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- ItemFluorosolvatochromism of furanyl- and thiophenyl-substituted acetophenones(London : RSC, 2015) Friebe, Nadine; Schreiter, Katja; Kübel, Joachim; Dietzek, Benjamin; Moszner, Norbert; Burtscher, Peter; Oehlke, Alexander; Spange, StefanA series of para-substituted acetophenones bearing a furanyl or a thiophenyl moiety show a large Stokes-shift, which is a function of various solvent properties. Photophysical properties such as emission lifetime of the compounds have been determined using time-correlated-single photon counting to secure the intrinsic fluorescence behaviour. The solvent dependent position of the UV/Vis emission band [small nu, Greek, tilde]max,em of the compounds has been measured in 26 various solvents. The influence of the solvent on [small nu, Greek, tilde]max,em is of very complex nature and mathematically analysed by multiple square linear solvation energy (LSE)-correlation analysis using Catalán's four-solvent parameter set. Solvent acidity has a strong influence on the bathochromic shift of 2,5-disubstituted furan derivatives compared to the non-5-substituted furan and thiophene derivatives, which show a contrary behaviour. Therefore, the 5-cyanofuranyl-substituted acetophenone derivative is useful as a probe for measuring environmental properties by fluorescence spectroscopy.
- ItemHole injection dynamics from two structurally related Ru-bipyridine complexes into NiOx is determined by the substitution pattern of the ligands(Cambridge : RSC Publ., 2015) Bräutigam, Maximilian; Kübel, Joachim; Schulz, Martin; Vos, Johannes G.; Dietzek, BenjaminThe dyes bis[2,2′-bipyridine][4,4′-dicarboxy-2,2′-bipyridine]ruthenium(II) dihexafluorophosphate, [Ru(bpy)2dcb](PF6)2 (Ru1), and tris[4,4′-bis(ethylcarboxy)-2,2′-bipyridine]ruthenium(II) dihexafluorophosphate, [Ru(dceb)3](PF6)2 (Ru2), attached to NiOx nanoparticle films were investigated using transient absorption and luminescence spectroscopy. In acetonitrile solution the dyes reveal very similar physical and chemical properties, i.e. both dyes exhibit comparable ground state and long-lived, broad excited state absorption. However, when immobilized onto a NiOx surface the photophysical properties of the two dyes differ significantly. For Ru1 luminescence is observed, which decays within 18 ns and ultrafast transient absorption measurements do not show qualitative differences from the photophysics of Ru1 in solution. In contrast to this the luminescence of photoexcited Ru2 on NiOx is efficiently quenched and the ultrafast transient absorption spectra reveal the formation of oxidized nickel centres overlaid by the absorption of the reduced dye Ru2 with a characteristic time-constant of 18 ps. These findings are attributed to the different localization of the initially photoexcited state in Ru1 and Ru2. Due to the inductive effect (−I) of the carboxylic groups, the lowest energy excited state in Ru1 is localized on the dicarboxy-bipyridine ligand, which is bound to the NiOx surface. In Ru2, on the other hand, the initially populated excited state is localized on the ester-substituted ligands, which are not bound to the semiconductor surface. Hence, the excess charge density that is abstracted from the Ru-ion in the metal-to-ligand charge-transfer transition is shifted away from the NiOx surface, which ultimately facilitates hole transfer into the semiconductor.
- ItemIn situ spectroelectrochemical and theoretical study on the oxidation of a 4H-imidazole-ruthenium dye adsorbed on nanocrystalline TiO2 thin film electrodes(Cambridge : RSC Publ., 2015) Zhang, Ying; Kupfer, Stephan; Zedler, Linda; Schindler, Julian; Bocklitz, Thomas; Guthmuller, Julien; Rau, Sven; Dietzek, BenjaminTerpyridine 4H-imidazole-ruthenium(II) complexes are considered promising candidates for use as sensitizers in dye sensitized solar cells (DSSCs) by displaying broad absorption in the visible range, where the dominant absorption features are due to metal-to-ligand charge transfer (MLCT) transitions. The ruthenium(III) intermediates resulting from photoinduced MLCT transitions are essential intermediates in the photoredox-cycle of the DSSC. However, their photophysics is much less studied compared to the ruthenium(II) parent systems. To this end, the structural alterations accompanying one-electron oxidation of the RuIm dye series (including a non-carboxylic RuIm precursor, and, carboxylic RuImCOO in solution and anchored to a nanocrystalline TiO2 film) are investigated via in situ experimental and theoretical UV-Vis absorption and resonance Raman (RR) spectroelectrochemistry. The excellent agreement between the experimental and the TDDFT spectra derived in this work allows for an in-depth assignment of UV-Vis and RR spectral features of the dyes. A concordant pronounced wavelength dependence with respect to the charge transfer character has been observed for the model system RuIm, and both RuImCOO in solution and attached on the TiO2 surface. Excitation at long wavelengths leads to the population of ligand-to-metal charge transfer states, i.e. photoreduction of the central ruthenium(III) ion, while high-energy excitation features an intra-ligand charge transfer state localized on the 4H-imidazole moiety. Therefore, these 4H-imidazole ruthenium complexes investigated here are potential multi-photoelectron donors. One electron is donated from MLCT states, and additionally, the 4H-imidazole ligand reveals electron-donating character with a significant contribution to the excited states of the ruthenium(III) complexes upon blue-light irradiation.
- ItemResonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways(Basel : MDPI, 2019) Zedler, Linda; Krieck, Sven; Kupfer, Stephan; Dietzek, BenjaminElectron transfer reactions play a key role for artificial solar energy conversion, however, the underlying reaction mechanisms and the interplay with the molecular structure are still poorly understood due to the complexity of the reaction pathways and ultrafast timescales. In order to investigate such light-induced reaction pathways, a new spectroscopic tool has been applied, which combines UV-vis and resonance Raman spectroscopy at multiple excitation wavelengths with electrochemistry in a thin-layer electrochemical cell to study [RuII(tbtpy)2]2+ (tbtpy = tri-tert-butyl-2,2′:6′,2′′-terpyridine) as a model compound for the photo-activated electron donor in structurally related molecular and supramolecular assemblies. The new spectroscopic method substantiates previous suggestions regarding the reduction mechanism of this complex by localizing photo-electrons and identifying structural changes of metastable intermediates along the reaction cascade. This has been realized by monitoring selective enhancement of Raman-active vibrations associated with structural changes upon electronic absorption when tuning the excitation wavelength into new UV-vis absorption bands of intermediate structures. Additional interpretation of shifts in Raman band positions upon reduction with the help of quantum chemical calculations provides a consistent picture of the sequential reduction of the individual terpyridine ligands, i.e., the first reduction results in the monocation [(tbtpy)Ru(tbtpy•)]+, while the second reduction generates [(tbtpy•)Ru(tbtpy•)]0 of triplet multiplicity. Therefore, the combination of this versatile spectro-electrochemical tool allows us to deepen the fundamental understanding of light-induced charge transfer processes in more relevant and complex systems.
- ItemInvestigating light-induced processes in covalent dye-catalyst assemblies for hydrogen production(Basel : MDPI, 2020) Bold, Sebastian; Straistari, Tatiana; Muñoz-García, Ana B.; Pavone, Michele; Artero, Vincent; Chavarot-Kerlidou, Murielle; Dietzek, BenjaminThe light-induced processes occurring in two dye-catalyst assemblies for light-driven hydrogen production were investigated by ultrafast transient absorption spectroscopy. These dyads consist of a push-pull organic dye based on a cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) bridge, covalently linked to two different H2-evolving cobalt catalysts. Whatever the nature of the latter, photoinduced intramolecular electron transfer from the excited state of the dye to the catalytic center was never observed. Instead, and in sharp contrast to the reference dye, a fast intersystem crossing (ISC) populates a long-lived triplet excited state, which in turn non-radiatively decays to the ground state. This study thus shows how the interplay of different structures in a dye-catalyst assembly can lead to unexpected excited state behavior and might open up new possibilities in the area of organic triplet sensitizers. More importantly, a reductive quenching mechanism with an external electron donor must be considered to drive hydrogen production with these dye-catalyst assemblies. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemYield—not only Lifetime—of the Photoinduced Charge-Separated State in Iridium Complex–Polyoxometalate Dyads Impact Their Hydrogen Evolution Reactivity(Weinheim : Wiley-VCH, 2020) Luo, Yusen; Maloul, Salam; Schönweiz, Stefanie; Wächtler, Maria; Streb, Carsten; Dietzek, BenjaminCovalently linked photosensitizer–polyoxometalate (PS-POM) dyads are promising molecular systems for light-induced energy conversion processes, such as “solar” hydrogen generation. To date, very little is known of their fundamental photophysical properties which affect the catalytic reactivity and stability of the systems. PS-POM dyads often feature short-lived photoinduced charge-separated states, and the lifetimes of these states are considered crucial for the function of PS-POM dyads in molecular photocatalysis. Hence, strategies have been developed to extend the lifetimes of the photoinduced charge-separated states, either by tuning the PS photophysics or by tuning the POM redox properties. Recently, some of us reported PS-POM dyads based on cyclometalated IrIII complexes covalently linked to Anderson-type polyoxometalate. Distinct hydrogen evolution reactivity (HER) of the dyads was observed, which was tuned by varying the central metal ion M of the POMM (M=Mn3+, Co3+, Fe3+). In this manuscript, the photoinduced electron-transfer processes in the three Ir-POMM dyads are investigated to rationalize the underlying reasons for the differences in HER activity observed. We report that upon excitation of the IrIII complex, ultrafast (sub-ps) charge separation occurs, leading to different amounts of the charge-separated states (Ir.+-POMM.−) generated in the different dyads. However, in all dyads studied, the resulting Ir.+-POMM.− species are short-lived (sub-ns) when compared to reference electron acceptors (e.g. porphyrins or fullerenes) reported in the literature. The reductive quenching of Ir.+-POMM.− by a sacrificial donor, triethyl amine (1 m), to generate the intermediate Ir-POMM.− is estimated to be very efficient (70–80 %) for all dyads studied. Based on this analyses, we conclude that the yield instead of the lifetime of the Ir.+-POMM.− charge-separated state determines the catalytic capacity of the dyads investigated. This new feature in the PS-POM photophysics could lead to new design criteria for the development of novel PS-POM dyads. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemBlock Copolymers Featuring Highly Photostable Photoacids Based on Vinylnaphthol: Synthesis and Self-Assembly(Weinheim : Wiley-VCH, 2020) Wendler, Felix; Tom, Jessica C.; Sittig, Maria; Biehl, Philip; Dietzek, Benjamin; Schacher, Felix H.The synthesis of a photoresponsive amphiphilic diblock quarterpolymer containing 5-vinyl-1-naphthol (VN) as a photostable photoacidic comonomer is presented. The preparation is realized via a sequential reversible addition fragmentation chain transfer (RAFT) polymerization starting from a nona(ethylene glycol) methyl ether methacrylate (MEO9MA/“O”) hydrophilic block, which is then used as a macro-RAFT agent in the terpolymerization of styrene (S), 2-vinylpyridine (2VP), and TBS-protected VN (tVN). The terpolymerization proceeds in a controlled fashion and two diblock quarterpolymers, P(Om)-b-P(Sx-co-2VPy-co-VNz), with varying functional comonomer compositions are prepared. These diblock quarterpolymers form spherical core-corona micelles in aqueous media according to dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM). Upon irradiation, the photoacids within the micellar core experience a drastic increase in acidity causing a proton transfer from the photoacid to neighboring 2VP units. As a result, the hydrophilic/hydrophobic balance of the entire assembly is shifted, and the encapsulated cargo is released. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemWater-Soluble Polymeric Carbon Nitride Colloidal Nanoparticles for Highly Selective Quasi-Homogeneous Photocatalysis(Weinheim : Wiley-VCH, 2019) Krivtsov, Igor; Mitoraj, Dariusz; Adler, Christiane; Ilkaeva, Marina; Sardo, Mariana; Mafra, Luis; Neumann, Christof; Turchanin, Andrey; Li, Chunyu; Dietzek, Benjamin; Leiter, Robert; Biskupek, Johannes; Kaiser, Ute; Im, Changbin; Kirchhoff, Björn; Jacob, Timo; Beranek, RadimHeptazine-based polymeric carbon nitrides (PCN) are promising photocatalysts for light-driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom-up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi-homogeneous conditions. The superior performance of water-soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4-methoxybenzyl alcohol and benzyl alcohol or lignocellulose-derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re-dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemPhotophysics of BODIPY dyes as readily designable photosensitisers in light-driven proton reduction(Basel : MDPI, 2017) Dura, Laura; Wächtler, Maria; Kupfer, Stephan; Kübel, Joachim; Ahrens, Johannes; Höfler, Sebastian; Bröring, Martin; Dietzek, Benjamin; Beweries, TorstenA series of boron dipyrromethene (BODIPY) dyes was tested as photosensitisers for light-driven hydrogen evolution in combination with the complex [Pd(PPh3)Cl2]2 as a source for catalytically-active Pd nanoparticles and triethylamine as a sacrificial electron donor. In line with earlier reports, halogenated dyes showed significantly higher hydrogen production activity. All BODIPYs were fully characterised using stationary absorption and emission spectroscopy. Time-resolved spectroscopic investigations on meso-mesityl substituted compounds revealed that reduction of the photo-excited BODIPY by the sacrificial agent occurs from an excited singlet state, while, in halogenated species, long-lived triplet states are present, determining electron transfer processes from the sacrificial agent. Quantum chemical calculations performed at the time-dependent density functional level of theory indicate that the differences in the photocatalytic performance of the present series of dyes can be correlated to the varying efficiency of intersystem crossing in non-halogenated and halogenated species and not to alterations in the energy levels introduced upon substitution.
- ItemStructure of Ni(OH)2 intermediates determines the efficiency of NiO-based photocathodes – a case study using novel mesoporous NiO nanostars(Cambridge : RSC, 2019) Wahyuono, Ruri Agung; Dellith, Andrea; Schmidt, Christa; Dellith, Jan; Ignaszak, Anna; Seyring, Martin; Rettenmayr, Markus; Fize, Jennifer; Artero, Vincent; Chavarot-Kerlidou, Murielle; Dietzek, BenjaminWe report the wet chemical synthesis of mesoporous NiO nanostars (NS) as photocathode material for dye-sensitized solar cells (DSSCs). The growth mechanism of NiO NS as a new morphology of NiO is assessed by TEM and spectroscopic investigations. The NiO NS are obtained upon annealing of preformed β-Ni(OH)2 into pristine NiO with low defect concentrations and favorable electronic configuration for dye sensitization. The NiO NS consist of fibers self-assembled from nanoparticles yielding a specific surface area of 44.9 m2 g-1. They possess a band gap of 3.83 eV and can be sensitized by molecular photosensitizers bearing a range of anchoring groups, e.g. carboxylic acid, phosphonic acid, and pyridine. The performance of NiO NS-based photocathodes in photoelectrochemical application is compared to that of other NiO morphologies, i.e. nanoparticles and nanoflakes, under identical conditions. Sensitization of NiO NS with the benchmark organic dye P1 leads to p-DSSCs with a high photocurrent up to 3.91 mA cm-2 whilst the photoelectrochemical activity of the NiO NS photocathode in aqueous medium in the presence of an irreversible electron acceptor is reflected by generation of a photocurrent up to 23 μA cm-2 © 2019 The Royal Society of Chemistry.
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