Abstract: The effects of pressure, solvent on the intramolecular spin exchange of biradicals having two nitroxide fragments linked by a long flexible chain were studied by means of highpressure EPR technique. It was found that the intramolecular exchange interaction between nitroxides of biradical took place through the direct contact between them. By analyzing the observed EPR spectra, we have estimated the ratio(Τ_in/Τ_out) value of the average lifetime of the radical fragments inside a cage(Τ_in) to that outside the cage(Τ_out). The Τ_in/Τ_out values decreased with decreasing temperature, increasing pressure. The results suggest that the nearly cyclic conformation in a cage is favorable in solution. Fur ther, the rotational correlation time of individual nitroxide was estimated from the anisotropic EPR signal,, the information on the segmental motion of the nitroxide group in biradical was obtained.

Key words: EPR, Spin exchange, Pressure effect, Rotational correlation time