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.