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UV/Vis Spectroscopy

Many prominent colors known from everyday life like the "Ferrari red" or "indigo blue", the color of jeans, can be related to electrical transitions of particular molecules. Furthermore, these distinct color characteristics of organic substances can be utilized for scientific investigations. UV/Vis spectroscopy is a very fast measurement method, to determine the absorption in the ultraviolet and the visible spectral range of materials in the gaseous, liquid and solid phase. This allows conclusions on the crystalline structure and orientation as well as the chemical configuration of the investigated substance.

If molecules are illuminated with electromagnetic radiation, they can absorb a photon by exciting an electron to a higher energy level. This process is the more effective the more the photon energy corresponds to the difference of the involved energy states. Therefore, aromatic hydrocarbons are particularly suitable for UV/Vis investigations (e.g. pentacene) as their energetic gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) typically amounts to several eV.

If such a molecule chemically bonds to a foreign atom or forms a crystalline solid, the charge density distribution changes thus causing an energy shift of HOMO and LUMO, which can be observed in the absorption spectrum [3,4].

Furthermore, in heterostructures of different materials, excitations of an electron from one into the other molecule can be observed. These are the so-called charge-transfer excitons [1].

A particular example illustrating the benefit of UV/Vis measurements is the characterization of different molecular orientations within an organic thin film. In the following example, a lying configuration of perfluoropentacene on potassium chloride (right) leads to a significantly different color appearance than an upright-standing configuration on sodium fluoride (left) [2].

In this context, polarization-analyzed measurements are also applied [4]:



Some exemplary publications of our group where UV/vis spectroscopy has been utilized are:

  • [1] Thermally activated intermixture in pentacene-perfluoropentacene heterostructures.
    Tobias Breuer and Gregor Witte
    J. Chem. Phys. 138 (11), 114901 (2013)
    Full Text
  • [2] Epitaxial growth of perfluoropentacene films with predefined molecular orientation: A route for single-crystal optical studies.
    Tobias Breuer, Gregor Witte
    Physical Review B 83 (15), 155428 (2011)
    Full Text
  • Growth of aromatic molecules on solid substrates for applications in organic electronics.
    Gregor Witte and Christof Wöll
    J. Mater. Res. 19 (7), 1889-1916 (2004)
    Full Text
  • Vinyl-functionalized gold nanoparticles as artificial monomers for the copolymerization with methyl methacrylate.
    Katharina Gries, Mira El Helou, Gregor Witte, Seema Agarwal, Andreas Greiner
    Polymer 53 (8), 1632-1639 (2012)
    Full Text
  • Immobilization of quantum dots via conjugated self-assembled monolayers and their application as a light-controlled sensor for the detection of hydrogen peroxide.
    W. Khalid, M. El Helou, T. Murböck, Z. Yue, J.-M. Montenegro, K. Schubert, G. Göbel, F. Lisdat, G. Witte and W. J. Parak
    ACS Nano 5 (12), 9870-9876 (2011)
    Full Text

Other sources:

  • [3] Optical properties of pentacene and perfluoropentacene thin films, Journal of Chemical Physics.
    A. Hinderhofer, U. Heinemeyer, A. Gerlach, S. Kowarik, R. M. J. Jacobs, Y. Sakamoto, T. Suzuki and F. Schreiber
    J. Chem. Phys. 127 (19), 194705 (2007)
    Full Text
  • [4] Preparation and Characterization of thin films of organic semiconductors and their heterostructures.
    Tobias Breuer
    Dissertation (2013)