Thermoelectric Properties of 2,7-Dipyridylfluorene Derivatives in Single-Molecule Junctions.

A series of 2,7-dipyridylfluorene derivatives have been synthesized with different substituents (2H, 2Me, 2OMe, 2CF3, and O) at the C(9) position. Experimental measurements on gold|single-molecule|gold junctions, using a modified scanning tunneling microscope-break-junction technique, show that the C(9) substituent has little effect on the conductance, although there is a more significant influence on the thermopower, with the Seebeck coefficient varying by a factor of 1.65 within the series. The combined experimental and computational study, using density functional theory calculations, provides insights into the interplay of conductance and thermopower in single-molecule junctions and is a guide for new strategies for thermopower modulation in single-molecule junctions.

Anisotropic chemical pressure effects in single-component molecular metals based on radical dithiolene and diselenolene gold complexes.

On the basis of the reported radical neutral complex [Au(Et-thiazdt)(2)] (Et-thiazdt = N-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate), a series of single-component conductors derived from [Au(Et-thiazdt)(2)], also noted as [AuS(4)(═S)(2)], has been developed, by replacing the outer sulfur atoms of the thiazoline-2-thione rings by oxygen atoms and/or by replacing the coordinating sulfur atoms by selenium atoms toward the corresponding diselenolene complexes. Comparison of the X-ray crystal structures and transport properties of the four isostructural complexes, noted as [AuS(4)(═S)(2)], [AuS(4)(═O)(2)], [AuSe(4)(═S)(2)], and [AuSe(4)(═O)(2)], shows that the oxygen substitution on the outer thiazoline ring actually decreases the conductivity by a factor of 100, despite a contracted unit cell volume reflecting a positive chemical pressure effect. On the other hand, the S/Se substitution increases the conductivity by a factor of 100, and the pressure needed to transform these semiconductors into the metallic state is shifted from 13 kbar in [AuS(4)(═S)(2)] to only ≈6 kbar in [AuSe(4)(═S)(2)]. Analysis of unit cell evolutions and ab initio band structure calculations demonstrates the strongly anisotropic nature of this chemical pressure effect and provides an explanation for the observed changes in conductivity. The greater sensitivity of these neutral single-component conductors to external pressure, as compared with "classical" radical salts, is also highlighted.

Multiredox tetrathiafulvalene-modified oxide-free hydrogen-terminated Si(100) surfaces.

Tetrathiafulvalene (TTF) monolayers covalently bound to oxide-free hydrogen-terminated Si(100) surfaces have been prepared from the hydrosilylation reaction involving a TTF-terminated ethyne derivative. FTIR spectroscopy characterization using similarly modified porous Si(100) substrates revealed the presence of vibration bands assigned to the immobilized TTF rings and the Si-C═C- interfacial bonds. Cyclic voltammetry measurements showed the presence of two reversible one-electron systems ascribed to TTF/TTF(.+) and TTF(.+)/TTF(2+) redox couples at ca. 0.40 and 0.75 V vs SCE, respectively, which compare well with the values determined for the electroactive molecule in solution. The amount of immobilized TTF units could be varied in the range from 1.7 × 10(-10) to 5.2 × 10(-10) mol cm(-2) by diluting the TTF-terminated chains with inert n-decenyl chains. The highest coverage obtained for the single-component monolayer is consistent with a densely packed TTF monolayer.