We studied the electronic and conductance properties of two thiophene-curcuminoid molecules, 2-thphCCM (1) and 3-thphCCM (2), where the only structural difference is the position of the sulfur atoms in the thiophene terminal groups. We used electrochemical techniques as well as UV-vis absorption studies to obtain the values of the HOMO-LUMO band gap energies, showing that molecule 1 has lower values than 2. Theoretical calculations show the same trend. Self-assembled monolayers (SAMs) of these molecules were studied using electrochemistry, showing that the interaction with gold reduces drastically the HOMO-LUMO gap in both molecules to almost the same value. Single-molecule conductance measurements show that molecule 2 has two different conductance values, whereas molecule 1 exhibits only one. Based on theoretical calculations we conclude that the lowest conductance value, similar in both molecules, corresponds to a van der waals interaction between the thiophene ring and the electrodes. The one order of magnitud higher conductance value for molecule 2 corresponds to a covalent interaction between the sulfur atoms and the gold electrodes.
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