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Supramolecular chemistry based on redox-
active components and cucurbit[n]urils
Samir Andersson
Doctoral Thesis
Stockholm 2010
Akademisk avhandling som med tillstånd av Kungl Tekniska Högskolan i Stockholm
framlägges till offentlig granskning för avläggande av doktorsexamen i kemi med
inriktning mot organisk kemi fredagen den 15 oktober kl 10.00 i sal F3, KTH,
Lindstedtsvägen 26, Stockholm. Avhandlingen försvaras på engelska. Opponent är
Professor Werner M. Nau, Jacobs University, Germany.
Samir Andersson, 2010: Supramolecular chemistry based on redox-active
components and cucurbit[n]urils, KTH Chemical Science and Engineering,
Royal Institute of Technology, SE-100 44 Stockholm, Sweden.
Abstract
This thesis describes the host-guest chemistry between Cucurbit[7]uril (CB[7])
and CB[8] and a series of guests including bispyridinium cations, phenols and
napthalenes. These guests are bound to ruthenium polypyridine complexes or
ruthenium based water oxidation catalysts (WOCs). The investigations are
based upon utilizing the covalently linked photosensitizer and the electronic
effects and chemical processes are investigated.
The first part involves a careful study of the exited state for the Ruthenium
viologen dyad. It was shown that the complex could form a long-lived exited
state radical as well as a light driven formation of rotaxane.
The second part are based upon a charge-transfer system, investigating the
possible formation of a trimer inside the CB[8] utilizing viologen and a
ruthenium-phenol dyad.
The third part involves a study of a ruthenium photosensitizer and two
different viologen units, where a chemical- or light-driven process is used to
create a psuedorotaxane and a rotaxane.
The fourth part concerns the interactions between a napthol-viologen triad with
CB[7,8] and -CD.
The fifth part contains the investigation of CB[8] in a non-aqueous
environment. The viologen-CB[8] chemistry was investigated and a novel
template based synthesis was designed.
The sixth part is an investigation of water oxidation catalysts and the allosteric
mechanism of such compounds when encapsulated in a CB host. It was found
that the host could regulate the activity of the catalyst, depending on
placement.
Keywords: Cucurbit[n]uril; Molecular motor; Light driven; Water oxidation,
Redox-active; Viologen.
Abbreviations
CB[n] Cucurbit[n]uril, where n=5, 6, 7, 8, 10
CT Charge-transfer
DHN Dihydroxynapthalene
WOC Water Oxidation Catalyst
OEC Oxygen Evolving Complex
Bpy 2,2'-bipyridine
Dba 2,2'-bipyridine-6,6'-dicarboxylic acid
MV2+
N,N-dimethyl-4,4’-bipyridinium (methylviologen)
V2+
N,N-R,R’-4,4-bipyridinium Viologen, R=alkyl chains
DMV2+
N,N-dimethyl-3,3’dimethyl-4,4’-bipyridinium
DV2+
N,N-R,R’-3,3’dimethyl-4,4’-bipyridinium, R=alkyl chains
DAP2+
Diazapyrenium
EC Electrochemistry
TEA Triethylamine
TEOA Triethanolamine
β-CD β-Cyclodextrin
TA Transient absorption
CV Cyclic voltammogram
LFTA Laser Flash Transient Absorption
Np Napthol
List of Publications
This thesis is based on the following papers, referred to in the text by their
Roman numerals I-IX:
I. The photoinduced long-lived charge-separated state of Ru(bpy)3-
methylviologen with cucurbit[8]uril in aqueous solution
Shiguo Sun, Rong Zhang, Samir Andersson, Jingxi Pan; Bjorn
Åkermark and Licheng Sun
Chem. Commun. 2006, 40, 4195-4197
II. Host-guest chemistry and light driven molecular lock of Ru(bpy) 3-
viologen with cucurbit[7-8]urils
Shiguo Sun, Rong Zhang, Samir Andersson, Jingxi Pan; Bjorn
Åkermark and Licheng Sun
J. Phys. Chem. B, 2007, 111(47), 13357-63
III. Unusual partner radical trimer formation in a host complex of
cucurbit[8]uril, ruthenium(II) tris-bipyridine linked phenol and
methyl viologen
Shiguo Sun, Samir Andersson, Rong Zhang and Licheng Sun
Chem. Commun. 2010, 46(3), 463-465
IV. Selective positioning of CB[8] on two linked viologens and
electrochemically driven movement of the host molecule
Samir Andersson, Dapeng Zou, Rong Zhang, Shiguo Sun, Bjorn
Åkermark and Licheng Sun
Eur. J. Org. Chem. 2009, 8, 1163-1172.
V. Light driven formation of a supramolecular system with three
CB[8]s locked between redox-active Ru(bpy)3 complexes
Samir Andersson, Dapeng Zou, Rong Zhang, Shiguo Sun, Bjorn
Åkermark and Licheng Sun
Org. Biom. Chem. 2009, 7(17), 3605-3609
VI. Selective binding of cucurbit[7]uril and β-Cyclodextrin with a
redox-active molecular triad Ru(bpy)3-MV2+
-napthol
Dapeng Zou, Samir Andersson, Rong Zhang, Shiguo Sun, Bjorn
Åkermark and Licheng Sun
Chem. Commun. 2007, 45, 4734-4736
VII. A host-induced intramolecular charge-transfer complex and light-
driven radical cation formation of a molecular triad with
cucurbit[8]uril
Dapeng Zou, Samir Andersson, Rong Zhang, Shiguo Sun, Bjorn
Åkermark and Licheng Sun
J. Org. Chem. 2008, 73(10), 3775-3783.
VIII. Isolated supramolecular Ru(bpy)3-viologen- Ru(bpy)3 complexes
with trapped CB[7,8] and photoinduced electron transfer study in
non-aqueous solution
Thitinun K. Monhaphol, Samir Andersson, and Licheng Sun
Manuscript
IX. An efficient water oxidation system based on a supramolecular
assembly of molecular catalyst and cucurbit[7]uril.
Samir Andersson and Licheng Sun
Manuscript
Paper not included in this thesis:
I. Synthesis, electrochemical, and photophysical studies of
multicomponent systems based on porphyrin and ruthenium(II)
polypyridine complexes
Xien Liu, Jianhui Liu, Jingxi Pan, Samir Andersson and Licheng Sun
Tetrahedron. 2007, 63(37), 9195-9205.
II. Chemical and Photochemical Water Oxidation Catalyzed by
Mononuclear Ruthenium Complexes with a Negatively Charged
Tridentate Ligand
Lele Duan, Yunhua Xu, Mikhail Gorlov, Lianpeng Tong, Samir
Andersson and Licheng Sun
Chem. Eur. J. 2010, 16(15), 4659-4668.
Table of Contents
Abstract
Abbreviations
List of publications
1. Introduction ..................................................................................... 1 1.1. Supramolecular chemistry ..................................................................... 1 1.2. Cucurbit[n]uril ........................................................................................ 2
1.2.1. Synthetic studies ....................................................................................... 3 1.2.2. Inclusion complexes.................................................................................. 4 1.2.3. Molecular machines .................................................................................. 5 1.2.4. Catalysis of reactions and other improvements ......................................... 6 1.2.5. Drug delivery ............................................................................................ 6
1.3. Our interest in the CB[n] supramolecular chemistry............................... 6 1.4. The aim of this thesis ............................................................................. 7 1.5. General synthetic considerations ........................................................... 7
1.5.1. Mixed-chelate ruthenium complexes ......................................................... 7 2. The Ruthenium-Viologen System ................................................... 8
2.1. Introduction ............................................................................................ 8 2.1.1. The CB[7-8]-viologen system .................................................................... 9 2.1.2. Aim of this study ....................................................................................... 9 2.1.3. General synthetic procedures ..................................................................10 2.1.4. Characterization of the host-guest system ...............................................10
2.2. Laser flash transient absorption measurements .................................. 11 2.2.1. Transient absorption measurements of 1 .................................................12 2.2.2. Transient absorption measurements of 1CB[8] ......................................12 2.2.3. Transient absorption measurements of 1CB[7] ......................................13 2.2.4. Light-driven formation of a [3]-rotaxane ....................................................14 2.2.5. Conclusions .............................................................................................15
3. The Ruthenium-Phenol System .................................................... 16 3.1. Introduction .......................................................................................... 16
3.1.1. The aim of this study ................................................................................16 3.1.2. Synthetic considerations. .........................................................................17 3.1.3. Investigation of the guests with CB[8] ......................................................17 3.1.4. Conclusion ...............................................................................................19
4. The Ruthenium-DV2+
-V2+
System ................................................. 20 4.1. Introduction .......................................................................................... 20
4.1.1. Aim of this study ......................................................................................20 4.1.2. Synthetic procedures ...............................................................................21 4.1.3. Host-guest analysis .................................................................................22
4.1.4. Molecular dyads 3 and 4 ..........................................................................24 4.1.5. Light driven formation of a [5]rotaxane .....................................................25 4.1.6. LFTA of triad 5 (unpublished works) ........................................................27 4.1.7. Conclusion ...............................................................................................28
5. The Ruthenium-Viologen-Napthol System ................................... 29 5.1. Introduction .......................................................................................... 29
5.1.1. The aim of this study ................................................................................29 5.1.2. Synthetic procedures ...............................................................................30 5.1.3. Interaction with cucurbit[7-8] and CD. ....................................................31 5.1.4. Photochemical investigation with the guest and CB[8] .............................33 5.1.5. Conclusions .............................................................................................35
6. The Ruthenium-Viologen-Ruthenium Rotaxane System
Encapsulating CB[7-8] ............................................................................. 36 6.1. Introduction .......................................................................................... 36
6.1.1. Aim of this study ......................................................................................36 6.1.2. Synthetic procedure and characterization of the host-guest interaction ....36 6.1.3. Non-aqueous photoinduced viologen radical formation ............................40 6.1.4. Conclusions .............................................................................................41
7. An Efficient Water Oxidation System Based on Supramolecular
Assembly of Molecular Catalyst and Cucurbit[7]uril. ............................... 42 7.1. Introduction .......................................................................................... 42
7.1.1. The aim of this study ................................................................................44 7.1.2. Design and synthetic procedures .............................................................44 7.1.3. Host-guest interaction ..............................................................................46 7.1.4. Catalytic water oxidation ..........................................................................46 7.1.5. Theoretical calculations ...........................................................................48 7.1.6. Conclusions .............................................................................................48
8. Concluding Remarks .................................................................... 49
Acknowledgements
Appendices
1
1. Introduction
As our daily life becomes more complex, the needs on the chemical industry to
produce both smaller and more advanced active compounds are increasing.
Examples of such systems are smaller transistors, more energy efficient
monitors.
As these compounds become more advanced the complexity of the synthesis
also increases. This increases costs as well as time to manufacture the
compounds needed for a specific process. Here is where the advantage of
supramolecular systems can be implemented, where the relative simplicity of
forming complexes via non-covalent interactions instead of chemical bonds, is
utilized.
This can, for example, be used in multicomponent redox-active systems,
commonly used to mimic and understand biological processes as well as
forming molecular machines. Modifying these complex systems via self-
assembly to supramolecular systems is a simple way of altering the properties
of the systems.
While practical industrial use within this field is still in its infancy,
supramolecular chemistry offers possibilities to improve not only our life but
also our understanding on how the nature works.
1.1. Supramolecular chemistry
Jean-Marie Lehn classified supramolecular chemistry as “chemistry of
molecular assemblies and of the intermolecular bond”.1 The cucurbit[n]uril
(CB[n]) is a macrocyclic host, defined as a molecular receptor that is arranged
as a ring, where the guest (or substrate) bind in the interior of the host. By
classification the CB[n] is generally a cavitand or endoreceptor compared to
exoreceptors or clathrands where the guest binds to the exterior of the host.
When small substrate such as an alkali metal is said to be encapsulated inside
the host, a simple host-guest complexation is formed. However with larger and
more complex systems, the guests can be composed of mechanically
interlocked components. Of these there are 3 basic systems defined, see
Scheme 1.2-3
Rotaxanes are defined as “interlocked dumbbell architecture”.
This means that the macrocyclic host is bound on a rod shaped structure with
2
two bulky groups covalently connected on each side preventing the separation
of the components. Catenanes are ring formations mechanically interlocked. In
either case the systems cannot be separated back to their components unless
chemical bonds are broken. Psuedorotaxanes on the other hand have no such
stoppers and the mechanical interaction is as such dependent on non-covalent
interactions, often a precursor to the rotaxanes and catenanes.
Scheme 1. Schematic representation of Psuedorotaxanes(left), rotaxanes (middle)
and catenane(right).
1.2. Cucurbit[n]uril
The name of the cucurbit[n]uril comes from the Latin word “cucurbitaceace”
which is the family name of the pumpkin plants. The CB[n] molecule has
received considerable attention and has been the focus of many different
reviews over the years.4 The molecule is cylinder formed and has a cavity
accessible from the exterior by two polar portals.
Figure 1. Picture of the electrostatic potential of the CB[7] left and molecular
structure right.
Similarly to cyclodextrin the interior is hydrophobic and can bind to
hydrocarbon substrates. Unlike the cyclodextrin, however, the host can bind to
protonated complexes due to the externally pointing carbonyls.
3
As our work began, there was already a broad interest towards this field of
study. The research can be said to be broadly aimed towards three different
fields; (1) synthetic studies; (2) new inclusion complexes and the chemically
induced movement of a guest molecule; (3) the host being used to catalyse
chemical processes and reactions.
1.2.1. Synthetic studies
The CB[n] is self-assembled from the acid-catalyzed condensation reaction of
glycoluril and formaldehyde where the cucurbit[6]uril (CB[6]) host was the
first synthesized in 1905 by Behrend and coworkers.5 They were, however, not
able to fully understand the chemical structure, and it was referred to as
“Behrends polymer”. Full characterization was not reported until 1981 by
Mock and coworkers.6 The first study was host-guest interactions, where the
recognition behavior of CB[6] with and variety of guests were explored and
the tight binding with ammonium ions was found in water.7
Day and Kim continued to explore synthetic procedures in milder conditions
forming new homologues n=5, 7 and 8.8 Isaacs and coworkers has continued
the work towards understanding the formation of the CB[n], showing that the
pathway is based upon a step growth cyclopolymerisation.9
The research into improving the solubility or make other adjustment to the
supramolecular properties of the host have also received a significant
attention,4i, 10
with several classes of derivatives including functionalized,
inverted,11
and nor-seco-CB[n].12
Kim and coworkers have for example shown
how addition of reactive groups has been used to attach the host onto solid
surface,10
which can be important for practical industrial use.
4
1.2.2. Inclusion complexes
The cavity of the CB[5-8] hosts varies from 4.4 to 8.8 Å in diameter and have,
compared to crown ethers (CE) and CD, a very rigid structure, see table 1. This
varying cavity and portal size is what gives the host its remarkable molecular
recognition properties.4k
Table 1. Cavity sizes of CB[5] to CB[8].
CB[5] CB[6] CB[7] CB[8]
Outer diameter a 13.1 14.4 16.0 17.5
Cavity (inner) b 4.4 5.8 7.3 8.8
Cavity (entrance) c 2.4 3.9 5.4 6.9
Height d 9.1 9.1 9.1 9.1
This difference in size has yielded a wide range of publications investigating
the properties of the homologues. CB[5] has a very small cavity and can
mainly bind at the cavity edge with substrates such as ammonia, lead and
alkali metal ions.13
The CB[6] compound with its slightly larger cavity has been extensively
explored to hold substrates such as diaminoalkanes and alkali group ions.14
Solvents like THF, benzene and xylene derivates have also been investigated,15
as well as thiols and dyes.16
It has also found to bind to transition metals17
as
well as lanthanides.18
CB[7] have a slightly larger cavity size compared to -CD, hence it can bind to
a much wider range of guests compared to the smaller CB[5-6]. These include
aromatic compounds such as imidazoles,19-20
stilleben,21
viologen and
analogues,22-26
and pyridinium27-28
among others.
CB[8] is similar in size to the -CD and can also hold a wide range of
positively charged compounds,4 as well as even larger guests such as cyclen
and cyclam.29
The host has even been found to work as exoreceptor to
fullrene,30
where two guests were bound to the portals. The CB[8]s large cavity
is even large enough to hold two substrates simultaneously.31-52
Even though
5
CB[7] in rare cases can hold two guests,53
the larger cavity of CB[8] gives it
the ability to bind to a larger variety.
The two guests bound to CB[8] can be homo-guest pairs, as observed with
naphthalene derivatives,31
stilbene derivatives,32
cinnamic acids,43-44
coumarins,40
viologen derivatives,33-34
tetrathiafulvalenes36
and acridines.47
A
second option is hetero-guest pairs, usually comprising of an electron poor
guest, such as viologen and an electron rich guest such as naphthalene,
dihydroxybenzene or similar. 48-52
The different amounts of units in the host and thus size give considerable
difference in its affinities for the guests, see Figure 2.
Figure 2. Examples of host guest interactions of CB[6-8].
1.2.3. Molecular machines
Molecular machines have been defined as “an assembly of discrete
components designed to perform mechanical-like movements (output) as a
consequence of appropriate external stimuli (input)”.54
The earliest example
containing CB[n] was published by Mock and coworker in the early 1990,55
where a CB[6] was shuttled along a triamine string by changing the pH value.
Several other groups have continued exploring this amine CB[6-7] system
using different stimuli such as UV/Vis and fluorescence. Kaifer and coworker
have been exploring the ferrocence- or acid-linked viologen with CB[7] as
host, utilizing the electrochemical reduction or pH as stimuli to control the
movement of the host.56-57
The charge-transfer interactions have been used to control intermolecular
folding processes, and have been used to create different types of shuttles.58-59
Dendrimer-type structures bound to viologen and phenols have also been
found to form charge-transfer type complexes, and the substrates could shift
controlled by electrochemical stimuli.60
Other examples include formation of
hydrogel or displacing guests via the addition of salt.61-62
6
1.2.4. Catalysis of reactions and other improvements
Mock and coworkers were once more the first to recognize the ability of CB[6]
to catalyze a chemical reaction.63
They could increase the rates of 1,3
cycloadditions between alkynes and alkyl halides by a factor of 5.5x104.63
CB[7-8] have been employed to catalyze different types of photocycloaddition
reactions as well as being able to hydrolyze amides.64-71
Isaacs and coworkers
recently reported biological catalysis, utilizing CB[7] to regulate an inhibitor
with DNA.72
1.2.5. Drug delivery
Considerable interest has also been devoted to the use of the host for drug
delivery.73
The drugs investigated with the host are within a broad field.
Examples include medicines used against cancer, leukemia and even for the
stomach. The effect of the host are among others the ability to improve
solubility,73f
reduce toxicity,73b
improve stability or even activate a drug.73e
Recently Nau and coworkers published an in vitro study of CB showing a very
low toxicity.74
1.3. Our interest in the CB[n] supramolecular chemistry
We became interested in the cucurbit[n]uril compounds in early 2005 in our
group due to its unique properties. Its ability to stabilize viologen radical
dimers led to the investigation of CB[n]‟s effect on the electron transfer
mechanisms (paper I and II). As previously presented, the polar cavity edges
can readily form hydrogen-bonds to substrates as well as clathrands (externally
bound guests).
As the main focus of our group is on alternate energy sources, the
understanding of the Oxygen evolving complex (OEC) that exist in the PS II
system is of great importance. This complex system is still one of the mysteries
of nature, and the foundation that gives plants and other photosynthetic
organisms the ability to convert sunlight to chemical energy. The ability to
design systems via self-assembly is a very good way of altering and studying
the PSII system, and more specifically Water Oxidation Catalysts (WOCs).
7
1.4. The aim of this thesis
The aim of this thesis is to develop new tools and methods in supramolecular
chemistry with cucurbit[n]uril, and more specifically explore the bindings of
the CB[n] host in systems with photosensitizer bound redox active
components.
1.5. General synthetic considerations
1.5.1. Mixed-chelate ruthenium complexes
The synthetic approach to preparing the ruthenium complexes is shown in
figure 3. For the complexes in this publication two precursors were prepared,
Ru(DMSO)4Cl275
and cis-Ru(bpy)2Cl2,76
in either case the complex RuCl3∙H2O
were used as starting material. The advantage of DMSO ligands are that they
are easily displaced, allowing milder conditions compared to the starting
material.
Figure 3. Synthetic approach to the preparation of mixed-chelate ruthenium
complexes.
8
2.
The Ruthenium-Viologen System
(Paper I-II)
2.1. Introduction
To study the photosystem II (PSII) model system and the electron transfer (ET)
mechanisms, where ruthenium polypyridine complexes connected to an
acceptor are commonly used. Bispyridinium ions are some of the most popular,
reversible electron acceptors, where methylviologen (N,N-dimethyl-4,4-
bipyridinium, MV2+
) is most often used.77
Figure 4. The structure of methylviologen (above) and complex 1 (below) used to
explore the long-lived charged separate state.
An earlier study by Mallouk and coworkers investigated photoinduced
intramolecular ET reactions of ruthenium trisbipyridyl-viologen molecules
[(2,2-bipyridine)2Ru(4-CH3-2,2-bipyridine-4)(CH2)n(4,4-bipyridinium-
CH3)4+
] (n=1-8). The lifetime of the photo induced charge separated state was
explored yielding slight changes in the lifetime due to the difference in chain
length.78
Scheme 2. The pathway of the photoinduced charge-separated state.
9
The pathway is light driven formation of the Metal to Ligand Charge Transfer
(MLCT) state of the complex followed by an intra or intermolecular electron
transfer to the acceptor, see scheme 2. This unstable system then reverts via
charge recombination to the ground state. β-CD has been used to encapsulate
the spacer chain in order to control the transient state decay rates.78
As MV2+
was known to be completely encapsulated by the CB[7-8] host,25-26,33
the
investigation of its effect on ET mechanism was a natural starting point.
2.1.1. The CB[7-8]-viologen system
In 2002 Kaifer and Kim and coworkers presented the interaction of viologen
with the hosts CB[7] and CB[8] as previously mentioned.25-26,33
In the case of
CB[8], it was found that the substrate could strongly bind inside the cavity
with a Ka= 1.1 x 105. When reduced to its radical form, the MV
+∙ can either
form monomer radical or complex into its dimer radical form. The CB[8]s
voluminous cavity is however large enough to accommodate two
methylviologen and was found to enhance this dimerization process by a factor
of 105, see scheme 3.
CB[7] on the other hand is not large enough to form
dimer radical form and was instead found to hinder the formation of radical
dimer. It was concluded that quantitative control of the addition of host
allowed control of the stoichiometry of the host-guest complex.
Scheme 3. Formation of the viologen radical dimer inside the CB[8] cavity.
2.1.2. Aim of this study
The goal of this study was to explore how the cucurbit[n]uril host would affect
the photoinduced charge-separate state of the Ru-MV system. A second goal
was to investigate the possibility of forming a [3]-rotaxane between two guests
and CB[8].
10
2.1.3. General synthetic procedures
Triad 1 was synthesized following the known route as depicted in scheme 4.78
Commercially available 4,4‟-bipyridine was methylated with iodomethane,
affording b after ion exchange to PF6- counterions.
78 4,4‟-dimethyl-
2,2‟bipyridine was reacted with freshly prepared LDA (lithium
diisopropylamine) under nitrogen at -78 0C for 1 h, addition of 1,3-
dibromopropane and purification gave precursor d. Stirring precursor d in
DMF with methylated 4,4′-bipyridine (b) and subsequent purification gave the
bi-dentate ligand e. Coordination with cis-Ru(bpy)2Cl2∙2H2O gave the target
complex 1. In general the cationic complexes described in this thesis were
purified by column chromatography on silica gel with a mixture of
H2O/CH3CN/KNO3 as eluent.
Scheme 4. Synthetic route for complex 1
2.1.4. Characterization of the host-guest system
In order to understand the effect of the host, careful investigation of the
inclusion complexes of 1 and either of the hosts CB[8] and CB[7] was
performed, see scheme 5. It was found that the CB[8] host present itself
squarely on the viologen, while the CB[7] has a more dynamic position. The
latter host could be detected both partly on the inner part of the viologen and
on the carbon linker, see scheme 5 (top figure). This difference in binding
11
modes is well established and CB[7]s preference to the linker has been
thoroughly investigated by other groups.79
Scheme 5. Schematic illustration for the 1:1 inclusion complexes of 1 with CB[7] and CB[8].
Confirmation of the inclusion was done by HRMS and UV/Vis titration
confirming the 1:1 binding mode. The stability of the complexes was found to
be very high, with a binding constant of 1.2×105 M
-1 for 1 and CB[7] and
3×105 M
-1 for 1 and CB[8]. These types of strong interactions are consistent
with the above mentioned different binding modes.
2.2. Laser flash transient absorption measurements
To investigate the effect of the CB host on photoinduced ET and charge
recombination, laser flash transient absorption (LFTA) measurements are
commonly used.80
The 1:1 inclusion complexes as well as the well-known
complex 1 were irradiated in water solution. Laser Irradiation was done
utilizing an 8ns laser pulse at 532nm. The absorption spectrum is then
followed on the nanosecond timescale. Any bleaching or negative peaks would
indicate that compound is disappearing from the system, such as changing
oxidation state. Any positive peaks conversely indicate that something is
formed.
The changes of the MV2+
moiety can be observed via bleaching at 253 nm (the
absorption band of the viologen) and peak formations at 400 and 600nm, due
12
to the MV2+
→MV+∙
process. Ru2+
(bpy)3, as a photosensitizer, is well
investigated and have both an intense ligand-centered absorption band at 301
nm and a d→* MLCT absorption band at 450 nm.80, 81
The excited state
ruthenium moiety can then be indicated by an absorption from the reduced
bpy∙- ligand in the
3MLCT state at a maximum of 370 nm, and a bleaching at
450 nm.
2.2.1. Transient absorption measurements of 1
Complex 1 alone yielded a transient absorption spectrum dominated by a
bleaching band around 450 nm, as well as a positive absorption band around
370 nm. No clear signals for the formation of the MV+•
radical (at 600 nm or
400 nm) could be observed due to the short lifetime, as seen in figure 5. For
complex 1, the Ruthenium recovery is due to charge recombination was
observed from the 450 nm bleaching and is fitted well with a single-
exponential decay curve with a time constant of 10 ns.78
300 400 500 600 700-10
-5
0
5
10
15
A
/nm
Figure 5. Time-resolved absorption spectra for complex 1 in aqueous solution at
room temperature, the data were recorded at delays of 15 ns, 32 ns, 49 ns, 66 ns, 83 ns, 100 ns following excitation of 532 nm laser pulse.
2.2.2. Transient absorption measurements of 1CB[8]
Figure 6 shows the nanosecond transient absorption spectra of the 1CB[8]
system at different time domains after laser flash. The spectra is dominated by
a bleaching band around 450 nm, together with a positive absorption around
370 nm and also a emission band at 650 nm indicating the excitation of the
sensitizer, as well as a absorption peak at ca 400 nm from the MV+•
radical.
13
-2
0
2
4
300 400 500 600 700
A
/nm Figure 6. Transient absorption spectra of complex 1 in the presence of 1 equiv. of CB[8] in aqueous solution at room temperature, data recorded at delays of 170 ns,
335 ns, 500 ns, 665 ns and 830 ns following the excitation of 532 nm laser pulse.
With the CB[8] host the decay curve at 400nm fitted well to a double
exponential process with time constants of 2060 ns (99.85%) and 30 ns
(0.15%), see figure 6. The recovery kinetic at 450 nm could be fitted with a
double-exponential decay by time constants of 424 ns (88%) and 12 ns (12%).
It was found as the host-interacted systems where measured, double
exponential processes were observed. The faster recovery (12 ns) agreed well
to the measurements made before host inclusion, which indicate that the
smaller component most likely was due to dissociation of the 1:1 inclusion
complex. This also showed that the bleaching at 450 nm and subsequently the
formation of the Ru2+
-(bpy)3 ground state was faster compared to the lifetime
of the viologen moiety.
2.2.3. Transient absorption measurements of 1CB[7]
300 400 500 600 700
-10
-5
0
5
10
15
A
/nm
Complex 1 + CB[7]
Figure 7 Time-resolved absorption spectra for 1:1 inclusion complex of 1+CB[7] in
water solution at room temperature, data recorded at delays of 17 ns(■), 34 ns(●), 51 ns(▲), 68 ns(♦), 100 ns(▼) following the laser excitation at 532 nm
14
The time resolved spectra for the 1:1 binding mode of 1-CB[7] is displayed in
figure 7. In this case, however, with CB[7] host, no clear signals for the
formation of the MV+•
radical (at 450nm or 600 nm) could be observed as had
been the case in the presence of CB[8]. This result can most likely be
explained by the different binding modes of CB[7] and CB[8] with the MV2+
as previously shown. This indicates that the complete encapsulation is vital for
the long-lived charge separate state.
For this system as in dyad 1 alone, the decay of the bleaching at 450 nm was
instead investigated. The recovery of Ru2+
for 1-CB[7] could, as in the case of
1-CB[8], be fitted with to bi-exponential process with two time constants (11
ns (24%) and 280 ns (76%)). The faster recovery rate constant agreed with the
value for the molecular dyad 1 alone (24%). The main component (76%)
represents the relatively slow charge recombination of Ru3+
-MV+•
due to the
inclusion of 1 into the cavity of CB[7].
This shows that the charge separated state Ru3+
-MV+•
generated after
photoinduced ET can be stabilized to some extent by the binding of the
viologen radical to CB[7]. However when compared to the CB[8] system,
CB[7] is considerably less efficient in stabilizing the charge separated state.
2.2.4. Light-driven formation of a [3]-rotaxane
To investigate the possibility of forming a [3]-rotaxane, and to utilize the high
binding constant of the viologen radical dimer inside the CB[8] cavity, an
external sacrificial electron donor was added. Triethanolamine (TEOA) is a
common and well known sacrificial electron donor,82
and can stabilize the
system and the formed MV+•
radical by reducing the photo-oxidized sensitizer.
The irradiated solution of 1 with CB[8] was investigated with UV/Vis and 1H-
NMR. Slight shifts could be observed from the viologen protons as well as a
broadening of the peaks indicating a paramagnetic complex. The color change
that of the solution indicates that TEOA does sufficiently slow down charge
recombination between Ru3+
and MV+•
so that the viologen radicals can
undergo dimerization, forming a stable dimer leaving the excess CB[8] in
solution.
UV/Vis showed the typical peaks of the radical dimer in the spectral changes.
After addition of 1 equiv. CB[8], two absorption peaks at 370 nm and 540 nm
were observed33,83
The peak intensity increased as the irradiation time
prolonged, and confirmed the radical dimer formation (see figure 8). 1 alone,
when irradiated, gave only the viologen monomeric radical peaks on the
15
UV/Vis while the proton peaks completely shifted outside the normal NMR
window confirming the rotaxane formation.
Figure 8. Absorption spectra of 1 (4.0×10-5 M) and TEOA (5×10-2 M) in the absence
of CB[8] (left), and in the presence of 1 equiv. CB[8] (4.0×10-5 M) (right), (a) before
and (b) after 5 min, (c) after 20 min light irradiation. Inset: (d) after 20 min light irradiation, (e) after exposing the cuvette to air (oxygen), the spectral curve returns to the original one.
2.2.5. Conclusions
The binding modes between CB[7] and CB[8] differed, as evidenced by 1HNMR, ESI-MS, and UV/Vis measurements. Due to these different modes of
encapsulation the intramolecular ET gave rise to vastly different long-lived
charged separate states.
In the presence of the sacrificial electron donor, the photoinduced long-lived
charge separated state of molecular dyad 1 with CB[8] gave a very stable
Ru2+
−MV+•
radical, which could be used to create a light driven molecular
“lock”.
16
3.
The Ruthenium-Phenol System
(Paper III)
3.1. Introduction
A phenol analogue has been used in conjunction with viologen and CB[8]
previously. 60
It was shown that the system worked well when the guests are
connected to dendrimer type systems, wherein the formed charge transfer
system can be bound together followed by the viologen radical dimer system
see figure 9. Theoretical calculations showed that the cavity of CB[8] indeed
was big enough to hold more than two viologens. In the former cases, the
systems were designed so that only two substrates could be included in the
system. The question we asked was: what would happen if the system was
designed so that a trimer is possible?
Figure 9. A charge transfer systems based upon dialkoxybenzene and viologen.60
3.1.1. The aim of this study
The aim of this study was to investigate the phenol-viologen system when
linked to a photosensitizer and study the radical dimer formation process.
17
3.1.2. Synthetic considerations.
The dyad 2 was synthesized following earlier published route as depicted in
scheme 6.110
The commercially available 4,4‟-dimethyl-2,‟bipyridine was
reacted with freshly prepared LDA (lithium diisopropylamine) followed by
addition of 4-(chloromethyl)phenyl acetate yielding bi-dentate ligand h.110
Coordination with cis-Ru(bpy)2Cl2∙2 H2O gave the target complex 2.
Scheme 6. Synthetic route for 2.
3.1.3. Investigation of the guests with CB[8]
No interaction between complex 2 and CB[8] was observed on 1H NMR
experiments. This was due to destabilization of the polar hydroxyl group with
the polar cavity edges of the CB[8] host. As one equivalent of MV2+
was added
into the solution CB[8] and 2, all protons of the ethyl chain and the phenol ring
in 2 underwent an up-field shift indicating an interaction with the cavity of the
host. The protons of the MV2+
moiety underwent a similar up-field shift
indicating an interaction with the cavity of the host, as well confirmed by
HRMS.
The stoichiometry for the binding of 2, MV2+
and CB[8] was established by
UV-Vis absorption titration measurements, giving a 1 : 1 : 1 binding model
with an association constant of 3.8 × 105 M
−1
The complex 2 has three reduction peaks attributed to the reductions of the
three bipyridine ligands, and one oxidation peak at 0.896 V attributed to the
oxidation of Ru(II), which are similar to the ruthenium(II) tris-bipyridine
18
complex.84
Electrochemical investigation showed that the oxidation potential of
phenol is 1.208 V from DPV. As the oxidation potentials of Ru(II) is at 0.896 V
vs. Ag/Ag+ which is a clear indication that Ru(III) cannot oxidize the phenol.
The photoinduced ET processes in this system were studied via NMR and
UV/Vis. The 1: 1: 1 host–guest reaction process was monitored by 1H NMR.
No spectral changes were observed, even after several hours of light
irradiation, with no free 2 being observed by 1H NMR. To obtain additional
information three systems were investigated; (1)the documented Ru(bpy)3 +
MV2+
+ CB[8]; (2) the 1:1:1 system as described above with different amounts
of CB[8] and (3) the 2 + MV2+
system see figure 10.
Figure 10. 1H NMR of the three systems a), b), and c) after addition of 10 equiv. of
TEOA and irradiated for 15 min.
It could be concluded that the phenol moiety of 2 indeed still interacts
dynamically with the CB host dynamically, due to the fact that no free 2 could
be detected.
UV-Vis spectra were measured to understand the ET process. It was found that
that a shift in the peaks could be observed. System b gave peaks at 368, 546
and 853 nm. In the case of system a these peaks occurred at 367, 540 and 880
nm. The ca 30 nm shift was consistent and repeatable, and clearly showed that
a third moiety as the phenol in this case, was interfering slightly with the
geometry of the MV radical dimer pair inside the host.85
This behavior can be
19
explained in the following fashion. The energy levels of the two interacting
radicals inside the host are due to the electron rich phenol are merged into a
higher ground state and a lower excited state level. The difference between
these two energy levels and therefore the CT excitation energy becomes higher,
see scheme 7.
Scheme 7. A proposed structure for the formation of the stable 2–(MV+∙)2–CB[8] partner radical trimer after photoinduced ET.
3.1.4. Conclusion
In conclusion, accompanied by the positively charged MV2+
, the phenol moiety
of guest molecule 2 can be inserted into the cavity of CB[8], forming a stable
1:1:1 inclusion complex 2-MV2+
-CB[8]. This is an interaction that agrees with
previous published interactions as described above.60
However, when
irradiated in the presence of sacrificial electron donor, a stable 2-(MV+•
)2-
CB[8] partner radical dimer in the CB[8] cavity can be generated with light
and the structure for this radical dimer has been proposed. Molecular oxygen
can quench the radical dimer and regenerate the original 1:1:1 inclusion
complex. These results may indicate a new type of interactions and can
hopefully provide some fundamentals to new types of designs of molecular
devices.
20
4.
The Ruthenium-DV2+
-V2+
System
(Paper IV-V)
4.1. Introduction
During the past years, several groups have reported the dynamic interactions of
a CB[n] host to different positions of a guest using either CDs, proton or
electron transfer reactions.86
These had been performed on mainly systems
such as donor-acceptor complexes, ferrocence-acceptor or by proton transfer
from amide based polymers.
Continuing our earlier work with the Ru(bpy)3-MV2+
and CB[7,8], we decided
to expand this area by introducing a secondary viologen derivate to the system
with CB[8]. Dimethyl methylviologen (N,N-dimethyl-3,3‟-dimethyl-4,4‟-
bipyridinium, (DMV2+
) also function as an electron acceptor, but is more
difficult to reduce compared to the parent viologen.87
The properties of DMV and MV when linked have been investigated
previously.88-89
Crown ether was introduced and the changes in the long lived
charged-separate state was investigated.88
The authors concluded that the
crown ether decreased the reorganization energy which according to classic
Marcus theory would increase the lifetime of the charge separate state.
Stoddart and coworkers also designed a molecular “abacus”89
in which crown
ether could shift to the viologen unit when reduced to its radical form.
4.1.1. Aim of this study
The goal of this study was to explore a new guest molecule as well as to
explore if the host could shift position with two different electron acceptors.
Also the photochemical [5]-rotaxane is explored (see scheme 8).
21
Scheme 8. The chemical structures of compounds MV2+, DMV2+, 3, 4 and 5.
4.1.2. Synthetic procedures
The dyad 3, 4 and triad 5 was synthesized following the route as depicted in
scheme 10. N,N-dimethyl-4,4‟-bipyridinium was synthesized according to a
procedure of Rebek and coworker.90
A two-step process with sodium based
reductive dimerization utilizing TMSCl, followed by oxidation with KMnO4.
The dyads 3 and 4 were synthesized stepwise where crude purification of the
precursor I, j was performed via precipitation and ion exchange. The synthesis
of triad 5 was performed from a modified procedure of an analogue 88
as seen
below (scheme 9, 10).
Scheme 9. Synthetic route for DMV2+ and precursor i, j
22
Scheme 10. Synthetic route for dyad 3, 4 and triad 5
4.1.3. Host-guest analysis
The inclusion complex between the guest DMV2+
and CB[8] can be observed
on 1H NMR spectral changes, and is consistent with similar systems.
33, 86 The
inclusion was as well confirmed via HRMS and UV/Vis titration. A binding
23
constant of (2.6±0.3) × 105 L mol
1 was obtained, indicating a preference of the
CB[8] on the DMV2+
moiety compared to the MV2+
moiety.33
This difference in binding constants was observed by dissolving equivalent
amounts MV2+
, DMV2+
in water. The CB[8] host was incrementally added and
a strong preference was found for the DMV2+
-CB[8] interaction. At the 1:1:1
(MV2+
:DMV2+
: CB[8]) system, a broadening of the MV2+
signals was
observed, suggesting a weak dynamic interaction.
The reduction potential of DMV2+
was found to be 400 mV lower compared to
the MV2+
in aqueous solution, indicating that a selective reduction was
possible. When included in a CB[8] host in the presence of 0.5 and 1
equivalents of CB[8] a slight negative shift of a total of 15 mV was detected,
indicating a similar effect as in the MV2+
-CB[7] system.25-26
Spectroelectrochemistry was performed to further analyse the host-guest
interaction see figure 11. The radical cation was clearly observed at 390 and
790 nm87-89
in both aqueous and acetonitrile solution. When performed in
aqueous solution a third peak at 315 nm was detected.
The exact origin of this last peak could not be determined, but can be derived
from either the doubly reduced specie DMV0 or possibly the DMV
+∙ radical
pair. With 1 equivalent of CB[8] added to the solution of DMV2+
the intensity
of the peak at 315 was strongly reduced while the peaks at 390 and 790 nm
were essentially unaffected.
Figure 11. UV/Vis spectroelectrochemistry of DMV
2+ at 5 mM concentration (solid
line), with 1 equiv. CB[8] (dashed line) and baseline (dotted) at 1.1 V vs. Ag/AgCl.
Even though the measurements were conducted at several concentrations of
DMV2+
and amounts of CB[8] no new spectral peaks were found, leading to
the conclusion that the DMV+∙
cannot form a radical dimer inside the CB[8]
host. The same conclusion has been reached with other larger derivate of the
viologen like DQ2+
, DAP2+
and DPT2+
(shown in scheme 10).91
24
4.1.4. Molecular dyads 3 and 4
After the interactions of the DMV2+
substrate had been determined, the dyads
followed the same investigation procedure with NMR, HRMS,
electrochemistry and spectroelectrochemistry as mentioned above.
1H NMR analysis indicated a steric hindrance when two equivalents CB[8] was
added to dyad 3. This was not observed on the second dyad (4) where a six
carbon chain was separating the two viologens. Similar dynamics have been
observed on other [2]-psuedorotaxane with dual CB hosts. 75
The linkage as well gave electrostatic interactions between the viologen
moieties, lowering the potential of the MV2+
→MV+·
by up to 100mV. When
CB[8] was added to the system, by one half equivalents a small shift towards
the positive region could be detected on the first reduction wave. This effect
was more strongly seen on the oxidation peak but also the reduction peak
shifted. The electrochemistry was performed at different speeds giving
different shifts of the reduction peak, all in all indicating a chemical process.
However the shift of the V2+
→V+∙
was less pronounced compared to the MV2+
-CB[8] system.33
Confirmation was obtained via spectroelectrochemistry
where the systems was investigated with and without host (see figure 12).
Figure 12. UV/Vis spectro-electrochemistry of 3 (left) and 4 (right) in water solution,
potential applied at 0.56 and 0.66 V vs. SCE respectively. Compound only
(dashed line) and with 0.5 equivalents CB[8] (solid line) and before potential was applied (base line).
The dyads were reduced at a potential of 0.56V vs. SCE, selected so that only
the MV2+
moiety could be reduced. In the absence of CB[8] only the MV+·
radical was detected in both cases with the characteristic peaks at 397 and 600
nm on UV/Vis.
25
As one half equivalent of CB[8] were added to the solution peaks at 397 nm or
600 nm were not observed. Instead very strong peaks at 370, 550 and 880 nm
were observed in the UV/Vis spectrum, attributed to the formation of MV+·
radical dimers inside the cavity of the CB[8] (Figure 12, top).33
Stability
measurements were performed by repeating the tests and showed that the
systems were stable.
4.1.5. Light driven formation of a [5]rotaxane
To form rotaxane in a similar way as we previously performed92-93
for 1, the
dyad 3 was covalently linked to a Ru(bpy)3 unit, working both as a stopper and
sensitizer, see scheme 10. When the CB[8] host was included into the system
and investigated via 1H NMR it was found that triad 5 behaved slightly
differently compared to dyad 3. As the second host was introduced the peaks
from the MV2+
moiety shifted and became sharp, indicating strong interaction
compared to the weak interaction discussed above.
Investigation showed that in this instance the CB[8] moved slightly away from
the DMV moiety instead, “making room” for full inclusion of the second host
on the MV moiety. And indeed reported data support this where it has been
found that the CB host shows an increased ability to move away from the
center of an alkylated viologen compared to methylviologen, as previously
discussed.79
Electrochemistry measurements indicated a very complex system but do show
the classic indication of the dimer formation when one half or more
equivalents CB[8] are added to the system. The light driven investigations for
the complex alone showed as expected only the peaks of the free
Ru2+
―DMV2+
―MV+•
monomeric radical, (see Fig. 10b). As two equivalents
of CB[8] to the solution of 5 and TEOA, only a very strong peak at 370 nm and
a broad peak around 540 nm and a broad peak at 870 nm were observed. These
peaks are attributed to the formation of MV+·
radical dimers inside the cavity
of the CB[8], as in the case of the dyads 3 and 4 above.
26
Figure 13. UV/Vis spectra of 5 (4 × 10-5) in water solution, with TEOA as sacrificial electron donor. a) before irradiation (solid); (b) after 50 min light irradiation (dashed); c) with 2 equiv. CB[8] before irradiation (dotted); d) after 50 min light irradiation with 2.0 equivalents CB[8] (dotted- dashed).
NMR investigations as well showed that even when irradiated, the DMV units
were still found to be inside a CB[8] host, confirming that the [5]-rotaxane can
be formed as shown in scheme 11.
27
Scheme 11: schematic representation of the light driven interaction between 5 and
CB[8], forming the [5]-rotaxane.
§
4.1.6. LFTA of triad 5 (unpublished works)
Triad 5 was investigated via laser flash transient absorption with the CB host,
as discussed in 2.3. The MV+∙
could not be detected at 600 nm, instead for
comparison the decay of the *Ru
2+ (bpy)3 ground-state bleaching at 450 nm
was used. Without any host included, the lifetime of the charge separated
system was 20.3 ± 2.1 ns, which is similar to earlier reported analouges.88
When one equivalent CB[8] was positioned on the DV2+
moiety, the lifetime
was increased to 303 ± 30 ns while when two equivalents of CB[8] were used
a lifetime of 378 ns ± 10 ns was observed. Inclusion into one equivalent CB[7]
has also been investigated yielding a lifetime of 397 ns ± 2 ns.
28
4.1.7. Conclusion
The DMV-MV system does work with the CB[8] host. Since the DMV as a
substrate cannot form a radical dimer inside the host, selective movement of
the CB[8], can be performed to the Ru2+
―DV2+
―V+•
monomeric radical,
forming a V+•
radical dimer. Further TA investigations are ongoing, as shown
in the unpublished material section, and indicate that the host greatly stabilizes
the long lived charge-separate state.
29
5.
The Ruthenium-Viologen-Napthol System
(Paper VI-VII)
5.1. Introduction
Charge transfer complexes have been used to create molecular machines and
other systems with the CB[8] due to its ability to form a high strength
encapsulation that easily can be broken due to the hosts strong interaction with
the radical dimer. Kim et al made the first system in 200149
and several
systems have since been investigated (scheme 12).58-59
Scheme 12. Example of an earlier published molecular loop, utilizing the charge-transfer system of napthol-viologen vs. the viologen radical dimer-pair.58
5.1.1. The aim of this study
The goal of this study was to explore the dual host-interaction on a guest
complex as well as to explore the stability of the charge-transfer complex when
covalently bound to a photo sensitizer.
30
Scheme 13. The structures of Ru2+-MV2+-DHN (triad 6), β-CD and CB[7-8]
5.1.2. Synthetic procedures
The triad 6 was synthesized following the synthetic route as depicted in
scheme 14. A solution of 6-methoxy-naphthalen-2-ol and 1, 6-dibromo-hexane
was stirred in acetonitrile in the presence of K2CO3 was stirred for 24 h at 80
ºC to form precursor m, Several solvent, temperature and bases were tested and
these reaction conditions gave sufficiently good solubility of the base for
optimal yields. Following reaction of m with viologen gave n. The ligand o
was formed by reacting precursor n with d in DMF. The triad 6 was formed by
coordinating the bi-dentate ligand o with cis-Ru(bpy)Cl2•2H2O
31
Scheme 14. Synthetic route to triad 6
5.1.3. Interaction with cucurbit[7-8] and CD.
To study the inclusion complex formation of the triad 6 with β-CD and CB[7],
different equivalents β-CD and CB[7] were added to the solution of triad 6, see
scheme 15. After preliminary addition of either host the free positions could be
determined via 1H NMR, indicating that the CB[7] host binds to the viologen
moiety in the complex, see scheme 15,a.
Similar tests upon addition of β-CD, also showed a 1:1 inclusion complex, as
well as revealed that the intermolecular host exchange rate between the free
guest and the β-CD-bound guest is fast on the NMR time scale. The 1H NMR
demonstrated that the β-CD in the 1:1 inclusion complex is positioned on the
naphthalene unit, see 15, b.
32
The addition of β-CD into a 1:1 inclusion complex of 6 and CB[7] revealed the
the CB[7] had moved further towards the ruthenium moiety while the β-CD
was mainly positioned at the six carbon chain (15,c). The shift of the CB[7]
was even more pronounced than detected in the 1:1 system of 1-CB[7]. Even
the 2, 2′-bipyridinium ring were partly inside the cavity of the CB[7]. Addition
of CB[7] to the 1:1 inclusion complex of 6 and β-CD yielded the same final
product, showing the two possible pathways, as shown in scheme 15.
These observations indicated that β-CD can be displaced by CB[7] from the
1:1 inclusion complex of 6 + β-CD, forming the 1:1 transit-complex of triad 6-
CB[7]. We conjecture that both hosts moving closer to “stopper” is due to the
shielding effect of the CB[7] on the MV2+
positive charges when partly
included in the cavity. This lessens the cationic influence on the β-CD, which
then can shift position to the six-carbon chain, tightening the „„nut‟‟ on the
„„bolt‟‟. In addition, the positive charges of Ru(II) might also have some
influence on this process. This phenomena is in agreement with earlier
observations from Liu and co-workers with the -CD and CB[7].86
Scheme 15. Schematic illustration of the interaction of A: 6 with CB[7] (1:1 equiv.); B: 6 with β-CD (1:1 equiv.); C: changes of interaction by addition of equivalent β-CD into a 1:1 inclusion complex of 6 and CB[7]; D: formation of the 1:1:1 ternary
inclusion complex by addition of equivalent CB[7] into a 1:1 inclusion complex of 6 and β-CD.
The 6CB[8] system with β-CD was also investigated. The well-known CT
complex was formed,49
where the 2,6-dihydroxynaphthalene moiety of 6 folds
back with viologen moiety inside the cavity of CB[8]. This interaction could
also be readily detected via UV/Vis where the complex show a distinctive peak
(λmax =566 nm) as discovered by Kim and co-workers (figure 14).49
33
400 500 600 700 8000,00
0,02
0,04
0,06
0,08
0,10
0,12
0,14
0,16
0,18A
bs
Wavelength (nm)
400 500 600 700 800
0,0
0,1
500 600 700 800
0,00
0,01
0,02
Abs
Wavelength (nm)
Figure 14. Absorption spectra of ligand (left) and triad 6 (right) in the absence (solid
line) and in the presence of 1.0 equiv. (dashed line) of CB[8], inset showing the enlarged CT-band area.
The experiments were controlled by addition of β-CD host to the substrate 6
forming the above mentioned 1:1 host-guest complex, however when CB[8]
was added the complex yet again formed the charge-transfer complex and no
interaction could be detected between the complex and the β-CD.
Stoichiometry measurements of triad 6 with CB[8] by UV-Vis absorption
titration measurements verified this interaction. The data fitted to a 1:1 binding
model with a binding constant of 2.62×105 L mol
1. This value is very close to
the binding constant (3×105 L mol
1) of the molecular dyad where Ru(bpy)3 is
covalently linked to MV2+
by a four carbon chain as previously presented.92-93
5.1.4. Photochemical investigation with the guest and CB[8]
To study the possible photoinduced electron transfer processes in a system
containing the triad 6 without the presence of CB[8], an external sacrificial
electron donor, TEOA, was added to the solution of 6 as previously described. 1H NMR spectral changes clearly supported the formation of the Ru
2+- MV
+•
radical in triad 6. The broadened 1H NMR peaks are due to the paramagnetic
effect of the MV+•
radical. The measurements were repeated with addition of
CB[8]. Again, when the NMR tube was degassed with argon and irradiated for
3 h with light, the color of the solution changed from brown to dark green. 1H
NMR showed no proton shift in the NMR spectra before or after irradiation.
In contrast, the formation of V+•
radical dimer inside CB[8] has been observed
for a similar system as mentioned in the introduction. We concluded that the
steric factors designed into the system can withhold the formation of a radical
dimer, even with external viologen added to the solution.
The light-induced formation of the stable Ru2+
-MV+•
monomeric radical was
further confirmed by UV/Vis absorption. The two absorption peaks at 399 and
605 nm were readily observed while prolonged irradiation gave no further
34
peaks. These two peaks are well known and due to formation of the monomeric
radical.25-26
The CB[8] induced CT complex gave rise to the two absorption peaks at 401
and 607 nm. The small redshift can be attributed to the electron rich DHN moiety
affecting the viologen. When comparing the spectrums, no other significant effect
on the spectrum could be found, see scheme 16.
Scheme 16. The formation of MV+• radical in 1:1 inclusion complex of 6 + CB[8] after photoinduced intramolecular electron transfer.
35
5.1.5. Conclusions
The Molecular triad 6 can form a stable 1:1 inclusion complex with CB[7] and
β-CD. Two self-sorting system are found, where the β-CD either can be
released from the DHN moiety by the addition of CB[8] or shifted in position
in the case of CB[7]. Light irradiation in the presence of CB[8] only yields the
monomeric MV+•
. The formation and behaviors of the 1:1 inclusion complex
of the photoactive molecular triad 6 with CB[8] can provide basic
understanding for the future design of more advanced systems.
36
6.
The Ruthenium-Viologen-Ruthenium Rotaxane System Encapsulating CB[7-8]
(Paper VIII)
6.1. Introduction
In contrast to cyclodextrin, the solubility of the CB[n] hosts is low. Especially
the CB[8] has a very low solubility even in aqueous solutions unless it
encapsulates a guest molecule. While few literatures have reported on the
inclusion of CB[7] in organic media,94
however none has been presented with
CB[8]. Work has instead been focused on modifying the CB host itself.11
6.1.1. Aim of this study
The goal of this study was to explore the possibility of moving the
cucurbit[8]uril complex into a purely non aqueous solution to investigate if the
host still was able to promote the dimer formation.
6.1.2. Synthetic procedure and characterization of the host-guest interaction
A dual ruthenium complex was designed with a central viologen moiety, upon
which the CB host could be encapsulated. A four carbon linker was used for its
known ability to have different binding interactions between the two CB[7,8]
hosts and sensitizer,93-93
see scheme 17.
37
Scheme 17. Chemical structures of ligand p and complexes 7, 7CB[7], 7CB[8].
Complex 7 was synthesized, by reacting ligand p with 2.5 equivalents
Ru(bpy)2Cl2 in degassed H2O and stirring the solution overnight at 900
C. The
synthesis with CB[7] followed the same procedure, but with addition of CB[7]
to ligand p before reaction with Ru(bpy)2Cl2. The interaction between the
ligand and CB[7] host was easily determined both by NMR and ESMS.
Synthesis of complex 7CB[8] could not proceed in a similar fashion. Instead
a mono-ruthenium psuedorotaxane was produced with the unbound 2,2‟-
bipyridine chelate forming a charge transfer interaction with viologen moiety
inside the host cavity.
38
Scheme 18. Synthetic pathway of 7CB[8]
Investigating the interaction between ligand p and CB[8], via NMR and
UV/Vis, showed that the intramolecular stabilized charge-transfer (CT)
complex between 2,2‟-bipyridine and the viologen moiety (scheme 18, p-q)
had been formed.[5d]
This CT complex was strong enough to stop the rotaxane
formation. The issue was solved via introducing a stronger electron donor, 2,6-
dihydroxynapthalene (DHN) (scheme 18, r). DHN subsequently formed a CT-
complex with viologen inside the CB[8] cavity. This interaction could easily be
confirmed via ESMS, NMR and UV/Vis measurements, where in the latter
39
case, the CT band between viologen and DHN was found at 340 nm. Reaction
with cis-Ru(bpy)2Cl2 produced complex s. DHN was removed using sephadex
LH20 column chromatography. Additional cleanup steps yielded the pure
7CB[8].
Characterization of the complexes, surprisingly, showed that the host in either
case was dynamically positioned upon the carbon linker. In the case of CB[8]
some of the protons of the bi-dentate ligands were also found to interact with
the cavity of the host, see protons 9-11 marked in scheme 18. Such an
interaction with the ligands is not surprising considering the size of the host.
Electrochemistry of 7 in acetonitrile gave two successive one-electron
reduction waves, corresponding to the two redox-peaks of the viologen moiety.
For the complex 7, the first and second reduction are observed at -0.68 and -
1.10 V whereas -0.86 and -1.37 V were observed for both 7CB[7] and
7CB[8]. This similar reduction potential for both hosts indicates that no
viologen radical dimer could be created with only the formed complex.
The oxidation potential of 7 was found at 0.93V vs. Ag+/AgNO3. Surprisingly
the complexes with host gave dual oxidation potentials at 0.93 V and at a lower
potential. Ca 100 and 200 mV lower for CB[8]-[7] respectively. A likely reason
for this observation is that host positions itself close to one metal center during
oxidation as depicted in scheme 19. Measurements were performed at different
speeds from 0.01 to 3 V/s but showed no significant changes. This is assumed
to be due to the dynamic shift of the host is very fast, also indicated from the
NMR investigations.
Scheme 19. Selective positioning of the CB[7,8] host during the oxidation of
7CB[7,8] in acetonitrile.
40
6.1.3. Non-aqueous photoinduced viologen radical formation
The possibility of viologen radical dimer formation based on complex
7CB[8], with and without CB[8] was one of the main reasons for this project
and was analyzed via 1H NMR, electrochemistry and photochemistry.
1 equivalent MV2+
and TEOA were added directly into the acetonitrile solution
containing the 7CB[8] and irradiated. During irradiation for 50 min, the color
of solution changed slowly from orange to dark brown. Due to the excess of
TEOA the complex retained its Ru(II) oxidation state, during the following 1H
NMR experiments, as seen in figure 15.
Figure 15. 1H NMR in CD3CN of 7CB[8]: MV2+ 1 equiv. (8.26×10-4 M) in the
presence of TEOA 5x10-2 M (a) before irradiation, (b) irradiation for 50 min and (c) expose to the air (O2).arrows indicate the shift of the atoms on the chelate
(numbered 9-11) in scheme 18.
It was observed by 1H NMR experiments, when irradiated with TEOA and
external viologen, that the CB[8] host shifted away from the metal centers
towards a more central position as the viologen units were reduced, as seen in
figure 15. This effect were not observed when free viologen was absent from
the system. Electrochemistry measurements, although complex, also indicated
a chemical process during the irradiation.
To confirm that both 1H NMR and electrochemical shift were due to 7CB[8]-
V+
+ MV+
radical dimer formation, UV/Vis spectroscopy was performed (see
figure 16). Irradiation for a total of 40 minutes, gave a typically brown colored
solution, both the radical monomer at 397 and 600nm as well as the radical
dimer absorption peaks at 370 and 555 nm and broad absorption band
around 800-1100 nm were detected (Amax=970 nm).85
41
Figure 16. Absorption spectra of 7CB[8] in the presence of MV2+ 1 equiv. (6.4×10-
5 M) and TEOA (5×10-2 M) in acetonitrile before irradiation (black), irradiate for 25
(red), 40 minutes (green) and expose to the air (O2) (blue)
.
6.1.4. Conclusions
A template-based synthesis has been presented to form a rotaxane which is
able to provide the possibility to move the CB[8] host-guest complex into non-
aqueous solution with high solubility. It was found that the host could stabilize
the radical dimer system in non-aqueous solution, however preliminary results
suggest that the stabilization of the radical dimer is weaker compared to
aqueous solution.
42
7.
An Efficient Water Oxidation System Based on Supramolecular Assembly of Molecular
Catalyst and Cucurbit[7]uril.
(Paper IX)
7.1. Introduction
The Oxygen evolving complex (OEC) that exist in the PS II system has
received much attention but is still one of the mysteries of nature. This
complex system is the foundation that gives plants and other photosynthetic
organisms the ability to convert sunlight to chemical energy. With the current
need to find new green energy sources, this process has received considerable
attention, as such possible energy source.96
In nature PSII is a large
homodimeric protein-cofactor complex where the water oxidation catalytic
center is a Mn4OxCa cluster surrounded by different types of proteins.97
The
precise reason of the proteins is not known but there have been suggested that
they facilitate the electron transfer via tyr z. Inspired by the OEC in PSII,
significant improvements on the turnover frequency [TOF] of Water Oxidation
Catalysts (WOC), using alternative transition metals, such as ruthenium,98-108
since the “blue dimer” was first reported by Meyer and coworkers. 98
Scheme 20. Meyers “blue dimer” (left)98 and example of a WOC with dimeric
pathway (middle) 107 and monomeric pathway (right).105
In our group we have investigated WOCs that have a TOF of up to 4/s using
Ce(IV) in pH 1 solutions. These complexes are based upon a negatively
charged backbone ligand and the ability to form dimeric complexes in water
during the water oxidation process confirmed by crystal structure and by
43
theoretical calculations as seen in scheme 20.108
One problem with the WOCs
of today is the low Turn Over Number (TON), which needs a significant
increase for any practical use. For the 2,2‟-bipyridine-6,6‟-dicarboxylic acid
(dba)-based complexes, experiments have shown that, the oxidation of the two
mono dentate axial ligands is one possible reason for the degradation of the
WOCs and the low TONs.107
To solve this issue as well as investigate the
dimeric pathway; the CB[n] looked to be a prime component.
Scheme 21. Proposed mechanism for water oxidation via the bimolecular process, red arrows indicating the removal of protons.
As previously mentioned the host family has been used as catalysts to good
effect in different types of reactions.63-72
For the WOCs a host-guest interaction
with the CB[n] could have several effects. The first would be to shield the
sensitive axial mono-dentate ligands from oxidation. A secondary effect would
be to act as a second sphere ligand,109
helping in the proton transfer mechanism
away from the Ru-O…
O-Ru center, via hydrogen bonding between the
carbonyl groups on the CB[n] cavity edges and the bound water molecule
increasing the proton transfer rate.
44
7.1.1. The aim of this study
The aim of this study was to explore the possibilities of regulation of a WOC
catalyst, by inclusion into a CB[7] host.
7.1.2. Design and synthetic procedures
The principle was designing two different WOC‟s with the ability to bind
towards CB[7]. By modifying two known catalysts105-107
different binding
modes towards a host were created. The host CB[7] can bind either axial close
to the reaction center (WOC 8), or away from the reaction center on the
tridentate backbone ligand (WOC 9) as shown in scheme 22.
Scheme 22. Supramolecular systems of WOC 8∙2CB[7] and 9∙CB[7].
The backbone tetra-dentate ligand for 8 was formed by oxidizing 6,6‟-
dimethyl-2,2‟-bipyridine with sodium dichromate in conc. Sulphuric acid to
form the biscarboxyl backbone ligand according to literature.110
coordination
with Ru(DMSO)4 and methylated axial ligand gave the finished product.105
The
Synthetic procedure of complex 9 was a seven step route as shown in scheme
23 below. Cheldamic acid was reacted with PBr5 forming the 4-
Bromopyridine-2,6-dicarboxylic acid dimethyl ester.111
The pyridine was
introduced by Suzuki coupling.112
Different reaction conditions (catalyst, time,
temperature, solvent) were investigated. The best choice of catalyst was
45
Pd(dppf)Cl2∙CH2Cl2 due to stability. Under microwave irradiation, this reaction
showed good yields. Methylation in CH2Cl2 gave intermediate y, followed by
hydrolysis. Coordination to Ru(DMSO)4Cl2 was conducted in methanol with
TEA as base, followed by a second coordination step with 4-piccoline which
yielded WOC 9.
Scheme 23. Synthetic pathway of WOC 9
46
7.1.3. Host-guest interaction
Both complexes were investigated and characterized via NMR and HRMS and
as designed, showing the possibility to hold two (WOC 8) or one (WOC 9)
CB[7] host(s). As potential water catalysts the electrochemistry is of high
importance. Cyclic voltammetry was used to determine the redox potentials of
the catalysts. WOC 8 in a PH 1 solution of TFA (Trifluorosulphonic acid) show
a reversible redox couple, assigned to RuII/Ru
III process with E1/2 being 0.76 V
vs. NHE. A slight oxidation ridge can be detected at around 1.23 V vs. NHE.20
Complex 9 shows as its analogue107
two oxidation potentials at 100mv/s
depending on solvent interaction, When CB[7] was added to the solutions a
slight decrease of the oxidation potential of 0.03-0.04 V was detected for the
RuII/III
redox potential in both cases.
7.1.4. Catalytic water oxidation
The catalytic water oxidation was performed utilizing a simple two-component
molecular system consisting catalyst and Ce(IV)(aq.) as an oxidant in pH 1.0
aqueous solution, see scheme 24.
Scheme 24. Catalytic water oxidation using molecular catalysts 8 and 9.
The oxygen formation through water oxidation was measured with an oxygen
sensor, while continuous GC measurements were performed to measure the
amount of oxygen generated.106-107
Table 2. Dioxygen evolution rates using Ce(IV) as oxidant
complex TON[a]
Rate[b]
8 600 0.99[c]
8∙2CB[7] 1486 2.11[c]
9 300 1.20[c]
9∙CB[7] 300 1.18[c]
[a] Measured by oxygen sensor and calibrated by GC. [b] Initial rate in turnover/s. [c] A linear fitting of the first 3 minutes for 8 and 5
minutes in the case of 9 was used to obtain the rate.
47
The expectations from this inclusion were to find a lower TOF due to change
in mechanism, where the host-guest system is unable to form the dimer with
the bulky host, as well as an increased TON due to the protected axial ligands.
In the case of 8∙2CB[7] not only did we detect a better TON but also a faster
TOF compared to 8 alone. In the case of 9∙CB[7] and 9 however, no
significant changes could be seen, see table 2 and figure 17.
0 500 1000 1500 2000
0
20
40
60
80
100
120
140
mic
rom
ol O
2 fo
rme
d
Time/s
82CB[7]
8
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5
-10
0
10
20
30
40
50
60
mic
rom
ol O
2
Time (h)
9
9CB[7]
Figure 17. (left) Oxygen evolution of complex 8 and 8∙2CB[7] recorded in gas phase
by oxygen sensor and calibrated by GC. (Right) Oxygen evolution of complex 9 and 9∙CB[7] recorded in gas phase by oxygen sensor and calibrated by GC.
To understand the high TON and TOF of the 8∙2CB[7] assembly, it was
investigated kinetically via UV/Vis where the absorption decay of Ce(IV)
could be monitored at different concentrations, see figure 19,106-107
when
combined into one graph the rate dependence on concentration could be
observed, as shown in figure 18.
0,5 1,0 1,5 2,0 2,5 3,0
0,5
1,0
1,5
2,0
2,5
3,0
Ko
bs x
10
6 (
Ms
-1)
[1] x 10-6 [M]
pure
2 equiv
Figure 18. Plot of the initial rate (kobs) vs. concentration of complex 8 with and without CB[7]. The initial kobs was calculated from the first 60 seconds data of the
kinetic experiments
The kinetic measurements indicated that there are two different pathways. In
the case of 8 a second order mechanism was observed, as its analogue (scheme
17, middle). In the case of 8∙2CB[7] a first order mechanism was detected.
48
Possibly due to the size of the CB[7] hindering the dimerization, between two
8∙2CB[7] systems.
7.1.5. Theoretical calculations
To understand the higher TOF of 8∙2CB[7] above 8 alone, theoretical analysis
was performed on a simplified system of 8∙1CB[7], as shown below in figure
19. The theoretical calculations did indeed show a significant hydrogen
bonding interaction. The distance between the CB7 carbonyl oxygen and
hydroxyl proton was calculated to 1.8 Å indicating a significant hydrogen
bonding interaction.
Figure 19. Optimized geometry (M06-L/LACVP)113 of a model complex containing one tmebipy+Cl- ligand binding to a CB7.
7.1.6. Conclusions
The theoretical data, based on a simplified structure clearly show that the
system could hinder the dimerization process. Kinetic measurements indicate a
possibility for the complex to proceed via a mono-mechanism compared to
dimer based mechanism without host. That complex 9 has no increase or
change in TOF or TON proves the importance of the positioning of the host.
Further research is in progress with the supramolecular catalytic systems.
49
8. Concluding Remarks
The number of possible uses of the cucurbit[n]uril host is almost infinite. In the
introduction I have described several different types of uses, like catalysis,
drug treatment as well as molecular machines.
My work, presented here, has been based upon the goal of understanding the
host‟s interaction with different ruthenium based photosensitizers. As always
there are many paths within even this narrow field that are yet to be explored.
We have explored the ET transfer and stabilizing abilities of the host when
ruthenium is connected with phenols, viologens to name a few. The host is also
clearly stabilizing these complexes, preventing degradation. We have also
shown several instances of molecular machines, however the supramolecular
machine in its classic sense is still far away from practical uses.
It is hard to debate where the first practical industrial use of the host will
emerge. However due to the low toxicity, I believe that the pharmaceutical
industry will be the first to benefit from the host.
Catalytic uses are I assume already in use in laboratories due to the multitude
of publications within this field. It is my hope that our work with water
oxidation catalysts, catalyzed and stabilized by CB[7] will be one of the
processes that are industrialized.
.
50
Acknowledgements
I would like to express my gratitude to:
Professor Licheng Sun. Thank you for accepting me as a PhD student and for
your guidance as well as your happiness.
The Swedish Research Council, Swedish Energy Agency and the K & A
Wallenberg foundation for financial support of this study.
The Aulin Erdmann foundation for funding to make it possible for me to go to
several conferences.
Also past and present group members and for fruitful discussions and
collaborations; Yunhua Xu, Daniel Hagberg, Peng Quin, Lele Duan, Shiguo
Sun, Dapeng Zou, Martin Karlsson, Erik Gabrielsson, Lianpeng Tong, Mikhail
Gorlov, Haining Tian, Fuyu Wen, Xien Liu. This is truly a wonderful group of
people.
Also Jingxi Pan and Rong Zhang for help with kinetic measurements as well as
HRMS.
A special thank you to professor Torbjörn Norin, Juho Bah, Lele Duan and
Yunhua Xu for valuable comments of this thesis.
Lena Skowron, Henry Challis and Ulla Jacobsson for help with practical
matters.
All past and present coworkers on the Organic chemistry department
My friends; for the good times.
Mom, Dad and my brothers and sister and my older brothers daughter Lea.
51
Appendix A
The following is a description of my contribution to Publications I to IX, as
requested by KTH.
Paper I: Minor contribution to the formulation of the research problems.
Performed parts of the synthesis as well as performed a majority of the UV/Vis
electrochemistry measurements and wrote parts of the manuscript.
Paper II: Minor contribution to the formulation of the research problems.
Performed parts of the synthesis as well as performed a majority of the UV/Vis
measurements and wrote parts of the manuscript.
Paper III: contributed to the formulation of the research problems. Performed
parts of the synthesis and measurements as well as wrote the parts of the
manuscript.
Paper IV: Major contribution to the formulation of the research problems.
Performed the synthesis and measurements as well as wrote the manuscript.
Paper V: Major contribution to the formulation of the research problems.
Performed the synthesis and measurements as well as wrote the manuscript.
Paper VI: Major contribution to the formulation of the research problems.
Performed parts of the synthesis and measurements as well as wrote parts of
the manuscript
Paper VII: Major contribution to the formulation of the research problems.
Performed parts of the synthesis and measurements as well as wrote parts of
the manuscript.
Paper VIII: Major contribution to the formulation of the research problems.
Performed parts of the synthesis and measurements as well as wrote the parts
of the manuscript.
Paper IX: Major contribution to the formulation of the research problems.
Performed the synthesis and measurements as well as wrote the manuscript.
52
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