Dendrimers for light harvesting and conversion

Since their discovery, dendrimers have attracted widespread interest due to their unique structural characteristics. The introduction of convergent dendrimer synthesis in 1990 opened the door to a large number of different dendrimer structures and architectures, and allowed for the placement of functionality at the core, the periphery, or the internal monomer units within the macromolecule. By taking advantage of this architectural control, it has been possible to utilize dendrimers as models for a wide variety of biological molecules and systems, including enzymes and proteins in which active complexes (porphyrins, metal clusters) are encapsulated by a well-defined polypeptide chain. As an extension of the trend toward bio-mimetic dendrimers, this dissertation describes a novel family of chromophore-functionalized dendrimers prepared for the purpose of mimicking photosynthesis. The dendrimers are specifically functionalized with numerous energy donor chromophores at the periphery and a single energy acceptor chromophore at the core. The peripheral chromophores act as a light-harvesting antenna from which energy is funneled to the single core acceptor with nearly quantitative efficiency up to the fourth generation. Once this energy is received at the core, it can be utilized to enable various processes, depending on the nature of the core chromophore.; After a brief overview of the energy-transfer process and some early examples of energy transfer within dendrimers (Chapter 1), the synthesis and full spectroscopic characterization of a number of different chromophore-functionalized dendrimers will be described (Chapters 2, 3 and 4). Several key concepts are introduced and demonstrated in these chapters. These include the accelerated modular synthesis of chromophore-labeled dendrimers using a novel hypermonomer, the specific attributes of chromophores necessary for efficient energy transfer, the ability to utilize a light-harvesting antenna for light amplification, and the ability to vary the acceptor chromophore while retaining efficient energy transfer, thereby allowing for emission wavelength tunability. Chapter 5 describes detailed time-resolved data that enabled the measurement of energy transfer times within the dendrimers. Comparison of these values with theoretical predictions was important in elucidating the mechanism of energy transfer. This data represents the first example of energy transfer within dendrimers where the observed energy-transfer rate constants matched those predicted by Förster Theory. Extension of this work toward self-assembled monolayers (SAMs) on silicon surfaces is described in Chapter 6. It was possible to show that energy-transfer efficiency could be modulated by varying the ratio of adsorbates on the silicon surface, and this resulted in the first demonstration of energy transfer within SAMs. The preparation and characterization of chromophore-functionalized linear polymers as less “perfect” but more easily accessible analogs of the dendrimers is described in Chapter 7. Finally, implications of light-harvesting dendrimers on processes involving electron transfer, and the area of photovoltaics are outlined in Chapter 8.