Water-Promoted Molar-Level Photocatalysis and Spontaneous Product Separation with Near-Unity Quantum Efficiency

Photocatalysis provides a sustainable toolbox for challenging redox chemistry, showing potential for the selective synthesis of value-added chemicals. The light-induced reactions are generally performed in selected organic solvents to achieve optimum performance, yet the relatively low concentration of reactants and the regular efficiency result in a modest yield of desired products, thus requiring extensive product separation and casting concerns for practical applications. Here, we demonstrate how the addition of water boosts the photocatalytic dehydrogenative condensation of amines, enabling the synthesis of imines at practical concentrations with outstanding quantum efficiencies (QE) in the absence of organic solvents. Water not only lowers the adsorption energy of generated hydrogen atoms to facilitate the liberation of molecular hydrogen, but also manipulates the conversion of the generated benzimine intermediate through an energy-favorable pathway. Representatively, a complete conversion of 20 vol % (1.9 M) benzylamine is achieved using a Ru/TiO₂ photocatalyst with an average QE of 20% at a 100 mL scale, yielding ∼20 g (62 mmol) of benzylidenebenzylamine exclusively in a spontaneous separation manner. A near-unity QE is achieved for the conversion of 70 vol% (6.7 M) benzylamine employing a Rh/TiO₂ photocatalyst. The water-promoted photocatalytic conversion of amines exhibits a broad substrate scope, excellent scalability, and stability with a minimum cost of purification, providing an economical and sustainable solution for practical applications.