In Situ STM Investigation of Aromatic Poly(azomethine) Arrays Constructed by “On-Site” Equilibrium Polymerization

Abstract

Two-dimensional (2D) arrays of π-conjugated aromatic polymers produced by surface-selective Schiff base coupling reactions between an aromatic diamine and an aromatic dialdehyde were investigated in detail using in situ scanning tunneling microscopy. Surface-selective coupling was achieved for almost all diamine/dialdehyde combinations attempted, although several combinations did not proceed even in homogeneous aqueous alkaline solution. Most of the combinations of an aromatic diamine and a dialdehyde, except the combinations of 4,4'-azodianiline with mono/bithiophenedicarboxaldehyde, formed highly ordered π-conjugated polymer arrays on an iodine-modified Au(111) surface in aqueous solution at a suitable pH. The simplest polymer of the various combinations tested, obtained from the combination of 1,4-diaminobenzene with terephthaldicarboxaldehyde, gave a 2D array consisting of linearly connected benzene units. Poly(azomethine) adlayers caused a positive shift in the electrochemical potential of the butterfly shaped oxidative adsorption and reductive desorption of iodine. The acceleration of the reductive desorption of iodine suggests the existence of a weak interaction between the polymer layer and iodine. Not only the first polymer adlayers but also partially adsorbed secondary adlayers with "on-top" epitaxial behavior were frequently observed for all polymer systems. The alignment of the polymer chains in the adlayers possessed a certain regularity in terms of a regular interval between polymer chains because of repulsive interpolymer interactions.