Comprehensive Picture of the Excited State Dynamics of Cu(I)- and Ru(II)-Based Photosensitizers with Long-Lived Triplet States

Ru(II)- and Cu(I)-based photosensitizers featuring the recently developed biipo ligand (16H-benzo-[4',5']-isoquinolino-[2',1',:1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one were comprehensively investigated by X-ray crystallography, electrochemistry, and especially several time-resolved spectroscopic methods covering all time scales from femto- to milliseconds. The analysis of the experimental results is supported by density functional theory (DFT) calculations. The biipo ligand consists of a coordinating 1,10-phenanthroline moiety fused with a 1,8-naphthalimide unit, which results in an extended pi-system with an incorporated electron acceptor moiety. In a previous study, it was shown that this ligand enabled a Ru(II) complex that is an efficient singlet oxygen producer and of potential use for other light-driven applications due to its long emission lifetime. The goal of our here presented research is to provide a full spectroscopic picture of the processes that follow optical excitation. Interestingly, the Ru(II) and Cu(I) complexes differ in their characteristics even though the lowest electronically excited states involve in both cases the biipo ligand. The combined spectroscopic results indicate that an emissive (MLCT)-M-3 state and a rather dark (LC)-L-3 state are populated, each to some extent. For the Cu(I) complex, most of the excited population ends up in the (LC)-L-3 state with an extraordinary lifetime of 439 mu s in the solid state at 20 K, while a significant population of the (MLCT)-M-3 state causes luminescence for the Ru(II) complex. Hence, there is a balance between these two states, which can be tuned by altering the metal center or even by thermal energy, as suggested by the temperature-dependent experiments.

Automated summary

Ru(II)- and Cu(I)-based photosensitizers utilizing the biipo ligand were thoroughly studied using various spectroscopic methods. Results showed differences in characteristics between the Ru(II) and Cu(I) complexes, despite both involving the same lowest electronically excited states. The emissive (MLCT)-M-3 state and the dark (LC)-L-3 state were found to be populated in both complexes, with a balance between the two states that can be tuned by altering the metal center or thermal energy.