Solid State Molecular Device Based on a Rhenium(I) Polypyridyl Complex Immobilized on TiO2 Films

The photochemical and photophysical behaviors of fac-[Re(CO)(3)(phen)(trans-stpyCOOH)(+) (phen = 1,10-phenanthroline, trans-stpyCOOH = 4-[trans-(pyridin-4-yl-vinyl)]benzoic acid) in acetonitrile solution and adsorbed on a TiO2 film have been investigated. The trans-to-cis photoisomerization at 404 nm irradiation of coordinated stpyCOOH occurs efficiently in fluid solution as shown by quantum yield determined spectrophotometrically (Phi(UV-vis) = 0.37 +/- 0.04) and, more accurately, by H-1 NMR (Phi(NMR) = 0.48 +/- 0.04), following the photoproduct signals in the distinct region of the reactant. For the first time, the trans-to-cis isomerization is also reported for the complex adsorbed on the TiO2 surface (Phi(UV-vis) = 0.23 +/- 0.03). The photoproduct,fac[Re(CO)(3)(phen)(cis-stpyCOOH)](+), is emissive in acetonitrile (phi = 0.032), but its radiative decay is highly quenched on the oxide surface by electron photoinjection into the semiconductor, leading to an increasing photocurrent as the trans-to-cis isomerization takes place. Therefore, the photoinduced trans-to-cis isomerization of coordinated ligand immobilized on TiO2 films acts as a trigger for the electron injection process. This system exemplifies the use of photoinduced molecular motion to yield electrical current, which can be used as a "proof of concept" for molecular machines/devices.