Quantum Coherence for Light Harvesting

Abstract: Popular Abstract in English The main reason that we humans exist at all is because of photosynthesis. The food we eat, the oxygen we breath, most of the energy that warms our homes and powers our electronics are all products of photosynthesis. Over millions of years, plants, algae and some bacteria have developed biomachinery to harness solar energy and thrive. They do this by fixing carbon into chemical compounds, which can turn into either nutrition or fossil fuels. Photosynthetic organisms use pigment molecules embedded in proteins to absorb photons. These absorb only part of the visible spectrum, with the remaining transmitted/reflected light giving photosynthetic organisms their color — often a multitude of shades of green. If the pigments were completely isolated from one another, the absorbed energy would be lost within a few nanoseconds (10 −9 s) in the form of heat, which can not be further utilized. Hence, for efficient operation, the absorbed energy must be transformed into a more stable form — e.g. electrons and protons — within this time-frame. To this extent, the pigments interact to form a network which directs the energy through the various proteins within the photosynthetic membrane towards a special place called reaction center. In this pigment-protein complex, the absorbed energy is transformed into charges. To prevent charge recombination, the protons and the electrons are spatially separated and transferred to opposite sides of the photosynthetic membrane. The resulting proton-motive force is then used to drive the enzymatic reactions which fix carbon and power the cells. The first steps of the light-harvesting processes described above are close to 100% efficient. One of the main reasons for this high efficiency is the ultrafast timescale in which energy is transferred between the pigments. This can range from femtoseconds to picoseconds (10−15 − 10−12 s). In order to study light harvesting processes, we therefore need to resolve the excitation energy flow both in energy and time. Coherent two-dimensional electronic spectroscopy (2DES) is a recently developed technique capable of extracting the most complete information on light-matter interactions within complex multi-pigment systems such as light-harvesting complexes. Using 2DES, we can resolve processes down to 10 fs time scale and follow their evolution on the energy map. On such timescales, the purely quantum-mechanical properties of matter — similar to Schrödinger’s cat being simultaneously dead and alive — can be observed. It was suggested that these so called quantum coherences were facilitating ultrafast energy transfer and charge separation in photosynthesis, thereby enhancing light-harvesting efficiency. These hypotheses were tested using advanced spectroscopic techniques, namely 2DES. Several light-harvesting systems were investigated. Coherent dynamics in the reaction centers of the naturally occurring Rhodobacter sphaeroides — purple photosynthetic bacterium — were studied. We explored the power and limitations of 2DES, and demonstrated for the first time a novel coherence shift process. Inspired by natural photosynthetic light-harvesting antennas, artificially formed bacteriochlorophyll aggregates were subsequently investigated at temperatures close to absolute zero. The basic spectroscopic parameters which define the function of the aggregate were extracted. Finally, a purely artificial light-harvesting system was studied, comprised of well defined tubular aggregates of cyanine dyes. We identified complex interplay between excitation and vibrational motions which seems to be involved in energy transfer. The introductory part of the thesis reviews the investigated systems, introduces 2DES as implemented in this work and describes the working principles of the new photophysical process; “Energy Transfer Induced Coherence Shift”. The second part contains the author’s original work of published papers and manuscripts relevant to light-harvesting processes.