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

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.