Rigid Fused Oligoporphyrins as Potential Versatile Molecular Wires. 2. B3LYP and SCF Calculated Geometric and Electronic Properties of 98 Oligoporphyrin and Related Molecules Jeffrey R. Reimers,* ,† Lachlan E. Hall, † Maxwell J. Crossley, † and Noel. S. Hush †,‡ School of Chemistry and Department of Biochemistry, The UniVersity of Sydney, Sydney, NSW 2006, Australia ReceiVed: June 16, 1998; In Final Form: December 7, 1998 Over 100 oligoporphyrin (porphyrin molecules fused to each other through rigid acene-type bridges) molecules have now been synthesized, their long rigid π-bonded structures making them very suitable as molecular wires while their synthetic flexibility offers the possibility of tailoring their structural and electronic properties to match specific needs. To examine their basic operational principles and to explore synthetic possibilities, we optimize the geometry of 85 oligoporphyrin and related molecules including porphyrin dimers and trimers using the accurate B3LYP density-functional technique. Also, a scheme is developed by which accurate geometries of oligoporphyrins of arbitrary size can be estimated, and this is applied to determine the geometries of a further 13 porphyrin trimers and tetramers. At these geometries we analyze SCF orbital properties in order to determine the superexchange electronic couplings within the oligoporphyrins. Couplings are monitored for bridge-length dependence and interpreted in terms of a detailed description involving bridge-porphyrin orbital resonances, as well as in terms of a simpler picture in which π-electron delocalization is seen as a prerequisite for strong intramolecular coupling. Variations of the coupling with the nature of the bridge (e.g., naphthalene, anthracene, free-base or protonated 1,4,5,8-tetraazaanthracene, tetracene, pyrene, coronene, biphenylene, dicyclobuta[a,d]benzene, dicyclobuta[b,g]naphthalene, dicyclobuta[b,h]biphenylene, and bridges additionally fused to porphyrin meso positions) and porphyrin (e.g., porphyrin or bacteriochlorin, -substituents such as methoxy and cyano, Mg, Zn, Ru(CO) 2 , and free-base porphyrins) units are considered, and the physical origin of quinonoid switching is determined. Terminal “alligator clips” such as fused phenanthroline, here complexed with Cu I Cl 2 , are also considered. 1. Introduction Fused rigid oligoporphyrins have many natural advantages 1,2 in potential applications as molecular wires, 3-9 molecules which can communicate electronic coupling and/or transfer electronic charge over macroscopically large distances, distances suf- ficiently large to span biological membranes and nanoelectrode gaps. These molecules can be thought of as comprised of various basic building blocks, chemically combined to produce the desired structure and function. Building blocks which to date have been synthesized 1,10-13 into oligoporphyrins include, of course, the basic porphyrin unit itself, inter-porphyrin bridge units such as 1,4,5,8-tetraazaanthracene (TAA) and its deriva- tives, and end groups such as phenanthrolines and thiophenes to serve as molecular “alligator clips”, 12,14 connecting wires to the outside world. To date, 13 over 100 oligoporphyrins have been synthesized in our laboratories, including linear porphyrin tetramers and octamers having end to end spans of ca. 56 Å and ca. 118 Å, respectively. In principle, it is straightforward to produce longer chains than this as the synthetic strategy allows for doubling of the length with each step, and a large range of molecules with nonlinear topologies have also been made. In part 1 of this series, 15 we considered TAA-bridged oligoporphyrins and showed that, at the simplest level, electronic properties are controlled by the degree of π delocalization. Areas within the molecules in which the π-system forms discrete single and double bonds localize the π-electron wave functions either inside or outside of these regions, effectively insulating electrons on either side from each other, while delocalized regions support long-distance electronic communication. This simple idea may become a key design principle in molecular wire technology and has already been exploited with the synthesis of chemically controllable oligoporphyrin molecular switches: 13 in these, the central anthracene ring is made to be quinonoidal, with conversion of the quinonoid (or quinonoid dioxime) forms to the corresponding hydroquinone effectively converting a π-lo- calized ring to a delocalized one and hence greatly enhancing the through-bridge coupling. Optical switches of this type are also possible. 16 As calculated π electron distributions are sensitive to the details of the geometries at which they are evaluated, in quantitative studies it is essential to use the best-possible estimate of the geometry. Not only is it important to correctly describe gross features such as porphyrin inner-hydrogen tautomerization, but also it is important to describe subtle features such as -′ pyrrole bond length variations. In part 1 we used what was then thought to be the best method available for determining these properties, the semiempirical PM3 method. While subsequent comparison with extensive ab initio MP2 calculations for free-base porphyrin 17 suggests that PM3 pro- vides an adequate description of the relative energies of different tautomers, it is clear that PM3 geometries could be improved upon, especially for structures of low point-group symmetry. * Corresponding author. † School of Chemistry. ‡ Department of Biochemistry. 4385 J. Phys. Chem. A 1999, 103, 4385-4397 10.1021/jp982650j CCC: $18.00 © 1999 American Chemical Society Published on Web 05/18/1999