Photodetachment of gaseous multiply charged anions, copper phthalocyanine tetrasulfonate tetraanion: Tuning molecular electronic energy levels by charging and negative electron binding

We report photodetachment photoelectron :spectroscopy (PES) of gaseous copper phthalocyanine (CuPc) tetrasulfonate quadruply charged anions, [CUPC(SO3)(4)](4-), and its monoprotonated and -sodiumated triply charged anions, [CuPc(SO3)(4)H](3-) and [CuPc(SO3)(4)Na](3-). The [CUPC(So(3))(4)](4-) tetraanion was found to possess a negative electron binding energy of -0.9 eV, whereas-the trianions have binding energies of 1.0 and 1.2 eV for the sodiumated and protonated species; respectively. The PES spectral features of the three multiply charged anions were observed to be similar to that of the parent CuPc neutral molecule, except that the anions have lower binding energies due to the presence of the negative charges (-SO3-). The data thus suggested a stepwise tuning of the molecular electronic energy levels of the CuPc molecule through charging, wherein the molecular orbital energies of the parent molecule were systematically pushed up by the negative charges. We further carried out semiempirical calculations, which provided insight into the nature of the localized charges on the peripheral -SO3- groups and the intramolecular electrostatic interactions in the multiply charged anions and confirmed the interpretation of the stepwise tuning of molecular energy levels by charging. Photon energy-dependent studies revealed, the effects of the repulsive Coulomb barriers on the photodetachment PES spectra of the multiply charged anions. The barrier heights were estimated to be about 3.5 and 2.5 eV for the tetra- and trianions, respectively.: We also observed excited states for the multiply charged anions and resonant tunneling through the:repulsive Coulomb barriers via the excited states.