Dimensionality of spin fluctuations in highly anisotropic TCNQ salts

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Abstract
An anisotropic three‐dimensional random walk model is developed and used to describe the frequency dependence of proton spin–lattice relaxation rates in several TCNQ salts. The triplet exciton system (φ3AsCH3) (TCNQ)2 at 145 °K is shown to display two‐dimensional diffusive behavior. Measurements of the proton relaxation rate in NMP–TCNQ at room temperature are reported and interpreted as displaying two‐dimensional diffusive behavior with a transition to three‐dimensional behavior. These results suggest that the nuclear resonance time scale is so long as to preclude observing one‐dimensional behavior even in highly anisotropic one‐dimensional systems. The random walk model is extended to display the dependence of relaxation rate upon charge transfer and thus explain the unusually rapid relaxation in NMP–TCNQ.