Photoelectron spectroscopy of s-triazine anion clusters: Polarization-induced electron binding in aza-aromatic molecule

Abstract

Photoelectron spectroscopy was carried out for the mass-selected cluster anions of s-triazine molecule, Tzn− (n=1–6). The mass spectrum and vibrationally resolved photoelectron spectrum of Tz− showed that unlike pyridine and pyrazine, Tz binds an electron and thus becomes the first molecule in the azabenzene series with a positive electron affinity (0.03 eV). This indicates that the local charge polarization in the aromatic ring by the three nitrogen atoms is large enough to facilitate electron binding to a homologue of benzene. A Jahn–Teller distortion was proposed to explain the vibrational progressions of the photoelectron spectrum of Tz−. A series of Ar-solvated clusters of Tz−, Tz−⋅Arm (m=1–7), have been also studied. Their photoelectron spectra showed a drop in the incremental electron binding energy when going from m=4 to 5, indicating the closure of a solvation shell with four Ar atoms. In the mass abundance spectrum of Tzn−, a distinctly high intensity for Tz2− indicated its exceptional stability, which was also manifested by the large increase by more than 0.5 eV in the vertical detachment energy of the photoelectron spectrum. Theoretical calculations were carried out to obtain optimized geometries of the neutral and anion of Tz and Tz2. We confirmed the Jahn–Teller distortion in Tz− and also addressed the role of hydrogen bonding in determining the geometries of Tz2−. A common feature for the two most stable forms of Tz2− with comparable energies was that they achieve their unique stability through equal sharing of the negative charge between their two molecular constituents. A new photoelectron band was found to emerge from Tzn− for n⩾2 by the 355 nm light, in addition to the photoelectron band at lower electron binding energy observed for n⩾1 at 532 nm. The relative intensity of this new band decreased as n increased, and its position was 1.6–1.8 eV above the first band. Photodetachment to an electronically excited state was suggested to give rise to the second photoelectron band. The nature of the excited state should be cluster-derived, not molecular, and it is highly likely that it is dimeric. A parallel-displaced geometry for Tz2− may upon photodetachment lead to a neutral excited state with a similar geometry and a strong excimer type character.