Sequential Proton-Coupled Electron Transfer Mediates Excited-State Deactivation of a Eumelanin Building Block.

Skin photoprotection is commonly believed to rely on the photochemistry of 5,6-dihydroxyindole (DHI)- and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-based eumelanin building blocks. Attempts to elucidate the underlying excited-state relaxation mechanisms have been partly unsuccessful due to the marked instability to oxidation. We report a study of the excited-state deactivation of DHI using steady-state and time-resolved fluorescence accompanied by high-level quantum-chemistry calculations including solvent effects. Spectroscopic data show that deactivation of the lowest excited state of DHI in aqueous buffer proceeds on the 100 ps time scale and is 20 times faster than in methanol. Quantum-chemical calculations reveal that the excited-state decay mechanism is a sequential proton-coupled electron transfer, which involves the initial formation of a solvated electron from DHI, followed by the transfer of a proton to the solvent. This unexpected finding would prompt a revision of current notions about eumelanin photophysics and photobiology.

Ultrafast Dynamics of Hole Injection and Recombination in Organometal Halide Perovskite Using Nickel Oxide as p-Type Contact Electrode.

There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organometal halide perovskite-based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the subpicosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination time between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of picoseconds to a few nanoseconds. Because of the low conductivity of NiO(np), holes are pinned at the interface, and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination.

Superior photoprotective motifs and mechanisms in eumelanins uncovered.

Human pigmentation is a complex phenomenon commonly believed to serve a photoprotective function through the generation and strategic localization of black insoluble eumelanin biopolymers in sun exposed areas of the body. Despite compelling biomedical relevance to skin cancer and melanoma, eumelanin photoprotection is still an enigma: What makes this pigment so efficient in dissipating the excess energy brought by harmful UV-light as heat? Why has Nature selected 5,6-dihydroxyindole-2-carboxylic acid (DHICA) as the major building block of the pigment instead of the decarboxylated derivative (DHI)? By using pico- and femtosecond fluorescence spectroscopy we demonstrate herein that the excited state deactivation in DHICA oligomers is 3 orders of magnitude faster compared to DHI oligomers. This drastic effect is attributed to their specific structural patterns enabling multiple pathways of intra- and interunit proton transfer. The discovery that DHICA-based scaffolds specifically confer uniquely robust photoprotective properties to natural eumelanins settles a fundamental gap in the biology of human pigmentation and opens the doorway to attractive advances and applications.

Towards longer-lived metal-to-ligand charge transfer states of iron(II) complexes: an N-heterocyclic carbene approach.

A 9 ps (3)MLCT lifetime was achieved by a Fe(II) complex based on C(NHC)^N(py)^C(NHC) pincer ligands. This is the longest known so far for any kind of complexes of this abundant metal, and increased by almost two orders of magnitude compared to the reference Fe(II) bis-terpyridine complex.

Control of the size and shape of TiO2 nanoparticles in restricted media.

Template-capped TiO2 nanostructures have been synthesized. In certain template conditions, TiO2 hexagons are found to form. These hexagonal structures can be effectively sensitized by fluorescein dye without any change in the protonation state of the dye. Bare TiO2 nanoparticles are not so useful for sensitization with dyes like fluorescein as they alter the dye protonation state. The novelty of this work is twofold-the hitherto elusive hexagonal phase of TiO2 nanoparticles has been stabilized and the synthesis of TiO2 in the rutile phase has been achieved under mild conditions.

Excited-State Proton-Transfer Processes of DHICA Resolved: From Sub-Picoseconds to Nanoseconds.

Excited-state proton transfer has been hypothesized as a mechanism for UV energy dissipation in eumelanin skin pigments. By using time-resolved fluorescence spectroscopy, we show that the previously proposed, but unresolved, excited-state intramolecular proton transfer (ESIPT) of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) occurs with a time constant of 300 fs in aqueous solution but completely stops in methanol. The previously disputed excited-state proton transfer involving the 5- or 6-OH groups of the DHICA anion is now found to occur from the 6-OH group to aqueous solvent with a rate constant of 4.0 × 10(8) s(-1).

Bottom-up approach to eumelanin photoprotection: emission dynamics in parallel sets of water-soluble 5,6-dihydroxyindole-based model systems.

The molecular mechanisms by which the black eumelanin biopolymers exert their photoprotective action on human skin and eyes are still poorly understood, owing to critical insolubility and structural heterogeneity issues hindering direct investigation of excitation and emission behavior. Recently, we set up strategies to obtain water-soluble 5,6-dihydroxyindole (DHI)-based polymers as useful models for disentangling intrinsic photophysical properties of eumelanin components from aggregation and scattering effects. Herein, we report the absorption properties and ultrafast emission dynamics of two separate sets of DHI-based monomer-dimer-polymer systems which were made water-soluble by means of poly(vinyl alcohol) or by galactosyl-thio substitution. Data showed that dimerization and polymerization of DHI result in long-lived excited states with profoundly altered properties relative to the monomer and that glycosylation of DHI imparts monomer-like behavior to oligomers and polymers, due to steric effects hindering planar conformations and efficient interunit electron communication. The potential of S-glycation as an effective tool to probe and control emission characteristics of eumelanin-like polymers is disclosed.

Photophysics of indole-2-carboxylic acid in an aqueous environment studied by fluorescence spectroscopy in combination with ab initio calculations.

The photo-physics and -chemistry of indoles are known to be highly complex and strongly dependent on their precise molecular structure and environment. Combination of spectroscopic analysis with quantum chemical calculations should be a powerful tool to unravel precise excited state deactivation mechanisms. At the same time, combined studies are seldom and likely far from trivial. In this work we explore the feasibility of combining spectroscopic and quantum-chemical data into one consistent model. The molecule of choice is indole-2-carboxylic acid (ICA) in aqueous media. Excited state dynamics are determined by time-resolved fluorescence experiments, while excited state reaction pathways of ICA-H(2)O clusters are explored by ab initio calculations.

Matrix-isolation and Ab initio study of the HKrCl...HCl complex.

The HKrCl complex with HCl is characterized by IR spectroscopy in a Kr matrix and by ab initio calculations. The HKrCl...HCl complex exhibits a strong blue shift of the H-Kr stretching mode in comparison with the HKrCl monomer, which indicates stabilization of the H-Kr bond upon complexation. The obtained maximal shift of ca. +300 cm(-1) is probably the largest blue shift experimentally observed for 1:1 molecular complexes. The HCl absorptions are found to be strongly red-shifted upon complexation with HKrCl (up to ca. -500 cm(-1)). In the HKrCl synthesis procedure, an HCl/Kr matrix was first photolyzed at 193 nm to yield H and Cl atoms in a Kr matrix and then annealed at about 30 K to activate mobility of H atoms and to promote the H + Kr + Cl reaction. The HKrCl...HCl complex is mainly formed from the Cl...HCl intermediate complex that is produced by photolysis of HCl dimers. Bands of the HKrCl...(HCl)(2) complex are tentatively identified with a very large shift of ca. +700 cm(-1) for the H-Kr stretching mode. These experimental observations are supported and explained by ab initio calculations.