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 8C. 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 (s/I~8 10-13 S cmW-1 with E=10 Vmm-1 at l=370 nm). Moreover, complex 2 shows a normalized photoconductivity s/I=8.510-13 S cmW-1 at l=760 nm. Organic photoconductors in such a high wavelength spectral range are not common and are usually assembled by mixing dyes with organic semiconductors.