Ultraviolet Photoelectron Spectroscopy and Metastable Atom Electron Spectroscopy of a Sashlike Polydiacetylene (Atomic Sash) Monolayer: Observation of the π Electronic Structures Peculiar to Two Major Conformers

Abstract

Atomic sash (AS) is a single sheet of a sashlike polydiacetylene (PD) made from conjugated alkadiyne molecules laid flat on a graphite (0001) surface. When they are irradiated with UV light, one of the major AS conformers, AS-I, with the PD chain held higher than the alkyl chains in contact with the substrate is formed first but transformed to another conformer AS-II with all of the carbon atoms placed in a common plane. Changes in the π electronic structures upon polymerization and structural transformation are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES), and the spectra are compared with the results of our preceding calculations. The π electronic structures peculiar to each conformer are successfully discriminated: UPS detects differences in the density of states, whereas MAES probes a drastic decrease in the local electron distribution of the π wave functions at the monolayer surface due to the foundering of the PD chain in the lying alkyl chains. The threshold ionization potential 5.0 eV obtained for the PD of the AS-I exposed outside the surface corresponds to the highest HOMO energy in an ideal isolated PD chain.