Cornelia Damm

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.

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.

Photo-physics of surface-treated titanium dioxides

Abstract The photoelectric properties of commercially used TiO2 pigments were studied by transient photo-electromotive-force (PEMF) measurements. The influence of the crystal structure (anatase or rutile) and the pigment coating (and other) on the PEMF parameters of TiO2 was investigated. Pure anatase type pigments show in the microsecond time range a creation of a positive PEMF signal. In the millisecond time range the PEMF decay starts with a positive sign. This means that anatase pigments behave as typical n-type photoconductors. However, pure rutile type pigments show in the microsecond time range a negative PEMF signal or a signal starting with negative sign. This means that rutile pigments act as typical p-type photoconductors during the first few microseconds. Coating of rutile or anatase pigments decrease the PEMF maximum voltages Umax in comparison to untreated pigments. However, in some cases coating creates an increase of the PEMF decay rate which is explained by an increase of the charge carrier recombination. Rutile pigments epitaxially coated with Al2O3 and SiO2 may show very complex PEMF signals with two negative maximum voltages. Such types of signals may indicate heterojunctions between the TiO2 core and the coating shell. In this way the PEMF parameters are an excellent tool to characterise TiO2 pigments.

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.

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.