An EPR study of triplet excitons in crystalline anthracene–tetracyanobenzene: Exciton dynamics, long-range order parameter, and critical exponent

We report new measurements of the EPR spectra of triplet excitons in the charge-transfer complex anthracene–tetracyanobenzene. The spectra show exchange phenomena arising from exciton motion over a lattice undergoing an order–disorder phase transition. The interpretation of these spectra requires a four-site exchange model from which we estimate the rates of interchain and intrachain motion for the excitons, as well as the temperature dependence of the long-range order parameter and the critical exponent β. This value β=0.34, the first for an order–disorder transition in a molecular crystal, is in agreement with the predictions of renormalization group theory.