Probing the Electronic Stability of Multiply Charged Anions: Sulfonated Pyrene Tri- and Tetraanions

The strong intramolecular Coulomb repulsion in multiply charged anions (MCAs) creates a potential barrier that provides dynamic stability to MCAs and allows electronically metastable species to be observed. The 1-hydroxy-3,6,8-pyrene-trisulfonate {[Py(OH)(SO3)(3)](3-) or HPTS3-} was recently observed as a long-lived metastable MCA with a large negative electron binding energy of -0.66 eV. Here we use Penning trap mass spectrometry to monitor the spontaneous decay of HPTS3- -> HPTS center dot 2- + e(-) and have determined the half-life of HPTS3- to be 0.1 s. To explore the limit of electronic metastability, we tried to make the related quadruply charged pyrene-1,3,6,8-tetrasulfonate {[Py(SO3)(4)](4-)}. However, only its decay product, the triply charged radical anion [Py(SO3)(4)](center dot 3-), as well as the triply charged ion-pairs [Py(SO3)(4)H](3-) and [Py(SO3)(4)Na](3-), was observed, suggesting that the tremendous intramolecular Coulomb repulsion makes the [Py(SO3)(4)](4-) anion extremely short-lived. Photoelectron spectroscopy data showed that (Py(SO3)(4)](center dot 3-) is an electronically stable species with electron binding energies of +0.5 eV, whereas [Py(SO3)(4)H](3-) and [Py(SO3)(4)Na](3-) possess electron binding energies of 0.0 and -0.1 eV, respectively. Ab initio calculations confirmed the stability of these triply charged species and further predicted a large negative electron binding energy (-2.78 eV) for [Py(SO3)(4)](4-), consistent with its short lifetime.