Photoactive Neutral Three-Coordinate Cu(I) Complexes of Anionic N‑Heterocyclic Carbenes

Three-coordinate Cu-(I) complexes are promising candidates for photoactive compounds, but their application in photocatalysis remains largely unexplored. Here, we report the synthesis and comprehensive characterization of four novel three-coordinate Cu-(I) complexes featuring an anionic N-heterocyclic carbene ligand with a weakly coordinating tris-(pentafluorophenyl)-borate moiety (WCA-NHC) and different methyl substituted dipyridylamine-based N,N'-ligands. This ligand design significantly improves the stability and photophysical properties of these complexes in solution. Steady-state and time-resolved spectroscopy, electrochemical measurements, temperature-dependent emission studies and quantum chemical calculations were used to elucidate the electronic and excited-state properties of these complexes. Our results demonstrate metal-to-ligand charge transfer absorption and thermally activated delayed fluorescence (TADF), leading to extended excited-state lifetimes (up to 8.6 μs) and high excited-state energies (≈2.7 eV). All four complexes efficiently photosensitize the norbornadiene-to-quadricyclane photoisomerization, a key reaction for molecular solar thermal energy storage (MOST). By demonstrating that careful ligand selection allows the design of three-coordinate Cu-(I) complexes with excellent photophysical and photocatalytic properties, this study expands the scope of Cu-(I) photosensitizers and lays the foundation for further applications in photochemistry.