Time Dependent Density Functional Theory (TD-DFT) has been used to assist the design and synthesis of a series thioxan-thone triplet sensitizers. Calculated energies of the triplet excited state (ET) informed both the type and position of auxo-chromes placed on the thioxanthone core, enabling fine-tuning of the UV-Vis absorptions and associated triplet energies. The calculated results were highly consistent with experimental observation in both the order of the λmax and ET values. The synthesised compounds were then evaluated for their efficacies as triplet sensitizers in a variety of UV and visible light pre-parative photochemical reactions. The results of this study exceeded expectations; in particular [2+2] cycloaddition chemis-try that had previously been sensitized in the UV was found to undergo cycloaddition at 455 nm (blue) with a 2 to 9-fold increase in productivity (g/h) relative to input power. This study demonstrates the ability of powerful modern computational methods to aid the design of successful and productive triplet sensitized photochemical reactions.