Enhanced Visible Light Absorption in Heteroleptic Cuprous Phenanthrolines

• Published in: Inorganic chemistry 2024-01, Vol.63 (3), p.1692-1701
• Main Authors: Rosko, Michael C., Wheeler, Jonathan P., Alameh, Reem, Faulkner, Adrienne P., Durand, Nicolas, Castellano, Felix N.
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.3c04024

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 (εMLCTmax = 7430–9980 M–1 cm–1) and significantly broadened across the UVA and visible regions of the spectrum compared to 1 (εMLCTmax = 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.