A molecular beam of europium-cyclooctatetraene sandwich nanowires Eu(n)()(COT)(m)() was produced by a laser vaporization synthesis method. The formation mechanism of the nanowires was quantitatively revealed by photoelectron and photoionization spectroscopies of the Eu-COT species, together with supporting theoretical calculations. From these results, it is confirmed that growth processes extending the length of Eu-COT nanowires involve a series of elementary reactions in which efficient charge transfer occurs at the terminal reaction sites. In every elementary step, the reaction proceeds between one reactant having low ionization energy and the other reactant having high electron affinity, probably via a "harpoon" mechanism.

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Organoeuropium sandwich clusters, comprising europium (Eu) and 1,3,5,7-cyclooctatetraene (COT) (Eu(n)(COT)(m)), were produced in the gas phase using a laser vaporization synthesis method. Photoionization mass spectra revealed an exclusive Eu(n)(COT)(m) formation with three compositions: m = n + 1, m = n, and m = n - 1, which, we propose, correspond to full-sandwich, half-sandwich, and inverted-sandwich structures, respectively. The charge distributions, metal-ligand bonding characteristics, and electronic structures of the clusters were comprehensively investigated by photoionization measurements of Eu(n)(COT)(m) neutrals and by photoelectron spectroscopy of Eu(n)(COT)(m)(-) and isoelectronic Ba(n)(COT)(m)(-) anions. The results confirmed that (1) highly ionic metal-ligand bonding is formed between Eu(2+) and COT(2-) within the sandwich structure (at the termini, ionic forms are Eu(+) and COT(-)) and (2) size dependence of orbital energy can be explained by the Coulombic interaction of simple point charge models between the detaching electrons and dipoles/quadrupoles. When the terminus of the sandwich clusters is Eu(2+), COT(2-), or Eu(0), the orbital energy of the electron detachment channel at the opposite terminus strongly depends on the cluster size. In this case, the molecular stack behaves as a one-dimensionally aligned dipole; otherwise, it behaves as a quadrupole, and the relationship between cluster size and electron detachment energy is much weaker. The study also reports on the 4f orbital energy in Eu ions and the formation mechanism of organoeuropium sandwich nanowires up to 12 nm in length. The nanowires are formed by successive charge transfer at the terminal part, Eu(+) and COT(-), which reduces the ionization energy and increases the electron affinity, respectively.