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 Cubiipo ([(xantphos)­Cu­(biipo)]­PF6; biipo = 16H-benzo-[4′,5′]-isoquinolino-[2′,1′:1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one). As a key feature, biipo 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 Cudmbiipo 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 Cubiipo (2.6 μs). In addition, Cudmbiipo was compared with the well-established reference complex Cubcp ([(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 Cubcp enable Cudmbiipo to be more active in exemplary photocatalytic applications, i.e., in the formation of singlet oxygen and the isomerization of (E)-stilbene.

Bichromophoric Photosensitizers: How and Where to Attach Pyrene Moieties to Phenanthroline to Generate Copper(I) Complexes

“…The syntheses of the heteroleptic copper­(I) PS CuPyr1 , CuPyr2 , and CuPyr3 were conducted following a well-known one-pot two-step procedure (Scheme ) starting from [Cu­(MeCN) 4 ]­PF 6 (MeCN = acetonitrile). ,,,,, To ensure a slow and precise addition of the diimine ligand to the reaction mixture, an automatic syringe pump was utilized for the preparation of CuPyr1 and CuPyr3 . In contrast, due to its low solubility in CH 2 Cl 2 , Pyr2 was added directly as a solid under inert conditions at −20 °C for the synthesis of CuPyr2 .…”

“…The efficient capture and usage of solar energy provides a sustainable alternative to the burning of fossil fuels. − In this context, photosensitizers (PS) offer a viable tool for harvesting sunlight and converting solar energy directly into chemical energy. This has been successfully demonstrated for the conversion of small molecules such as the photocatalytic production of hydrogen from water, − the photoreduction of CO 2 to CO, − or the generation of reactive singlet oxygen. − Furthermore, PS also enable a wide range of organic transformations like the E → Z isomerization of different alkenes, − cyanation , or even the challenging alkylation , and arylation , of various organic substrates. − …”

“…In this context, especially heteroleptic copper­(I) complexes of the type [Cu­(N^N)­(P^P)] + bearing a diimine and diphosphine ligand enable long-lived excited states, sufficient quantum yields, and high excited state redox potentials. ,,,− Moreover, they have already successfully demonstrated their competitiveness with noble metal-based Ir­(III) and Ru­(II) complexes in photocatalysis. ,,,, As the absorptivity of the heteroleptic Cu­(I) complexes in the visible region is rather limited, and with the aim of further improving the excited state properties, several studies have targeted this challenge by modifying the diimine ligand. ,,, One promising strategy is the introduction of extended π-systems in the backbone of the diimine moiety by directly fusing an additional organic chromophore to form large and highly conjugated ligand scaffolds. ,− It was proven that the implementation of conjugated groups in this way can strongly increase the excited state lifetimes through the introduction of long-lived ligand-centered triplet ( 3 LC) states. ,,, However, this sometimes comes at the cost of significantly shifting reduction potentials toward less negative values and therefore a loss in possible driving force in catalytic transformations. , …”

Trendbericht: Photochemie

“…Eine Erweiterung des chromophoren Systems (Abbildung 1), 16,17) beispielsweise durch eine kovalent gebundene oder direkt kondensierte Naphthalimideinheit, trägt zur Optimierung von heteroleptischen Diimin-Diphosphin-Cu I -Komplexen des Typs [Cu(N^N)(P^P)] + bei. Synthetisiert wurden der Cu(NIphen)-(NIphen 5-(1,8-Naph thal imid)-1,10phen an thro lin)-und der Cu(biipo)-Komplex (biipo Benzo-isochinolino-imidazo-phenanthrolin-on).…”

Reaching strong absorption up to 700 nm with new benzo[g]quinoxaline-based heteroleptic copper(i) complexes for light-harvesting applications

“…By now, there are still a few cases of heteroleptic diimine-diphosphine copper(I) complexes showing an absorption at longer wavelengths than 450 nm. [20][21][22][23][24][25] In this work, we aimed at new heteroleptic Cu(I) complexes of type (NN)Cu(PP) with a broad and intense absorption in the visible range. We used bis[(2-diphenylphosphino) phenyl] ether (DPEPhos) as chelating diphosphine and as diimine ligands, 4-(benzo[g]quinoxal-2′-yl)-1,2,3triazole-derivatives.…”