The current state, achievements, problems and prospects of the intramolecular electron spin exchange interaction as a function of the nitroxide biradical composition and structure, type of the radical ring, temperature and the solvent nature are considered on the basis of the literature data and the results of our own experiments.

Regularities of the spin exchange coupling through a bridge in nitroxide biradicals

Conformational structure of nitroxide biradicals use of biradicals as spin probes

Intramolecular electron spin exchange as a function of temperature and solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two rigid nitroxide biradicals existing in one spatial conformation only. Temperature variations of the isotropic hyperfine splitting constanta and exchange integral value |J/a| were measured from EPR spectra and subsequently analyzed. The interaction of polar solvent molecules with >N-O fragments of nitroxide groups led to a slight decrease of the |J/a| value with the increase of temperatureT. In contrast, the interaction of polar solvent molecules with functional groups inside the bridge resulted in a noticeable increase of |J/a| vs.T. In the last case, a coverse relationship between the values of |J/a| and the hyperfine splitting constanta has been observed for solvents with different polarity.

Effect of solvent nature on spin exchange in rigid nitroxide biradicals

The synthesis of dinuclear derivatives of trovacene (η7-C7H7)V(η5-C5H5) (1•) is reported, in which ethynyl (6••), butadiynyl (7••), and 1,4-di(ethynyl)phenyl (8••) groups serve as spacers between paramagnetic (S = 1/2) [5]trovacenyl units. The mononuclear precursors [5]trovacenylcarbaldehyde (2•) and [5]trovacenylacetylene (4•) are also described. Structural characterization by X-ray diffraction has been performed for 4•, 6••, 7••, and 8••. Electronic communication as gleaned from cyclic voltammetry only manifests itself in the reduction processes where redox splitting δE1/2(0/1−, 1−/2−) is resolved for 6•• (δE1/2 = 150 mV) and indicated for 7•• (δE1/2 ≲ 80 mV). Magnetocommunication leads to exchange coupling of the two electron spins which reside in vanadium centered orbitals. The values JEPR(6••) = (−)0.92, JEPR(7••) = (−)0.56, and JEPR(8••) = (−)0.005 cm-1 are derived from the 51V hyperfine patterns. Accordingly, attenuation of exchange interaction by oligoalkyne spacers is weak, corresponding to a fac...

Di([5]trovacenyl)ethyne, di([5]trovacenyl)butadiyne, and di-1,4-([5]trovacenylethynyl)benzene: electrocommunication and magnetocommunication mediated by --C triple bond C--, --C triple bond C--C triple bond C-- and --C triple bond C--C6H4--C triple bond C-- spacers.

Intramolecular electron spin exchange as a function of temperature and solvent viscosity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two short-chain flexible nitroxide biradicals. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. The process of spin exchange in these biradicals dissolved in six different alcohols is compared with that in a nonpolar solvent (toluene) and with the thermodynamic characteristics of the solvents. No correlation is found on a macroscopic level (solvent viscosity) but on a microscopic level (solvent longitudinal relaxation times) a distinct correlation is seen. The original results are compared with literature data.

M.G. Goldfield

Papers

The iminoxyl biradical with unusual spin-exchange properties