Transport and transport-related phenomena in gated junctions based on molecules with floppy vibrational degrees of freedom

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Molecular electronics represents a field of great current interest aiming at a deeper understanding of the interplay of chemistry, physics, and engineering at the nanoscale, which should allow to fabricate molecular circuits with desired functionality. Extensive work done in collaboration with the experimental group of Prof. C. D. Frisbie (University of Minnesota) in the first funding period of this project yielded many results providing important insight into transport and transport-related phenomena in nanojunctions based on floppy molecules. Building on those results, this proposal renewal has two objectives. First, stimulated by findings from the first period, we plan further joint theoretical-experimental studies on molecular junctions under mechanical stretching. These studies are strongly motivated by preliminary (unpublished) results revealing a highly puzzling nanoelastic behavior of nanojunctions based on floppy molecules. In view of this completely unusual behavior, we strongly expect that our work will become a new, valuable contribution to nanoelasticiy altogether. Second, in a joint theoretical-experimental effort we aim at investigating for the first time in the literature the charge transport through nanojunctions based on floppy molecules under electrolyte gating using ionic liquids/gels instead of ordinary electrolytes. This experimental approach was recently pioneered by Frisbie's group in organic electronics. The theoretical studies planned in this part differ from the vast majority of existing studies on nanotransport, which assume molecules at frozen geometry. To account for the presence of reorganizable vibrational modes, we will perform ensemble averaging of the transport properties computed at given geometry. The approach to be employed in this project will extend a recent applicant's approach based on a single-step description of nanotransport. That approach, which received recently recognition in the biochemistry and electrochemistry communities, is different from the consensus description based on a two-step mechanism. The ensemble average to be performed is nontrivial because the weight function needed in calculations requires the Gibbs free energy of a junction out of equilibrium and because reorganization related to floppy modes is qualitatively different from that of common solvents. Stronger fluctuation effects due to floppy modes and their temperature (T) dependence are expected in cases where the dominant molecular orbital can be brought closer to electrode Fermi levels. This can be achieved by electrochemical gating, which is the most efficient setup to control energetic alignments. Still, T-ranges accessible using ordinary electrolytes are very limited. Using ionic liquids/gels, a considerably broader T-range can be explored. The theoretical results obtained in this project, to be compared with data emerging from companion experimental studies, will provide new insight into charge transport at the nanoscale.