Water: An underappreciated reaction medium for photodimerizations

Abstract

Organic solvents have been the sought-after medium to achieve light initiated transformations in laboratories and industries. Given the current emphasis on green chemistry and rising awareness of environmental pollution it may be necessary for us to utilize abundantly freely available, non-toxic and environmentally friendly water as the medium to perform photoreactions. Although water has attracted the attention of organic synthetic chemists, it is yet to receive the indispensable attention of photochemists. In this article we present examples of photocycloaddition reactions of four molecules namely coumarin, indene, cinnamic acid and acenaphthylene that are sparingly soluble in water. Photodimerization of these molecules in water is much faster and occurs at much lower concentrations than in organic solvents. Aggregation probably forced by hydrophobic association is suggested to be the cause for the increased reactivity even at lower concentrations. The dimer distribution in water is different from that in organic solvents. Further work is required to fully understand the mechanism of photodimerization in water. Although the poor solubility of organic molecules in water requires one to irradiate large volumes to collect enough useful amounts, availability of flow reactors and LEDs as light sources should help one overcome the challenges.