Controlling Multivalent Interactions in Triply-Threaded Two-Component Superbundles

Article abstract of DOI:10.1002/chem.200304979

We have investigated the ¹H NMR spectra, the absorption spectra, the fluorescence spectra and decays, and the electrochemical properties of i) a tritopic receptor in which three benzo[24]crown-8 macrorings are fused onto a triphenylene core, ii

Full text of DOI:10.1002/chem.200304979

FULL PAPER
Controlling Multivalent Interactions in Triply-Threaded Two-Component
Superbundles
Vincenzo Balzani,*[a] Miguel Clemente-Leo¬n,[a, d] Alberto Credi,*[a] James N. Lowe,[b, e]
Jovica D. Badjic¬,[b] J. Fraser Stoddart,*[b] and David J. Williams*[c]
Abstract: We have investigated the
1H NMR spectra, the absorption spec-
tra, the fluorescence spectra and decays,
and the electrochemical properties of
i) a tritopic receptor in which three
benzo[24]crown-8 macrorings are fused
onto a triphenylene core, ii) a trifurcated
trication wherein three dibenzylammo-
nium ions are linked 1,3,5 to a central
benzenoid core, and iii) their 1:1 adduct
which constitutes a triply-threaded, two-
component supramolecular bundle.
X-Ray crystallography has established
the precise geometry of this paucivalent
recognition motif in the solid state. In
chloroform and acetonitrile). The in-
tense fluorescence emissions of the two
recognition components are quenched
upon association with the concomitant
appearance of a lower energy, broad
fluorescence band originating from the
superbundle, such units are not interact-
ing. In acetonitrile solution, dethread-
ing/rethreading of the 1:1 adduct can be
controlled quantitatively by addition of
base and acid. Dethreading and re-
1
threading is also observed by H NMR
p
p stacking in the 1:1 adduct of the
spectroscopy when dimethylsulfoxide is
added to a solution of the 1:1 adduct in
equal volumes of acetonitrile and
aromatic cores in the two matching
components. Titration experiments, in-
cluding Job plots, establish the 1:1 stoi-
chiometry of the adduct, an observation
which is also confirmed by electrochem-
ical experiments. The electrochemical
results show that, both in the tritopic
receptor and in the superbundle itself,
the first oxidation process is associated
with the hexaalkoxytriphenylene core.
The successive oxidation processes of
the peripheral dioxybenzene units are
affected by charge-transfer interactions
in the tritopic receptor, whereas, in the
chloroform.
A
trifurcated trication
where methyl groups are located on
the para positions of the three dibenzyl-
ammonium ions, which are linked 1,3,5
to the neutral benzenoid core, has been
employed to demonstrate that dethread-
ing of the 1:1 adduct involves doubly-
threaded and singly-threaded species,
that is, the paucivalent site is dismem-
bered in a sequence of logical steps
involving stable intermediates. This mo-
lecular recognition system is a rare
example of a supramolecular entity
based on a cooperative binding motif
that can be switched on and off by
chemical means.
‡
addition to [N -H ¥¥¥ O] hydrogen bond-
ing and [C-H ¥¥¥ O] interactions between
‡
the NH2 centers on the three dibenzyl-
ammonium ion containing arms of the
trication and the three crown ether rings
in the tritopic receptor, there is a
stabilizing [p ¥¥¥ p] stacking interaction
between the two aromatic cores. Mass
spectrometry and 1H NMR spectroscopy
have confirmed the integrity of the 1:1
adduct beyond the solid state, provided
the solvents are relatively apolar (e.g.,
Keywords: hydrogen
luminescence ¥ molecular machines
molecular recognition
self-assembly
bonds
¥
¥
¥
¬
[a] Prof. V. Balzani, Dr. A. Credi, Dr. M. Clemente-Leon
[c] Prof. D. J. Williams
Dipartimento di Chimica ™G. Ciamician∫
Chemical Crystallography Laboratory
Department of Chemistry, Imperial College
South Kensington, London, SW7 2AY (UK)
Fax : (‡44)207-594-5835
¡
Universita di Bologna
Via Selmi 2, 40126 Bologna (Italy)
Fax : (‡39)051-029-9456
E-mail: vbalzani@ciam.unibo.it
acredi@ciam.unibo.it
¬
[d] Dr. M. Clemente-Leon
Current address:
¡
Instituto de Ciencia Molecular, Universitat de Valencia
Dr. Moliner 50, 46100 Burjassot (Spain)
¬
[b] Prof. J. F. Stoddart, Dr. J. N. Lowe, Dr. J. D. Badjic
Department of Chemistry and Biochemistry
University of California, Los Angeles
[e] Dr. J. N. Lowe
405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA)
Fax : (‡1)310-206-1843
Current address:
Department of Chemistry, University of Pennsylvania
231 South 34th Street, Philadelphia, PA 19104-6323 (USA)
E-mail: stoddart@chem.ucla.edu
5348
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200304979
Chem. Eur. J. 2003, 9, 5348 5360

5348 5360
rotaxanes. These activities also led to the design of molecular-
Introduction
10]
level devices and machines activated by acid base inputs,[8
The concept of threading a secondary dialkylammonium
as well as to the modification of surfaces and to the control of
the aggregation of silver nanocrystals.[11]
‡
‡
(RCH2NH2 CH2R) ion containing an NH2 center through
the cavity of a macrocyclic polyether with a [24]crown ether
constitution minimally was first demonstrated[1, 2] in 1995. This
In an effort to achieve enhanced association using this
particular recognition motif, we have recently launched an
investigation[12] into the complexation properties of matching
components, each containing multiple recognition sites. This
clustering of weak interactions is a well known phenomen-
on[13] in biological systems and is variously referred to[14] as the
cluster effect or multivalency. For our initial investigations on
multivalency in a wholly unnatural supramolecular system, we
have chosen, as the matching components, i) a tritopic
receptor in the form of a tris-crown ether derivative 1, which
consists of three benzo[24]crown-8 macrorings fused onto a
triphenylene core, and ii) the trifurcated trication [2-H3]3‡, as
its hexafluorophosphate salt,[15] wherein three dibenzylam-
monium ions are linked 1,3,5 to a central benzenoid core.
Matching the components gives rise (Scheme 1) to the
‡
recognition motif, which relies primarily upon [N -H ¥¥¥ O]
hydrogen bonds and [C-H ¥¥¥ O] interactions,[3] has been
6]
exploited extensively[4 in the noncovalent synthesis[7] of
supermolecules (complexes including pseudorotaxanes) and
in supramolecular assistance[7] to the covalent synthesis of
interlocked molecular compounds such as catenanes and
1
Abstract in Italian: Sono stati studiati gli spettri H NMR, gli
spettri di assorbimento elettronico, gli spettri ed i decadimenti
¡
di fluorescenza e le proprieta elettrochimiche di i) un recettore
tritopico formato da tre macrocicli benzo[24]corona-8 costruiti
su un gruppo trifenilene centrale, ii) un tricatione ramificato in
¡
cui tre ioni dibenzilammonio sono legati ad un×unita benzenica
centrale in posizione 1,3,5 e iii) il loro addotto 1:1. Quest×ulti-
mo costituisce una sorta di ™fascio∫ supramolecolare, in cui
¡
¡
tritopico che nell×addotto e attribuibile all×unita centrale
esaossitrifenilene. I successivi processi di ossidazione delle
¡
ciascun ramo del componente tricationico e infilato in un
¡
unita diossibenzene periferiche nel recettore tritopico sono
macrociclo del recettore tritopico. L×analisi cristallografica ai
raggi X ha permesso di stabilire la geometria dell×addotto allo
stato solido e di evidenziare le interazioni fra i suoi compo-
influenzati da interazioni di trasferimento di carica, mentre tali
¡
unita non interagiscono tra loro quando si trovano nell×addot-
‡
¡
to. In acetonitrile l×addotto 1:1 puo essere sfilato e reinfilato in
nenti. Oltre a legami a idrogeno [N -H ¥¥¥ O] e [C-H ¥¥¥ O] tra
maniera quantitativa e controllata mediante l×aggiunta di acido
e di base. I processi di sfilamento e reinfilamento si osservano
anche aggiungendo dimetilsolfossido ad una soluzione del-
l×addotto in acetonitrile/cloroformio 50:50 (v/v), attraverso la
spettroscopia 1H NMR. Utilizzando un tricatione ramificato in
cui si sono introdotti gruppi metilici in posizione para nelle tre
gli ioni dibenzilammonio di un componente e gli eteri corona
dell×altro, si osservano interazioni di stacking [p ¥¥¥ p] tra le
¡
¡
due unita aromatiche centrali. L×addotto 1:1 e stabile anche in
soluzione, come dimostrato da esperimenti di spettrometria di
1
¬
massa e di spettroscopia H NMR, purche in solventi relativa-
mente poco polari come il cloroformio o l×acetonitrile.
L×intensa fluorescenza dei due componenti molecolari viene
spenta in seguito alla loro associazione, con la concomitante
comparsa di una banda di fluorescenza larga e a bassa energia
¡
¡
unita dibenzilammonio, e stato possibile dimostrare che lo
sfilamento dell×addotto 1:1 coinvolge specie doppiamente e
singolarmente infilate. In altre parole, l×addotto si disassembla
secondo una sequenza logica che passa attraverso specie
intermedie stabili. Questo sistema di riconoscimento moleco-
lare costituisce un raffinato esempio di architettura supramo-
lecolare basata su interazioni che cooperano tra loro e che
possono essere attivate e disattivate mediante stimoli chimici.
¡
dovuta alle interazioni [p ¥¥¥ p] tra le due unita aromatiche
¡
centrali nell'addotto. La stechiometria 1:1 dell×addotto e
confermata da titolazioni, Job plot ed esperimenti elettrochi-
mici. I risultati di questi ultimi esperimenti mostrano che il
primo processo di ossidazione osservato sia nel recettore
Scheme 1. The equilibrium between the tris-crown ether 1 and the tris-ammonium ion [2-H3]3‡ which lies very much over to the right in favor of the 1:1
adduct [1 ꢀ 2-H3]3‡ in solvents such as MeCN and CH2Cl2. In DMSO, the 1:1 adduct becomes completely dissociated into its components. The graphical
‡
representation of the 1:1 adduct portrays the good surface-to-surface match between the two interpenetrating components. The dots indicate the [N -H ¥¥¥ O]
hydrogen bonds between the NH2 centers (indicated by the symbol H placed inside a circle) of [2-H3]3‡ and the fused benzo[24]crown-8 rings in 1 and the
horizontal dashes, the [p ¥¥¥ p] stacking interactions between the central aromatic core units.
‡
‡
Chem. Eur. J. 2003, 9, 5348 5360
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¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5349

V. Balzani, A. Credi, J. F. Stoddart, D. J. Williams et al.
FULL PAPER
formation of a 1:1 adduct [1 ꢀ 2-H3]3‡ in the shape of a triply
threaded, two-component supramolecular bundle or super-
bundle with C3v symmetry.
In this paper, we describe 1) the synthesis of 1 and a
derivative [3-H3][PF6]3 of the parent[14] trifurcated tricationic
salt [2-H3][PF6]3, 2) the characterization of the 1:1 adducts
formed between 1 and [2-H3]3‡, and 1 and [3-H3]3‡ in solution
1
by H NMR spectroscopy and mass spectrometry, and 3) the
solid-state superstructure of [1 ꢀ 2-H3][PF6]3, obtained by
X-ray crystallography on a single crystal of the 1:1 adduct. On
account of their extended aromatic structures, both 1 and [2-
H3]3‡ are expected to be photo- and redox-active. Stimulated
Scheme 2. The synthesis of the tris-ammonium salts [2-H3][PF6]3 and
[3-H3][PF6]3.
by the growing interest in the excited-state and redox
10, 16
properties of supramolecular systems,[8
23] we have inves-
tigated the 4) photophysical and 5) electrochemical proper-
ties of the interwoven superbundle [1 ꢀ 2-H3]3‡ and its free
components 1 and [2-H3]3‡. We have also shown that
6) assembling/disassembling of the superbundle, which in-
volves a triple threading/dethreading process, can be rever-
sibly and quantitatively controlled by the addition of acid and
base to an acetonitrile solution. Some of the results reported
in this paper have been the subject of a preliminary
communication.[24, 25]
tion conditions. HHTP (5) was prepared according to
literature procedures.[27] The bistosylate 7 was obtained[28]
from catechol in two steps–its bisalkylation (K2CO3/KI/
MeCN) with 2[2(2-chloroethoxy)ethoxy]ethanol to give a diol
6 which, on tosylation (TsCl/CH2Cl2), gave the desired
product 7.
Characterization of the parent superbundle in solution: Fast
atom bombardment (FAB) mass spectrometry performed on
Results and Discussion
Synthesis of the tritopic recog-
nition components: Of the
three components–namely 1,
[2-H3][PF6]3, and [3-H3][PF6]3–
the synthesis of the tris-ammo-
nium salt has been described
previously,[15] starting from the
known[26] trialdehyde 4 as de-
picted in Scheme 2. The deriv-
ative [3-H3][PF6]3 of [2-H3]-
[PF6]3 with three terminal
p-methylbenzyl groups was ob-
tained (Scheme 2) by condens-
ing with p-methylbenzyla-
Scheme 3. The synthesis of the tris-crown ether derivative 1.
4
mine in PhMe in a Dean
Stark apparatus to afford a
trisimine, which was subse-
quently reduced (NaBH4/THF/
MeOH) to form a trisamine.
The trisamine was protonated
(HCl/THF/MeOH/H2O) and
the counterions were ex-
changed (NH4PF6/H2O) to give
[3-H3][PF6]3.
The tris-crown ether deriva-
tive 1 was obtained (Scheme 3)
in 10% yield following a triple
macrocyclization
(Cs2CO3/
DMF) between 2,3,6,7,10,11-
hexahydroxytriphenylene
Figure 1. 1H NMR Spectra [500 MHz, CDCl3/CD3CN (1:1), 7.5 mm, 300 K] recorded on a) the 1:1 adduct [1 ꢀ
2-H3][PF6]3, b) the tris-crown ether 1, and c) the tris-ammonium salt [2-H3][PF6]3.
(HHTP, 5) and the appropriate
bistosylate 7 under high dilu-
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Chem. Eur. J. 2003, 9, 5348 5360

Two-Component Superbundles
5348 5360
a sample obtained from an equimolar solution of the tris-
crown ether derivative 1 and the tris-ammonium salt [2-
H3][PF6]3 revealed intense signals in the spectrum at m/z 2295,
‡
2149, and 2004 corresponding to the [1‡2-H3‡2PF6] , [1‡2-
‡
‡
H3‡PF6] , and [1‡2-H3] ions, respectively. This evidence for
a strong 1:1 adduct being formed in relatively apolar solvents
was supported by 1H NMR spectroscopy. A spectrum,
recorded in CDCl3/CD3CN (1:1) of an equimolar mixture
(7.5 mm) of 1 and [2-H3][PF6]3 reveals (Figure 1a) a very
highly dispersed array of almost isolated resonances. A
comparison of this spectrum with those recorded under the
same conditions for pure 1 (Figure 1b) and pure [2-H3][PF6]3
(Figure 1c) indicates that there are no signals for these free
1
species present in the H NMR spectrum of the 1:1 adduct.
Indeed, only when the concentration of [1 ꢀ 2-H3][PF6]3 is
lowered from 7.5 to 0.05 mm is there any evidence of free
species being present in the CDCl3/CD3CN (1:1) solution.
1
1
With the aid of H,1H COSY (Figure 2) and 2D H NMR
T-ROESY (Figure 3) experiments,[29, 30] it is possible to assign
the resonances as shown in Figure 1a, to a single C3v sym-
metrical adduct–namely, the [1 ꢀ 2-H3]3‡ superbundle–that
is kinetically stable on the H NMR timescale.[31] The large
1
downfield shifts (Table 1) and multiplicities of the resonances
for the benzylic methylene protons Hd and He adjacent to the
NH2 centers are characteristic[32] of the formation of a 1:1
‡
complex between dibenzo[24]crown-8 (DB24C8) and the
‡
dibenzylammonium ion ([DBA] ). In addition, substantial
changes in the chemical shifts of the aromatic core protons–
that is, Hf (À0.55 ppm) in 1 and Ha (À0.44 ppm) in [2-H3]3‡–
are observed, a phenomenon which suggests that these cores
are [p p] stacked with respect to each other. On complex-
ation with the [2-H3]3‡ trication, the OCH2 protons, such as Hi/
Hi' and Hj/Hj' in the tris-crown ether derivative separate into
two different sets of signals as a consequence of 1 losing its
plane of symmetry orthogonal to the principal axis in the
molecule. Stated another way, the pairs of protons in each
OCH2 group that are directed toward the bound [2-H3]3‡
trication become diastereotopic with respect to those which
are directed away from the [2-H3]3‡ trication. The diaster-
eotopicities of the OCH2 protons Hi/Hi' and Hj/Hj' in 1 can be
observed (Figure 1a) in the 1H NMR spectrum of the 1:1
adduct. In fact, for Hj/Hj', the anisochronous effect is quite
dramatic: in comparison with the free tris-crown ether
derivative 1, the resonances for one set of protons are shifted
by ‡0.2 ppm (downfield) while those for the other set are
shifted by À0.5 ppm (upfield). These observations lend
further support to the conclusion that the 1:1 adduct is a
highly stable supramolecular entity in CDCl3/CD3CN (1:1)–
and the fact that complexation decomplexation is slow on
1
the H NMR timescale.
Solid-state superstructure of the superbundle: A single crystal
X-ray investigation[33] confirmed the formation in the solid
state of a 1:1 adduct in which the three unique cationic arms of
the triply charged guest are inserted through the centers of the
three crown ether components of the neutral host. In contrast
with the time-averaged C3v symmetry observed in solution for
the 1:1 adduct, here, in the solid state, a geometry that
approximates more closely to Cs is adopted (Figure 4).
Figure 2. Selected areas of the 1H,1H COSY NMR spectrum [500 MHz,
CDCl3/CD3CN (1:1), 7.5 mm, 300 K] of the 1:1 adduct [1 ꢀ 2-H3][PF6]3.
Chem. Eur. J. 2003, 9, 5348 5360
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V. Balzani, A. Credi, J. F. Stoddart, D. J. Williams et al.
FULL PAPER
Table 1. The chemical shifts (d) of the protons[a] in the uncomplexed tris-
crown ether 1, the uncomplexed tris-ammonium salt [2-H3][PF6]3, and the
superbundle [1 ꢀ 2-H3][PF6]3–and the change (Dd) in chemical shift on
complexation.
Proton[b]
d(uc)
d(c)
Dd
Ha
Hb
Hc
Hd/He
Ph
Hf
Hh/Hg
Hi
Hi'
Hj
7.89
7.58
7.86
4.27
7.46
7.86
6.85
4.09
4.09
4.37
4.37
7.45
7.59
7.63
4.80 4.90
7.58 & 7.42
7.35
6.88 6.94
4.13
À 0.44
‡ 0.01
À 0.23
‡ 0.5 0.6
À 0.51
‡ 0.03 0.09
‡ 0.04
4.20
3.90
4.57
‡ 0.11
À 0.47
Hj'
‡ 0.20
[a] The 1H NMR spectra were recorded at 500 MHz in CDCl3/CD3CN (1:1)
at a concentration of 7.5 mm at ambient temperature. [b] The proton
designations are shown in Scheme 1.
Figure 4. Ball-and-stick representation of the solid-state superstucture of
the 1:1 adduct [1 ꢀ 2-H3]3‡
.
Figure 3. Selected areas of the T-ROESY NMR spectrum [500 MHz,
CDCl3/CD3CN (1:1), 7.5 mm, 300 K] of the 1:1 adduct [1 ꢀ 2-H3][PF6]3.
Figure 5. Space-filling representation of the solid-state superstucture of
the 1:1 adduct [1 ꢀ 2-H3]3‡. The lighter shaded molecule is the tris-crown
ether component and the darker shaded one, the tris-ammonium ion
component. The discoid-shaped superbundle has a surface radius of
approximately 14 ä and thickness of 16 ä, i.e., overall dimensions of ca.
2.8 Â 1.6 mm.
Although the C3 symmetry of the tri-cationic component is
essentially retained, two distinctly different conformations are
evident for the crown-ether components of the host: two of
these macrocycles have a U-shaped conformation whereas the
third adopts an extended conformation with a pseudo-Ci
symmetry (Figure 5). The two-component superbundle is
of nonplanar conformations by the three biphenylene units of
the trication. Here, the local symmetry is C3, the planes of the
three radial phenylene rings being inclined (by between ca. 15
and 238) in a propeller-like manner with respect to the central
pivotal benzenoid ring in the guest. Accompanying these
twists, there is a commensurate but smaller, distortion of the
triphenylene unit of the host. The only inter-superbundle
interaction of note is a [p ¥¥¥ p] stacking of one of the radial
‡
stabilized by a combination of [p ¥¥¥ p], [N -H ¥¥¥ O] and
[C-H ¥¥¥ O] interactions as discussed previously.[24] The two
components overlay each other with the planes of their
central aromatic rings nearly parallel and a centroid-centroid
separation of 3.6 ä, consistent with stabilizing [p ¥¥¥ p] inter-
actions. A secondary conformational feature is the adoption
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¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chem. Eur. J. 2003, 9, 5348 5360

Two-Component Superbundles
5348 5360
phenylene rings of the tricationic component with its Ci-
related counterpart (mean interplanar separation of 3.65 ä).
zene subunits (lmax ꢁ 310 nm)[10b] is observed. The [2-H3]3‡
tricationic component shows the characteristic strong fluo-
rescence band[35] of its 1,3,5-triphenylbenzene core with lmax
356 nm and t ˆ 17.5 ns; no emission typical (lmax ˆ 337 nm) of
ˆ
Absorption and fluorescence spectra: The absorption and
‡
the [DBA] ion is observed. In the emission spectrum of the
[34]
fluorescence data for the compounds recorded in CH2Cl2
1:1 mixture, the fluorescence bands of the two components
are no longer present, a result which is to be expected if a 1:1
adduct had been formed. The superbundle [1 ꢀ 2-H3]3‡
exhibits a broad unstructured luminescence band at longer
wavelengths (lmax ˆ 410 nm) which decays according to a
single exponential with a lifetime of 12.5 ns. The excitation
spectrum of [1 ꢀ 2-H3]3‡, recorded at lmax ˆ 410 nm, reveals
that the excitation of both components leads quantitatively to
the fluorescent excited state of the superbundle. By contrast,
the fluorescence spectrum of DB24C8 is hardly affected[10d] by
are given in Table 2. Figure 6 shows the absorption spectra of
Table 2. Absorption and fluorescence data for the compounds examined in
CH2Cl2 at room temperature.
Absorption
lmax [nm]
Fluorescence
Compound
e [mÀ1 cmÀ1
]
lmax [nm]
t [ns]
1
278
262
270
88000
69000
122000
384
356
410
10
17.5
12.5
[2-H3]3‡
[1 ꢀ 2-H3]3‡
‡
its being threaded by [DBA] ions. The dramatic changes in
the spectra observed upon titration of 1 and [2-H3]3‡ indicate
that the two components are tightly connected in the resulting
1:1 adduct which, in fact, behaves as a novel fluorophoric unit.
The new fluorescence emission observed in the superbundle
most likely originates from a lower lying excited state, which
results from the [p p] stacking of the aromatic cores of the
two components (Scheme 1). This is evident from the solid-
state superstructure where the centroid centroid separation
was observed to be 3.6 ä. In MeCN solution, the fluorescence
spectra and lifetimes of this tris-crown ether derivative 1 (t ˆ
9.6 ns) and the tris-ammonium trication [2-H3]3‡ (t ˆ 17 ns)
and the 1:1 adduct (t ˆ 11.6 ns) are very similar to those
observed in CH2Cl2 solution.
Figure 6. Absorption spectra of the 2.0 Â 10À5 molLÀ1 CH2Cl2 solutions of
3‡
* * * *
the tris-crown ether 1 (
), the tris-ammonium ion [2-H3]
(
) and the
superbundle [1 ꢀ 2-H3]3‡ (––).
Titration experiments: In order to establish that the associ-
ation of 1 with [2-H3]3‡ is strong and also to confirm the 1:1
stoichiometry of the supramolecular species, we have carried
out numerous titration experiments. Figure 8 shows the
changes in the emission spectra (lexc ˆ 252 nm) observed on
free 1, free [2-H3]3‡, and their 1:1 mixture, that is, of the
superbundle [1 ꢀ 2-H3]3‡ in CH2Cl2 solution. It is evident that
the formation of the superbundle causes relatively small
changes in the absorption spectrum. The spectra obtained in
MeCN do not differ significantly from those recorded in
CH2Cl2.
The emission spectra of free 1, free [2-H3]3‡ and the
superbundle [1 ꢀ 2-H3]3‡ in CH2Cl2 are illustrated in Figure 7.
The tris-crown ether derivative 1 shows a relatively strong and
structured emission band, quite similar to that observed[35] for
its triphenylene core with lmax ˆ 384 nm and t ˆ 10 ns. No
fluorescence band typical of DB24C8 or of its 1,2-dioxyben-
Figure 8. Changes in the emission spectra (lexc ˆ 252 nm) of a 2.0 Â
10À5 molLÀ1 CH2Cl2 solution of the tris-ammonium ion [2-H3]3‡ on titration
with the tris-crown ether 1. The numbers of equivalents of tris-crown ether
added are 0 (a), 0.13 (b), 0.29 (c), 0.45 (d), 0.59 (e), 0.75 (f), 0.92 (g), 1.01
(h). The inset shows the corresponding titration curve obtained by reading
the luminescence intensity at 490 nm.
the addition of small volumes of a concentrated CH2Cl2
solution of the tris-crown ether derivative 1 to a 2.0 Â
10À5 molLÀ1 CH2Cl2 solution[34] of the tris-ammonium salt
[2-H3][PF6]3. A decrease in the intensity of the emission band
(lmax ˆ 356 nm) of [2-H3]3‡ is accompanied by an increase in
the intensity of the band (lmax ˆ 410 nm) of the superbundle.
Figure 7. Emission spectra (lexc ˆ 252 nm) of 2.0  10À5 molLÀ1 CH2Cl2
3‡
* * * *
solutions of the tris-crown ether 1 (
), the tris-ammonium ion [2-H3]
(
), and the superbundle [1 ꢀ 2-H3]3‡ (––).
Chem. Eur. J. 2003, 9, 5348 5360
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V. Balzani, A. Credi, J. F. Stoddart, D. J. Williams et al.
FULL PAPER
From the titration curve (inset in Figure 8) obtained at lem
ˆ
[2-H3]3‡–and then of 1. The former does not exhibit any
redox processes in the potential window examined whereas
the latter was found to behave in a complex manner,
exhibiting three irreversible oxidation processes. The poten-
tial values, obtained from differential pulse voltammogram
(DPV) measurements on 1 were ‡0.99, ‡1.29 and ‡1.44 V in
CH2Cl2 and ‡1.00, ‡1.23 and ‡1.38 V in MeCN. The second
peak is more appropriately described as a shoulder; the
second and third peaks are more intense than the first one.
The superbundle [1 ꢀ 2-H3]3‡ exhibits only two oxidation
peaks–at ‡1.01 and ‡1.44 V in CH2Cl2 and at ‡1.08 and
‡1.43 V in MeCN. In an attempt to assign these observed
processes, and associate them with structural features, we
have also examined the electrochemical behavior of some
model compounds. We have found that 1) in CH2Cl2,
2,3,6,7,10,11-hexamethoxytriphenylene–a model compound
for the central triphenylene core of the tris-crown ether 1–
reveals an irreversible process at ‡1.02 V, 2) in MeCN,
DB24C8 exhibits two irreversible oxidation peaks at ‡1.32
and ‡1.47 V, a result that can be attributed to a donor ac-
ceptor interaction between the two dioxybenzene subunits
arising when the first one is oxidized[10e] and 3) in MeCN, the
490 nm,[36] a Ka value in excess of 107 LmolÀ1 can be
estimated. Titration experiments in MeCN solutions yielded
a Ka value of 106 LmolÀ1. Such very high association constants,
compared with those[31] for the [2]pseudorotaxane formed
between DB24C8 and [DBA][PF6]–27000 and 420 LmolÀ1 in
CHCl3 and MeCN, respectively–proved that association of 1
with [2-H3]3‡ is strongly favored by cooperative action[37
39]
between the three binding sites, emphasizing the highly
complementary nature of the two components. Not surpris-
ingly, the 1:1 adduct is even more stable[40] in CH2Cl2 than in
MeCN.
We have also constructed Job plots for the association
process by measuring the emission light intensities of the 1:1
adduct at 490 nm[36] in CH2Cl2 (Figure 9) and MeCN solutions
‡
[2]pseudorotaxane formed between DB24C8 and the [DBA]
ion displays only one oxidation process at ‡1.46 V, an
observation which can be assigned to the two equivalent
‡
dioxybenzene subunits separated by the threaded [DBA]
ion.[10e] These results allow us to assign the first oxidation
process (‡1.01 V) involving the superbundle [1 ꢀ 2-H3]3‡ to
the oxidation of the hexaoxytriphenylene core and the second
one (‡1.44 V) to the oxidation of the three peripheral
dioxybenzene subunits. Since all the processes are irreversi-
ble,[41] it does not seem appropriate to overspeculate. We
Figure 9. Job plot of the intensity of the emission at 490 nm of different
mixtures of the tris-crown ether 1 and the tris-ammonium ion [2-H3]3‡ as a
function of the molar fraction of [2-H3]3‡. Excitation was performed at
270 nm, where the two components exhibit the same molar absorption
coefficient (Figure 6). The solvent was CH2Cl2 and the sum of the
concentrations of the two components was 4.0 Â 10À5 molLÀ1 in all cases.
The curve simulating the formation of a 1:1 adduct with a stability constant
of 1.5 Â 107 LmolÀ1 is also shown.
containing different molar frac-
tions of the two components. The
maximum observed at x ˆ 0.5
proves conclusively that the
complex has 1:1 stoichiometry.
Moreover, the sharpness of the
curve, which is more pronounced
in the CH2Cl2 experiment, con-
firms the association constant is
extremely high, that is, Ka ˆ
1.5 Â 107 LmolÀ1.
Electrochemical behavior: We
have performed electrochemical
investigations on the tris-crown
ether 1, the tris-ammonium ion
[2-H3]3‡ and the 1:1 adduct [1 ꢀ
2-H3]3‡ both in MeCN and in
CH2Cl2. Correlations between
these species and some model
compounds and complexes are
Figure 10. Correlation diagram of the potential values for the oxidation processes of the 2,3,6,7,10,11-
hexamethoxytriphenylene model compound, the tris-crown ether 1 and the supramolecular bundle [1 ꢀ 2-H3]3‡ in
CH2Cl2 solution. The potential values for the oxidation processes of DB24C8 and its 1:1 complex formed with the
shown in Figure 10. To begin
with, we investigated the electro-
chemical behavior in MeCN of
‡
[DBA] ion in MeCN solution are also shown for comparison. For more details, see the text.
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Two-Component Superbundles
5348 5360
would only like to make the point that, after the first oxidation
process, the oxidized hexaoxytriphenylene core becomes an
electron acceptor. Therefore, in the free tris-crown ether 1, the
oxidized hexaoxatriphenylene core may interact with the
electron-donating peripheral dioxybenzene subunits, affect-
ing their oxidation potentials. Further donor acceptor inter-
actions, involving the dioxybenzene subunits may occur upon
successive oxidations of these subunits as observed[10e] for
DB24C8. However, donor acceptor interactions arising from
successive oxidation processes are likely to be prevented in
the superbundle [1 ꢀ 2-H3]3‡ because the [2-H3]3‡ component
rigidifies the superstructure and keeps the redox-active units
separated from each other. Under such conditions, the three
dioxybenzene peripheral units become equivalent and hence
are oxidized at the same potential, just as in the case
(Figure 10) of the two dioxybenzene subunits present in the
ponents. These resonances must result from the intermediate
complexes on the pathway between these two extreme
situations. They are probably doubly- and singly-threaded
1:1 complexes which are formed as the three arms of the [2-
H3]3‡ trication dissociate progressively from the three macro-
cyclic polyethers present in 1. The 1H NMR spectra are
exceedingly complicated, when these numerous species are all
present under conditions of slow-exchange.
For this reason, we decided to use a simple 1H NMR probe
by locating methyl groups at the para-positions of the three
benzyl groups in [2-H3]3‡. The CD3SOCD3-driven decom-
plexation of the resulting superbundle [1 ꢀ 3-H3]3‡ was
followed by monitoring (Figure 12) the resonances for the
different methyl groups as aliquots of CD3SOCD3 were added
to [1 ꢀ 3-H3][PF6]3 dissolved initially in CDCl3/CD3CN (1:1).
In principle, during the dissociation of [1 ꢀ 3-H3]3‡ to the free
species via the doubly- and singly-threaded complexes there
are six different methyl environments which could possibly
‡
[2]pseudorotaxane formed when DB24C8 binds the [DBA]
ion. The values of the peak potentials for the oxidation
processes indicate that one-electron oxidation of the super-
bundle, which takes place on the central hexaoxytriphenylene
unit, does not lead to dethreading. Such a result, unexpected
at a first glance, suggests that the removal of one electron
from the triphenylene unit of the tris-crown ether component
1
lead to the observation of six separate singlets in H NMR
spectra. The triply-, doubly- and singly-threaded trications
each provide different ™complexed methyl∫ environments,
while the doubly-, singly-threaded and free trications each
provide different ™uncomplexed methyl∫ environments. In
the event, four different singlets are observed (Figure 12) at d
2.350 (B), 2.345 (C), 2.336 (D), and 2.311 (A) in the 1H NMR
spectrum recorded with 14% CD3SOCD3 present. It seems
not unreasonable that the methyl groups located in all of the
bound side arms will resonate with identical or near identical
chemical shifts (A), irrespective of whether they are located in
singly, doubly or triply threaded complexes. On this assump-
tion, it has to be the methyl groups in the unbound arms of the
free, singly and doubly threaded species that give rise
(Figure 13) to the signals C, B, and D, respectively. It is
evident from inspection of the partial spectra portrayed in
Figure 12 that, as the proportion of CD3SOCD3 is raised, the
intensity of the resonance C increases, while those for
‡
does not weaken considerably the [N -H ¥¥¥ O] hydrogen-
bonding interactions that provide the principal stabilization to
the superbundle.
Solvent-induced dissociation of the superbundle: Since it is
known[2, 31] that DB24C8 and [DBA][PF6] do not form a
complex in CD3SOCD3 solution, it was not surprising to us
that addition of CD3SOCD3 to an equimolar mixture (7.5 mm)
in CDCl3/CD3CN (1:1) causes the superbundle [1 ꢀ 2-H3]3‡ to
undergo complete decomplexation. This observation[42] is
undoubtedly a result of the fact that CD3SOCD3 is a much
better hydrogen-bond acceptor than CDCl3 or CD3CN. Thus,
CD3SOCD3 competes successfully with the crown ether×s
oxygen atoms to hydrogen bond
‡
to the NH2 centers in the tris-
ammonium trication. The de-
complexation was monitored
by recording
a
series of
1H NMR spectra (Figure 11),
while gradually increasing the
proportions of CD3SOCD3 pres-
ent in the mixed solvent system,
namely CDCl3/CD3CN (1:1).
These spectra reveal that a com-
plete transition takes place from
the fully complexed species, that
is, the superbundle [1 ꢀ 2-H3]3‡
to the completely free tris-
crown ether derivative 1 and
trisammonium trication [2-
H3]3‡ at 33% CD3SOCD3. A
closer inspection of the spectra
leads to the identification of
resonances which correspond
to neither the triply-threaded
Figure 11. A series of 1H NMR spectra (500 MHz, 300 K) recorded on addition of CD3SOCD3 to a 7.5 mm
CDCl3/CD3CN (1:1) solution of the 1:1 adduct [1 ꢀ 2-H3][PF6]3 showing the progressive decomposition of the
1:1 complex nor the free com- superbundle to give the tris-crown ether 1 and the tris-ammonium salt [2-H3][PF6]3.
Chem. Eur. J. 2003, 9, 5348 5360
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V. Balzani, A. Credi, J. F. Stoddart, D. J. Williams et al.
FULL PAPER
some certainty: the dissociation of [1 ꢀ 3-H3]3‡ is a stepwise
process.
Dethreading/rethreading with base/acid inputs: An important
feature of pseudorotaxanes based on hydrogen-bonding
‡
interactions between crown ethers and NH2 ion centers is
the possibility of controlling the dethreading/rethreading
processes using base/acid inputs.[8, 9, 10a,b,d,f] We have therefore
investigated the possibility of dethreading the [1 ꢀ 2-H3]3‡
[34]
superbundle in CH2Cl2
by addition of tri-n-butylamine
(NBu3). In an experimental foray, we examined the behavior
of [2-H3]3‡ on addition of NBu3. It is well known[44] that
deprotonation of aromatic ammonium salts often causes
quenching of their fluorescence because of the appearance of
an n p* excited state below the fluorescent p p* level. We
established that addition of a stoichiometric amount of NBu3
to a solution of [2-H3]3‡ does not cause fluorescence quench-
ing. The possibility that NBu3 is not sufficiently strong a base
‡
to deprotonate the NH2 ion centers in [2-H3]3‡ was dis-
Figure 12. Partial 1H NMR spectra [500 MHz, (100 À x)% CDCl3/CD3CN
(1:1) ‡ x% CD3SOCD3, x ˆ a) 0, b) 7.4, c) 13.8, d) 19.4, e) 24.3, f) 28.6, and
g) 33.3] showing the resonances corresponding to the methyl groups in an
equimolar mixture of the tris-crown ether 1 and the tris-ammonium salt
[3-H3][PF6]3.
counted[45] by the fact that we found that addition of one
equivalent of NBu3 does cause complete quenching of the
‡
fluorescence of the [DBA] ion (lmax ˆ 337 nm). Thus, we
have to conclude that NBu3 is strong enough to deprotonate
the NH2 ion centers in [2-H3]3‡ and that the fluorescence of
‡
[2-H3]3‡ is not quenched upon deprotonation because, in
CH2Cl2 solution, the n p* excited state of 2 is higher in
energy than the emitting p p* level. On the basis of this
result, dethreading of [1 ꢀ 2-H3]3‡ caused by deprotonation of
the [2-H3]3‡ component implies the disappearance of the
broad [1 ꢀ 2-H3]3‡ emission (lmax ˆ 410 nm) and the re-
instatement of both the fluorescence of the tris-crown ether
(lmax ˆ 384 nm) and the (deprotonated) tris-ammonium ion
(lmax ˆ 356 nm). Upon titration of [1 ꢀ 2-H3]3‡ with NBu3,
changes in the shape and intensity of the fluorescence spectra
in the expected directions were indeed observed–but to a
lesser extent than expected, which indicates that dethreading
was not complete. Apparently, the stabilization caused by
hydrogen bonding in the [1 ꢀ 2-H3]3‡ adduct makes deproto-
‡
nation of the NH2 center in the threaded compound more
difficult than in the case of the free [2-H3]3‡ component. An
alternative explanation would be that the deprotonation of
the NH2 centers in the bound [2-H3]3‡ component is not a
‡
sufficient condition for complete dethreading because of a
significant contribution from the p p stacking interaction to
the stability of the 1:1 adduct. This hypothesis, however, can
be rejected since no association between the tris-crown ether
1 and the deprotonated thread 2 is observed in solution.
Contrary to what happens in CH2Cl2 solution, addition of
NBu3 to an MeCN solution of [2-H3]3‡ causes the quenching
of its fluorescence band with lmax ˆ 356 nm. The different
fluorescence behavior of the deprotonated compound 2 in the
two solvents is most likely related to the greater stabilization
of the n p* charge-transfer excited state in the more polar
MeCN solvent. Titration experiments show that addition of
one equivalent of base quenches the fluorescence of [2-H3]3‡
completely, indicating that, as expected, deprotonation of
Figure 13. A schematic representation of the equilibration between the
tris-crown ether 1 and the tris-ammonium ion [3-H3]3‡ involving species
that are triply-, doubly-, and singly-threaded, as well as free.
resonances B and D decrease. Hence, C would appear to be
the resonance for the free trication. Resonances B and D can
only be assigned tentatively.[43] Since resonance B, in general,
has a greater intensity than that of D, then resonance B
probably corresponds to the methyl groups in the unbound
arms of the singly-threaded complex, leaving resonance D to
correspond to the methyl groups in the unbound arm of the
doubly-threaded complex. One conclusion can be drawn with
‡
only one NH2 ion center is sufficient to cause the formation
of a low energy n p* excited state. According to the above
results, in MeCN solution, dethreading of [1 ꢀ 2-H3]3‡ by
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Two-Component Superbundles
5348 5360
deprotonation of the [2-H3]3‡ component should lead to the
disappearance of the broad emission typical of the 1:1 adduct
(lmax ˆ 410 nm) and the re-instatement of the fluorescence of
the tris-crown ether (lmax ˆ 384 nm). This behavior is exactly
what was observed upon titration of [1 ꢀ 2-H3]3‡ with NBu3.
After addition of three equivalents of NBu3 to [1 ꢀ 2-H3]3‡,
the fluorescence spectrum was the same as that observed
(Figure 14) for the tris-crown ether 1 alone and no further
resonances of the central aromatic-core protons Ha (d ˆ 7.48
in the [2-H3]3‡ component) and Hf (d ˆ 7.40 in the 1
component), influenced by the [p p] stacking interaction of
the two aromatic systems, are shifted downfield to d ˆ 7.87
and d 7.86, respectively, clearly demonstrating the absence of
any [p p] stacking upon the addition of base. Moreover, the
diastereotopicities of the o-methylene protons (Hi/i' and Hj/j')
expressed by their pronounced anisochronous behavior in the
1H NMR spectrum (Figure 15a) of the [1 ꢀ 2-H3]3‡ trication,
are lost (Figure 15b) completely on addition of base. All of the
1H NMR spectroscopic evidence points unambiguously to the
complete dethreading of the superbundle and the presence of
only two separate species–the tris-crown ether 1 and the tris-
amine 2–in the CD3CN solution treated with NEtiPr2.
Subsequent addition of a slight excess of trifluoroacetic acid
compared with the previously added amine brought about
complete rethreading as evidenced by the ™restored∫ 1H NMR
spectrum (Figure 15c) of the [1 ꢀ 2-H3][PF6]3 superbundle.
1
The results of these H NMR experiments–on top of the
fluorescence studies–demonstrate the reversibility of the
dethreading and rethreading of [1 ꢀ 2-H3][PF6]3 and the fact
that it can be controlled (Scheme 4) by inputs of base and
acid, respectively.
Figure 14. Fluorescence spectra (lexc ˆ 307 nm) of a MeCN solution of
[1 ꢀ 2-H3]3‡ (a), after addition of three equivalents of NBu3 (b), and after
successive addition of three equivalents of triflic acid (c).
change was observed on addition of an excess of the amine.
The occurrence of dethreading was also shown by changes in
the absorption spectrum, which–at the end of the titration–
matches exactly the sum of the spectra of the free components
1 and 2. After the addition of three equivalents of NBu3,
successive addition of three equivalents of triflic acid caused
the restoration of the initial absorption and emission spectra
(Figure 14), indicating that the dethreading-rethreading proc-
ess is fully reversible (Scheme 4).
Conclusions
An artificial recognition motif, wherein one reasonably flat
and rigid component with three trigonally arranged orifices is
triply penetrated by the three trigonally disposed tentacles of
a slightly more flexible component has been demonstrated by
numerous spectroscopic, photophysical and electrochemical
experiments to be, both kinetically and thermodynamically,
extremely stable in appropriate solvents.[46] More specifically,
a tritopic receptor in the shape of the tris-crown ether 1 is the
near-perfect match for certain trifurcated trications–namely
[2-H3]3‡ and [3-H3]3‡–to form [2]pseudorotaxanes three
times over within the same 1:1 adducts. The availability of
single crystals, suitable for X-ray crystallography, of one of
these adducts, that is, [1 ꢀ 2-H3][PF6]3 has established the
The dethreading/rethreading process has also been inves-
1
1
tigated (Figure 15) by H NMR spectroscopy. The H NMR
spectrum (Figure 15a) of an equimolar mixture of the tris-
crown ether 1 and the tris-ammonium salt [2-H3][PF6]3 in
CD3CN revealed (cf. Figure 1a) the quantitative formation of
the [1 ꢀ 2-H3]3‡ superbundle. Upon addition of slightly more
than three equivalents of diisopropylethylamine (NEtiPr2) to
the CD3CN solution of the superbundle, significant changes
were observed in the spectrum (Figure 15b). The resonances
for the benzylic methylene Hd/e protons in [1 ꢀ 2-H3]3‡ are
shifted upfield from about d ˆ 4.9 to about d ˆ 3.8, indicating
complete deprotonation of the [2-H3]3‡ trication to give the
tris-amine 2. The chemical shifts of all the other protons in
both the 1 and [2-H3]3‡ components of the superbundle
change (Figure 15b) quite dramatically as well. In particular,
‡
stabilizing cooperative actions of a few key strong [N -H ¥¥¥ O]
hydrogen bonds in addition to [C-H ¥¥¥ O] and [p ¥¥¥ p] stacking
interactions, leading to the formation of a compact and
efficient binding site.
While complexation can be suppressed completely in
solvents such as dimethylsulfoxide, which competes favorably
as a hydrogen-bond acceptor for hydrogen-bond donors, other
Scheme 4. A graphical representation of the reversible dethreading and rethreading caused by base and acid, respectively.
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V. Balzani, A. Credi, J. F. Stoddart, D. J. Williams et al.
FULL PAPER
(Merck 9385, 230 400 mesh). Melting
points were determined on an Electro-
thermal 9100 apparatus and are uncor-
rected. 1H and 13C NMR spectra were
recorded on a Bruker ARX400 spec-
trometer (at 400 MHz and 100 MHz,
respectively) and on either Bruker
ARX500 or Bruker Avance 500 spec-
trometers (at 500 MHz and 125 MHz,
respectively) at ambient temperature.
They were referenced using their resid-
ual signals as internal standards. The
chemical shifts are expressed in ppm
and the coupling constants from the
1H NMR spectra in Hertz (Hz). Elec-
tron impact mass spectra (EI-MS) were
obtained from
a VB Prospec mass
spectrometer. Fast atom bombardment
mass spectra (FAB-MS) were obtained
on a VB ZAB-SE spectrometer equip-
ped with a krypton primary atom beam
using a 3-nitrobenzyl alcohol matrix.
Matrix-assisted laser desorption ioniza-
tion time-of-flight (MALDI-TOF-MS)
were recorded on an PerSeptives Bio-
systems instrument using a trans-indole
acrylic acid matrix and an average of 50
laser shots per sample. The absorption
spectra, fluorescence spectra and life-
times, and electrochemical experiments
were carried out[34] in CH2Cl2/MeCN
Figure 15. 1H NMR spectra (500 MHz, CD3CN, 148 mm, 298 K) of a) the superbundle [1 ꢀ 2-H3][PF6]3, i.e., an
equimolar mixture of 1 and [2-H3][PF6]3, b) the deprotonated tris-amine 2 and the free tris-crown ether 1 upon
addition of 5 mL (3.2 equiv) of diisopropylethylamine to the superbundle, and c) the rethreaded superbundle [1 ꢀ
2-H3][PF6]3 upon addition of 2.2 mL (3.2 equiv) of trifluoroacetic acid (TFA).
organic solvents, such as dichloromethane, chloroform, and
even acetonitrile, can sustain very strong superbundle for-
mation. In the particular case of the [1 ꢀ 2-H3]3‡ superbundle,
the 1:1 adduct is only slightly more stable (Ka > 107 molLÀ1) in
CH2Cl2 than it is (Ka ꢂ 106 molLÀ1) in the more polar MeCN.
The photophysical properties of this superbundle are partic-
ularly interesting because both the separated free components
exhibit characteristic fluorescent bands. These properties
helped to establish beyond any doubt the 1:1 stoichiometry
of the superbundle in solution. Fluorescence titration experi-
ments have produced spectacular Job plots in support of this
1:1 adduct formation, a result which has been confirmed by
both electrochemical experiments and 1H NMR spectroscopy.
Complete and reversible dethreading/rethreading of the 1:1
adduct can be achieved in acetonitrile using base/acid inputs.
This particular supramolecular species represents a prime
example of a cooperative, multivalent recognition site[47] that
can be switched on and off by chemical means. As such, it
lends itself to the construction of molecular machines[7, 8] of
nanometer-scale dimensions, as well as to the self-assembly of
supramolecular polymers[48] of considerable size and stability
with rigid architectures that could be extremely sensitive to
pH and solvent polarities.
(99:1)–hereafter referred to as CH2Cl2–and MeCN solutions at room
temperature. The concentration of each component was 2.0 Â 10À5 molLÀ1
for the spectroscopic measurements and 5.0 Â 10À4 molLÀ1 for the electro-
chemical experiments. Absorption and luminescence spectra were record-
ed with a Perkin Elmer l16 spectrophotometer and a Perkin Elmer LS-
50 spectrofluorometer, respectively. Luminescence lifetimes were meas-
ured with an Edinburgh DS 199 single photon counting instrument,
equipped with a deuterium-filled arclamp (excitation in the range of 250
320 nm). In the titration experiments, the luminescence data were
corrected for the luminescence intensity versus absorbance instrumental
response.[50] Cyclic voltammetric (CV) and differential pulse voltammetric
(DPV) experiments were carried out in argon purged solutions at room
temperature with an Autolab 30 multipurpose instrument interfaced at a
PC. All potentials are referred to SCE; the potential window examined was
À2 to ‡1.5 and À2 to ‡2 V versus SCE in CH2Cl2 and MeCN, respectively.
The working electrode was a glassy carbon electrode (Amel, 0.08 cm2) and
the counter electrode was a Pt spiral contained in a fine glass frit. A silver
wire was employed as a quasi-reference electrode. Tetraethylammonium
hexafluorophosphate (5 Â 10À2 molLÀ1) was added as supporting electro-
lyte and ferrocene was used as an internal standard for the potential values.
CVs were obtained at sweep rates of 20, 50, 100, 200, and 500 mVsÀ1. DPVs
were performed with scan rates of 20 mVsÀ1, a pulse height of 75 mV, and a
duration of 40 ms. Experimental errors are estimated to be: wavelengths,
Æ1 nm; luminescence lifetimes, Æ5%; redox potential values, Æ10 mV.
Tris-crown ether 1: An oven-dried 2 L three-necked, round-bottomed flask,
equipped with a stirrer bar, a gas inlet tube, an addition funnel, and a
condenser, was purged with argon for 10 min and then charged with
anhydrous DMF (300 mL). Cs2CO3 (30.1 g, 92 mmol) was added to the
flask and the white suspension stirred vigorously and heated to 1008C. The
addition funnel was charged with a solution of 2,3,6,7,10,11-hexahydroxy-
triphenylene[27] (5) (2.5 g, 7.7 mmol) and 1,2-bis(2-(2-(2-(2-p-tolylsulfonyl-
oxy)ethoxy)ethoxy)ethoxy) benzene[28] (7) (15.8 g, 23 mmol) in anhydrous
DMF (500 mL). This solution was added dropwise during 24 h to the
suspension which was heated under reflux for an additional 6 d. On cooling
the reaction mixture down to room temperature, the suspension was
filtered off and the residue was washed with CHCl3 (250 mL). The filtrate
and CHCl3 washings were concentrated under reduced pressure. The
resulting dark tar was dissolved in CH2Cl2 (500 mL) and washed with 10%
aqueous K2CO3 (2 Â 300 mL) and finally H2O (300 mL). The organic layer
was dried (MgSO4), filtered, and concentrated under reduced pressure to
Experimental Section
General methods: All solvents and reagents were used as received unless
stated otherwise. All solvents were dried prior to use according to standard
literature procedures.[49] For the spectroscopic and electrochemical experi-
ments, respectively, CH2Cl2 and MeCN were Merck Uvasol and Romil Hi-
Dry and were used as received. Reactions were monitored by thin-layer
chromatography on aluminum sheets coated with silica gel 60F (Merck
5554). The plates were inspected by UV light and, if required, developed in
iodine vapor. Column chromatography was carried out by using silica gel 60
5358
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Chem. Eur. J. 2003, 9, 5348 5360

Two-Component Superbundles
5348 5360
afford a crude product which was subjected to column chromatography
(silica gel; gradient elution CHCl3/iPrOH 25:1 ! 4:1) to yield the pure tris-
crown ether 1 as an off-white solid (1.03 g, 10%). M.p. 110 1128C;
1H NMR (400 MHz, CDCl3): d ˆ 3.87 3.94 (m, 36H), 4.03 4.08 (m, 12H),
4.15 4.21 (m, 12H), 4.39 4.42 (m, 12H), 6.85 (m, 12H), 7.80 (s, 6H);
13C NMR (100 MHz, CDCL3): d ˆ 69.3, 69.8, 70.0, 71.3, 71.3, 107.4, 114.1,
200, 5 52; f) S. J. Cantrill, A. R. Pease, J. F. Stoddart, J. Chem. Soc.
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‡
121.4, 123.8, 148.7, 148.9; FAB-MS: m/z: 1338 [M ]; HRMS (MALDI-
TOF): calcd for C72H90O24Na: 1361.5720; found: 1361.5927.
Tris-ammonium salt [3-H3][PF6]3: A mixture of 1,3,5-tris(4-formylphenyl)-
benzene[26] (4) (100 mg, 0.26 mmol) and p-methylbenzylamine (93 mg,
0.77 mmol) in PhMe (40 mL) was heated under reflux overnight and the
H2O was collected in a Dean Stark apparatus. The solution was allowed to
cool down to room temperature and the solvent was evaporated off under
reduced pressure to give 1,3,5-tris[(4-tolyliminomethyl)phenyl]benzene as
an off-white solid (170 mg, 95%). 1H NMR (200 MHz, CDCl3): d ˆ 2.36 (s,
9H), 4.84 (s, 6H), 7.16 7.30 (m, 12H), 7.76 (d, J ˆ 8 Hz, 6H), 7.86 (d, J ˆ
8 Hz, 6H), 7.93 (s, 3H), 8.45 (s, 3H). This solid was dissolved in dry THF
(5 mL) and dry MeOH (5 mL). After the portionwise addition of NaBH4,
the reaction mixture was left to stir overnight. Aqueous 5n HCl was added
to the reaction mixture until the pH was just less than 1. This mixture was
concentrated in vacuo and the residue was partitioned between aqueous 2n
NaOH solution (40 mL) and CH2Cl2 (40 mL). The aqueous layer was then
extracted further with CH2Cl2 (30 mL). The combined organic extracts
were washed with H2O (30 mL) and then dried (MgSO4). Filtration,
followed by evaporation, gave, 1,3,5-tris[(4-tolylaminoethyl)phenyl]ben-
zene (3) (134 mg, 79%) as a clear oil. 1H NMR (200 MHz, CDCl3): d ˆ 2.35
(s, 9H), 3.82 (s, 6H), 3.86 (s, 6H), 7.15 (d, J ˆ 8 Hz, 6H), 7.27 (d, J ˆ 8 Hz,
6H), 7.45 (d, J ˆ 8 Hz, 6H), 7.65 (d, J ˆ 8 Hz, 6H), 7.74 (s, 3H). This oil was
heated overnight under reflux in 12n HCl. On cooling, the reaction mixture
was filtered and the residue washed with H2O (20 mL) and Me2CO (5 mL).
The resulting white solid was dissolved in hot H2O (80 mL) and saturated
aqueous NH4PF6 solution was added. The resulting suspension was
extracted with MeNO2 (2 Â 50 mL). The combined extracts were washed
with H2O (2 Â 100 mL), dried (MgSO4), filtered and the solvent was
removed under reduced pressure to give the trisammonium salt [3-
H3][PF6]3 as an off-white solid (187 mg, 86%). M.p. 1108C (decomp);
1H NMR (400 MHz, CD3CN):d ˆ 2.36 (s, 9H), 4.23 (m, 6H), 4.29 (m, 6H),
7.12 (br s, 6H), 7.28 (d, J ˆ 8 Hz, 6H), 7.36 (d, J ˆ 8 Hz, 6H), 7.57 (d, J ˆ
8 Hz, 6H), 7.89 (d, J ˆ 8 Hz, 6H), 7.95 (s, 3H); 13C NMR (100 MHz,
CD3CN): d ˆ 20.3, 51.0, 51.4, 125.3, 125.3, 127.9, 129.7, 130.0, 130.2, 130.8,
¬
¬
b) P. R. Ashton, R. Ballardini, V. Balzani, M. Gomez-Lopez, S. E.
Lawrence, M. V. MartÌnez-DÌaz, M. Montalti, A. Piersanti, L. Prodi,
J. F. Stoddart, D. J. Williams, J. Am. Chem. Soc. 1997, 119, 10641
10651; c) M. V. MartÌnez-DÌaz, N. Spencer, J. F. Stoddart, Angew.
Chem. 1997, 109, 1991 1994; Angew. Chem. Int. Ed. Engl. 1997, 36,
1904 1907; d) P. R. Ashton, R. Ballardini, V. Balzani, M. C. T. Fyfe,
M. T. Gandolfi, M. V. MartÌnez-DÌaz, M. Morosini, C. Schiavo, K.
Shibata, J. F. Stoddart, A. J. P. White, D. J. Williams, Chem. Eur. J.
1998, 4, 2332 2341; e) P. R. Ashton, R. Ballardini, V. Balzani, I.
‡
140.1, 141.5, 141.6; FAB-MS: m/z: 852 [M À PF6 À HPF6] .
Acknowledgement
This research was supported by the National Science Foundation (CHE-
9910199 and CHE-9974928) in the United States and MURST (Supra-
molecular Devices Project) in Italy. The European Union is gratefully
acknowledged for a Marie-Curie Fellowship (To M.C.-L.) and for support
under the auspices of the Molecular-Level Devices and Machines Network
(HPRN-CT-2000 00029). We thank Dr. R. Ballardini and Professors M. T.
Gandolfi and M. Venturi for helpful discussions.
¬
¬
Baxter, A. Credi, M. C. T. Fyfe, M. T. Gandolfi, M. Gomez-Lopez,
M. V. MartÌnez-DÌaz, A. Piersanti, N. Spencer, J. F. Stoddart, M.
Venturi, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 1998, 120,
11932 11942; f) E. Ishow, A. Credi, V. Balzani, F. Spadola, L.
Mandolini, Chem. Eur. J. 1999, 5, 984 989; g) J. Cao, M. C. T. Fyfe,
J. F. Stoddart, G. R. L. Cousins, P. T. Glink, J. Org. Chem. 2000, 65,
1937 1946; h) P. R. Ashton, V. Baldoni, V. Balzani, A. Credi,
H. D. A. Hoffmann, M.-V. MartÌnez-DÌaz, F. M. Raymo, J. F. Stod-
dart, M. Venturi, Chem. Eur. J. 2001, 7, 3482 3493; i) R. Ballardini, V.
¬
Balzani, M. Clemente-Leon, A. Credi, M. T. Gandolfi, E. Ishow, J.
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5359

V. Balzani, A. Credi, J. F. Stoddart, D. J. Williams et al.
FULL PAPER
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MeCN (99:1), as well as in neat CH2Cl2. The Ka values obtained,
¬
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namely 6 Â 105 and 9 Â 105 LmolÀ1
, respectively, show that the
association constant in CH2Cl2/MeCN (99:1) is large and indeed very
close to that measured in neat CH2Cl2. Therefore, for the sake of
simplicity, the mixed CH2Cl2/MeCN (99:1) solvent will be referred to
hereafter as CH2Cl2.
[35] I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic
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[36] Since light emitted in this spectral region comes exclusively from the
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hydrogen bond acceptor. See C. Reichardt, Solvents and Solvent
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¬
Gomez-Kaifer, Supramolecular Electrochemistry, Wiley-VCH, Wein-
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[41] Chemically irreversible oxidation processes are known to occur in
aromatic ethers. See, e.g., G. W. Morrow, in Organic Electrochemistry
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[42] The dissociation is not a result of diluting the sample which was
diluted over the same concentration range with CDCl3/CD3CN (1:1).
No change was observed in 1H NMR spectra recorded for increasingly
dilute solutions.
[43] The doubly-threaded complex illustrated in Figure 13 has averaged Cs
symmetry with an unbound arm hovering over the free crown ether in
what might be described as a syn co-conformation which has the virtue
of being stabilized by [p p] stacking interactions. It is possible to
envisage another doubly-threaded complex which also has averaged
Cs symmetry but in which the unbound arm is oriented anti in a co-
conformational sense from the free crown ether. This complex is likely
to be more stable than the singly-threaded one. The possibility of
intermediate complexes in which the stoichiometries between 1 and
[2-H3]3‡ are other than 1:1 is a very real one as well, particularly at
higher absolute concentrations of the host and guest. Unraveling the
ins and outs of this highly dynamic supramolecular system offers a
veritable maze for the contemporary physical organic chemist.
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dibenzylammonium hexafluorophosphate and dibenzo-[24]crown-8 in
CDCl3/CD3CN (1:1) is in slow exchange on the 1H NMR timescale
with its free components at room temperature. See: P. R. Ashton,
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[45] The possibility that NBu3 is not
a
sufficiently strong base to
‡
deprotonate the three NH2 ion centers was also discounted by
1H NMR spectroscopy. In the 1H NMR spectrum of [2-H3][PF6]3 in
CD3COCD3, the resonances of the benzylic methylene protons next to
‡
the NH2 ion centers move upfield from d ˆ 4.71 and 4.69 to d ˆ 3.90
and 3.88, respectively, on the addition of 3.1 equivalents of NBu3,
confirming complete deprotonation of the [2-H3]3‡ trication to give
the trisamine 2.
[46] F. Kotzyba-Hibert, J.-M. Lehn, K. Saigo, J. Am. Chem. Soc. 1981, 103,
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[47] The kinetics and thermodynamics associated with this kind of
multivalent recognition site are under active investigation in our
laboratory. The results of these investigations will be reported in the
fulness of time.
[48] Supramolecular Polymers (Ed.: A. Ciferri), Marcel Dekker, New
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[32] S. Vidal, D. G. Amirsakis, T. Chang, A. R. Williams, J. F. Stoddart,
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[33] The crystallographic data for [1 ꢀ 2-H3][PF6]3 were deposited with the
original communication (see ref. [24]) and have the CCDC compound
code QIZMIV. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (‡44)1223-336033; or deposit@ccdc.cam.uk).
[34] The tris-ammonium salt [2-H3][PF6]3 is insoluble in neat CH2Cl2. The
salt was initially dissolved in the minimum amount of MeCN and then
this solution was added to CH2Cl2. The final composition of the
solution was CH2Cl2/MeCN (99:1 v/v). In order to explore the effect of
the presence of 1% MeCN in the CH2Cl2 solvent in the present
[50] A. Credi, L Prodi, Spectrochim. Acta A 1998, 54, 159 170.
Received: March 24, 2003 [F4979]
5360
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chem. Eur. J. 2003, 9, 5348 5360