Direct Observation of Anion Formation in the Electronic Structure of Tetracyanoquinodimethane (TCNQ) Charge-Transfer Salts

Abstract

New insights into the states involved in an organic charge-transfer reaction have been obtained by using in situ ultraviolet-photoemission-spectroscopy measurements to monitor valence-state binding energies during the reaction. For example, in forming ${\mathrm{Cs}}_{2}$-${(\mathrm{TCNQ})}_{3}$, the transferred electrons are found to be strongly localized, with neither evidence for them occupying extended states nor evidence for fractional ionization of TCNQ. The ${b}_{1\mathrm{u}}\ensuremath{-}{b}_{2\mathrm{g}}$ energy gap of TCN${\mathrm{Q}}^{0}$ is greatly reduced (about 2 eV) upon anion formation.