Enhanced Visible Light Absorption in Heteroleptic Cuprous Phenanthrolines

This work presents a series of Cu(I) heteroleptic 1,10-phenanthroline chromophores featuring enhanced UVA and visible-light-harvesting properties manifested through vectorial control of the copper-to-phenanthroline charge-transfer transitions. The molecules were prepared using the HETPHEN strategy, wherein a sterically congested 2,9-dimesityl-1,10-phenanthrolne (mesPhen) ligand was paired with a second phenanthroline ligand incorporating extended π-systems in their 4,7-positions. The combination of electrochemistry, static and time-resolved electronic spectroscopy, 77 K photoluminescence spectra, and time-dependent density functional theory calculations corroborated all of the experimental findings. The model chromophore, [Cu(mesPhen)(phen)]⁺ (1), lacking 4,7-substitutions preferentially reduces the mesPhen ligand in the lowest energy metal-to-ligand charge-transfer (MLCT) excited state. The remaining cuprous phenanthrolines (2-4) preferentially reduce their π-conjugated ligands in the low-lying MLCT excited state. The absorption cross sections of 2-4 were enhanced (ε_MLCTmax = 7430-9980 M^-1 cm^-1) and significantly broadened across the UVA and visible regions of the spectrum compared to 1 (ε_MLCTmax = 6494 M^-1 cm^-1). The excited-state decay mechanism mirrored those of long-lived homoleptic Cu(I) phenanthrolines, yielding three distinguishable time constants in ultrafast transient absorption experiments. These represent pseudo-Jahn-Teller distortion (τ₁), singlet-triplet intersystem crossing (τ₂), and the relaxed MLCT excited-state lifetime (τ₃). Effective light-harvesting from Cu(I)-based chromophores can now be rationalized within the HETPHEN strategy while achieving directionality in their respective MLCT transitions, valuable for integration into more complex donor-acceptor architectures and longer-lived photosensitizers.