Time-resolved EPR spectra of short biradicals (1,3- to 1,8-) in solution. The interconnecting roles of the external magnetic field, dipolar coupling, exchange interaction, and molecular motion in the spin polarization and relaxation mechanisms

Time-resolved electron paramagnetic resonance spectra of biradicals in solution with chain lengths of 3-8 carbon atoms are reported. Spin polarization arises from the spin-correlated radical pair (SCRP) mechanism and in some cases also from the triplet mechanism (TM). The observation of spectra at two magnetic fields, Z-band (3315 G) and Q-band (12 500 G), provides qualitative estimates of the relative importance of each mechanism for a given biradical. The influence of several magnetic properties, such as the spin-spin coupling J, the dipolar coupling D, and the electron-nuclear hyperfine interactions a H , on the time dependence and spectral shape is discussed