Ru(II)-polypyridine complexes covalently linked to electron acceptors as wires for light-driven pseudorotaxane-type molecular machines

Article abstract of DOI:10.1002/(SICI)1521-3765(19981204)4:12<2413::AID-CHEM2413>3.0.CO;2-A

An investigation has been performed on the design of light-driven, pseudorotaxane-type, mechanical molecular machines based on wires made up of an electron-transfer photosensitizer covalently linked to an electron acceptor. Compounds (2,2'-bipyridine)₂Ru(2,2'-bipyridine-5-(CH₂)-1-(4,4'- bipyridinium)-1'-CH₂-R)⁴⁺ (1⁴⁺), (4,4'-(Me)₂-2,2'-bipyridine)₂Ru(2,2'- bipyridine-5-(CH₂)₄-1-(4,4'-bipyridinium)1'-CH₂-Me)⁴⁺ (2⁴⁺), and (2,2':6',2''-terpyridine)Ru(2,2'.6',2''-terpyridine-4'-phenylene-2-(2,7- diazapyrenium)-7-CH₂-R)⁴⁺ (3⁴⁺), where R = -C₆H₄-(O-CH₂-CH₂)₂-O- Ph) have been prepared and their photochemical and photophysical processes have been investigated in butyronitrile fluid solution (room temperature) and rigid matrix (77 K). At room temperature the triplet metal-to-ligand charge- transfer (³MLCT) excited state of the Ru-based unit of 1⁴⁺ is quenched by a very fast (k(q) > 5 x 10⁹ s^-1) electron-transfer process. For 2⁴⁺, where the Ru-based and electron-acceptor units are separated by four methylene groups, the value of the quenching constant is 6.2 x 10⁸ s^-1. In 3⁴⁺, the potentially fluorescent S₁ excited state of the diazapyrenium unit is quenched by the Ru-based moiety with a rate constant ≥1 x 10¹¹ s^{- 1}. In rigid matrix at 77 K, the ³MLCT excited state of the Ru-based moiety is not quenched by the bipyridinium or diazapyrenium moiety, whereas both the fluorescence and phosphorescence of the diazapyrenium moiety of 3⁴⁺ are completely quenched by the MLCT levels of the Ru-based moiety through energy transfer. Excitation spectra of the Ru-based emission show that, in a rigid matrix at 77 K, the excitation of the bipyridinium moiety leads to population of the ³MLCT excited state of the Ru-based moiety. The above wires and a crown ether (1/5DN38C10) containing two 1,5-dioxynaphthalene electron-donor units self-assemble to give pseudorotaxane systems. Light-induced dethreading of a pseudorotaxane has been achieved and valuable information has been gathered concerning the design of more efficient systems. A spin-off of these studies has been the design of pseudorotaxanes in which the dethreading/rethreading process can be controlled by chemical stimuli.

Full text of DOI:10.1002/(SICI)1521-3765(19981204)4:12<2413::AID-CHEM2413>3.0.CO;2-A

FULL PAPER
RuII-Polypyridine Complexes Covalently Linked to Electron Acceptors as
Wires for Light-Driven Pseudorotaxane-Type Molecular Machines**
Peter R. Ashton, Roberto Ballardini,* Vincenzo Balzani,* Edwin C. Constable,
Alberto Credi, Oldrich Kocian, Steven J. Langford, Jon A. Preece, Luca Prodi,
Emma R. Schofield, Neil Spencer, J. Fraser Stoddart,* and Sabine Wenger
Abstract: An investigation has been
performed on the design of light-driven,
pseudorotaxane-type, mechanical mo-
lecular machines based on wires made
up of an electron-transfer photo-
sensitizer covalently linked to an elec-
tron acceptor. Compounds (2,2'-bipyri-
dine)2Ru(2,2'-bipyridine-5-(CH2)-1-
(4,4'-bipyridinium)-1'-CH2-R)4‡ (14‡),
(4,4'-(Me)2-2,2'-bipyridine)2Ru(2,2'-bi-
pyridine-5-(CH2)4-1-(4,4'-bipyridinium)-
1'-CH2-Me)4‡ (24‡), and (2,2':6',2''-
terpyridine)Ru(2,2':6',2''-terpyridine-4'-
phenylene-2-(2,7-diazapyrenium)-
(3MLCT) excited state of the Ru-based
quenched by the MLCT levels of the
Ru-based moiety through energy trans-
fer. Excitation spectra of the Ru-based
emission show that, in a rigid matrix at
77 K, the excitation of the bipyridinium
moiety leads to population of the
3MLCT excited state of the Ru-based
moiety. The above wires and a crown
ether (1/5DN38C10) containing two 1,5-
dioxynaphthalene electron-donor units
self-assemble to give pseudorotaxane
systems. Light-induced dethreading of
a pseudorotaxane has been achieved
and valuable information has been gath-
ered concerning the design of more
efficient systems. A spin-off of these
studies has been the design of pseudor-
otaxanes in which the dethreading/re-
threading process can be controlled by
chemical stimuli.
unit of 14‡ is quenched by a very fast
1
(kq > 5 Â 109 s ) electron-transfer proc-
ess. For 24‡, where the Ru-based and
electron-acceptor units are separated by
four methylene groups, the value of the
1
quenching constant is 6.2 Â 108 s . In
34‡, the potentially fluorescent S1 excited
state of the diazapyrenium unit is
quenched by the Ru-based moiety with
1
a rate constant ꢀ1 Â 1011 s . In rigid
matrix at 77 K, the 3MLCT excited state
of the Ru-based moiety is not quenched
by the bipyridinium or diazapyrenium
moiety, whereas both the fluorescence
and phosphorescence of the diazapyre-
nium moiety of 34‡ are completely
7-CH2-R)4‡
(34‡),
where
R ˆ
C6H4 (O CH2 CH2)2 O Ph) have
been prepared and their photochemical
and photophysical processes have been
investigated in butyronitrile fluid solu-
tion (room temperature) and rigid ma-
trix (77 K). At room temperature the
triplet metal-to-ligand charge-transfer
Keywords: charge transfer ´ energy
transfer ´ molecular devices ´ self-
assembly ´ rotaxanes
Since such interactions can be switched on and off by redox
processes, it occurred to us to use photoinduced electron-
transfer reactions to bring about the mechanical displace-
ments.[1±3]
Because of a unique combination of chemical stability,
redox properties, luminescence intensity, and excited-state
lifetime, the complexes of the polypyridine-type family
Introduction
Currently, we are engaged in the design and construction of
simple mechanical molecular machines[1±5] based on pseudor-
otaxanes, rotaxanes, and catenanes[6] whose structure is
essentially controlled by p-electron-deficient and p-electron-
rich aromatic-stacking and hydrogen-bonding interactions.[7]
[*] Prof. V. Balzani, Dr. A. Credi, Dr. L. Prodi
Dr. O. Kocian, Dr. S. J. Langford, Dr. J. A. Preece, Dr. N. Spencer,
Dr. S. Wenger, P. R. Ashton, E. R. Schofield
School of Chemistry, University of Birmingham
Edgbaston, Birmingham B15 2TT (UK)
Fax : (‡ 44)121-414-3531
Prof. E. C. Constable
Á
Dipartimento di Chimica G. Ciamician, Universita di Bologna
Via Selmi 2, I-40126 Bologna (Italy)
Fax : (‡ 39)051-259456
E-mail: vbalzani@ciam.unibo.it
Dr. R. Ballardini
Istituto FRAE-CNR, Via Gobetti 101, I-40129 Bologna (Italy)
Fax : (‡ 39)051-6399844
Institut für Anorganishe Chemie, Universität Basel
CH-4056 Basel (Switzerland)
Fax : (‡ 41)61-267-1015
Prof. J. F. Stoddart
Department of Chemistry and Biochemistry
University of California at Los Angeles
405 Hilgard Avenue, Los Angeles CA 90095 (USA)
Fax : (‡ 1)310-206-1843
[**] Supporting information for this contribution is available on the WWW
under http://www.wiley-vch.de/home/chemistry/
Chem. Eur. J. 1998, 4, No. 12
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R. Ballardini, V. Balzani, J. F. Stoddart et al.
FULL PAPER
(prototype: [Ru(bpy)3]2‡, in which bpy ˆ 2,2'-bipyridine) have
been used extensively during the past 20 years as photo-
sensitizers in a variety of intermolecular electron-transfer
processes.[8±11] More recently, these compounds have also been
used to obtain photoinduced energy- or electron-transfer
processes in suitably-designed supramolecular (multicompo-
nent) systems.[12±14]
phane containing two 4,4'-bipyridinium electron-acceptor
units, we found that light excitation of the photosensitizer in
the presence of a sacrificial electron donor [e.g., triethanol-
amine (TEOA)] causes single-electron transfer to the elec-
tron-acceptor unit of the ring component (Figure 1a). This
Our first attempt[2, 3] to design a photochemically driven,
mechanical molecular machine was based on the use of an
external electron-transfer photosensitizer ([Ru(bpy)3]2‡ or
9-anthracenecarboxylic acid). Starting from the pseudorotax-
ane constituted by a wire containing the 1,5-dioxynaphthalene
electron-donor unit threaded through the cavity of a cyclo-
Abstract in Italian: Nellꢀambito degli studi intrapresi sulla
progettazione e sintesi di macchine molecolari fotochimiche
basate su pseudorotassani, sono state preparate tre molecole
filiformi costituite da un sensibilizzatore di trasferimento
elettronico [un complesso polipiridinico di RuII] legato cova-
lentemente ad un gruppo elettron accettore (4,4'-dipiridinio o
2,7-diazapirenio). Le proprietà fotofisiche e fotochimiche di
queste tre specie e di opportuni composti modello sono state
esaminate in butirronitrile a temperatura ambiente e in matrice
rigida a 77 K. A temperatura ambiente, lo stato eccitato di
tripletto MLCT del complesso di RuII nel composto (2,2'-
dipiridile)2Ru(2,2'-dipiridile-5-(CH2)-1-(4,4'-dipiridinio)-1'-C-
H2-R)4‡ [in cui R ˆ -C6H4(OCH2CH2)2OPh] viene disattivato
mediante un processo di trasferimento elettronico molto veloce
Figure 1. Schematic representation of the light-induced dethreading of a
pseudorotaxane by use of a) an external and b) an incorporated photo-
sensitizer. The photosensitizer P is an excited-state electron donor (e.g.,
[Ru(bpy)3]2‡ 42‡); Red is a sacrificial reductant (e.g., triethanolamine);
Prod are species originating from irreversible oxidation of Red; A and D
are electron-acceptor and electron-donor units, respectively. Rethreading
can be obtained by oxidation of the photoreduced electron-acceptor unit.
For more details, see text.
reduction decreases the donor± acceptor interaction between
the component parts of the pseudorotaxane, which is the
driving force for the self-assembly. As a consequence, the
thread component dethreads from the cavity of the cyclo-
phane. If oxygen is allowed to enter the irradiated solution,
which is stored in the dark, the reduced ring is reoxidized, the
donor± acceptor interaction restored, and the thread compo-
nent rethreads through the cyclophane once more.
Currently our attention is focused on the design of a
second-generation mechanical machine that is based upon a
pseudorotaxane in which the photosensitizer is incorporated
into a wire,[15] which in turn contains a p-electron-deficient
unit (which also acts as a single-electron acceptor), and the
ring component contains p-electron-rich moieties (Figure 1b).
To explore the feasibility of this project, we have synthesized
the wire-type systems 14‡, 24‡, and 34‡ shown in Figure 2a. As
the photosensitizer, we have chosen either i) [Ru(bpy)3]2‡
derivatives (42‡; Figure 2b), because of their well-defined
photophysical and redox properties,[9] or ii) [Ru(tpy)2]2‡ (52‡)
derivatives, which exhibit less favorable photophysical prop-
erties (very short excited-state lifetimes),[14] but offer several
synthetic and structural advantages for the construction of
wire-type systems.[14, 16, 17] As an electron acceptor, besides the
well-known 4,4'-bipyridinium unit 62‡,[18] we have used the 2,7-
diazapyrenium unit 72‡ because of its high association
constants with p-electron-rich crown ethers such as 8 and
9[4] and its interesting luminescence properties.[19]
1
(kq > 5 Â 109 s ). Per (4,4'-(Me)2-2,2'-dipiridile)2Ru(2,2'-dipi-
ridile-5-(CH2)4-1-(4,4'-dipiridinio)-1'-CH2-Me)4‡, in cui il
complesso di RuII e lꢀunità elettron accettrice sono separati da
quattro gruppi metilene, il valore della costante di spegnimento
1
›
6.2 Â 108 s . Nel composto (2,2':6',2''-terpiridile)Ru-
(2,2':6',2''-terpiridile-4'-fenilene-2-(2,7-diazapirenio)-7-CH2-
R)4‡ (R come sopra), lo stato eccitato S1 dellꢀunità diazapire-
nio, potenzialmente fluorescente, › disattivato dal complesso
terpiridinico di RuII (kq ꢀ 1 Â 1011 s ). In matrice rigida, lo
1
3
stato eccitato MLCT del complesso metallico non › influen-
zato dai gruppi dipiridinio o diazapirenio, mentre sia la
fluorescenza che la fosforescenza di (2,2':6',2''-terpiridile)-
Ru(2,2':6',2''-terpiridile-4'-fenilene-2-(2,7-diazapirenio)-7-C-
H2-R)4‡ sono completamente spente a causa di un processo di
trasferimento di energia ai livelli MLCT dellꢀunità contenente
RuII. Lo spettro di eccitazione dellꢀemissione dovuta al
complesso di RuII mostra che, a 77 K, si ha popolazione del
suo stato eccitato 3MLCT in seguito ad eccitazione del gruppo
dipiridinio. I composti filiformi esaminati si assemblano
spontaneamente con un etere corona (1/5DN38C10) contenen-
te unità elettron donatrici 1,5-diossinaftalene formando pseu-
dorotassani. Per uno di questi pseudorotassani › stato possibile
indurre lo sfilamento dei componenti molecolari mediante
irradiazione con luce visibile. Gli studi riportati si sono rivelati
di grande utilità per la progettazione di macchine molecolari
In this paper, we report the results of an investigation on the
luminescence and photoinduced intramolecular energy- and
electron-transfer processes that occur on excitation of the
systems shown in Figure 2a, both in a fluid solution at room
temperature and in a rigid matrix at 77 K. We have also
investigated the self-assembly of these wires with crown
ethers 8 and 9 containing two hydroquinone and two 1,5-
Ï
fotochimiche piu efficienti, nonchØ per la costruzione di
pseudorotassani nei quali i processi di sfilamento/infilamento
dei componenti si possono controllare mediante stimoli di tipo
chimico.
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Chem. Eur. J. 1998, 4, No. 12

Molecular Machines
2413±2422
the ring, iii) the system must be chemically stable under the
operating conditions, and iv) the threading/dethreading
process must result in an easily monitorable change of some
properties (the most useful ones are absorption spectra, and
luminescence spectra and lifetimes).
a)
O
O
O
N
N
N
N
N
+
+
RuII
N
N
The newly designed machine, shown in Figure 1b, differs
from the previous one[2, 3] (illustrated in Figure 1a) in several
respects. Besides having covalently incorporated the photo-
sensitizer into the wire, we have exchanged the position of the
p-electron-deficient and p-electron-rich aromatic units in the
pseudorotaxane structure, such that the p-electron-deficient
aromatic bipyridinium or diazapyrenium unit is now incorpo-
rated into the thread and the p-electron-rich aromatic units
into the macrocyclic component. A consequence of this
modification is that there is only one electron-acceptor
moietyÐthe organic viologen-like dication unitsÐwhich
equates with the single-electron transfer caused by photo-
excitation. Thus, the noncovalent bonding interactions be-
tween the viologen dication and the crown ether will be
switched off upon photoinduced electron transfer from the
[Ru(bpy)3]2‡ or [Ru(tpy)2]2‡ sensitizer to the viologen dica-
tion. The 4,4'-bipyridinium unit was chosen as an electron
acceptor because of its good reversible electrochemical
behavior[18] and its affinity for crown ethers such as 8 and 9.
Additionally, we have also explored the possibility of using
the 2,7-diazapyrenium unit since i) it binds crown ethers such
as 8 and 9 more strongly than the analogous bipyridinium unit
and ii) it exhibits a strong fluorescence.[4, 19] As the macro-
cyclic component for constructing the pseudorotaxane super-
structure, we have chosen crown ethers containing two p-
electron-rich units because such compounds i) bind strongly
to bipyridinium and diazapyrenium dications and ii) show a
strong fluorescence, particularly in the case of the naphtha-
lene-containing crown ether 9.[4]
The photochemical operation of the mechanical machines
shown in Figure 1 is based on the presence of a sacrificial
electron donor, which is capable of reducing the oxidized
photosensitizer by an intermolecular process more rapidly
than the intermolecular (Figure 1a) or intramolecular (Fig-
ure 1b) back-electron-transfer reaction from the reduced
organic cation to the oxidized photosensitizer (vide infra).
The most efficient sacrificial reductants are amines (especially
TEOA) or polycarboxylate anions (e.g., oxalate anions).[22]
Polycarboxylate anions are soluble only in water. Since crown
ethers and the three wire-type compounds (as their hexa-
fluorophosphate salts) shown in Figure 2a are only moder-
ately soluble in water, but are soluble in polar organic solvents
(such as MeCN and Me2CO), we employed the organic
soluble TEOA as the sacrificial electron donor.
14+
N
+
N
N
N
N
N
+
24+
RuII
N
N
N
+
N
O
O
O
N
N
N
+
N
RuII
N
N
34+
N
+
+
b)
+
N
N
N
N
N
N
N
N
RuII
N
RuII
N
N
N
N
N
N
N
+
72+
42+
52+
62+
O
O
O
O
O
O
O
O
O
O
O
O
9
8
O
O
O
O
O
O
O
O
Figure 2. Structure formulas of a) the novel wire-type systems, and b) the
reference compounds for the luminescent and redox-active units of the
wire-type systems, and of the crown ether used as a ring for obtaining
pseudorotaxanes.
dioxynaphthalene p-electron-rich units, respectively, to give
pseudorotaxane superstructures. We have subsequently at-
tempted to cause light- and chemically-induced dethreading
of such systems and have been successful in our attempts in
two cases. Wire-type compounds similar to those in Figure 2a
have recently been investigated for other purposes by
Mallouk et al.[20] and by Kelly and Rodgers.[21]
2. Synthesis: The synthesis of 1 ´ 4PF6 is illustrated in
Scheme 1. Phenol was alkylated with 2-chloro(ethoxy)ethanol
under basic conditions (K2CO3) to afford the alcohol 10,
which was tosylated to give 11. 4-Hydroxybenzyl alcohol was
then alkylated selectively with the tosylate 11, affording the
benzylic alcohol 12. Next, the benzylic alcohol 12 was
converted (SOCl2) into the benzylic chloride 13, which was
used without further purification to monoalkylate 4,4'-bipyr-
idine that, after counterion exchange (NH4PF6), yielded 14 ´
Results and Discussion
1. Strategy: The design of photochemically driven, mechanical
molecular machines of the types illustrated in Figure 1 is not a
trivial problem. It is clear that i) each component must exhibit
suitable spectroscopic and redox properties, ii) there must be
appropriate noncovalent bonding interactions between the
various components to allow the self-assembly of the wire and
Chem. Eur. J. 1998, 4, No. 12
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R. Ballardini, V. Balzani, J. F. Stoddart et al.
FULL PAPER
K2CO3
MeCN
TsCl
CH2Cl2
NEt3
O
OTs
O
O
OH
O
Cl
+
O
OH
OH
11
10
K2CO3
MeCN
HO
OH
O
N
N
SOCl2
1.
O
O
O
O
O
CH2Cl2
C6H5N
Cl
OH
2. NH4PF6
Me2CO / H2O
MeCN
12
13
1.
O
O
O
+
N
N
O
O
O
+
N
N
N
N
Br
15
.
16 2PF6
.
2PF6
14 PF6
2. NH4PF6
PF6
Me2CO / H2O
+
N
N
1.
[Ru(bpy)2(Me2CO)2][PF6]2
Me2CO
∆
.
1 4PF6
2. NH4PF6
Me2CO / H2O
Scheme 1.
4PF6. A second quaternization was then performed on the
pyridine moiety of 14 ´ PF6 with the benzylic bromide 15
affording 16 ´ 2PF6 after counterion exchange with (NH4PF6).
[Ru(bpy)2Cl2] was heated under reflux in Me2CO with an
equimolar proportion of AgPF6 to generate the pure solvo-
complex, [Ru(bpy)2(Me2CO)2][PF6]2. The precipitated AgCl
was removed by filtration and the resulting filtrate was added
slowly to a solution of 16 ´ 2PF6 in Me2CO, yielding the desired
tetracationic 14‡ as its tetrakis(hexafluorophosphate) salt in
68% yield.
The synthesis of 2 ´ 4PF6 is illustrated in Scheme 2. 4,4'-
Bipyridine was monoalkylated with EtI to afford the mono-
quat derivative 17 ´ PF6,[23] after counterion exchange. The
anion of 5-methyl-2,2'-bipyridine, formed by deprotonation
with BuLi, was alkylated with 1,3-dibromopropane[20a] to give
the bromide 18. The monoquat derivative 17 ´ PF6 was
quaternized with the alkylbromide 18 to afford, after counter-
ion exchange (NH4PF6), the dicationic salt 19 ´ 2PF6. [Ru-
(Me2bpy)2Cl2][24] was converted to its solvo-complex (Me2CO
and AgPF6), before being added slowly to a solution of 19 ´
2PF6 in Me2CO, giving the desired tetracationic salt 24‡ in
55% yield.
The synthesis of 3 ´ 4PF6 is illustrated in Scheme 3. [Ru-
(tpy)Cl3][25] was treated with 4'-(p-bromomethylphenyl)-
2,2':6',2''-terpyridine[26] to afford, after counterion exchange
(NH4PF6), the ruthenium complex 20 ´ 2PF6. 2,7-Diazapyr-
ene[27] was monoalkylated with 20 ´ 2PF6 that, after counterion
exchange (NH4PF6), yielded 21 ´ 3PF6. The monoquat deriv-
ative 21 ´ 3PF6 was quaternized with benzylic chloride 13
yielding the desired tetracationic salt 3 ´ 4PF6 in 49% yield.
excess
BuLi
THF
+
Br
Br
N
N
+
+
N
N
N
[Ru(Me bpy) (Me CO) ][PF ]
1.
Me2CO
∆
N
N
2
2
2
2
6 2
1.
CH3CH2I
MeCN
4
N
N
Br
18
.
2PF6
2 4PF6
PF6
2. NH4PF6
Me2CO / H2O
2. NH4PF6
Me2CO / H2O
N
+
4
.
17 PF6
N
N
.
19 2PF6
Scheme 2.
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Chem. Eur. J. 1998, 4, No. 12

Molecular Machines
2413±2422
.
20 2PF6
N
N
N
N
N
Br
RuII
N
3PF6
N
N
N
N
N
N
+
2PF6
MeCN
RuII
N
N
NH4PF6
Me2CO / H2O
+
N
.
21 3PF6
N
13
Cl
1.
O
O
O
MeCN
.
3 4PF6
2. NH4PF6
Me2CO / H2O
Scheme 3.
3. Pseudorotaxanes: When progressively more and more bis-
p-phenylene-34-crown-10 (BPP34C10; 8) or 1,5-dinaphtho-
38-crown-10 (1/5DN38C10; 9) is added to solutions of the
viologen dications such as 162‡ and 192‡ in MeCN, a deep
orange or purple color, respectively, appears as a result of the
charge-transfer interactions between the p-electron-rich ar-
omatic rings of the crown ether and the p-electron-deficient
viologen unit incorporated within the thread components.
Qualitative evidence for the pseudorotaxane formation was
obtained from mass spectrometry, in which it was evident that
there were peaks corresponding to the loss of one then two
hexafluorophosphate counterions from the pseudorotaxanes.
Table 1 lists the FABMS data for selective photoactive wires
and model viologen compounds, and their respective pseu-
dorotaxanes.
Further qualitative evidence for the formation of a
pseudorotaxanelike complexation was obtained from changes
in the chemical-shift values (Dd) in the 1H NMR spectra of the
photoactive wires before and after the addition of an
equimolar amount of the crown ethers. For example, the
aromatic protons of uncomplexed 8 resonate at d ˆ 6.75.
However, on complexation with 162‡, an upfield shift of 0.28
ppm is observed.
The association constants for several pseudorotaxane-type
structures obtained from electron-acceptor threads and
electron-donor rings (or vice versa) have been previously
reported.[2±6, 28] The values of the association constants be-
tween 24‡ and 9 in ethanol and MeCN, or between 34‡ and 9 in
1
ethanol, have been estimated to be 2 Â 104 m 1 and 7 Â 105 m ,
respectively, by fluorescence-quenching measurements (see
Experimental Section). More details will be given in Sec-
tion 6.
Table 1. LSIMS[a] data for the threads 16 ´ 2PF6, 1 ´ 4PF6 and 2 ´ 4PF6, and
their inclusion complexes[b] with the crown ethers 8 and 9.
4. Reference compounds: In order to understand the photo-
chemical and photophysical properties of a multicomponent
species, it is important to know the behavior of the isolated
components.[13] As far as the photosensitizer is concerned, for
both 14‡ and 24‡ we can take [Ru(bpy)3]2‡ (42‡) as a reference
compound since its properties are only slightly affected by
methyl substitution.[9] For 34‡, the reference compound is
[Ru(tpy)2]2‡ (52‡), whose properties are not strongly affected
by phenyl substitution in the 4'-tpy position.[14a] For the
electron-acceptor units, suitable reference compounds are the
1,1'-dimethyl-4,4'-bipyridinium dication (62‡) and the N,N'-
dimethyl-2,7-diazapyrenium dication (72‡).
‡
‡
M[c]
[M PF6]
[M 2PF6]
16 ´ 2PF6
[16 ´ 8][PF6]2
[16 ´ 9][PF6]2
887
1422
1522
741
1277
±
596
1132
±
1 ´ 4PF6
[1 ´ 8][PF6]4
[1 ´ 9][PF6]4
1590
2127
2227
1445
1982
±
1300
1836
±
2 ´ 4PF6
[2 ´ 8][PF6]4
[2 ´ 9][PF6]4
2000
±
±
±
±
1837
1937
1694
1794
[a] Liquid secondary ion mass spectometry (LSIMS) was carried out on a
VG-Zab Spec mass spectrometer (accelerating voltage, 8 kV; resolution,
2000). Spectra were recorded in the positive-ion mode at a scan speed of 5 s
per decade. Samples were dissolved in a small volume of 3-nitrobenzyl
alcohol. [b] Samples were prepared in MeCN (0.5 mL) with 1 equiv of
crown ether and 1 equiv of thread (3 ± 6 mmol). [c] The molecular weight of
the appropriate hexaflourophosphate salt.
5. Absorption spectra, luminescence properties, and photo-
induced processes of the wire-type compounds: Absorption
and luminescence data for the wire-type compounds 14‡ ± 34‡
Chem. Eur. J. 1998, 4, No. 12
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R. Ballardini, V. Balzani, J. F. Stoddart et al.
FULL PAPER
and for the reference compounds 42‡, 52‡, 62‡, and 72‡, all
represented in Figure 2, are collected in Table 2.
moderately intense band in the UV region. The absorption
spectrum of 14‡ is practically equal to the sum of the spectra of
the two component units. As shown in Table 2, 62‡ is not
luminescent, whereas 42‡ shows a relatively strong emission
Table 2a. Absorption and luminescence properties at 298K[a] of the wires
14‡, 24‡, and 34‡, and reference compounds.
3
from its lowest energy MLCT level. In butyronitrile fluid
solution at room temperature, the strong luminescence of the
Absorption[b]
lmax emax
[nm] [Lmol 1 cm
Fluorescence[c]
Phosphorescence[d]
lmax Irel
[nm] [ns]
Ru-based unit of 14‡ is quenched by a factor of at least 103.
lmax
t
Irel
t
1
The rate constant of the quenching process (kq > 5 Â 109 s )
1
]
[nm] [ns]
evaluated from the luminescence-intensity quenching [Eq. (6)
in the Experimental Section] is not inconsistent with the value
14‡
24‡
34‡
286 79000
451 11000
287 94000
457 14500
279 54000
308 65000
421[e] 14400[e]
482 20300
287 72000
451 12000
±
±
< 1
ꢁ 5
1
(kq ˆ 5.9  1010 s ) obtained by Mallouk, et al.[20] from pico-
615 1.6
second transient-spectroscopy experiments carried out
in MeCN solution on the very similar [(bpy)2Ru-
(Mebpy) CH2 viologen CH3]4‡ system. These researchers
have also shown that the quenching process is due, as
±
±
< 1
±
±
±
3
42‡
610 190 1000
629 0.25 < 0.1
expected, to electron transfer from the MLCT excited state
of the Ru-based unit to the 4,4'-bipyridinium unit and that the
back electron-transfer reaction is very fast (kq ˆ 4.0 Â
52‡[f] 270 46000
307 28000
1
1010 s ).
474 10400
260 19000
418[g] 15000[g]
62‡
72‡
In a rigid matrix at 77 K, the luminescence of the Ru-based
moiety of 14‡ is not quenched at all (Table 2). Under such
conditions, in fact, the electron-transfer quenching process is
most likely endoergonic because of the lack of solvent
repolarization (vide infra).[29] The excitation spectrum of the
Ru-based emission in a rigid matrix at 77 K shows that
excitation of the 4,4'-bipyridinium unit leads to emission from
the Ru-based unit. This observation indicates that an energy-
transfer process takes place from the bipyridinium to the Ru-
based unit, but the efficiency of the process is difficult to
evaluate because an intense absorption band of the Ru-based
component overlaps with the absorption band of the
bipyridinium unit (Figure 3).
428 9.7
1000
[a] Butyronitrile solution, unless otherwise noted. [b] MeCN solution.
[c] DAP-based. [d] Ru-based. [e] lmax and lowest energy feature of a
structured band of the DAP2‡ subunit. [f] Data from ref. [14a]. [g] lmax and
lowest energy feature of a structured band.
Table 2b. Luminescence properties at 77K[a] of the wires 14‡, 24‡, and 34‡
,
and reference compounds.
Fluorescence[b]
lmax [nm] t [ns]
Phosphorescence[b] Phosphorescence[c]
lmax [nm] t [ms] lmax [nm] t [ms]
14‡
600
597
632
580
607
6.6
5.6
12
6.1
10.6
24‡
In 24‡, the presence of four methylene groups between the
Ru-based unit and the 4,4'-bipyridinium unit slows down the
rate of the photoinduced- (forward) and back-electron-trans-
fer processes. In a fluid solution at room temperature the
luminescence of the Ru-based unit of this system can be
clearly seen (Table 2). Its intensity, however, is strongly
quenched compared with that of the reference compound
42‡ and its lifetime is 1.6 nsÐ120 times shorter than that of 42‡.
By use of Equation (5) [see Experimental Section], a rate
34‡
±
±
±
±
42‡
52‡[d]
72‡
424
10.7
586
900
[a] Butyronitrile rigid matrix. [b] DAP-based. [c] Ru-based. [d] Data from
ref. [14a].
The absorption spectra at room temperature of 14‡, and of
the 42‡ and 62‡ reference compounds are shown in Figure 3.
The RuII complex 42‡ shows the well-known, very intense
ligand-centered (LC) bands in the UV region and moderately
intense metal-to-ligand charge-transfer (MLCT) bands in the
visible region. The viologen dication 62‡ shows only a
1
constant of 6.2 Â 108 s is obtained from the excited-state
1
quenching, which compares well with the value of 6.5 Â 108 s
obtained by Mallouk et al.[20] for an analogous compound not
carrying methyl-substituents on the bpy ligands. In a rigid
matrix at 77 K, the luminescence of the Ru-based moiety of
24‡ is not quenched (Table 2) and excitation of the 4,4'-
bipyridinium unit leads to emission of the Ru-based unit, as
described for the system 14‡.
The absorption spectra of 34‡, and of the 52‡ and 72‡
reference compounds at room temperature in MeCN solution
are shown in Figure 4. [Ru(tpy)2]2‡ exhibits very intense LC
bands in the UV region and moderately intense, broad MLCT
bands in the visible, whereas 72‡ shows, besides a very intense
band around 260 nm, two moderately intense, structured
bands in the 300 ± 450 nm region. The absorption spectrum of
34‡ almost matches the sum of the spectra of 52‡ and 72‡. The
lack of exact matching can be attributed to the fact that the
reference compounds are slightly different from the two
Figure 3. Absorption spectra in MeCN solution at room temperature of
14‡ (Ð), 42‡ (- - -), and 62‡ ( ´´´ ).
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2413±2422
Figure 4. Absorption spectra in MeCN solution at room temperature of
34‡ (Ð), 52‡ (- - - ), and 72‡ ( ´´´ ).
chromophoric units of the wire-type compound. The emission
spectra of the three compounds in butyronitrile rigid matrix
are shown in Figure 5. The diazapyrenium dication 72‡ shows
both fluorescence and phosphorescence. The 3MLCT band of
Figure 6. Energy-level diagram for 34‡ in a simplified representation in
which P (photosensitizer) and A (electron acceptor) stand for the two
component units. The most relevant absorption (full lines), emission
(dashed lines), and radiationless (wavy lines) processes are indicated. The
‡
horizontal dotted line represents an estimation of the P
A energy level
in rigid matrix at 77 K. For more details, see text.
1
Section], a lower limiting value of 1 Â 1011
s
is obtained for
the rate constant of this quenching reaction. As illustrated by
the schematic energy-level diagram of Figure 6, quenching of
the fluorescence of the diazapyrenium-based unit can occur
by energy or electron transfer.
Figure 5. Emission spectra in butyronitrile rigid matrix at 77 K of 34‡ (Ð),
52‡ (- - -), and 72‡ (fluorescence, ´´´ ; phosphorescence, - ´ - ´ - ´ ).
In a rigid matrix at 77 K, the luminescence of the Ru-based
unit of 34‡ is not affected by the presence of the diazapyre-
nium-based unit, confirming that the electron-transfer
52‡ lies at slightly lower energy than the phosphorescence
band of 72‡. One can see that in the emission spectrum of 34‡
at 77 K only one band is present, which is very similar to that
of the reference compound 52‡.
3
quenching process of the MLCT level cannot occur under
‡
such conditions because of the destabilization of the P
A
species (Figure 6). It is interesting to note, however, that the
quenching of the S1 excited state of the diazapyrenium-based
unit occurs very rapidly also in rigid matrix at 77 K (kq ꢀ 1 Â
The photoinduced processes taking place in 34‡ can be
discussed with reference to the energy-level diagram shown in
1
1011 s ). Although the exoergonicity of the electron-transfer
3
Figure 6. The energy values for the MLCT, S1, and T1 levels
quenching process on excitation of S1 is so large that it is
possible that it could take place even in rigid matrix,[29] the
strict similarity between absorption and excitation spectra
indicates that the quenching process takes place by energy
transfer. It should also be noticed that at 77 K the T1 excited
state of the diazapyrenium-based unit is quenched by the
have been obtained from the highest energy feature of the
emission spectra at 77 K; as a rough estimation of the energy
of 1MLCT, we have taken the maximum of the corresponding
‡
absorption band. The energy of the P
A charge-transfer
level is assumed to be that obtained from the reduction
potentials of [Ru(tpy)2]3‡ (‡1.30 V, vs SCE)[14a] and 72‡
3
lower lying MLCT level of the Ru-based unit (Table 2).
(
0.44 V, vs Ag/AgCl)[27] species. As mentioned when
discussing the previous wire-type compounds, one can expect
6. Photoinduced dethreading of the pseudorotaxane struc-
tures
‡
that the P
A level to be somewhat destabilized in rigid
matrix at 77 K because of the lack of solvent repolarization.[29]
As schematically depicted in Figure 6, at room temperature
excitation of the Ru-based unit (P) leads to formation of its
3MLCT level, which is expected to transfer an electron to the
6.1. Strategy: The photoinduced dethreading of a pseudoro-
taxane as illustrated in Figure 1b can only occur if light
excitation disrupts the donor± acceptor interaction. As al-
ready described, light excitation of wires 14‡ and 24‡ causes an
electron-transfer process with oxidation of the photosensitizer
and reduction of the electron acceptor (in the following
equations, the photosensitizer and electron-acceptor moieties
of the wires will be schematically indicated by Ru2‡ and A2‡,
respectively).
‡
DAP-based unit (A) to give the P A species. Although this
process is expected to be relatively fast, its occurrence cannot
be proved by luminescence measurements, since the isolated
Ru-based unit (i.e., the reference compound 52‡) is already
substantially nonemissive (Table 2). The efficiency of the
coupling of the two units (P and A) is shown by the quenching
of the strong fluorescence of the diazapyrenium-based unit
(Table 2 and Figure 5). From Equation (6) [see Experimental
The photoinduced electron-transfer process [Eqs. (1) and
(2)] is followed by a very fast back-electron-transfer reaction
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R. Ballardini, V. Balzani, J. F. Stoddart et al.
FULL PAPER
[Eq. (3)]. Therefore, the occurence of the photoinduced
electron-transfer process in the wire of a pseudorotaxane
Ru2‡ A2‡ ‡ hn !*Ru2‡ A2‡
(1)
(2)
(3)
‡
*Ru2‡ A2‡ !Ru3‡
A
‡
Ru3‡
A
!Ru2‡ A2‡
system cannot cause dethreading of the crown ether, since this
process, requiring extensive nuclear motions, is much slower.
Dethreading can only occur if the photoinduced process leads
to a permanent reduction of the electron acceptor. This result
can be obtained when a suitable sacrifical electron donor
(Red) is present in the solution. In such a case, a fast reaction
between the sacrifical electron donor and the oxidized
photosensitizer unit of the wire [Eq. (4)] can compete with
the intra-wire back-electron transfer reaction [Eq. (3)].
Figure 7. Spectral changes upon irradiation of an EtOH solution contain-
ing 3  10 5 m 24‡, 3  10 5 m 9, and 0.01m TEOA: a) initial spectrum;
b) spectrum at the end of the irradiation (complete reduction of the
electron-acceptor unit).
electron-acceptor unit. When the irradiation was stopped and
oxygen was allowed to enter the irradiated solution, the
spectral changes were reversed and the original absorption
spectrum was again obtained. Therefore, this system seemed
suitable to be used as a photoactive wire of a mechanical
machine of the type shown in Figure 1b.
‡
‡
Ru3‡
A
‡ Red !Ru2‡
A
‡ Products
(4)
Permanent reduction of the electron-acceptor unit of the
wire can thus be obtained and dethreading can follow
(Figure 1b). The most suitable sacrifical electron donors are
amines, particularly TEOA. Introduction of amines into our
systems, however, is problematic. The wire type compounds
14‡, 24‡, and 34‡ are stable in ethanol, acetonitrile, and
dichloromethane solution both in the dark and under light
excitation. In aprotic solvents, however, addition of amines
causes the decomposition of 14‡ and 24‡, as a result of a
nucleophilic attack by the amine nitrogen on the electrophilic
methylene groups adjacent to the bipyridinium units; this
breaks the covalent bond between the Ru-complex and the
electron acceptor. This decomposition process is clearly
shown by luminescence measurements. For 14‡ and 24‡ in
MeCN solution at room temperature, the emission of the
Ru2‡-based moieties are strongly quenched by the covalently-
linked bipyridinium electron acceptor (A2‡). Upon addition
of TEOA, while the absorption spectrum of the solution
remains unaffected, the Ru-based luminescence progressively
increases, indicating the separation of the two moieties; this
process, however, is not observed in ethanol. In the case of 34‡,
addition of TEOA to an MeCN solution causes completely
different spectral changes.[4] Upon subsequent addition of
CF3COOH the changes in the absorption spectrum are fully
reversed and no fluorescence of free diazapyrenium-based
units is observed. This behavior is consistent with the amine
forming an adduct with the diazapyrenium unit and not
chemically degrading 34‡.
Since 62‡ and crown ethers such as 8 and 9 self-assemble
into pseudorotaxane structures,[30] we prepared an EtOH
solution containing 3  10 5 m 24‡ and 3  10 5 m 9. It is
known[4] that when the crown ether 9 is complexed in the
rotaxane structure, the fluorescence of the 1,5-dioxynaphtha-
lene units is quenched because of the presence of low-energy
excited states due to the interaction with the electron-
acceptor 4,4'-bipyridinium unit of the wire. From the residual
fluorescence intensity of 9 (compared with that of a reference
solution containing only 9), we determined that in the two
component solution 30% of 9 was complexed with the 24‡
wire.[31] Light excitation of this system under the same
conditions as those described above for 24‡ alone led to the
permanent, almost quantitative reduction of the 4,4'-bipyr-
idinium units of the wire (as shown by the formation of the
characteristic absorption bands around 400 and 600 nm,
Figure 7). Under such conditions, we expected dethreading
of crown ether 9 from 24‡ as schematically indicated in
Figure 1b, with a corresponding increase in the fluorescence
intensity of the crown ether (lmax ˆ 345 nm). An increase in
the fluorescence intensity was indeed observed, indicating
that the photoinduced dethreading process does take place.
If oxygen is allowed to enter the irradiated solution, the
4,4'-bipyridinium unit is oxidized (as shown by the disappear-
ence of the visible absorption bands) and rethreading takes
place (as shown by the decrease of the fluorescence intensity
of the crown ether). Quantitatively, however, the increase in
the fluorescence intensity was about two-thirds that expected
for quantitative dethreading. In principle, this incomplete
recovery of the crown ether fluorescence intensity could be
due to a quenching of the excited state of 9 by the reduced
acceptor unit of 24‡. However, this possibility can be ruled out
in our system because i) it is implausible that 9 remains
assembled around the reduced acceptor, and ii) the short
excited-state lifetime of the 1,5-dioxynaphthalene units (7 ns
in MeCN) and the very low concentration of the wire do not
allow the occurrence of a bimolecular dynamic quenching
process.
6.2. Pseudorotaxane of the 24‡ wire: Since the back electron-
1
transfer reaction in 24‡ is not too fast (3.2  109 s in the
analogous Malloukꢁs compound),[20] and it is slower than in
14‡, it appeared feasible to obtain permanent reduction of the
4,4'-bipyridinium unit under continuous-irradiation condi-
tions in EtOH in the presence of TEOA as sacrificial electron
donor [Eq. (4)]. Irradiation with visible light (l > 350 nm) of a
3  10 5 m EtOH solution of 24‡ (as PF6 salt) containing 0.01m
TEOA caused noticeable spectral changes (Figure 7), indicat-
ing more than 90% reduction of the 4,4'-bipyridinium
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Molecular Machines
2413±2422
In an attempt to elucidate the lack of quantitative recovery
of the crown fluorescence, we performed experiments which
showed that 9 gives an adduct not only with 62‡, but also with
42‡, and that the fluorescence of 9 is quenched even in the case
of its adduct with 42‡. The values of the association constant of
the adducts of the crown ether 9 with 42‡ and 62‡ are about
driven pseudorotaxane-type mechanical machines based on
wires made of an electron-transfer photosensitizer covalently-
linked to an electron acceptor (Figure 1b) is not simple in
practice, because of incompatibilities between the various
components of the system. A wire like 24‡ is appropriate from
a photochemical viewpoint, but the association constant
between the 4,4'-bipyridinium electron-acceptor unit of the
wire and the electron-donor moieties of the ring 9 is not
sufficiently large to allow quantitative formation of pseudor-
otaxane species. Under such conditions, however, photo-
induced dethreading of the fraction of pseudorotaxane
species present in the solution has been demonstrated. When
the 4,4'-bipyridinium electron-acceptor unit was replaced by
the 2,7-diazapyrenium one, which is much more appropriate
from the viewpoint of association, we found that the system
was incompatible with the sacrificial electron donor used in
the photochemical cycle. As a spin-off of these studies, we
have discovered that one can design pseudorotaxane systems
in which the dethreading/rethreading process can be con-
trolled by chemical stimuli.
1
5000 and 15000m , respectively. Therefore, in the ethanol
solution containing 24‡ and 9, only about three-quarters of the
quenching of the fluorescence of 9 was due to the formation of
a pseudorotaxane-type structure (i.e., association of 9 with the
4,4'-bipyridinium moiety of 24‡), whereas the remaining
quenching was due to association of 9 with the photosensitizer
moiety. It follows that upon photoreduction of 4,4'-bipyridi-
nium moiety of 24‡ at most three-quarters of the quenching
can be recovered. This is in good agreement with the results
obtained (two-thirds recovery), if one considers the exper-
imental uncertainity of the fluorescence measurements.
Clearly, to obtain more precise results one should start with
a system exhibiting a much larger association constant
between the electron-donor ring and the electron-acceptor
unit of the wire.
We have also shown that, in the first two wire-type systems
3
displayed in Figure 2a, the MLCT excited state of the Ru-
6.3. Pseudorotaxane of the 34‡ wire: This system was
synthesized in an attempt to explore the possibility of using
the 2,7-diazapyrenium unit as an electron-acceptor compo-
nent of the wire. Such a unit is known to give a pseudorotax-
ane-type adduct with the crown ether 9, which exhibits a very
large association constant.[4] Titration of an MeCN solution of
34‡ (1.0  10 4 m) with 9 caused an increase in absorbance in
the visible region (e.g., at 490 nm) and a decrease in the
fluorescence intensity of 9 (compared with a reference
solution containing only 9), as expected when the formation
of the pseudorotaxane occurs.[4, 37] From the titration curve, a
based unit is quenched by very fast electron transfer to the
bipyridinium or diazapyrenium unit in fluid polar solvents,
whereas this process does not take place in rigid matrix at
77 K because of the lack of solvent repolarization. In 34‡, the
strong fluorescence of the DAP unit is strongly quenched
1
(kqx1 Â 1011 s ) by the Ru-based unit both in fluid solution
at room temperature and in rigid matrix at 77 K. In the latter
case, the excitation spectrum shows that the quenching
process takes place by an energy-transfer mechanism.
1
value of about 7 Â 105 m was obtained for the association
Experimental Section
constant, showing that in a solution containing 5  10 5 m 34‡
and 1  10 4 m 9 almost 100% of the 34‡ molecules are
complexed in a pseudorotaxane structure. This favorable
equilibrium was an excellent starting point. However, when
we added 0.01m TEOA to the solution to perform the
photochemical experiments, we observed a strong color
change (from orange to greenish) that was found to be due
to the formation of an adduct beween the diazapyrenium-unit
of the 34‡ wire and TEOA, as described previously. Other
amines (e.g., hexylamine and tributylamine) were found to
behave in a similar way. The adduct between the DAP unit
and amines is so strong that the addition of the amine causes
dethreading of the pseudorotaxane structure in the dark.
While this result prevented us from performing photochemi-
cally induced dethreading experiments, it opened the way to
the design of systems in which dethreading and rethreading
can be chemically controlled, as described in detail else-
where.[4, 32]
General methods: Chemicals were purchased from Aldrich and used as
received. Solvents were dried (MeCN and CH2Cl2 were distilled over CaH2
under nitrogen) and reagents were purified where necessary by literature
methods.[33] All the syntheses were carried out with no protection against
ambient light. [Ru(tpy)Cl3],[25] 4'-(p-bromomethylphenyl)-2,2':6',2''-terpyr-
idine,[26] 2,7-diazapyrene,[27] 5-methyl-2,2'-bipyridine,[34] and the crown
ethers 8[35] and 9[36] were prepared according to published literature
procedures. Thin-layer chromatography (TLC) was carried out on alumi-
nium sheets precoated with Merck 5735 Kieselgel 60F. Developed plates
were dried and examined under UV light. Column chromatography was
carried out with Kieselgel 60 (0.040 ± 0.063 mm mesh, Merck 9385). Melt-
ing points were determined with an Electrothermal 9200 melting point
apparatus and are uncorrected. Microanalyses were performed by the
University of Sheffield Microanalytical Service. Low-resolution mass
spectra (MS) were obtained from a Kratos Profile instrument operating
in electron impact (EIMS) or chemical ionization (CI) modes, while fast
atom bombardment (FABMS) were recorded on a Kratos MS80 spec-
trometer operating at 8 keV in the positive-ion mode at a scan speed of 10 s
per decade (with a krypton or xenon primary atom beam in conjunction
with a 3-nitrobenzyl alcohol (NOBA) matrix). 1H NMR spectra were
recorded on a Bruker AC300 (300 MHz) (with the deuterated solvent as
lock and residual solvent or tetramethylsilane as internal reference)
spectrometers. 13C NMR spectra were recorded on a Bruker AC300
(75 MHz) spectrometer with the JMOD pulse sequence. The description of
the experimental procedure and characterization by mass spectroscopy, 1H
and 13C NMR spectroscopy, and elemental analysis are given as Supporting
Information.
Conclusions
We have synthesized three wire-type compounds containing a
photosensitizer and an electron-acceptor unit. The investiga-
tions have shown that the theoretically ideal design of light-
The absorption spectra were recorded in MeCN (Merck UvasolTM
solution. The luminescence experiments were carried out in MeCN or
)
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R. Ballardini, V. Balzani, J. F. Stoddart et al.
FULL PAPER
butyronitrile (Fluka) solution at room temperature and in butyronitrile
rigid matrix at 77 K. The concentration of the solution was of the order of
10 5 m. Absorption and emission spectra were recorded with a Perkin ±
Elmer l6 spectrophotometer and a Perkin ± Elmer LS-50 spectrofluorim-
eter, respectively. Fluorescence lifetimes were measured with an Edin-
burgh 199 single-photon counting instrument. Photochemical experiments
were carried out with a tungsten halogen lamp (150 W, 24 V, l > 350 nm).
The photoinduced electron-transfer reactions, including the dethreading
experiments on the pseudorotaxane based on 24‡ in the presence of a
sacrificial electron donor, were performed in degassed EtOH (Merck
UvasolTM) solutions. The chemical dethreading of the pseudorotaxane
based on 34‡ was performed in MeCN solution. For more details on the
choice of the experimental conditions see the Results and Discussion
section. In the measurement of the luminescence intensity, a correction for
the fraction of absorbed light by the relevant chromophoric unit had to be
performed.[37] Experimental errors are as follows: luminescence intensity,
15%; luminescence lifetimes, 10%. Association constants were evaluated
by titration methods following changes in the absorption or emission
bands.[3, 4, 37] The values of association constants between bipyridinium-
Reddington, N. Spencer, J. F. Stoddart, C. Vicent, D. J. Williams, J.
Am. Chem. Soc. 1992, 114, 193.
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[11] K. Kalyanasundaram, Photochemistry of Polypyridine and Porphyrin
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[12] F. Scandola, M. T. Indelli, C. Chiorboli, C. A. Bignozzi, Top. Curr.
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8 and 9 are given as Supporting
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1
t
1
t
1
kq ˆ
kq ˆ
(5)
(6)
t0
ꢀ
ꢁ
I0
1
I
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Acknowledgments: This research was supported by the European Com-
munity (contract FMRX-CT96-0076), in Italy by MURSTand University of
Bologna (Funds for Selected Research Topics), and in the United Kingdom
by the Engineering and Physical Sciences Research Council (EPSRC). We
wish to acknowledge the Ramsay Memorial Trust for providing S.J.L. with a
Ramsay Memorial Postdoctoral Fellowship.
Received: May 11, 1998 [F1148]
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