RIGID FUSED OLIGOPORPHYRINS AS POTENTIAL VERSATILE MOLECULAR WIRES .1. GEOMETRY AND CONNECTIVITY OF 1,4,5,8-TETRAAZAANTHRACENE-BRIDGED SYSTEMS

We investigate theoretically a class of rigid, fused oligoporphyrin molecules proposed as potential ''molecular wires'', composed of substituted and/or chelated porphyrins fused together with conjugated bridging units such as 1,4,5,8-tetraazaanthracene (TA). Our long-term goal is to investigate the effects of porphyrin or bridge substitution, metal chelation, oxidation state, and related properties on the conductivity and spectroscopic characteristics of these molecules, leading to predictions of systems most likely to have useful molecular electronic potential. Here, the primary aim is to develop an accurate empirical method by which the geometry of an arbitrary oligoporphyrin, extending perhaps over say 50-100 Angstrom in length, can be readily constructed. This is achieved through the semiempirical quantum-chemical study of a set of basic ''building blocks'', which may be reliably combined by using a deduced set of rules to predict the structure and relative energy of an arbitrary molecule. The most important effects occur for and about the porphyrin-TA fused bond, and the consequences of this for the degree of pi delocalization and hence the interporphyrin communication are discussed. We also consider possible isomerization effects associated with the location of the two inner hydrogens per free base porphin unit, including the height of interconversion barriers. Finally, we consider briefly some effects of porphyrin substitution or chelation on the calculated geometries and bond orders.