Synthesis of an azobenzene derivative bearing two stable nitronyl nitroxide radicals as substituents and its magnetic properties

3,4'-Bis(4,5-dihydro-4,4,5,5-tetramethyl-3-oxido-1-oxyl-3-imidazolio-2-yl)azobenzene (1) was synthesized starting from 3,4'-dimethylazobenzene via the diformyl derivative. Biradical 1 crystallized as dark blue-green needles; its crystal structure was determined by the X-ray crystallography. X-band ESR spectra of biradical 1 in the degassed toluene solution were measured in the temperature range 10-300 K. At 300 K the ESR spectrum showed nine lines centered at g = 2.006 (a(N) = 3.7 G). At 10 K the ESR spectrum showed a fine structure with zero-field splitting (zfs) parameters \D/hc\ = 0.0026 cm(-1) and \E/hc\ = 0.00014 cm(-1). Magnetic susceptibility measurements revealed that the two radical centers coupled in a ferromagnetic way and the through-bond exchange interaction was determined to be 2J/k(B) = 8.36+/-0.26 K with a Weiss constant theta = -0.39+/-0.01 K. Photochromic reaction was monitored by ESR and UV-vis spectroscopy.