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

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 Cs₂-(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 b₁ᵤ-b2g energy gap of TCNQ⁰ is greatly reduced (about 2 eV) upon anion formation.