Transformation of Alkatetrayne Monolayers into Nanoflatcables Studied by Ultraviolet Photoelectron Spectroscopy and Metastable Atom Electron Spectroscopy

Nanoflatcables (NFCs) are an array of all-trans pi-conjugated chains periodically bridged with and maintained at regular intervals by alkyl chains. The prototype NFC containing alternately arranged polydiacetylenes and poly acetylenes is constructed by the photopolymerization of alkatetrayne molecules physisorbed and laid flat on graphite. Ultraviolet photoelectron spectra (UPS) and metastable atom electron spectra (MAES) before and after polymerization are compared with the first-principles calculations to reveal transformation in the electronic structure caused by the drastic recombination of covalent bonds in the extrathin (0.4 nm) films. Unlike related films so far reported, minute features corresponding well to the calculated density of states are observed in the UPS owing to regularity in monomer arrangement and (polyacetylene pi) band and that of the HOMO - 1 (polydiacetylene NFC periodicity, and the top of the HOMO band are detected distinctly for NFC, giving a threshold ionization potential of 3.6 eV. Changes in the MAES are explained by orbital consanguinity, large dispersions for specific bands, V-shape valley effect for indiscernible NFC pi orbitals, and orbital interaction/mixing between the same and different type(s) of chains detectable in the deflated or inflated distributions of pseudo-pi orbitals. These findings enable us to make detailed band assignments in the whole C 2p-derived valence region.