Artificial Light-Harvesting Arrays for Energy Transduction

Abstract

ABSTRACT Natural systems make use of light-driven reactions for many diverse purposes, the most important being photosynthesis, vision and enzymatic repair processes. In all cases, the natural system is equipped with an effective light-harvesting unit to increase the flux of photon uptake. The light harvesting is usually engineered from readily available materials that can be recycled at the end of the season and that are assembled without the need for covalent chemistry. The main advantage of this strategy is that the active catalyst does not need to be in high concentration and can therefore be constructed from more expensive or rare materials. Artificial photosystems, in marked contrast, try to utilise the same compound for photon collection, accumulation and catalysis. This complicates the speedy identification of appropriate building blocks and leads to chemical instability. There have been some notable attempts to construct artificial light-harvesting units but, by-and-large, this is a relatively unexplored area of photochemistry. In this thesis, we examine some possible molecular materials for accretion into photon concentrators and consider the possibility of building some kind of useful device based on luminescent compounds. Chapter 1 provides an introduction to the field and develops a standard procedure for discussing molecular photophysics. Examples of selected functional molecules are highlighted and the special effects of using solid-state materials are considered. A brief comparison is made between natural and artificial light-harvesting arrays. Experimental techniques used to characterise the photon collectors are described, together with methods for data analysis. The main requisites for a successful artificial device are listed and a few potential types of molecular modules are selected. The types of application that might benefit from the availability of inexpensive photon concentrators are reviewed. Chapter 2 serves to introduce the compounds used throughout this work. These were invariably synthesized by other research groups and provided as small samples of highly purified material. The chapter also describes the experimental methods used and the protocols applied for data analysis. Chapter 3 begins the scientific discussion by introducing the subject of boranils. These are simple boron(III) chelates that can be synthesized easily in a one-pot process and on a large scale. In solution, the boranils are weakly luminescent due to the onset of intramolecular light-induced charge transfer. Only in non-polar solvents do we see significant levels of fluorescence. This is not the case for crystalline samples of boranils. Consequently, the chapter focuses on the photophysics of crystals formed from several different substituted boranils. The luminescence properties are related to the crystal structure determined by X-ray crystallography. Further insight into the properties of these compounds is derived from resonance structure analysis. Chapter 4 continues the work with boron(III) chelates but focuses on a small series of bis-boranils which vary according to the topology at the connecting phenyl bridge. The ortho derivative is subject to considerable electronic coupling between the closely spaced boranils and this compound is weakly fluorescent. The para derivative shows unusual behaviour in that fluorescence is strong in all solvents and the loss of emission found for the mono-boranil does not occur for this species. The meta derivative is more emissive than is the mono-boranil and the effect of polar solvent is less pronounced but more significant than found for the para isomer. Chapter 5 continues the above theme and describes the photophysical properties of two derivatives of boron dipyrromethene (BODIPY) having aryl rings replacing the conventional BF2 groups. Rotation of the aryl rings causes structura