Construction of spin-1/2 Heisenberg ferromagnetic-antiferromagnetic alternating chains with various exchange couplings

Magnetic properties of the Heisenberg alternating chains with ferromagnetic and antiferromagnetic couplings have been investigated. Three related organic biradicals with intramolecular ferromagnetic couplings (J(F)'s) were synthesized. The magnitude of J(F) can be controlled by chemical modifications. All of them crystallize to form chain structures with intermolecular antiferromagnetic couplings (J(AF)'S). The exchange couplings are estimated to 2J(F)/k(B) = 638 K and 2J(AF)/k(B) = -14.5 K for PNNNO, 2J(F)/k(B) = 357 K and 2J(AF)/k(B) = -43 K for F2PNNNO, and 2J(F)/k(B) = 216 K and 2J(AF)/k(B) = -12K for PIMNO, where PNNNO = 2-[p-(N-tert-butyl-N-oxyamino)phenyl]-4,4,5,5-tetramethyl-4,5- dihydro-1H-imidazol-1-oxyl-3-oxide, F2PNNNO = 2-[o-difluoro-p-(N-tert-butyl-N-oxyamino)phenyl]-4,4.5,5- tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide and PIMNO = 2-[p-(N-tert-butyl-N-oxyamino)phenyl]-4,4,5,5-tetramethyl-4,5- dihydro-1H-imidazol-1-oxyl. (C) 1998 Elsevier Science B.V. All rights reserved.