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

Semiempirical PM3-RHF-CI calculations were used to probe structure-exchange coupling relationships in radical-substituted Zn(II) porphyrins. The resul ts support a number of important design elements for creating high-spin mol ecules from metalloporphyrins and the corresponding pi-cation radicals. The calculations showed that inactive porphyrin-active phenoxy union mode prov ides stronger exchange coupling than active porphyrin-inactive phenoxy unio n mode. Connecting radicals to the metalloporphyrin core via an ethynyl lin kage eliminates severe torsion, permitting a coplanar alignment of the two pi systems. Metalloporphyrins could be excellent redox-activated exchange c ouplers: porphyrin radical cation exchange couples attached radicals more e ffectively than the neutral porphyrin. Finally, the magnitude of exchange c oupling between a metalloporphyrin pi-cation radical depends on the nature of the attached radical. The results of the calculations were explained usi ng a coupler spin analysis. Copyright (C) 1999 John Wiley gr Sons, Ltd.