Series of aminoxyl radicals (TEMPO or nitronyl nitroxide radicals) bearing phenyl benzoate, troponoid, or biphenylcarbonitrile as mesogenic cores with long alkyl substituents were prepared. Although most aminoxyl radicals showed only weak antiferromagnetic interactions due probably to the remote spin centers as clarified by the X-ray analysis of 4a and no appreciable mesogenic phase was observed in each compound, an unusual magnetic transition from an original Curie-Weiss phase to another magnetic phase well-expressed by a singlet-triplet (ST) model was disclosed through the thermal transition in the 4'-undecyloxy-4-biphenylcarbonitrile derivative with oxocarbonyl-TEMPO 12b.