Singlet-Triplet Energy Inversion in Carbon Nitride Photocatalysts.

Time-resolved EPR (TR-EPR) demonstrates the formation of well-defined spin triplet excitons in carbon nitride. This permits to experimentally probe the extent of the triplet wavefunction which delocalizes over several tri-s-triazine units. Analysis of the temperature dependence of the TR-EPR signal reveals the mobility of the triplet excitons. By employing monochromatic light excitation in the range 430-600 nm, the energy of the spin triplet is estimated to be ≈0.2 eV above the conduction band edge, proving that the triplet exciton lies above the corresponding singlet. Comparison between amorphous and graphitic forms establishes the singlet-triplet inversion as a general feature of carbon nitride materials.

Morphology and Light-Dependent Spatial Distribution of Spin Defects in Carbon Nitride.

Carbon nitride (CN) is a heterogeneous photocatalyst that combines good structural properties and a broad scope. The photocatalytic efficiency of CN is associated with the presence of defective and radical species. An accurate description of defective states-both at a local and extended level-is key to develop a thorough mechanistic understanding of the photophysics of CN. In turn, this will maximise the generation and usage of photogenerated charge carriers and minimise wasteful charge recombination. Here the influence of morphology and light-excitation on the number and chemical nature of radical defects is assessed. By exploiting the magnetic dipole-dipole coupling, the spatial distribution of native radicals in CN is derived with high precision. From the analysis an average distance in the range 1.99-2.34 nm is determined, which corresponds to pairs of radicals located approximately four tri-s-triazine units apart.

Carbon Vacancies Steer the Activity in Dual Ni Carbon Nitride Photocatalysis.

The manipulation of carbon nitride (CN) structures is one main avenue to enhance the activity of CN-based photocatalysts. Increasing the efficiency of photocatalytic heterogeneous materials is a critical step toward the realistic implementation of sustainable schemes for organic synthesis. However, limited knowledge of the structure/activity relationship in relation to subtle structural variations prevents a fully rational design of new photocatalytic materials, limiting practical applications. Here, the CN structure is engineered by means of a microwave treatment, and the structure of the material is shaped around its suitable functionality for Ni dual photocatalysis, with a resulting boosting of the reaction efficiency toward many CX (X = N, S, O) couplings. The combination of advanced characterization techniques and first-principle simulations reveals that this enhanced reactivity is due to the formation of carbon vacancies that evolve into triazole and imine N species able to suitably bind Ni complexes and harness highly efficient dual catalysis. The cost-effective microwave treatment proposed here appears as a versatile and sustainable approach to the design of CN-based photocatalysts for a wide range of industrially relevant organic synthetic reactions.

Radical defects modulate the photocatalytic response in 2D-graphitic carbon nitride.

Graphitic carbon nitride (gCN) is an important heterogeneous metal-free catalytic material. Thermally induced post-synthetic modifications, such as amorphization and/or reduction, were recently used to enhance the photocatalytic response of these materials for certain classes of organic transformations, with structural defects possibly playing an important role. The knowledge of how these surface modifications modulate the photocatalytic response of gCN is therefore not only interesting from a fundamental point of view, but also necessary for the development and/or tuning of metal-free gCN systems with superior photo-catalytic properties. Herein, employing density functional theory calculations and combining both the periodic and molecular approaches, in conjunction with experimental EPR measurements, we demonstrate that different structural defects on the gCN surface generate distinctive radical defect states localized within the electronic bandgap, with only those correlated with amorphous and reduced gCN structures being photo-active. To this end, we (i) model defective gCN surfaces containing radical defect states; (ii) assess the interactions of these defects with the radical precursors involved in the photo-driven alkylation of electron-rich aromatic compounds (namely perfluoroalkyl iodides); and (iii) describe the photo-chemical processes triggering the initial step of that reaction at the gCN surface. We provide a coherent structure/photo-catalytic property relationship on defective gCN surfaces, elaborating how only specific defect types act as binding sites for the perfluoroalkyl iodide reagent and can favor a photo-induced charge transfer from the gCN surface to the molecule, thus triggering the perfluoroalkylation reaction.