Unexpected Boost in Activity of a Cu(I) Photosensitizer by Stabilizing a Transient Excited State

Abstract

In this study, we present a slight but surprisingly successful structural modification of the previously reported heteroleptic Cu(I) photosensitizer <b>Cubiipo</b> ([(xantphos)Cu(biipo)]PF₆; <b>biipo</b> = 16<i>H</i>-benzo-[4',5']-isoquinolino-[2',1':1,2]-imidazo-[4,5-<i>f</i>]-[1,10]-phenanthrolin-16-one). As a key feature, <b>biipo</b> bears a naphthalimide unit at the back, which is directly fused to a phenanthroline moiety to extend the conjugated π-system. This ligand was now altered to include two additional methyl groups at the 2,9-positions at the phenanthroline scaffold. Comparing the novel <b>Cudmbiipo</b> complex to its predecessor, ultrafast transient absorption spectroscopy reveals the efficient suppression of a major deactivation pathway by stabilization of a transient triplet state. Furthermore, quantitative measurements of singlet oxygen evolution in solution confirmed that a larger fraction of the excited-state population is transferred to the photocatalytically active ligand-centered triplet ³LC state with a much longer lifetime of ∼30 μs compared to <b>Cubiipo</b> (2.6 μs). In addition, <b>Cudmbiipo</b> was compared with the well-established reference complex <b>Cubcp</b> ([(xantphos)Cu(bathocuproine)]PF₆) in terms of its photophysical and photocatalytic properties by applying time-resolved femto- and nanosecond absorption, step-scan Fourier transform infrared (FTIR), and emission spectroscopies. Superior light-harvesting properties and a greatly enhanced excited-state lifetime with respect to <b>Cubcp</b> enable <b>Cudmbiipo</b> to be more active in exemplary photocatalytic applications, <i>i.e</i>., in the formation of singlet oxygen and the isomerization of (<i>E</i>)-stilbene.