Organised Functional Liquids for Photon Upconversion

<p>Photon upconversion is a process by which lower energy photons are converted to higher energy photons, which can be achieved by the interaction of two triplet excited states. This process holds potential for wavelength shifting solid films in photovoltaic cells. Not all wavelengths emitted by the sun have sufficient energy to be utilized by such devices. Typical solar cells have a band gap of around 1 µm, however there is a significant amount of energy output by the sun that falls below this threshold. Upconversion could lead to more efficient use of energy by converting these lower energy wavelengths to wavelengths that could be directly absorbed by the solar panel. Upconversion has thus far been harnessed in solution, where diffusion is the limiting factor for the efficiency of the process. However, for technological applications it would be better to create thin solid films. In these films, molecules would have to be brought within the distance on which upconversion occurs, as the process would no longer be defined by diffusion. One way to achieve this would be to create liquid crystalline derivatives of upconversion emitter molecules. This would provide ordering in the system, which would enhance electronic coupling and bring molecules within the scale on which upconversion occurs.  The work of this thesis has focused on the synthesis of these organised functional liquids: liquid crystals of common upconversion emitter molecules. 9,10-diphenylanthracene (DPA) and 9,10-bis(phenylethynyl)anthracene (BPEA) are popular emitter molecules, and derivatives of these molecules were synthesized. A variety of alkyl chains were attached with or without phenyl linkers. The alkyl chains would provide entropy to the system in order to induce the formation of liquid crystalline phases. The resulting phase behaviour of these derivatives was studied using differential scanning calorimetry (DSC) and polarised optical microscopy (POM).  Eight novel derivatives of DPA and BPEA were synthesized. New information was gained as to the requirements of inducing liquid crystallinity in these dye molecules. Direct addition of chains symmetrically to the central dye molecules did not result in the formation of liquid crystalline phases. Through extension of the central core by an extra phenyl ring a liquid crystalline behaviour was observed. A synthesized derivative of DPA exhibited extreme supercooling, which is one of a few derivatives of 9,10-diphenylanthracene to exhibit a liquid state at room temperature. A derivative of BPEA was synthesized that exhibited formation of a mesophase (liquid crystal phase). These two derivatives were investigated for potential use as a material for upconversion.</p>