Copper(I) complexes are prominent candidates to replace noble metal-based photosensitizers. We recently introduced a three-coordinate design for copper(I) charge-transfer chromophores that pair β-diketiminate ligands with aryl isocyanides. The excited-state lifetime in these compounds can be extended using a bichromophoric "triplet reservoir" strategy, which comes at the expense of a decrease in excited-state energy and reducing power. In this work, we introduce a complementary, sterically driven strategy for increasing the excited-state lifetimes of these photosensitizers, which gives a higher-energy, more strongly reducing charge-transfer triplet state than does the bichromophore approach. The compounds presented (-) have the general formula Cu(CyNacNac)(CN-Ar), where CyNacNac is a cyclohexyl-substituted β-diketiminate and CN-Ar is an aryl isocyanide with a variable steric profile. Their structural features and electrochemical and photophysical properties are described. The complexes with sterically encumbered 2,6-diisopropylphenyl or -terphenyl isocyanide ligands (-) exhibit prolonged excited-state lifetimes relative to those of the parent 2,6-dimethylphenyl isocyanide compound . Specifically, one of the -terphenyl isocyanide compounds, , displays an excited-state lifetime of 276 ns, approximately 30 times longer than that of (9.3 ns). The photoluminescence quantum yield of (0.09) also increases by two orders of magnitude compared to that of (0.0008). The strong excited-state reducing power (* = -2.4 V vs Fc) and long lifetime of lead to higher yields in photoredox and photocatalytic isomerization reactions, which include dehalogenation and/or hydrodgenation of benzophenone substrates, C-O bond activation of a lignin model substrate, and photocatalytic / isomerization of stilbene.

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http://dx.doi.org/10.1021/jacs.4c05180DOI Listing

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