Potassium Poly(Heptazine Imide): Transition Metal-Free Solid-State Triplet Sensitizer in Cascade Energy Transfer and [3+2]-cycloadditions

Polymeric carbon nitride materials have been used in numerous light-to-energy conversion applications ranging from photocatalysis to optoelectronics. For a new application and modelling, we first refined the crystal structure of potassium poly(heptazine imide) (K-PHI)-a benchmark carbon nitride material in photocatalysis-by means of X-ray powder diffraction and transmission electron microscopy. Using the crystal structure of K-PHI, periodic DFT calculations were performed to calculate the density-of-states (DOS) and localize intra band states (IBS). IBS were found to be responsible for the enhanced K-PHI absorption in the near IR region, to serve as electron traps, and to be useful in energy transfer reactions. Once excited with visible light, carbon nitrides, in addition to the direct recombination, can also undergo singlet-triplet intersystem crossing. We utilized the K-PHI centered triplet excited states to trigger a cascade of energy transfer reactions and, in turn, to sensitize, for example, singlet oxygen (1O2) as a starting point to synthesis up to 25 different N-rich heterocycles

All-organic Z-scheme photoreduction of CO2 with water as the donor of electrons and protons

Carbon nitrides and flavins are two classes of easy accessible transition metal-free, photoactive materials. Their photocatalytic efficiency to enable a variety of chemical reactions is well documented. Here, we report on their combination in one photocatalytic system by designing biomimetic non-spherical core-shell architectures comprising micro-sized crystals of flavins decorated by potassium poly(heptazine imide) nanoparticles on the surface. The designed non-spherical core-shell composites are tested in the photocatalytic CO2 reduction to CH4, MeOH, EtOH and CO using water vapor as a donor of electrons and protons at the gas-solid interface. The forged Z-scheme heterojunction between these two materials allows increasing of their photocatalytic performance. Whereby excited states lifetimes are extended by interface charge recombination and photoredox processes are boosted because of an overall wider band gap