Synthesis And Electrical Properties Of D-A-D Type Molecular Wires With Thienyl Terminal Groups

During the past half century, the study on molecular electronics has grown rapidly. It is the primary focus in the molecular electronics field to measure molecular conductance with different techniques. Most of these measurement processes are based on the construction of metal-molecule-metal junctions.By using Scanning Tunneling Microscope to form break junctions and theoretical calculations, we have studied the molecular conductance of a series of molecular wires with different donor-accepter-donor structures, which contain thienyl anchoring groups. Furthermore, we also tested the photoresponse of the molecular wires in light excitation conditions. The main results of the thesis are listed as following:1. We have synthesized molecules with thienyl-fluorene-thienyl backbone and studied their molecular conductance. It has been found from the measurement that the single molecule conductance changes with the different donor or accepter unites. The four model molecules have the same molecular length and anchoring groups, and are substituted by H, dimethyl, carbonyl and diallylcyan groups (named as T-F-T, T-DMF-T, T-FO-T and T-FCN-T respectively). We have tested their single molecule conductance in darkness, and obtained the I-S curves in one and two dimensional histograms. The results show that molecules with strong D-A-D structures have larger conductance than weak D-A-D ones. The molecular conductance order is T-FCN-T>T-DMF-T>T-FO-T>T-F-T. We have also tested the T-FO-T, T-F-T and T-DMF-T molecules in darkness and under visible light. It is interesting that the T-FO-T molecule showed photoresponse, whose molecular conductance increased by70%. But the weak D-A-D structure T-F-T and T-DMF-T molecule exhibited no light response. The study has simulated the operating principle of small molecule solar cells, and may have guiding significance in future design of molecular devices.2. We also use theoretical calculations to analyze the experimental results. By Gaussian and ATK software, we have studied their molecular orbitals and transmission spectra. 3. The molecular conductance of different molecules with TTF and DPP backbone were measured. Symmetrically molecule terminated with four pyridyls contains TTF backbone shows a steric effect. The molecules contain pridyl groups with a meta-position N atom have reproducible break-junction steps in I-S curves and their conductance values could be detected. In contrast, the ortho-position one does not show any conductance steps. Meanwhile, molecules with four cyan groups could form molecular junctions. One DPP molecule with thienyl terminal groups have been tested and showed clear break junction steps.