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Infrared spectroscopy (FTIR)

Vibrations of molecules and molecular crystals can be excited upon exposure to infrared radiation if the corresponding displacement pattern is not perfectly symmetric. Since the vibrational energies depend distinctly on the present elements and binding situations in the investigated sample, the analysis of infrared absorption spectra allows a precise investigation of the chemical composition.

As the different investigated energies are not selected dispersively (by means of a monochromator) but continuously using a Michelson interferometer, this technique typically combines a very good signal-to-noise-ratio with fast data acquisition. Due to its high chemical sensitivity this technique is furthermore well suited to observe contaminations and decomposition over time or upon introduction of thermal energy.

Figure 1: Infrared absorption of perfluoropentacene thin films in different orientations. The spectra of lying (b) and uprightly oriented molecules (c) yield in superposition the spectrum of statistically oriented molecules (a). Dependent on the molecule orientation some of these modes are visible, while others are not. More details in [T. Breuer et al, J. Phys. Chem. C, 116, 14491 (2012), see especially also Supporting Information].

Further analyses are enabled when the polarization dependency of the vibrational modes is exploited. The different displacement patterns of the individual vibration modes result in variably oriented associated dipole moments. In order to effectively activate a vibration, the polarization of the incident infrared light has to be compatible with this dipole moment, i.e., pointing in the same direction. This can be used to determine the orientation of the molecules by comparing the intensities of modes with different dipole moments.

Such assignments require, however, precise knowledge of the present displacement patterns. Especially for larger aromatic molecules and molecular crystals such an assignment is not trivial, as the vibrations cannot directly be assigned to an isolated bond, but the entire molecule vibrates. Therefore, in order to perform significant analyses, the vibrational modes have to be reliably assigned either by theoretical description or appropriate experiments. This can be achieved via investigation of well-ordered crystalline thin films. If these exhibit unique vertical and lateral alignment, the orientation of all dipole moments can be determined by polarization-resolved measurements.

Figure 2: Visualization of vibrational modes in PFP crystal.
More details in [T. Breuer et al, J. Phys. Chem. C, 116, 14491 (2012)].

By theoretical approaches, such descriptions are possible applying Density Functional Theory (DFT) methods.
Unfortunately, these methods feature systematical problems in such simulations like difficulties in the computation of mode intensities, systematic energy errors, and no intrinsic consideration of the stabilizing van-der-Waals forces in molecular crystals. Therefore, these simulations have to be correlated to appropriate experiments to allow their verification and further methodical development. If this is accomplished, detailed effects such as differences between isolated vibrations of single molecules and those in the molecular solid can be understood and the corresponding displacement patterns can be visualized in a distinct way.

Figure 3: Different absorption spectra of single molecules and molecules in solids. Note that some of the single molecule modes split into two so-called Davydov components, others, however, do not. More details in [T. Breuer et al, J. Phys. Chem. C, 116, 14491 (2012)].

Since our research group does not possess its own infrared spectrometer, IR studies in Marburg are performed in collaboration with the Surface Science Group (→ Surface Spectroscopy, Prof. Dr. P. Jakob).

Some exemplary publications of our group where IR spectroscopy has been utilized are:

• Vibrational Davydov-Splittings and Collective Mode Polarizations in Oriented Organic Semiconductor Crystals.
  Tobias Breuer, Mehmet Ali Celik, Peter Jakob, Ralf Tonner, and Gregor Witte
  Journal of Physical Chemistry C 116 (27), 14491-14503 (2012)
  Full Text
• Selenium as a Key Element for Highly Ordered Aromatic Self-Assembled Monolayers.
  Asif Bashir, Daniel Käfer, Jan Müller, Christof Wöll, Andreas Terfort, and Gregor Witte
  Angewandte Chemie Int. Ed. 47 (28), 5250-5252 (2008)
  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
• A Comprehensive Study of Self-Assembled Monolayers of Anthracenethiol on Gold:  Solvent Effects, Structure, and Stability.
  D. Käfer, G. Witte*, P. Cyganik, A. Terfort, and Ch.Wöll
  J. Am. Chem. Soc. 128 (5), 1723-1732 (2006)
  Full Text
• Structural Characterization of Organothiolate Adlayers on Gold:  The Case of Rigid, Aromatic Backbones.
  Claus Fuxen, Waleed Azzam, Ralf Arnold, Gregor Witte, Andreas Terfort, Christof Wöll*
  Langmuir 17 (12), 3689-3695 (2001)
  Full Text