Dr. Bernd Strehmel Profile

Dr. Bernd Strehmel

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SPIE Journal Paper | 2 January 1996 Open Access

Photophysical properties of fluorescence probes I: dialkylamino stilbazolium dyes

Bernd Strehmel, Wolfgang Rettig

JBO, Vol. 1, Issue 01, (January 1996) https://doi.org/10.1117/12.10.1117/12.227538

KEYWORDS: Luminescence, Bioalcohols, Quantum efficiency, Temperature metrology, Molecules, Absorption, Polymers, Spectroscopy, Rhodamine, Picosecond phenomena

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The photophysical and photochemical behavior of different isomers of dimethylamino-stilbazolium dyes was investigated in n-alcohols at different temperatures by stationary and time-resolved fluorescence measurements. At room temperature, an efficient nonradiative deactivation knr was observed, which increases in high-polarity and low-viscosity solvents. For viscosities < 10 cP, knr possesses a linear viscosity dependence. At low temperatures, the fluorescence spectra show considerable blue shifts, which are absent in solvent polarity changes relative to room temperature, and a spectral narrowing. From the combined quantum yield and lifetime measurements, knr is shown to decrease strongly with decreasing temperature, by more than two orders of magnitude, and kf is shown to increase. These facts and the observation of an initial lifetime lengthening by increasing the temperature from 77 K indicate several simultaneously emitting states with a temperature-dependent population efficiency. These states were tentatively assigned to several possible single-bond twisted excited conformers. This assignment is supported by the near absence of isomerization.

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Proceedings Article | 16 May 1994 Paper

Characterization of photochemical cured acrylates with calorimetric methods

Bernd Strehmel, Dirk Anwand, Henrik Wetzel

Proceedings Volume 2195, (1994) https://doi.org/10.1117/12.175393

KEYWORDS: Polymers, Polymerization, Luminescence, Oxygen, Polymethylmethacrylate, Glasses, Photopolymers, Argon, Crystals, Sodium

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Radical polymerization kinetics of different kinds of diacrylates was investigated in linear polymers (binders) by using an isoperibolic calorimeter. For all experiments benzoin compounds were added as photoinitiator. The ester between acrylic acid and bisphenol-A-diglycidylether (DDGDA) and hexamethylenediacrylate were used as monomers. Both compounds have a high limiting conversion and a large polymerization rate in the binders investigated. Additionally, three kinds of termination reaction were observed: first order, second order, and primary radical termination. The last reaction was mainly found in the case of using the hexamethylenediacrylate monomer. The materials were investigated by DSC to determine the phase behavior. Both monomers form one phase with the binder (polymethylmethacrylate, PMMA). In contrast, a phase separation was observed between the crosslinked hexamethylenediacrylate and PMMA. Formations of semi- interpenetrating networks were found in the case of crosslinked DDGDA and PMMA. The glass transition temperatures were determined at different polymerization degrees also. The obtained results indicate that most of the network formation occurred in the glassy state. Fluorescence probe technique was applied to study changes in the mobility during network formation. The fluorescence probe crystal violet (CV) was used because this compound shows a strong free volume-dependent fluorescence. It was found that in the glassy state, where most of networks were formed, a large variation of the molecular mobility was observed during irradiation of the photopolymers. This result was in agreement with the observations during DSC experiments.

Proceedings Article | 16 May 1994 Paper

Photochemical curing of epoxies in the liquid-crystalline state

Veronika Strehmel, Bernd Strehmel, Jorg Trempler

Proceedings Volume 2195, (1994) https://doi.org/10.1117/12.175391

KEYWORDS: Crystals, Liquid crystals, Liquids, Epoxies, Glasses, Polymers, Polarization, Microscopy, Polarizers, Information operations

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In this work the cationic photoinduced crosslinking of the diglycidylether of 4-hydroxyphenyl-4'-hydroxybenzoate was investigated in the liquid crystalline state as well as in the isotropic state. The photochemical curing technique was applied because only small amounts of the initiating species were necessary to form networks of epoxy compounds. As the photoinitiator system, a mixture of (alpha) , (alpha) '- dimethoxydeoxybenzoin and dicumyliodonium-hexafluorophosphate proved to be the most effective. FTIR-spectra of the networks were compared with those of the starting material. Only small amounts of nonreacted epoxy groups were found in the networks synthesized either in the isotropic state or in the liquid crystalline state. Furthermore, different results were obtained with DSC-measurements. Networks prepared in the liquid crystalline state showed two glass transition temperatures. In contrast, networks synthesized in the isotropic state have only one glass transition temperature. Moreover, polarization microscopy was used to find ordered structures. The networks formed in the liquid crystalline state showed frozen ordered structures. Heating of such networks above the melting point of the starting material did not lead to a change of the frozen ordered structures. Additionally, differences between the frozen ordered structure of the networks and the crystalline structure of the starting compound were observed by this method. Finally, the material properties of networks with ordered structures are different from common networks with disordered structures.

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