Using antiferromagnetic couplers for high-spin ground states in ion radicals

The presence of negatively spin-polarized peripheral sites in delocalized r adical ions is exploited to propose a type of direct linkage leading to sta bilization of high-spin states. Ion monoradicals of anthracene, pyrene, azu lene and s-indacene are taken as spin-bearing units. Pairs with close charg ing potentials are outlined as likely to form diradicals. Triplet stabiliza tion is envisaged to occur from the exchange coupling of opposite-signed br idgehead a-spin densities belonging to individual radical units, e.g., 2(9' -anthryl)pyrene and 2,2'-bipyrenyl di-ions. Results from AM1-ROHF-CI (CAS[1 0,10], CAS[9,9]) computations are reported, where the computational approac h is first checked for adequacy with regard to the spin-density distributio n of the monoions, and then employed to assess the spin states of several i onized dimers and trimers, some of which are synthetically available. The h igh-spin states are found to be stabilized by up to similar to 300 cal mol( -1), with the singlet-triplet or doublet-quartet gaps increasing for nonequ ilibrium conformations approaching planarity. (C) 2000 Elsevier Science B.V . All rights reserved.