The directed assembly of molecular building blocks into larger structures that remain organized at near-molecular scales is probably the single biggest barrier to developing nanotechnological systems. Approaches based on self-assembly probably hold the greatest promise in the short term, but modulating self-assembly processes to yield intricate 3D structures is a major challenge.
We have devised a simple system that suggests how light could be used to modulate and direct molecular assembly. Near-UV irradiation of an aqueous ethanolic solution of ferricenium cation ((C5H5)2Fe+, Fc+) containing suspended particles of a wide-bandgap semiconductor (e.g., TiO2) that has appropriate surface functionalization causes reduction of the Fc+ to ferrocene (dicyclopentadienyliron(II), Fc), with its concomittant absorption into the surface layer on the semiconductor. The change is striking visually; under illumination the blue ferricenium solution becomes colorless, while the white, suspended TiO2 particles become rusty brown due to immobilized Fc. Under appropriate conditions this system will spontaneously reverse in the dark from re-oxidation of the Fc to Fc+ by atmospheric O2.
The particles of TiO2 are rendered hydrophobic by reaction with octadecyltrichlorosilane (C18H37SiCl3, ODS), which coats the TiO2 with a hydrocarbon monolayer. Absorption of photons with energy greater than the TiO2 band gap (<~390 nm) generates electron-hole pairs. Charge separation occurs in the surface depletion layer, with the holes driven toward the surface. These holes are strong oxidizing agents and in aqueous solution will even oxidize water to hydrogen ions and O2:
(1) 2 h+ + H2O ==> 2 H+ + _ O2.
The electrons left behind by this reaction are moderately powerful reducing agents that can reduce solution species such as ferricenium:
(2) Fc+ (aq) + e- ==> Fc .
The ferrocene, being strongly hydrophobic, is extracted quantitatively into the hydrocarbon layer on the TiO2. After several hours in the dark under highly acidic conditions (pH < 1), the ferrocene will reoxidize and return to the solution as ferricenium.
This system exploits a wide disparity in polarity between the reduced and oxidized forms of a species to control its quantitative extraction out of solution, with the redox change mediated by a photoactive semiconductor. Although crude, it suggests how illumination could be used in molecular construction using redox-active species as building blocks. Moreover, functionalization of the redox species by appropriate side groups could result in cross-linking and covalent bond formation on precipitation. In addition, the photogenerated holes could conceivably also be exploited to drive molecular construction at the semiconductor surface, rather than be merely discarded into the solvent as in the present system.