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

Extent of carbon nitride photocharging controls energetics of hydrogen transfer in photochemical cascade processes

Abstract Graphitic carbon nitride is widely studied in organic photoredox catalysis. Reductive quenching of carbon nitride excited state is postulated in many photocatalytic transformations. However, the reactivity of this species in the turn over step is less explored. In this work, we investigate electron and proton transfer from carbon nitride that is photocharged to a various extent, while the negative charge is compensated either by protons or ammonium cations. Strong stabilization of electrons by ammonium cations makes proton-coupled electron transfer uphill, and affords air-stable persistent carbon nitride radicals. In carbon nitrides, which are photocharged to a smaller extent, protons do not stabilize electrons, which results in spontaneous charge transfer to oxidants. Facile proton-coupled electron transfer is a key step in the photocatalytic oxidative-reductive cascade – tetramerization of benzylic amines. The feasibility of proton-coupled electron transfer is modulated by adjusting the extent of carbon nitride photocharging, type of counterion and temperature