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, X-band (3375 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. The sip and magnitude of D in the precursor triplet state play a strong role in the degree of TM polarization observed in the biradical EPR spectrum. In the biradical, the ratio of J to the Zeeman energy and that of a(H) to J determine the sign and the magnitude of SCRP polarization. Molecular orbital calculations support the observation that the value of J is lowered by delocalization of electron density. The biradical also has a dipolar interaction, and it is shown how this term and the rotational orientation time tau(c) can contribute to spin relaxation. Observation of 1,4-bis(benzyl) and photoenol type biradicals was hindered by intermolecular reactions. The effects of conformational flexibility and tumbling on the polarization and relaxation mechanisms are discussed. The Closs biradical 7 is a special case, with a triplet ground state and an encounter rate k(en) defined by a vibrational normal mode rather than a series of conformational jumps.