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- ItemPhotophysics of Anionic Bis(4H-imidazolato)CuI Complexes(Weinheim : Wiley-VCH, 2022) Seidler, Bianca; Tran, Jens H.; Hniopek, Julian; Traber, Philipp; Görls, Helmar; Gräfe, Stefanie; Schmitt, Michael; Popp, Jürgen; Schulz, Martin; Dietzek‐Ivanšić, BenjaminIn this paper, the photophysical behavior of four panchromatically absorbing, homoleptic bis(4H-imidazolato)CuI complexes, with a systematic variation in the electron-withdrawing properties of the imidazolate ligand, were studied by wavelength-dependent time-resolved femtosecond transient absorption spectroscopy. Excitation at 400, 480, and 630 nm populates metal-to-ligand charge transfer, intraligand charge transfer, and mixed-character singlet states. The pump wavelength-dependent transient absorption data were analyzed by a recently established 2D correlation approach. Data analysis revealed that all excitation conditions yield similar excited-state dynamics. Key to the excited-state relaxation is fast, sub-picosecond pseudo-Jahn-Teller distortion, which is accompanied by the relocalization of electron density onto a single ligand from the initially delocalized state at Franck-Condon geometry. Subsequent intersystem crossing to the triplet manifold is followed by a sub-100 ps decay to the ground state. The fast, nonradiative decay is rationalized by the low triplet-state energy as found by DFT calculations, which suggest perspective treatment at the strong coupling limit of the energy gap law.
- 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.
- ItemZnO nanoflowers-based photoanodes: aqueous chemical synthesis, microstructure and optical properties(Berlin : de Gruyter, 2016) Wahyuono, Ruri Agung; Schmidt, Christa; Dellith, Andrea; Dellith, Jan; Schulz, Martin; Seyring, Martin; Rettenmayr, Markus; Plentz, Jonathan; Dietzek, BenjaminWe have developed an efficient, low temperature, synthetic route for ZnO nanoflowers (NFs) as photoanode material. This alternative route yields small flowerlike nanostructures, built from densely self-assembled tip-ended rod structures. The obtained ZnO NFs possess a large bandgap of 3.27 - 3.39 eV, enabling the generation of an average open current voltage of 0.56 V. Additionally, they show a high internal light harvesting of 14.6•10-7A-mol-1. The growth mechanism and self-assembly of ZnO NFs were studied in detail by joint spectroscopic-TEM investigations. It is shown that the ZnO crystallite size increases with increasing annealing temperatures and that the stress and the improved crystallinity are induced by annealing and reduce the lattice strain and the dislocation density. The bandgaps of ZnO are affected by the lattice strain revealing an optimal region of lattice strain to gain high bandgap energies. The properties of the synthesized ZnO NFs are compared with other morphologies, i.e. ZnO spherical aggregates (SPs) and ZnO nanorods (NRs), and are tested as electrode materials in dye-sensitized solar cells.