Photochemical Synthesis in Water

Water as the medium for photochemical synthesis may, at present, be an unusual choice. This unique solvent offers more than being environmentally friendly: it can lead to increased or different reactivity and selectivity compared to organic solvents.

Although water is perceived as a benign and green solvent,¹⁾ it is often neglected when developing synthetic methodologies, in particular those based on photochemistry. More common are solvents such as dichloromethane (DCM) or acetonitrile (MeCN). However, water is not only an environmentally friendly but a unique solvent: for example, water contains little dissolved oxygen.²⁾ Being a triplet quencher,³⁾ oxygen can hamper triplet-based photochemistries, which many photochemical synthesis protocols rely on.

Most organic substrates are not water-soluble. To dissolve into aqueous solution they must be either chemically modified or assisted by for example supramolecular hosts. An alternative strategy uses the lack of solubility of organic substrates as an advantage. This has led to the concept of on-water synthesis. The following sections provide representative examples that focus on water in either

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