UV/Vis to NIR photoconduction in cyclopalladated complexes.

The incorporation of a rigid core, formed by a cyclopalladated azobenzene fragment bonded to an ancillary Schiff base ligand, into molecules with 12 or 11 peripheral alkyl chains has been successfully achieved. These new complexes, 1 and 2, respectively, are columnar liquid crystals between room temperature and about 50 degrees C. Both cyclometallated and ancillary ligands have been polyalkylated through either aryl ester (electron-withdrawing group) or aryl ether (electron-releasing group) linkages, in order to tune the HOMO/LUMO energy levels. The photoconductive properties of 1 and 2 have been studied as a function of their absorption properties before and after annealing, from the UV/Vis to NIR region. Compared with the reference compounds, tris-alkynyl benzene discotics, these new materials gave similar performances (sigma/I approximately 8x10(-13) S cm W(-1) with E = 10 V microm(-1) at lambda = 370 nm). Moreover, complex 2 shows a normalized photoconductivity sigma/I = 8.5x10(-13) S cm W(-1) at lambda = 760 nm. Organic photoconductors in such a high wavelength spectral range are not common and are usually assembled by mixing dyes with organic semiconductors.

Wetting properties of flat and porous silicon surfaces coated with a spiropyran.

We report the immobilization and characterization of a spiropyran (SP) derivative (1) on smooth Si(100) and porous H-terminated silicon surfaces through a thermal hydrosilylation protocol. Under visible light exposure the SP is in a closed, hydrophobic form, whereas under UV irradiation it converts to a polar, hydrophilic open form named merocyanine (MC). The SP-MC photoinduced isomerization gives a small contact angle (CA) change of 9 degrees for smooth Si(100) samples under sequential irradiation cycles with white and UV light. Irradiation of porous silicon (PS) surfaces, under the same conditions, gave a CA change of 11 degrees. Treatment of PS surfaces, bearing the MC form of chromophore 1, with cobalt(II) ions enhances the wettability switching of the PS surface to a much larger extent, giving rise to a CA variation as high as 32 degrees.

Wetting behavior of porous silicon surfaces functionalized with a fulleropyrrolidine.

We report the immobilization of a fulleropyrrolidine, bearing a dec-9-ynyl functionality, on silicon surfaces through a thermal hydrosilylation protocol. Contact angle measurements on porous silicon (PS) surfaces reveal an unusual dependence of the angle with the PS roughness that apparently contradicts Wenzel's formula. This result has been explained by an extension of Wenzel's model in which the critical angle, which discriminates between the hydrophilic/hydrophobic character of a solid material, is substantially reduced below 90 degrees by surface roughness.

High charge-carrier mobility in an amorphous hexaazatrinaphthylene derivative.

An isomeric mixture of a tris(pentafluorobenzyl ester) derivative of hexaazatrinaphthylene forms stable amorphous films with an effective charge-carrier mobility of 0.02 cm2/Vs, while the pure 2,8,15-isomer exhibits widely differing morphologies and carrier mobilities (0.001-0.07 cm2/Vs), depending critically on the processing conditions.