COMBINED EFFECT OF ISOTOPIC-SUBSTITUTION, TEMPERATURE, AND MAGNETIC-FIELD ON THE LIFETIMES OF TRIPLET BIRADICALS

Lifetimes τ for decay of triplet biradicals derived from 2-phenylcycloalkanones were measured by nanosecond transient absorption under conditions designed to probe the contribution of intersystem crossing and chain dynamics to the observed decay. The temperature dependence of τ was measured at both 0 and 2 kG magnetic field, and also in the presence of 0.004 M MnCl2. Under a variety of conditions of temperature, solvent viscosity, and magnetic field, the lifetime of the biradical derived from 2-phenylcyclododecanone was compared with its perdeuterated analogue and with a 1:1 mixture of 1,2-13C2 and 1,12-13C2 isotopomers. The magnetic isotope effect on τ and the temperature dependence of Mn2+ quenching support chain dynamics as the rate-limiting step at -85°C and intersystem crossing as the rate-limiting step at room temperature. However, the magnetic field effect on τ reaches a maximum around -50°C and persists even at -99°C, in contrast to the magnetic isotope effect and Mn2+ quenching, which are absent at low temperature. The significance of this observation is discussed. © 1990 American Chemical Society.