Quantum Coherence for Light Harvesting

Abstract

Almost all life on Earth depends on the products of photosynthesis — the

biochemical process whereby solar energy is stored as chemical-rich compounds.

The energy of captured photons is transferred through a network of pigment-protein complexes towards the reaction center. The reaction center is responsible

for trans-membrane charge separation, which generates a proton motive force

which drives all subsequent biochemical reactions. The ultrafast (femtosecond)

nature of the primary processes in photosynthesis is the main reason for its

astonishing efficiency. On this timescale, quantum effects start to play a role and can appear in measured spectra as oscillations. It has been hypothesized that

these are evidence of wave-like energy transfer.

To unveil the fundamental principals of ultrafast excitation energy transfer

in both natural and artificial light-harvesting systems, advanced spectroscopy

techniques have been utilized. Coherent two-dimensional electronic spectroscopy

is a state of the art technique which allows the most complete spectroscopic

and temporal information to be extracted from the system under study. This

technique has allowed us to identify a new photophysical process where the

coherence of the initially excited state is shifted to the ground state upon an

energy transfer step. Coherence dynamics caused by this process bear most of

the signatures of pure electronic coherences, and can therefore be easily mistaken for coherent energy transfer. Our results imply that the wave-like energy transfer hypothesis should be reconsidered and tested against coherence shift mechanisms. Furthermore, we have demonstrated that the mixing between vibrational and electronic motions seem to be a general phenomenon in light-harvesting systems.

Abstract (Swedish)

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.

• author: Palecek, David
• supervisor: Donatas Zigmantas
• opponent: Jonas, David, Department of Chemistry and Biochemistry, University of Colorado, Boulder, USA
• organization: Chemical Physics
• publishing date: 2015
• type: Thesis
• publication status: published
• subject: Physical Chemistry
• keywords: Coherent two-dimensional spectroscopy, Quantum coherence, Light harvesting, Photosynthetic reaction center, Energy transfer
• pages: 185 pages
• defense location: Hall C, Kemicentrum, Getingevagen 60, Lund
• defense date: 2016-01-28 13:15:00
• ISBN: 978-91-7422-425-2
• ISBN: 978-91-7422-426-9
• language: English
• LU publication?: yes
• id: 2eb421dc-3bd6-45e7-a9d4-d2d2d207536f (old id 8409766)
• date added to LUP: 2016-04-04 11:15:53
• date last changed: 2020-04-29 12:53:12