Charge-Transfer Triplet Chromophores: Adding Efficiency to Organic Photosensitisers

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Light-induced chemical transformation is ubiquitous in nature and central to the function of many life processes; photosynthesis of bacterial or plant and phototropism of plant or fungi, among many others, are two examples of longstanding interest and importance. In these events, light-absorbing molecules are photo-excited and store this energy transiently before transferring the excess energy (or electron) to the substrates for subsequent chemical or physical processes. Since the reactive species in the latter process often cannot interact with solar radiation directly, these otherwise inactive components are said to be 'sensitised' by the light-absorbing molecules. The involvement of solar power as the ultimate energy input not only suggests the sustainability of these processes, but also hints at the fact that photoexcitation can supply an ample amount of energy to enable high barrier, energy-demanding process to occur--photocatalytic water splitting to generate hydrogen as solar fuel is one example that attracts much recent attention due to the increasing demands for sustainable chemistry.Inspired by this possibility, the use of photosensitizers under mild conditions for synthetic chemistry, photocatalytic waste/toxin decomposition, photodynamic therapy and photovoltaics has found widespread applications. The selection of photosensitizers, however, is largely dominated by the transition-metal-based materials due to the long lifetime and strong redox power in their excited states. While the development of less toxic organic photosensitizers would enhance the sustainability of this process, along with the energetic and structural diversity, it presents considerable challenges for optimising the favourable kinetic and thermodynamic parameters seen in transition-metal materials. The popularity of metal-based photosensitizers stems primarily from the efficient formation of the triplet excited state, where two electrons in different molecular orbitals have a parallel spin orientation. The difference between the spin configurations in the triplet excited state and the ground state (antiparallel spins, singlet) impedes energy dissipation, thus allowing sufficient time for inter-molecular energy (or electron) transfer from excited photosensitizers to occur.To unlock the full potential of metal-free organic photosensitisers, I present in this project a renovated molecular design based on fundamental quantum chemical considerations. I will introduce a small functional group into commercial dyes to bias the orbital orientation of their high-energy electrons. I will also engineer the electron density across the molecular skeleton such that the electron distribution will be dissimilar between the ground and excited states. These two features would guarantee fast triplet formation with minimal energy loss from the spin flipping process. I will quantitatively analyse the kinetic process after photoexcitation to gauge the energy conversion efficiency of the new photosensitizers in the absence and presence of reactive substrates. Special attention will be paid to ensure energy tuning and high photostability for practical applications. Furthermore, since several spin states of small energy separation are present in these excited photosensitisers, I will elucidate the influence of the nearby energy levels on the spin flipping mechanism. This proposal is built upon my research expertise in the synthetic, analytical, and theoretical aspects of photo- and electrochemically active materials. The outcome of the project will immediately offer new photocatalysis tools to enhance reaction efficiency. The fundamental understanding of the energy conversion mechanism will provide insight into any spin-involving processes, such as electroluminescence, photovoltaics and spintronics.