Morphology Dependent Conductivity and Photoconductivity of Ionic Porphyrin Crystalline Assemblies

The influence of morphology on the photoconductive properties of binary ionic porphyrin assemblies is examined. Micro-to millimeter sized crystalline rods and complex hyperbranched structures, or sheaves, were grown in a controlled manner by combining stoichiometric amounts of meso-tetra(4-aminophenyl)porphyrin, H2[H2TAPP]2+, and meso-tetra(4-sulfonatophenyl)porphyrin, [H4TSPP]2−, metal-free ionic tectons. The H2TAPP:H4TSPP structures were characterized by X-ray diffraction, microscopic methods, optical spectroscopy, conductivity and photoconductivity measurements. The two different H2TAPP:H4TSPP morphologies (rods and sheaves) exhibit similar molecular organization and crystal structure but possess different optoelectronic properties. The rods and sheaves exhibit weak dark conductivity and become more conducting upon illumination into the tetrapyroles Soret and Q bands. The primary charge carriers in these solids upon photoexcitation are electrons and the charge recombination mechanism follows monomolecular kinetics. The sheaves also display some persistent photoconductivity, as well as a long "grow in" period of the photoconductivity upon initial illumination. Both of these actions may be attributed to the presence of defects in the sheaves that form during their growth. The measured photoresponse of the H2TAPP:H4TSPP sheaves is higher than that of the rods. This work provides evidence that optoelectronic properties of organic semiconductors can be effectively tuned by controlling their molecular organization and growth morphology.