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Characterization of the neutral -to -ionic transition in the charge -transfer complex perylene -TCNE
Near-normal specular reflection spectroscopy was used to obtain the polarized electronic spectra from single crystals of the mixed-stack charge-transfer complex perylene-tetracyanoethylene (P-TCNE) in the spectral region from 7.5 kK to 16.5 kK. Spectra were measured over the temperature range 10 K to 170 K. Three solid-state phase-transitions were observed through electronic spectral changes and confirmed by differential scanning and AC-temperature calorimetry. Four phases exist in P-TCNE: a neutral phase between 330 K and ∼120 K, a neutral triadic phase between ∼120 and 25 K, a neutral mixed random/triadic/dyadic phase between 25 K and 10.4 K, and an ionic dimerized phase below 10.4 K. The first observation of more than one phase transition that involves electrons, an aromatic and neutral-to-ionic transition (NIT) in the same complex, are made. CT absorption bands were used to find the degree of charge transfer, and verify the NIT. A model is presented that rationalizes each phase of the P-TCNE complex. Trimer and dimer structures in the stacks of CT molecules provide a mechanism for a given stack to increase its number of ionic species, yet remain neutral. The NIT in P-TCNE is observed to have a discontinuous change in ionicity. These findings require a new NIT mechanism that may be more general than those now available. P-TCNE has the unique property to perturb the Coulombic interactions through either or both of its phase transitions involving changes in electronic states and, with this added flexibility, may be the model system for study of the NIT. ^
Albro, Rick, "Characterization of the neutral -to -ionic transition in the charge -transfer complex perylene -TCNE" (2005). ETD collection for University of Nebraska - Lincoln. AAI3186846.