Synthesis, self-assembly and applications of tribenzopenta-phene derivatives and triphenylene-based conjugated foldamers
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Polycyclic aromatic hydrocarbons (PAHs) are interesting discotic liquid crystalline materials. Their often planar or near-planar geometry and extended π-delocalization render them appealing π-π stacking building blocks which can be utilized to dictate controlled self-assembly processes. In addition, the potentially high charge mobility along the stacked PAH column may find applications in sensors, field effect transistors and photovoltaic cells. In this dissertation, efficient synthetic approaches to tribenzopentaphene (TBP) derivatives are reported and their optical, electrochemical, self-assembly, and photovoltaic properties are studied. It is shown that TBP derivatives are good electron donors which, when coated on pre-grown perylenediimide (PDI) nanofibers can significantly enhance the photocurrent response of the PDI nanofibrils. The peripheral substituents on the TBP core are found to exhibit significant effect on its self-assembly properties which in turn affects the photocurrent response of the PDI/TBP heterojunction nanofibrils. Heterjunction solar cells using TBP derivatives as the donor and PDI or PCBM as the acceptor have also been fabricated and a power conversation efficiency of 0.12% has been achieved. Conjugated oligomers and polymers which can self-organize into a helical conformation have been extensively studied. Such studies have not only led to much better understanding of the principles behind noncovalent folding, but also brought about a number of exciting applications from selective and specific guest binding, functional group organization, to reactive sieves. The major driving forces responsible for the folding of π-conjugated systems are π-π stacking and solvophobic interactions. The strength of the π-π interaction depends on the size and the electronic nature of the aromatic systems. In general, the larger the size of the planar aromatic system is, the stronger the π-π interaction will be. Interestingly, studies on conjugated foldamers have so far been limited to single-ring aromatic systems, in which the mere existence of π-π interactions is questionable. Polycyclic aromatic systems (PCA), as far as we know, have not been utilized in conjugated foldamers. It is envisioned that, with PCAs in the conjugated polymer backbone, the polymer will not only exhibit much stronger propensity for folding due virtually to their stronger π-π stacking interactions, but also may lead to conjugated systems with very interesting electronic and optical properties. In the second part of this dissertation, we report the synthesis and optical properties of triphenylene-containing conjugated foldamers. Systematic optical property studies show that such polymers indeed fold in a variety of solvents. Depending on the solvents, the polymer may also adopt a random coil conformation or interchain aggregation. Among the three conformations, the folding conformation is found to exhibit the highest fluorescence quantum yield.
Table of Contents
Abstract -- Illustrations -- Tables -- Acknowledgements -- Synthesis and characterizations of new discotic tribenzopentaphene derivatives -- Photoconductivity and photovolataic applications of tribenzopentaphene derivatives -- Synthesis, self-assembley, and applications of a diphenyl-substituted tribenzopentaphene -- Syntheses and optical properties of triphenylene-containing conjugated foldamers -- References