CHARGE MIGRATION IN SUPRAMOLECULAR STACKS OF PERIPHERALLY SUBSTITUTED PORPHYRINS

PORPHYRIN derivatives play a central part in energy- and electron-transfer processes in natural systems, in which they occur as individual entities or, commonly, as oligomers or supramolecular assemblies. These compounds have also been proposed for use in conducting and photoconducting bulk materials and as conductive and capacitive elements in molecular electronic devices. Much effort has therefore been devoted towards understanding the factors that control energy and charge transport within porphyrin assemblies. Here we describe studies of charge migration along one-dimensional columnar stacks-of porphyrin molecules bearing peripheral hydrocarbon groups. In both the solid phase and the relatively plastic liquid-crystalline mesophase, in which the hydrocarbon groups are mobile, charge is transferred between adjacent porphyrin groups with a jump time of a few picoseconds or less. The isotropic liquid phase, on the other hand, is not conductive. The formation of supramolecular structures therefore seems to be necessary to support and direct charge and energy migration in these systems.