Electron photoejection from dianion of an expanded phthalocyanine

Photoelectron ejection from the dianion of pentabenzotriazasmaragdyrin (PBTAS) which is an expanded phthalocyanine, is observed using femtosecond transient absorption spectroscopy. The PBTAS dianion was produced by chemical reduction using excess cobaltocene as a reductant, which was confirmed by their steady-state absorption spectrum as compared to the absorption spectrum obtained by electrochemical reduction, and characterized by magnetic circular dichroism (MCD). Upon photoexcitation of the PBTASdianion, the generation of a radical anion was confirmed by the characteristic absorption peaks that are observed at 950 and 1000 nm. The radical anion was relaxed to form a dianion with a time constant of 0.7 ps.

Multipole-bound states of succinonitrile and other dicarbonitriles

Very recently two anionic states of succinonitrile have been observed, and these two states have been interpreted as a dipole-bound state of the gauche and a quadrupole-bound state of the anti conformer. Here we study the electron binding properties of succinonitrile using high-level ab initio methods. While the dipole-bound state can be investigated using well established approaches, studying the quadrupole-bound state is more challenging owing to the multiconfiguration character of its wave function. The standard methods typically applied to dipole-bound anions fail, and we employ direct electron propagator based and equation-of-motion coupled-cluster methods. Since there is no experience with this type of quadrupole-bound state, various basis set related and methodological aspects are examined in detail. According to our results the quadrupole moment as such plays only a minor role in binding the extra electron, whereas electron correlation effects are decisive. Our best fixed-nuclei electron binding energy is 11 meV. In view of the small binding energy the influence of the nuclear motion on the electron binding properties is examined, in particular, the torsional motion around the central carbon-carbon bond, since it is a very soft mode and the dipole and quadrupole moment depend strongly on it. Our results provide a firm basis to interpret the experimental findings and support the experimental assignments. Moreover, we discuss molecules that possess only a quadrupole-bound state, and preliminary results for dicarbonitriles of bicyclopentane and cubane are presented.

A fresh look at aromatic dianions

Isolated dianions of cyclooctatetraene and cyclobutadiene are, despite their aromatic character, unstable with respect to electron loss. Here we present the first ab initio study of the autodetachment lifetimes of several "aromatic" dianions. In particular, we investigate whether aromatic pi systems are associated with an enhanced stability with respect to electron loss and whether cross-conjugated (Y-aromatic) hydrocarbon dianions possess longer lifetimes than their linear-conjugated isomers. Moreover, our results are compared with bound state calculations for these metastable dianions, allowing us to analyze the validity of the latter approach. All considered, hydrocarbon dianions are found to possess surprisingly short autodetachment lifetimes on the order of a few femtoseconds, and therefore only electron scattering techniques seem to be capable of observing these species in the gas phase. The trends of the computed energies and lifetimes are best understood in terms of the mutual Coulomb repulsion of the extra charges. Small gas phase dianions can only be stable if the excess electrons can localize at least at two subgroups of high local electron affinity. Adding both extra electrons into one delocalized pi system leads to a very unstable species, and thus especially those structures associated with pronounced aromaticity are found to show high energies and short lifetimes.