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 approximate to 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. The use of carbon nitride as a photocatalyst requires to understand its response to light and the nature of all the photoexcited species formed. Here we devise an original approach based on time-resolved magnetic resonance to provide the spin state, the energy and wavefunction delocalisation of carbon nitride photoexcited states. We directly measure the energy of the triplet state and demonstrate that it is higher than the corresponding singlet.image

Automated summary

Time-resolved EPR technique was used to study the formation of well-defined spin triplet excitons in carbon nitride. The mobility of the triplet excitons was revealed through the analysis of temperature-dependent TR-EPR signal. It was found that the energy of the spin triplet is approximately 0.2 eV higher than the conduction band edge, suggesting the inversion of singlet-triplet in carbon nitride materials. This study provides important insights into the response of carbon nitride to light and the nature of photoexcited states.