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. © 1975 The American Physical Society.