Two-dimensional photopolymerization at adsorbed monolayers on gold

Abstract

Disulfide-bearing ammonium amphiphiles with different counter-anions, Cl^–(1Cl) styrene-p-sulfonate (1SS), and poly(styrene-p-sulfonate)(1PSS), and a photoinitiator bearing a dithiocarbonate group (2) have been prepared. They formed monolayers on gold by spontaneous adsorption from organic solutions. Surface properties of these monolayers have been estimated by contact-angle and electrochemical measurements. Both techniques showed that there are distinct differences in surface propertias among counter-anions. The surface free energy calculated from the results of water and methylene iodide contact angle decreased in the order: 1Cl > 1SS > 1PSS, dependent upon the hydrophobicity of the counter-anions. Reverse trends (1Cl < 1SS < 1PSS) were observed in the electrochemical barrier capability of the monolayers toward Fe(CN)₆^4–. Photopolymerizations of 1SS monolayers have been studied by cyclic voltammetry using Fe(CN)₆^4– as an electroactive species. The polymerization process could be traced on the basis of reduction of the redox peak currents. For the 1SS–2([2]:[1SS]= 0.5%) monolayer, polymerization proceeded smoothly upon uv irradiation, with completion of the polymerization after ca. 3 h. The pure 1SS monolayer (without 2) showed only a slight decrease in the redox peak current, indicating that effective polymerization did not occur.

Binary monolayers composed of 1SS and a fluoroalkanethiol (3) could also be formed on gold from solution, and their photopolymerization behaviour has also been studied. With decreasing 1SS content in the mixture, the redox peak currents increased with UV irradiation, in contrast to the results of the single-component (1SS) monolayer, suggesting formation of a boundary void between phase-separated domains of polymerized 1SS and 3.