A novel stable organic radical, 2,2-diphenyl-1,2-dihydrobenzo[g]quinoline-N-oxyl (2), has been synthesized and structurally characterized in order to examine the ring extension effect on the known derivative, 2,2-diphenyl-1,2-dihydroquinoline-N-oxyl (1). Based on SQUID measurement, 2 exhibited a ferromagnetic interaction obeying the Bleaney-Bowers equation with 2J = +4.6 cm(-1). The X-ray analysis revealed an intermolecular edge-edge association of the aryl ring and N-oxyl moiety in the crystal of 2. The bifurcated edge-edge contacts between the alpha-spin site (nitroxyl O atom) and the beta-spin site (aryl-H atoms) are responsible for the ferromagnetic interaction of 2. (C) 2004 Elsevier B.V. All rights reserved.