Rigid fused oligoporphyrins as potential versatile molecular wires. 2. B3LYP and SCF calculated geometric and electronic properties of 98 oligoporphyrin and related molecules

Over 100 oligoporphyrin (porphyrin molecules fused to each other through ri gid acene-type bridges) molecules have now been synthesized, their long rig id pi-bonded structures making them very suitable as molecular wires while their synthetic flexibility offers the possibility of tailoring their struc tural and electronic properties to match specific needs. To examine their b asic operational principles and to explore synthetic possibilities, we opti mize the geometry of 85 oligoporphyrin and related molecules including porp hyrin dimers and trimers using the accurate B3LYP density-functional techni que. Also, a scheme is developed by which accurate geometries of oligoporph yrins of arbitrary size can be estimated, and this is applied to determine the geometries of a further 13 porphyrin trimers and tetramers. At these ge ometries we analyze SCF orbital properties in order to determine the supere xchange electronic couplings within the oligoporphyrins. Couplings are moni tored for bridge-length dependence and interpreted in terms of a detailed d escription involving bridge-porphyrin orbital resonances, as well as in ter ms of a simpler picture in which pi-electron delocalization is seen as a pr erequisite for strong intramolecular coupling. Variations of the coupling w ith the nature of the bridge (e.g., naphthalene, anthracene, free-base or p rotonated 1,4,5,8-tetraazaanthracene, tetracene, pyrene, coronene, biphenyl ene, dicyclobuta[a,d]benzene, dicyclobuta[b,g]naphthalene, dicyclobuta[b,h] biphenylene, and bridges additionally fused to porphyrin meso positions) an d porphyrin (e.g., porphyrin or bacteriochlorin, beta-substituents such as methoxy and cyano, Mg, Zn, Ru(CO)(2), and free-base porphyrins) units are c onsidered, and the physical origin of quinonoid switching is determined. Te rminal "alligator clips" such as fused phenanthroline, here complexed with (CuCl2)-Cl-I, are also considered.