Spectroscopic analysis of electronic energy transfer in molecular cassettes formed around boron dipyrromethene dyes

Abstract

Photosynthesis, in its many diverse forms, has provided inspiration for countless researchers over several centuries and continues to spring surprises and new concepts. At the simplest level, photosynthesis can be considered to store sunlight in the form of chemical (or electrochemical) potential. As such, it is often proposed as a model for artificial systems aimed at the conversion and storage of solar energy. One of many key components of photosynthesis concerns the collection of sunlight by various pigments and the transfer of the resultant exciton to a reaction centre, where fuel formation can take place. In this thesis, we examine chemical systems that facilitate electronic energy transfer (EET) between chromophores arranged in rather simple molecular architectures built around boron dipyrromethene (Bodipy) dyes. These latter compounds are taken from an ever-expanding family of robust, highly fluorescent synthetic reagents developed originally as laser dyes and bio-labels. Chapter 1 gives a brief introduction to the general field of EET and covers a few basic concepts special to the photosynthetic apparatus. This is followed by a brief consideration of Förster theory, which is the staple mechanism underpinning much of the work covered in later chapters, and mention of the alternative Dexter theory for EET. By way of acknowledging that we are not the only researchers to explore this type of work, we provide a few key examples of molecular systems designed to probe various aspects of intramolecular EET. These examples cover Bodipy-based arrays and certain bio-inspired molecular systems. In Chapter 2, we describe the behaviour of certain sterically unhindered Bodipy dyes as fluorescent probes for rheology changes, most notably variations in viscosity under ambient conditions. This situation depends on changing the degree of (micro) friction between an appended meso-aryl ring and the surrounding medium. In order to vary in a systematic manner the resistance to gyration of the aryl ring, the photophysical properties of the dye have been recorded in different media and as functions of temperature and pressure. Local viscosity is also affected by the presence of an inert polymer. Extending the system to include an unusual bichromophore where the linkage is through boron-oxygen bonds switches off the sensory action due to light-induced electron transfer. Chapter 3 includes a critical comparison of EET within two disparate molecular types; namely, covalently-linked and non-covalently-linked molecular dyads bearing ii identical subunits drawn from the Bodipy family. Here, the intention is to explore how the binding motif affects the likelihood of intramolecular EET between the subunits. Both systems, which consist of a yellow Bodipy dye as a donor and a blue Bodipy dye as the complementary acceptor, show highly efficient EET. Again, the probability of EET has been probed as a function of applied pressure and temperature to better expose the mechanism. The non-covalently-linked system, which makes use of electrostatic binding between charged species, forms a liquid crystalline state upon heating and it is notable that efficacious EET occurs within this phase. Chapter 4 looks at the nano-mechanical properties of molecular-scale bridges in linear donor-spacer-acceptor compounds by monitoring the probability of intramolecular EET as a function of bridge length. The bridge (or spacer) consists of 1 to 5 ethynylene-carborane units that allow the centre-to-centre distance between the donor and acceptor to be varied systematically from 38 to 115 Å. Interestingly, the probability of EET is higher than the predicted value for all systems except the shortest bridge. On cooling to 77K, the agreement between theory and experiment agrees much better but depends on applied pressure in fluid solution at room temperature. We rationalise these various results in terms of structural distortion of the longer bridges, thereby allowing determination of the strain energy and Young’s modulus for the spacer unit. In Chapter 5, we report on a study of intramolecular EET in a molecular triad where the highest-energy donor is situated in the centre and there are two disparate Bodipy at the terminals. Overall, the probability of EET exceeds 95% and the individual EET steps can be resolved; the rate of EET follows the order of spectral overlap integrals. By selective protonation of one of the Bodipy-based terminals, it is possible to change the relative ordering of the spectral overlap integrals and thereby switch the direction of EET. This chapter also includes an investigation of the general photophysical behaviour of the symmetric triads, where the same Bodipy dye is present at each terminal, in addition to the spectroscopic properties of the isolated chromophores. Experimental variations include changes in solvent polarity, effect of lowing the temperature, moving from fluid to solid phases and applying high pressure to the fluid medium are discussed in this chapter. Finally, Chapter 6 provides a brief summary of the experimental approaches used throughout the work, including instrumentation and chemicals. In addition, the many mathematical equations and computer programs employed are mentioned here.