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

Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements

Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump–probe transient absorption and charge extraction of photogenerated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 × 10^–12 cm^–3 s^–1. This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed

Exceptional Excited-State Lifetime of an Iron(II)–<i>N</i>‑Heterocyclic Carbene Complex Explained

Earth-abundant transition-metal complexes are desirable for sensitizers in dye-sensitized solar cells or photocatalysts. Iron is an obvious choice, but the energy level structure of its typical polypyridyl complexes, featuring low-lying metal-centered states, has made such complexes useless as energy converters. Recently, we synthesized a novel iron–<i>N</i>-heterocyclic carbene complex exhibiting a remarkable 100-fold increase of the lifetime compared to previously known iron­(II) complexes. Here, we rationalize the measured excited-state dynamics with DFT and TD-DFT calculations. The calculations show that the exceptionally long excited-state lifetime (∼9 ps) is achieved for this Fe complex through a significant destabilization of both triplet and quintet metal-centered scavenger states compared to other Fe^II complexes. In addition, a shallow ³MLCT potential energy surface with a low-energy transition path from the ³MLCT to ³MC and facile crossing from the ³MC state to the ground state are identified as key features for the excited-state deactivation

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 × 10⁸ s^–1

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

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Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals Single-Trap Control of Luminescence

Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH₃NH₃PbI₃) nanocrystals of size ∼200 × 30 × 30 nm³. The photoluminescence blinking amplitude caused by a single quencher was a hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to nonradiative recombination. These trapping sites can be activated and deactivated by light

Electron Transfer in Quantum-Dot-Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz Study

Photoinduced electron injection dynamics from CdSe quantum dots to ZnO nanowires is studied by transient absorption and time-resolved terahertz spectroscopy measurements. Ultrafast electron transfer from the CdSe quantum dots to ZnO is proven to be efficient already on a picoseconds time scale (τ = 3–12 ps). The measured kinetics was found to have a two-component character, whose origin is discussed in detail. The obtained results suggest that electrons are injected into ZnO via an intermediate charge transfer state

Role of Adsorption Structures of Zn-Porphyrin on TiO₂ in Dye-Sensitized Solar Cells Studied by Sum Frequency Generation Vibrational Spectroscopy and Ultrafast Spectroscopy

Several Zn-porphyrin (ZnP) derivatives were designed to build highly efficient dye-sensitized solar cells (DSC). It was found that solar cell efficiencies normalized for surface coverage (η_rel) are affected by the molecular spacer connecting the porphyrin core to the TiO₂ surface, the sensitization conditions (solvent and time), and, to a lesser extent, the nature of the terminal group of the ZnP. Ultrafast transient absorption spectroscopy shows that electron transfer rates are strongly dependent on spacer and sensitization conditions. To understand this behavior at a molecular level, surface-sensitive vibrational spectroscopy, sum frequency generation (SFG), has been employed to investigate the adsorption geometries of these ZnP derivatives on the TiO₂ surface for the first time. The average tilt angles and adsorption ordering of the ZnP molecules on the TiO₂ surface were measured. A simple linear correlation between adsorption geometry of the adsorbed ZnP molecules, η_rel, and the concentration of long-lived electrons in the conduction band of TiO₂ was shown to exist. The more perpendicular the orientation of the adsorbed ZnP (relative to the TiO₂ surface), the higher the concentration of long-lived electrons in the conduction band, which contributes to the increase of photocurrent and solar cell efficiency. This result indicates that the electron transfer between ZnP and TiO₂ occurs “through-space” rather than “through the molecular spacer”. It is also revealed that the sensitization solvent (methanol) may affect adsorption geometry and adsorption ordering through coadsorption and modify the electron transfer dynamics and consequently solar cell efficiency. Aggregation effects, which were observed for the longer sensitization times, are also discussed in relation to adsorption geometry and radiationless quenching processes. With the work reported here we demonstrate a novel strategy for DSC material characterization that can lead to design and manufacturing of photoactive materials with predictable and controlled properties

Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite

Solar cells based on organometal halide perovskites have seen rapidly increasing efficiencies, now exceeding 15%. Despite this progress, there is still limited knowledge on the fundamental photophysics. Here we use microwave photoconductance and photoluminescence measurements to investigate the temperature dependence of the carrier generation, mobility, and recombination in (CH₃NH₃)­PbI₃. At temperatures maintaining the tetragonal crystal phase of the perovskite, we find an exciton binding energy of about 32 meV, leading to a temperature-dependent yield of highly mobile (6.2 cm²/(V s) at 300 K) charge carriers. At higher laser intensities, second-order recombination with a rate constant of γ = 13 × 10^–10 cm³ s^–1 becomes apparent. Reducing the temperature results in increasing charge carrier mobilities following a T^–1.6 dependence, which we attribute to a reduction in phonon scattering (Σμ = 16 cm²/(V s) at 165 K). Despite the fact that Σμ increases, γ diminishes with a factor six, implying that charge recombination in (CH₃NH₃)­PbI₃ is temperature activated. The results underline the importance of the perovskite crystal structure, the exciton binding energy, and the activation energy for recombination as key factors in optimizing new perovskite materials