Molecular electronic properties of fused rigid porphyrin-oligomer molecular wires

Porphyrin-derived materials may be linked together by fusion to rigid coplanar aromatic bridges such as substituted anthracenes to form linear or branched oligomers. Here, we consider linear oligomers of free-base porphin and meso-tetra-aryl derivatives, such as the tetramer synthesized by Crossley and Burn. A number of semi-empirical quantum-chemical methods have been used to determine the geometries and electronic structures of the ground and excited electronic states. The inter-ring coupling responsible for electron or hole conduction is discussed as a function of oligomer size and the predicted molecular-wire characteristics outlined. Comparison with properties of other linear molecular wires are summarized, and possibilities of introducing switching capacity are indicated. The edge-to-edge length of the tetramer is about 56 Å, sufficient for trans-membrane spanning; this length may be e.g. doubled in one synthetic step, producing an octamer of length about 120 Å, sufficient for electrode spanning.