Chain Dynamics Cause the Disappearance of Spin-Correlated Radical Pair Polarization in Flexible Biradicals

Abstract

X-band time-resolved electron paramagnetic resonance (TREPR) spectra of 1,16- and 1,21- acylalkyl biradicals, obtained in toluene solution at low temperatures (187−253 K), are reported. The spectra show a strong temperature dependence in their patterns of chemically induced electron spin polarization. The spin-correlated radical pair (SCRP) mechanism dominates at high temperatures, while the radical pair mechanism (RPM) is the main pattern at lower temperatures. It is postulated that the SCRP spectrum is suppressed at the lower temperature due to molecular motion on a time scale which modulates the exchange interaction (J) between the unpaired electrons. Simultaneously the RPM is enhanced by this motion. Average values of J calculated from end-to-end distance distributions for the C21 biradical show that in the absence of this dynamic effect, an intense SCRP spectrum would be present at the lower temperature. The results demonstrate that the interpretation of average J couplings may be difficult in temperature regions where these motional effects dominate the appearance of the spectrum.