Femtosekundenspektroskopie im mittleren Infraroten: Energierelaxation in para-Nitroanilin und Konformationsdynamik von Azobenzolpeptiden

Time-resolved vibrational spectroscopy (Raman and IR) provides
insight into energy relaxation processes and information about
structural dynamics of molecules in solution. An extremely
sensitive pump-probe laser-spectrometer has been built for
time-resolved IR-spectroscopy in the coursework of this thesis. It
allows one to initiate photochemical processes in the visible
spectral region and to observe the changes in the mid-infrared. The
achieved time resolution (delta tau_(FWHM) < 100 fs) makes it
possible to follow extremely fast structural changes. With this
setup dynamical processes can be resolved covering 4 orders of
magnitude (from 100 fs to 4 ns). The setup is tunable in the range
of 3-10 micrometer (1000-3300 cm^-1) and covers the whole relevant
spectral range of vibrational structure analysis. It is known from
chemical synthesis, that reaction rates can depend on the chosen
solvent. This might be due to partially solvent dependent energy
redistribution processes. Such processes are important for chemical
reactions, e.g. in stabilizing product states. The molecule
para-Nitroaniline (pNA) is especially suitable to investigate such
processes. pNA returns via fast (< 400 fs) internal conversion
to a hot electronic ground state after excitation by a laser pulse
(lambda = 400 nm). The vibrational energy relaxation can be
followed subsequently. This relaxation has been observed by the
symmetrical NO_2 stretch mode. Due to anharmonic coupling this low
frequency mode has been an exceptionally sensitive sensor for the
energy relaxation to a solvent. The observed relaxation time
constants for pNA are 3,4 ps in deuterated dimethylsulfoxide, 1,9
ps in methanol and 1,5 ps in water. Cooling behavior correlates
well with macroscopic thermal conduction and heat capacity of the
used solvent. In the microscopic picture we assume a connection to
the forming of hydrogen bonds. As chemical reaction rates depend
strongly on the internal temperature of a molecule, these results
improve the understanding of solvent effects. With the same method
the question of how fast structural changes can occur in a peptide
of 8 amino acids has been explored by a collaboration with the
group of prof. Hamm in Zurich. Due to the addition of an azobenzene
switch in this peptide, which subsequently closes the cycle of the
peptide chain, an ultrafast conformational change has been
triggered by a light pulse. In a driven phase the peptide backbone
reacts within the first 20 ps to the fast (< 2 ps) change in
length of the switch. Weakly directed diffuse processes occur on a
wide range of time scales, which lead to the final structure
change. The results of the measurements gave new insights into the
complexity of fast protein folding. A hierarchy of time scales is a
typical signature, which can be observed when moving on a rough
energy landscape.