Enhancing the Visible-Light Absorption and Excited-State Properties of Cu(I) MLCT Excited States

A computationally inspired Cu(I) metal-to-ligand charge transfer (MLCT) chromophore, [Cu(sbmpep)(2)](-) (sbmpep = 2,9-di(sec-butyl)-3,8-dimethyl-4,7-di-(phenylethynyl)-1,10-phenanthroline), was synthesized in seven total steps, prepared from either dichloro- or dibromophenanthroline precursors. Complete synthesis, structural characterization, and electrochemistry, in addition to static and dynamic photophysical properties of [Cu(sbmpep)(2)](+), are reported on all relevant time scales. UV-Vis absorption spectroscopy revealed significant increases in oscillator strength along with a concomitant bathochromic shift in the MLCT absorption bands with respect to structurally related model complexes (epsilon = 16 500 M-1 cm(-1) at 491 nm). Strong red photoluminescence Phi = 2.7%, lambda(max) = 687 nm) was observed from [Cu(sbmpep)(2)](+), which featured an average excited-state lifetime of 1.4 ps in deaerated dichloromethane. Cyclic and differential pulse voltammetry revealed similar to 300 mV positive shifts in the measured one-electron reversible reduction and oxidation waves in relation to a Cu(I) model complex possessing identical structural elements without the pi-conjugated 4,7-substituents. The excited-state redox potential of [Cu(sbmpep)(2)](+) was estimated to be -1.36 V, a notably powerful reductant for driving photoredox chemistry. The combination of conventional and ultrafast transient absorption and luminescence spectroscopy successfully map the excited-state dynamics of [Cu(sbmpep)(2)](+) from initial photoexcitation to the formation of the lowestenergy MLCT excited state and ultimately its relaxation to the ground state. This newly conceived molecule appears poised for photosensitization reactions involving energy and electron-transfer processes relevant to photochemical upconversion, photoredox catalysis, and solar fuels photochemistry.