Synthesis, structural elucidation and photophysical examination of a series of bio-inspired molecular constructs

Abstract

Natural photosynthesis continues to provide great inspiration to the molecular chemist and innumerable efforts have been made to mimic certain features of the natural organism using artificial materials. In terms of light-to-energy conversion, photovoltaic devices offer far more promise than photochemical systems based on complex organic chromophores. Nonetheless, interest in the latter remains unabated and many new aspects of molecular photophysics have been exposed during their study. The work reported here concerns the putative development of simple light-harvesting materials based on oligo-proline spacer groups. Unlike artificial lightharvesting units based on the accretion of rigid aryl hydrocarbons into short chains, oligoprolines can adopt various structures able to interchange on short timescales. This thesis reports our attempts to understand and control the factors influencing this fascinating structural diversity. Chapter 1 sets out the research area and objectives, starting with an introduction to the field of artificial photosynthesis and a brief discussion of electronic energy transfer (EET) processes. This is followed by an overview of Förster theory, this being the mechanism underpinning much of the work described in subsequent chapters. The introduction is expanded to cover the synthesis and post-synthetic modification of boron dipyrromethene (BODIPY) fluorophores. The latter are used as light-active labels throughout our work. Proteins and peptides are living systems able to change structure and length in response to external stimuli in a way that is simply not possible with artificial analogues. In part, this structural diversity arises from the special properties associated with the amide bond as reviewed in Chapter 2. Despite their extensive application in biological systems, there are surprisingly few artificial light-harvesting systems that utilise amide connections. This chapter, supported by later work, describes how the geometry of simple amides can be modulated by changes in temperature, solvent polarity, nature of substituents and length. The study depends heavily on sophisticated NMR spectroscopy and quantum chemical calculations. Critical comparison is made with simpler benzamides to identify key trends and to “benchmark” the calculations. Chapter 3 describes the preparation and characterization of a crystalline sample of a BODIPY derivative bearing a cis-proline residue at the meso-position. This dye crystallizes in the form of platelets with astonishingly strong red fluorescence. A key building block for the crystal is a pseudo-dimer where hydrogen bonding aligns the proline groups and separates the terminal chromophores by ca. 25Å. The latter favours the establishment of filaments rich in chromophore which, in turn, promotes excitonic coupling between nearby BODIPY residues. Fluorescence, which decays with a lifetime of 2.2 ns, is attributed to a delocalised and (slightly) super-radiant BODIPY dimer situated at the interface and populated via electronic energy transfer from the interior. Chapter 4 explains the dynamics of EET along a proline-based octamer decorated with emissive terminal chromophores. To this end, a new molecular dyad, PY-P8-PER, was synthesised and fully characterised, comprising a proline octamer sandwiched between pyrene and perylene terminals. Because of a unique signal, NMR spectroscopy could be used to assign the geometry at the pyrene-amide connection and how this was affected by changes in solvent or temperature. Circular dichroism was used to establish that the dyad formed a helix while quantum chemical calculations allowed recognition of the location of cis-amides along the chain. Moreover, by combining experimental and computational methods, the dynamics of EET events of different conformers of PY-P8-PER were measured and tuned by the solvent. Time-resolved fluorescence decay profiles were complex, deviating significantly from exponential processes, but simulation studies permitted exploration of the underlying conformational landscale. This chapter might be considered as a first step towards the construction of an artificial light-harvesting antenna based on oligoproline spacer units. Chapter 5 presents two isomeric molecular dyads that were synthesized where the lightactive terminals were formed by -extended BODIPY units of differing conjugation length. These dyads contrast with respect to the relative positioning of the chromophores while retaining the same overall composition. In Chapter 5 we consider how these changes affect the probability of intramolecular EET along the connecting bis-proline spacer. The short spacer favours a wide distribution of conformers due to amide isomerisation and inversion at the connecting point. Interestingly, the transition dipole moment vector is of comparable length to the separation between the terminals. A further consequence of this unusual feature is that the bis-proline geometry is locked in place such that the families of conformers interconvert only slowly. Chapter 6 examines the decoration of the polyproline backbone with complementary BODIPY fluorophores to accommodate efficient intrastrand EET. Three mono and di-styrylBODIPY dyes have been selected based on their known optical properties. Absorption maxima are varied systematically from 503 nm to 573 nm and finally to 646 nm. This strategy is continued with specific reference to the synthesis of a long helical polyproline backbone (14 prolines) attached to terminal BODIPY chromophores and with a rigid adamantane-based relay in the middle. This chapter includes a brief conclusion on the status of our attempts to construct bio-inspired artificial light harvesters. Chapter 7 discuss the future work planned to be done within the thesis. Finally, Chapter 8 presents the experimental techniques and synthetic protocols used throughout the work and provides a full characterization of the new compounds