Professor Gerd Kothe
Professor emeritus
Department of Physical Chemistry
University of Freiburg
Albertstrasse 21
79104 Freiburg
Office 3005 FB
Phone +49-761-203-6193
eMail gerd.kothe@physchem.uni-freiburg.de
Academic Background
Gerd Kothe studied Chemistry at the Ludwig Maximilian University of Munich (diploma 1966) and obtained his PhD in Physical Chemistry in 1971 at the University of Freiburg (Professor H. Zimmermann). After postdoctoral studies at the Washington University in St. Louis/USA with Professor S.I. Weissman in 1974, he returned to Freiburg for further postdoctoral qualification (habilitation 1977). In 1981, he was offered a chair of Physical Chemistry at the University of Stuttgart which he accepted. After declining an offer at Darmstadt University of Technology in 1990, he was appointed Professor of Physical Chemistry at the University of Freiburg in 1994.
Research interests
NMR studies of the viscoelastic properties of liquid crystals
Center of interst is the development and application of new NMR methods to characterize mesophases over an extremely large length and timescale range. Of particular interest is the identification and characterization of ultraslow motions inherent to liquid crystals. These low-frequency processes reflect the relaxation of mesoscopic structures determined by the viscoelastic properties of the liquid crystal. New methodical developments in NMR enable a detailed study of a number of mesophases including biological membranes and liquid crystal polymers. Analysis of pulse frequency dependent transverse relaxation times provides values for the bending elastic modulus of a variety of biomembranes containing cholesterol and proteins.
Measurement of the spin-lattice relaxation time as a function of temperature, director orientation and Larmor frequency enables one to characterize the complex motions in liquid crystals. Using pulse frequency dependent transverse relaxation times, it is possible to study collective motions and viscoelastic properties of these systems at common high magnetic fields. Relaxation time measurements combined with rheo-NMR techniques have been employed to evaluate virtually all eight viscoelastic parameters of a nematic polymer. Slow sample rotation can be used to identify biaxial nematics. It is our aim to employ NMR as powerful tool in designing new functional liquid crystals with tailored material properties.
EPR studies of the primary events of photosynthesis
The primary events of photosynthesis proceed via light-induced radical pairs as short-lived intermediates. Using transient and pulsed EPR at 9.5 GHz (X-band), 24 GHz (Q-band) and 94 GHz (W-band), we explore the structure and kinetics of these species. Particular emphasis is given to quantum oscillations detectable in the radical pairs of the electron transfer chain. From the results, we expect to deduce the origin of the exceptional high quantum yield of natural photosynthesis, unattained by any model system.
The formation of quantum oscillations in the radical pairs of photosynthesis is a consequence of the ultrafast primary charge separation initiated by a short light pulse. In a two-dimensional transient EPR experiment, the frequency of the zero-quantum electron coherences varies across the powder spectrum. The pronounced variation can be used to evaluate the three-dimensional structure of the radical pair in the photosynthetic membrane (structure determination on a nanosecond time scale). Using pulsed X-band EPR, it is possible to unravel the mechanism for the formation of single quantum nuclear coherences in the radical pairs of photosynthesis. These coherences are directly connected with photochemically induced dynamic nuclear polarization (photo-CIDNP) which provides detailed information of the electronic structure of the cofactors in the electron transfer chain.
Quantum coherence studies of the intersystem crossing in organic molecules
Since a couple of years, we are interested in the mechanism of the intersystem crossing (ISC) of organic molecules. It has been generally believed that only electron spins participate in this process. However, our quantum coherence studies show that also nuclear spins play an active role. Analysis reveals a mixing of the zero-field triplet functions during the ISC which gives rise to large NMR signal enhancement at high magnetic fields. This opens new perspectives for the analysis of photo-CIDNP in mechanistic studies of photoactive proteins. If, initially, a singlet radical pair is formed, the observed photo-CIDNP can be analyzed using a conventional radical pair mechanism. In case of a triplet radical pair, however, a more comprehensive analysis is required involving the new triplet mechanism.
Recently, we introduced a new level anti-crossing (LAC) experiment for photo-excited triplet states, designed to create multiqubit entangled nuclear spin states in molecular solids. In this experiment, a laser pulse generates the triplet state and initiates entanglement between an electron spin and N hyperfine coupled nuclear spins. This gives rise to huge nuclear spin polarization during a first evolution period. Then, a resonant high-power microwave pulse disentangles the electron spin from the nuclear spins. Simultaneously, 2N N-qubit entangled nuclear spin states are formed. Using scalable global or parallel quantum gates, one is able to address and manipulate the entangled spin states in the time domain. We therefore expect that the new LAC experiment paves the way for large-scale quantum information processing with more than 1000 multiqubit entangled states. The total of these states depends on the number of I = ½ nuclei in an organic molecule which can be tailored by chemical synthesis.