Degree of Carbon Nitride Photocharging Controls Energetics of Hydrogen Transfer in Photochemical Cascade Processes

Photocharging of graphitic carbon nitrides (g-CN) is a process of electrons and charge-compensating cations accumulation in the material that is triggered by irradiating a mixture of the semiconductor and an electron donor with light. Although this process has been applied in sensing, energy storage and organic synthesis, the energetics of g-CN discharging has not been studied in details. Herein, we investigate transfer of e‒/H+ from g-CN photocharged with electrons and either protons (H+) or ammonium cations (NH4+) to an oxidant, such as O2 and imine. NH4+ exerts a strong stabilizing effect, which makes e‒/H+ transfer uphill. Especially in aqueous environment, NH4+ yields air-stable photocharged sodium poly(heptazine imide). In mildly-reduced g-CN, H+ do not stabilize electrons, which results in spontaneous transfer of e‒/H+ to oxidants. Facile transfer of e‒/H+ is a key step in a photocatalytic oxidative-reductive cascade – tetramerization of benzylic amines, which involves two photocatalytic events: i) oxidation of two benzylic amine molecules to the imine with a concomitant storage of 2e‒/2H+ in g-CN and ii) reduction of the imine to α-aminoalkyl radical that involves 1e‒/1H+ transfer.