Structure and photophysical properties of copper(I) photosensitizers: a computational study of substituent effects

Context: Developing efficient and sustainable photosensitizers is crucial for advancing solar energy conversion and light-driven chemical processes. Ruthenium(II) and iridium(III) complexes have traditionally dominated this field due to their favorable photophysical properties, but their high cost has prompted the search for alternatives using earth-abundant metals. Copper(I) complexes are promising because they are cost-effective, have a tunable electronic structure, and feature a d¹⁰ d 10 configuration that avoids low-lying metal-centered excited states. However, further improvements are needed to match the performance of Ru(II) and Ir(III) complexes, with ligand engineering playing a key role. Although there has been progress, the systematic effect of R substituents on the N ^ ∧ N R ligand in heteroleptic Cu(N ^ ∧ N R )(NacNac) complexes remains underexplored. Methods: We investigated a series of Cu(I) complexes of the type Cu(N ^ ∧ N R ) (NacNac Me ), where N ^ ∧ N is 2,2 $$'$$ ′ -biquinoline and NacNac is β β -diketiminate, using electronic structure calculations. The N ^ ∧ N R ligand was systematically substituted with R groups including -NH ₂ 2 , -OH, -CH ₃ 3 , -H, -Cl, -COOCH ₃ 3 , and -CF ₃ 3 to examine how these modifications affect the structure and photophysical properties of copper(I) photosensitizers. Ground- and excited-state properties were evaluated using density functional theory (DFT) and time-dependent DFT (TD-DFT), with excited-state character quantified using the TheoDORE program. We analyzed structural parameters, frontier orbital energies, and electronic transitions to correlate substituent electronic character with photophysical behavior. Local energy decomposition (LED) analyses based on DLPNO-CCSD(T) calculations were also performed to study metal-ligand interactions. The results indicate that strong electron-withdrawing groups such as -COOCH ₃