Observation of Vacuum Deposited Thin Films of Topochemically Photopolymerizable Conjugated Aromatic Compounds by Scanning Tunneling Microscopy

Abstract

A conjugated aromatic compound, 1, 4-bis[β-pyridyl-(2)-vinyl]benzene (P2VB), can be photopolymerized by UV irradiation (topochemical photopolymerization). This reaction is known to be sensitive to the features of the crystal structures of starting compounds. Ultra thin films of P2VB monomer deposited on highly oriented pyrolytic graphite (HOPG) and Au (111) substrates in high vacuum were studied at room temperature in air by scanning tunneling microscopy (STM). On the HOPG substrate, a spindle-like island structure and its molecular structure were observed. On the Au (111) substrate, the for-mation mechanism of the P2VB thin film was of the nucleation and growth type, in the case that the substrate temperature was kept at room temperature during deposition, but was of the monolayer overgrowth type with the substrate temperature up to 40°C. The molecular axis of the first monolayer was parallel to the step direction of Au substrate, indicating that the formation of monolayer is governed by the adsorption of these molecules to the step edge. In spite of the well-ordered structure, however, the obtained lattice constants did not satisfy the topochemical photopolymerization condition probably due to the strong interaction between the molecules and Au (111) reconstructed structure. The result obtained by “simultaneously UV-irradi-ated vacuum deposition” was also presented. In this case, we could observe a structure which may be assigned to poly-P2VB.