The solid-state low-temperature luminescence spectrum of an uncoordinated imino nitroxide radical, 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl, is reported. The luminescence maximum is at 16300 cm(-1) and the band has a width at half height of 3140 cm(-1) with a prominent vibronic progression interval of 1150 cm(-1). Corresponding spectroscopic results are shown for the nitronyl nitroxide analog, where a lower-energy luminescence maximum at 14 700 cm-1 and a narrower band with a width at half height of 1900 cm(-1) are observed. These differences are analyzed in terms of the different delocalized pi-systems of the radicals. (c) 2005 Elsevier B.V. All rights reserved.