Intramolecular spin-interactions between two nitronyl nitroxide or imino nitroxide radical units linked to thieno[2,3- b]thiophene and thieno[2,3- b]thieno[3′,2′- d]thiophene chromophores

Abstract

Diradical species 1N, 1I, 2N, and 2I, in which two nitronyl nitroxide ( N) or imino nitroxide ( I) radical centers are attached to thieno[2,3- b]thiophene ( 1) and thieno[2,3- b]thieno[3′,2′- d]thiophene ( 2) chromophores, respectively, were prepared and their intramolecular exchange interactions were investigated in frozen solutions by means of ESR spectroscopy and magnetic susceptibility measurements at cryogenic temperature. Temperature dependence of the ESR signal intensities due to the triplet state revealed that bis(nitronyl nitroxide) radicals, 1N and 2N, are ground-state singlet species with quite small singlet-triplet gaps. In bis(imino nitroxide) radical 1I the spin interaction was antiferromagnetic and the singlet-triplet gap was estimated to be 2J k B =−18 K . On the other hand, bis(imino nitroxide) 2I was revealed to be a singlet-triplet degenerate species. These results are discussed in terms of the intramolecular NO---S contact along with corroboration by the semi-empirical MO calculations.