In recent years, research dealing with organic paramagnetic compounds such as stable radicals and high-spin systems has been focused on applied aspects. Several key trends have formed in the application and, hence, in the function-oriented synthesis of organic radicals and polyradicals. This review addresses one of such trends in which the dominant role is played by so-called conjugated nitroxides. Their specific feature is the presence of any unsaturated moiety (C=C, C=N or C=O multiple bond; aromatic or heteroaromatic ring; fused polyaromatic system) adjacent to the nitroxide group. The achievements of the chemistry of conjugated nitroxides are presented, and their physicochemical properties, magneto-structural correlations and practical applications are discussed. The bibliography includes 641 references.

It was shown recently on the basis of DFT calculations (N. P. Gritsan and E. A. Pritchina, Mendeleev Commun., 2001, 11, 94) that the singlet states of aroylnitrenes undergo tremendous stabilization due to an extra N–O bonding interaction. To test experimentally the multiplicity and the structure of the lowest state of benzoylnitrenes we performed a study of their photochemistry in Ar matrices at 12 K. Formation of two species was observed on irradiation of benzoyl azide (1b) and its 4-acetyl derivative (1c). One of these species has an IR spectrum, which is consistent with that of isocyanate (2b,c). The IR and UV spectra of the second intermediate are in very good agreement with the calculated spectra of the singlet species (3b,c), whose structure is intermediate between that of a carbonylnitrene and an oxazirene. We further examined the photochemistry of benzoyl azide in solution at ambient temperatures by nanosecond time-resolved IR methods and obtained additional evidence for the singlet ground state of benzoylnitrene as well as insight into its reactivity in acetonitrile, cyclohexane, and dichloromethane. The above experiments were accompanied by quantum chemical calculations which included also a thorough investigation of the parent species, formylnitrene, at different levels of theory.