Functionality of epidermal melanin pigments: current knowledge on UV-dissipative mechanisms and research perspectives

So far it is not known whether epidermal melanins are solely photoprotective or also phototoxic. Also largely unknown are underlying UV-induced mechanisms and impact of the chemical structure. This perspective will focus on the current insights into the UV-dissipative mechanisms in melanin model systems and the implications for the in vivo pigments. We will also identify future research perspectives towards a full understanding of the functionality of epidermal pigments.     

Fluorescence excitation and excited state intramolecular proton transfer of jet-cooled naphthol derivatives: Part 1. 1-Hydroxy-2-naphthaldehyde

The S(0) → S(1) fluorescence excitation spectrum of jet-cooled 1H2N with origin at 25484 cm(-1) has been measured. Twelve totally symmetric modes and five non-totally symmetric modes have been assigned in the excitation spectrum. Theoretical calculations at DFT B3LYP/6-31G** and CIS/6-31G** levels indicate that the 1H2N molecular geometry is more planar in the S(1) state than in the ground state. The geometry of the naphthalene ring changes upon excitation and promotes a number of totally symmetric ring stretching modes, in the excitation spectrum. As a result of the geometry change upon excitation a number of non-totally symmetric modes gain intensity. Based on a rotational envelope fitting procedure the average excited rotational state lifetime was estimated to be between 7 and 16 ps for 0 ≤E≤ hc × 800 cm(-1) (E is excess energy above the S(1) origin). The decay rate coefficients, k, of the rotational S(1) states, are not constant over this range of excess energies. By applying a Golden Rule model, it was determined that internal conversion to S(0) is unlikely to be the sole non-radiative process contributing to the decay of the excited states. It was concluded that excited state intramolecular proton transfer (ESIPT) plays a role in the observed behaviour of the rate co-efficient with excess energy. The observation of (i) a sharp increase in rate coefficient, k, above an excess energy of ～550 cm(-1), and (ii) a significant number of high intensity fluorescence excitation spectrum features above an excess energy of ～700 cm(-1), may indicate the presence of an energy barrier of ～550 cm(-1), between the enol and keto geometries in the S(1) state. This result supports the conclusions of S. De, S. Ash, S. Dalai and A. Misra, J. Mol. Struc. Theochem, 2007, 807, 33-41, who estimated a barrier to ESIPT of ～750 cm(-1). It was concluded that ESIPT occurs in 1H2N, across an energy barrier with a rate constant, k(pt)≤ 10(11) s(-1). Hence, at low excess energies (≤ 550 cm(-1)), the observed emission band originates predominantly from the keto tautomer. Above an excess energy of ～1600 cm(-1), 1H2N decays predominantly via a non-radiative mechanism.     

Echolocation call intensity and directionality in flying short-tailed fruit bats, Carollia perspicillata (Phyllostomidae)

The directionality of bat echolocation calls defines the width of bats' sonar "view," while call intensity directly influences detection range since adequate sound energy must impinge upon objects to return audible echoes. Both are thus crucial parameters for understanding biosonar signal design. Phyllostomid bats have been classified as low intensity or "whispering bats," but recent data indicate that this designation may be inaccurate. Echolocation beam directionality in phyllostomids has only been measured through electrode brain-stimulation of restrained bats, presumably excluding active beam control via the noseleaf. Here, a 12-microphone array was used to measure echolocation call intensity and beam directionality in the frugivorous phyllostomid, Carollia perspicillata, echolocating in flight. The results showed a considerably narrower beam shape (half-amplitude beam angles of approximately 16° horizontally and 14° vertically) and louder echolocation calls [source levels averaging 99 dB sound pressure level (SPL) root mean square] for C. perspicillata than was found for this species when stationary. This suggests that naturally behaving phyllostomids shape their sound beam to achieve a longer and narrower sonar range than previously thought. C. perspicillata orient and forage in the forest interior and the narrow beam might be adaptive in clutter, by reducing the number and intensity of off-axis echoes.     

Substitution kinetics of biphenol at dichlorobis(acetylacetonato-O,O′)titanium(IV): Isolation,characterization, crystal structure and enhanced hydrolytic stability of the product bis(acetylacetonato-O,O′)(biphenyldiolato-O,O′)titanium(IV)

Novel bis(acetylacetonato-O,O′)(biphenyldiolato-O,O′)titanium(IV) is synthesized and characterized by X-ray crystallography and other physical methods. The kinetics of substitution of bidentate 2,2′-biphenyldiolato for the two monodentate Cl⁻ ligands in Ti(CH₃COCHCOCH₃)₂Cl₂ proceeds via a 7-coordinated transition state according to an associative mechanism. The Ti(CH₃COCHCOCH₃)₂biphen complex exhibits high hydrolytic stability.     

Syntheses,crystal structure and theoretical modelling of tetrahedral mono-β-diketonato titanocenyl complexes

A comparative investigation on three novel bis(cyclopentadienyl) mono(β-diketonato) titanium(IV) complexes, [Cp₂Ti^IV(R¹COCHCOR²)]⁺ClO₄⁻ (Cp = η⁵-C₅H₅), i.e. [Cp₂Ti(tfba)]⁺, [Cp₂Ti(tfth)]⁺ and [Cp₂Ti(tfba)]⁺ where tfba = CF₃COCHCOC₆H₅⁻, tfth = CF₃COCHCOC₄H₃S⁻ and tffu = CF₃COCHCOC₄H₃O⁻, has been performed based on structural data and DFT calculations. The preparation of [Cp₂Ti^IV(β-diketonato)]⁺ClO₄⁻ involves the reaction of Cp₂TiCl₂ with AgClO₄ and the respective β-diketones. The crystal structures show that the structures are isomorphous. All the complexes exhibit π-stacking between one Cp ring and the aromatic R-group ring, i.e. the C₆H₅, C₄H₃S and C₄H₃O fragments, respectively. The DFT calculations show that the formal 16-electron count of these d⁰ titanium(IV) complexes is increased via Ti ← O π bonding. The bonding mode in the [Cp₂Ti(β-diketonato)]⁺ complexes is different from that in Cp₂Ti(OR)₂ and Cp₂Ti(dioxolene) complexes.     

Picosecond calorimetry: time-resolved x-ray diffraction studies of liquid CH2Cl2

Liquid phase time-resolved x-ray diffraction with 100 ps resolution has recently emerged as a powerful technique for probing the structural dynamics of transient photochemical species in solution. It is intrinsic to the method, however, that a structural signal is observed not only from the photochemical of interest but also from the embedding solvent matrix. To experimentally characterize the x-ray diffraction signal deriving from the solvent alone we performed time-resolved diffraction studies of a pure liquid sample over a time domain from -250 ps to 2.5 micros. Multiphoton excitation was used to rapidly heat liquid CH(2)Cl(2) using UV pulses of 100 fs duration. A significant x-ray diffraction signal is visible prior to the onset of thermal expansion, which characterizes a highly compressed superheated liquid. Liquid CH(2)Cl(2) then expands as a shock wave propagates through the sample and the temporal dependence of this phenomenon is in good agreement with theory. An unexpectedly slow initial release of energy into the liquid as heat is observed from multiphoton excited CH(2)Cl(2), revealing the presence of a metastable state of multiphoton excited CH(2)Cl(2).     

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