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.

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.

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 .SECTION: Spectroscopy, Photochemistry, and Excited States T he 5,6-dihydroxyindole-2-carboxylic acid (DHICA), a key product of tyrosine metabolism in cutaneous melanocytes, plays important roles in skin homeostasis as a major precursor with 5,6-dihydroxyindole (DHI) of eumelanin biopolymers, the dark pigments of human skin, hair, and eyes, 1 as an antioxidant, 2 and as a central mediator in cell−cell communication. 3 Eumelanins contain various proportions of DHICA-derived units ranging from only a few % up to >50% depending on the phenotype and organism. While eumelanins appear to be complex biopolymers in which the various units concur to determine the macroscopic properties at several levels of structural organization, redox states, and disorder, a detailed understanding of the photophysical behavior of key building blocks is crucial if structure−property−function relationships are to be drawn. The peculiar absorption features of eumelanin 4,5 and its former monomer DHI and DHICA might play a role in protecting the skin against UV radiation, though the UV-induced reaction mechanisms are largely unknown. Recent time-resolved spectroscopy work on DHICA 6−8 revealed a rich, pH-dependent photochemistry in aqueous buffer solution. At a pH where the carboxyl group of the molecule is fully protonated (e.g., pH 2.5), a red-shifted fluorescence band (λ max ≈ 430 nm) with a relatively short lifetime of 240 ps was observed and attributed to a zwitterionic species formed as a result of rapid excited-state intramolecular proton transfer (ESIPT) from the COOH group toward the NH group. 6 However, the actual transfer time and decay of the original excited state of the fully protonated molecule could not be resolved with the temporal resolution of the employed streak camera technique. By using femtosecond fluorescence upconversion (FU), we can now directly resolve this reaction step and show that it indeed proceeds on the sup-picosecond time scale.The deprotonated carboxylate anion DHICA − was observed to have a long (∼1.6 ns) lifetime in neutral (pH 7) buffer solution representing decay to a new species with a red-shifted fluorescence spectrum (λ max ≈ 450 nm) and a 2.4 ns lifetime. 6 This species was assigned to a complex between the excited DHICA − and a buffer species formed through a diffusion process. 6 On the other hand, calculations by Olsen et al. 9 suggested that...

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.

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 watersoluble 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.

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.

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.

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.