Dynamics of triplet excitons in the simple tetracyanoquinodimethane (TCNQ) salts of rubidium, potassium, and tri-methyl-benzimidazol

The ESR spectra of ${\mathrm{Rb}}^{+}$TCN${\mathrm{Q}}^{-}$, of ${\mathrm{K}}^{+}$TCN${\mathrm{Q}}^{-}$, and of TM${\mathrm{B}}^{+}$ (tri-methyl-benzimidazolium) TCN${\mathrm{Q}}^{-}$ were measured. The observed dipolar splittings unequivocally prove that the ESR signals derive from triplet excitons. From the behavior of the linewidths as a function of temperature and orientation of the crystalline axes with respect to the magnetic field, the complete dynamics of the excitons can be derived. They are found to be diffusive, with hopping activation energies of 0.08 eV for ${\mathrm{Rb}}^{+}$TCN${\mathrm{Q}}^{-}$ and 0.16 eV for TM${\mathrm{B}}^{+}$TCN${\mathrm{Q}}^{-}$. The anisotropy of the exciton motion is ${10}^5$; 1 for ${\mathrm{Rb}}^{+}$TCN${\mathrm{Q}}^{-}$ and 30:1 for TM${\mathrm{B}}^{+}$TCN${\mathrm{Q}}^{-}$, which is in complete agreement with the crystal structures of these compounds.