PHOTOSYN-STM · Single-Molecule studies of photo-conductance on photosynthetic molecular systems by SPM break-junction measurements

Periodic Report Summary 1 - Photosyn-STM (Single-Molecule studies of photo-conductance on photosynthetic molecular systems by SPM break-junction measurements)

The main objective of this project was centered on measuring molecular photo-conductance at the single molecule level and applying the acquired knowledge to the study of more complex bio-molecular system. As the partial objectives, first we wanted to design methodologies to reliably build and measure single-molecule junctions between two metal electrodes based on the STM break-junction technique. Then, design a proper set-up which incorporates a photo-detection system to both illuminate as well as detect emission to/from the molecular junction. Then characterize charge transport on complex synthetic molecules with potential interest to perform photo-conductance measurement. Finally, Photo-emission experiments from this particular single-molecule junction are pursued (see Figure 1 for a summarizing sketch). %During the outgoing phase, the researcher has been working on different methodologies that allowed the formation of single Metal-molecule-Metal junctions and measurement of charge transport through them. New designs for the experimental array as well as the exploration of AC-modulated signals were the main focus. The studied molecular systems were: low optical band-gap perylene derivatives, diblock molecular diodes and poly-conjugated molecules with large pi-electron system. Charge transport at the single-molecule level has been explored with special emphasis on rectification behavior, electrochemical gate, pi-electrons lateral coupling and photo-emission. So far, a robust methodology for detection of single-molecule junction formation has been developed. Using this methodology, a number of new properties have been successfully demonstrated on a few particular molecular systems: - A strong diode behavior has been demonstrated in a single-molecule junction by using symmetric stable binding to both electrodes of the junction. - A pronounce electrochemical gate effect has been also demonstrated on different poly-aromatic fused rings systems like perylene and coronene derivatives which suggested a relative low-band gap behavior on the molecular bridge. - The magnitude of the pi-electron coupling on previous poly-aromatic compounds has been quantified by modifying the actual tilt angle of the molecule within the single-molecule junction with respect to the metal terminals. This is an important achievement to understand how lateral molecular interactions may facilitate electron transport through more complicated bio-molecular networks. - The transport mechanism through the same poly-aromatic molecular wires has also been studied as a function of the molecular length in order to determine the transition on the transport mechanism from direct tunneling to indirect hopping. - First single-molecule photo-emission data using perylene derivatives and a diblock molecular diode has been collected. The results achieved during the outgoing phase have already had a significant impact in the field of Molecular Electronics as it is reflected by the amount and the quality of the scientific publications derived from this work (see publication list in section 5). We foresee even higher impact on the final results of this project through two parallel paths: - Final demonstration of single-molecule photo-emission would by itself open up a complete new topic within the Molecular Electronics field and would constitute a new conception for the future design of optoelectronic devices. Its impact would extend to the field of biochemistry through the study of photo charge separation processes in nature. - Second, using single-molecule charge transport methods to investigate the different electron pathways through complex redox bio-molecular architectures would indeed be a high impact result to be developed at the host institution.

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