The emitting state of the imino nitroxide radical

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 16 300 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 π-systems of the radicals.