There is strong motivation for the detailed exploration of physical processes that present the potential for enhancing the power conversion efficiencies of solar photovoltaic devices. One key assumption underlying the theoretical Shockley-Queisser limit of the efficiency of photovoltaic devices is that the absorption of one photon can only produce a single electron-hole pair; all excess energy above the bandgap of the semiconductor is lost to thermalisation. Singlet fission (SF) is a physical process in certain organic materials that enables the production of two excited states, each with triplet multiplicity, from a lone photoexcited singlet state. It has been widely recognised that integration of an appropriately energetic and efficient singlet fission material into a photovoltaic device may facilitate circumvention of the Shockley-Queisser efficiency limit. This requires that the energy of both SF-born triplets can be harvested independently and transferred to the semiconductor.

In covalently connected assemblies of a singlet fission active chromophore, intramolecular singlet fission (iSF) can generate two triplet states upon a single molecule. The singlet fission properties of the molecule can be tailored by synthetic engineering of the interchromophore connectivity. Only a modest sub-set of the chromophores that have been identified to undergo SF in condensed phases have been investigated in molecular systems for iSF activity.

1,6-Diphenylhexa-1,3,5-triene (DPH) is a promising chromophore that is known to undergo singlet fission in the solid state. The high triplet energy of DPH (~ 1.5 eV) makes it a particularly attractive chromophore for further studies, given the identification of high triplet energy as a critical requirement for the viable application of SF in photovoltaic systems. In the work described in this thesis, the first studies of iSF activity in DPH materials were conducted.

This thesis describes the design and chemical synthesis of the initial dimeric DPH derivatives for investigation as iSF candidates. The optical characterisation of these materials using transient absorption spectroscopy is reported and two of the initial five materials are found to form an ultrafast equilibrium between singlet and triplet-pair states. Following the initial studies, several subsequent generations of DPH dimers and oligomers are investigated. Facile iSF to generate a triplet-pair state is achieved for several materials but reducing triplet-pair annihilation, in order to achieve persistent pairs of triplets, proves elusive. Nevertheless, significant progress is achieved with the formation of spatially separated triplet-pairs being identified for contiguously connected oligomers, presented in the final chapter. Stabilisation of these spatially separated triplet populations remains the outstanding challenge for the generation of persistent pairs of triplets.