Multiscale Approach to the Study of the Electronic Properties of Two Thiophene Curcuminoid Molecules.

We studied the electronic and conductance properties of two thiophene-curcuminoid molecules, 2-thphCCM (1) and 3-thphCCM (2), in which 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 by 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 magnitude higher conductance value for molecule 2 corresponds to a coordinate (dative covalent) interaction between the sulfur atoms and the gold electrodes.

A theoretical study of conformational aspects and energy transfer between terthiophene and quinquethiophene in perhydrotriphenylene inclusion compounds.

A theoretical study of models with supramolecular architecture of co-inclusion compounds based on the host perhydrotriphenylene and guests terthiophene and quinquethiophene (PHTP:T3,T5) is carried out to elucidate in detail the conformational aspects of the oligomeric guest species in the PHTP matrix host. The factors that direct the geometry, location and separation of terthiophene and quinquethiophene within the channels of the PHTP host have been studied using semi-empirical and ab initio calculations. The movement of the guests inside the channel is subject to constraints preventing free rotations or axial displacements along the nanochannel. Optimal arrangement and the general trend of the relative order between T3 and T5 in the (PHTP:T3,T5) co-inclusion compound is obtained. Furthermore, excited state calculations allow the explanation of the spectral shifts of the included species in terms of the planarization of their geometries. An analysis of the energy transfer processes between the T3-T5 donor-acceptor pair based on the configurational details of the co-inclusion compound conclude that efficient transfer proceeds only through two different and perpendicular windows for the T3 --> T5 transfer. The results emphasize the importance for better understanding of the directional details of the energy transfer mechanisms in this kind of one-dimensional systems.