Volume 228, Issue 2-3 - Special Issue dedicated to Professor Thomas Wolff on the occasion of his 65th birthday

This contribution discusses photoinitiated crosslinking of multifunctional acrylic esters in polymeric binders based on digital imaging using the Computer to Plate (CtP) technology applying laser exposure at 830 nm in the near infrared (NIR). All coating components (NIR photoinitiator system, monomers, binder, adhesion promoter, contrast dye, and oxygen barrier material) were applied as thin double layer film (1–2 μm dry film thickness) on Al-plates with an anodized surface. Materials exposed exhibit a sensitivity between 30–200 mJ/cm 2 depending on the NIR-photoinitiator composition. This was processed in a weak aqueous alkaline bath to obtain the image. Generation of initiating radicals occurs by electron transfer from the excited state of the NIR-sensitizer to the radical generator; that is either an onium salt or neutral electron deficient compound. Redox potentials were determined for the NIR-sensitizers and the radical generator. These data allow a rough estimate regarding the free energy of electron transfer of the excited state of the sensitizer and radical generator in NIR-photosensitive imaging material. Photoinduced electron transfer plays a major function to generate initiating radicals by a sensitized mechanism but thermal events also influence sensitivity of the coating. Particular the non-radiative deactivation of the NIR-sensitizer possesses a major function to release selectively the heat. Absorption data of many NIR-sensitizers used exhibit molar extinction coefficient of more than 200000 M – 1  cm – 1 .

A dual probe was investigated by UV-Vis, fluorescence, and ESR spectroscopy. It comprises the pyrene chromophore and the paramagnetic 2,2,6,6-tetramethylpiperidinyl- N -oxyl radical that are covalently linked together via an ester bridge. The dual probe was used to investigate molecular solvents of different polarity as well as ionic liquids bearing either imidazolium or pyrrolidinium cations and various anions, such as bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, tris(pentafluoroethyl)trifluorophosphate, or dicyanamide. The dual probe does not show solvatochromism that is typical for some pyrenes. Furthermore, the dual probe is considerable less mobile compared to 2,2,6,6-tetramethylpiperidinyl- N -oxyl (TEMPO) without additional substituent as detected by ESR spectroscopy. This is caused by the bulky pyrenyl substituent bound at the dual probe resulting in a reduced mobility of the dual probe.

In this work we summarize last achievements of the colloidal synthesis and applications of different amphiphilic nanoparticles, among which are semiconductor, metal and metal oxide materials. All these nanoparticles have been obtained employing polyethylene glycol based stabilizers which provide compatibility with a wide variety of media from nonpolar organics to water. Owing to their versatile solubility, the materials obtained have the ability of spontaneous multiphase transfer. Moreover, they have been shown to permeate through cell membranes avoiding endocytosis which makes them particularly interesting for bio-applications.

We report on fluorescence quenching experiments of CdSe quantum dots, which were encapsulated for biocompatibility using different strategies. Since their fluorescence is very sensitive against water and other quenching molecules reaching their surface, quenching experiments allow conclusions on particle shielding towards the surrounding medium. It turns out that best shielded particles with comparable chemical composition of their ligand shell have also outstanding properties in respect of undesired unspecific cellular uptake.

Using single-pulse and two-pulse two-colour laser flash photolysis (308 nm and 355 nm), we have quantitatively studied the effect of sodium dodecyl sulfate (SDS), at concentrations above and below the critical micellar concentration (cmc), on the cyclic photoionization of the ruthenium(tris)bipyridine dication in aqueous medium. In contrast to reports in the literature (Chem. Phys. Lett. 315 (1999) 416) about a superincrease of the electron yield by SDS concentrations in the range between 3.0 · 10 – 4  M and 4.5 · 10 – 4  M we found that the observed absorption increases are entirely due to background signals in the vicinity of an insolubility range, as can be proved by difference experiments (argon-saturated vs.  N 2 O-saturated solutions); SDS has no appreciable effect on the reaction below the cmc, and causes a moderately increased electron yield above the cmc. By varying the excitation intensities and analyzing the intensity dependences, we have shown that that increase is not due to a facilitation of the photoionization proper by the micelles but to a more efficient photoreduction of the oxidized complex by SDS in that cyclic mechanism.

Hydrogels are cross-linked, water-soluble polymers which are swollen in water or in aqueous solution. Some of these hydrogels (smart hydrogels) show a drastic change of their properties, particularly the degree of swelling and the modulus, in dependence on the properties (temperature, pH, ionic strength, and composition) of the swelling agent. The application of smart gels requires a defined and fast swell kinetic. Therefore, there is an interest in the synthesis of thin patterned films with good mechanical performance. Photolithography is a powerful method to synthesize films and pattern in the μm-range. Nanocomposite (NC) hydrogels exhibit excellent mechanical properties, even in the swollen state. The photochemical reaction may be stopped easily by switching off the light source. This allows characterizing the structure of molecules formed during the reaction. On example of synthesis of nanocomposite hydrogels the monitoring of photochemical induced network formation by dynamic light scattering is demonstrated. Application relevant properties of NC-gels and the phase diagram are reported. Gels were patterned by UV irradiation through a mask. The patterning process is visualized by FT-IR imaging.

Two dibenzobarrelene derivatives were synthesized that are covalently linked to a ( S )-BINOL unit through an oxymethylene ( 1b ) or ethylenedioxymethylene ( 1c ) unit, and their photoreactivity was examined. Irradiation of 1b in acetone resulted in a di- π -methane rearrangement to give diastereoselectively one dibenzosemibullvalene isomer. Experimental and theoretical data point to a directing effect of the BINOL auxiliary based on steric repulsion. The derivative 1c with a longer linker between the BINOL unit and the dibenzobarrelene is photoinert, presumably because of fast competing relaxation processes in the excited state.

In this contribution, we add theoretical investigations to an experimental study on the photocleavage of coumarin, 6-methylcoumarin, 6-dodecylcoumarin monomers and several syn or anti , head-to-head or head-to-tail isomers. Using a density-functional tight-binding method together with a linear-response extension, we were able to calculate the photoexcitation spectra of all molecules in their optimized geometries. Additional molecular-dynamics simulations of the molecules in the gas phase, gave access to the structural changes due to thermal vibrations and to the influence on the excitation spectra. The sampled photoexcitation spectra are comparable to the experimentally measured data. Moreover, we could show that the dimerization reaction can be monitored by recording the changes of the spectra.

Thermoresponsive microgels based on poly( N -vinylcaprolactam), poly( N -isopropylacrylamide) and poly( N -isopropylmethacrylamide) with core-shell morphology were synthesized by two-step seed precipitation polymerisation. The microgel shell thickness was controlled by increasing the monomer amount added to the core seed during the second polymerisation step. 1 H nuclear magnetic resonance and Raman spectroscopy were used to determine the real molar ratio between the components of core and shell. The volume phase temperature transition of microgels was investigated with dynamic light scattering and differential scanning calorimetry. The influence that the intrinsic critical solution temperature of the core and shell polymers has on the volume phase transition temperature of the microgels is discussed.

Recently a Markov chain model has been proposed to predict the composition of alloys as a function of the electrolyte composition. The essential parameters of this model are selectivity constants, describing the selectivity of the growth centers, the kink site positions, for the ions of the metal components in the electrolyte. In this paper application of the simple model to deposition of intermetallic compounds will be discussed. The Markov chain model can be successfully applied to the formation of the intermetallic compound NiSn. Whereas the model can describe well the deposition of Ni rich Zn-Ni alloys at low current densities, it fails at high current densities with anomalous alloy composition.

Polymer brushes are representing a versatile tool to adjust and control surface phenomena like wetting, adsorption and adhesion. Moreover, certain complementary surface modification methods can support improving their intended applications. A promising strategy is the combination of the grafting-to method under initiator and solvent free conditions with an additional electron-beam irradiation step which can lead to polymer brushes with high-selective properties. The experiments were focused on the investigation of the surface reactions taking place during the dose-dependent irradiation of the thin polymer brush layers with the aim to modify their properties. A comprehensive and detailed study of the initiated functionalization reactions associated with their influence on the alteration of surface properties of different homo and binary polymer brush layers was performed using sensitive surface characterization techniques such as XPS, contact angle measurements, and ellipsometry.

Photoinduced electron transfer reactions from the triplet erythrosin (Ery 2− ) dianion to various quinones (1,4-benzoquinone, 2,5-di-t-butyl-1,4-benzoquinone, duroquinone, 2,5-dichloro-1,4-benzoquinone, chloranil, bromanil, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and 9,10-anthraquinone) and other organic acceptors like 7,7,8,8-tetracyanoquinodimethane, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and with tetracyano-1,4-benzoquinone (cyanil), one of the strongest oxidizing agent reported in literature ( E ○  = 0.90 V vs. SCE), were studied by laser flash photolysis in acetonitrile/water mixtures at room temperature. Quenching rate constants are obtained from Stern-Volmer plots. The measured bimolecular quenching rate constants are close to the diffusion controlled rates. Excitation of a contact radical-pair or a triplet exciplex as an intermediate in the photoinduced electron transfer reaction in a slightly polar solvent (1-propanol) is confirmed by the observation of the net absorptive chemically induced dynamic electron polarization (CIDEP) spectra. The unusual net–absorptive CIDEP spectrum is explained by spin-orbit coupling interactions due to presence of four iodine atoms in the structure of erythrosin (heavy atom effect). The dependence of the electron transfer rate constants, k et , on the driving force, ΔG et is slightly parabolic and indicates the Marcus Inverted Region.