Positional Tuning of Photophysics and Catalysis in Methoxy-Substituted Heteroleptic Copper(I) Complexes

A series of heteroleptic copper(I) photosensitizers based on methoxy-substituted 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline ligands was synthesized to investigate the influence of substitution patterns on structure and function. Methoxy groups were introduced in ortho-, meta-, and para-positions of the phenyl rings. Single-crystal X-ray diffraction and DFT calculations confirmed the expected tetrahedral geometry with position-dependent aryl torsion. Photophysical studies reveal that ortho/para-substitution enhances absorptivity, emission quantum yields, and excited-state lifetimes compared to the meta/unsubstituted complexes. The ortho-substituted complex shows the strongest electron-donating effect, reflected in the most cathodic ligand reduction $(E_{1/2}^{\mathrm{red}}=-2.11V)$ and the least oxidizing excited state potential (E* = 0.46 V). Temperature-dependent luminescence and emission lifetimes are consistent with thermally activated delayed fluorescence (TADF) across the series and reveal substitution-controlled singlet-triplet energy gaps ΔE_ST. Complemented by step-scan FTIR studies, the predominant excited state was identified and analyzed, highlighting the impact of spin density location on both energy- and electron-transfer reactivity. The photocatalytic relevance was demonstrated in three benchmark reactions: singlet oxygen generation (energy transfer, demonstrated by the photooxidation of diphenylfuran to cis-dibenzoylethylene), hydrogen evolution from water, and reductive dehalogenation of aryl halides (electron transfer). In hydrogen evolution, the para isomer gave the highest initial rate and a TON of 590 at 20 h, while the ortho isomer remained active up to 36 h with a TON of 530. Stern-Volmer quenching in THF with TEA confirms a reductive pathway under these conditions. In the photocatalytic dehalogenation, activity trends were substrate-dependent, reflecting a balance between excited-state driving force (E*) and ground-state reducing power $\left(E_{1/2}^{\mathrm{red}}\right)$. Together, these results establish clear position-property-performance relationships to guide Cu(I) photosensitizer design.