Effective Quenching and Excited-State Relaxation of a Cu(I) Photosensitizer Addressed by Time-Resolved Spectroscopy and TDDFT Calculations

Homogenous photocatalytic systems based on copper photosensitizers are promising candidates for noble metal free approaches in solar hydrogen generation. To improve their performance a detailed understanding of the individual steps is needed. Here, we study the interaction of a heteroleptic copper (I) photosensitizer with an iron catalyst by time-resolved spectroscopy and ab-initio calculations. The catalyst leads to rather efficient quenching of the 3MLCT state of the copper complex, with a bimolecular rate being about three times smaller than the collision rate. Using control experiments with methyl viologen an appearing absorption band is assigned to the oxidized copper complex demonstrating that electron transfer from the sensitizer to the iron catalyst occurs and the system reacts along an oxidative pathway. However, only about 60% of the quenching events result in an electron transfer while the other 40% experience deactivation indicating that the photocatalytic performance could be improved by optimizing the intermolecular interaction.