Reverting the effect of magnetic couplers in bridged di- and polyradicals

The direct C-C connection of two delocalized monoradicals through positions of opposite spin densities is expected to yield triplet ground states. Mor eover, for ethylene and phenylene bridging units, the usual spin-coupling f unctions are reverted for such unsymmetric connection of delocalized radica ls. 1,2-ethylene and 1,4-phenylene, known as antiferromagnetic couplers, ar e then predicted to produce triplet biradicals, whereas the nominally ferro magnetic bridges, 1,1-ethylene and 1,3-phenylene, destabilize the triplet s tates. The concept inferred initially from simple spin-polarization rules i s illustrated by AM1 quantum-chemical calculations with extended configurat ion interaction. Phenalenyl or perinaphthenyl (PNT) is taken as a model rad ical with positions of alternating spin densities on the periphery. Series of differently bridged perinaphthenyl biradicals are studied further with r espect to the ordering of the spin states and the spin-density distribution for the triplet. (C) 1999 Elsevier Science B.V. All rights reserved.