Controlled Grafting of Tetrathiafulvalene (TTF) Containing Diacetylenic Units on Hydrogen-Terminated Silicon Surfaces: From Redox-Active TTF Monolayer to Polymer Films

Abstract

A tetrathiafulvalene (TTF)-terminated butadiyne derivative was synthesized and used for the preparation of redox-active TTF-modified hydrogen-terminated oxide-free silicon (Si–H) surfaces. TTF monolayer-modified silicon surfaces were produced when low grafting temperatures were used (typically 45 °C), whereas higher temperatures (90 °C) led to TTF polymer-modified surfaces. IR spectroscopy characterization provided evidence that TTF units bound to the surface through the formation of enyne linkers via hydrosilylation of the terminal alkyne bond. The TTF monolayers prepared at 45 °C were densely packed with a surface coverage of ca. 5.4 × 10^–10 mol of TTF per cm². For such systems, electrochemical measurements showed the redox signature of the bound TTF centers characterized by two reversible one-electron systems at ca. 0.40 and 0.73 V versus saturated calomel electrode (SCE). High values of electron-transfer rate constants were determined (>200 s^–1) and ascribed to the presence of the conjugated bridge between the attached redox-active center and the underlying silicon surface. The TTF polymer-modified surfaces prepared at 90 °C resulted from the direct grafting of polymeric structures on Si–H and/or the postattachment functionalization of the preformed TTF monolayer. Polymerization process of the TTF-terminated butadiyne derivative was also investigated in solid state by means of differential scanning calorimetry and diffuse reflectance IR spectroscopy measurements