Combined effect of isotopic substitution, temperature, and magnetic field on the lifetimes of triplet biradicals

Abstract

Lifetimes {tau} 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 {tau} was measured at both 0 and 2 kG magnetic field, and also in the presence of 0.004 M MnCl{sub 2}, 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-{sup 13}C{sub 2} and 1,12-{sup 13}C{sub 2} isotopomers. The magnetic isotope effect on {tau} and the temperature dependence of Mn{sup 2+} quenching support chain dynamics as the rate-limiting step at -85{degree}C and intersystem crossing as the rate-limiting step at room temperature. However, the magnetic field effect on {tau} reaches a maximum around -50{degree}C and persists even at -99{degree}C, in contrast to the magnetic isotope effect and Mn{sub 2+} quenching, which are absent at low temperature. The significance of this observation is discussed.