Amplified excited-state diverse effects arising from ligand regioisomerism in Cu(i) complexes for advanced applications

Designing earth-abundant photoactive coordination complexes that combine molecular tunability with device-relevant performance is essential to advance next-generation optoelectronic and solar-energy technologies. In this work, two heteroleptic Cu(I) complexes were synthesised using regioisomeric diimine ligands obtained through lactam–lactim tautomerism of a common precursor. Despite their close structural similarity, the complexes exhibit pronounced differences in photophysical and electrochemical behaviour. In the solid state, both compounds show phosphorescent emission, whereas at room temperature one complex displays some characteristics consistent with thermally activated delayed fluorescence. Quantum-chemical calculations reveal a rather large singlet–triplet energy gap for the charge-transfer states, and a ligand-centred triplet state is supposed to play a decisive role. Preliminary photoelectrochemical measurements further demonstrate that simple physisorption of the complexes onto TiO2 yields measurable photocurrent generation. These results highlight the significant impact of ligand regiochemistry on the excited-state properties and functional performance of Cu(I) complexes.