Browsing by Author "Tschierlei, Stefanie"

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    The coordination behaviour of CuI photosensitizers bearing multidentate ligands investigated by X‐ray absorption spectroscopy
    (2020) Rentschler, Martin; Iglesias, Sirma; Schmid, Marie‐Ann; Liu, Cunming; Tschierlei, Stefanie; Frey, Wolfgang; Zhang, Xiaoyi; Karnahl, Michael; Moonshiram, Dooshaye
    A systematic series of four novel homo‐ and heteroleptic CuI photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers.
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    Exploring the full potential of photocatalytic carbon dioxide reduction using a dinuclear Re2Cl2 complex assisted by various photosensitizers
    (2021) Giereth, Robin; Obermeier, Martin; Forschner, Lukas; Karnahl, Michael; Schwalbe, Matthias; Tschierlei, Stefanie
    Photosensitizing units have already been applied to enable light‐driven catalytic reduction of CO2 with mononuclear rhenium complexes. However, dinuclear catalytic systems that are able to activate CO2 in a cooperative bimetallic fashion have only rarely been combined with photosensitizers. We here present detailed studies on the influence of additional photosensitizers on the catalytic performance of a dirhenium complex (Re2Cl2) and present correlations with spectroscopic measurements, which shed light on the reaction mechanism. The use of [Ir(dFppy)3] (Ir, dFppy=2‐(4,6‐difluorophenyl)pyridine)) resulted in considerably faster CO2 to CO transformation than [Cu(xant)(bcp)]PF6 (Cu, xant=xantphos, bcp=bathocuproine). Emission quenching studies, transient absorption as well as IR spectroscopy provide information about the electron transfer paths of the intermolecular systems. It turned out that formation of double reduced species [Re2Cl2]2- along with an intermediate with a Re-Re bond ([ReRe]) can be taken as an indication of multi‐electron storage capacity. Furthermore, under catalytic conditions a CO2‐bridged intermediate was identified.
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    How the way a naphthalimide unit is implemented affects the photophysical and -catalytic properties of Cu(I) photosensitizers
    (2022) Yang, Yingya; Doettinger, Florian; Kleeberg, Christian; Frey, Wolfgang; Karnahl, Michael; Tschierlei, Stefanie
    Driven by the great potential of solar energy conversion this study comprises the evaluation and comparison of two different design approaches for the improvement of copper based photosensitizers. In particular, the distinction between the effects of a covalently linked and a directly fused naphthalimide unit was assessed. For this purpose, the two heteroleptic Cu(I) complexes CuNIphen (NIphen = 5-(1,8-naphthalimide)-1,10-phenanthroline) and Cubiipo (biipo = 16H-benzo-[4′,5′]-isoquinolino-[2′,1′,:1,2]-imidazo-[4,5-f]-[1,10]-phenanthroline-16-one) were prepared and compared with the novel unsubstituted reference compound Cuphen (phen = 1,10-phenanthroline). Beside a comprehensive structural characterization, including two-dimensional nuclear magnetic resonance spectroscopy and X-ray analysis, a combination of electrochemistry, steady-state and time-resolved spectroscopy was used to determine the electrochemical and photophysical properties in detail. The nature of the excited states was further examined by (time-dependent) density functional theory (TD-DFT) calculations. It was found that CuNIphen exhibits a greatly enhanced absorption in the visible and a strong dependency of the excited state lifetimes on the chosen solvent. For example, the lifetime of CuNIphen extends from 0.37 µs in CH2Cl2 to 19.24 µs in MeCN, while it decreases from 128.39 to 2.6 µs in Cubiipo. Furthermore, CuNIphen has an exceptional photostability, allowing for an efficient and repetitive production of singlet oxygen with quantum yields of about 32%.