absorption of 15 at 538 nm
(e
=
8140 dm3 mol-1 cm-1). 7-(4-dimethylaminophenyl)-
cyclohepta-1,3,5-triene was used as a secondary actinometer
(Ur313nm = 0.45, EtOH, 296 K)21 in order to determine the intensity
of incoming light.
Scheme 13 Switching cycle of the rotaxane 43 by light and by acid–base
titration.
The thermal reactions were followed by UV-Vis-spectroscopy
using the kinetics-program of the spectrometer. The decay curves
were analysed by nonlinear regression fits (Origin 6.0, Microcal
Software, Inc.).
Binding studies were carried out by means of 1H NMR titrations
of solutions of 26 usually at a concentration of 1 mmol. Increasing
amounts of 19, 22 or 25 were added up to ring: axle of 0.2:1–10:1
ratio (R) (20–80% saturation). Upfield shifts of the resonances of
different protons of the ring were monitored in order to calculate
binding constants K and the upfield shift of the protons of the
Conclusions
One- and two-station rotaxanes incorporating the 9-aryl-
acridinium station and the corresponding 9-methoxy-acridane
station, respectively, have been synthesised. Two station rotaxanes
contain an alternate anisol or phenol ester recognition station.
Due to charge-transfer and edge-to-face interactions, the acridane
interacts more strongly with the tetracationic cyclophane 26 than
the alternate station. The situation is reversed in the acridinium
state of the two-station rotaxanes. Despite the positive charges
ring fully saturated by the axle dGC
.
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Titration data points d = f(R) were fitted to the equation
addition of methanol (0.1 mL). The suspension was stirred for
several hours until decolouration of the solution. NaHCO3 was
filtered off. The solution was evaporated. The remaining oily solid
was extracted with chloroform (3 ¥ 5 mL). The remaining solid
was used without further purification.
1
22
d = dG - Dd 2 b - b2 - 4R
b =1+ R +
with
and
(
)
(
)
K H
[
]
0
H
G
[
[
]
]
where H = host, G = guest, and Dd = dG - dGC
.
R =
Rotaxane 40. Prepared according to the general procedure
from rotaxane 29 (0.030 g), yield 0.26 g, 93%), dark blue solid,
m.p. 170–176 ◦C; lmax(MeCN)/nm 261.5 (e/dm3 M-1 cm-1 62600)
and 313 (sh) (12400). dH (400 MHz, CD3OD/CD3CN 5:1, TMS)
1.7 (6 H, br m, adamantane), 1.8 (6 H, br s, adamantane), 1.9
(5 H, br m, adamantane, phenyl), 2.1 (2 H, br m, NCH2), 3.6
(4 H, m, ethoxy), 3.8 (7 H, m, NMe, ethoxy), 4.0 (2 H, m,
CH2O(CO)), 4.47 (2 H, d, J 8.8, phenyl), 5.77, 5.80, 5.81, 5.86
(8 H, m, ring 26), 7.52 (2 H, d, J 8.3, acridane), 7.60–7.73 (22 H,
m, acridane, ring 26), 8.85 (8 H, br s, ring 26); m/z (ESI) 710.2653
Best-fit parameters K and dGC were obtained in a nonlinear least-
square fitting procedure using the curve fitting program “Sigma
Plot” (Jandel Scientific Software).
Syntheses
Full experimental data are given in the ESI†. Typical procedures
are listed below.
The model compounds 2 and 3, the rotaxane 1 and the axle 25
were available from previous studies.16,20
[M - 2 PF6 ]2+. C74H78F12N6O5P2 requires 710.2659, 425.1889
-
Rotaxane 29. A solution of 21 (ESI) (0.28 g, 0.61 mmol)
dissolved in MeCN (4 mL), 26 (0.80 g, 0.73 mmol) and 2,6-
di-t-butyl-4-methylpyridine (0.13 g, 0.62 mmol) was stirred for
30 min under an argon atmosphere. An acetonitrile solution
(1.5 mL) of adamantane-1-carbonyl chloride (0.12 g, 0.61 mmol)
was then added. The resulting suspension was stirred at room
temperature for 3 days. A white precipitate was removed and
the filtrate was evaporated. The resulting oil was extracted with
dichloromethane in order to dissolve compounds 21 and 30. The
residue was treated with 5 mL of a solution of NH4PF6 in MeCN
(400 mL), ethyl acetate (200 mL) and cyclohexane (100 mL). Free
26 was filtered off. The solution was chromatographed (NH4PF6
in MeCN/ethylacetate/cyclohexane 2:1:0.5). The red fraction was
collected and the solvents were removed. The resulting red solid
was washed with water and dried in vacuum to give 29 (0,055 g,
[M - 3 PF6 ]3+. C74H78F6N6O5P requires 425.1886.
-
Rotaxane 37. An acetonitrile solution (4 mL) of 24 (ESI)
(0.233 g, 0.39 mmol), 26 (0.425 g, 0.39 mmol) and 2,6-di-t-butyl-4-
methylpyridine (0.080 g, 0.39 mmol) was stirred for 1 h under an ar-
gon atmosphere. An acetonitrile solution (0.7 mL) of adamantane-
1-carbonyl chloride (0.084 g, 0.42 mmol) was added. The solution
was stirred at room temperature for 4 days. A white precipitate was
removed and the filtrate was evaporated. The residue was extracted
with dichloromethane. The remaining violet solid was treated with
1 mL of a solution of NH4PF6 (7 g) in MeCN (400 mL), ethyl
acetate (200 mL) and cyclohexane (100 mL). Free 26 (0.189 g)
was filtered off. The solution was chromatographed (NH4PF6
(7 g) in 400 mL MeCN/ethylacetate/cyclohexane 2:1:0.5). The
violet fractions were collected and the solvents were removed. The
resulting solid was washed with water and dried in vacuum to
give 37 (0,173 g, 22%), violet solid, m.p. 218◦C; lmax (MeCN)/nm
259 (e/mol-1 dm3 cm-1122000), 359 (18000), 412 (3700), 553 nm
(9500); dH (400 MHz, CD3CN, TMS) 1.66 (2 H, m, CH2), 1.8
(6 H, m, adamantane), 2.0 (8 H, br m, adamantane, CH2), 2.1
(3 H, br s, adamantane), 2.92 (2 H, d, J 8.2, Ph), 3.0 (2 H, br s,
ethyleneoxy), 3.1 (3 H, s, NCH3), 3.7 (2 H, br s, NCH2), 3.75
(2 H, m, br, ethyleneoxy), 3.9 (2 H, m, ethyleneoxy), 3.94, 3.96
(4 H, m, ethyleneoxy), 4.12 (2 H, t, J 6.8, CH2), 4.52 (2 H, d,
J 8.2, Ar), 4.77 (3 H, s, N+Me), 5.74 (8 H, m, ring 26, benzyl-
H), 6.99 (2 H, br d, J 8.0, Ar), 7.37 (2 H, br d, J 8.5, Ar), 7.8
(10 H, m, ring 26, acridinium, H-2,7), 7.9 (8 H, br s, ring 26),
8.17 (d, J(H,H) = 8.8 Hz, 2 H; acridinium, H-1,8), 8.32 (m, 2 H;
acridinium, H-3,6), 8.52 (d, J(H,H) = 9.2 Hz, 2 H; acridinium, H-
4,5), 8.88 (d, J(H,H) = 5.3 Hz, 8 H; ring 26); m/z (ESI) 841.2818
◦
5%), m.p. 205–208 C, lmax (MeCN)/nm 258 (e/dm3 mol-1 cm-1
85100), 360 (13900), 405 (2500), 507 nm (8500); dH (400 MHz,
CD3CN, TMS) 0.48 (2 H, br s, N-CH2), 1.71 (6 H, m, adamantane),
1.76 (6 H, br s, adamantane), 1.9 (3 H, br s, adamantane), 2.70
(2 H, d, J 8.8, Ar), 3.46 (2 H, br t, J 4.3, OCH2), 3.70–3.76 (4 H,
m, ethyleneoxy), 3.85 (2 H, br s, ethyleneoxy), 4.08 (2 H, br t, J
4.7, OCH2), 4.87 (3 H, s, NMe), 5.74, 5.78, 5,80, 5.84 (8 H, m, ring
26, benzyl-H), 6.32 (2 H, d, J 8.2, Ar), 7.65 (2 H, dd, J 1 and 7.8,
acridinium, H-1,8), 7.75 (8 H, s, ring 26), 8.1 (br s, ring 26), 8.26
(2 H, m, acridinium, H-2,7), 8.52 (2 H, m, acridinium, H-3,6), 8.68
(2 H, d, J 9.3, acridinium, H-4,5), 8.97 (8 H; d, J 7.0, ring 26); m/z
(ESI) 767.2378 [M - 2PF6 ]2+. C73H75F18N6O4P3 requires 767.2382,
-
-
463.1706 [M - 3PF6 ]3+. C73H75F12N6O4P2 requires 463.1706.
The dichloromethane solution was evaporated to give the
mixture of compounds 21 and 30. The separation and purification
was performed by column chromatography (1 g NH4PF6 in 440 mL
acetone/cyclohexane 10:1). Besides 21 (0.070 g) 30 (0.10 g, 23%)
was obtained as oily compound; dH (400 MHz, CDCl3, TMS) 1.6
(6 H, m, adamantane), 1.8 (6 H, br s, adamantane), 1.9 (3 H; br s,
adamantane), 3.5 (2 H, m, OCH2), 3.6 (6 H, m, ethoxy), 3.7 (2 H,
m, ethoxy), 3.8 (2 H, m, N-CH2), 4.83 (3 H, s, NMe), 7.54 (2 H, d, J
8.3, Ar), 7.70 (2 H, d, J 8.3, Ar), 7.85 (2 H, m, acridinium, H-2,7),
8.02 (2 H, dd, J 1.1 and 7.6 Hz acridinium, H-1,8), 8.39 (2 H, m,
acridinium, H-3,6), 8.61 (2 H, d, J 9.2, acridinium, H-4,5); m/z
-
[M - 2 PF6 ]2+. C83H87F18N6O5P3 requires 841.2826, 512.5331
-
[M - 3PF6 ]3+. C83H87F12N6O5P2requires 512.5335, 348.1588 [M -
-
4PF6 ]4+. C83H87F6N6O5P requires 348.1589.
The dichloromethane solution was evaporated to give the com-
pound 38. The separation and purification was performed with the
help of column chromatography [MeOH/H2O/NH4Cl (saturated
aqueous solution) 200:60:1]. The blue fractions were collected,
the solvents were evaporated, and the remaining blue solid
was dissolved with a mixture of H2O (30 mL)/dichloromethane
(30 mL)/NH4PF6 (0.2 g). The organic phase was dried. The solvent
was evaporated to give compound 38 (0.044 g, 16%) as blue viscous
oil; dH (400 MHz, CD3CN, TMS) 1.7 (6 H, m, adamantane), 1.8
(8 H, m, adamantane, CH2), 1.9 (3 H, br s, adamantane), 2.50
(ESI) 579.3219 [M - PF6 ]+. C37H43N2O4 requires 579.3217.
-
Acridane rotaxanes.
General procedure. NaHCO3 (0.2 g) was added to the acri-
dinium rotaxane (0.02 mmol) dissolved in acetonitrile (4mL) under
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(2 H, t, J 7.4, CH2), 3.1 (3 H, s, NCH3), 3.6 (4 H, m, ethyleneoxy),
3.7 (2 H, m, ethyleneoxy), 3.72 (4 H, m, ethyleneoxy), 3.88 (2 H,
t, J 6.3, O(CO)CH2), 3.99 (2 H, m, ethyleneoxy), 4.71 (3 H, s,
N+Me), 6.73 (2 H; d, J 8.3, Ph), 6.98 (2 H, d, J 8.5 Hz, Ph),
7.02 (2 H, br d, J 8.8, Ar), 7.36 (2 H, d, J 8.7, Ar), 7.8 (2 H, m,
acridinium, H-2,7), 8.23 (2 H, d, J 8.7, acridinium, H-1,8), 8.29
(2 H, m, acridinium, H-3,6), 8.52 (2 H, d, J 9.2 Hz, acridinium,
H-4,5), 8.50 (2 H, d, J 9.2, acridinium, H-4,5); m/z (ESI) 727.4105
References
1 J. F. Stoddart, Acc. Chem. Res., 2001, 34, 410; C. A. Schalley, K. Beizai
and F. Vo¨gtle, Acc. Chem. Res., 2001, 34, 465–476; V. Balzani, M.
Venturi, and A. Credi, Molecular Devices and Machines, Wiley-VCH,
Weinheim 2003; E. R. Kay, D. A. Leigh and F. Zerbetto, Angew. Chem.
Int. Ed. Engl., 2007, 46, 72.
2 M. C. T. Fyfe, P. T. Glink, S. Menzer, J. F. Stoddart, A. J. P. White and
D. J. Williams, Angew. Chem. Int. Ed. Engl., 1997, 36, 2068.
3 W. Clegg, C. Gimenez-Saiz, D. A. Leigh, A. Murphy, A. M. Z. Slawin
and S. J. Teat, J. Am. Chem. Soc., 1999, 121, 4124.
[M - PF6 ]+. C47H55N2O5 requires 727.4099.
-
4 R. A. Bissel, E. Cordova, A. E. Kaifer and J. F. Stoddart, Nature, 1994,
369, 133.
Rotaxane 43. N-Ethyldiisopropylamine (0.27 g, 2.1 mmol)
was added to an acetonitrile solution (5 mL) containing methanol
(0.5 mL) of the rotaxane 37 (0.037 g, 0.019 mmol). The solution
was stirred for 1 h. After evaporating the solvents the remaining
solid was washed with dry chloroform to give 43 (0.030 g,
86%) as dark solid, m.p. 188–192 ◦C (dec.); dH (400 MHz,
CD3OD/CD3CN 5:1) 1.7 (6 H, br m, adamantane), 1.9 (8 H;
br m, adamantane, CH2), 2.0 (3 H, br m, adamantane), 2.1 (3 H,
br s, NCH3), 2.4 (2 H, br t, NCH2), 2.5 (2 H; br s, appears at
343 K, aryl), 2.54 (2 H; t, J 7.5, CH2), 3.3 (3 H, s, OCH3) 3.4
(2 H, br t, OCH2), 3.7 (5 H, m, ethyleneoxy, NCH3), 3.9 (4 H,
m, ethyleneoxy), 4.0 (2 H, t, J 6.3, (C(O)OCH2), 4.1 (2 H, m,
ethyleneoxy), 4.4 (2 H; br s, aryl), 5.68, 5.71, 5.78, 5.81 (8 H, m,
ring 26), 6.3 (2 H, br s phenyl), 6.8 (2 H, br s phenyl), 7.51 (2 H,
d, J 8.3, acridane, H-4,5), 7.64 (2 H, d, J 7.0, acridane, H-1,8),
7.7–8.1 (20 H, m, acridane, ring 26), 8.8 (6 H, br s, ring 26), 9.09
(2 H, br d, ring 26). dH (400 MHz, CD3OD/CD3CN 5:1, 233K)
1.7 (6 H, br m, adamantane), 1.9 (8 H, br m, adamantane, CH2),
2.0 (3 H, br m, adamantane), 2.2 (2 H, br d, aryl), 2.5 (2 H, br s,
NCH2), 2.54 (2 H, br t, CH2), 3.2 (3 H, s, OCH3), 3.5 (2 H br s,
ethyleneoxy), 3.7 (5 H, br s, ethyleneoxy, NCH3), 3.8 (4 H, m,
ethyleneoxy), 4.0 (t, br, (C(O)OCH2), 4.1 (m, 2 H; ethyleneoxy),
4.24 (2 H, br d, 2 H, aryl), 5.7 (8 H, br m, ring 26), 6.09 (2 H, d, J
8.3, phenyl), 6.82 (2 H, d, J 8.3, phenyl), 7.1 (2 H br s, ring 26), 7.2
(2 H, br s, ring 26), 7.4 (2 H, br s, acridane, H-4,5), 7.51 (2 H; d,
J 8.3, acridane, H-4,5), 7.7 (2 H br m, acridane, H-2,7), 7.7 (2 H,
br s, acridane, H-1,8), 7.3, (2 H, br s, ring 26), 7.81, (2 H, br s, ring
26), 7.78 (4 H, br s, ring 26), 8.2 (2 H, br s, ring 26), 8.3 (2 H, br s,
ring 26), 8.6 (4 H, br s, ring 26), 9.04 (2 H, br s, ring 26), 9.18 (2 H,
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We gratefully acknowledge the Deutsche Forschungsgemeinschaft
for financial support.
154 | Org. Biomol. Chem., 2009, 7, 142–154
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