Push and Pull: the Impact of Amino and Nitro Substituents on Heteroleptic Cu(I) Photosensitizers

This study presents the synthesis and characterization of four novel heteroleptic Cu(I) complexes incorporating nitro- and amino-substituted phenanthroline ligands. Their molecular structures, featuring a substituent at the 5-position of the phenanthroline backbone, were elucidated by X-ray crystallography and density functional theory (DFT) calculations. Spectroscopic and electrochemical investigations revealed distinct mesomeric (M) and inductive (I) effects of the NO₂ and NH₂ substituents on the photophysical and electronic properties. The electron-withdrawing NO₂ group (-M, -I) stabilizes the lowest unoccupied molecular orbital (LUMO), shifts the reduction potential by +0.9 V and completely quenches emission via nonradiative processes. In contrast, the electron-donating NH₂ group (+M, -I) induces a moderate redshift (≈ 25 nm) in absorption and leads to long-lived, nonemissive excited states, particularly in CunNH₂ (29.1 µs). Photocatalytic tests revealed no hydrogen evolution, but NH₂-substituted complexes efficiently generated singlet oxygen (¹O₂) with quantum yields of up to 39.8%. These findings demonstrate how small structural modifications can fundamentally alter the photophysics and catalytic behavior of Cu(I) complexes, providing valuable design principles for next-generation photosensitizers (PSs).