Concentration dependent relaxation times in organic radical solids

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Abstract
The two‐pulse electron spin echo envelope decays have been studied in rigid glass matrices of sulfuric acid and sulfuric acid‐d 2 containing the randomly oriented radical cations of anthracene and anthracene‐d 10, respectively. The sources of the time dependent local fields responsible for the shapes of the two‐pulse echo envelope decays and the phase memory times have been identified. In the anthracene cation: sulfurie acid system, where the phase memory times are independent of the radical concentration, these local fields arise from spin flips of the protons on the matrix molecules which are a result of spin‐spin relaxation within this proton spin system. In the deuterated system, where the phase memory times are strongly dependent on the radical concentration, the time dependent local fields have three sources: (1) the surrounding A electron spins which are flipped by the microwave pulses; (2) the surrounding B electron spins which are flipped as a result of spin‐spin and spin‐lattice relaxation processes within the electron spin system; and (3) the matrix deuterium spins which are flipped as a result of spin‐spin relaxation processes within the matrix deuterium spin system. The electron spin‐lattice relaxation timesT 1 have been measured at 77°K as a function of radical concentration C using an echo method. The data for both systems fit the expression 1/T 1 = 1.2× 10 7 C 2 +28(± 5) where T 1 is in second−1 and C in mole per liter. The lack of any dependence on the nature of the radical nuclei indicates that the concentration independent term is due to a spin‐robit, orbit‐lattice mechanism. The concentration dependent term is attributed to either a Wailer mechanism involving a modulation of the electron magnetic dipole‐dipole interaction by the lattice vibrations or to the participation of clusters of radicals (pairs or triads) in the spin‐lattice relaxation process. The shape of the recovery of the two‐pulse echo following an adiabatic fast passage is multiexponential and its exact form is dependent on τ, the interpulse time. This is interpreted as indicating the presence of regions with different radical concentrations which probably form when the matrix solidifies.