Salts of the donor molecule, bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET), with pentafluorothiomethylsulfonate (SF 5 CX 2 SO 3 -, X ) H or F) anions have been prepared. Three phases, β′′-(ET) 2 SF 5 CH 2 SO 3 , β′-(ET) 2 SF 5 CF 2 SO 3 , and β′′-(ET) 2 SF 5 CHFSO 3 were obtained by electrocrystallization with the corresponding LiSF 5 CX 2 SO 3 electrolytes. The structures of these salts were determined by single-crystal X-ray diffraction, and their physical properties were examined by electrical resistivity measurements as well as by ESR and Raman spectroscopy. The β′′-(ET) 2 SF 5 CH 2 SO 3 , β′′-(ET) 2 SF 5 CHFSO 3 and β′-(ET) 2 SF 5 CF 2 SO 3 salts are considerably different in their crystal structures, physical properties, and electronic structures despite the similarity in the structures of the SF 5 CX 2 SO 3 -(X ) H, F) anions. The β′′-(ET) 2 SF 5 -CH 2 SO 3 salt has two kinds of ET donor molecules with considerably different charge densities. The electronic structure of β′′-(ET) 2 SF 5 CHFSO 3 has both one-dimensional (1D) and twodimensional (2D) Fermi surfaces which are similar to those found in the organic superconductor β′′-(ET) 2 SF 5 CH 2 CF 2 SO 3 . The ESR data for the β′-(ET) 2 SF 5 CF 2 SO 3 salt indicate that it opens a spin gap below 45 K. The differences in the physical properties of the three salts were analyzed by calculating the HOMO-HOMO interaction energies between nearestneighbor ET molecules in their donor molecule layers.

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A series of thiophene-pyrrole-derived annulenes containing either 6 (9a-c) or 10 (14a,band 17) heterocyclic units and 2 vinylene bridges have been synthesized from the appropriate dialdehydes or tetraaldehydes by the McMurry coupling reaction. All of these annulenes except 17 contain one or two intramolecular aliphatic chains linking the pyrrole units. Spectroscopic properties of these annulenes show no evidence for overall aromaticity or antiaromaticity, although the nature of the aliphatic bridge has a small but definite effect on the UV-visible spectra and electrochemical properties of the compounds.

L=2S pm W=3 mm 2.5 2 1.5 1 0.5 0.01 0.1 1 10 100Voltage (V) Fig. 4. Open circles: Log-log plot of the current-voltage characteristic of a 6T single crystal equipped with two planar gold electrodes. The full line gives the slope of the curve. A slope of 1 corresponds to a linear variation, and a slope of 2 to a quadratic regime.without the gate electrode). At low voltages, the variation is linear. At voltages greater than about 1 V, the variation becomes quadratic, which corresponds to the well-known space-charge limited current (SCLC).[251 It is worth pointing out that a transition between linear and SCLC regimes located close to 1 V indicates that the linear regime is also space-charge limited.[2s1 Accordingly, the slope of the low-voltage linear regime only gives an upper limit of the conductivity of the 6T single crystal, here lo-' S/cm. We note that the onset of the I d -v d characteristics also exhibits a quadratic shape, which provides evidence that the lack of a linear regime is due to current losses through the crystal thickness.The quadratic regime can be used to estimate an effective mobility pscLc, Equation 3, where jscLc is the space-charge limited current density and L the electrode distance. (3)We find a mobility of 0.07 cm2V-'s-', in very good agreement with the field-effect mobility. Combining this value with that of the conductivity, we get a free carrier density n of 10" cmP3, which is one thousand times lower than the dopant concentration. We have already reported on such a difference in vacuum-evaporated 6T FETs [~~] (although in that case both densities were much higher), and have attributed it to traps that could be located at crystal defects.In conclusion, we have fabricated a field-effect transistor with a sexithiophene single crystal and a PMMA insulating layer. The field-effect mobility is 0.075 cm2 Vs -I, and the on-off ratio above lo4. The latter is mainly limited by leaks through the insulator. Current-voltage measurements on this FET, together with conductivity measurements on a single crystal, gave us a dopant concentration of 0.2 ppm, and a free carrier density of 10" cm-', which are consistent with the very high purity of the material in its crystalline form. Work is presently underway to realize a structure where the source and drain electrodes are located at the semiconductor-insulator interface, which would give a device that could be used at low drain voltages.

A novel technique was developed to control the deposition of electrospun polyurethane fibers using a silicone collector substrate patterned with soft lithography. This method can be used to control selective fiber deposition with broad pattern dimensions (50–500 µm) over a large area. The combination of ease of use, low cost, tunability, and generation of relatively large fiber mats available with this technique is expected to advance our ability to mimic the orientation and anisotropic properties of native tissues to generate improved tissue engineering scaffolds.