Effect of the bridge alkylene chain on adlayer structure and property of functional oligothiophenes studied with scanning tunneling microscopy and spectroscopy.

Five dual-quinquethiophene self-assemblies are prepared on a highly oriented pyrolytic graphite (HOPG) surface. The dual-quinquethiophenes are quinquethiophenes (5T)-di to hexamethylene (n, 2-6)-quinquethiophenes (5T), abbreviated as 5T-n-5T (n = 2-6). The effect of the bridge alkylene chains on the structure and property of the five assemblies are investigated by scanning tunneling microscopy (STM). It is found that all 5T-n-5T molecules form ordered adlayers on a HOPG surface with stripe feature. The alkylene bridge part in a molecule appears in a dark contrast in an STM image. Intriguingly, the thiophene backbones of individual molecules in the adlayer always keep an angle with the direction of molecular stripes. With alkylene bridge length increasing, different structures are found in 5T-5-5T and 5T-6-5T assemblies. To understand the effect of bridge chains on single molecular property, scanning tunneling spectroscopy is used to probe the electronic property of the different adlayers. The results will be important in surface engineering by self-assembly and molecular device fabrication with oligothiophenes.