School of Chemistry - Theses

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Synthesis and antioxidant capacity of chalcogen-containing furanose derivatives

Velichenko, Yuliia ( 2021)

Cardiovascular diseases are known to be a leading cause of death around the globe. Oxidative damage of endothelium plays a crucial role in development and progression of hypertension, atherosclerosis and other pathological cardiovascular conditions. Therefore, antioxidants that can effectively remove reactive oxygen species, represent promising therapeutic approaches. Sulfur, selenium and tellurium compounds act as effective oxidant scavengers due to their fast reaction rates with typical biological oxidants. Previous studies have shown that a novel selenofuranose derivative, specifically 1,4-anhydro-seleno-D-talitol, can modulate oxidative damage of biomolecules in plasma and exhibits wound healing activity. It should be noted tissue repair is extremely important in the treatment of vascular pathologies. Little is known about furanose analogues of seleno-D-talitol. Their antioxidant capacity and biological action were not investigated while knowledge of those could provide guidance for the development of novel selenosugar based therapeutics. In this thesis, approaches to the synthesis of selenium and tellurium furanose derivatives are described. Selenium and tellurium sugars were prepared from a range of different starting materials such as monosaccharides, lactones and sugar alcohols. Selenide and telluride moieties were inserted into the sugar core using sodium selenide and sodium telluride prepared in situ. Final furanose analogues were synthesised in up to 8 steps with good yields. Unlike selenium containing furanose derivatives, most of their tellurium counterparts were unstable light sensitive compounds presumably due to the larger chalcogen atom causing strain in the sugar ring. Additionally, thio-D-talitol and D-talitol with oxygen in the sugar ring were prepared for comparison in antioxidant studies. Antioxidant activity of the synthesised compounds was determined in Chapter 3. Cyclic voltammetry and linear sweep voltammetry were used for investigation of oxidation of chalcogen-containing selenofuranose derivatives. It was shown that the primary oxidation of the sugars is an irreversible process which, presumably, occurs through oxygen transfer reaction. The oxidation potentials were determined and the half-wave potentials for electrochemical oxidation of the chalcogen-containing sugars were calculated. In Chapter 4 the in vitro vascular studies were carried out to investigate the effectiveness of the synthesised chalcogen-containing furanose derivatives, toward the prevention of endothelial damage in mouse aorta under oxidative stress conditions. The in vitro studies revealed that the stereo configuration of the sugar core significantly influences the biological activity of the chalcogen-containing furanose analogues of seleno-D-talitol. In the range of selenofuranose derivatives, seleno-D-talitol was shown to prevent endothelial dysfunction more effectively than its analogues. Furthermore, the structural isomer, hexose sugar selenogalactitol demonstrated different effects on mouse aorta than the selenopentoses, by presumably inhibiting endothelial nitric oxide synthase. As expected, thio-D-talitol exhibited a lower ability to protect the endothelium in mouse aorta from oxidative stress, than its selenium counterpart, seleno-D-talitol. Telluro-D-talitol appeared to be toxic to cells in mouse aorta.
Photocatalytic carbon dioxide reduction with water

Agnihotri, Shruti ( 2021)

Abstract New catalyst discovery to enhance photocatalytic reduction of CO2 with H2O is a materials space problem, where determining the ideal catalyst composition, catalyst processing conditions, and optimising the reaction conditions is a “wicked” problem. To examine more of the materials space, we need to develop new high throughput screening methods. A new continuous flow photocatalytic reactor with enhanced capabilities to discover robust photocatalyst for the photocatalytic reduction of CO2 is thus designed in this project. A key component in the new reactor is in-situ thermal monitoring of the activated catalyst. A more detailed analysis is made on selected catalysts that promote a greater temperature drop under constant illumination, that is more active catalysts will require more energy for this highly endothermic reaction. This reactor has the potential to accelerate the discovery/optimization process of required catalysts, when fully automated. Additionally, its continuous gas flow configuration, flexibility to test varied set of reaction parameters with precise control over input parameters makes it distinguished from conventionally used reactors. Validation of this custom-designed photocatalytic reactor setup, which is based on entirely new metric to identify reactivity of an active catalyst was commenced using benchmark catalyst material TiO2. Further, extended experiments using TiO2/C3N4 as photocatalysts confirmed the usefulness of in-situ temperature monitoring screening method, as detected enhanced CO2 reduction was linear to monitored drop in catalyst’s temperature. This newly built photocatalytic reactor set up was then used to explore photocatalytic CO2 reduction reactions over range of prepared catalysts. Catalyst designing was focussed to suppress recombination losses during photocatalytic reaction, which is reported as key responsible factor of low obtained product yields in the field. For this, nanoparticle – nanocomposites of C3N4 with various transition metal complexes were synthesized in this work and subsequently tested as photocatalysts. C3N4 based composite catalysts were found to be effective in enhancing photocatalytic CO2 reduction reaction, when compared to only single catalyst systems. Also, controlled syngas formation was made possible when modified C3N4 was combined with cobalt phosphate using layer by layer assembly method. Obtained results were also exemplifying that composite photocatalyst’s performance is also sensitive to used catalyst preparation method, which relates to provisioning more active sites over catalyst surface to enhance photocatalytic reaction rates. In addition to C3N4, another low-cost semiconductor option- FexOy has also been explored as photocatalyst for CO2 reduction reaction. Low conduction band edge energy of Fe2O3 was addressed by structural and surface modifications, where electrospinning method was used for fibrous catalyst preparation with additional doped metal cations. These strategies were helpful in enhancing selective CO formation as CO2 reduction product, significantly. Structural and morphological parameters of explored nano composites effecting catalytic performance were investigated using X-ray absorption spectroscopy and Electron microscopy.
Spectroelectrochemistry of Semiconductor Nanocrystals

Ashokan, Arun ( 2021)

Semiconductor nanocrystals exhibit well-known, size-dependent optical and electronic properties. Control over the charge carriers in semiconductor nanocrystals enables the possibility to tune the optical response. One way to achieve this is through electrochemistry. Carrier modulation through electrochemical methods allows more precise control over electron transfer compared to methods such as photocharging and chemical redox reactions. By combining electrochemistry with spectroscopic techniques, the charged states in semiconductor nanocrystals can be studied in detail. A spectroelectrochemical setup has been developed to study the charging of semiconductor nanocrystals in solution and its influence on absorption and photoluminescence (PL). A negative trion state can be generated in CdSe quantum dots (QDs) and stabilised for hours under an applied cathodic potential. By monitoring both the absorbance and fluorescence changes, one can determine whether charge carriers are free or trapped. The total number of electrons injected into the QDs can be estimated from current and coulometry measurements. Hole injection into CdSe QDs induces corrosion of the lattice, whereas injection into nanocrystals shelled with CdS induces bleaching. Coupling the spectroelectrochemical setup with time-resolved PL measurements reveals the trion lifetime of CdSe/CdS QDs as a function of shell thickness. In the last section of the thesis, the effects of charge injection on CdSe nanoplatelets (NPLs) is explored. In contrast to QDs, hole injection into the NPLs enhances the photoluminescence.
Amphiphilic Block Copolymers for Morphological Control in Bulk Heterojunction Solar Cells

Ahluwalia, Gagandeep ( 2021)

Organic solar cell (OSC) is a type of solar cell that utilise a solution-processable organic semiconductor material as a photoactive component. The use of less toxic, flexible organic semiconductors in solar cells leads to a formation of a lightweight device with a short energy payback time. Along with this, it is easy to fabricate organic solar cells on a large scale by roll-to-roll printing, which is the driving force for industrialisation. The recent efforts on organic materials such as small molecules or polymers have pushed the OSCs towards a milestone efficiency of more than 18 %. However, achieving the desired control over the morphology by blending of the donor (electron-rich) and acceptor (electron-deficient) materials and reproducibility for industrial scope has remained a challenge, based on the fact that often deposition techniques used in the laboratory are different to those in continuous operation, i.e. spin coating compared to slot die deposition. Moreover, the use of chlorinated solvents for processing restricts OSCs large-scale manufacturing scope. In order to increase the commercial applicability of OSCs, a bi-continuous interpenetrating network of the donor and acceptor block is required, where donor and acceptor materials should have 10-20 nm domain sizes with continuous interfaces to promote the charge generation/separation process. Also, the photoactive component should show solubility in non-chlorinated or industrial relevant solvents. Fully conjugated block copolymers, where a conjugated donor and acceptor block are covalently linked with each other, have an ability to attain a defined morphology by manipulating the Flory–Huggins segment–segment interaction parameter. The BCP materials could self-assemble into a thermodynamically favoured morphology and offers continuous interface, control over domain size, long term stability, and reproducibility. However, the BCP’s have failed to deliver a high performing OSC so far. The primary reason is the difficulty of achieving a clean phase separation in BCP’s due to the high inter-block interactions parameter between donor and acceptor block, which enhances the charge recombination over charge separation process. Moreover, developing a fully conjugated block copolymer with a specific molecular weight, block ratio, and high purity is lacking behind due to the unavailability of the synthetic approach. The synthesis of BCP via a well-known step-growth polymerisation usually ends-up containing a mixture of the desired BCP along with a significant number of polymer contaminants, which impacts device performance. Furthermore, the BCP’s reported in the literature usually require chlorinated solvents for the processing, which limits its industrial scope. For industrial adoption of OSCs, we have developed a strategy to control the synthesis of an amphiphilic di-block copolymer containing high performing push-pull donor and acceptor blocks to achieve a clean phase separation and solubility in non-chlorinated solvents. In order to obtain control synthesis of a block copolymer, firstly, we have developed a strategy to control the molecular weight/ end-group functionality of homopolymers. The strategy involves designing and synthesising of asymmetric functionalised push-pull monomers that undergo a Suzuki or a Stille pseudo-living polymerisation. Herein, studies on four different homopolymers, i.e., p(BDT-BT), p(BDT1-BT), p(IID-TT), p(NDI-TT), were performed and control over the p(BDT-BT), p(BDT1-BT) polymer via Suzuki catalyst transfer polymerisation and p(NDI-TT) via Stille catalyst transfer polymerisation was achieved. Two fully conjugated amphiphilic di-block copolymers with a specific molecular weight and block ratio (1:1) were synthesised via a stepwise or one-pot procedure. In a stepwise method, TfO-p(NDI-TT) was initially synthesised using a pseudo-living Stille polymerisation with a single triflate (OTf) end group and specific molecular weight. Subsequently, TfO-p(NDI-TT) was used as a macro-initiator to grow an amphiphilic di-block copolymer (BCP1) via a grafting-into approach, where the donor polymer block was grown using a pseudo-living Suzuki polymerisation. Moreover, the one-pot synthesis of an amphiphilic di-block copolymer (BCP2) with the sequential addition of donor and acceptor monomer was performed utilising similar optimised conditions developed for stepwise block copolymer synthesis. Furthermore, the preliminary morphological behaviour of BCP1 and BCP2 was investigated using X-ray scattering techniques. At last, a device containing a BCP1 as an active layer achieved an efficiency of 3.2 %, whereas BCP2 showing a maximum efficiency of 2.7 % was reported. This work has demonstrated a potential route of utilising asymmetrically functionalised push-pull monomers to achieve control over di-block copolymer synthesis containing high performing donor and acceptor polymer.
Spectral Engineering of QD-organic Hybrid Systems

Wu, Na ( 2021)

Colloidal semiconductor nanoparticles, which are commonly referred to as quantum dots (QDs), are inorganic crystals with sizes in the nanometre regime. QDs have been intensively studied because of their potential applications in diverse fields including light harvesting systems, light emitting devices, optoelectronics, sensing, and biological applications. Colloidal QDs consist of an inorganic core which is coated with an organic shell layer for stabilization. The inorganic core exhibits typical size-, shape-, and composition-dependent optoelectronic properties, while the organic surface ligands control the physical and chemical properties. The hybrid organic-inorganic structure makes them multidimensional materials which can be further functionalised by covalent attachment of molecules to the nanocrystal surface. This thesis aims to advance our understanding of this surface functionalization and how it can be used for spectral engineering. By combining QDs and functionalized organic molecules, the resulting hybrid systems have unique optical properties and are of great interest for applications in solar energy conversion and photosynthetic light harvesting. This thesis consists of the following sections: Firstly, blue emitting QDs were used as down-shifting materials for UV LEDs to afford targeted, narrow and stable, blue light sources. The solid-state QD-polymer composites were fabricated by blue-emitting alloyed CdZnSeS/ZnS and CdZnS/ZnS QDs in poly(methyl methacrylate) (PMMA) films. Functionalized PMMA molecules were employed as surface ligands, which could improve the compatibility of QDs with PMMA matrix. The photophysical properties of QD-PMMA composite films were studied with the controlling of polymer ligands, including functional group and polymer ligand binding ratio. The thiol group-based polymer ligands (SH-PMMA) quenched the photophysical properties of QDs in composites due to hole trapping by the thiol groups. Conversely, the carboxylic acid group-based poly(methyl methactylate-co-methacrylic acid) (P(MMA-co-MAA)) improved the dispersion of QDs in PMMA without altering the photophysical properties. The new ligands enable excellent solubility of QDs in a PMMA matrix. The photostability of CdZnS/ZnS QDs in PMMA was studied under UV light irradiation. QDs exhibited quenched PL intensity due to photooxidation of the semiconductor lattice catalysed by oxygen in the film. By using air-free CdZnS/ZnS QDs and appropriate encapsulation methods with a UV epoxy based polymer, the lifetime of QDs under UV light illumination was significantly extended. These results demonstrated that UV epoxy encapsulated air-free CdZnS/ZnS QD-PMMA film is extremely photostable, which is promising for down-shifting UV light to afford bright and narrow blue light. Then the investigation of QDs in light harvesting was performed by the fabrication of QD-organic fluorophore hybrid materials in PMMA film based on efficient surface functionalization. Three different emitting QDs were used as energy donors with the addition of two perylene diimide (PDI) dyes, bPDI3 and LR305, as energy acceptors to form QD-bPDI3-LR305 three-chromophore hybrid system in PMMA. The Forster resonance energy transfer (FRET) process was studied in these hybrid systems with three different emitting QDs. By tuning the concentration of bPDI-3, a two-step FRET process was identified with energy flowing from the QDs to LR305 via bPDI-3 in the PMMA film. This was the dominant energy transfer pathway in these three-chromophore systems. The photoluminescence quantum yield (PLQY) of QD-based films was significantly increased with the presence of efficient energy transfer process under UV light excitation. In this case, the UV light harvesting was broadened with the introduction of blue emitting QDs in organic hybrid system and exhibited bright red light emission. Regarding their light harvesting performance, the internal quantum efficiency of QD-bPDI3-LR305 LSC devices was quite close to that of bPDI3-LR305 LSC devices. This is attributed to the efficient energy transfer process and near unity PLQY of hybrid systems. These results indicated that the QD-organic hybrid system provides a promising way for broadening the light harvesting range, and the spectral engineering of QDs in solid-state was successfully achieved with near unity two-step FRET process. Finally, the interaction between QDs and surface ligands was studied with the fabrication of QD-organic fluorophore composites in solution. PDI derivatives, containing amine and carboxylic acid anchoring groups and two different anchoring chain lengths, have been designed and synthesized. In QD-PDI composites, PDI derivatives were used as energy acceptor with CdZnS/ZnS QDs as the energy donor. The effects of molecular geometry on both PDI binding and the energy transfer behaviour have been investigated in the composites. It was observed that stable carboxylic acid binding and longer anchoring chain length were essential to achieve high PDIs loading ratios on the QD surface. In addition, the FRET process was also closely related to the anchoring group and side chain length in QD-PDI composites. The colloidal stability of QD-PDI composites was investigated. Desorption of PDIs from the QD surface occurred and the FRET process efficiency decreased with increasing donor-acceptor distance. Due to the bulky structure of the PDI derivatives, oleic acid molecules could displace them on the QD surface and this exchange process slowed down QDs aggregate formation in solution. Among these composites, the QD-PDI-C11-COOH composites exhibited the best colloidal stability. These results demonstrated that both strong ligand binding and long anchoring chain length are essential to achieve good FRET-efficient and colloidally stable QD-PDI composites. Based on the results presented here, the understanding of QDs surface chemistry is advanced and the development of QD-based light conversion systems with minimum energy loss is further broadened.
Direct Assembly of Single Nanocrystal Arrays

Zhang, Heyou ( 2021)

Nanocrystals are a remarkable class of materials, which exhibit size-dependent optical and electronic properties. Numerous applications have been proposed for these materials but they suffer from a key handicap. Nanocrystals are generally made in solution, rendering integration into devices very challenging. Therefore, there is an increasing demand for new fabrication methods to transfer nanocrystals from solution to a solid-state substrate. In this thesis, we develop a new, direct assembly method based on electrophoretic deposition (EPD), which we call Surface Templated Electrophoretic Deposition (STEPD). Our research starts by demonstrating the successful assembly of single gold nanocrystal arrays. The strength of the applied electric field and the electrolyte concentration are key parameters that control the assembly process. The orientation of asymmetric gold nanorods can also be controlled by carefully designing the template. With further experimental and theoretical investigation, we find that the electrically induced dipole moment on asymmetric nanorods plays a major role in orientating the nanorod during EPD assembly. Our STEPD method is not only able to assemble gold nanorods in designed orientation horizontally but also able to assemble gold nanorods vertically with respect to the substrate. The universality of STEPD is also demonstrated by the successful assembly of gold nanocrystals with different sizes and shapes, magnetic Fe$_3$O$_4$ nanocrystals, fluorescent organic nanoparticles and semiconductor quantum dots with different photoluminescence. During our investigation, we find that there is a minimum particle size limit in STEPD. Very small nanocrystals (i.e. < 20 nm in size) are difficult to assemble via EPD due to their smaller total charge and stronger Brownian motion. Finally, we propose three potential applications derived from STEPD assembled arrays including hydrogen gas sensing, single particle addressing and anti-reflective metasurfaces. Through this thesis, we believe the direct STEPD assembly method holds great potential as an efficient and versatile assembly method for large-area, single nanocrystal arrays.
Visible-Light Promoted Carbonylation of Unactivated Alkyl Halides and Inert C(sp3)-H Bonds in Continuous Flow

Forni, Jose Augusto ( 2021)

Multicomponent carbonylations with carbon monoxide gas is an increasingly important strategy to generate carbonyl-rich scaffolds, such as amides and ketones. Traditionally, carbonylation reactions are catalysed by transition metals in a 2-electron pathway. While the carbonylation of aromatic substrates is well established, methodology to C(sp3) hybridised substrates remains underdeveloped. The p-acidity of carbon monoxide renders the metal catalyst less reactive towards an oxidative addition to alkyl halides and completive b-hydride elimination precludes product formation. Successful C(sp3) carbonylation has been achieved via transition-metal catalysed C(sp3)-H activation. These reactions, however, require elevated temperatures which could negatively affect selectivity and cause degradation of the starting materials and/or products. Additionally, this approach generally requires prolongated reaction times and stoichiometric amounts of external oxidants. Radical chemistry has emerged as an important alternative to overcome the limitation of the 2- electron carbonylation reactions. In this approach, the reactivity of carbon centred radicals towards carbon monoxide to form carbonyl compounds is explored. In the context of carbonylation reactions, alkyl radicals have been generated mainly from alkyl halides by means of explosive and toxic radical initiators or UV radiation. Direct C(sp3)-H alkylation is also reported, however, available methodologies required the use of toxic lead salts and prolonged reaction times. With the advent of visible light photoredox catalysis, safer and milder conditions to generate radicals have emerged. In the context of carbonylation reactions, the low redox potential of alkyl halides (Ered = -1.90 to -2.90 V vs SCE) greatly limited the use of this technique. This constrain of traditional photoredox methods can be overcome by the recently developed multiphoton catalytic systems and electrochemical methods, which both remain underexplored. Additionally, a visible light photocatalysed method for direct carbonylation of C(sp3)-H bonds remains unreported. In this PhD thesis, visible light multiphoton photoredox catalysis was explored to discover new C(sp3) carbonylation reactions. Implementation of continuous flow technology allowed for short residence times, safe handling of toxic gases and ready scalability. In chapter one, an introduction to transition-metal catalysed and radical C(sp3) carbonylation is provided. In this section, the current examples and limitations are highlighted. Next, the principles of conventional and multiphoton photoredox catalysis are discussed. The general objectives of this PhD thesis are subsequently elaborated followed by an introduction to flow chemistry. In chapter two, a new visible light promoted aminocarbonylation of unactivated alkyl iodides with carbon monoxide gas is reported. The reaction harnesses the highly reducing capabilities of the multiphoton tandem catalytic cycle of the [Ir(ppy)2(dtb-bpy)]+ photocatalyst to engage energy demanding substrates in an oxidative quenching cycle. The reaction presented excellent functional group tolerance and a broad substrate scope. This new methodology allowed for the late-stage functionalisation of natural products, in which five new cholesterol derivatives were synthetized in good to excellent yields. Steady state quenching experiments confirmed operation of the tandem photocatalytic cycle and DFT calculations led to the conclusion that the reaction proceeds via radical chain propagation. The use of continuous flow allowed short reaction times, safe handling of CO gas and scalability of the reaction. In chapter three, the tandem photoredox of the [Ir(ppy)2(dtb-bpy)]+ photocatalyst was explored in the first visible-light promoted radical carbonylative hydroacylation of alkenes with unactivated alkyl iodides and bromides. Optimisation studies showed this transformation benefits from the presence of water in the reaction mixture. The reaction showed broad substrate scope, good functional group tolerance and was high yielding. Deuterium labelling experiments studies demonstrated that this transformation proceeds via a radical polar cross over mechanism. The anionic intermediate was subsequently trapped with carbon dioxide leading to the synthesis of 1,4-keto esters and furanones via a 4 component multi-gas reaction. In chapter four, various substrates and photocatalytic systems were investigated to promote the carbonylation of inert C(sp3)-H bonds via an intramolecular hydrogen atom transfer process. The desired product could be obtained, however in low yield. The limitation of the photocatalytic methods motivated investigation of an electrochemical approach. During this study, it was discovered a new unexpected electrochemical arylation of inert C(sp3)-H bonds, which was fully developed in the subsequent chapter. In chapter five, a new electrochemical and catalyst-free arylation of arylation of inert C(sp3)-H bonds of aryl sulfonamides is reported. This reaction harnesses the ability of electron deficient cyanoarenes to function as both mediators and arylating reagent. Mechanistic studies demonstrated that the cathodic cyanoarene radical anion promotes single electron reduction of the halogenated aryl sulfonamide leading to aryl radical formation. Subsequent intramolecular hydrogen atom transfer led to the arylation of the remote C(sp3)-H arylation. An extensive study was conducted to optimise this reaction. Substrate scope was limited as the reaction was very sensitive to the nature of the cyanoarene and substitution on the sulfonamide.
Investigations into Chalcogen Bonding

Fellowes, Thomas Joseph Haldane ( 2021)

Chalcogen bonding (Ch-bonding) is a non-covalent attractive interaction between a Lewis base and a positively polarised group 6 element, typically sulfur, selenium, or tellurium. It has been recognised as complementary to the related phenomena of hydrogen bonding and halogen bonding, and it has potential applications in fields as diverse as crystal engineering, anion sensing, and medicinal chemistry. This work was originally motivated by the design of a Ch-bonding DNA binder. Throughout the design and synthesis of this molecule, we investigated some simpler systems to gain a better understanding of the Ch-bond. X-ray diffraction analysis of co-crystals with Lewis bases afforded data which showed the Ch-bond length was inversely related to the electron density at the selenium atom. We also observed a lengthening of the endocyclic bond opposite to the Ch-bond, indicative of partial covalent character of the interaction. Experimental electron density was analysed within the QTAIM framework, which showed that critical points associated with Ch-bonds actually bear more similarity to closed-shell, electrostatic interactions. The unique NMR properties of the 77Se nucleus were used to investigate Ch-bonding through solid state NMR. Due to the large degree of polarisation of the selenium, an enormous chemical shift anisotropy is observed. Measurement of the chemical shift tensor in a single crystal of a Ch-bonded complex showed that this is due to the approach of the Lewis base. In a departure from selenium chemistry, we observed a O...O short contact in a o-nitro-O-aryl oxime which showed characteristics consistent with a Ch-bond. This piqued our interest, as oxygen is usually considered to be insufficiently polarisable to form Ch-bonds. We prepared and studied a series of derivatives, and found sufficient evidence to claim Ch-bonding is occurring in this system. This may have implications for our understanding of reactive oxygen species such as peroxides and nitrate radicals.
Exploring the applicability of Density Functional Theory approximations

Mehta, Nisha ( 2021)

The development of density functional approximations (DFAs) is a very active research field, with the number of approximations continuously growing. At the same time, there is a lack of published work that gives a fair evaluation and comparison of DFAs on an equal basis. Considering the vast and important Density Functional Theory (DFT) user community, the introduction of such a variety of techniques can lead to confusion and a low acceptance rate of newly proposed methods in general. In fact, from a user's point of view, it would be beneficial to have access to only a handful of methods that distinguish themselves through their accuracy, reliability, and general robustness in benchmark studies and subsequent applications. Therefore, the primary objective of this thesis is to identify ways to the dizzying array of DFAs. Throughout this thesis, a range of topics will be discussed. They originate from the idea of making computational techniques more reliable. The findings and applications of reliable and efficient computational techniques are at the heart of this thesis. The first project of the thesis is concerned with the thorough evaluation of the double-hybrid density functionals on a large and diverse GMTKN55 database for general main-group thermochemistry, kinetics, and noncovalent interactions. Our study represents one of the largest and most thorough double hybrid studies ever conducted and can serve as an important benchmark informing method developers and users alike. We then shed light on the communication gap between method developers and users, and show that despite tremendous scientific progress in the area, people's perceptions of their favourite approaches have not changed much. The third project provides the most comprehensive database for chalcogen bonding interactions (CHAL336), with high-level wave function based interaction energies. This is followed by the first thorough evaluation of the performance of DFAs for such interactions. Fourthly, we present a proof-of-concept study with the aim of reducing the cost of double hybrid DFT calculations. In this framework, we present the suitability of Grimme's geometric counterpoise correction scheme for basis set superposition errors in double hybrids. Finally, we utilise some of the most reliable and computationally efficient DFT methods to gain insights into cyclotricatecylene-based molecular capsules that can selectively extract ions from solution. Overall, this thesis encourages a shift in the computational procedures surrounding thermochemical problems, allowing users to make more informed decisions about using DFT methodologies in computations.
Development and Assessment of new Time-Dependent Long-Range Corrected Double-Hybrid Density Functionals for Excited States

Casanova Paez, Marcos Andres ( 2021)

This thesis presents the implementation, assessment, and applicability of the currently most accurate density functional approximations (DFAs) for the calculation of vertical excitation energies by means of linear-response time-dependent density functional theory (TD-DFT). Firstly, we demonstrate how the inclusion of a single parameter, which controls the interplay between interlectronic short- and long-regimes, improves global double-hybrid density functional approximations (DHDFAs) when it comes to long-range excitations; such as Rydberg states and charge-transfer (CT) transitions, without much loss of accuracy for local-valence ones. In this context, we define the two first long-range corrected (LC) DHDFAs optimised for excitation energies, namely, wB2PLYP and wB2GP-PLYP, being the best TD-DFT methods until then. Furthermore, we include an additional analysis of the CT problem after noticing some misconceptions by some in the developer community regarding the application of global DHDFAs as an alternative to the LC ones. In the second part, we investigate the performance of the best DHDFAs for singlet-singlet transitions in the Tamm-Dancoff Approximation (TDA) but, more importantly, we revive the applicability of DHDFAs for singlet-triplet excitations after 12 years of slumber. Herein, we demonstrate once again that our LC-DHDFAs are the best-performing methods not only for singlet but also for triplet excitations, being the most accurate and robust methods until then. Finally, inspired by the precursory work done by Schwabe and Goerigk in 2017, we extend our two previous studies by combining our methods with the Spin-Component Scaling (SCS) and Spin-Opposite Scaling (SOS) techniques, with the main difference being the inclusion of LC-DHDFAs and singlet-triplet excitations in the context of DHDFAs. We also defined the currently two best LC-DHDFAs optimised for excitation energies on the market, namely, wB88PP86 and wPBEPP86. We particularly note that the SCS and SOS variants our new functionals delivered the best TD(A)-DHDFA results to date for 1La and 1Lb transitions in Policyclic Aromatic Hydrocarbons (PAHs). Finally, SCS/SOS-wB88PP86 and SCS/SOS-wPBEPP86 have been demonstrated to be the most accurate and robust approximations for the calculation of singlet and triplet excitations regardless of the type of transition, i.e., they describe local-valence, Rydberg states, and CT transitions with the same excellent accuracy, and we highly recommend them for future applications. Note that during the peer-review of this work, new SCS/-SOS methods were also published in the very same journal. Therefore, we also included them after some minor comments from the reviewers. Nevertheless, our newly proposed methods are still the best-performing SCS/SOS-DHDFAs for the calculation of excitation energies. All the methods presented in this thesis are implemented into the ORCA5 quantum package, which is free for academics.

School of Chemistry - Theses

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