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.