Influence of Imidazole Substituent Bulkiness on [CuI(PPh 3 ) 2 N] Complexes with TADF Blue Solid-State Emission

High Resolution Image Download MS PowerPoint Slide Copper(I) heteroleptic complexes with phosphine- and nitrogen-containing ligands are well-known thermally activated delayed fluorescence (TADF) emitters and are of interest for OLED applications due to their cost-effectiveness. In this work, we report the design and synthesis of a series of novel Cu(I) complexes featuring triphenylphosphine- and imidazole-based ligands functionalized with varying alkyl chains. This allowed for the evaluation of purely monodentate ligand systems and the impact of the ligand bulkiness on the photophysical properties. Time-resolved spectroscopy, combined with temperature-dependent measurements, revealed blue solid-state emission and confirmed the TADF behavior across the series. Room-temperature lifetimes reached as low as 0.5 μs, facilitated by small singlet–triplet energy gaps Δ E (S1–T1) between 110 and 60 meV. Bulky ligands led to the smallest S–T gaps. A decrease in phosphorescence lifetimes with increasing alkyl chain size suggested enhanced spin–orbit coupling (SOC), attributed to a more tetrahedral coordination environment. Quantum chemical calculations revealed that the imidazole moiety does not participate directly in the frontier molecular orbitals, indicating that the alkyl chains primarily influence the emission by modulating the geometry around the Cu(I) center.