Research

Kinetics and reaction dynamics of transient radicals in the gas-to-liquid region, supercritical fluids, and in ionic liquids

In the framework of the prestigious Sofja Kovalevskaja Program from Humboldt Foundation (2001-2006) and DFG-funded projects for the support of young international research investigators in Germany, we carried out various studies of transient large organic radicals and their recombination kinetics in various environments, such as gas-phase, supercritical fluids, and their mixtures with ionic liquids. We developed new experimental systems for high pressures and high temperatures up to 1000 bar and 250-900 K. Time-resolved laser spectroscopic techniques based on ns-UV-laser systems as well as ultrafast transient lens experiments have been employed.

• Alexander-von-Humboldt-Foundation Sofja-Kovalevskaja-Award: Pressure and Temperature Dependent Kinetics of Large Organic Radicals in Supercritical Fluids
• DFG OU58/2-1 and OU58/2-2: Kinetics and Dynamics of Molecular Aggregates in Supercritical Fluids
• DFG OU58/3-1 and 3-2, Priority Program SPP1191 „Ionic Liquids“: Time-resolved laser spectroscopy of reactive and nonreactive processes in ionic liquids and their mixtures with carbon dioxide
• Fonds der Chemischen Industrie Stipend (2007-2009) for Florian Ehlers: Photoinduzierte Prozesse großer organischer Moleküle in Lösungsmitteln, auf Oberflächen und in inversen Mizellen

• Selected relevant publications:
  

Lohse P. W, Bürsing R., Lenzer T. and Oum K., "Exploring 12'-apo-β-carotenoic-12'-acid as an ultrafast polarity probe for ionic liquids", J. Phys. Chem. B, 112, 3048 (2008).

Kopczynski M., Lenzer T., Oum K., Seehusen J., Seidel M. T. and Ushakov V. G., "Ultrafast transient lens spectroscopy of various C₄₀ carotenoids: lycopene, β-carotene, (3R, 3'R)-zeaxanthin, (3R,3'R,6'R)-lutein, echinenone, canthaxanthin and astaxanthin", Phys. Chem. Chem. Phys., 7, 2793 (2005).

Luther K., Oum K. and Troe J., "The role of the radical-complex mechanism in the ozone recombination/dissociation reaction", Phys. Chem. Chem. Phys., 7, 2764 (2005).  

Luther, K., Oum, K., Sekiguchi, K. and Troe, J., "Recombination of benzyl radicals: dependence on the bath gas, temperature, and pressure", Phys. Chem. Chem. Phys., 6, 4133 (2004)

Oum, K., Luther, K. and Troe, J., "High-pressure studies of radical-solvent molecule interactions in the CCl₃ and bromin combinaiton reactions of CCl₃", J. Phys., Chem., A, 108, 2690 (2004).

Ultrafast photoinduced processes of large organic molecules (Carotenoids, optical switches, fluorescent dyes, solar cell dyes, etc.)

Carotenoids provide a variety of colours in nature and fulfill important functions such as light-harvesting in plants and light-driven signalling in living organisms. Various synthesized dye molecules in the chemical and biological systems show interesting and complex excited state dynamics upon photoexcitation by short laser pulses. We investigate the kinetics and dynamics of primary photophysical and photochemical processes of such dyes by using time-resolved laser spectroscopic techniques (transient absorption, transient Raman, transient FTIR) on time-scales from femtoseconds to milliseconds. Especially, our interest lies in influence of the solvent and interfacial environments in photoinduced processes of such dye systems. Development and carrying out global kinetic modelling of transient absorption spectra belong to our major activities.  

• Photoinduced dynamics of chiral molecular systems (DFG-Project (2017-currently), PIs: K. Oum and T. Lenzer)

• Ultrafast photoinduced charge separation and migration in thin films for photovoltaics (DFG project (2014-currently): PIs K. Oum and T. Lenzer)

• Interactions of ionic liquids with embedded probe molecules (project in the framework of the SPP 1191 of the DFG (2008-2012), PIs K. Oum and T. Lenzer)

• Investigation of photochemical processes in industrial applications (BASF - University of Siegen project (2016), PIs K. Oum and T. Lenzer) 

• Selected relevant publications:
  

Scholz, M., Flender, O., Oum, K. and Lenzer, T., "Ultrafast excited-state dynamics of all-trans capsanthin in organic solvents", J. Chem. Phys. A., under review (2017).

Wirtz, M., Jung, G., Klein, J. R., Scholz, M., Oum, K. and Lenzer, T., "Excited-state dynamics of α- and β-styryl-BODIPY chromophores", Phys. Chem. Chem. Phys., to be submitted (2017).

Klein, J. R., Scholz, M., Oum, K. and Lenzer, T., "Photoinduced dynamics of the hole-transport material in organic solvents and on mesoporous Al₂O₃ and TiO₂ thin films", Phys. Chem. Chem. Phys., 19, 21748  (2017).

Tietze, L.F., Waldecker, B., Ganapathy, D., Eichhorst, C., Lenzer, T., Oum, K., Reichmann, S.O. and Stalke, D., "Four- and Sixfold Tandem-Domino Reactions Leading to Dimeric Tetrasubstituted Alkenes Suitable as Molecular Switches", Angewandte Chemie International Edition, 54, 10317  (2015). 

Chen Z., Lee C., Lenzer T. and Oum K., "Solvent effects on the S₀-S₂ transition of β-carotene, echinenone, canthaxanthin and astasanthin in supercritical CO₂ and CF₃H", J. Phys. Chem. A, 110, 11291 (2006)

Nano-/ micrometer particle generation and optical characterization

  RESS (Rapid Expansion of Supercritical Solutions)

- On-line and in situ optical characterization of particles by laser-based optical spectroscopy: 3-WEM (3-wavelength extinction measurement) and LABS (laser-based shadowgraphy)

- Dynamic Light Scattering (DLS), Step-scan FTIR imaging

• Selected relevant publications:
  

Harrison J. J., Lee C,  Lenzer T. and Oum K., "On-line in-situ characterization of CO₂ RESS processes for benzoic acid, cholesterol and aspirin", Green Chemistry, 9, 351 (2007)

Oum, K., Harrison, J. J., Lee, C., Wild, D. A., Luther, K. and Lenzer, T., "On-line and in situ optical detection of particles of organic molecules formed by rapid expansion of supercritical solutions (RESS) of CO₂", Phys. Chem. Chem. Phys., 5, 5467 (2003).