Abstract
Time Dependent Density Functional Theory has been used to assist the design and synthesis of a series thioxanthone triplet sensitizers. Calculated energies of the triplet excited state (ET) informed both the type and position of auxochromes 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 synthesized compounds were then evaluated for their efficacies as triplet sensitizers in a variety of UV and visible light preparative photochemical reactions. The results of this study exceeded expectations; in particular [2 + 2] cycloaddition chemistry 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 in the design of successful and productive triplet sensitized photochemical reactions.
Highlights
Excited state photochemistry1 involves the formation of bonds by the reaction of molecules in an electronically excited state
The overall efficiency, and synthetic utility, of a photochemical reaction is often dependent on the lifetime of these states relative to the rate of chemical reaction, which in turn is determined by the physical properties of the compound
This study has demonstrated that modern DFT methods can be used to inform the design of a family of thioxanthone sensitizers that span the UV-Vis region (300-450nm)
Summary
Excited state photochemistry involves the formation of bonds by the reaction of molecules in an electronically excited state. Its purest embodiment occurs without catalysts or reagents by irradiation with UV light, most commonly in the region of 250-400 nm. This ‘reagentless’ approach to synthesis is highly desirable in an ever more economic and green focused society. Reactions typically occur from either the lowest energy excited singlet (S1) or triplet (T1) states, the latter accessed via intersystem crossing (ISC) from the singlet. These ‘electronic isomers’ of the ground state species can react in exotic ways to give rapid access to complex or highly strained species with favorable properties for drug discovery or further reactivity.. Whilst the short-lived S1 state is inherently difficult to control, the much longer lifetime of the T1 state can be exploited to significantly enhance the overall quantum efficiency of a reaction
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