Gunter Israel

Luminescence and photoconductivity in mononuclear ortho-platinated metallomesogens

The electronic properties of series of mononuclear ortho-platinated and -palladated metallomesogens were investigated by means of UV-vis absorption and fluorescence spectroscopy in solution, fluorescence spectroscopy in the solid state as well as time-resolved photo-emf measurements. The organoplatinum compounds show luminescence in solution and in the solid state as well as photoconductivity caused by Pt⋯Pt closed shell interactions in comparison to the non-luminescent and non-photoconductive organopalladium compounds. A significant influence of the substitution pattern regarding luminescence and photoconductivity of the materials (number, length and distribution of attached alkyl chains) was found for the organoplatinum phenylpyrimidine and phenylpyridine series 1 and 3.

Synthesis and photoswitching properties of bent-shaped liquid crystals containing azobenzene monomers

Three novel bent-shaped monomers, namely 1,3-phenylene bis-{4-[4-(n-allyloxyalkyloxy)phenylazo]benzoate} 5a–c, containing azobenzene as side arms, resorcinol as central units and terminal double bonds as polymerisable functional groups were synthesised and characterised. The mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements and it was found that all three compounds display SmAintercal mesophases. These bent-shaped molecules exhibit strong photoisomerisation behaviour in solutions in which trans to cis isomerisation takes about 50 seconds whereas the reverse process takes almost 31 hours.

1,2-Dithiines and precursors, XVI: Synthesis, structure, and reactivity of non-anellated 1,2-dithiines

Various monocyclic 1,2-dithiines 6a,b,d-t were prepared via (Z,Z)-1,4-difunctionalized butadienes (4–11,19,20). A twisted cyclic structure A is unequivocally proved rather than of the ringopened valence isomer B. The reactivity of these 1,2-dithiines is described. Thermal as well as day-light induced sulfur extrusion is an important feature of their chemistry. The latter mode of sulfur extrusion depends to a significant extent on absorption in the visible region.

Synthesis and mesophase behaviour of ionic liquid crystals

N-Alkyl-pyridinium derivatives 1–3 and N-alkyl-stilbazolium halides 4–6 with hydroxy, methoxy and hydrogen 4′-substituents and long non-branched alkyl chains (n = 14, 16, 18, 20, 22) were synthesized and characterized by polarizing microscopy, differential scanning calorimetry and X-ray measurements. The compounds exhibit liquid crystalline SmA phases when heated above their melting point. Phenyl substitution in the 3- and 4-position of the pyridinium ring causes a large tendency to decrease the clearing temperature in comparison to the 4-methyl substituted N-alkyl-pyridinium salts. By elongation of the pyridinium ring to the 4′-substituted stilbazolium unit the clearing points of the new compounds 4–6 increase drastically up to temperatures ≥200 °C in which partial decomposition of the compounds sets in. The length of the alkyl chains and the type of counter ions have large influences on the stability of the mesophase. Elongation of the alkyl chain length n increases the temperature range of the liquid crystalline phase. The counter ions increase the stability of the SmA phase in order Cl− > Br− > I− > BPh4−. When CH3–C6H5SO3− is introduced as an anion no liquid crystalline phase can be observed. UV/Vis measurements indicate the presence of a charge-transfer complex between the pyridinium cation and the iodide anion. Differences in the liquid crystalline behaviour of N-alkyl-4′-substituted stilbazolium halides 4–6 compared with N-alkyl-3- and -4-substituted-pyridinium derivatives 1–3 may be explained by additional intramolecular charge-transfer and resulting strong dipole–dipole interactions between stilbazolium compounds.

Structural influences on the photoelectric properties of TiO2

Abstract The Influence of crystal structure and surface coating on the photoelectric properties of TiO2 pigments was studied using the transient photo-EMF. Amorphous TiO2 did not show any photo-EMF signal showing that it is not a photoconductor. The crystalline TiO2 modifications, anatase and rutile, behave like n-type photoconductors. In comparison to rutile, the maximum photo-EMF of anatase was significantly larger. While the photo-EMF signal of the rutile changed under repetitive laser flash exposure, the signal of anatase did not depend on number of flashes. This shows that only rutile contains a small amount of very deep traps. So in rutile a small part of charge carriers live longer than in anatase. Coating anatase with the photoelectrically inert Ba(OH)2∗nH2O or BaCO3 reduced the photo-EMF signal. The epitaxial coating of rutile with SiO2 and Al2O3 resulted in a complex photo-EMF signal that displayed two negative maximum voltages. Such behaviour suggests that heterojunctions may exist in the substrate.

Preparation and characterization of core-shell structured TiO2–BaCO3 particles

Abstract Preparation of core-shell structured TiO 2 –BaCO 3 particles as precursor of BaTiO 3 genesis, proceeds using a two step procedure, by first coating the TiO 2 core by Ba(OH) 2 shell followed by conversion of the shell region with CO 2 gas by the formation of BaCO 3 . Straightforward experimental results reveal environmental scanning electron microscopy (ESEM) and scanning transmission electron microscopy (STEM) as suitable methods for analytical characterization of the core and shell regions from individual TiO 2 –BaCO 3 grains. Evidence of coating the whole ensemble of TiO 2 particles is possible using Photo Electro Motive Force (Photo EMF, PEMF) measurements. This method is able to indicate very sensitively changes of surface properties of TiO 2 after coating with Ba(OH) 2 and BaCO 3 , respectively. PEMF measurements were used for the first time with concern to this topic.

Photo‐degradation of melatonin: influence of argon, hydrogen peroxide, and ethanol

Abstract:  When organic compounds are irradiated with UV light at 254 nm, part of their covalent bonds can dissociate if the compound absorbs light at that wavelength. Therefore, photo‐degradation depends strongly on the wavelength used. The energy of a light quanta at 254 nm amounts to approximately 110 kcal/mol quanta, which is in many cases higher than the binding energy of a variety of covalent bonds. As a consequence, the absorbing molecule is degraded. As melatonin absorbs light at 254 nm, this compound is vulnerable to UV light. In order to minimize undesired effects of other absorbing substances, we used as solvent mostly pure water and analyzed the influence of λirr = 254 nm on the disappearance of the educt (melatonin) as well as on the appearance of products derived from melatonin in the presence of oxygen, argon, hydrogen peroxide, and ethanol by UV–VIS spectroscopy and high‐performance liquid chromatography (HPLC) technique. N1‐acetyl‐N2‐formyl‐5‐methoxykynuramine (AFMK) appears to be the main, but obviously not the only product of photo‐degradation of melatonin, independently of whether the system contains oxygen or not. If the system contains additionally hydrogen peroxide, a very strong oxidant, the hydroxyl radical (•OH), is formed. Under such conditions, melatonin is not solely photo‐degraded but also attacked by the formed •OH which interact similarly with the formed main photo‐product AFMK. Ethanol, as a potent scavenger of •OH, efficiently blocks the effect of this aggressive radical even at low concentrations of that scavenger (0.1% v/v) but is less effective in preventing photo‐degradation of melatonin.

Crystal structures and spectroscopic properties of platina-β-diketones — a new mode of stacking in platinum complexes

Abstract Platina-β-diketones [Pt2{(COR)2H}2(μ-Cl)2] (R=Me 1a, Et 1b, n-Pent 1c, n-Hex 1d, (CH2)4Ph 1e) are prepared by reaction of hexachloroplatinic acid in n-butanol with silyl-substituted acetylenes R′CCSiMe3 (R′=RCH2). Complexes 1 are characterized by NMR, IR and UV–vis spectroscopies. The crystal structures of complexes 1a, b, e reveal discrete dinuclear molecules linked via Pt⋯Pt interactions [d(Pt⋯Pt)=3.318(1)–3.352(1) A] in such a way that polymeric chain-like structures are formed. These complexes exhibit only an emission in the solid state (λmax 555 nm) that is ascribed to a d8–d8 electronic excited state. No emission is observed in solution. The distinct differences in the absorption and emission spectra in solution and in the solid state, as well as the lack of photoconductivity, are ascribed to the columnar structures in the solid state with the mode of stacking of the platinum centers with alternating Pt⋯Pt interactions and PtClPt bridges.

Pigment polymer layers as sensitizers for the photopolymerization of trimethylolpropane triacrylate

Abstract Layers of organic dye pigments dispersed in poly(vinyl butyral) can be used as heterogeneous sensitizers for the photopolymerization of trimethylolpropane triacrylate. Reactions of the bulk monomer with different 3, 4, 9, 10-perylenetetracarboxylic diimides and copper phthalocyanine were studied by means of real-time IR spectroscopy. Compared to all polymerizations sensitized with perylene diimide layers, the relative rate of polymerization r p of reactions sensitized with the phthalocyanine layer is higher. In all cases, an increase of the pigment concentration leads to higher r p values. No simple relationship exists between incident light intensity and r p values. The initiating activity of the pigments correlates with the maximum voltage U max of their photo-emf.

Photoinduced Pyridine Cleavage-Closure in Viscous Polymer Solutions

We have found that photoinduced pyridine ring cleavage-closure occurs in polymeric viscous solution, because in certain polymer solutions, pyridine can serve as a photo-modulated crosslinker. We suggest this reaction as a way to control a polymers optical properties. Irradiation of the system: poly(4-vinyl pyridine)/pyridine/water with 250-nm wavelength range leads to the appearance of a new absorption band centered at 360 nm, new red-shifted emission, and HOMO-LUMO band gap changes. The subsequent irradiation with 360 nm (the new absorption band maximum) leads to reversion almost to the initial stage. A main active product of the photoreaction is aldehyde enamine, which has two active groups: primary amine and aldehyde, which can associate with the polymer molecules to form a physical crosslinked supramolecular structure. We evaluated the activation energy of the pyridine ring cleavage and back reaction depending on the polymer/pyridine/water ratio and by changing the polymer structure. The activation energy of pyridines ring cleavage in viscous polymeric solutions is in the range of 0.6–3.2 Kcal/mol. The activation energy of the back reaction is significantly lower and is in the range of 0.05–0.15Kcal/mole.