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Tetratopic and hexatopic terpyridine ligands as building blocks in coordination polymers
Full text linkThe work described in this thesis concerns the synthesis of tetratopic bis(3,2':6',3''-tpy) and
bis(4,2':6',4''-tpy), hexatopic tris(3,2':6',3''-tpy) and tris(4,2':6',4''-tpy) ligands and their capability to
form 1D, 2D and 3D coordination polymers. The ligands were reacted with [Cu(hfacac)2]∙H2O,
[Zn(hfacac)2]∙2H2O and Co(NCS)2 under single-crystal growth conditions and the resulting crystals were
analysed by X-ray diffraction analysis. Good solubility in common organic solvents of the resulting
terpyridine building blocks was crucial for the successful crystallisation of the final assemblies. A variety
of solubilising substituents have been used throughout this thesis.
Chapter 1 introduces the discipline of coordination polymers. After a few definitions and an overview
of classic synthetic approaches to these materials, there follows an important section dealing with the
topological description of coordination networks. This is accompanied by an overview of the typically
used metal centres and polytopic ligands as building blocks for the assembly of coordination polymers,
followed by a focus on the terpyridine compounds. Finally, the chapter concludes with some selected
examples of applications to illustrate to the reader the great potential that this class of materials
possesses.
Chapter 2 deals with the synthesis and coordination chemistry of V-shaped bis(3,2':6',3''-tpy) and
bis(4,2':6',4''-tpy) ligands. In previous works of Newkome and coworkers, analogous V-shaped
2,2':6',2''-isomers have been shown to be valuable precursors for the synthesis of even highly
sophisticated molecular constructs. However, the V-shaped bis(3,2':6',3''-tpy) and bis(4,2':6',4''-tpy)
ligands used in this thesis, did not prove to be effective for the synthesis of coordination polymers,
specifically the growth of single-crystals suitable for X-ray characterisation.
In Chapter 3, a combination of tetratopic bis(3,2':6',3''-tpy) ligands and [M(hfacac)2]∙xH2O (M = Cu, x =
1; M = Zn, x = 2) led to a series of 2D coordination polymers propagating in two dimensions. These
networks all share the same underlying (4,4) topology regardless of the nature of the solubilising
substituent, crystallisation solvent and configuration of the metal centres.
Chapter 4 describes the reaction of tetratopic bis(3,2':6',3''-tpy) ligands with a 4-connecting metal
centre, Co(NCS)2. Layering setups using MeOH and chlorobenzene led to the formation of 3D
assemblies with a cds topology. In contrast, a solvent switch from chlorobenzene to CHCl3 results in a
topological change to a 3D trinodal self-penetrating network. A comparison is then made with the most
recent findings obtained by a former Master's candidate in the Constable-Housecroft research group.
An extension beyond the usual 2-4 connecting terpyridine ligands is made in Chapter 5. Several
hexatopic tris(3,2':6',3''-terpyridine) and tris(4,2':6',4''-terpyridine) compounds were prepared and
reacted with [Cu(hfacac)2]∙H2O. The 3,2':6',3''- and 4,2':6',4''-isomers possess different coordination
flexibilities yielding an unexpected 1D coordination polymer and a predictable 2D network,
respectively. The former is the first single-crystal X-ray characterisation of a tris(terpyridine)-based
coordination polymer.
The general results of this thesis, with a brief outlook on the projects, are given in Chapter 6
Ligand design strategies for photoactive first-row transition metal complexes
No full textPhotochemistry attracts increasing attention, but still most of the used photoactive transition metal complexes are based on metals of the second and third row. The scarcity of the popular metals like ruthenium and iridium prohibits their widespread use in applications. The transition towards more sustainable photochemistry using photoactive complexes based on abundant first-row transition metals is an attractive research goal nowadays. This thesis aims at introducing different strategies towards this goal and highlights two projects, where new first-row transition metal complexes have been investigated. In the first project complexes of 3d10 electronic configuration have been used for photochemical applications (Chapter 3) and in the second project new 3d6 and 3d5 transition metal complexes were synthesized and studied (Chapter 4).
The careful design of a charge transfer ligand bearing an electron-donating bis(4-methoxyphenyl)amine unit and an electron-accepting benzoxazole backbone leads to two constitutionally isomeric zinc(II) complexes with fundamentally different photophysical properties in the first project (Chapter 3). Their favorable properties enabled their use in different photocatalytic reactions, the transfer of an electron to a suitable substrate and as sensitizer in a sensitized triplet triplet annihilation upconversion. Photostability studies provided insights into the main decomposition pathways of the two complexes.
In the second project (Chaper 4), new synthetic approaches towards geometrically optimized N-heterocyclic carbene (NHC) ligands were investigated. Additionally, in the course of this project, two tridentate ligands with a central cyclometalating phenyl ring framed by two NHC side arms and a bidentate ligand with a pyridine and a NHC binding site were successfully synthesized. The complexation of the tridentate cyclometalating ligand to Co(III) and Fe(III) resulted in two homoleptic complexes that were investigated. Preliminary results of photophysical measurements allowed a first insight into the excited state properties of these complexes
Design and properties of lanthanoid chelating tags
Full text linkThe work presented in this thesis is centred around the magnetic anisotropy of lanthanoids and how it can be harnessed for the study of bio macromolecules. Following the introduction about paramagnetic NMR spectroscopy, lanthanoids and lanthanoid chelating tags are three individual chapters regarding the properties, design and synthesis of lanthanoid chelating tags.
The first chapter comprises a published study concerning the anisotropy of the magnetic susceptibility exhibited by lanthanoid (III) ions within lanthanoid chelating tags (LCTs). LCTs are widely used to induce pseudocontact shifts (PCSs) or residual dipolar couplings (RDCs) on bio macromolecules. The size of the observed PCSs or RDCs is dependent on the anisotropy of the magnetic susceptibility of the lanthanoid (III) ion. The anisotropy of the magnetic susceptibility can be described by the anisotropy parameters, which are for LCTs commonly determined from PCS observed on a conjugated protein. Because the PCS observed on the protein are inevitably reduced by motional averaging, the anisotropy parameters determined from them describe always only a fraction of the anisotropy of the magnetic susceptibility a lanthanoid ion exhibits within an LCT. This study presents for the first time the intrinsic anisotropy parameters for the full lanthanoid series determined from shifts observed on the LCT itself. The strongly shifted proton spectra could no longer be assigned using conventional 2D- NMR assignment strategies, due to the extremely short T2 times. Instead, the 1D proton spectra were fully assigned using extensive, site-specific 2H and 13C labelling in combination with combinatorial methods. The full assignments were used to determine the anisotropy parameters, which deliver an upper limit for future PCS applications relying on this coordination polyhedron as well as new insights into future LCT designs. Surprisingly, we observed an, at least at room temperature, unprecedented correlation between the oblate or prolate f-electron distribution of the lanthanoid and the orientation of the main magnetic axis. Furthermore, a comparison of different ligands revealed that the size of the anisotropy parameters depends on the interaction between the ligand and the lanthanoid ion.
In the second chapter, the focus lies on the development of a new single-arm LCT, which could provide predictable anisotropy parameters. For current single-arm LCTs the averaging of the observed PCS on a conjugated protein is not only dependent on the LCT but also on the tagging site. Therefore, the averaging of the PCS is different for each tagging site, requiring the determination of the anisotropy parameters in each case. It would greatly ease the application of PCS NMR spectroscopy if a single-arm LCT would provide predictable anisotropy parameters. A priori knowledge of the anisotropy parameters would allow the choice of the best-suited tagging site for a given application and facilitate the assignment of paramagnetic signals. Based on the insights gained from determining the intrinsic anisotropy parameters it was hypothesized that coaxiality between the rotation axis and z-axis of the tensor frame could provide predictable anisotropy parameters. The sought coaxiality can be achieved with a symmetric LCT that is tethered via a thioether to the protein. In order to develop a new LCT, two scaffolds were tested for their suitability as an LCT. The second scaffold provided a new symmetric LCT, which was successfully tethered to ubiquitin S57C. Determination of the anisotropy parameters and comparison to the estimated intrinsic anisotropy parameters showed that the PCS induced by the new LCT are barely affected by averaging. The current results were not sufficient to ascertain whether the new LCT is able to provide predictable anisotropy parameters but they hold great promise for further research.
The third chapter describes the synthesis of a new building block for DOTA-M8-based LCTs. DOTA-M8 provides excellent properties for the development of lanthanoid chelating tags but has the inherent problem that it is difficult to synthesize. The main challenge in the synthesis towards DOTA-M8 is the synthesis of M4-cyclen. In this chapter, a new cyclen building block is presented, which should be simpler to synthesise but still provides all the necessary properties for high performance LCTs. A convenient synthetic route towards the new building block was tested. The encountered obstacles revealed which changes would be necessary for the future success of the proposed synthetic strategy
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Towards increased performance of iron(II)-based dye sensitized solar cells
Full text linkToday, because of the current climate situation, renewable energy sources are of high importance. Over the last few decades, solar cells have become a vital technology due to their ability to convert sunlight into electrical energy. Most of the commercially available solar cells are made of crystalline silicon. Silicon-based solar cells are already well known in the market, but their costly and tedious fabrication motivates the examination of alternative systems. Since the pioneering work of O’Regan and Grätzel in 1991, dye-sensitized solar cells (DSCs) have become a promising substitute. The sandwich-type DSC structure is easy to manufacture, and a broad variety of sensitizers offers lower costs of materials. However, the use of metals such as ruthenium with low natural abundances still significantly increases the price and reduces the sustainability of DSCs.
In this work, the focus is set on n-type DSCs, which combine the use of Earth abundant iron(II) coordination complexes as dyes and the advantageous effects of using different additives in electrolytes. Tuning of electrolyte composition can also remarkably enhance the photoconversion efficiency (PCE) and, as shown for other dyes, has the potential to make iron-sensitizers a promising alternative to ruthenium-based compounds. It has been demonstrated that both the redox couple and the components of the electrolyte have a critical influence on the PCE, and this effect originates from its role as a charge transfer medium. The effects of lithium salts, ionic liquids with different counter-ions and solvents while retaining an I-/I3- redox shuttle are presented.
Sometimes small changes might lead to significant progress. The design of an alternative iron(II) based dye is proposed with a corresponding synthetic route. The synthesis towards the target complex is presented.
Moreover, a statistical study of electrochemical impedance (EIS) measurements was performed. EIS offers a possibility to study complex electronic systems and is commonly used for solar cells, but there is a general tendency in the literature to present impedance data only for one device. At the same time, the current density–voltage plots can illustrate that measurements may vary within one set of DSCs with identical components. The multiple DSCs impedance measurements are presented on the example of two dyes and provide the statistical analysis for their reproducibility between the cells
The effects of sulfur on heteroleptic copper (I) complexes for the potential application in light emitting electrochemical cells
Full text linkThe aim of this project was the synthesis and characterisation of heteroleptic copper(I) compounds that emit light in the visible spectrum (400–800 nm), with the goal to incorporate these complexes as emitting species in light emitting electrochemical cells (LECs). LECs have several advantages when compared to organic light emitting diodes (OLEDs) and light emitting diodes (LEDs), and some challenges that need to be overcome before large scale applications can be considered. These advantages and challenges will be discussed in Chapter 1: Introduction, together with a brief overview over the history of lighting and an introduction to LEDs and OLEDs.
Typically, the cation in such a light emitting copper(I) complex consists of the copper centre, a diphosphane – in the case of this project the commercially available bis(2-(diphenylphosphano)phenyl) (POP) and 4,5-bis(diphenylphosphano)-9,9-dimethylxanthene (xantphos) – and a diimine such as 2,2’-bipyridine (bpy) or 1,10-phenantroline (phen). Often, the bpy or phen carries alkylic or aromatic substituents.
The overarching theme of this project was the incorporation of heteroatom substituents into these complexes. In Chapter 2 sulfur replaces one of the chelating nitrogens, and thereby takes an active role in binding to the Cu(I) centre. The first series of complexes with only one aromatic ring (pyridine), did not show significant luminescence. The second series, which used a 2-(thiophen-2-yl)pyridine as a chelating ligand, featured a yellow emission in solid-state with photoluminescence quantum yields (PLQYs) up to 10.8% and a blue emission, with significant ligand-based contributions, in solution with PLQYs up to 33.2%.
Chapter 3 focuses on classical [Cu(P^P)(N^N)][PF6] compounds, with N^N being a dibromo-1,10-phenanthroline. In this series of complexes, the influence of substituents in the 2- and 9-positions of the phen on the emission wavelength and PLQY are clearly visible. The complexes do not feature a PLQY >1% in deaerated CH2Cl2 solution, but in the solid state they are yellow to orange emitters with a PLQY up to 45%.
Chapter 4 investigates the influence of chalcogen substituents in the 1,10-phenanthroline backbone. The introduction of either alkylsulfanyl or alkoxy substituents has significant influence on the photophysical properties of the complexes. The complexes are yellow to orange emitters with a PLQY of up to 9.4% in deaerated CH2Cl2 solution and up to 60% in the solid state. 2,9-Alkylsufanyl substituted phenanthrolines feature a notable blue shift compared to 3,8- and 4,7-subtituted ones with a significantly higher PLQY. In the solid state these complexes exhibit excited state lifetimes in the μs regime with the longest being 19 μs. To investigate the nature of these emissions further measurement in a frozen matrix of 2-methyltetrahydrofuran at 77 K were carried out, where most of the complexes exhibited a considerable blue-shift and dual emission.
At the end of this work a short summary of the work at hand and a glance into the future of light emitting Cu(I) complexes and LECs awaits
One- and two-dimensional coordination assemblies incorporating 3,2’:6’,3”-terpyridine linkers
Full text linkIn the field of material sciences, coordination polymers represent a topic of interest, due to their extended structures and the associated properties. To date, the formation of these networks is a fascinating but not fully understood phenomenon, and the evidence of structure-property relationships makes the control of this process an issue of major concern. Towards this end, systematic studies in which one parameter is varied at one time could offer a meaningful contribution to gain a better comprehension of the factors that influence the assembly of one kind of structure over the other possibilities. This is especially true when flexible building blocks are chosen. Although a substantial part of the effort in this area has been centered on the use of rigid ligands, there is scope for developing metal coordination assemblies with more flexible components. In particular, in this PhD thesis, homologous series of ditopic 3,2’:6’,3”-terpyridine ligands have been synthesized following the one-pot procedure by Wang and Hanan, fully characterized (as described in Chapter 2) and their coordination chemistry has been explored focusing on two metal salts. Copper(II) acetate (which typically forms dinuclear paddle-wheel motifs) directed the assembly of 1D-chains (Chapter 3), while cobalt(II) thiocyanate led to the formation of 2D-nets with a (4,4) topology (Chapter 4); only one exception for each category is reported. All the obtained coordination polymers have been characterized via single-crystal XRD, solid state FT-IR and powder X-ray diffraction (PXRD), the latter to confirm whether the crystal chosen for structural determination was representative of the bulk sample. A structural determination and description is reported for each of the coordination assemblies, including comparisons to underline similarities and differences between the networks, in terms of topology, packing interactions and lattice solvents. Thermogravimetric analysis coupled with mass spectrometry (TGA-MS) has been conducted on some of the obtained polymers, with the purpose of identifying and quantifying the solvent released from the lattice under mild condition, i.e. without the net losing its crystallinity. Further TGA-MS measurements have been carried out to check whether the process of solvent extraction is reversible and if the removed solvent could be replaced by other small molecules.
In Chapter 5, the notion of “expanded ligands” is introduced, referring to analogs of conventional ligands in which the donor sites are separated by metal-containing groups. With the aim of combining this concept of metalloligands with terpyridine-based building blocks, in the last part of the thesis asymmetric bis(terpyridines) have been prepared, containing the 2,2’:6’,2”-terpyridine bis(chelating) unit on one end and the divergent ditopic 3,2’:6’,3”- or 4,2’:6’,4”- terpyridine unit on the other. Homoleptic iron(II) complexes of the asymmetric bisterpyridine compounds have been synthesized; these are tetratopic metal-containing molecules, which represent a new type of expanded ligands, i.e. the expanded version of the purely organic bis(terpyridines), and are potential building blocks for the assembly of coordination polymers. All the intermediates and the final compounds have been fully characterized, and several crystallization experiments with different metal salts have been carried out in attempts to obtain supramolecular architectures incorporating the newly-prepared expanded ligands
Optical enhancement in water-splitting photoelectrodes: modeling and application to deposited nanoparticles
Full text linkProducing dihydrogen at large scale using sustainable, carbon neutral technologies is one of the main challenges facing the dual requirements of satisfying the global energy demand and minimizing the climate change. Photoelectrochemical water-splitting electrodes made with stable metal oxides have a great potential in this regard, but efficiency enhancement strategies must be found. Material nanostructuring and optical phenomena at the nanoscale such as plasmonic effects have been widely studied for improving the solar-to-hydrogen efficiency. The present thesis focuses on the theoretical and experimental understanding of such effects with an emphasis on strategies involving nanoparticle deposition which offer the advantages of being up-scalable and easy to implement. A method for analyzing the contribution of optical effects from nanostructures in enhancing the performance of photoelectrodes is described. The model involves electromagnetic simulations as well as modeling of the transport and transfer of photogenerated charges to the electrolyte. Some physical parameters can be determined by fitting the model to experimental measurements, which enable the investigation of various strategies for enhancing the performance of a fabricated electrode. This method is validated using published experimental data, and can be used to analyze experimental results and discriminate between optical and nonoptical (e.g. catalytic) enhancement. Next, improved performances obtained after nanoparticle deposition on the photoelectrode surface are investigated in detail. Bismuth vanadate, which has shown the highest reported performances as a metal oxide photoanode, is chosen as the active semiconductor material. Noble metal nanoparticles (plasmonic) are considered with different photoelectrode morphologies, for various position of the nanoparticles with respect to the active material and illumination directions. Nanoparticles of different materials are also compared. Both theoretical analysis and experiments are performed. It is found that the effects of the nanoparticles on the performances is strongly dependent on the illumination direction for a given morphology. Moreover, far-field effects are found to be predominant compared to near-field effects. Finally, an innovative strategy for enhancing the performance of water-splitting photoanodes illuminated from the electrolyte side is presented. It involves high refractive index nanoparticles such as titania. The performances of a bismuth vanadate photoelectrode are found to be enhanced by about 10% after deposition of nanoparticles. Theoretical and experimental analysis reveal that the effect originates mainly from reduced reflection losses arising from the interaction of light with the nanoparticles. Simulations with the previously described method suggest that this strategy could be applied to various photoelectrode materials where reflection losses are problematic such as iron oxide (hematite). It can potentially be used to improve further the performances of the best-performing reported devices. Compared to noble metal nanoparticles, high refractive index materials such as titania are economic and highly resistant in wide range of pH
Asymmetrical DπA 2,2'-diimines for homoleptic copper(I)-based dyes in dye-sensitised solar cells
Full text linkWe live in times of grave precariousness for our environment. It is a proven fact that global warming is mainly caused by the uncontrolled human emissions of CO2 in the atmosphere. The Earth as we know it may irreparably change if we as humans do not take action. The prospects for the future are in rapid change and the temperatures increase must be contained at all costs. In order to do so, new policies must be enforced to promote a conversion of our primary energy sources away from fossil fuels and greatly develop non-emissive sources. Hydroelectric, wind and solar are at the forefront of the renewable energies. The solar sector has been expanding quite rapidly in the last twenty years and will continue to do so. However, the high production costs of silicon solar panels are yet a limiting factor.
As described in Chapter 1, the invention of the dye-sensitised solar cell (DSC) paved the way to the development of a rising technology. The DSC is easy to manufacture and its inexpensive production costs are appealing. State-of-the-art devices are sensitised with organic or ruthenium-based dyes. However, ruthenium is a noble metal, its low abundance is responsible of the prohibitive costs, and together with its general toxicity it is unsuitable for commercial applications.
Copper(I) is a metal with similar photophysical properties to ruthenium(II), and it is widely available thanks to a greater abundance on the Earth’s crust. In Chapter 2, heteroleptic bis(diiminine) copper(I)-based dyes are initially investigated in the classical push-pull architecture, where two 2,2'-bipyridine ligands are functionalised with an anchoring and electron-donor units. The focus of this chapter is the effects of introducing alkynyl spacers in the ancillary ligand.
In Chapters 3–5, an attempt has been made to overcome known issues such as ligand dissociation and to improve the photoconversion efficiencies, by making a shift from the heteroleptic copper(I)-based dyes with the push-pull design to homoleptic copper(I)-based dyes with the alternative [Cu(DπA)2]+ design. Organic dyes bearing a coordinating domain are presented as new asymmetrical DπA 2,2'-bipyridine ligands. The old and the new design are compared thanks to the study of pairs of structural isomers. It is demonstrated that the properties of the [Cu(DπA)2]+ dyes are superior through a thorough analysis of the DSC performances, corroborated by DFT and TD-DFT studies. The properties of these dyes are investigated by structural changes at the 2,2'-bipyridine scaffold. Finally, the properties of the [Cu(DπA)2]+ design are further studied by broadening the scope with 2,2'-biquinoline ligands. Furthermore, one of the presented [Cu(DπA)2]+ dye nears 50% of relative photoconversion efficiency to the reference N719 ruthenium dye, being among the best efficiencies recorded for copper(I)-based dyes
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