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Next Generation Cuprous Phenanthroline MLCT Photosensitizer Featuring Cyclohexyl Substituents

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journal contribution
posted on 07.06.2021, 17:04 by Michael C. Rosko, Kaylee A. Wells, Cory E. Hauke, Felix N. Castellano
A new long-lived, visible-light-absorbing homoleptic Cu­(I) metal-to-ligand charge transfer (MLCT) photosensitizer, [Cu­(dchtmp)2]­PF6 (dchtmp = 2,9-dicyclohexyl-3,4,7,8-tetramethyl-1,10-phenanthroline), has been synthesized, structurally characterized, and evaluated in terms of its molecular photophysics, electrochemistry, and electronic structure. Static and time-resolved transient absorption (TA) and photoluminescence (PL) spectroscopy measured on the title compound in CH2Cl2 (τ = 2.6 μs, ΦPL = 5.5%), CH3CN (τ = 1.5 μs, ΦPL = 2.6%), and THF (τ = 2.0 μs, ΦPL = 3.7%) yielded impressive photophysical metrics even when dissolved in Lewis basic solvents. The combined static spectroscopic data along with ultrafast TA experiments revealed that the pseudo-Jahn–Teller distortion and intersystem crossing dynamics in the MLCT excited state displayed characteristics of being sterically arrested throughout its evolution. Electrochemical and static PL data illustrate that [Cu­(dchtmp)2]­PF6 is a potent photoreductant (−1.77 V vs Fc+/0 in CH3CN) equal to or greater than all previously investigated homoleptic Cu­(I) diimine complexes. Although we successfully prepared the cyclopentyl analog dcptmp (2,9-dicyclopentyl-3,4,7,8-tetramethyl-1,10-phenanthroline) using the same C–C radical coupling photochemistry as dchtmp, the corresponding Cu­(I) complex could not be isolated due to the steric hindrance presented at the metal center. Ultimately, the successful preparation of [Cu­(dchtmp)2]+ represents a major step forward for the design and discovery of novel earth-abundant photosensitizers made possible through a newly conceived ligand synthetic strategy.