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. Moreover, for ethylene and phenylene bridging units, the usual spin-coupling functions are reverted for such unsymmetric connection of delocalized radicals. 1,2-ethylene and 1,4-phenylene, known as antiferromagnetic couplers, are then predicted to produce triplet biradicals, whereas the nominally ferromagnetic bridges, 1,1-ethylene and 1,3-phenylene, destabilize the triplet states. The concept inferred initially from simple spin-polarization rules is illustrated by AM1 quantum-chemical calculations with extended configuration interaction. Phenalenyl or perinaphthenyl (PNT) is taken as a model radical with positions of alternating spin densities on the periphery. Series of differently bridged perinaphthenyl biradicals are studied further with respect to the ordering of the spin states and the spin-density distribution for the triplet. (C) 1999 Elsevier Science B.V. All rights reserved.