Semiempirical computational assessment of porphyrins as building blocks for molecule-based magnets: spin-spin coupling in radical-substituted metalloporphyrins

Abstract

Semiempirical PM3-RHF-CI calculations were used to probe structure–exchange coupling relationships in radical-substituted Zn(II) porphyrins. The results support a number of important design elements for creating high-spin molecules from metalloporphyrins and the corresponding pi-cation radicals. The calculations showed that inactive porphyrin–active phenoxy union mode provides stronger exchange coupling than active porphyrin–inactive phenoxy union mode. Connecting radicals to the metalloporphyrin core via an ethynyl linkage eliminates severe torsion, permitting a coplanar alignment of the two pi systems. Metalloporphyrins could be excellent redox-activated exchange couplers: porphyrin radical cation exchange couples attached radicals more effectively than the neutral porphyrin. Finally, the magnitude of exchange coupling between a metalloporphyrin pi-cation radical depends on the nature of the attached radical. The results of the calculations were explained using a coupler spin analysis. Copyright © 1999 John Wiley & Sons, Ltd.