Biomolecular spectroscopy in charged helium nanodroplets

Three billion years ago, our world changed completely when cells with light-absorbing molecules found a way to capture and transform light into chemical energy. Photosynthesis became the foundation of life on Earth, providing food and energy that keeps almost every organism alive. In photosystems, light is absorbed by special pigments, which respond to different wavelengths of visible light. The primary pigment in photosystems is chlorophyll. It absorbs red and blue light most strongly and reflects green light, making plants appear green. In plants, the first step of photosynthesis starts in the chloroplasts, where light is absorbed by chlorophyll pigments. The absorbed energy is then transferred by an array of chlorophyll molecules and proteins to the reaction center, where charge separation occurs. Electrons are released into an electron transport chain, leading in the following to the synthesis of ATP (adenosine triphosphate), the energy currency of cells. Oxygen is produced as a side product. In natural photosystems, the light-to-charge conversion efficiency exceeds 95%, far higher than that in man-made photovoltaic devices. The fundamental mechanism responsible for the high efficiency of this process is still unknown. Unlocking the design principle of natural photosystems promises to improve the development of bio-inspired technologies, which mimic the natural process of photosynthesis. In this project, a new experimental setup is assembled to study systematically the influence of the interaction of chlorophyll pigments and larger aggregates (nearby pigments, water molecules, proteins) on the absorption energies for getting a deeper understanding of the energy transport as well as the light-to-charge conversion process. One-by-one, chlorophyll pigments are put together to larger aggregates and are embedded inside helium nanodroplets. Helium nanodroplets provide an ultracold (<1K) and non-viscous environment and are ideal to study molecules and molecular clusters in isolation. The new experimental setup is combined with a laser source and a suitable detection system (mass-selective detection) and opens a playground to study a wide range of larger (bio)molecular systems in a well-defined and controllable environment.

Photoactive organic molecules open up a fascinating field of research exploring the interaction of light with matter at the molecular level. Due to their unique ability to absorb light and convert it into various forms of energy, these molecules play a key role in many biological processes such as photosynthesis, as well as in the development of solar cells, and find application in photonics and sensing, photodynamic therapy, and the production of high-performance optical materials. To gain a deep understanding of their intrinsic photophysical properties and to investigate the influence of the environment on the electronic structure of these molecules, precise spectroscopic studies in a well-defined environment are essential. During the Hertha-Firnberg project, an innovative experimental setup was developed to produce cold molecular ions in the gas phase and study their interaction with light. The setup utilizes tiny helium droplets to trap molecules and cool them to temperatures below one degree Kelvin. Through controlled collisions with helium atoms, the molecules are gently released from the droplets, resulting in molecules with some helium atoms attached, which are then irradiated with a laser beam. Upon photoabsorption, the weakly bound helium atoms are lost, leading to the formation of photofragments that are subsequently detected. One particularly feature of this method is the ability to produce both positively and negatively charged molecular ions and systematically modify them, such as through the controlled attachment of individual water molecules or clustering into larger molecular complexes. By further adapting the apparatus, the research possibilities have recently been expanded. Now, in addition to producing singly charged ions, the production of multiply charged monoatomic or cluster ions is also possible, opening up new possibilities to explore their reactions with other atoms, molecules, and photons. The new experimental setup has been used to study the photoactive molecule phthalocyanine in more detail. This macrocyclic compound with an alternating nitrogen-carbon ring structure is sought after not only for its deep blue color in the dye industry but also for its unique electronic structure, making it a model system for the biologically relevant chlorophyll. Stepwise modifications of the molecules through ionization, protonation, and the attachment of individual water molecules allowed for systematic investigation of their photophysical properties.