Advances in ruthenium-based light harvesting materials and photocatalysis: From molecular photosensitizers to photoactive frameworks

Hennessey, Seán
The presented manuscript deals with the areas of light harvesting and photocatalysis in solar chemistry. The work presented is divided into several sections, beginning with an introductory chapter on the main topics involved in the investigational work. The experimental research chapters are divided into five separate works, moving from molecular photosensitising systems to photoactive heterogeneous frameworks. Chapter 1 - Solar-driven technologies are very promising to the scientific community, in particular that of solar chemistry. In recent years researchers have developed photocatalysts that are capable of producing chemicals by using sunlight as a source of energy; consequently causing a surge in interest into new development of the technologies in this area. The state-of-the-art of these solar chemical processes is discussed in this chapter; from the inspiration of natural light-harvesting systems to the design and construction of molecular photo-responsive systems and photoactive heterogeneous materials. In particular, the photochemical processes of CO2 reduction, water-splitting and organic oxidations in the most recent literature are highlighted. The drawbacks of such systems are discussed, and the areas of improvement, which this research aims to address, are highlighted. Chapter 4 - Starting from previous work and experience in the ChemLight group, Chapter 4 focuses on the preparation of photocatalytic molecular systems in the form of dinuclear ruthenium complexes or ‘dyads’. Although many examples of photocatalytic dyads are reported, improvements in the area are in constant progress by tuning the auxiliary and bridging ligands to enhance electron transport. Herein, a bridging ligand, 2,2-(pyrazole-3,5-diyl)dipyridine (bpp), was used to synthetize a novel family of dinuclear ruthenium dyads with both a light-harvesting and a catalytic unit. The performance of the dyads was investigated by functionalisation of the bridging ligand, as well as the catalytic moiety (Ru(tpy)Cl). The modifications have been performed to investigate their influence on the electronic communication between the two metal centres for future photocatalytic systems. Chapter 5 - Moving from molecular systems to heterogeneous materials, Chapter 5 looks into the formation of new light-harvesting metal-organic frameworks (MOFs) using novel diphenyl and diphenylacetylene ligands functionalised with hydroxy, thiol and carboxylate groups. To improve the electron transport properties of the ligands, electron-rich alkyne spacer units were used. These ligands were utilised in conjunction with metal ions such as zinc and zirconium, for a proof-of-concept of the synthetic process, to then apply in the utilisation of metal clusters such as silver, with the aim to create new photoactive MOFs. The knowledge acquired in the synthesis of the ligands, and of the frameworks, is used as inspiration to build new photoactive metalloligands for their use in the formation of coordination polymers and MOFs. Chapter 6 - One of the key goals in this thesis is the use of photoactive ruthenium polypyridyl complexes for improving both homogeneous and heterogeneous light-harvesting systems. Chapter 6 focuses on the synthesis and characterisation of a library of ‘linear’ functionalised [Ru(tpy)2]-type complexes for further use as metalloligands. The most known carboxylate and much less explored thiol and hydroxy functional groups were used to explore the properties of these complexes and modes of connectivity with metal nodes. The combination of the synthetized metalloligands with transition metal ions has been performed to develop photoactive porous coordination polymers and multicomponent metal-organic frameworks (MMOFs). Chapter 7 - Building on the work in Chapter 6 with the use of metalloligands in supramolecular structures, Chapter 7 describes the successful isolation of a new photoactive 1D coordination polymer. This has been achieved by the synthesis of a luminescent ruthenium metalloligand bearing two 2,6-di(quinolin-8-yl)-pyridine carboxylic acid (dqpCOOH) ligands, which has been successfully synthesised, fully characterised and single crystals of the compound isolated. The new metalloligand has been coordinated to zinc ions using a one-step solvothermal method, to give a crystalline multimetallic photoactive 1D coordination polymer, Ru-(dqpCOO)-Zn-(OOCH)2. Single crystals of the coordination polymer were developed, and the crystal structure of the material determined by X-ray diffraction. Furthermore, advanced microscopy, electrochemistry, as well as cutting-edge photophysical analyses, have been used to extensively characterise the new RuZn coordination polymer. The development of this material gives a new strategy in the design of novel photoactive polymers as multimetallic building blocks for their use in light-based applications such as photovoltaics and photo(electro)catalysis. Chapter 8 - In order to increase the stability of these ruthenium polymeric systems, covalent framework principles were utilised with ruthenium metalloligands to build unique multidimensional metal-covalent organic frameworks (MCOFs). Chapter 8 looks at exploiting photoactive [Ru(tpy)2]-type complexes bearing aldehyde functionalities for the development of unprecedented photoactive MCOFs. This has been achieved by reaction with a tetra-substituted amino-bearing pyrene ligand to develop a new luminescent Schiff-base MCOF. The material has been characterised by an extensive range of electrochemical, thermal and photophysical techniques. In addition, an array of microscopy studies, including the rarely used in MCOF analysis, scanning tunnelling microscopy (STM), have helped recognize the periodicity of the material. Moreover, a series of molecular ruthenium complexes bearing pyrene moieties have been synthesised for photophysical comparative analyses with the MCOF.
NUI Galway
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland