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A critical step in photocatalytic water dissociation is the hole‐mediated oxidation reaction. Molecular‐level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time‐resolved, surface‐specific vibrational sum frequency generation spectroscopy to study the photo‐induced reaction directly at the interface of the photocatalyst TiO2 in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub‐20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H2O into Ti−OH groups and the deprotonation of pre‐existing Ti−OH groups. This study provides unique experimental insights into the early steps of the photo‐induced dissociation processes at the photocatalyst‐water interface, relevant to the design of improved photocatalysts. [ABSTRACT FROM AUTHOR]

Increasing the metal‐to‐ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo‐ and heteroleptic complexes [Fe(cpmp)2]2+ (12+) and [Fe(cpmp)(ddpd)]2+ (22+) with the tridentate ligands 6,2"‐carboxypyridyl‐2,2'‐methylamine‐pyridyl‐pyridine (cpmp) and N,N'‐dimethyl‐N,N'‐di‐pyridin‐2‐ylpyridine‐2,6‐diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal‐ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six‐membered chelate rings. The push‐pull ligand cpmp provides intra‐ligand and ligand‐to‐ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 12+ and 22+ were accessed by X‐ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X‐ray emission spectroscopy, static and time‐resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations. [ABSTRACT FROM AUTHOR]

Polymer light‐emitting diodes based on two poly(p‐phenylene vinylene) derivatives are aged at a constant current density, leading to the formation of hole traps. Time‐resolved photoluminescence spectroscopy (TRPL) measurements show that the degraded polymer light‐emitting diodes (PLEDs) also demonstrate a decrease in exciton lifetime. The amount of nonradiative exciton quenching sites in the aged devices is quantified by Monte Carlo simulations. It is found that the number of hole traps obtained from electrical charge transport measurements matches the number of newly formed nonradiative quenching sites determined from the TRPL experiments. The results reveal the origin for the apparent different behavior of the electroluminescence and photoluminescence upon PLED degradation. The decrease of the electroluminescence is governed by recombination of free electrons with trapped holes, whereas the photoluminescence is reduced by nonradiative quenching processes between excitons and hole traps. [ABSTRACT FROM AUTHOR]